Rajeev Dattani

Toward Improved Lifetimes of Organic Solar Cells under Thermal Stress: Substrate-Dependent Morphological Stability of PCDTBT:PCBM Films and Devices

Morphological stability is a key requirement for outdoor operation of organic solar cells. We demonstrate that morphological stability and lifetime of polymer/fullerene based solar cells under thermal stress depend strongly on the substrate interface on which the active layer is deposited. In particular, we find that the stability of benchmark PCDTBT/PCBM solar cells under modest thermal stress is substantially increased in inverted solar cells employing a ZnO substrate compared to conventional devices employing a PEDOT:PSS substrate. This improved stability is observed to correlate with PCBM nucleation at the 50 nm scale, which is shown to be strongly influenced by different substrate interfaces. Employing this approach, we demonstrate remarkable thermal stability for inverted PCDTBT:PC70BM devices on ZnO substrates, with negligible (<2%) loss of power conversion efficiency over 160 h under 85 °C thermal stress and minimal thermally induced “burn-in” effect. We thus conclude that inverted organic solar cells, in addition to showing improved environmental stability against ambient humidity exposure as widely reported previously, can also demonstrate enhanced morphological stability. As such we show that the choice of suitable substrate interfaces may be a key factor in achieving prolonged lifetimes for organic solar cells under thermal stress conditions.

Fullerene oxidation and clustering in solution induced by light

We investigate the environmental stability of fullerene solutions by static and dynamic light scattering, FTIR, NMR and mass spectroscopies, and quantum chemical calculations. We find that visible light exposure of fullerene solutions in toluene, a good solvent, under ambient laboratory conditions results in C60 oxidation to form fullerene epoxides, and subsequently causes fullerene clustering in solution. The clusters grow with time, even in absence of further illumination, and can reach dimensions from ≈100 nm to the μm scale over ≈1 day. Static light scattering suggests that resulting aggregates are fractal, with a characteristic power law (d(f)) that increases from approximately 1.3 to 2.0 during light exposure. The clusters are bound by weak Coulombic interactions and are found to be reversible, disintegrating by mechanical agitation and thermal stress, and reforming over time. Our findings are relevant to the solution processing of composites and organic photovoltaics, whose reproducibility and performance requires control of fullerene solution stability under storage conditions.

A general mechanism for controlling thin film structures in all-conjugated block copolymer:fullerene blends

Block copolymers have the potential to self-assemble into thermodynamically stable nanostructures that are desirable for plastic electronic materials with prolonged lifetimes. Fulfillment of this potential requires the simultaneous optimisation of the spatial organisation and phase behaviour of heterogeneous thin films at the nanoscale. We demonstrate the controlled assembly of an all-conjugated diblock copolymer blended with fullerene. The crystallinity, nanophase separated morphology, and microscopic features are characterised for blends of poly(3-hexylthiophene-block-3-(2-ethylhexyl) thiophene) (P3HT-b-P3EHT) and phenyl-C61-butyric acid methyl ester (PCBM), with PCBM fractions varying from 0–65 wt%. We find that PCBM induces the P3HT block to crystallise, causing nanophase separation of the block copolymer. Resulting nanostructures range from ordered (lamellae) to disordered, depending on the amount of PCBM. We identify the key design parameters and propose a general mechanism for controlling thin film structure and crystallinity during the processing of semicrystalline block copolymers.

Microstructure and dynamics of Janus particles in a phase separating medium

The evolution of interactions and dynamics of Janus colloidal particles suspended in quasi-binary liquid mixtures undergoing phase separation is presented. The experimental system consisted of silica-nickel Janus particles dispersed in mixtures of 3-methylpyridine, water, and heavy water. Colloidal microstructure and dynamics were probed by ultra-small-angle X-ray scattering and ultra-small-angle X-ray photon correlation spectroscopy, respectively. The observed static and dynamic behaviors are significantly different from those found for Stöber silica colloids in this mixture. The Janus particles manifest a slow aggregation below the coexistence temperature and become strongly attractive upon phase separation of the solvent mixture. In the two-phase region, particles tend to display surfactant-like behavior with silica and nickel surfaces likely preferring water and 3-methylpyridine rich phases, respectively. While the onset of diffusiophoretic motion is evident in the dynamics, it is gradually suppressed by particle clustering at the investigated colloid volume fractions.

Rapid Precipitation: An Alternative to Solvent Casting for Organic Solar Cells

Rapid precipitation, immersion of a liquid formulation into a nonsolvent, is compared with drop casting for fabricating organic solar cells. Blends comprising poly-3-hexylthiophene (P3HT), phenyl-C61-butyric acid methyl ester (PCBM), and chlorobenzene were processed into bulk samples by using two distinct routes: rapid precipitation and drop casting. The resulting structure, phases, and crystallinity were analyzed by using small-angle neutron scattering, X-ray diffraction, differential scanning calorimetry, and muon spin resonance. Rapid precipitation was found to induce a finely structured phase separation between PCBM and P3HT, with 65 wt % crystallinity in the P3HT phase. In contrast, solvent casting resulted in a mixed PCBM/P3HT phase with only 43 wt % P3HT crystallinity. The structural advantages conferred by rapid precipitation were shown to persist following intense thermal treatments.