Department of Physics; Institute of Advanced Material
A detailed investigation of the impact of molecular weight distribution of a photoactive polymer, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), on photovoltaic device performance and carrier transport properties is reported. It is found that different batches of as-received polymers have substantial differences in their molecular weight distribution. As revealed by gel permeation chromatography (GPC), two peaks can generally be observed. One of the peaks corresponds to a high molecular weight component and the other peak corresponds to a low molecular weight component. Photovoltaic devices fabricated with a higher proportion of low molecular weight component have power conversion efficiencies (PCEs) reduced from 5.7% to 2.5%. The corresponding charge carrier mobility at the short-circuit region is also significantly reduced from 2.7 × 10-5 to 1.6 × 10-8 cm2 V-1 s-1. The carrier transport properties of the polymers at various temperatures are further analyzed by the Gaussian disorder model (GDM). All polymers have similar energetic disorders. However, they appear to have significant differences in carrier hopping distances. This result provides insight into the origin of the molecular weight effect on carrier transport in polymeric semiconducting materials. Batch-to-batch variation of the photovoltaic performance of devices based on commercial samples of the polymer poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5- (4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) is reported, with efficiency ranging from 5.7% to 2.5%. As revealed by gel permeation chromatography, bimodal distributions are observed in the molecular weight. Charge transport data suggest that low molecular weight components increase the average hopping distance, resulting in lower mobility and poorer photovoltaic performance.
charge transport, hopping distance, mobility, molecular weight, photovoltaic devices, polymeric materials
Source Publication Title
Advanced Energy Materials
This is the peer reviewed version of the following article: Lee, Harrison Ka Hin, Zhao Li, Iordania Constantinou, Franky So, Sai Wing Tsang, and Shu Kong So. "Batch-to-batch variation of polymeric photovoltaic materials: Its origin and impacts on charge carrier transport and device performances." Advanced Energy Materials 4.16 (2014): 14007681, which has been published in final form at http://dx.doi.org/10.1002/aenm.201400768. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.
Shu Kong So & Harrison Ka Hin Lee acknowledge supports from the Research Grant Council under grants #211412E and the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. [T23-713/11]). The GIXRD used in this work was supported by the Institute of Advanced Materials with funding from the Special Equipment Grant SEG-HKBU06. Sai Wing Tsang acknowledges the funding support from CityU start-up grant #7200372.
Link to Publisher's Edition
Lee, Harrison Ka Hin, Zhao Li, Iordania Constantinou, Franky So, Sai Wing Tsang, and Shu Kong So. "Batch-to-batch variation of polymeric photovoltaic materials: Its origin and impacts on charge carrier transport and device performances." Advanced Energy Materials 4.16 (2014): 14007681.