• Energy Research

AbstractThe growth of n-type μc-SiOx:H layers in the thickness range of up to 700nm is investigated. We show that the electrical and structural properties of μc-SiOx:H are thickness dependent but to a weaker extend in comparison with n-type μc-Si:H. μc-SiOx:H layers with low dark conductivity in the order of 10-11 S/cm, measured in the planar direction, can be successfully used as n-layers in amorphous silicon solar cells, resulting in an improved cell performance due to reduced parasitic absorption in the n-type layer. The results suggest anisotropic electrical transport in μc-SiOx:H materials, supported by an observation of elongated crystalline regions in high resolution TEM microscopy.

Abstract We present the study of high efficiency single and multi-junction solar cells, focusing on the stability against degradation under illumination. In both single and multijunction solar cells, the thickness of the a-Si:H absorber layer was varied over a wide range up to 790 nm. While single junction a-Si:H solar cells show reduced stability against prolonged light illumination with an increase in layer thickness, themultijunction solar cells are significantly more stable. In these cells the total thickness of the a-Si:H absorber layers (first and second sub-cells) can be significantly increased up to 790 nm while keeping the degradation level low (below 13%) after 1000 hrs of light soaking. The possible origins of an improved stability against degradation are addressed in the paper.

Abstract The fabrication process of high performance quadruple junction thin film silicon solar cells is described and the application of the solar cells in an integrated photoelectrochemical water splitting device is demonstrated. It is shown that the performance of solar cells can be adjusted by varying the process parameters and the thickness of the absorber layers of the individual sub cells and by integrating microcrystalline silicon oxide as intermediate reflecting layers. Thereby current matching of the sub cells was improved and a high open-circuit voltage of 2.8 V was achieved. Furthermore, the solar cell stability against light-induced degradation was investigated. Efficiencies of 13.2% (initial) and 12.6% (after 1000 h of light-soaking) were achieved. Bias-free water splitting with a solar-to-hydrogen efficiency of 7.8% was demonstrated in an integrated photovoltaic–electrochemical device using the developed quadruple junction photocathode. Finally, it is shown that in the case of quadruple junction solar cells the light-induced degradation has a lower effect on the photovoltaic–electrochemical efficiency as on the photovoltaic efficiency.

Abstract To optimize the optical response of a solar cell, specifically designed materials with appropriate optoelectronic properties are needed. Owing to the unique microstructure of doped nanocrystalline silicon oxide, nc-SiO x :H, this material is able to cover an extensive range of optical and electrical properties. However, applying nc-SiO x :H thin-films in photovoltaic devices necessitates an individual adaptation of the material properties according to the specific functions in the device. In this study, we investigated the detailed microstructure of doped nc-SiO x :H films via atom probe tomography at the sub-nm scale, thereby, for the first time, revealing the three-dimensional distribution of the nc-Si network. Furthermore, n- and p-type nc-SiO x :H layers with various optical and electrical properties were implemented as a window, back contact, and an intermediate reflector layer in silicon heterojunction and multi-junction thin-film solar cells with a focus on the key aspects for adapting the material properties to the specific functions. Here, nc-SiO x :H effectively reduced the parasitic absorption and opened new possibilities for the photon management in the solar cells, thereby, demonstrating the versatility of this material. Remarkably, using our adapted nc-SiO x :H layers in distinct functions enabled us to achieve a combined short circuit current density of 15.1 mA cm −2 for the two a-Si:H sub-cells in an a-Si:H/a-Si:H/µc-Si:H triple-junction thin-film solar cell and an active area efficiency of 21.4% was realized for a silicon heterojunction solar cell.

Ultraviolet (UV)‐induced degradation (UVID) poses a significant challenge for the prospective mass production of silicon heterojunction (SHJ) solar cells, known for their high efficiency. In this study, the magnified impact of UV radiation when employing a silicon carbide (SiC)‐based transparent passivating contact (TPC) on the front side of SHJ solar cells is reported. A reduction in open‐circuit voltage (VOC), short‐circuit current (JSC), and fill factor of 12%, 6%, and 11%, respectively, is observed after UV exposure. Conventional UVID mitigation measures, UV‐blocking encapsulation, are assessed through single‐cell TPC laminates, revealing an unavoidable tradeoff between current loss and UVID. Alternatively, the utilization of ultraviolet‐downshifting (UV‐DS) encapsulants is proposed to convert UV radiation into the visible light spectrum. An optical simulation method, conducted via OPAL2, is presented to evaluate UV‐DS encapsulants for diminishing UVID in SHJ solar cells with different front contacts. A simple methodology is proposed to mimic the optical property of UV‐DS encapsulants. In the simulation results, additional current gains of up to 0.33 mA cm−2 achievable with suitable UV‐DS encapsulants are highlighted. The factors related to the UV‐DS effects are evaluated and the optimization pathway for UV‐DS encapsulants is elucidated.

A major challenge for achieving the energy transition and transforming the current energy model into distributed production is the development of efficient artificial leaf devices made of earth-abundant materials for sustainable fuel production.

AbstractSurface morphology of ZnO layers with as-deposited thickness between 550 to 1050nm was varied over wide range by changing etching time. The surface morphology of these ZnO layers was measured by atomic force microscopy (AFM) and statistically analyzed in terms of root mean square roughness and the diameter and depth of the craters. The texture-etched ZnO covered with a Ag and ZnO layers were applied into n-i-p μc-Si:H solar cells as back reflectors. The effects of the back reflector morphologies on solar cell performances were investigated. The link between the morphology, the scattering properties of ZnO layers, and the solar cell performance is discussed.