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polymer materials that possess a strong and broad absorption to harvest more sunlight, a high charge mobility, and low-lying HOMO energy levels, while keeping the band gap between 1.2 and 1.9 eV.13–15 The aforementioned criteria can be fullled only when employing a donor–acceptor (D–A) approach with alternating electron-rich and electron-decient moieties along the polymeric backbone.13,16–19 Also, the band gaps of these polymers can be easily tuned by controlling intramolecular charge transfer (ICT) between the donor and acceptor moieties. Interestingly, a common feature of many of these efficient low band gap donor–acceptor type conjugs the employment of the benzo[1,2-b:4,5-b0]dithiophene (BDT) unit. The polymers based on BDT showed exceptional performance as a common unit in PSCs that can achieve PCEs of up to 10%.20–22
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Introduction
In recent years, considerable progress has been made on the development of conjugated polymers for photovoltaic applications. Subsequently, polymer solar cells (PSCs) with a bulk heterojunction (BHJ) structure have been researched extensively.1,2 To date, BHJ solar cells, with an active layer comprising an interpenetrating network of p-type conjugated polymers and n-type fullerene derivatives, have been comprehensively studied
23760 | RSC Adv., 2016, 6, 23760–23774
This journal is © The Royal Society of Chemistry 2016
Published on 18 February 2016. Downloaded by Institute of Chemistry, CAS on 03/03/2016 01:49:27.
Ranjith Krishna Pai,*a Ahipa T. N.b and Hemavathi B.b
The rational design of conjugated polymers is crucial. As photovoltaic materials they need to be optimized for high-performance polymer solar cells (PSCs). We present a concise review of conjugated polymers based on benzodithiophenes (BDTs). In this account, we have discussed the conjugated polymeric designs of various architectures; consisting of electron rich donor and electron deficient acceptor moieties in the main chain, as well as in the side chains, to facilitate effective intra-molecular charge transfer. We summarize that the application of these polymeric materials drastically influences the power conversion efficiency (PCE) of polymeric solar cells. As of now, PCEs of over 10% are reported for these polymeric materials along with fullerene derivatives.
Cite this: RSC Adv., 2016, 6, 23760
Received 8th January 2016 Accepted 15th February 2016 DOI: 10.1039/c6ra00651e /advances
Rational design of benzodithiophene based conjugated polymers for better solar cell performance
and have displayed the highest efficiencies of over 10%. A widely used polymer as a donor material in PSCs is poly(3hexylthiophene) (P3HT),3–11 which has produced power conversion efficiencies (PCEs) of up to $5% when combined with a soluble fullerene derivative, (6,6)-phenyl C61-butyric acid methyl ester (PCBM), as the acceptor.7,12 However, further improvement of the efficiency of P3HT-based PSCs is found to be difficult because of its intrinsic limited absorption in the solar spectrum (energy band gap of 1.9 eV) and the relatively small energy difference between its highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PCBM. Hence, there is a quest for new structural entity based polymers to obtain high performance polymer solar cells and it is imperative to design new electron-donating
Dr Ranjith Krishna Pai received a Ph.D. degree in Natural Sciences, from Dr Othmar Marti’s group, Ulm University, Germany, in 2005. He is a Senior Scientist at the Ministry of Science and Technology, Department of Science and Technology, Government of India, New Delhi, India. From 2006 to 2007, he was a Postdoctoral Researcher at the University of Chile, Santiago. From 2007 to 2009, he spent two years as a Post-Doctoral Scientist at Stockholm University, Stockholm, Sweden. From 2009 to 2011, he was a Research Scientist at CFN, Brookhaven National Laboratory, New York, USA. Dr Pai spent another 2 years 4 months (2011–2013) as a Research Scientist at INL, Braga, Portugal. From 2013 to 2015, he was an Associate Professor and Group Leader at Jain University, Bangalore, India. His research interests include energy conversion technologies, including low cost photovoltaics (organic and hybrid solar cells), electrical energy storage (batteries and supercapacitors), synthesis of semiconducting polymers and polymer nanostructures and their application to organic transistors, solar cells, light emitting diodes, and other photonic applications, syntheses, characterizations and applications of carbon and inorganic nanotubes, and modelling of the electronic properties of nanostructured semiconductors. He has been awarded the outstanding scientic research output award (Science and Technology) from the Vice Chancellor of the Jain University, Bangalore on 29th September 2015. On 19th December 2015, he was selected for the “Outstanding Scientist Award” in the category of Thin Film Solar Cells (Science) by the Venus International Foundation, Chennai, India.