Laser induced flash-evaporation printing CH3NH3PbI3 thin films for

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Laser induced flash-evaporation printing CHNHPbI thin films for high performance planar solar cells

Meiqian Tai, Xingyue Zhao, Haoming Wei, Guang Wang, Feng Hao, Xin Li, Xuewen Yin, Yu Zhou, Jianhua Han, Yang Wei, Kaili Jiang, and Hong Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05918 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Applied Materials & Interfaces

Laser

induced

flash-evaporation

printing

CH3NH3PbI3 thin films for high performance planar solar cells Meiqian Tai1, Xingyue Zhao1, Haoming Wei2, Guang Wang2, Feng Hao3, Xin Li1, Xuewen Yin1, Yu Zhou1, Jianhua Han1, Yang Wei4*, Kaili Jiang2,4* and Hong Lin1* 1

State Key Laboratory of New Ceramics and Fine Processing, School of Materials

Science and Engineering, Tsinghua University, Beijing, China 2

State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics

and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing, China 3

School of Materials and Energy, University of Electronic Science and Technology of

China, Chengdu, China 4

Collaborative Innovation Center of Quantum Matter, Beijing, China

Corresponding Author *E-mail: [email protected] (H.L), [email protected] (Y.W), [email protected] (K.J)

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Abstract Organic-inorganic hybrid perovskites have emerging as promising light harvesting materials for high efficiency solar cells recently. Compared to solution-based methods, vapor-based deposition technologies are more suitable in preparing compact, uniform and large-scale perovskite thin films. Here, we utilized flash-evaporation printing (FEP), a laser induced ultrafast single source evaporation method employing a carbon nanotube (CNT) evaporator, to fabricate high quality methylammonium lead iodide perovskite thin films. Stoichiometric films with pure tetragonal perovskite phase can be achieved using a controlled MAI to PbI2 ratio in evaporation precursors. The film crystallinity and crystal grain growth could further be promoted after post-annealing. Planar solar cells (0.06 cm2) employing these perovskite films exhibit a champion power conversion efficiency (PCE) of 16.8% with insignificant hysteresis, which is among the highest reported PCEs using vapor-based deposition methods. Large area (1 cm2) devices based on such perovskite films also achieved a stabilized PCE of 11.2%, indicating the feasibility and scalability of our FEP method in fabricating large area perovskite films for other optoelectronic applications.

Keywords: single source evaporation, stoichiometry, crystallinity, perovskite solar cells, carbon nanotube evaporator


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Table of Contents Image



Introduction Organic-inorganic hybrid perovskites MAPbX3 (MA=CH3NH3, X=Cl, Br, I) have been extensively investigated as light harvesters for photovoltaics due to their superior optoelectronic properties such as appropriate bandgaps, long charge carrier diffusion lengths and strong absorption in wide wavelength range1-6. The efficiency of perovskite solar cells has shown a rapid increase from 3.8% to 22.7% since the first report in 20097,8, indicating their potential for the next generation solar cells in the near future. Perovskite thin films can be prepared by various solution- and vapor-based deposition technologies including one-step spin-coating9,10, two-step deposition11,12, doctor blading13,14, spray coating15, vapor-assisted solution processes16, dual source co-evaporation17,18, and sequential vapor deposition19,20. Solution-based methods, especially one-step spin-coating, have been widely used for their low-cost and simplicity. However, it is difficult to form compact films by spin-coating without solvent engineering, especially on a large scale3,17. In contrast, vapor-based methods can easily obtain high purity, flat, and homogenous films with little concern about precursor solubility and substrate hydrophilicity21. The crucial point in vapor-based methods is to control the stoichiometry of different components to form pure phase. Dual source co-evaporation was the first reported for depositing perovskite films among several vapor-based methods17. High efficiency solar cells can be prepared based on uniform perovskite films via c-evaporation of PbX2 and MAI17,18, while monitoring and controlling the MAI evaporation rate remains a challenge due to the


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high vaporization pressure of MAI22,23. Perovskite films fabricated by sequential vapor deposition have also been investigated, while its thickness was limited by the reaction diffusion length of MAI19,20. An alternative method is single source thermal evaporation, such as flash evaporation, in which target materials are placed in a metal foil and heated to extremely high temperature (>1000 °C) instantly (within seconds) by a large current24. Since its heating process is ultrafast, organic and inorganic components can be evaporated and reach the substrate simultaneously. Therefore, the evaporated amount of organic component and the stoichiometry of obtained film are easier to control in flash evaporation than that in co-evaporation method. 2D perovskite materials have been synthesized successfully using this method in 2001 and lately flash evaporation of 3D MAPbI3 films have also been studied24,25, leading to solar cells with efficiency exceeding 12%. Our group has recently reported a modified methodology, named flash-evaporation printing (FEP), to deposit inorganic PbI2 films followed by immersing in MAI-IPA solution in fabricating perovskite solar cells with a power conversion efficiency (PCE) of ~10.3%, which applies a carbon nanotube (CNT) sheet as an evaporator and an yttrium aluminum garnet (YAG) laser as the heating source26. This efficiency is not so satisfactory probably due to the MAI dipping method after the evaporation of PbI2. The obtained perovskite film showed a rough surface with obvious defects and some unreacted PbI2 remained, which limited the performance of solar cells. Therefore, depositing perovskite films using the FEP method directly is highly desirable to further simplify the fabrication process and also



improve the as-obtained film quality.

Here, we extended the FEP methodology to depositing organic and inorganic hybrid perovskites from a single step. By adjusting the precursor ratios and post-annealing temperatures we achieved stoichiometric and uniform MAPbI3 films. The simple post-annealing process also led to large grains and increased crystallinity of as-obtained perovskite films. We then fabricated planar solar cells with the structure of FTO/TiO2-PCBM/MAPbI3/Spiro-OMeTAD/Au using the evaporated perovskite films, which showed the potential of FEP method in preparing hybrid perovskite films and efficient photovoltaics. Large area (1 cm2) devices based on these evaporated perovskite films were also fabricated, indicating the feasibility and scalability of our FEP method.

Results and discussion Fig. 1 shows the schematic view of FEP process for MAPbI3 thin film. The substrate was placed at the bottom of a vacuum chamber, and the perovskite precursor coated CNT was placed 1 mm above the substrate with the perovskite side face down. A YAG laser with a wavelength of 1.06 µm is used as the heating source. The laser beam scanned the carbon nanotube sheet in a speed of 1 m·s-1 along a programmed route. The laser power was optimized as 12 W, under which the CNT can be heated to over 1000 °C immediately. Perovskite precursor was then evaporated and transferred to a 2 cm x 2 cm substrate instantly within several seconds. The ultrafast deposition is proved to be a convenient and efficient methodology for thin films preparation.


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Fig. 1 Schematic of the flash evaporation printing (FEP) equipment and process.

In evaporation methods, precise control of the film stoichiometry is vital to obtain high quality perovskite films. For co-evaporation, it is achieved by carefully optimizing evaporation rates of two source materials18, while for our FEP method, it can be realized by simply changing the molar ratio of PbI2 and MAI in precursors. Fig. 2a and b show the XRD spectra of as-evaporated and after-annealing (100 °C, 30 min) perovskite films using different precursor ratios. With an MAI to PbI2 molar ratio of 1, the normal stoichiometry of MAI and PbI2, the PbI2 (001) peak of 12.6° appears in addition to the main peaks of 14.2°, 28.3° and 31.8° corresponding to the (110), (220) and (311) planes of tetragonal perovskite27, which may result from the different



evaporate distribution of two components: the evaporated PbI2 particles distribute within a small angle while MAI is gas-like and distribute everywhere in the chamber, and then a loss of MAI is inevitable28. Therefore, the MAI amount transported to substrate is less than in the precursor, and then 1:1 ratio in precursor leads to a film with excess PbI2 on substrate. It is notable that the positive effects of PbI2 on reducing hysteresis and suppressing ion migration have been reported in several works utilizing spin-coating method to fabricate perovskites29.30, in which PbI2 was controlled to be slightly excess and was found to be located at grain boundaries of perovskites, leading to the passivation of defects. However, excess PbI2 was observed as dendrites in our evaporated perovskite films (Figure S1), which is unfavorable to device performance. Therefore, obtaining stoichiometric perovskite film is of great significance for our FEP method, which is the same as other evaporation methods21,22. In addition, the intensity of PbI2 (001) peak becomes stronger after annealing, indicating the partly degradation of perovskite films with excess PbI2 during annealing process. When the MAI: PbI2 ratio increases to 1.5, we obtained a pure tetragonal phase without any impurity XRD peak. If the ratio further increases to 2 or 2.5, the XRD spectra of an as-deposited film almost remain a perovskite tetragonal pattern, while a small 11.4° peak also appeared, demonstrating the existence of MAxPbI3 (x>1) due to excess MAI, which is often observed in evaporation methods21,31. After annealed at 100 °C for 30 min, the film with 1.5: 1 MAI to PbI2 ratio still remains pure tetragonal phase. However, a new peak at 9.7° appears in XRD patterns of films with MAI to PbI2 ratio of 2 or 2.5, indicating the presence of crystalline MAI in films21. The excess


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MAI can be removed with prolonged annealing (e.g. 120 °C for 60 min), while the perovskite (110) peak intensity would be lower than that of films with 1.5: 1 MAI to PbI2 ratio (Figure S2), demonstrating the deteriorated film crystallinity. Thus, these results confirm the importance of precursor ratio control in the single source evaporation process to obtain pure tetragonal phase perovskite film.

Fig. 2 XRD spectra of (a) as-evaporated and (b) annealed at 100 °C for 30 min perovskite films using precursors of different MAI to PbI2 molar ratio: 1, 1.5, 2 and 2.5.

With a 1.5:1 molar ratio of MAI to PbI2 in precursor, the as-deposited film exhibits pure phase of tetragonal perovskite but low XRD peak intensity. Since the FEP method is ultrafast and the laser-induced evaporation is completed within a few milliseconds for the ultralow heat capacity of the CNT evaporators26, the two components can only collide and react till they arrived at the substrate and time for crystallizing may not be sufficient. In addition, the substrates remain room



temperature during the evaporation process, as-deposited perovskite is difficult to grow and recrystallize without enough activation energy. Therefore, post-annealing processes were conducted to promote perovskite film crystallization, and the effect of different annealing conditions on the film crystallinity is systematically studied. Fig. 3a shows the XRD spectra of films with balanced stoichiometry (using precursor ratio of 1.5:1) after different annealing process (without annealing, 100 °C for 30 min, 100 °C, 110 °C and 120 °C for 60 min, respectively). When the annealing temperature further increased to 130 °C, the films would turn from dark brown to yellow within 40 min, implies the degradation of perovskite and emergence of PbI2. From Fig. 3a tetragonal perovskite XRD patterns can be observed after different annealing process, indicating that the films remain a pure phase all the time. The increased (110) peak intensity as annealed at higher temperature demonstrates increased crystallinity, which is beneficial to reduce bulk defect density for high efficiency solar cells32,33. Besides enhanced crystalline quality, post-annealing can also promote grain growth, as validated by SEM observations (Fig. 3b-f). All films show compact surface without any pinhole, which is one of the advantages of evaporation methods. The grains in as-evaporated films are only several tens nanometers, while after post-annealing process, grains grow bigger and the film become smoother. The grain size distributions after different annealing processes are counted from SEM images (20k X) and given in Figure S3. The average grain size increases from 92 nm to 385 nm when annealed at 120 °C for 60 min, and the sizes of more than 75% grains are over 300 nm, which is beneficial to cell performance for reduced grain boundaries32.


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Fig. 3 (a) XRD spectra and (b-e) surface SEM images of perovskite films (with MAI: PbI2 precursor ratio of 1.5:1) after different annealing processes in N2-filled glove box. The annealing conditions are without annealing (b), 100 °C for 30 min (c), 100 °C for 60 min (d), 110 °C for 60 min (e) and 120 °C for 60 min (f).

The element composition of optimized perovskite films (using precursor ratio of 1.5:1, annealed at 120 °C for 60 min) was further confirmed by EDS and XPS measurement (Figure S4). The average atomic ratios of I/Pb are measured as 3.04, close to the ideal ratio of 3 (Table S1). Figure S5a shows ultraviolet-visible (UV-Vis) absorption spectra and steady-state photoluminescence (PL) emission spectra of these perovskite films, evidencing a bandgap of about 1.6 eV as literatures3,28. We then prepared perovskite solar cells (PSCs) with the structure of FTO/TiO2 -PCBM/MAPbI3/Spiro-OMeTAD/Au (Fig. 4a and b). Current density-Voltage (J-V) characteristics of solar cells using perovskite films annealed at different temperatures are shown in Fig. 4c and their photovoltaic performance distributions are shown in



Figure S6. As the annealing temperature increases, the solar cells exhibit higher PCE, with their open-circuit voltage (VOC) have little change while short-circuit current density (JSC) and fill factor (FF) improved significantly. It is consistent with the increase of crystallinity and grain size for decreasing number of grain boundaries and trap state density, which have been verified in solution-based methods33,34. Besides, larger grains possess longer diffusion length and higher mobility, which can reduce the loss by trapping or recombination during carrier migration25,32. The reduced charge recombination can be further validated by time-resolved PL characterization using perovskite/TiO2-PCBM/FTO/Glass samples(Figure S5b). The PL lifetime was fitted by a biexponential function involving a fast decay and a slow decay process and the fitted decay time ( for fast and for slow process) were summarized in Table S2. The fast decay process is attributed to carrier transportation from perovskite to electron or hole transport materials, which in our samples reflects the quenching effect at the perovskite/TiO2-PCBM interface, and the slow process is considered to be the result of radiative decay, which is largely influenced by defects and trap states35. The fast decay lifetime was 0.71 ns for the perovskite film without annealing, and almost remained the same after different annealing process. The slow decay time increased from 4.90 ns to 7.66 ns after annealed at 120 °C for 60 min, indicating the reduce of trap states in the perovskite domain. Our champion device was acquired using the film annealed at 120 °C for 60 min, exhibiting a PCE of 16.8% under reverse scan (from reverse to forward bias), with JSC, VOC and FF of 23.1 mA·cm-2, 0.98 V and 75% (Fig. 4d). It also generates a PCE of 14.8% under forward scan (from forward to reverse


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bias), showing relatively less hysteresis in normal planar solar cells. This is among the best reported efficiencies in PSCs fabricated via vapor-based methods23,36-38. Since the perovskite thin films prepared by FEP method are compact and relatively uniform, we also fabricated large area (1 cm x 1 cm) PSCs. The best device achieves an open-circuit voltage of 0.99 V and up to 18.5 mA·cm-2 short-circuit current density, results in 12.1% PCE under reverse scan with a fill factor of 66%. The corresponding J-V characteristic and steady-state photocurrent measurement (holding at maximum power point with applied voltage at 0.75 V) are plotted in Fig. 4e and f. The decrease of PCE compared to smaller area solar cells is mainly due to the decreased JSC and FF, which may result from large sheet resistance of FTO and increased recombination at interfaces due to more defects in non-optimized TiO2 compact layer in large area39. The device yields a stable photocurrent density of 15.0 mA·cm-2 in the steady-state measurements and leading to a stabilized power output (SPO) of 11.25%, which is almost in the middle of the reverse scan and forward scan value. This encouraging result indicates that the efficient FEP method have the potential in large scale device fabrication.



Fig. 4 (a) Schematic view and (b) cross-sectional SEM image of planar PSC with the configuration of FTO/TiO2-PCBM/MAPbI3/Spiro-OMeTAD/Au. (c) J-V curves of PSCs based on perovskite films annealed at different temperature with 0.06 cm2 active area. (d) J-V curves of the champion device with 0.06 cm2 active area. (e) J-V curves (the inset shows the photo of PSC with 1 cm2 active area from the glass side and from the Au electrode side) and (f) steady-state photocurrent measurement of the best performance PSC with 1 cm2 active area. The SPO J-V point is shown in (e) by an asterisk.


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Conclusions In summary, we reported the preparation of uniform and flat perovskite films for high efficient planar solar cells using a single step ultrafast laser induced FEP method. Our results further showed that the stoichiometric MAPbI3 films can be obtained by optimizing precursor ratio, and film crystallinity were greatly improved after post-annealing. Besides, annealing process is also beneficial to grain growth. Planar solar cells based on these perovskite films deposited by FEP and annealed at 120 °C for 60 min exhibit a maximum PCE of 16.8%, which is among the highest reported PCEs using vapor-based deposition methods. We also fabricated large area (1 cm2) solar cells with a stabilized 11.25% PCE, indicating the feasibility and scalability of our FEP method. This work paves the way for future investigation of FEP method in fabricating high-quality thin films, including but not limited to organic-inorganic hybrid perovskite, for efficient and up-scaling photovoltaics and other optoelectronic applications.



Experimental Section Materials MAI was purchased from Dyesol, other chemicals were purchased from Alfa Aesar or Sigma-Aldrich. All these materials were used without purification. Perovskite film evaporation PbI2 and MAI with different molar ratios were dissolved in DMF (N, N-dimethylformamide) to form a series of perovskite precursors. Freestanding carbon nanotube (CNT) sheets used as evaporators in this work were prepared by low-pressure chemical vapor deposition as previously reported40,41. The perovskite precursors were then spin-coated on the CNT sheets at 1500 rpm for 30 s, and dried at 100 °C for 5 min. Next, CNT sheets with precursors and substrates were transferred into a vacuum chamber, and a rapid evaporation was carried out by laser scan under a vacuum environment of 3x10-3 Pa. The evaporated films were then used either as-deposited or after annealed at various conditions. Solar cell fabrication The solar cells were fabricated on 2 cm x 2 cm fluorine-doped tin oxide (FTO) coated glasses (7 Ω/sq) which were partially etched by Zn powder and 2 M HCl. The etched substrates were then cleaned with deionized water, acetone, ethanol and isopropanol sequentially. The TiO2 compact layers were deposited by spin-coating a solution of titanium isopropoxide in ethanol at 2000 rpm for 60 s, and annealed at 500 °C for 30 min

[20]

. Then a 10 mg/mL PCBM solution in chlorobenzene was spin-coated on the

top of TiO2 compact layer at 4000 rpm for 40 s in a N2-filled glove box. The


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perovskite layers were deposited on FTO/TiO2/PCBM substrates in a vacuum chamber by FEP, after which a spiro-OMeTAD solution was spin-coated at 4000 rpm for 40 s to form hole transport layers. The composition of spiro-OMeTAD solution is 72.3 mg spiro-OMeTAD (2, 2, 7, 7’-tetrakis-(N, N-di-p-methoxyphenyl-amine)-9, 9’-spirobifluorene), 28.8 µL TBP (4-tert-butylpyridine) and 17.5 µL LiTFSI solution (520 mg/mL bis(trifluoromethane) sulfonamide lithium salt in acetonitrile) in 1 mL chlorobenzene. Finally, 100 nm Au electrodes were thermally evaporated on the top of hole transport layers to form whole devices. Characterization The crystalline structures of perovskite films were characterized by X-ray diffraction (XRD, D8 Advance diffractometer, Bruker, Germany, Cu-Kα, λ=1.5406 Å). The morphology of films were observed with a scanning electron microscope (SEM, Merlin, Zeiss, Germany). Ultraviolet-visible absorption spectra were measured by a UV-Vis-NIR spectrophotometer (Lambda 950, Perkin Elmer, USA) in the 300-800 nm wavelength range at room temperature. X-ray photoelectron spectroscopy (XPS) spectra were characterized with an X-ray photoelectron spectrometer (ESCALAB 250 Xi, Thermo Fisher SCIENTIFIC INC., USA). Steady-state and time-resolved PL spectra were detected by a fluorescence spectrometer (FLS920, Edinburgh Instruments, U.K.). The steady-state PL spectra were collected by illuminating the sample with a monochromatic xenon lamp source ( =460 nm). The time-resolved PL spectra were acquired with samples photoexcited by a pulsed laser beam (405 nm, 10 MHz, pulse duration of < 100 ps) under a time-correlated single photon counting



(TCSPC) mode. PL lifetime were obtained by fitting the measured transient curves with a biexponential decay function of the form:

I t = exp −/ + exp −/ The photocurrent-voltage characteristics of perovskite solar cells were tested by a calibrated (by a standard silicon solar cell) solar simulator (91192, Oriel, USA) with a digital source meter (2400, Keithley Instruments, USA) under AM 1.5G illumination (100 mW·cm2), and the active area of tested solar cells were defined as 0.06 cm2 and 1 cm2 using metal masks. A 450 W xenon lamp (69920, Newport, USA) was used for steady-state power measurements.

Supporting Information SEM image, XRD spectra, Grain size distributions, XPS spectra, EDS results, PL spectra, summary of PL decay time, photovoltaic performance distributions (Voc, Jsc, FF, PCE)

Author Information Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected] *E-mail: [email protected] ORCID Hong Lin: 0000-0002-3382-5960


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Notes The authors declare no competing ﬁnancial interest.

Acknowledgements This work was supported by the Projects of International Cooperation and Exchanges NSFC (51561145007), the National Natural Science Foundation of China NSFC (51772166, 51702038, 51472142, 61774090) and the Ministry of Science & Technology, P. R. China: Sino-Italy International Cooperation on Innovation (2016YFE0104000).



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Laser induced flash-evaporation printing CH3NH3PbI3 thin films for

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