A. Khairy Aboul-Soud, Moataz M. Soliman, Alaa S. Hafez
Faculty of Engineering, Alexandria University, Egypt.

* Institute of Graduate Studies and Research, Alexandria University, Egypt.

ABSTRACT:

The aim of this paper is to determine the optimum parameters associated with the use of the quantum well for high efficiency Alx Ga1-x As/GaAs solar cells.

Two types of cells are considered, single quantum well and multi quantum wells solar cells. For SQW the optimized parameters are: well location in the intrinsic region, well width and aluminum mole fraction x. It is found that the optimum location of the quantum well should be away from the high recombination zone around the center of the space charge region and closer to the p region. The optimum well width is found to be 200A due to the enhancement of quantum confined stark effect for the thick well. Mole fraction of 0.2 is an optimum value taking into consideration the trade off between the open circuit voltage and the short circuit current.

For the MQW solar cell, the optimized parameters are the barrier to well width ratio and the spacer thickness. It is found that barrier to well width ratio should be less than 0.25 to enhance the resonant tunnelling between adjacent wells in order to maximize carrier collection through electron tunnelling. The optimum spacer thickness is about 300 A to prevent the impurity scattering effect into the space charge region.

1- INTRODUCTION:

Quantum well solar cells were proposed [1], as a method to increase the solar cell efficiency. Several models were introduced to reduce the dark current in QWSC [2-3] in order to increase the efficiency of the solar cell. It is found out that the current depends on quantum well position contrary to spectral response. Other models [4-5] studied the effect of quantum well width and aluminum mole fraction x on the output voltage of the QWSC. The study of barrier to well ratio [6-7] and the spacer thickness also discussed [8]. These models study different parameters for SQW and MQW solar cells but didn’t optimize these parameters in order to achieve maximum photovoltaic conversion efficiency. Figure (1) shows the energy band diagram of the quantum well solar cell under illumination. An intrinsic region is inserted into a conventional p-n solar cell to extend the field-bearing region. The quantum wells (QWs) extend the absorption below the bulk band-gap Eg to threshold Ea. If the field is maintained across the i-region, the carriers produced by the extra photons absorbed in the well escape to the bulk cell and contribute extra current with high quantum efficiency at room temperature.