Correlating nanoscale morphology with device performance in conventional and inverted PffBT4T-2OD:PC71BM polymer solar cells

Nanoscale morphology has been established as one of the controlling factors in the device performance of bulk heterojunction polymer solar cells. We report in this work morphology changes in both lateral and vertical directions in PffBT4T-2OD:PC71BM solar cells, as well as their effects on device performance. Thermal annealing was found to increase the crystallinity of PffBT4T-2OD and domain size of PC71BM clusters without any observable impact on vertical component redistribution, while methanol rinsing reduces the crystallinity of PffBT4T-2OD, which encourages the migration of PC71BM toward the mixed polymer-rich phase as well as toward the film surface on both PEDOT:PSS and TiO2 substrates. The polymer-rich surface region in vacuum- and thermal-annealing-treated conventional devices obstructs electron injection toward the cathode, and reduces the maximum achievable device efficiency, while this polymer-rich surface region is beneficial in the inverted devices. However, although a PC71BM-rich region will locate at the cathode or anode interface upon methanol-rinsing treatment in conventional and inverted devices, respectively, holes can still be effectively injected from both sides of the device to ensure effective charge transport, as supported by a number of optoelectronic property investigations.