Inorganic lead-free Cs2AgBiBr6 double perovskite solar cell with a photoelectronic conversion efficiency of 6.37%

We synthesize a hydrogenated Cs2AgBiBr6 perovskite to fabricate the stable and environmentally friendly perovskite solar cell with higher photoelectronic performance. The photoelectronic conversion efficiency of hydrogenated Cs2AgBiBr6 perovskite solar cell could reach up to 6.37% (record value).
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Zeyu Zhang, Doctor, Beijing Key Laboratory of Microstructure and Properties of Solids, Faculty of Materials and Manufacturing, Beijing University of Technology

Written by Zeyu Zhang, Yue Lu and Manling Sui.

Recently, organic-inorganic halide perovskite (OIHP) solar cell has attracted extensive attentions due to its low cost and high photoelectronic conversion efficiency (PCE). However, the structural instability caused by volatile organic components and environmental pollution caused by lead toxicity will limit its industrial application. To solve these problems, lots of efforts have been tried to explore the lead-free inorganic perovskite materials as the absorbent layer in solar cell. As one of the most promising candidates, inorganic lead-free Cs2AgBiBr6 double perovskite material has become an alternative to replace OIHP materials. Nevertheless, the broadly indirect bandgap limits its performance, and the highest PCE of Cs2AgBiBr6-based perovskite solar cells was only lower than 4.23% as so far. Here, we report hydrogenated lead-free inorganic perovskite solar cells with enhanced power conversion efficiency.

As shown in Figure 1, after the hydrogenation treatment, the absorption performance of Cs2AgBiBr6 perovskite film has been largely optimized (the color of films turned from yellow to black). And the bandgap of a 1200 s hydrogenated Cs2AgBiBr6 perovskite film could be decreased from initial 2.18 eV to 1.64 eV. At the same time, the carrier mobility, carrier concentration and carrier lifetime have been improved after the 1200 s hydrogenation process.

 

Figure 1. Fabrication of the hydrogenated Cs2AgBiBr6 perovskite films and the photoelectrical property characterization. a The preparation method of Cs2AgBiBr6 perovskite films during the hydrogenation treatment in hydrogen gas plasma. b X-ray Diffraction (XRD) patterns of the Cs2AgBiBr6 perovskite films with different hydrogenation time (inserts show the optical pictures of the corresponding Cs2AgBiBr6 films). c Scanning electron microscopy (SEM) images of the pristine Cs2AgBiBr6 film and the 1,200 s hydrogenated Cs2AgBiBr6 film. d Ultraviolet-visible absorption spectra (UV-vis) of the Cs2AgBiBr6 perovskite films with different hydrogenation time. e Photoluminescence (PL) spectra of the Cs2AgBiBr6 perovskite films with different hydrogenation time. f Carrier mobility and carrier concentration of the Cs2AgBiBr6 perovskite films with different hydrogenation time. g Time-resolved photoluminescence (TRPL) and the measurement of carrier lifetime (inset) of Cs2AgBiBr6 perovskite films with different hydrogenation time.

Subsequently, the hydrogenated Cs2AgBiBr6 perovskite solar cells were successfully fabricated as shown in Figure 2. With the hydrogenation time of 1200 s, PCE of Cs2AgBiBr6 perovskite solar cell has been largely improved from 0.55% to 6.37% (a record PCE value in Cs2AgBiBr6 perovskite solar cells as so far). Meanwhile, the short-circuit current density of solar cells presents an enormous enhancement, which may be ascribed to the improvement of light absorption ability of hydrogenated Cs2AgBiBr6 perovskite film (Figure 1). Furthermore, the hydrogenated Cs2AgBiBr6 perovskite solar cells also exhibited excellently environmental stability.

 

Figure 2. Photoelectrical performance and environmental stability of hydrogenated Cs2AgBiBr6 perovskite solar cell. a Current density-voltage (J-V) curve of the champion Cs2AgBiBr6 PSCs with different hydrogenation time (0 s, 600 s and 1,200 s). Inset shows the schematic of the layered perovskite solar cell. b J-V curves under reverse and forward bias of 1,200 s hydrogenated Cs2AgBiBr6 PSC. c Steady-state photocurrents of 1,200 s hydrogenated Cs2AgBiBr6 PSCs at bias voltages of 0.64 V near the maximum power output. d The average photoelectric conversion efficiency (PCE) distribution of Cs2AgBiBr6 PSCs with different hydrogenation time (0 s, 600 s and 1,200 s). The outliers, middle line, upper/lower box limits and upper/lower whiskers in the box plot indicate the single points, median, 25th/75th quartiles, and maximum/minimum, respectively. e EQE spectrum of and integrated current density of the Cs2AgBiBr6 PSCs with different hydrogenation time (0 s, 600 s and 1,200 s). f Long-term stability of 1,200 s hydrogenated Cs2AgBiBr6 PSCs under light illumination, 85 oC, 85 oC plus light illumination and double 85 condition of 85% humidity at 85 oC, respectively.

In order to explore the effect of atomic hydrogen on adjusting the bandgap in Cs2AgBiBr6 perovskite material, we analyzed the chemical environment of hydrogen atoms in hydrogenated Cs2AgBiBr6 perovskite films through XPS and DFT calculations (Figure 3). The results show that hydrogen atom would occupy the interstitial sites in Cs2AgBiBr6 lattice. According to the DFT calculation results, hydrogen atom could couple with cation to form a fully occupied energy level, which raises the maximum of valence band. At the same time, the couple between hydrogen and anion would form a new energy level to reduce the minimum of conduction band.

Figure 3. Analyses on the chemical environment of Cs2AgBiBr6 during hydrogenation treatment. a-c X-ray photoelectron spectroscopy (XPS) spectra of Cs 3d, Ag 3d and Bi 4f in Cs2AgBiBr6 films with different hydrogenation time (0 s, 600 s and 1,200 s). d The formation energies of H1(in), H2(in) and H3(in). Here, Hn(in), where n=1, 2, or 3, presents the H atom in the interstitial position of the Hn polyhedrons surrounded by Cs-Br-Ag (H1), Cs-Br-Bi (H2), and Cs-Br-Cs (H3), respectively. Cs, Ag, Bi, Br and H atoms are represented by cyan, light grey, purple, brown and red dots, respectively. e Schematic band coupling models of Hn(in) showing energy level positions when the H 1s orbital is coupled to anion (forming donor) and cation (forming acceptor) levels (HA (coupled from interstitial Hn(in) and anion) and HC (coupled from interstitial Hn(in) and cation)). f Bader charge variations of Ag, Bi, and Cs atoms in the host and next to the interstitial H* in the Hn(in) polyhedrons. g The variation tendency of binding energy of Cs 3d3/2, Ag 3d3/2 and Bi 4f5/2 peaks as the increase of hydrogenation time.

For more details, please check out our paper “Hydrogenated Cs2AgBiBr6 for Significantly Improved Efficiency of Lead-Free Inorganic Double Perovskite Solar Cell” in Nature Communications (DOI 10.1038/s41467-022-31016-w).

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