Interfacial Strain Engineering in Wide-Bandgap GeS Thin Films for Photovoltaics

Scanning Tunneling Spectroscopy Method for the Prediction of Semiconductor Heterojunction Performance as a Prequel for Device Development

The prediction of semiconductor device performance is a persistent challenge in materials science, and the ability to anticipate useful specifications prior to construction is crucial for enhancing the overall efficiency. In this study, we investigate the constituents of a solar cell by employing scanning tunneling microscopy (STM) and spectroscopy (STS). Through our observations, we identify a spatial distribution of the dopant type in thin films of materials that were designed to present major p-doping for germanium sulfide (GeS) and dominant n-doping for tin disulfide (SnS2). By generating separate STS maps for each semiconductor film and conducting a statistical analysis of the gap and doping distribution, we determine intrinsic limitations for the solar cell efficiency that must be understood prior to processing. Subsequently, we fabricate a solar cell utilizing these materials (GeS and SnS2) via vapor phase deposition and carry out a characterization using standard J-V curves under both dark/illuminated irradiance conditions. Our devices corroborate the expected reduced efficiency due to doping fluctuation but exhibit stable photocurrent responses. As originally planned, quantum efficiency measurements reveal that the peak efficiency of our solar cell coincides with the range where the standard silicon solar cells sharply decline. Our STS method is suggested as a prequel to device development in novel material junctions or deposition processes where fluctuations of doping levels are retrieved due to intrinsic material characteristics such as the occurrence of defects, roughness, local chemical segregation, and faceting or step bunching.

In Situ Formation of 2D Perovskite Seeding for Record-Efficiency Indoor Perovskite Photovoltaic Devices

With 40% efficiency under room light intensity, perovskite solar cells (PSCs) will be promising power supplies for low-light applications, particularly for Internet of Things (IoT) devices and indoor electronics, shall they become commercialized. Herein, β-alaninamide hydrochloride (AHC) is utilized to spontaneously form a layer of 2D perovskite nucleation seeds for improved film uniformity, crystallization quality, and solar cell performance. It is found that the AHC addition indeed improves film quality as demonstrated by better uniformity, lower trap density, smaller lattice stress, and, as a result, a 10-fold increase in charge carrier lifetime. Consequently, not only does the small-area (0.09 cm2 ) PSCs achieve a power conversion efficiency of 42.12%, the large-area cells (1.00 cm2 , and 2.56 cm2 ) attain efficiency as high as 40.93%, and 40.07% respectively. All of these are the highest efficiency values for indoor photovoltaic cells with similar sizes, and more importantly, they represent the smallest efficiency loss due to area scale-up. This work provides a new method to fabricate high-performance indoor PSCs (i-PSCs) for IoT devices with great potential in large-area printing technology.

Optical properties of orthorhombic germanium sulfide: unveiling the anisotropic nature of Wannier excitons

To fully explore exciton-based applications and improve their performance, it is essential to understand the exciton behavior in anisotropic materials. Here, we investigate the optical properties of anisotropic excitons in GeS encapsulated by h-BN using different approaches that combine polarization- and temperature-dependent photoluminescence (PL) measurements, ab initio calculations, and effective mass approximation (EMA). Using the Bethe-Salpeter Equation (BSE) method, we found that the optical absorption spectra in GeS are significantly affected by the Coulomb interaction included in the BSE method, which shows the importance of excitonic effects besides it exhibits a significant dependence on the direction of polarization, revealing the anisotropic nature of bulk GeS. By combining ab initio calculations and EMA methods, we investigated the quasi-hydrogenic exciton states and oscillator strength (OS) of GeS along the zigzag and armchair axes. We found that the anisotropy induces lifting of the degeneracy and mixing of the excitonic states in GeS, which results in highly non-hydrogenic features. A very good agreement with the experiment is observed.

Preparation of Mixed Few-Layer GeSe Nanosheets with High Efficiency by the Thermal Sublimation Method

Two-dimensional (2D) GeSe has been proven promising in fast and broadband optoelectronic applications for its complicated band structure, inert surface property, and excellent stability. The major challenge is the deficiency of the effective technique for controllably prepared large-scale few-to-monolayer GeSe films. For this purpose, a layer-by-layer thinning method by thermal sublimation for manufacturing large-scale mixed few-layer GeSe with direct bandgaps is proposed, and an optimized sublimation temperature of 300 °C in vacuum is evaluated by atomic force microscopy. Scanning electron microscopy, transmission electron microscopy, energy-dispersive spectra, and fluorescence mapping measurements are performed on the thinned GeSe layers, and results are well-indexed to the orthorhombic lattice structure with direct bandgaps with an atomic ratio of Ge/Se ≈ 5:4. Raman and fluorescence spectra show an α-type crystalline structure of the thinned GeSe films, indicating the pure physical process of the sublimation thinning. Both the bulk and few-layer GeSe films demonstrate broadband absorption. Conductivity of the few-layer GeSe device indicates the overall crystalline integrity of the film after thermal thinning. Given the convenience and efficiency, we provide an effective approach for fabrication of large-scale 2D materials that are difficult to be prepared by traditional methods.

New Approach to Low-Power-Consumption, High-Performance Photodetectors Enabled by Nanowire Source-Gated Transistors

Power consumption makes next-generation large-scale photodetection challenging. In this work, the source-gated transistor (SGT) is adopted first as a photodetector, demonstrating the expected low power consumption and high photodetection performance. The SGT is constructed by the functional sulfur-rich shelled GeS nanowire (NW) and low-function metal, displaying a low saturated voltage of 0.61 V ± 0.29 V and an extremely low power consumption of 7.06 pW. When the as-constructed NW SGT is used as a photodetector, the maximum value of the power consumption is as low as 11.96 nW, which is far below that of the reported phototransistors working in the saturated region. Furthermore, benefiting from the adopted SGT device, the photodetector shows a high photovoltage of 6.6 × 10^-1 V, a responsivity of 7.86 × 10¹² V W^-1, and a detectivity of 5.87 × 10¹³ Jones. Obviously, the low power consumption and excellent responsivity and detectivity enabled by NW SGT promise a new approach to next-generation, high-performance photodetection technology.

Indoor photovoltaics awaken the world's first solar cells

Selenium (Se) solar cells were the world's first solid-state photovoltaics reported in 1883, opening the modern photovoltaics. However, its wide bandgap (~1.9 eV) limits sunlight harvesting. Here, we revisit the world's oldest but long-ignored photovoltaic material with the emergence of indoor photovoltaics (IPVs); the absorption spectrum of Se perfectly matches the emission spectra of commonly used indoor light sources in the 400 to 700 nm range. We find that the widely used Te adhesion layer also passivates defects at the nonbonded Se/TiO₂ interface. By optimizing the Te coverage from 6.9 to 70.4%, the resulting Se cells exhibit an efficiency of 15.1% under 1000 lux indoor illumination and show no efficiency loss after 1000 hours of continuous indoor illumination without encapsulation, outperforming the present IPV industry standard of amorphous silicon cells in both efficiency and stability. We further fabricate Se modules (6.75 cm²) that produce 232.6 μW output power under indoor illumination, powering a radio-frequency identification-based localization tag.

Solution-processed Ge (II)-based chalcogenide thin films with tunable bandgaps for photovoltaics

Solution processes have been widely used to construct chalcogenide-based thin-film optoelectronic and electronic devices that combine high performance with low-cost manufacturing. However, Ge(ii)-based chalcogenide thin films possessing great potential for optoelectronic devices have not been reported using solution-based processes; this is mainly attributed to the easy oxidation of intermediate Ge(ii) to Ge(iv) in the precursor solution. Here we report solution-processed deposition of Ge(ii)-based chalcogenide thin films in the case of GeSe and GeS films by introducing hypophosphorous acid as a suitable reducing agent and strong acid. This enables the generation of Ge(ii) from low-cost and stable GeO₂ powders while suppressing the oxidation of Ge(ii) to Ge(iv) in the precursor solution. We further show that such solution processes can also be used to deposit GeSe_1−xS_x alloy films with continuously tunable bandgaps ranging from 1.71 eV (GeS) to 1.14 eV (GeSe) by adjusting the atomic ratio of S- to Se-precursors in solution, thus allowing the realization of optimal-bandgap single-junction photovoltaic devices and multi-junction devices.

Solution-processed Ge(ii)-based chalcogenide films are achieved by introducing hypophosphorous acid as a suitable reducing agent and strong acid and demonstrated for photovoltaic application.

Investigation of the sublimation mechanism of GeSe and GeS

GeSe and GeS have emerged as promising light-harvesting materials for photovoltaics due to their attractive optoelectronic properties, non-toxic and earth-abundant constituents, and excellent stability. Here we unveil the diatomic molecule sublimation mechanism of GeSe and GeS that directly guides the optimization of GeSe and GeS solar-cell fabricated via the close-space sublimation method.

Surface-Defect States in Photovoltaic Absorber GeSe

GeSe is an emerging promising light-harvesting material for photovoltaics due to its excellent optoelectronic properties, nontoxic and earth-abundant constituents, and high stability. In particular, perovskite-like antibonding states at the valence band maximum arising from Ge-4s and Se-4p coupling enable the bulk-defect-tolerant properties in GeSe. However, a fundamental understanding of surface-defect states in GeSe, another important factor for high-performance photovoltaics, is still lacking. Here, we investigate the surface-defect properties of GeSe through first-principle calculations. We find that different from common semiconductors possessing numerous surface dangling bonds, some GeSe surfaces are prone to reconstruction, thus eliminating the dangling bonds. The rearranged armchair edges exhibit unexpected benign defect properties, similar to those of bulk GeSe, arising from the formation of bulk-like [GeSe₃] tetrahedrons. We further show that the stable exposed (111) surfaces are hard to reconstruct due to the stiff structure but are effectively passivated by the addition of H.