Layered optimization strategy enables over 17.8% efficiency of layer-by-layer organic photovoltaics

A ternary strategy of halogen-free solvent processing can open up a promising pathway for the preparation of polymer solar cells (PSCs) on a large scale and can effectively improve the power conversion efficiency with an appropriate third component. Herein, the green solvent o-xylene (o-XY) is used as the main solvent, and the non-fullerene acceptor Y6-DT-4F as the third component is introduced into the PBB-F:IT-4F binary system to broaden the spectral absorption and optimize the morphology to achieve efficient PSCs. The third component, Y6-DT-4F, is compatible with IT-4F and can form an "alloy acceptor", which can synergistically optimize the photon capture, carrier transport, and collection capabilities of the ternary device. Meanwhile, Y6-DT-4F has strong crystallinity, so when introduced into the binary system as the third component can enhance the crystallization, which is conducive to the charge transport. Consequently, the optimal ternary system based on PBB-F:IT-4F:Y6-DT-4F achieved an efficiency of 15.24%, which is higher than that of the binary device based on PBB-F:IT-4F (13.39%).

Two Compatible Acceptors as an Alloy Model with a Halogen-Free Solvent for Efficient Ternary Polymer 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.

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

Inhibiting excessive aggregation and migration of active layer molecules is essential to improve the stability and performance of organic solar cells (OSCs). Herein, the star-shaped nitrogen heterocyclic-ring acceptor TF1 featuring...

Inhibiting Excessive Molecular Aggregation to Achieve Highly Efficient and Stabilized Organic Solar Cells by Introducing Star-Shaped Nitrogen Heterocyclic-Ring Acceptor

The rapid development of low bandgap polymer acceptors has promoted the efficiency up to ≈17% for all‐polymer solar cells (all‐PSCs). Nevertheless, the polymeric blend film, core to the photoelectric conversion of all‐PSCs, has not been thoroughly understood in terms of the influence and regulatory factors of mechanical properties, which hinders the advances in flexible and wearable applications. Herein, a range of characterization methods is combined to investigate the mechanical properties, miscibility, and film microstructure of the blends based on several representative polymer donors (PTzBI‐Si, PTVT‐T, PM6 and PTQ10) and a benchmark polymer acceptor N2200, and to further reveal the miscibility‐property relationships of the miscibility property. The results stress that fracture behaviors and elastic moduli of these blends with varied compositions show different changing trends, which are affected by molecular interactions and aggregated structure of the blends. The elastic moduli of the four all‐polymer blends can be nicely predicted by different models that are deduced from macromolecular mechanics. Most crucially, the correlations between elastic modulus, morphology, and miscibility of all‐polymer blends are elucidated for the first time. The derived relationships is validated with another high‐efficiency blend and will be the key to the successful fabrication of mechanically robust and stretchable all‐PSCs with high efficiency.

Unraveling the Correlations between Mechanical Properties, Miscibility, and Film Microstructure in All‐Polymer Photovoltaic Cells

The flexibility and stability of organic solar cells (OSCs) are becoming hotspot research due to their practical applications. Molecular arrangement and network morphology of the active layer are important factors affecting the flexibility and stability of OSCs. Here, two polymer donors (PBB1-Cl and PBB2-Cl) with different side chains were investigated as the third component to finely regulate morphology and molecular accumulation of the active layer to improve the performance of OSCs. Both third components show complementary absorption spectra and good compatibility. The good planarity of PBB1-Cl and PBB2-Cl significantly increases intermolecular overlap to promote the charge transfer in the active layer. In addition, the steric hindrance of the one-dimensional side chain of PBB1-Cl enhanced the intermolecular close packing and entanglement of the active layer. As a result, the tensile endurance of PM6:PBB1-Cl:Y6-BO-4Cl ternary blend film reached 4.6 times (26.86%) of the PM6:Y6-BO-4Cl binary blend film. Furthermore, the power conversion efficiency (PCE) increased from 15.83% for PM6:Y6-BO-4Cl based binary OSCs to 17.36% for PBB1-Cl based rigid ternary OSCs, and the flexible OSCs increased from 13.44% to 14.96%, respectively. 17.36% efficiency of PBB1-Cl based ternary OSCs is the highest value in OSCs with elongation at the break of active layer exceeding 25% so far. Further, the PCEs of PBB1-Cl-based flexible ternary OSCs still remain above 74% within 500 cycles of bending with a diameter of 10 mm or annealing at 100 °C for 24 hours, which is mainly attributed to the smaller surface energy difference between PBB1-Cl-based ternary blend film and transport layers, higher glass transition temperature and larger elongation at break of the active layer. All results suggest that enhancing intermolecular overlapping fasten packing in the active layer has great potential in improving the photovoltaic performance, mechanical and thermal stability of OSCs.

Overlapping fasten packing enables efficient dual-donor ternary organic solar cells with super stretchability

Thanks to the development of acceptor–donor–acceptor (A–D–A) type electron acceptors, organic solar cells (OSCs) have achieved marvelous progress in recent years. However, a systematic investigation about the structure–efficiency relationship is still highly desired to better understand the working mechanisms inside a bulk heterojunction (BHJ). In this study, new acceptors with the synergistic effect of side chain and unilateral π‐bridge strategy are designed, accounting for lower energy loss, expanded absorption, modulated intermolecular interactions and BHJ morphology. As a consequence, the resultant A–D–π–A acceptor ID‐C6Ph‐ST‐4F based binary solar cell receives an impressive efficiency up to 15.36%, much greater than the A–D–A analog ID‐C6Ph‐4F (10.75%), and ranks the best among all small molecular acceptors with symmetric or asymmetric π‐bridges. The results highlight the promising manipulation of phenylalkyl side chain and unilateral π‐bridge on intermolecular interactions among acceptor molecules and interactions between donor and acceptor (D/A) molecules. Superior interactions among acceptor molecules are an essential precondition to ensure preferable molecular assembly for charge transport, and meanwhile, moderately enhanced D/A interactions are also crucial for proper phase‐separation networks with efficient exciton dissociation and charge generation.

Manipulating the Intermolecular Interactions through Side Chain Engineering and Unilateral π‐Bridge Strategy for Efficient Small Molecular Photovoltaic Acceptor

Vertical Stratification Engineering of Insulating Poly(aryl ether)s Enables 18.6% Organic Solar Cells with Improved Stability

Side-chain engineering has been acknowledged as one ingenious method to regulate material crystallinity, miscibility and microstructure to achieve favorable photovoltaic performance. Subtle changes of the side chains would have a...

Side-chain engineering improve molecular stacking and miscibility for efficient fullerene organic solar cells

Employing Symmetry-breaking Strategy to Reduce Self-aggregation of Donor in Blend Films to Control Morphology for All Small Molecular Organic Solar Cells

Yuanwei Wu

Papers

Overlapping fasten packing enables efficient dual-donor ternary organic solar cells with super stretchability

Manipulating the Intermolecular Interactions through Side Chain Engineering and Unilateral π‐Bridge Strategy for Efficient Small Molecular Photovoltaic Acceptor

Vertical Stratification Engineering of Insulating Poly(aryl ether)s Enables 18.6% Organic Solar Cells with Improved Stability

Side-chain engineering improve molecular stacking and miscibility for efficient fullerene organic solar cells

Employing Symmetry-breaking Strategy to Reduce Self-aggregation of Donor in Blend Films to Control Morphology for All Small Molecular Organic Solar Cells