Effects of selenophene substitution on the mobility and photovoltaic efficiency of polyquaterthiophene-based organic solar cells

We investigated thin films comprised of a blend of poly(5,5′-bis(3-dodecylthiophene-2-yl)2-2′-biselenophene)/[6,6] and phenyl C61 butyric acid methyl ester (PDT2Se2/PCBM) for use in bulk heterojunction photovoltaic cells. The charge transport characteristics of PDT2Se2 and its analog, poly(3,3′′′-didodecyl quaterthiophene) (PQT-12), were elucidated through analysis of the space charge limited current behavior at various temperatures. PDT2Se2 showed higher mobilities, lower field activation parameters, and a lower temperature dependence of these parameters than did PQT-12, indicating better charge transport in PDT2Se2. Optimization of the PDT2Se2:PCBM composition ratio produced a bicontinuous donor-acceptor network with domain sizes on the order of 10 nm, which afforded power conversion efficiencies of 1.4%, a short circuit current density of 4.3 mA cm^-2, an open circuit voltage of 0.69 V, and a fill factor of 47%. This performance was much better than the performance achieved previously using PQT-12:PCBM blend systems. Selenophene substitution appears to be an effective strategy for enhancing the photovoltaic effect of thiophene-based polymeric semiconductors for high performance organic solar cells (OSCs).