Elizabeth von Hauff

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

Polymeric semiconductors are materials where unique optical and electronic properties often originate from a tailored chemical structure. This allows for synthesizing conjugated macromolecules with ad hoc functionalities for organic electronics. In photovoltaics, donor-acceptor co-polymers, with moieties of different electron affinity alternating on the chain, have attracted considerable interest. The low bandgap offers optimal light-harvesting characteristics and has inspired work towards record power conversion efficiencies. Here we show for the first time how the chemical structure of donor and acceptor moieties controls the photogeneration of polaron pairs. We show that co-polymers with strong acceptors show large yields of polaron pair formation up to 24% of the initial photoexcitations as compared with a homopolymer (η=8%). π-conjugated spacers, separating the donor and acceptor centre of masses, have the beneficial role of increasing the recombination time. The results provide useful input into the understanding of polaron pair photogeneration in low-bandgap co-polymers for photovoltaics.

Correlation between charge transfer exciton recombination and photocurrent in polymer/fullerene solar cells

We correlate carrier recombination via charge transfer excitons (CTEs) with the short circuit current, Jsc, in polymer/fullerene solar cells. Near infrared photoluminescence spectroscopy of CTE in three blends differing for the fullerene acceptor, gives unique insights into solar cell characteristics. The energetic position of the CTE is directly correlated with the open-circuit voltage, Voc, and more important Jsc decreases with increasing CTE emission intensity. CTE emission intensity is discussed from the perspective of blend morphology. The work points out the fundamental role of CTE recombination and how optical spectroscopy can be used to derive information on solar cell performances.

How intermolecular geometrical disorder affects the molecular doping of donor-acceptor copolymers

Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. However, the widely reported low efficiency of doping remains a crucial limitation to obtain high performance. Here we investigate how charge transfer between dopant and donor-acceptor copolymers is affected by the spatial arrangement of the dopant molecule with respect to the copolymer repeat unit. We p-dope a donor-acceptor copolymer and probe its charge-sensitive molecular vibrations in films by infrared spectroscopy. We find that, compared with a related homopolymer, a four times higher dopant/polymer molar ratio is needed to observe signatures of charges. By DFT methods, we simulate the vibrational spectra, moving the dopant along the copolymer backbone and finding that efficient charge transfer occurs only when the dopant is close to the donor moiety. Our results show that the donor-acceptor structure poses an obstacle to efficient doping, with the acceptor moiety being inactive for p-type doping.

Investigations of the effects of tempering and composition dependence on charge carrier field effect mobilities in polymer and fullerene films and blends

In this study we report on field effect mobilities of charge carriers in materials used in polymer photovoltaics. The field effect mobilities of charge carriers in polymer and fullerene films and in various blend compositions were investigated for the effects of tempering. Contact resistances were considered for more accurate mobility estimations. The field effect mobilities of holes in films of the conjugated polymer poly(3-hexylthiophene) (P3HT) were found to be in the 10−3cm2∕Vs range and the field effect mobilities of electrons in films of the methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were found to be in the order of 10−2cm2∕Vs. The concentration of PCBM was then varied in P3HT:PCBM blends, and blend compositions with PCBM contents of 0%, 33%, 40%, 50%, 60%, 67%, and 100% weight percents were measured before and after tempering. Electron mobilities were not detected in untempered blend compositions with PCBM content lower than 67% PCBM. Tempering led to a large increase in the...

Current-limiting mechanisms in polymer diodes

We examined the current-voltage (JV) characteristics of poly[2-methoxy,5-(3,7-dimethyloctyloxy)]- 1,4phenylenevinylene (MDMO-PPV) diodes in the framework of the model for space-charge- limited currents (SCLCs). The thickness of the MDMO-PPV layer was varied between samples, and the effect of using different metals for the cathode on the device performance was investigated. Since the SCLC model alone could not explain the experimental data, modifications to it were individually investigated: (1) a double Gaussian density of states to account for deep traps, (2) injection of electrons from the back contact resulting in a recombination current, and (3) the influence of the built-in potential resulting from the asymmetric work functions of the electrodes. Although the JV curves of organic diodes are often discussed in literature in the context of SCLC in the presence of traps, the introduction of deep traps into the SCLC model could not explain the observed cathode dependence. Considering recombination proces...

High-Permittivity Conjugated Polyelectrolyte Interlayers for High-Performance Bulk Heterojunction Organic Solar Cells

Conjugated polyelectrolyte (CPE) interfacial layers present a powerful way to boost the I-V characteristics of organic photovoltaics. Nevertheless, clear guidelines with respect to the structure of high-performance interlayers are still lacking. In this work, impedance spectroscopy is applied to probe the dielectric permittivity of a series of polythiophene-based CPEs. The presence of ionic pendant groups grants the formation of a capacitive double layer, boosting the charge extraction and device efficiency. A counteracting effect is the diminishing affinity with the underlying photoactive layer. To balance these two effects, we found copolymer structures containing nonionic side chains to be beneficial.

Detailed investigation of the conducting channel in poly(3-hexylthiophene) field effect transistors

In this study, the conducting channel in poly(3-hexylthiophene) (P3HT) organic field effect transistors (OFETs) was investigated. The effect of varying the P3HT layer thickness on the OFET parameters was studied. The threshold voltage and the field effect mobility were determined from both the linear and saturation regime of the OFET output characteristics for all film thicknesses and the results are compared and discussed. A gated four probe technique was used to investigate the formation and evolution of the conducting channel by monitoring changes in potential at different points in the channel during measurement. It was found that the device performance of the OFETs was significantly influenced by the thickness of the P3HT layer. Bulk currents were found to dominate device performance for thicker P3HT layers.

Spectral signatures of polarons in conjugated co-polymers.

We study electronic and optical properties of the low-bandgap co-polymer PCPDT-BT (poly-cyclopentadithiophene-co-benzothiadiazole) and compare it with the corresponding homo-polymer PCPDT (poly-cyclopentadithiophene). We investigate the linear absorptivity in these systems for neutral molecules and for their singly charged ions based on quantum-chemical calculations and experiments. One of our main findings is that the ions of the homo-polymer show a polaron absorption that is symmetric between anion and cation, whereas for polaron excitations in the co-polymer this symmetry is strongly lifted. We demonstrate that this asymmetry can be attributed to the absence of symmetry between the high-lying occupied and low-lying unoccupied molecular orbitals in the co-polymer with type-II orbital alignment between the moieties constituting the chain. This notion is of importance for the qualitative and quantitative interpretation of spectroscopic polaron data and is not specific to the system studied here but similarly applies to other co-polymers.

Increasing organic solar cell efficiency with polymer interlayers

We demonstrate how organic solar cell efficiency can be increased by introducing a pure polymer interlayer between the PEDOT:PSS layer and the polymer:fullerene blend. We observe an increase in device efficiency with three different material systems over a number of devices. Using both electrical characterization and numerical modeling we show that the increase in efficiency is caused by optical absorption in the pure polymer layer and hence efficient charge separation at the polymer bulkheterojunction interface.

Investigations of electron injection in a methanofullerene thin film transistor

In this study we investigate charge injection into a methanofullerene. The temperature and electric field dependent source-drain currents from contact limited [6,6]-phenyl C61-butyric acid methyl ester (PCBM) thin film transistors (TFTs) were analyzed. A form for the temperature and field dependent behavior of the parasitic contact resistances between metal and semiconductor was proposed based on a diffusion limited thermionic emission (DLTE) injection current and accounting for the disorder in the system. The temperature dependent current-voltage characteristics were initially modeled with a model for field effect behavior in amorphous organic semiconductors from the literature to determine material parameters. The initial fit resulted in discrepancies between the experimental data and the data predicted by the model. Modifying the model to account for the effects of contact resistances led to much better agreement between the experimental and predicted data. Based on these results, the DLTE injection cu...