Charge-Carrier Mobility and Localization in Semiconducting Cu₂AgBiI₆ for Photovoltaic Applications

Abstract

Lead-free silver–bismuth semiconductors have become increasingly popular materials for optoelectronic applications, building upon the success of lead halide perovskites. In these materials, charge-lattice couplings fundamentally determine charge transport, critically affecting device performance. In this study, we investigate the optoelectronic properties of the recently discovered lead-free semiconductor Cu2AgBiI6 using temperature-dependent photoluminescence, absorption, and optical-pump terahertz-probe spectroscopy. We report ultrafast charge-carrier localization effects, evident from sharp THz photoconductivity decays occurring within a few picoseconds after excitation and a rise in intensity with decreasing temperature of long-lived, highly Stokes-shifted photoluminescence. We conclude that charge carriers in Cu2AgBiI6 are subject to strong charge-lattice coupling. However, such small polarons still exhibit mobilities in excess of 1 cm2 V–1 s–1 at room temperature because of low energetic barriers to formation and transport. Together with a low exciton binding energy of ∼29 meV and a direct band gap near 2.1 eV, these findings highlight Cu2AgBiI6 as an attractive lead-free material for photovoltaic applications.

Chemical Control of the Dimensionality of the Octahedral Network of Solar Absorbers from the CuI–AgI–BiI3 Phase Space by Synthesis of 3D CuAgBiI5

“…We also note the presence of some subgap absorption between approximately 950−700 nm, as measured by Fourier transform infrared (FTIR) spectroscopy, which is possibly due to subgap defect states similar to those observed by Photo-Thermal Deflection Spectroscopy (PDS) in Cu 2 AgBiI 6 , although we cannot rule out scattering of longwavelength light from the rough film surfaces leading to lower light transmission in this region but still allowing for the red transmitted light observed when the films are backlit (Figure S7f). We measure steady-state photoluminescence (PL) spectra on the same thin film under continuous wave excitation in vacuum, with CuAgBiI 5 showing weak, broad emission peaking at approximately 760 nm (1.63 eV), giving a Stokes shift of 390 meV, similar to, though slightly larger than, that observed in Cu 2 AgBiI 6 and characteristic of emission from localized charge-carrier states, which has been proposed as the source of PL emission in Cu 2 AgBiI 6 , 25 as well as for Cs 2 AgBiBr 6 (Figure 7a). 6,38,39 Compared to Cu 2 AgBiI 6 , the PL peak of CuAgBiI 5 is slightly shifted to lower energies (1.71 eV for Cu 2 AgBiI 6 ) with a slightly higher Stokes shift (350 meV for Cu 2 AgBiI 6 ).…”

“…We measure steady-state photoluminescence (PL) spectra on the same thin film under continuous wave excitation in vacuum, with CuAgBiI 5 showing weak, broad emission peaking at approximately 760 nm (1.63 eV), giving a Stokes shift of 390 meV, similar to, though slightly larger than, that observed in Cu 2 AgBiI 6 and characteristic of emission from localized charge-carrier states, which has been proposed as the source of PL emission in Cu 2 AgBiI 6 , as well as for Cs 2 AgBiBr 6 (Figure a). ,, Compared to Cu 2 AgBiI 6 , the PL peak of CuAgBiI 5 is slightly shifted to lower energies (1.71 eV for Cu 2 AgBiI 6 ) with a slightly higher Stokes shift (350 meV for Cu 2 AgBiI 6 ). The PL peak of the CuAgBiI 5 thin film is fitted to a Gaussian peak shape with an fwhm of 312(6) meV, slightly wider than the fwhm of 289 meV extracted for Cu 2 AgBiI 6 films.…”

“…CuBiI 4 has been processed into devices with a maximum PCE of 1.1%; , however, we previously found this composition to be an unstable phase, and it decomposes back into CuI and BiI 3 at room temperature . To stabilize a Cu-containing material, we previously synthesized Cu 2 AgBiI 6 and fabricated a preliminary device with a PCE of 0.43%. , The band gap was found to be 2.06(1) eV, which was modeled to pair efficiently with a crystalline silicon solar absorber in a lead-free tandem cell. Little is known about the other Cu-containing compound Cu 2 BiI 5 , which has been reported to crystallize in a hexagonal unit cell, with no crystal structural solution or devices reported yet; but it has a band gap of 1.53–1.74 eV …”

Doping of Sn-based two-dimensional perovskite semiconductor for high-performance field-effect transistors and thermoelectric devices

“…The extracted mobility and its temperature dependence are presented in Figure 3 I, which illustrates an increase of μ with decreasing temperature. In general, the temperature dependence of charge-carrier mobility is closely related to the charge scattering mechanism in the semiconductors and can be expressed as ( Biewald et al., 2019 ; Wright et al., 2016 ), where γ reflects the magnitude of charge scattering, with larger γ representing stronger scattering in the perovskite ( Buizza et al., 2021 ; Herz, 2017 ; Senanayak et al., 2017 ). The pristine device shows a γ value of 1.47 between 200 and 280 K, which seems to indicate the charge transport in this regime is dominated by acoustic phonon scattering ( Biewald et al., 2019 ; Wright et al., 2016 ; Yi et al., 2016 ).…”

Halide Double-Perovskite Semiconductors beyond Photovoltaics

“…In contrast, no change or minor red-shifts in photoluminescence have been observed for Cs 2 AgBiBr 6 , accompanied by a slight narrowing of the emission line width (∼20 meV). 39 For some of the halide double perovskites (e.g., Cu 2 AgBiI 6 , 40 Rb 4 Ag 2 BiBr 9 41 ), even more complex low-temperature spectra have been observed, showing multiple peaks also in the near-infrared region of the spectrum. The origin of the emission feature is still heavily under debate.…”