Silicon quantum dot based solar cells: addressing the issues of doping, voltage and current transport

• Published in: Progress in photovoltaics 2011-11, Vol.19 (7), p.813-824
• Main Authors: Conibeer, Gavin, Green, Martin A., König, Dirk, Perez-Wurfl, Ivan, Huang, Shujuan, Hao, Xiaojing, Di, Dawei, Shi, Lei, Shrestha, Santosh, Puthen-Veetil, Binesh, So, Yong, Zhang, Bo, Wan, Zhenyu
• Format: Article
• Language: English
• Subjects: Applied sciences; band gap engineering; Crystallization; Doping; Energy; Exact sciences and technology; modulation doping; Natural energy; nucleation; Oxides; Photovoltaic cells; Photovoltaic conversion; Quantum confinement; Quantum dots; Silicon; Solar cells; Solar cells. Photoelectrochemical cells; Solar energy; tandem; Volatile organic compounds
• ISSN: 1062-7995; 1099-159X; 1099-159X
• DOI: 10.1002/pip.1045

Silicon quantum dot (Si QD) solar cells offer the potential to tune the effective band gap through quantum confinement and hence allow fabrication of optimised tandem devices in one growth run in a thin film process. Previous work in our group has shown how such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. Doping multilayers with P and B allows formation of a rectifying junction with an effective band gap of 1.8 eV, which can give an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. In addition P and B have big but opposite effects on QD crystallisation, with P(B) forming larger (smaller) QDs than for undoped material. Two possible models for the doping mechanisms in these materials are explored: one relying on doping of a sub‐oxide region around the Si QDs and the other based on the differing nucleation effects of P and B. In addition initial results on hetero‐interfaces in the QD superstructure are assessed as a means to improve vertical current transport, and the effects of hydrogenation on improving VOC by passivating defects. Routes to incorporating these explanations for doping and other structural improvements are discussed as means of optimising the performance of these Si QD cells. Copyright © 2010 John Wiley & Sons, Ltd. Quantum confinement in Silicon quantum dots can be used to engineer optimal effective band gaps for tandem cells which can be grown using thin film vapour phase processes such as sputtering or PECVD. However, very large resistances due to low tunnelling probabilities between quantum dots and an incomplete understanding of doping in these materials need to be addressed to improve device performance. Progress on reducing series resistance using nitride interlayers and a new model for ‘modified modulation’ doping are presented.