A fluorescent sensor to detect lead leakage from perovskite solar cells

Abstract

Hybrid perovskites have been considered a hot material in the semiconductor industry; included as an active layer in advanced devices, from light emitting applications to solar cells, where they lead as a new strategic solution, they promise to be the next generation high impact class of materials. However, the presence - in most cases - of lead in their matrix, or lead byproducts as a consequence of material degradation, such as PbI₂, is currently hindering their massive deployment. Here, we develop a fluorescent organic sensor (FS) based on the Pb-selective BODIPY fluorophore that emits when the analyte - lead in this case - is detected. We carried out a fluorimetric analysis to quantify the trace concentration of Pb²⁺ released from lead-based perovskite solar cells, exploring different material compositions. In particular, we immersed the devices in rainwater, to simulate the behavior of the devices under atmospheric conditions when the sealing is damaged. The sensor is studied in a phosphate buffer solution (PBS) at pH 4.5 to simulate the pH of acidic rain, and the results obtained are compared with ICP-OES measurements. We found that with fluorometric analysis, lead concentration could be calculated with a detection limit as low as 5 μg l^-1, in agreement with ICP-OES analysis. In addition, we investigated the possibility of using the sensor on a solid substrate for direct visualization to determine the presence of Pb. This can constitute the base for the development of a Pb-based label that can switch on if lead is detected, alerting any possible leakage.

Fig. 1. Generic crystal structure of an organic–inorganic hybrid perovskite where A is the organic cation, B is the inorganic cation and X is the anion, usually halogen (a); XRD patterns of the MAPbI₃ perovskite stored under high humidity conditions (RH 75%), at different times (b); a schematic representation of the different degradation pathways and byproducts released by the MAPbI₃ perovskite (c).

Scheme 1. (a) ethyl bromoacetate, KI, DIPEA, CH₃CN, reflux under Ar, 16 h, rt; (b) POCl₃, DMF, dry pyridine, reflux under Ar, rt, 1 h and warmed up to 75 °C, 45 min; (c) (1) 2,4-dimethylpyrrole-TFA, DCM, reflux under Ar, 14 h, rt; (2) DDQ, reflux under Ar, 4 h, rt; (3) NEt₃, BF₃·OEt₂, reflux under Ar, 2 h, rt; (d) diethanolamine, CH₃CN, reflux under Ar, 24 h, rt.

Fig. 2. (a) Fluorescence emission spectra (λ_ex = 470 nm) of FS (5.5 μM) in PBS buffer (0.1 M) pH 4.5 with different concentration of Pb²⁺ (0–2.3 μM); (b) fluorescent intensity at 512 nm as a function of [Pb²⁺].

Fig. 3. schematic representation of Pb²⁺ release mechanism from PSCs: (A) CsPbI₃; (B) PSCs of FA_0.8MA_0.2PbI₃; (C) FA_0.8MA_0.2PbI₃ perovskite film (a); sensor in liquid phase and supported on solid substrate for fluorimetric measurements (b); and sensor complexation and fluorescence enhancement (c).