A–D–A-type S,N-heteropentacene-based hole transport materials for dopant-free perovskite solar cells

Abstract

This work reports the design and synthesis of acceptor-donor-acceptor (A-D-A) type low band gap hole transport materials (HTM) comprising S,N-heteropentacene central units for solid-state perovskite-based solar cells. The optical and electrochemical properties were tuned by the insertion of thiophene or ethylenedioxythiophene units in the molecular backbone. These HTMs showed strong absorption in the visible region and suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies with respect to the CH3NH3PbI3 perovskite. Mesoscopic solid state perovskite solar cells prepared by solution-2 processing using these HTMs generated power conversion efficiencies (PCE) of 10.3-11.4% without the use of any additive or dopant. The charge transfer behavior between photoexcited perovskite and the HTMs was further investigated by photo-induced absorption spectroscopy. Introduction Recently, organometal halide CH3NH3PbX3 (X = Cl, Br, I) perovskites 1, 2 have received considerable attention in solid-state photovoltaics due to their several interesting features, such as intense light absorption across the visible spectrum, high charge carrier mobilities, direct band gap, longer carrier diffusion length, and small exciton binding energy. 3-8 Miyasaka and coworkers were the first to report the incorporation of CH3NH3PbX3 (X = Br, I) perovskite as sensitizer in liquid electrolyte-based dye-sensitized solar cells reaching power conversion efficiencies (PCE) of 3.8%. 9 Later, Park et al. reported efficient and stable solid-state CH3NH3PbI3based perovskite solar cells with PCEs as high as 9.7%. 3 Since then, by extensive optimization of the metal oxide scaffolds and device processing conditions PCEs have been improved to over 19% using state of the art 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-MeOTAD) as hole transporting material. 10-12 Seok and co-workers reported a series of spiro-MeOTAD derivatives showing an enhanced PCE of 16.7% for the o-OMe substituted derivatives compared to the conventional p-OMe counterpart. 13 However, due to low hole mobility and conductivity of spiro-MeOTAD in the pristine form, additives, such as Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) 14 and cobalt complexes as p-type dopants, 15 are necessary to improve the device performance. Although PCEs up to 12.8% were reported for HTM-free perovskite devices, 16-19 the most efficient solar cells usually employed organic HTMs which play a key role for the hole transport and reduce charge recombination processes. 20 In this respect, a wide range of material classes consisting of pyrene, 21 thiophene, 22,Heteropentacene-based A-D-A type hole transport materials with suitable frontier orbital energy levels were synthesized and implemented in perovskite-based solar cells generating promising power conversion efficiencies up to 11.4%. Table of ContentHeteropentacene-based A-D-A type hole transport materials with suitable frontier orbital energy levels were synthesized and impl...

Regioregular Phthalocyanines Substituted with Bulky Donors at Non‐Peripheral Positions

“…Furthermore, the cross‐shaped 1 and 2 showed an excellent solubility in organic solvents and can form uniformly thin layers through their slow crystallization. In this study, the hole‐transporting performance of 1 and 2 thin films was evaluated to apply HTM layers to PSCs by means of the space‐charge limited current (SCLC) method (Figure a) . The hole‐mobility of 1 and 2 was determined to be 2.4×10 −6 and 1.1×10 −5 cm V −1 s −1 , respectively.…”

Enhancement of charge transport properties of small molecule semiconductors by controlling fluorine substitution and effects on photovoltaic properties of organic solar cells and perovskite solar cells

“…32–34 This type of D–A small molecule material often exhibits a high hole mobility and good conductivity with potential utility as a dopant-free hole transport material (HTM) in efficient perovskite solar cells (PSCs), which have recently emerged as promising photovoltaic devices. 35–38 Therefore, the rational molecular design of D–A structures has given rise to high-performance materials with physical properties that are appropriate for use in SM-OPVs as well as in PSCs. Thus far, a range of material designs have been presented and synthetic efforts have been applied toward controlling the optical, electrical and solid state assembly properties of a material by varying the D and A subunits to have different electron donating and accepting abilities.…”

High‐Efficiency Perovskite Solar Cells Employing a S,N‐Heteropentacene‐based D–A Hole‐Transport Material

“…The mostf requently used HTM is the wide band gap 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'spirobifluorene( spiro-MeOTAD) and has so far provent ob e the most effective HTM. [25,30] Besides acting as the HTM, these oligomers also displayed strongl ight absorption in the visible and near-infrared region, thus forming ad ual light absorbing system with the perovskite. [15][16][17][18][19][20][21][22][23] In order to reduce the processing ando ptimization time and thereby overall device cost, several dopant-free low band gap HTMs were developed;d evices employing these HTMs yielded PCEs in the range of 10-15 %.…”

“…[15][16][17][18][19][20][21][22][23] In order to reduce the processing ando ptimization time and thereby overall device cost, several dopant-free low band gap HTMs were developed;d evices employing these HTMs yielded PCEs in the range of 10-15 %. [25,30] Besides acting as the HTM, these oligomers also displayed strongl ight absorption in the visible and near-infrared region, thus forming ad ual light absorbing system with the perovskite. [29] We recently employed dicyanovinyl( DCV)-capped A-D-A oligomers comprising dithieno[2,3-d:2',3'-d']thieno[3,2-b:4,5-b']dipyrrole (S,N-heteropentacene) as the HTM in CH 3 NH 3 PbI 3 -based perovskite devices achieving PCEs up to 11.4 %.…”