Efficient bulk heterojunction solar cells based on low band gap bisazo dyes containing anthracene and/or pyrrole units

2,5-Bis(2-anthracenyldiazo)-1H-pyrrole (dye D1) and 2,2'-[1,4-phenylenebis(azo)]bis-1H-pyrrole (dye D2) were synthesized by four- and two-step reactions, respectively. Their absorption spectra were broad with thin film absorption onset at 829 and 738 nm corresponding to an optical band gap of 1.39 and 1.68 eV for D1 and D2, respectively. The solution processed bulk heterojunction (BHJ) photovoltaic devices fabricated from D1 and D2 as donor blended with PCBM as an acceptor showed power conversion efficiency (PCE) up to 2.13% and 1.59%, respectively. The PCE has been further improved up to 2.42% and 2.18%, after thermal annealing of the D1:PCBM and D2:PCBM layers, respectively. The higher PCE of the photovoltaic devices based on the D1:PCBM blend as compared to D2:PCBM is attributed to the higher hole mobility and lower band gap of D1 relative to D2. The combination of D1 with D2 and PCBM allows not only a broad absorption, but also tuning of the inter energy level leading to a higher short circuit current (J(sc)) and open circuit voltage (V-oc). The best performance device exhibited PCE of about 3.61% with the thermally annealed D1:D2:PCBM photoactive layer. Such a high PCE using small molecules is due to the increased values of both J(sc) and V-oc. (C) 2010 Elsevier B.V. All rights reserved.