Abstract
The van der Waals material GeSe is a potential solar absorber, but its optoelectronic properties are not yet fully understood. Here, through a combined theoretical and experimental approach, the optoelectronic and structural properties of GeSe are determined. A fundamental absorption onset of 1.30 eV is found at room temperature, close to the optimum value according to the Shockley–Queisser detailed balance limit, in contrast to previous reports of an indirect fundamental transition of 1.10 eV. The measured absorption spectra and first-principles joint density of states are mutually consistent, both exhibiting an additional distinct onset ∼0.3 eV above the fundamental absorption edge. The band gap values obtained from first-principles calculations converge, as the level of theory and corresponding computational cost increases, to 1.33 eV from the quasiparticle self-consistent GW method, including the solution to the Bethe–Salpeter equation. This agrees with the 0 K value determined from temperature-dependent optical absorption measurements. Relaxed structures based on hybrid functionals reveal a direct fundamental transition in contrast to previous reports. The optoelectronic properties of GeSe are resolved with the system described as a direct semiconductor with a 1.30 eV room temperature band gap. The high level of agreement between experiment and theory encourages the application of this computational methodology to other van der Waals materials.
Highlights
Conventional commercial solar cells based upon Si, Cu(In,Ga)Se2, and CdTe present both high stability and high device efficiency.[1]
In the PBE +D3-optimized structure, the valence band maximum (VBM) occurs away from the high symmetry points and GeSe is predicted to be an indirect semiconductor, in line with previous generalized gradient approximation (GGA) calculations; with the HSE06+D3 structure, the VBM occurs at Γ, meaning the fundamental band gap is direct
Under the assumption of a direct band gap, experimental absorption spectra from thin films indicate a value of 1.301 ± 0.004 eV at 300 K, somewhat larger than the widely quoted value of 1.1−1.2 eV for the fundamental band gap
Summary
Conventional commercial solar cells based upon Si, Cu(In,Ga)Se2, and CdTe present both high stability and high device efficiency.[1]. The compound has low-toxicity, a lower raw cost than Sb2Se3, and germanium is over 6 times more earth-abundant than antimony.[10] Solar cells incorporating GeSe as the absorber, deposited by thermal sublimation, have been reported with a power conversion efficiency of 1.48%,11 an impressive result for such an understudied material It has been reported for single crystal electrical transport measurements that conductivity exhibits weak anisotropy and p-type transport character.[12,13] the literature available on GeSe regarding optical properties is highly conflicted with neither the nature nor magnitude of the fundamental band gap being reconciled. Chen et al performed a combined experimental-theoretical study on GeSe films.[28] Optical absorption measurements were analyzed under the assumption of an indirect band gap, giving a room-temperature value of 1.25 eV. VASP and Questaal, and phonon dispersion curves, made use of the sumo package.[49]
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