Stability issues of conjugated polymer/fullerene solar cells from a chemical viewpoint

The efficiency of energy conversion and the stability or lifetime of 'plastic' photovoltaic cells, based on conjugated polymer/ fullerene blends, are the two main issues to be improved for this type of devices. The stability of these PV cells depends potentially on a large number of factors. A brief layer-by-layer overview of these factors is given, with main emphasis on the factors possibly playing a role in the active photovoltaic layer consisting of the interpenetrating network of a conjugated polymer and a fullerene derivative. Complicated sets of photochemical processes can take place in the pure materials and in the donor-acceptor blends, both in the absence and in the presence of oxygen. Especially, photochemical [2+2] cycloaddition and cycloreversion processes have been observed for fullerene derivatives and in certain mixtures containing an oligomer and a fullerene derivative. These and other (photo) chemical processes are very likely to have an influence on the performance of the photovoltaic cell

Silole derivatives with a high and non-dispersive electron mobility, and a 100 % photoluminescence quantum efficiency

Non-dispersive and fast electron transport was realized for an amorphous, vapor deposited film of 2,5-bis(2′,2″-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopent adiene in ambient and inert atmospheres. An electron mobility of 2×10-4 cm2/Vs was measured by time-of-flight at an electric field of 6×105 V/cm. This mobility is more than two orders of magnitude larger than that of the most widely used electron transporter, tris(quinolin-8-olato) aluminum (III), in molecular organic light-emitting devices (MOLEDs). Another silole derivative, namely 1,2-bis (1-methyl-2,3,4,5,-tetraphenylsilacyclopentadienyl)ethane, exhibits bright fluorescent blue-green light with an absolute quantum yield close to 100% in the solid state. MOLEDs composed of stacked neat films of these two silole derivatives and a hole transporter show a significantly low operating voltage and an external quantum efficiency of 4.8%, close to the theoretical limit

Non-dispersive and air-stable electron transport in an amorphous organic semiconductor

We report the electron transport properties of an amorphous organic semiconductor based on silole derivatives. The observed non-dispersive and fast electron transport suggests that electron trapping due to energetic disorder is very small. Based on the mobility measurements in air we conclude that oxygen does not function as a significant electron trap. The observed excellent electron transport properties of this silole derivative are attributed to a large electron affinity originating from its σ*-π* conjugation and a high aromaticity of its anionic species. © 2001 Elsevier Science B.V

Nondispersive electron transport in Alq<inf>3</inf>

We have studied room temperature electron transport in amorphous films of tris (8-hydroxyquinolinolato) aluminum (III) (Alq3) with the time-of-flight technique. Nondispersive photocurrent transients indicate the absence of intrinsic traps in well-purified films. Exposure of the films to ambient atmosphere results in highly dispersive transport, indicating that oxygen is a likely candidate for a trapping site. The mobility was found to obey the Poole-Frenkel law. We use the correlated disorder model to determine an effective dipole moment for Alq3, and the corresponding meridional to facial isomeric ratio. © 2001 American Institute of Physics

Modeling the photocurrent of poly-phenylene vinylene/fullerene-based solar cells

We have studied the photocurrent data of 20:80 wt% blends of poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C-61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. Two cases have been investigated: When only drift of charge carriers is taken into account, a voltage-independent photocurrent is expected, corresponding to the extraction of all generated charges. It is demonstrated that the experimental data are in disagreement with this prediction. However, when both drift and diffusion of charges are taken into account, the predicted photocurrent shows a different behavior: At low electric fields a linear behavior is predicted, which results from the diffusion of charges, followed by saturation at high fields. The agreement between the numerical result and the experimental data obtained from MDMO-PPV:PCBM cells is satisfactory when a charge carrier generation rate of G=1.6 x 10(27) m(-3)s(-1) is used showing the importance of diffusion at low fields, i.e., near the open-circuit voltage

Hole transport in poly(p-phenylene vinylene) based light-emitting diodes revisited

Understanding of the charge transport properties is of great importance for the operation and the efficiency of polymer based light-emitting diodes (LEDs). We investigate the charge transport in hole-only diodes based on poly(p-phenylene vinylene) (PPV) as function of temperature T, charge carrier density p and electric field E. At low voltages the hole mobility is independent on the electric field and charge carrier density. At high voltages both the charge-carrier density and electric-field dependence of the mobility have to be taken into account to describe the hole transport in polymer LEDs

Stimulated emission from films of conjugated polymers and oligomers

Oligomers and block copolymers structurally related to PPV were investigated under intense optical excitation. Their well-defined molecular structure allows a better control of emission properties than is usually feasible in semiconducting polymers. Stimulated emission is demonstrated in single crystals of PPV-type oligomers, and also in thin films obtained by spincasting of copolymers containing PPV-type blocks. Waveguiding is shown to provide the length of interaction required for mirrorless laser generation. Thin films of oligomers obtained through deposition from the vapor phase are polycrystalline, and the optical losses in the as-deposited films are too large for lasing to be achieved. These films show stimulated emission only after the domain size has been increased by annealing. Lasing occurs within individual crystalline domains with a threshold value comparable to that found for optically clear amorphous films of conjugated polymers

Stimulated emission from films of conjugated polymers and oligomers

Oligomers and block copolymers structurally related to PPV were investigated under intense optical excitation. Their well-defined molecular structure allows a better control of emission properties than is usually feasible in semiconducting polymers. Stimulated emission is demonstrated in single crystals of PPV-type oligomers, and also in thin films obtained by spincasting of copolymers containing PPV-type blocks. Waveguiding is shown to provide the length of interaction required for mirrorless laser generation. Thin films of oligomers obtained through deposition from the vapor phase are polycrystalline, and the optical losses in the as-deposited films are too large for lasing to be achieved. These films show stimulated emission only after the domain size has been increased by annealing. Lasing occurs within individual crystalline domains with a threshold value comparable to that found for optically clear amorphous films of conjugated polymers

Electron and hole transport in poly(para-phenylene vinylene): methanofullerene bulk heterojunction solar cells

We have measured the electron and hole mobility in blends of poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C-61-butyric acid methyl ester (PCBM) with varying MDMO-PPV/PCBM composition. It is shown that the electron mobility in the PCBM-rich phase gradually increases up to 80 wt.% PCBM, due to an increased number of percolated pathways from bottom to top electrode. In contrast to the expectations the hole mobility in the MDMO-PPV phase shows a similar behavior as a function of fullerene concentration; Starting at 40 wt.% with the value of pristine MDMO-PPV the hole mobility strongly increases and saturates beyond 67 wt.% at a value which is more than two order of magnitude higher. The large enhancement of the hole mobility and its saturation is related to recent findings on the film morphology of this material system

Nonlinear optical properties of C-60 with explicit time-dependent electron dynamics

An explicit electron dynamics approach has been used to calculate the nonlinear optical properties of C-60 and its radical anion. An external perturbation, in the form of an oscillating electric field, induces the time-evolution of the molecular wavefunction. The time-averaged instantaneous dipole moment of the systems gives the molecular response to perturbations of varying field intensities and frequency of oscillation. The polarizabilities and the second-order hyperpolarizabilties have been calculated and are in good qualitative agreement with experimentally available data. In line with previous theoretical and experimental studies, the nonlinear effect is enhanced for the radical species