Philipp Tockhorn

Perovskite-silicon tandem solar cells offer the possibility of overcoming the power conversion efficiency limit of conventional silicon solar cells. Various textured tandem devices have been presented aiming at improved optical performance, but optimizing film growth on surface-textured wafers remains challenging. Here we present perovskite-silicon tandem solar cells with periodic nanotextures that offer various advantages without compromising the material quality of solution-processed perovskite layers. We show a reduction in reflection losses in comparison to planar tandems, with the new devices being less sensitive to deviations from optimum layer thicknesses. The nanotextures also enable a greatly increased fabrication yield from 50% to 95%. Moreover, the open-circuit voltage is improved by 15 mV due to the enhanced optoelectronic properties of the perovskite top cell. Our optically advanced rear reflector with a dielectric buffer layer results in reduced parasitic absorption at near-infrared wavelengths. As a result, we demonstrate a certified power conversion efficiency of 29.80%.

We present a highly efficient monolithic perovskite/silicon tandem solar cell and analyze the tandem performance as a function of photocurrent mismatch with important implications for future device and energy yield optimizations.

Abstract Formamidinium iodide (FAI) based perovskite absorbers have been shown to be ideal candidates for highly efficient and operationally stable perovskite solar cells (PSC). A major challenge for formamidinium lead iodide (FAPbI 3 ) is to suppress the phase transition from the photoactive black phase into yellow nonperovskite δ‐phase. Several approaches to stabilize the black phase have been developed for solution‐based perovskites, whereas so far, vacuum‐deposited FAPbI 3 has rarely been reported. This study demonstrates the preparation of FAPbI 3 by co‐evaporation and discusses the influence of the subjacent hole transporting layer (HTL) on its phase stability. By using FAI excess in the evaporation process in combination with phosphonic acids groups from the HTL, the black perovskite phase is stabilized at room temperature. Further addition of 32–59% methylammonium iodide (MAI) during the co‐evaporation process leads to good absorption properties and high PSC efficiencies of 20.4%. In addition, excellent stability is achieved for optimized MAI to FAI ratios, maintaining 100% of the initial PSC performance after 1000 h under constant operation. This highly stable perovskite composition enables the first monolithic fully textured perovskite/silicon tandem solar cells with co‐evaporated perovskite absorbers. Due to the conformally covered pyramid texture, these tandem cells show minimal reflection losses and reach an efficiency of 24.6%.

For methylammonium lead iodide perovskite solar cells prepared by co-evaporation, power conversion efficiencies of over 20% have been already demonstrated, however, so far, only in n-i-p configuration. Currently, the overall major challenges are the complex evaporation characteristics of organic precursors that strongly depend on the underlying charge selective contacts and the insufficient reproducibility of the co-evaporation process. To ensure a reliable co-evaporation process, it is important to identify the impact of different parameters in order to develop a more detailed understanding. In this work, we study the influence of the substrate temperature, underlying hole-transport layer (polymer PTAA versus self-assembling monolayer molecule MeO-2PACz), and perovskite precursor ratio on the morphology, composition, and performance of co-evaporated p-i-n perovskite solar cells. We first analyze the evaporation of pure precursor materials and show that the adhesion of methylammonium iodide (MAI) is significantly reduced with increased substrate temperature, while it remains almost unaffected for lead iodide (PbI2). This substrate temperature-dependent evaporation behavior of MAI is also transferred to the co-evaporation process and can directly influence the perovskite composition. We demonstrate that the optimal substrate temperature window for perovskite deposition is close to room temperature. At high temperature, not enough MAI for precise stoichiometry is incorporated even with very high MAI rates. While, at temperatures below −25 °C, the conversion of MAI with PbI2 is inhibited, and an amorphous yet unreacted film is formed. We observe that perovskite composition and morphology vary widely between the organic hole-transport layers (HTLs) PTAA and MeO-2PACz. For all substrate temperatures, MeO-2PACz enables higher solar cell PCEs than PTAA. Through the combination of vapor-deposited perovskites and a self-assembled monolayer, we achieve a stabilized power conversion efficiency of 20.6%, which is the first reported PCE above 20% for evaporated perovskite solar cells in p-i-n architecture.

active area is demonstrated with fully scalable processes.