Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

Abstract

ABSTRACT:Recently, there has been an explosive growth in research based on hybrid lead-halide perovskites for photovoltaics owing to rapid improvements in efficiency. The advent of these materials for solar applications has led to widespread interest in understanding the key enabling properties of these materials. This has resulted in renewed interest in related compounds and a search for materials that may replicate the defect-tolerant properties and long lifetimes of the hybrid lead-halide perovskites. Given the rapid pace of development of the field, the rises in efficiencies of these systems have outpaced the more basic understanding of these materials. Measuring or calculating the basic properties, such as crystal/electronic structure and composition, can be challenging because some of these materials have anisotropic structures, and/or are composed of both heavy metal cations and volatile, mobile, light elements. Some consequences are beam damage during characterization, composition change under vacuum, or compound effects, such as the alteration of the electronic structure through the influence of the substrate. These effects make it challenging to understand the basic properties integral to optoelectronic operation. Compounding these difficulties is the rapid pace with which the field progresses. This has created an ongoing need to continually evaluate best practices with respect to characterization and calculations, as well as to identify inconsistencies in reported values to determine if those inconsistencies are rooted in characterization methodology or materials synthesis. This article describes the difficulties in characterizing hybrid lead-halide perovskites and new materials, and how these challenges may be overcome. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 challenges discussed. The focus in this article is on crystallography, composition measurements, photoemission spectroscopy and calculations on perovskites and new, related absorbers. We suggest how some of the important artifacts could be avoided and how the reporting for each technique could be streamlined between groups to ensure reproducibility as the field progresses.

Strongly Enhanced Photovoltaic Performance and Defect Physics of Air‐Stable Bismuth Oxyiodide (BiOI)

“…We checked to ensure that there was no beam damage and negligible charging during these measurements (Section S7, Supporting Information). [29] We found the VB-E F of BiOI to decrease with increasing thickness from 1.3 to 0.9 eV over the thickness range studied ( Figure 5 and Table 3). The work function of BiOI also increased from 4.6 to 5.1 eV over the same thickness range (Table 3).…”

“…This agreed with the photoluminescence peak centered at 1.9 eV (Figure 1c), red appearance of the films (Figure 1b), and previous reports. [32,33] Our semilogarithmic plot of the absorption coefficient against photon energy [29] also yielded a bandgap of 1.93 eV ( Figure S1, Supporting Information). A bandgap of 1.9 eV is highly suitable for a top-cell absorber with silicon in tandem photovoltaics.…”

“…This process is similar to vapor transport deposition systems used industrially. [28] 2D diffraction patterns showed the films to be polycrystalline and isotropic (Figure 1a), [29] and 1D line scans (Figure 1b) showed that all peaks matched those of tetragonal BiOI (P4/nmm, space group 129). [23,30] No additional peaks were observed, apart from amorphous broadening from the glass substrate between 20° and 28°, indicating our material was phase-pure down to the detection limits of our diffractometer.…”

“…Using a method reported by Kraut et al, [29,39,40] we convolved our calculated density of states for BiOI with a Gaussian representing instrument broadening. This convolved model was fit to the leading edge of our measured XPS spectrum (Figure 2c).…”

Research Update: Bismuth-based perovskite-inspired photovoltaic materials

“…4,5 However, there is debate over the environmental and commercial impact of the lead content, 6 motivating efforts to find lead-free alternatives. 7 Many groups searching for lead-free alternatives to lead-halide perovskites have investigated chemical substitution of Pb 2+ for neighboring elements, such as Sn 2+ , Ge 2+ , Sb 3+ , and Bi 3+ . These efforts are well-documented in recent reviews, and the compounds are compositionally analogous to lead-halide perovskites, e.g., methylammonium tin iodide, methylammonium germanium iodide, and methylammonium bismuth iodide.…”