Single crystals of lead-free preovskite materials show a decrease in current when exposed to simulated sunlight, a study reveals1.
The crystals, when kept in the dark, showed a slow and steady increase in current, surpassing the levels of light-induced current, physicists from the Roorkee-based Indian Institute of Technology have found.
Such optical properties of the crystals, termed as negative photoconductivity, could be utilised to fabricate highly sensitive light detectors, the researchers say.
Light exposure usually causes an increase in current in crystals of perovskite materials. In contrast, the lead-free perovskite crystals show a drop in light-generated current. To explore the cause of such a phenomenon, the scientists shone simulated sunlight on millimetre-sized single crystals of lead-free perovskites materials.
The researchers, led by Soumitra Satapathi, found that a light exposure of 15 minutes caused a significant reduction in current in the crystals. In their analysis, they revealed that the light exposure created virtual defect states in the crystals that trap charge carriers, forming charged islands. This, in turn, increases resistance, reducing light-induced current.
The crystals, when kept in the dark for three hours, healed their defects, causing the disappearance of charge-trapping states and the release of the charge carriers. This contributes to an increase in dark current.
The light exposure didn’t cause any structural defects in the crystals, suggesting that the charge-trapping defect states contributed to the drop in light-induced current.
The crystals, being lead-free, are non-toxic and eco-friendly. Their light-detecting efficiency is comparable to other lead-containing photodetectors whose current increases on exposure to light, says Satapathi.
References
1. Tailor, N. K. et al. Dark self-healing-mediated negative photoconductivity of a lead-free Cs3Bi2Cl9 perovskite single crystal. J. Phys. Chem. Lett. 12, 2286-2292 (2021)
