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**20** 

**Towards High-Efficiency Organic Solar Cells:** 

Enwei Zhu1, Linyi Bian1, Jiefeng Hai1, Weihua Tang1,\* and Fujun Zhang2,\*

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

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

**1. Introduction** 

cells for harvesting exciton in the visible light range.

**Polymers and Devices Development** 

*1Nanjing University of Science and Technology,* 

*2Beijing Jiaotong University People's Republic of China* 

