Nonequal current generation in the cells of a photovoltaic module, e.g., due to partial shading, leads to operation in reverse bias. This quickly causes a significant efficiency loss in perovskite solar cells. We report a more quantitative investigation of the reverse bias degradation. Various small reverse biases (negative voltages) were applied for different durations. After normalizing the applied voltages with the breakdown voltages, we found similar dependences of the reverse bias current and the degradation rate. We draw conclusions regarding possible degradation mechanisms and propose a way to increase the comparability of degradation rates for comparing different perovskite solar cells.

@en

Partial shading of PV modules can lead to degradation of the shaded cells. The degradation originates from a reverse bias voltage over the shaded cells. In order to mitigate reverse bias damage in Cu(In, Ga)Se2 (CIGS) modules, a good understanding of the fundamental mechanisms governing the reverse characteristic is required. In this study, a model is introduced that describes this behavior for CIGS cells. In this model, the low and non-Ohmic leakage current is accounted for by the space charge limited current component. A sharp increase in current that is typically observed in the CIGS reverse characteristics can be described by Fowler-Nordheim tunneling. This model has been validated against measurements performed at different temperatures and illumination intensities, and is able to describe the dependencies of the reverse bias behavior on both temperature and illumination.

@en

Partial shading of monolithically interconnected Cu(In,Ga)Se₂ (CIGSe) modules can lead to the formation of reverse bias induced defects. These localized defects permanently reduce the output of the PV module. The formation and propagation mechanisms of these defects is studied. Understanding these mechanisms can help to prevent or mitigate damage due to partial shading of CIGSe PV modules. A propagation mechanism is proposed based on both compositional changes found at the edges of the reverse bias induced defects and differences in observed propagation patterns caused by the lateral voltage drop over the TCO layer.

@en

When a PV module is partially shaded, the shaded solar cells operate in a reverse bias condition. For Cu(In,Ga)Se2 cells this condition can cause defects that irreversibly reduce the output of these cells and the full module. In order to design robust shade-tolerant CIGS modules details need to be known of the conditions at which these defects will be formed. In this study a large number of cells were exposed to different reverse bias conditions. By using simple statistics the probability of the occurrence of defects as a result of reverse bias at any given voltage has been determined. Based on our experiments we have found that the absorber thickness is one of the main parameters that affects the shade-tolerance: the thicker the absorber, the more shade tolerant the CIGS module will be.

@en

Partial shading of Cu(In,Ga)(Se,S)2 (CIGS) photovoltaic (PV) modules is getting more attention, as is witnessed by the increase in publications on this topic in recent years. This review will give an overview of shading tests executed on CIGS modules and focuses on the more fundamental aspects that are often studied on cells. Generally, CIGS modules display very attractive performance under predictable row-to-row shading. However, potential damage could occur under nonoptimal shading orientations: module output after shading tests could reduce due to the formation of local shunts, often called wormlike defects. The influence of many factors on the formation of these defects, including the internal currents and voltages and the shape and intensity of the shade, will be discussed. This review allows an increased insight in the degradation mechanisms caused by partial shading, which would ultimately lead to the introduction of more shade-tolerant CIGS PV products in the future.

@en

Partial shading of Cu(In,Ga)Se₂ modules can lead to the formation of reverse bias induced wormlike defects. These wormlike defects act as local shunts and permanently decrease module output. A good understanding of the formation and propagation mechanisms of these defects is needed in order to mitigate the negative effects, or to prevent these defects from forming. In this article, wormlike defects were formed on small nonencapsulated cells by exposing them to reverse bias conditions. Scanning electron microscopy-energy-dispersive X-ray spectroscopy measurements showed a rearrangement of elements: Indium, gallium, and copper were replaced by cadmium, whereas selenium was replaced by sulfur in the area around the defect. Moreover, additional electronic-defect levels were found in that area with spectrally resolved photoluminescence spectroscopy. Based on the material changes in the area close to the wormlike defects, a propagation mechanism is proposed. The model assumes a chemical reaction as the driving force for propagation instead of melting because of ohmic heating.

@en

Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening positron annihilation spectroscopy (DB-PAS) depth profiling demonstrate pronounced growth of vacancy clusters at the grain boundaries of as-deposited Al-doped ZnO films deposited as transparent conductive oxide (TCO) on Cu(In,Ga)Se2 (CIGS) solar cells upon accelerated degradation at 85∘C/85% relative humidity. Quantitative fractions of positrons trapped either in the vacancy clusters at the grain boundaries or in Zn monovacancies inside the grains of ZnO:Al were obtained by detailed analysis of the PALS data using a positron trapping model. The time and depth dependence of the positron Doppler depth profiles can be accurately described using a planar diffusion model, with an extracted diffusion coefficient of 35nm2/hour characteristic for in-diffusion of molecules such as H2O and CO2 into ZnO:Al TCO films via the grain boundaries, where they react with the ZnO:Al. This leads to increased open volume at the grain boundaries that imposes additional transport barriers and may lead to charge carrier trapping and nonradiative recombination. Simultaneously, a pronounced increase in series resistance and a strong reduction in efficiency of the ZnO:Al capped CIGS solar cells is observed on a remarkably similar timescale. This strongly indicates that these atomic-scale processes of molecular in-diffusion and creation of open volume at the grain boundaries play a key role in the degradation of the solar cells.@en

Positron annihilation depth-profiling is used as an innovative tool to monitor the evolution of vacancy defects in two series of ZnO:Al transparent conductive oxide (TCO) layers for Cu(In,Ga)Se2 (CIGS) solar cells under accelerated degradation at 85??C/85% relative humidity. The first series of ZnO:Al layers are deposited directly on flat glass substrates, leading to low densities of (extended) grain boundaries in the ZnO:Al. These ZnO:Al layers only show an increase in open volume upon degradation in the near-surface range. The second series of ZnO:Al layers are deposited on the more corrugated surface of CdS/CIGS/Mo solar cells, and show, on the other hand, a pronounced formation of open volume throughout the layer. Its depth-dependence is consistent with in-diffusion of molecules such as H2O and CO2 into the ZnO:Al layer via the grain boundaries, as primary driver for the degradation. The detected time-dependence of the growth of open volume at the grain boundaries in the ZnO:Al TCO layer matches the time scale of the observed reduction in solar cell efficiency and series resistance, suggesting that the generated open volume induces a significant barrier against charge carrier transport.@en

Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening positron annihilation spectroscopy DB-PAS) depth profiling demonstrate pronounced growth of vacancy clusters at the grain boundaries of as-deposited Al-doped ZnO films deposited as transparent conductive oxide (TCO) on Cu(In, Ga)Se2 (CIGS) solar cells upon accelerated degradation at 85 ◦C/85% relative humidity. Quantitative fractions of positrons trapped either in the vacancy clusters at the grain boundaries or in Zn monovacancies inside the grains of ZnO:Al were obtained by detailed analysis of the PALS data using a positron trapping model. The time and depth dependence of the positron Doppler depth profiles can be accurately described using a planar diffusion model, with an extracted diffusion coefficient of 35 nm2/hour characteristic for in-diffusion of molecules such as H2O andCO2 into ZnO:Al TCO films via the grain boundaries, where they react with the ZnO:Al. This leads to increased open volume at the grain boundaries that imposes additional transport barriers and may lead to charge carrier trapping and nonradiative recombination. Simultaneously, a pronounced increase in series resistance and a strong reduction in efficiency of the ZnO:Al capped CIGS solar cells is observed on a remarkably similar timescale. This strongly indicates that these atomic-scale processes of molecular in-diffusion and creation of open volume at the grain boundaries play a key role in the degradation of the solar cells. PhySH: Solar Cells, Positron Annihilation Spectroscopy, Grain Boundaries, Vacancies, Thin Films, Diffusion, Electrical Properties, Solid State Chemistry, Optoelectronics@en