Ian L. Braly

Highly Efficient Perovskite–Perovskite Tandem Solar Cells Reaching 80% of the Theoretical Limit in Photovoltage

Organic-inorganic hybrid perovskite multijunction solar cells have immense potential to realize power conversion efficiencies (PCEs) beyond the Shockley-Queisser limit of single-junction solar cells; however, they are limited by large nonideal photovoltage loss (V oc,loss ) in small- and large-bandgap subcells. Here, an integrated approach is utilized to improve the V oc of subcells with optimized bandgaps and fabricate perovskite-perovskite tandem solar cells with small V oc,loss . A fullerene variant, Indene-C60 bis-adduct, is used to achieve optimized interfacial contact in a small-bandgap (≈1.2 eV) subcell, which facilitates higher quasi-Fermi level splitting, reduces nonradiative recombination, alleviates hysteresis instabilities, and improves V oc to 0.84 V. Compositional engineering of large-bandgap (≈1.8 eV) perovskite is employed to realize a subcell with a transparent top electrode and photostabilized V oc of 1.22 V. The resultant monolithic perovskite-perovskite tandem solar cell shows a high V oc of 1.98 V (approaching 80% of the theoretical limit) and a stabilized PCE of 18.5%. The significantly minimized nonideal V oc,loss is better than state-of-the-art silicon-perovskite tandem solar cells, which highlights the prospects of using perovskite-perovskite tandems for solar-energy generation. It also unlocks opportunities for solar water splitting using hybrid perovskites with solar-to-hydrogen efficiencies beyond 15%.

Photoluminescence and Photoconductivity to Assess Maximum Open-Circuit Voltage and Carrier Transport in Hybrid Perovskites and Other Photovoltaic Materials

Photovoltaic (PV) device development is much more expensive and time-consuming than the development of the absorber layer alone. This Perspective focuses on two methods that can be used to rapidly assess and develop PV absorber materials independent of device development. The absorber material properties of quasi-Fermi level splitting and carrier diffusion length under steady effective 1 Sun illumination are indicators of a materials ability to achieve high VOC and JSC. These two material properties can be rapidly and simultaneously assessed with steady-state absolute intensity photoluminescence and photoconductivity measurements. As a result, these methods are extremely useful for predicting the quality and stability of PV materials prior to PV device development. Here, we summarize the methods, discuss their strengths and weaknesses, and compare photoluminescence and photoconductivity results with device performance for four hybrid perovskite compositions of various bandgaps (1.35-1.82 eV), CISe, CIGSe, and CZTSe.

Nonlinear photocarrier recombination dynamics in mixed-halide CH3NH3Pb(I1− x Br x )3 perovskite thin films

Mixed-halide perovskites, whose bandgap energies can be widely controlled through choice of composition, are promising for various optoelectronic applications. Herein, we report the photocarrier recombination dynamics in mixed-halide CH3NH3Pb(I1− x Br x )3 perovskite films with different Br contents. We observed small changes in the single-carrier trapping rate with respect to the Br content. In contrast, the two-carrier radiative and three-carrier Auger recombination rates increased significantly with the Br content. These increases in the multicarrier recombination rates likely originated from the enhancement of the Coulomb interactions between electrons and holes caused by incorporating Br. Our findings are useful for designing mixed-halide perovskite-based optoelectronic devices.

Quasi-Fermi level splitting, stability, and healing of high bandgap hybrid perovskites using photoluminescence, composition spread libraries, and post-synthesis treatments

We show the optoelectronic quality and the stability of thousands of compositions of hybrid perovskites (including the range of halide substitutions) and methods to re-grow hybrid perovskites to improve their optoelectronic quality. Hyperspectral maps of steady-state absolute intensity photoluminescence (AIPL) are used to determine the quasi-Fermi level splitting (QFLS) for the combinatorial libraries. For methyl ammonium iodobromides, the QFLS upon first illumination increases with bandgap and reaches a maximum of 1.27 eV under 1 Sun illumination intensity for a bandgap of 1.75 eV However, the optoelectronic quality, defined as the ratio of the QFLS to the maximum theoretical QFLS for bandgap, decreases with bandgap from around 88% for 1.60 eV bandgap down to 82% for 1.84 eV bandgap. We show the effects of water exposure, air exposure and the effects of re-growing the films under several conditions.