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Four new donor–acceptor conjugated polymers based on the acceptor block 8,10-bis(2-octyldodecyl)-8H-dithieno[3′,2′:5,6;2′′,3′′:7,8]naphtho[2,3-d]imidazole-9(10H)-one and various donor moieties were synthesized. Binary and ternary composites comprising new polymers, PC71BM fullerene and non-fullerene acceptors based on indacenodithiophene, were developed. The mobility of electrons and holes in thin films of the ternary composites is balanced in contrast to that in binary composite films. This is explained by less aggregation in the ternary composite films and good matching of the frontier orbital energy levels of the composite components. [ABSTRACT FROM AUTHOR]

A new non-fullerene small molecule acceptor (MPU6) containing a diketopyrrolopyrrole (DPP) acceptor unit as the core, connected to dicyanomethylene-3-ethylrhodanine end-capped units via thienylethynylselenophene bridges, has been designed and synthesized and its optical and electrochemical properties have been examined. MPU6 has been used as an acceptor along with a wide bandgap polymer (TDTBTA) as the donor for solution-processed bulk heterojunction polymer solar cells, thus resulting in a power conversion efficiency (PCE) of 13.14%, which is higher than that obtained when PC71BM is used as the acceptor (6.69%). The increase in PCE for the MPU6-based polymer solar cell is due to the increase in both VOC and JSC, while the FF is lower than that for PC71BM. In addition, PC71BM has been used as a second acceptor to form ternary PSCs, thus achieving a PCE of 15.60%, which is significantly higher than those for the binary counterparts. The increase in PCE for ternary PSCs may be linked to better charge transport and energy transfer, which leads to better utilization of excitons and also leads to the enhancement of JSC and FF. [ABSTRACT FROM AUTHOR]

In fullerene-free single-junction organic solar cells despite a small interfacial energy offset between the donor and acceptor materials efficient hole transfer occurs, resulting in power conversion efficiencies (PCE) >16%. Here, we demonstrate efficient organic solar cells by tuning of the electronic properties through simple structural modulation of coumarin-based electron donors. The OSCs with C2:BThIND-Cl:Y6 ternary blends exhibit a PCE of 13.54%, with an increased open circuit voltage (VOC) compared to the C1:BThIND-Cl:Y6-based devices (11.27%). Interestingly, an efficient charge transfer is revealed despite a reduced HOMO orbital offset of 0.02–0.04 eV between the C2 donor and BThIND-Cl and Y6 acceptors, resulting in a lower energy loss of 0.45 eV in ternary devices while still maintaining a maximum EQE of ∼80%@820 nm. The work demonstrates the importance of simple coumarin-based donors and their role in performance improvement for fullerene-free ternary OSCs by facilitating charge carrier transport with suppressed recombination. [ABSTRACT FROM AUTHOR]

Using a ternary blend consisting of two donors/one acceptor or one donor/two acceptors is a potential strategy to increase the power conversion efficiency (PCE) of polymer solar cells (PSCs). Herein, ternary PSCs based on PBDB-T:IT-4F:O6T-4F blends have been investigated by employing narrow bandgap O6T-4F as a second acceptor. Our studies indicate that introduction of 20% O6T-4F in the acceptor moiety can increase the PCE from 9.5% for the binary PBDB-T:IT-4F (1 : 1) cell to 10.5% for the ternary PBDB-T:IT-4F:O6T-4F (1 : 0.8 : 0.2) cell. The enhancement is attributed to a compromise between the increased short-circuit current (JSC) and open-circuit voltage (VOC) and the decreased fill factor (FF) with addition of the O6T-4F acceptor. It is shown that O6T-4F lowers the energy loss (Eloss) occurring in the binary PBDB-T:IT-4F (1 : 1) blend, leading to a higher VOC of the ternary PSCs with regard to the binary PBDB-T:IT-4F PSCs. Eloss is decreased from 0.80 eV for the binary PBDB-T:IT-4F (1 : 1) blend to 0.61 eV for the ternary PBDB-T:IT-4F:O6T-4F (1 : 0.8 : 0.2) blend. At the same time, the addition of the O6T-4F acceptor reduces the aggregation of the PBDB-T donor in the ternary blends. Ultra-fast transient absorption demonstrates that the hole transfer and subsequent carrier recombination occur faster in the ternary blend than in the PBDB-T:IT-4F blend, which accounts for a slightly decreased FF in the ternary PSCs. [ABSTRACT FROM AUTHOR]

In order to realize high photocurrent generation in the low-energy region of the solar spectrum, two conjugated A–D–A oligomers, PY-1 and DCI-2 comprising a central dithieno[3,2-b:2′,3′-d]pyrrole donor (D) unit and end-capping acceptors (A) 1-butyl-4-methyl-2,6-dioxopyridine-3-carbonitrile (PY) or 3-dicyanomethyleneindan-1-one (DCI) have been synthesized and characterized. The oligomers showed strong absorptions in the red and near-IR region in solution and in the solid state. As a consequence of the strong electron-accepting character, the absorption band of DCI-2 in thin films is significantly red-shifted compared to that of PY-1 resulting in a low optical energy gap of 1.23 eV. In optimized solution-processed bulk-heterojunction solar cells using a polymeric donor P1, the new non-fullerene acceptor DCI-2 provides an excellent power conversion efficiency of 6.94% which is noticeably higher than that of PY-1-based devices (PCE = 4.89%). Most importantly, a high open-circuit voltage (VOC) of ∼0.8 V with unprecedented energy losses, between the optical energy gap and the open-circuit voltage, between 0.39 and 0.43 eV concomitant with excellent external quantum efficiencies of 69%@880 nm in the NIR-regime have been achieved for DCI-2-based devices. [ABSTRACT FROM AUTHOR]

A new non-fullerene small molecule acceptor, MPU2, that incorporates dicyano-rhodanine moieties on the edges of thiophene–diketopyrrolopyrrole was designed and prepared. In films MPU2 showed a broad absorption in the 550–850 nm region with an optical energy gap of 1.59 eV. MPU1 and MPU2 were used as small molecule acceptors to construct solar cells. DTS(QxHTh2)2, which has a D1–A–D2–A–D1 structure and complementary absorption in the 470–700 nm region of the solar spectrum, was selected as the electron-donor. All small molecule solution-processed bulk heterojunction organic solar cells were prepared using these small molecules as acceptors along with a small molecule donor. In these devices, MPU2 showed higher power conversion efficiency than MPU1. The morphology of the active layer was improved by applying thermal annealing and vacuum drying methodologies. The devices for which the latter methodology was used showed 6.28% (MPU1) and 7.76% (MPU2) power conversion efficiencies (PCEs), with the latter value being one of the highest PCEs for all small molecule fullerene-free solar cells. The results of this study show that vacuum drying treatment of the active layer is better than thermal annealing to achieve high PCE values. [ABSTRACT FROM AUTHOR]