Invited Paper TF+NC-MoM10
Structured Photoanodes for Enhanced Electron Transport in Dye Sensitized Solar Cells (DSSC) with Atomic Layer Deposition (ALD)Functionalization

Monday, October 20, 2008, 11:20 am, Room 302

Inexpensive, efficient solar energy conversion requires synthetic methodology capable of creating structures that resolve the conflict between the long lengths required for efficient photon absorption with the short photoelectron diffusion lengths found for all but the most perfect crystalline materials. DSSC’s photoanodes utilize titania nanocrystals sensitized with monolayers of dye to achieve efficient photon absorption. However, DSSC conversion efficiencies have been limited to ~10% by a charge collection time of milliseconds at the maximum power point. Thus efficient charge extraction is only possible with an iodide/triiodide (I-/I₃^-) redox shuttle with both the high driving force needed to regenerate dyes at acceptable rates (and the consequent >500 mV energy cost/photon)and a remarkably inefficient I₃^- electron interception rate. We have been addressing this problem using the ability of ALD to conformally grow nm thick, pin-hole free layers on nanoporous, high surface area supports. The charge collection times can be improved either by reducing the dimensionality of the photoanode or by reducing the electron collection distances. We address the former with precision films grown on nanoporous substrates such as anodic aluminum oxide (AAO) or silica aerogel supports. The AAO, for instance, has a regular array of 200 nm straight channels that traverse its 60 @smicron@ thick membrane. ALD is used to conformally coat with either ZnO or TiO₂ (achieving peak efficiencies at ~ 4-5 nm thickness) the membrane channels. One side of the membrane is coated with a thick transparent conducting oxide such as ITO. The pore walls are then sensitized with a dye and the resulting photoanode is assembled into a photocell. The linear tubes thus created form a one-dimensional network of 60 m long tubes with excellent charge collection times. Dramatically reduced electron collection distances can be tested with a more complex structure grown by first coating the AAO substrate with a transparent conducting oxide such as ITO, then adding a wide band gap oxide (TiO₂), before completing the cell with dye attachment and assembly. In these cells, electron diffusion occurs radially through the thin (~5 nm) TiO₂ layer into the TCO, rather than along the pore. Again the electron diffusion times are dramatically reduced. The wide palate, precision composition, and conformal nature of ALD synthesized films enable the exploration of these complex structures improving our understanding of the factors limiting solar energy conversion. *The work at Northwestern is supported by the U.S. Department of Energy, Basic Energy Sciences Program, under Grant DE-FG02-87ER13808. Work at Argonne is supported by the U.S. Department of Energy, BES-Materials Sciences under Contract W-31-109-ENG-38.