A new sequential-vapour-deposition method is demonstrated for the growth of high-quality CH 3 NH 3 PbI 3 perovskite films. This has enabled the all-vapour, low-temperature fabrication of hole-conductor-free planar perovskite solar cells consisting of only a CH 3 NH 3 PbI 3 /C 60 bilayer sandwiched between two electrical contacts, with a power conversion efficiency of 5.4%.Organometallic trihalide perovskites with the general formula (RNH 3 )MeX 3 (where R is an organic group, Me is Pb or Sn, and X is a halogen I, Br, or Cl) have recently emerged as new generation light harvesting materials in excitonic solar cells.1-3 In particular, methylammonium (MA) lead triiodide (CH 3 NH 3 PbI 3 or MAPbI 3 ) has attracted great deal of attention since it was rst applied as the light absorber in mesoscopic solar cells.4-6 Within a short period of time the power conversion efficiency (PCE) of MAPbI 3 -based solar cells has shot up dramatically to 16.2%. 4,6,7 This rapid rise in the performance is the result of the innate desirable properties of MAPbI 3 , including favourable direct band gap, large adsorption coefficient, high carrier mobilities and long carrier (balanced) diffusion lengths. 1,5,6,8 With regards to the latter, Xing et al.8 showed that the carrier diffusion length in solution-processed MAPbI 3 thin lms is at least 100 nm, despite the non-ideal nature of the solution-spun MAPbI 3 lms. Higher PCEs of $12% were obtained by Malinkiewicz et al. 9 and Chen et al.10 in planar (non-mesoscopic) solar cells based on better quality MAPbI 3 lms of $300 nm thickness. This suggests that maximizing the quality (coverage, crystallinity, texture) of MAPbI 3 lms can lead to carriers diffusion lengths >300 nm, which is the key to realizing planar perovskite-based solar cells with high efficiencies.
8,9However, reliable deposition of high-quality lms of phasepure MAPbI 3 perovskites with full coverage and high crystallinity still remains a challenge.5,6 One-step solution-spun MAPbI 3 generally results in lms with pinholes due to high reaction rate between MAI and PbI 2 .4,10 To address this issue Burschka, et al.4 developed a two-step method, where mesoporous TiO 2 is rst inltrated by solution-processed PbI 2 , followed by dipping it in a MAI solution for the in situ formation of MAPbI 3 . However, in the case of planar solar cells, where a mesoporous TiO 2 scaffold is not used, several hours are needed for the complete conversion of MAPbI 3 , which can result in the peeling of the lms.4 In another study involving planar solar cells, the dipping process was replaced by extended annealing of the solution-processed PbI 2 lm in a MAI-vapour-rich N 2 atmosphere at 150 C. 10 However, this process may be not amenable to producing uniform MAPbI 3 lms on organic substrates.10 Meanwhile, Liu et al. 11 and Malinkiewicz et al.
9used dual-source co-evaporation deposition to prepare uniform, pinhole-free lms of MAPbI 3 or chorine-doped MAPbI 3 , where the resulting planar solar cells delivered one of the highest PCEs. However, car...