MTL Annual Research Report 2011

Photovoltaic cells containing nanoscale phase separated mixtures of a poly(thiophene) donor and a fullerene acceptor in the active layer have proven to be a attractive class of low-cost solar energy harvesting devices. The benchmark polymer solar cell (PSC) is one fabricated with poly(3-hexylthiophene) (P3HT) and PC_nBM (n = 61, 71, Figure 1), which displays power conversion efficiencies of approximately 5% ^[1] . Extensive optimization of the polymeric donor component has lead to the evolution of PSCs with power conversion efficiencies of approximately 8% ^[2] ; however, soluble fullerene derivatives remain the primary choice for the donor component in bulk heterojunctions. We have demonstrated that select small-molecule acceptors can serve as fullerene substitutes in P3HT/PC₆₁BM bulk heterojunctions and can increase the power conversion efficiencies of the resulting solar cells (Figure 2). Ternary mixtures containing 6,6-dicyanofulvenes ^[3] , such as DCF (Figure 1), yield average power conversion efficiencies of 4%. Moreover, ternary mixtures containing stable organic radicals, such as TEMPO and DPPH, also augment the performance of P3HT–PC₆₁BM solar cells and result in power conversion efficiencies of up to 3.4%.