Solution-Processed Perovskite Active Materials for Photovoltaics and High-Energy Radiation Detection

Abstract

Solution-processed semiconductor materials offer the ability of greatly reducing the fabrication cost of semiconductor devices, while also offering a more facile pathway to scaling device production, especially for devices with a large area or volume of the active material. One particularly promising solution processed semiconductor material family is that of organic-inorganic hybrid perovskites (OIHPs), which have been extensively investigated as the active material for photovoltaic (PV) applications. OIHP-based PV devices have been extensively studied, with over 5,000 peer-reviewed articles published in 2019. While the progress in OIHP PV devices has been rapid and extensive, with device power conversion efficiencies (PCEs) increasing from 2.2 % to over 20 % between 2007 and 2017, material stability and compatibility issues have limited the device lifetime to under 300 hours in most cases. While the bulk trap defect density of perovskite is low (< 1010 cm^-3), the trap density near a heterojunction interface between the OIHP active material and selective transport layers (STLs) is much higher (> 1016 cm-3), leading to material performance deficiencies and ion-induced field degradation known as “hysteresis” behavior or “polarization”.

The initial work involved demonstrating semi-transparent, colored OIHP PV cells. Using a Fabry-Perot cavity, OIHP-PV cells were demonstrated with PCEs > 4.5 % and which maintained their color to over 60 degrees from normal incidence. Then, the fabrication processes for thin-film, large-area scalable OIHP devices were optimized to result in improved device performance and consistency, while the interface properties were investigated. For PV cells, altering the deposition techniques of the polymer PEDOT:PSS hole transport layer (HTL) resulted in an average PCE and open-circuit voltage increase of the PV devices from 8.34 % to 12.2 % and from 0.84 V to 0.95 V, respectively, the latter indicating improved band alignment of the PV device.

The work then focused on developing high-energy spectroscopic photon (x-rays and gamma rays) detectors using OHIPs. While PV device structures showed promise, the higher interaction depth required thicker devices, using perovskite single crystals (PSCs), grown by inverse-temperature crystallization (ITC). By optimizing ITC growth, PSC detectors with an energy resolution of 15 % were produced, though these suffered from poor repeatability. To improve the repeatability, a solution-based surface alkylamine treatment (SAAT) method was used to coat the PSC surface with a 2-D Ruddlesden-Popper (R-P) surface layer, resulting in improved average yield and energy resolution from 43.8 % and 39.9 % to 100 % and 24.9 %, respectively, a consequence of the greatly minimized surface recombination velocity (620 cm/s to 307 cm/s). To demonstrate the scalability of the ITC process for large detectors, seeded PSC growth methods were studied. An axial-flow crystallization (AFC) method was developed, enabling seed PSCs to grow by over 280 times their initial volume in 40 hours to final volumes around 0.5 cm3. The AFC method was optimized and computer-controlled, resulting in greatly increased PSC growth rates (3.0 to 13.3 mm3/hr). The drift time limitations of standard detection electronics was also addressed with the slow pulse processing (SPP) method, which enabled large PSC detectors to show spectroscopic features. The combination of AFC growth and SAAT post-treatment will enable high-performance, large-volume PSC detectors to be produced at over 2 orders of magnitude lower cost than existing room-temperature semiconductor detectors, enabling broader applications of radiation spectroscopy from its current niche areas.