Efficient Charge Photogeneration by the Dissociation of PC₇₀BM Excitons in Polymer/Fullerene Solar Cells

Abstract

ABSTRACT:The role of PC 70 BM excitons in driving charge photogeneration in low bandgap polymer/fullerene bulk heterojunction solar cells has been studied. Both transient absorption spectroscopy (TAS) of charge generation yields in blend films as a function of excitation energies and photocurrent quantum efficiency spectra of the corresponding devices indicate that charge generation in this system results primarily from direct optical excitation of PC 70 BM. Blend composition studies of photocurrent density and photoluminescence quenching indicate that the efficiency of photocurrent generation is primarily determined by the limited efficiency of PC 70 BM exciton diffusion to the polymer, due to the formation of PC 70 BM domains (≥ 5nm). This limitation becomes more severe as the PC 70 BM content is increased above 50%. Despite this limitation, and despite the poor charge photogeneration from polymer excitons, organic solar cells fabricated using this photoactive blend layer yielded device photocurrents of 7.1 mA/cm 2 , maximal EQE's of 41% and a device efficiency of 3.1%.The most efficient organic photovoltaic (OPV) devices are now reaching power conversion efficiencies of 8%.1-4 Their active layer consists of a bicontinuous network of a small bandgap polymer and a soluble fullerene that act as an electron donor and acceptor (D/A) pair.5 Charge photogeneration in these devices is typically considered to be initiated by photon absorption by the polymer followed by electron transfer from the polymer to the fullerene. 5,6 However increasing attention is being placed upon the role of the acceptor as an additional light absorbing element of the device, motivated in particular by the increased use of C 70 based acceptors such as [6,6]-phenyl-C71-butyric acid methyl ester (PC 70 BM) which exhibit significant visible light absorption. 7,8 Charge generation from fullerene excitons can proceed either by direct hole transfer from the fullerene to the polymer (sometimes referred to as reverse electron transfer), or by exciton energy transfer from the fullerene to the polymer followed by the conventional electron transfer from the polymer exciton to the fullerene.9-11 Neither of these charge separation pathways have received much attention to date compared to the more conventional charge generation pathway initiated by polymer light absorption in such blend films. Time-resolved spectroscopy experiments have estimated that hole transfer in polymer/PC 60 BM films is an ultrafast process with efficiencies limited by phase segragation in the blends. 9,10,12 In this letter we report our investigations on the contribution of photoinduced hole transfer to the device photocurrent for an OPV device with an active layer comprising a blend film of the small bandgap polymer BTT-DPP and PC 70 BM. By means of transient absorption spectroscopy we demonstrate that the BTT-DPP exciton is unable to drive efficient charge separation, with charge photogeneration in the device active layer resulting primarily from PC 70 BM light absorption, ...

Charge-Transfer State Dynamics Following Hole and Electron Transfer in Organic Photovoltaic Devices

“…Figure 1a shows the chemical structures of the donor and acceptor molecules involved, and figure 1b presents the previously reported positions of their energy levels. [28][29][30] We used well-studied donor-acceptor blends with similarly large band offsets both for the hole and the electron transfer (MDMO shows substantial optical absorption, particularly near 500 nm, in contrast to PC 60 BM. The spectra were measured for films of each material separately and later normalised to obtain a reasonable estimate of the donor and acceptor contributions to the absorption in the blend at different wavelengths.…”

Ultrafast electron and hole transfer in bulk heterojunctions of low-bandgap polymers

“…Initial processes of photon absorption, intramolecular chargetransfer (ICT) exciton formation and their following dissociation/ recombination form the basis for the device operation. These early-time dynamics in pristine pushpull polymers are quite complex and include a number of competitive processes [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. The initial ultrafast photophysics of polymer-based bulk heterojunctions (BHJs) with fullerenes are even more complicated and depend on a number of factors such as the particular polymer structure, acceptor type, donor-acceptor phase separation known as BHJ morphology [25] etc.…”

“…However, efficiency of the charge generation processes of these polymers in BHJs differs substantially. BTT-DPP features extremely inefficient electron transfer to [70]PCBM which is usually explained by a too low LUMO-LUMO offset of 0.1-0.3 eV [12][13][14]39]. Agreeably, this offset might be lower than the exciton binding energy -but in the PCPDTBT:PCBM system where the PCPDTBT LUMO energy is only~0.2 eV higher than that of BTT-DPP, the electron transfer is highly efficient [15,20].…”

Exciton and Hole-Transfer Dynamics in Polymer: Fullerene Blends

“…It has been shown that the mirror process in which charges are generated by dissociation of an acceptor exciton is very efficient as well [1]. Although the importance of hole transfer was recognised only recently, the first devices functioning primarily on hole transfer have already been demonstrated [2].Charge generation processes are but one of the processes which determine the eventual efficiency of devices. For the efficiency, one of the most important parameters is the morphology of the bulk heterojunction architecture.…”

“…It has been shown that the mirror process in which charges are generated by dissociation of an acceptor exciton is very efficient as well [1]. Although the importance of hole transfer was recognised only recently, the first devices functioning primarily on hole transfer have already been demonstrated [2].…”