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 CIGS modules can lead to permanent damage of the module in the shaded area. This is caused by harmful reverse bias voltages in the shaded area which lead to reverse bias induced defects, also known as wormlike defects. A lot is already known about the origin and propagation of wormlike defects. However, the fundamental question; why is CIGS so sensitive to reverse bias damage? has not yet been answered. In this study we show that CIGS semiconductor material in the presence of an electric field will spontaneously decompose.

@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

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

Two types of Cu(In,Ga)Se₂ (CIGS) solar cells, both designed for implementation in CIGS modules, were subjected to temperatures between 25C and 105C. Simultaneous exposure to AM1.5 illumination allowed the measurement of their electrical parameters at these temperatures. These two types of solar cells, produced with different deposition routes on soda lime glass (SLG) and polyimide (PI) substrates, showed large variations in the temperature dependency of their electrical parameters. It was shown that the temperature dependency of the open circuit voltage (V_oc) was dependent on its room temperature value: a high V_oc at 25 °C led to a slower loss of V_oc when the temperature was increased. For the V_oc, the normalised temperature dependency varied between -0.28%/°C and -0.47%/°C, which is in agreement with the literature. The temperature dependency of the short circuit current density (J_sc) showed more surprising results: while the PI samples had the expected positive temperature dependency (0.03 to 0.32%/°C), the SLG samples showed a small negative impact of increasing temperature (-0.01 to -0.05%/°C). A correlation between the temperature dependencies of the J_sc and the ideality factor n was observed. Therefore, this difference in the temperature dependence of the J_sc could be caused by increased recombination for the SLG samples. Furthermore, the temperature coefficients of the fill factor (negative), efficiency (negative), and the series (slightly negative) and shunt (negative) resistances were calculated.

@en

Unencapsulated CIGS solar cells were simultaneously exposed to damp heat and illumination. In-situ monitoring of their electrical parameters demonstrated a rapid decrease of the efficiency, mainly driven by changes in the series and shunt resistances. The non-degraded and degraded solar cells were studied by SIMS and XRD to investigate the material changes leading to efficiency loss. SIMS showed the migration of sodium and potassium, likely leading to changes in the shunt resistance and output voltage. Extensive XRD measurements showed that molybdenum oxide was formed and that the in-plane stress in the ZnO:Al film increased. The stress increase is most likely due to the incorporation of species like hydroxide in the grain boundaries. These phenomena could lead to the observed increased series resistance in the solar cells.

@en

Degradation rates and mechanisms for molybdenum back contacts and ZnO:Al front contacts exposed to damp heat were obtained from literature and experiments. It was found that molybdenum back contacts with a higher density and covered by a MoSe₂ film are more stable than their low density, bare counterparts. For ZnO:Al front contacts, various material changes, including thickness, deposition temperatures and doping concentration increase as well as post-deposition treatments can lead to a more stable material. Moreover, the degradation rate of encapsulated and non-encapsulated CIGS solar cells and modules exposed to damp heat were determined It was observed that in general, exposure to damp heat led to a reduction in efficiency, mostly caused by a reduction in Voc and FF. However, it was found that very large differences exist between samples.

@en

The degradation behavior of Mo/MoSe₂ layers have been investigated using damp heat exposure. The two studied molybdenum based films with different densities and microstructures were obtained by lifting off Cu(In,Ga)Se₂ layers from a bilayer molybdenum stack on soda lime glass. Hereby, a glass/Mo/MoSe₂ was obtained, which resembles the back contact as present in Cu(In,Ga)Se₂ solar cells. The samples were degraded for 150 h under standard damp heat conditions and analyzed before, during and after degradation. It was observed that the degradation resulted in the formation of needles and molybdenum oxide layers near the glass/Mo and the Mo/Cu(In,Ga)Se₂ interfaces. X-ray Photoelectron Spectroscopy measurements have shown that the sodium was also present at the surface of the degraded material and it is proposed that the degraded material consists mostly of MoO₃ with intercalated Na⁺. This element has likely migrated from the soda lime glass. This intercalation process could have led to the formation of Na_xMoO₃ ‘molybdenum bronze’ following this redox reaction: xNa⁺ + MoO₃ + xe⁻ ↔ Na_xMoO₃ It is proposed that the formed oxide layer contains Na_xMoO₃ with different Na⁺ contents and different grades of conductivity. This intercalation process can also explain the high mobility of Na⁺ via the grain boundaries in molybdenum. It was also observed that the molybdenum film with a top layer deposited at a high pressure is more susceptible for damp heat degradation.

@en

Non-encapsulated CIGS solar cells with different contents of sodium (Na) and potassium (K) were simultaneously exposed to damp heat and illumination. The solar cells with higher alkali (Na, K) content exhibited higher initial conversion efficiencies, but degraded severely within 100 hours, while samples with a lower alkali content retained their efficiency longer. The degradation was likely caused by the migration of alkali atoms, leading to shunting of the solar cells and the formation of alkali rich spots.

@en

A 'hybrid' degradation setup, which allows the use of humidity, temperature and illumination as loads in order to accelerate degradation of solar cells and modules, has been designed and constructed. In this setup, the current voltage output of photovoltaic samples is automatically logged and the electrical parameters are calculated. This allows the study of the impact of illumination and damp heat induced degradation by in-situ monitoring. Additionally, this setup also allows the determination of temperature dependency of solar cells by a simple procedure.

@en

CIGS solar cells were exposed to liquid water purged with the atmospheric gases carbon dioxide (CO2), oxygen (O2), nitrogen (N2) and air in order to investigate their chemical degradation behavior. The samples were analyzed by electrical, compositional and optical measurements before, during and after exposure in order to follow the degradation behavior of these solar cells in time. The solar cells showed a rapid decrease in conversion efficiency when exposed to water purged with a combination of CO2 and N2 as well as to water purged with air, while their efficiency was slowly reduced in unpurged water and water purged with N2 or O2. Cross-section SEM showed that the exposure of samples to H2O with large concentrations of CO2 led to the dissolution of the ZnO:Al layer, likely starting from the grain boundaries. This resulted in an increased series resistance, which is likely related to an increase in resistivity of the ZnO:Al layer. It also led to a very rapid decrease of the short-circuit current of these samples. Therefore, the conversion efficiency was rapidly lost.@en