Quantum well and quantum dot research at the U.S. Army Research Laboratory (ARL)

Abstract

This paper reviews the work of several teams at ARL. In the first section a summary of the work done by the photovoltaic devices team is presented, the team is using quantum dots to enhance the efficiency of solar photovoltaic devices. We have discovered that doping the quantum dots is critical in enhancing the efficiency of the solar cells. In the quantum well arena we are developing type II SLS detector material for high performance focal plane array applications, so far we have observed that the minority carrier lifetimes have been short. This presents a major barrier towards the realization of high performance focal plane arrays. This paper discusses some of the details of type II SLS material studies as they pertain to minority carrier lifetime studies. Quantum Dots for Solar Photovoltaic DevicesOne of the central ideas for creating next generation IR imaging systems and solar cell photovoltaic devices is to increase the photoresponse to IR radiation [1]. To enhance the IR photoreponse, it is necessary to: (i) improve electron coupling to IR radiation and (ii) increase the photocarrier lifetimes, i.e. to suppress recombination losses. However, it is extremely challenging to increase the IR absorption without enhancement of recombination losses, because by introducing electron levels that provide strong IR transitions, we inevitably create additional channels for inverse processes that increase recombination losses.This trade-off between IR absorption and recombination processes are well understood for a number of technologies and corresponding materials. For example, in the early sixties, semiconductors with impurities which provide electron levels inside the semiconductor bandgap, and induce the IR transitions from localized impurity states to conducting states received significant attention. However, midgap impurities drastically enhance the recombination processes, i.e. Shockley-Read-Hall recombination, and deteriorate the photovoltaic conversion efficiency [2,3]. To accommodate the solar spectrum and utilize its IR portion, modern photovoltaic technology mainly employs multi-junction cells with different electron bandgaps [4]. According to the theoretical predictions, a multi-junction solar cell with five or more junctions has the ability to achieve a photovoltaic efficiency that may exceed 70%. However, current technology enables only triple-junction cells with the maximum conversion efficiency of ~ 40% for concentrator cells. Strong technological limitations are caused the high density of interfacial defects that result from the mismatch in lattice constants, strain that result from differences in the coefficient of thermal expansion and the inability to match the photocurrent from individual cells.Recently, quantum-dot (QD) structures have attracted much attention due to their ability to enhance absorption of IR radiation via multiple energy levels introduced by QDs [5-7]. In QDs, the carriers are confined in all three dimensions. Electron states in separate dots can be connected via manageable tunnel...