CH3NH3PbI3 crystal orientation and photovoltaic performance of planar heterojunction perovskite solar cells

CH3NH3PbI3 crystal orientation and photovoltaic performance of planar heterojunction perovskite solar cells

Solar Energy Materials & Solar Cells 160 (2017) 77–84 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepag...

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Solar Energy Materials & Solar Cells 160 (2017) 77–84

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

CH3NH3PbI3 crystal orientation and photovoltaic performance of planar heterojunction perovskite solar cells Seunghwan Baea, Joon-Suh Parkb, Il Ki Hanb, Tae Joo Shinc, Won Ho Joa,

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a

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea Nanophotonics Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea UNIST Central Research Facilities & School of National Science, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea

b c

A R T I C L E I N F O

A BS T RAC T

Keywords: CH3NH3PbI3 Perovskite solar cells Crystal orientation Charge transfer

The effect of crystal orientation of CH3NH3PbI3 on photovoltaic properties has rarely been studied due to the lack of method to fabricate perovskite films with well-controlled crystal orientation in specific direction. Here, we have controlled the orientation of CH3NH3PbI3 crystal by changing organic precursors (CH3NH3I and CH3NH3Cl) and successfully fabricated highly oriented CH3NH3PbI3 crystals with two different orientations to the substrate. When we investigated the effect of crystal orientation on photovoltaic performance using the films with two different crystal orientations, we have realized that the crystal orientation of perovskite is directly related to power conversion efficiency (PCE). The PCE difference due to different crystal orientation is interpreted by the charge transfer lifetime. Faster charge transfer from perovskite layer to charge transport layer exhibits higher JSC and PCE.

1. Introduction The most important issue of photovoltaics for practical application is to fabricate the cost-effective solar cells. Although crystalline silicon, cadmium telluride and other inorganics have been developed for high efficiency light absorbers of solar cells, the fabrication cost is still expensive due to vacuum deposition and/or high temperature process. To reduce the fabrication cost, solution processes such as spin-coating, slot-die coating, blade-coating and spray coating have widely been used. However, these processes may not be easily applied to fabrication of inorganic-based solar cells [1,2]. Recently, organometal halide perovskites have intensively been studied as a promising light absorber of solar cells because those have superior properties such as high absorption coefficient (~104 cm−1) [3,4], low exciton binding energy ( < 100 meV) [5–9], long charge carrier diffusion length (102–105 nm) [10–13] and high charge carrier mobility (10–102 cm2 V−1 s−1) [14– 17]. Furthermore, organometal halide perovskites are suitable for solution and low temperature processing because the precursor materials have good solubility in several organic solvents (dimethyl formamide, dimethyl sulfoxide, N-methylpyrrolidone, γ-butyrolactone) [18–21]. Particularly, the power conversion efficiency (PCE) of perovskite solar cells has increased dramatically from the first report of 3.8% [22] in 2009 to the current record efficiency of 22.1% [23].



The PCE of perovskite solar cells can be enhanced by minimizing charge recombination in bulk perovskite film and/or at the interfaces between perovskite and charge transport layers (CTLs). The reduction of charge recombination in bulk film requires high quality perovskite film with full surface coverage [24–26], large crystal size [21,27,28] and low defect density [29–31], while the reduction of charge recombination at the interfaces between perovskite and CTLs needs to develop new hole and electron transport materials which can facilitate the charge transport from perovskite layer to the corresponding electrode [32–36]. According to a recent simulation study [37], hole and electron transfers at the interfaces in perovskite solar cells are significantly affected by the type of perovskite crystal plane contacting with CTLs. Therefore, the charge recombination at the interface can also be reduced when the perovskite crystal plane is properly oriented to the CTL surface so as to exhibit good charge transfer at the interface. Herein, we report the effect of crystal orientation on the photovoltaic performance of perovskite solar cells. The CH3NH3PbI3 films with two different crystal orientations ((002)/(110) and (112)/(200) planes parallel to the substrate, respectively) are fabricated by the use of two different organic precursors (CH3NH3Cl and CH3NH3I) [38– 43]. Since the planar heterojunction device has simple interfaces between perovskite and CTLs, the crystal plane of CH3NH3PbI3 contacting with CTLs is easily specified, while the CH3NH3PbI3 crystal

Corresponding author. E-mail address: [email protected] (W.H. Jo).

http://dx.doi.org/10.1016/j.solmat.2016.10.019 Received 3 June 2016; Received in revised form 23 September 2016; Accepted 7 October 2016 0927-0248/ © 2016 Elsevier B.V. All rights reserved.

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vacuum ( < 10−6 Torr) through a shadow mask to give a device area of 0.1 cm2. For fabrication of normal cells, TiO2 solution was spin-coated at 3000 rpm for 40 s onto the ITO and the substrate was then annealed at 150 °C for 30 min in ambient condition. After annealing, the substrate was immediately transferred to a N2-filled glove box. CH3NH3PbI3 and spiro-OMeTAD layer were deposited in a N2-filled glove box. The fabrication procedures of CH3NH3PbI3 films were the same as described above in fabrication of inverted cells. SpiroOMeTAD solution (EM index, 99.9%, 72.3 mg/mL in chlorobenzene) with 17.5 µl of Li-bis(trifluoromethanesulfonyl)imide (520 mg/mL in acetonitrile) and 28.8 µl 4-tert-butylpyridine was spin-coated on the perovskite layer at 3000 rpm for 30 s. Finally, Au electrodes was deposited by thermal evaporation under vacuum ( < 10−6 Torr) through a shadow mask to give a device area of 0.1 cm2.

plane contacting with mesoporous TiO2 in mesoscopic heterojunction device is difficult to be specified. Hence, we fabricate planar heterojunction devices to clearly observe the effect of crystal orientation on photovoltaic performance of CH3NH3PbI3-based solar cells. Planar heterojunction solar cells can be fabricated with two different device structures, viz. normal structure and inverted structure. When CH3NH3PbI3 films with different crystal orientations were introduced into planar heterojunction devices with normal and inverted structures, respectively, we have found that the crystal orientation to the substrate affects strongly the photovoltaic performance of perovskite solar cells due to difference in charge transfer between CH3NH3PbI3 and CTL. 2. Material and methods 2.1. Preparation of materials

2.3. Characterization Methylammonium iodide (CH3NH3I) was synthesized by reacting equimolar methylamine (40% in methanol, TCI) with hydroiodic acid (HI, Alfa Aesar, 57% w/w aqueous solution stabilized with 1.5% hypophosphorous acid) at 0 °C. HI was added dropwise to methylamine solution under constant stirring. After a reaction time of 2 h, solvent was evaporated using a rotary evaporator and the product was recrystallized from ethanol and diethyl ether. After filtration, the precipitate was washed with diethyl ether and dried at 60 °C in vacuum oven for 24 h. To prepare the methylammonium chloride (CH3NH3Cl), HI was replaced by hydrochloric acid (HCl, Samchun Chemical, 35– 37%). The purification procedure was the same as described above. Lead iodide (PbI2, Acros Organics, 99%) was dried in vacuum oven at 240 °C for 12 h. To prepare the perovskite precursor solution, 1.18 mmol of organic precursor (CH3NH3I or CH3NH3Cl) with equimolar PbI2 were dissolved in 1 mL of dimethyl sulfoxide. The solution was stirred at 60 °C for 3 h and cooled to room temperature before use. TiO2 nanoparticles were synthesized as reported in literatures [44,45]. 0.5 mL of TiCl4 (Sigma Aldrich, 99.9%) was added slowly to 2 mL of ethanol under stirring, and then the whole content was mixed with 10 mL of anhydrous benzyl alcohol (Acros Organics, 98+%). After the solution was heated at 80 °C for 6 h, translucent suspension was obtained. 3 mL of suspension was precipitated in 27 mL of diethyl ether. To isolate TiO2 nanoparticles, the precipitate was centrifuged at 5000 rpm and then re-dispersed in ethanol (6 mg/mL). 14.7 µl of titanium diisopropoxide bis(acetylacetonate) (Aldrich, 75 wt% in isopropanol) was added to 1 mL of re-dispersed TiO2 nanoparticle solution. The final solution was diluted 2 times for normal cells and 4 times for inverted cells with ethanol.

XRD patterns were obtained by X-ray diffractometer (D8 Advance, Bruker) using Cu-Kα radiation (λ=1.5418 Å) at a scan rate of 2° min−1 and morphology of perovskite films were characterized using fieldemission scanning electron microscopy (FESEM, SU70, Hitachi). Information of tetragonal CH3NH3PbI3 crystal structure was obtained in literature [15] and VESTA was used to visualize the crystal structure. Grazing incidence X-ray diffraction (GIXD) measurements were performed at PLS-II 9 A U-SAXS beam line of Pohang Accelerator Laboratory in Korea. The X-rays coming from the in-vacuum undulator (IVU) are monochromated (wavelength, λ=1.12 Å) using a Si(111) double-crystal monochromator and focused horizontally and vertically (300 (H)×30 (V) µm2 in FWHM at sample position) using K-B type mirror system. GIXD sample stage is equipped with a 7-axis motorized stage for the fine alignment of sample and the incidence angle of X-ray beam was set to 0.08°. GIXD pattern were recorded with a 2D CCD detector (Rayonix SX165) and X-ray irradiation time was 1.5 s. Diffraction angles were calibrated by a pre-calibrated sucrose (Monoclinic, P21, a=10.8631 Å, b=8.7044 Å, c=7.7642 Å, β=102.938°) [46]. and the sample-to-detector distance was about 226.6 mm. J–V curves of photovoltaic cells were obtained using a computer-controlled Keithley 4200 source measurement unit under AM 1.5G (100 mW cm−2) simulated by an Oriel solar simulator (Oriel 91160A). The light intensity was calibrated using a NREL-certified photodiode prior to each measurement. The EQE was measured using Polaronix K3100 measurement system (McScience). The light intensity at each wavelength was calibrated with a standard single-crystal Si cell. PL lifetimes were measured at 400 nm, second harmonic generated Ti:Sapphire femto-second laser (MaiTai, Spectra Physics) as excitation source and time-correlated single photon counting module (MPD-PDM Series DET-40 photon counting detector and Pendulum CNT-91 frequency counter) combined with monochromator as detector. The parameters of PL decay were obtained by fitting the spectra with a biexponential decay function, I(t)=A1exp (−t/τ1)+A2exp(−t/τ2). Average lifetime, τave were calculated by equation, τave=A1τ2+A2τ2 [53].

2.2. Device fabrication The ITO coated glass (10 Ω sq−1, KETI) was cleaned by stepwise sonication in acetone and isopropanol for 30 min each. After complete drying at 120 °C, the ITO coated glass was treated with UV-ozone for 15 min. For fabrication of inverted cells, PEDOT:PSS (Heraeus Clevios P VP AI 4083) was spin-coated at 3000 rpm for 40 s onto the ITO and the substrate was annealed at 150 °C for 30 min in ambient condition. After annealing, the substrate was immediately transferred to a N2filled glovebox and CH3NH3PbI3 and PCBM layers were successively deposited in a N2-filled glovebox. DMSO precursor solution was spincoated at 3000 rpm for 15 s and the substrate was immediately transferred on a hot plate at 110 °C for 30 s. For fabrication of perovskite film from CH3NH3Cl and PbI2, CH3NH3I solution (5 mg/ mL in isopropanol) was additionally spin-coated on CH3NH3PbI3 film at 3000 rpm for 30 s and the substrate was annealed at 110 °C for 120 s. CH3NH3I was treated twice to remove residual CH3NH3PbCl3 in CH3NH3PbI3. PC61BM solution (Nano-C, 99.5%, 10 mg/mL in chloroform) was spin-coated onto the perovskite layer at 3000 rpm for 30 s and TiO2 solution was spin-coated on the PCBM layer at 3000 rpm for 30 s. Finally, Al electrodes was deposited by thermal evaporation under

3. Results and discussions Several methods to fabricate perovskite film with preferentially oriented perovskite crystals have been reported using Cl-containing precursors [38–42]. When CH3NH3Cl or lead chloride as precursor was used to fabricate perovskite film, the (110) plane of perovskite crystal is preferentially oriented parallel to the substrate. For fabrication of perovskite crystals oriented to other directions, Liang et al. [47] used the selective solvent annealing (SSA) method and reported partial change of crystal orientation from (110) to (200) after SSA treatment. However, precise investigation of crystal orientation effect on photovoltaic performance needs to fabricate perovskite films with highly oriented crystals. We have recently developed a new method to fabricate high quality CH3NH3PbI3 film by controlling the type of 78

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Fig. 1. XRD patterns of films fabricated from (a) two different organic precursors (CH3NH3I and CH3NH3Cl) and (b) the change of XRD patterns of I2Cl films according to CH3NH3I treatment. CH3NH3I is treated on I2Cl film several times to minimize the residual amount of CH3NH3PbCl3 in CH3NH3PbI3 crystal. XRD patterns are measured by θ–2θ mode.

the crystal orientation is controlled almost over the whole sample area. Ball-and-stick representations of CH3NH3PbI3 crystals oriented along (002), (110), (112) and (200) directions with respect to the substrate are shown in Fig. 2e. According to the previous report [48], the (110) and (002) planes of CH3NH3PbI3 crystal form nonpolar surface which consists of alternative stacking of neutral [CH3NH3I]° and [PbI2]° planes, while the (200) and (112) planes are polar surface consisting of alternative stacking of [CH3NH3PbI]2+ and [I2]2− planes. Morphologies of I3 and I2Cl films as observed by scanning electron microscopy (SEM) reveal that the morphologies of I3 and I2Cl films are similar and additional CH3NH3I treatment does not affect significantly the film morphology (Fig. 3), indicating that the film morphology may not be related to photovoltaic performance. To identify the effect of preferred orientation of CH3NH3PbI3 crystals on photovoltaic performance, two types of planar heterojunction solar cells (inverted and normal cells) are fabricated with I3 and I2Cl films. Normal cell has the device configuration of ITO/TiO2/ CH3NH3PbI3/spiro-OMeTAD/Au, and inverted cell has the device configuration of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/TiO2/Al. J–V curves of solar cells are shown in Fig. 4, and photovoltaic parameters are listed in Table 1. Two types of device have standard deviation less than 1, indicating that the devices are quite reproducible. The efficiency of inverted cell made of I3 film is higher than that of the inverted cell fabricated from I2Cl film mainly due to higher short circuit current (JSC) and higher fill factor (FF) of I3-based cell, whereas the efficiency of normal cell prepared with I2Cl film is much higher than that of the inverted cell made of I3 film. The JSC difference between I3 and I2Cl films can be analysed by comparing the optical absorption with external quantum efficiency (EQE). I3 and I2Cl films show similar UV–vis spectra, but, I3 film absorbs light slightly more than I2Cl film in short wavelength region ( < 550 nm). It should be noted here that in contrast to the optical absorption, the inverted cell fabricated from I3 film exhibits higher JSC (the value from EQE spectrum (Fig. S6)) than I2Cl film, which arises from higher EQE of I3 film in the range of 650– 800 nm, while the normal cell fabricated from I2Cl film exhibits higher EQE over the whole absorption range than that fabricated from I3 film. Therefore, an increase of JSC in the same type of device may not come from higher photon absorption of perovskite film, but it must attribute to other factors such as the crystal orientation of I3 and I2Cl films. Considering that the crystals in I3 and I2Cl films exhibit different orientation to the substrate, we assume that the orientation of CH3NH3PbI3 crystal in film affects largely the charge transfer between CH3NH3PbI3 and CTL. The charge transfer between CH3NH3PbI3 film and CTL is evaluated by measuring the photoluminescence (PL) lifetime: Shorter PL lifetime indicates faster charge transfer between CH3NH3PbI3 film and

solvent, spin-coating time and annealing temperature [43]. Furthermore, when we fabricated CH3NH3PbI3 films from CH3NH3I and PbI2 as precursor using DMSO as a processing solvent, we have found that (112)/(200) planes in the crystals are highly oriented parallel to the substrate. In this study, we fabricate perovskite films with another crystal orientation, i.e., (002)/(110) planes being highly oriented parallel to the substrate, by changing the organic precursor from CH3NH3I to CH3NH3Cl. In other word, when we used CH3NH3Cl instead of CH3NH3I as organic precursor, we have found that (002)/ (110) planes of perovskite crystal in the film are highly oriented parallel to the substrate. X-ray diffraction (XRD) pattern of CH3NH3PbI3 film fabricated from CH3NH3I and PbI2 (hereafter denoted as I3 film) shows only strong (112) and (200) peaks, while the XRD pattern of CH3NH3PbI3 film fabricated from CH3NH3Cl and PbI2 (hereafter denoted as I2Cl film) shows strong (002) and (110) peaks with an additionally weak peak at 15.6° corresponding to (100) reflection of CH3NH3PbCl3 crystal, indicating that two films have different crystal orientations (Fig. 1a) and the crystal orientation of the films is independent of substrates (Fig. S1). Since it has been known that CH3NH3PbI3 (I2Cl film) contains a small amount of CH3NH3PbCl3 crystal after the film is prepared from Cl precursors, we treated several times CH3NH3I on the I2Cl film to remove the residual CH3NH3PbCl3 crystal. After the I2Cl film is treated two or three times by CH3NH3I, the peak intensity of CH3NH3PbCl3 crystal mostly disappeared (Fig. 1b). Mole percent of residual CH3NH3PbCl3 in I2Cl film after CH3NH3I treatment are precisely calculated by Rietveld analysis from the powder XRD patterns (Fig. S1 and Table S1). The orientation of CH3NH3PbI3 perovskite crystals within I3 and I2Cl films is characterized by 2D grazing-incidence X-ray diffraction (2D GIXD). Since 2D GIXD patterns of I3 and I2Cl films show spot-like reflections with strong intensity, as shown in Fig. 2a and b, the CH3NH3PbI3 crystals are highly oriented. The azimuthal angle (χ) scan is a powerful method to assess the crystal orientation. Considering that the peak at χ=90° indicates preferential orientation of the corresponding crystal plane parallel to the substrate, we realize that the CH3NH3PbI3 crystals in I3 film are oriented such (112) and (200) planes that are parallel to the substrate while those in I2Cl film are oriented such (002) and (110) planes that are parallel to the substrate, because the χ scans of (112) and (200) reflections of I3 film show peaks at 90° while the χ scans of (002) and (110) reflections of I2Cl film reveal the peaks at 90°. All peaks are assigned in Fig. S3. To precisely demonstrate whether the crystal orientation is controlled over the whole sample area, I3 and I2Cl films are divided into four pieces and XRD pattern of each piece is collected (Fig. S4). Four pieces of I3 film and I2Cl film exhibit identical diffraction patterns, demonstrating that

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Fig. 2. GIXD patterns of (a) I3 and (b) I2Cl film and (c, d) azimuthal angle (χ) scans. CH3NH3PbI3 crystals in I3 film are oriented such (112) and (200) planes that are parallel to the substrate while those in I2Cl film are oriented such (002) and (110) planes that are parallel to the substrate. (e) Ball-and-stick representation of CH3NH3PbI3 crystal in which (002), (110), (112) and (200) planes are parallel to the substrate. The C, N, I and Pb atoms are represented by brown, blue, purple and black spheres, respectively, and dark grey octahedron represent PbI6 unit. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

CTL. PL decays, as shown in Fig. 5, were fitted with bi-exponential decay curves to determine the lifetimes of carriers [49,50], and the fitting parameters are listed in Table S2. From the relation 1/(τave,HJ) =1/(τave,Perov)+1/(τCT), we can estimate the charge carrier transfer time (τCT) by measuring average lifetimes of pristine CH3NH3PbI3 film (τave,Perov) and CH3NH3PbI3 film contacting with CTL (τave,HJ) [10]. When CH3NH3PbI3 contacts with CTLs (PEDOT:PSS, spiroOMeTAD, PCBM and TiO2), the PL lifetimes become shorter than the PL lifetime of pristine CH3NH3PbI3, indicating that charges are effectively transferred from CH3NH3PbI3 to CTL. For the purpose to examine the effect of crystal orientation of perovskite on the charge

transfer, we compare the charge transfer time from I3 to CTL with that from I2Cl to CTL. Note here that the crystals in I3 are oriented such that (112) and (200) planes are parallel to the substrate while the crystals in I2Cl are oriented along (002) and (110) directions. Comparison of average lifetimes reveals that hole and electron are more easily transferred from I3 to PEDOT:PSS and to PCBM than from I2Cl to PEDOT:PSS and to PCBM, respectively, indicating that (112) and (200) planes contacting with the CTL have more benefit for charger transfer than (002) and (110) planes. On the other hand, the charge carriers are more easily transferred from I2Cl to spiro-OMeTAD and to TiO2 than from I3 to spiro-OMeTAD and to TiO2, respectively, indicating that

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Fig. 3. SEM images of films fabricated from different organic precursors: (a) I3 film, (b) I2Cl, and I2Cl films after CH3NH3I is treated (c) one and (d) two times. Top-view and crosssection images with high magnification are placed on right side of top-view image with low magnification.

Another feature to note here is that the charge transfer from CH3NH3PbI3 to spiro-OMeTAD or TiO2 is slow as to compared the charge transfer to PEDOT:PSS or to PCBM. Especially, the charge transfer time to TiO2 (τCT=203.8 ns) is much longer than other cases and thus the charge transfer efficiency (16%) is the lowest. Since it has been known that photo-induced charges accumulate at the interface and thus cause J–V hysteresis when hole and electron extractions from perovskite layer are delayed and imbalanced [51,52], it is not strange to observe strong hysteresis in J–V curves of normal cells due to slow and imbalanced charge transfer to spiro-OMeTAD and TiO2 whereas the

(002) and (110) planes contacting with the CTL are more favorable for the charge transfer. Since perovskite layer contacts with PEDOT:PSS and PCBM in inverted cell, we may expect that I3-based inverted solar cell exhibits higher efficiency than I2Cl-based inverted cell. On the other hand, the perovskite layer contacts with spiro-OMeTAD and TiO2 in normal solar cell, and therefore I2Cl-based normal cell is expected to exhibit higher efficiency than I3-based normal solar cell. In short, the orientation of CH3NH3PbI3 crystals on the CTL is directly related to the photovoltaic performance of perovskite solar cells through the anisotropy of charge transfer in perovskite crystal.

Fig. 4. Device structures and J–V curves of (a) inverted cells and (b) normal cells. The devices are fabricated from I3 and I2Cl films. J–V curves are measured under AM 1.5G condition.

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charge transfers from perovskite layer to PEDOT:PSS and PCBM (Fig. 6). Sanchez et al. [53] have proposed the hysteric index (HI) as defined by the following equation to quantify the hysteresis effect in J– V curve:

Table 1 Photovoltaic properties of CH3NH3PbI3 solar cells made of different organic precursor and different device structure. Device structure

Organic precursor

VOC [V]

JSC [mA cm−2]

FF [%]

PCEa [%]

σb

Invertedc

CH3NH3I

0.86

21.34

74.1

0.53

Invertedc

CH3NH3Cl

0.86

18.54

70.6

CH3NH3I

0.92

12.43

34.7

CH3NH3Cl

1.02

18.28

69.1

13.60 (12.85) 11.26 (10.72) 3.97 (3.18) 12.88 (11.36)

Normal

d

Normald

a b c d

HI =

0.45

JSC,rev (VOC /2) − JSC,fwd (VOC /2) JSC,rev (VOC /2)

where JSC,rev(VOC/2) and JSC,fwd(VOC/2) are the photocurrent at VOC/2 bias for reverse scan and forward scan, respectively. A value of HI=0 represents a device without hysteresis, while the value of 1 represents a device showing hysteresis as high as the photocurrent at VOC/2. The HI values obtained from J–V curves of inverted cells are very small while the values obtained from J–V curves of normal cells are large. Since the HI value is inversely proportional to the charge transfer time and the ratio of hole (electron) to electron (hole) transfer time, our result supports previous reports that J–V hysteresis originates from slow and imbalanced charge transfer [51,52].

0.98 0.47

Average PCE values based on 12 devices are given in parentheses. Standard deviation. ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/TiO2/Al. ITO/TiO2/CH3NH3PbI3/spiro-OMeTAD/Au.

inverted cells do not show hysteresis because of fast and balanced

Fig. 5. (a) Energy levels of materials used for solar cells; transient photoluminescence spectra: (b) pristine CH3NH3PbI3 film, (c) CH3NH3PbI3/PEDOT:PSS, (d) CH3NH3PbI3/PCBM, (e) CH3NH3PbI3/spiro-OMeTAD and (f) CH3NH3PbI3/TiO2. All samples for PL are prepared on quartz glass. (CTE: Charge transfer efficiency). Bi-exponential function is used for fitting PL decays.

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Fig. 6. J–V hysteresis of (a, b) inverted and (c, d) normal cells. The devices are fabricated from (a, c) I3 and (b, d) I2Cl films. J–V curves are measured under AM 1.5G condition.

4. Conclusions

Acknowledgements

We successfully fabricated CH3NH3PbI3 films with two different crystal orientations with respect to the substrate CTL by using two different organic precursors (CH3NH3I and CH3NH3Cl): Crystals in I3 film are oriented such (112) and (200) planes are parallel to the substrate while those in I2Cl film are oriented along (002) and (110) directions with respect to the substrate. We have clearly demonstrated that the crystal orientation of perovskite is closely related to the photovoltaic performance of planar heterojunction solar cells, where the orientation effect on photovoltaic performance becomes different depending upon the type of substrate: The efficiency of inverted cell made of I3 film is higher than that of the cell fabricated from I2Cl film, while the efficiency of normal cell made of I2Cl film is higher than that of the device prepared from I3 film. The efficiency difference arises mainly from orientation difference of CH3NH3PbI3 crystals through anisotropy of charge transfer in CH3NH3PbI3 crystal. Strong hysteresis as observed in J–V curves of normal cells are interpreted in terms of slow and imbalanced charge transfers from perovskite to CTLs. In short, we have found that the crystal orientation of CH3NH3PbI3 is directly related to the photovoltaic performance due to anisotropy of charge transfer in perovskite crystal, suggesting that control of crystal orientation is one of important methods to improve the photovoltaic performance of perovskite solar cells with planar heterojunction structure.

The authors thank the Ministry of Education,South Korea for financial support through Global Research Laboratory (GRL). Experiment at the PLS-II 9A U-SAXS beamline was supported in part by MIST and POSTECH. The authors also thank Dr. Hyungju Ahn for GIXD measurement. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2016.10.019. References [1] D.A.R. Barkhouse, O. Gunawan, T. Gokmen, T.K. Todorov, D.B. Mitzi, Device characteristics of a 10.1% hydrazine-processed Cu2ZnSn(Se,S)4 solar cell, Prog. Photovoltaics: Res. Appl. 20 (2012) 6–11. [2] T.K. Todorov, O. Gunawan, T. Gokmen, D.B. Mitzi, Solution-processed Cu(In,Ga) (S,Se)2 absorber yielding a 15.2% efficient solar cell, Prog. Photovoltaics: Res. Appl. 21 (2013) 82–87. [3] N.-G. Park, Perovskite solar cells: an emerging photovoltaic technology, Mater. Today 18 (2015) 65–72. [4] X. Ziang, L. Shifeng, Q. Laixiang, P. Shuping, W. Wei, Y. Yu, Y. Li, C. Zhijian, W. Shufeng, D. Honglin, Y. Minghui, G.G. Qin, Refractive index and extinction coefficient of CH3NH3PbI3 studied by spectroscopic ellipsometry, Opt. Mater. Express 5 (2015) 29–43. [5] T. Ishihara, Optical properties of PbI-based perovskite structures, J. Lumin. 60–61 (1994) 269–274. [6] W. Zhang, M. Saliba, S.D. Stranks, Y. Sun, X. Shi, U. Wiesner, H.J. Snaith, Enhancement of perovskite-based solar cells employing core–shell metal nanoparticles, Nano Lett. 13 (2013) 4505–4510. [7] Q. Lin, A. Armin, R.C.R. Nagiri, P.L. Burn, P. Meredith, Electro-optics of perovskite solar cells, Nat. Photonics 9 (2015) 106–112. [8] A. Miyata, A. Mitioglu, P. Plochocka, O. Portugall, J.T.-W. Wang, S.D. Stranks,

Notes The authors declare no competing financial interest. 83

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