2005 很好Potentially modulated multi-quantum wells for high-efficiency solar cell applications 1-s2.0-S0927024804002065-main.pdfVIP

ARTICLE IN PRESS Solar Energy Materials Solar Cells 85 (2005) 143–152 Potentially modulated multi-quantum wells for high-efficiency solar cell applications Yoshitaka Okada*, Naoyuki Shiotsuka, Toru Takeda Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan Received 6 November 2003 Available online 15 June 2004 Abstract The expected high performance of multi-quantum well (MQW) solar cells depends not only on the material quality ensuring minimum non-radiative recombination losses at the heterointerfaces, but also on the escape rate of photogenerated carriers out of QWs into the ‘‘base’’ region having minimum radiative losses within QWs. For this, we propose and investigate potentially modulated MQW solar cell structures, for which the MQW structure is modified from a series of conventional square-shaped QWs to ‘‘step-like’’ potentially modulated QWs. We have studied ‘‘two-step’’ and ‘‘three-step’’ MQWs introduced into the intrinsic region of GaAs p–i–n solar cells. Compared with a square-shaped MQW cell, a three- step modulated MQW cell having the same average potential well depth showed photoluminescence characteristics with a smaller thermal activation energy and higher average carrier temperature. The dark current characteristics also indicated that photo- generated carriers escape out of QWs more efficiently in this type of structure. r 2004 Elsevier B.V. All rights reserved. Keywords: Quantum well solar cells; GaAs/InGaAs heterostructures; Molecular beam epitaxy; Photo- luminescence 1. Introduction Recently, the multi-quantum well (MQW)-based III–V solar cells have gained an increasing