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

Abstract

In order to facilitate the move from small to large area devices it is important that the mechanisms governing charge generation and cell operation are well understood. In particular, the infl uence of blend morphology upon device performance is known to be critical, and therefore understanding the relationship between molecular structure and blend morphology is a crucial step towards controlling the microstructure. A particularly successful approach to the design of new, higher effi ciency donor polymers has been the donor-acceptor approach, in which electron rich donor monomers are co-polymerized with electron defi cient aromatics. [ 4,5 ] This has been demonstrated to lead to low optical band gaps by hybridization of the molecular orbitals.Within the class of donor-acceptor polymers, bridged bithiophenes with fi vemember fused rings in the central core have proven to be a promising class of donor. The bridging atom keeps the bithiophene highly co-planar ensuring effective overlap of the conjugated systems, and additionally serves as a point of attachment for the required solubilizing groups. One such example polymer is poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) (PCPDTBT), in which the bithiophene donor (cyclopentadithiophene, CPT) is bridged by a carbon atom. [6][7][8] Blends of PCPDTBT with [6,6]-phenyl C 70 butyric acid methyl ester (PC 70 BM) have reached a PCE around 5%. Since this initial promising solar cell performance of PCPDTBT blends, various approaches have been undertaken to further improve the efficiency. One interesting approach was to substitute the bridging C atom of CPT with different group IV heteroatoms. For example the analogous polymer incorporating a bridging silicon (Si) atom, poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSiBTBT, Scheme 1 a) showed an enhanced OPV device performance upon thermal treatment (unlike PCPDTBT) which was attributed to higher crystallinity of the Si based polymer. [ 9 ] The enhanced crystallinity of PSiBTBT over PCPDTBT has been associated with the C-Si bond being longer than the C-C bond of the central bridging fi ve-membered ring, which changes the geometry of the fused The effects of heteroatom substitution from a silicon atom to a germanium atom in donor-acceptor type low band gap copolymers, poly[(4,4′-bis(2ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSiBTBT) and poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d] germole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PGeBTBT), are studied. The optoelectronic and charge transport properties of these polymers are investigated with a particular focus on their use for organic photovoltaic (OPV) devices in blends with phenyl-C 70 -butyric acid methyl ester (PC 70 BM).It is found that the longer C-Ge bond length, in comparison to C-Si, modifi es the molecular conformation and leads to a more planar chain conformation in PGeBTBT than PSiBT...

Donor Conjugated Polymers with Polar Side Chain Groups: The Role of Dielectric Constant and Energetic Disorder on Photovoltaic Performance

“…The structural change is kept minimal within each polymer pair to avoid significant impact on polymer solubility and molecular packing. The idea of introducing a minimal structural change, which includes important aspects of identical polymerization and isolation methods, is an important approach when studying structure–property–performance relationships in complex materials . The location of the polar functional groups a significant distance from the polymer backbone is expected to minimize their impact on the polymer electronic structure, where the two different side chain lengths are chosen to allow investigation of the impact of polar group location in terms of their distance from the backbone.…”

Solution‐Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

“…Replacing C bridging atoms with Si or Ge resulted in a reduction of the intramolecular bend angle and the steric hindrance. This led to improved intermolecular ordering of the resulting polymers, PSBTBT or PGeBTBT, while exhibiting a negligible effect on the bandgap and the PCE in the range of 4.2–5.3% . Studies on the effect of bridging atom replacement on structural characteristics comparing CPDT, dithieno[3,2‐ b :2′,3′‐ d ]silole (DTS), and dithieno[3,2‐ b :2′,3′‐ d ]germole (DTG) have been intensively explored in polymeric and oligomeric molecules .…”

A Nonfullerene Small Molecule Acceptor with 3D Interlocking Geometry Enabling Efficient Organic Solar Cells

“…SF‐PDI4 displayed an intense UV absorption band centered at 531 nm ( α max = 4.8 × 10 4 cm −1 ) due to the π–π* transition of the chromophore, which complemented the absorption bands of P4T2FBT and PV4T2FBT. The absorption spectra of the two copolymers contain two spectral features, high‐energy bands attributed to the localized π–π* transition and lower‐energy bands ascribed to the intramolecular charge transfer (ICT) transition . PV4T2FBT exhibited better absorption properties compared to P4T2FBT, probably due to the enhanced ICT transitions resulting from the stronger electron‐donating properties of the TVT unit compared to the bithienyl (2T) unit .…”