Sourabh Dongaonkar

Universality of non-ohmic shunt leakage in thin-film solar cells

We compare the dark current-voltage (IV) characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon (a-Si:H) p-i-n cells, organic bulk heterojunction (BHJ) cells, and Cu(In,Ga)Se2 (CIGS) cells. All three device types exhibit a significant shunt leakage current at low forward bias (V<∼0.4) and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage (Ish), across all three solar cell types considered, is characterized by the following common phenomenological features: (a) voltage symmetry about V=0, (b) nonlinear (power law) voltage dependence, and (c) extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from the measured data and discuss its implications on dark IV parameter extraction. We propose a space charge limited (SCL) current model for capturing all these features of the shunt leakage in a consistent framework and discuss possible physical origin of the parasitic paths responsible for this shunt current mechanism.We compare the dark current-voltage (IV) characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon (a-Si:H) p-i-n cells, organic bulk heterojunction (BHJ) cells, and Cu(In,Ga)Se2 (CIGS) cells. All three device types exhibit a significant shunt leakage current at low forward bias (V<∼0.4) and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage (Ish), across all three solar cell types considered, is characterized by the following common phenomenological features: (a) voltage symmetry about V=0, (b) nonlinear (power law) voltage dependence, and (c) extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from the measured data and discuss its implications on dark IV parameter extraction. We propo...

Performance and Reliability Implications of Two-Dimensional Shading in Monolithic Thin-Film Photovoltaic Modules

We analyze the problem of partial shading in monolithically integrated thin-film photovoltaic (TFPV) modules, and explore how the shape and size of the shadows dictate their performance and reliability. We focus on the aspects of the shading problem unique to monolithic TFPV, arising from thin long rectangular series-connected cells, with partial shadows covering only a fraction of the cell area. Using calibrated 2-D circuit simulations, we show that due to the cell shape, the unshaded portion of partially shaded cell experiences higher heat dissipation due to redistribution of voltages and currents across the cells. We then use thermal imaging techniques to compare our results with module behavior under shade in realistic situations. We also analyze the effect of shadow size and orientation by considering several possible shading scenarios. We find that thin edge shadows can cause potentially catastrophic reverse bias damage, depending on their orientation. Finally, we show that external bypass diodes cannot protect the individual cells from shadow-induced reverse stress, but can limit the string output power loss for larger shadows.

On the Nature of Shunt Leakage in Amorphous Silicon p-i-n Solar Cells

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.

Correlation of Built-In Potential and I–V Crossover in Thin-Film Solar Cells

Thin-film solar cells often show a crossover between the illuminated and dark I-V characteristics. Several device specific reasons for crossover exist and have been discussed extensively. In this paper, we show that a low contact-to-contact built-in potential can produce a voltage-dependent photocurrent that leads to I-V crossover at a voltage that is almost exactly the device built-in potential. This mechanism can produce crossover in the absence of carrier trapping or recombination. It can be a contributing factor to crossover, but when an anomalously low contact-to-contact built-in potential exists, it can be the dominant factor. Using numerical simulations, we examine a variety of model solar cell structures with low contact-to-contact built-in potential and show a strong correlation of the crossover and built-in potential voltages. These simulations also suggest that a plot of the illuminated minus dark current may help identify when a low Vbi is limiting device performance.

End to end modeling for variability and reliability analysis of thin film photovoltaics

We present an end-to-end modeling framework, spanning the device, module and also system levels, for analyzing thin film photovoltaics (PV). This approach is based on embedding a detailed, statistically relevant, physics based equivalent circuit into module and array level simulations. This approach enables us to analyze key variability and reliability issues in thin film PV, and allows us to interpret their effect on process yield and intrinsic module lifetimes. Our results suggest that the time-zero gap between cell and module efficiencies, a key variability concern for thin-film PV, can be attributed to process-related shunts with log-normal PDF distributed randomly across the cell surface. Similarly, this end-to-end simulation approach allows us to investigate the reliability issues caused by partial shadowing in thin film modules, especially in context of array configurations. These results provide important insights into its nature and consequences of shadow degradation on long term system performance. This work showcases the importance of an integrated analysis in case of thin film PV, because traditional approaches used to Silicon PV to tackle reliability/variability issues cannot be applied directly to such systems.

Identification, characterization, and implications of shadow degradation in thin film solar cells

We describe a comprehensive study of intrinsic reliability issue arising from partial shadowing of photovoltaic panels (e.g., a leaf fallen on it, a nearby tree casting a shadow, etc.). This can cause the shaded cells to be reverse biased, causing dark current degradation. In this paper, (1) we calculate the statistical distribution of reverse bias stress arising from various shading configurations, (2) identify the components of dark current, and provide a scheme to isolate them, (3) characterize the effect of reverse stress on the dark current of a-Si:H p-i-n cells, and (4) finally, combine these features of degradation process with shadowing statistics, to project ‘shadow-degradation’ (SD) over the operating lifetime of solar cells. Our results establish shadow degradation as an important intrinsic reliability concern for thin film solar cell.

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.

Physics and Statistics of Non-Ohmic Shunt Conduction and Metastability in Amorphous Silicon p–i–n Solar Cells

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.

Intrinisic reliability of amorphous silicon thin film solar cells

In this paper, we have discussed three intrinsic reliability issues of thin-film -Si∶H solar cells; space charge limited shunt conduction through localized metal-semiconductor-metal structures; shadow degradation in series connected cells in a module, and light induced degradation. Despite their distinct external manifestation, these intrinsic reliability issues appear to share common physical phenomena. For example, the light induced and the shadow degradation may be related because they are described by very similar time-exponents (see Fig. 4c and 6a). While the physics of G are different (e.g. photon induced dissociation for LID and (possibly) electron-hole recombination induced dissociation for shadow degradation), it is likely that they both break SiH bonds and are subsequently follow similar diffusive kinetics. Finally, analogies to CMOS reliability; e.g., shunt conduction related to non uniform conduction through oxides, shadow degradation to bulk defect generation and TDDB in gate dielectric, and light induced degradation to NBTI in PMOS transistors; may help illuminate many aspects of the degradation processes.

In-Line Post-Process Scribing for Reducing Cell to Module Efficiency Gap in Monolithic Thin-Film Photovoltaics

The gap between cell and module efficiency is a major challenge for all photovoltaic (PV) technologies. For monolithic thin-film PV modules, a significant fraction of this gap has been attributed to parasitic shunts and other defects, distributed across the module. In this paper, we show that it is possible to contain or isolate these shunts using state-of-the-art laser scribing processes, after the fabrication of the series-connected module is finished. We discuss three alternatives, and quantify the performance gains for each technique. We demonstrate that using these techniques, it is possible to recover up to 50% of the power lost to parasitic shunts, which results in 1-2% (absolute) increase in module efficiencies for typical thin-film PV technologies.