Improving the Performance of Layer-by-Layer Processed Organic Solar Cells via Introducing a Wide-Bandgap Dopant into the Upper Acceptor Layer.

The layer-by-layer (LbL) solution-processed organic solar cells (OSCs) are conductive to achieve vertical phase separation, tunable donor-acceptor (D/A) interfaces, and favorable charge-transport pathways. In this work, a wide-bandgap component poly(9-vinylcarbazole) (PVK) is added to the upper electron acceptor layer to improve the performance of LbL-processed OSCs. Results show that the PVK component can adjust the film morphology, dope the electron acceptor, increase the electron concentration, and improve charge transport. Such n-type doping is verified by Seebeck coefficient measurement, ultraviolet photoelectron spectroscopy, and electron paramagnetic resonance characterization. In addition, the fluorescence intensity and exciton lifetime of the PVK-doped acceptor film are increased, thus being beneficial for exciton diffusion to the D/A interface. Therefore, the power conversion efficiency (PCE) of LbL OSCs increases when 2.50 wt.% PVK is employed in the electron acceptor layer of commonly-used high-efficiency system and a maximum value of 19.05% can be achieved. The role of PVK played in the active layer is different from those of additives and ternary components reported previously, so the results provide an alternative way to enhance the device performance of LbL-processed OSCs.