Insertion of InGaAs/GaAsP strain-balanced multiple quantum wells (MQWs) into i-regions of GaAs p-i-n solar cells show several advantages against GaAs bulk p-i-n solar cells. Particularly under high-concentration sunlight condition, enhancement of the open-circuit voltage with increasing concentration ratio in thin-barrier MQW cells has been reported to be more apparent than that in GaAs bulk cells. However, investigation of the MQW cell mechanisms in terms of <i>I-V</i> characteristics under high-concentration sunlight suffers from the increase in cell temperature and series resistance. In order to investigate the mechanism of the steep enhancement of open-circuit voltage in MQW cells under high-concentration sunlight without affected by temperature, the quasi-Fermi level splitting was evaluated by analyzing electroluminescence (EL) from a cell. Since a cell under current injection with a density <i>J<sub>inj</sub></i>has similar excess carrier density to a cell under concentrated sunlight with an equivalent short-circuit current <i>J<sub>sc</sub></i> = <i>J<sub>inj</sub></i>, EL measurement with varied <i>J<sub>inj</sub></i> can approximately evaluate a cell performance under a variety of concentration ratio. In addition to the evaluation of quasi-Fermi level splitting, the external luminescence efficiency was also investigated with the EL measurement. The MQW cells showed higher external luminescence efficiency than the GaAs reference cells especially under high-concentration condition. The results suggest that since the MQW region can trap and confine carriers, the localized excess carriers inside the cells make radiative recombination more dominant.
Multiple quantum well (MQW) solar cells have been explored as one promising next-generation solar cells toward high conversion efficiency. However, the dynamics of photogenerated carriers in MQWs are complicated, making it difficult to predict the device performance. Our purpose of this study is to investigate a model for the photocurrent component characteristics of MQW cells based on experimental findings. Using our proposed carrier time-of-flight technique, we have found that the carrier averaged drift velocity has linear dependence on the internal field regardless of complicated carrier cascade dynamics in MQW. This behavior is similar to carriers in bulk materials, allowing us to approximate the MQW region as a quasi-bulk material with specific effective drift mobility. With the effective drift mobility and equivalent material parameters such as effective density of states, the quasi-bulk approach reduces the device complexity, and the characteristics of such MQW cells can be simulated using the conventional drift-diffusion model. We have confirmed this model with experimentally obtained photocurrent characteristics. The simulation of carrier collection efficiency (CCE)—normalized photocurrent—based on the effective mobility approximation, or quasibulk approximation, agrees well with the experimental results when the carrier lifetime is set to be in the order of hundred nanoseconds. This simplified model enhances our understanding of the MQW cell operation and helps design the optimal structure for better performance.