Less is More: Dopant‐Free Hole Transporting Materials for High‐Efficiency Perovskite Solar Cells

Abstract

years, the demands for reducing the fabrication costs and the energy payback time of photovoltaic devices led to the invention of emerging third-generation PV technologies, such as dye-sensitized solar cells (DSSCs), organic photovoltaic (OPV), quantum dots solar cells, and the latest perovskite solar cells (PSCs). Among them, PSC is the only technology that combines both merits of low processing energy cost and high power conversion efficiency (PCE), which makes it possible to replace the silicon-based PV technology.Since the first reports by Bach and Grätzel et al. about the use of 2,2′,7,7′tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) as the hole-transporting material (HTM) in solid-state DSSCs (ssDSSC), spiro-OMeTAD becomes the benchmark of all the new HTMs developed for ssDSSC and PSCs. Contrary to the popular myth, spiro-OMeTAD is not completely amorphous but a semi-crystalline organic p-type semiconductor with a glass transition temperature of 121 °C and a melting point of 246 °C. [1] Its chemical structure is shown in Figure 1a and Grätzel's lab first revealed the crystal structure in 2014 [2] ( Figure 1b). It has a large bandgap of 2.98 eV and yields almost colorless thin films when deposited from solution in organic solvents such as toluene or chlorobenzene. The first oxidation potential of spiro-OMeTAD is found to be approximately −5.1 eV as derived from electrochemical and photoelectron spectroscopy measurements. [3] In the case of the solid-state PSCs reported since 2012, [4,5] the PCE of lab-made devices has increased from 9.7% [6] to above 21% [7] in 2017, owing to the improvement in the perovskite composition and device engineering. However, on account of several desirable properties, (i) favorable glass transition temperature; (ii) reasonable solubility; (iii) proper ionization potential; (iii) low visible light absorption; and (iv) appropriate solid-state morphology, spiro-OMeTAD has remained as the material of choice when high PCEs are demanded. Due to the fact that spiro-OMeTAD suffers from a relatively low conductivity in its pristine form, all the eyecatching PCEs were achieved by adding additives and/or dopants-a standard technique used to tune the electrical properties of both organic and inorganic semiconductors. Typical additives including 4-tert-butylpyridine (TBP) and lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI) were added to the spin-coating formulation of spiro-MeOTAD and Perovskite solar cells have delivered power conversion efficiency beyond 22% in less than seven years, implying the potential for the paradigm shift of lowcost photovoltaics with high efficiency and low embedded energy. Besides the "perovskite fever," the development of new hole transport materials (HTM), especially dopant-free HTMs, is another research hotspot. This is because the currently used HTMs, such as spiro-OMeTAD derivatives, require additional chemical doping process to ensure sufficient conductivity and proper ionic potential level for efficient hole transport and colle...

LiTFSI‐Free Spiro‐OMeTAD‐Based Perovskite Solar Cells with Power Conversion Efficiencies Exceeding 19%

“…Novel additives, such as H 3 PO 4 [28] and fluorine-containing Lewis acid, [29] are also employed to enhance the HTM conductivity while reducing the hysteresis in devices due to the lack of extrinsic ion migration such as Li ion. [31][32][33] Another approach is the use of dicationic salts of spiro-OMeTAD, the mechanism of which is shown in Equation (3) [34,35] spiro-OMeTAD spiro-OMeTAD 2 spiro-OMeTAD 2 + → + + [18,30] The by-products of the oxidation, such as reduced metal cations and counter ions, remain as impurities in the final devices with still undetermined effects on device performance.…”

Enhanced Stability of Perovskite Solar Cells Incorporating Dopant‐Free Crystalline Spiro‐OMeTAD Layers by Vacuum Sublimation

“…[13,21,22] Other dopants such as the Co complex FK209 (tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt (III) tri[bis(trifluoromethane) sulfonimide]) further increase the photovoltaic parameters when are doping the Spiro-OMeTAD layer in combination with LiTFSI and TBP, although the mechanism for this enhanced performance is still under debate. [23,24] The issue is so severe that several authors have aimed at the development of PSCs in a hole-transport layer free configuration. Unfortunately, one of the main challenges of perovskite-based solar cells is their stability and reproducibility, which is partially hindered by the use of dopants or additives.…”

Spiro‐Linked Molecular Hole‐Transport Materials for Highly Efficient Inverted Perovskite Solar Cells

“…The most successful application of the spiro‐based materials was implemented as HTLs in normal (n‐i‐p) architecture devices, which necessarily requires a thick film of around 200–300 nm to fully cover the perovskite layer underneath . However, it is noticeable that pristine spiro‐based materials without doping are unqualified in delivering highly efficient PSCs . This is primarily because the low conductivity of the undoped spiro‐based materials can result in large internal series resistance .…”

“…However, it is noticeable that pristine spiro‐based materials without doping are unqualified in delivering highly efficient PSCs . This is primarily because the low conductivity of the undoped spiro‐based materials can result in large internal series resistance . As a consequence, normal structure PSCs prepared with undoped spiro HTMs generally exhibited a rather low fill factor (FF) and open‐circuit voltage ( V OC ) .…”