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 Al₂O₃ recombination barriers in solid-state dye-sensitized solar cells (ss-DSSCs) employing an organic hole transport material (HTM) for the first time. Al₂O₃ recombination barriers of varying thickness were incorporated into efficient ss-DSSCs utilizing the Z907 dye adsorbed onto a 2 μm-thick nanoporous TiO₂ active layer and the HTM spiro-OMeTAD. The impact of Al₂O₃ barriers was also studied in devices employing different dyes, with increased active layer thicknesses, and with substrates that did not undergo the TiCl₄ surface treatment. In all instances, electron lifetimes (as determined by transient photovoltage measurements) increased and dark current was suppressed after Al₂O₃ deposition. However, only when the TiCl₄ 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 Al₂O₃ ALD and the TiCl₄ surface treatment whereas the insulating properties of Al₂O₃ hinder charge injection and lead to current loss in TiCl₄-treated devices. The impact of Al₂O₃ 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 Al₂O₃ 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.