Novel Materials for Perovskite Photovoltaics
Perovskite thin-film photovoltaics promise to reduce dramatically the cost of next-generation photovoltaics. While perovskite photovoltaics demonstrated greatly improved power conversion efficiencies over the past decade, the technology’s future prospects rely on a breakthrough in the development of stable and long-lifetime perovskite materials of high optoelectronic quality. Moreover, lead-free alternatives are desired to facilitate the acceptance of this technology. Given the countless compositions of perovskite semiconductors, a knowledge-based material design approach is needed. For this reason, our team not only examines a large variety of perovskite semiconductors (e.g. “multi-cation”, “inorganic”, “lead-free”, “two-dimensional (2D)”), but is also strongly engaged in the analysis of optoelectronic characteristics such as charge carrier dynamic , photon-recycling  , compositional variation  , and thin-film morphology .
Figure 1: Cross sectional scanning electron image of a thin-film perovskite solar cell and associated energy diagram.
Two-dimensional (2D) perovskite semiconductors and 2D/3D perovskite heterostructures:
Not only three-dimensional (3D) perovskite semiconductors, but also their 2D analogues show excellent optoelectronic properties. 2D perovskite semiconductors have demonstrated promising stability against environmental factors and act as platforms for realizing lead-free alternatives. However, since the 2D structure complicates charge transport, tailor-made large cations need to be invented and synthesised. 2D/3D heterostructures demonstrated already excellent performance in laboratory-scale solar cells. Our team showed that 2D/3D heterostructures can strongly reduce non-radiative recombination losses, leading to astonishingly high open-circuit voltages in perovskite solar cells over a wide range of bandgaps (see Figure 2,  ). These device architectures are perfectly suited for perovskite/Si and perovskite/perovskite tandem photovoltaics  .
Figure 2: Schematic illustration of a 2D/3D perovskite heterostructure within the layer stack of a perovskite solar. The enhanced power conversion efficiency of the 2D/3D perovskite heterostructure is compared to the 3D perovskite reference solar cell.
Stable and high-quality perovskite semiconductors by compositional engineering:
Compositional engineering of defect-tolerant perovskite semiconductors describes the variation of the organic and inorganic components in the crystal structure. Progress in this field over recent years led to multi-cation perovskite semiconductors that demonstrate improved, but still not sufficient, stability and power conversion efficiency. In order to advance further the stability, we research the underlying mechanisms of light-induced  , temperature-induced  , and moisture-induced degradation. We develop guidelines for the material design and device architecture with regard to the composition of multi-cation perovskites , passivation layers , and charge transport layers  .
Figure 3: Effect of thermal stress on perovskite thin films with respect to their performance in perovskite solar cells.
Tailored bandgaps of perovskite semiconductors:
Tailored bandgaps are of paramount importance to the realization of high-efficiency perovskite-based tandem photovoltaics. While the optimum bandgap for a perovskite top solar cell is 1.7-1.8 eV in tandem architecture with crystalline silicon (c-Si) or copper indium gallium selenide (CIGS) solar cell, the optimal bandgap of the perovskite bottom solar cell in all-perovskite tandem solar cells is below 1.3 eV. By applying a novel passivation strategy, we demonstrated excellent performance of wide-bandgap perovskite solar cells (~1.7eV). In addition, we research new routes to control the crystallization of low-bandgap perovskite thin films (~1.26 eV) for efficient all-perovskite tandem solar cells   .
Figure 4: The bandgap of the perovskites can be tuned by compositional engineering.