Hole Transport Materials with Low Glass Transition Temperatures and High Solubility for Application in Solid-State Dye-Sensitized Solar Cells

We present the synthesis and device characterization of new hole transport materials (HTMs) for application in solid-state dye-sensitized solar cells (ssDSSCs). In addition to possessing electrical properties well suited for ssDSSCs, these new HTMs have low glass transition temperatures, low melting points, and high solubility, which make them promising candidates for increased pore filling into mesoporous titania films. Using standard device fabrication methods and Z907 as the sensitizing dye, power conversion efficiencies (PCE) of 2.94% in 2-μm-thick cells were achieved, rivaling the PCE obtained by control devices using the state-of-the-art HTM spiro-OMeTAD. In 6-μm-thick cells, the device performance is shown to be higher than that obtained using spiro-OMeTAD, making these new HTMs promising for preparing high-efficiency ssDSSCs

TiO₂ Conduction Band Modulation with In₂O₃ Recombination Barrier Layers in Solid-State Dye-Sensitized Solar Cells

Atomic layer deposition (ALD) was used to grow subnanometer indium oxide recombination barriers in a solid-state dye-sensitized solar cell (DSSC) based on the spiro-OMeTAD hole-transport material (HTM) and the WN1 donor-π-acceptor organic dye. While optimal device performance was achieved after 3–10 ALD cycles, 15 ALD cycles (∼2 Å of In₂O₃) was observed to be optimal for increasing open-circuit voltage (<i>V</i>_OC) with an average improvement of over 100 mV, including one device with an extremely high <i>V</i>_OC of 1.00 V. An unexpected phenomenon was observed after 15 ALD cycles: the increasing <i>V</i>_OC trend reversed, and after 30 ALD cycles <i>V</i>_OC dropped by over 100 mV relative to control devices without any In₂O₃. To explore possible causes of the nonmonotonic behavior resulting from In₂O₃ barrier layers, we conducted several device measurements, including transient photovoltage experiments and capacitance measurements, as well as density functional theory (DFT) studies. Our results suggest that the <i>V</i>_OC gains observed in the first 20 ALD cycles are due to both a surface dipole that pulls up the TiO₂ conduction band and recombination suppression. After 30 ALD cycles, however, both effects are reversed: the surface dipole of the In₂O₃ layer reverses direction, lowering the TiO₂ conduction band, and mid-bandgap states introduced by In₂O₃ accelerate recombination, leading to a reduced <i>V</i>_OC