2 results
Suppression of Edge Recombination in InAs/InGaAs DWELL Solar Cells
- Tingyi Gu, Mohamed A El-Emawy, Kai Yang, Andreas Atintz, Luke F Lester
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- Journal:
- MRS Online Proceedings Library Archive / Volume 1210 / 2009
- Published online by Cambridge University Press:
- 31 January 2011, 1210-Q02-07
- Print publication:
- 2009
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- Article
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The InAs/InGaAs DWELL solar cell grown by MBE is a standard pin diode structure with six layers of InAs QDs embedded in InGaAs quantum wells placed within a 200-nm intrinsic GaAs region. The GaAs control wafer consists of the same pin configuration but without the DWELL structure. The typical DWELL solar cell exhibits higher short current density while maintaining nearly the same open-circuit voltage for different scales, and the advantage of higher short current density is more obvious in the smaller cells. In contrast, the smaller size cells, which have a higher perimeter to area ratio, make edge recombination current dominant in the GaAs control cells, and thus their open circuit voltage and efficiency severely degrade. The open-circuit voltage and efficiency under AM1.5G of the GaAs control cell decrease from 0.914V and 8.85% to 0.834V and 7.41%, respectively, as the size shrinks from 5*5mm2 to 2*2mm2, compared to the increase from 0.665V and 7.04% to 0.675V and 8.17%, respectively, in the DWELL solar cells. The lower open-circuit voltage in the smaller GaAs control cells is caused by strong Shockley-Read-Hall (SRH) recombination on the perimeter, which leads to a shoulder in the semi-logarithmic dark IV curve. However, despite the fact that the DWELL and GaAs control cells were processed simultaneously, the shoulders on the dark IV curve disappear in all the DWELL cells over the whole processed wafer. As has been discussed in previous research on transport in QDs, it is believed that the DWELL cells inhibit lateral diffusion current and thus edge recombination by collection first in the InGaAs quantum well and then trapping in the embedded InAs dots. This conclusion is further supported by the almost constant current densities of the different area DWELL devices as a function of voltage.
GaAs Based InAs/InGaAs Quantum Dots-in-a-Well Solar Cells and Their Concentration Applications
- Kai Yang, Mohamed A El-Emawy, Tingyi Gu, Andreas Stintz, Luke F Lester
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- Journal:
- MRS Online Proceedings Library Archive / Volume 1211 / 2009
- Published online by Cambridge University Press:
- 31 January 2011, 1211-R03-02
- Print publication:
- 2009
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- Article
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Quantum dot (QD) solar cells have been actively investigated recently since they have been theoretically shown to have the potential to realize high conversion efficiencies. However, very little research has analyzed the effect the dots have on the transport or recombination effects in the device. In this paper, we report the I-V and spectral response characteristics of InAs/InGaAs “dots-in-a-well” (DWELL) solar cells and compared them with GaAs control cells. The QD cells show higher short circuit density (Jsc) and better long-wavelength efficiency compared to the control cell. By comparing the dark current behavior of the QD cells to the GaAs control cells, we have conservatively estimated the concentration level at which the QD solar cells would surpass GaAs control devices.
The quantum dot solar cells are grown by molecular beam epitaxy using the DWELL technique and a standard pin structure. The control cell structure is similar to the QD one except that there are no InAs dots or surrounding InGaAs quantum wells. The light I-V characteristics were measured under AM1.5G at 100 mW/cm2 illumination. The control cell has a Voc of 0.89V and a Jsc of 9.1 mA/cm2. The InAs QD solar cell has a Voc of 0.68 V and a Jsc of 12.2 mA/cm2. The QD cell has about a 33% larger short circuit current density compared to the GaAs control cell, which is mainly due to the higher photon absorption rate related to the DWELL structure. The spectrum response data show that the GaAs control cell and the QD cell have similar external quantum efficiency (EQE) in the visible to near-IR range (400-870nm). Beyond the GaAs absorption edge (870nm), the QD solar cell shows extended response with much higher measured EQE up to ˜1200 nm. This is strong evidence of the contribution from the InAs QDs and InGaAs QWs, the latter being the primary contributor to the increased Jsc.
We calculated the “local” ideality factor from measured dark IV data, and then substituted it into a single diode equation to get the “local” reverse saturation current. Whereas the GaAs control shows the typical monotonically decreasing ideality from 0.3 to 0.8V, a linearly increasing ideality is observed in the QD cell. Based on the measured dark currents, and neglecting series resistance, we extrapolated the IV curves to higher voltages and found that they intercept at ˜2×104 mA/cm2. Dividing the intercept point Jdark by the Jsc of the QD cell conservatively estimates the light concentration (˜1400×) above which the QD cell would have a higher Voc than the GaAs cell assuming additivity applies. This result is mainly attributed to the unique carrier transport properties that are introduced into the solar cell devices that utilize QDs.