The Non‐Innocent Role of Hole‐Transporting Materials in Perovskite Solar Cells

Abstract

Spiro-OMeTAD (2,2 0 ,7,7 0 -tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 0 -spirobifluorene, from now on simply Spiro) is the most used and applied hole-transporting material (HTM) in perovskite solar cells (PSCs). The reason is straightforward: after opportune formulation (i.e., addition of 4-tert-butylpyridine, tBP, and lithium bis(trifluoromethylsulfonyl)-imide, LiTFSI, as dopants/additives), the planar PSCs normally achieve the highest power conversion efficiencies (PCEs) at lab scale (up to 24%). [1] However, this result is strongly overestimated when compared with the longterm stability of the device: it has been already largely proved by several works that the necessary doping of the organic material augments the hygroscopic character of the layer, thus favoring moisture uptake from the atmosphere and subsequent perovskite film decomposition. [2,3] Combined with its notable cost (nowadays slightly decreasing due to the presence of more producers on the market), [4,5] we can assert that Spiro is only a model/benchmark HTM useful for lab-scale PSC production and testing. This is a pity indeed, because it possesses all the properties required for a good performing HTM: the high glass transition temperature due to the Spiro-type bond, the smooth film morphology that it forms, the relative simple processing, the electrochemical stability, the optical transparency in the visible window, the amorphous nature, the optimal band alignment with the perovskite valence band, and the chemical compatibility with dopants. Despite these very promising properties therefore, the high costs and very low inherent hole conductivity hugely reduce the possible commercialization of Spiro-based PSCs and force scientists to identify new avenues for obtaining long-term stability and for simplifying device processing methods.

Bio-Inspired Graphitic Carbon-Based Large-Area (10 × 10 cm2) Perovskite Solar Cells: Stability Assessments under Indoor, Outdoor, and Water-Soaked Conditions

“…However, they are currently considered the best candidates for light-harvesting applications due to their high optoelectronic properties and their photovoltage spectral response in the visible region . However, the contemplating requirements of PSCs from the golden triangle in PV technologies (including the efficiency, lifetime (or stability), and cost) limit their practical commercial viability largely because of two concerns: (1) stability issues within the material and in the device layers and (2) cost issues related to the use of unstable organic hole-transporting materials (HTMs) (typically Spiro-MeOTAD (∼$ 500/g) and PTAA (∼$ 3000/g) in most cases of n – i – p and p – i – n structured devices, respectively), , which have low crystallization temperature, which is the reason for the weak interaction between the perovskite layer and the back contact electrode layer, and this sequentially deteriorates the PSC performance. The energy-intensive and expensive noble metal (typically gold in most cases) used as a counter electrode (CE) in PSC devices can eventually diffuse through the hole-transporting layers (HTLs), exhibiting a low-temperature (at ∼70 °C) ion migration nature, which tends to react with the halides of the active absorbing perovskite layer forming a metal halide barrier layer and further diminishes the device performance …”

“…(2) cost issues related to the use of unstable organic holetransporting materials (HTMs) (typically Spiro-MeOTAD (∼$ 500/g) and PTAA (∼$ 3000/g) in most cases of n−i− p and p−i−n structured devices, respectively), 4,5 which have low crystallization temperature, which is the reason for the weak interaction between the perovskite layer and the back contact electrode layer, and this sequentially deteriorates the PSC performance. The energy-intensive and expensive noble metal (typically gold in most cases) used as a counter electrode (CE) in PSC devices can eventually diffuse through the holetransporting layers (HTLs), exhibiting a low-temperature (at ∼70 °C) ion migration nature, 6 which tends to react with the halides of the active absorbing perovskite layer forming a metal halide barrier layer and further diminishes the device performance.…”

A Modified Sequential Deposition Route for High-Performance Carbon-Based Perovskite Solar Cells under Atmosphere Condition

“…As known to us all, the moisture instability of the perovskite layer reduces the lifetime of the solar cells and, thus, limits their application outdoors. Under the combined action of humidity, oxygen, and ultraviolet light, the perovskite active layer will rapidly decompose [7][8][9][10]. Moreover, the majority of PSCs use hole transport material (HTM) to facilitate hole extraction.…”

An Efficient Modification at the Hole Transporting Layer/Electrode Interface for High‐Performance Ag‐Electrode‐Based Perovskite Solar Cells

“…Nowadays, most PSCs with high PCEs are based on a conventional n‐i‐p configuration, where the perovskite absorber layer is underneath the hole transporting layer (HTL), and the anode is deposited on the top. [ 10,11 ] In this configuration, Au is the standard back contact electrode material due to its decent conductivity and appropriate work function. [ 10 ] However, Au is extremely expensive, which dramatically increases the device fabrication cost.…”