An alloy-like model based on Y6 and its derivative BTP-M is constructed to fabricate ternary organic solar cells, leading to a best efficiency of 17.03%.

composed of both electron donor (D) and acceptor (A), finally, transformed to bulk-heterojunction (BHJ) cells formed between D and A, and BHJ structures were further optimized by introducing additional charge-transporting and/or interfacial layers. [27-29] The objectives of the above modifications in device structure are the same: charges can be generated and extracted more efficiently via appropriate spatial alignment of functional components. Currently, BHJ structure is still the dominant configuration for OPV devices, because it can create sufficient D/A interfaces for charge separation, but exist the risk of obvious charge recombination. [30-36] Since charges are transported and collected in vertical direction within OPV devices, forming a preferred vertical phase distribution to some extent, like donor-enrichment at the anode and acceptor-enrichment at the cathode, is a more ideal morphology to reduce the charge recombination and promote the charge collection efficiencies. [37-39] However, it's a tough task for BHJ structure to form well vertical phase distribution. Especially for non-fullerene acceptor-based OPVs, due to the high similarity in chemical structures between donors and nonfullerene acceptors, it will make thermodynamically D and A mix too well. [40] Therefore, it's a tough challenge for fullerenefree OPVs to realize the desirable vertical phase distribution. To achieve the vertical phase distribution, researchers have developed layer-by-layer (LbL) processing method by sequential depositing D and A layers, so as to form a p-in like morphology. [41-45] There are mainly two ways to construct p-in like morphology: 1) sequentially dissolving and processing D and A components in orthogonal solvents; [46] 2) adopting the blade-coating film-forming technology. [47] Due to the similarity of conjugated backbones and side chains of donors and acceptors, it is challenging to find a pair of orthogonal solvents applicable to various donors and acceptors. However, in most labs, spin-coating is still the most common film processing method, which possesses particular advantages in tuning the BHJ morphology due to the larger shear stress. Thereafter, LbL-type OPVs are less studied and their efficiencies are also largely lagging behind those of BHJ-type counterparts. [48] However, LbL-type OPVs might be more suitable for large-area or roll-to-roll fabrications, because they can offer precise control over the morphology of each layer in mass production. [49-51] Obtaining a finely tuned morphology of the active layer to facilitate both charge generation and charge extraction has long been the goal in the field of organic photovoltaics (OPVs). Here, a solution to resolve the above challenge via synergistically combining the layer-by-layer (LbL) procedure and the ternary strategy is proposed and demonstrated. By adding an asymmetric electron acceptor, BTP-S2, with lower miscibility to the binary donor:acceptor host of PM6:BO-4Cl, vertical phase distribution can be formed with donor-enrichment at the anode and accept...

Herein, we investigated a series of fullerene-free organic solar cells (OSCs) based on six different donor:acceptor (D:A) blends with varied highest occupied molecular orbital (HOMO) offsets from −0.05 to 0.21 eV. First, to verify the energetic compatibility of a specific D:A pair, especially for HOMO offsets, we established a simple method to estimate the hole transfer tendencies between D and A by using bilayer hole-only devices. It reveals that the asymmetrical diode effect of the bilayer hole-only devices can correlate with the FF and J sc of the relevant OSCs. Second, to find out whether HOMO offset is the main restriction of hole transfer, we measured transient absorption spectra and examined the hole transfer behavior in the blends, revealing that the occurrence of hole transfer is independent of the HOMO offsets and ultrafast in the time scale of ≤4.6 ps for those blends with ≥0 eV HOMO offsets. In contrast, a negative HOMO offset can significantly slow down the hole transfer with a half-time of ∼400 ps. Furthermore, we compare the device parameters under varied light intensities and discover that the bimolecular recombination should be one of the main restrictions for high device performance. Surprisingly, small HOMO offsets of 0 and 0.06 eV can also enable high PCEs of 10.42% and 11.75% for blend 2 (PTQ10:HC-PCIC) and blend 3 (PBDB-TF:HC-PCIC), respectively. Overall, our work demonstrates not only the validity of high-performance OSCs operating at the near zero HOMO offsets but also the charge dynamic insights of these blends, which will help gain understanding on the further improvement of OSCs.

Most nonfullerene acceptors developed so far for high-performance organic solar cells (OSCs) are designed in planar molecular geometry containing a fused-ring core. In this work, a new nonfullerene acceptor of DF-PCIC is synthesized with an unfused-ring core containing two cyclopentadithiophene (CPDT) moieties and one 2,5-difluorobenzene (DFB) group. A nearly planar geometry is realized through the F···H noncovalent interaction between CPDT and DFB for DF-PCIC. After proper optimizations, the OSCs with DF-PCIC as the acceptor and the polymer PBDB-T as the donor yield the best power conversion efficiency (PCE) of 10.14% with a high fill factor of 0.72. To the best of our knowledge, this efficiency is among the highest values for the OSCs with nonfullerene acceptors owning unfused-ring cores. Furthermore, no obvious morphological changes are observed for the thermally treated PBDB-T:DF-PCIC blended films, and the relevant devices can keep ≈70% of the original PCEs upon thermal treatment at 180 °C for 12 h. This tolerance of such a high temperature for so long time is rarely reported for fullerene-free OSCs, which might be due to the unique unfused-ring core of DF-PCIC. Therefore, the work provides new idea for the design of new nonfullerene acceptors applicable in commercial OSCs in the future.

Figure 1. a) The synthetic routes of IC-Br(in) and IC-Br(out). b) Chemical structures of PY-IT, PY-OT and PY-IOT.