Organic bulk heterojunction solar cells using poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2,-b]thiophene)

By transitioning to semicrystalline polymers, the performance of polymer-based solar cells has recently increased to over 5% [W. Ma et al., Adv. Fund. Mater. 15, 1665 (2005); G. Li et al., Nat. Mater. 4, 864 (2005); M. Reyes-Reyes et al., Org. Lett. 7, 5749 (2005); J. Y. Kim et al., Adv. Mater. (Weinheim, Ger.) 18, 572 (2005); J. Peet et al., Nat. Mater. 6, 497 (2007)]. Poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-$b$ ]thiophene) (pBTTT) has caused recent excitement in the organic electronics community because of its high reported hole mobility ($0.6{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ ) that was measured in field effect transistors and its ability to form large crystals. In this letter, we investigate the potential of pBTTT as light absorber and hole transporter in a bulk heterojunction solar cell. We find that the highest efficiency of 2.3% is achieved by using a 1:4 blend of pBTTT and[6,6]-phenyl ${\mathrm{C}}_{61}$ -butyric acid methyl ester. The hole mobility as measured by space charge limited current modeling was found to be $3.8\times {10}^{-4}{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ in this blend.