Influence of the Electron Deficient Co‐Monomer on the Optoelectronic Properties and Photovoltaic Performance of Dithienogermole‐based Co‐Polymers

Abstract

particularly successful strategy has been the donor-acceptor approach, in which an electron rich donor is co-polymerized with an electron defi cient acceptor. The co-polymerization has shown to assist the reduction of the band gap by molecular orbital hybridization. [1][2][3][4][5] Computational density functional theory (DFT) calculations on donor-acceptor co-polymers have typically shown that the HOMO is delocalized over the conjugated backbone whilst the LUMO is more strongly localized on the acceptor co-monomer. [6][7][8] Therefore, tuning of either the electron rich or electron defi cient unit is an effective method to engineer the band gap and energetic levels of the resulting co-polymers.Electron [9][10][11][12][13] Previously, we have reported the synthesis of high molecular weight PDTG-BT and demonstrated PCE of 4.5% in OPV devices with promisingly high short circuit currents ( J sc ) of 18.6 mA cm −2 after annealing. [ 13 ] At the same period of time, other groups have also reported the synthesis of lower molecular weight PDTG-BT that exhibits somewhat The PCE of PDTG-PT is further improved to 6.6% when the device architecture is modifi ed from normal to inverted. Therefore, PDTG-PT is an ideal candidate for application in tandem solar cells confi guration due to its high effi ciency at very low band gaps ( E g opt = 1.32 eV). Finally, the 6.6% PCE is the highest reported for all the co-polymers containing bridged bithiophenes with 5-member fused rings in the central core and possessing an E g opt below 1.4 eV.

Germanium‐ and Silicon‐Substituted Donor–Acceptor Type Copolymers: Effect of the Bridging Heteroatom on Molecular Packing and Photovoltaic Device Performance

“…[13][14][15] By substituting the bridging carbon atom in PCP-DTBT with Ge, the analogous Ge substituted polymer poly[(4,4′bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]germole)-2,6-diyl-alt-(2,1,3benzothiadiazole)-4,7-diyl] (PGeBTBT, Scheme 1 b) was demonstrated to exhibit a reduced optical band gap. [ 13,14,16,17 ] We also found that the incorporation of the bridging Ge atom resulted in much higher stability to aqueous base than the Si analogue. [ 13 ] This was important because we were able to produce highly purifi ed monomers which afforded high molecular weight polymer by a Suzuki cross-coupling procedure.…”

Marriage of heavy main group elements with π-conjugated materials for optoelectronic applications

“…4; 21-41). [57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] The dithienogermole containing polymers are generally obtained by metal-catalyzed cross-coupling methodologies such as the Stille, Suzuki-Miyaura and Sonogashira reactions, and very high number average molecular weights (M n ) of up to 133000 g mol -1 have been occasionally reported. Due to d-block contraction, 73,74 the electronegativity of germanium is slightly closer to carbon than the related silicon species, 75 therefore reducing the polarization of the Ge-C bond and rendering aryl germanes more tolerable towards bases and nucleophiles than the related aryl silanes.…”

A fluorine-induced high-performance narrow bandgap polymer based on thiadiazolo[3,4-c]pyridine for photovoltaic applications

“…32 Compared with their BT counterparts, the PT-containing polymers showed smaller bandgaps and decreased lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. 36 The PT-containing polymer exhibited the smallest bandgap and the largest short-circuit current density (J sc = 19.6 mA/cm 2 ), in comparison with the BTz-containing polymer (J sc = 6.17 mA/cm 2 ) and the ffBT-containing polymer (J sc = 5.74 mA/cm 2 ). In 2013, Heeney and coworkers synthesized three polymers based on benzo[d] [1,2,3]triazole (BTz), 5,6difluorobenzo[c] [1,2,5]thiadiazole (ffBT), and PT, respectively.…”

“…32,[36][37][38][39][40][41][42][43] Therefore, it is necessary and meaningful to further investigate the structure-property relationships of PT-containing polymers. However, to the best of our knowledge, none of the reported PCEs of PTcontaining polymers is over 7%, implying that there is still considerable room to improve the photovoltaic performance.…”