Facile fabrication of large-grain CH3NH3PbI3-xBrx films for high-efficiency solar cells via CH3NH3Br-selective Ostwald ripening

Abstract

Organometallic halide perovskite solar cells (PSCs) have shown great promise as a low-cost, high-efficiency photovoltaic technology. Structural and electro-optical properties of the perovskite absorber layer are most critical to device operation characteristics. Here we present a facile fabrication of high-efficiency PSCs based on compact, large-grain, pinhole-free CH3NH3PbI3-xBrx (MAPbI3-xBrx) thin films with high reproducibility. A simple methylammonium bromide (MABr) treatment via spin-coating with a proper MABr concentration converts MAPbI3 thin films with different initial film qualities (for example, grain size and pinholes) to high-quality MAPbI3-xBrx thin films following an Ostwald ripening process, which is strongly affected by MABr concentration and is ineffective when replacing MABr with methylammonium iodide. A higher MABr concentration enhances I-Br anion exchange reaction, yielding poorer device performance. This MABr-selective Ostwald ripening process improves cell efficiency but also enhances device stability and thus represents a simple, promising strategy for further improving PSC performance with higher reproducibility and reliability.

Figure 1. Impact of MABr treatment on the structural and electro-optical properties of MAPbI₃ thin film.

Top view of the SEM images of (a) MAPbI₃ and (b) MABr-treated MAPbI₃ films. Scale bars, 1 μm. Comparison of (c) UV-vis absorption spectra and (d) X-ray diffraction patterns of MAPbI₃ films with and without MABr treatment.

Figure 2. Device characteristics and stability comparison.

Comparison of (a) J–V characteristics and (b) EQE spectra for planar perovskite solar cells based on MAPbI₃ thin films with and without MABr treatment. (c) Comparison of the photographs of as-prepared MAPbI₃ thin films (with and without MABr treatment) versus those subjected to 15 h exposure to 60% relative humidity (R.H.) at 95 °C. (d) Device stability comparison of perovskite solar cells based on MAPbI₃ thin films with and without the MABr treatment. The error bars represent the s.d.'s from eight devices.

Figure 3. MABr concentration effect on pinhole-free MAPbI₃ thin films.

Top view of SEM images of (a) a pinhole-free high-quality MAPbI₃ thin film and those treated with (b) 8 mg ml⁻¹, (c) 4 mg ml⁻¹, and (d) 2 mg ml⁻¹ MABr solutions. Scale bars, 1 μm.

Figure 4. MABr concentration effect on absorption and structure of MAPbI₃ thin films.

(a) UV-vis absorption spectra with (b) a zoom-in view near the absorption edge, and (c) X-ray diffraction patterns of the high-quality MAPbI₃ thin films without and with MABr treatment at different concentrations, as indicated.

Figure 5. Device characteristics based on high-quality pinhole-free MAPbI₃ thin films with and without MABr treatment.

(a) J–V curves, (b) EQE spectra and (c) stabilized photocurrent density and power conversion efficiency biased near the maximum power point. MABr (2 mg ml⁻¹) solution was used for the MABr treatment.

Figure 6. Effect of MABr treatment on XPS core-level analysis and film morphology.

(a) XPS spectra of the Br 3d core-level region for MAPbI₃ thin films and those treated with low (2 mg ml⁻¹) and high (8 mg ml⁻¹) concentration MABr solutions. Top-view SEM images of (b) a high-quality MAPbI₃ thin film and those treated with 2 mg ml⁻¹ (c) MABr and (d) MAI solutions. Scale bars, 1 μm.