Sulfur vs. tellurium: the heteroatom effects on the nonfullerene acceptors

Reducing Interfacial Recombination in Inverted Perovskite Solar Cells With Selenophene-Substituted PCBM: Comparison With Thiophene and Furan Substitution

Reducing the interfacial recombination and improving the charge transfer capability of charge transport layers are effective strategies to enhance the efficiency and stability of perovskite solar cells (PSCs). This study evaluates, for the first time, the effects of selenophene substitution in the chemical structure of phenyl-butyric acid methyl ester (PCBM) on the performance and stability of inverted PSCs. Selenophene substitution was compared to thiophene and furan substitutions, and the reference PCBM without chalcogenophene moiety. Additionally, this study investigates the differences between using the fullerene cages C70 and C60 in the PCBM chemical structure. The photovoltaic results demonstrate that, with an adequate control of the thickness of the electron transport layer (ETL), incorporating the selenophene moiety in the structures of fullerenes enhances the photovoltaic parameters of PSCs. This improvement results from the reduction in trap-assisted recombination, an increase in electron mobility, and the improved charge extraction processes. The use of C70, as opposed to C60, allows for the preparation of a series of ETLs with comparable thicknesses, although slightly lower efficiencies. This feature facilitates a systematic comparative analysis focused on variations in the electron properties of ETLs, thereby avoiding the inclusion of issues related to thickness and charge recombination processes.

Binding Energy of Triplet Excitons in Nonfullerene Acceptors: The Effects of Fluorination and Chlorination

One strategy to improve the photovoltaic properties of nonfullerene acceptors (NFAs), employed in state-of-art organic solar cells, is the rational fluorination or chlorination of these molecules. Although this modification improves important acceptor properties, little is known about the effects on the triplet states. Here, we combine the polarizable continuum model with an optimally tuned range-separated hybrid functional to investigate this issue. We find that fluorination or chlorination of NFAs decreases the degree of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) overlap along these molecules. Consequently, the energy gap between T₁ and S₁ states, ΔE_ST = E_S1 - E_T1, also decreases. This effect reduces the binding energy of triplet excitons, which favors their dissociation into free charges. Furthermore, the reduction of ΔE_ST can contribute to mitigating the losses produced by the nonradiative deactivation of the T₁ excitons. Interestingly, although Cl has a lower electronegativity than F, chlorination is more effective to reduce ΔE_ST. Since the chlorination of NFAs is easier than fluorination, Cl substitution can be a useful approach to enhance solar energy harvesting using triplet excitons.

Triplet Acceptors with a D-A Structure and Twisted Conformation for Efficient Organic Solar Cells

Triplet acceptors have been developed to construct high-performance organic solar cells (OSCs) as the long lifetime and diffusion range of triplet excitons may dissociate into free charges instead of net recombination when the energy levels of the lowest triplet state (T1 ) are close to those of charge-transfer states (3 CT). The current triplet acceptors were designed by introducing heavy atoms to enhance the intersystem crossing, limiting their applications. Herein, two twisted acceptors without heavy atoms, analogues of Y6, constructed with large π-conjugated core and D-A structure, were confirmed to be triplet materials, leading to high-performance OSCs. The mechanism of triplet excitons were investigated to show that the twisted and D-A structures result in large spin-orbit coupling (SOC) and small energy gap between the singlet and triplet states, and thus efficient intersystem crossing. Moreover, the energy level of T1 is close to 3 CT, facilitating the split of triplet exciton to free charges.