Open access

Bulk Heterojunction Solar Cells — Opportunities and Challenges

Written By

Qun Ye and Jian Wei Xu

Submitted: 26 April 2014 Published: 22 October 2015

DOI: 10.5772/58924

From the Edited Volume

Solar Cells - New Approaches and Reviews

Edited by Leonid A. Kosyachenko

Chapter metrics overview

2,883 Chapter Downloads

View Full Metrics

1. Introduction

Due to the rising concerns over the exhaust of fossil fuel and the associated environmental consequence of the carbon emission problem, search for renewable energy has become a hot research topic worldwide. Organic semiconducting materials based photovoltaic (PV) technology developments have attracted tremendous attention from both the academic community and the industry. In principle, the organic solar cells employ organic material based light absorbing functional layer to convert sunlight to electricity. Typically the light absorbing layer is made of a blend of donor material and a fullerene based acceptor material. The observation of photovoltaic effect on organic materials began in 1986 with the “Tang cell” [1], which was a two-layer device with a structure of ITO/copper phthalocyanine/perylene diimide/gold. Later in the early 1990s, the discovery of ultrafast charge transfer from polymer to fullerene [2] initiated the research field of bulk hetero-junction (BHJ) solar cells. Over the past two decades, substantial research progress have been made in the development of more robust light harvesting materials, the further modification of the modelling theory of the OPV physics, better understanding and elucidation of the light-to-electricity process and the continuous optimization of the device fabrication process with new strategies employed. This process can be witnessed by the fast growing efficiency data of the OPV cells (Figure 1) and the vast amount of literatures published annually on the topic of OPV technology. Concurrently, the industrial attention is mainly focused on development of robust materials with long lifetime and good efficiency in large scale application, production technique optimization and market exploration for OPV technology. The fast growing research activity on OPV technology involves the collaborative consolidation of knowledge from synthetic chemistry, especially on π-chemistry, semiconductor physics and device engineering. Figure 1 [3] depicts the certified record PCE data of various types of PV technologies that have been continuously optimized in the past decade. Compared with inorganic PV technologies, organic solar cells have achieved magnificent improvement in terms of efficiency. Currently the record holder is Mitsubishi Chemicals who has demonstrated a reproducible 10.7 ± 0.3% organic solar cell.

Compared with the existing mature inorganic based photovoltaic technology, a list of proposed advantages of OPV technology should be mentioned which include 1) short energy payback time [4]; 2) lower production cost compared with inorganic PVs; 3) potential fabrication via continuous printing tools; 4) new market opportunities, such as flexible PV, wearable PV, semitransparent PV window, etc.; 5) low weight and easy integration of the organic PV products. Nevertheless, inorganic based PV technologies, such as silicon, cadmium telluride (CdTe), III-V group semiconductors and copper indium gallium selenide (CIGS), are still dominating the PV market.[5] The reasons why the OPV are short of the market are mainly due to the inferior power conversion efficiency (PCE) and the poor stability of the organic solar cell devices compared with their inorganic analogs. As the ultimate goal of materials research is to apply the material based science and technology into material based products, the applicability and the competitiveness of the technology should always be buried in mind. The economic aspects of the OPV technology have attracted more attention as many companies start to step into the OPV market and create opportunities. The optimization of the OPV device, both in terms of device stability and power conversion efficiency, has become a synergistic work between the academia and the industries. Moreover, the industrial production of OPV modules, which is very different from the lab-scale production step, is also being optimized. All these efforts will be highlight in this Chapter.

With both the advantages and existing disadvantages of OPV technology in mind, we plan to give an overview of how to transform a molecule to a material and finally to a product for OPV technology and we organize this Chapter in the following way. Firstly we will summarize the existing strategies to prepare new and better light harvesting materials by synthetic chemists. Due to the limited space, we focus on the most commonly used polymeric donor-acceptor (D-A) type materials. In the following part, we summarize the recently developed device engineering methods to improve the performance of the OPV materials. This part deserves its own merit because the OPV device engineering process is essential to demonstrate the full potential of a new polymer molecule as a functional light harvesting material. Then we will summarize some aspects of OPV materials which are important for the production development, such as the lifetime/stability of the material and production techniques. Finally we will end up our discussion with a summary and perspective on the future research. There have been many excellent summary works dealing with various aspects of OPV technology, such as the working mechanism and physics of the OPV device [6-9], design principles and synthesis of new light absorbing materials [9-19], thin film morphology control and characterization [20-21], new device architecture development [22-24], interface engineering [25], quantum chemical calculations [26], economical aspects [27-29] and many insightful overviews and perspectives [30-42]. There are also a series of Photovoltaics Literature Survey papers by Santosh Shrestha [43] in Progress in Photovoltaics: Research and Applications which are useful for readers to catch up current research progress in various aspects regarding PV technology.

Figure 1.

Best Research Cell Efficiencies for all types of PV technology. Data from National Renewable Energy Laboratory (NREL).[3]


2. Development of new materials

2.1. Chromophores with new π-structure

In the concept of BHJ solar cells, the active layer is comprised of a blend of electron-donating material and electron-withdrawing material. The electron-donating material can be small molecules [12, 13] or polymeric materials [10,11,14-19] while polymeric materials are more commonly used in the literature. Both types of materials typically follow a Donor-Acceptor (D-A) design principle, in which the conjugated backbone is constructed by covalent linkage of a series of electron rich moieties and electron deficient moieties in an alternative way. This design strategy is especially useful in tuning the physical properties (absorption, frontier orbital energy levels, etc.) of the final materials due to the vast stock of electron rich and electron deficient building blocks. Guo and co-workers [44] have presented a thorough summary of current prevailing donor and acceptor species, which include about 45 donor and 60 acceptor backbones. Note that there are also variations on the solubilizing chains and spacer groups. Hence the actual number of such building blocks would be much larger than the summarized numbers. Given the wide choice of building blocks, there is no surprise that a huge structural diversity of OPV materials exists and a large amount of new materials are coming out every year in the literature. The design principle of the donor material in the BHJ blend has been summarized [45]. Basic considerations include light absorption range, frontier orbital energy levels, charge carrier mobility, favorable blend morphology, stability and solubility.

Concurrently new building blocks are being synthesized to provide more possibilities to further optimize the optical and electronic properties of the final material. Take benzothiadiazole (BT) as an example (Figure 2). Benzothiadiazole (BT) is one of the most commonly used building blocks for the construction of D-A type conjugated polymers. By carrying out structural modification of the backbone, a series of new electron-deficient moieties with different electronic properties can be prepared. One strategy is to replace the sulfur atom in the thiadiazole hetero-cycle with other elements such as carbon [46], oxygen [47,48], nitrogen [49], selenium [50,51], etc. The new building blocks have various electron withdrawing properties and hence are useful in tuning the properties of the polymer materials. The second strategy is to introduce substitutions on the BT unit, such as fluorine atoms [52-60], alkoxy groups [61-65], or replacement of C-H with imine nitrogen [66-69]. More building blocks can be prepared with the combination of these two strategies [70-75]. The third strategy is to extend the π-conjugation of the BT unit to prepare π-extended moieties [76-81]. Extension of the π-backbone is a versatile route to tune the electronic properties of the BT unit; however, the new building blocks typically exhibit poorer solubility and have to be prepared in longer synthetic steps. For example, by fusion of one more thiadiazole ring to the BT unit, bis-benzothiadiazole (BBT) can be prepared and possess much higher electron deficiency but poorer solubility.

Figure 2.

Evolution of benzothiadiazole (BT) based electron withdrawing moieties for the preparation of D-A type light absorbing polymers.

Out of all these strategies, the substitution of fluorine atoms on the aromatic backbone turns out to be a very efficient approach to achieve high-performance OPV materials. Introduction of fluorine substitutions has minor influence on the absorption behavior of the polymer; however, it induces a decrease of the frontier orbital energy levels due to its strong electronegativity and consequently the fluorinated polymers typically exhibit higher open circuit voltage (Voc). The advantages of fluorine in OPV polymer have been demonstrated by Zhou et al. [82] Polymer PBnDT-DTBT (Figure 3) exhibits a HOMO and LUMO energy level at-5.20 eV and-2.92 eV, respectively. After addition of two fluorine atoms on the BT unit, the HOMO and LUMO energy level of PBnDT-DTffBT decreases to-5.30 eV and-2.97 eV, respectively. PBnDT-DTBT/PC61BM based solar cell device exhibits the best PCE=5.0% with Jsc=10.03 mA cm-2, Voc=0.87 V and FF=0.57. For PBnDT-DTffBT, the best device exhibits PCE=7.2% with Jsc=12.91 mA cm-2, Voc=0.91 V and FF=0.61. It is found that after attachment of the fluorine atoms on the repeating unit, the short circuit current, the open circuit voltage and the factor are all enhanced.

Figure 3.

Chemical structures of PBnDT-DTBT and PBnDT-DTffBT.

The longer synthesis steps for the more complicated building blocks are also a concern if they are to attract industrial attention. Although achieving materials in a cost-effective way is generally neglected in the academic publication and in many cases the complicated synthesis of a monomer is considered as novelty of the work, a simple, high yield and easily scalable synthesis of materials is highly desirable in the industry from the application point of view. For example, 4,7-dibromobenzo[c][1,2,5]thiadiazole can be prepared in three steps from 1,2-phenylenediamine (Scheme 1). As this building block is so commonly used nowadays, it has become commercially available by vendors such as Sigma Aldrich. While for alkylated 4,8-dibromo-[1,2,5]thiadiazole[3,4-f]benzotriazole [83], which is a BT unit fused with a triazole hetero-cycle, is prepared with four more synthetic steps from 4,7-dibromobenzo[c][1,2,5]thiadiazole. According to a recent estimation [84], the cost per gram of the final material increases linearly with the number of synthetic steps needed for the synthesis. The extended synthesis would definitely reduce the potential applicability of the material, as the value of a material is a compromise between the performance and the cost.

Sheme 1.

Synthetic routes of 4,7-dibromobenzo[c][1,2,5]thiadiazole and alkylated 4,8-dibromo-[1,2,5]thiadiazole[3,4-f]benzotriazole.

A list of D-A type polymers that have demonstrated PCE values > 7% are shown in Figure 4 [85]. It should be highlighted that the high performance of the polymers does not necessarily mean that the embedded building blocks are superior. The power conversion efficiency is determined by a number of factors and the chemical structure of the polymer is just one of them. Even for a classical polymer P3HT, after careful optimization of the device condition, the PCE can also reach 7.4% [86]. Many other factors, like the fabrication conditions, also play a significant role in determining the overall efficiency of the cell. These factors will be discussed in the following text.

Figure 4.

Chemical structures of polymers that exhibit PCE > 7%.

2.2. Side chain engineering

Side chains are attached on the rigid aromatic π-backbones to form “hairy rod” type polymers with suitable solubility to allow solution based processing techniques viable. In fact, the role of the side chains are far beyond the solubility concerns. Other physical properties of the polymer, such as absorption, emission, energy levels, molecular packing, charge transport and the morphology of the thin film are critically affected by the side chains attached in many cases. Commonly used side chains include linear alkyl side chains (n-CnH2n+1), branched alkyl side chains, electron donating side chains (-OR,-SR,-NHR, etc.), electron withdrawing side chains (-C(=O)R,-SO2R, etc.), aromatic side chains (4-alkoxyphenyl, etc.), functional side chains (e.g. with cleavable groups on the side chains), ionic side chains, oligoether side chains, fluorinated side chains and so on. A comprehensive discussion on various types of side chains has been presented by Mei and Bao [87]. Side chain engineering has become a routinely used strategy to modify the physical properties, especially the self-assembly of the materials in the thin film, and thus to optimize the light absorbing materials in the OPV device. Given the importance of side chain engineering, it should be noted that as the side chains do not contribute to the light absorption or charge transport in the thin films, a trade-off between the solubility and the performance of the final polymer must be made.

2.3. New synthetic methodology

Currently, the D-A type polymers are typically synthesized via palladium catalyzed cross coupling reactions such as Stille coupling [88] and Suzuki coupling reactions [89]. Stille coupling involves C-C bond formation between trialkylstannyl species and aromatic halide species and has been routinely used for the preparation of a large number of high performance polymers. However, the high toxicity of the tin reagent and the associated environmental issue of the generated tin wastes inhibit its wide industrial applications. Recently, a new polymerization method involving direct heteroarylation polymerization (DHAP) between aryl C-H bond and aromatic halides has been developed as a promising greener alternative of Stille coupling for the preparation of conjugated polymers (Scheme 2). Berrouard et al. [90] has demonstrated that the DHAP reaction between 5-alkyl[3,4-c]thienopyrrole-4,6-dione and 5,5’-dibromo-4,4’-dioctyl-2,2’-bithiophene is as efficient as the corresponding Stille approach. As in this direct coupling reaction no organo-tin or organo boron reagents are needed, it shortens the synthesis of final polymer by at least two steps. This strategy has been successfully implemented for the synthesis of OPV polymers [91,92], OFET polymers [92] and EC polymers [93] with reasonable molecular weight and polydispersity after judicious optimization of the coupling condition. Nevertheless, as this polymerization technique is still in its infancy, the reaction is still difficult to control for some substrates and the final polymer might be branched due to unselective C-H activation in the substrate [94,95]. The reaction conditions of the reaction including the catalyst, ligand, base, additive, solvent, temperature and duration have to be carefully controlled and optimized in order to achieve the highest molecular weight.

Sheme 2.

Synthetic approaches of direct heteroarylation polymerization (DHAP) and conventional Stille coupling reaction.

2.4. Molecular weight and purity of the polymer

The molecular weight and the purity of the polymers are issues beyond the molecular architecture of the semiconducting polymers. But both factors have been demonstrated as essential parameters to ensure the good performance of the prepared polymers within the device. A high molecular weight increases the regularity of thin film and in many cases induces enhanced charge carrier transport in the transistor device [96,97] and power conversion efficiency in the BHJ solar cell device [98]. For instance, P1 (Figure 5) [99,100] with a low molecular weight (Mn < 10 kg mol-1) exhibits a charge carrier mobility of μ=5.2 × 10-5 cm2 V-1 s-1 and power conversion efficiency of η=2.7% with Jsc=4.2 mA cm-2, Voc=0.64 V, and FF=0.35. For P1 with high molecular weight (Mn > 34 kg mol-1), it exhibits an enhanced mobility of μ=3.6 × 10-2 cm2 V-1 s-1 and power conversion efficiency of η=5.9% with Jsc=17.3 mA cm-2, Voc=0.57 V, and FF=0.61. Similar phenomenon is also observed for P2 [98]. P2 with a low molecular weight (Mn ~ 46 kg mol-1) exhibits an ambipolar behavior with μh=2 × 10-3 cm2 V-1 s-1 and μe=5.2 × 10-5 cm2 V-1 s-1 and a PCE η=5.48% with Jsc=12.1 mA cm-2, Voc=0.90 V, and FF=0.50. For P2 with high molecular weight (Mn ~ 61.8 kg mol-1), the mobility increases to μh=0.15 cm2 V-1 s-1 and μe=0.064 cm2 V-1 s-1 and an enhanced PCE η=6.79% with Jsc=13.7 mA cm-2, Voc=0.89 V, and FF=0.56. The improved mobility for high molecular weight samples is ascribed to improved π-π stacking, thin-film formation properties and increased inter-chain interactions. The increased Jsc and fill factor are mainly because of the improved hole mobility of the polymer, which facilitates the charge collection and inhibit charge recombination in the blend.

The purity [101-103] and the end group effect [104-106] on the performance of transistor materials and OPV materials have also been investigated. However, as the exact determination of “contaminant” or “purity level” of a given material, especially for polymers, is very difficult to achieve, the attempts to correlate the performance of an “impure” material to the existence of some extrinsic impurity would be questionable. Even though the end capping strategy has been found efficient to improve the performance of the polymer [104-106], it is still not commonly adopted by research groups, even not routinely used by the groups who claimed the positive effect. Questions such as how the end group influences the performance of the polymer, what kinds of impurities are detrimental to the performance and what kinds of impurities serve as friendly dopants still remain unaddressed. More research effort, for example, intentional doping [107,108], is in need to solve the impurity issue of organic semiconductors in both the theoretical aspect and the practical aspect. But it is commonly believed that tedious and labor-intensive purification processes, such as Soxhlet extraction and silica gel column chromatography is always necessary to ensure sufficient purity of the sample for characterization.

Figure 5.

Chemical structures of P1 and P2.

2.5. Acceptor

The other important active species in the BHJ blend is the acceptor. The benchmark acceptors are fullerene based derivatives, mainly [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) (Figure 6) [109,110]. The key features of these sphere-shaped acceptors are their low internal reorganization energy, high polarizability, relatively high dielectric constant, favorable LUMO energy level, reversible redox properties, good electron transport properties and anisotropic charge transport behavior [109,110]. The superior performance of these two acceptors in the BHJ devices renders them as the first choice for most of the newly developed donor materials. Whereas PC61BM absorbs minimal amount of light in the visible region, PC71BM is strongly blue and green light absorbing acceptor and is more useful when the absorption of it is complementary to that of the donor so that more sunlight can be captured [111]. Nevertheless, some drawbacks of these fullerene derivatives would hamper their wide application in the industrial production. One is the high production energy cost of these fullerene based acceptors. For PC71BM, the production energy is approximately 90 GJ kg-1 [112]. For comparison, the production energy of P3HT is only about 1.9 GJ kg-1[113]. The other concern is the relative high price of these fullerene derivatives. A recent analysis by Lewis and Nocera [114] indicates that the OPV system should cost no more than $10 per m2 to compete with fossil fuels for energy production. The cost of PCBM at roughly $500-1000 per m2 [42] makes the BHJ based PV technology with PCBM problematic for commercialization. Use of technical grade PCBM (~80% PC61BM and 20% PC71BM) [115] might help relieve the stress but is far away from the desired price range. In fact, various types of small molecule based [116] or polymer based [117] acceptors have been tested to replace fullerene derivatives in OPV cells. But yet the efficiency of these acceptors still cannot surpass that of fullerene based derivatives.

Figure 6.

Chemical structures of PC61BM and PC71BM.


3. Morphology and device engineering

3.1. Characterization of morphology

Along with the research effort to prepare D-A type conjugated polymers in BHJ solar cells to achieve the world record efficiency value, studies revealing the importance of the morphology of the polymer/fullerene blend have been carried out and the experience gained on controlling the morphology has become a valuable tool to explore the full potential of a new polymer as light harvesting materials. The thin film morphology characterization tools include grazing incidence wide-angle X-ray scattering, grazing incidence small angle X-ray scattering, resonant soft X-ray scattering, small-angle neutron scattering, transmission electron microscopy, atomic force microscopy, solid-state nuclear magnetic resonance, dynamic secondary ion mass spectrometry, near-edge X-ray absorption fine structure and scanning transmission X-ray microscopy. These analysis techniques are comprehensively summarized by Huang et al. [21]. As so much work has been done to investigate the morphology of the thin film, a rational question to ask is: what is the best morphology? Unfortunately, so far a precise answer to this question has not been achieved. One reason is because every characterization technique only sees the film from one aspect and a thorough mapping of the material distribution in the film still remains a challenge [118-119]. Another reason may be due to the fact that the reported polymers with the highest power conversion efficiency values do not really share exactly the same morphology profile. As a result, the optimal morphology and the engineering method to achieve the best performance are case-by-case and mostly obtained in a trial-and-error approach. General descriptions like homogeneous and interpenetrating networks with nanoscale phase-separated domains are routinely used to describe the morphology in the cells with distinct performance.

3.2. Morphology optimization by device engineering

A series of parameters that will influence the morphology of the polymer/fullerene thin film are listed in Figure 7. The physical properties of the polymer such as the π-backbone, side-chains, the molecular weight, the identity of the fullerene acceptors used and the mass ratio between the two etc., are factors related to the materials. The determination of the ideal D:A ratio for a new polymer material has been a matter of trial and error, with the ratio 1:1 to 1:4 most commonly used. The solubility of the polymer and the fullerene derivative should be sufficient in the processing solvent. Halogenated solvents (chloroform, chlorobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene) are routinely used as they possess high solubility for both the donor polymer and the fullerene acceptors. For many reported polymers [20,21], judicious choice of the processing solvent has to be made in order to achieve the optimal morphology and power conversion efficiency. For a specific new polymer, or a new polymer/fullerene blend, the choice of optimal processing solvent is not trivial, normally based on trial-and-error investigations. Recently, more research has been focused on the replacement of halogenated processing solvents with more environmentally friendly solvents, such as toluene, xylenes and long alkanes [120]. This is especially important for the industrial production of OPV modules [121].

Figure 7.

Summary of parameters that influence the morphology of the BHJ active thin film.

Processing additives with low vapor pressure and high boiling point are commonly added in the solvent to optimize the morphology of the polymer/fullerene blend [85]. Commonly used additives include 1,8-diiodooctane, 1,8-octanedithiol and 1-chloronaphthalene (Figure 8). General guideline for selection of the additive is that the solvent additive should be less volatile with higher boiling points than the host solvents. The mechanism how the additive influence the morphology and the overall performance of the device has been discussed by Lee et al. [122]. It should be noted that solvent additives added during the fabrication process might remain in the solar cell and behave as contaminants to hamper the performance of the solar cell. In some case, addition of additives shows no effect [123,124] while in some case the addition of additives is detrimental to the performance [125]. These results indicate that the solvent additive is not an elixir to enhance the PCE performance of all polymers and the detailed effect and mechanism should be made case by case. Moreover, a specific process step must be added to remove the residual deleterious compounds which are obviously unfavorable in the industrial application. Hence, it is more desirable to design high performance polymer/fullerene system with no such additives needed to achieve the good performance. During the production of OPV modules, some other additives, such as rheology modifiers, anti-foaming agents and surface tension modifiers would be added into the ink formulation to make it more suitable for printing technology. The effect of these additives on the solar cell performance has not yet been well studied. There are research attempts to incorporate various non-solvent additives, such as nanoparticles [126], carbon nanotubes [127], small molecules [128] and polymers [129], to create a ternary blend BHJ solar cell. These strategies have demonstrated how a third component in the polymer/fullerene blend influences the overall morphology and performance of the solar cell, which could be used as a potent routine method to enhance the performance of the device [130].

Figure 8.

Chemical structures of solvent additives for BHJ solar cells.

Thermal annealing is an alternative method for controlling the BHJ morphology and improving the PCEs. By applying thermal energy to the thin film, it helps the reorganization of the polymer/fullerene blends and increases the crystallinity of the film [131]. This processing method has been routinely used to optimize the thin films for transistors [132] and BHJ solar cells [20,21]. The annealing temperature, the annealing duration and the cooling rate are key parameters to optimize the performance of the device and the optimal combination of the thermal treatment is material-dependent. Even for the same material, e.g., P3HT/PCBM blend, the optimal annealing condition differs from lab to lab [20], presumably due to different device structure, different solvent, different purity, different molecular weight, or even different operator. So far there is no general guideline to predict the optimal condition for a new OPV material and the optimal condition is obtained via a tedious trial-and-error approach.

Solvent annealing refers to the treatment of the BHJ thin film with solvent vapor, typically in a petri dish contained with the solvent. Parameters to optimize include the solvent type and the treatment duration. By exposing the coated thin film with solvent vapors, there is reorganization and further morphological evolution over time. This method has been demonstrated to enhance the morphology, the hole mobility of P3HT [133] and the PCE performance of the P3HT/PCBM solar cell [134]. Treatment of the thin film with polar solvents, such as methanol and ethanol, is also found to improve the morphology and render higher PCE values for P3HT/PCBM [135].

In summary, the morphology of the active polymer/fullerene blend in the BHJ solar cell can be tuned by a number of factors, such as the materials, choice of solvent, solvent additives, annealing condition, etc. The optimal condition to achieve the best device performance is typically material dependent and achieved in a trial-and-error approach. Sometimes, a minor modification of the processing condition can introduce magnificent enhancement of the device performance, for example, addition of processing additives. The complexity of the morphology control and the tedious optimization process would account for the phenomenon that why so many promising polymer materials in the literature with suitable absorption, energy level and solubility possess inferior device performance. Even for the same polymer motif, the OPV performance would vary significantly by different processing methods [136]. This again highlights the importance of device engineering work to explore the full potential of a new polymer in the BHJ solar cell.


4. Industrial concerns

As the ultimate goal of any material related research and technology development is to apply the material and to fulfill the promises of the material, such stress on the large scale manufacturing and product development has also been witnessed for OPV technology. With the fast performance improvement of OPV cells in research labs, the application of OPV technology as a renewable energy source has become more appealing. However, it is never trivial to translate a lab-based technology into a large volume production process. A large number of difficulties and problems have to be overcome to ensure the successful commercialization of the technology. In this session, we will highlight some aspects related to the industrialization of OPV technology.

4.1. The stability and the lifetime issue

The stability issue of the light harvesting material in the solar cell device should be brought into discussion as the OPV technology is aimed to generate electricity from sunlight for a long period of service time. The materials used to construct the OPV module, which include the active layer, the electrode materials and the encapsulating materials, should be robust under the outdoor condition and the performance of the OPV module should be maintained to ensure the power generation efficiency of the technology. The currently known degradation mechanisms of the solar cell device, including morphology degradation, photo-oxidation, interface degradation, physical and mechanical degradation, have been well discussed in a number of review articles [137-139]. It should be highlighted that the active layers and the metal electrode materials are especially prone to degrade upon contact with water and oxygen. Therefore, in real practice the encapsulation of the device is mandatory to guarantee the long-term stability of the device. The water and oxygen transmission properties of the encapsulant materials are thus essential to ensure the stability of the OPV module [29]. The growing concerns over the stability issue on OPV technology and fulfillment of the promise of OPV as a renewable energy technology has initiated the “International Summits on Organic Photovoltaic Stability” (ISOS) [140] to stimulate the research effort to address these issues.

Figure 9.

Typical decay curve of a polymer solar cell employing a standard device architecture. The lifetime is defined by the point at which the efficiency has dropped by 20% from the start of the linear decay period. [141] Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The degradation profile of an OPV module typically follows a trend shown in Figure 9 [141]. The device suffers a burn-in degradation process at the early working life which is characterized by an exponential loss in efficiency and then a linear decay process. The lifetime of the device is defined as the time at which the efficiency drops to 80% of the efficiency after burn-in process. By appropriate encapsulation, solar cell devices based on poly[9’-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-thienyl-2’,1’,3’-benzothiadiazole)] and PC71BM blend have demonstrated a lifetime of ca. six years [141]. This lifetime is marvelous in reported lifetime of OPV solar cells [142]. However, such lifetime is still considered insufficient if OPV technology is aimed to compete with the mainstream Si-based PV technology. A silicon based solar cells typically lasts on the order of 25 years and much higher PCE (Figure 1). In this regard, there is still a huge space for OPV technology to improve in order to survive in the PV market.

4.2. Processing technique

As one of the potential advantages, solution based processing methods, such as roll-to-roll printing and ink-jet printing, are suitable for organic solar cell materials [143-145]. So far in the lab scale, the BHJ solar cells are typically fabricated by spin-coating method. As spin-coating turns out to be suitable for the reproducible formation of homogeneous thin films, it is difficult to scale up and a large amount of inks is wasted during the spin-coating process. Wet-printing with a roll to roll production process is a more favorable processing technique for large scale OPV module fabrication. These printing techniques include gravure printing, flexographic printing, screen printing, rotary screen-printing, knife coating, slot die coating, and so on. More details regarding these printing techniques can be found in [144, 145]. Demonstration of large scale printing of OPV modules has been done by Krebs et al. [146] and currently OPV based solar parks have been established to explore the potential this new energy technology. One issue related to the printing technology is the uniformity and reproducibility of the final OPV module since the OPV performance of BHJ blend is very sensitive to the morphology of the thin film whereas the morphology is very sensitive to the processing condition. The materials used and the processing technique should be able to provide an OPV module with lifetime > 10 years with an average power conversion efficiency > 10% to compete with the mature inorganic PV products in the mainstream market. Even though the current start-of-the-art efficiency can reach >10% for small devices in the research lab, the efficiencies of the large area devices by solution processing methods are still low (< 3.5%) [144]. Further optimization of both the materials and the processing methods is urgently needed to fulfill all the advantageous claims of OPV technology.

4.3. The economic potential of OPV technology

Figure 10.

Top: A plot of the power conversion efficiency versus the active area. Bottom: The PCE values obtained versus the publication year. [30] Copyright Elsevier 2013.

One concern regarding the OPV technology is that how cheap the electricity generated by this renewable technology can be. More anxiety appears after the business failures of endeavors in the OPV market [147,148]. As a matter of fact, in the past years only a number of companies (Solarmer Energy Inc., Ossila, DyeSol, Heliatek, G24 Power, Eight19, Mitsubishi, Plextronics, Sharp Solar, Solaronix, SolarPrint, etc.) have ventured into the OPV technology related business, and are struggling to survive in the market. A number of articles have addressed the economic potential of OPV technology to estimate the cost of the OPV electricity [4, 27-29, 149-150]. One key factor that dominates the cost is definitely the power conversion efficiency. Figure 10 shows the PCE values of the reported OPV devices vs. the area (top) and the year (bottom) by Krebs et al.[30] It is found that most of the highly promising efficiency data are only achievable with a device area < 1 cm2. Furthermore, even though there are promising efficiency data (~7-10%) of some hero polymers, the majority of the research work exhibits a power conversion efficiency of less than 3%. As the area of the solar cell device increases, the efficiency is expected to be lower. With the current efficiency number for large scale OPV modules (< 3.5%), the applicability as mainstream power generation technology is rather dim. Some potential market and niche products for OPV technology include portable, low weight charger for electronics, PV covered uniforms, backpacks and tents for military usage and OPV integrated windows and walls. One intriguing idea about OPV usage is to serve as top cell in a tandem device with an inorganic bottom cell [151]. The tandem cell design strategy for all organic based materials has been proven efficient to improve the power conversion efficiency [22,24]. Key to the success of this tandem organic/inorganic strategy is the development of OPV modules with comparable lifetimes so that the technology can be used in a time of 5-10 years range. Another aspect is the energy pay-back time. For crystalline silicone PV technology, the energy pay-back time is estimated to be 4.12-2.38 year while for OPV it is about 2.02 to 0.79 year [28]. An even more optimistic estimation of the energy pay-back time of OPV technology is only 1 day [4]. As currently all promising data about OPV technology come from research labs and theoretical work, it is still difficult to conclude on the future and fate of OPV technology. Efficiency and stability are two major obstacles, but may also become opportunities for new business players.


5. Summary and outlook

The concept of bulk hetero junction solar cells has been continuously developed over the past two decades. Enormous achievements have been witnessed over the journey and currently the record efficiency of BHJ solar cells has reached over 10% (Figure 1). New materials, especially the donor materials in the blend, have been developed in an expanding rate, with new design strategies, new building blocks and new polymerization methods at the same time. For the acceptor part, fullerene based derivatives, PC61BM and PC71BM, are still the first choice for researchers. As the energy conversion process involves charge transfer over the donor/acceptor interface, the morphology of the donor/acceptor is therefore essential for an efficient power conversion process. The morphology of the thin film, however, is very sensitive to the processing conditions, such as the materials used, solvent, solvent additives, annealing, spin-coating conditions, etc. A tedious but worthwhile optimization process of all these parameters has to be carried out to explore the full potential of any newly synthesized polymer donor material or any new acceptor material. So far, the choice of the best condition is still based on a trial-and-error approach. Furthermore, problems arise as the OPV technology is translated from the lab-scale to industrial scale, e.g., how to achieve the optimal morphology of a cm2 device in the industrial scale, how to optimally process the OPV module, and how to improve the device stability by suitable encapsulation. The solutions to these questions are by no means trivial. Most probably a rediscovery process has to be carried out to optimize all the parameters associated with the industrial scale production.

As the OPV technology has gradually become business relevant and quite a number of companies are currently active in the OPV market to cash the promises of OPV researchers, more creative breakthroughs are in urgent need to solve the intrinsic efficiency and stability issues of current OPV technology. Other than further development of more efficient light harvesting materials, some new concepts such as ternary solar cell [23, 130], and modification of the solar cell structures [22,24], e.g., inverted solar cells, tandem solar cells, or tandem organic/inorganic solar cells would pave new ways to improve the efficiency of the solar cell. Further development of encapsulant materials with lower water/oxygen transmission rate would help the solar cells survive longer under ambient conditions [29]. The continuous optimization effort on the industrial roll-to-roll printing techniques would help minimize the gap between the best efficiency data from lab devices and the large scale OPV modules. These developments have to be fast, as tremendous work is spent to optimize current inorganic PV technology as well (Figure 1). Furthermore, BHJ based solar cells also have to compete with other organic material based PV technology, such as dye-sensitized solar cells [152] and perovskite solar cells [153]. There are also issues regarding the marketing of OPV based technology. As it is envisaged that OPV will not be able to compete with inorganic PV technology in the mainstream energy production market in the coming 5 to 10 years [28], niche markets, such as portable electronics chargers, flexible PV and wearable PV, are therefore sought in the short term. The light weight and the flexibility of OPV technology would become advantageous to survive in the market.

To end up our discussion, we will emphasize the nature of OPV research and related materials development. Any science and technology development, if it is aimed at large scale application, it should be robust, reproducible, affordable and efficient in its claimed function. The materials used in the device should be accessed in an easy and cheap way and the production process should be cost-effective. And more importantly, the commercial products should have attractive features to survive in the market. There are still a lot of obstacles for OPV researchers to conquer, but more opportunities in the future.



The authors would like to acknowledge the financial support from the Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR).


  1. 1. Tang CW. Two-layer organic photovoltaic cell. Applied Physics Letters 1986; 48 (2) 183-185.
  2. 2. Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995; 270 (5243) 1789-1791.
  3. 3. NREL efficiency chart. (accessed 01 July 2014).
  4. 4. Espinosa N, Hösel M, Angmo D, Krebs FC. Solar cells with one-day energy payback for the factories of the future. Energy &Environmental Science 2012; 5 (1) 5117-5132.
  5. 5. Green MA, Emery K, Hishikawa Y, Warta W, Dunlops ED. Solar cell efficiency tables (version 44). Progress in Photovoltaics: Research and Applications 2014; 22 (7) 701-710.
  6. 6. Dou L, You J, Hong Z, Xu Z, Li G, Street RA, Yang Y. 25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research. Advanced Materials 2013; 25 (46) 6642-6671.
  7. 7. Schlenker CW, Thompson ME. The molecular nature of photovoltage losses in organic solar cells. Chemical Communications 2011; 47 (13) 3702-3716.
  8. 8. Proctor CM, Kuik M, Nguyen TQ. Charge carrier recombination in organic solar cells. Progress in Polymer Science 2013; 38 (12) 1941-1960.
  9. 9. Clarke TM, Durrant JR. Charge Photogeneration in Organic Solar Cells. Chemical Reviews 2010; 110 (11) 6736-6767.
  10. 10. Bian L, Zhu E, Tang J, Tang W, Zhang F. Recent progress in the design of narrow bandgap conjugated polymers for high-efficiency organic solar cells. Progress in Polymer Science 2012; 37 (9) 1292-1331.
  11. 11. Rasmussen SC, Evenson SJ. Dithieno[3,2-b:2’,3’-d]pyrrolo-based materials: Synthesis and application to organic electronics. Progress in Polymer Science 2013; 38 (12) 1773-1804.
  12. 12. Coughlin JE, Henson ZB, Welch GC, Bazan GC. Design and Synthesis of Molecular Donors for Solution-Processed High-Efficiency Organic Solar Cells. Accounts of Chemical Research 2014; 47 (1) 257-270.
  13. 13. Chen Y, Wan X, Long G. High Performance Photovoltaic Applications Using Solution-Processed Small Molecules. Accounts of Chemical Research 2013; 46 (11) 2645-2655.
  14. 14. Boudreault PLT, Najari A, Leclerc M. Processable Low-Bandgap Polymers for Photovoltaic Applications. Chemistry of Materials 2011; 23 (3) 456-469.
  15. 15. Cheng YJ, Yang SH, Hsu CS. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chemical Reviews 2009; 109 (11) 5868-5923.
  16. 16. Li Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Accounts of Chemical Research 2012; 45 (5) 723-733.
  17. 17. Duan C, Huang F, Cao Y. Recent development of push-pull conjugated polymers for bulk-heterojunction photovoltaics: rational design and fine tailoring of molecular structures. Journal of Materials Chemistry 2012; 22 (21) 10416-10434.
  18. 18. Son HJ, He F, Carsten B, Yu L. Are we there yet? Design of better conjugated polymers for polymer solar cells. Journal of Materials Chemistry 2011; 21 (47) 18934-18945.
  19. 19. Kularatne RS, Magurudeniya HD, Sista P, Biewer MC, Stefan MC. Donor-Acceptor Semiconducting Polymers for Organic Solar Cells. Journal of Polymer Science Part A: Polymer Chemistry 2013; 51 (4) 743-768.
  20. 20. Dang MT, Hirsch L, Wantz G, Wuest JD. Controlling the Morphology and Performance of Bulk Heterojunctions in Solar Cells. Lessons Learned from the Benchmark Poly(3-hexylthiophene):[6,6]-Phenyl-C61-Butyric Acid Methyl Ester System. Chemical Reviews 2013; 113 (5) 3734-3765.
  21. 21. Huang Y, Kramer EJ, Heeger AJ, Bazan GC. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chemical Reviews 2014 DOI: 10.1021/cr400353v.
  22. 22. Sista S, Hong Z, Chen LM, Yang Y. Tandem polymer photovoltaic cells-current status, challenges and future outlook. Energy & Environmental Science 2011; 4 (5) 1606-1620.
  23. 23. Ameri T, Khoram P, Min J, Brabec CJ. Organic Ternary Solar Cell: A Review. Advanced Materials 2013; 25 (31) 4245-4266.
  24. 24. You J, Dou L, Hong Z, Li G, Yang Y. Recent trends in polymer tandem solar cells research. Progress in Polymer Science 2013; 38 (12) 1909-1928.
  25. 25. Po R, Carbonera C, Bernardi A, Camaioni N. The role of buffer layers in polymer solar cells. Energy & Environmental Science 2011; 4 285-310.
  26. 26. Risko C, McGehee MD, Brédas JL. A quantum-chemical perspective into low optical-gap polymers for highly-efficient organic solar cells. Chemical Science 2011; 2 (7) 1200-1218.
  27. 27. Dennler G, Scharber MC, Brabec CJ. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Advanced Materials 2009; 21 (34) 1323-1338.
  28. 28. Darling SB, You F. The case for organic photovoltaics. RSC Advances 2013; 3 (39) 17633-17648.
  29. 29. Po R, Bernardi A, Calabrese A, Carbonera C, Corso G, Pellegrino A. From lab to lab: how must the polymer solar cell materials design change? – an industrial perspective. Energy & Environmental Science 2014; 7 (3) 925-943.
  30. 30. Jørgensen M, et al. The state of organic solar cells – A meta analysis. Solar Energy Materials & Solar Cells 2013; 119 (1) 84-93.
  31. 31. Dennler G and co-signatories. The value of values. Materials Today 2007; 10 (11) 56.
  32. 32. Krebs FC, Espinosa N, Hösel M, Søndergaard RR, Jørgensen M. 25th Anniversary Article: Rise to Power – OPV-Based Solar Parks. Advanced Materials 2013; 26 (1) 29-39.
  33. 33. Krebs FC, Jorgensen M. Polymer and organic solar cells viewed as thin film technologies: What it will take for them to become a success outside academia. Solar Energy Materials & Solar Cells 2013; 119 (1) 73-76.
  34. 34. Henson ZB, Müllen K, Bazan GC. Design strategies for organic semiconductors beyond the molecular formula. Nature Chemistry 2012; 4 (9) 699-704.
  35. 35. Janssen RAJ, Nelson J. Factors Limiting Device Efficiency in Organic Photovoltaics. Advanced Materials 2013; 25 (13) 1847-1858.
  36. 36. Thompson BC, Fréchet JMJ. Polymer-Fullerene Composite Solar Cells. Angewandte Chemie International Edition 2008; 47 (1) 58-77.
  37. 37. Helgesen M, Søndergaard R, Krebs FC. Advanced materials and processes for polymer solar cell devices. Journal of Materials Chemistry 2010; 20 (1) 36-60.
  38. 38. Brabec CJ, Gowrisanker S, Halls JJM, Laird D, Jia S, Williams SP. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Advanced Materials 2010; 22 (34) 3839-3856.
  39. 39. Beaujuge PM, Fréchet JMJ. Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications. Journal of the American Chemical Society 2011; 133 (50) 20009-20029.
  40. 40. Brunetti FG, Kumar R, Wudl F. Organic electronics from perylene to organic photovoltaics: painting a brief history with a broad brush. Journal of Materials Chemistry 2010; 20 (15) 2934-2948.
  41. 41. Chiechi RC, Havenith RWA, Hummelen JC, Koster LJA, Loi MA. Modern plastic solar cells: materials, mechanisms and modeling. Materials Today 2013; 16(7/8) 281-289.
  42. 42. Scharber MC, Sariciftci NS. Efficiency of bulk-heterojunction organic solar cells. Progress in Polymer Science 2013; 38 (12) 1929-1940.
  43. 43. Burke DJ, Lipomi DJ. Green Chemistry for organic solar cells. Energy & Environmental Science 2013; 6 (7) 2053-2066.
  44. 44. Shrestha S. Literature Survey Photovoltaics Literature Survey (No. 112). Progress in Photovoltaics: Research and Applications 2014; 22 (8) 933-936.
  45. 45. Guo X, Baumgarten M, Müllen K. Designing π-conjugated polymers for organic electronics. Progress in Polymer Science 2013; 38 (12) 1773-2070.
  46. 46. Ye Q., Chi C. Conjugated Polymers for Organic Solar Cells. In: Kosyachenko LA. (ed.) Solar Cells – New Aspects and Solutions. Rijeka: InTech; 2011. P453-474.
  47. 47. Song S, Kim J, Shim JY, Kim G, Lee BH, Jin Y, Park SH, Kim I, Lee K, Suh H. Synthesis and characterization of dimethyl-benzimidazole based low bandgap copolymers for OPVs. Synthetic Metals 2012; 162 (11-12) 988-994.
  48. 48. Hoven CV, Dang XD, Coffin RC, Peet J, Nguyen TQ, Bazan GC. Improved Performance of Polymer Bulk Heterojunction Solar Cells Through the Reduction of Phase Separation via Solvent Additives. Advanced Materials 2010; 22 (8) E63-E66.
  49. 49. Bijleveld JC, Shahid M, Gilot J, Wienk MM, Janssen RAJ. Copolymers of Cyclopentadithiophene and Electron-Deficient Aromatic Units Designed for Photovoltaic Applications. Advanced Functional Materials 2009; 19 (20) 3262-3270.
  50. 50. Balan A, Baran D, Toppare L. Benzotriazole containing conjugated polymers for multipurpose organic electronic applications. Polymer Chemistry 2011; 2 (5) 1029-1043.
  51. 51. Hou J, Park MH, Zhang S, Yao Y, Chen LM, Li JH, Yang Y. Bandgap and Molecular Energy Level Control of Conjugated Polymer Photovoltaic Materials Based on Benzo[1,2-b:4,5-b’]dithiophene. Macromolecules 2008; 41(16) 6012-6018.
  52. 52. Zhao W, Cai W, Xu R, Yang W, Gong X, Wu H, Cao Y. Novel conjugated alternating copolymer based on 2,7-carbazole and 2,1,3-benzoselenadiazole. Polymer 2010; 51 (14) 3196-3202.
  53. 53. Qin T, Zajaczkowski W, Pisula W, Baumgarten M, Chen M, Gao M, Wilson G, Easton CD, Müllen K, Watkins SE. Tailored Donor-Acceptor Polymers with an A-D1-A-D2 Structure: Controlling Intermolecular Interactions to Enable Enhanced Polymer Photovoltaic Devices. Journal of the American Chemical Society 2014; 136 (16) 6049-6055.
  54. 54. Xu YX, Chueh CC, Yip HL, Ding FZ, Li YX, Li CZ, Li X, Chen WC, Jen AKY. Improved Charge Transport and Absorption Coefficient in Indacenodithieno[3,2-b]thiophene-based Ladder-Type Polymer Leading to Highly Efficient Polymer Solar Cells. Advanced Materials 2012; 24 (47) 6356-6361.
  55. 55. Kim J, Yun MH, Kin GH, Lee J, Lee SM, Ko SJ, Kim Y, Dutta GK, Moon M, Park SY, Kim DS, Kim JY, Yang C. Synthesis of PCDTBT-Based Fluorinated Polymers for High Open-Circuit Voltage in Organic Photovoltaics: Towards an Understanding of Relationships between Polymer Energy Levels Engineering and Ideal Morphology Control. ACS Applied Materials & Interfaces 2014; 6 (10) 7523-7534.
  56. 56. Albrecht, Janietz S, Schindler W, Frisch J, Kurpiers J, Kniepert J, Inal S, Pingel P, Fostiropoulos K, Koch N, Neher D. Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells. Journal of the American Chemical Society 2012; 134 (36) 14932-14944.
  57. 57. Chang HH, Tsai CE, Lai YY, Chiou DY, Hsu SL, Hsu CS, Cheng YJ. Synthesis, Molecular and Photovoltaic Properties of Donor-Acceptor Conjugated Polymers Incorporating a New Heptacyclic Indacenodithieno[3,2-b]thiophene Arene. Macromolecules 2012; 45 (23) 9282-9291.
  58. 58. Li Y, Zou J, Yip HL, Li CZ, Zhang Y, Chueh CC, Intemann J, Xu Y, Liang PW, Chen Y, Jen AKY. Side-chain effect on cyclopentadithiophene/fluorobenzothiadiazole-based low band gap polymers and their application for polymer solar cells. Macromolecules 2013; 46 (14) 5497-5503.
  59. 59. Dou L, Chen CC, Yoshimura K, Ohya K, Chang WH, Gao J, Liu Y, Richard E, Yang Y. Synthesis of 5H-Dithieno[3,2-b:2’,3’-d]pyran as an electron rich building block for donor-acceptor type low bandgap polymers. Macromolecules 2013; 46 (9) 3384-3390.
  60. 60. Bronstein H, Frost JM, Hadipour A, Kim Y, Nielsen CB, Ashraf RS, Rand BP, Watkins S, McCulloch I. Effect of Fluorination on the Properties of a Donor-Acceptor Copolymer for Use in Photovoltaic Cells and Transistors. Chemistry of Materials 2013; 25 (3) 277-285.
  61. 61. Zhang Y, Chien SC, Chen KS, Yip HL, Sun Y, Davies JA, Chen FC, Jen AKY. Increased open circuit voltage in fluorinated benzothiadiazole-based alternating conjugated polymers. Chemical Communications 2011; 47 (39) 11026-11028.
  62. 62. Almeataq MS, Yi H, Al-Faifi S, Alghamdi AAB, Iraqi A, Scarratt MW, Wang T, Lidzey DG. Anthracene-based donor-acceptor low bandgap polymers for applications in solar cells. Chemical Communications 2013; 49 (22) 2252-2254.
  63. 63. Alghamdi AAB, Watters DC, Yi H, Al-Faifi S, Almeataq MS, Coles D, Kingsley J, Lidzey DG, Iraqi A. Selenophene vs. thiophene in benzothiadiazole-based low energy gap donor-acceptor polymers for photovoltaic applications. Journal of Materials Chemistry A 2013; 1 (16) 5165-5171.
  64. 64. Gong X, Li C, Lu Z, Li G, Mei Q, Fang T, Bo Z. Anthracene-containing Wide-Band-Gap Conjugated Polymers for High-Open-Circuit-Voltage Polymer Solar Cells. Macromolecular Rapid Communications 2013; 34 (14) 1163-1168.
  65. 65. Liu X, Wen W, Bazan GC. Post-Deposition Treatment of an Arylated-Carbazole Conjugated Polymer for Solar Cell Fabrication. Advanced Materials 2012; 24 (33) 4505-4510.
  66. 66. Ku SY, Liman CD, Burke DJ, Treat ND, Cochran JE, Amir E, Perez LA, Chabinyc ML, Hawker CJ. A Facile Synthesis of Low-Band-Gap Donor-Acceptor Copolymers Based on Dithieno[3,2-b:2’,3’-d]thiophene. Macromolecules 2011, 44 (24) 9533-9538.
  67. 67. Zhou H, Yang L, Price SC, Knight KJ, You W. Enhanced Photovoltaic Performance of Low-Bandgap Polymers with Deep LUMO levels. Angewandte Chemie International Edition 2010; 49 (43) 7992-7995.
  68. 68. Tseng HR, Phan H, Luo C, Wang M, Perez LA, Patel SN, Ying L, Kramer EJ, Nguyen TQ, Bazan GC, Heeger AJ. High-Mobility Field-Effect Transistors Fabricated with Macroscopic Aligned Semiconducting Polymers. Advanced Materials 2014; 26 (19) 2993-2998.
  69. 69. Sun Y, Chien SC, Yip HL, Zhang Y, Chen KS, Zeigler DF, Chen FC, Lin B, Jen AKY. High-mobility low-bandgap conjugated copolymers based on indacenodithiophene and thiadiazole[3,4-c]pyridine units for thin film transistor and photovoltaic applications. Journal of Materials Chemistry 2011; 21 (35) 13247-13255.
  70. 70. Wen W, Ying L, Hsu BBY, Zhang Y, Nguyen TQ, Bazan GC. Regioregular pyridyl[2,1,3]thiadiazole-co-indacenodithiophene conjugated polymers. Chemical Communications 2013; 49 (65) 7192-7194.
  71. 71. Wang Y, Liu Y, Chen S, Peng R, Ge Z. Significant Enhancement of Polymer Solar Cell Performance via Side-Chain Engineering and Simple Solvent Treatment. Chemistry of Materials 2013; 25 (15) 3196-3204.
  72. 72. Tumbleston JR, Stuart AC, Gann E, You W, Ade H. Fluorinated Polymer Yields High Organic Solar Cell Performance for a Wide Range of Morphologies. Advanced Functional Materials 2013; 23 (27) 3463-3470.
  73. 73. Uy RL, Yan L, Li W, You W. Tuning Fluorinated Benzotriazole Polymers through Alkylthio Substitution and Selenophene Incorporation for Bulk Heterojunction Solar Cells. Macromolecules 2014; 47 (7) 2289-2295.
  74. 74. Price SC, Stuart AC, Yang L, Zhou H, You W. Fluorine Substituted Conjugated Polymer of Medium Band Gap Yields 7% Efficiency in Polymer-Fullerene Solar Cells. Journal of the American Chemical Society 2011; 133 (12) 4625-4631.
  75. 75. Jiang JM, Yang PA, Hsieh TH, Wei KH. Crystalline Low-Band Gap Polymers Comprising Thiophene and 2,1,3-benzooxadiazole Units for Bulk Heterojunction Solar Cells. Macromolecules 2011; 44 (23) 9155-9163.
  76. 76. Min J, Zhang ZG, Zhang M, Li Y. Synthesis and photovoltaic properties of a D-A copolymer of dithienosilole and fluorinated-benzotriazole. Polymer Chemistry 2013; 4 (5) 1467-1473.
  77. 77. Fan J, Yuen JD, Wang M, Seifter J, Seo JH, Mohebbi AR, Zakhidov D, Heeger AJ, Wudl F. High-Performance Ambipolar Transistors and Inverters from an Ultralow Bandgap Polymer. Advanced Materials 2012; 23 (16) 2186-2190.
  78. 78. Keshtov ML, Marochkin DV, Kochurov VS, Khokhlov AR, Koukaras EN, Sharma GD. Synthesis and characterization of a low band gap quinoxaline based D-A copolymer and its application as a donor for bulk heterojunction polymer solar cells. Polymer Chemistry 2013; 4 (14) 4033-4044.
  79. 79. Zha D, Chen L, Wu F, Wang H, Chen Y. Modulation of the molecular geometry of carbazolebis[thiadiazole]-based conjugated polymers for photovoltaic applications. Polymer Chemistry 2013; 4 (8) 2480-2488.
  80. 80. Zhou P, Zhang ZG, Li Y, Chen X, Qin J. Thiophene-Fused Benzothiadiazole: A Strong Electron-Acceptor Unit to Build D-A Copolymer for Highly Efficient Polymer Solar Cells. Chemistry of Materials 2014; 26 (11) 3495-3501.
  81. 81. Wang M, Hu X, Liu P, Li W, Gong X, Huang F, Cao Y. Donor-Acceptor Conjugated Polymer Based on Naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole for High-Performance Polymer Solar Cells. Journal of the American Chemical Society 2011; 133 (25) 9638-9641.
  82. 82. Wang M, Hu X, Liu L, Duan C, Liu P, Ying L, Huang F, Cao Y. Design and Synthesis of Copolymers of Indacenodithiophene and Naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) for Polymer Solar Cells. Macromolecules 2013; 46 (10) 3950-3958.
  83. 83. Zhou H, Yang L, Stuart AC, Price SC, Liu S, You W. Development of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of 7% Efficiency. Angewandte Chemie International Edition 2011; 50 (13) 2995-2998.
  84. 84. Dong Y, Cai W, Wang M, Li Q, Ying L, Huang F, Cao Y. [1,2,5]Thiadiazolo[3,4-f]benzotriazole based narrow band gap conjugated polymers with photocurrent response up to 1.1 μm. Organic Electronics 2013; 14 (10) 2459-2467.
  85. 85. Osedach TP, Andrew TL, Bulović V. Effect of synthetic accessibility on the commercial viability of organic photovoltaics. Energy & Environmental Science 2013; 6 (3) 711-718.
  86. 86. Liao HC, Ho CC, Chang CY, Jao MH, Darling SB, Su WF. Additives for morphology control in high-efficiency organic solar cells. Materials Today 2013; 16 (9) 326-336.
  87. 87. Gao X, Cui C, Zhang M, Huo L, Huang Y, Hou J, Li Y. High efficiency polymer solar cells based on poly(3-hexylthiophene)/indene-C70 bisadduct with solvent additive. Energy & Environmental Science 2012; 5 (7) 7943-7949.
  88. 88. Mei J, Bao Z. Side Chain Engineering in Solution-Processable Conjugated Polymers. Chemistry of Materials 2014; 26 (1) 604-615.
  89. 89. Carsten B, He F, Son HJ, Xu T, Yu L. Stille Polycondensation for Synthesis of Functional Materials. Chemical Reviews 2011; 111 (3) 1493-1528.
  90. 90. Miyaura N, Suzuki A. Palladium-catalysed cross-coupling reactions of organoboron compounds. Chemical Reviews 1995; 95 (7) 2457-2483.
  91. 91. Berrouard P, Najari A, Pron A, Gendron D, Morin PO, Pouliot JR, Veilleux J, Leclerc M. Synthesis of 5-Alkyl[3,4-c]thienopyrrole-4,6-dione-Based Polymers by Direct Heteroarylation. Angewandte Chemie International Edition 2012; 51 (9) 2068-2071.
  92. 92. Rudenko AE, Khlyabich PP, Thompson BC. Random Poly(3-hexylthiophene-co-3-cyanothiophene) Copolymers via Direct Arylation Polymerization (DArP) for Organic Solar Cells with High Open-Circuit Voltage. ACS Macro Letters 2014; 3 (4) 387-392.
  93. 93. Kuwabara J, Yasuda T, Choi SJ, Lu W, Yamazaki K, Kagaya S, Han L, Kanbara T. Direct Arylation Polycondensation: A Promising Method for the Synthesis of Highly Pure, High-Molecular-Weight Conjugated Polymers Needed for Improving the Performance of Organic Photovoltaics. Advanced Functional Materials 2014; 24 (21) 3226-3233.
  94. 94. Estrada LA, Deininger JJ, Kamenov GD, Reynolds JR. Direct (Hetero)arylation Polymerization: An Effective Route to 3,4-Propylenedioxythiophene-Based Polymers with Low Residual Metal Content. ACS Macro Letters 2013; 2 (10) 869-873.
  95. 95. Okamoto K, Zhang J, Housekeeper JB, Marder SR, Luscombe CK. C-H Arylation Reaction: Atom Efficient and Greener Syntheses of π-Conjugated Small Molecules and Macromolecules for Organic Electronic Materials. Macromolecules 2013; 46 (20) 8059-8078.
  96. 96. Mercier LG, Leclerc M. Direct (Hetero)Arylation: A New Tool for Polymer Chemists. Accounts of Chemical Research 2013; 46 (7) 1597-1605.
  97. 97. Kline RJ, McGehee MD, Kadnikova EN, Liu J, Fréchet JMJ. Controlling the Field-Effect Mobility of Regioregular Polythiophene by Changing the Molecular Weight. Advanced Materials 2003; 15 (18) 1519-1522.
  98. 98. Tsao HN, Cho DM, Park I, Hansen MR, Mavrinskiy A, Yoon DY, Graf R, Pisula W, Spiess HW, Müllen K. Ultrahigh Mobility in Polymer Field-Effect Transistors by Design. Journal of the American Chemical Society 2011; 133 (8) 2605-2612.
  99. 99. Intemann JJ, Yao K, Yip HL, Xu YX, Li YX, Liang PW, Ding FZ, Li X, Jen AKY. Molecular Weight Effect on the Absorption, Charge Carrier Mobility, and Photovoltaic Performance of an Indacenodiselenophene-Based Ladder-Type Polymer. Chemistry of Materials 2013; 25 (15) 3188-3195.
  100. 100. Coffin RC, Peet J, Rogers J, Bazan GC. Streamlined microwave-assisted preparation of narrow-bandgap conjugated polymers for high-performance bulk heterojunction solar cells. Nature Chemistry 2009; 1 (8) 657-661.
  101. 101. Tong M, Cho S, Rogers JT, Schmidt K, Hsu BBY, Moses D, Coffin RC, Kramer EJ, Bazan GC, Heeger AJ. Higher Molecular Weight Leads to Improved Photoresponsivity, Charge Transport and Interfacial Ordering in a Narrow Bandgap Semiconducting Polymer. Advanced Functional Materials 2010; 20 (22) 3959-3965.
  102. 102. Nikiforov MP, Lai B, Chen W, Chen S, Schaller RD, Strzalka J, Maser J, Darling SB. Detection and role of trace impurities in high-performance organic solar cells. Energy & Environmental Science 2013; 6 (5) 1513-1520.
  103. 103. Nielsen KT, Bechgaard K, Krebs FC. Removal of Palladium Nanoparticles from Polymer Materials. Macromolecules 2005; 38 (3) 658-659.
  104. 104. Leong WL, Welch GC, Kaake LG, Takacs CJ, Sun Y, Bazan GC, Heeger AJ. Role of trace impurities in the photovoltaic performance of solution processed small-molecule bulk heterojunction solar cells. Chemical Science 2012; 3 (6) 2103-2109.
  105. 105. Park JK, Jo J, Seo JH, Moon JS, Park YD, Lee K, Heeger AJ, Bazan GC. End-Capping Effect of a Narrow Bandgap Conjugated Polymer on Bulk Heterojunction Solar Cells. Advanced Materials 2011; 23 (21) 2430-2435.
  106. 106. Kim JS, Lee Y, Lee JH, Park JH, Kim JK, Cho K. High-Efficiency Organic Solar Cells Based on End-Functional Group Modified Poly(3-hexylthiophene). Advanced Materials 2010; 22 (12) 1355-1360.
  107. 107. Kim Y, Cook S, Kirkpatrick J, Nelson J, Durrant JR, Bradley DDC, Giles M, Heeney M, Hamilton R, McCulloch I. Effect of the End Group of Regioregular Poly(3-hexylthiophene) Polymers on the Performance of Polymer/Fullerene Solar Cells. The Journal of Physical Chemistry C 2007; 111 (23) 8137-8141.
  108. 108. Kaake L, Dang XD, Leong WL, Zhang Y, Heeger AJ, Nguyen TQ. Effects of Impurities on Operational Mechanism of Organic Bulk Heterojunction Solar Cells. Advanced Materials 2013; 23 (12) 1706-1712.
  109. 109. Cowan SR, Leong WL, Banerji N, Dennler G, Heeger AJ. Identifying a Threshold Impurity Level for Organic Solar Cells: Enhanced First-Order Recombination Via Well-Defined PC84BM Traps in Organic Bulk Heterojunction Solar Cells. Advanced Functional Materials 2011; 21 (16) 3083-3092.
  110. 110. He Y, Li Y. Fullerene derivatives acceptors for high performance polymer solar cells. Physical Chemistry Chemical Physics 2011; 13 (6) 1970-1983.
  111. 111. Li CZ, Yip HL, Jen AKY. Functional fullerenes for organic photovoltaics. Journal of Materials Chemistry 2012; 22 (10) 4161-4177.
  112. 112. Wienk MM, Kroon JM, Verhees WJH, Knol J, Hummelen JC, Hal PA, Janssen RAJ. Efficient Methano[70]fullerene/MDMO-PPV Bulk Heterojunction Photovoltaic Cells. Angewandte Chemie International Edition 2003; 42 (29) 3371-3375.
  113. 113. Anctil A, Babbitt CW, Raffaelle RP, Landi BJ. Material and Energy Intensity of Fullerene Production. Environmental Science & Technology 2011; 45 (6) 2353-2359.
  114. 114. Anctil A, Babbitt CW, Raffaelle RP, Landi BJ. Cumulative energy demand for small molecule and polymer photovoltaics. Progress in Photovoltaics: Research and Applications 2013; 21 (7) 1541-1554.
  115. 115. Lewis NS, Nocera DG. Powering the planet: Chemical challenges in solar energy utilization. Proceedings of the National Academy of Sciences of the United States of America 2006; 103 (43) 15729-15735.
  116. 116. Solaris: (access 1 July 2014).
  117. 117. Anthony JE. Small-Molecule, Nonfullerene Acceptors for Polymer Bulk Heterojunction Organic Photovoltaics. Chemistry of Materials 2011; 23 (3) 583-590.
  118. 118. Facchetti A. Polymer donor-polymer acceptor (all-polymer) solar cells. Materials Today 2013; 16 (4) 123-132.
  119. 119. Kouijzer S, Michels JJ, Berg M, Gevaerts VS, Turbiez M, Wienk MM, Janssen RAJ. Predicting Morphologies of Solution Processed Polymer:Fullerene Blends. Journal of the American Chemical Society 2013; 135 (32) 12057-12067.
  120. 120. Meng L, Shang Y, Li Q, Li Y, Zhan X, Shuai Z, Kimber RGE, Walker AB. Dynamic Monte Carlo Simulation for Highly Efficient Polymer Blend Photovoltaics. The Journal of Physical Chemistry B 2010; 114 (1) 36-41.
  121. 121. Chueh CC, Yao K, Yip HL, Chang CY, Xu YX, Chen KS, Li CZ, Liu P, Huang F, Chen Y, Chen WC, Jen AKY. Non-halogenated solvents for environmentally friendly processing of high-performance bulk-heterojunction polymer solar cells. Energy & Environmental Science 2013; 6 (11) 3241-3248.
  122. 122. Schmidt-Hansberg B, Sanyal M, Grossiord N, Galagan Y, Baunach M, Klein MFG, Colsmann A, Scharfer P, Lemmer U, Dosch H, Michels J, Barrena E, Schabel W. Investigation of non-halogenated solvent mixtures for high throughput fabrication of polymer-fullerene solar cells. Solar Energy Materials & Solar Cells 2012; 96 (1) 195-201.
  123. 123. Lee JK, Ma WL, Brabec CJ, Yuen J, Moon JS, Kim JY, Lee K, Bazan GC, Heeger AJ. Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells. Journal of the American Chemical Society 2008; 130 (11) 3619-3623.
  124. 124. Hammond MR, Kline RJ, Herzing AA, Richter LJ, Germack DS, Ro HW, Soles CL, Fischer DA, Xu T, Yu L, Toney MF, DeLongchamp DM. Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells. ACS Nano 2011; 5 (10) 8248-8257.
  125. 125. Collins BA, Li Z, Tumbleston JR, Gann E, McNeill CR, Ade H. Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC71BM Solar Cells. Advanced Energy Materials 2013; 3 (1) 65-74.
  126. 126. Guo X, Cui C, Zhang M, Huo L, Huang Y, Hou J, Li Y. High efficiency polymer solar cells based on poly(3-hexylthiophene)/indene-C70 bisadduct with solvent additive. Energy & Environmental Science 2012; 5 (7) 7943-7949.
  127. 127. Kim K, Carroll DL. Roles of Au and Ag nanoparticles in efficiency enhancement of poly(3-octylthiophene)/C60 bulk heterojunction photovoltaics. Applied Physics Letters 2005; 87 (20) 203113.
  128. 128. Berson S, Bettignies R de, Bailly S, Guillerez S, Jousselme B. Elaboration of P3HT/CNT/PCBM Composites for Organic Photovoltaic Cells. Advanced Functional Materials 2007; 17 (16) 3363-3370.
  129. 129. Burke KB, Belcher WJ, Thomsen L, Watts B, McNeill CR, Ade H, Dastoor PC. Role of Solvent Trapping Effects in Determining the Structure and Morphology of Ternary Blend Organic Devices. Macromolecules 2009; 42 (8) 3098-3103.
  130. 130. Tsai JH, Lai YC, Higashihara T, Lin CJ, Ueda M, Chen WC. Enhancement of P3HT/PCBM Photovoltaic Efficiency Using the Surfactant of Triblock Copolymer Containing Poly(3-hexylthiophene) and Poly(4-vinyltriphenylamine) Segments. Macromolecules 2010; 43 (14) 6085-6091.
  131. 131. Yang L, Yan L, You W. Organic Solar Cells beyond One Pair of Donor-Acceptor: Ternary Blends and More. Journal of Physical Chemistry Letters 2013; 4 (11) 1802-1810.
  132. 132. Yang X, Loos J, Veenstra SC, Verhees WJH, Wienk MM, Kroon JM, Michels MAJ, Janssen RAJ. Nanoscale Morphology of High-Performance Polymer Solar Cells. Nano Letters 2005; 5 (4) 579-583.
  133. 133. Dong H, Fu X, Liu J, Wang Z, Hu W. 25th Anniversary Article: Key Points for High-Mobility Organic Field-Effect Transistors. Advanced Materials 2013; 25 (43) 6158-6183.
  134. 134. Mihailetchi VD, Xie H, Boer B, Popescu LM, Hummelen JC, Blom PWM, Koster LJA. Origin of the enhanced performance in poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester solar cells upon slow drying of the active layer. Applied Physics Letters 2006; 89 (1) 012107.
  135. 135. Li G, Shrotriya V, Huang J, Yao Y, Moriarty T, Emery K, Yang Y. High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nature Materials 2005; 4 (11) 864-868.
  136. 136. Nam S, Jang J, Cha H, Hwang J, An TK, Park S, Park CE. Effects of direct solvent exposure on the nanoscale morphologies and electrical characteristics of PCBM-based transistors and photovoltaics. Journal of Materials Chemistry 2012; 22 (12) 5543-5549.
  137. 137. Beaupré S, Leclerc M. PCDTBT: en route for low cost plastic solar cells. Journal of Materials Chemistry A 2013; 1 (37) 11097-11105.
  138. 138. Jørgensen M, Norrman K, Krebs FC. Stability/degradation of polymer solar cells. Solar Energy Materials & Solar Cells 2008; 92 (7) 686-714.
  139. 139. . Jørgensen M, Norrman K, Gevorgyan SA, Tromholt T, Andreasen B, Krebs FC. Stability of Polymer Solar Cells Advanced Materials. 2012; 24 (5) 580-612.
  140. 140. Lee JU, Jung JW, Jo JW, Jo WH. Degradation and stability of polymer-based solar cells. Journal of Materials Chemistry 2012; 22 (46) 24265-24283.
  141. 141. ISOS: (access 10 July 2014).
  142. 142. Peters CH, Sachs-Quintana IT, Kastrop JP, Beaupré S, Leclerc M, McGehee MD. High Efficiency Polymer Solar Cells with Long Operating Lifetimes. Advanced Energy Materials 2011; 1 (4) 491-494.
  143. 143. Hauch JA, Schilinsky P, Choulis SA, Childers R, Biele M, Brabec CJ. Flexible organic P3HT:PCBM bulk-heterojunction modules with more than 1 year outdoor lifetime. Solar Energy Materials & Solar Cells 2008; 92 (7) 727-731.
  144. 144. Arias AC, MacKenzie JD, McCulloch I, Rivnay J, Salleo A. Materials and Applications for Large Area Electronics: Solution-Based Approaches. Chemical Reviews. 2010; 110 (1) 3-24.
  145. 145. Søndergaard R, Hösel M, Angmo D, Larsen-Olsen TT, Krebs FC. Roll-to-roll fabrication of polymer solar cells. Materials Today 2012; 12 (1-2) 36-49.
  146. 146. Krebs FC. Fabrication and processing of polymer solar cells: A review of printing and coating techniques. Solar Energy Materials & Solar Cells 2009; 93 (4) 394-412.
  147. 147. Krebs FC, Espinosa N, Hösel M, Søndergaard RR, Jørgensen M. 25th Anniversary Article: Rise to Power – OPV-Based Solar Parks. Advanced Materials 2014; 26 (1) 29-39.
  148. 148. Konarka Technology: (access 15 July 2014).
  149. 149. Solyndra: (access 15 July 2014)
  150. 150. Yue D, Khatav P, You F, Darling SB. Deciphering the uncertainties in life cycle energy and environmental analysis of organic photovoltaics. Energy & Environmental Science 2012; 5 (11) 9163-9172.
  151. 151. Lizin S, Passel SV, Schepper ED, Maes W, Lutsen L, Manca J, Vanderzande D. Life cycle analyses of organic photovoltaics: a review. Energy & Environmental Science 2013; 6 (11) 3136-3149.
  152. 152. Beiley ZM, McGehee MD. Modelling low cost hybrid tandem photovoltaics with the potential for efficiencies exceeding 20%. Energy & Environmental Science 2012; 5 (11) 9173-9179.
  153. 153. Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-Sensitized Solar Cells. Chemical Reviews 2010; 110 (11) 6595-6663.
  154. 154. Snaith HJ. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. The Journal of Physical Chemistry Letters 2013; 4 (21) 3623-3630.

Written By

Qun Ye and Jian Wei Xu

Submitted: 26 April 2014 Published: 22 October 2015