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.

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

The design and synthesis of a stable and efficient hole-transport material (HTM) for perovskite solar cells (PSCs) are one of the most demanding research areas. At present, 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-MeOTAD) is a commonly used HTM in the fabrication of high-efficiency PSCs; however, its complicated synthesis, addition of a dopant in order to realize the best efficiency, and high cost are major challenges for the further development of PSCs. Herein, various diketopyrrolopyrrole-based small molecules were synthesized with the same backbone but distinct alkyl side-chain substituents (i.e., 2-ethylhexyl-, n-hexyl-, ((methoxyethoxy)ethoxy)ethyl-, and (2-((2-methoxyethoxy)ethoxy)ethyl)acetamide, designated as D-1, D-2, D-3, and D-4, respectively) as HTMs. The variation in the alkyl chain has shown obvious effects on the optical and electrochemical properties as well as on the molecular packing and film-forming ability. Consequently, the power conversion efficiency (PCE) of the PSC under one sun illumination (100 mW cm^-2) is shown to increase in the order of D-1 (8.32%) < D-2 (11.12%) < D-3 (12.05%) < D-4 (17.64%). Various characterization techniques reveal that the superior performance of D-4 can be ascribed to the well-aligned highest occupied molecular orbital energy level with the counter electrode, the more compact π-π stacking with a higher coherence length, and the excellent hole mobility of 1.09 × 10^-3 cm² V^-1 s^-1, thus providing excellent energetics for effective charge transport with minimal charge-carrier recombination. Furthermore, the addition of the dopant Li-TFSI in D-4 is shown to deliver a remarkable PCE of 20.19%, along with a short-circuit current density (J_SC), open-circuit voltage (V_OC), and fill factor (FF) of 22.94 mA cm^-2, 1.14 V, and 73.87%, respectively, and superior stability compared to that of other HTMs. These results demonstrate the effectiveness of side-chain engineering for tailoring the properties of HTMs, thus offering new design tactics to fabricate for the synthesis of highly efficient and stable HTMs for PSCs.

Synthesis, characterization, photophysical properties, and computational studies on N-hexylphenothiazine/cyanopyridine based π-conjugated copolymers

In this paper, π-conjugated copolymers, namely N-hexylphenothiazine/cyanopyridine/phenyl/benzothiadiazole, N-hexylphenothiazine/cyanopyridine/phenyl/9,9-dihexylfluorene, and N-hexylphenothiazine/cyanopyridine/phenyl/9,9-diethylhexylfluorene were readily synthesized via Pd-catalyzed Suzuki cross-coupling reaction. The polymer structures and their photophysical properties were characterized by elemental analysis, 1H NMR, GPC, TGA, XRD, UV-vis absorption and PL spectroscopy measurements. The coupling agent effect on photophysical properties of copolymers was investigated to rationally design polymers with particular physical properties to be employed in optoelectronic devices. The UV-vis absorption spectroscopy of copolymers showed λmax at a range of ∼334–474 nm and red-shifted in their films to a range of ∼342–381 nm. These copolymers displayed highly intense fluorescence in their solutions and films. The PL spectra of copolymers indicated red and near-infrared light, rendering them a prospect for being red and near-infrared light-emitting materials for PLEDs. XRD analysis demonstrated a d-spacing range of ∼3.79–4.32 Å, reflecting π-π stacking and some degree of crystallinity in some polymers, and only P1 and P2 showed peaks in the small-angle region, indicating lamellar structures. To understand the relationship between molecular structures of target materials and their photophysical and photovoltaic properties, density functional theory (DFT) and its time-dependent form (TD-DFT) were employed.

Design Principles and Synergistic Effects of Chlorination on a Conjugated Backbone for Efficient Organic Photovoltaics: A Critical Review

The pursuit of low-cost, flexible, and lightweight renewable power resources has led to outstanding advancements in organic solar cells (OSCs). Among the successful design principles developed for synthesizing efficient conjugated electron donor (ED) or acceptor (EA) units for OSCs, chlorination has recently emerged as a reliable approach, despite being neglected over the years. In fact, several recent studies have indicated that chlorination is more potent for large-scale production than the highly studied fluorination in several aspects, such as easy and low-cost synthesis of materials, lowering energy levels, easy tuning of molecular orientation, and morphology, thus realizing impressive power conversion efficiencies in OSCs up to 17%. Herein, an up-to-date summary of the current progress in photovoltaic results realized by incorporating a chlorinated ED or EA into OSCs is presented to recognize the benefits and drawbacks of this interesting substituent in photoactive materials. Furthermore, other aspects of chlorinated materials for application in all-small-molecule, semitransparent, tandem, ternary, single-component, and indoor OSCs are also presented. Consequently, a concise outlook is provided for future design and development of chlorinated ED or EA units, which will facilitate utilization of this approach to achieve the goal of low-cost and large-area OSCs.

Stable P3HT: amorphous non-fullerene solar cells with a high open-circuit voltage of 1 V and efficiency of 4%

A non-fullerene small molecule acceptor, SF-HR composed of 3D-shaped spirobifluorene and hexyl rhodanine, was synthesized for use in bulk heterojunction organic solar cells (OSCs). It possesses harmonious molecular aggregation between the donor and acceptor, due to the interesting diagonal molecular shape of SF-HR. Furthermore, the energy level of SF-HR matches well with that of the donor polymer, poly(3-hexyl thiophene) (P3HT) in this system which can affect efficient charge transfer and transport properties. As a result, OSCs made from a P3HT:SF-HR photoactive layer exhibited a power conversion efficiency rate of 4.01% with a high V_OC of 1.00 V, a J_SC value of 8.23 mA cm⁻², and a FF value of 49%. Moreover, the P3HT:SF-HR film showed superior thermal and photo-stability to P3HT:PC₇₁BM. These results indicate that SF-HR is specialized as a non-fullerene acceptor for use in high-performance OSCs.

A 3D-shaped SF-HR was designed and synthesized for use in non-fullerene organic solar cells. Owing to the aligned energy levels, the P3HT:SF-HR system exhibited a high efficiency of 4.01% with good thermal stability and photostability.

Effects of incorporated pyrazine on the interchain packing and photovoltaic properties of wide-bandgap D–A polymers for non-fullerene polymer solar cells

The impact of using pyrazine as an acceptor core for designing donor–acceptor (D–A) based wide bandgap polymers for application in non-fullerene solar cells was evaluated.