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We have designed and synthesized two wide bandgap new donor‐acceptor (D‐A) copolymers consisting of the same alkylthiazole‐substituted benzo[1,2‐b;4,5‐b′]dithiophene (BDTTz) donor unit and but different acceptor units, i.e., thiazolo[5,4‐d]thiazole (TTZ) (P122) and 1,3,‐4 thiadiazole (TDz) (P123) and investigated their optical and electrochemical properties. We have employed these copolymers as donor and fullerene (PC71BM) and narrow bandgap non‐fullerene (Y6) as acceptor, to fabricate binary and ternary bulk heterojunction polymer solar cells (PSCs). The overall power conversion efficiency (PCE) of optimized binary bulk heterojunction PSCs based on P122:Y6 and P123:Y6 is 12.60% and 13.16%, respectively. The higher PCE for PSCs based on P123 than P122 counterparts may be associated with the broader absorption profile of the P123 and more charge carrier mobilities than that for the P122 active layer. With the incorporation of small amount of PC71BM into either P122:Y6 or P123:Y6 binary blend, the corresponding ternary PSCs showed an overall PCE of 14.89% and 15.52%, respectively, which is higher than the binary counterparts using either Y6 or PC71BM as acceptor. Incorporating the PC71BM in the binary host blend increases the absorption in the 300–500 nm wavelength region, generating more excitons in the active ternary layer and helping to dissociate the excitons into free charge carriers more effectively. The more appropriate nanoscale phase separation in the active ternary layer than the binary counterpart may be one of the reasons for higher PCE. [ABSTRACT FROM AUTHOR]

Two D‐A copolymers consisting of fused ring pyrrolo‐dithieno‐quinoxaline acceptors are synthesized with different donor units, i.e., benzodithiophene (BDT) with alkylthienyl (P134) and 2‐ethylhexyloxy (P117) side chains. These copolymers are used as donors and a narrow bandgap acceptor Y6 to fabricate bulk heterojunction polymer solar cell devices. Owing to the strong electron‐deficient fused ring pyrrolo‐bithieno‐quinoxaline and weak alkyl thienyl side chains in BDT, the polymer solar cells (PSCs) based on P134:Y6 attain the power conversion efficiency (PCE) of 15.42%, which is higher than the P117:Y6 counterpart (12.14%). The superior value of PCE for P134:Y6 can be associated with more well‐adjusted charge transport, weak charge recombination, proficient exciton generation, and dissociation into free charge carriers and their subsequent charge collection owing to the dense π–π stacking distance and more considerable crystal coherence length for the P134:Y6 thin films. This investigation confirms the great potential of a strong acceptor‐weak donor tactic for developing efficient D‐A copolymers consists of quinoxaline acceptor for PSCs. [ABSTRACT FROM AUTHOR]

A new acceptor unit anthra[1,2‐b: 4,3‐b': 6,7‐c'']trithiophene‐8,12‐dione (А3Т) (A2) is synthesized and used to design D‐A1‐D‐A2 medium bandgap donor copolymers with same thiophene (D) and A2 units but different A1, i.e., fluorinated benzothiadiazole (F‐BTz) and benzothiadiazole (BTz) denoted as P130 and P131, respectively. Their detailed optical and electrochemical properties are examined. The copolymers show good solubility in common organic solvents, broad absorption in the visible spectral region from 300 to 700 nm, and deeper HOMO levels of −5.45 and −5.34 eV for P130 and P131, respectively. Finally, an optimized polymer solar cell (PSC) based on P131 as the donor and narrow bandgap non‐fullerene small molecule acceptor Y6 demonstrated a power conversion efficiency (PCE) of >11.13%. To further improve the efficiency of the non‐fullerene PSC, the P130 is optimized by introducing a fluorine atom into the BTz unit, F‐BTz acceptor unit, and PCE PSC based on P130: Y6 active layer increased to >15.28%, which is higher than that for the non‐fluorinated analog P131:Y6. The increase in the PCE for former PSC is attributed to the more crystalline nature and compact π–π stacking distance, leading to more balanced charge transport and reduced charge recombination. These remarkable results demonstrate that A3T‐based copolymer P130 with F‐BTz as the second acceptor is a promising donor material for high‐performance PSCs. [ABSTRACT FROM AUTHOR]

• Effect of the polymer M n on the TADF property was explored for the first time. • More compacted stacking and better energy transfer is observed in low M n polymers. • Low M n polymer achieves a record-high maximum EQE of 21.2 % at 628 nm. Thermally activated delayed fluorescence (TADF) polymers have made remarkable progress in solution processed organic light-emitting diodes (OLEDs) due to their superior film-forming stability. However, the effect of some factors such as the molecular weight and end groups on the optoelectronic properties is never explored so far. Herein, a set of the TADF conjugated polymers with the different number-average molecular weights (M n s) of 5.4, 24.8, 41.8 and 146.1 kg/mol are synthesized and their M n effect on the photoluminescence (PL) and electroluminescence (EL) is systematically investigated. The microstructure models of the spin-coated polymer films based on the M n and chain length are proposed for realizing efficient non-doped OLEDs. Low M n produces more compacted polymer chain stacking and smoother film morphology, leading to restricted non-radiative loss and more effective energy transfer. In particular, the polymers with low M n s of 5.4 and 24.8 kg/mol achieve the higher PL quantum yields and more efficient reverse intersystem cross from triplet to singlet excited state, as well as the record external quantum efficiencies of 21.2 % and 18.4 % with the EL emission peaks over 620 nm. In contrast, the PL and EL performances eventually deteriorate with further increasing M n s due to the sparser chain stacking and larger phase separation. The results show that the lower M n s enable the rigid polymers to form perfect amorphous films and render the luminescent unit more uniform distribution, similar to the classical doped systems, improving the EL behaviors of the polymers. [ABSTRACT FROM AUTHOR]