High Polymer/Fullerene Ratio Realized in Efficient Polymer Solar Cells by Tailoring of the Polymer Side‐Chains

Abstract

polymer side-chains can prevent fullerene intercalation and result in a low optimal fullerene ratio. In their following work, the existence of bimolecular crystals was confi rmed by the use of X-ray diffraction, molecular simulations, and spectroscopy. [ 7 ] Note that the polymers studied in their original work were based on different backbones, which introduced extra complexity and might weaken their conclusions. Additionally, no following work on molecular design has been performed to strengthen their theory. Therefore, inspired by their work, we used alter- [ 8 ] This popular backbone was intentionally chosen for the side-chain modifi cation to demonstrate that this strategy may become a simple and universal way to reduce the use of PCBM for existing effi cient polymers. The free volume between side-chains can be adjusted by substituting alkyl chains with alkylthienyl or alkylphenyl ones, which are less fl exible and more bulky than alkyl chains. In addition, the introduction of conjugated side chains to BDT-based D-A copolymers can result in more planar confi guration and fi netuned energy levels, leading to improved device performance in both PSCs and organic fi eld effect transistors (OFETs). [ 9 ] It is worth noting that we also appended alkyl aromatic side chains to the acceptor comonomer to further adjust the sidechain spacing, while the density of side-chains was altered by incorporating conjugated π-bridge to the polymer backbone. In this work, aiming to reduce the use of fullerene, we performed systematical side-chain modifi cation on BDT-TPD based polymers ( Scheme 1 ). A novel D-A copolymer PTP8 with alkyl aromatic side-chains on both donor and acceptor comonomer was fi rstly synthesized. A high V oc of 0.96 V and PCE of 6.18% were achieved for PTP8:PC 71 BM based BHJ PSCs at a low blend ratio of 1:0.8. Furthermore, the device performance showed a weak dependence on the PCBM concentration, demonstrating a relatively high PCE even at an extremely low blend ratio of 1:0.3. Interestingly, with lower PCBM concentrations, less solvent additive was required to achieve the optimal morphologies, suggesting a more environment-friendly fabrication process. As

Side‐Chain Engineering for Enhancing the Thermal Stability of Polymer Solar Cells

“…One conjecture is that the conjugated side chains of polymer facilitate the retention of bulk-heterojunction morphology since the introduction of conjugated side chains could increase co-planarity of the polymer, which is benefi cial for improving its crystallinity. [ 13 ] As the conjugated length of side chains increases, the π-π interaction becomes stronger, the morphology of the blend fi lms becomes more stable upon thermal annealing and thus the stability of the corresponding device is improved.…”

Side‐Chain Engineering for Enhancing the Properties of Small Molecule Solar Cells: A Trade‐off Beyond Efficiency

“…Nevertheless, the meta-alkylthio substitution seems to positively influence the thermal stability of the BHJ composites' microstructure. [30,59] Optical microscopy ( Figure S19 in the SI) and AFM ( Figure S20 in the SI) were further used to study the origin of the thermal instability but did not give strong evidences. The surface morphology of BDTT-TR, BDTT-O-TR, and BDTT-S-TR blends show distinct but small changes during thermal annealing (24 and 80 h) but no evidence of major phase separation.…”

Fine Control of Side Chains in Random π‐Conjugated Terpolymers for Organic Photovoltaics

“…Possible interpretations are that fewer F8 units of terpolymers, less side chains and hence more free spacing between them, allow for more PCBM to occupy and interact with terpolymer backbones. This suggests that tuning the composition ratio in PF8TBT enables a fine‐control of side‐chains and thus prevents the PCBM intercalation on the molecular level . Notably, the PF8TBT14:PCBM blend shows a low ϕ q value less than 40% at blend ratio of 1:1 and only 85% at a high blend ratio of 1:2.…”