Guiting Chen

Regulating aggregation and molecular packing of small-molecule cathode interlayer (CIL) materials is a significant but imperceptible issue in the development of high-performance organic solar cells (OSCs). For the celebrity PDINN small molecule, the strong aggregation tendency of the perylene diimide molecular backbone leads to excessive crystallinity when films form, ultimately affecting the morphology and charge transport ability of the films. Herein, we address this issue by developing a hydroxyl-induced anti-aggregation strategy by introducing a suitable amount of hydroxypropyl cellulose (HPC) into the solution of PDINN, and a careful balance is achieved between the film-forming quality and the aggregation of the material. Taking two commercially available active layer systems, PM6/Y6 and D18/L8-BO, as examples, the introduction of HPC significantly increases the J_SC and FF values of the devices. Therefore, power conversion efficiency risen from 17.38% to 18.25% for the PM6/Y6 system and from 18.45% to 19.73% for the D18/L8-BO system, and it was proved that the thickness tolerance of the HPC hybrid interface was improved significantly. This hydroxyl-induced anti-aggregation strategy has demonstrated efficiency in other active layer systems. This work provides a simple and effective method to solve the aggregation problem of small molecule CIL materials, which is conducive to the commercial development of OSCs.

We developed two copolymers composed of BDD-Ph unit. PBF-C8 exhibited red-shifted absorption and stronger face-on stacking than PBF-EH, NF-PSCs based on PBF-C8 achieved a high PCE of 16.44%, significantly outperforming PBF-EH-based device (3.21%).

Organic solar cells (OSCs) based on non-fused ring electron acceptors (NFREAs) have shown significant potential for practical application owing to their easily tunable chemical structures and short synthesis routes. The merging of side-chain engineering and terminal-group halogenation has facilitated the optimization of NFREAs properties. Thus, in this study, we developed four NFREAs using a naphthalenediimide (NDI) unit with different alkyl side-chains (–C₆C₁₀ or –C₈C₁₂) as the building block and two different electron-withdrawing groups 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (IC) and chlorinated IC (IC-2Cl) as the terminal groups. These NFREAs were designated as NDI610-H, NDI610-Cl, NDI812-H, and NDI812-Cl, respectively. All four NFREAs exhibited similar optical bandgaps and molecular conformations. Under the synergistic effects of side-chain modification and terminal-group halogenation, the NFREAs-based active-layer morphologies were significantly different. The NDI610-Cl-based blended film demonstrated favorable phase-separation morphology, facilitating exciton dissociation and charge transport in the corresponding OSCs. Thus, the PM6:NDI610-Cl-based OSCs achieved a maximum power conversion efficiency (PCE) of 8.35%. Moreover, using D18 as the polymer donor further enhanced the device performance, with the D18:NDI610-Cl-based device yielding a higher PCE of 10.25% due to improved short-circuit current and fill factor. Our findings indicate that selecting an NDI unit as the building block and combining tailored alkyl side-chains with altered terminal groups are promising strategies for constructing highly efficient NFREAs.

Unfused-ring non-fullerene acceptors (UF-NFAs) have garnered substantial attention because of their simple chemical structure and easy synthesis, making them promising candidates for advancing the application of organic solar cells (OSCs). The design and synthesis of UF-NFAs with upshifted lowest unoccupied molecular orbitals (LUMOs) can improve the open-circuit voltage (V_OC) and thus the photovoltaic performance of devices. Herein, we developed three UF-NFAs, namely, BPD-H, BPD-F, and BPD-Cl, using the phthalimide electron-deficient unit as the building block. Because of the stronger electron-withdrawing effect of chlorine atoms, BPD-Cl exhibited higher molar extinction coefficients and red-shifted absorption than BPD-H or BPD-F. Additionally, the PM6:BPD-Cl-based blended film displayed highly efficient exciton separation, better charge transportation, and an appropriate active layer morphology. Thus, PM6:BPD-Cl-based OSCs achieved a higher PCE of 10.72%, outperforming PM6:BPD-H-based (6.44%) and PM6:BPD-F-based (7.98%) devices. Our findings indicate that choosing the phthalimide electron-deficient unit as the building block and modifying terminal groups are promising strategies for fabricating efficient UF-NFAs.

The design and synthesis of polymer donors with a low-lying highest occupied molecular orbital (HOMO) level are crucial for increasing open-circuit voltages (V_OC) and achieving high-performance non-fullerene polymer solar cells. Here, we developed two copolymers using non-fluorinated or fluorinated thienyl-conjugated benzodithiophenes as electron donor units, and difluoro-quinoxaline with a naphthalimide substituent (DNB) as the electron acceptor unit. These copolymers, namely PDNB and PDNB-2F, exhibited deep HOMO levels owing to the strong electron-withdrawing ability of the naphthalimide substituent. Density-functional theory calculations demonstrated that the skeletons of the two copolymers featured good coplanarity. Owing to the fluorination, PDNB-2F displayed an increased absorption coefficient and deeper HOMO level than PDNB. Moreover, the blended film based on PDNB-2F:Y6 demonstrated enhanced carrier mobility, decreased bimolecular recombination as well as favorable phase-separation regions. Consequently, the PDNB-2F:Y6-based device yielded a superior power conversion efficiency (PCE) of 12.18%, whereas the device based on PDNB:Y6 showed a comparatively lower PCE of 8.83%. These results indicate that difluoro-quinoxaline with a naphthalimide substituent is a prospective electron-deficient building block to develop donor polymers with low-lying HOMO levels to achieve efficient non-fullerene polymer solar cells.

A novel conjugated molecule (PBSON) based on a main chain composed of bis­(dibenzothiophene-S,S-dioxide) fused cyclopentadiene and side chains containing amino groups is presented as an efficient cathode buffer material (CBM) for organic solar cells (OSCs). PBSON showed a deep highest occupied molecular orbital (HOMO) energy level of −6.01 eV, which was beneficial for building hole-blocking layers at the cathodes of OSCs. The energy bandgap of PBSON reached 3.17 eV, implying high transmittance to visible and near-infrared light, which meant PBSON should be suitable for the applications to most inverted OSCs. The scanning Kelvin probe microscopy measurement and theoretical calculation on the PBSON/cathode interfacial interaction proved the excellent work function-regulating abilities of PBSON for various cathodes, suggesting that PBSON could promote the formation of Ohmic contacts at the cathodes and thus improve the transport and collection of electron carriers for OSCs. The characterization of electron-only devices demonstrated the good electron-transporting performance of PBSON at the cathodes. In the conventional OSCs, it was hinted that PBSON might restrain the infiltrations of evaporated cathode atoms into the active films, consequently reducing the reverse leakage currents. As a result, PBSON was able to boost the power conversion efficiencies (PCEs) by 58.2 and 56.4% for both conventional and inverted OSCs of the typical PTB7:PC71BM system, respectively, as compared to the unadorned devices. In terms of the classical PTB7-Th:PC71BM system, substantial increases in PCEs could also be found with PBSON interlayers, which were 54.7 and 59.8% for the conventional device and inverted device, respectively. Therefore, PBSON is a kind of promising CBM for realizing both conventional and inverted OSCs of high performance.

All-polymer solar cells (all-PSCs) with mechanical and thermal stability have potential for applications in flexible devices. Polymer acceptors based on naphthalene diimide (NDI) have been widely studied because of their strong electron affinity, high electron mobility, and high mechanical reliability. However, controlling the film morphology of the polymer–polymer blends of NDI-based all-PSCs is difficult. Consequently, all-PSCs based on NDI building blocks exhibit a low fill factor (FF) and a lower power-conversion efficiency (PCE) than state-of-the-art polymer solar cells. In this work, we added a small amount of dicyanodistyrylbenzene (DCB) unit to the NDI-based polymer acceptor N2200 through random copolymerization and synthesized a series of NDI-based terpolymer acceptors PNDIx, where x is the molar concentration of DCB units relative to NDI units. PNDI5 and PNDI10, corresponding to 5% and 10% molar concentrations of DCB, respectively, showed lower crystallization and good miscibility with PBDB-T, a widely used electron-donating copolymer, than the terpolymer based on DCB-free N2200. Moreover, compared to the PBDB-T:N2200 device, the PNDI5-based device exhibited a much higher PCE (8.01%), and an enhanced FF of 0.75 in all-PSCs. These results indicate that ternary random copolymerization is a convenient and effective strategy for optimizing the film morphology of NDI-based polymers, and that the resulting terpolymer acceptor is a promising n-type acceptor for constructing high-performance all-PSCs.

A wide-bandgap polymer donor with improved efficiency plays an important role in improving the photovoltaic performance of polymer solar cells (PSCs). In this study, two novel wide-bandgap polymer donors, PBDT and PBDT-S, were designed and synthesized based on a dicyanodivinyl indacenodithiophene (IDT-CN) moiety, in which benzo[1,2-b:4,5-b′]dithiophene (BDT) building blocks and IDT-CN are used as electron-sufficient and -deficient units, respectively. In our study, the PBDT and PBDT-S polymer donors exhibited similar frontier-molecular-orbital energy levels and optical properties, and both copolymers showed good miscibility with the widely used narrow-bandgap small molecular acceptor Y6. Non-fullerene polymer solar cells (NF-PSCs) based on PBDT:Y6 exhibited an impressive power conversion efficiency of 10.04% with an open circuit voltage of 0.88 V, a short-circuit current density of 22.16 mA cm⁻² and a fill factor of 51.31%, where the NF-PSCs based on PBDT-S:Y6 exhibited a moderate power conversion efficiency of 6.90%. The enhanced photovoltaic performance, realized by virtue of the improved short-circuit current density, can be attributed to the slightly enhanced electron mobility, higher exciton dissociation rates, more efficient charge collection and better nanoscale phase separation of the PBDT-based device. The results of this work indicate that the IDT-CN unit is a promising building block for constructing donor polymers for high-performance organic photovoltaic cells.

The synthesis and application as a cathode interfacial material (CIM) for organic solar cells (OSCs) of a fluorene-based conjugated small molecule (3FPy) with pyridyl functional side chains are presented. 3FPy is soluble in MeOH containing trace AcOH due to the interaction between pyridyl and AcOH, offering opportunities to fabricate multilayer OSCs by orthometric solution methods. When 3FPy was introduced into the conventional OSCs of both poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithio-phene-2,6-diyl-altethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7):[6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) and poly[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl)] (PTB7-Th):PC₇₁BM systems, significant simultaneous improvements in open-circuit voltage (V_oc) and fill factor (FF) were achieved. It was found that pyridyl groups could lessen the cathode's work functions by interfacial dipoles, subsequently enlarging the built-in potentials and V_oc. For the FF enhancements, the working mechanisms for 3FPy consisted of suppressing the devices’ dark currents, reducing the charge transport loss by n-doping PC₇₁BM, and optimizing the contacts at the active layer/cathode interfaces. Therefore, this kind of novel small molecule is a promising CIM for high-performance OSCs.