Design and Synthesis of Low Band Gap Ambient Stable Organic Semiconductors for Photovoltaics

Abstract

Organic photovoltaics is a fast-developing technology and has the potential to revolutionize the energy market. In the recent years, high photoconversion efficiency has been achieved through continuous efforts on design and synthesis of newer organic semiconductors. However, enormous scope of development and research in this area remains. BODIPY based organic semiconductors for examples have only recently started catching up with other class of organic materials for solar cell applications. In this thesis, therefore we focus our efforts on designing newer BODIPY materials and more importantly investigating the structure-function correlation. Understanding the structure-function correlation in this class of materials will help provide guidelines for designing newer materials with better performance. In Chapter 1, Introduction, we discuss the development of organic semiconductors and their application in solar cells reported in the literature. We find both small molecules and polymers used for both donor and acceptor applications in OPVs. While historically more research have been done on donor materials, in recent years development of acceptors to replace fullerenes is catching up. In the same chapter, we also discuss some of the examples of BODIPY based OPV materials and decide on the different aspects of structure-function correlation that we would be looking into in this thesis. In Chapter 2, we have discussed the major characterization techniques we use in the thesis, to study the new BODIPY molecules synthesized here. The results of this thesis are laid down in the subsequent 4 chapters. In Chapter 3, we look into the effect of trifluoromethyl substituent at the meso position of BODIPY on the properties of BODIPY based A-D-A molecules. We designed and synthesized two pairs of A-D-A molecules with terminal BODIPY (with either a 4-methylphenyl meso group or a 4-trifluoromethylphenyl at the meso position) and Fluorene (S1 and S2) and Benzodithiophene (S3 and S4) as the central donor unit. The effect of the trifluoromethyl group on the photophysical properties of these molecules were thoroughly characterized using steady state and femtosecond transient absorption spectroscopy. We further look into charge transfer from these molecules into PC60BM using Time resolved microwave conductivity (TRMC) measurements and study the effect of the trifluoromethyl group on the charge transfer mechanism. Based on our finding we could suggest that incorporation of the trifluoromethyl group in BODIPY based donor small molecules leads to a poorer charge transfer into fullerene acceptor. In Chapter 4, we look into the effect of trifluoromethyl substituent at the meso position of BODIPY on the properties of BODIPY based D-A polymers. We could synthesize a set of BODIPY-fluorene polymers (P7 and P8) which had sufficiently low HOMO and low band gap allowing absorption throughout the visible region of the solar spectra. Through TRMC measurements we could demonstrate that these polymers could be used as non-fullerene acceptors with PTB7-Th donor polymer for all polymer solar cells. The trifluoromethyl group is seen to clearly render better electron acceptor property to the polymer. Organic solar cells are also fabricated with the pair of BODIPY polymer (with and without trifluoromethyl group) as electron acceptors and the trifluoromethylated polymer (P8) is seen to perform better than its counterpart. In Chapter 5, we design and synthesize another set of BODIPY based polymers (P9-P12). The BODIPY subunits in these polymers have been modified by fusing thiophene to the core and hence the conjugation length is extended. This leads to a narrower band gap and the resulting polymers absorb in the NIR region up to 1200 nm. Using TRMC measurements we demonstrate that these polymers could be used as non-fullerene acceptors. These polymers (P11 and P12) can be used for development of fullerene free polymer photodetectors with NIR response. In Chapter 6, we study the effect of structural isomerism in BODIPY based D-A polymers. BODIPY can be connected to the polymer backbone through either the α or the β position. However, in literature most of the BODIPY based D-A polymers used for OPV applications are β-connected. In this chapter we design and synthesize a pair of structural isomeric BODIPY polymers (α-connected and β-connected). The properties of the α-connected (P1 and P3) and β-connected polymers (P2 and P4) are compared. The optoelectronic properties change drastically without affecting the charge carrier mobility. However, the α-connected polymers perform better than the β-connected polymers as electron acceptors. This is shown by the photovoltaic performance of all polymer solar cells fabricated with P3HT donor and the BODIPY polymers a well as by Time resolved photoluminescence spectroscopy. In summary (Chapter 7), we have designed and synthesized a library of new BODIPY based organic semiconductors. We have also demonstrated for the first time, the possibility of using BODIPY based polymers as non-fullerene acceptors. More importantly, we have investigated three major aspects of structure function correlation in this class of materials, previously overlooked. The results from this thesis help us conclude that incorporation of trifluoromethyl group improved the electron mobility in BODIPY polymers. Also, we see that α-connected polymers and not β-connected polymers could perform better for photovoltaic applications