The Fabrication and Characterization of Double-Gyroid Photovoltaics
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. Our research group hopes to decrease this cost by increasing solar cell efficiency 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. To create DG semiconductors, a DG-silica template was filled by electrodepositing metal chalcogenide films, in particular lead selenide. Using new applied potential techniques and additives in solution, decreased roughness and greater nucleation density was observed for PbSe. Bulk PbSe devices showed very nice diode behavior.
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 calculated by our transit time model 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). Additional calculations using the detailed balance limit but including a non-radiative recombination term 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, suggesting that lead chalcogenides are not ideal for DG devices.