Efficient and stable organic solar cells via a sequential process

Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted considerable attention in the last two decades. Sequentially solution processed BHJ (s-BHJ) have been developed in recent years. s-BHJ not only maintain some advantages of mixed BHJ (m-BHJ), but also exhibit other advantages over m-BHJ. However, to date, s-BHJ OSCs exhibit relatively lower efficiency and have received much less attention compared with m-BHJ OSCs. Moreover, there have been rare systematic comparisons between m-BHJ and s-BHJ OSCs. In this work, we systematically compare the m-BHJ and s-BHJ OSCs based on a classical system PTB7-TH/PC71BM in terms of film morphology, domain size and purity, molecular orientation and aggregation, vertical phase separation, charge transport, efficiency and stability. The s-BHJ OSCs without additives exhibit efficiencies as high as 8.6%, which is similar to that of m-BHJ OSCs with additives (8.5%) and is the highest reported for sBHJ OSCs. More importantly, the s-BHJ OSCs show much better device stability than the m-BHJ OSCs. This study demonstrates that employing s-BHJ is a promising strategy towards efficient and stable OSCs.