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.