Indacenodithiophene-co-benzothiadiazole Copolymers for High Performance Solar Cells or Transistors via Alkyl Chain Optimization

Abstract

I nitially, the role of an alkyl chain on a conjugated polymer was to confer solubility of the rigid-rod-like structure in common organic solvents. It is now apparent that the choice of solubilizing chain can vastly affect the properties of the polymer, as it can be used to tune a materials' crystallinity in the solid state as well as its miscibility with other materials in thin film blends. The latter of these is of extreme importance in the field of OPV where the extent of phase separation between a donor conjugated polymer and a fullerene acceptor is a critical factor in determining solar cell performance. 1À5 Yang et al. studied the effects of side chain variation on a naphtho[2,1-b:3,4-b 0 ]dithiophene-co-4,7-di(thiophen-2-yl)benzothiadiazole system 6 and observed differences in the V oc and J sc . These were quantitatively correlated with a pre-exponential dark current term, which they claimed accounts for the intermolecular interactions in the polymer/fullerene blend. Biniek et al. studied the effect of side chain variation and site of attachment on a benzo[1,2-b:4,5-b 0 ] dithiophene-co-thieno[3,4-b]thiophene polymer. 7 They found an increased solar cell efficiency using ethylhexyl chains over linear dodecyl chains. Szarko et al. very recently reported on the effect of side chain variation on thieno[3,-b]thiophene-co-benzodithiophene polymer systems, which have given record breaking (∼8%) PCEs. 8 They found that varying the degree of alkyl chain branching affected the πÀπ stacking distance which they suggested allowed a varying amount of fullerene intercalation to occur. A study on the variation of the side chain on the acceptor portion of a Nalkylthieno[3,4-c]pyrrole-4,6-dione polymer demonstrated that the highest solar cell efficiency was achieved with a linear chain. 9 Recently, we reported the synthesis and high hole mobility of an indacenodithiophene-co-benzothiadiazole (IDTÀBT) copolymer with hexadecyl alkyl chains P4. 10 The long linear alkyl chains were attributed to impart some degree of crystallinity in thin film. Chen et al. demonstrated the high performance of a similar IDTÀBT copolymer bearing phenylhexyl side chains in solar cell devices. 11 Replacement of benzothiadiazole on this system with alternative acceptors has also yielded efficient solar cells. 12 We were interested in investigating the role the solubilizing chains play in the performance of this low-band-gap polymer in both TFT and OPV devices. The effect of varying the alkyl chain from short and branched (methylbutyl) to long and linear (hexadecyl) on the optical and electronic properties is presented here.Copolymers P1ÀP4 were synthesized via Suzuki couplings using previously reported conditions. 10 It was not possible to achieve high molecular weights with the methylbutyl side chains P1 due to precipitation of the polymer during polymerization, a consequence of its lower solubility. The remaining polymers were synthesized in high molecular weights, as expected, due to the enhanced solubilizing effect of the longer alkyl chains...

Role of Polymer Fractionation in Energetic Losses and Charge Carrier Lifetimes of Polymer: Fullerene Solar Cells

“…Recently, indacenodithiophene (IDT) copolymers have shown promising field-effect transistor mobilities and solar cell performance. [18][19][20][21][22] Enhanced co-planarity of the polymer backbone has been hypothesized to lead to reduced energetic disorder. 23,24 IDT-co-benzothiadiazole polymers have achieved up to 6.5% PCE with device optimization and material purification.…”

Exploring the origin of high optical absorption in conjugated polymers

“…To test the proposal that persistence length dominates optical extinction in solution we identified additional polymers with long monomers and high expected co-linearity (small mon), namely, an indacenodithiophene-co-benzothiadiazole polymer IDTBT 35 We now address the MW dependence of the extinction of DPP-TT-T. Examining the gel permeation chromatography data, we find that for both low and high MW fractions, the majority of the MW distributions lie at MWs beyond the point where the calculated specific extinction begins to saturate (Figure S1.1).…”

Naphthacenodithiophene Based Polymers—New Members of the Acenodithiophene Family Exhibiting High Mobility and Power Conversion Efficiency

“…[14][15][16][17] In addition to this backbone rigidity, alkyl chains can be easily attached and varied, having both a profound effect on performance, but also dramatically increasing solubility, thus facilitating dissolution in nonchlorinated solvents. [19] Although the transport properties of this polymer class are outstanding, solar cell device performance has remained modest, with less than 7% PCE achieved. [20] In order to simultaneously widen the bandgap and lower the HOMO energy level, the central phenyl unit of IDT was replaced with a naphthyl unit.…”