Improving Organic Solar Cells Efficiency Through a Two-Step Method Consisting of Solvent Vapor Annealing and Thermal Annealing

The realization of high-efficiency organic small molecule: Fullerene solar cells are challenging but become more and more feasible due to the rapid development in donor materials and device fabrication techniques. In the past several years, the optimization in the processing techniques, such as, solvent vapor annealing (SVA), the use of solvent additives had led to superior improvement in the device performance of organic solar cells from different donor systems, but at an expense of reduction in the open-circuit voltage (
$V_{{\rm OC}}$
). In this paper, we report that the overall device performance of the organic small molecule solar cells (SMSCs) can be significantly enhanced through a two-step process consisting of SVA and thermal annealing (TA) (short for SVA+TA), especially the reduction in
$V_{{\rm OC}}$
can be effectively avoided in case of exclusive use of SVA. The carrier dynamics is determined by transient photovoltage and transient photocurrent measurements, which can provide information on the origin of enhanced device performance upon the two-step annealing. The observed
$V_{{\rm OC}}$
recovery is attributed to the preferable change in charge dynamic, thus, leading to a superior overall device performance. Furthermore, the SVA and the consequential TA are complementary to each other; thus, the two-step annealing method represents a feasible route to simultaneously improve the
$V_{{\rm OC}}$
,
$J_{{\rm SC}}$
, FF, and PCE of SMSCs.