The anti-correlation effect of alkyl chain size on the photovoltaic performance of centrally extended non-fullerene acceptors.

Contrary to previous results, a unique anti-correlation effect of the alkyl chain size on the photovoltaic performance of acceptors was observed. For a centrally-extended acceptor, replacing linear alkyl chains (n-undecyl for CH-BBQ) on the thienothiophene unit with branched ones (2-butyloctyl for CH-BO) leads to a plunge in the power conversion efficiency of organic solar cells (18.12% vs. 11.34% for binary devices), while the largely shortened ones (n-heptyl for CH-HP) bring a surge in performance (18.74%/19.44% for binary/ternary devices). Compared with CH-BO, the more compact intermolecular packing of CH-HP facilitates carrier transport. The characterization of organic field effect transistors and carrier dynamics also echoes the above results. Molecular dynamics simulations indicate that the encounter of the branched alkyl chains and the extended central core hinders the effective interfacial interaction of polymer donors and acceptors, thus deteriorating the device performance. This work suggests that the conventional strategy for alkyl chain engineering of Y-series acceptors might need to be reconsidered in other molecular systems.

Fully Fused Indacenodithiophene-Centered Small-Molecule n-Type Semiconductors for High-Performance Organic Electronics.

Developing novel n-type organic semiconductors is an on-going research endeavour, given their pivotal roles in organic electronics and their relative scarcity compared to p-type counterparts. In this study, a new strategy was employed to synthesize n-type organic semiconductors featuring a fully fused conjugated backbone. By attaching two sets of adjacent amino and formyl groups to the indacenodithiophene-based central cores and triggering a tandem reaction sequence of a Knoevenagel condensation-intramolecular cyclization, DFA1 and DFA2 were realized. The solution-processed organic field effect transistors based on DFA1 exhibited unipolar n-type transport character with a decent electron mobility of ca. 0.10 cm2 V-1 s-1 (ca. 0.038 cm2 V-1 s-1 for DFA2 based devices). When employing DFA1 as a third component in organic solar cells, a high power conversion efficiency of 19.2 % can be achieved in ternary devices fabricated with PM6 : L8-BO : DFA1. This work provides a new pathway in the molecular engineering of n-type organic semiconductors, propelling relevant research forward.

Dipole Moments Regulation of Biphosphonic Acid Molecules for Self-assembled Monolayers Boosts the Efficiency of Organic Solar Cells Exceeding 19.7.

PEDOT: PSS has been widely used as a hole extraction layer (HEL) in organic solar cells (OSCs). However, their acidic nature can potentially corrode the indium tin oxide (ITO) electrode over time, leading to adverse effects on the longevity of the OSCs. Herein, we have developed a class of biphosphonic acid molecules with tunable dipole moments for self-assembled monolayers (SAMs), namely, 3-BPIC(i), 3-BPIC, and 3-BPIC-F, which exhibit an increasing dipole moment in sequence. Compared to centrosymmetric 3-BPIC(i), the axisymmetric 3-BPIC and 3-BPIC-F exhibit higher adsorption energies (Eads) with ITO, shorter interface spacing, more uniform coverage on ITO surface, and better interfacial compatibility with the active layer. Thanks to the incorporation of fluorine atoms, 3-BPIC-F exhibits a deeper highest occupied molecular orbital (HOMO) energy level and a larger dipole moment compared to 3-BPIC, resulting in an enlarged work function (WF) for the ITO/3-BPIC-F substrate. These advantages of 3-BPIC-F could not only improve hole extraction within the device but also lower the interfacial impedance and reduce nonradiative recombination at the interface. As a result, the OSCs using SAM based on 3-BPIC-F obtained a record high efficiency of 19.71%, which is higher than that achieved from the cells based on 3-BPIC(i) (13.54%) and 3-BPIC (19.34%). Importantly, 3-BPIC-F-based OSCs exhibit significantly enhanced stability compared to that utilizing PEDOT:PSS as HEL. Our work offers guidance for the future design of functional molecules for SAMs to realize even higher performance in organic solar cells.

Balancing Flexible Side Chains on 2D Conjugated Acceptors Enables High-Performance Organic Solar Cell.

Balancing the rigid backbones and flexible side chains of light-harvesting materials is crucially important to reach optimized intermolecular packing, micromorphology, and thus photovoltaic performance of organic solar cells (OSCs). Herein, based on a distinctive CH-series acceptor platform with 2D conjugation extended backbones, a series of nonfullerene acceptors (CH-6F-Cn) are synthesized by delicately tuning the lengths of flexible side chains from n-octyl to n-amyl. A systemic investigation has revealed that the variation of the side chain's length can not only modulate intermolecular packing modes and crystallinity but also dramatically improve the micromorphology of the active layer and eventual photovoltaic parameters of OSCs. Consequently, the highest PCE of 18.73% can be achieved by OSCs employing D18:PM6:CH-6F-C8 as light-harvesting materials.

What is the Limit Size of 2D Conjugated Extension on Central Units of Small Molecular Acceptors in Organic Solar Cells?

2D conjugated extension on central units of small molecular acceptors (SMAs) has gained great successes in reaching the state-of-the-art organic photovoltaics. Whereas the limit size of 2D central planes and their dominant role in constructing 3D intermolecular packing networks are still elusive. Thus, by exploring a series of SMAs with gradually enlarged central planes, it is demonstrated that, at both single molecular and aggerated levels, there is an unexpected blue-shift for their film absorption but preferable reorganization energies, exciton lifetimes and binding energies with central planes enlarging, especially when comparing to their Y6 counterpart. More importantly, the significance of well-balanced molecular packing modes involving both central and end units is first disclosed through a systematic single crystal analysis, indicating that when the ratio of central planes area/end terminals area is no more than 3 likely provides a preferred 3D intermolecular packing network of SMAs. By exploring the limit size of 2D central planes, This work indicates that the structural profiles of ideal SMAs may require suitable central unit size together with proper heteroatom replacement instead of directly overextending 2D central planes to the maximum. These results will likely provide some guidelines for future better molecular design.

Binary All-polymer Solar Cells with a Perhalogenated-Thiophene-Based Solid Additive Surpass 18 % Efficiency.

Morphological control of all-polymer blends is quintessential yet challenging in fabricating high-performance organic solar cells. Recently, solid additives (SAs) have been approved to be capable in tuning the morphology of polymer: small-molecule blends improving the performance and stability of devices. Herein, three perhalogenated thiophenes, which are 3,4-dibromo-2,5-diiodothiophene (SA-T1), 2,5-dibromo-3,4-diiodothiophene (SA-T2), and 2,3-dibromo-4,5-diiodothiophene (SA-T3), were adopted as SAs to optimize the performance of all-polymer organic solar cells (APSCs). For the blend of PM6 and PY-IT, benefitting from the intermolecular interactions between perhalogenated thiophenes and polymers, the molecular packing properties could be finely regulated after introducing these SAs. In situ UV/Vis measurement revealed that these SAs could assist morphological character evolution in the all-polymer blend, leading to their optimal morphologies. Compared to the as-cast device of PM6 : PY-IT, all SA-treated binary devices displayed enhanced power conversion efficiencies of 17.4-18.3 % with obviously elevated short-circuit current densities and fill factors. To our knowledge, the PCE of 18.3 % for SA-T1-treated binary ranks the highest among all binary APSCs to date. Meanwhile, the universality of SA-T1 in other all-polymer blends is demonstrated with unanimously improved device performance. This work provide a new pathway in realizing high-performance APSCs.

Multiarmed Aromatic Ammonium Salts Boost the Efficiency and Stability of Inverted Organic Solar Cells.

Inverted organic solar cells (OSCs) have attracted much attention because of their outstanding stability, with zinc oxide (ZnO) being commonly used as the electron transport layer (ETL). However, both surface defects and the photocatalytic effect of ZnO could lead to serious photodegradation of acceptor materials. This, in turn, hampers the improvement of the efficiency and stability in OSCs. Herein, we developed a multiarmed aromatic ammonium salt, namely, benzene-1,3,5-triyltrimethanaminium bromide (PhTMABr), for modifying ZnO. This compound possesses mild weak acidity aimed at removing the residual amines present within ZnO film. In addition, the PhTMABr could also passivate surface defects of ZnO through multiple hydrogen-bonding interactions between its terminal amino groups and the oxygen anion of ZnO, leading to a better interface contact, which effectively enhances charge transport. As a result, an efficiency of 18.75% was achieved based on the modified ETL compared to the bare ZnO (PCE = 17.34%). The devices utilizing the modified ZnO retained 87% and 90% of their initial PCE after thermal stress aging at 65 °C for 1500 h and continuous 1-sun illumination with maximum power point (MPP) tracking for 1780 h, respectively. Importantly, the extrapolated T80 lifetime with MPP tracking exceeds 10 000 h. The new class of materials employed in this work to modify the ZnO ETL should pave the way for enhancing the efficiency and stability of OSCs, potentially advancing their commercialization process.