Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development

The effective conversion of solar energy into electricity has attracted intense scientific interest in solving the rising energy crisis. Organic solar cells (OSCs), a kind of green energy source, show great potential application due to low production costs, mechanical flexibility devices by using simple techniques with low environmental impact and the versatility in organic materials design (Beal, 2010). In the past years, the key parameter, power conversion efficiencies (PCE), is up to 7% under the standard solar spectrum, AM1.5G (Liang et al., 2010). The PCE of solar cells are co-determined by the open circuit voltage (Voc), the fill factor (FF) and the short circuit density (Jsc). Researchers have made great efforts in both developing new organic materials with narrow band gap and designing different structural cells for harvesting exciton in the visible light range. Solution processing of π-conjugated materials (including polymers and oligomers) based OSCs onto flexible plastic substrates represents a potential platform for continuous, largescale printing of thin-film photovoltaics (Krebs, 2009; Peet, 2009). Rapid development of this technology has led to growing interest in OSCs in academic and industrial laboratories and has been the subject of multiple recent reviews (Cheng, 2009; Dennler, 2009; Krebs, 2009; Tang, 2010). These devices are promising in terms of low-cost power generation, simplicity of fabrication and versatility in structure modification. The structure modification of πconjugated materials has offered wide possibilities to tune their structural properties (such as rigidity, conjugation length, and molecule-to-molecule interactions) and physical properties (including solubility, molecular weight, band gap and molecular orbital energy levels). This ability to design and synthesize molecules and then integrate them into organic–organic and inorganic–organic composites provides a unique pathway in the design of materials for novel devices. The most common OSCs are fabricated as the bulkheterojunction (BHJ) devices, where a photoactive layer is casted from a mixture solution of polymeric donors and soluble fullerene-based electron acceptor and sandwiched between two electrodes with different work functions (Yu et al., 1995). When the polymeric donor is excited, the electron promoted to the lowest unoccupied molecular orbital (LUMO) will lower its energy by moving to the LUMO of the acceptor. Under the built-in electric field caused by the contacts, opposite charges in the photoactive layer are separated, with the holes being transported in the donor phase and the electrons in the acceptor. In this way, the blend can be considered as a network of donor–acceptor heterojunctions that allows efficient


Introduction
The effective conversion of solar energy into electricity has attracted intense scientific interest in solving the rising energy crisis.Organic solar cells (OSCs), a kind of green energy source, show great potential application due to low production costs, mechanical flexibility devices by using simple techniques with low environmental impact and the versatility in organic materials design (Beal, 2010).In the past years, the key parameter, power conversion efficiencies (PCE), is up to 7% under the standard solar spectrum, AM1.5G (Liang et al., 2010).The PCE of solar cells are co-determined by the open circuit voltage (V oc ), the fill factor (FF) and the short circuit density (J sc ).Researchers have made great efforts in both developing new organic materials with narrow band gap and designing different structural cells for harvesting exciton in the visible light range.Solution processing of π-conjugated materials (including polymers and oligomers) based OSCs onto flexible plastic substrates represents a potential platform for continuous, largescale printing of thin-film photovoltaics (Krebs, 2009;Peet, 2009).Rapid development of this technology has led to growing interest in OSCs in academic and industrial laboratories and has been the subject of multiple recent reviews (Cheng, 2009;Dennler, 2009;Krebs, 2009;Tang, 2010).These devices are promising in terms of low-cost power generation, simplicity of fabrication and versatility in structure modification.The structure modification of πconjugated materials has offered wide possibilities to tune their structural properties (such as rigidity, conjugation length, and molecule-to-molecule interactions) and physical properties (including solubility, molecular weight, band gap and molecular orbital energy levels).This ability to design and synthesize molecules and then integrate them into organic-organic and inorganic-organic composites provides a unique pathway in the design of materials for novel devices.The most common OSCs are fabricated as the bulkheterojunction (BHJ) devices, where a photoactive layer is casted from a mixture solution of polymeric donors and soluble fullerene-based electron acceptor and sandwiched between two electrodes with different work functions (Yu et al., 1995).When the polymeric donor is excited, the electron promoted to the lowest unoccupied molecular orbital (LUMO) will lower its energy by moving to the LUMO of the acceptor.Under the built-in electric field caused by the contacts, opposite charges in the photoactive layer are separated, with the holes being transported in the donor phase and the electrons in the acceptor.In this way, the blend can be considered as a network of donor-acceptor heterojunctions that allows efficient donating moiety in D-A narrow band gap polymers' design.Besides, feasible dialkylation at 9-position and selective bromination at the 2,7-positions of fluorene allow versatile molecular manipulation to achieve good solubility and extended conjugation via typical Suzuki or Stille cross-coupling reactions.By using 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) as electron accepting unit and didecylated FL as donating unit, Slooff (Slooff et al., 2007) developed P1 (P1-12 structure in Chart 1) with extended absorption spectrum ranging from 300 to 800nm.Spin-coated from chloroform solution, the device ITO/PEDOT:PSS/P1:PCBM(1:4, w/w)/LiF/Al harvested a extremely high PCE of 4.2% (Table 1).An external quantum efficiency (EQE) of 66% was achieved in the active layer with a film thickness up to 140 nm, and further increasing the film thickness did not increase the efficiency due to limitations in charge generation or collection.For 4.2% PCE device, a maximum EQE of about 75% was calculated, indicating efficient charge collection.By using quinoxaline as electron accepting unit, P2 was synthesized with an E g of 1.95eV (Kitazawa et al., 2009).The device performance is dependent upon the ratio of chloroform(CF)/chlorobenzene(CB) in co-solvent for blend film preparation and a maximal J sc is achieved with CF/CB (2:3 v/v) co-solvent.The optimized device showed 5.5% PCE by inserting 0.1nm LiF layer between BHJ active layer and Al cathode with the structure ITO/PEDOT:PSS/P2:PC71BM/LiF/Al.Similarly structured P3 achieved 3.7% PCE by blending with PC71BM (1:3 w/w) (Gadisa et al., 2007).Different from the common linear D-A alternating polymer design, Jen and his coworkers designed a series of novel two-dimensional narrow band gap polymers, whose backbone adopts high hole transporting fluorene-triarylamine copolymer (PFM) and is grafted with malononitrile (P4) and diethylthiobarbituric acid (P5) through a styrylthiophene π-bridge (Huang et al., 2009).Both of them show two obvious absorption peaks, where the first absorption peaks at ~385 nm are corresponding to the π-π * transition of their conjugated main chains and the others are corresponding to the strong ICT characters of their side chains.Two polymers show narrowed down E g (<2eV) and present similar HOMO energy level as that of PFM, while these two polymers exhibit much lowered LUMO levels (-3.43 and -3.50eV for P4 and P5, respectively) than PFM,.The devices with structure of ITO/PEDOT:PSS(40nm)/P4-5:PC71BM(1:4 w/w, 85nm)/Ca(10nm)/Al(100nm) exhibit a PCE of 4.74% and 4.37% for P4 and P5, respectively (refer to Table 1).Determined by using space-charge-limited-current (SCLC) method, the hole mobility of these two polymer/P71BM blend films was found to be 5.27×10 -4 cm 2 V -1 s -1 for P4 and 1.16×10 -3 cm 2 V -1 s -1 for P5, respectively, which is even higher tha n t h a t o f P 3 H T w i t h s i m i l a r d e v i c e configuration (G.Li, 2005).These two-dimensional polymers are thought to possess better isotropic charge transport ability than linear polymers, which is beneficial for PSC applications.By replacing FL in P4 and P5 with silafluorene (2,7-dibenzosilole) unit, similarly structured two-dimensional conjugated polymers P6 and P7 were developed (Duan et al., 2010) developed.With an E g of 1.83 and 1.74eV, P6 and P7 OSCs harvest high PCE as 2.5% and 3.51%, respectively, with device configuration ITO/PEDOT:PSS/P6-7:PC71BM/Ba/Al.It should be noted that the silofluorene polymers display lower hole transport ability than FL-based P4 and P5.By alternating silafluorene and DTBT units, copolymer P8 was synthesized (Boudreault et al., 2007a).With an E g of 1.82eV, P8 films presented an absorption spectrum blend covered the range from 350 to 750nm.P8 device displayed a PCE of 1.6% under AM 1.5 (90 mW/cm 2 ) illumination with the structure ITO/PEDOT:PSS/P8:PCBM(1:4)/Al.By using similar polymer, P8:PCBM (1:2 w/w) blend film delivered as high as PCE of 5.4% under AM 1.5 (80 mW/cm 2 ) (Zhou et al., 2004).By using DTBT as electron accepting unit, Leclerc (Allard et al., 2010) developed germafluorene based polymer P9 whose device displayed a good PCE of 2.8% with the structure of ITO/PEDOT-PSS/P9:PC71BM/LiF(20nm)/Al.In order to extend π-conjugation of polymer backbone, the ladder-type oligo-p-phenylenes (indenofluorene) consisting of several "linearly overlapping" fluorene was developed.Compared to FL unit, indenofluorene presents a broader more intense absorption band.Solubilizing alkyl chains can be easily introduced into this unique molecular backbone, which may provide a better solution processability of the polymers.Zheng (Zheng et al., 2010) reported three alternating D-A copolymers (P10-12) combining indenofluorene as the donor and DTBT or 5,8-dithien-2-yl-2,3-diphenyl quinoxaline (DTQX) as the acceptor unit.By spin-coating from chlorobenzene (CB):o-dichlorobenzene(DCB) (4:1 v/v) co-solvent, P10 with decylated indenofluorene presents good solubility and its device achieved 2.44% PCE with the configuration ITO/PEDOT:PSS/P10:PCBM/Cs 2 CO 3 /Al (Veldman et al., 2008).In comparison with P10, P12 with one more phenylene group in indenofluorene unit presents greatly improved device PCE (3.67%) with the same configuration, due to great improvement of J sc .By blending with PC71BM in 1:4 weight ratio, P18 OSC showed significantly improved PCE (4.5%).The DTQX-containing P11 presents a good PCE of 2.32%, which was attributed to broad absorption spectra and high J sc .

Carbazole-based conjugated polymers
Structurally analogous to FL, carbazole has been also known as 9-azafluorene.The central fused pyrrole ring makes tricyclic carbazole fully aromatic and electron-rich with its donating nitrogen.The alkyl chains introduced into nitrogen of carbazole improve the solubility.Carbazole derivatives have been widely used as electron-donor materials due to their excellent thermal and photochemical stabilities as well as relatively high charge mobility and good solubility with 9-position alkylation.When two carbazoles fused together to form indolo[3,2-b]carbazole (IC) unit, it exhibits even stronger electron-donating properties, higher hole mobility and better stability than carbazole (Boudreault, 2007b;Li, 2006).With bulky heptadecanyl modified carbazole to improve solubility, Leclerc (Blouin et al., 2007a) developed DTBT-based conjugated poly(N-alkyl-2,7-carbazole) (PC) P13 (P13-26 structure in Chart 2), which exhibits excellent thermal stability, relatively high molecular weight and good solubility.Spin-coated from CF solution, P13:PCBM(1:4 w/w) OSC delivered a high PCE of 3.6% (Table 2).Such high PCE and excellent stability show its potential application in OSC.By using longer alkyl chain, Hashimoto (Zou et al., 2010) reported a similar PC P14, whose device based on P14:PCBM (1:3w/w) showed 3.05% PCE, with all photovoltaic parameters similar to those of P13.By using 4,7-dithien-2-yl-2,1,3benzothiadiazoxaline (DTBX) as accepting unit, Leclerc (Blouin et al., 2007b) synthesized P15.Due to its symmetrical backbone, P15 showed good structural organization, which leads to good hole mobility and thus resultant improvement of J sc and FF for its OSC.P15 based OSC achieved a good PCE of 2.4%.

Chart 2. Carbazole based narrow band gap polymers
In order to improve the solubility and close packing of polymer backbone, Zhang (Qin et al., 2009) designed P16 with planar polymer conformation by introducing two octyloxy chains onto benzothiazole (BT) ring and an octyl chain onto carbazole ring.P16 showed good solubility at elevated temperature and an E g of 1.95eV.Spin-coated from DCB mixture solutions with 2.5% 1,8-diiodooctane (DIO), P16:PC71BM device showed extremely high PCE of 5.4% with the structure ITO/PEDOT:PSS/P16:PC71BM(1:2.5 w/w)/LiF/Al.By replacing the BT unit with benzotriazole, Cao (Zhang et al., 2010) developed PC P17, with an E g of 2.18eV, which was broader than that of P3HT.The optimized device of P17 achieved improved PCE from 1.51% to 2.75% by sandwiching a thin layer of PFN between Al cathode and active layer.On the basis of successful design of two-dimensional PF P4 and P5, Cao [Duan et al., 2010b] developed a series of carbazole-based two-dimensional polymers P18-23, with their E g ranging from 1.7 to 1.9eV.P18-23 present similar HOMO energy levels as the conjugated backbone while their LUMO levels were mainly determined by narrow band gap sidechains.Among them, P22 and P23 exhibited stronger absorption intensity and broader absorption spectra.The photovoltaic performance of P22 and P23 devices indicates the 2ethylhexyl chains had little influence on the V oc of OSC while the bulky heptadecanyl chains on carbazole led to a significant decrease in J sc and FF.In comparison with that of P4 and P5, P18 devices exhibited excellent PCE of 4.16%.P18-23 OSCs possessed high V oc (>0.8eV) values, which was relevant with their relatively low HOMO levels.As known, V oc is related to the offset between the HOMO levels of donor materials and the LUMO levels of acceptor materials.The alkyl chains on carbazole unit had important influence on the holetransporting ability of the resultant polymers: P18 and P19 with short side-chains demonstrated hole mobility high as 2.4×10 -4 and 1.1×10 -3 cm 2 V -1 s -1 , respectively, which are much higher than those of P20-P23 with bulkier alkyl side-chains.The bulky alkyl chains possibly are thought to result in imbalanced electron transporting and consequently decreased FF of polymer.Definitions for all parameters are the same as those in Table 1.
Table 2.The optical, electrochemical, hole mobility, and PSC characteristics of P13-26 It is widely accepted that the formation of strong intermolecular π-π stacking is necessary to achieve high charge mobility in OSC (Payne, 2004;Mcculloch, 2006).Rigid coplanar fused aromatic rings can efficiently enhance this interaction and hence improve charge mobilities.Indolo[3,2-b]carbazole (IC) has large-size fused aromatic ring, making closer packing of conjugated structure.Hashimoto (Zou et al., 2009) developed narrow band gap (1.90eV) PIC P24 by alternating IC donating unit and DTBT accepting unit.The optimized P24 device ITO/PEDOT:PSS/P24:PCBM1:3(w/w)/Al demonstrated a PCE of 2.07%.The study reveals that the formation of an interpenetrated network of donors and acceptors inside active layer is critical for obtaining high-performance OSCs.Such interpenetrated networks can be built by tuning device processing parameters such as the ratio of polymer to PCBM in the blend and post-annealing treatment at suitable temperatures.Besides, a simple and effective method to improve the photovoltaic performance of OSCs is to increase the number of unsubstituted thienyl units in D-A copolymers consisting of DTBT as acceptor.The V oc of P24 is similar with those of some DTBT based narrow band gap polymers with donating units like FL (1.0eV) (Slooff et al., 2007) dibenzosilole (0.9-0.97eV) (Boudreault et al., 2007) or carbazole (0.88 eV) (Blouin et al., 2007) but higher than those of D-A derivatives with donating units such as cyclopenta[2,1-b:3,4-b']dithiophene (0.6eV), dithienosilole (0.44eV) (Liao et al., 2007) or dithieno[3,2-b:2',3'-d] pyrrole(0.52eV)(Zhou et al., 2008).By adopting DTTP as accepting unit, the same group developed P25, with an E g of 1.91 Hashimoto (Zhou et al., 2010a).Spin-coating from CF solution, P25:PCBM (1:4, w/w) based OSC contributed a PCE of 1.56% and was further improved to 2.54% by using DCB instead of CF.By using hexylthiophene moiety in DTBT accepting unit and shorter alkyl chains in IC unit, Movileanu (Lu et al., 2008) developed P24 analogous PIC P26.Long solubilizing side-chains generally resulted in lower charge mobility, balance of the alkyl chains in conjugated polymers was important for charge mobility.P26 film showed pronounced peak broadening and significant red shift (100nm) in absorption spectra than its solution.P26 device ITO/PEDOT:PSS/P26:PCBM(1:2w/w)/LiF/Al achieved a high PCE of 3.6%.
As discussed above, both PC and PF exhibit relatively low-lying HOMO level and thus high V oc for OSCs via copolymerizing with accepting unit including DTBT, DTTP and DTQX.In contrast with other accepting units, DTBT-containing polymers may maintain deep HOMO and narrow the band gap through reducing LUMO, which results in a high V oc .However, PCs devices generally display low J sc and FF, which was similar to PFs.

Thiophene-based conjugated polymers
Polythiophenes (PTs) are considered as the most important conjugated polymers for a broad spectrum of optoelectronic applications such as light-emitting diodes, field-effect transistors, and OSCs due to their excellent optical and electrical properties as well as exceptional thermal and chemical stability (Roncali, 1992).
Chart 3. Thiophene-based narrow band gap polymers www.intechopen.com By alternating quaterthiophene and diketopyrrolopyrrole (DPP) units, Janssen (Wienk et al., 2008) developed PT P27 (P27-36 structure in Chart 3), with an E g of 1.4eV (Table 3).The absorption spectrum of P27 in DCB showed stronger red-shift than its solution in CF.Fast evaporation of CF did not allow P27 to crystallize, which results in poor morphology when spin-coating P27:PCBM blend film from CF solution.Certain degree of crystallization could be achieved by heating the film at 130 o C. Spin-coated from CF:DCB (4:1 v/v) mixture solution, P27:PCBM (1:2 w/w) optimized OSC reached highest PCE of 4%, while OSC achieving only 1.1% PCE using CF as blending solvent and good PCE of 3.2% when prepared from DCB.
Furans have been used as an alternative to thiophenes in organic dyes for dye-sensitized solar cells and have shown very similar optical and electronic properties (R. Li et al., 2009).
The first furan-containing DPP-based low band gap polymers P28-29 were developed (Claire et al., 2010).By spin-coating from CB solution with 1vol% high-boiling-point 1chloronaphthalene (CN), P28 device achieved slightly improved PCE from 3.4% to 3.8% with the structure ITO/PEDOT:PSS/P28:PCBM(1:3w/w)/LiF/Al, due to optimized blend morphology.The best P28:PC71BM (1:3w/w) OSC contributed a PCE of 5.0% with the addition of 9% CN in CB.The device with blend P29:PC71BM (1:3 w/w) a PCE of 4.1% with 5% CN addition in CB.The addition of CN led to much finer phase separation between polymer and fullerene to form a fiber-like inter-penetrating morphology at the length scale of 20 nm, which is close to the ideal domain size, assuming an exciton diffusion length of 5-10 nm (Markov et al., 2005;Scully et al., 2006;Shaw et al., 2008).By alternating bithiophene donating unit and DPP accepting unit, Janssen (Johan et al., 2009)  Definitions for all parameters are the same as those in Table 1.
For the former, the maximum EQE was over 60% at 650 nm and was 50% in the range of 550-750 nm; the latter exhibited a higher EQE, almost all over 60% in the range from 400 to 750 nm.By changing side-chain substituents, P37 analogous polymers P38-41 were developed (Liang et al., 2009b).All polymers showed very close between 2.6×10 -4 and 4.7×10 -4 cm 2 V -1 s -1 and similar absorption spectra, covering the whole visible region.
Chart 4. BDT based narrow band gap polymers.
The substitution of octyloxy to octyl group lowered the HOMO energy level of P38 from -4.94 to -5.04 eV (P39).With electron-withdrawing fluorine in polymer backbone, P40 had significantly lower HOMO level in comparison to P41.The devices based on ITO/PEDOT: PSS/P38-41:PCBM (1:1 w/w)/Ca/Al exhibited a PCE of 5.10%, 5.53%, 3.20% and 3.02%,respectively, with active layer spin-coated from DCB solution.By using DCB/DIO (97:3 v/v) mixture solvent, PCE of P39-P41 can be dramatically improved to 5.85%, 5.90% and 4.10%, respectively.By replacing octyl group in P40 into 2-ethylhexyl, P52 was further developed (Liang et al., 2010).Spin-coated from DCB solution, P51:PC71BM harvested a PCE of 5.74%, 6.22% and 5.58%, respectively at their D/A weight ratio as 1:1, 1:1.5 and 1:2.The best device contributes PCE as high as 7.4% with the mixture solvent of CB/DIO(97:3 v/v), where uniform morphology of blend film was prepared.BDT-containing polymers can finely tune the HOMO level and efficiently narrow the band gap (Pan, 2007;Liang, 2008;Liang, 2009).Besides, alkoxy groups have been proved to be beneficial to reduce the HOMO level of polymer due to its stronger electron-donating property (Liang et al., 2009b).By removal of the oxygen atom on ester group in thieno [3,4-b]thiophene unit, ketone-substituted polymer P43 (Chen et al. 2009) was developed to further reduce the HOMO level.The best OSC based on P43:PC71BM showed a high PCE of 6.58%.By alternating BDT and TPD, Leclerc (Zou et al, 2010) presented polymer P44, with an E g of 1.8eV and HOMO/LUMO energy levels -5.56eV/-3.75eV,respectively.The device ITO/PEDOT:PSS/P53:PC71BM/LiF/Al showed a high PCE of 5.5%.By incorporating BDT with DTBT, You (Price et al., 2010) (Peet et al., 2008).However, for P50, the addition of DIO led to only slight improvements, which was attributed to a high level of order of P50 in the blend without DIO.It was concluded that alkyl-substituted groups had much impact on the PCE of P48-50.The use of DIO is effective in promoting the packing of the polymer by preventing excessive crystallization of PCBM.Dithieno[3,2-b:2',3'-d]pyrrole (DTP) has good planarity and stronger electron-donating ability of nitrogen atoms.Geng (Yue et al. 2009) synthesized BT-based P(DTP)s P51-52 (Chart 5), with an E g ~1.43eV.Owing to different alkyl chain, P52 exhibited stronger absorption in the range of 600-900nm compared to P51.P51-52 devices showed a PCE of 2.06% and 2.8%, respectively with the structure ITO/PEDOT:PSS/P51-52:PCBM(1:3 w/w)/LiF/Al, at the optimized blend film thickness of 100 nm for P51 and 90 nm for P52.The device of P51-52 delivered a V oc in the range of 0.4-0.54V,leading to relatively low PCE.By adopting DTP and DTBT units as the donor and acceptor segments, Hashimoto (Zhou et al., 2008) synthesized P53 with an E g of 1.46eV, which was the lowest compared with other D-A copolymers based on DTBT acceptor segment with other donor segments such as FL, silafluorene, carbazole, DTS, and CPDT (Slooff, 2007;Blouin, 2007;Moulle, 2008).P53 device exhibited a PCE of 2.18% and a high J sc of 9.47mA/cm 2 , which was attributed to its broad absorption spectrum.However, P53 device presented a V oc of 0.52V, which is lower than that of the devices based on DTBT-containing D-A copolymers with FL (1V), silafluorene (0.9V) or carbazole (0.9V) as donating unit.3,pyrrole-1,4-dione (DTDPP) emerged as a promising electron acceptor in the design of photovoltaic polymers (Wienk et al., 2008).With DTDPP as accepting unit, Hashimoto (Zhou et al., 2010b) presented P54, with an E g lowed down to 1.13eV.P54 presented a hole mobility of 0.05cm 2 V -1 s -1 , 1-2 orders of magnitude higher than that of other D-A polymers.Spin-coated with mixing solution of CF and DCB (4:1 v/v), P54 device with structure of ITO/PEDOT:PSS/P54:PCBM(or PC71BM)/LiF/Al showed a PCE of 2.34% with PCBM and 2.71% with PC71BM as acceptor in the blend films.The PCE of P54 was lower that of P3HT, which was mainly attributed to the lower V oc , resulting from its high-lying HOMO level (-4.90eV).It was expected that the device performance would be improved by further reducing the HOMO and LUMO level.
Chart 5. Fused Thiophene based narrow band gap polymers.
Recently, You (H.Zhou, 2010b) proposed a design strategy for narrow band gap polymers by alternating "weak donor" and "strong acceptor" to approach ideal polymers with both low HOMO energy level and small E g in order to achieve both high V oc and J sc (H.Zhou, 2010e).By alternating DTBT or thiadiazolo[3,4-c]pyridine as strong acceptor,with naphtho[2,1-b:3,4-b']-dithiophene (NDT), dithieno[3,2-f:2',3'-h]quinoxaline (QDT) or BDT as weak donor, polymers P55-61 were developed (Yang, 2010;H. Zhou, 2009;H. Zhou, 2010bH. Zhou, , 2010c)).For NDT-containing P55-56, a 4-(2-ethylhexyl)thiophene unit in DTBT was used to reduce the steric hindrance of polymer backbone and hence to achieve near identical E g and energy levels (H.Zhou, 2010a).It was believed that E g and energy levels of a conjugated polymer were primarily determined by the molecular structure of conjugated backbone, while the solubilizing alkyl chains should have a negligible impact on these properties and hence a little impact on device's J sc and V oc .However, Yu's study showed variations of length and shape of alkyl chains in NDT had significant influence on device performance: long and branched side-chains would weaken the intermolecular interaction, benefiting for improvement of V oc ; while short and straight side-chains would promote the intermolecular interaction, rendering a large J sc .The desirable balance between V oc and J sc could be tuned through adopting suitable short and branched side-chains.P55-56:PCBM (1:1,w/w) devices showed a best PCE of 3.0% and 3.36%, respectively.(Price, 2010;H. Zhou, 2010c).
The device with ITO/PEDOT:PSS/P57-59:PCBM/Ca/Al achieved a PCE of 6.20% for P57, 5.57% for P58, and 6.32%% for P59.By alternating DTBT acceptor with NDT or QDT weak donor, You (H.Zhou, 2010e) developed P60-61, with an E g of P60-61 of 1.61eV and 1.70eV, respectively.The devices based on P60-61:PCBM achieved a PCE of 5.1% and 4.3%, respectively.The donor unit with electron-withdrawing atoms was demonstrated to lead to a lower HOMO level while have little impact on the LUMO level.You thought that future research should be focused on employment of even weaker donors and stronger acceptors via innovative structural modification in order to concurrently achieve a higher V oc and a higher J sc .By alternating DPP and benzo[1,2-b:4,5-b']dithiophene, Yang (Huo et al., 2009) synthesized P62.Its device with the structure of ITO/PEDOT:PSS/P62:PC71BM(1:2 w/w)/Ca/Al achieved improved PCE from 4.31% to 4.45% after device annealing under 110 o C for 30 min.P62 was expected to a very promising candidate for highly efficient OSCs owing to relatively low E g and suitable HOMO level.Ladder-type coplanar thiophene-phenylene-thiophene (TPT) structure tends to lead to strong molecular π-π interaction and thus exhibits remarkable hole mobility (Wong et al., 2006).Poly(TPT)s demonstrated an E g ~2.1eV and hole mobility as high as 10 -3 cm 2 V -1 s -1 (Chan et al., 2008).By alternating TPT donor and DTBT acceptor, Ting (Chen et al., 2008) developed P63-64, which exhibited an E g of 1.76eV and 1.70eV, and absorption band in the range of 350-700nm and 350-730nm, respectively.P64 displayed a hole mobility up to 3.4×10 -3 cm 2 V -1 s -1 .Both of them showed good solubility at room temperature in organic solvents.Using DCB as blending solvent, the devices ITO/PEDOT:PSS/P63-64:PCBM(1:3 w/w)/Ca/Al harvested a PCE of 2.0% and 2.5%, respectively, a little lower than that (3.9% PCE) of P3HT:PCBM(1:1 w/w) device under the same conditions.The optimized device www.intechopen.comSolar Cells -New Aspects and Solutions 446 with P63-64:PC71BM (1:3 w/w) showed a PCE of 3.9% for P63 and 4.3% for P64, higher than that of the device based on P3HT:PC71BM (1:1 w/w) (3.4%) under the same conditions.

Conclusions
Narrow band gap polymers P1-P64 developed by alternating donor (ca.fluorene, carbazole and thiophene) and acceptor (ca.benzothiadiazole, quinoxaline and diketopyrrolopyrrole) units in recent 4 years are summarized, with their fullerene blend-based BHJ OSCs contributing PCE over 3%.The design criteria for ideal polymer donors to achieve high efficiency OSCs is: (1) a narrow E g (1.2-1.9eV) with broad absorption to match solar spectrum; (2) a HOMO energy level ranging from -5.2 to -5.8 eV and a LUMO level ranging from -3.7 to -4.0eV to ensure efficient charge separation while maximizing V oc ; and (3) good hole mobility to allow adequate charge transport.Besides, device structure and morphology optimizations of polymer:fullerene blend film have been extensively demonstrated to be crucial for PCE improvement in OSCs.The current endeavors boosted OSCs PCEs up to 7% would encourage further efforts toward a next target of efficiency in excess of 10%.