Publications

Durch Selbstorganisation des Blockcopolymers entsteht für die Ladungstrennung eine nanostrukturierte Grenzfläche. Eine Solarzelle aus diesem Blockcopolymer weist einen zehnfach höheren Kurzschlussstrom und eine beträchtlich höhere Leerlaufspannung als das analoge Polymerblendsystem auf. Diese beachtliche Steigerung kann mit dem nanostrukturierten Bulk-Heteroübergang korreliert werden, der in Blockcopolymeren gebildet wird. (siehe TEM-Querschnitt einer Solarzelle).

A flexible organic dyad consisting of a perylene bisimide antenna covalently linked to a [60]fullerene has been synthesized and studied by electrochemistry, steady-state spectroscopy, and time-resolved spectroscopy. We found that the energy absorbed by the perylene bisimide is transferred to the fullerene with an efficiency close to 100%. The fullerene in turn undergoes intersystem crossing followed by triplet energy transfer back to the perylene bisimide with an efficiency of at least 20%. Hence the perylene bisimide unit acts as an antenna for the fullerene, i.e., effectively extending the fullerene absorption far into the visible spectral range, while at the same time the fullerene acts as a triplet sensitizer for the perylene bisimide. This has severe consequences for the exploitation of the dye antenna-fullerene concept for light harvesting in solar cells.

Unlike the conventional p-doping of organic semiconductors (OSCs) using acceptors, here, an efficient doping concept for diketopyrrolopyrrole-based polymer PDPP[T]₂ -EDOT (OSC-1) is presented using an oxidized p-type semiconductor, Spiro-OMeTAD(TFSI)₂ (OSC-2), exploiting electron transfer from HOMO_OSC-1 to HOMO_OSC-2 . A shift of work function toward the HOMO_OSC-1 upon doping is confirmed by ultraviolet photoelectron spectroscopy (UPS). Detailed X-ray photoelectron spectroscopy (XPS) and UV-vis-NIR absorption studies confirm HOMO_OSC-1 to HOMO_OSC-2 electron transfer. The reduction products of Spiro-OMeTAD(TFSI)₂ to Spiro-OMeTAD(TFSI) and Spiro-OMeTAD is also confirmed and their relative amounts in doped samples is determined. Mott-Schottky analysis shows two orders of magnitude increase in free charge carrier density and one order of magnitude increase in the charge carrier mobility. The conductivity increases considerably by four orders of magnitude to a maximum of 10 S m^-1 for a very low doping ratio of 8 mol%. The doped polymer films exhibit high thermal and ambient stability resulting in a maximum power factor of 0.07 µW m^-1 K^-2 at a Seebeck coefficient of 140 µV K^-1 for a very low doping ratio of 4 mol%. Also, the concept of HOMO_OSC-1 to HOMO_OSC-2 electron transfer is a highly efficient, stable and generic way to p-dope other conjugated polymers.

The dependence of zero field charge carrier mobilities obtained from time-of-flight (TOF) measurements for a series of doped triphenyldiamines on trap depth and concentration were determined. The influence of these mobilities on the solar cell characteristics have been studied by measuring I–V characteristics and IPCE values using the same doped hole conductors in solid-state dye-sensitized TiO2 solar cells. The traps exhibit only a weak influence on the solar cell characteristics, because the traps which are responsible for low charge transport mobilities in the TOF experiment are filled under steady state illumination conditions. In consequence the effective mobility is raised almost to that of the pristine hole conductor, thus allowing the use of doped hole conductor systems in solar cells without any adverse effects.

In an endeavor to correlate the optoelectronic properties of π-conjugated polymers with their structural properties, we investigated the aggregation of P3HT in THF solution within a temperature range from 300 to 5 K. By detailed steady-state, site-selective, and time-resolved fluorescence spectroscopy combined with Franck–Condon analyses, we show that below a certain transition temperature (265 K) aggregates are formed that prevail in different polymorphs. At 5 K, we can spectroscopically identify two H-type aggregates with planar polymer backbones yet different degree of order regarding their side chains. Upon heating, the H-character of the aggregates becomes gradually eroded, until just below the transition temperature the prevailing “aggregate” structure is that of still phase-separated, yet disordered main and side chains. These conclusions are derived by analyzing the vibrational structure of the spectra and from comparing the solution spectra with those obtained from thin films that were cooled slowly from the melting temperature to room temperature and that had been analyzed previously by various X-ray techniques. In addition, site selectively recorded fluorescence spectra show that there is—dependent on temperature—energy transfer from higher energy to lower energy aggregates. This suggests that they must form clusters with dimensions of the exciton diffusion length, i.e., several nanometers in diameter.

The backbone conformation of conjugated polymers affects, to a large extent, their optical and electronic properties. The usually flexible substituents provide solubility and influence the packing behavior of conjugated polymers in films or in bad solvents. However, the role of the side chains in determining and potentially controlling the backbone conformation, and thus the optical and electronic properties on the single polymer level, is currently under debate. Here, we investigate directly the impact of the side chains by studying the bulky-substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) and the common poly(3-hexylthiophene) (P3HT), both with a defined molecular weight and high regioregularity, using low-temperature single-chain photoluminescence (PL) spectroscopy and quantum-classical simulations. Surprisingly, the optical transition energy of PDOPT is significantly (∼2,000 cm^-1 or 0.25 eV) red-shifted relative to P3HT despite a higher static and dynamic disorder in the former. We ascribe this red shift to a side-chain induced backbone planarization in PDOPT, supported by temperature-dependent ensemble PL spectroscopy. Our atomistic simulations reveal that the bulkier 2,5-dioctylphenyl side chains of PDOPT adopt a clear secondary helical structural motif and thus protect conjugation, i.e., enforce backbone planarity, whereas, for P3HT, this is not the case. These different degrees of planarity in both thiophenes do not result in different conjugation lengths, which we found to be similar. It is rather the stronger electronic coupling between the repeating units in the more planar PDOPT which gives rise to the observed spectral red shift as well as to a reduced calculated electron-hole polarization.

Novel semiconductor block copolymers were synthesized using nitroxide-mediated radical polymerization (NMRP). They are comprised of a hole conductor block carrying tetraphenylbenzidine pendant units (PVDMTPD) and a second poly(4-vinylpyridine) (P4VP) block suitable for the preferential incorporation of n-type semiconductor nanocrystals. The conditions of NMRP for both monomers were optimized in order to get macroinitiators with well-defined molecular weights and very low polydispersity (<1.2). The resulting block polymers exhibit a lamellar morphology due to microphase separation. Furthermore, semiconductor nanocomposites were prepared using these diblock copolymers and light harvesting CdSe:Te nanocrystals, and their bulk morphologies were characterized by TEM. This new hybrid nanocomposite material maintains the original lamellar structure in which the hole conductor domains are separated from electron conducting/light harvesting nanocrystals that are confined in the P4VP domains. Thus, the challenging task of applying the block copolymer strategy to obtain fully functionalized semiconductor hybrid nanocomposites with morphological control and stability has been realized.

Altering the sample temperature in a photorefractive material changes the rotational mobility of the chromophores. A change of three orders of magnitude in the response times over a temperature change of 12 K has been observed. In the photorefractive experiment, however, the chromophore orientation is induced by the non-instantaneous change of the space charge field. The finite speed of the latter causes the chromophore answer to be different from their normal relaxation behaviour to an instantaneous change. This effect is most pronounced when both time constants are in the same range.