Facile synthesis of low band-gap DPP–EDOT containing small molecules for solar cell applications

Abstract

Four donor–donor–acceptor–donor–donor (D1–D2–A–D2–D1) type small molecules (SM1, SM2, SM3 and SM4), in which diketopyrrolopyrrole (DPP) was used as an acceptor (DPP) core and 3,4-ethylenedioxythiophene (EDOT), triphenylamine (TPA) or alkyl thiophene (AT) acted as a donor, were synthesized (by direct alkylation) for photovoltaic applications. The benzene flanked DPPs (SM1 and SM2) show more limited absorption and a lower-lying HOMO energy level compared to the thiophene flanked DPPs (SM3 and SM4, respectively), due to benzene being a poor donor compared to a thiophene moiety. SM3 and SM4 have absorptions extending to 750 nm (in CH₂Cl₂) and can be regarded as low band-gap small molecules. The inverted small molecule solar cell (using PCBM as an acceptor, and ZnO and MoO₃ as the electron and hole transporters, respectively) based on SM2 has the highest value of V_oc (0.82 V) due to SM2 having the lowest HOMO level. An SM3 based device achieves the highest value of J_sc (5.56 mA cm⁻²), due to the wide absorption profile and proper LUMO level. The poor photovoltaic performances of SM1 and SM4 (in which D1 is a triphenyl amine group) are due to the serious self-aggregation of the molecules in the SMx/PCBM active films. Two acceptor–donor–acceptor type (A–D–A) molecules (SM5 and SM6) were also synthesized by Stille coupling, in which DPP and EDOT were used as the acceptor and donor, respectively, for the structure–photovoltaic performance study. The SM5 and SM6 films, having on-set absorption of up to 1000 nm, can also be regarded as low band-gap small molecules. SM5 and SM6 are structural isomers, however the inverted cell based on SM6 has a much higher PCE than that based on SM5, due to SM6 having better solubility and a lower HOMO energy level. This study provides a guideline for the rational design and preparation of conjugated small molecules for organic solar cells.