Solution Processable Direct Bandgap Copper‐Silver‐Bismuth Iodide Photovoltaics: Compositional Control of Dimensionality and Optoelectronic Properties

Abstract

The search for lead‐free alternatives to lead‐halide perovskite photovoltaic materials resulted in the discovery of copper(I)‐silver(I)‐bismuth(III) halides exhibiting promising properties for optoelectronic applications. The present work demonstrates a solution‐based synthesis of uniform CuxAgBiI4+x thin films and scrutinizes the effects of x on the phase composition, dimensionality, optoelectronic properties, and photovoltaic performance. Formation of pure 3D CuAgBiI5 at x = 1, 2D Cu2AgBiI6 at x = 2, and a mix of the two at 1 < x < 2 is demonstrated. Despite lower structural dimensionality, Cu2AgBiI6 has broader optical absorption with a direct bandgap of 1.89 ± 0.05 eV, a valence band level at ‐5.25 eV, improved carrier lifetime, and higher recombination resistance as compared to CuAgBiI5. These differences are mirrored in the power conversion efficiencies of the CuAgBiI5 and Cu2AgBiI6 solar cells under 1 sun of 1.01 ± 0.06% and 2.39 ± 0.05%, respectively. The latter value is the highest reported for this class of materials owing to the favorable film morphology provided by the hot‐casting method. Future performance improvements might emerge from the optimization of the Cu2AgBiI6 layer thickness to match the carrier diffusion length of ≈40–50 nm. Nonencapsulated Cu2AgBiI6 solar cells display storage stability over 240 days.

Perovskite-inspired Cu2AgBiI6 for mesoscopic indoor photovoltaics under realistic low-light intensity conditions

“…2b) is markedly the highest FF ever achieved for a CABI solar cell fabricated with the same metal electrode. 18,20,24,25 The external quantum efficiency (EQE) values of this device are ∼29% and 18% at 370 nm and 580 nm, respectively (Fig. S3†), which are comparable to the values reported earlier for CABI.…”

“…, Au). 18,24,25 The FF value of 72% of the CABI-Meso device (Fig. 2b) is markedly the highest FF ever achieved for a CABI solar cell fabricated with the same metal electrode.…”

“…The champion CABI-Meso device delivers a PCE of 1.26%, which is comparable with the highest PCE values reported for the CABI planar and mesoscopic solar cells fabricated with the same metal electrode (i.e., Au). 18,24,25 The FF value of 72% of CABI-Meso device (Figure 2b) is markedly the highest FF ever achieved for a CABI solar cell fabricated with the same metal electrode. 18,20,24,25 The external quantum efficiency (EQE) values of this device are ⁓29% and 18% at 370 nm and 580 nm, respectively (Figure S3), which are comparable to the values reported earlier for CABI.…”

“…18,24,25 The FF value of 72% of CABI-Meso device (Figure 2b) is markedly the highest FF ever achieved for a CABI solar cell fabricated with the same metal electrode. 18,20,24,25 The external quantum efficiency (EQE) values of this device are ⁓29% and 18% at 370 nm and 580 nm, respectively (Figure S3), which are comparable to the values reported earlier for CABI. 18,24 The short-circuit current density (J SC ) value (from current density (J)-voltage (V) scans) of 3.0 mA cm -2 of the champion device closely matches the value extracted from its EQE spectrum (2.9 mA cm -2 ).…”

“…Cu 2 AgBiI 6 (CABI) is a newly proposed PIM with favorable optoelectronic properties that has rapidly received great interest from the solar cell community. 18,[20][21][22][23][24][25] Very recently, we successfully improved the lm morphology and increased the surface coverage of CABI lms. 18 As a result of the reduced interfacial recombination, we achieved IPVs with a promising PCE(i) of 4.7% under 1000 lux WLED illumination, which is already approaching that of commercially available a-Si:H IPVs.…”