Medium Bandgap Nonfullerene Acceptor for Efficient Ternary Polymer Solar Cells with High Open-Circuit Voltage.

We have designed a new medium bandgap non-fullerene small-molecule acceptor consisting of an IDT donor core flanked with 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]-thiophene-4-ylidene) malononitrile (TC) acceptor terminal groups (IDT-TC) and compared its optical and electrochemical properties with the IDT-IC acceptor. IDT-TC showed an absorption profile from 300 to 760 nm, and it has an optical bandgap of 1.65 eV and HOMO and LUMO energy levels of -5.55 and -3.83 eV, respectively. In contrast to IDT-IC, IDT-TC has an upshifted LUMO energy level, which is advantageous for achieving high open-circuit voltage. Moreover, IDT-TC showed higher crystallinity and high electron mobility than IDT-IC. Using a wide bandgap D-A copolymer P as the donor, we compared the photovoltaic performance of IDT-TC, IDT-IC, and IDT-IC-Cl nonfullerene acceptors (NFAs). Polymer solar cells (PSCs) using P: IDT-TC, P: IDT-IC, and P:IDT-IC-Cl active layers achieved a power conversion efficiency (PCE) of 14.26, 11.56, and 13.34%, respectively. As the absorption profiles of IDT-IC-Cl and IDT-TC are complementary to each other, we have incorporated IDT-TC as the guest acceptor in the P: IDT-IC-Cl active layer to fabricate the ternary (P:IDT-TC: IDT-IC-Cl) PSC, demonstrating a PCE of 16.44%, which is significantly higher than that of the binary BHJ devices. The improvement in PCE for ternary PSCs is attributed to the efficient exploitation of excitons via energy transfer from IDT-TC to IDT-IC-Cl, suitable nanoscale phase separation, compact stacking distance, and more evenly distributed charge transport.

Role of Exciton Lifetime, Energetic Offsets, and Disorder in Voltage Loss of Bulk Heterojunction Organic Solar Cells.

Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has significantly progressed with a rapid increase from 10 to 19% due to state-of-the-art research on nonfullerene acceptor molecules and various device processing strategies. However, OSCs still exhibit significant open circuit voltage loss (ΔVOC ∼ 0.6 V) due to high energetic offsets and molecular disorder. In this work, we present a systematic investigation to determine the effects of energetic offset and disorder on different recombination losses in open circuit voltage (VOC) using 13 different photoactive layers, wherein the PCE and ΔVOC vary in the ranges of 2.21-14.74% and 0.561-1.443 V, respectively. The detailed voltage loss analysis of all these devices was carried out, and voltage losses were correlated with energetic offset and disorder. This has enabled us to identify the key features for minimizing the voltage loss like: (1) a low energy offset between the donor and acceptor molecular states is essential to attain a nonradiative voltage loss (ΔVOC, nrad) as low as ∼200 meV and (2) Urbach energy, which is a measure of the materials' disorder and packing, should be low for the minimization of the radiative voltage loss (ΔVOC, rad). In addition, time-resolved photoluminescence spectroscopy was employed to further understand the exciton dynamics of pristine materials and donor-acceptor blends. It was observed that the absorbers with ultralong exciton lifetime (∼1000 ps) produce higher efficiencies. The current study emphasizes the importance of simultaneously testing photovoltaic performance and active layer exciton dynamics for rational device optimization and opens new prospects for designing novel molecules with fine-tuning of energetic offset and disorder with longer exciton lifetime which is the effective strategy to boost the efficiency of OSCs to their modified Shockley-Queisser (SQ) limit by minimizing radiative and nonradiative voltage losses.

New Medium Bandgap Donor D-A1 -D-A2 Type Copolymers Based on Anthra[1,2-b: 4,3-b":6,7-c"'] Trithiophene-8,12-dione Groups for High-Efficient Non-Fullerene Polymer Solar Cells.

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.