Introduction of Siloxane-Terminated Side Chains into Semiconducting Polymers To Tune Phase Separation with Nonfullerene Acceptor for Polymer Solar Cells

Abstract

In this work, five PTB7-Th-based conjugated polymers (PTB7-Th, PTBSi20, PTBSi25, PTBSi33, and PTBSi100) with different contents of siloxane-terminated pentyl side chain were synthesized, and properties of corresponding blend films with narrow band gap nonfullerene IEICO-4F acceptor were extensively investigated. According to the contact angle testing, the PTB7-Th with 100% alkyl side chain and PTBSi100 100% siloxane-terminated side chain on the benzodithiophene unit showed surface energy values of 40.04 and 34.52 mJ/m2, respectively. The results demonstrate that relative to alkyl side chain in PTB7-Th, the siloxane-terminated side chain could effectively reduce the surface energy of a resulting polymer. Based on Flory–Huggins interaction parameter estimations, the miscibility between the polymer and IEICO-4F would vary in an order of PTB7-Th > PTBSi20 > PTBSi25 > PTBSi33 > PTBSi100, suggesting that siloxane-terminated side chain would afford a tunable driving force for phase separation. Transmission electron microscopy and Raman mapping could confirm large bulk domains inside the PTBSi100:IEICO-4F blend film. In polymer solar cells, the blend film of the PTBSi100 with the lowest miscibility to IEICO-4F showed an undesirable power conversion efficiency (PCE) of 8.52%, which was significantly lower than that of 11.23% for PTB7-Th, suggesting that too large phase separation driving force is not beneficial for the device performance. Side-chain random copolymers PTBSi20, PTBSi25, and PTBSi33 for fine tuning could display PCEs of 11.94, 12.61, and 11.80%, respectively, all higher than that of PTB7-Th. Our results not only reveal the big surface energy difference between the siloxane-terminated side chain and the common alkyl side chain but also provide a guideline for side chain engineering of conjugated polymer donors with tunable morphology and optimal matching with a nonfullerene acceptor.

Nonhalogenated Solvent‐Processed Thick‐Film Ternary Nonfullerene Organic Solar Cells with Power Conversion Efficiency >13% Enabled by a New Wide‐Bandgap Polymer

“…The higher roughness leads to insufficient donor–acceptor interfaces for exciton dissociation. [ 39–44 ] In contrast, a uniform smooth surface was observed for the ternary thick active layer, which maintained the FF (Table 1). The P2:IT‐4F:BTP‐4F and P2:IT‐4F:BTP‐4Cl films exhibited more balanced miscibilities after the introduction of the siloxane‐functionalized side chains with BTP‐4F and BTP‐4Cl.…”

“…The incorporation of siloxane‐functionalized side chains improves the solubility without increasing the steric repulsion, increases the charge‐carrier transport, and thus increases the PCEs. [ 42–44 ] Furthermore, the siloxane side chain can be used to tune polymer molecular stacking orientations and down‐shift the HOMO levels. Thus, it contributes to a well‐balanced charge‐carrier transport with an effective driving force.…”

On the Impact of Linear Siloxanated Side Chains on the Molecular Self‐Assembling and Charge Transport Properties of Conjugated Polymers

“…[ 21–24,27 ] Such a controlled siloxane functionalization favorably offered promising photovoltaic devices with efficiencies over 10% with subtly mixed alkyl/siloxane side‐chains polymer systems. [ 19,28–31 ]…”

Improved Average Figure‐of‐Merit of High‐Efficiency Nonfullerene Solar Cells via Minor Combinatory Side Chain Approach

“…It should be noted that a descending trend in surface energy is observed as the content of siloxane-terminated combinatory side chain increases (Figure 3), consistent with our recent work. [58] When blended with IT-4F, the surface energy of the blend film is increased for all the polymers, also showing the decreased γ tendency for a higher content of the siloxane-terminated combinatory side chain. Considering low γ of pristine polymer in an order of PQSi05 < PQSi10 < PQSi25 and the same D:A blend ratio, the unexpected declined trend in the γ increments of blend films demonstrates that polymer with higher content of siloxaneterminated combinatory side chain is prone to occur on the blend top.…”