Isovalent Alloying and Heterovalent Substititution as Routes to Accelerate the Development and Optimization of Super-Efficient Halide Perovskite PV Solar Cells
Halide perovskites represent one of the fastest growing photovoltaic (PV) device technologies in the last decade, with research efficiencies among the highest confirmed. However, the research needed to advance tandem perovskite solar cells requires a specialized understanding base, but there is no such base for this young group of materials with the same depth as in semiconductor alloy research. Whereas IV-IV, II-V and II-VI semiconductors have a sophisticated science foundation refined over decades due of their use in electronics, lasers, LEDs and night vision applications, this is not yet the case for halide perovskites. At the same time, these materials behave rather differently than conventional semiconductors. Within the rapidly unfolding field of PV, the emerging halide-perovskite alloy research has largely been based on empiricism. Clearly, a concerted and comprehensive application to halide perovskite alloys of the modern quantum theory of solids brought us over the years a broad understanding of the critical factors enabling conventional inorganic PV could leapfrog the Edisonian, trial-and-error search for solutions to critical problems that are poorly understood. These theory approaches are required to accelerate the progress toward commercially ready hybrid-perovskites by providing a theory-guided experiment component to the EERE investments in this high-priority R&D. During this project we have studied three different rungs on the ladder of knowledge about hybrid perovskites. This kind of single halide perovskites represent the first level of development (rung 1) of solar cells, and includes materials such as APbI 3 , ASnI 3 , APbBr 3 , ASnBr 3 with different A cations, such as methylammonium (MA) with formamidinium (FM). Rung 2 is obtained by mixing B cations (Sn and Pb), and X anions (Cl and Br or I) producing a disordered alloy (A, A’)(M,M’)(X,X’) 3 -space. The most advanced rung corresponds to heterovalent-substitution of halide perovskites, creating an ordered double-perovskite compound. Although the focus of the project was on Rungs 2 and 3, we have made important discoveries regarding the nature of pure perovskites. We’ve observed that cubic perovskites should be modeled by a polymorphous network, instead of the basic single unit cell approach. This leads to the conclusion that perovskites have a fluid-like nature. An article regarding this work has been submitted to Nature Materials. Using the approach described above, we have also simulated alloys of halide perovskites, representing rung 2. We have focused on mixtures of [Cs,FA][Sn,Pb]I 3 with different concentrations, being able to disentangle the structural and electronic factors that determines the variation (bowing) of properties such as band gaps, mixing enthalpies and decomposition energies. For rung 3 compounds, we’ve closely interacted with experimental groups clarifying the chemical routes to obtain a certain compound, and what should be the most stable structure for them. As the final product, and consequent dissemination of the research, we’ve submitted two papers for publication, that are currently under evaluation. The interaction with experimental groups was also an important focus. We have participated in several local, national and international meetings, and reached out more closely to groups from CU Boulder and Arkansas.