A New Polythiophene Derivative for High Efficiency Polymer Solar Cells with PCE over 9%

Abstract

compared with that of [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) or [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) (−3.90 eV), then the PSCs based on P3HT:IC 70 BA shows the improved PCE of ≈7.4% with increased V oc of 0.87 V compared to the PSCs based on P3HT:PC 71 BM (PCE of ≈4.4% and V oc of 0.61 V).[23] Another strategy is to design and synthesize new PT derivatives with deep HOMO energy level by modification of side chains. [3,25,32,33] For instance, Zhang et. al. synthesized a PT derivative, PDCBT, by attaching electron-withdrawing carboxylate substituents on PT backbone, and the HOMO level of PDCBT (−5.26 eV) decreased by 0.36 eV in comparison with P3HT (−4.90 eV). [25] As a result, the corresponding PSCs showed an enhanced PCE of 7.2% with increased V oc of 0.91 V. In addition, to construct the D-A type PT derivatives by introducing the thiophene-based acceptor units has been proved to be an effective way to modulate the energy levels of PT derivatives. [24,34] The PT derivative PBT1 with the thiophene fused benzodithiophene-4,8-dione (BDD) acceptor unit was reported and showed a lower HOMO level (−5.13 eV), and an improved PCE of 6.88% was achieved with V oc of 0.83 V in the PBT1-based PSCs. [24] More interestingly, this method can simultaneously reduce the bandgap of PT derivatives, resulting in the broadening of absorption, which benefits for increasing J sc and PCE. Therefore, the performance of PSCs based on the PT derivatives with D-A structure still has great room for improvement.Recently, fluorination has attracted much attention due to its multiple effects on the photophysical properties of conjugated polymers, i.e., the strong electron-withdrawing ability of fluorine atom can effectively modulate the energy levels of polymers with minor effect on their bandgaps and induce the strong dipole along C F bond, resulting in the strong inter/ intramolecular interaction which benefits the excitons dissociation and charge carries transport. [35][36][37][38][39][40][41][42][43][44][45] Hence, in this work, we designed and synthesized a novel D-A structured PT derivative, PBDD-ff4T, based on BDD as acceptor unit and difluorinated bithiophene (ff2T) as donor unit. By introducing two strong electron-withdrawing fluorine atoms to the bithiophene unit in the polymer backbone, PBDD-ff4T showed lower HOMO level of −5.42 eV, smaller optical bandgap (E g opt ) of 1.74 eV, and a higher crystallinity compared to the analog polymer PBT1 without fluorine substituent (HOMO = −5.13 eV and E g opt = 1.77 eV). [24] The lower HOMO level and smaller bandgap could be beneficial for improving the V oc and J sc in PSCs. As a result, the PBDD-ff4T/PC 71 BM-based PSCs under optimal conditions exhibit the highest PCE of 9.2% with V oc of 0.95 V, J sc of 13.2 mA cm −2 , and FF of 73%. This value of PCE is one of the highest values reported in the literature so far for the PSCs As a promising next generation energy source technology, the bulk heterojunction (BHJ) polymer solar cells (PSCs) based on conjugated ...

Direct arylation polymerization toward efficient synthesis of benzo[1,2‐c:4,5‐c'] dithiophene‐4,8‐dione based donor‐acceptor alternating copolymers for organic optoelectronic applications

“…The former enhances the flexibility of the polymer chains due to their non‐rigid structure. These structural differences have been especially highlighted in previous reports of BDTD ‐quarterthiophene polymers which are analogues of P3 .…”

“…BDTD has a rigid and planar structure which enhances interchain packing and is beneficial in tuning the energy levels of polymers by pairing with appropriate comonomers . However, majority of BDTD polymers have been synthesized via Stille coupling which requires tedious pre‐functionalization of monomers with highly toxic organotin intermediates.…”

Fluorinated Photovoltaic Materials for High‐Performance Organic Solar Cells

“…Thiophene is the basic unit of organic photovoltaic materials, and its fluorination is one of the most studied types . Beaujuge et al.…”

“…Compared with its non‐fluorinated analogue, PBDT(T)2F obtained a higher PCE of 9.8 %, owing to improved V oc , short‐circuit current density ( J sc ), and fill factor (FF) values in OSCs. Moreover, the 3,3′‐difluoro‐2,2′‐bithiophene (ff2T) unit has emerged for the design of high‐performance photovoltaic materials . Compared with the non‐fluorinated 2T unit, the ff2T unit shows improved planarity and a more stable conformation, owing to the reduced dihedral angle and increased rotational energy barrier of these thienyl rings.…”

“…[11,12] Therefore, molecular fluorination will significantly affect the photoelectronic properties of materials.F or example, comparedw ith their non-fluorinated analogues,f luorinatedm olecules show improved planarity,e nhanced extinctionc oefficients, better molecular orientation, higher chargec arrierm obility,a nd improvedm orphological miscibility. [13][14][15] For these reasons, great effortsh ave been devoted to the design and synthesis of fluorinated photovoltaic materials and their applicationsi n OSCs. [10][11][12][13][14][15][16][17] Recently,O SCs have achieved aP CE of over 16 %, owing to thes ynergistic effect of fluorination on both donor and acceptor materials.…”