Him Cheng Wong

Performance enhancement of fullerene-based solar cells by light processing.

A key challenge to the commercialization of organic bulk heterojunction solar cells is the achievement of morphological stability, particularly under thermal stress conditions. Here we show that a low-level light exposure processing step during fabrication of blend polymer:PC60BM solar cells can result in a 10-fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerization of PC60BM that effectively hinders their diffusion and crystallization in the blend. We thus suggest that light processing may be a promising, general and cost-effective strategy to optimize fullerene-based solar cell performance. The low level of light exposure required suggests not only that this may be an easily implementable strategy to enhance performance, but also that light-induced PC60BM oligomerization may have inadvertently influenced previous studies of organic solar cell device behaviour.

Morphological Stability and Performance of Polymer–Fullerene Solar Cells under Thermal Stress: The Impact of Photoinduced PC60BM Oligomerization

We report a general light processing strategy for organic solar cells (OSC) that exploits the propensity of the fullerene derivative PC60BM to photo-oligomerize, which is capable of both stabilizing the polymer:PC60BM active layer morphology and enhancing the device stability under thermal annealing. The observations hold for blends of PC60BM with an array of benchmark donor polymer systems, including P3HT, DPP-TT-T, PTB7, and PCDTBT. The morphology and kinetics of the thermally induced PC60BM crystallization within the blend films are investigated as a function of substrate and temperature. PC60BM nucleation rates on SiOx substrates exhibit a pronounced peak profile with temperature, whose maximum is polymer and blend-composition dependent. Modest illumination (<10 mW/cm(2)) significantly suppresses nucleation, which is quantified as function of dose, but does not affect crystalline shape or growth, in the micrometer range. On PEDOT:PSS substrates, thermally induced PC60BM aggregation is observed on smaller (≈ 100 nm) length scales, depending upon donor polymer, and also suppressed by light exposure. The concurrent thermal dissociation process of PC60BM oligomers in blend films is also investigated and the activation energy of the fullerene-fullerene bond is estimated to be 0.96 ± 0.04 eV. Following light processing, the thermal stability, and thus lifetime, of PCDTBT:PC60BM devices increases for annealing times up to 150 h. In contrast, PCDTBT:PC70BM OSCs are found to be largely light insensitive. The results are rationalized in terms of the suppression of PC60BM micro- and nanoscopic crystallization processes upon thermal annealing caused by photoinduced PC60BM oligomerization.

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.

Synergetic enhancement of organic solar cell thermal stability by wire bar coating and light processing

We demonstrate that organic solar cells can exhibit different morphological and performance stability under thermal stress depending upon the processing technique employed, without compromising initial device efficiency. In particular, we investigate benchmark PCDTBT:PC60BM solar cells fabricated by wire bar coating (a technique attractive for commercial manufacture) and the more widely employed, lab scale, technique of spin coating. For this system, wire bar deposition results in superior device stability, with lifetime improvements in excess of 20-fold compared to spun cast devices. Neutron reflectivity reveals that the enhanced PC60BM segregation to the top interface in the slower, wire bar, casting process is likely responsible for the hindered PC60BM nucleation at tens of nm length scale, characterized by atomic force microscopy (AFM), and thus enhanced morphological stability. Modest light exposure of the active layer (at approximately 10 mW cm−2), known to reversibly photo-oligomerize fullerenes and thus impart morphological stability, is found to further improve device stability by a factor of 10. The combined effects of wire bar coating and light processing are highly synergetic, resulting in solar cells which are overall 200 times more stable than devices prepared by spin casting without light processing.

Nanoparticle aggregation behaviour in polymer nanocomposites: bulk vs. thin films

We report on the stability and cluster formation in C60 fullerene-polystyrene mixtures, using a combination of small angle neutron scattering, optical and atomic force microscopy. Bulk nanocomposites are found to be stable for C60 loadings up to 1-2% mass fraction in PS of Mw = 270 kg/mol and approximately 160?C. At larger fullerene concentration, the coherent scattering intensity grows with time, within 10s of min, and is well described by an asymptotic relationship with rate compatible with Arrhenius temperature dependence. The nanocomposite structure can be described by fractal scattering with a broad range of dimensions Df ~1.85-2.4 and polydisperse fractal sizes, up to hundreds of nm, depending on annealing temperature and time. Confinement in thin films of tens to hundreds of nm is shown to result in well-defined surface topographies, ranging from isolated clusters to percolated spinodal-like undulations with tuneable wavelength and amplitude. These results provide insight into the control of dispersion and self-assembly of nanoparticles in polymer matrices and thus their exploitation in functional materials, including organic photovoltaics and thin film coatings.