1 © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com www.MaterialsViews.com High-Performance Graphene-Based Hole Conductor-Free Perovskite Solar Cells: Schottky Junction Enhanced Hole Extraction and Electron Blocking Keyou Yan, Zhanhua Wei, Jinkai Li, Haining Chen, Ya Yi, Xiaoli Zheng, Xia Long, Zilong Wang, Jiannong Wang, Jianbin Xu, and Shihe Yang* cell performance degraded quickly due probably to chemical etching of Au by iodide. Therefore, carbon electrodes, a chem- ically and thermally stable material, such as 0D carbon black, candle soot, and 1D carbon nanotube, were also employed to serve as low cost hole extraction electrode due to their suit- able Fermi level at ≈-5.0 eV, and to eliminate both the HTM layer and the vacuum-deposited noble metal electrode. [9,10] At the same time, we employed candle soot electrode as a hole extractor to achieve 2% PCE at first and then 11% after chemical modification of the interface between perovskite and carbon electrode, making a key step toward metal elec- trode-free PSC. [11] Compared to HTM based PSC, carbon electrode displayed outstanding long-term performance sta- bility when operating for more than 1000 h under illumina- tion, demonstrating commercial potential. [10] To date, however, the nature of the interface between per- ovskite and nanocarbon hole extraction electrodes has not been addressed and until now, the fill factor (FF) of carbon based PSC was still less than 0.70. [10] In this work, we employ flexible reduced graphene oxide [12] to yield conducting gra- phene as hole extraction electrode. Single-layered graphene (SG) and multilayered graphene (MG) are employed in the solar cells, which actually have different work functions (4.8 eV for SG and 5.0 eV for MG). This energetic difference will result in ohmic contact (SG/perovskite) and Schottky junction (MG/perovskite) with perovskite (Fermi level at -4.73 eV) if well-assembled, and will help to find out the suitable interfacial contact regime for hole extraction and charge separation. The assembly of graphene toward final perovskite is realized by the chemically reactive paste made of graphene and CH 3 NH 3 I, which can release the organic component of CH 3 NH 3 I to PbI 2 at the interface between graphene and PbI 2 and transform PbI 2 to CH 3 NH 3 PbI 3 per- ovskite to reduce pinholes, [11,13,14] thereby yielding a perfect contact for efficient hole extraction. Indeed, MG/perovskite interface was successfully assembled as a Schottky junction, with a rectifying characteristic, which afford hole extraction and electron rejection effectively, whereas the SG/perovskite interface shows an ohmic contact behavior, whose charge- selective capability is not nearly as well as MG. This is con- firmed by time resolved photoluminescence (TRPL), which shows that MG has higher hole extraction rates (5.1 ns -1 ) than SG (<3.7 ns -1 ). MG is a champion for hole extraction, DOI: 10.1002/smll.201403348 Perovskite Solar Cells Dr. K. Yan, Z. Wei, J. Li, Dr. H. Chen, X. Zheng, Dr. X. Long, Z. Wang, Prof. S. Yang Department of Chemistry Hong Kong University of Science and Technology Clear Water Bay Kowloon, Hong Kong, China E-mail: chsyang@ust.hk Dr. K. Yan, Prof. J. Xu Department of Electronic Engineering Chinese University of Hong Kong Shatin, Hong Kong, China Y. Yi, Prof. J. Wang Department of Physics Hong Kong University of Science and Technology Clear Water Bay Kowloon, Hong Kong, China In recent years, perovskite solar cells (PSCs) have shown great promise for their solution process and high efficiency, which have overtaken organic and quantum dot solar cells in a short time span in power conversion efficiency (PCE), vigorously catching up with crystalline silicon solar cells. [1] In the standard configuration, mesoporous TiO 2 or Al 2 O 3 are employed as a scaffold for perovskite absorbers, with hole transporting materials (HTMs) atop. [2,3] Such configuration has achieved a 20.2% certified PCE. [3,4] However the HTM layer, usually the spiro-OMeTAD, is not air-stable for out- door application. Although the problem can be ameliorated by encapsulation as for dye-sensitized solar cell (DSSC), this greatly increases the cost. [5] Another problem relates to the ponderous vacuum deposition of noble metal electrode as current collector for the cathode. Thus the present p– i– n architecture still needs to be simplified so that it is HTM-free. This is possible considering that some of the most amazing advantages of the perovskite are the long, balanced electron- hole diffusion lengths, up to 1 μm, [6] so that even without electron transporting material (ETM) and/or HTM, the perovskite in itself permits effective charge separation and collection when selective contacts are made to it. [7] Indeed, a HTM-free PSC with Au electrode has already been dem- onstrated with efficiency up to 6%–10%. [8] In this case, how- ever, vacuum deposition of noble metal was still required and small 2015, DOI: 10.1002/smll.201403348