Influence of Fullerene Ordering on the Energy of the Charge-Transfer State and Open-Circuit Voltage in Polymer:Fullerene Solar Cells

Abstract

The influence of fullerene side chain functionalization on both the morphology and electro-optical properties of bulk-heterojunction polymer:fullerene solar cells is discussed through a systematic investigation of material blends consisting of the conjugated polymer poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) as donor and fullerene molecules with different side chains as the acceptor. The varying side chain of the fullerenes was found to induce morphological changes as confirmed by different analytical techniques such as Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), and Nuclear Magnetic Resonance (NMR). The fullerene with the shorter side chain, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), forms crystalline nanophases in the blend, while this is not the case for the alternative diphenylmethanofullerene acceptor, [6,6]-1,1-bis(4,4′-dodecyloxyphenyl)methanofullerene (DPM-12). The introduction of NMR allows us to estimate the fraction of crystalline fullerene. The morphological changes have a profound effect on the characteristics of charge transfer states (CT) formed at the polymer:fullerene interfaces. Crystallization of fullerene molecules shifts the energy of the CT state. This shift in energy is directly manifested in the open-circuit voltage of solar cells based on the fullerene acceptors under investigation.

Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells

“…Consistent with previous observations, we observe a redshift of E CT with increasing C 60 fraction [10][11][12][13][14] , but find that modelling based on the accompanying change in dielectric constant via the solid-state solvation effect (SSSE) 15 can only explain the data at high (450%) C 60 loading. We find that E CT at low fullerene concentration is higher than expected from the change in dielectric constant and attribute this discrepancy to increased localization of the electron component of the CT state because of a reduction in average C 60 crystallite size below B4 nm.…”

“…In addition to dielectric constant, the degree of C 60 crystallinity in the BHJs reduces with decreasing C 60 concentration as evident from glancing incidence X-ray diffraction data shown in Fig. 3b and well-documented in polymer-fullerene systems 13 Based on these observations, we hypothesize that the increase in CT emission energy at low C 60 concentration, also noted by Piersimoni et al 13 and Muller et al 14 in polymer-fullerene blends, is due to confinement of the wavefunction as C 60 crystallite size becomes comparable to the natural spatial extent of the CT state (that is, that for large crystallites where CT volume is not influenced by the crystallite boundaries), similar to the manner in which exciton energy increases with decreasing inorganic semiconductor quantum dot size 23 . In this scenario, the electron component of the CT wavefunction in 1:9 and 1:4 BHJs is delocalized over two or three C 60 molecules within a crystallite, whereas the hole remains localized on an adjacent NPD molecule.…”

Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

“…15,[30][31][32] Even if the determination of energy levels is done with one consistent method in pristine materials, the results may not be transferable to blend films and devices, because effects such as aggregation, crystallization and interface dipoles can shift energy levels by up to 0.5 eV. [33][34][35][36][37] These uncertainties in energy levels are relevant for material design rules and for the estimation of efficiency potentials as done by Scharber et al 15 The present paper uses electroluminescence (EL) and photoluminescence (PL) measurements of polymer:fullerene devices and films to study the relationship between interfacial energetics and charge generation and recombination. EL spectroscopy probes the emissive states directly in working devices and allows a quantitative determination of the energy of the CT state by measuring its emission.…”

Polarization anisotropy of charge transfer absorption and emission of aligned polymer:fullerene blend films

“…32 In this respect, the study of CT absorption and emission has already proven useful for the determination of CT state energies for different polymer:fullerene combinations. 21 Furthermore, donor-acceptor separation distance 33 and crystallization of either polymer 34 or fullerene 35 phase have a pronounced influence on the spectral position and strength of CT absorption bands.…”