Advances in Stretchable Organic Photovoltaics: Flexible Transparent Electrodes and Deformable Active Layer Design.

Stretchable organic photovoltaics (OPVs) have attracted significant attention as promising power sources for wearable electronic systems owing to their superior robustness under repetitive tensile strains and their good compatibility. However, reconciling a high power-conversion efficiency and a reasonable flexibility is a tremendous challenge. In addition, the development of stretchable OPVs must be accelerated to satisfy the increasing requirements of niche markets for mechanical robustness. Stretchable OPV devices can be classified as either structurally or intrinsically stretchable. This work reviews recent advances in stretchable OPVs, including the design of mechanically robust transparent electrodes, photovoltaic materials, and devices. Initially, an overview of the characteristics and recent research progress in the areas of structurally and intrinsically stretchable OPVs is provided. Subsequently, research into flexible and stretchable transparent electrodes that directly affect the performances of stretchable OPVs is summarized and analyzed. Overall, this review aims to provide an in-depth understanding of the intrinsic properties of highly efficient and deformable active materials, while also emphasizing advanced strategies for simultaneously improving the photovoltaic performance and mechanical flexibility of the active layer, including material design, multi-component settings, and structural optimization.

Ultra Robust and Highly Efficient Flexible Organic Solar Cells with Over 18 % Efficiency Realized by Incorporating a Linker Dimerized Acceptor.

The wearable application of flexible organic solar cells (f-OSCs) necessitates high power conversion efficiency (PCE) and mechanical robustness. However, photoactive films based on efficient non-fullerene small molecule acceptors (NF-SMAs) are typically brittle, leading to poor mechanical stability in devices. In this study, we achieved a remarkable PCE of 18.06 % in f-OSCs while maintaining ultrahigh mechanical robustness (with a crack-onset strain (COS) of higher than 11 %) by incorporating a linker dimerized acceptor (DOY-TVT). Compared to binary blends, ternary systems exhibit reduced non-radiative recombination, suppressed crystallization and diffusion of NF-SMAs, and improved load distribution across the chain networks, enabling the dissipation of the load energy. Thus, the ternary f-OSCs developed in this study achieved, high PCE and stability, surpassing binary OSCs. Moreover, the developed f-OSCs retained 97 % of the initial PCE even after 3000 bending cycles, indicating excellent mechanical stability (9.1 % higher than binary systems). Furthermore, the rigid device with inverted structure based on the optimal active layer exhibited a substantial increase in efficiency retention, with 89.6 % after 865 h at 85 °C and 93 % after more than 1300 h of shelf storage at 25 °C. These findings highlight the potential of the linker oligomer acceptor for realizing high-performing f-OSCs with ultrahigh mechanical robustness.

Ductile Oligomeric Acceptor-Modified Flexible Organic Solar Cells Show Excellent Mechanical Robustness and Near 18% Efficiency.

High power conversion efficiency (PCE) and mechanical robustness are key requirements for wearable applications of organic solar cells (OSCs). However, almost all highly efficient photoactive films comprising polymer donors (PD ) and small molecule acceptors (SMAs) are mechanically brittle. In this study, highly efficient (PCE = 17.91%) and mechanically robust (crack-onset strain [COS] = 11.7%) flexible OSCs are fabricated by incorporating a ductile oligomeric acceptor (DOA) into the PD :SMA system, representing the most flexible OSCs to date. The photophysical, mechanical, and photovoltaic properties of D18:N3 with different DOAs are characterized. By introducing DOA DOY-C4 with a longer flexible alkyl linker and lower polymerization, the D18:N3:DOY-C4-based flexible OSCs exhibit a significantly higher PCE (17.91%) and 50% higher COS (11.7%) than the D18:N3-based device (PCE = 17.06%, COS = 7.8%). The flexible OSCs based on D18:N3:DOY-C4 retain 98% of the initial PCE after 2000 consecutive bending cycles, showing greater mechanical stability than the reference device (maintaining 89% of initial PCE). After careful investigation, it is hypothesized that the enhancement in mechanical properties is mainly due to the formation of tie chains or entanglement in the ternary blend films. These results demonstrate that DOAs have great potential for achieving high-performance flexible OSCs.

Modulation of Molecular Stacking via Tuning 2-Ethylhexyl Alkyl Chain Enables Improved Efficiency for All-Small-Molecule Organic Solar Cells.

There is always a dilemma between strong π-π stacking/crystallinity and suitable domain size for all-small-molecule organic solar cells (ASM-OSCs), which puts forward higher requirements for the design of molecular donors. In this work, a series of novel molecular donors with different positional 2-ethylhexy (EH) attachments are designed and synthesized, named SM-R, SM-REH, SM-EH-R, and SM-EH-REH. It is found that EH-substitution on end groups (SM-REH) enables improved π-π interaction and crystallinity but with decreased solubility and phase size, leading to the improved efficiency of 15.6% as compared to 14.0% of SM-R. In contrast, EH-substitution on the π-bridge (SM-EH-R) significantly suppresses π-π stacking and increases the solubility, resulting in the lower efficiency of 11.9%. The further EH-substitution on end-groups of SM-EH-R, namely, SM-EH-REH, recovers the π-π stacking strength and obtains a moderate efficiency of 14.4%. Despite the higher crystallinity and increased π-π stacking in some molecules, the blend films show the gradually decreased domain size in the sequence of SM-R, SM-REH, SM-EH-R, and SM-EH-REH owing to the steric hindrance of the EH-chain. Overall, this work indicates that obtaining the higher π-π stacking/crystallinity and decreased domain size is achievable by tuning the EH-chain substitution, which paves the way to further improve the photovoltaic performance of ASM-OSCs.

Modulation of the Fluorination Site on Side-Chain Thiophene Improved Efficiency in All-Small-Molecule Organic Solar Cells.

Fine-tuning the phase-separated morphology is of great importance to achieve efficient all-small-molecule organic solar cells (ASM-OSCs). In this work, a pair of isomers are designed and synthesized, namely, BDT-UF and BDT-DF, in which the fluorine atom in BDT-UF is close to the alkyl chain of side-chain thiophene, while that in BDT-DF is close to the center core. Owing to the noncovalent interaction between fluorine and hydrogen, BDT-DF shows a smaller dihedral angle between the thiophene side chain and the BDT core, which causes better molecular planarity. When mixed with N3, BDT-UF shows better miscibility, higher crystallinity, and more ordered molecule stacking in the blend film. Finally, the device of BDT-DF:N3 gains a power conversion efficiency (PCE) of 14.5%, while that of BDT-UF:N3 shows an increase in Voc, Jsc, and FF and gains a PCE of 15.1%. Our work exhibits a way of adjusting the substitution site of fluorine atoms to improve the PCE of ASM-OSCs.

Asymmetric Substitution of End-Groups Triggers 16.34% Efficiency for All-Small-Molecule Organic Solar Cells.

Asymmetric substitution of end-groups is first applied in molecular donors. Three commonly used end-groups of 2-ethylhexyl cyanoacetate (CA), 2-ethylhexyl rhodanine (Reh), and 1H-indene-1,3(2H)-dione (ID) are combined to construct a series of symmetric and asymmetric donors. Correspondingly, the asymmetric donors SM-CA-Reh and SM-CA-ID show largely increased dipole moments (2.14 and 3.39 D, respectively) and enhanced aggregation propensity, as compared to those of symmetric donors of SM-CA, SM-Reh, and SM-ID. Using N3 as acceptor, interestingly, SM-CA-Reh integrates the photovoltaic characteristics of high fill factor (FF) for SM-CA and high short-circuit current density for SM-Reh, and delivers a record power conversion efficiency (PCE) of 16.34% with a high FF of 77.5%, which is much higher than 15.41% for SM-CA and 14.76% for SM-Reh. However, SM-CA-ID and SM-ID give the lower PCE of 8.20% and 2.76%. Characterization results suggest that the π-π interaction mainly dictates the packing morphology of blend films instead of dipole effect or crystallinity. Mono-substitution of Reh facilitates the molecular demixing appropriately but keeps the characteristic of the fine bicontinuous network of SM-CA:N3. SM-CA-Reh:N3 shows more efficient exciton extraction, higher hole transport, and better miscibility. These results well explain the merits integration and improved photovoltaic performance.