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

Abstract

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.

Electronic structure, photovoltage, and photocatalytic hydrogen evolution with p-CuBi2O4 nanocrystals

“…31 Depending on the direction of charge transport, this produces either positive or negative photovoltage. Spectra obtained in this way provide information about the carrier type, 32 mid gap states, 33,34 defects, 35 and electrochemical reactions at interfaces, 36 38 Previous studies indicate a dependence of the maximum photovoltage on the absorber film thickness. 32,39 In order to study this effect, the CuBi 2 O 4 film thickness on FTO was varied.…”

Petascale Simulations of the Morphology and the Molecular Interface of Bulk Heterojunctions

“…Because charge separation mainly occurs at the P3HT/PCBM interface, 42 a complementary parameter that is equally important is the interfacial area-to-volume ratio, γ. Both d and γ are related to the P3HT volume fraction, ϕ P3HT , as γ ≈ ϕ P3HT (1 − ϕ P3HT )/d for systems having smooth interfaces between the phases.…”

Electronic transitions and band offsets in C60:SubPc and C60:MgPc on MoO3 studied by modulated surface photovoltage spectroscopy

“…The corresponding values of E SB (C 60 ), E SB (SubPc), E SB (SubPc:C 60 ), E SB (MgPc), E SB (MgPc:C 60 ), and E SB (MgPc:C 60 ) are 1.85, 0.96, 0.96, 0.75, and 1.00 eV, respectively. The origin of SPV signals below the HOMO-LUMO transition has been recently discussed by Osterloh et al 37,38 Localization of charge carriers in layers of conjugated molecules causes the formation of charge transfer states within the HOMO-LUMO gap.…”