Yifan Zhao

Synergistic Effect of Polymer and Small Molecules for High‐Performance Ternary Organic Solar Cells

A ternary blend system with two donors and one acceptor provides an effective route to improve the performance of organic solar cells. A synergistic effect of polymer and small molecules is observed in ternary solar cells, and the power conversion efficiency (PCE) of the ternary system (8.40%) is higher than those of binary systems based on small molecules (7.48%) or polymers (6.85%).

Oligomeric Donor Material for High‐Efficiency Organic Solar Cells: Breaking Down a Polymer

Dr. L. Yuan, Dr. J. Zhang, Dr. Y. Zhang, Dr. L. Zhu, Dr. K. Lu, Prof. Z. Wei National Center for Nanoscience and Technology Beijing 100190 , China E-mail: lvk@nanoctr.cn; weizx@nanoctr.cn Dr. L. Yuan, Prof. Z. Wei University of Chinese Academy of Sciences Beijing 100049 , China Dr. Y. Zhao, Prof. W. Yan Department of Environmental Science and Engineering Xi’an Jiaotong University Xi’an 710049 , China

Enhancing Performance of Large-Area Organic Solar Cells with Thick Film via Ternary Strategy

Large-scale fabrication of organic solar cells requires an active layer with high thickness tolerability and the use of environment-friendly solvents. Thick films with high-performance can be achieved via a ternary strategy studied herein. The ternary system consists of one polymer donor, one small molecule donor, and one fullerene acceptor. The small molecule enhances the crystallinity and face-on orientation of the active layer, leading to improved thickness tolerability compared with that of a polymer-fullerene binary system. An active layer with 270 nm thickness exhibits an average power conversion efficiency (PCE) of 10.78%, while the PCE is less than 8% with such thick film for binary system. Furthermore, large-area devices are successfully fabricated using polyethylene terephthalate (PET)/Silver gride or indium tin oxide (ITO)-based transparent flexible substrates. The product shows a high PCE of 8.28% with an area of 1.25 cm2 for a single cell and 5.18% for a 20 cm2 module. This study demonstrates that ternary organic solar cells exhibit great potential for large-scale fabrication and future applications.

Small molecules incorporating regioregular oligothiophenes and fluorinated benzothiadiazole groups for solution-processed organic solar cells†

Small molecules incorporating regioregular oligothiophenes and fluorinated benzothiadiazole groups were synthesized for solution-processed organic solar cells. The length of oligothiophene units showed profound influences on light absorption, energy levels, crystalline behaviors and active layer morphologies. The photovoltaic performances were tested using mixed-solvent processing methods. When blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active layer, the shorter molecule, 4TA4T, achieved a power conversion efficiency (PCE) of 3.81%, with an open circuit voltage (Voc) of 0.83 V, a short current density (Jsc) of 8.02 mA cm−2 and a Fill Factor (FF) of 0.57. The longer molecule, 6TA6T, showed a lower PCE of 3.21% due to a lower Voc of 0.74 V. This study provides detailed insights for the designing of small molecules by subtle changes in donor lengths.

Evolution of morphology and open-circuit voltage in alloy-energy transfer coexisting ternary organic solar cells

The oligomer-type small molecule PDT2FBT-ID is applied in a polymer/small molecule/fullerene ternary system. The PTB7-Th/PDT2FBT-ID/PC71BM ternary active layer shows complementary absorption spectra, enhanced face-on orientation and energy transfer properties. Compared to the binary system, the FF of the ternary system is improved from 66.72% to 76.14% and the Jsc is improved from 17.90 mA cm−2 to 18.92 mA cm−2. The maximum PCE of 11.1% is obtained in the ternary system with a common inverted device structure. Additionally, an alloy-like domain structure and energy transfer between the two donor materials are found to coexist in the ternary system. Based on the combined model, the variation in morphology and Voc is discussed in detail and compared with previous publications. An in-depth interpretation for the selection of a third compound in high performance ternary organic solar cells is provided. Finally, this facile design strategy could also be widely used in other systems.

Large-area, flexible polymer solar cell based on silver nanowires as transparent electrode by roll-to-roll printing

Conventional organic solar cell’s (OSC) architectures, including rigid transparent substrate (Glass), conductive electrode (Indium tin oxide, ITO) and small working areas, are widely utilized in organic photovoltaic fields. However, such a structure as well as conventional spin-coating method obviously restrict their industrial application. In this article, we report the deposition of silver nanowires (AgNWs) on the flexible substrate by slot-die printing. The obtained AgNWs films exhibited a high transmittance and a low resistance, and were further used as the transparent conductive electrode of OSCs. A typical conjugated polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c] [1,2,5]thiadiazole)] (PPDT2FBT), was used as the active material to fabricate large-area (7 cm2 solar cells by a slot-die coating process. The power conversion efficiency (PCE) could reach 1.87% initially and further increased to 3.04% by thermal annealing. Compared to the performance of reference cell on ITO substrate, the result indicated that the AgNWs could be developed as an alternative substitute of conductive electrode to fabricate the large-area flexible OSCs by roll-to-roll printing.

Understanding effects of two different acceptors in one small molecule for solution processable organic solar cells

Introducing two different electron-withdrawing groups into the molecular backbone is one of the effective methods to extend absorption and lower the highest occupied molecular orbital (HOMO) energy level for a high open-circuit voltage (Voc). However, examples of two-acceptor type small molecules are less studied. In order to understand the effect of two acceptors, two kinds of small molecules named as BDT(dFBT-TT)2 with a single acceptor unit and BDT(dFBT-ON)2 with two acceptor units have been synthesized, respectively. However, the power conversion efficiency (PCE) of the BDT(dFBT-ON)2 device is lower (3.54%) than that of BDT(dFBT-TT)2 (5.55%). Grazing incidence wide angle X-ray scattering (GIWAXS) and the intensity dependence of current–voltage characteristics were used to study the spatially periodic order and charge recombination of these small molecules. Although the Voc of BDT(dFBT-ON)2 is higher than that of BDT(dFBT-TT)2, the short circuit current (Jsc) of BDT(dFBT-ON)2 is lower due to a higher ratio of charge carrier recombination. For the sake of getting a high Voc and Jsc simultaneously in the future, this work would supply valuable insights for designing multi-acceptor-type small molecules.