Matthew E. Sykes

Ultrafast Charge-Transfer Dynamics at the Boron Subphthalocyanine Chloride/C60 Heterojunction: Comparison between Experiment and Theory.

Photoinduced charge-transfer (CT) processes play a key role in many systems, particularly those relevant to organic photovoltaics and photosynthesis. Advancing the understanding of CT processes calls for comparing their rates measured via state-of-the-art time-resolved interface-specific spectroscopic techniques with theoretical predictions based on first-principles molecular models. We measure charge-transfer rates across a boron subphthalocyanine chloride (SubPc)/C60 heterojunction, commonly used in organic photovoltaics, via heterodyne-detected time-resolved second-harmonic generation. We compare these results to theoretical predictions based on a Fermis golden rule approach, with input parameters obtained using first-principles calculations for two different equilibrium geometries of a molecular donor-acceptor in a dielectric continuum model. The calculated rates (∼2 ps(-1)) overestimate the measured rates (∼0.1 ps(-1)), which is consistent with the expectation that the calculated rates represent an upper bound over the experimental ones. The comparison provides valuable understanding of how the structure of the electron donor-acceptor interface affects the CT kinetics in organic photovoltaic systems.

Recovering lost excitons in organic photovoltaics using a transparent dissociation layer

In organic photovoltaic (OPV) cells, photocurrent generation relies on exciton diffusion to the donor/acceptor heterojunction. Excitons that fail to reach the heterojunction are lost to recombination via quenching at the electrodes or relaxation in the bulk. Bulk recombination has been mitigated largely through the use of bulk heterojunctions, while quenching at the metal cathode has been previously circumvented through the introduction of exciton blocking layers that “reflect” excitons. Here, we investigate an alternative concept of a transparent exciton dissociation layer (EDL), a single layer that prevents exciton quenching at the electrode while also providing an additional interface for exciton dissociation. The additional heterojunction reduces the distance excitons must travel to dissociate, recovering the electricity-generating potential of excitons otherwise lost to heat. We model and experimentally demonstrate this concept in an archetypal subphthalocyanine/fullerene planar heterojunction OPV, generating an extra 66% of photocurrent in the donor layer (resulting in a 27% increase in short-circuit current density from 3.94 to 4.90 mA/cm2). Because the EDL relaxes the trade-off between exciton diffusion and optical absorption efficiencies in the active layers, it has broad implications for the design of OPV architectures and offers additional benefits over the previously demonstrated exciton blocking layer for photocurrent generation.

An all-conjugated gradient copolymer approach for morphological control of polymer solar cells

This work introduces fully π-conjugated gradient copolymers as promising materials to control and stabilize the nanoscale morphology of polymer:fullerene solar cells. Gradient and block sequence copolymers of 3-hexylselenophene (3HS) and 3-hexylthiophene (3HT) are utilized as the donors (D) in bulk-heterojunction (BHJ) solar cells with phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor (A). We show that for the same overall copolymer composition, the ordering of molecular constituents along the copolymer chain (copolymer sequence) significantly influences the nanoscale morphology and phase separation behavior of π-conjugated copolymer:fullerene devices. In addition, energy-filtered transmission electron microscopy (EFTEM) of the blends revealed that relative to the block copolymer:PCBM, the gradient copolymer:PCBM sample formed a more uniform, continuous and interconnected network of polymer fibrils within the acceptor-rich phase, associated with a large D/A interface. Charge extraction of photogenerated carriers by linearly increasing voltage (photo-CELIV) shows that the gradient copolymer:PCBM device possesses the highest initial carrier density, n(0) = (3.92 ± 0.3) × 1018 cm−3, consistent with a larger D/A interfacial area suggested by the observed morphology, albeit at the expense of increased carrier recombination rate. Accelerated degradation studies show that the gradient copolymer:PCBM system maintains the highest efficiency over prolonged heat treatment.

Reduction of open circuit voltage loss in a polymer photovoltaic cell via interfacial molecular design: Insertion of a molecular spacer

Loss to the open circuit voltage (Voc) in organic photovoltaic cells is a critical bottleneck to achieving high power conversion efficiency. We demonstrate that the insertion of multilayers of a poly(phenylene ethynylene) spacer into the planar heterojunction between poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester incrementally escalates the Voc of a polymer solar cell from 0.43 V to 0.9 V. Through a combination of light intensity and temperature dependent measurements, we show that this control over the molecular structure local to the interface increases Voc by raising the polaron pair energy and by suppressing the dark-diode current.

Heterodyne-detected and ultrafast time-resolved second-harmonic generation for sensitive measurements of charge transfer

In organic photovoltaics many key ultrafast processes occur at the interface between electron donor and acceptor molecules. Traditional ultrafast spectroscopies, such as pump-probe or time-resolved fluorescence, are not ideal for studying the interface because most of their signal is from the bulk material. Time-resolved second-harmonic generation (TRSHG) spectroscopy solves this problem by only generating signal from the interface. We demonstrate an optically heterodyned TRSHG to reduce the impact of stray light, enhance sensitivity, and detect the full complex signal field.

Influence of Exciton Lifetime on Charge Carrier Dynamics in an Organic Heterostructure

Interactions between charge carriers and excitons, as well as between excitons and optical cavity modes in organic optoelectronic devices are fundamental to their operational limits and chief in preventing the realization of certain phenomena, such as electrically pumped organic lasing. We uncovered a previously unreported phenomenon, wherein optical cavity-modulated exciton decay rate leads to a concomitant modulation in the electrical current of an archetypal NPD/Alq3 organic light emitting device operated in forward bias. The magnitude of this variation is sensitive to the local dielectric environment of the device and is found to be as large as 15%.