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Partial shading in thin film solar panels can result in reverse bias stress across shaded cells. Therefore, it is important to understand the effect of such reverse stress in commercially competitive PV technologies such as CIGS. In this paper, we systematically investigate the effect of moderate reverse bias on solution-processed CIGS solar cells. We subject the solar cells to varying degrees of reverse biases and continuously monitor the impact of the stress on dark current. We also explore the relaxation behavior of dark current following passive storage and the long term effect of the shadow stress on power output of the cell. We find that the reverse stress affects only the localized shunt current paths, without affecting the bulk device characteristics. The shunt current exhibits a metastable change with reverse stress, and can increase or decrease on application of reverse stress. We analyze this phenomenon in detail, and discuss the hypothesis that can explain its characteristic features.

We analyze the problem of partial shading of thin film photovoltaic (TFPV) panels, using full two dimensional circuit simulations. By accounting for the panel structure and typical array configurations, we can accurately account for the effect of various shading configurations at the cell and panel level. We demonstrate the limitation of external bypass diodes in protecting shaded cells from reverse breakdown, and explore the whole range of shading scenarios and their impact on reverse stress experienced by shaded cells. Based on the analysis, we identify the key aspects of shading problem, and formulate design rules for shadow aware geometrical design of panels. Finally, we present a new radial design for a thin film PV panel, which not only improves the cell reliability under partial shading, but also enhances the array output current.

In this paper, we present a physical model of the non- Ohmic shunt current ISH in amorphous silicon (a-Si:H) p–i–n solar cells and validate it with detailed measurements. This model is based on space-charge-limited (SCL) transport through localized p–i–p shunt paths. These paths can arise from n-contact metal incorporation in the a-Si:H layer, causing the (n)a-Si:H to be counterdoped to p-type. The model not only explains all the electrical characteristics of preexisting shunts but also provides insight into the metastable switching that is observed in the shunt-dominated region of dark current as well.We first verify the SCL model using simulations and statistically robust measurements, and then use it to analyze our systematic observations of nonvolatile switching of the low-bias dark characteristics. This study interprets broad experimental observations regarding shunt behavior, and suggests possible techniques for alleviating shunt-induced performance and reliability issues.

We present a physical model of non-ohmic shunt current in a-Si:H p-i-n solar cells, and validate it with detailed measurements. This model is based on space-charge-limited (SCL) transport through localized p-i-p shunt paths, which can arise from contact metal incorporation in a-Si:H layer. This model explains both the electrical characteristics and the metastable switching behavior of the shunts within an integrated framework. We first verify the SCL model using simulations and statistically robust measurements, and then use this picture to analyze our systematic observations of non-volatile switching in these shunts. Our work not only resolves broad experimental observations on shunt behavior, but also suggests possible techniques for alleviating shunt induced performance and reliability issues.

We describe a comprehensive study of intrinsicreliability issue arising from partial shadowing of photovoltaicpanels (e.g., a leaf fallen on it, a nearby tree casting a shadow,etc.). This can cause the shaded cells to be reverse biased, causingdark current degradation. In this paper, (1) we calculate thestatistical distribution of reverse bias stress arising from variousshading configurations, (2) identify the components of darkcurrent, and provide a scheme to isolate them, (3) characterizethe effect of reverse stress on the dark current of a-Si:H p-i-ncells, and (4) finally, combine these features of degradationprocess with shadowing statistics, to project ‘shadow-degradation’ (SD) over the operating lifetime of solar cells. Ourresults establish shadow degradation as an important intrinsicreliability concern for thin film solar cell.

In this letter, we investigate the nature of shunt leakage currents in large-area (on the order of square centimeters) thin-film a-Si:H p-i-n solar cells and show that it is characterized by following universal features: 1) voltage symmetry; 2) power-law voltage dependence; and 3) weak temperature dependence. The voltage symmetry offers a robust empirical method to isolate the diode current from measured “shunt-contaminated” forward dark IV . We find that space-charge-limited current provides the best qualitative explanation for the observed features of the shunt current. Finally, we discuss the possible physical origin of localized shunt paths in the light of experimental observations from literature.