Evidence of Sequential Carrier Escape in III-V p-i-n Multi-Quantum Well Solar Cells
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1031-H13-03
Evidence of Sequential Carrier Escape in III-V p-i-n Multi-Quantum Well Solar Cells Andenet Alemu, Jose A. H. Coaquira, and Alex Freundlich Photovoltaics and Nanostructures Laboratories, Center for Advanced Materials, University of Houston, 724 Science & Research Building 1, Houston, TX, 77204-5002 ABSTRACT Several InAsP/InP p-i-n Multi-Quantum Well (MQW) solar cells, only differing by their MQW region composition and geometry, were investigated. For each sample, the Arrhenius plot of the temperature related variation of the photoluminescence intensity was used to deduce the radiative recombination activation energy. The electron and holes confinement energy levels in the quantum wells and the associated effective potential barriers seen by each carrier were theoretically calculated. Carrier escape times were also estimated for each carrier. The fastest escaping carrier is found to display an effective potential energy barrier equal to the experimentally determined photoluminescence activation energy. This not only shows that the temperature related radiative recombination extinction process is driven by the carrier escape mechanism but also that the carriers escape process is sequential. Moreover, a discrepancy in device performance is directly correlated to the nature of the fastest escaping carrier. INTRODUCTION Recent advances in nanotechnology have invigorated efforts in overcoming conversion efficiency limitations of conventional single junction solar cells [1,2]. In this race for higher efficiencies researchers have proposed various promising device designs incorporating nanoscale components. Nevertheless, to this date, none of the proposed methods have shown any substantial efficiency improvement as theoretically predicted. In fact, most devices display performances far below their conventional counterparts. Multiquantum well solar cells have been proposed as far back as the early nineties [3]. Much discussion has followed on their possible use for solar cell efficiency improvement. By extending the absorption threshold of the device towards the infrared, quantum wells come in handy in increasing the cells photocurrent output. This feature has been shown as being extremely attractive to facilitate current matching in multijunction solar cells and hence to improve the efficiency and radiation tolerance of these devices [4, 5]. Despite obvious advantages for multijunction applications the inclusion of quantum wells is inevitably associated with some amount of open circuit voltage (Voc) degradation. In general, confined states in nanostructured solar cells could act as recombination centers when carriers are not rapidly extracted. Thus, an efficient escape and collection of photogenerated carriers from the well potentials is of critical importance and a necessary path toward higher efficiency devices.
In p-i-n type solar cells, such a condition is satisfied when a large built-in potential exists across the nanostructure containing iregion of the cell [6]. This imposes an upper limit on the thickn
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