Organic solar cells power conversion efficiencies

Organic solar cells have reached efficiencies exceeding 4%. In fact power conversion efficiencies of organic solar cells have reached an impressive 5%. This has been possible because of the discovery of bulk heterojunction solar cells. Solar cells are discussed in Chapter 5. 

Rectification and photovoltaic effects in organic p-n junctions were first reported by Kearns and Calvin [101] and by Meier [3]. The combination of rhodamines or triphenylmethane dyes (both n-type) with merocyanines or phthalocyanines (both p-type) generated photovoltages up to 200 mV and photocurrents of about 10 8 A at low light intensity, with power conversion efficiency much less than 1%. More recent studies have been performed on merocyanine and malachite green [89,90] and on phthalocyanines and TPyP (a porphyrin derivative) [102,103]. These devices showed stronger spectral sensitization and better spectral match to a solar spectrum than those of Schottky barrier cells using only one component. 

To help the design of optimized polymeric materials for BHJ solar cells, several models have been recently proposed [87-89]. The combination of these models and DFT calculations has recently led to the synthesis of several other poly(2,7-carbazole) derivatives (P17, P19-P22). Symmetric polymers (P17-P19) show better structural organization than asymmetric polymers (P20-P22), resulting in higher hole mobilities and power conversion efficiencies. Moreover, their low HOMO energy levels (ca. (- 5.6)—(— 5.4)eV) provide an excellent air stability and relatively high Voc values (between 0.71-0.96 V). 

In this section, we review the basic device physics of organic donor-acceptor solar cells and identify the key material and device parameters that should be addressed in order to improve power conversion efficiency. 

Various alternative acceptor components for organic BHJ solar cells have been tried in an attempt to improve cell performance. Fullerene derivatives such as C70 PCBM and Cg4 PCBM have been used in place of Ceo PCBM, because the lower molecular symmetry compared with Ceo PCBM enables stronger light absorption by the fullerene. The C70 PCBM derivatives were relatively successful, leading to 3% power conversion efficiency for devices made with MDMO-PPV polymer (Wienk et al, 2003). The Cg4 derivatives resulted in rather poor device efficiencies, attributed to the unfavourable film morphology resulting from the immiscibility of Cs4 derivatives with typical organic solvents (Kooistra et al, 2006). 

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