Polymer/Polymer Blend Solar Cells Improved by Using High-Molecular-Weight Fluorene-Based Copolymer as Electron Acceptor

The highest power conversion efficiency (PCE) of 2.7% has been achieved for all polymer solar cells made with a blend of poly(3-hexylthiophene) (P3HT, electron donor) and poly[2,7(9,9-didodecylfluorene)-alt-5,5-(4',7'-bis(2-thienyl)-2',1',3'-benzo-thiadiazole)] (PF12TBT, electron acceptor). The PCE of the P3HT/PF12TBT solar cells increases from 1.9% to 2.7% with an increase in the molecular weight (M-w) of PF12TBT from 8500 to 78 000 g mol(-1). In a device with high-molecular-weight PF12TBT, efficient charge generation is maintained even at high annealing temperatures because of the small phase separation on the length scale of exciton diffusion due to an increase in the glass transition temperature (T-g) and a reduced diffusional mobility of the PF12TBT chains above T-g. On the other hand, efficient charge transport is also achieved through the formation of interconnected networks of PF12TBT-rich domains, which is facilitated by the high molecular weight of PF12TBT, and the ordering of P3HT chains in P3HT-rich domains, which is a result of high temperature annealing. Thus, when high-molecular-weight PF12TBT is used, an optimal blend morphology that supports efficient charge generation as well as charge transport can be obtained by thermal annealing, and consequently, the highest PCE reported so far for an all polymer solar cell is achieved.