Temperature dependent behaviour is described below, but first it is useful to note that the current – voltage (IV) characteristics of p-i-n solar cells (Lancaster processed) with 5 or 10 layers of Quantum rings and the GaAs control device were reported at Lancaster under 1 sun illumination using a 150 W Oriel solar simulator [74]. The area of the devices is 7.54x10-3 cm-2 with ring like mask design. A Keithley 2400 source meter was used to measure the IV curves of the samples. In this experiment voltage was used as the source and the current was measured. The solar cells containing 5 and 10 QR layers exhibit a lower open circuit voltage of Voc ~0.6V compared with Voc ~0.95V
in the GaAs control cell without Quantum rings (Refer Fig 3.7 b.).The increase in photocurrent for 10 layer QRs is accompanied by a small decrease in Voc compared to 5
layer QRs but with an increase in Jsc. The increase in the short circuit current is due to
the absorption of extended wavelength photons by quantum rings.
Figure 5.8: Schematic representation of band diagram of GaSb/GaAs structure showing drifting of holes
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The photo-generated minority holes from the base region undergo drift across the depletion region and are captured by the QRs (thereby reducing the short-circuit current. These trapped holes then act as recombination centres, decreasing the open-circuit voltage. Figure 5.8 shows the schematic representation of the holes drift from the base region when the quantum rings are placed in the depletion region.
5.2.1. Temperature dependent current-voltage characteristics
using AM1.5
Temperature dependent I-V measurements were done by placing the device (Sheffield processed) inside a variable temperature cryostat capable of cooling to 80 K. (The area of the device is 0.096 cm2 with spider web like mask design.) Solar cells are sensitive to temperature. Increases in temperature reduce the band gap of a semiconductor, thereby affecting the solar cell parameters viz., short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), efficiency (η) and hence the performance of solar cells
[123]. As shown in figure Fig 5.9. ISC increases slightly, while VOC decreases more
significantly.
Fig 5.9: Current density- voltage curves for the GaAs control and GaSb/GaAs QR solar cells obtained
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A small variation of short circuit current density (JSC) with temperature is primarily
due to the change in bandgap energy with temperature. As the cell heats up, the bandgap decreases, and hence the cell responds to longer wavelength portions of the spectrum, and therefore the short circuit current actually increases with temperature. Hence, the JSC variation term is roughly proportional to the incident spectral intensity at
wavelengths near the band edge [124]. Figure 5.10 shows a summary of the temperature dependence of the representative solar cell characteristics under 1 sun concentration, that is, short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF) and
efficiency. Both the QR sample and the control sample show a constant increase of JSC
with increasing temperature, resulting from the increased photo-absorption associated with the GaAs bandgap narrowing.
Figure 5.10: Temperature dependence of the representative solar cell characteristics for the GaAs control
and GaSb quantum ring (QR) cells under a solar concentration equivalent to 1 sun; (a) JSC, b)VOC, c)FF
and d) efficiency (-error bars lie within the thickness of the points).
At low temperature (100 K), JSC for the GaSb QR cell is much lower than that of the
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GaAs control cell above 230 K. At the same time, VOC for the GaSb QR cell shows a
slight change of slope, and the FF starts to decrease rapidly above 180 K, resulting in a net reduction of the efficiency.
5.2.2.
Temperature dependent dark current-voltage characteristics
Figure 5.11 shows the temperature dependent dark J-V characteristics measured from the control and QR SCs. Both the devices do not display a steady increase in dark current with temperature. The change in the shape of the dark J-V curves with temperature depends on the concentration of different types of defects present in the sample with different temperature dependent carrier capture cross sections and tunnelling effects [125] [126].Figure 5.11: Temperature dependent dark J-V characteristics measured from the (a) GaAs control and
(b) GaSb QR solar cells.
In the QR device, the presence of QRs in the depletion region introduces additional recombination paths via QR states to contribute to dark current, the amount of which largely depends on the carrier capture and recombination processes under different biases and temperatures. The temperature dependence of the ideality factors are presented in figure 5.12.
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Figure 5.12: Temperature dependence of the ideality factor for (a) GaAs control and (b) GaSb QR solar
cells.
At low temperatures the ideality factor is close to unity, indicating that diffusion current dominates, with the increase to 1.8 at room temperature and an increase in non-radiative recombination via trap levels in the depletion region.At low temperatures, the ideality factor of the QR SC is slightly higher than that of the control sample, but increases more slowly with temperature, reaching a similar value at room temperature. This behaviour has been observed previously in InGaAs/GaAs QD SCs [125] and was attributed to an increase in recombination of injected minority carriers with majority carriers trapped in the QDs. For the GaSb QR SCs we can assume that under forward bias, hole injection results in state filling within the QR layers close to the highly doped p-type emitter. Under these conditions, the QRs act as efficient recombination centres, capturing holes that recombine with injected minority electrons. (Also there is recombination via midgap traps in the depletion region which is characterised by an ideality factor of 2). Therefore in the QR SC, the combination of these different recombination paths gives rise to an ideality factor of 1.7.
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