Exceptionally Stable CH3NH3PbI3 Films in Moderate Humid Environmental Condition

An unprecedentedly stable CH3NH3PbI3 film synthesized by a modified chemical vapor transport method is demonstrated. The results show that the crystal structure, light absorption, and device efficiency do not degrade after storing for 100 d in air with 40% relative humidity, while the conventional solution‐processed perovskites are usually stable for less than 20 d in similar conditions.


DOI: 10.1002/advs.201500262
prepared as condensed phase by spin-coating PbI 2 solution (in N , N -dimethylformamide, DMF) onto the TiO 2 -compact-layercoated fl uorine-doped tin oxide (FTO) and dried at 100 °C for 10 min. Scanning electron microscope (SEM) characterization shows that the fi lm is composed of small crystallites hosting many randomly distributed pores (Figure 1 b and Figure S1, Supporting Information). Afterward, the PbI 2 fi lm is transferred to the tube furnace containing CH 3 NH 3 I powder. The reaction between PbI 2 fi lm and CH 3 NH 3 I is conducted at 140 °C at a pressure of 1 mbar using Ar as carrier gas (Figure 1 a). Optimizations show that appropriate reaction time is 2-3 h. Reaction less than 2 h cannot lead to complete transformation to CH 3 NH 3 PbI 3 while elongated reaction brings forth poorer device effi ciency ( Figure S2, Supporting Information).
SEM image (Figure 1 c) of the as-prepared fi lm exhibits tightly packed crystals without pinholes throughout the whole fi lm (denoted as sample 1). A large-area SEM image is provided in Figure S3 (Supporting Information), showing pinhole and crack free across the surface. Obviously, the volume expansion upon the formation CH 3 NH 3 PbI 3 is responsible for diminishing pores in the original PbI 2 fi lm. Previously, the solid-gas reactions in either static gas atmosphere or two-zone furnace generate pinholes or cracks in between the perovskite nanocrystals. [ 11 ] Here the mCVT reaction in isothermal chamber with carrier gas is thus advantageous. The other distinct feature of the mCVT approach is that the fl owing gas could more efficiently deplete excess CH 3 NH 3 I deposition on the surface of the as-prepared fi lm than the static gas atmosphere or the twozone apparatus.
The phase purity of the as-prepared fi lm is characterized by X-ray diffraction (XRD), displaying typical perovskite structure of CH 3 NH 3 PbI 3 without impurity peaks ( Figure 2 a). To gain clear and reliable conclusions regarding the moisture stability of perovskite, we record the phase changes in 40% RH in air under darkness in order to rule out other potential infl uences such as UV light. As a result, the XRD characterizations show identical patterns after storing for 30, 45, and 100 d (Figure 2 a). The UV-visible absorption characterizations also show nearly the same spectra after storage (Figure 2 d). It should be noted that the use of PbCl 2 and CH 3 NH 3 I as precursors in mCVT system can generate identical fi lm morphologies ( Figure S4, Supporting Information) with the same stability to that synthesized with PbI 2 and CH 3 NH 3 I as precursors.
To uncover the reason why the mCVT-synthesized perovskite exhibits unusual stability, we prepare perovskite by a solution process with the same precursors for comparative analysis. In brief, PbI 2 is fi rst spin-coated on the TiO 2 /FTO substrate; it is then dipped into the CH 3 NH 3 I solution. Afterward, the fi lm is taken out and heated at 100 °C for 10 min. This is the conventionally applied "two-step sequential deposition" method This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Methylammonium lead iodide (CH 3 NH 3 PbI 3 ) based perovskite materials have drawn intense interests due to the excellent photovoltaic energy conversion capability; the power conversion effi ciency (PCE) of perovskite solar cell has been boosted to 20.1%. [ 1 ] Recent investigations have gained comprehensive understandings about the materials and operational principles as well as accumulated rich experiences in the device fabrication. [ 2,3 ] It has been increasingly acknowledged that the major concern regarding this technology is the poor device stability. [ 4 ] Most recently, an extensive investigation shows that in 25% relative humidity (RH) the absorbance of the perovskite CH 3 NH 3 PbI 3 fi lm decays to half of its original value in 57 d ( t 1/2 ). While in moderate moisture (40% RH), the t 1/2 is only 26 d. The high sensitivity to moisture poses severe challenge in terms of the practical applications. [ 5 ] To improve the stability, one way is to protect the perovskite from water molecule attachment. In this regard, Niu et al. utilized Al 2 O 3 as an interfacial coating layer to resist the moisture permeation; the stability can be improved by a few days. [ 6 ] Han's group employed a thick layer of carbon materials as both hole transporting materials (HTMs) and protection layer to improve the moisture stability; the fi nal PCE reached 12.3% and maintained for over 40 d. [ 7 ] Inorganic HTMs such as CuI and NiO x could also enhance the stability by a few days. [ 8 ] The other approach is to improve intrinsic stability of the perovskite. Apparently, this solution is of fundamental signifi cance and can alleviate the reliance on the stringent encapsulation, thus reducing the fabrication and installing expenses of solar panels. In this perspective, Seok's group found that doping Br in the iodide-perovskite to form CH 3 NH 3 PbI 3x Br x could noticeably improve the stability to 20 d. [ 9 ] Most recently, a layered perovskite (C 6 H 5 (CH 2 ) 2 NH 3 ) 2 (CH 3 NH 3 ) 2 [Pb 3 I 10 ] was synthesized which was stable for 45 d in ambient condition; the device delivered a PCE of 4.7%. [ 10 ] Here we demonstrate that the intrinsic stability of CH 3 NH 3 PbI 3 fi lm can be dramatically improved by tailoring the compositional purity and morphology of the perovskite fi lm through a modifi ed chemical vapor transport (mCVT) reaction approach ( Figure 1 a). In this process, PbI 2 fi lm is fi rstly www.MaterialsViews.com www.advancedscience.com Adv. Sci. 2016, 3,1500262 (denoted as sample 2). [ 2 ] The fi nal fi lm is composed of cuboid nanocrystals (Figure 1 d). XRD characterization shows typical pattern of CH 3 NH 3 PbI 3 (Figure 2 b). The moisture stability of the fi lm is examined by storing with 40% RH at the same condition as sample 1. According to the XRD analysis, the diffraction peaks from PbI 2 increase substantially after 20 and 30 d, indicating that the perovskite starts to decompose in less than 20 d. The degradation speed is in agreement with the literature reports. [ 5,10 ] We also monitor the degradation process of the perovskite fi lm under irridiation and compared it with that of the mCVTprepared one. It is found that the mCVT-prepared fi lm is still much more stable than the two-step prepared one except that the decomposition rate of both the fi lms is more quickly under irridiation than in darkness.
The difference in the synthesis of sample 1 and sample 2 is that the reaction for sample 2 is in DMF solution followed by annealing for 10 min at 100 °C. A fact is that DMF can coordinate with Pb 2+ and it is verifi ed that the DMF molecule is prone to intercalating between the perovskite nanocrystals or adsorbed onto the surface of solution processed perovskite fi lm. [ 12,13 ] The intercalation of DMF molecules could create microgaps onto the perovskite nanocrystals, generating more available sites for water molecules attachment ( Scheme 1 a). Therefore, the decomposition by means of hydrolysis can be considerably expedited. To substantiate this assumption, we prepare CH 3 NH 3 PbI 3 fi lm by the two-step method and intentionally dry it at lower temperature (70 °C) with reduced annealing time. Trace amount of DMF is detected in the perovskite fi lm with Fourier transform infrared spectroscopy at this condition ( Figure S5, Supporting Information). Lower temperature annealing could lead to more DMF retaining in the fi lm and thus more available    1), b) two-step solution processing using PbI 2 and CH 3 NH 3 I (sample 2), and c) one-step processing using PbCl 2 and CH 3 NH 3 I as reactants (sample 3). The samples are stored in air with 40% RH under darkness. d) UV-vis absorption spectra of the mCVT prepared CH 3 NH 3 PbI 3 storing in air with 40% RH for up to 100 d.
site for water attachment. The as-prepared perovskite shows identical crystal structure to that annealed at 100 °C for 10 min, while the decomposition rate is much faster than that annealed at high temperatures ( Figure S5, Supporting Information), it decomposes nearly completely after 30 d. On the other hand, if we prolong the annealing time to detach the DMF molecules, the stability can be improved to certain extent depending on the annealing time.
Furthermore, since DMF molecule possesses both O and N, these two atoms can form hydrogen bonding with H 2 O molecule (Scheme 1 b), which promotes H 2 O attachment. Hence, the existence of the DMF in the perovskite brings forth two negative effects: (1) intercalating between the perovskite crystals to generate more available areas of the perovskite fi lms for H 2 O attachment and (2) accumulating H 2 O molecule through hydrogen bonding, on both the surface of perovskite fi lm and gaps generated (Scheme 1 a).
On the contrary, in the mCVT method, the 2 h reaction in high-vacuum condition at 140 °C is able to detach the DMF much more effi ciently from the fi lm; the formation of this kind of structural fl aw can be effi ciently suppressed (Scheme 1 c). In addition, the overall exposed area of sample 1 is much smaller than that of sample 2 (Figure 1 c,d), this is also a signifi cant factor slowing down the hydrolysis process. The phenomenon that the compact morphology of the perovskite fi lm is benefi cial for the stability enhanced is also reported. [ 14 ] Here, we fi nd that the phase purity of the perovskite fi lm also prominently affects the fi lm's stability.
Conventionally, the planar perovskite fi lm is prepared by a casting-and-annealing processing approach; here we also prepare the fi lm for a comprehensive study. A mixture solution of PbCl 2 and CH 3 NH 3 I in DMF is usually used as the reaction precursors, which is spin-coated onto the TiO 2 compact layer. It is then heated at 100 °C for 45 min. SEM image (Figure 1 e) shows relatively uniform fi lm with a considerable number of pinholes (sample 3), which is in agreement with the literature report. [ 15 ] Obviously, the existing pinholes can serve as channels for water attachment. On the other hand, it is proved that the reaction between PbCl 2 and CH 3 NH 3 I releases gas in the forms of CH 3 NH 3 Cl, CH 3 NH 2 , or HCl, [ 13,16 ] which can induce micropores in the crystals. Therefore, the surface area for water adsorption is substantially increased. It is also plausible that there are DMF molecules on the fi lm surface and unreacted chlorine remained in sample 2 ( Figure S9, Supporting Information) that aid the water attachment. Therefore, sample 3 degrades very quickly (Figure 2 c) and changes to translucent after only 5 d in air with 40% RH. The appearance of diffraction at 10.54° indicates the formation of (CH 3 NH 3 ) 4 PbI 6 ·2H 2 O as a result of water adsorption. [ 5 ] The device (with samples 1, 2, and 3 as the light absorbing layers) performance was examined on the ground of planar heterojunction architectures as shown in Figure 3 a,b, where spiro-OMeTAD is utilized as HTM and thermally evaporated Ag is employed as metal contact. A cross-section of the device based on mCVT-synthesized CH 3 NH 3 PbI 3 is shown in Figure 3b. The initial PCE of sample 1 is 12.23%. The devices based on samples 2 and 3 generate PCE of 12.11% and 12.74%, respectively (Figure 3 c). It is observed that the V oc of device based on mCVT fabricated fi lm is 0.95 V, which is slightly lower than the twostep and one-step solution processed ones which are 0.97 and 1.00 V, respectively. It is possible that defect states are generated at elevated temperature in an I-rich environment in the mCVT synthesis, which usually leads to slight voltage loss. [ 17 ] To exclude the infl uence of the other components such as the HTM on the device stability and gain a clear conclusion regarding the stability of perovskite fi lm, we test the photovoltaic performance of the aged perovskite fi lms by using fresh HTM every time. The performance evolution of the mCVTsynthesized perovskite is shown in Figure 3 c,d. After storing the perovskite fi lm for 12 d in ambient condition, the performance has no obvious change, showing PCE of 12.25%. The PCE (12.68%) is even higher after storing the fi lm for up to 30 d with the V oc increasing substantially from 0.95 to 1.05 V, the J sc slightly drops 5%, and there is no much variation in fi ll factor (FF) on average. These alternations fi nally render an increment on the overall PCE. Remarkably, after storing for 70 and 100 d, the devices show PCEs of 12.59% and 12.71%, respectively. The J-V curves are provided in Figure S6 (Supporting Information).
Notably, the device effi ciency is increased in the fi rst 30 d and the highest PCE of 15.15% is obtained (Figure 3 e). There are dual mechanisms for the effi ciency improvement. First, the perovskite fi lm is prone to decomposing to PbI 2 even though with negligible concentration. The formation of PbI 2 is favorable for a larger V oc , which has been confi rmed by fabricating perovskite with residual unreacted PbI 2 ( Figure S2, Supporting Information). The type-I heterojunction between PbI 2 and perovskite is the reason for the enlarged V oc . [ 18 ] Another mechanism contributing to the PCE improvement might originate from the defect state diminishing. As discussed before, the defect states form at high temperature in an iodine-rich environment and they might self-heal after storing for a long period. [ 17 ] www.MaterialsViews.com www.advancedscience.com Adv. Sci. 2016, 3, 1500262 Scheme 1. a) Illustration of DMF molecules intercalated into or on the surface of CH 3 NH 3 PbI 3 nanocrystal which is synthesized via conventional solution process using DMF as the solvent. The red arrows indicate possible sites for water attachment. b) The formation of the hydrogen bond between water molecule and DMF. c) DMF-free CH 3 NH 3 PbI 3 nanocrystals synthesized by the mCVT approach. The CH 3 NH 3 + is omitted in (a) and (c) for clarifi cation.
The photovoltaic performance of the device based on solution processed perovskite degrades quite fast. After storing for 12 d, the PCE of the one-step perovskite drops to less than 20% of its initial value (Figure 3 c). The V oc and FF show signifi cant reduction. In the two-step-prepared fi lm, the device effi ciency degrades to less than 75% of the initial value after 30 d (Figure 3 c), which is more stable than the one-step-solution-processed fi lm while still much worse than the mCVT-prepared fi lm.
In conclusion, for the fi rst time we show that perovskite fi lm is stable for 100 d in air with 40% RH. The synthesis by an mCVT reaction in an isothermal furnace is able to generate high-quality perovskite fi lm when compared with the two-zone furnace or the solid-gas reaction in static atmosphere. The conventional solution processed perovskite fi lm reduces its absorption intensity to a half of its initial value even in 0% RH after 76 d. [ 9 ] Thus, our research is a quantum leap in the stability improvement of organic-inorganic perovskite-based solar cells. We discover that the phase purity is important for the fi lm stability; the fi lm morphology and arrangement of the perovskite crystallites synergistically contribute to the enhanced stability. The mCVT approach is also adaptable. It has been initially confi rmed by preparing lead-free CH 3 NH 3 SnI 3 via the reaction between SnI 2 fi lm and CH 3 NH 3 I vapor, showing tight arrangement of the CH 3 NH 3 SnI 3 nanocrystals without pinholes across the fi lm ( Figure S8, Supporting Information). In all, this research provides a new, low-cost, and adaptable fabrication method for the perovskite fi lm synthesis with excellent stability. The mechanistic understandings regarding the intrinsic stability of the perovskite fi lm would benefi t further improvement on the life time of the devices for practical applications.

Experimental Section
The fi lm (CH 3 NH 3 PbI 3 ) synthesis is conducted by a chemical vapor transport synthesis in an isothermal furnace (Figure 1 a). Excess amount of CH 3 NH 3 I powder is placed at the upstream in the tube furnace together with the PbI 2 -covered substrate. The reaction is performed at 140 °C and 1 mbar with Ar as carrier gas. The details are included in the Supporting Information. It is worth noting that the reaction taking place in low pressure and isothermal environment is critical for obtaining high-purity perovskite fi lm.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author. Figure 3. a) Device confi guration of the perovskite solar cells, in which the perovskite fi lms were prepared by either the mCVT method or the conventional solution processing. b) A cross-section image of the solar cell based on mCVT synthesized perovskite fi lm. c)Photocurrent-voltage characteristics of the device based on perovskite fi lm fabricated by the mCVT, two-step deposition method, and one-step deposition method; the solid and dashed lines indicate initial device performance with respect to the fi lm stored for 100, 30, and 12 d, respectively. d) The evolutions of normalized PCE, V oc , J sc , and FF of the devices based on mCVT prepared fi lms. e) J-V curve of the best performance solar cell based on mCVT-fabricated CH 3 NH 3 PbI 3 fi lm after storing in air with 40% RH for 30 d.