In an endeavor to examine how optical excitation of C60 and PCBM contribute to the photogeneration of charge carriers in organic solar cells, we investigated stationary photogeneration in single-layer C60 and PCBM films over a broad spectrum as a function of the electric field. We find that intrinsic photogeneration starts at a photon energy of about 2.25 eV, i.e., about 0.4 eV above the first singlet excited state. It originates from charge transfer type states that can autoionize before relaxing to the lower-energy singlet S1 state, in the spirit of Onsager’s 1938 theory. We analyze the internal quantum efficiency as a function of electric field and photon energy to determine (1) the Coulombic binding and separation of the electron–hole pairs, (2) the value of the electrical gap, and (3) which fraction of photoexcitations can fully separate at a given photon energy. The latter depends on the coupling between the photogenerated charge transfer states and the eventual charge transporting states. It is by a factor of 3 lower in PCBM. Close to the threshold energy for intrinsic photoconduction (2.25 eV), the generating entity is a photogenerated electron–hole pair with roughly 2 nm separation. At higher photon energy, more expanded pairs are produced incoherently via thermalization.

Organic solar cells (OSCs) have achieved much attention and meanwhile reach efficiencies above 10%. One problem yet to be solved is the lack of long term stability. Crosslinking is presented as a tool to increase the stability of OSCs. A number of materials used for the crosslinking of bulk heterojunction cells are presented. These include the crosslinking of low bandgap polymers used as donors in bulk heterojunction cells, as well as the crosslinking of fullerene acceptors and crosslinking between donor and acceptor. External crosslinkers often based on multifunctional azides are also discussed. In the second part, some work either leading to OSCs with high efficiencies or giving insight into the chemistry and physics of crosslinking are highlighted. The diffusion of low molar mass fullerenes in a crosslinked matrix of a conjugated polymer and the influence of crosslinking on the carrier mobility is discussed. Finally, the use of crosslinking to make stable interlayers and the solution processing of multilayer OSCs are discussed in addition to presentation of a novel approach to stabilize nanoimprinted patterns for OSCs by crosslinking.

While it has been argued that field‐dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT‐type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p‐DTS(FBTTh2)2 layer as donor material and C60 as acceptor. The authors measure JV‐characteristics as a function of intensity and charge‐extraction‐by‐linearly‐increasing‐voltage‐type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (Lp) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p‐DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s‐shape of the JV‐curves and the concomitant additional decrease of the FF with increasing layer thickness.

We use bilayer devices with a series of three fullerene acceptors differing in order and intermolecular coupling to systematically explore the influence of electron delocalization in the acceptor phase on the dissociation efficiency of charge transfer states. Structural information from GIWAXS measurements is combined with the results of optical and electrical characterization as well as theoretical modeling.Our results indicate that an increase in CT-dissociation efficiency is directly coupled to an enhancement in electron delocalization that is particularly prominent for C 60 which forms crystalline domains. Therefore, our results substantiate the concept of delocalization of electrons taking a positive role in the charge separation process, and of acceptor crystallinity being crucial in this respect.

We show that the performance of an organic solar cell can be increased by the introduction of an additional polymeric exciton blocking layer. In order to realize this, the novel polymer PFTPDAc with pendant acrylate groups is developed. Thin films are coated from a PFTPDAc solution and subsequently crosslinked by irradiation. Thereby, the film becomes completely insoluble and allows spincoating of a second polymer layer on top. We realize a three layer solar cell which contains a crosslinked PFTPDAc interlayer on top of the molybdenum oxide anode and layers of the low-bandgap polymer PCDTBT and C 60 . In comparison with a reference cell without the interlayer, the EQE is significantly increased in the spectral region between 400 and 650 nm. From current-voltage measurements a power conversion efficiency of 1.8% is determined. PL measurements show that the increase of solar cell performance is attributed to exciton blocking by the PFTPDAc interlayer.