Compositional and Electric Field Dependence of the Dissociation of Charge Transfer Excitons in Alternating Polyfluorene Copolymer/Fullerene Blends

Abstract

The electro-optical properties of thin films of electron donor-acceptor blends of a fluorene copolymer (PF10TBT) and a fullerene derivative (PCBM) were studied. Transmission electron microscopy shows that in these films nanocrystalline PCBM clusters are formed at high PCBM content. For all concentrations, a charge transfer (CT) transition is observed with absorption spectroscopy, photoluminescence, and electroluminescence. The CT emission is used as a probe to investigate the dissociation of CT excited states at the donor-acceptor interface in photovoltaic devices, as a function of an applied external electric field and PCBM concentration. We find that the maximum of the CT emission shifts to lower energy and decreases in intensity with higher PCBM content. We explain the red shift of the emission and the lowering of the open-circuit voltage (V(OC)) of photovoltaic devices prepared from these blends with the higher relative permittivity of PCBM (epsilon(r) = 4.0) compared to that of the polymer (epsilon(r) = 3.4), stabilizing the energy (E(CT)) of CT states and of the free charge carriers in blends with higher PCBM concentration. We show that the CT state has a short decay time (tau = ca. 4 ns) that is reduced by the application of an external electric field or with increasing PCBM content. The field-induced quenching can be explained quantitatively with the Onsager-Braun model for the dissociation of the CT states when including a high electron mobility in nanocrystalline PCBM clusters. Furthermore, photoinduced absorption spectroscopy shows that increasing the PCBM concentration reduces the yield of neutral triplet excitons forming via electron-hole recombination, and increases the lifetime of radical cations. The presence of nanocrystalline domains with high local carrier mobility of at least one of the two components in an organic heterojunction may explain efficient dissociation of CT states into free charge carriers.

Influence of blend microstructure on bulk heterojunction organic photovoltaic performance

“…Recently, the existence of an intermediate charge transfer (CT) state between the exciton and the free charges in some blend systems based on polyfluorene has been reported. [62][63][64] Understanding the nature of such an intermediate state and its dependence on blend composition is important in understanding the effect of microstructure on charge generation. As an example, Veldman et al confirmed the existence of a charge transfer (CT) state in the blend of a fluorene copolymer (PF10TBT) ''poly[2,7-(9,9-dialkylfluorene)-alt-5,5-(4 0 ,7 0 -di-2-thienyl-2 0 ,1 0 ,3 0 -benzothiadiazole)] with PCBM.…”

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…As a result, the measured spectrum is usually composed of the PL from both the pristine materials and CT states. [45][46][47] …”

“…CT state EL was first reported by Kim et al, 48 and then also observed by other groups (Figure 2c). 46,47,49 Although the EL of the CT state is in the same spectral range as the PL, the EL peak usually shows slight redshift compared with the PL peak. 50 In addition, the EL peak could also show slight blueshift with increasing injection current or applied voltage.…”

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

“…The question, of what the polaron pair generation yield in pristine donor-acceptor co-polymers actually is, remains largely unanswered while being of very large relevance for understanding and improving photovoltaics. For example, discrepancies between theoretically predicted solar cell efficiencies, based on Onsager-Braun models, and the higher measured values may originate from the previously disregarded contribution of polaron pairs forming directly in the polymer 2,7 .…”

“…Light absorption by the polymer has been largely improved towards the near infrared (NIR) spectral region by the introduction of low-bandgap co-polymers 4,5 . Exciton dissociation, however, remains to be understood and has recently been the subject of several investigations [6][7][8][9][10] .…”