Single Carrier Resonant Tunneling Design for Improving Carrier Collection in Quantum Confined Solar Cells
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1102-LL07-10
Single Carrier Resonant Tunneling Design for Improving Carrier Collection in Quantum Confined Solar Cells Andenet Alemu, and Alex Freundlich Photovoltaics and Nanostructures Laboratories, Center for Advanced Materials, University of Houston, 724 Science & Research Building 1, Houston, TX, 77204-5002 Promising nanostructured device concepts with staggering theoretical efficiencies where quantum confined states are embedded in the intrinsic region of conventional p-i-n solar cells have been proposed. However, practical realizations remain inefficient as these devices suffer from an inherent difficulty in the extraction of photo-generated carriers from the confined states. Within the framework of a "single particle in the box" theory, such shortcomings could be addressed by the use of resonant quantum tunneling designs that can expedite carrier escape. Nonetheless, in material systems studied thus far, the implementation of such design becomes elusive as band offsets between the nanostructure and the host material are distributed between the conduction and valence band leading to the confinement of both holes and electrons (i.e. two particle problem). Our studies of such p-i-n Multi-Quantum Well (MQW) solar cells, only differing by their MQW region composition and geometry, have shown a strong dependence of device performance on quantum wells composition and thickness. Leveraging on the special property of dilute nitrides and using a carefully chosen material system and device design we show the possibility of circumventing this problem by separating the optimization of the valence and conduction band and reducing the issue to a single particle problem. Band structure calculations including strain effects, band anti-crossing models and transfer matrix methods are used to theoretically demonstrate optimum conditions for enhanced vertical transport. High electron tunneling escape probability, together with a free movement of quasi-3 D holes, is predicted to result in enhanced PV device performance. Furthermore, the increase in electron effective mass due to the incorporation of N translates in enhanced absorptive properties, ideal for PV application.
INTRODUCTION Photovoltaic devices containing nanostructured components have been touted as offering one of the most attractive and promising routes for photo-conversion efficiency improvement.[1,2] They not only offer new possibilities due to their size-tunable optoelectronic properties [3] but can also enhance the performance of conventional devices[4]. The use of multi-quantum wells in the intrinsic region of p-i-n solar cells to increase the photocurrent output by extending the absorption threshold of their conventional counterpart towards the infrared have been proposed as far back as 1990 [5]. Their possible application for radiation hardness enhancement in space applications [6] and for improvement in current matching conditions in multi-junction devices has also been investigated with promising results [4]. The extension in absorption ranges and the pos
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