Unravelling the Effects of Cl Addition in Single Step CH3NH3PbI3

Subscriber access provided by MICHIGAN STATE UNIVERSITY | MSU LIBRARIES

Article

Unravelling the Effects of Cl Addition in Single Step CHNHPbI Perovskite Solar Cells 3

3

3

Natalia Yantara, Yanan Fang, Shi Chen, Herlina Arianita Dewi, Pablo P. Boix, Subodh G Mhaisalkar, and Nripan Mathews Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm502710r • Publication Date (Web): 18 Feb 2015 Downloaded from http://pubs.acs.org on February 19, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Unravelling the Effects of Cl Addition in Single Step CH3NH3PbI3 Perovskite Solar Cells Natalia Yantara1, Fang Yanan1, Chen Shi2,4, Herlina Arianita Dewi3, Pablo P. Boix3, Subodh G. Mhaisalkar1,3, and Nripan Mathews*1,3,4 1. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore. 2. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore. 3. Energy Research Institute@NTU (ERI@N), Research TechnoPlaza, X-Frontier Block, Level 5, 50 Nanyang Drive, 637553, Singapore. 4. Singapore-Berkeley Research Initiative for Sustainable Energy, 1 Create Way, 138602, Singapore.

ABSTRACT: The effect of film morphology on the photovoltaic performances is illustrated. Pin holes free and uniform films could be formed by slowing down the crystallization rate through additional of excess CH3NH3+. Though better film coverage is obtained, the device series resistance increases as excess CH3NH3I increases. Thermogravimetric analysis confirms the presence of residual excess CH3NH3I in the final film. When chlorine is added to the CH3NH3+ rich system, CH3NH3Cl is formed and can easily sublime during annealing at 100°C. A little trace of excess CH3NH3Cl can be detected from thermogravic analysis and the residual chlorine atoms matches with XPS reading.

Methylammonium lead halide (CH3NH3PbX3, X = I, Cl, or Br) perovskites have become the most efficient solution processed solar cell technology with power conversion efficiencies of almost 18% reported.1 Remarkably CH3NH3PbI3 can be deposited by solution based process, vapor assisted deposition, as well as thermal evaporation with high efficiencies reported for solar cells fabricated from each of the previous processes.2–5 The high performance of perovskite solar cells stem from the long diffusion lengths6–8, small exciton binding energies9,10, low defect densities11 as well as low recombination rates12 observed in this material. An interesting question that has arisen during the development of this field is the role of chlorine within the organic inorganic halide perovskite. Snaith and coworkers have utilised precursors involving Cl (PbCl2) and termed the resultant compound as CH3NH3PbI3-xClx. Micron long diffusion lengths have also been noted in the mixed composition, explaining why such a composition can be employed in MSSC where Al2O3 is used as a scaffold.5,8,12 However, Hagfeldt and co workers have demonstrated a similar configuration which employed ZrO2 and CH3NH3PbI3 with comparable performances to the aforementioned MSSCs.13 The effect of chlorine had been tentatively attributed to a doping effect caused by the substitution of iodine by the chlorine at the apical positions.14 However, the high efficiencies noted in both

the compositions (with and without chlorine incorporation) further raise the question on the exact role played by the chlorine . A critical factor when comparing the two different compositions is the role played by the morphology of the deposited films as well as the role of the post deposition annealing protocols. As Eperon et al. and Dualeh et al. have shown, both annealing time and temperature can result in variation of performances due to morphological evolution.15,16 Herein, we have compared the procedures and compositions commonly utilised in the formation of the single step CH3NH3PbI3 and CH3NH3PbI3-xClx. We denote films prepared from an equimolar solution of PbI2 and CH3NH3I as CH3NH3PbI3, while films prepared from solutions of PbCl2 and 3CH3NH3I are denoted as CH3NH3PbI3-xClx. The role of exact precursors and their concentration on the film morphology has been elucidated. X-ray photoelectron spectroscopy (XPS) and Energy dispersive X-ray (EDX) measurements indicate that Cl content in the films are miniscule and the morphological evolution may be attributable to slow crystallisation induced by the excess CH3NH3+. Furthermore, the intermediates occuring during the formation of CH3NH3PbI3-xClx is studied by examining the film evolution during the deposition. Thermogravimetric analysis supports the XPS measurements in indicating a very low content of Cl within the final CH3NH3PbI3-xClx

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 8

Figure 1. The FESEM images (A), XRD spectra (B), absorbance (C) of CH3NH3PbI3-xClx and CH3NH3PbI3 film together with the representative J-V characteristic (D) of the devices made from those films. The recombination resistance extracted from the impedance measurement was also presented (E). The absorbance spectrum of CH3NH3PbI3 is shifted for better comparison.

film. The morphological and crystallographical characteristics of CH3NH3PbI3-xClx can be duplicated by utilising CH3NH3Cl as the starting precursor. CH3NH3PbI3 and CH3NH3PbI3-xClx films were prepared from single precursor solutions using the most commonly utilized procedures.16 CH3NH3PbI3 samples turn black immediately upon drying (70°C), while CH3NH3PbI3-xClx samples require 30 minutes of drying at 1000C before the films turn completely black. The CH3NH3PbI3-xClx film undergoes a color evolution from brown to yellow to finally black. The reasons for the color evolution will be discussed in a later section. A comparison of the physical properties of the films spuncoat on top of mesoporous TiO2 reveals that the final morphologies of the film are completely different. Both the cross section and top view images (Figure 1A) of CH3NH3PbI3 films showed the formation of fibrous structure with significant exposed mesoporous TiO2. However, CH3NH3PbI3-xClx solutions tend to form more uniform and smooth polycrystalline films with less pin holes that exposed the mesoporous TiO2. The XRD spectra (Figure 1B) of both CH3NH3PbI3xClx and CH3NH3PbI3 matched well with a tetragonal I4/mcm structure of CH3NH3PbI3 as reported previously.17 While CH3NH3PbI3 forms a polycrystalline film with no preferred orientation, CH3NH3PbI3-xClx films showed a preferred orientation along the (110) direction. No peak shift or broardening is observed, indicating almost negligible Cl incorporation inside the CH3NH3PbI3 crystal structure. Optical absorption measurements (Figure 1C) indicate a similar bandgap (1.55eV) for both the compositions. Moreover, both films possess identical

features at 480 nm and 760 nm which was identified as the signatures of the valence band 2 to conduction band (VB2-CB) and valence band 1 to conduction band (VB1CB) transitions respectively.6 Photovoltaic devices with TiO2 mesoporous structure and spiro-OMeTAD were made from both films. Representative solar cell data is indicated in Figure 1D. Power conversion efficiencies of 7.69% and 10% were attained with CH3NH3PbI3 and CH3NH3PbI3-xClx based devices respectively. The power conversion efficiency, Jsc, Voc, and FF distribution of devicess with varied deposition solutions are reported in Figure S1. Relatively small hysteresis are observed throughout all sample regardless of its morphology (Figure S6). Relatively higher Voc and FF resulted in the better photovoltaic performance of the CH3NH3PbI3-xClx based devices. Other single step CH3NH3PbI3 devices showed comparable photocurrents to their CH3NH3PbI3-xClx counterparts but still suffered from a reduced VOC. As a significant amount of pin holes on the CH3NH3PbI3 film was previously noted, the exposed TiO2 might came into contact with the spiro-OMeTAD which increases the recombination and reduces both Voc and FF. Moreover, the non-uniformity of the CH3NH3PbI3 film may hinder the formation of uniform spiro-OMeTAD film which increases the probability of shorting between CH3NH3PbI3 and the electrode. As a consequence, CH3NH3PbI3 solar cells could display a higher recombination rate. In order to confirm this effect, impedance spectroscopy measurements were conducted on the samples under illumination. The obtained spectra (see example in Figure S1) were fitted by means of an


Page 3 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. The XRD diffraction pattern (A) and top morphology (B) of CH3NH3PbI3 films formed from solution with varied excess CH3NH3I together with the representative J-V characteristic of devices made from the films (C).

equivalent circuit18 which comprises a transmission line(TL), from which it is possible to extract the recombination resistance (Rrec) of the devices, as well as a high frequency capacitance (Chf) and resistance (Rhf) commonly attributed to the hole transport19 and a series resistance (Rs) which are not affected by the modification of the perovskite (inset Figure S1). The comparison of Rrec (Figure 1E) confirms the reduction of the recombination when the Cl is introduced, as expected from the better coverage of the TiO2 by the perovskite overlayer in this case. In order to confirm the presence of the chlorine within the CH3NH3PbI3-xClx samples, XPS measurements of the films were conducted. The widescan of CH3NH3PbI3 and CH3NH3PbI3-xClx in Figure S2. indicated no significant difference between the two samples. Due to the low concentration of chlorine, Cl 2p peak is not distinguishable in the widescan. In narrow scans, Cl 2p peaks can be found in CH3NH3PbI3-xClx. The peak area of Cl 2p was measured using peak fitting and normalized according to its sweeps and atomic sensitivity factor. All other elements in perovskite are also measured and normalized in the same way. The atomic concentration of

chlorine in CH3NH3PbI3-xClx film is found to be about 1 at% in line with a previous report by Yang Yang and coworkers.20 EDX measurements were unable to pick up Cl content indicating the low concentrations of the element in the film (Figure S2). This is much lesser than the starting concentration of 6.67 at% (calculated from PbCl2+3CH3NH3I) and the possible final composition (where the excess CH3NH3I is not present) of 16.67 at%. The low concentration of Cl noted in these samples indicates that the favorable morphology noted in CH3NH3PbI3-xClx films may originate from alternate reasons. When comparing the two precursor “recipes” used to fabricate CH3NH3PbI3 and CH3NH3PbI3-xClx, it is important to note that excess CH3NH3I is utilized in the latter case. Attempts to decrease the CH3NH3I content in the PbCl2-CH3NH3I system lead to insolubility of the PbCl2. Thus the effect of excess CH3NH3I on the CH3NH3PbI3 film deposition was studied in the PbI2 CH3NH3I system. Excess CH3NH3I was added to the initially equimolar PbI2 - CH3NH3I solution (i.e. 16 mol%, 33 mol%, 50 mol%, 100 mol%, and 200 mol% excess CH3NH3I). The films as spin-coated showed a reddish color. The higher the concentration of excess CH3NH3I,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the longer it took to transform into black films. For films with excess CH3NH3I ≥100 mol%, the transformation occurred only when it was annealed >130°C indicating the slower crystallization rate of CH3NH3PbI3 with excess CH3NH3I. The XRD spectra (Figure 2A) of all films show the formation of CH3NH3PbI3 after drying. Films with excess CH3NH3I ≥100 mol% had a preferred orientation towards (110) direction similar to that obtained for CH3NH3PbI3-xClx. To eliminate the possible effect of annealing in inducing the preferred orientation, CH3NH3PbI3 films were also annealed at 130°C. No preferred orientation was noted, while an additional PbI2 was observed due to loss of CH3NH3I.21 Hence, the slower crystallization rate of the films due to excess CH3NH3+ seems to be the underlying cause for the preferred growth in the (110) crystal direction. The excess CH3NH3I also plays a critical role in the morphology. The surface morphology of films with different excess CH3NH3I content are presented in Figure 2B. Samples with excess CH3NH3I shows smoother surface with interconnected island morphology. Thus the advantageous morphology and preferred alignment associated with CH3NH3PbI3-xClx, is a consequence of the excess CH3NH3I added to the precursor solution which slows down crystallization kinetics. Despite the improved morphologies for samples with excess CH3NH3I, the device efficiency of the samples was hampered by the lower short circuit currents (Figure 2C). Higher series resistance was observed with increasing amounts of excess CH3NH3I. The Voc of devices with excess CH3NH3I was higher due to better CH3NH3PbI3 coverage. The open circuit voltage increment however saturates when excess CH3NH3I reach 50 mol%. Above 50 mol% excess CH3NH3I, the Voc and FF of the devices decrease which may indicate higher recombination. This indicates that excess CH3NH3I acts deleteriously in the CH3NH3PbI3 system while it does not do the same when PbCl2 is utilized as the Pb source. In order to understand the difference between these two precursor systems, the composition evolution during the preparation of the samples was studied. The composition evolution of the CH3NH3PbI3-xClx samples was monitored through XRD analysis. Samples were transferred out of the glovebox in a sealed sample holder for performing XRD measurements of the constituents. The XRD patterns of the film after spin coating and drying were presented in Figure 3. As indicated previously, the as-spin coated CH3NH3PbI3-xClx films have a brownish appearance. The XRD pattern of the as spin-coated films can be indexed to contain CH3NH3PbI3/CH3NH3PbI3xClx, PbI2, CH3NH3PbCl3, and traces of CH3NH3Cl. Distinguishing between CH3NH3PbI3- xClx and CH3NH3PbI3 is not possible due to the identical XRD spectra. When the films are annealed at 100°C for approximately 10 minutes, it transforms from a brownish to a yellowish film. Analysis of the yellow film showed that it consists of CH3NH3Cl and CH3NH3PbCl3. An unassigned peak at 12.1°, which was also observed in 33 mol% and 50 mol% excess CH3NH3I films, may be attributed to solvent-perovskite complexes.15 Annealing of

Page 4 of 8

Figure 3. Evolution of film composition as gleaned through XRD measurements of CH3NH3PbI3-xClx film. The insets show the optical images of the film.

the samples transform the yellowish film to black. The XRD spectra of the black film shows the final composition of the CH3NH3PbI3/CH3NH3PbI3-xClx with a preferred growth direction of the (110) plane. Based on the intermediates noted, an estimation of the chemical transformations occurring during film formation can be performed. The as-spin coated CH3NH3PbI3-xClx film contained CH3NH3PbI3, PbI2, CH3NH3PbCl3, CH3NH3Cl. To maintain the mass balance before and after reaction CH3NH3I should be present in the film. The hump in the XRD spectra between 10° to 17° indicates the present of disordered materials which may be attributed to the CH3NH3I. Hence the reaction for the initial brown film formation is hypothesized as reaction 1. 3PbCl2 + 9CH3NH3I → CH3NH3PbI3 + 3CH3NH3Cl + CH3NH3PbCl3 + PbI2 + 4CH3NH3I (1) With annealing at 100ºC for 10 minutes, the CH3NH3PbI3 start to dissociate or react (brown film transforms to yellow), leaving behind the CH3NH3Cl, CH3NH3PbCl3, and PbI2. This points to CH3NH3Cl reacting with CH3NH3PbI3 to form CH3NH3PbCl3. Such intersubstitution of the halides has been seen previously in the iodide/bromide system where the excess halide precursor determines the final perovskite formed.22 The intersubstitution of Cl and I in the CH3NH3PbX3 compounds has also been demonstrated here (Figure S3) CH3NH3PbI3 + 3CH3NH3Cl ↔ 3CH3NH3I + CH3NH3PbCl3 (2) Due to the excess CH3NH3I in the system, further annealing at 100ºC would transform the film to black indi-


Page 5 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cating the presence of CH3NH3PbI3/CH3NH3PbI3-xClx (as also seen from XRD). 2CH3NH3PbCl3 + PbI2 + 7CH3NH3I → 3CH3NH3PbI3 + 6CH3NH3Cl (3) Thus the excess CH3NH3I added into the precursor solution transforms into CH3NH3Cl in the case that PbCl2 is present in the precursor system. However, although significant amount of Cl atoms was present in the initial precursors, Cl could be hardly detected in the film (XPS measurements), pointing towards the loss of CH3NH3Cl during the preparation process. Thermogravic analysis (TGA) of CH3NH3Cl and CH3NH3I powder confirmed that both of them undergo sublimation, (Figure S4) with sublimation temperatures of 193.8°C and 216.9°C for CH3NH3Cl and CH3NH3I respectively. The high sublimation temperatures measured, indicates that a high concentration of chlorine within the CH3NH3PbI3-xClx films should still be present. To investigate the composition of the final film, TGA of CH3NH3PbI3 and CH3NH3PbI3-xClx films after annealing at 100°C for 40 minutes was performed (Figure 4) . This pre-annealing is the exact parameters utilized in the fabrication of the respective solar cells. When CH3NH3PbI3 films were heated above 280oC, ~ 27 wt% loss was observed, attributable to the loss of CH3NH3I from the film (theoretical value ~25.6 wt%) in agreement with the previous report by Kanatzidis and coworkers.17 No mass loss was observed at 216.9oC (sublimation temperature of CH3NH3I) indicating absence of excess CH3NH3I in the films. Further weight reduction were observed at temperatures above 400°C, in agreement with the melting point of PbI2. The TGA of CH3NH3PbI3shows similarities to CH3NH3PbI3. Similar xClx decomposition with a mass loss after 280oC closely

Figure 4. TGA result for CH3NH3PbI3 and CH3NH3PbI3-xClx film after annealing at 100°C for 45 minutes together with CH3NH3PbI3 with 200 mol% excess CH3NH3I after 130°C annealing for 15 minutes.

corresponding to CH3NH3I loss is noted. However, a slight weight loss starting from about 195.2°C, which is not observed in the pure CH3NH3PbI3 sample is also noted. This temperature is consistent with the sublimation of CH3NH3Cl (193.8°C), further supporting the formation of CH3NH3Cl (reaction 3) in the film. The mass loss at 195.2°C (1.96 wt%) corresponds to only 1.3 at% of Cl content consistent with the low concentrations noted by XPS. Films were also prepared with excess CH3NH3I (200 mol% excess CH3NH3I) compositions as described previously and measured after 15 minutes of 130°C preannealing. The TGA of such compositions revealed a 6 wt% reduction close to the evaporation temperature of CH3NH3I (212°C). Increasing the temperature above 280°C resulted in a 37 wt% reduction significantly higher than the estimated theoretical value of CH3NH3I from CH3NH3PbI3 (~24 wt%). By assuming that only PbI2 exists at 390°C, the presence of at least 24 wt% which corresponds to 125 mol% of excess CH3NH3I (compared to the initial 200 mol%) can be expected to be present in the solar cells. This excess CH3NH3I reduces the device performance. The lower final concentrations of the excess CH3NH3I and CH3NH3Cl in the films despite the high initial concentrations in the precursor solution seems to indicate that there is a large loss either in the initial state of film drying or during the annealing step through sublimation. As CH3NH3Cl possess lower enthalpy of sublimation (~25%) than CH3NH3I, CH3NH3Cl is easier to sublime than CH3NH3I.23 The small traces of CH3NH3Cl does not seem to negatively affect the photovoltaic performance. The effect of excess CH3NH3Cl was put to test by adding excess CH3NH3Cl to the stoichiometric CH3NH3PbI3 precursor (PbI2+CH3NH3I+CH3NH3Cl). The as spun coated films showed no transformation to yellow upon annealing at 100°C. This can be explained by the equations describing the chemical transformation (Supporting information reactions S1-S2). The absence of yellow phase in this system supports the validity of reaction 2 where excess CH3NH3Cl is neccessary to react with CH3NH3PbI3 to form CH3NH3I and CH3NH3PbCl3. Similar to the excess CH3NH3I case, additional CH3NH3Cl slowed down the CH3NH3PbI3 crystallization rate and enhanced the growth in the (110) direction (supporting information). The slow crystallization also resulted in a planar morphology (Figure 5) similar to CH3NH3PbI3-xClx case. Devices that are made out of these films show the enhanced power conversion efficiencies as compared to CH3NH3PbI3 due to better film coverage while similar devices performances are noted with CH3NH3PbI3-xClx. Excess CH3NH3Cl does not hamper the device short circuit current. In conclusion, the effect of precursors (i.e. PbI2 and PbCl2) and the mechanism of CH3NH3PbI3 formation from the respective precursors were scrutinized. Slowing the crystallization rate by introducing excess CH3NH3I or CH3NH3Cl resulted in a smooth polycrystalline film with favorable crystal growth in the (110) direction. Excess



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 8

posited by spin coating the solution at 4000rpm for 30 second. The deposition solution was consist of 2,2’,7,7’tetrakis-(N,N-di-p-methoxyphenylamine) 9, 9’spirobifluorene (spiro-OMeTAD), 4-tert-butylpyridine (TBP), LiTFSI and FK102 dopant in chlorobenzene. To complete the device fabrication, 0.2 cm2 of Au film was deposited by thermal evaporator as a back contact.

Figure 5. Top morphology (A) of films deposited with excess CH3NH3Cl together with the representative J-V characteristic of the devices made from the films (B).

CH3NH3Cl could be introduced to the system by direct addition or reaction between PbCl2 and CH3NH3I. Though excess CH3NH3I produced favorable morphology, it harmed the device performance as the remaining secondary CH3NH3I phase might increase the series resistance of the device. On the other hand, much of the excess CH3NH3Cl easily leaves the film during annealing and the small trace of excess CH3NH3Cl did not significantly alter the device series resistance. Due to better film morphology with minimum effect on the series resistance of the device, CH3NH3PbI3 based photovoltaic devices made with excess CH3NH3Cl or with PbCl2 precursors showed better performance than PbI2 counterpart when deposited by a single step procedure.

EXPERIMENTAL SECTION Device fabrication: Laser etched Fluorine doped tin oxide (FTO, TEC 15) coated glasses were used as the substrates. The FTO were cleaned by sonication in decon soap, DI water, and ethanol for 10 minutes. TiO2 compact layer were formed by spray pyrolysis at 450°C. Approximately 350 nm thick mesoporous TiO2 layer were deposited by spin coating TiO2 paste (Dyesol-30NRD). Chemical treatment with 40 mM TiCl4 solution at 70oC was done followed by annealing at 500oC. CH3NH3PbI3 or CH3NH3PbI3-xClx was infiltrated inside the mesoporous film by spin coating the solutions at 2000rpm for 60 seconds. Various solutions with varied concentrations were used and reported in Table 1. The as-spin coated films were dried at 70oC for 10 minutes and annealed at 100oC for 45 minutes. The whole CH3NH3PbI3 or CH3NH3PbI3xClx processing was done inside the glovebox due to the high humidity of the environment that could harm the film stability. Hole transporting material (HTM) was de-

Characterizations: The current density-voltage (J-V) curves were measured by using Keithley model 2612A source meter with and without illumination (San-EI Electric, XEC-301S) under AM 1.5 and 100 mW.cm-2. The statistics of the solar cell performances were calculated from batches of 6 solar cells. The crystallographic information of the films were analyzed by Bruker AXS (D8 ADVANCE) X-Ray Diffractometer with Cu Kα radiation. The X-Ray diffraction patterns were documented from films deposited on top of FTO coated with planar TiO2 film substrates to mimic the solar cell deposition condition. The topographical and cross sectional images were recorded by Field Emission Scanning Electron Microscopy (FESEM, JEOL, JSM 7600F). The XPS measurements were done in a home-build analytical chamber with monochromatic Xray source. The photon energy was set to Al K alpha (hv=1486.7eV) and photoelectrons were detected using a hemispherical electron analyzer (Omicron EA125). Thermo gravimetric analysis (TGA) was performed on a TGA Q950 (TA Instruments) in an interval from 25°C to 500°C at a ramp rate of 10°C/min under Nitrogen with flow rates of 40ml/min. Approximately 10mg of sample was used in each experiment. Impedance spectroscopy measurements were carried out using AutoLab PGSTAT302N. The cells were illuminated with a white LED and different bias potentials were applied ranging from 0.05 V to opencircuit voltage. A voltage perturbation with 20mV of amplitude was applied at frequencies between 1 MHz and 0.1 Hz. The results were fitted with the software Z-View. Table 1. List of film names and the chemical compositions of the solutions utilized to fabricate the same Sample and solutions name

Descriptions

CH3NH3PbI3

0.88M PbI2 + 0.88M CH3NH3I

16 mol% excess CH3NH3I

0.88M PbI2 + 1.02M CH3NH3I


0.88M PbI2 + 1.17M CH3NH3I


0.88M PbI2 + 1.32M CH3NH3I


0.88M PbI2 + 1.76M CH3NH3I


0.88M PbI2 + 2.64M CH3NH3I

CH3NH3PbI3-xClx

0.88M PbCl2 + 2.64M CH3NH3I

PbI2+CH3NH3I+CH3NH3Cl

0.88M PbI2 + 0.88M CH3NH3I + 0.88M CH3NH3Cl

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]


Page 7 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACKNOWLEDGMENT

(13)

We acknowledge Dr. Tom Baikie for the valuable scientific discussion. Funding from National Research Foundation (NRF), Singapore, is acknowledged through CRP Award No.: NRF-CRP4-2008-03 and the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE programme.

Bi, D.; Moon, S.-J.; Häggman, L.; Boschloo, G.; Yang, L.; Johansson, E. M. J.; Nazeeruddin, M. K.; Grätzel, M.; Hagfeldt, A. RSC Adv. 2013, 3, 18762.

(14)

Mosconi, E.; Amat, A.; Nazeeruddin, K.; Gra, M.; Angelis, F. De J. Phys. Chem. C 2013, 117, 13902–13913.

(15)

Dualeh, A.; Tétreault, N.; Moehl, T.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Adv. Funct. Mater. 2014, 24, 3250–3258.

(16)

Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J. Adv. Funct. Mater. 2014, 24, 151–157.

(17)

Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Inorg. Chem. 2013, 52, 9019–38.

(18)

Gonzalez-Pedro, V.; Juarez-Perez, E. J.; Arsyad, W.-S.; Barea, E. M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J. Nano Lett. 2014, 14, 888–93.

(19)

Krishna, A.; Sabba, D.; Li, H.; Yin, J.; Boix, P. P.; Soci, C.; Mhaisalkar, S. G.; Grimsdale, A. C. Chem. Sci. 2014.

(20)

You, J.; Hong, Z.; Yang, Y. M.; Chen, Q.; Cai, M.; Song, T.; Chen, C. ACS Nano 2014, 8, 1674–1680.

(21) Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.-H.; Liu, Y.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2014, 136, 622–5.

Supasai, T.; Rujisamphan, N.; Ullrich, K.; Chemseddine, a.; Dittrich, T. Appl. Phys. Lett. 2013, 103, 183906.

(22)

Kulkarni, S. a.; Baikie, T.; Boix, P. P.; Yantara, N.; Mathews, N.; Mhaisalkar, S. J. Mater. Chem. A 2014, 2, 9221–9225.

Liu, M.; Johnston, M. B.; Snaith, H. J. Nature 2013, 501, 395– 8.

(23)

Dualeh, A.; Gao, P.; Seok, S. Il; Nazeeruddin, M. K.; Grätzel, M. Chem. Mater. 2014, 26, 6160–6164.

SUPPORTING INFORMATION The intersubstition of CH3NH3PbX3 (X=I,Cl), the reactions for CH3NH3Cl rich system, Figure S1-S6. This information is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES (1)

Research Cell Efficiency Records http://www.nrel.gov/ncpv/images/efficiency_chart.jpg.

(2)

Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Nature 2013, 499, 316–9.

(3)

(4)

(5)

Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science. 2012, 338, 643–7.

(6)

Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Science. 2013, 342, 344–7.

(7)

Edri, E.; Kirmayer, S.; Mukhopadhyay, S.; Gartsman, K.; Hodes, G.; Cahen, D. Nat. Commun. 2014, 5, 3461.

(8)

Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Science . 2013, 342, 341–4.

(9)

Ishiharat, T. J. Lumin. 1994, 61, 269.

(10)

Sun, S.; Salim, T.; Mathews, N.; Duchamp, M.; Boothroyd, C.; Xing, G.; Sum, T. C.; Lam, Y. M. Energy Environ. Sci. 2014, 7, 399.

(11)

Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Nat. Mater. 2014, 13, 476–480.

(12)

Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. Adv. Mater. 2014, 26, 1584–1589.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 8

Table of Content


8

Unravelling the Effects of Cl Addition in Single Step CH3NH3PbI3

Recommend Documents