No abstract

Perovskite solar cells have rapidly advanced to the forefront of solution-processable photovoltaic devices, but the CH3NH3PbI3 semiconductor decomposes rapidly in moist air, limiting their commercial utility. In this work, we report a quantitative and systematic investigation of perovskite degradation processes. By carefully controlling the relative humidity of an environmental chamber and using in situ absorption spectroscopy and in situ grazing incidence X-ray diffraction to monitor phase changes in perovskite degradation process, we demonstrate the formation of a hydrated intermediate containing isolated PbI6(4-) octahedra as the first step of the degradation mechanism. We also show that the identity of the hole transport layer can have a dramatic impact on the stability of the underlying perovskite film, suggesting a route toward perovskite solar cells with long device lifetimes and a resistance to humidity.

The rapid development of organometal halide perovskite solar cells has led to reports of power conversion efficiencies of over 20%. Despite this excellent performance, their instability remains the major challenge limiting their commercialization. In this report, we systematically investigate the origin of the thermal instability of perovskite solar cells fabricated using ZnO electron transport layers. Through in situ grazing incidence X-ray diffraction experiments and Density Functional Theory calculations, we show that the basic nature of the ZnO surface leads to proton-transfer reactions at the ZnO/CH 3 NH 3 PbI 3 interface, which results in decomposition of the perovskite film. The decomposition process is accelerated by the presence of surface hydroxyl groups and/or residual acetate ligands; calcination of the ZnO layer results in a more thermally stable ZnO/CH 3 NH 3 PbI 3 interface, albeit at the cost of a small decrease in power conversion efficiency.

The recent breakthrough of organometal halide perovskites as the light harvesting layer in photovoltaic devices has led to power conversion efficiencies of over 16%. To date, most perovskite solar cells have adopted a structure in which the perovskite light absorber is placed between carrier-selective electron- and hole-transport layers (ETLs and HTLs). Here we report a new type of compact layer free bilayer perovskite solar cell and conclusively demonstrate that the ETL is not a prerequisite for obtaining excellent device efficiencies. We obtained power conversion efficiencies of up to 11.6% and 13.5% when using poly(3-hexylthiophene) and 2,2',7,7'-tetrakis(N,N-di(4-methoxyphenyl)amino)-9,9'-spirobifluorene, respectively, as the hole-transport material. This performance is very comparable to that obtained with the use of a ZnO ETL. Impedance spectroscopy suggests that while eliminating the ZnO leads to an increase in contact resistance, this is offset by a substantial decrease in surface recombination.

Recent advances in the development of perovskite solar cells based on CH 3 NH 3 PbI 3 have produced devices with power conversion efficiencies of >15%. While initial work in this area assumed that the perovskitebased cells required a mesoporous TiO 2 support, many recent reports have instead focused on the development of planar heterojunction structures. A better understanding of how both cell architecture and various design parameters (e.g., perovskite thickness and morphology) affect cell performance is needed. Here, we report the fabrication of perovskite solar cells based on a ZnO nanoparticle electron transport layer, CH 3 NH 3 PbI 3 light absorber, and poly(3-hexylthiophene) (P3HT) hole transport layer. We show that vapor-phase deposition of the PbI 2 precursor film produces devices with performances equivalent to those prepared using entirely solution-based techniques, but with very precise control over the thickness and morphology of the CH 3 NH 3 PbI 3 layer. Optimization of the layer thickness yielded devices with efficiencies of up to 11.3%. The results further demonstrate that a delicate balance between light absorption and carrier transport is required in these planar heterojunction devices, with the thickest perovskite films producing only very low power conversion efficiencies.