The evolution of triphenylamine hole transport materials for efficient perovskite solar cells

Enhancement of Efficiency of Perovskite Solar Cells with Hole-Selective Layers of Rationally Designed Thiazolo[5,4-d]thiazole Derivatives

We introduce thiazolo[5,4-d]thiazole (TT)-based derivatives featuring carbazole, phenothiazine, or triphenylamine donor units as hole-selective materials to enhance the performance of wide-bandgap perovskite solar cells (PSCs). The optoelectronic properties of the materials underwent thorough evaluation and were substantially fine-tuned through deliberate molecular design. Time-of-flight hole mobility TTs ranged from 4.33 × 10-5 to 1.63 × 10-3 cm2 V-1 s-1 (at an electric field of 1.6 × 105 V cm-1). Their ionization potentials ranged from -4.93 to -5.59 eV. Using density functional theory (DFT) calculations, it has been demonstrated that S0 → S1 transitions in TTs with carbazolyl or ditert-butyl-phenothiazinyl substituents are characterized by local excitation (LE). Mixed intramolecular charge transfer (ICT) and LE occurred for compounds containing ditert-butyl carbazolyl-, dimethoxy carbazolyl-, or alkoxy-substituted triphenylamino donor moieties. The selected derivatives of TT were used for the preparation of hole-selective layers (HSL) in PSC with the structure of glass/ITO/HSLs/Cs0.18FA0.82Pb(I0.8Br0.2)3/PEAI/PC61BM/BCP/Ag. The alkoxy-substituted triphenylamino containing TT (TTP-DPA) has been demonstrated to be an effective material for HSL. Its layer also functioned well as an interlayer, improving the surface of control HSL_2PACz (i.e., reducing the surface energy of 2PACz from 66.9 to 52.4 mN m-1), thus enabling precise control over perovskite growth energy level alignment and carrier extraction/transportation at the hole-selecting contact of PSCs. 2PACz/TTP-DPA-based devices showed an optimized performance of 19.1 and 37.0% under 1-sun and 3000 K LED (1000 lx) illuminations, respectively. These values represent improvements over those achieved by bare 2PACz-based devices, which attained efficiencies of 17.4 and 32.2%, respectively. These findings highlight the promising potential of TTs for the enhancement of the efficiencies of PSCs.

Para-selective nitrobenzene amination lead by C(sp2)-H/N-H oxidative cross-coupling through aminyl radical

Arylamines, serving as crucial building blocks in natural products and finding applications in multifunctional materials, are synthesized on a large scale via an electrophilic nitration/reduction sequence. However, the current methods for aromatic C-H amination have not yet attained the same level of versatility as electrophilic nitration. Here we show an extensively investigated transition metal-free and regioselective strategy for the amination of nitrobenzenes, enabling the synthesis of 4-nitro-N-arylamines through C(sp2)-H/N-H cross-coupling between electron-deficient nitroarenes and amines. Mechanistic studies have elucidated that the crucial aspects of these reactions encompass the generation of nitrogen radicals and recombination of nitrobenzene complex radicals. The C(sp2)-N bond formation is demonstrated to be highly effective for primary and secondary arylamines as well as aliphatic amines under mild conditions, exhibiting exceptional tolerance towards diverse functional groups in both nitroarenes and amines (>100 examples with yields up to 96%). Notably, this C(sp2)-H/N-H cross-coupling exhibits exclusive para-selectivity.

In Situ Photogenerated Radicals of Hydroxyl Substituted Pyrene-Based Triphenylamines with Enhanced Transport and Free Doping/Post-Oxidation for Efficient Perovskite Solar Cells

The high-performance hole transporting material (HTM) is one of the most important components for the perovskite solar cells (PSCs) in promoting power conversion efficiency (PCE). However, the low conductivity of HTMs and their additional requirements for doping and post-oxidation greatly limits the device performance. In this work, three novel pyrene-based derivatives containing methoxy-substituted triphenylamines units (PyTPA, PyTPA-OH and PyTPA-2OH) are designed and synthesized, where different numbers of hydroxyl groups are connected at the 2- or 2,7-positions of the pyrene core. These hydroxyl groups at the 2- or 2,7-positions of pyrene play a significantly role to enhance the intermolecular interactions that are able to generate in situ radicals with the assistance of visible light irradiation, resulting in enhanced hole transferring ability, as well as an enhanced conductivity and suppressed recombination. These pyrene-core based HTMs exhibit excellent performance in PSCs, which possess a higher PCE than those control devices using the traditional spiro-OMeTAD as the HTM. The best performance can be found in the devices with PyTPA-2OH. It has an average PCE of 23.44% (PCEmax = 23.50%), which is the highest PCE among the reported PSCs with the pyrene-core based HTMs up to date. This research offers a novel avenue to design a dopant-free HTM by the combination of the pyrene core, methoxy triphenylamines, and hydroxy groups.

Morphology, dynamic disorder, and charge transport in an indoloindole-based hole-transporting material from a multi-level theoretical approach

The exponential effort in the design of hole-transporting materials (HTMs) during the last decade has been motivated by their key role as p-type semiconductors for (opto)electronics. Although structure-property relationships have been successfully rationalized to decipher optimal site substitutions, aliphatic chain lengths or efficient aromatic cores for enhanced charge conduction, the impact of molecular shape, material morphology and dynamic disorder has been generally overlooked. In this work, we characterize by means of a multi-level theoretical approach the charge transport properties of a novel planar small-molecule HTM based on the indoloindole aromatic core (IDIDF), and compare it with spherical spiro-OMeTAD. Hybrid DFT calculations predict moderate band dispersions in IDIDF associated to the main transport direction characterized by π-π stacked molecules, both between the indoloindole cores and the thiophene groups. Strongly coupled dimers show relevant non-covalent interactions (NCI), indicating that NCI surfaces are a necessary but not exclusive requirement for large electronic couplings. We evidence remarkable differences in the site energy standard deviation and electronic coupling distributions between the conduction paths of IDIDF and spiro-OMeTAD. Despite the spherical vs. planar shape, theoretical calculations predict in the static crystal strong direction-dependent charge transport in the two HTMs, with ca. one-order-of-magnitude higher mobility (μ) for IDIDF. The dynamical disorder promoted by finite temperature effects in the crystal leads to a reduction in the hole transport properties in both HTMs, with maximum μ values of 2.42 and 4.2 × 10-2 cm2 V-1 s-1 for IDIDF and spiro-OMeTAD, respectively, as well as a significant increase in the transport anisotropy in the latter. Finally, the impact of the material amorphousness in the hole mobility is analysed by modelling a fully random distribution of HTM molecules. An average (lower-bound) mobility of 1.1 × 10-3 and 4.9 × 10-5 cm2 V-1 s-1 is predicted for planar IDIDF and spherical spiro-OMeTAD, respectively, in good accord with the experimental data registered in thin-film devices. Our results demonstrate the strong influence of molecular shape, dynamic structural fluctuations and crystal morphology on the charge transport, and pose indoloindole-based HTMs as promising materials for organic electronics and photovoltaics.

An efficient asymmetric structured hole transport material for perovskite solar cells

Hole transport materials (HTMs) play a crucial role in achieving efficient perovskite solar cells (PSCs). In this work, an HTM MF-ACD with an asymmetric structure is designed by introducing two different peripheral end groups. The asymmetric feature increases the molecular dipole of MF-ACD, and endows MF-ACD with good stability and film formation properties, higher hole mobility and conductivity. Consequently, the MF-ACD-based PSC shows a high efficiency of 23.1%, which is much higher than that of the symmetric counterpart. The results show that the asymmetric configuration might be a potential choice to develop more efficient HTMs.

Donor engineering of a benzothiadiazole-based D-A-D-type molecular semiconductor for perovskite solar cells: a theoretical study

Due to the easy formation of compact molecular packing arrangements and the favorable photophysical and electrochemical properties, donor-acceptor-donor (D-A-D)-type small molecule hole-transporting materials (HTMs) have been widely synthesized and researched to improve the efficiency and stability of perovskite solar cells (PSCs). The main approach in recent experiments has been to seek good acceptors, whereas the influence of the electron-donating units has been less reported. In this work, six new benzothiadiazole-based D-A-D-type HTMs are tailored by employing the ethyl-substituted phenoxazine (POZ), phenothiazine (PTZ) and carbazole (CZ) as the donors. To obtain an elementary understanding of new HTMs, the electronic, optical, hole-transporting and interfacial properties are simulated with quantum chemistry methods. The results indicate that all tailored HTMs exhibit suitable energy alignment compared with the band structures of the perovskite, and the continuous highest occupied molecular orbital (HOMO) levels will be helpful for interfacial energy regulation. In comparison with the YN1, the maximum absorption wavelengths of the newly designed HTMs are red-shifted due to the decreased excitation energies from the ground-state to the first singlet excited-state. Importantly, the hole mobilities of all designed HTMs are distinctly higher than the referenced YN1, which is contributed by the better planarity of the molecular skeleton and the easier orbital overlapping between adjacent molecules. The interfacial simulations manifest that the FAPbI3/SM37 system displays a more stable adsorption configuration and greater charge redistributions at the interface compared to YN1, which further promotes the separation of photogenerated electron-hole pairs. Moreover, larger Stokes shifts and better solubility are also acquired for the new HTMs. In summary, our calculations not only propose several potential highly efficient HTMs, but also provide useful insights at the atomic level for the experimental synthesis of new D-A-D-type HTMs.

Challenges in the design and synthesis of self-assembling molecules as selective contacts in perovskite solar cells

Self-assembling molecules (SAMs), as selective contacts, play an important role in perovskite solar cells (PSCs), determining the performance and stability of these photovoltaic devices. These materials offer many advantages over other traditional materials used as hole-selective contacts, as they can be easily deposited on a large area of metal oxides, can modify the work function of these substrates, and reduce optical and electric losses with low material consumption. However, the most interesting thing about SAMs is that by modifying the chemical structure of the small molecules used, the energy levels, molecular dipoles, and surface properties of this assembled monolayer can be modulated to fine-tune the desired interactions between the substrate and the active layer. Due to the important role of organic chemistry in the field of photovoltaics, in this review, we will cover the current challenges for the design and synthesis of SAMs PSCs. Discussing, the structural features that define a SAM, (ii) disclosing how commercial molecules inspired the synthesis of new SAMs; and (iii) detailing the pros- and cons- of the reported synthetic protocols that have been employed for the synthesis of molecules for SAMs, helping synthetic chemists to develop novel structures and promoting the fast industrialization of PSCs.

Benzothieno[3,2-b]thiophene-Based Noncovalent Conformational Lock Achieves Perovskite Solar Cells with Efficiency over 24

Organic semiconductors with noncovalently conformational locks (OSNCs) are promising building blocks for hole-transporting materials (HTMs). However, lack of satisfied neighboring building blocks negatively impacts the optoelectronic properties of OSNCs-based HTMs and imperils the stability of perovskite solar cells (PSCs). To address this limitation, we introduce the benzothieno[3,2-b]thiophene (BTT) to construct a new OSNC, and the resulting HTM ZS13 shows improved intermolecular charge extraction/transport properties, proper energy level, efficient surface passivation effect. Consequently, the champion devices based on doped ZS13 yield an efficiency of 24.39 % and 20.95 % for aperture areas of 0.1 and 1.01 cm2 , respectively. Furthermore, ZS13 shows good thermal stability and the capability of inhibiting I- ion migration, thus, leading to enhanced device stability. The success in neighboring-group engineering can triggered a strong interest in developing thienoacene-based OSNCs toward efficient and stable PSCs.

Inhibition of Ion Migration for Highly Efficient and Stable Perovskite Solar Cells

In recent years, organic-inorganic halide perovskites are now emerging as the most attractive alternatives for next-generation photovoltaic devices, due to their excellent optoelectronic characteristics and low manufacturing cost. However, the resultant perovskite solar cells (PVSCs) are intrinsically unstable owing to ion migration, which severely impedes performance enhancement, even with device encapsulation. There is no doubt that the investigation of ion migration and the summarization of recent advances in inhibition strategies are necessary to develop "state-of-the-art" PVSCs with high intrinsic stability for accelerated commercialization. This review systematically elaborates on the generation and fundamental mechanisms of ion migration in PVSCs, the impact of ion migration on hysteresis, phase segregation, and operational stability, and the characterizations for ion migration in PVSCs. Then, many related works on the strategies for inhibiting ion migration toward highly efficient and stable PVSCs are summarized. Finally, the perspectives on the current obstacles and prospective strategies for inhibition of ion migration in PVSCs to boost operational stability and meet all of the requirements for commercialization success are summarized.

Redox: Organic Robust Radicals and Their Polymers for Energy Conversion/Storage Devices

Persistent radicals can hold their unpaired electrons even under conditions where they accumulate, leading to the unique characteristics of radical ensembles with open-shell structures and their molecular properties, such as magneticity, radical trapping, catalysis, charge storage, and electrical conductivity. The molecules also display fast, reversible redox reactions, which have attracted particular attention for energy conversion and storage devices. This paper reviews the electrochemical aspects of persistent radicals and the corresponding macromolecules, radical polymers. Radical structures and their redox reactions are introduced, focusing on redox potentials, bistability, and kinetic constants for electrode reactions and electron self-exchange reactions. Unique charge transport and storage properties are also observed with the accumulated form of redox sites in radical polymers. The radical molecules have potential electrochemical applications, including in rechargeable batteries, redox flow cells, photovoltaics, diodes, and transistors, and in catalysts, which are reviewed in the last part of this paper.

Effect of steric hindrance and number of substituents on the transfer and interface properties of Y-shaped hole-transporting materials for perovskite solar cells

Alkyl sulfoxide groups were introduced into the branch chain terminals of a hole-transporting material (HTM) Z34 with different numbers and positions to design four new Y-shaped HTMs: ZT1, ZT2, ZT3 and ZT4. The effects of steric hindrance and number of substituents on the transfer and interface properties of the Y-shaped HTMs were investigated theoretically. Calculations reveal that the introduction of alkyl sulfoxide increases the distribution of intramolecular holes and orbital overlap between the HOMOs of the dimers. The electronic coupling was greatly improved owing to the increased distribution of holes and orbital overlap. ZT1 shows small steric hindrance when one alkyl sulfoxide is introduced into the top branch chain, which leads to translation π-π stacking. ZT2 and ZT4 show slightly greater steric hindrance when two or four alkyl sulfoxide groups are introduced into the side branch chains, which leads to face-to-face stacking. While ZT3 shows large steric hindrance when three alkyl sulfoxide groups are introduced into the top and side branch chains, which causes head-to-head stacking. With the increase in number of alkyl sulfoxide groups, the steric hindrance of the molecule increases and the hole mobility decreases. ZT1 achieves the highest hole mobility (2.63 × 10-2 m2 V-1 s-1) that is two orders of magnitude higher than that of Z34 (1.36 × 10-4 m2 V-1 s-1) owing to the optimal balance between the number of alkyl sulfoxide groups and steric hindrance. The HTM/CH3NH3PbI3 adsorbed system was also simulated to characterize the interface properties. Enhanced interface interaction was achieved in the HTM/perovskite systems of ZT2 and ZT3. The orbital distribution of the HTM/perovskite cluster indicates that the new HTMs can promote hole migration and prevent internal electron-hole recombination. The present work not only evaluates the reliable relationship between the structure and properties of new HTMs, but also provides a valuable design strategy for efficient Y-shaped HTMs.

Synthesis, Properties, and Application of Small-Molecule Hole-Transporting Materials Based on Acetylene-Linked Thiophene Core

Three small molecule organic compounds based on conjugated acetylene-linked methoxy triphenylamine terminal groups with different substituted thiophene cores were synthesized and firstly applied as hole-transporting materials (HTMs). The electron-deficient acetylene linkers can tune the energy levels of frontier molecular orbitals. The physical property measurements show that the HTMs (CJ-05, CJ-06, and CJ-07) possess good stability, hydrophobicity, and film-forming ability. Further, the HTMs were applied in the MAPbI₃-based perovskite solar cells (PSCs), and the best power conversion efficiency (PCE) of 6.04%, 6.77%, and 6.48% was achieved, respectively, which implies that they exhibit great potential in photovoltaic applications.

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

Perovskite solar cells (PSCs) have great potential for future application. However, the commercialization of PSCs is limited by the prohibitively expensive and doped hole-transport materials (HTMs). In this regard, small molecular dopant-free HTMs are promising alternatives because of their low cost and high efficiency. However, these HTMs still have a lot of space for making further progress in both efficiency and stability. This review firstly provides outlining analyses about the important roles of molecular orientation when further enhancements in device efficiency and stability are concerned. Then, currently studied strategies to control molecular orientation in small molecular HTMs are presented. Finally, we propose an outlook aiming to obtain optimized molecular orientation in a cost-effective way.

Simple and robust phenoxazine phosphonic acid molecules as self-assembled hole selective contacts for high-performance inverted perovskite solar cells

For inverted perovskite solar cells (PSCs), the interfacial defects and mismatched energy levels between the perovskite absorber and charge-selective layer restrain the further improvement of photovoltaic performance. Interfacial modification is a powerful tool for defect passivation and energy level turning by developing new charge-selective materials. Herein, we report three new molecules, 2BrCzPA, 2BrPTZPA, and 2BrPXZPA as self-assembled hole selective contacts (SA-HSCs) by an economical and efficient synthetic procedure. Benefiting from the stronger electron-donating ability of phenothiazine and phenoxazine compared to that of carbazole, 2BrPTZPA and 2BrPXZPA showed more matched energy levels and decreased energy loss. In addition, the ITO substrate coated with 2BrPTZPA and 2BrPXZPA could induce higher-quality perovskite crystal growth without obvious grain boundaries in the vertical direction. Consequently, the corresponding inverted PSCs with decreased trap state density achieved high power convention efficiencies (PCEs) of 22.06% and 22.93% (certified 22.38%) for 2BrPTZPA and 2BrPXZPA, respectively. Furthermore, the 2BrPXZPA-based device with encapsulation retained 97% of the initial efficiency after 600 h of maximum power point tracking under one sun continuous illumination. Finally, 2BrPXZPA was also used for the surface modification of NiO_x, and the inverted PSC based on the NiO_x/2BrPXZPA bilayer achieved a higher PCE of 23.66% with an open circuit voltage of 1.21 V. This work extends the design strategy of SA-HSCs for efficient and stable inverted PSCs and promotes the commercialization process.