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Water/alcohol soluble cathode interfacial materials (CIMs) are playing important roles in optoelectronic devices such as organic light emitting diodes, perovskite solar cells and organic solar cells (OSCs). Herein, n-doped solution-processable single-wall carbon nanotubes (SWCNTs)-containing CIMs for OSCs are developed by dispersing SWCNTs to the typical CIMs perylene diimide (PDI) derivatives PDIN and PDINO. The Raman and X-ray photoelectron spectroscopy (XPS) measurement results illustrate the n-doped behavior of SWCNTs by PDIN/PDINO in the blend CIMs. The blended and n-doped SWCNTs can tune the work function and enhance the conductivity of the PDI-derivative/SWCNT (PDI-CNT) composite CIMs, and the composite CIMs can regulate and down-shift the work function of cathode, reduce the charge recombination, improve the charge extraction rate and enhance photovoltaic performance of the OSCs. High power conversion efficiency (PCE) of 17.1% and 17.7% are obtained for the OSCs based on PM6:Y6 and ternary PM6:Y6:PC71BM respectively with the PDI-CNT composites CIMs. These results indicate that the n-doped SWCNT-containing composites, like other n-doped nanomaterials such as zero dimensional fullerenes and two dimensional graphenes, are excellent CIMs for OSCs and could find potential applications in other optoelectronic devices.

Three small-molecule donors based on dithieno[2,3-d:2ʹ,3ʹ-dʹ]-benzo[1,2-b:4,5-bʹ] dithiophene (DTBDT) unit were designed and synthesized by side chain regulation with chlorinated or/and sulfurated substitutions (namely ZR1, ZR1-Cl, and ZR1-S-Cl respectively), along with a crystalline non-fullerene acceptor IDIC-4Cl with a chlorinated 1,1-dicyanomethylene-3-indanone (IC) end group. Energy levels, molar extinction coefficients and crystallinities of three donor molecules can be effectively altered by combining chlorination and sulfuration strategies. Especially, the ZR1-S-Cl exhibited the best absorption ability, lowest higher occupied molecular orbital (HOMO) energy level and highest crystallinity among three donors, resulting in the corresponding all-small-molecule organic solar cells to produce a high power conversion efficiency (PCE) of 12.05% with IDIC-4Cl as an acceptor.

Organic solar cells have attracted academic and industrial interests due to the advantages like lightweight, flexibility and roll-to-roll fabrication. Nowadays, 18% power conversion efficiency has been achieved in the state-of-the-art organic solar cells. The recent rapid progress in organic solar cells relies on the continuously emerging new materials, device fabrication technologies, and the deep understanding on film morphology, molecular packing and device physics. Donor and acceptor materials are the key materials for organic solar cells since they determine the device performance. The past 25 years have witnessed an odyssey in developing high-performance donors and acceptors. In this review, we focus on those star materials and milestone work, and introduce the molecular structure evolution of the key materials. These key materials include homopolymer donors, D-A copolymer donors, A-D-A small molecule donors, fullerene acceptors and nonfullerene acceptors. At last, we outlook the challenges and very important directions in key materials development.

Interface engineering is a critical factor in achieving high photovoltaic performances for large-area, printable, flexible and low cost organic solar cells (OSCs). Herein, we developed a solution-processable n-doped graphene cathode interfacial material (CIM) PDINO–G for OSCs by adding graphene to the classic material PDINO ((N,N-dimethyl-ammonium N-oxide)propyl perylene diimide). The dispersed graphene in PDINO–G was n-doped by the nitroxide radical of N-oxide in PDINO, which was certified by the ESR, Raman spectroscopy and XPS results. The n-doped PDINO–G CIM possessed increased conductivity, lower work function, reduced charge recombination and increased charge extraction rate. Previously, n-doped graphene was prepared only on the surface of chemical vapor-deposited (CVD) graphene films without solution processability. The effect of the CIM on the photovoltaic performance of OSCs was studied by selecting the photovoltaic model system of PTQ10 as the donor and IDIC-2F as the acceptor. The OSCs based on PTQ10:IDIC-2F with PDINO–G CIM demonstrated the best power conversion efficiency (PCE) of 13.01%, which was significantly enhanced in comparison with that of the devices without graphene modification on the PDINO CIM (12.23%). The universality of the high performance PDINO–G CIM was verified in two other photovoltaic systems based on PTQ10:IDIC and PM6:Y6. Especially, the single junction bulk heterojunction OSCs based on PM6:Y6 with PDINO–G CIM displayed a high PCE of 16.52% (average PCE of 16.3 ± 0.2%). The results indicate that the n-doped PDINO–G is an excellent CIM for OSCs. Furthermore, this PDINO-doped graphene can be applied in other optoelectronic devices such as tandem OSCs, organic light-emitting diodes, and high-performance perovskite solar cells as a cathode interfacial material considering its advantages of solution processability, low work function and high conductivity.

An alcohol soluble hybrid organic–inorganic three-dimensional material 1,3,5,7,9,11,13,15-(9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-octavinylpentacyclo-octasiloxane (POSS-FN) has been synthesized and assessed as a cathode interlayer within organic solar cells consisting of a PBDT-BT:PC61BM bulk heterojunction. For comparison, we also studied another two linear interfacial materials: a typical conjugated polymer poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene] (PFN) and an insulating polymer poly(4-N,N-dimethylamino-styene) (PStN) in the same system. The hybrid interlayer caused a significant improvement to the device power conversion efficiency by 32%, comparable to the other two interlayers. We found that there are two kinds of interfacial dipoles formation: one weak but unfavourable between the interlayer and the active layer, and the other larger, favourable and significant between the interlayer and the cathode. This latter factor maximized the built-in electric field across the interlayer-modified devices, which provides one of the major reasons for the improved performance. The thermodynamics study revealed that the driving force for the dipole formation could be ascribed to the amino groups.

An electrically modulated single-/dual-color imaging photodetector with fast response speed is developed based on a small molecule (COi 8DFIC)/perovskite (CH3 NH3 PbBr3 ) hybrid film. Owing to the type-I heterojunction, the device can facilely transform dual-color images to single-color images by applying a small bias voltage. The photodetector exhibits two distinct cut-off wavelengths at ≈544 nm (visible region) and ≈920 nm (near-infrared region), respectively, without any power supply. Its two peak responsivities are 0.16 A W-1 at ≈525 nm and 0.041 A W-1 at ≈860 nm with a fast response speed (≈102 ns). Under 0.6 V bias, the photodetector can operate in a single-color mode with a peak responsivity of 0.09 A W-1 at ≈475 nm, showing a fast response speed (≈102 ns). A physical model based on band energy theory is developed to illustrate the origin of the tunable single-/dual-color photodetection. This work will stimulate new approaches for developing solution-processed multifunctional photodetectors for imaging photodetection in complex circumstances.

Four low-cost copolymer donors of poly(thiophene-quinoxaline) (PTQ) derivatives are demonstrated with different fluorine substitution forms to investigate the effect of fluorination forms on charge separation and voltage loss (Vloss ) of the polymer solar cells (PSCs) with the PTQ derivatives as donor and a A-DA'D-A-structured molecule Y6 as acceptor. The four PTQ derivatives are PTQ7 without fluorination, PTQ8 with bifluorine substituents on its thiophene D-unit, PTQ9, and PTQ10 with monofluorine and bifluorine substituents on their quinoxaline A-unit respectively. The PTQ8- based PSC demonstrates a low power conversion efficiency (PCE) of 0.90% due to the mismatch in the highest occupied molecular orbital (HOMO) energy levels alignment between the donor and acceptor. In contrast, the devices based on PTQ9 and PTQ10 show enhanced charge-separation behavior and gradually reduced Vloss , due to the gradually reduced nonradiative recombination loss in comparison with the PTQ7-based device. As a result, the PTQ10-based PSC demonstrates an impressive PCE of 16.21% with high open-circuit voltage and large short-circuit current density simultaneously, and its Vloss is reduced to 0.549 V. The results indicate that rational fluorination of the polymer donors is a feasible method to achieve fast charge separation and low Vloss simultaneously in the PSCs.