Nexuses Between the Chemical Design and Performance of Small Molecule Dopant‐Free Hole Transporting Materials in Perovskite Solar Cells

Abstract

interface defects, energy alignments, device stability, and charge extraction and transportation. [3][4][5][6][7][8] Since 2009, Miyasaka and colleagues have attempted to use perovskite structures as light-harvesting layers of dye synthesized solar cells and achieved a PCE of 3.8% with an iodide/ triiodide-based redox shuttle and liquid electrolyte. [9] In 2011, Park and co-workers enhanced the PCE of the liquid-based perovskite solar cell to 6.5% by modifying the titania surface and the processing method for deposition of the perovskite absorber layer. [10] However, these devices were unstable and degraded quickly because perovskite compounds dissolved in liquid electrolytes. [11] Both Park and Snaith groups demonstrated that the liquid electrolyte could be replaced with 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenyla m i n e ) -9 , 9 ′ -s p i r o b i f l u o r e n e ( s p i r o -OMeTAD), which serves as the hole transport material (HTM). The corresponding solid-state PSCs exhibited enhanced efficiencies of 9.7% and 10.9%. [3,4] Since then, PSCs have become an international hot topic, and this direction has also entered a stage of rapid development.Generally, PSCs are categorized in three different architectures: planar n-i-p, mesoporous n-i-p, and inverted planar p-i-n, as shown in Figure 1a,b, and c, respectively. HTM layer in n-i-p devices is exposed to oxygen, moisture, while in p-i-n devices, it acts as the base surface for perovskite layer deposition. At both positions, HTM plays an essential role in device stability and controls the interface defects, energy alignments, charge extraction, and transportation. In p-i-n PSCs, the inorganic HTMs, poly (3, 4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT: PSS) and poly[bis(4-phenyl)(2,5,6-trimethylphenyl)amine] (PTAA) are commonly incorporated as HTMs where inorganic HTMs usually show good device stability. Still, the achievable PCE is limited by their relatively low intrinsic conductivity and surface defects. [12] Polymeric HTMs result in good morphological properties but inferior reproducibility and stability, particularly when dopants are needed for high efficiency devices. [13] On the other side, the n-i-p type is the most studied, with spiro-OMeTAD as primary HTM. In the earlier development of PSCs, spiro-OMeTAD as HTM sets several PCE records due to its ease of processing, somehow

Phthalocyanine in perovskite solar cells: a review

“…Nevertheless, another big concern is replacing the conventional hole transport materials (HTMs) ((2,2 0 ,7,7 0 -tetrakis(N,N 0 -di-pmethoxyphenyl-amine)-9,9 0 -spirobifluorene (spiro-OMeTAD), poly[bis(4-phenyl)(2,5,6-trimethylphenyl)-amine] (PTAA), and poly-(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT:PSS)) with new materials to enhance the cost-effectiveness and stability of the device, where small molecules can play a vital role. 9 Small molecules enable facile design modifications to achieve the required properties, such as dopant-free HTMs, interlayers, organic-inorganic hybrid HTMs or electron transport materials (ETMs), or as additives to improve the formation of perovskite crystals. 9,10 Conclusively, small molecules are beneficial to enhance the properties of the bottom, top, and perovskite layers.…”

“…9 Small molecules enable facile design modifications to achieve the required properties, such as dopant-free HTMs, interlayers, organic-inorganic hybrid HTMs or electron transport materials (ETMs), or as additives to improve the formation of perovskite crystals. 9,10 Conclusively, small molecules are beneficial to enhance the properties of the bottom, top, and perovskite layers. However, before the application of small molecules in PSCs, several factors need to be considered, such as their reproducibility, cost-effectiveness, solubility, stability, hydrophobicity, energy levels, and light absorption.…”

“…11 Generally, most of these properties are acquired through the selection of an appropriate molecular core, and then further design modifications are applied with regard to the specific device. 9 Besides these properties, the formation of a thin film with the desired structural and morphological characteristics is one of the main priorities, where conjugated p systems play a significant role. 12 Phthalocyanines (Pcs), which are aromatic heterocyclic compounds derived from porphyrins, are among the most comprehensive p-conjugated system-based molecules.…”

Controlling Molecular Orientation of Small Molecular Dopant-Free Hole-Transport Materials: Toward Efficient and Stable Perovskite Solar Cells

“…In a previous review focusing on dopant-free small molecular HTMs, molecular orientation was briefly discussed from the view point of molecular structure [ 45 ]. However, a certain molecular orientation is the result of synergy effects between the molecular structure and external factors such as substrates and processing methods [ 46 ].…”

Thermally stable inverted perovskite solar cells using an electropolymerized Zn-porphyrin film as a dopant-free hole-transporting layer

“…Among them, the development of dopant-free HTMs provides an effective means to improve the long-term stability of devices. 16,17 Currently, the commercialized polymers poly[bis(4phenyl)(2,4,6-trimethylphenyl) amine] (PTAA) and poly (3,4ethylene dioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) are routinely used in p-i-n-structured PSCs as the HTM due to their superior hole mobilities. 18,19 However, the biggest limitation of PTAA is the batch-to-batch variability and high cost.…”