6.5% efficient perovskite quantum-dot-sensitized solar cell† Jeong-Hyeok Im, Chang-Ryul Lee, Jin-Wook Lee, Sang-Won Park and Nam-Gyu Park * Received 19th July 2011, Accepted 15th August 2011 DOI: 10.1039/c1nr10867k Highly efficient quantum-dot-sensitized solar cell is fabricated using ca. 2–3 nm sized perovskite (CH 3 NH 3 )PbI 3 nanocrystal. Spin- coating of the equimolar mixture of CH 3 NH 3 I and PbI 2 in g-butyrolactone solution (perovskite precursor solution) leads to (CH 3 NH 3 )PbI 3 quantum dots (QDs) on nanocrystalline TiO 2 surface. By electrochemical junction with iodide/iodine based redox electrolyte, perovskite QD-sensitized 3.6 mm-thick TiO 2 film shows maximum external quantum efficiency (EQE) of 78.6% at 530 nm and solar-to-electrical conversion efficiency of 6.54% at AM 1.5G 1 sun intensity (100 mW cm 2 ), which is by far the highest efficiency among the reported inorganic quantum dot sensitizers. Introduction Semiconductor quantum dots (QDs) have attracted considerable attention due to their unique opto-electronic properties. 1 Size tunable absorption, 2 high optical extinction coefficient 3 and multiple exciton generation 4 characteristics in semiconductor QDs are particularly suitable for light absorbers in solar cells. Three different QD solar cell configurations, a QD-arrayed p-i-n junction solar cell, a QD-sensi- tized solar cell and a QD-embedded polymer solar cell, were proposed, 5 where the QD-sensitized solar cell is an effective archi- tecture to utilize QDs as absorbers because of using high surface area metal oxide supports for photosensitizing QDs. The QD-sensitized solar cell configuration is analogous to the dye-sensitized solar cell, 6 where the dye is replaced by QDs. Since the experimental verification of charge separation of photo-excited QD by n-type metal oxide and redox electrolyte, 7 numerous QD materials have been tested to confirm their photovoltaic performance in QD-sensitized solar cell structure [see Table 1 in ref. 8]. However, photovoltaic performance was mostly poor with conversion efficiency less than 1% at standard 1 sun illumination, until the report on CdS QD-sensitized solar cell with efficiency exceeding 1% in 2008. 9 Soon after, rapid progress has been made and, as a result, 4–5% efficiencies were recently achieved by using metal chalcogenides. 10 Although metal chalcogenide QDs seem to be excellent inorganic sensitizers for QD-sensitized solar cells, recombination and chemical instability have been issued in ill-defined sulfur-based redox electro- lyte. 11 To overcome such problems in metal chalcogenide QDs, lead iodide based perovskite-type sensitizers were recently introduced. 12 Perovskite ABX 3 (X ¼ halogens) structure consists of organic components in cuboctahedral A site and inorganic components in octahedral B site and the chemistry of the organic and inorganic components can be tailored to tune the optical, electronic, magnetic, and mechanical properties of hybrid materials. 13 The (CH 3 NH 3 )PbI 3 perovskite used for sensitizer demonstrated solar-to-electrical energy conversion efficiency of 3.81% at 1 sun illumination in iodide-based redox electrolyte. 12 Despite excellent optical property of perovskite lead halide, 14 solution-based in situ deposition of (CH 3 NH 3 )PbI 3 perovskite sensitizer on TiO 2 surface led to maximum external quantum efficiency of about 50%, 12 which is, however, much lower than those of organometallic ruthenium dye and metal chalcogenide QDs. The low EQE from (CH 3 NH 3 )PbI 3 sensitizer is probably due to lack of detailed studies on its opto-electronic properties. Therefore, systematic and detailed investigations are required to get optimal performance from perovskite sensitizer. In this paper, we report 6.54% efficient QD-sensitized solar cell based on perovskite (CH 3 NH 3 )PbI 3 sensitizer, which is, as far as we know, the highest efficiency among the reported inorganic QD sensitizers. Energy dispersive X-ray spectroscopy (EDS) and trans- mission electron microscopy (TEM) studies were performed to investigate 3-dimentional distribution of perovskite QD sensitizers in nanocrystalline TiO 2 film and adsorption of QD on TiO 2 surface. Effects of perovskite coating solution concentration and post- annealing temperature on photovoltaic performance were investi- gated. Electron transport and lifetime of perovskite QD-sensitized TiO 2 film were compared to those of a conventional ruthenium dye. Experimental (CH 3 NH 3 )PbI 3 was formed on TiO 2 surface from the g-butyr- olactone (Aldrich) solution containing equimolar mixture of CH 3 NH 3 I and PbI 2 . CH 3 NH 3 I was synthesized by reacting 27.86 mL methylamine (40% in methanol, TCI) and 30 mL of hydroiodic acid (57 wt% in water, Aldrich) in 250 mL round bottomed flask at 0  C for 2 h with stirring. The precipitate was recovered by evaporation at 50  C for 1 h. The product, methyl ammonium iodide CH 3 NH 3 I, was washed with diethyl ether by stirring the solution for 30 min, which was repeated three times, and then finally dried at 60  C in vacuum oven for 24 h. The synthesized CH 3 NH 3 I powder was mixed with PbI 2 (Aldrich) at 1 : 1 mol ratio in g-butyrolactone at 60  C for 12 h, followed by filtering twice using 13 mm diameter and 0.45 mm pore School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon, 440-746, Korea. E-mail: npark@ skku.edu; Fax: +82-31-290-7272; Tel: +82-31-290-7241 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c1nr10867k 4088 | Nanoscale, 2011, 3, 4088–4093 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2011, 3, 4088 www.rsc.org/nanoscale COMMUNICATION Published on 07 September 2011. Downloaded by Indian Institute of Science on 28/05/2013 11:53:02. View Article Online / Journal Homepage / Table of Contents for this issue