Abstract
The efficiency of a solar cell can be substantially increased by opening new energy gaps within the semiconductor band gap. This creates additional optical absorption pathways which can be fully exploited under concentrated sunlight. Here we report a new approach to opening a sizable energy gap in a single junction GaAs solar cell using an array of InAs quantum dots that leads directly to high device open circuit voltage. High resolution imaging of individual quantum dots provides experimentally obtained dimensions to a quantum mechanical model which can be used to design an optimized quantum dot array. This is then implemented by precisely engineering the shape and size of the quantum dots resulting in a total area (active area) efficiency of 18.3% (19.7%) at 5 suns concentration. The work demonstrates that only the inclusion of an appropriately designed quantum dot array in a solar cell has the potential to result in ultrahigh efficiency under concentration.
Highlights
The cost of electricity produced from a photovoltaic system depends directly on the efficiency of the solar cells
The most studied experimental IBSC prototype is based on incorporating an array of self-assembled InAs semiconductor quantum dots within the intrinsic region of a GaAs pin diode.[6−12] This gives rise to the quantum dot intermediate band solar cell (QD-IBSC)
EIB,conduction band (CB) is less than ideal (∼5−120 meV) for this material system, a realistically achievable solar energy conversion of 34% under concentration is predicted[13] and it is notable that the first demonstration of a photovoltaic concentrator module using QD-IBSCs was recently reported.[14]
Summary
This behavior is attributed to VB to IB optical transitions.[21] The important point to note here, is the difference between the EQE data for the quantum engineered QD-IBSC compared with the reference QD-IBSC While both solar cells exhibit a InAs wetting layer characteristic around 1.35 eV, only the quantum engineered QD-IBSC presents a shoulder at approximately 1.19 eV. Both the GaAs control and the quantum engineered QD-IBSC included a window layer and this is clearly evident as a significant increase in Jsc, which is 18.3, 18.9,. Calculations based on experimentally obtained quantum dot dimensions show that this opens up a clear second energy gap in the device which leads directly to high Voc. The results demonstrate that nanometre variations in the material used to create the IB have a profound effect on the macroscopic device performance.
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