Prof Henry Snaith FRS

Abstract:

The family of organic-inorganic tri-halide perovskites including MA (MethylAmmonium)PbI3, MAPbI3-xClx, FA (FormAmidinium)PbI3 and FAPbBr3 are having a tremendous impact on the field of photovoltaic cells due to the combination of their ease of deposition and high energy conversion efficiencies. Device performance, however, is known to be still significantly affected by the presence of inhomogeneities. Here we report on a study of temperature dependent micro-photoluminescence which shows a strong spatial inhomogeneity related to the presence of microcrystalline grains, which can be both bright and dark. In all of the tri-iodide based materials there is evidence that the tetragonal to orthorhombic phase transition observed around 160 K does not occur uniformly across the sample with domain formation related to the underlying microcrystallite grains, some of which remain in the high temperature, tetragonal, phase even at very low temperatures. At low temperature the tetragonal domains can be significantly influenced by local defects in the layers or the introduction of residual levels of chlorine in mixed halide layers or dopant atoms such as aluminium. We see that improvements in room temperature energy conversion efficiency appear to be directly related to reductions in the proportions of the layer which remain in the tetragonal phase at low temperature. In FAPbBr3 a more macroscopic domain structure is observed with large numbers of grains forming phase correlated regions.

Abstract:

In this Letter, we demonstrate a planar n–i–p perovskite solar cell design with a steady-state efficiency of up to 18.8% in the absence of any electronic dopants. In the device stack, solution-processed SnO2 is used as an electron-accepting n-type layer. The absorber layer is a perovskite with both mixed organic A-site cations and mixed halides (FA0.83MA0.17Pb(I0.83Br0.17)3). The hole-transporting p-type layer is a double-layer structure of polymer-wrapped single-walled carbon nanotubes and undoped spiro-OMeTAD. We show that this approach can deliver steady-state efficiencies as high as and even higher than those of traditionally doped spiro-OMeTAD, providing a pathway for dopant-free perovskite solar cells crucial for long-term stability.

Abstract:

Air-stable doping of the n-type fullerene layer in an n-i-p planar heterojunction perovskite device is capable of enhancing device efficiency and improving device stability. Employing a (HC(NH2 )2 )0.83 Cs0.17 Pb(I0.6 Br0.4 )3 perovskite as the photoactive layer, glass-glass laminated devices are reported, which sustain 80% of their "post burn-in" efficiency over 3400 h under full sun illumination in ambient conditions.