Influence of π-bridge conjugation on the electrochemical properties within hole transporting materials for perovskite solar cells

Stable Perovskite Solar Cells Using Molecularly Engineered Functionalized Oligothiophenes as Low-Cost Hole-Transporting Materials

Triarylamine-substituted bithiophene (BT-4D), terthiophene (TT-4D), and quarterthiophene (QT-4D) small molecules are synthesized and used as low-cost hole-transporting materials (HTMs) for perovskite solar cells (PSCs). The optoelectronic, electrochemical, and thermal properties of the compounds are investigated systematically. The BT-4D, TT-4D, and QT-4D compounds exhibit thermal decomposition temperature over 400 °C. The n-i-p configured perovskite solar cells (PSCs) fabricated with BT-4D as HTM show the maximum power conversion efficiency (PCE) of 19.34% owing to its better hole-extracting properties and film formation compared to TT-4D and QT-4D, which exhibit PCE of 17% and 16%, respectively. Importantly, PSCs using BT-4D demonstrate exceptional stability by retaining 98% of its initial PCE after 1186 h of continuous 1 sun illumination. The remarkable long-term stability and facile synthetic procedure of BT-4D show a great promise for efficient, stable, and low-cost HTMs for PSCs for commercial applications.

Boosting the performance of D–A–D type hole-transporting materials for perovskite solar cells via tuning the acceptor group

Phenanthrothiadiazole (PT) and triphenylenobisthiadiazole (TBT) are proposed as the acceptor groups of D–A–D-type HTMs, and compared with the benzothiadiazole (BT) unit, three small molecule HTMs are investigated theoretically.

Tailoring of the core structure towards promising small molecule hole-transporting materials for perovskite solar cells: a theoretical study

The influence of the core on the performance of small molecule HTMs is investigated by using quantum chemistry methods, and potential HTMs are proposed.

Quantum Interference and Substantial Property Tuning in Conjugated Z-ortho-Regio-Resistive Organic (ZORRO) Junctions

Coherence is a significant factor in nanoscale electronic insulator technology and necessitates an understanding of the structure-property relationship between constructive and destructive quantum interference. This is particularly important in organic dielectric circuitry, which is the subject of this work. It is known that molecular wires composed of (i) meta-substituted phenylene rings, (ii) cross-conjugated double bonds (orthogonal to the molecular long axis), and (iii) single bonds can dramatically reduce electrical transmission. Here we add to these tools the use of an unexplored molecular shape to create strong and counterintuitive interference: a fully conjugated molecular wire with a structure that is forced back on itself in a Z shape, thereby exhibiting remarkably low conductance (G = 0.43 × 10^-9 S) even though the phenylene arrangements are ortho- rather than meta-disposed. We call these Z-shaped molecules having ultralow conduction Z-ortho-regio-resistive organics (ZORROs). Here we analyze a series of ZORRO molecules and find them to have significant insulating properties in the coherent electron-transport regime due to interfering transmission pathways in the phenylene rings. Importantly, we find that both electron-withdrawing (fluorine) and electron-donating (methoxy) substituents enhance the transmission, which is not desirable. The former is due to the suppression of the destructive quantum interference at the F site, thereby enhancing the overall transmission, much like a Büttiker probe. The latter is due to a methoxy unit resonance additive effect, akin to oxygen doping, and positively contributes to the transmission. We then examine the effects of replacing the phenylene rings with 4,5- and 3,4-disubstituted thiophenes and how this ZORRO modification further reduces the transmission. An ultralow conductance of 0.13 × 10^-9 S and a relatively high dielectric constant (ε_r) of ∼5 are predicted for the 3,4-thiophene ZORRO derivative, which closely resembles two cross-conjugated units, making it an intriguing candidate for a gate dielectric material.

Optimizing electron-rich arylamine derivatives in thiophene-fused derivatives as π bridge-based hole transporting materials for perovskite solar cells

Based on the observations of thienothiophene derivatives as π-bridged small molecule hole transporting materials (HTMs), adjusting their electron-rich arylamine derivatives is an effective approach to obtain the alternative HTMs for perovskite solar cells (PSCs). In this work, starting from a new electron-rich arylamine derivative and different π-bridged units of thienothiophene derivatives, a series of arylamine derivative-based HTMs were designed, and their properties were investigated using density functional theory combined with the Marcus charge transfer theory. Compared with the parental Z26 material, the designed H01-H04 exhibit appropriate frontier molecular orbitals, good optical properties, better solubility, good stability and higher hole mobilities. H01-H04 materials with high hole mobility (∼× 10-2) can serve as promising HTMs for improving the efficiency of PSCs. The results confirm that the design strategy of adjusting the electron-rich arylamine derivatives in thienothiophene derivatives as π-bridged HTMs is a reliable approach to obtain the promising HTMs for PSC applications.

Probing effects of molecular conformation on the electronic and charge transport properties in two- and three-dimensional small molecule hole-transporting materials: a theoretical investigation

Thiophene/benzene-fused π-conjugated systems are normally employed as the core units of two- and three-dimensionally expanded small molecule hole-transporting materials (HTMs) to improve their electronic and charge transport properties, whereas comparison studies between two-dimensional and three-dimensional core conformations are less reported. To further find useful clues for the design of highly-efficient small molecule HTMs and to find new core units, in this work, four HTM molecules are designed by employing triphenylene, benzotrithiophene, triptycene, and thiophenetriptycene as the core units, and simulated with density functional theory combined with the Marcus hopping model. Our results show that all the considered HTMs display appropriate molecular energy levels, less optical absorption in the visible light region and large Stokes shifts, and high hole mobilities (9.80 × 10-2 cm2 V-1 s-1). Compared with the two-dimensional core structures, the three-dimensional cores exhibit evident superiorities with the same chemical components. Meanwhile, we also find that the quasi-degenerate HOMO energy levels will be helpful to enlarge the transfer integrals between adjacent molecules, and further to promote the hole transport in HTMs. By considering the various elements simultaneously, these investigated HTMs (S-1-S-4) with thiophene- and benzene-fused cores can be expected as potential promising candidates to help create more efficient solar cells.

Novel adamantane-based hole transport materials for perovskite solar cells: a computational approach

Adamantane derivatives have been subjected to quantum mechanical calculations to determine their capabilities as potential hole transport materials (HTMs) in perovskite solar cells (PSCs). Adamantane has been modified in two ways: multiple arm substitution and outermost substituent variation. It has been shown that tetra-substitution of adamantane gave the best characteristics as a HTM. Further modification showed tetra-ethyl substituted adamantane (ad-EtTPA) has the lowest HOMO, a small hole reorganization energy (λh), absorption in the UV region, and good stability. These appropriate properties mean that ad-EtTPA could be a promising HTM in PSCs. In addition, the relationship between λh and the electrostatic potential (ESP) maps of the cationic geometry has been studied. Three outcomes were drawn from the investigation: (1) the high positive potential in the ESP map is the region where more geometric distortions are occurring in going from the neutral to the cationic state, (2) large geometric distortions in this region lead to high λh, and (3) for tetra-substituted adamantane derivatives, delocalization of the positive potential leads to lower λh. The results showed that ESP maps can give insight into the molecular engineering of HTMs.

Germanium Fluoride Nanocages as Optically Transparent n-Type Materials and Their Endohedral Metallofullerene Derivatives

Carbon- and silicon-based n-type materials tend to suffer from instability of the corresponding radical anions. With DFT calculations, we explore a promising route to overcome such challenges with molecular nanocages which utilize the heavier element Ge. The addition of fluorine substituents creates large electron affinities in the range 2.5-5.5 eV and HOMO-LUMO gaps between 1.6 and 3.2 eV. The LUMOs envelop the surfaces of these structures, suggesting extensive delocalization of injected electrons, analogous to fullerene acceptors. Moreover, these Ge _nF _n inorganic cages are found to be transparent in the UV-visible region as probed with their excited states. Their capacitance, linear polarizabilities, and dielectric constants are computed and found to be on the same order of magnitude as saturated oligomers and some extended π-organics (azobenzenes). Furthermore, we explore fullerene-type endohedral isomers, i.e., cages with internal substituents or guest atoms, and find them to be more stable than the parent exohedral isomers by up to -206.45 kcal mol^-1. We also consider the addition of Li, He, Cs, and Bi, to probe the utility of the exo/ endo cages as host-guest systems. The endohedral He/Li@F₈@Ge₆₀F₅₂ cages are significantly more stable than their parent exohedral isomers He/Li@Ge₆₀F₅₂ by -182.46 and -49.22 kcal mol^-1, respectively. The energy of formation of endohedral He@F₈@Ge₆₀F₅₂ is exothermic by -10.4 kcal mol^-1, while Cs and Bi guests are too large to be accommodated but are stable in the exohedral parent cages. Conceivable applications of these materials include n-type semiconductors and transparent electrodes, with potential for novel energy storage modalities.

Exploring the electrochemical properties of hole transporting materials from first-principles calculations: an efficient strategy to improve the performance of perovskite solar cells

Perovskite solar cells (PSCs) have been achieved with impressively dynamic improvement in power conversion efficiency (PCE), becoming the hottest topic in photovoltaics. One of the hot topics is to develop inexpensive and efficient hole transporting materials (HTMs). In the present work, we systematically investigated the impact of different atoms in the heteromerous structure on the performance of perovskite solar cells. In addition, the influence of the structural modification of the HTM molecular building blocks was also revealed. To further understand the relationship between the charge-transport properties and the structural modification, the electronic properties, reorganization energy, and hole transporting properties of a series of organic hole transporting materials were investigated using first-principles calculations combined with Marcus theory. Moreover, the orientation function μ_Φ (V, λ, r, θ, γ; Φ) was applied to quantitatively evaluate the overall carrier mobility of HTMs in PSCs. It is revealed that this model predicts the hole mobility of HTMs correctly. The calculated results indicate that hole transporting materials with heteroatoms and larger dimensional structures show higher hole mobility, which may significantly improve the photovoltaic performance of PSCs.

Influence of Nonfused Cores on the Photovoltaic Performance of Linear Triphenylamine-Based Hole-Transporting Materials for Perovskite Solar Cells

The core plays a crucial role in achieving high performance of linear hole transport materials (HTMs) toward the perovskite solar cells (PSCs). Most studies focused on the development of fused heterocycles as cores for HTMs. Nevertheless, nonfused heterocycles deserve to be studied since they can be easily synthesized. In this work, we reported a series of low-cost triphenylamine HTMs (M101-M106) with different nonfused cores. Results concluded that the introduced core has a significant influence on conductivity, hole mobility, energy level, and solubility of linear HTMs. M103 and M104 with nonfused oligothiophene cores are superior to other HTMs in terms of conductivity, hole mobility, and surface morphology. PSCs based on M104 exhibited the highest power conversion efficiency of 16.50% under AM 1.5 sun, which is comparable to that of spiro-OMeTAD (16.67%) under the same conditions. Importantly, the employment of M104 is highly economical in terms of the cost of synthesis as compared to that of spiro-OMeTAD. This work demonstrated that nonfused heterocycles, such as oligothiophene, are promising cores for high performance of linear HTMs toward PSCs.