COMMUNICATION © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 5) 1401855 wileyonlinelibrary.com High-Efficiency Solution-Processed Planar Perovskite Solar Cells with a Polymer Hole Transport Layer Dewei Zhao, Michael Sexton, Hye-Yun Park, George Baure, Juan C. Nino, and Franky So* Dr. D. Zhao, M. Sexton, Dr. H.-Y. Park, G. Baure, Prof. J. C. Nino, Prof. F. So Department of Materials Science and Engineering University of Florida 100 Rhines Hall, Gainesville, FL 32611, USA E-mail: fso@mse.ufl.edu DOI: 10.1002/aenm.201401855 In this work we demonstrate a high-efficiency solution- processed inverted CH 3 NH 3 PbI 3 perovskite solar cell, which is free of PEDOT:PSS and high-temperature processed metal oxides ( Figure 1a). We use poly[ N, N′-bis(4-butylphenyl)- N, N′- bis(phenyl)benzidine] (poly-TPD) as the HTL and electron blocking layer for the perovskite cells. In previous reports, poly- TPD was used as an HTL in vacuum deposited perovskite solar cells. [14] Here, the perovskite film was formed by sequential deposition of lead iodide (PbI 2 ) and methyl ammonium iodide (CH 3 NH 3 I). We found that the resulting film consisted of large crystallites with a complete coverage on the poly-TPD surface, and the average efficiency of the final devices reach a value of 13.8% and a maximum value as high as 15.3%. To deposit the perovskite film on the poly-TPD surface, a concentrated solution of PbI 2 was first spin-coated and then heated to partially evaporate the solvent and crystallize PbI 2 . Subsequently, a dilute solution of CH 3 NH 3 I is spin-coated on top of the PbI 2 layer and CH 3 NH 3 PbI 3 is formed by interdif- fusion of the precursors. As shown in Figure 1b, a composite layer of spin-coated [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 60 BM), and thermally evaporated C 60 and 2,9-dimethyl- 4,7-diphenyl-1,10-phenanthroline (BCP) is deposited on top of the CH 3 NH 3 PbI 3 layer to planarize the surface of the perovskite layer, and to facilitate electron extraction and hole blocking. [17] More details on device fabrication can be found in the Experi- mental Section. To better understand the device characteristics, devices were also fabricated with PEDOT:PSS as the HTL for comparison. The average current density–voltage ( J–V) characteristics of the devices with poly-TPD or PEDOT:PSS as the HTL under 100 mW cm –2 illumination (AM1.5G) are shown in Figure 2a. As shown in the figure, the poly-TPD devices perform sig- nificantly better than the PEDOT:PSS devices. The poly-TPD devices have an average PCE of 13.8% with a short-circuit cur- rent density ( J sc ) of 20.01 mA cm –2 , a V oc of 0.99 V, and a fill factor (FF) of 69.55% ( Table 1). As shown in the histogram of the poly-TPD device data in Figure S1a (Supporting Informa- tion), the highest PCE of the poly-TPD device is 15.3%. The dependence of perovskite solar cell performance on the poly- TPD thickness is also plotted in Figure S1b,c (Supporting Information). The results show that both J sc and V oc are not dependent on the poly-TPD thickness, while the FF is signifi- cantly reduced with increasing the poly-TPD thickness up to 100 nm due to an increase in series resistance. An optimum thickness of 40 nm was used for the devices in this study. How- ever, the PEDOT:PSS devices produce a significantly lower PCE of 4.63% with a J sc of 9.41 mA cm –2 , a V oc of 0.80 V, and a FF of 61.8%. The external quantum efficiency (EQE) spectra meas- ured with and without white light bias (WLB) are shown in Organometallic halide perovskite solar cells are rapidly becoming a promising technology for solar energy conver- sion. Organic/inorganic hybrid perovskite materials have sev- eral unique properties for photovoltaic applications, such as strong absorption across the visible spectrum, [1] long carrier diffusion length (100–1000 nm), [2,3] solution processability, and insensitivity to defect formation. [4–6] In most perovskite cells, compact or mesoporous metal oxides are used as the electron transport layers (ETLs). [7] These ETLs usually require high-temperature processing to achieve efficient carrier trans- port and the resulting devices are not stable with hysteresis in the current–voltage characteristics. [8–11] On the other hand, the most commonly used hole transport layer (HTL) for perovskite cells is 2,2′,7,7′-Tetrakis (N,N-di-p-methoxyphenylamino)-9,9′- spirobifluorene (spiro-OMeTAD) which requires a complex- doping mechanism to promote oxidation of spiro-OMeTAD and degrades the device stability and repeatability. [12] An alternative to this architecture is to place the HTL on the transparent electrode in the so-called “inverted” struc- ture. [13] Most inverted devices employ either poly(3,4-ethylen edioxythiophene):polystyrene sulfonate (PEDOT:PSS) or solu- tion-processed nickel oxide (NiO x ) as the HTL, which present their own issues for perovskite solar cells. [14–19] PEDOT:PSS corrodes the indium-doped tin oxide (ITO) electrode, and causes migration of indium into PEDOT:PSS. [20] The hygro- scopic nature of PEDOT:PSS is prone to degrade the resulting organic devices due to the water uptake. [20] This is specifically problematic for perovskite cells because the perovskite mate- rial methyl ammonium lead iodide (CH 3 NH 3 PbI 3 ) is vulner- able to decomposition upon water exposure. [21,22] While the efficiency of inverted devices with NiO x has reached a power conversion efficiency (PCE) value as high as 11.6%, NiO x requires high-temperature or high-vacuum processing. Poor wetting of the perovskite film on NiO x leads to formation of crystallite islands resulting in a rough surface with shunting paths and hence a lower open-circuit voltage ( V oc ). [19] Addi- tionally, NiO x also forms trap states at the perovskite inter- face leading to significant carrier recombination affecting the device performance. [23,24] Therefore, it is highly desired to develop alternative low-temperature solution-processed HTL materials for perovskite solar cell applications. Adv. Energy Mater. 2014, 1401855 www.MaterialsViews.com www.advenergymat.de