Electronic Structure and Optoelectronic Properties of Bismuth Oxyiodide Robust against Percent‐Level Iodine‐, Oxygen‐, and Bismuth‐Related Surface Defects

Abstract

In the search for nontoxic alternatives to lead-halide perovskites, bismuth oxyiodide (BiOI) has emerged as a promising contender. BiOI is air-stable for over three months, demonstrates promising early-stage photovoltaic performance and, importantly, is predicted from calculations to tolerate vacancy and antisite defects. Here, whether BiOI tolerates point defects is experimentally investigated. BiOI thin films are annealed at a low temperature of 100 °C under vacuum (25 Pa absolute pressure). There is a relative reduction in the surface atomic fraction of iodine by over 40%, reduction in the surface bismuth fraction by over 5%, and an increase in the surface oxygen fraction by over 45%. Unexpectedly, the Bi 4f 7/2 core level position, Fermi level position, and valence band density of states of BiOI are not significantly changed. Further, the charge-carrier lifetime, photoluminescence intensity, and the performance of the vacuum-annealed BiOI films in solar cells remain unchanged. The results show BiOI to be electronically and optoelectronically robust to percent-level changes in surface composition. However, from photoinduced current transient spectroscopy measurements, it is found that the as-grown BiOI films have deep traps located ≈0.3 and 0.6 eV from the band edge. These traps limit the charge-carrier lifetimes of BiOI, and future improvements in the performance of BiOI photovoltaics will need to focus on identifying their origin. Nevertheless, these deep traps are three to four orders of magnitude less concentrated than the surface point defects induced through vacuum annealing. The charge-carrier lifetimes of the BiOI films are also orders of magnitude longer than if these surface defects were recombination active. This work therefore shows BiOI to be robust against processing conditions that lead to percent-level iodine-, bismuth-, and oxygen-related surface defects. This will simplify and reduce the cost of fabricating BiOI-based electronic devices, and stands in contrast to the defect-sensitivity of traditional covalent semiconductors.

Lead‐Free Perovskite‐Inspired Absorbers for Indoor Photovoltaics

“…With regard to BiOI, while a slope equal of kT / q in the V oc versus light intensity plot has been customarily attributed to band‐to‐band recombination, this is not consistent with our previous results, in which we showed the fluence‐independence of the decay in photoluminescence (from time‐correlated single photon counting measurements) and ground‐state bleach (from transient absorption spectroscopy measurements). [ 45,58,74 ] Furthermore, we measured the PLQE of BiOI, which we found to be <0.1%. These results suggest that recombination in BiOI is dominated by nonradiative processes.…”

“…As shown in Figure 2e, devices based on BiOI and Cs 3 Sb 2 Cl x I 9‐ x have a power conversion efficiency (PCE) of 0.9% and 1.2% under AM 1.5G illumination respectively, which is in agreement with the median efficiency of these materials in prior reports. [ 45,56,58 ] However, under FL and WLED illumination, the PCE values increased to the range of 4–5%, peaking at 4.4% for BiOI and 4.9% for Cs 3 Sb 2 Cl x I 9‐ x under FL lighting (Figure 2d,f). These PCEs have already entered the range of efficiencies reported for commercial a‐Si:H IPV (4.4–9.2%).…”

Assessing the Impact of Defects on Lead‐Free Perovskite‐Inspired Photovoltaics via Photoinduced Current Transient Spectroscopy

“…[21][22][23][24][25][26][27][28][29][30][31] In view of the key role that defect tolerance has played in determining the success of lead-halide perovskites, identifying how defect tolerance arises and how it can be designed into a material has been a key driving force for PIM development. [7,8,23,[32][33][34] While the concept of defect tolerance has not been rigorously and quantitatively defined, with different interpretations being proposed, [8,23,32,35,36] an absorber is generally regarded as defect tolerant if its compositional and structural defects result in electronic states (so-called defect levels) that are either "shallow," or with small capture cross-sections, or that fall within the energy bands (Figure 1a,b). [7,9,12,15,32,35] Indeed, a shallow defect level (see below) would easily re-emit a captured carrier to the nearest energy band due to thermal excitation (Figure 1b).…”

Controlling the preferred orientation of layered BiOI solar absorbers

“…Up to four glass/ITO substrates coated with B15 nm thick NiO x (see ref. 17 for spincoating procedure) were placed several centimeters away from the crucible. The temperature of the crucible and substrates were controlled independently by changing the temperatures of zones 1 and 2 individually in the furnace.…”

“…All elements are sufficiently abundant for commercialization and Bi-based compounds have demonstrated very little evidence of toxicity. 15,16 We have recently shown BiOI to tolerate vacancy and anti-site defects, 16,17 and found the unencapsulated films to be phasestable in ambient air for 4197 days. 16 With a band gap of 1.9 eV and absorption coefficient 410 4 cm À1 , BiOI has suitable optical properties for photovoltaics, indoor solar cells, photocatalysis [18][19][20][21] and photodetectors.…”