Effects of pressure on nano- and micro-scale morphological changes in conjugated polymer photovoltaic cells
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Egidius Rutatisbwa Rwenyagila Department of Materials Science and Engineering, African University of Science and Technology, Abuja, Nigeria
Joseph Asare Department of Theoretical and Applied Physics, African University of Science and Technology, Abuja, Nigeria
Martiale Gaetan Zebaze Kana Department of Materials Science and Engineering, Kwara State University, Ilorin, Kwara State, Nigeria
Winston O. Soboyejoa) Department of Materials Science and Engineering, African University of Science and Technology, Abuja, Nigeria; and Princeton Institute of Materials Technology (PRISM), Department of Mechanical and Aerospace Engineering, Princeton University, New Jersey, USA (Received 7 May 2016; accepted 19 August 2016)
This paper presents the results of an experimental study of the effects of pressure on polymer chain alignments in poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) blends that are used in bulk heterojunction organic photovoltaic cells (OPVs). The P3HT:PCBM blends on glass were subjected to pressure and annealing at 140 °C. The surface morphologies, nano-/micro-structures and the chain alignments were analyzed using atomic force microscopy techniques and grazing incidence x-ray scattering. The current–voltage characteristics of the resulting devices are also shown to change significantly with changes in the nano-/micro-structures. The polymer chains were aligned in the direction of the applied pressure (edge-on), which reduced the lamellae spacing between the polymer units and increased the degree of crystallinity. The increased crystallinity plays significant role in the current–voltage enhancements. The implications of the study are discussed for the design and control of the nano/ microstructures of bulk heterojunction organic solar cells.
I. INTRODUCTION
Large area solar energy conversion is possible using organic photovoltaic (OPV) cells due to the availability of low cost and sustainable energy materials sources. Blends of conjugated polymers and fullerene have been used to produce OPVs from low cost sustainable energy materials sources.1–7 OPVs with bulk heterojunction structures consisting of mixtures of P3HT polymers and fullerene based acceptors have recorded efficiency values ranging from ;6 to 7%.4,5,8 However, these efficiencies degrade with time, making it challenging to adopt OPVs in practical applications. The relatively high hole mobilities of ;10 4 cm2/(V s) (Refs. 9 and 10) and high electric field mobilities [;0.1–0.2 cm2/(V s)]11–14 of bulk heterojunction OPVs have been attributed to improved
Contributing Editor: Sarah Morgan a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.344
charge hopping characteristics on the composite bulkheterojunction structures,15 as well as the effects of active layer crystallinity.16–21 There is, therefore, a considerable interest in the development of a fundamental understanding of how the structure and morphology of P3HT:PCBM blends contribute to their electrical and optical pr
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