Deciphering the Role of Chalcogen-Containing Heterocycles in Nonfullerene Acceptors for Organic Solar Cells

Abstract

The field of organic solar cells has experienced paradigm-shifting changes in recent years because of the emergence of nonfullerene acceptors (NFAs). It is critically important to gain more insight into the structure–property relationship of the emerging A′-DAD-A′-type NFAs. In this Letter, a family of NFAs named BPF-4F, BPT-4F, and BPS-4F incorporating various chalcogen-containing heterocycles, i.e., furan, thiophene, and selenophene, respectively, was designed and synthesized. These NFAs exhibited dramatic differences in their photovoltaic performances with device efficiencies of 16.8% achieved by the thiophene-based cells, which was much higher than the furan-based ones (12.6%). In addition, the selenophene-based NFA showed a red-shifted absorption relative to the furan- and thiophene-based ones and obtained a decent efficiency of 16.3% owing to an improved J SC. The reasons why these NFAs performed differently are systematically studied by comparing their optoelectronic properties and film morphology, which provides new understandings of the molecular design of high-performance NFAs.

A Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Acceptors Enables Polymer Solar Cells with 17.5 % Efficiency

“…The relatively obvious stronger and gradually bathochromic shift absorption spectra of A‐WSSe‐Cl and S‐WSeSe‐Cl could be attributed to the strengthened ICT effect by the introduction of one or two election‐rich selenophene in the DAD central core, which is beneficial for capturing more photons. Compared to their absorption in diluted solution, these spectra of S‐YSS‐Cl , A‐WSSe‐Cl , and S‐WSeSe‐Cl in thin films (Figure 1 d) exhibit obviously wider and red‐shifted absorption with enhanced vibration shoulder peaks and the maximum molar absorption peaks move to 835, 840, and 853 nm, along with the maximum absorption coefficient of 1.56×10 5 cm −1 at λ =835 nm, 1.63× 10 5 cm −1 at λ =840 nm, 1.94×10 5 cm −1 at λ =853 nm, respectively, indicating much enhanced intermolecular π–π interactions and more pronounced aggregation characteristics in thin films, which can effectively reflect the molecular packing behaviour and agree with the data of their single crystals [11] . Meanwhile, the optical band gaps of S‐YSS‐Cl , A‐WSSe‐Cl , and S‐WSeSe‐Cl determined from the onset of film absorption gradually decreased, which are 1.34 eV, 1.33 eV, and 1.30 eV, respectively.…”

“…In the past several years, non‐fullerene small molecular acceptors (NF‐SMAs) have attracted more and more attention in the field of PSCs due to their unique advantages such as easily modified chemical structures, highly tunable optoelectronic properties and out‐performing device performances superior to their fullerene counterparts [4–6] . Recently, many A‐D‐A and A‐DA′D‐A type NF‐SMAs have been developed for high performance PSCs by rational molecular design strategies such as side chain engineering, [7] end group engineering [8] and fused central core engineering [9–11] . To date, the PSCs based on A‐D‐A/A‐DA′D‐A type NF‐SMAs and polymer donors have presented over 17 % efficiency [12] .…”

Side‐Chain Engineering on Y‐Series Acceptors with Chlorinated End Groups Enables High‐Performance Organic Solar Cells

“…As shown in Figure 3d-f, the quenching efficiency of the PM6:BTP1O-4Cl-C12 film (95.7%) is higher than that of PM6:BTP1O-4Cl-C8 (93.1%) and PM6:BTP1O-4Cl-C10 blend ones (92.8%), demonstrating more efficient exciton dissociation, hole transfer and the highest EQE and J SC of this material combination. [41] In order to probe the charge mobility of the three blends, the space-charge-limited current (SCLC) methods were employed (Figure S5a,b: Supporting Information) to calculate the hole and electron mobility (μ h and μ e ). As summarized in Table S2 (Supporting Information), both μ h and μ e increase from PM6:BTP1O-4Cl-C8 (5.81 × 10 −4 and 3.05 × 10 −4 cm 2 V −1 s −1 ), PM6:BTP1O-4Cl-C10 (6.10 × 10 −4 and 3.57 × 10 −4 cm 2 V −1 s −1 ) to PM6:BTP1O-4Cl-C12 (6.28 × 10 −4 and 3.81 × 10 −4 cm 2 V −1 s −1 ).…”

“…As shown in Figure 3d–f, the quenching efficiency of the PM6:BTP1O‐4Cl‐C12 film (95.7%) is higher than that of PM6:BTP1O‐4Cl‐C8 (93.1%) and PM6:BTP1O‐4Cl‐C10 blend ones (92.8%), demonstrating more efficient exciton dissociation, hole transfer and the highest EQE and J SC of this material combination. [ 41 ]…”

“…[30][31][32] All the three parts can be reasonably designed and regulated to fine-tune the molecular properties of Y-series acceptors. [33][34][35][36][37][38][39][40][41] For example, if the chlorine atoms are used to replace the fluorine atoms in one of the famous Y-series acceptors named Y6, [42] the resulting molecule BTP-4Cl not only exhibits red-shifted absorption but also reduced voltage loss, leading to the BTP-4Cl-based devices showing an increased power conversion efficiency (PCE) of 16.5% with enhanced open-circuit voltage (V OC ) and short-circuit current density (J SC ) compared to those of the Y6-based devices. [43] It is also promising to modify the inner branched side chains which determine the solubility and crystallinity of Y-series acceptors.…”