Incompatible length scales in nanostructured Cu2O solar cells
2012, Musselman, Kevin P., Marin, Andrew, Schmidt-Mende, Lukas, MacManus-Driscoll, Judith L.
Electrodeposited Cu2O-ZnO heterojunctions are promising low-cost solar cells. While nanostructured architectures improve charge collection in these devices, low open-circuit voltages result. Bilayer and nanowire Cu2O-ZnO heterojunction architectures are systematically studied as a function of the Cu2O layer thickness, ZnO nanowire length, and nanowire seed layer. It is shown that a thick depletion layer exists in the Cu2O layer of bilayer devices, owing to the low carrier density of electrodeposited Cu2O, such that the predominant charge transport mechanisms in the Cu2O and ZnO are drift and diffusion, respectively. This suggests that the low open-circuit voltage of the nanowire cells is due to an incompatibility between the nanostructure spacing required for good charge collection (<1 μm) and the heterojunction thickness necessary to form the full built-in potential that inhibits recombination (>2 μm). The work shows the way to improve low-cost Cu2O cells: increasing the carrier concentration or mobility in Cu2O synthesized at low temperatures.
Macroscopically uniform electrodeposited ZnO films on conducting glass by surface tension modification and consequent demonstration of significantly improved p–n heterojunctions
2011, Musselman, Kevin P., Gershon, Talia, Schmidt-Mende, Lukas, MacManus-Driscoll, Judith L.
An alternative water–ethanol zinc nitrate solution is demonstrated to completely eliminate macroscopic defects that are normally prevalent in ZnO films electrochemically deposited from aqueous zinc nitrate solutions. The inclusion of 25% ethanol (by volume) reduces the surface tension of the mixture and eliminates bubble formation on the conducting glass surface during deposition. To demonstrate the importance of film uniformity, the ZnO films are employed in ZnO–Cu2O n–p heterojunctions and an order of magnitude improvement in diode behaviour is observed.
A novel buffering technique for aqueous processing of zinc oxide nanostructures and interfaces, and corresponding improvement of electrodeposited ZnO-Cu2O photovoltaics
2011, Musselmann, Kevin, Marin, Andrew, Wisnet, Andreas, Scheu, Christina, MacManus-Driscoll, Judith, Schmidt-Mende, Lukas
A novel buffering method is presented to improve the stability of zinc oxide processed in aqueous solutions. By buffering the aqueous solution with a suitable quantity of sacrifi cial zinc species, the dissolution of functional zinc oxide structures and the formation of unwanted impurities can be prevented. The method is demonstrated for ZnO fi lms and nanowires processed in aqueous solutions used for the selective etching of mesoporous anodic alumina templates and the electrochemical deposition of Cu 2 O. In both cases, improved ZnO stability is observed with the buffering method. ZnO-Cu 2 O heterojunction solar cells (bilayer and nanowire cells) synthesized using both traditional and buffered deposition methods are characterized by impedance spectroscopy and solar simulation measurements. Buffering the Cu 2 O deposition solution is found to reduce unwanted recombination at the heterojunction and improve the photovoltaic performance.