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-ofthe-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 (CuO x , CuSCN, CuI, etc.), nickel-based materials (NiO x ), and twodimensional layered materials (MoS 2 , WS 2 , 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.

Planar heterojunction perovskite solar cells with a high efficiency up to 17.76% are fabricated by modifying the compact TiO2 (c-TiO2) with a [6,6]-phenyl-C61-butyric acid (PCBA) monolayer. High quality CH3NH3PbI3 films can be easily fabricated on PCBA-modified c-TiO2 substrates by a one-step solution processing method. Significant improvements of the device parameters are observed after PCBA modification. A high open-circuit voltage (Voc) of 1.16 V has been achieved, indicating that the PCBA monolayer can act as a hole blocking layer to reduce the trap site density atop the c-TiO2 and the hole recombination at the c-TiO2 /perovskite interface. The enhancement of the fill factor, as well as the partial quenching of the fluorescence of perovskite after modification with PCBA, reveals that the charge extraction is improved.

Intramolecular S–O interactions were adopted for designing HTMs with high hole mobility and a suitable HOMO energy level in PVSK.

A series of crosslinked diphenylamine derivatives have been developed and employed as hole transport materials in inverted p–i–n planar perovskite solar cells, which exhibit the significantly improved device efficiency and stability.

To develop novel hole-transport materials (HTMs) with less synthetic steps is still a great challenge. Here, a small molecule hexakis[4-(N,N-di-p-methoxyphenylamino)phenyl]benzene (F-1) was successfully synthesized by a relatively simple scenario. F-1 exhibits a deep highest occupied molecular orbital energy level of -5.31 eV. Notably, F-1 also features 2 times higher hole mobility of 4.98 × 10 cm V s than that of the mostly used 2,2',7,7'-tetrakis(N,N-bis(4-methoxyphenyl)amino)-9,9'-spirobifluorene (spiro-OMeTAD). Consequently, F-1-based perovskite solar cells (PSCs) show markedly improved performance compared with spiro-OMeTAD-based ones. These results indicate such a material can be a promising HTM candidate to boost the overall performance of the PSC.

Two conjugated polymers (P1, P2) were designed and used as donor materials for polymer solar cells. The open-circuit voltage was significantly improved to 0.85 V. The synergistic effect of combined carboxylate and fluoro functional groups promoted the PCE to as high as 9.26%.

A new 1,8-naphthalimide based planar small molecular acceptor and two benzothiadiazole based wide band gap (WBG) polymer donors P1 and P2 were synthesized for nonfullerene organic photovoltaic cells (OPVs). Devices based on fluorinated polymer P2 achieved a highly improved PCE of 3.71% with an open circuit voltage (V(oc)) of 1.07 V, which is beyond the currently known levels for nonfullerene OPVs with the V(oc) higher than 1 V.

No abstract