Abstract
PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) is a highly soluble C60 derivative that is extensively used in organic solar cells, enabling power conversion efficiencies above 10%. Here we report, for the first time to the best of our knowledge, the photoluminescence of high-quality solvent-free PCBM crystals between room temperature and 4 K. Interestingly, the PL spectra of these crystals become increasingly structured as the temperature is lowered, with extremely well-resolved emission lines (and a minimum line width of ∼1.3 meV at 1.73 eV). We are able to account for such a structured emission by means of a vibronic coupling model including Franck–Condon, Jahn–Teller and Herzberg–Teller effects. Although optical transitions are not formally forbidden from the low-lying excited states of PCBM, the high symmetry of the electronically active fullerene core limits the intensity of the 0–0 transition, such that Herzberg–Teller transitions which borrow intensity from higher-lying states represent a large ...
Highlights
TPhCeBmMos(t[6im,6p]o-PrthaenntyCl C6061debruivtyartiicvea,c1,i2domweinthgytloesttheer)eiffis cciuernrceyntolyf charge transfer from nearby photoexcited molecular states, which makes it the elective electron acceptor in highly efficient bulk heterojunction (BHJ) solar cells, and to other advantages, such as lower synthetic costs compared to C70 derivatives
Given the similarities between C60 and PCBM PL and the success of Sassara et al.’s empirically tuned QC-based approach applied to C60, here we develop a model of PCBM PL based on Sassara et al.’s diabatic-state model of C60, making some empirical adjustments of our own to account for the observed differences between PCBM and C60 spectra
We are able to account for such a structured emission by means of a vibronic coupling model including FC, Jahn− Teller (JT) and HT effects
Summary
TPhCeBmMos(t[6im,6p]o-PrthaenntyCl C6061debruivtyartiicvea,c1,i2domweinthgytloesttheer)eiffis cciuernrceyntolyf charge transfer from nearby photoexcited molecular states, which makes it the elective electron acceptor in highly efficient bulk heterojunction (BHJ) solar cells, and to other advantages, such as lower synthetic costs compared to C70 derivatives. A recent study by Lanzani et al.[5] found that the primary process upon photoexcitation is ultrafast energy transfer from poly(3-hexylthiophene-2,5-diyl) (P3HT) to PCBM, challenging the accepted view of the dominance of ultrafast electron transfer at the polymer/PCBM interface, and suggesting that an improved understanding of the low-lying excited states of PCBM should be a prerequisite for exploiting approaches to engineer the energy transfer process and increase device efficiencies Both the molecular packing and the details of the nanostructure of fullerene-based BHJs have been shown recently to have a crucial impact on the device efficiency.[6] OPV devices based on P3HT and PCBM blends show better performance upon formation of aggregate phases in either components following thermal annealing.[7] In particular, PCBM aggregates have been proven to assist ultrafast long-range charge separation.[8,9] the crystal structure of PCBM turns out to be strongly dependent on the solvent from which the crystals are grown and that is present in the crystals themselves as an inclusion.[10] Only recently, solventfree PCBM single-crystals were reported with a monoclinic structure following solvent extraction in vacuum[11] or via a thermal treatment,[12] thereby allowing fundamental structure− property investigations without the solvent dependence.[13] In particular, crystallite geometries are fundamental to investigate the electronic states involved in charge separation.[14]. We are able to describe such an evolution quantitatively by using a combined approach in which Jahn− Teller (JT) and Franck−Condon (FC) progressions are built upon Herzberg−Teller (HT) intensity-borrowing transitions
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