Donor–Acceptor Semiconducting Polymers Containing Benzodithiophene with Bithienyl Substituents

Abstract

Synthesis and photovoltaic properties of two donor−acceptor polymers containing benzodithiophene with 3,3′,5-trihexylbithienyl substituents are reported. Benzo[c]-[1,2,5]thiadiazole and 5-hexylthieno [3,4-c]pyrrole-4,6-dione were used as acceptor building blocks for the synthesis of donor−acceptor polymers. The photovoltaic properties of the synthesized donor−acceptor polymers were investigated in bulk heterojunction solar cells with [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) acceptor. ■ INTRODUCTIONBenzodithiophene semiconducting polymers have garnered considerable interest in the past few years due to their good performances in both organic field-effect transistors and polymeric solar cells. 1,2 In addition to having a planar backbone, the benzodithiophene core offers the flexibility of attaching different substituents on the central benzene core to fine-tune the energy levels of the polymer. 2−4 Various research groups have exploited this property and attached different substituents such as alkyl, alkoxy, and thioalkyl to the benzene core of benzodithiophene. 2,3,5−9 Our group reported the synthesis and electronic properties of benzodithiophene polymers with alkyl phenylethynyl substituents. 10 More recently, the Yang group reported the synthesis of donor−acceptor polymers containing benzodithiophene with a conjugated thienyl substituent. 11 Huo showed that replacing the alkoxy with alkylthienyl substituents on the BDT core increases the thermal stability, gives broader absorption spectra, lowers the HOMO and LUMO energy levels, and improves the photovoltaic properties of the polymers. 12−14 Furthermore, the attachment of thienyl substituents to the BDT can result in increasing the solubility of the polymer as it provides the ability to attach a larger number of alkyl substituents on the thiophene ring. 15 For example, Cao attached four alkyl chains on the BDT building block which improved the solubility of the resulting polymer. 15 Recent reports have shown that benzodithiophene semiconducting polymers displayed power conversion efficiencies (PCE) in excess of 7% in bulk heterojunction (BHJ) solar cells. 16,17 Donor−acceptor polymers with lower band gaps are highly desirable for organic solar cell applications. Additionally, the ability to improve the open-circuit voltage (V oc ) has attracted much attention. Since the V oc of a BHJ solar cell is directly proportional to the difference between the HOMO of the donor and the LUMO of the acceptor, lowering the HOMO level of the donor polymer will be a crucial factor in enhancing the V oc . 18 Moreover, a donor−acceptor copolymer with a deeper HOMO level will enhance the oxidative stability of the polymer. Hou and co-workers synthesized a donor−acceptor polymer with 5-alkylthiophene-substituted BDT with a deeplying HOMO of −5.3 eV and obtained an air-stable donor polymer that gave a V oc of 0.88 V in bulk heterojunction solar cells. 12 To generate highly soluble semiconducting polymers with higher molecular weight and broader absorption in the visible re...

Side‐Chain Engineering for Fine‐Tuning of Energy Levels and Nanoscale Morphology in Polymer Solar Cells

“…[ 10,26,27 ] As for PBDT2FBT-T and PBDT2FBT-T-O, it is obvious that the introduction of the thienyl side chains leads to red-shifted band edges and broadened absorption bands due to the extension of the conjugation length that results from the 2D-conjugated polymers. [ 32 ] In most reported cases, the band gaps of conjugated polymers are reduced by incorporating additional thienyl units between the donor and the acceptor segments because of the resulting increases in the extension and delocalization of the π-electron system. [ 33 ] Note that similar results are evident when the thienyl units are added to the BDT units as side chains in this systems.…”

Poly(3-Hexylthiophene) Nanostructured Materials for Organic Electronics Applications

“…Typical field-mobilities for conjugated polymers are in the range of 10 −1 -10 −7 cm 2 /Vs, [103][104][105][106] with P3HT having reported mobilities in the range of 0.05-0.1 cm 2 /Vs. 64 107 However, P3HT mobilities can vary by a few orders of magnitude depending on the processing conditions.…”

Donor–acceptor semiconducting polymers for organic solar cells

“…Because of the deeplying HOMO a very high V OC of 1.00 V was measured for polymer 66 with a PCE of 2.52% for a weight ratio of 66:PC 61 BM ¼ 1:3 (J SC ¼ 5.36 mA cm À2 , FF ¼ 0.47). 146 However, for these polymers the addition of the DIO additive lowered the PCEs. This behavior was attributed to the difference in morphology of the active area of the polymer/PCBM blend, which changed from a smooth surface to a rougher surface with large domains upon the addition of DIO (Fig.…”

“…Since the presence of the fluorine atom also lowered the LUMO, a (66)(67). 146 Six alkyl substituents per each BDT repeating unit improved the solubility of polymers and allowed the synthesis of relatively high molecular weight polymers. Because of the conjugated bithienyl substituents, both the polymers showed a deep-lying HOMO of $-5.53 eV and a bandgap of 1.70 eV.…”

“…17). 146 Benzo[2,2-b:4,5-b 0 ]difuran D-A Polymers BDF is a structurally similar analog to BDT for which the thiophene is replaced by a furan ring. The smaller radii and the higher electronegativity of oxygen is expected to give BDF polymers a good packing, a more planar polymer backbone, and a deep-lying HOMO.…”