Photochemical Charge Separation in Poly(3-hexylthiophene) (P3HT) Films Observed with Surface Photovoltage Spectroscopy

Osterloh, Frank E.; Holmes, Michael A.; Chang, Lilian; Moulé, Adam J.; Zhao, Jing

doi:10.1021/jp409262v

Cited by 39 publications

(61 citation statements)

References 71 publications

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“…Lastly, the positive SPV signal at ≥2.4 eV does not originate from the polymer film but is an artifact of the instrument. 31 It is also observed in background spectra on non-coated substrates and will not be discussed further.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

P3HT:PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy

et al. 2014

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Surface photovoltage (SPV) spectra are reported for separate films of (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) and for regioregular and regiorandom poly(3-hexylthiophene) (P3HT):PCBM bulk heterojunctions, as a function of wavelength, film thickness, thermal annealing, and substrate. In PCBM films, two photovoltage features are observed at 1.1–1.4 eV (F1) and 1.4–2.3 eV (F2), which are assigned to excitation of charge transfer states at the interface (F1) and in the bulk (F2) of the film. In BHJ films, five different photovoltage features are observed at 0.75–0.9 eV (F1), 0.9–1.3 eV (F2), 1.3–1.8 eV (F3), 1.8–2.0 eV (F4), and 2.0–2.4 eV (F5). This data can be analyzed on the basis of optical absorbance and fluorescence spectra of the films, and using SPV spectra for PCBM and P3HT only films, and for a BHJ film containing P3HT nanofibers for comparison. SPV features are assigned to states at the polymer–substrate interface (F1 and F2), the P3HT:PCBM charge transfer state (F3), the self-ionized (CT) state of P3HT (F4), and the band gap transition of P3HT (F5). This interpretation is also consistent with molecular orbital energy diagrams and electron microscopy-derived topological maps of the films. Photovoltage sign and substrate dependence can be understood with the depleted semiconductor model. Features F1–4 are caused by polarization of electrostatically bound charge pairs by the built-in electric field at the substrate–BHJ interface, whereas F5 is due to transport of free charge carriers through the film and through the substrate film interface. This work will promote the understanding of photochemical charge generation and transport in organic photovoltaic films.

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Section: ■ Results and Discussionmentioning

confidence: 99%

P3HT:PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy

et al. 2014

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“…The higher sensitivity allows the observation of localized charge transfer states with small optical cross sections. 17,[21][22][23][24][25][26][27][28] As a model system we use single crystalline nanoparticles of Rh-doped SrTiO 3 . SrTiO 3 is an established photocatalyst for overall water splitting under UV light only, 18,21,[29][30][31] but it can be converted into a visible light responsive material upon doping with Cr, 32 Cr/Sb, 33,34 Cr/Ta, 35 Rh, 36,37 Ni/Ta, 38 and…”

Section: Introductionmentioning

confidence: 99%

Photochemical charge transfer observed in nanoscale hydrogen evolving photocatalysts using surface photovoltage spectroscopy

Wang

Zhao

Osterloh

2015

Energy Environ. Sci.

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The application of inorganic nanostructures for solar water splitting is currently limited by our understanding of photochemical charge transfer on the nanoscale, where space charge layers are less effective for carrier separation. Here we employ surface photovoltage spectroscopy to measure the internal photovoltages in single crystalline platinum/ruthenium-modified Rh-doped SrTiO 3 nanocrystals for the first time. Voltages of À0.88 V and À1.13 V are found between the absorber and the Ru and Pt cocatalysts, respectively, and a voltage of À1.48 V for a Rh:SrTiO 3 film on an Au substrate. This shows that the Pt and Ru cocatalysts not only improve the redox kinetics but also aid charge separation in the absorber. Voltages of +0.4 V, +0.6 V, and +1.2 V are found for hole injection into KI, K 4 [Fe(CN) 6 ], and methanol, respectively, and a voltage of À0.7 V for electron injection into K 3 [Fe(CN) 6 ]. These voltages correlate well with the photocatalytic performance of the catalyst; they are influenced by the built-in potentials of the donor-acceptor configurations, the physical separation of donors and acceptors, and the reversibility of the redox reaction.The photovoltage data also allowed the identification of a photosynthetic system for hydrogen evolution (80 mmol g À1 h À1) under visible light illumination (4400 nm) from 0.05 M aqueous K 4 [Fe(CN) 6 ]. Broader contextNanostructured light absorbers have advantages for solar water splitting, including shortened carrier collection pathways and improved light distribution. However, the application of nanoscale absorbers for artificial photosynthesis is currently limited by our understanding of photochemical charge transfer on the nanoscale, where space charge layers are not effective for charge separation. Here we employ surface photovoltage spectroscopy (SPS) to observe electron and hole transfer from single crystalline platinum/ruthenium-modified Rh-doped SrTiO 3 nanocrystals for the first time. We find that the absorber-Pt/Ru junctions promote electron-hole separation strongly, allowing open circuit voltages of over 1.0 V. This is comparable to the effect of molecular redox reagents at the absorber surfaces. Overall, we find that nanoscale charge transfer is controlled by the built-in potentials of the absorber/acceptor configuration, by the spatial separation between donors and acceptors and by the reversibility of the redox reactions. These observations aid the understanding of photochemical charge transfer on the nanoscale and contribute to the design of more efficient systems for artificial photosynthesis.

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“…Both contain two positive features (F1 and F2) below and near the bandgap of the blend, and a negative super bandgap feature (F3) at >2.0 eV. According to our previous analysis, 52,53 F1 and F2 are due to excitation of BHJ/PEDOT:PSS interfacial states in the PCBM rich and P3HT-PCBM mixed domains, respectively. The photovoltage arises from polarization of these electron-hole pairs by the built-in eld at the PEDOT:PSS\BHJ interface, as shown in Fig.…”

Section: -41mentioning

confidence: 79%

Effect of fractal silver electrodes on charge collection and light distribution in semiconducting organic polymer films

et al. 2014

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Photochemical Charge Separation in Poly(3-hexylthiophene) (P3HT) Films Observed with Surface Photovoltage Spectroscopy

Cited by 39 publications

References 71 publications

P3HT:PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy

P3HT:PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy

Photochemical charge transfer observed in nanoscale hydrogen evolving photocatalysts using surface photovoltage spectroscopy

Effect of fractal silver electrodes on charge collection and light distribution in semiconducting organic polymer films

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