Probing the nanoscale phase separation in binary photovoltaic blends of poly(3-hexylthiophene) and methanofullerene by energy transfer

Abstract

Images reproduced with permission of Michael GrätzelArticles published in this issue include: PERSPECTIVES:Introducing a dark reaction to photochemistryPhotocatalytic hydrogen from [FeFe] The generation of charge carriers in organic photovoltaic devices requires exciton diffusion to an interface of electron donor and acceptor materials, where charge separation occurs. We report a time resolved study of fluorescence quenching in films of poly(3-hexylthiophene) containing a range of fractions of the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We show that energy transfer from P3HT to PCBM helps to bring excitons to the interface, where they dissociate into charge carriers. Fluorescence quenching in blends with ≤50 wt% of PCBM is controlled by exciton diffusion in P3HT. This allows us to estimate the average size of PCBM domains to be about 9 nm in the 1:1 blend. The implications for polymer solar cells are discussed.

Charge Photogeneration in Organic Solar Cells

“…Indeed, it has been noted that if the domain size is smaller than the Coulomb capture radius then the charges will not be able to escape one another efficiently and geminate recombination may result. 200 We note that in this case the charges may not be confined by the Coulomb attraction but rather by the physical size of the domains. An ideal blend morphology will involve a bicontinuous interpenetrating network of donor and acceptor components with an optimized interfacial area for efficient exciton dissociation and with the scale of phase separation on the order of the exciton diffusion length and greater than the Coulomb capture radius.…”

Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum

“…Such a comprehensive and systematic approach has not been used in previously reported time-resolved emission investigations of P3HT. 16,17,21,23,24,37,44,54 Furthermore, those studies obtained either single-wavelength FU dynamics with a ∼200 fs time resolution or entire emission spectra with a lower time resolution of several picoseconds typical for streak cameras. Here, we use the FU dynamics obtained at many emission wavelengths to reconstruct the emission spectra with a ∼200 fs time resolution.…”

Molecular Weight Dependence of Exciton Diffusion in Poly(3‐hexylthiophene)

“…[1] Exciton diffusion length determines the fraction of excitons that split into electron-hole pairs whereas charge carrier mobility determines the fraction of charges that reach an electrode for photocurrent generation. Exciton diffusion is particularly important in planar heterojunction solar cells where it limits the exciton dissociation rate but it also defines the slow phase of charge generation in bulk heterojunction solar cells which extends to 100 ps [2,3] and sets limits on the acceptable length scale of phase separation. Solar cells based on a bulk heterojunction of a semi-crystalline polymer poly(3-hexylthiophene) (P3HT) and a fullerene derivative PCBM show high internal quantum efficiencies of about 0.8 and have been studied extensively as model materials.…”