Phenomena In Solar Cells Research Articles

Long-term stability of graphene/c-Si Schottky-junction solar cells

A long operational lifetime is required for the use of solar cells in real-life photovoltaic applications. The optimization of operational lifetimes is achieved through understanding the inherent degradation phenomena in solar cells. In this study, graphene/Si Schottky-junction solar cells were produced, utilizing liquid-phase-exfoliated graphene as an active surface. The operational and interface stability of these solar cells over a period of 5 years in ambient conditions (following ISOS-D protocols: dark storage/shelf life) was examined, and the origin of their degradation was reported. It was found that the dominant degradation mechanism could be attributed to the degradation of silver contacts. This was indicated by a decrease in shunt resistance, an increase in the ideality factor (due to a higher carrier recombination), and a constant defect density in graphene films for up to 4 years. Measurements across the solar cell's active area during the 5-year period revealed neither significant spatial inhomogeneity, nor shunt channel defects.

Multiscale prediction of localized hot-spot phenomena in solar cells

Hot-spot phenomena in photovoltaic (PV) devices may suppress the performance and cause irresistible damage. Traditional study focuses on hot spots caused by shade or faults of PV modules, discussing the systematic failure. Different from that, this study focuses on hot-spot phenomena due to nonuniform heat generation within a unit cell of the module, which is resulted from surface microstructures, locally enhanced absorption and uneven concentration of light. In this paper, the volume heat generation in a cell is derived by modeling interactions between photons, excited free electrons and lattice phonons. And significant unevenness of heat generation is found. Based on the 3D heat distribution, transient simulations of cell temperature are conducted. Although hot-spot phenomena are negligible in non-concentrated PV cells, in a 400 × (1000 × ) high concentrator photovoltaic(HCPV) cell, remarkable hot spot is found with maximum temperature difference of 68K(169K) and the local hottest spot reaches 372K(491K). More seriously, in cloudy or windy days, cells may be shaded constantly, leading to sharp and constant temperature variation at the hot spot area in tens of microseconds. All these results creatively reveal that localized hot-spot phenomena within a cell may significantly suppress the cell performance and shorten the lifespan.

From Process to Modules: End-to-End Modeling of CSS-Deposited CdTe Solar Cells

In this paper, we develop an end-to-end modeling framework to explore how various multiscale phenomena in solar cells translate from materials to module level. Specifically, the model captures the physics related to 1) the pressure-dependent grain growth of polycrystalline thin films (nanometers to micrometers), 2) averaging of the effects of grain-size distribution at the centimeter scale, and 3) effects of parasitic series and shunt resistance distributions on the efficiency of thin-film solar cell modules (centimeter to meter scale). As an idealized illustrative example, we consider a number of puzzling features that are associated with close space sublimated CdTe solar cells. The model explains both the increase in the grain size with deposition pressure, as well as the saturation of cell efficiency beyond a critical grain size. The analysis shows that grain geometry and grain-size distribution are unimportant for average grain sizes larger than 1 μm. The model attributes the significant efficiency loss at the module level to the series resistance and the operating point inhomogeneity caused by parasitic shunts. Overall, the model identifies opportunities for significant improvement at all length scales of thin-film solar cell technologies.

Lock-in thermography for investigating solar cells and materials

For more than 10 years lock-in thermography (LT) is being used for investigating leakage current phenomena in solar cells. In operation the p-n junction of a solar cell is forward-biased. So-called local shunts are leakage sites which cause losses in efficiency. They can be investigated by applying a pulsed forward-bias to the cell in the dark (typical lock-in frequency 10 Hz), where all local currents become visible as local pulsed heat sources. Such investigations allow the reliable localization of all leakage sites. This is the first step for investigating their physical nature and therefore very helpful for optimizing the efficiency of solar cells. In this contribution the practical realization of LT on solar cells is described and typical results of such investigations are shown. In the last years several special LT techniques have been developed for imaging special physical parameters of solar cells, some of them employing light irradiation. Also non-thermal LT techniques for imaging the minority carrier lifetime in semiconductor wafers have been developed. These special LT techniques are reviewed in the second part of this contribution. Thus, LT meanwhile implies a whole family of techniques for investigating solar cells, solar modules, and materials.

96/06215 Investigation of the current break-down phenomena in solar cells

96/06215 Investigation of the current break-down phenomena in solar cells

Investigation of the current break-down phenomena in solar cells

Observed reverse current-voltage characteristics of the single crystal silicon and gallium arsenide solar cells have been analyzed. Physical mechanisms behind the junction break-down in silicon cells and current break-down in gallium arsenide cells have been identified. Preliminary estimates of the diffusion capacitance in GaAs cells have been presented.