Fluorobenzotriazole (FTAZ)‐Based Polymer Donor Enables Organic Solar Cells Exceeding 12% Efficiency

Abstract

The fluorobenzotriazole (FTAZ)‐based copolymer donors are promising candidates for nonfullerene polymer solar cells (PSCs), but suffer from relatively low photovoltaic performance due to their unsuitable energy levels and unfavorable morphology. Herein, three polymer donors, L24, L68, and L810, based on a chlorinated‐thienyl benzodithiophene (BDT‐2Cl) unit and FTAZ with different branched alkyl side chain, are synthesized. Incorporation of a chlorine (Cl) atom into the BDT unit is found to distinctly optimize the molecular planarity, energy levels, and improve the polymerization activity. Impressively, subtle side chain length of FTAZ realizes a dramatic improvement in all the device parameters, as revealed by the short‐current density (Jsc) improved from 7.41 to 20.76 mA cm−2, fill‐factor from 36.3 to 73.5%, and even the open‐circuit voltage (Voc) from 0.495 to 0.790 V. The best power conversion efficiency (PCE) of 12.1% is obtained from the L810‐based device, which is one of the highest values reported for FTAZ‐based PSCs so far. Notably, the corresponding external quantum efficiency curve keeps a very prominent value up to 80% from 500 to 800 nm. The notable performance is discovered from the reduced energy loss, improved molecular face‐on orientation, the down‐shifted energy levels, and optimized absorption coefficient regulated by side‐chain engineering.

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

“…However, the maximum absorption coefficients of the five polymers in solution were somewhat different (Figure a), which were gradually decreased from 6.21 × 10 4 cm –1 at 699 nm for PTBSi20 to ∼5.67 × 10 4 cm –1 at 699 nm for PTBSi25 and PTB7-Th, and then to ∼4.65 × 10 4 cm –1 at ∼700 nm for PTBSi33 and PTBSi100, which implies a better light-harvesting ability of PTBSi polymers with small content (20–25%) of siloxane-terminated pentyl side chain. Similar results were reported for fluorobenzotriazole-based polymer donors and fluorenedicyclopentathiophene-based small molecular acceptors, in which the absorption coefficient of materials in solution have been enhanced by side chain engineering. , The optical band gaps ( E g opt ) of the polymers are calculated from the absorption edges of the film absorptions (Figure b). The PBTSi polymers possess comparable E g opt of 1.58 eV based on absorption edges (λ onset ) of ∼785 nm, with very little difference with that of polymer PTB7-Th (Table ).…”

“…Similar results were reported for fluorobenzotriazolebased polymer donors and fluorenedicyclopentathiophenebased small molecular acceptors, in which the absorption coefficient of materials in solution have been enhanced by side chain engineering. 52,53 The optical band gaps (E g opt ) of the polymers are calculated from the absorption edges of the film absorptions (Figure 1b). The PBTSi polymers possess comparable E g opt of 1.58 eV based on absorption edges (λ onset ) of ∼785 nm, with very little difference with that of polymer PTB7-Th (Table 1).…”

Side-chain engineering of copolymers based on benzotriazole (BTA) and dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophenes (DTBDT) enables a high PCE of 14.6%

“…The CV results imply that the increased length of the side chain is attributable to the decrease in the HOMO and LUMO energy levels of the copolymers [55]. Furthermore, the E LUMO/HOMO of J52-Cl was estimated to be −3.50/−5.44 eV via the same test methods and under the same conditions, which is in conformity with the reported values (figure S5) [28].…”

Diketopyrrolopyrrole-Based Dual-Acceptor Copolymers to Realize Tunable Charge Carrier Polarity of Organic Field-Effect Transistors and High-Performance Nonvolatile Ambipolar Flash Memories

“…34 Additionally, the alkoxyl side chains in BTz can up-shift the HOMO, whereas prolonging the alkyl chains in ffBTA can down-shift the HOMO. 35 These effects and the larger band gap of pDPPy-ffBTA resulted in the lower HOMO and higher LUMO energy levels.…”

“…Generally, the insertion of electron-donating thiophene will increase the HOMO; however, the strong electron-withdrawing effects of F atoms can neutralize the electron-donating capacity of thiophene . Additionally, the alkoxyl side chains in BTz can up-shift the HOMO, whereas prolonging the alkyl chains in ffBTA can down-shift the HOMO . These effects and the larger band gap of pDPPy-ffBTA resulted in the lower HOMO and higher LUMO energy levels.…”