The quest for sustainable and clean energy solutions has positioned solar power at the forefront of global research and development. Among the various technologies for harnessing sunlight, thin film solar panels offer distinct advantages, including reduced material usage, lower manufacturing costs, and the potential for flexible and lightweight applications. While established technologies like CdTe and CIGS have achieved notable commercial success, concerns regarding material toxicity, elemental scarcity, and long-term stability persist. This has spurred the exploration of novel absorber materials composed of earth-abundant, non-toxic elements. In this context, germanium monoselenide (GeSe) has emerged as a highly promising candidate for the next generation of thin film solar panels. Our research has been dedicated to unlocking the photovoltaic potential of this simple binary compound, pioneering its development from fundamental material studies to functional device fabrication.
The evolution of thin film solar panels has been marked by a continuous search for the optimal balance between efficiency, cost, sustainability, and stability. Crystalline silicon dominates the market, but its energy-intensive production process limits cost reduction. Thin-film alternatives like CdTe and CIGS have achieved efficiencies over 22%, yet they rely on toxic (Cd) or scarce (In, Te, Ga) elements. Perovskite solar cells have shown meteoric efficiency rises but face significant challenges related to lead toxicity and environmental instability. Materials such as Sb2(S,Se)3 and CZTSSe represent important steps towards greener alternatives, but they often contend with complex quaternary or penternary chemistry, narrow thermodynamic stability windows, or persistent efficiency bottlenecks. GeSe, with its binary composition, offers a pathway to circumvent these complexities. Our work was initiated by recognizing its unique combination of suitable optoelectronic properties and intrinsic material benignity, which we believe could redefine the landscape of low-cost, high-performance thin film solar panels.
The fundamental properties of a semiconductor dictate its ultimate potential as a photovoltaic absorber. GeSe possesses a compelling portfolio of characteristics ideal for thin film solar panels. Crystallographically, it adopts an orthorhombic structure (space group Pnma) with a pronounced two-dimensional, layered architecture. Within each layer, Ge and Se atoms are bound by strong covalent bonds, while adjacent layers are held together by weaker van der Waals forces. This structural anisotropy has profound implications for its defect physics and film growth. Optically, GeSe is an indirect bandgap semiconductor with a value remarkably close to its direct transition. Our measurements consistently yield a bandgap (Eg) of approximately 1.14 eV, which is near the ideal Shockley-Queisser limit for single-junction solar cells. The theoretical maximum efficiency (ηSQ) for a material with this bandgap can be calculated as:
$$
\eta_{SQ} \approx \frac{J_{sc} \times V_{oc} \times FF}{P_{in}}
$$
where the achievable short-circuit current density (Jsc) and open-circuit voltage (Voc) are intrinsically linked to Eg. For Eg = 1.14 eV, the theoretical limit exceeds 30%. Crucially, GeSe exhibits an exceptionally high absorption coefficient (α > 105 cm-1 at visible wavelengths), governed by dipole-allowed p→p transitions. This allows for near-complete light absorption with an ultra-thin layer, a key advantage for thin film solar panels. The absorption depth (d) required can be estimated by:
$$
d \approx \frac{2.303}{\alpha}
$$
For α ≈ 2×105 cm-1, d is only about 115 nm, meaning films of 500-1000 nm are more than sufficient.
Electrically, as-grown GeSe typically shows p-type conductivity due to the low formation energy of germanium vacancies (VGe). Hall effect measurements on single crystals reveal a high hole mobility (μh) exceeding 120 cm2V-1s-1, significantly higher than that of polycrystalline CdTe or CIGS. This high mobility, combined with a relatively high dielectric constant (εr ≈ 15.3), leads to weaker carrier trapping and lower non-radiative recombination rates. The most distinctive feature of GeSe, however, lies in its electronic structure and consequent defect properties. Unlike conventional semiconductors where bonding orbitals constitute the valence band maximum (VBM) and anti-bonding orbitals form the conduction band minimum (CBM), the VBM of GeSe is primarily composed of Ge 4s and Se 4p anti-bonding states. This electronic configuration raises the VBM energy and renders the dominant acceptor defect (VGe) shallow. Furthermore, the strong covalent bonding leads to high formation energies for deep-level defects, resulting in an intrinsically benign defect landscape. Deep-level transient spectroscopy (DLTS) confirms that the concentration of harmful deep traps is on the order of 1012 cm-3, which is remarkably low for a polycrystalline thin-film material. These properties are summarized in the table below.
| Property Category | Parameter | Value / Description | Significance for Thin-Film Solar Panels |
|---|---|---|---|
| Structural & Material | Crystal System | Orthorhombic (Pnma) | Layered growth, anisotropic properties. |
| Density | 5.56 g cm-3 | Moderate, enables thin layers. | |
| Melting/Sublimation Point | ~670 °C | Suitable for thermal deposition processes. | |
| Optical | Bandgap (Eg) | ~1.14 eV (indirect) | Near-ideal for single-junction efficiency limit (>30%). |
| Absorption Coefficient (α) | > 105 cm-1 @ visible | Enables ultrathin (~500 nm) fully absorbing layers. | |
| Refractive Index (n) | ~4 | Requires anti-reflection coating for optimal light in-coupling. | |
| Electrical | Conductivity Type | p-type (intrinsic) | Requires pairing with an n-type partner (e.g., CdS). |
| Hole Mobility (μh) | > 120 cm2V-1s-1 (single crystal) | High carrier drift velocity, reduces series resistance. | |
| Dielectric Constant (εr) | ~15.3 | Reduces Coulombic attraction, leading to shallow defects. | |
| Carrier Concentration (p) | ~1015 cm-3 | Moderate doping, suitable for photovoltaic junction formation. | |
| Defect | Deep-Level Trap Density (Nt) | ~1012 cm-3 | Intrinsically low, minimizes Shockley-Read-Hall recombination. |
The journey to realizing GeSe-based thin film solar panels began with the challenge of depositing high-quality, phase-pure films. Early attempts using methods like co-evaporation or sputtering often resulted in amorphous films or mixtures containing secondary phases like Ge or GeSe2, which are detrimental to device performance. Our breakthrough came from leveraging a quintessential property of GeSe: its propensity to sublime. We developed a close-space sublimation (CSS) process specifically tailored for GeSe. This method capitalizes on the significant difference in vapor pressure between GeSe and its common impurities (Ge, GeSe2) at typical deposition temperatures (400-500°C). The process can be described by the relation between vapor pressure (P) and temperature (T), approximated by the Clausius-Clapeyron equation:
$$
\ln P = -\frac{\Delta H_{sub}}{R} \cdot \frac{1}{T} + C
$$
where ΔHsub is the enthalpy of sublimation and R is the gas constant. At 400°C, the vapor pressure of GeSe is orders of magnitude higher than that of Ge or GeSe2, enabling intrinsic source purification during deposition. Furthermore, mass spectrometry confirmed that GeSe sublimes predominantly as diatomic (GeSe)g molecules. This molecular sublimation mode, where one short Ge-Se bond remains intact, minimizes the formation of point defects like inter-layer vacancies or anti-site disorders that could occur if individual atoms were deposited. The CSS process is not only effective but also highly compatible with industrial-scale manufacturing of thin film solar panels, similar to the established process for CdTe. The table below contrasts common thin-film deposition techniques as applied to GeSe.
| Deposition Method | Principle | Advantages for GeSe | Disadvantages/Challenges |
|---|---|---|---|
| Close-Space Sublimation (CSS) | Thermal sublimation of source in close proximity to substrate. | Intrinsic source purification; fast growth rate; high-quality polycrystalline films; scalable. | Requires precise temperature and pressure control. |
| Thermal Evaporation | Evaporation in high vacuum from a single source. | Good film uniformity; relatively simple setup. | Risk of stoichiometry deviation; slower rate; less effective purification. |
| Sputtering (Ge + Se target) | Ejection of material via plasma bombardment. | Excellent uniformity over large areas; good adhesion. | Often produces amorphous or mixed-phase films; requires post-selenization. |
| Solution Processing | Deposition from a precursor ink/solution. | Ultra-low cost; non-vacuum; potential for printing. | Extremely challenging for pure GeSe due to complex chemistry and precursor design; often contains carbon residues. |
With a reliable method for fabricating dense, polycrystalline GeSe films, we proceeded to construct the first functioning GeSe thin film solar panels. The initial device architecture employed a superstrate configuration: Glass/ITO/CdS/GeSe/Au. In this design, light enters through the glass and transparent ITO electrode, passes through the n-type CdS buffer layer, and is absorbed in the p-type GeSe layer. The key metric, power conversion efficiency (PCE), is given by:
$$
PCE (\eta) = \frac{J_{sc} \times V_{oc} \times FF}{P_{in}} \times 100\%
$$
Our first devices yielded a PCE of 1.48%, with Voc = 0.24 V, Jsc = 14.48 mA cm-2, and FF = 42.6%. While modest, this result was a critical proof-of-concept. However, detailed analysis revealed a major issue: significant inter-diffusion at the CdS/GeSe interface during the high-temperature GeSe deposition. Cadmium atoms from the CdS layer diffused into the GeSe, acting as deep donors and creating recombination centers. This interfacial degradation severely limited Voc and overall performance.
To circumvent this problem, we inverted the structure to a substrate configuration: Glass/Mo/GeSe/CdS/i-ZnO/ITO/Ag. Here, the GeSe absorber is deposited on the Mo-coated glass at high temperature first, followed by the low-temperature deposition of the CdS buffer layer and transparent conducting oxides (TCOs). This sequence entirely avoids subjecting the completed p-n junction to high temperatures, thereby suppressing Cd diffusion. Elemental mapping confirmed sharp, diffusion-free interfaces. This structural change led to a substantial performance improvement, boosting the champion device PCE to 3.1% (Voc=0.33 V, Jsc=20.1 mA cm-2, FF=47.1%). The substrate configuration is also strategically important for the future development of flexible thin film solar panels and for integration into tandem or multi-junction architectures.
While bulk GeSe has benign defects, the surface and interface properties became the new bottleneck. Theoretical calculations indicated that the preferred (111)-oriented surface of our CSS-grown films exposes Se dangling bonds that create deep trap states within the bandgap. To passivate these surface states, we introduced an ultra-thin layer of Sb2Se3 at the GeSe/CdS interface. This low-dimensional material bonds effectively with the GeSe surface, neutralizing the dangling bonds and delocalizing the remaining defect states. The effectiveness of passivation was quantified using capacitance-voltage (C-V) and drive-level capacitance profiling (DLCP) measurements. The interface defect density (Nit) plummeted from approximately 2 × 1012 cm-2 to below 3 × 1011 cm-2. This order-of-magnitude reduction directly translated to enhanced device parameters. The modified devices showed a dramatic increase in efficiency, with the best cell achieving a PCE of 5.2% as independently certified by a recognized laboratory (Voc=0.376 V, Jsc=24.6 mA cm-2, FF=56.3%). This progress, achieved within a few years of the first report, underscores the rapid development potential of GeSe-based thin film solar panels. The evolution of key performance parameters is summarized below.
| Device Stage / Innovation | Architecture | Voc (V) | Jsc (mA cm-2) | FF (%) | PCE (%) | Primary Limitation Addressed |
|---|---|---|---|---|---|---|
| Initial Proof-of-Concept | Superstrate (Glass/ITO/CdS/GeSe/Au) | 0.24 | 14.5 | 42.6 | 1.48 | Demonstration of photovoltaic effect in GeSe. |
| Substrate Configuration | Substrate (Mo/GeSe/CdS/i-ZnO/ITO) | 0.33 | 20.1 | 47.1 | 3.1 | Elimination of high-temperature Cd inter-diffusion. |
| Interface Passivation | Substrate with Sb2Se3 interlayer | 0.376 | 24.6 | 56.3 | 5.2 (certified) | Reduction of interfacial recombination loss. |
| Theoretical S-Q Limit | Ideal Single Junction | ~0.85* | ~45* | ~90* | > 30* | Target for future material and device optimization. |
* Approximate ideal values for a 1.14 eV bandgap under AM1.5G spectrum.
For any photovoltaic technology to be viable, especially for thin film solar panels intended for long-term outdoor deployment, stability is as crucial as efficiency. We have conducted systematic stability tests on unencapsulated GeSe devices following international protocols (e.g., IEC 61646). The results are highly encouraging. GeSe films themselves show no signs of oxidation or phase degradation when stored in ambient air for extended periods, a stark contrast to many metal-halide perovskites. This inherent stability stems from the strong covalent Ge-Se bonds and the low chemical reactivity of the constituent elements. Device stability tests, including continuous illumination under maximum power point tracking, exposure to ultraviolet light, and thermal cycling between -40°C and 85°C, have shown minimal performance degradation over hundreds of hours. This robust intrinsic stability provides a solid foundation for the further development of durable GeSe thin film solar panels.
In conclusion, our work has established GeSe as a serious new contender in the field of thin film solar panels. Its compelling properties—an ideal bandgap, high absorption, high mobility, intrinsically benign defects, and elemental abundance—are complemented by a scalable and purifying deposition process. From the first functional device at 1.48% to a certified 5.2% efficiency through interface engineering, the progress has been rapid and demonstrates the significant headroom for improvement. The current efficiency, while promising, remains far from the material’s theoretical potential. The primary losses can be attributed to non-ideal band alignment at the heterojunction, limited carrier collection length potentially due to grain boundaries, and still non-optimum film morphology. Future research directions are clear and multifaceted. Firstly, further optimization of the CSS process to achieve larger grains and more controlled orientation could reduce bulk recombination. Secondly, exploration of alternative buffer layers (e.g., Zn(O,S), (Zn,Sn)O, or organic molecules) with better band alignment to minimize interface recombination and boost Voc is essential. The relationship between band offset (ΔEC, ΔEV) and Voc loss is critical:
$$
V_{oc,loss} \approx \frac{E_g}{q} – V_{oc} = \phi_n + \phi_p + \Delta \phi_{interface} + \frac{kT}{q} \ln\left(\frac{J_{00}}{J_{sc}}\right)
$$
where φn and φp are the Fermi-level positions in the n- and p-type materials, and Δφinterface represents losses from interface defects and band misalignment. Thirdly, the development of a suitable, stable back contact and hole-transport layer for a true p-i-n structure could further enhance charge extraction. Finally, given its near-ideal bandgap, GeSe is a perfect candidate for the bottom cell in a tandem configuration with a wider-bandgap top cell (e.g., perovskite), offering a pathway to efficiencies beyond 30%. With continued dedicated research addressing these challenges, GeSe-based thin film solar panels have the potential to evolve into a high-performance, sustainable, and cost-effective photovoltaic technology, contributing significantly to the global clean energy transition.