Thermocleavable materials for polymer solar cells with high open circuit voltage-a comparative study.

The search for polymer solar cells giving a high open circuit voltage was conducted through a comparative study of four types of bulk-heterojunction solar cells employing different photoactive layers. As electron donors the thermo-cleavable polymer poly-(3-(2-methylhexyloxycarbonyl)dithiophene) (P3MHOCT) and unsubstituted polythiophene (PT) were used, the latter of which results from thermo cleaving the former at 310 degrees C. As reference, P3HT solar cells were built in parallel. As electron acceptors, either PCBM or bis-[60]PCBM were used. In excess of 300 solar cells were produced under as identical conditions as possible, varying only the material combination of the photo active layer. It was observed that on replacing PCBM with bis[60]PCBM, the open circuit voltage on average increased by 100 mV for P3MHOCT and 200 mV for PT solar cells. Open circuit voltages approaching 1 V were observed for the PT:bis[60]PCBM solar cells and a maximum conversion efficiency of 1.3% was obtained for solar cells with P3MHOCT:PCBM as the photoactive material. For the reference solar cells maximum efficiencies of 2.1 and 2.4% were achieved for P3HT:PCBM and P3HT:bis[60]PCBM, respectively. Despite special measures taken in terms of substrate design and device processing, a substantial spread in the photovoltaic properties was generally observed. This spread could not be correlated with the optical properties of the solar cells, the thickness of the photo active layer or the electrode deposition conditions of the aluminum top electrode.

Two-dimensional simulations of CuPc-PCTDA solar cells: the importance of mobility and molecular pi stacking.

An existing two-dimensional microkinetical model for the photovoltaic effect of molecular-based solar cells has been extended to include electron-hole pair recombination between donor and acceptor sites. Simulations of the short circuit current for simple two-dimensional model heterojunction structures composed of copper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) are presented. The short circuit current was investigated as a function of the thickness of the photoactive layer for different choices of mobility for CuPc and PTCDA. The hole mobility of CuPc and/or the electron mobility of PTCDA limits the photovoltaic performance if chosen below a certain threshold determined by the net electron-hole generation rate at the CuPc-PTCDA interface. Also, the mobilities should be of the same order of magnitude. The effect of changing the interplanar separation alpha between the pi stacking molecules was investigated, and it was found that increasing alpha from 0.33 to 0.6 nm increases the short circuit current up to 5 orders of magnitude. This was rationalized in terms of the charge separation energetics of geminate electron-hole pairs and its dependence on alpha. As mobilities decrease with increasing alpha and thus opposes this effect, an optimum for alpha approximately 0.66 nm was found for the CuPc-PTCDA heterojunction model structures. The simulations are interpreted in a simple kinetic picture of an electron-hole pair generation step at the CuPc-PTCDA interface and subsequent transport in the CuPc and PTCDA domains. It is argued that an optimal device configuration involves an amorphous region at the CuPc-PTCDA interface and a gradual increase of the molecular order as the electrodes are approached in the respective CuPc and PTCDA transport regions.

Simplified charge separation energetics in a two-dimensional model for polymer-based photovoltaic cells.

An extension of our two-dimensional working model for photovoltaic behavior in binary polymer and/or molecular photoactive blends is presented. The objective is to provide a more-realistic description of the charge generation and charge separation processes in the blend system. This is achieved by assigning an energy to each of the possible occupation states, describing the system according to a simple energy model for exciton and geminate electron-hole pair configurations. The energy model takes as primary input the ionization potential, electron affinity and optical gap of the components of the blend. The underlying photovoltaic model considers a nanoscopic subvolume of a photoactive blend and represents its p- and n-type domain morphology, in terms of a two-dimensional network of donor and acceptor sites. The nearest-neighbor hopping of charge carriers in the illuminated system is described in terms of transitions between different occupation states. The equations governing the dynamics of these states are cast into a linear master equation, which can be solved for arbitrary two-dimensional donor-acceptor networks, assuming stationary conditions. The implications of incorporating the energy model into the photovoltaic model are illustrated by simulations of the short circuit current versus thickness of the photoactive blend layer for different choices of energy parameters and donor-acceptor topology. The results suggest the existence of an optimal thickness of the photoactive film in bulk heterojunctions, based on kinetic considerations alone, and that this optimal thickness is very sensitive to the choice of energy parameters. The results also indicate space-charge limiting effects for interpenetrating donor-acceptor networks with characteristic domain sizes in the nanometer range and high driving force for the photoinduced electron transfer across the donor-acceptor internal interface.

Refractive index of liquid water in different solvent models.

We present a combined molecular dynamics/quantum chemical perturbation method for calculating the refractive index of liquid water at different temperatures. We compare results of this method with the refractive index obtained from other solvent models. The best agreement with the experimental refractive index of liquid water and its temperature dependence is obtained using correlated gas-phase polarizabilities in the classical Lorentz-Lorenz expression. Also, the iterative self-consistent reaction field approach in the semicontinuum implementation matches the experimental refractive index reasonably well.