Optoelectronic characteristics comparison for silicon-based films deposited at different temperatures.
The concept for inorganic-organic device is an attractive technology to develop devices with better characteristics and functionality due to the complementary advantages of inorganic and organic materials. This chapter provides an overview of the principal requirements for organic and inorganic semiconductor properties and their fabrication processes and focus on the compatibility between low temperature plasma enhanced chemical vapor deposition (PECVD) and polymer organic materials deposition. The concept for inorganic-organic device was validated with the fabrication of three hybrid thin film photovoltaic structures, based on hydrogenated silicon (Si:H), organic poly(3-hexythiophene): methano-fullerenephenyl-C61-butyric-acid-methyl-ester (P3HT:PCBM), and poly(3,4ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) films. Optoelectronic characteristics, performance characteristics, and interfaces of the different configurations aspects are discussed. Hybrid ITO/PEDOT:PSS/(i)Si:H/(n)Si:H structure results in a remarkably high short circuit current density as large as 17.74 mA/cm2, which is higher than the values in organic or inorganic reference samples. Although some hybrid structures demonstrated substantial improvement of performance, other hybrid structures showed poor performance, further R&D efforts seem to be promising, and should be focused on deeper study of organic materials and related interface properties.
- organic semiconductors
- plasma deposited materials
- thin film devices
- hybrid devices
- solar cells
Polymer-based organic semiconductors have attracted considerable attention due to their excellent light absorption properties and compatibility with the solution processing technologies enabling new approaches to fabricate cost-effective, organic thin-film solar cells by printable process. On the other hand, hydrogenated silicon (Si:H) prepared by plasma enhanced chemical vapor deposition (PECVD) is a widely known inorganic semiconductor with rather mature deposition technology, which has been successfully implemented in several commercial optoelectronic devices, including photovoltaics. Important advantages of Si:H and related semiconductors are their high photocarrier generation, good transport properties of photocarriers, possibility to vary optical gap in the range of 1.2–3 eV, and acceptable stability under sun irradiation. Organic solar cells based on conjugated polymers as electron-donor materials blended with small molecule sensitizer as an electron acceptor have achieved 7–9% conversion efficiency in small area single bulk heterojunction devices. The efficiency of single-junction Si:H solar cells is around 10% even with optimal optical design. A few attempts have been undertaken to merge the beneficial properties of both materials in the so-called hybrid silicon-organic heterojunction solar cells. The main idea behind this concept is based on the possibilities to enhance photovoltaic performance characteristics by engineering suitable material and improving interface properties and design. Rather developed deposition and doping process of amorphous silicon and related thin films (intrinsic and p-n-doped) allow to tune the important structural and electronic parameters (such as degree of order, optical gap, mobility, Fermi level position, etc.) of the material which determines the device performance characteristics. Thus, the combination of organic and inorganic materials would be a promising approach for developing new types of large area, lightweight, thin film efficient solar cells.
We present a concept for silicon-organic heterojunction structures for photovoltaic devices, based on mainly PECVD noncrystalline silicon (can be also extended and involve crystalline semiconductors) describe fabrication of both plasma deposited films and organic materials and their relevant electronic properties, fabrication of hybrid device structures, and their characteristics comparing with those reported in literature.
2. Fabrication and electronic properties of plasma deposited and organic materials
2.1. Inorganic materials based on silicon deposited by PECVD
Technology based on amorphous silicon semiconductors has become an important part in the multibillion business in electronics in the last decades. There was a time, when the theory of the physical properties of solids seems to be impossible without considering a stable and periodic structure, Ziman . However, nowadays, the theory of amorphous materials has not only been developed but also technology based on these materials has become omnipresent in wide range of electronic applications. The breakthrough of amorphous silicon materials was directly related to the development of a new method called GD that allowed for the first time to deposit intrinsic hydrogenated amorphous silicon (a-Si:H) films [2, 3] with high photoconductivity and low concentration of ESR centers, p-type and n-type doped amorphous silicon films, Spear and Le Comber , with sufficient quality (low DOS) to be used in functional p-n junctions .
The modern approach of the GD system used for first time is now well-known (PECVD). The actual PECVD technique has reached the versatility to deposit not only amorphous silicon but also silicon materials in their entire range of structure: nanostructured , microcrystalline [7, 8], epitaxial crystalline silicon , and alloys such as SiGe:H, SiC:H, SiO:H, SiN:H, etc. The optoelectronic properties of these materials depend on the deposition parameters such as pressure, flow rate, substrate temperature, power, and excitation frequency. Good electronic properties for interesting high Ge-content nanostructured SixGey:H films deposited at 300°C, high H-dilution ratio, and low frequency have been systematically reported in Ref. . On the other hand, one of the main characteristics of the PECVD technique to take advantage for actual applications is the relative low temperature process in the range of 100–300°C. Low temperature process is a requirement for new applications such as large area, lightweight, flexible devices on low-cost plastic substrates, and, more recently, hybrid organic-inorganic devices based on polymers. It was general to think that reduction of temperature below 200°C leads to the increase of dangling bonds (defects) and results in films with electronic properties that are generally poor . However, it has been found that deterioration of electronic properties in materials deposited at temperature lower than
A-Si:H films deposited at reference temperature (
|a-Si:H||300||–||1.8||8 × 10−10||–||0.60||–|||
|220||0.93||1.72||1.1 × 10−9||1.8 × 10−4||0.93||RF|||
|120||–||1.92||4 × 10−11||–||–||–|||
|75||–||1.90||9 × 10−11||–||–||–|||
|75||–||–||3 × 10−7||–||–||–|||
|µc-Si:H||100||0.12||2.14||3 × 10−5||1.7 × 10−5||0.31||RF|||
|25||0.09||2.10||8.6 × 10−9||5.9 × 10−8||0.63||RF|||
|GeSi:H||300||2.2||0.94||2.5 × 10−4||–||0.28||LF|||
|160||1.3||0.95||1.3 × 10−4||7.5 × 10−6||0.32||LF|||
2.2. Polymer organic materials
Organic materials based on polymer semiconductors have been in focus during the last decade. The most studied materials such as the bulk heterojunction P3HT:PCBM and the conductive semiconductor PEDOT:PSS have been widely implemented in new organic thin-film solar cells . One important advantage of these materials is their solution type processing techniques for fabrication that allows printable deposition of the films at room temperature and at atmospheric pressure in comparison to inorganic material deposition that usually requires high substrate temperature and complex high vacuum process. The film-forming processes for organic materials can be classified in (1) coating process methods that include spin coating, blade coating, spray coating, and painting and (2) printing process methods thus include screen printing, offset printing, and inkjet printing . Spin coating method has been one of the important techniques for the deposition of polymer materials. The final characteristics of the films depend on the specific material and its solvent at a given concentration, see, for example, Figure 3, it is shown that the thickness dependence as a function of the rotation velocity for PEDOT:PSS films deposited by spin coating at different IPA solvent concentrations. Spin coating deposition is a complex process that also depends among others properties of viscosity, molecular weight, diffusivity, and volatility of the material; however, it is a highly reproducible process . Nevertheless, there are some aspects that limit the application of the technique for industrial production such as no possibility for deposition on large areas, serial production, and problems with patterning. A review of alternatives to spin coating method for the deposition of polymer materials is presented in Ref. , just to recognize that the “ideal” process for the deposition of organic materials is still unresolved and an urgent task for the future of this technology.
Despite the large number of developed organic materials, there are a few materials that have been usefully applied in promising optoelectronic devices, i.e., PEDOT:PSS is a very attractive conductive polymer semiconductor for electronic and photovoltaic applications (not only for organic electronics) due to its optical transparency, high conductivity, and processing techniques. The use of PEDOT:PSS in hybrid organic-inorganic solar cells and devices has attracted the attention in recent years specially as a candidate for substitution of p-type a-SiC:H:B layer in silicon-based photovoltaic devices [28–30]. The Si/PEDOT interface has shown interesting properties such as effective formation of blocking barrier for electrons, acting similar to an “ideal” barrier . Other advantage of PEDOT:PSS versus traditional inorganic p-type a-Si:H is the flexibility of processing; Figure 4 shows the DC conductivity,
2.3. Transparent contact issues
For several years, TCOs were a unique option used as transparent contacts in optoelectronic applications due to their transparency in visible range (0.4 <
Recently with the introduction of organic materials and new technologies, new options have been studied for substitution of traditional TCOs to overcome some disadvantages as low transmission in blue region, high cost, complex high vacuum process, mechanical characteristics, and scarcity. To achieve this, here are some proposals: thin film grids , high carbon-based materials , and organic polymers [38–40]. PEDOT:PSS has been one of the most investigated organic materials as alternative to TCOs, the polymer has shown higher transparency and conductivity than traditional ITO contacts. Additionally, PEDOT:PSS can be deposited by dispersion in water which is compatible with large area, flexible, and low-cost substrates . The electrical conductivity of PEDOT:PSS can be increased by orders of magnitude using secondary “dopants” as solvents, polyols, surfactants, among others . By solvent dilution and post-treatment method the conductivity of PEDOT:PSS films has reached values of 1418 Ω−1 cm−1, low sheet resistance below 65 Ω/□, and transmission in visible range >80%, which have been reported in . In Ref. , silver nanostructures were used to enhance conductivity of PEDOT:PSS contacts using a codeposition of spray and inject printing techniques, and the lower sheet resistance achieved by this method was 26.2 Ω−1 cm−1 with an average transmission of 80.7%.
2.4. Flexible substrates issues
Hybrid inorganic-organic devices seem to be an attractive approach for large area, lightweight flexible solar cells. Flexible electronics also opens the landscape to new applications such as flexible displays, X-ray sensors, electronic textiles, and electronic skin . Many materials have been investigated as flexible substrates: thin glass substrates , metal foils , and different polymeric films (PET, PEN, Polyimide – KAPTON®, etc.) . The substrate-specific requirements depend strongly of the application, process fabrication, and technology: optical properties, surface roughness, thermal properties, chemical properties, mechanical properties, and electrical and magnetic properties . In the case of applications in organic hybrid solar cells, special attention should be paid to thermal properties, optical properties, and surface roughness. Table 2 presents a comparison of the main properties of flexible substrates. One of the most attractive materials for their flexibility, low cost, and transparency are the plastic substrates. Figure 5 shows the spectral dependence of the optical transmittance in the visible range of plastic substrates. PEN and PET are most desirable materials for solar cells application due to the optical transmittance and relative low cost. Another interesting material is PTFE or TEFLON®; the comparison of transmittance spectra presented in Figure 5 shows that TEFLON transparent thin film is an attractive candidate for application in solar cell application with transparency from 250 to 900 nm and high transmittance (~96%) of above 350 nm. However, TEFLON has a nonpolar surface or a relative low surface energy to the polymer and inorganic materials, which result in low adhesion . Polymer substrates require relatively low processing temperature, below
|Substrate||Temperature process (°C)||Transparency (%)||Flexibility||Planarization|
3. Concept for inorganic-organic hybrid device structure
A concept for inorganic-organic device (CIOD) structures can be developed taking into account the following material aspects: (a) compatibility of deposition technologies for inorganic-organic semiconductors and electrode materials, (b) complementary of electronic properties of these materials resulting in improvement of device performance, and (c) some technological advantages in device fabrication, e.g., organic materials can be deposited by printable PC controlled technique resulting in simplification of patterning. Inorganic semiconductors can be presented by both crystalline materials and PECVD noncrystalline materials. The latter is applicable for large area flexible devices. In general, CIOD includes various combinations of crystalline and noncrystalline materials together with organic materials. In our further consideration, we shall be focused on PV applications based on PECVD films and organic materials because of their promise for large area flexible devices, although for achieving very high efficiency more complex (crystalline inorganic)-(noncrystalline inorganic)-(organic) material combinations would be of much interest.
3.2. Strategies for CIOD structures aimed at performance improvement
Performance of PV device is determined by: (a) generation of photo-induced carriers controlled by light absorption, (b) transportation of photo-induced carriers controlled by built-in electric field and diffusion, (c) life time of photocarriers that determine both concentration and transport of photocarriers, and (d) properties of interfaces, which control generation-recombination near interface and charge transport through the interface. We select p-i-n configuration as basic PV device structure, where p-layer is p-doped semiconductor, i-layer is intrinsic (nondoped) semiconductor, and n-layer is n-type semiconductor. Internal built-in electric field in this structure is created because of difference in work functions for p- and n-layer. PECVD silicon is conventionally used as intrinsic semiconductor (nondoped), boron-doped as p-layer and phosphor-doped as n-layer. Optical gap for Si:H is typical
Most of mature solar cell technologies are being developed with silicon-based materials. The stability with sufficient efficiency of these solar cells has been demonstrated for several years. However, in comparison to new thin film organic materials, they have complex fabrication process, high relative cost, and do not suit well for large area flexible PV devices. New technologies such as organic polymer materials bring new approaches of fabrication techniques and materials that can be molecular designed and tailored with specific optoelectronic properties but efficiency and stability are relatively low in comparison to inorganic counterpart. Thus, the combination of both technologies is looking for devices that can take advantage of the strong optical absorption of organic polymer materials, high charge carrier mobility, and stability of inorganic materials while looking for simplification of process fabrication. Different approaches have been proposed to combine both technologies, the versatility of organic materials results in many possibilities for forming hybrid devices structures . Silicon remains a good candidate for hybrid devices due to favorable and well-known electronic properties. Different ways have been reported to combine silicon with polymers in hybrid solar cell: hybrid crystalline silicon-PEDOT:PSS solar cell , nanowire-Si/polymer solar cells , and a-Si:H/PEDOT:PSS pin solar cell . Figure 6 shows examples of some ideas for application of organic and inorganic materials in hybrid solar cells. In the structure labeled H1 with configuration (ITO)(p) SiC:H/P3HT:PCBM/(n) Si:H), P3HT:PCBM bulk heterojunction film is used as active absorber film and conventional a-Si:H and a-SiC:H are used as n-type and p-type layers, respectively. The structure labeled H2 (ITO/PEDOT:PSS/(i) Si:H/(n) Si:H) is an a-Si:H p-i-n structure, where the p-type SiC:H layer was substituted by an organic conductive film (PEDOT:PSS). Finally, the structure labeled H3 is a double-absorber layer structure (ITO/PEDOT:PSS/P3HT:PCBM/(i) Si:H/(n) Si:H).
3.3. On electron energetical diagrams for hybrid device structures
Electron energetic diagram is a conventional and convenient instrument applied to the analysis of device structures and their performance. Such diagram can be created if electronic properties (optical gap, work function, electron affinity, etc.) are known (well measured) that does not exist even in crystalline semiconductors. Moreover, knowledge of bulk electronic characteristics is not sufficient because information on interfaces is required as well. As far as organic semiconductors are concerned, data reported on bulk electronic properties are very poor and those for interfaces have not been reported. In many publications, “energetic diagrams” look as a set of energetic sketches for different materials with different Fermi levels and some arrows denoted hypothetical electron transitions. This is misleading because in multilayered device structure without illumination a thermal equilibrium is suggested (i.e., exchange of particles and their energies between materials) meaning the same Fermi level in the system, rather than different ones in isolated materials. Under illumination, electronic diagram becomes significantly more complex and can only be created based on special experimental study of device structure. Nevertheless, design of energetic diagram based on available experimental data (even with some shortages and errors in details) is strongly motivated. In Figure 7, such hypothetical electron energetic diagrams in thermal equilibrium for H1, H2, and H3 structures are represented, more details about them can be found in Ref. . In H1, energetic diagram (Figure 7a) shows that the light absorption occurs in the organic bulk heterojunction formed by P3HT and PCBM. The creation of an exciton (bounded electron-hole pair) in P3HT is followed by separation of the charge at the internal interfaces of the heterojunction P3HT/PCBM. The inorganic p- and n-type silicon-based layers are used to create an internal electrical field to transport the “free” charge to electrodes. In H2 structure the PEDOT:PSS layer acts as a heavily doped p-type layer and its comparable optical gap (~1.6 eV) to amorphous silicon (~1.7 eV) forms an optimal internal build-in-field and prevents back diffusion of electrons at the frontal interface. In H3 structure improvement of light absorption is expected by using double active absorber layers (a-Si:H and P3HT:PCBM), the energetic diagram shows that one obstacle of this is that the alignment of energetic band between organic and inorganic materials is complex and may create barriers, which oppose to electron and hole transport.
4. Experimental results on fabrication and characterization of different configurations of silicon-organic p-i-n heterojunction structures
The fabrication flow chart for the H2 structure (ITO/PEDOT:PSS/(i) Si:H/(n) Si:H) is shown in Figure 8. The process begins with a coated ITO substrate. The bottom electrode was defined by etching using hydrochloric acid and then the substrate is cleaned by ultrasonic cleaning in acetone and then isopropyl alcohol. The PEDOT:PSS solution was prepared with a 1:6 weight ratio, filtered using a pore size filter of 0.45 µm and deposited by a spin-coating in N2 ambient with a rotation speed of 2500 rpm that results in a thickness of the film of
Figure 9 shows the DC current density—voltage characteristics J(U) under AM1.5 standard illumination for nonoptimized hybrid structures presented in Figure 6. As seen, the structure H1 with organic semiconductor as nondoped intrinsic material demonstrate low
Figure 10 presents the spectral dependence of short circuit current density
Some sort of “optimization” (see details in ) has been performed for H2 type of structure. Figure 11 shows the current voltage characteristics of an improved hybrid H2* structure in comparison to those for organic and inorganic solar cells. It is most interesting that
|Frontal interface||PCE (%)|
|a-SiC:H:B/(i) a-Si:H efficiency record||16.36||0.89||71||10.2|
5. Hybrid structures on flexible substrates
Flexible solar cells on plastic substrates have several advantages such as lightweight and relative low cost. Inorganic and organic semiconductor technologies have demonstrated to be compatible with flexible substrates [12, 33–35]. However, the new concept of hybrid PECVD-polymer solar cell on flexible substrates has not been reported until now. A hybrid solar cell ((AZO/PEDOT:PSS/(i) Si:H/(n) Si:H//Ag) on PEN and polyimide substrate shown in Figure 12 have been fabricated and studied. A transparent electrode of AZO was deposited on the flexible substrate by sputtering method. The inorganic silicon layers were deposited by RF PECVD process at substrate temperature of
Similar structures on PEN with different AZO-film thickness were compared; it was found that the performance characteristics of the cell were strongly dependent on the thickness of the AZO film. The short circuit current density increased from
6. Summary and conclusion
In the beginning, a brief overview of polymer organic and PECVD inorganic material semiconductor technologies has been made to focus on optoelectronic characteristics and fabrication process at low substrate temperature for application on hybrid photovoltaic devices. Inorganic semiconductor materials deposited by PECVD have been reported in literature to be a mature technology compatible with large area flexible devices; while organic semiconductors and their technology are significantly less studied. On other hand, organic materials have shown potential as PV materials with several advantages in the fabrication process such as room temperature deposition and reduction of complex vacuum stages for deposition but the “ideal” process for the deposition of organic materials is still unresolved.
A concept of inorganic-organic photovoltaic devices mainly for PECVD inorganic materials has been considered. The concept is based on compatibility of both technologies and complementary electronic properties. Three types of hybrid structures have been fabricated and studied: (1) with OS as intrinsic absorber and p-, n-Si:H layers creating built-in electric field (H1 structures), (2) ITO/PEDOT:PSS/(i) Si:H/(n) Si:H structures (H2 type), and (3) structures with double intrinsic absorber such as ITO/PEDOT:PSS/P3HT:PCBM/(i) Si:H/(n) Si:H (H3). All the structures showed PV performance with different characteristics discussed and compared with those reported in the literature. Performance of the structures including those fabricated on flexible plastic substrates confirmed clear compatibility OS and PECVD technologies. The device diagnostics of the studied structures has been performed by measurements of photocurrent spectral dependencies. The measurements of subgap photocurrent revealed, that electronic quality of intrinsic OS materials, was sufficient, but worse comparing to that of Si:H films. The structures with OS frontal interfaces showed better short wavelength photocurrent response indicating better electronic properties of the interfaces comparing to reference samples. All the structures suffered from photocurrent suppression at voltages near
Ismael Cosme Bolaños acknowledges support from “CONACyT Cathedra” project No.2734. Hiram Enrique Martinez and Antonio Olivares acknowledge support from CONACyT through the scholarships number 362152 and 363344, respectively.
|PECVD||Plasma enhanced chemical vapor deposition|
|CVD||Chemical vapor deposition|
|DOS||Density of states|
|HIT||Heterojunction intrinsic thin configuration|
|VRH||Variable range hopping|
|TCO||Transparent conductive oxide|
|PLD||Pulsed laser deposition|
|AZO||Aluminum zinc oxide|
|CIOD||Concept for inorganic-organic device|
|AM1.5||Air mass spectrum 1.5|
|SEM||Scanning electron microscopy|
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