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The development of wide bandgap p-type conjugated polymers to pair with low bandgap nonfullerene n-type acceptors (NFAs) has emerged as an important topic in polymer solar cells. Naphtho[1,2-d:5,6-d’]bis([1,2,3]triazole (NTz), in which the central nitrogen bears an aliphatic side chain, can serve as a mild electron acceptor to generate wide bandgap p-type conjugated polymers. To this end, we synthesized two donor-acceptor alternating copolymers, PBDTNTz1 and PBDTNTz2, based on naphthobistriazole (NTz) as the acceptor and benzodithiophene (BDT) as the donor. The two polymers had optical bandgaps of ca. 1.9 eV with absorption spectra complementary to that of the typical nonfullerene n-type acceptor ITIC. Side-chain engineering was implemented by introducing different combinations of alkyl groups on the BDT and NTz units in PBDTNTz1 and PBDTNTz2. According to theoretical calculations, PBDTNTz1 has higher backbone coplanarity with a face-on π-π stacking orientation. Consequently, a device with the PBDTNTz1:ITIC combination delivered a high PCE of 7.37% with a Vocof 0.94 V, a Jscof 14.04 mA/cm2and an FF of 55.8%. In addition, organic field-effect transistors (OFETs) using PBDTNTz1 and PBDTNTz2 exhibited hole mobilities of 2.2 × 10−2cm2V−1s−1and 5.0 × 10−2cm2V−1s−1with on/off ratios of 1 × 104and 9 × 102, respectively.

We discovered a unique synthetic route to construct 2H-pyran-containing tetracyclic dithienocyclopentapyran (DTCP) and dibenzocyclopentapyran (DBCP) architectures. The synthesis involves an acid-induced dehydration cyclization followed by a [1,5] hydride-shift isomerization to form a cyclopentanone moiety which was converted to the pyran-embedded tetracyclic products by a CuI-catalyzed intramolecular C–O bond formation in good yield. DTCP was used as a building block to prepare an acceptor–donor–acceptor (A–D–A) type n-type material DTCP-BC leading to a solar cell efficiency of 9.32%.

A new zwitterionic small molecule 4-(dimethyl(pyridin-2-yl)ammonio)butane-1-sulfonate (PAS), synthesized from 2-dimethylaminopyrindine (2-DMAP), was developed for the ITO cathode modifier. PAS and 2-DMAP dissolved in methanol can form a thin layer on ITO cathode by a simple spin-coating process. The heteroatom moieties in 2-DMAP (sp2and sp3nitrogen) and PAS (sp2nitrogen and sulfonate ion) can coordinate to the ITO surface and decrease the ITO work function by the induced surface dipoles. The fullerene-based (PBDTT-FTTE:PC71BM) inverted OSCs using PAS and 2-DMAP interlayer can achieve PCEs of 8.95 and 8.26%, respectively, which are superior to the devices without a modifier (PCE = 3.25%) and comparable to the corresponding ZnO-based device (PCE = 8.57%). Nevertheless, 2-DMAP, like other nitrogen-containing polymer interlayer materials, turns out to be not applicable to inverted organic solar cells (I-OSCs) with IT-4F as the n-type electron acceptor because the amino group of 2-DMAP can act as a nucleophile to attack the end-group of IT-4F at the interface. The decomposition of IT-4F by 2-DMAP was carefully proved to be via retro-aldol condensation. As a result, the device (PBDBT-F:IT-4F) modified with 2-DMAP displayed a low PCE of 7.34%. The zwitterionic PAS with reduced nucleophilicity and basicity can modify the ITO surface without decomposing IT-4F. The PBDBT-F:IT-4F-based device modified with PAS maintained a high PCE of 11.41%. Most importantly, the PAS-based device using the well-known Y6 acceptor (PBDBT-F:Y6) can achieve a PCE of 13.82%. This new interfacial material can be universally applied to I-OSCs employing various A–D-A-type acceptors installed with the electrophilic 1,1-dicyanamethylene-5,6-difluoro-3-indanone (FIC) end-group.

Two-dimensional alkylated αβ-TTNand βα-TTNwere designed and synthesized. The cross-shaped αβ-TTNand βα-TTNmoieties were polymerized to afford two isomeric polymers, Pαβ-TTNFBT40and Pβα-TTNFBT40. Pαβ-TTNFBT40using an α-aNDT unit in the main chain exhibited more red-shifted absorption and a smaller HOMO/LUMO band gap than Pβα-TTNFBT40using β-aNDT in the main chain. Compared to Pβα-TTNFBT40showing a curved backbone, density functional theory (DFT) calculations revealed that Pαβ-TTNFBT40exhibited a quasi-linear polymeric backbone that induces stronger intermolecular interactions and higher molecular ordering in the solid state. Pαβ-TTNFBT40exhibited an organic field-effect transistor (OFET) hole mobility of 1.12 × 10–2cm2V–1s–1, which outperformed Pβα-TTNFBT40by 1 order of magnitude (1.60 × 10–3cm2V–1s–1). Through side-chain engineering and optimization, Pαβ-TTNFBT24using a shorter 2-butyloctyl side chain further strengthened the intermolecular interactions. From grazing incidence wide-angle X-ray scattering (GIWAXS), Pαβ-TTNFBT24adopted an edge-on π–π stacking orientation in a thin film, leading to an improved mobility of 1.75 × 10–1cm2V–1s–1, which has exceeded the mobility of the one-dimensional α-aNDT-based PaNDTDTFBT(4.70 × 10–2cm2V–1s–1). This research demonstrated that the two-dimensional structure is an effective strategy to achieve higher OFET mobility, and the geometry of the main chain played a more significant role than that of the side chain in determining the physical and packing properties of polymers.

Benzene-based 1,1-dicyanomethylene-3-indanone (IC) derivatives have been widely utilized as the end-group to construct acceptor–donor–acceptor type nonfullerene acceptors (A–D–A type NFAs). The extension of the end-group conjugation of nonfullerene acceptors (NFAs) is a rational strategy to facilitate intermolecular stacking of the end-groups which are responsible for efficient electron transportation. A bicyclic benzothiophene-based end-group acceptor, 2-(3-oxo-2,3-dihydro-1H-benzo[b]cyclopenta[d]thiophen-1-ylidene)malononitrile, denoted as α-BC was designed and synthesized. The Knoevenagel condensation of the unsymmetrical 1,3-diketo-precursor with one equivalent of malononitrile selectively reacts with the keto group attached at the α-position of the thiophene unit, leading to the isomerically pure benzothiophene-fused α-BC. The well-defined α-BC with extended conjugation was condensed with three different ladder-type diformylated donors to form three new A–D–A NFAs named BDCPDT-BC, DTCC-BC, and ITBC, respectively. The corresponding IC-based BDCPDT-IC, DTCC-IC, and ITIC model compounds were also synthesized for comparison. The incorporation of the electron-rich benzothiophene unit in the end-group upshifts the lowest unoccupied molecular orbital energy levels of the NFAs, which beneficially enlarges the Vocvalues. On the other hand, the benzothiophene unit in α-BC not also imparts an optical transition in the shorter wavelengths around 340–400 nm for a better light harvesting ability but also promotes the antiparallel π–π stacking of the end-groups for efficient electron transport. The organic photovoltaic cell devices using a PBDB-T polymer and BC-based NFAs all showed the improved Vocand Jscvalues. The BDCPDT-BC- and DTCC-BC-based devices exhibited a power conversion efficiency (PCE) of 10.82 and 10.74%, respectively, which outperformed the corresponding BDCPDT-IC-, and DTCC-IC-based devices (9.33 and 9.25%). More importantly, the ITBC-based device delivered the highest PCE of 12.07% with a Jscof 19.90 mA/cm2, a Vocof 0.94 V, and an fill factor of 64.51%, representing a 14% improvement relative to the traditional ITIC-based device (10.05%).

In this research, we developed six new selenophene-incorporated naphthobisthiadiazole-based donor–acceptor polymers PNT2Th2Se-OD, PNT2Se2Th-OD, PNT4Se-OD, PNT2Th2Se-DT, PNT2Se2Th-DT, and PNT4Se-DT. The structure–property relationships have been systematically established through the comparison of their structural variations: (1) isomeric biselenophene/bithiophene arrangement between PNT2Th2Se and PNT2Se2Th polymers, (2) biselenophene/bithiophene and quarterselenophene donor units between PNT2Th2Se/PNT2Se2Th and PNT4Se polymers, and (3) side-chain modification between the 2-octyldodecylthiophene (OD)- and 2-decyltetradecyl (DT)-series polymers. The incorporation of selenophene units in the copolymers induces stronger charge transfer to improve the light-harvesting capability while maintaining the strong intermolecular interactions to preserve the intrinsic crystallinity for high carrier mobility. The organic field-effect transistor device using PNT2Th2Se-OD achieved a high hole mobility of 0.36 cm2V–1s–1with an on/off ratio of 1.9 × 105. The solar cells with PNT2Th2Se-OD:PC71BM exhibited a power conversion efficiency of 9.47% with a Vocof 0.68 V, an fill factor of 67%, and an impressive Jscof 20.69 mA cm–2.

We performed frequency-modulated (AC) and steady-state (DC) surface photovoltage spectroscopy (SPS) measurements on a bilayer structure consisting of an organic semiconductor (P3HT, P3HT:PC61BM, or PFBT2Se2Th:PC71BM) on top of a ZnO electron-transport layer. The AC spectra overlap with the absorption spectra of the organic layer, providing evidence that AC SPS corresponds to band-to-band transitions. The DC spectra are generally broader than the AC spectra, with responses extended below the absorption edge. Thus, DC SPS also probes transitions between band states and trap states within the band gap in addition to band-to-band transitions. When a hole-transport layer (HTL) is deposited on top of the organic layer, the DC spectra of P3HT and P3HT:PC61BM are narrower than those without the HTL, suggesting that the sub-band gap states exist at the surface of these organic semiconductors. In contrast, PFBT2Se2Th:PC71BM does not show signature of surface states or optically active trap states in the band gap. External quantum efficiency and capacitance measurements are employed to explain the nature of sub-band gap states that contribute to surface photovoltage signals and the differences between the two bulk heterojunction systems.

An angular-shaped 4,9-didodecylnaphthodithiophene-based octacyclic ladder-type structure was developed. This new ladder-type donor (LD) was coupled with electron-withdrawing IC and FIC units to form two A-LD-A type non-fullerene acceptor materials, NCIC and NCFIC. NCIC and NCFIC as the n-type acceptors are blended with a p-type donor PBDB-T to form the complementary absorption and suitable energy level alignments. The binary-blend PBDB-T:NCIC and PBDB-T:NCFIC devices achieved an efficiency of 7.3% and 7.5%, respectively. PC71BM was incorporated to form D1:A1:A2ternary blends which further strengthen the absorption at shorter wavelengths. Introduction of PC71BM not only efficiently improves the absorption but also provides multiple channels for exciton dissociation and electron transport to dramatically improve Jsc. It is interesting to find that the Vocof the ternary-blend devices is reversely in proportional to the added amount of PC71BM. The device using the PBDB-T:NCIC:PC71BM (1:1:1 in wt %) showed an improved PCE of 8.32%. Moreover, the optimized device using the PBDB-T:NCFIC:PC71BM (1:1:1.5 in wt %) blend achieved the highest efficiency of 9.18%.

A two‐dimensional heptacyclic tetrathienyl‐TBDT (TBDT‐4T) was synthesized by palladium‐catalyzed C─H benzannulation. The stannlyated TBDT was copolymerized with FBT and FTT monomers to produce PTBDT‐FBT and PTBDT‐FTT with excellent thermal property and air stability. The inverted PSC devices using PTBDT‐FBT and PTBDT‐FTT showed a PCE of 4.16 and 3.98%, respectively. A new two‐dimensional tetrathienyl‐TBDT (TBDT‐4T) was synthesized by employing palladium‐catalyzed C─H benzannulation between an iodobiaryl and 1,2‐dithienylacetylene. The stannylated TBDT‐4T was copolymerized with di(thiophenyl)difluorobenzothiadiazole (FBT) and fluorothienothiophene (FTT) monomers to yield PTBDT‐FBT and PTBDT‐FTT, which showed high decomposition temperatures (421 and 408°C) and deep‐lying HOMO levels (−5.80 and −5.84 eV). The inverted solar cell device using PTBDT‐FBT:PC71BM (1:1 in wt%) blend showed a power conversion efficiency (PCE) of 4.16%, while the device using PTBDT‐FTT:PC71BM (1:2 in wt%) blend showed a PCE of 3.98%.

A new class of additive materials bis(pentafluorophenyl) diesters (BFEs) where the two pentafluorophenyl (C6F5) moieties are attached at the both ends of a linear aliphatic chain with tunable tether lengths (BF5, BF7, and BF13) were designed and synthesized. In the presence of BF7to restrict the migration of fullerene by hand-grabbing-like supramolecular interactions induced between the C6F5groups and the surface of fullerene, the P3HT:PC61BM:BF7device showed stable device characteristics after thermal heating at 150 °C for 25 h. The morphologies of the active layers were systematically investigated by optical microscopy, grazing-incidence small-angle X-ray scattering (GISAXS), and atomic force microscopy. The tether length between the two C6F5groups plays a pivotal role in controlling the intermolecular attractions. BF13with a long and flexible tether might form a BF13–fullerene sandwich complex that fails to prevent fullerene’s movement and aggregation, while BF5with too short tether length decreases the possibility of interactions between the C6F5groups and the fullerenes. BF7with the optimal tether length has the best ability to stabilize the morphology. In sharp contrast, the nonfluorinated BP7analogue without C6F5–C60physical interactions does not have the capability of morphological stabilization, unambiguously revealing the necessity of the C6F5group. Most importantly, the function of BF7can be also applied to the high-performance PffBT4BT-2OD:PC71BM system, which exhibited an original PCE of 8.80%. After thermal heating at 85 °C for 200 h, the efficiency of the PffBT4BT-2OD:PC71BM:BF7device only decreased slightly to 7.73%, maintaining 88% of its original efficiency. To the best of our knowledge, this is the first time that the thermal-driven morphological evolution of the high-performance PffBT4BT-2OD polymer has been investigated, and its morphological stability in the inverted device can be successfully preserved by the incorporation of BF7. This research also demonstrates that BF7is not only effective with PC61BM but also to PC71BM.

A formylated benzodi(cyclopentadithiophene) (BDCPDT) ladder-type structure with forced coplanarity is coupled with two 1,1-dicyanomethylene-3-indanone (IC) moieties via olefination to form a non-fullerene acceptor, BDCPDT-IC. The BDCPDT-IC, as an acceptor (A1) with broad light-absorbing ability and excellent solution processability, is combined with a second PC71BM acceptor (A2) and a medium band gap polymer, PBDB-T, as the donor (D) to form a ternary blend with gradient HOMO/LUMO energy alignments and panchromatic absorption. The device with the inverted architecture using the D:A1:A2ternary blend has achieved a highest efficiency of 9.79% with a superior Jscof 16.84 mA cm–2.

Bis-adduct fullerenes surrounded by two insulating addends sterically attenuate intermolecular interaction and cause inferior electron transportation. In this research, we have designed and synthesized a new class of bis-adduct fullerene materials, methylphenylmethano-C60bis-adduct (MPC60BA), methylthienylmethano-C60bis-adduct (MTC60BA), methylphenylmethano-C70bis-adduct (MPC70BA), and methylthienylmethano-C70bis-adduct (MTC70BA), functionalized with two compact phenylmethylmethano and thienylmethylmethano addends via cyclopropyl linkages. These materials with much higher-lying lowest unoccupied molecular orbital (LUMO) energy levels successfully enhanced the Vocvalues of the P3HT-based solar cell devices. The compact phenylmethylmethano and thienylmethylmethano addends to promote fullerene intermolecular interactions result in aggregation-induced phase separation as observed by the atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of the poly(3-hexylthiophene-2,5-diyl) (P3HT)/bis-adduct fullerene thin films. The device based on the P3HT/MTC60BA blend yielded a Vocof 0.72 V, a Jscof 5.87 mA/cm2, and a fill factor (FF) of 65.3%, resulting in a power conversion efficiency (PCE) of 2.76%. The unfavorable morphologies can be optimized by introducing a solvent additive to fine-tune the intermolecular interactions. 1-Chloronaphthalene (CN) having better ability to dissolve the bis-adduct fullerenes can homogeneously disperse the fullerene materials into the P3HT matrix. Consequently, the aggregated fullerene domains can be alleviated to reach a favorable morphology. With the assistance of CN additive, the P3HT/MTC60BA-based device exhibited enhanced characteristics (a Vocof 0.78 V, a Jscof 9.04 mA/cm2, and an FF of 69.8%), yielding a much higher PCE of 4.92%. More importantly, the additive-assisted morphological optimization is consistently effective to all four compact bis-adduct fullerenes regardless of the methylphenylmethano or methylthienylmethano scaffolds as well as C60or C70core structures. Through the extrinsic additive treatment, these bis-adduct fullerene materials with compact architectures show promise for high-performance polymer solar cells.