Thermally Stable Perovskite Solar Cells by Systematic Molecular Design of the Hole-Transport Layer

Abstract

With metal halide perovskite solar cells (PSCs) now reaching device efficiencies >23%, more emphasis must now shift toward addressing their device stability. Recently, a triarylamine-based organic hole-transport material (HTM) doped with its oxidized salt analogue (EH44/EH44-ox) led to unencapsulated PSCs with high stability in ambient conditions. Here we report criteria for triarylamine-based organic HTMs formulated with stable oxidized salts as hole-transport layer (HTL) for increased PSC thermal stability. The triarylamine-based dopants must contain at least two para-electron-donating groups for radical cation stabilization to prevent impurity formation that leads to reduced PSC performance. The stability of unencapsulated devices prepared using these new HTMs stressed under constant load and illumination far outperforms that of both EH44/EH44-ox and Li+-doped spiro-OMeTAD controls at 50 °C. Furthermore, the ability to mix and match these dopants with a nonidentical small-molecule-based HTL matrix broadens the design scope for highly stable and cost-effective PSCs without sacrificing performance.

Fluorine‐Substituted Benzotriazole Core Building Block‐Based Highly Efficient Hole‐Transporting Materials for Mesoporous Perovskite Solar Cells

“…However, the complicated synthesis procedures and low yield of Spiro‐OMeTAD make its price very expensive, triggering the researchers over the world to search for/develop efficient and low‐cost alternative HTMs. To date, many alternative HTMs constructed with different conjugated core units, such as phenothiazine, thiophene, phenoxazine, silolothiophene, triazines, and tetraphenylethene, have been reported . Benzotriazole (BTA) is a cheap and commercially available material, which has been widely used in designing sensitizers for dye‐sensitized solar cells by reason of the outstanding electron‐accepting property.…”

Understanding the Degradation of Spiro‐OMeTAD‐Based Perovskite Solar Cells at High Temperature

“…[ 18,23,24 ] The chemical structure of spiro‐OMeTAD is shown in Figure a. Although other small molecules, [ 25–27 ] polymers, [ 28–30 ] and inorganics [ 31–33 ] have been developed for the HTLs of PSCs, spiro‐OMeTAD is still excellent among the reported HTL materials in terms of PCEs. PSCs with spiro‐OMeTAD are comparably stable at room temperature.…”

Choose Your Own Adventure: Fabrication of Monolithic All‐Perovskite Tandem Photovoltaics

“…[ 118–120,125 ] Spiro‐OMeTAD can be processed at room temperature out of orthogonal solvents (chlorobenzene) but is known to have poor temperature stability, because its dopants are too mobile, and will crystallize if heated, potentially causing mechanical deformations to the recombination stack and thus decreasing the solvent barrier properties. [ 165,166 ] This highlights the fact that each CL chosen brings their own complications to tandem fabrication, either through potential processing incompatibilities or instabilities.…”

“…[ 19,178,179 ] In n–i–p devices, Spiro‐OMeTAD is one of the most commonly used HTLs to reach high‐efficiency in a single junction, but has poor device operational and temperature stability and is likely the cause for the ≈100 °C ceiling in n–i–p tandems. [ 13,165 ] There are ample studies that explain the poor stability of spiro‐OMeTAD, [ 13,120,165,180 ] but the ISOS testing protocols (ISOS‐L‐1, 2, 3) [ 152–154 ] used are likely much more rigorous than what is required strictly for tandem fabrication, although all tandems should be made with the goal of passing these ISOS tests. This 100 °C ceiling does not limit perovskite fabrication but does limit the use of more traditional ETLs, such as TiO 2 or SnO 2 , for the CL3 because of the required higher annealing temperatures.…”