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Reporting period: 2015-06-02 to 2017-06-01

Solar energy is the most important energy source on Earth and the natural choice for renewable and environmentally safe energy. In one year the Earth receives 3,850,000 exajoules, orders of magnitude more than the annual global human energy consumption of 550 exajoules in 2010. However, the cost of solar energy is still higher than carbon-based and nuclear energy mainly due to the high cost of crystalline silicon used in commercial solar cells. Therefore, developing higher-efficiency solar cells with lower cost materials is an important requirement for clean and sustainable energy.

The objective of the project was to combine the remarkable optical and electronic properties of two-dimensional (2-D) materials and semiconducting quantum dots (QDs) to obtain higher photovoltaic efficiencies. QDs have excellent light absorption that can be tuned and optimized for sunlight spectrum. In the case of 2-D atomic materials (e.g. graphene, MoS2), they are semi-transparent, with high charge mobility and strong optoelectronic properties. The key idea behind this proposal is combining the advantages of these materials into a hybrid 2-D/QD solar cell to fully exploit the properties of these materials for solar energy photovoltaic harvesting. In this project, we set two specific objectives: 1) Study graphene as a layer to enhance the current collection at the interface between the quantum dots and the metallic electrode, and 2) Develop a novel architecture hybrid solar cells with a Graphene/QuantumDots/Graphene configuration, using QDs as light absorbers and graphene as charge extractor and electric conductor.

Our results concluded that graphene can enhance the current collection since we observed an increase in the short circuit current from 14 mA/cm2 to 18 mA/cm2. The power efficiency increase using graphene is from 4.5% to 5.5%.
We fabricated devices using graphene in between the QD layers and gold contacts to enhance the current collection in the devices under solar illumination. The devices were fabricated using glass slides with conductive ITO layers. Then a ZnO n-type layer was fabricated by spin coating of ZnO nanoparticles. Then, we spin coating the light absorbing layer of lead sulfide (PbS) quantum-dots. Then the critical step is depositing a graphene layer on the PbS QDs before depositing the gold contact. The graphene is deposited by a wet transfer on the QDs, involving an aqueous process that can damage the surface chemistry of the QDs. After graphene transfer, we evaporated the Au contacts. To characterize the cells we use an AM 1.5 solar simulator with a 2mm aperture pin to avoid light collection outside the cell. We observe an increase in short circuit current from 19 mA/cm2 (no graphene layer) to 13 mA/cm2 (with graphene intermediate layer).

We prepared large area devices consisting of PbS QD layers in between two graphene layers. Each graphene layer had contacts allowing to measure the current transport in each individual graphene layer as well as the current across layers through the QDs. Our measurements for top and graphene showed the typical graphene characteristics, with a maximum in resistance corresponding to the Dirac point as the gate voltage is changed. However, the current from across QDs (top to bottom graphene) showed a very similar behavior and no trace of rectification regardless of bottom gate applied.

MIT 2013 Energy and Climate Outlook; Morton, O. Nature 443, p. 19-22 (2006)
We have obtained hybrid graphene/quantum dot devices proving the feasibility to pursue this technology and identify that graphene can enhance the current collection in photovoltaic devices involving quantum dots. For the first time, we also realized Gr/QD/Gr devices, going one step further from current Gr/QD photodetector configuration. Further development and integration of hybrid Graphene and Quantum Dot semiconductors can open new routes for tunable photoresponse detectors in terms of electrical and spectral response. The use of nanoscale materials can lead to improved performance in terms of power consumption and flexible platforms.