A technique for spatially resolved optical characterization of CdS/CdTe thin film solar cells has been developed using electroluminescence (EL). In EL, excess minority carriers are injected via forward biasing. Light produced in radiative carrier recombination is collected with a CCD camera. Because EL intensity depends upon radiative vs. non-radiative recombination lifetimes, EL provides insight into material quality.
Spatial resolution is a key benefit of EL as it provides insight into the non-uniformities of polycrystalline CdTe. At high magnification the resolution is diffraction limited, but coarser measurements of up to several millimeters in range may also be made. Non-uniformities in emission have been observed throughout this range.
Further benefits of EL as a characterization technique are as follows: EL probes the region of most interest, namely the CdTe near the main junction. Also, it is observable at room temperature and data acquisition is fast. Finally, EL is observable at very low carrier injection rates, comparable to short circuit current. (Though more structure is often revealed at higher injection rates.) This low injection means that EL can be a non-destructive probe. This fact, along with the aforementioned ease of observation, means that EL could possibly be used for quality control and in situ testing of modules.
Data gathered from CdS/CdTe cells from various institutions deposited using different methods such as close spaced sublimation, vapor transport, and sputtering are presented. In addition to changes in deposition technique, changes in processing parameters were observed to affect EL emission. Furthermore, overall EL emission decreased noticeably with stress at various biases and elevated temperature, with non-uniformity increasing in many cases. Changes in EL become apparent before changes in parameters acquired with standard current-voltage measurements, suggesting that this technique can be used as an early indicator for degrading cells. Finally, some dramatic changes in EL with stress suggest highly non-uniform degradation of the back contact.