Recent progress in development of diverse kinds of hole transport materials for the perovskite solar cells: A review

Currently, several kinds of solar cells are developed and among them, organic-inorganic perovskite solar cells (PSCs) have received substantial interest because they have shown panchromatic light absorption capacities using low amounts of earth abundant materials, low exciton recombination rate, high carrier mobility and high solar to electrical power conversion efficiencies (PCEs) which can exceed 22%. In most PSCs, methylammonium lead iodide (CH3NH3PbI3 or MAPbI(3)) semiconductor perovskite is sandwiched between an electron transport material (ETM, n-type material) and a hole transport material (HTM, a p-type material). Two kinds of n-i-p and p-i-n configurations are fabricated which depends on the relative positions of the ETM and HTM layers in the device. The ETM in the n-i-p solar cell is usually a planar or mesoporous TiO2 thin film in which the perovskite solution is infiltrated. A disadvantage of the n-i-p cell is using very expensive HTM, 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9'-spirobifluorene (spiro-OMeTAD). Although numerous HTMs have been introduced, spiro-OMeTAD is the most efficient HTM. Nonetheless, spiro-OMeTAD must be ultra-pure to produce high performance and this increases its price which is not cost-effective from economical viewpoint. Additionally, its pristine form performs badly and doping is necessary to improve its hole-mobility and conductivity. Also, the common dopants including 4-tert-butylpyridine, lithium salts and cobalt complex are corrosive and very hygroscopic which result in the device instability. In the p-i-n cells, poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT:PSS) is extensively utilized as the HTM which is UV-unstable and hydrophilic that makes the device unstable. Furthermore, it is only deposited through solution coating process and shows acidic property that reacts with its underlying transparent conductive oxide. Thus, many attempts have been done to substitute the spiro-OMeTAD with other materials to find HTMs suitable for commercialization of PSCs. In this review, recent researches performed to improve the PCEs of PSCs using different classes of HTMs are studied.