Jinru Cao

Bio: Jinru Cao is an academic researcher from Nanjing University of Science and Technology. The author has contributed to research in topics: Organic solar cell & Materials science. The author has an hindex of 11, co-authored 19 publications receiving 304 citations.

Molecular engineering of central fused-ring cores of non-fullerene acceptors for high-efficiency organic solar cells

Abstract: Organic solar cells (OSCs) using bulk-heterojunction (BHJ) blends of polymer donors and non-fullerene acceptors (NFAs) have witnessed significant progress in recent years. NFAs, especially, fused-ring electron acceptors (FREAs) adopting acceptor–donor–acceptor (A–D–A) structures have contributed most high-efficiency OSCs, pushing the power conversion efficiency (PCE) to over 15% and 17% for single-junction and tandem devices, respectively. The vibrant development of novel FREAs is largely attributed to their versatility in manipulating energy levels and molecular ordering via chemical modification. FREAs typically feature coplanar aromatic fused-rings as D cores and two electron-deficient A units as end caps. In this review, we try to summarize the recent advancement in molecular engineering of central fused-ring cores of FREAs for high-efficiency OSCs. The impact of such core engineering on the light absorption, energy levels, electron mobility, and photovoltaic performance of the resultant FREAs is discussed. Some guidelines for future molecular design are suggested from the aspects of improving light absorption, the fill factor, the driving force and voltage loss. Finally, we give an outlook on the remaining challenges and promising directions towards the commercialization of OSCs.

Tuning of the conformation of asymmetric nonfullerene acceptors for efficient organic solar cells

Abstract: In this work, three dithieno[3,2-b:2′,3′-d]pyrrol fused-ring electron acceptors (IPT-2F, IPTT-2F, and IPTTT-2F) have been successfully developed as efficient asymmetric nonfullerene acceptors (NFAs) for organic solar cells (OSCs). The molecular conformation of these NFAs can be subtly tuned by extending the donating cores with thiophene rings. Experimental and theoretical studies indicate the crucial role of the conformation change in asymmetric NFAs played in the aggregation of their blend films with PBDB-T. Indeed, the blend films with S-shaped IPT-2F and IPTTT-2F reveal less trap-assisted recombination and better microphase separation compared with C-shaped IPTT-2F. Decent power conversion efficiency (PCE) values of 14% and 12.3% were achieved for IPT-2F- and IPTTT-2F-based OSCs, respectively. Our results indicate the S-shaped conformation of asymmetric NFAs locked via S⋯O interactions is advantageous to fine-tune the morphology in the active layer for more efficient OSCs.

Modification on the Indacenodithieno[3,2-b]thiophene Core to Achieve Higher Current and Reduced Energy Loss for Nonfullerene Solar Cells

Abstract: Modification on the indacenodithieno[3,2-b]thiophene (IT) core was used in the central phenyl moiety to explore efficient fused-ring acceptors (FRAs). By replacing two free H atoms with weak and sm...

Molecular engineering of acceptors to control aggregation for optimized nonfullerene solar cells

Abstract: Dual molecular engineering of alkyl side chains and halogen accepting ends of asymmetric fused-ring acceptors has been proposed for controlling aggregation for optimized organic solar cells (OSCs). Fluorination or chlorination on end-capped groups are explored along with linear octyl (C8) or branched 2-butyl-1-octyl chain (BO) substitution on the donating core. The inherent features of the larger Cl atom and longer C–Cl bond markedly extend the backbone stacking area and thus enhance molecular aggregation, while bulky BO chain exert a heavier steric shielding effect on backbone stacking. Consequently, IPTBO-4Cl shows properly weakened intermolecular interaction for balanced molecular aggregation. IPTBO-4Cl when blended with a PM6 polymer donor delivers a highest power conversion efficiency (PCE) of 15% and a 72.6% fill factor (FF). Expectedly, fluorinated IPT-4F bearing shorter C8 chains outputs a good PCE nearing 15% with a 74.2% FF. To the best of our knowledge, the PCE of 15% is by far the highest for asymmetric FRA based OSCs. By contrast, IPT-4Cl and IPTBO-4F with either excessively strong or weak aggregation result in relatively low photovoltaic performance. Our results demonstrate controlling aggregation via delicate molecular engineering is an undeniably effective way to achieve efficient OSCs.

TD-DFT benchmark for UV-visible spectra of fused-ring electron acceptors using global and range-separated hybrids

Abstract: Non-fullerene acceptors, especially acceptor-donor-acceptor structured fused-ring electron acceptors (FREAs), have attracted widespread attention in organic solar cells because of their versatile molecular design in fine-tuning light absorption and energy levels. We report the accuracy of Time-Dependent Density Functional Theory (TD-DFT) for FREAs by comparing their theoretically predicted vertical absorption wavelength (λver-abso) with the experimental maximum absorption (λmax). The λver-abso values of 50 molecules obtained from major types of FREAs have been investigated using TD-DFT by considering the solvent effects. The values of λver-abso predicted with a pure density functional (PBE), global hybrids (B3LYP and PBE0) and range-separated schemes (CAM-B3LYP and LC-ωPBE) follow the exact exchange percentage included at an intermediate inter-electronic distance. Global hybrids outperform all other schemes. The mean absolute error provided is 22 nm by PBE0 and 38 nm by B3LYP for the whole set of molecules. The maximum deviation of 92 nm provided by B3LYP and 69 nm provided by PBE0 confirms that PBE0 is more appropriate for predicting the absorption wavelengths when designing new FREAs. By applying linear regression analysis to obtain the calibration curve, we found that the range-separated methods provide an equal or even more consistent description of FREA excited states. For the whole set of molecules, linearly corrected data yield an average error of 25 and 27 nm for CAM-B3LYP and LC-ωPBE, respectively. Consequently, when a statistical analysis technique is applicable for a certain series of FREAs, a theoretical method permits a chemically comprehensive and empirically good explanation of UV/Vis spectra for newly-designed FREAs.

Achieving 19% Power Conversion Efficiency in Planar‐Mixed Heterojunction Organic Solar Cells Using a Pseudosymmetric Electron Acceptor

Abstract: A record power conversion efficiency (PCE) of over 19% is realized in planar‐mixed heterojunction (PMHJ) organic solar cells (OSCs) by adopting the asymmetric selenium substitution strategy in making a pseudosymmetric electron acceptor, BS3TSe‐4F. The combined molecular asymmetry with more polarizable selenium substitution increases the dielectric constant of the D18/BS3TSe‐4F blend, helping lower the exciton binding energy. On the other hand, dimer packing in BS3TSe‐4F is facilitated to enable free charge generation, helping more efficient exciton dissociation and lowering the radiative recombination loss (ΔE2) of OSCs. As a result, PMHJ OSCs based on D18/BS3TSe‐4F achieve a PCE of 18.48%. By incorporating another mid‐bandgap acceptor Y6‐O into D18/BS3TSe‐4F to form a ternary PMHJ, a higher open‐circuit voltage (VOC) can be achieved to realize an impressive PCE of 19.03%. The findings of using pseudosymmetric electron acceptors in enhancing device efficiency provides an effective way to develop highly efficient acceptor materials for OSCs.

Renewed Prospects for Organic Photovoltaics.

Abstract: Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (Eloss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller Eloss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.