Microscopic Theory of Coupled Quantum Well Structures in Photovoltaics
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Microscopic Theory of Coupled Quantum Well Structures in Photovoltaics Urs Aeberhard1, and Rudolf Morf2 1 Condensed Matter Theory, Paul Scherrer Institute, WHGA 127, Villigen PSI West, Villigen, 5232, Switzerland 2 Condensed Matter Theory, Paul Scherrer Institute, WHGA 129, Villigen PSI West, Villigen, 5232, Switzerland
ABSTRACT We use our recently developed microscopic approach to a quantum theory of photovoltaic processes in nanostructures [1] to investigate geometry effects in quantum well photovoltaics, focusing on the role of asymmetry and inter-well coupling in double quantum well (DQW) systems. For that purpose, the I-V and power characteristics for DQW systems with different asymmetry and degree of coupling are calculated numerically in the vicinity of the maximum power point. In order to assess the transport properties of a specific QW structure, we isolate these from the effects of absorption by normalizing to the absorptivity. The results obtained from this procedure reveal the escape regime dominating at room temperature and are in agreement with previous experimental observations. INTRODUCTION For many years now, quantum well solar cells have been investigated as a potential high efficiency concept in photovoltaics [2]. Experimental observations showed that the gain in photocurrent due to the extension of the absorption range by inserting quantum wells in a wide band gap material can overcompensate the associated loss in open circuit voltage as compared to the host cell, resulting in a higher efficiency [3]. Since the insertion of quantum wells affects not only the absorption, but also the transport properties of the structure, the question arises if the photovoltaic device performance cannot be optimized by making use of such design degrees of freedom like geometry and band-offsets, which can be controlled by suitable choice of alloys and alloy-fractions. Several experiments were dedicated to explore the role of specific QW geometry parameters in the determination of photovoltaic properties: photocurrent measurements for a regular superlattice and asymmetrically coupled multi-DQW systems found a strong effect of geometry on the temperature dependence of the efficiency in the photovoltaic regime, favouring the asymmetrical structure [4]; photocurrent enhancement due to resonant tunneling escape was also detected as a geometrical factor influencing the device performance ([5], [6]). These findings, among many others, support the idea of a geometry related efficiency enhancement via design optimization. A theoretical investigation of the impact of QW geometry on the photovoltaic device performance requires a microscopic picture of optical excitation and transport processes in nanoscale semiconductor heterostructures. In this paper we briefly outline a theoretical framework capable of providing such a picture and discuss its qualitative answer to the question under consideration.
MICROSCOPIC THEORY AND NUMERICAL SIMULATION The theoretical approach is based on the non-equilibrium Green's fu
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