The importance of dye chemistry and TiCl4 surface treatment in the behavior of Al2O3 recombination barrier layers deposited by atomic layer deposition in solid-state dye-sensitized solar cells

Abstract
Atomic layer deposition (ALD) was used to fabricate Al 2O 3 recombination barriers in solid-state dye-sensitized solar cells (ss-DSSCs) employing an organic hole transport material (HTM) for the first time. Al 2O 3 recombination barriers of varying thickness were incorporated into efficient ss-DSSCs utilizing the Z907 dye adsorbed onto a 2 μm-thick nanoporous TiO 2 active layer and the HTM spiro-OMeTAD. The impact of Al 2O 3 barriers was also studied in devices employing different dyes, with increased active layer thicknesses, and with substrates that did not undergo the TiCl 4 surface treatment. In all instances, electron lifetimes (as determined by transient photovoltage measurements) increased and dark current was suppressed after Al 2O 3 deposition. However, only when the TiCl 4 treatment was eliminated did device efficiency increase; in all other instances efficiency decreased due to a drop in short-circuit current. These results are attributed in the former case to the similar effects of Al 2O 3 ALD and the TiCl 4 surface treatment whereas the insulating properties of Al 2O 3 hinder charge injection and lead to current loss in TiCl 4-treated devices. The impact of Al 2O 3 barrier layers was unaffected by doubling the active layer thickness or using an alternative ruthenium dye, but a metal-free donor-π-acceptor dye exhibited a much smaller decrease in current due to its higher excited state energy. We develop a model employing prior research on Al 2O 3 growth and dye kinetics that successfully predicts the reduction in device current as a function of ALD cycles and is extendable to different dye-barrier systems. © This journal is the Owner Societies 2012.

Citation
Brennan TP, Bakke JR, Ding I-K, Hardin BE, Nguyen WH, et al. (2012) The importance of dye chemistry and TiCl4 surface treatment in the behavior of Al2O3 recombination barrier layers deposited by atomic layer deposition in solid-state dye-sensitized solar cells. Physical Chemistry Chemical Physics 14: 12130. Available: http://dx.doi.org/10.1039/c2cp42388j.

Acknowledgements
This publication was based on work supported by the Center for Advanced Molecular Photovoltaics (Award No. KUS-C1-015-21), made by King Abdullah University of Science and Technology (KAUST). The development of the ALD reactor was funded as part of the Center on Nanostructuring for Efficient Energy Conversion at Stanford University, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001060. T.P.B. would like to thank the Albion Walter Hewlett Fellowship for financial support. The authors would like to thank Zhenan Bao for support in the synthesis of YE05. We would like to thank the Stanford Nanocharacterization Laboratory (SNL) staff for their support.

Publisher
Royal Society of Chemistry (RSC)

Journal
Physical Chemistry Chemical Physics

DOI
10.1039/c2cp42388j

PubMed ID
22850593

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