Abstract
We have used Soxhlet solvent purification to fractionate a broad molecular weight distribution of the polycarbazole polymer PCDTBT into three lower polydispersity molecular weight fractions. Organic photovoltaic devices were made using a blend of the fullerene acceptor PC71BM with the molecular weight fractions. An average power conversion efficiency of 5.89% (peak efficiency of 6.15%) was measured for PCDTBT blend devices with a number average molecular weight of Mn = 25.5 kDa. There was significant variation between the molecular weight fractions with low (Mn = 15.0 kDa) and high (Mn = 34.9 kDa) fractions producing devices with average efficiencies of 5.02% and 3.70% respectively. Neutron reflectivity measurements on these polymer:PC71BM blend layers showed that larger molecular weights leads to an increase in the polymer enrichment layer thickness at the anode interface, this improves efficiency up to a limiting point where the polymer solubility causes a reduction of the PCDTBT concentration in the active layer.
Highlights
We have used Soxhlet solvent purification to fractionate a broad molecular weight distribution of the polycarbazole polymer PCDTBT into three lower polydispersity molecular weight fractions
Under controlled laboratory conditions, solar cell lifetimes approaching seven years have been estimated for PCDTBT:PC71BM Organic Photovoltaic (OPV) devices; a value double that of P3HT:PCBM, making PCDTBT a favourable material for the scale-up of single or tandem-junction OPV devices[4,6,7]
The process of fractionation reduces the polydispersity and lowers the Mw relative to the unfractionated material. This resulted in a significant variation in Mn with values of 15 kDa, 21.5 kDa and 34.9 kDa respectively for the CH, CB and DCB Soxhlet fractions respectively, and provided a relatively wide range over which to test the effect of molecular weight on thin film OPV devices
Summary
Molecular weight dependent vertical composition profiles of PCDTBT:PC71BM blends for organic photovoltaics. When blended with the fullerene derivative PC71BM in a bulk-heterojunction (BHJ) architecture, power conversion efficiencies (PCEs) above 7% have been achieved[2] with open-circuit voltages VOC in the range of 0.9 V These champion efficiency metrics have since been surpassed by replacing PCDTBT with alternative low bandgap semiconductors[3], it remains an attractive polymer for practical applications due to its promising thermal stability, ease of processability and reduced susceptibility to oxidation due to a deep highest occupied molecular orbit (HOMO) level[4] when compared to polymers like PTB7, which suffer rapid photo-oxidative damage[5]. We study vertical stratification in a PCDTBT:PC71BM BHJ blend thin film as a function of the molecular weight of the copolymer This is correlated to the performance of PCDTBT:PC71BM OPV devices. Detection method, which was calibrated with a series of narrow polystyrene standards (Polymer Laboratories)
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