Bingyuan Huang

Effect of thickness-dependent microstructure on the out-of-plane hole mobility in poly(3-hexylthiophene) films.

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a widely used donor material for bulk heterojunction polymer solar cells. While much is known about the structure and properties of RR-P3HT films, important questions regarding hole mobilities in this material remain unresolved. Measurements of the out-of-plane hole mobilities, μ, of RR-P3HT films have been restricted to films in the thickness regime on the order of micrometers, beyond that generally used in solar cells, where the film thicknesses are typically 100 to 200 nm. Studies of in-plane carrier mobilities have been conducted in thinner films, in the thickness range 100-200 nm. However, the in-plane and out-of-plane hole mobilities in RR-P3HT can be significantly different. We show here that the out-of-plane hole mobilities in neat RR-P3HT films increase by an order of magnitude, from 10(-4) cm(2)/V·s, for a 80 nm thick film, to a value of 10(-3) cm(2)/V·s for films thicker than 700 nm. Through a combination of morphological characterization and simulations, we show that the thickness dependent mobilities are not only associated with the differences between the average morphologies of thick films and thin films, but specifically associated with changes in the local morphology of films as a function of distance from the interfaces.

Nanoscale orientation effects on carrier transport in a low-band-gap polymer

We show that due to a substrate-induced orientation of the local morphology of thin supported conjugated polymer films of poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-...

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.