**1. Introduction**

432 Solar Cells – New Aspects and Solutions

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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

Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development 435

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.

blending with PC71BM (1:3 w/w) (Gadisa et al., 2007).

μh cm2 V-1s-1

*Eg* eV

apolymer:PC71BM; bpolymer:PCBM in weight ratio.

Polymer λmaxabs

nm

By using quinoxaline as electron accepting unit, **P2** was synthesized with an *Eg* 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 *Jsc* 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

> HOMO /LUMO, eV

**P1** - - - 1:4 7.7 1.0 0.54 4.2 **P2** 540 1.95 - -5.37/- 1:4a 9.72 0.99 0.57 5.5 **P3** 542 1.94 - -6.30/-3.60 1:3 6.00 1.00 0.63 3.7 **P4** 545 1.87 5.3×10-4 -5.30/-3.43 1:4 9.62 0.99 0.5 4.74 **P5** 580 1.76 1.2×10-3 -5.26/-3.50 1:4 9.61 0.99 0.46 4.37 **P6** 541 1.83 1.8×10-4 -5.32/-3.49 1:4a 6.69 0.85 0.37 2.50 **P7** 579 1.74 2.1×10-4 -5.35/-3.61 1:4a 6.22 0.90 0.45 3.15 **P8** 565 1.82 1.0×10-3 - 1:2 9.50 0.90 0.51 5.4 **P9** 580 1.79 1.1×10-4 -5.58/-3.91 - 6.9 0.79 0.51 2.8 **P10** 543 1.97 4.2×10-3 -5.47/-3.44 1:3.0 6.10 1.00 0.40 2.44 **P11** 541 2.00 9.5×10-4 -5.45/-3.36 1:3.5a 7.57 1.00 0.40 3.04 **P12** 531 1.96 9.7×10-3 -5.45/-3.45 1:4.0a 10.3 1.04 0.42 4.50 λmaxabs: maximum absorption peak in lm; *Eg*: optical band gap; μh: hole mobility; *Jsc*: short-circuit current density; *Voc*: open-circuit voltage; FF: ll factor; PCE: power conversion efciency;

Table 1. The optical, electrochemical, hole mobility, and PSC characteristics of **P1-12**

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 *Eg* (<2eV) and present similar HOMO energy

Polymer: PCBMb

*Jsc*  mA/cm2 *Voc*

<sup>V</sup>FF PCE

charge separation and balanced bipolar transport throughout its whole volume. Remarkably, the power conversion efficiency (PCE, defined as the maximum power produced by a photovoltaic cell divided by the power of incident light) of the OSCs has been pushed to more than 7% from 0.1% after a decade's intensive interdisciplinary research. The current workhorse materials employed for PSCs are regioregular poly(3 hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). This material combination has given the highest reported PCE values of 4%~5% (G. Li, 2005). Theoretically, the PCE of polymer solar cells can be further improved (ca 10%) (Scharber et al., 2006) by implementing new materials (Cheng, 2009; Peet, 2009; Tang, 2010) and exploring new device architecture (Dennler, 2008; Ameri, 2009; Dennler, 2009) after addressing several fundamental issues such as bandgap, interfaces and charge transfer (Li, 2005; Chen, 2008; Cheng, 2009).

In this account, we will update the recent 4 years progress in pursuit of high performance BHJ OSCs with newly developed conjugated polymers, especially narrow bandgap polymers from a viewpoint of material chemists. The correlation of polymer chemical structures with their properties including absorption spectra, band gap, energy levels, mobilities, and photovoltaic performance will be elaborated. The analysis of structureproperty relationship will provide insight in rational design of polymer structures and reasonable evaluation of their photovoltaic performance.
