Improved efficiency of polymer solar cells by plasmonically enhanced photon recycling
In spite of the many favorable characteristics of polymer solar cells (PSCs), the critical issue of their relatively low efficiency stemming from a low carrier mobility and short exciton diffusion length is still a main hurdle for full commercialization. Herein we present a simple route to improve the efficiency of PSCs by exploiting plasmonically assisted photon recycling. With embedded gold nanorods (AuNRs) in the hole injection layer of PSCs, photon radiation from excitons is promoted via localized surface plasmons resonating at the band-edge energy of the active material, and hence photon recycling is enhanced. While this enhancement of photon recycling causes a slight increase of the open-circuit voltage as expected from previous studies, surprisingly it also provokes a substantial improvement of the short-circuit current without additional photon absorption. This exceptional improvement of the photocurrent in PSCs seems to be due to the effect of the photon recycling on extending the effective diffusion length of excitons. Together with another gain of electrical ohmic loss reduction, introducing AuNRs boosts the power conversion efficiency by ∼11.4% and ∼11.7% for devices with wide and narrow bandgap materials, respectively. Details of the experimental characterization of various devices provide evidence for the elucidation of the photon recycling effects in PSCs.