Unraveling the High Open Circuit Voltage and High Performance of Integrated Perovskite/Organic Bulk-Heterojunction Solar Cells Shiqi Dong, † Yongsheng Liu,* ,†,‡ Ziruo Hong, † Enping Yao, † Pengyu Sun, † Lei Meng, † Yuze Lin, § Jinsong Huang, § Gang Li, † and Yang Yang* ,† † Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States ‡ Key Laboratory of Functional Polymer Materials, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China § Department of Mechanical and Materials Engineering and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States * S Supporting Information ABSTRACT: We have demonstrated high-performance in- tegrated perovskite/bulk-heterojunction (BHJ) solar cells due to the low carrier recombination velocity, high open circuit voltage (V OC ), and increased light absorption ability in near- infrared (NIR) region of integrated devices. In particular, we find that the V OC of the integrated devices is dominated by (or pinned to) the perovskite cells, not the organic photovoltaic cells. A Quasi-Fermi Level Pinning Model was proposed to understand the working mechanism and the origin of the V OC of the integrated perovskite/BHJ solar cell, which following that of the perovskite solar cell and is much higher than that of the low bandgap polymer based organic BHJ solar cell. Evidence for the model was enhanced by examining the charge carrier behavior and photovoltaic behavior of the integrated devices under illumination of monochromatic light-emitting diodes at different characteristic wavelength. This finding shall pave an interesting possibility for integrated photovoltaic devices to harvest low energy photons in NIR region and further improve the current density without sacrificing V OC , thus providing new opportunities and significant implications for future industry applications of this kind of integrated solar cells. KEYWORDS: Perovskite, photovoltaic, bulk heterojunction, Fermi level M etal halide based organic-inorganic hybrid perovskite solar cells have been attracting increasing attention in recent years due to the rapid progress in terms of increased efficiency. 1,2 It has become a promising next-generation photovoltaic technology due to its potential to be lightweight, mechanically flexible and manufactured in a cost-effective manner. The power conversion efficiency (PCE) of perovskite solar cells (PSC) has risen from 3.8% to over 20% in the past few years. 3-7 Such a rapid increase in efficiency is attributed to the unique physical properties of the organic-inorganic hybrid perovskite, such as the excellent light absorption coefficient, long exciton diffusion length, ambipolar transport properties, and low cost fabrication of large area devices. 8-10 In addition, the accumulated knowledge in the organic photovoltaic (OPV) and dye-sensitized solar cells (DSSC) also have played an important role in such rapid progress. 11-16 Moreover, several perovskite-based photovoltaic device architectures, including both conventional and inverted architectures, have successfully demonstrated high PCE due to the intrinsic properties of perovskite materials. 17-20 Current organic-inorganic hybrid perovskite materials using organic cations, such as CH 3 NH 3 + or NH 2 CHNH 2 + , show an onset light response limited between 800 to 850 nm, which hinders near-infrared (NIR) light harvesting and thus further impedes PCE improvement. Thus, one important strategy to further enhance the photovoltaic performance of perovskite photo- voltaic devices lies in broadening the light absorption to include the NIR region. To use the NIR part of the solar spectrum, tin halide-based low band gap perovskites, such as CH 3 NH 3 SnI 3 , have been used as light harvesters for solar cell applications. 21,22 Although the onset photocurrent response in the light absorbing tin halide based perovskites extends to over 1000 nm, it is a challenge to further use these materials for future industry applications due to the poor Sn(II) stability and low PCE. 21-24 An even greater problem is that lowering the band gap will result in an smaller open circuit voltage (V OC ). 25 One of the main applications of low band gap polymers in solar cells is to fabricate tandem devices that better utilize the sunlight from the visible to NIR region. It has been reported that integrated perovskite/BHJ photovoltaic device is an efficient Received: June 15, 2017 Revised: July 15, 2017 Published: July 20, 2017 Letter pubs.acs.org/NanoLett © 2017 American Chemical Society 5140 DOI: 10.1021/acs.nanolett.7b02532 Nano Lett. 2017, 17, 5140-5147 Downloaded via UNIV OF NORTH CAROLINA on January 27, 2020 at 17:19:59 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.