Sven Huettner

Iodine Migration and its Effect on Hysteresis in Perovskite Solar Cells.

The migration and accumulation of iodide ions create a modulation of the respective interfacial barriers causing the hysteresis in solar cells based on methylammonium lead iodide perovskites. Iodide ions are identified as the migrating species by measuring temperature dependent current-transients and photoelectron spectroscopy. The involved changes in the built-in potential due to ion migration are directly measured by electroabsorption spectroscopy.

Ternary photovoltaic blends incorporating an all-conjugated donor-acceptor diblock copolymer.

We present a new fully conjugated diblock copolymer, P3HT-b-PFTBTT, containing donor and acceptor blocks with suitably positioned energy levels for use in a solar cell. This is the first block copolymer to be based on an existing high-performance polymer:polymer blend. We observe phase separation of the blocks and self-assembly behavior. In ternary blends with the respective homopolymers the diblock copolymer introduces lateral nanostructure without restricting P3HT crystallization in the charge transport direction, resulting in standing lamellae. By adding the diblock to the homopolymer blend as a compatibilizer, we prevent phase separation at elevated temperatures and benefit from a dramatic increase in P3HT ordering, allowing us to demonstrate polymer blend photovoltaics where the nanostructure is thermodynamically, rather than kinetically, controlled.

On the Role of Single Regiodefects and Polydispersity in Regioregular Poly(3-hexylthiophene): Defect Distribution, Synthesis of Defect-Free Chains, and a Simple Model for the Determination of Crystallinity

Identifying structure formation in semicrystalline conjugated polymers is the fundamental basis to understand electronic processes in these materials. Although correlations between physical properties, structure formation, and device parameters of regioregular, semicrystalline poly(3-hexylthiophene) (P3HT) have been established, it has remained difficult to disentangle the influence of regioregularity, polydispersity, and molecular weight. Here we show that the most commonly used synthetic protocol for the synthesis of P3HT, the living Kumada catalyst transfer polycondensation (KCTP) with Ni(dppp)Cl(2) as the catalyst, leads to regioregular chains with one single tail-to-tail (TT) defect distributed over the whole chain, in contrast to the hitherto assumed exclusive location at the chain end. NMR end-group analysis and simulations are used to quantify this effect. A series of entirely defect-free P3HT materials with different molecular weights is synthesized via new, soluble nickel initiators. Data on structure formation in defect-free P3HT, as elucidated by various calorimetric and scattering experiments, allow the development of a simple model for estimating the degree of crystallinity. We find very good agreement for predicted and experimentally determined degrees of crystallinities as high as ∼70%. For Ni(dppp)Cl(2)-initiated chains comprising one distributed TT unit, the comparison of simulated crystallinities with calorimetric and optical measurements strongly suggests incorporation of the TT unit into the crystal lattice, which is accompanied by an increase in backbone torsion. Polydispersity is identified as a major parameter determining crystallinity within the molecular weight range investigated. We believe that the presented approach and results not only contribute to understanding structure formation in P3HT but are generally applicable to other semicrystalline conjugated polymers as well.

Donor-acceptor block copolymers for photovoltaic applications

Extensive research activities in polymer synthesis and device engineering have been devoted to the development of donor–acceptor (D–A) bulk heterojunction solar cells in the last years. In such devices, several photophysical processes occur all of which have to be optimized for efficient operation. First, excitons created upon light absorption need to reach the D/A interface within their exciton diffusion length (10– 20 nm), where they may dissociate into holes and electrons. Subsequent charge transport and finally charge collection at the electrodes can occur, given that co-continous pathways of donor and acceptor domains are provided. Owing to the small exciton diffusion lengths and the required optical absorption length of 100–200 nm, vertically aligned pathways with a high aspect ratio of either phase should percolate through the film. The morphologies resulting from this ideal situation resemble those of vertically oriented microphase separated block copolymer thin films, and hence suggest the importance of D–A block copolymers for organic photovoltaics. Furthermore, the covalent bond between the donor and acceptor blocks is not only desired to improve morphology control, but also to enhance long term stability of the device. The potential of block copolymers with electronic functionality to microphase separate into well-defined microstructures with several tens of nanometers in size thus addresses the morphological requirements mentioned above. This article gives an overview of donor–acceptor block copolymers and summarises recent developments of this field.

What Controls the Rate of Ultrafast Charge Transfer and Charge Separation Efficiency in Organic Photovoltaic Blends

In solar energy harvesting devices based on molecular semiconductors, such as organic photovoltaics (OPVs) and artificial photosynthetic systems, Frenkel excitons must be dissociated via charge transfer at heterojunctions to yield free charges. What controls the rate and efficiency of charge transfer and charge separation is an important question, as it determines the overall power conversion efficiency (PCE) of these systems. In bulk heterojunctions between polymer donor and fullerene acceptors, which provide a model system to understand the fundamental dynamics of electron transfer in molecular systems, it has been established that the first step of photoinduced electron transfer can be fast, of order 100 fs. But here we report the first study which correlates differences in the electron transfer rate with electronic structure and morphology, achieved with sub-20 fs time resolution pump-probe spectroscopy. We vary both the fullerene substitution and donor/fullerene ratio which allow us to control both aggregate size and the energetic driving force for charge transfer. We observe a range of electron transfer times from polymer to fullerene, from 240 fs to as short as 37 fs. Using ultrafast electro-optical pump-push-photocurrent spectroscopy, we find the yield of free versus bound charges to be weakly dependent on the energetic driving force, but to be very strongly dependent on fullerene aggregate size and packing. Our results point toward the importance of state accessibility and charge delocalization and suggest that energetic offsets between donor and acceptor levels are not an important criterion for efficient charge generation. This provides design rules for next-generation materials to minimize losses related to driving energy and boost PCE.

Control of intrachain charge transfer in model systems for block copolymer photovoltaic materials.

We report the electronic properties of the conjugated coupling between a donor polymer and an acceptor segment serving as a model for the coupling in conjugated donor-acceptor block copolymers. These structures allow the study of possible intrachain photoinduced charge separation, in contrast to the interchain separation achieved in conventional donor-acceptor blends. Depending on the nature of the conjugated linkage, we observe varying degrees of modification of the excited states, including the formation of intrachain charge transfer excitons. The polymers comprise a block (typically 18 repeat units) of P3HT, poly(3-hexyl thiophene), coupled to a single unit of F8-TBT (where F8 is dioctylfluorene, and TBT is thiophene-benzothiadiazole-thiophene). When the P3HT chain is linked to the TBT unit, we observe formation of a localized charge transfer state, with red-shifted absorption and emission. Independent of the excitation energy, this state is formed very rapidly (<40 fs) and efficiently. Because there is only a single TBT unit present, there is little scope for long-range charge separation and it is relatively short-lived, <1 ns. In contrast, when the P3HT chain and TBT unit are separated by the wider bandgap F8 unit, there is little indication for modification of either ground or excited electronic states, and longer-lived charge separated states are observed.

Tunable Charge Transport Using Supramolecular Self-Assembly of Nanostructured Crystalline Block Copolymers

Electronically functionalized block copolymers, combining covalently linked p-type and n-type blocks, show switching behavior of charge transport in organic field effect transistors (OFETs). The electronically active subunits self-assemble into continuous microdomains in a nanoscale regime, thereby forming percolation channels for holes or electrons or both depending on the composition and processing conditions. Here, we establish a charge transport-morphology relation for donor-acceptor block copolymers with two crystalline blocks. The n-type and p-type blocks self-assemble into two-dimensional lattices of π-π stacks and main chain polymer lamellae, respectively, over a broad composition range. Controlling the crystallization preferences of the two blocks by thermal annealing allows controlling the OFET polarity. Depending on the block ratio, the charge transport can be tuned from p-type to n-type or p-type to ambipolar, respectively. The impact of nanostructured phase separation is further delineated by X-ray diffraction, time-resolved spectroscopy, and scanning electron microscopy studies.

Charge separation and recombination in self-organizing nanostructured donor–acceptor block copolymer films

A series of donor–acceptor diblock copolymers with varying molecular weight are studied in thin film and compared with an ‘equivalent’ blend formed from donor and acceptor homopolymers. Steady-state and transient spectroscopies are used to demonstrate a correlation between low molecular weight block copolymers and increased photoluminescence quenching (up to 99%) leading to higher yields of long-lived free charges. Such block copolymers are shown, by electron microscopy, to exhibit phase-segregated micrdomains whose size and periodicity are determined by their molecular weight. Photovoltaic devices made using these materials show a peak efficiency of 0.11% and correlate with our spectroscopic results, subject to a trade-off between charge generation and tranpsort/collection.

Polymer Crystallization as a Tool To Pattern Hybrid Nanostructures: Growth of 12 nm ZnO Arrays in Poly(3-hexylthiophene)

Well-ordered hybrid materials with a 10 nm length scale are highly desired. We make use of the natural length scale (typically 10-15 nm) of the alternating crystalline and amorphous layers that are generally found in semicrystalline polymers to direct the growth of a semiconducting metal oxide. This approach is exemplified with the growth of ZnO within a carboxylic acid end-functionalized poly(3-hexylthiophene) (P3HT-COOH). The metal-oxide precursor vapors diffuse into the amorphous parts of the semicrystalline polymer so that sheets of ZnO up to 0.5 μm in size can be grown. This P3HT-ZnO nanostructure further functions as a donor-acceptor photovoltaic system, with length scales appropriate for charge photogeneration.

Visualizing excitations at buried heterojunctions in organic semiconductor blends

Interfaces play a crucial role in semiconductor devices, but in many device architectures they are nanostructured, disordered and buried away from the surface of the sample. Conventional optical, X-ray and photoelectron probes often fail to provide interface-specific information in such systems. Here we develop an all-optical time-resolved method to probe the local energetic landscape and electronic dynamics at such interfaces, based on the Stark effect caused by electron-hole pairs photo-generated across the interface. Using this method, we found that the electronically active sites at the polymer/fullerene interfaces in model bulk-heterojunction blends fall within the low-energy tail of the absorption spectrum. This suggests that these sites are highly ordered compared with the bulk of the polymer film, leading to large wavefunction delocalization and low site energies. We also detected a 100 fs migration of holes from higher- to lower-energy sites, consistent with these charges moving ballistically into more ordered polymer regions. This ultrafast charge motion may be key to separating electron-hole pairs into free charges against the Coulomb interaction.