If solar cell voltage exceeds open-circuit voltage VOC owing to an external source, a current Ґ > 0 flows through the diodes in the solar cell affected, thus causing the diode to begin operating in the passband.
This phenomenon can also be referred to as reverse current in connection with current direction I under normal solar cell operating conditions.
A 1995 diploma thesis at the PV Lab investigated the quadrant 1 operating characteristics of various monocrystalline solar cells [4.1]. Analogue measurements of a Wurth CIS module were also realized in 2006, at which time the solar cells, which were in a dark state, were subjected to successively higher passband currents I’ for 15 minutes at a time (see Figure 3.13). The characteristic curves of these solar cells were then transferred to a solar simulator, where changes in the characteristic curves were investigated. The filling factor FF was particularly sensitive to solar cell damage. In the solar simulator investigations, all cells tolerated an IR — 3 ■ ISc-stc reverse current for 15 min periods without any measurable characteristic curve change (ISC-STC — short-circuit current at STC power output). On being subjected to IR — 3 ■ ISC-STC, at which time 800 to 900 W/m2 of area-specific power loss occurred in the solar cells, the temperature in the solar cells rose to 25 °C above ambient temperature. Some of the cells only began showing characteristic curve changes on being exposed to diode passband current and reverse current ranging from 4.5 ■ ISC-STC to 6 ■ ISC-STC, although other cells exhibited no measurable characteristic curve change despite having been exposed to these currents. In addition, some modules were exposed to 3 ■ ISC-sTC for up to 30 min without exhibiting any characteristic curve change.
These measurements were realized with the solar cells in the dark at an ambient temperature ranging from 20 to 25 °C. But of course when solar cells at 1 kW/m2 insolation are subjected to such reverse currents, cumulative temperature increases occur. Insolation results in a cell temperature increase of 20-40 °C. The area-specific loss secondary to the reverse current induced further temperature increases With 3 ■ ISC-STC, owing to the larger temperature differences resulting from slightly higher insolation, the temperature increased by around 20 °C more. Hence, normally the temperature of a solar cell subjected to such operating conditions will increase by some 50 °C above ambient temperature. At a maximum allowable cell operating temperature of 90-100 °C (as for the temperatures indicated in some manufacturers’ module datasheets), an ambient temperature of 40-50°C is allowable for this scenario.
In similar Fraunhofer Institut investigations where the modules were heated to more than 150 °C, no damage was observed in modules that were briefly exposed to up to 7 ■ ISC-sTC short-circuit current [4.15]. But in modules that were exposed to such temperatures for longer periods, damage was observed in the embedding material (EVA film) as there are no vendor-specified modules for such temperatures. Moreover, there is no guarantee that internal module connections can reliably conduct such currents over a lengthy period as the modules are not rated for these operating conditions.
Hence it is safe to assume that in the first power quadrant all commercial modules can withstand short-circuit current ISC-STC at STC power output, including as passband current (and as reverse current IR based on the normal current direction for solar cell operation), without being damaged. As the measurements demonstrated, solar cells can also withstand IR — 2 ■ ISC-STC to 3 ■ ISC-STC without any difficulty. That said, the PV system design process would be much simpler if exact figures concerning the maximum allowable module reverse current IR (and possibly at various temperatures as well) were included in module datasheets. For example, IR — 3.67 ■ ISC-STC reverse current is indicated for the Shell Ultra 85-P module. In some cases, IR is indirectly indicated for this module via indication of a maximum value Imax for series-connected fuses, in which case IR ~ 1.1 ■ Imax can be presupposed. Such information is essential for determining how many module strings can be wired to each other in parallel without using string fuses.