Intermediate Band Challenge in InAs/GaAs Quantum Dot Solar Cell at Cryogenic Temperatures
Yangyi Yao & Mario Dagenais
University of Maryland, College Park, MD, United States

The world record for the power conversion efficiency of a single-junction GaAs solar cell has now reached 29.1%, which is closest to the Shockley-Queisser limit of 33.5%. More than 20 years ago, Luque and Marti introduced the concept of intermediate band (IB) as a way to enhance the efficiency of solar cells beyond the Shockley-Queisser limit. Quickly after, it was suggested that III-V quantum dots (QDs) could form the IB and enhance the efficiency of the single-junction solar cell. The intermediate band solar cell based on InAs/GaAs and InGaAs/GaAs quantum dots have been extensively studied, but no one has demonstrated any efficiency enhancement over the optimized single-junction solar cell. Previously we have demonstrated that the IB two-photon absorption (2PA) in InAs/GaAs quantum dot solar cell (QDSC) has nearly no contribution to the photocurrent enhancement at room temperature. In the present work, we are studying the carrier dynamics of InAs/GaAs QDSC at cryogenic temperatures to investigate the proportion of the photocurrent generation, which is due to a background 2PA and to the saturation of the QDs ground state (GS) transition. We have measured the QDs transition energy from room temperature to 8K. We have studied the photocurrent response and extracted the saturation intensity of the QDs GS from room temperature to 170K with a modified on-resonance z-scan system. We observed a substantial decrease of the saturation intensity as the temperature is decreased, but this intensity is still much higher than one-sun intensity and is about 1.8 times higher than the sun intensity at maximum concentration at 170K. The IB 2PA contribution to the photocurrent enhancement is extremely small even at 170K. Our results challenge the concept of intermediate state for enhancing the conversion efficiency of quantum dot solar cells.