The fabrication and characterization of double-gyroid and thin film photovoltaics

Robert F McCarthy, Purdue University

Abstract

Renewable energy can offer cleaner sources of electricity, and photovoltaics have perhaps the greatest potential due to the large quantity of solar power that hits the earth's surface. However, the cost of electricity from photovoltaics is still too high. There are two paths for reducing this cost, the first being cheaper manufacturing. Our research group explores this option by looking at Cu2ZnSn(S,Se)4 (CZTSSe) thin film devices deposited by nanocrystal inks, which use earth abundant materials, and can be made using cheap deposition processes. The second path is increasing solar cell efficiency. We've explored this route using double-gyroid (DG) nanostructure devices that should be capable of multiple exciton generation (MEG), a kind of photophysics that allows for increased photocurrent and device efficiency. Electrical characterization of our thin film devices shows a back contact barrier of 75 meV exists in our CIGSe device. CZTSSe devices have a defect level at 57 meV above its valence band, most likely due to a copper on zinc antisite. This defect acts as the main acceptor state. The device's series resistance increases drastically with decreasing temperature. Interestingly, this correlates with increasing photoluminescence at an energy of 1.5 eV, correlating with a trap state in the CdS layer of the device. These results suggest that the CdS layer could be the source of increasing series resistance with temperature. With DG research, electrodeposited lead selenide was improved using new applied potential techniques and additives in solution, leading to less roughness and greater surface coverage. Bulk PbSe devices showed very nice diode behavior. CdSe and CdTe films were also electrodeposited, and films were improved by optimizing post-deposition techniques to make bulk CdTe devices with efficiencies over 4%. With DG films, an increased band gap was measured in DG PbS (0.11 eV increase) and CdSe films (0.05-0.14 eV increase), indicating quantum confinement. DG PbSe devices were electrodeposited by a number of techniques, but so far have shown only diode behavior and no photocurrent. Concerned that recombination was limiting our devices, a new, fully analytic model was constructed that allows us to consider the effect of a wide range of parameters on nanostructured devices. A voltage-dependent photocurrent is calculated that accounts for charge transport by both drift and diffusion. The dark current accounts for bulk and interfacial Shockley-Read-Hall recombination. Our model suggests that DG devices will require low interfacial recombination velocities (<104 cm/s). We also performed calculations using the detailed balance limit including a non-radiative recombination term. These calculations show that there is a significant shift in the ideal band gap for photovoltaics utilizing MEG. Even with a microsecond minority carrier lifetime, it shifts to 0.93 eV from 0.70 eV.

Degree

Ph.D.

Advisors

Agrawal, Purdue University.

Subject Area

Chemical engineering|Nanoscience|Materials science

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