Tutu, FKK; (2014) InAs/GaAs Quantum Dot Solar Cells. Doctoral thesis , UCL (University College London).

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Abstract

Self-assembled III-V quantum dots (QDs) have been intensely studied for potential applications in solar cell (SC) devices in order to increase power conversion efficiency. Due to their quantum confinement of carriers, QDs have been proposed as a means of implementing the intermediate band solar cell (IBSC). The IBSC concept is characterised by in an increase in photocurrent and a preservation of output voltage, resulting from an enhanced sensitivity to the solar spectrum. The work reported in this thesis is concerned with the development of InAs QDs in GaAs p-i-n solar cell structures, with the aim of realising of an IBSC. The work involves the design, epitaxial growth by molecular beam epitaxy (MBE), device processing and characterisation of the QDSCs. This thesis first investigates InAs/InGaAs dot-in-a-well (DWELL) solar cell structures grown under different conditions. The use of a high-growth-temperature GaAs spacer layers is demonstrated to significantly enhance the performance of the multilayer DWELL solar cells. Threading dislocations were observed for a 30-layer QD structure with GaAs spacer layers grown at a low temperature (510 oC). By growing the GaAs spacer layer at a higher temperature (580 oC), the formation of threading dislocations were suppressed, resulting in enhanced optical properties. The thesis then goes on to address the main challenges facing QD IBSCs, that is, the reduction in open-circuit voltage and the lack of significant increase in short-circuit current. To eliminate the wetting layer and enhance the open-circuit voltage of the QD solar cell, an AlAs cap layer technique was used. This resulted in an enhancement of the open-circuit voltage of a 20-layer InAs/GaAs QDSC from 0.69 V to 0.79 V. Despite a slight reduction in short-circuit current, for the QDSC with AlAs cap layer, the enhancement in the open-circuit voltage was enough to ensure that its efficiency is higher than the QDSC without AlAs cap layers. In an attempt to enhance the short-circuit current, an antimony-mediated growth approach was used to grow high-density QDs. After optimisation of the growth temperature and InAs coverage, a very high in-plane QD density of 1  1011 cm-2 was achieved by applying a few monolayers of antimony prior to QD growth. Compared with a reference QDSC without the incorporation of antimony, the high-density QDSC demonstrates a distinct improvement in short-circuit current from 7.4 mA/cm2 to 8.3 mA/cm2. This result shows that a significant increase in short-circuit current could potentially compensate for the drop in open-circuit voltage observed in InAs/GaAs QD solar cells. Ongoing work on the development of QDSCs with both AlAs capping and antimony-mediated growth have resulted in the simultaneous elimination of the wetting layer and increase in QD absorption in a single device. Overall, the studies in this thesis present important implications for the design and growth of InAs/GaAs QD solar cell structures for the implementation of IBSCs.

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