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

Abstract

The photocurrent available from a p-i-n solar cell can be increased by the addition of quantum wells ͑QWs͒ 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 efficiency of the device. Theoretical considerations indicate that nonradiative recombination, which is the dominant loss mechanism in Al x Ga 1Ϫx As/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 recombination is greatest at or near the center of the space charge region, this means locating the QWs closer to the doped regions. Spectral response should not be affected so long as the QWs are still located within the field bearing region. Thus, improved photovoltaic performance may be expected through strategic location of the QWs. We report on measurements on a series of Al 0.36 Ga 0.64 As p-i-n photodiodes, three of which contained a single 87 Å 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 response measurements confirm that for good quality samples photocurrent is independent of QW location within the depleted region. Contrary to expectations, 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 background doping in the intrinsic region.

Simulation of the effect of quantum dot location on a quantum dot p-i-n junction solar cell

“…This increase in efficiency is consistent with earlier experimental results. 21 We observe that QD should not be in direct contact with the edge of either the p or n region, in order to avoid the accumulation of the majority carriers that deteriorate the carrier transport at the shortcircuit condition. The variation of efficiency η with position x of the QD are similar for n i = 10 10 and N B = 10 13 .…”

Recent progress on quantum dot solar cells: a review

“…69 The reduction of the V oc was mainly attributed to SRH recombination, which showed the maximum value when the electron and hole density were similar at the place such as the center of the intrinsic region. 66 They have also fabricated three samples experimentally with the three QD locations mentioned above to verify the simulated results. 69 Figure 8(a) summarized the EQE results for the three samples along with the baseline GaAs cell.…”

“…41,[66][67][68] Driscoll et al 69 performed simulations on three positions for InAs QD located in the intrinsic region: (1) near the n-doped base, (2) exact center of i-region, and (3) near the p-doped emitter. 69 From the simulation, they found that J sc was nearly identical for the three positions as all were located in a high electric field region and carrier collection remained efficient for all three conditions.…”

Quantum wells for high-efficiency photovoltaics

“…Nelson et al found that the background doping of the intrinsic region played a key role on the process and that a careful placement of the QWs away from the region where minority carrier densities were similar helped to reduced non-radiative recombination 24 .…”