Florian S. U. Fischer

Anisotropic Charge Transport in Spherulitic Poly(3-hexylthiophene) Films

Poly(3-hexylthiophene) (P3HT) is one of the most important semiconducting polymers for organic photovoltaics and optoelectronics.[1] Yet, because of the structural complexity of semicrystalline polymers, comparatively little is known conclusively about the relationship between charge transport and microstructure in P3HT-containing semiconductors.[2–9] This lack of information arises mainly from an inability to manufacture samples with spontaneous long-range crystalline order approaching macroscopic dimensions, that allow characterisation in a similar fashion to inorganic single crystals. We have overcome this limitation and grown well-ordered P3HT spherulites, sufficiently large to measure charge transport within single oriented crystal domains, enabling measurements of the charge mobility both parallel and perpendicular to the characteristic π-stacked nanocrystalline lamellae and across spherulite boundaries. The importance of local order and molecular orientation in determining the electronic properties of polymer semiconductor films is now well established.[3] In the case of the poly(alkylthiophenes) (PAT)s an edge-on orientation of the molecules, such that the alkyl sidechains (a-axis) stand vertically and both the chain backbone (c-axis) and π-stacking direction (b-axis) lie in the plane of the film, is critical in achieving high in-plane

On the Efficiency of Charge Transfer State Splitting in Polymer:Fullerene Solar Cells

The field dependence and yield of free charge carrier generation in polymer:fullerene blends with varying energetic offsets is not affected when the excitation energy is varied from above band-gap to direct CT state excitation. Instead, the ability of the CT state to split is dictated by the energetic offset between the relaxed CT state and the charge separated (CS) state.

Highly Crystalline Films of PCPDTBT with Branched Side Chains by Solvent Vapor Crystallization: Influence on Opto‐Electronic Properties

PCPDTBT, a marginally crystallizable polymer, is crystallized into a new crystal structure using solvent-vapor annealing. Highly ordered areas with three different polymer-chain orientations are identified using TEM/ED, GIWAXS, and polarized Raman spectroscopy. The optical and structural properties differ significantly from films prepared by standard device preparation protocols. Bilayer solar cells, however, show similar performance.

Enhanced Photogeneration of Polaron Pairs in Neat Semicrystalline Donor–Acceptor Copolymer Films via Direct Excitation of Interchain Aggregates

We investigate the photogeneration of polaron pairs (PPs) in neat films of the semicrystalline donor-acceptor semiconducting copolymer PCPDTBT. Carefully selecting the solution-processing procedures, we obtain films with different amounts of crystallinity and interchain aggregation. We compare the photogeneration of PPs between the films by monitoring their photoinduced absorption in ultrafast pump-probe experiments, selectively exciting nonaggregated or aggregated polymer chains. The direct photoexcitation of interchain π-aggregates results in prompt (<100 fs) charge generation. Compared to the case where nonaggregated chains are excited, we find an 8-fold increase in the prompt PP to singlet-exciton ratio. We also show that highly crystalline lamellar nanostructures not containing π-stacked or any light-absorbing aggregates do not improve the efficiency of PP photogeneration. Our results show that light absorption from interchain aggregates is highly beneficial for charge photogeneration in semiconducting polymers and should be taken into account when optimizing film morphologies for photovoltaic devices.

Crosslinkable low bandgap polymers for organic solar cells

We present a number of polyfluorene based conjugated polymers with crosslinkable acrylate and oxetane units. These polymers can be crosslinked by free radical polymerization in the case of acrylates and by cationic ring opening polymerization for oxetanes. Upon polymerization densely crosslinked networks are formed which are completely insoluble. We show that the diffusion coefficient of C60 in polyfluorene is reduced by a factor of 1000 by crosslinking. MIS-CELIV measurements are used to monitor changes in the charge carrier mobility upon crosslinking. It shows that using appropriate conditions, e.g. low initiator concentrations or thermal crosslinking, the charge carrier mobility is not reduced by crosslinking. Solution processed three layer organic solar cells were realized with a crosslinkable fluorene based copolymer containing acrylate groups. The efficiency is increased from 1.4% for the reference to 1.8% in the three layer cell with a crosslinked exciton blocking layer. A critical issue of BHJ cells is the instability of the morphology of the polymer:fullerene blend over long operation times at elevated temperature. We present a crosslinkable derivative of the low bandgap polymer PFDTBT which contains oxetane units. BHJ cells with the crosslinked PFDTBT derivative and PCBM were tested in accelerated aging experiments at 100 °C for times up to 100 h. Stabilization was clearly observed in crosslinked BHJ cells compared to the non-crosslinked reference. We show for the first time that oxetane containing polymers can be thermally crosslinked without any added initiator. Initiator free crosslinking is particularly attractive as it avoids the formation of decomposition products, and thus potential electron traps and quenching sites from the initiator.