A New Acceptor for Highly Efficient Organic Solar Cells Feng Gao1,* Research into organic solar cells has gone from pure scientific curiosity to a topic of commercial relevance in the past few years, as a result of rapid development of non-fullerene acceptors. This transition is mainly driven by the development of new materials. Recently in Joule, Zou and co-workers developed a new acceptor material and reached a record efficiency for single-junction organic solar cells.
Light absorption in organic semiconductors generates strongly bound excitons. As a result, a heterojunction between two materials (donor and acceptor) is required to split excitons into free carriers in organic solar cells (OSCs). Progress in OSCs was dominated by the development of donor materials for more than two decades, since fullerene derivatives were believed to be the only option as acceptor materials in high-efficiency OSCs. Recent rapid development of nonfullerene OSCs has changed the situation. Zhan and co-workers pioneered the development of a novel acceptor named ITIC (Figure 1).1 ITIC and its derivatives have now become the superstar materials in state-of-the-art OSCs.2–4 The excitement of the community was further ignited by a unique donor material named PBDB-T (Figure 1),5 which was developed by Hou and co-workers. The uniqueness of PBDB-T lies in the fact that it shows appropriate aggregation, which is believed to control the formation of nanoscale microstructures and benefit the resulting morphology of the bulk heterojunction for optimal performance.4 The blend of ITIC and PBDB-T delivered a power conversion efficiency (PCE) up to 11.2%.5 Since that, both donor and acceptor materials have developed very quickly.
The new acceptor material developed by Zou and co-workers recently in Joule is named Y6 (Figure 1).6 Y6 follows the design strategies of the same group in their previous publications,7,8 i.e., to include ladder-type electron-deficientcore-based central fused ring, which is different from the electron-rich core in ITIC. In addition, the central fused core of Y6 is attached with alkyl side chains on the nitrogen atoms at the same side to prevent over aggregation of molecules and at the same time maintain an effective intramolecular contact for charge transport. The absorption of Y6 extends to 900 nm, which is optimal for balanced photocurrent and open-circuit voltage (VOC). For the selection of the donor materials, they followed the rules to minimize the energetic offset between donor and acceptor,9 with the motivation to enhance VOC. By coupling Y6 with a PBDB-T derivative named PM6 (Figure 1), the authors were able to make a breakthrough and demonstrate a high PCE up to 15.7%. The design of electron-deficient-core-based fused ring might open up totally new possibilities for high-performance acceptors. The device parameters are really encouraging. The VOC is 0.83 V, and the gap determined from the external quantum efficiency of the devices10 is 1.41 eV, indicating a small voltage loss of 0.58 V. Although this value is still
Joule 3, 908–919, April 17, 2019 ª 2019 Elsevier Inc.
large compared with high-efficiency perovskite and inorganic solar cells, it is among the lowest values for stateof-the-art OSCs. A high external quantum efficiency above 80% for a 150 nm active layer indicates very efficient charge separation and negligible recombination losses at short-circuit conditions. A high fill factor (FF) of 75% indicates efficient charge transport, which has also been evidenced by high efficiency of thick devices (13.6% for a 300-nm-thick device). Importantly, it indicates that there is no limitation of high FF in these OSCs with low photovoltage losses. It seems that this combination of PM6 and Y6 results in efficient optimization of all the parameters. It would be very interesting to have further investigations of device physics and material photophysics of this material combination, especially the processes related with charge separation and charge recombination. It might be able to rationalize this high efficiency and provide new insights from different perspectives. For the device performance, while marginal improvement of the short-circuit current and FF is possible by fine tuning Y6, the VOC might be the only avenue where significant improvement can be expected, so that the PCE can reach a new paradigm. This is certainly a challenging task. We previously proposed that, in order to decrease the photovoltage losses, it is important to improve the luminescent properties of the pristine material, especially that with the low gap (in this case, Y6).9 Unfortunately, rational design rules for enhancing the luminescent properties of organic semiconductors are still missing. I am not sure whether
of Physics, Chemistry and Biology (IFM), Linko¨ping University, 58183 Linko¨ping, Sweden *Correspondence: [email protected]
Figure 1. Chemical Structures of the Materials Discussed in This Paper
incremental modifications of Y6 can reach this goal or totally new acceptor materials are required. Although open questions remain, the development of Y6 is really encouraging for the community for different reasons. First, it reinforces our belief in the bright future of OSCs; second, it implies new possibilities beyond ITIC derivatives, which have dominated the development of non-fullerene acceptors during the past few years; third, it indicates that the next breakthrough might lie in the materials that can further decrease the voltage losses, a task that is challenging yet possible. 1. Lin, Y., Wang, J., Zhang, Z.-G., Bai, H., Li, Y., Zhu, D., and Zhan, X. (2015). An electron
acceptor challenging fullerenes for efficient polymer solar cells. Adv. Mater. 27, 1170– 1174. 2. Cheng, P., Li, G., Zhan, X., and Yang, Y. (2018). Next-generation organic photovoltaics based on non-fullerene acceptors. Nat. Photonics 12, 131–142. 3. Zhang, J., Tan, H.S., Guo, X., Facchetti, A., and Yan, H. (2018). Material insights and challenges for non-fullerene organic solar cells based on small molecular acceptors. Nat. Energy 3, 720–731. 4. Hou, J., Ingana¨s, O., Friend, R.H., and Gao, F. (2018). Organic solar cells based on nonfullerene acceptors. Nat. Mater. 17, 119–128. 5. Zhao, W., Qian, D., Zhang, S., Li, S., Ingana¨s, O., Gao, F., and Hou, J. (2016). Fullerene-free polymer solar cells with over 11% efficiency and excellent thermal stability. Adv. Mater. 28, 4734–4739. 6. Yuan, J., Zhang, Y., Zhou, L., Zhang, G., Yip, H.-L., Lau, T.-K., Lu, X., Zhu, C., Peng, H., Johnson, P.A., et al. (2019). Single-junction organic solar cell with over 15% efficiency
using fused-ring acceptor with electrondeficient core. Joule 3, this issue, 1140–1151. 7. Feng, L., Yuan, J., Zhang, Z., Peng, H., Zhang, Z.-G., Xu, S., Liu, Y., Li, Y., and Zou, Y. (2017). Thieno[3,2-b]pyrrolo-fused pentacyclic benzotriazole-based acceptor for efficient organic photovoltaics. ACS Appl. Mater. Interfaces 9, 31985–31992. 8. Yuan, J., Huang, T., Cheng, P., Zou, Y., Zhang, H., Yang, J.L., Chang, S.-Y., Zhang, Z., Huang, W., Wang, R., et al. (2019). Enabling low voltage losses and high photocurrent in fullerene-free organic photovoltaics. Nat. Commun. 10, 570. 9. Qian, D., Zheng, Z., Yao, H., Tress, W., Hopper, T.R., Chen, S., Li, S., Liu, J., Chen, S., Zhang, J., et al. (2018). Design rules for minimizing voltage losses in high-efficiency organic solar cells. Nat. Mater. 17, 703–709. 10. Wang, Y., Qian, D., Cui, Y., Zhang, H., Hou, J., Vandewal, K., Kirchartz, T., and Gao, F. (2018). Optical gaps of organic solar cells as a reference for comparing voltage losses. Adv. Energy Mater. 8, 1801352.
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