Physical Models for Charge Transfer at Single Crystal Oxide Semiconductor Surfaces as Revealed by the Doping Density Dependence of the Collection Efficiency of Dye Sensitized Photocurrents

Spectral Sensitization of n- and p-Type Gallium Phosphide Single Crystals with Single-Walled Semiconducting Carbon Nanotubes

The spectral sensitization of single-crystal p-GaP by semiconducting single-walled carbon nanotubes (s-SWCNT) via hole injection into the p-GaP valence band is reported. The results are compared to SWNCT sensitized n-type single-crystal substrates: TiO₂, SnO₂, and n-GaP. It was found that the sensitized photocurrents from CoMoCAT and HiPco s-SWCNTs were from a hole injection mechanism on all substrates, even when electron injection into the conduction band should be energetically favored. The results suggest an intrinsic p-type character of the s-SWCNTs surface films investigated in this work.

Optically Generated Free-Carrier Collection from an All Single-Walled Carbon Nanotube Active Layer

Semiconducting single-walled carbon nanotubes' (SWCNTs) broad absorption range and all-carbon composition make them attractive materials for light harvesting. We report photoinduced charge transfer from both multichiral and single-chirality SWCNT films into atomically flat SnO₂ and TiO₂ crystals. Higher-energy second excitonic SWCNT transitions produce more photocurrent, demonstrating carrier injection rates are competitive with fast hot-exciton relaxation processes. A logarithmic relationship exists between photoinduced electron-transfer driving force and photocarrier collection efficiency, becoming more efficient with smaller diameter SWCNTs. Photocurrents are generated from both conventional sensitization and in the opposite direction with the semiconductor under accumulation and acting as an ohmic contact with only the p-type nanotubes. Finally, we demonstrate that SWCNT surfactant choice and concentration play a large role in photon conversion efficiency and present methods of maximizing photocurrent yields.

Structural Effects on the Incident Photon-to-Current Conversion Efficiency of Zn Porphyrin Dyes on the Low-Index Planes of TiO2

The structural effects of substrates on the incident photon-to-current conversion efficiency (IPCE) of Zn porphyrin (ZnP) dyes (ZnP-ref, YD2, and ZnPBAT) have been studied on well-defined single-crystal surfaces of rutile TiO₂ (TiO₂(111), TiO₂(100), and TiO₂(110)). IPCE of ZnP-ref depends on the structure of the substrates remarkably: TiO₂(100) < TiO₂(110) < TiO₂(111). IPCE of ZnP-ref/TiO₂(111) is 13 times as high as that of ZnP-ref/TiO₂(100) at 570 nm. YD2 and ZnPBAT also give the highest IPCE on TiO₂(111). The relative coverages of the porphyrin dyes give the following order: TiO₂(111) < TiO₂(110) < TiO₂(100). This order is opposite to that of IPCEs. The orientation of the dyes is predicted using density functional theory calculations on simplified models of TiO₂ surfaces. The highest IPCE on TiO₂(111) is attributed to the high rate of electron transfer through the space due to the fluctuation of the tilt angle of the adsorbed dyes.

Probing the Relative Photoinjection Yields of Monomer and Aggregated Dyes into ZnO Crystals

Cyanine dyes, often used in dye-sensitized solar cells (DSSCs), form a range of molecular species from monomers to large H and J aggregates in both solution and when adsorbed at a photoelectrode surface. To determine the relative capability of the different dye species to inject photoexcited electrons into a wideband gap oxide semiconductor, sensitization at a single-crystal zinc oxide surface was studied by simultaneous attenuated reflection (ATR) ultraviolet-visible (UV-vis) absorption and photocurrent spectroscopy measurements. ATR measurements enable identification of the dye species populating the surface with simultaneous photocurrent spectroscopy to identify the contribution of the various dye forms to photocurrent signal. We study the dye 2,2'-carboxymethylthiodicarbocyanine bromide that is particularly prone to aggregation both in solution and at the surface of sensitized oxide semiconductors.