Side-Chain Engineering of Diketopyrrolopyrrole-Based Hole-Transport Materials to Realize High-Efficiency Perovskite Solar Cells

Advancing optoelectronic characteristics of Diketopyrrolopyrrole-Based molecules as donors for organic and as hole transporting materials for perovskite solar cells

A stable and efficient hole-transport material (HTM) is crucial for high-performance perovskite solar cells (PSCs). A 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-MeOTAD) being used widely to prepare highly efficient PSCs. However, Spiro-MeOTAD has some limitations due to its complex synthesis, which increases its cost, and it also requires dopants to improve its performance. Therefore, we designed thirteen unique small-molecule-based HTMs (MK1-MK13), which are easy to synthesize, highly cost-effective, and don't require dopants to prepare efficient PSCs. Their electrical and optical properties are then investigated theoretically using advanced quantum chemical approaches. The designed molecules showed lower energy gaps and improved optical and optoelectronic characteristics because of the improved phase inversion geometry. The detailed photo-physical and optoelectronic characteristics have been studied using density functional theory (DFT) and time-dependent (TD-DFT) calculations. Moreover, we investigated the impact of holes and electrons and the density of states, open-circuit voltage, frontier molecular orbital, transition density matrix, and other structural and photovoltaic characteristics of these materials. Among these, the MK3 molecule possesses the much narrower optical band gap of 1.04 eV and absorbance (λ max) of 684 nm, respectively. In addition, a profound investigation of the MK3/PC61BM blend shows excellent charge transfer at the acceptor-donor interface. Therefore, our proposed technique is necessary for generating appropriate photovoltaic materials for efficient optoelectronic devices and is helpful in further advancing the field.

Vinylene-linked diketopyrrolopyrrole chromophores for electrochromism

We report a novel series of vinylene-linked DPP compounds (1-5) formed via Knoevenagel condensation of dimethyl DPP (6) with various aromatic aldehydes. Incorporating the vinylene linkage and photo- and electro-active groups offered the distinct advantage of extending π-delocalization, resulting in deep-coloured solids with absorption maxima extending to 620-680 nm and low redox potentials. The DPP-triphenylamine compound (5) showed electrochromism in the near-infrared region. The colour of the solution changed from blue to green, and the absorption from 680 nm to 740 nm, leading to NIR-to-NIR absorption switching. This system exhibits rapid switching, swift response times (1.4 s and 1.2 s), and reversibility in electrochromic behaviour.

π-Conjugated Polymer with Pendant Side Chains as a Dopant-Free Hole Transport Material for High-Performance Perovskite Solar Cells

Dopant-free polymeric hole transport materials (HTMs) have attracted considerable attention in perovskite solar cells (PSCs) due to their high carrier mobilities and excellent hydrophobicity. They are considered promising candidates for HTMs to replace commercial Spiro-OMeTAD to achieve long-term stability and high efficiency in PSCs. In this study, we developed BDT-TA-BTASi, a conjugated donor-π-acceptor polymeric HTM. The donor benzo[1,2-b:4,5-b']dithiophene (BDT) and acceptor benzotriazole (BTA) incorporated pendant siloxane, and alkyl side chains led to high hole mobility and solubility. In addition, BDT-TA-BTASi can effectively passivate the perovskite layer and markedly decrease the trap density. Based on these advantages, dopant-free BDT-TA-BTASi-based PSCs achieved an efficiency of over 21.5%. Furthermore, dopant-free BDT-TA-BTASi-based devices not only exhibited good stability in N2 (retaining 92% of the initial efficiency after 1000 h) but also showed good stability at high-temperature (60 °C) and -humidity conditions (80 ± 10%) (retaining 92 and 82% of the initial efficiency after 400 h). These results demonstrate that BDT-TA-BTASi is a promising HTM, and the study provides guidance on dopant-free polymeric HTMs to achieve high-performance PSCs.

The Impact of Backbone Fluorination and Side-Chain Position in Thiophene-Benzothiadiazole-Based Hole-Transport Materials on the Performance and Stability of Perovskite Solar Cells

Perovskite solar cells (PSCs) currently reach high efficiencies, while their insufficient stability remains an obstacle to their technological commercialization. The introduction of hole-transport materials (HTMs) into the device structure is a key approach for enhancing the efficiency and stability of devices. However, currently, the influence of the HTM structure or properties on the characteristics and operational stability of PSCs remains insufficiently studied. Herein, we present four novel push-pull small molecules, H1-4, with alternating thiophene and benzothiadiazole or fluorine-loaded benzothiadiazole units, which contain branched and linear alkyl chains in the different positions of terminal thiophenes to evaluate the impact of HTM structure on PSC performance. It is demonstrated that minor changes in the structure of HTMs significantly influence their behavior in thin films. In particular, H3 organizes into highly ordered lamellar structures in thin films, which proves to be crucial in boosting the efficiency and stability of PSCs. The presented results shed light on the crucial role of the HTM structure and the morphology of films in the performance of PSCs.

Controlling the helicity of π-conjugated oligomers by tuning the aromatic backbone twist

The properties of π-conjugated oligomers and polymers are commonly controlled by side group engineering, main chain engineering, or conformational engineering. The last approach is typically limited to controlling the dihedral angle around the interring single bonds to prevent loss of π-conjugation. Here we propose a different approach to conformational engineering that involves controlling the twist of the aromatic units comprising the backbone by using a tether of varying lengths. We demonstrate this approach by synthesizing an inherently twisted building unit comprised of helically locked tethered acenes, bearing acetylene end-groups to enable backbone extension, which was applied in a series of nine helical oligomers with varying backbone length and twist. We find that the optical and electronic properties of π-conjugated systems may be determined by the additive, antagonistic, or independent effects of backbone length and twist angle. The twisted oligomers display chiral amplification, arising from the formation of secondary helical structures.

One approach to altering the properties of π-conjugated oligomers is conformational engineering, in which the degree of rotation around the bonds linking monomers is restricted. Here the authors apply the conformational engineering approach on individual monomers using tethers of varying lengths to twist the aromatic units, and study the effects of varying the angles.

Structure–property relationships of diketopyrrolopyrrole- and thienoacene-based A–D–A type hole transport materials for efficient perovskite solar cells

Two DPP-based hole-transporting materials with different aromatic π-bridges have been synthesized and tested for perovskite solar cells. Improved power conversion efficiency and stability were achieved by employing DPP-TT.