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Effect of crystallisation on the electronic energy levels and thin film morphology of P3HT:PCBM blends.

Tsoi, W C; Spencer, S J; Yang, L; Ballantyne, A M*; Nicholson, P G; Turnbull, A; Shard, A G; Murphy, C E; Bradley, D D C*; Nelson, J*; Kim, J S* (2011) Effect of crystallisation on the electronic energy levels and thin film morphology of P3HT:PCBM blends. Macromolecules, 44 (8). pp. 2944-2952.

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Abstract

Crystallization of regio-regular poly(3-hexylthiophene) (RR-P3HT) is crucial for the high device efficiency of RR-P3HT: [6,6]-phenyl C61 butyric acid methyl ester (RR-P3HT:PCBM) solar cells. However, there is a lack of systematical study to clarify the effects of the crystallization to the frontier electronic energy levels and thin film morphology which are important factors to determine the open circuit voltage (Voc), charge separation and charge transport in P3HT:PCBM devices. Here, by comparing crystalline RR-P3HT with amorphous regio-random P3HT using ultraviolet and (angle-resolved) X-ray photoelectron spectroscopy techniques, together with absorption, photoluminescence (PL) and optical microscopy, we show that the crystallization of P3HT reduces the highest occupied molecular orbital (HOMO) energy level with respect to the vacuum energy level and that the crystallization of P3HT occurs preferentially at the film-air interface. This preferential crystallization of P3HT at the film-air interface leads to an unfavorable vertical-like phase separation between P3HT and PCBM molecules allowing the formation of micron-size PCBM aggregates. The reduction in HOMO level of P3HT is expected to decrease Voc and increase hole trapping at the polymer/electrode interface. Suppressing this crystallization of P3HT at the film-air interface will therefore be an important factor to improve the device efficiency of P3HT:PCBM solar cells. Our result also show that both crystalline and amorphous phases of P3HT films have their own contributions to the device efficiency of P3HT:PCBM solar cells, so it is also important to quantify and optimize the relative ratio of these two components with different phases of P3HT in order to further improve the device efficiency.

Item Type: Article
Keywords: polymer solar cells, crystallization, electronic structure, film morphology, photoelectron spectroscopy
Subjects: Advanced Materials
Advanced Materials > Photovoltaics
Identification number/DOI: 10.1021/ma102841e
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/4962

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