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Perovskite solar cells (PSCs) have been brought into sharp focus in the photovoltaic field due to their excellent performance in recent years. The power conversion efficiency (PCE) has reached to be 25.2% in state-of-the-art PSCs due to the outstanding intrinsic properties of perovskite materials as well as progressive optimization of each functional layer, especially the active layer and hole transporting layer (HTL). In this review, we mainly discuss various hole transporting materials (HTMs) consisting of HTL in PSCs. The progress in PSCs is firstly introduced, then the roles of HTL playing in photovoltaic performance improvement of PSCs are emphasized. Finally, we generally categorize HTMs into organic and inorganic groups and demonstrate both their advantages and disadvantages. Specially, we introduce several typical organic HTMs such as P3HT, PTTA, PEDOT:PSS, spiro-OMeTAD, and inorganic HTMs such as copper-based materials (CuOx, CuSCN, CuI, etc.), nickel-based materials (NiOx), and two-dimensional layered materials (MoS2, WS2, etc.). On basis of reviewing the reported HTMs in recent years, we expect to provide some enlightenment for design and application of novel HTMs that can be used to further promote PSCs performance.

An alternating copolymer, poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)-9-octyl-9H-carbazole) (HXS-1), was designed, synthesized, and used as the donor material for high efficiency polymer solar cells. The close packing of the polymer chains in the solid state was confirmed by XRD. A Jsc of 9.6 mA/cm2, a Voc of 0.81 V, an FF of 0.69, and a PCE of 5.4% were achieved with HXS-1 and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as a bulk heterojunction active layer spin-coated from a solvent mixture of 1,2-dichlorobenzene and 1,8-diodooctane (97.5:2.5) under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW/cm2.

Two new fused ring electron acceptors (FREAs) IDT-IC-T and IDT-IC-B with thienyl or phenyl substituents at the terminal INCN unit are synthesized. Theoretical calculations indicate that the two acceptors dominantly favor an intermolecular π-π stacking between the flanking terminal groups. The twist angle between the aryl substituent and INCN unit has a significant influence on the π-π stacking distance of terminal unit. IDT-IC-T with a smaller twist angle has a shorter π-π stacking distance than that of IDT-IC-B with a larger twist angle. In addition, extending the conjugation also affects the blend film morphology. IDT-IC-T and IDT-IC-B based photoactive films show appropriate nanoscale phase separations; whereas, blend films based on the parent compound IDT-IC show large-size acceptor domains. As expected, PBDB-T : IDT-IC-T blend films show higher and more balanced electron and hole mobilities. Moreover, these two acceptors present a good charge-transport connectivity arising from the extended conjugation and the increased intermolecular overlapping. Ultimately, IDT-IC-T demonstrates the highest electron mobility (1.47×10−4 cm2 V−1 s−1) and the best power conversion efficiency (PCE) of 9.43%. As for IDT-IC, which only shows an electron mobility of 7.33×10−5 cm2 V−1 s−1 and a PCE of 5.82%. These findings provide a facile and effective way to improve the photovoltaic performance.

Bismuth-halide-based perovskites could be promising candidates for construction of lead-free and air-stable perovskites solar cells. Herein we report a simple strategy on the fabrication of the Ag3BiBr6 thin films. They were prepared by flexible spin-coating procedure and the solvent of N-butylamine is found to be critical to dissolving AgBr precursor solution under lower room temperatures. Experiment and calculation analysis both reveal it with hexagonal structure. Their physical properties and photovoltaic performance were both characterized. We demonstrate that Ag3BiBr6 films are capable of storing stably over 30 days in air. These results will inspire more research works on developing and applying Ag3BiBr6 beyond solar energy applications.

In this study, we successfully developed a novel method to create [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoscale aggregates using supercritical carbon dioxide (scCO2) annealing and fabricated bulk heterojunction (BHJ) solar cells with the nanoscale PCBM to improve device performance. PCBM forms nanoscale aggregates with a size of approximately 70 nm after scCO2 annealing at 11 MPa and 50 °C for 60 min. However, PCBM remains amorphous after thermal annealing (TA) at 150 °C for 5 min. The morphology, structure, and crystallinity of poly(3-hexylthiophene) (P3HT) in the scCO2-treated P3HT film are nearly the same as those in the TA-treated P3HT film. In the P3HT/PCBM blend, the formation of PCBM nanoscale aggregates by scCO2 treatment decreases the disturbance for P3HT crystallization and improves diffusion and regular packing of P3HT molecular chains. This increases the crystallinity of P3HT so that it becomes higher than that in the TA-treated blend film. The nanoscale aggregates of PCBM and the higher crystallinity of P3HT give the scCO2-treated P3HT/PCBM BHJ solar cells a maximum power conversion efficiency (PCE) of 2.74%, which is much higher than that of the as-cast device (PCE is 1.70%) and a little higher than the highest PCE (2.64%) of thermally annealed devices. These results indicate that scCO2 is an effective, mild, and environmental method to modulate the nanoscale aggregates of PCBM and to improve the PCE of BHJ solar cells. However, the size of the PCBM aggregates is a little larger than the most suitable size of the exciton diffusion length, leading to limited improvement of the PCE.

In the past few years, ternary polymer solar cells (PSCs) have emerged as a promising structure to simultaneously improve all solar cell parameters compared with traditional binary PSCs. The third component in ternary PSCs can play versatile functions to enhance the device performance. In this review, we summarize the design rules for fabricating high-performance ternary PSCs and introduce the recent progress in this field. In addition, the characterization methods for determining the role of the third component played in ternary PSCs are described.

Two conjugated polymers HXS-1 and PDFCDTBT were prepared by direct C-H activation and Suzuki polycondensation and their chemical structures were characterized by 1H NMR spectroscopy. The molecular weight of conjugated polymer synthesized by direct C-H activation is lower than the corresponding polymers prepared by Suzuki polycondensation. Conjugated polymers synthesized by direct C-H activation have considerable solubility in common organic solvents and form amorphous film. The photovoltaic property of conjugated polymers synthesized by direct C-H activation is inferior to the corresponding polymers synthesized by Suzuki polycondensation.

6,7-Dialkoxy-2,3-diphenylquinoxaline based narrow band gap conjugated polymers, poly[2,7-(9-octyl-9H-carbazole)-alt-5,5-(5,8-di-2-thinenyl-(6,7-dialkoxy-2,3-diphenylquinoxaline))] (PCDTQ) and poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(5,8-di-2-thinenyl-(6,7-dialkoxy-2,3-diphenylquinoxaline))] (PFDTQ), have been synthesized by Suzuki polycondensation. Their optical, electrochemical, transport and photovoltaic properties have been investigated in detail. Hole mobilities of PCDTQ and PFDTQ films spin coated from 1,2-dichlorobenzene (DCB) solutions are 1.0 × 10−4 and 4.1 × 10−4 cm2V−1 s−1, respectively. Polymer solar cells were fabricated with the as-synthesized polymers as the donor and PC61BM and PC71BM as the acceptor. Devices based on PCDTQ:PC71BM (1:3) and PFDTQ:PC71BM (1:3) fabricated from DCB solutions demonstrated a power conversion efficiency (PCE) of 2.5% with a Voc of 0.95 V and a PCE of 2.5% with a Voc of 0.98 V, respectively, indicating they are promising donor materials.

Two kinds of polyfluorenes bearing two lateral pyrene terminated alkyl chains and two alkyl chains per repeating unit were synthesized by Suzuki polycondensation and used to disperse single-walled carbon nanotubes (SWCNT) in organic solvents. Stable polymer-SWCNT complex can be formed via the multivalent π-π stacking interaction of the lateral pyrene functional groups and the polyfluorene backbone with the outer surface of carbon nanotubes; meanwhile the lateral alkyl chains can impart good solubility to the complex. As expected, polyfluorenes bearing lateral pyrene functional groups and octyl chains exhibited much higher carbon nanotube solubility in common organic solvents than the corresponding polyfluorenes bearing only octyl chains. Photophysical studies indicated that the formation of polymer-SWCNT complex can effectively quench the fluorescence of polyfluorenes.

Fluorene-based polymers containing perfluorooctyl moieties were designed and synthesized. The high electronegativity and F…H-C interactions are present in the fluorinated polymers, which slightly induce blue-shift of UV-Vis absorption in film. The fluoropolymer films prepared from fluorinated solvents show lager contact angles than those prepared in non-fluorinated solvents.

We report a newly synthesized polymer of a star-shaped porphyrin compound (TPA-FxP) with four oligofluorene arms at its meso positions with the pronounced enhancement of the two-photon properties and the generation of singlet oxygen by utilizing the two-photon excited fluorescence resonance energy transfer. The steady-state spectra and transient triplet-triplet absorption spectra give evidence that the enhanced two-photon absorption cross section results from not only the through-space energy transfer (Forster) but also the through-bond energy transfer between conjugated peripheral oligofluorene arms and the porphyrin core. The two-photon absorption cross section at 780 nm up to 3360 GM (1 GM = 10−50 cm4·s/photon) of TPA-FxP was obtained, which is comparable to the highest values reported from other similar chemically modified porphyrin core compounds. Furthermore, the enhanced production of singlet oxygen under two-photon absorption conditions is also reported.

The green emission (g-band) in polyfluorene-based conjugated materials is studied by various spec- troscopic methods on defined poly(9,9 0 -dioctylfluorene) with one foluorenone unit (P20), which can be seen as a model compound for polyfluorene emitting g-band. The absorption and emission properties of P20 in the film and solution (room temperature) reveal the optical properties of the green emission emerging in polyfluorene-type polymer. All the experimental evidence obtained demonstrates that g-band in polyfluorene is attributed to the mono-chain fluo- renone; and the aggregation of the chains further suppresses the blue emission and enhances g-band.

Porphyrin-containing conjugated polymers with fluorene or carbazole as spacer groups were prepared by Sonogashira cross-coupling reactions. The polymers were of high molecular weight and the flexible alkyl chains on fluorene or carbazole units made the conjugated polymers soluble in common organic solvents, such as THF and methylene chloride. The polymers could form high quality durable films from solution casting. Their optical and photocurrent responsive properties were investigated. It was found that the photocurrent response was directly proportional to the content of porphyrin. The incorporation of carbazole units into the polymer chains also gave positive contribution to the photocurrent generation in some extent.

A new kind of amphiphilic polyether dendrimer bearing eight alkyl chains at the periphery were synthesized step by step using the convergent method. Their structures were confirmed by FT-IR spectra, 1H NMR spectra and mass spectra etc. The π-A isotherms, hysteresis and molecular area-time curves at air water interface were reported. These results showed that they could form stable monolayers at water surface.