No abstract

We report a detailed study on APbX3 (A = formamidinium (FA+), Cs+; X = I–, Br–) perovskite quantum dots (PQDs) with combined A- and X-site alloying that exhibits both a wide bandgap and high open-circuit voltage (V oc) for the application of a potential top cell in tandem junction photovoltaic (PV) devices. The nanocrystal alloying affords control over the optical bandgap and is readily achieved by solution-phase cation and anion exchange between previously synthesized FAPbI3 and CsPbBr3 PQDs. Increasing only the Br– content of the PQDs widens the bandgap but results in shorter carrier lifetimes and associated V oc losses in devices. These deleterious effects can be mitigated by replacing Cs+ with FA+, resulting in wide-bandgap PQD absorbers with improved charge-carrier mobility and PVs with higher V oc. Although further device optimization is required, these results demonstrate the potential of FA1–x Cs x Pb­(I1–x Br x )3 PQDs for wide-bandgap perovskite PVs with high V oc.

Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate. Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite (HaP; CsPbBr3)-based device, expressed as reduction of open-circuit voltage and fill factor. We have prepared highly stable CsPbBr3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measurements, ruling out degradation as the origin of the observed phenomena. Reconstruction of current–voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance.

ABSTRACT:The use of self-assembled monolayers (SAMs) of fullerene derivatives reduces 11 the hysteresis of perovskite solar cells (PSCs