Organometal halide perovskites are promising photo-absorption materials in solar cells due to high extinction coefficient, broad light absorption range and excellent semiconducting properties. The highest power conversion efficiency (PCE) of perovskite solar cells (PrSCs) is now 20.1%. However, a high-temperature processed mesoscopic metal oxide (e.g., TiO 2 ) must be removed to realize flexible PrSCs on plastic substrates using low temperature processes. Although the planar heterojunction (PHJ) structure can be considered as the most appropriate structure for flexible PrSCs, they have shown lower PCEs than those with a mesoscopic metal oxide layer. Therefore, development of interfacial layers is essential for achieving highly efficient PHJ PrSCs, and necessary in fabrication of flexible PrSCs. This review article gives an overview of progress in PHJ PrSCs and the roles of interfacial layers in the device, and suggests a practical strategy to fabricate highly efficient and flexible PHJ PrSCs. We conclude with our technical suggestion and outlook for further research direction.

A self-organized hole extraction layer (SOHEL) with high work function (WF) is designed for energy level alignment with the ionization potential level of CH3 NH3 PbI3 . The SOHEL increases the built-in potential, photocurrent, and power conversion efficiency (PCE) of CH3 NH3 PbI3 perovskite solar cells. Thus, interface engineering of the positive electrode of solution-processed planar heterojunction solar cells using a high-WF SOHEL is a very effective way to achieve high device efficiency (PCE = 11.7% on glass).

Tailoring the interface energetics between a polymeric hole extraction layer (HEL) and a photoactive layer (PAL) in organic photovoltaics (OPVs) and organic–inorganic hybrid perovskite solar cells (PrSCs) is very important to maximize open circuit voltage (Voc), power conversion efficiency (PCE), and device lifetime.

mesoporous charge transfer layer, their application into the fl exible electronics would be limited due to their brittleness and the high-temperature (i.e., T > 450 °C) sintering process which damages plastic substrates. Otherwise, solutionprocessed planar heterojunction (SP-PHJ) PrSCs can be fabricated using low-temperature processable interlayers without mesoporous metal oxides; this approach enables the fabrication of fl exible PrSCs on plastic substrates. [7][8][9] Therefore, the development of a solution-processed and effi ciently charge-transporting interlayer material has been required recently to increase PCE of SP-PHJ PrSCs for the practical application of highly effi cient and fl exible PrSCs. In addition to conducting polymers (e.g., poly(3,4-ethylen edioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), [ 7,9,15,16 ] self-organized hole extraction layer (SOHEL) [ 9 ] ), several different hole transport materials (HTMs), including inorganic materials (graphene oxide, [ 17 ] reduced graphene oxide, [ 18 ] NiO x [ 15 ] ) and conjugated polymers (e.g., PolyTPD, [ 19 ] P3HT, [ 20 ] poly(2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2′,5′di(thien-2-yl)thieno[3,2-b]thiophene) (DPP-DTT), [ 21 ] PCP-DTBT, [ 21 ] and PCDTBT [ 21 ] ) have been used to increase the PCE in SP-PHJ PrSCs.Among these materials, polymeric HTMs have been intensively developed for highly effi cient SP-PHJ PrSCs because they can be fabricated by solution processing and exhibit better hole mobility compared to vacuum-processed small-molecule HTMs. [ 3 ] In the fi rst few papers reporting SP-PHJ PrSCs, they were based on the PEDOT:PSS HTM [ 16 ] because PEDOT:PSS is one of the most commonly used HTMs for organic photovoltaics [22][23][24][25][26][27] and organic light-emitting diodes (OLEDs). [ 28,29 ] However, work function (WF) of PEDOT:PSS (≈4.9-5.2 eV) is highly dependent on the ratio of the polymeric acid, PSS relative to PEDOT [ 25,28 ] and therefore may not be suffi ciently high to perfectly match the valence band maxima (VBM) of perovskite materials (e.g., -5.43 eV for methylammonium lead iodide (MAPbI 3 )) for ohmic contact and consequential effi cient charge extraction. [ 9,15,[19][20][21][22][23][24][25][26][27][28][29] Moreover, PEDOT:PSS is dispersed with a large particle size (≈60 nm) in solution; [ 30 ] it precipitates slowly from the solution during storage and is diffi cult to redisperse from the aggregated Organic-inorganic hybrid perovskite solar cells are fabricated using a watersoluble, self-doped conducting polyaniline graft copolymer based on poly(4styrenesulfonate)-g -polyaniline (PSS -g-PANI) as an effi cient hole-extraction layer (HEL) because of its advantages, including low-temperature solution processability, high transmittance, and a low energy barrier with perovskite photoactive layers. Compared with conventional poly(3,4-ethylenedioxythiop hene):poly(styrene sulfonate) (PEDOT:PSS) dispersed in water solution, PSSg-PANI molecules are dissolved in water because of the polymeric dopant covalently bonde...