Highly bright Li(Gd,Y)F4:Yb,Er upconverting nanocrystals incorporated hole transport layer for efficient perovskite solar cells

We report synthesis of highly bright Li(Gd,Y)F-4:Yb,Er upconversion (UC) nanophosphors and integrate them into the hole transport layers (HTLs) to examine their optical and electrical effects in perovskite solar cells (PSCs). Mono-disperse, single tetragonal-phase Li(Gd,Y)F-4:Yb,Er upconversion nanocrystals (UCNCs) were prepared by a thermal decomposition method. The grown Li(Gd,Y)F-4:Yb,Er UCNCs displayed tetragonal bipyramidal morphologies and intense UC green luminescence. The octahedral UCNCs were blended into the Spiro-OMeTAD-based HTL with different weight ratios. At an optimal weight ratio of UCNCs, the PSCs based on the upconverting transport hole layer achieved an average power conversion efficiency of 18.34% under AM1.5G illumination, an increase over 25% compared with conventional HTL based PSCs (14.69%). We systematically explore the optical and electrical enhancement effects of Li(Gd,Y)F-4:Yb,Er UCNCs via UV-visible absorption, diffused reflectance, external quantum efficiency (EQE), steady-state and time-resolved photoluminescence, photocurrent property and the space-charge-limited-current (SCLC) measurements. It was found that with the addition of the octahedral UCNCs in a proper proportion the composite HTL exhibits high charge carrier mobility and charge carrier transfer/collection capability. Besides, the embedded UCNCs boost the light harvesting of the PSCs over a large range of wavelengths from 400 nm to 800 nm. Furthermore, with a 980 nm near-infrared (NIR) laser irradiation, the embedded UCNCs acted as efficient upconverting centers to up-convert the NIR light for promisingly expanding the effective absorption spectrum for PSCs.