Silicon Nanowire/Polymer Hybrid Solar Cell-Supercapacitor: A Self- Charging Power Unit with a Total Efficiency of 10.5% Ruiyuan Liu, †,‡ Jie Wang, ‡,§ Teng Sun, † Mingjun Wang, ‡ Changsheng Wu, ‡ Haiyang Zou, ‡ Tao Song, † Xiaohong Zhang, † Shuit-Tong Lee, † Zhong Lin Wang,* ,‡,§ and Baoquan Sun* ,† † Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano and Soft Materials (FUNSOM), and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P. R. China ‡ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST), Beijing 100083, P. R. China * S Supporting Information ABSTRACT: An integrated self-charging power unit, combin- ing a hybrid silicon nanowire/polymer heterojunction solar cell with a polypyrrole-based supercapacitor, has been demon- strated to simultaneously harvest solar energy and store it. By efficiency enhancement of the hybrid nanowire solar cells and a dual-functional titanium film serving as conjunct electrode of the solar cell and supercapacitor, the integrated system is able to yield a total photoelectric conversion to storage efficiency of 10.5%, which is the record value in all the integrated solar energy conversion and storage system. This system may not only serve as a buffer that diminishes the solar power fluctuations from light intensity, but also pave its way toward cost-effective high efficiency self-charging power unit. Finally, an integrated device based on ultrathin Si substrate is demonstrated to expand its feasibility and potential application in flexible energy conversion and storage devices. KEYWORDS: Silicon nanowires, organic/inorganic hybrid solar cells, supercapacitors, integrated self-charging power unit T he rapid developments of modern electronics raise the demand for sustainable technologies by integrating multiple functions in one device. Compared with the intensive research on improving the power conversion efficiency (PCE) of various kinds of solar cells or energy storage devices, integrated systems combing energy conversion and storage functions are still far to be well investigated. Electrochemical systems including batteries and electrochemical supercapacitors designed for energy storage are specially favored since they are more sustainable and environmentally friendly. 1 Supercapaci- tors are outstanding due to their high power density, low weight, quick response to potential changes, high cycle life, and long-term stability that can easily exceed one million operation cycles. 2-4 Self-charging power unit consists of solar cells for energy conversion with supercapacitors for energy storage have attracted more attention than lithium-ion batteries (Table S1). 5-18 In this case, the supercapacitors can not only store the energy but also balance the solar electricity fluctuations by acting as both energy storage and output regulator. The current attempts are focusing on the dye-sensitized solar cells (DSSC) or organic solar cells (OSC) as the energy source part in either planar structure or fiber shape, which have shown their ability as self-charging units. However, the overall total efficiencies of the reported system are still ∼10% despite of the efforts put into the optimization of the integrated device structure and the usage of expensive materials, which are mainly restricted by the relatively low efficiency of the solar energy capturing part and the energy loss caused by the external electrical interconnec- tions in the energy storage process. Since achieving a PCE of over 10% for a single DSSC or OSC is still challenging and even lower in a fiber shape, 19-21 the efficiency of the integrated system is even lower. Pint’s group has pioneered the work using a porous silicon (Si) electrode energy storage system integrated with a DSSC to realize multifunctional platform, and a total efficiency of 2.1% was achieved. 18 Alternatively, an integrated system combining a fiber DSSC and a wire supercapacitor is a good design to achieve multifunctional system; however, the best reported overall energy conversion efficiency remains near ∼2.1%. 13 Wang and co-workers reported a power pack combing a new emerging perovskite solar cell with a polypyrrole-based supercapacitor for the first time with a high total efficiency of 10%. 22 The perovskite showed a highest PCE Received: March 17, 2017 Revised: May 17, 2017 Published: June 6, 2017 Letter pubs.acs.org/NanoLett © 2017 American Chemical Society 4240 DOI: 10.1021/acs.nanolett.7b01154 Nano Lett. 2017, 17, 4240-4247