Two novel side-chain fluorinated n-SMAs were synthesized to study the effect of fluoro-side-chain engineering on the thermal stability of OSCs.

No abstract

It is important to tune the energy levels of nonfullerene acceptors (NFAs) to achieve more balanced open‐circuit voltage (Voc) and short‐circuit current density (Jsc) to improve the device performance. Herein, two novel NFAs are designed via fusing fluorene or carbazole with two thieno[3,2‐b]thiophene and end capped with INIC‐2F, namely, 4TFIC‐4F and 4TCIC‐4F, respectively. The impact of the fluorene and carbazole unit on the PSC performance is systematically studied. Compared with 4TFIC‐4F, 4TCIC‐4F exhibits a higher lowest unoccupied molecular orbital (LUMO) energy level of −3.95 eV and a narrower optical bandgap of 1.51 eV owing to the stronger electron‐donating capacity of fused‐carbazole ring core. Consequently, the 4TCIC‐4F device achieves a high power conversion efficiencies (PCE) of 13.02% with a higher Voc of 0.94 V and a larger Jsc of 18.98 mA cm−2, whereas the 4TFIC‐4F device shows a PCE of 11.24%. The PCE of 13.02% is the highest value so far reported with the carbazole‐containing NFAs‐based PSCs. More importantly, the 4TCIC‐4F device shows good film thickness insensitive and long‐term thermal stability. The investigation demonstrates that the fused‐carbazole ring is a superior option to fused‐fluorene ring as electron‐donating core for designing high‐performance NFAs by improving Voc and Jsc simultaneously.

Herein, three carbazole‐based small molecule acceptors (SMAs), named 4TC‐4F‐C8C8, 4TC‐4F‐C6C8, and 4TC‐4F‐C16, are synthesized to study the influence of side chain conformation on SMAs. The three acceptors exhibit similar optical and electrochemical properties, but different crystallization properties. 4TC‐4F‐C16 shows a high crystallinity due to the small steric hindrance of linear n‐hexadecyl (C16) side chain. The large steric hindrance and free rotation for the 2‐hexydecyl (C6C8) side chain seriously disturb the molecular packing and result in a low crystallinity for 4TC‐4F‐C6C8. Despite the large steric hindrance for the 1‐octylnonyl (C8C8) side chain, 4TC‐4F‐C8C8 shows a moderate crystallinity due to the large torsion barrier restricting the rotation of C8C8 side chain. Attributing to the ideal morphology and better crystalline ordering in blend film, organic solar cell based on poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene‐alt‐N‐(2‐hexyldecyl)‐5′5‐bis[3‐(decylthio)thiophene‐2‐yl]‐2′2‐bithiophene‐3′3‐dicarboximide] (PBTIBDTT):4TC‐4F‐C8C8 displays a power conversion efficiency of 12.06%, higher than PBTIBDTT:4TC‐4F‐C6C8 (2.38%)‐ and PBTIBDTT:4TC‐4F‐C16 (9.53%)‐based devices. The work indicates that controlling the conformation of bulky side chain can tune the molecular packing of SMAs and the morphology of blend film, providing a new insight into the molecular design of SMAs.

Three S,N‐heteroacene nonfullenere small molecular acceptors (NF‐SMAs) with different branching positions on side chains named SNBDT1‐F, SNBDT2‐F, and SNBDT3‐F are synthesized to understand the relationship among branching positions, molecular orientations of NF‐SMAs, and photovoltaic performance of nonfullerene organic solar cells (OSCs). When the branching position is moved close to the conjugated backbone, the proportion of face‐on orientation increases gradually from 4% (SNBDT3‐F), 16% (SNBDT2‐F), to 44% (SNBDT1‐F), suggesting that the molecular orientation changes from edge‐on to face‐on, which has a positive influence on exciton dissociation and charge transport. As a result, the device based on PBDB‐T:SNBDT1‐F shows the highest exciton dissociation probability and carrier mobilities due to the highest proportion of face‐on orientation of SNBDT1‐F. Therefore, the highest power conversion efficiency (PCE) of 12.70% with a high short‐circuit current (Jsc) of 20.97 mA cm−2 and a fill factor of 70.4% is obtained for SNBDT1‐F‐based device, which is much higher than PCEs of SNBDT2‐F‐based (6.57%) and SNBDT3‐F‐based devices (5.87%). Overall, these results provide deep insight into the relationship among the branching positions, molecular orientation, and photovoltaic performance in nonfullerene OSCs, which is important for designing new NF‐SMAs with face‐on orientation and promoting the development of nonfullerene OSCs.

The acceptor Si4TIC-F, which incorporated a silicon bridging atom, was synthesized and gave a PCE of 10.20% with the donor PBTIBDTT.

Fine‐tuning the crystallinity and self‐aggregation features of donors/acceptor materials toward high‐efficiency organic solar cells (OSCs) is of crucial importance. Here, a convenient yet effective way to simultaneously control the crystallinity and self‐aggregation of the fused ring electron acceptor (FREA) is demonstrated by altering the length of the first‐position branched alkyl chain on the cyclic unit. Specifically, three carbazole‐based FREAs, 4TC‐4F‐C6C6, 4TC‐4F‐C8C8, and 4TC‐4F‐C10C10, are synthesized by changing the length of the first‐position branched alkyl chain on the carbazole unit. The crystallinity of the studied acceptors decreases as the branched alkyl chain is lengthened. The ability of the acceptors to undergo self‐aggregation decreases in the order 4TC‐4F‐C10C10, 4TC‐4F‐C6C6, and 4TC‐4F‐C8C8. The medium crystallinity and lower self‐aggregation properties of 4TC‐4F‐C8C8 result in favorable phase separation when blended with poly‐[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PM6), which is conducive to effective exciton dissociation and charge transport. Consequently, the OSC device based on PM6:4TC‐4F‐C8C8 delivers the best power conversion efficiency of 14.85%.