Efficient and Uniform Planar-Type Perovskite Solar Cells by Simple Sequential Vacuum Deposition

Transcription

1 Efficient and Uniform Planar-Type Perovskite Solar Cells by Simple Sequential Vacuum Deposition Chang-Wen Chen, Hao-Wei Kang, Sheng-Yi Hsiao, Po-Fan Yang, Kai-Ming Chiang, and Hao-Wu Lin * Organometal halide perovskites (CH NH PbI x Cl x and CH NH PbI ) have recently attracted tremendous attention as promising materials for solar energy conversion. The pioneer devices demonstrated a power conversion efficiency (PCE) of %, as reported by Miyasaka and co-workers in 9. [ ] The PCEs soon evolved to exceed % in a few years both in mesosuperstructure-type and planar-type cells. [ ] The planar-type device architecture is particularly interesting due to the simple cell configuration and possible low-temperature fabrication on flexible substrates. However, unlike meso-superstructure-type devices, in which perovskite can be scaffolded by mesoporous matrices, incomplete and non-uniform coverage of perovskite films was usually observed in planar-type perovskite solar cells and has been regarded as the major factor resulting in decreased device performance. [ 5 ] Many efforts have been made to control the morphology of perovskite thin films including optimization of the annealing time and temperature, [,6 ] selection of the under-layer material and thickness, [,,7 9 ] and the use of alternative deposition methods such as two-step deposition and vacuum sublimation. [9,,8, ] Among these methods, the vacuum thermal co-evaporation of CH NH I and PbCl (or PbI ) and the resulting perovskite thin films exhibited the most homogeneous morphology and the highest thin-film coverage, leading to a high performance of 5% PCE. [6,8 ] Despite the promising results, however, to date, only limited reports have utilized this vacuum sublimation technique to fabricate perovskite layers. [6,8 ] The main reason could be due to the small molecular weight of CH NH I, which results in a random diffusion of molecules inside the vacuum chamber and causes difficulty with the monitoring and control of the CH NH I deposition rate using quartz microbalance sensors. [6,8 ] In this paper, we demonstrate a novel method of perovskite thin-film deposition via a layer-by-layer sequential vacuum sublimation. The process is relatively simpler than the co-evaporation technique; however, surprisingly, very uniform perovskite thin films with high coverage can be produced. By incorporating these perovskite thin films with a poly(,-ethylenediox ythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole transporting layer and thermal evaporated C 6 /Bathophenanthroline (Bphen) electron transporting layers (ETLs), the cells with C.-W. Chang, H.-W. Kang, S.-Y. Hsiao, P.-F. Yang, K.-M. Chiang, Prof. H.-W. Lin Department of Materials Science and Engineering National Tsing Hua University Hsinchu, Taiwan hwlin@mx.nthu.edu.tw DOI:./adma.6 a simple device structure attain efficiencies as high as 5.% under simulated -Sun illumination. Because the devices are free of high-temperature-prepared metal oxide layers and the substrates are maintained under C throughout the fabrication, the deposition method is suitable for a wide variety of rigid and flexible applications. The procedure of layer-by-layer sequential vacuum deposition is illustrated in Scheme. A simple one-material-at-a-time deposition was used. First, PbCl films were thermally sublimed onto PEDOT:PSS-coated indium tin oxide (ITO) glass. The surface morphology of the vacuum-deposited PbCl was investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The PbCl films were extremely smooth with a root mean square roughness ( R rms ) of 7.8 nm. As illustrated in Figure a and Figure S in the Supporting Information, a nanocrystalline structure with to nm domain sizes and full surface coverage can be observed. CH NH I was then sublimed onto the PbCl layers. Upon CH NH I deposition, PbCl reacted with CH NH I in situ and formed perovskite thin films (CH NH PbI x Cl x ). The color of the thin film gradually changed from transparent to a dark reddish-brown appearance, as shown in Figure b. The thickness of the perovskite thin film was observed to be proportional to the thickness of the PbCl layer with a ratio of ca..9:. A post-annealing process at C in vacuum was applied for a short period (several minutes) to fully crystallize the perovskite film and remove the residual CH NH I on the surface. The prepared perovskite layers were further capped with layer-by-layer vacuum deposition of C 6 /Bphen ETLs and a Ca/Ag cathode to complete the devices. [ ] Note that we not only utilized a simple device structure but also low-cost pristine organic compounds (C 6, Bphen) to demonstrate their commercial potential. All the layers (PbCl, C6, Bphen, Ca, and Ag) were fabricated with constant deposition rates, which were monitored by quartz microbalance sensors, except for CH NH I layer. The small-molecular-weight CH NH I makes it difficult to accumulate the material onto the sensor head, and random pulsation of the deposition rate was observed. This observation partially explains the difficulty of controlling codeposition of PbCl (PbI ) and CH NH I to form a perovskite film. Consequently, the CH NH I layers were deposited with a constant source temperature of 85 C. Figure S in the Supporting Information shows the current voltage ( J V ) characteristics of perovskite solar cells of different PbCl thicknesses. The samples were maintained at room temperature during the entire vacuum sublimation process. It is clear that only the devices with PbCl of less than 5 nm exhibit reasonable photovoltaic behavior. This result indicates that the reaction interface area of PbCl and CH NH I is limited by the smooth surface of Adv. Mater., 6, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 667

2 Scheme. Schematic illustration of perovskite solar cells fabricated by sequential layer-by-layer vacuum deposition. the vacuum-sublimed PbCl layer, leading to a reaction penetration depth equal to or less than 5 nm. Intriguingly, we observed that the reaction penetration depth can be extended by simply increasing the substrate temperature while evaporating the CH NH I layer. Figure and Figure S in the Supporting Information show the morphology of perovskite thin films prepared from 5 nm PbCl seed layers at higher substrate temperatures (65 85 C). Similar to the vacuum-deposited PbCl, full surface coverage can be observed in all the perovskite films prepared at different substrate temperatures. All the films have smooth surfaces with R rms values of. nm,.7 nm and. nm for substrate temperatures of 65 C, 75 C and 85 C, respectively. The surface roughness is much lower than that of the previously reported solution processed films. [ ] The smooth perovskite thin films can be attributed to the ultra-smooth starting PbCl layer. Notably, even with the same low surface roughness and full surface coverage characteristics in all the perovskite thin films prepared at various substrate temperatures, we observe divergent crystalline morphologies in these films. The sample with the substrate temperature of 65 C exhibits a large domain size of ca. µm, which is times larger than the crystal domain size of the seed PbCl layer. Two distinct layer structures are also observed in the cross-sectional SEM image. Only the upper part of the PbCl thin film reacted with CH NH I to form the perovskite film, and the lower part of ca. 5 nm PbCl did not fully convert to the highly crystalline perovskite. More intriguing is that when the perovskite thin films were prepared at the 75 C substrate temperature, the crystalline structure fused together, and fuzzy domain boundaries were observed. The tilt-angle SEM image clearly illustrates the long-range fusion of the perovskite crystal domains all over the film (Figure g) and Figure S in the Supporting Information). When the perovskite films were fabricated at a substrate temperature of 85 C, vivid crystal domains reappeared but with much smaller domain sizes of ca. nm. The films were further characterized by X-ray diffraction (XRD) ( Figure a). For the sample with a substrate temperature of 65 C, CH NH PbI x Cl x signals were observed, while the signal intensities were weak. In addition, the pattern contains a small diffraction peak at approximately 5.7, which indicates the presence of CH NH PbCl (the CH NH PbCl () diffraction peak). [ 8 ] However, for the substrate temperature of 75 C, a highly oriented pure orthorhombic CH NH PbI x Cl x crystal was obtained. Four clear diffraction signals are identified at.5, 8.7,.5 and 59.5, which can be assigned to the,,, and peaks, respectively. [ 8 ] Interesting, if the films were sublimed at a higher temperature (85 C), regardless of the highly crystallized appearance observed in SEM images, the crystallization was not as pure as the films with substrate temperatures of 75 C. This finding could due to the violent reaction of CH NH I and PbCl at the 85 C hot sample surface, which may impair the crystal quality, which is not observed in the surface morphology but is clearly exhibited in the XRD patterns. The absorption spectra of these films Figure. a) AFM image ( µm by µm) of the vacuum-deposited PbCl thin fi lm. b) Photographs of the vacuum deposited PbCl (left) and perovskite (right) thin fi lms. 668 wileyonlinelibrary.com WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater., 6,

3 Adv. Mater., 6, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com promising photovoltaic performance by incorporating the 75 C perovskite thin films in the devices. Figure shows the J V characteristics of 5 nm PbCl ( nm perovskite) devices. The fabrication condition was kept the same as for the above-mentioned cells. The only control variables were the substrate temperature when the CHNHI layer deposition was performed. The highest-performing device was, as expected, fabricated at a 75 C substrate temperature. The cell delivered a shortcircuit current density (Jsc) of.9 ma/cm, an open-circuit voltage (Voc) of. V and a high fill factor (FF) of 7.%, leading to a very high PCE of 5.%. The efficiency is among one of the highest PCE values reported in vacuum-deposited perovskite solar cells, which is particularly remarkable given the simple layer-by-layer sequential fabrication and the complete omission of costly organic compounds and high-temperature processes. The sample with the substrate temperature of 65 C exhibits a kink in the J V curve and a lower Jsc of. ma/cm, which is caused by the incompletely converted PbCl film near the PEDOT:PSS interface. In contrast, the device with the substrate temperature of 85 C device exhibits kink-free characteristics but the lowest Jsc of 9. ma/cm and PCE of.5%. This result is quite surprising because the film exhibits similar absorbance as the 75 C substrate temperature films. The poor crystallization could be the main reason for the low efficiency. The external quantum efficiency (EQE) spectrum of the 75 C sample exhibits a high value of 8% across the entire 5 to 75 nm wavelength range, indicating full absorption and efficient carrier extraction in the nm-thick perovskite film. The smaller EQE value of 6 7% observed at ca. nm is due to the combination effect of the higher Figure. a c) Cross-sectional and d f) top view SEM images of the perovskite thin films reflection of the glass/air interface and the fabricated at substrate temperatures of 65 C (a,d); 75 C (b,e); and 85 C (c,f). g) Tilt-angle higher absorption of ITO and PEDOT:PSS SEM image of the perovskite thin film fabricated at 75 C substrate temperature (magnifica- at this wavelength. The integrated EQE over tion: 5 ). AM.5G solar photon flux estimates a Jsc of. ma/cm, which is consistent with the measured value obtained from solar simulators. Negliare presented in Figure b. The nm-thick 75 C and 85 C gible hysteresis of the J V characteristics was observed for samples exhibit a similar high absorption in the entire ultrathe reverse and forward scan directions (see Figure S5, Supviolet to 75 nm wavelength range with absorbance values as porting Information).[ 7] The Jsc of cells with and without a high as.7 5. The extremely high absorbance of > in the blue to green wavelength range indicates a pinhole-free film mask has been examined, and the difference was always less quality. The 65 C thin film exhibits a much lower absorption, than % (see Figure S6, Supporting Information). The high which again confirms the incomplete formation of the perovsperformance of champion devices can be attributed to the kite layer. With the combination of encouraging characteristics continuous low-contamination vacuum deposition process including full surface coverage, a large grain size, long-range throughout the device fabrication, the good crystallinity, and continuous and homogeneous thin-film morphology and a the unusual continuous, homogeneous, and pinhole-free perhigh level of crystallization phase purity, one can anticipate ovskite thin films. 669

4 (a) Intensity (counts) x 5 8x 6x x x 5 * () () * Intensity (Counts) 85 o C 65 o C 5 6 θ( ) () () * * 75 o C θ ( (b) ) 65 o C 75 o C 85 o C Current Density (ma/cm ) (a) o C - 75 o C 85 o C (b) 8 Voltage (V) Absorbance Wavelength (nm) Figure. a) XRD patterns and b) absorbance spectra of ITO/ PEDOT:PSS/perovskite ( nm) thin fi lms fabricated at various substrate temperatures. To further demonstrate the reproducibility of the sequential vacuum deposition technique, separate devices from different batches were fabricated and measured using the optimized fabrication conditions (75 C substrate temperature, nm perovskite). Figure S7 in the Supporting Information and Table show the histogram and the average performance, respectively. All the parameters including V oc, J sc, and FF were highly reproducible with low variation. Impressive.5%,.%, 5.7% and 5.9% relative standard deviations were observed for V oc, J sc, FF and PCE, respectively. The average PCE of ca. % is also encouraging, which is only slightly lower than the best PCE = 5.% cell. The promising results demonstrate the great advantage of this fabrication method in solar module production, of which sub-cells are series and parallel connected. Preliminary device stability results are shown in Figure S8 in the Supporting Information. An encapsulated device was stored in the dark and in ambient air at room temperature. The device showed a good stability over three months and the PCE value still remained at 86% of its initial efficiency. Compared with previous vacuum co-deposition fabrications of perovskite layer, [6,8,8 ] both the sequential layer-by-layer EQE (%) 6 65 o C 75 o C 85 o C Wavelength (nm) Figure. a) J V characteristics of perovskite solar cells fabricated at C substrate temperatures measured under sun AM.5G illumination (solid lines) and in the dark (dashed lines). b) The corresponding EQE spectra of the devices. deposition method proposed in this study and co-deposition of lead halide and methylammonium halide lead to pinhole-free perovskite films. However, the sequential vacuum deposition approach possesses three distinct advantages: i) It creates perovskite films with a much larger crystal domain size (micrometer grain size versus tens of nanometers in the co-deposition method), which may result in better carriertransport and carrier-diffusion properties; ii) the sequential Table. Performance of solar cells in this work. V OC [V] J SC [ma/cm ] FF [%] PCE [%] 65 C C (highest) a) C (average ± s.d.) b).98 ±..8 ± ±..9 ±.8 85 C a)the highest performing device; b) The average and standard deviation (s.d.) of the cells from different batches processed at the 75 C substrate temperature. 665 wileyonlinelibrary.com WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater., 6,

5 vacuum deposition method can be analogous to a two-step solution process: [,8,9 ] both sequential vacuum evaporation and the two-step solution process decouple the disposition of the reagent salts and offer an enhanced control over the perovskite morphology by individual manipulation of the lead halide and methylammonium halide deposition parameters; iii) the sublimation temperature of CH NH I in high vacuum (75 C) is much lower than that of lead halides, C 6, organic carrier transporting layers, and metals (all >5 C). Sequential layer-by-layer vacuum deposition offers the possibility of dedicated chambers for each layer, which can minimize the contamination issue of CH NH I resublimation when higherevaporation-temperature materials are evaporated. In conclusion, we have developed a novel sequential layerby-layer vacuum sublimation method to fabricate planar-type organometal halide perovskite solar cells. A clear fabricationcondition-morphology-device-performance correlation was observed. The sublimed PbCl converted to perovskite in situ upon CH NH I deposition, resulting in a uniform thin film with unusual large-scale homogeneous crystalline structures at an optimized substrate temperature. By sandwiching these vacuum-deposited perovskite films between the anode and vacuum sublimed electron-transporting layers and the cathode, the simple device architecture delivered a remarkable performance of V oc =. V, J sc =.89 ma/cm, FF = 7.% and PCE as high as 5.%. The cells also exhibited a promising reproducibility with an average PCE of ca. % and a relative standard deviation as small as 6%. We envision that the integration of a simplified device structure, simple layer-by-layer fabrication, a low-contamination and highly homogeneous sub- C vacuum process, low-cost raw materials and the compatibility of a matured mass-production infrastructure will make this particular method a promising technology that brings perovskite solar cells a large step closer to commercial production. Experimental Section Device and Thin-Film Preparation : The devices were prepared on cleaned ITO substrates by spin coating a PEDOT:PSS (Clevios AI 8) thin fi lm. Then, the substrates were loaded into a high vacuum chamber (base pressure < 6 Torr) to evaporate PbCl, CH NHI, C 6, Bphen, Ca, Ag thin fi lms layer-by-layer. The substrates were kept at room temperature, and all the layers (PbCl, C 6, Bphen, Ca, and Ag) were fabricated with constant deposition rates except the CH NH I layer. The substrates were controlled at specifi c temperatures (room temperature, 65 C, 75 C and 85 C), while the CH NH I layers were deposited with a constant source temperature of 85 C. The devices were confi gured as: glass substrate/ito (5 nm)/pedot:pss ( nm)/ch NH PbI x Cl x / C 6 ( nm)/bphen (6 nm)/ca ( nm)/ag ( nm). The devices were encapsulated using a UV-cured sealant (Everwide Chemical Co., Epowide EX) and a cover glass under an anhydrous nitrogen atmosphere after fabrication and subsequently characterized in air. Characterization : Perovskite thin fi lms for SEM, AFM, XRD and absorption measurements were prepared using the same fabrication conditions as for the solar cells with layer structures confi gured as: glass substrate/ito (5 nm)/pedot:pss ( nm)/ch NH PbI x Cl x. The SEM images were obtained using a Japan Electron Optics Laboratory Co., Ltd. (JEOL) JSM-7F scanning electron microscope. The AFM images were acquired using a Bruker Dimension Icon atomic force microscope. The X-ray diffraction was performed using a Rigaku TTRAX III instrument with Cu K α radiation. The absorption spectra were acquired using a SHIMADZU UV-6 UV vis spectrophotometer. The current density versus voltage ( J V ) characteristics of the devices were measured using a Keithley SourceMeter 66A in the dark and under AM.5G simulated solar illumination with an intensity of mw/cm ( sun, calibrated by a NREL-traceable KG5 fi ltered silicon reference cell). The ITO anode and the metal cathode were arranged in a cross-bar geometry and the device area was determined by the overlap of the ITO and the metal electrodes. Accurate device areas (ca..5 cm ) were measured device-by-device using a calibrated optical microscope. The EQE spectra were acquired by illuminating a chopped monochromatic light with a continuous-wave bias white light (from halogen lamp) on the solar cells. The monochromatic light intensities were measured with a NIST-traceable power meter (Ophir). 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7 Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 6969 Weinheim, Germany,. Supporting Information for Adv. Mater., DOI:./adma.6 Efficient and Uniform Planar-Type Perovskite Solar Cells by Simple Sequential Vacuum Deposition Chang-Wen Chen, Hao-Wei Kang, Sheng-Yi Hsiao, Po-Fan Yang, Kai-Ming Chiang, and Hao-Wu Lin*

8 Supporting Information Efficient and Uniform Planar-Type Perovskite Solar Cells by Simple Sequential Vacuum Deposition Chang-Wen Chen, Hao-Wei Kang, Sheg-Yi Hsiao, Po-Fan Yang, Kai-Ming Chiang and Hao- Wu Lin* Figure S. SEM image of the vacuum-deposited PbCl thin film. μm Current density (ma/cm ) nm 5 nm nm 5 nm 5 nm 75 nm nm 5 nm 5 nm Voltage (V) Figure S. J-V characteristics of room-temperature-deposited perovskite solar cells of various PbCl thicknesses under sun AM.5G illumination.

9 (a) (b) (c) Figure S. AFM images ( μm by μm) of perovskite thin films fabricated at substrate temperatures of (a) 65 C (b) 75 C (c) 85 C. μm Figure S. Tilt-angle SEM image of perovskite thin film fabricated at 75 C substrate temperature. (Magnification: X)

10 Current density (ma/cm ) Scan from negtive bias to positive bias Scan from positive bias to negtive bias Voltage (V) Figure S5. J-V characteristics of a high performance perovskite cell measured with different sweep directions. Current density (ma/cm ) Without an aperture With an aperture Voltage (V) Figure S6. J-V characteristics of a high performance perovskite cell measured with or without an aperture.

11 (a) Counts Counts Voc (V) (b) Current Density (ma/cm ) FF (%) Counts Counts Voltage (V) Jsc (ma/cm ) 6 PCE (%) Figure S7. (a) Histogram of cell parameters and (b) J-V characteristics of devices from different batches fabricated at 75 C substrate temperature. PCE (normalized %) Time (h) Figure S8. Long term power conversion efficiency (PCE) stability of a encapsulated high performance perovskite cell stored in the dark in ambient air.