Improving polymer based photovoltaic devices by reducing the voltage loss at the donor-acceptor interface

The costs of large area, organic photovoltaic devices are stronly related to their module efficiency. Even for niche markets, such as consumer electronics, efficiency is imperative since the available area is limited. Therefore, if polymer photovoltaics is to become a mature technology, it is key to increase the power conversion efficiency of the devices. In our contribution an analysis is given of the energy loss factors in P3HT:[C6O]PCBM cells. The main loss occurs as a voltage loss at the donor-accpetor interface. Since this loss factor is linked to the HOMO-LUMO levels of the system, it is impossilble to reduce this loss using the same material combination. We present polymer: [C6O]PCBM cells with similar optical properties but with a reduced voltage loss at the interface, leading to enhanced open circuit Voltage of 1.0 V (compared to 0.62 V for P3HT:[C6O]PCBM devices). The polymer is an alternating copolymer with fluorence and benzothiadiazole units (PFTBT). Well-characterised devices yield already an AM 1.5 efficiency of 4%, thus competing with state-of-the-art P3HT:PCBM devices

Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends

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 (VOC) of photovoltaic devices prepared from these blends with the higher relative permittivity of PCBM (?r = 4.0) compared to that of the polymer (?r = 3.4), stabilizing the energy (ECT) 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 (? = 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. © 2008 American Chemical Societ