Zhuohan Zhang

Bio: Zhuohan Zhang is an academic researcher from Nanjing University of Science and Technology. The author has contributed to research in topics: Organic solar cell & Polymer solar cell. The author has an hindex of 17, co-authored 29 publications receiving 1237 citations.

Dithieno[3,2-b:2',3'-d]pyrrol Fused Nonfullerene Acceptors Enabling Over 13% Efficiency for Organic Solar Cells.

Abstract: A new electron-rich central building block, 5,5,12,12-tetrakis(4-hexylphenyl)-indacenobis-(dithieno[3,2-b:2',3'-d]pyrrol) (INP), and two derivative nonfullerene acceptors (INPIC and INPIC-4F) are designed and synthesized. The two molecules reveal broad (600-900 nm) and strong absorption due to the satisfactory electron-donating ability of INP. Compared with its counterpart INPIC, fluorinated nonfullerene acceptor INPIC-4F exhibits a stronger near-infrared absorption with a narrower optical bandgap of 1.39 eV, an improved crystallinity with higher electron mobility, and down-shifted highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Organic solar cells (OSCs) based on INPIC-4F exhibit a high power conversion efficiency (PCE) of 13.13% and a relatively low energy loss of 0.54 eV, which is among the highest efficiencies reported for binary OSCs in the literature. The results demonstrate the great potential of the new INP as an electron-donating building block for constructing high-performance nonfullerene acceptors for OSCs.

A universal layer-by-layer solution-processing approach for efficient non-fullerene organic solar cells

Abstract: Layer-by-layer (LbL) solution processing is a cost-effective technology for the large-scale fabrication of organic solar cells (OSCs). In this work, LbL OSCs were fabricated using PTQ10/J71 as donors and ITC6-IC/IDIC/MeIC/ITCPTC/ITIC as non-fullerene acceptors (NFAs) without using orthogonal solvents and appropriate co-solvents. Compared with traditional bulk heterojunction (BHJ) OSCs, the corresponding solution-processed LbL devices exhibited higher or comparable power conversion efficiencies (PCEs), which had the advantages of reduced energy loss, stronger absorption spectra, better vertical phase separation, partially increased charge transport property and charge collection efficiency. Furthermore, taking the J71/ITC6-IC and PTQ10/IDIC LbL systems as examples, we fabricated large-area LbL OSCs using the doctor-blading process, which is closer to the roll-to-roll (R2R) technology. Importantly, both OSCs based on J71/ITC6-IC and PTQ10/IDIC LbL with an active area of 1.00 cm2 demonstrated encouraging PCEs of over 10%, which is the record efficiency for large-area LbL OSCs reported in the literature to date. Our work indicates that the solution-processed LbL approach not only presents good generality and high device performance, but also is a superior alternative to the BHJ method for the initial evaluation of photovoltaic materials and the industrial production of R2R OSCs.

A multi-objective optimization-based layer-by-layer blade-coating approach for organic solar cells: rational control of vertical stratification for high performance

Abstract: A major breakthrough in organic solar cells (OSCs) in the last thirty years was the development of the bulk heterojunction (BHJ) solution processing strategy, which effectively provided a nanoscale phase-separated morphology, aiding in the separation of Coulombically bound excitons and facilitating charge transport and extraction. Compared with the application of the layer-by-layer (LbL) approach proposed in the same period, the BHJ spin-coating technology shows overwhelming advantages for evaluating the performance of photovoltaic materials and achieving more-efficient photoelectric conversion. Thus, in this study, we have further compared the BHJ and LbL processing strategies via the doctor-blade coating technology because it is a roll-to-roll compatible high-throughput thin film fabrication route. We systematically evaluated multiple target parameters, including morphological characteristics, optical simulation, physical kinetics, device efficiency, and blend stability issues. It is worth emphasizing that our findings disprove the old stereotypes such as the BHJ processing method is superior to the LbL technology for the preparation of high-performance OSCs and the LbL approach requires an orthogonal solvent and donor/acceptor materials with special solubility. Our studies demonstrate that the LbL blade-coating approach is a promising strategy to effectively reduce the efficiency-stability gap of OSCs and even a superior alternative to the BHJ method in commercial applications.

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.

Non-fullerene acceptors with branched side chains and improved molecular packing to exceed 18% efficiency in organic solar cells

Abstract: Molecular design of non-fullerene acceptors is of vital importance for high-efficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach, as it may introduce an undesirable steric hindrance that reduces charge transport in non-fullerene acceptors. Here we show the design and synthesis of a highly efficient non-fullerene acceptor family by substituting the beta position of the thiophene unit on a Y6-based dithienothiophen[3,2-b]-pyrrolobenzothiadiazole core with branched alkyl chains. It was found that such a modification to a different alkyl chain length could completely change the molecular packing behaviour of non-fullerene acceptors, leading to improved structural order and charge transport in thin films. An unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of the branched alkyl chain topology in tuning the molecular packing and blend morphology, which leads to improved organic photovoltaic performance. Molecular design of acceptor and donor molecules has enabled major progress in organic photovoltaics. Li et al. show that branched alkyl chains in non-fullerene acceptors allow favourable morphology in the active layer, enabling a certified device efficiency of 17.9%.

High-efficiency organic solar cells with low non-radiative recombination loss and low energetic disorder

Abstract: Energy loss within organic solar cells (OSCs) is undesirable as it reduces cell efficiency1–4. In particular, non-radiative recombination loss3 and energetic disorder5, which are closely related to the tail states below the band edge and the overall photon energy loss, need to be minimized to improve cell performance. Here, we report how the use of a small-molecule acceptor with torsion-free molecular conformation can achieve a very low degree of energetic disorder and mitigate energy loss in OSCs. The resulting single-junction OSC has an energy loss due to non-radiative recombination of just 0.17 eV and a high power conversion efficiency of up to 16.54% (certified as 15.89% by the National Renewable Energy Laboratory). The findings take studies of organic photovoltaics deeper into a new regime, beyond the limits of energetic disorder and large energy offset for charge generation. An organic solar cell designed with minimal energetic disorder exhibits very low energy loss due to non-radiative recombination and highly efficient operation.

New Phase for Organic Solar Cell Research: Emergence of Y-Series Electron Acceptors and Their Perspectives

Abstract: With the recent emergence of a new class of high-performance nonfullerene acceptors (NFAs), organic solar cells (OSCs) have entered a new phase of research featuring high power conversion efficienc...

Eco-Compatible Solvent-Processed Organic Photovoltaic Cells with Over 16% Efficiency.

Abstract: Recent advances in nonfullerene acceptors (NFAs) have enabled the rapid increase in power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, this progress is achieved using highly toxic solvents, which are not suitable for the scalable large-area processing method, becoming one of the biggest factors hindering the mass production and commercial applications of OPVs. Therefore, it is of great importance to get good eco-compatible processability when designing efficient OPV materials. Here, to achieve high efficiency and good processability of the NFAs in eco-compatible solvents, the flexible alkyl chains of the highly efficient NFA BTP-4F-8 (also known as Y6) are modified and BTP-4F-12 is synthesized. Combining with the polymer donor PBDB-TF, BTP-4F-12 shows the best PCE of 16.4%. Importantly, when the polymer donor PBDB-TF is replaced by T1 with better solubility, various eco-compatible solvents can be applied to fabricate OPV cells. Finally, over 14% efficiency is obtained with tetrahydrofuran (THF) as the processing solvent for 1.07 cm2 OPV cells by the blade-coating method. These results indicate that the simple modification of the side chain can be used to tune the processability of active layer materials and thus make it more applicable for the mass production with environmentally benign solvents.