Effect of quantum well location on single quantum well p-i-n photodiode dark currents

The photocurrent available from a p-i-n solar cell can be increased by the addition of quantum wells (QWs) to the undoped region. At the same time the QWs reduce the open-circuit voltage by introducing areas of lower band gap where recombination is enhanced. This increase in recombination should be as small as possible for the most favorable effect on the photovoltaic effi ciency of the device. Theoretical considerations indicate that nonradiative recombination, which is the dominant loss mechanism in AlxGa1-xAs/GaAs QW structures, may be reduced by positioning the QWs away from the point where the electron-hole product is a maximum. For p-i-n diodes, where recombinat ion is greatest at or near the center of the space charge region, this mean s locating the QWs closer to the doped regions. Spectral response should no t be affected so long as the QWs are still located within the field bearing region. Thus, improved photovoltaic performance may be expected through st rategic location of the QWs. We report on measurements on a series of Al0.3 6Ga0.64As p-i-n photodiodes, three of which contained a single 87 Angstrom GaAs QW within the i region, and one which was a control sample with no QW. The three QW samples were grown with the QW located nearer to the p-doped layer, centrally, and nearer to the n-doped layer, respectively. Spectral r esponse measurements confirm that for good quality samples photocurrent is independent of QW location within the depleted region. Contrary to expectat ions, the dark current is highest for the sample with the QW located closer to the n region. We analyze these results in terms of structure and doping profile, and compare them with the predictions of a self-consistent model. The observed behavior is attributed to a relatively high unintentional bac kground doping in the intrinsic region. (C) 1999 American Institute of Phys ics. [S0021-8979(99)07822-6].