Efficient Perovskite Solar Cells by Temperature Control in Single and

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Efficient Perovskite Solar Cells by Temperature Control in Single and Mixed Halide Precursor Solutions and Films Devendra Khatiwada, Swaminathan Venkatesan, Nirmal Adhikari, Ashish Dubey, Abu Farzan Mitul, Lal Mohammad, Anastasiia Iefanova, Seth B Darling, and Qiquan Qiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08294 • Publication Date (Web): 28 Oct 2015 Downloaded from http://pubs.acs.org on October 29, 2015

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The Journal of Physical Chemistry

Efficient Perovskite Solar Cells by Temperature Control in Single and Mixed Halide Precursor Solutions and Films Devendra Khatiwada1, Swaminathan Venkatesan1, Nirmal Adhikari1, Ashish Dubey1, Abu Farzan Mitul1, Lal Mohammad1, Anastasiia Iefanova1, Seth B. Darling2,3, and Qiquan Qiao1* 1

Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, SD, USA Tel: 1-605-688-6965, [email protected] 2 Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439 Abstract Thermal annealing and precursor composition play critical roles in crystallinity control and morphology formation of perovskite thin films for achieving higher photovoltaic performance. In this study we have systematically studied the role of annealing temperature on the crystallinity of perovskite (CHNH3PbI3) thin films cast from single (without PbCl2) and mixed (with PbCl2) halide precursors. Higher annealing temperature leads to agglomeration of perovskite crystals. The effects of annealing temperature on the performance of perovskite solar cells are different in single and mixed halide processed films. It is observed that the perovskite crystallinity and film formation can be altered with the addition of lead chloride in the precursor solution. We report that single halide perovskite solar cells show no change in morphology and crystal size with increase in annealing temperature, which was confirmed by UV-vis absorption spectroscopy, x-ray diffraction (XRD) and atomic force microscopy (AFM). However, mixed halide perovskite (CH3NH3PbI3-xClx) solar cells show significant change in crystal formation in the active layer when increasing annealing temperature. In addition, heating perovskite precursor solutions at 150 oC can lead to enhancement in solar cell efficiency for both single and mixed halide systems. Perovskite solar cells fabricated using heated precursor solutions form dense film morphology, thus significantly improved fill factor up to 80% with power conversion efficiency exceeding 13% under AM 1.5 condition. Keywords: Perovskite solar cells, temperature control, crystallinity and morphology ___________________________________________________________________________________________ 3

Institute for Molecular Engineering, University of Chicago, 5640 S Ellis Ave, Chicago, IL 60637 1 ACS Paragon Plus Environment


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Introduction Hybrid organic-inorganic perovskite based solar cells have shown great promise with different device architectures (meso/planar structure) 1, ideal bandgap (1.5 eV), long carrier lifetimes, large diffusion length, high efficiency

1-5

, and rapid energy payback time 6. Adopting

device architecture similar to dye sensitized solar cells, perovskite solar cells were first fabricated with mesoporous structure that required high processing temperature (> 400 oC) 7. Low processing temperature, however, is necessary when using mechanically flexible substrates thereby enabling roll-to-roll manufacturing 8. Temperature control in forming perovskite crystals is crucial as it determines the crystal size and orientation as well as the film morphology 9-11 . Methylammonium lead triiodide (CH3NH3PbI3) is a widely reported single halide perovskite that has been used in fabricating efficient solar cells. Single halide perovskite possesses various advantages including broad absorption range (350-1100 nm), high electron and hole mobility (7.5 cm2V-1S-1 and 12.5-66 cm2V-1S-1, respectively), and high charge carrier diffusion length (100nm - 1µm) 12. Mixed halide (CH3NH3PbI3-xClx ) perovskite solar cells have also been of interest, possessing even higher electron mobility (~ 33 cm2V-1S-1) and longer diffusion length (1000-1500 nm)

13

. The addition of chlorine in single halide perovskite

precursor leads to large scale crystalline domains (> 200nm), which are correlated with increased charge transport with less charge recombination

5, 14-17

. In mixed halide perovskite solar cells,

different secondary phases (e.g., CH3NH3Cl, CH3NH3 PbCl3, and CH3NH3PbI3) of perovskite may be formed during the crystal formation, which can hinder photovoltaic performance 8. Moreover, perovskite solar cells often suffer strong hysteresis in current density-voltage (J-V) measurements

2, 18

. Hysteresis strongly depends on the perovskite crystal interfaces, size and

defects in both mesoporous and planar structures. Researchers are actively pursuing lowtemperature processing with no hysteresis and high efficiency. Under low-temperature processing, the temperature effects on perovskite crystal size in single and mixed halide films has to be understood. In addition, temperature effects on perovskite precursor solutions and the final device performance have not yet been clearly understood and correlated. In this work, the effects of annealing temperature on perovskite crystal formation, film morphology and their correlation with planar structure device performance in single and mixed halide processed solar cells were investigated. A series of experiments correlating annealing 2 ACS Paragon Plus Environment

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temperatures to solar cell performance were conducted. The single halide solution consists of methylammonium iodide and lead iodide whereas mixed halide contains methylammonium iodide, lead iodide, and a small amount of lead chloride. For the mixed halide precursor solutions heated at 60 oC, after processing into perovskite films, a trend in decreasing open circuit voltage and short circuit current density was observed with increasing annealing temperature of the perovskite film. However, for the single halide precursor solutions heated at 60 oC, no significant changes were found in solar cell performance. After increasing the temperature of single and mixed halide precursor solutions from 60 oC to 150 oC and processing into films a very high fill factor—close to 80%—was achieved. Therefore, the processing temperatures for preparation of both the precursor solutions and perovskite film are critical in achieving high performance cells with large fill factor. Experimental procedures Preparation of single and mixed halide perovskite solutions Methylammonium Iodide (CH3NH3I) was synthesized using a standard procedure

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.

Hydro-iodic acid (10 mL, 0.227 molar) and methylamine (9.266 mL, 0.273 molar) were stirred in ice bath in the air for 2 hr to obtain precipitate of CH3NH3I. Solvent was removed from the precipitate by rotating in a rotary evaporator at 50 oC until all the solvent evaporated. Yellowish raw product methylammonium iodide (CH3NH3I) was formed, which was purified by washing with diethyl ether and was filtered using a paper filter 15 cm in diameter in the air. After filtration, the solid precipitate CH3NH3I was collected and then dried in a vacuum oven at 60 °C for 24 hr. The prepared CH3NH3I was kept inside a nitrogen filled glove box. PbI2 and PbCl2 were purchased from Acros Organic and γ- Butyrolactone was ordered from Sigma Aldrich. Single halide perovskite was prepared from a precursor mixture solution of 209 mg CH3NH3I and 581 mg PbI2 in a binary solvent mixture made by 0.7 ml γ- Butyrolactone and 0.3 ml dimethyl sulfoxide (DMSO) inside a glovebox. For mixed halide perovskite solution preparation, a precursor mixture solution of 209 mg CH3NH3I, 581 mg PbI2 and 39 mg PbCl2 was prepared inside the glovebox with the same binary solvent as used in preparation of the single halide perovskite solution. Both single and mixed halide precursor solutions were kept stirring for 12 hr at 60 oC on a hot plate inside the nitrogen filled glove box. [6,6]-Phenyl-C613 ACS Paragon Plus Environment


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butyric acid methyl ester (PC61BM) was purchased from Nano-C material. 20 mg PC61BM was dissolved in1 ml chlorobenzene (CB) and kept stirring at 85 oC for 12 h. 0.5 mg Rhodamine 101 inner salt purchased from sigma Aldrich was dissolved in 1 ml iso-propane (IPA) and kept stirring for 1 hr without heating before spin coating. PC61BM and Rhodamine solution were also prepared inside the glove box in a nitrogen environment. Device fabrication Perovskite solar cells were fabricated on indium tin oxide (ITO) substrates. ITO substrates were thoroughly cleaned in detergent, DI water, acetone and IPA for 20 min each. Cleaned ITO was then plasma cleaned for 25 min. PEDOT:PSS as hole transport layer was spin coated on top of cleaned ITO at 4500rpm for 45 sec followed by annealing at 150 oC for 10 min to remove any excess water in the air. PEDOT:PSS coated films were moved into the nitrogen filled glove box and were again annealed at 100 oC. Both single and mixed halide perovskite precursor solutions (yellow in color) were heated at 60 oC on a hot plate. Then these hot yellow perovskite precursor solutions were spin coated on heated (100oC) PEDOT:PSS-coated ITO substrates at 750/4000 rpm for 20s and 60s, respectively. After 40 sec, 160µl of toluene was dripped onto the perovskite film. Toluene drip onto the film prepared from mixed solvent lead to uniform and dense perovskite layer through an intermediate phase of CH3NH3I-PbI2-DMSO resulting in low hysteresis

20

. Prior literature reveals that toluene reduces the solubility of

CH3NH3Pb3-xClx in a mixed solvent thereby promoting fast nucleation and crystal growth

21

.

Fabricated perovskite films from both single and mixed halide perovskite were then annealed on different hot plates at temperatures 80 oC, 90 oC, 100 oC, 110 oC, and 120 oC respectively for 20 min to form perovskite crystals. In the next step, single and mixed halide perovskite precursor solutions were again prepared inside a glove box following the same process as mentioned above. These yellow color perovskite precursor solutions inside the glove box were kept stirring overnight for 12 hr at temperature of 60 oC. At 5 minute before spin coating, these yellow color perovskite precursor solutions were further heated to a temperature of 150 oC in the glovebox till the color of precursor solutions changed from yellow to reddish brown for both single and mixed halide perovskite. Perovskite films were then fabricated using these reddish brown precursor solutions. Prepared perovskite films were annealed on different hot plates at temperature 80 oC, 90 oC, 100 4 ACS Paragon Plus Environment

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o

C, 110 oC, 120 oC, respectively, for 20 min. PC61BM was spin coated inside the glove box on

top of the perovskite active layer at 2000 rpm for 40 sec and then annealed at 100 oC for 10 min. Rhodamine was then spin coated inside the glove box at 4000rpm for 40 sec. Finally, silver was thermally evaporated as cathode with a thickness of 150 nm. The device cell area was 0.16cm2, as defined by a mask. Characterization An Agilent 4155 semiconductor parameter analyzer was used to characterize current density-voltage (J-V) characteristics. A silicon photo detector (NREL calibrated) was used as reference cell to calibrate the intensity. Device J-V measurements were performed with a Newport xenon lamp as a solar simulator (A.M 1.5). All perovskite solar cells were tested under the same condition. Fast scan was performed with a voltage step of 10mV and scan rate 1V/s. During fast reverse scan (open circuit to short circuit) capacitive charges together with photogenerated charges are extracted that reduces hysteresis. However, during fast forward scan (short circuit to open circuit), solar cells are partially charged by photo-generated charges, which reduced the total charges. Furthermore, due to the high density of defect states in the perovskite films, emptying and filling of trap states occur, which lead to hysteresis and are observed when scanned at a slower rate 2. Chemical or structural changes in the material due to formation of different secondary phases during fabrication also contributes to hysteresis 22. External quantum efficiency (EQE) measurements were performed using the same lamp that was attached to a Newport monochromator. An Agilent 8453 spectrophotometer was used to determine UV-Vis absorption. First, a blank scan was performed on a PEDOT:PSS film followed by a scan on the perovskite film to subtract absorbance due to glass/ITO and PEDOT:PSS. AFM images were obtained with an Agilent 5500 SPM (scanning probe microscope) in tapping mode. Silicon tips coated with Cr/Pt having a resonance frequency ~300 KHz were used. Results and discussion Perovskite solar cells were fabricated with device structure glass/ITO/PEDOT:PSS (hole transport

layer)/perovskite/PC61BM

(electron

transport

layer)/Rhodamine/Ag(thermal

evaporated). Figures 1a and b show the device structure and energy level diagram of perovskite 5 ACS Paragon Plus Environment


solar cells, respectively. Figure 1(c) and (d) show the absorbance spectra of perovskite solar cells fabricated with single halide and mixed halide precursors. All perovskite films on PEDOT:PSS at different temperatures 80 °C, 90 °C, 100 °C, 110 °C and 120 °C show broad absorbance. The absorbance of single halide perovskite films in the range 400-750 nm slightly increases with increase in annealing temperature. However, in mixed halide perovskite films, the absorbance in the range 400-750nm significantly increases with higher annealing temperature.

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0.4

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Figure 1. (a) Device structure and (b) energy level diagram of perovskite solar cells. Normalized absorbance spectra of (c) single halide (without PbCl2) and (d) mixed halide (with PbCl2) perovskite films at different annealing temperatures, 80 °C, 90 °C, 100 °C, 110 °C, and 120 °C processed from precursor solutions heated at 60 °C.

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(b)

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Figure 2. EQE of (a) single halide (without PbCl2) and (b) mixed halide (with PbCl2) perovskite solar cells at different film annealing temperatures 80 °C, 90 °C, 100 °C, 110 °C, and 120 °C processed from precursor solutions heated at 60 °C.

Figures 2a and b show external quantum efficiency (EQE) measurement for single and mixed halide perovskite solar cells, respectively. EQE results show broad spectral range from 350 – 800 nm. Integrated EQE for single and mixed halide perovskite solar cells are found to be 75% in average. The short circuit current density (Jsc) obtained from EQE integration over the AM 1.5 solar simulator spectrum is very close to the experimentally obtained J–V curves. 1400

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Figure 3. XRD spectra of (a) single halide (without PbCl2) and (b) mixed halide (with PbCl2) perovskite films at different annealing temperatures 80 °C, 90 °C, 100 °C, 110 °C and 120 °C processed from precursor solutions heated at 60 °C. Figures 3a and b show XRD measurements of single and mixed halide perovskite films respectively annealed each at different temperatures (80 °C, 90 °C, 100 °C, 110 °C and 120 °C). Strong peaks at 14.08°, 24.8°, 28.41°, 31.85°, and 43.19° are attributed to the formation of orthorhombic crystal structure of halide perovskite (CH3NH3PbI3/CH3NH3PbI3-xClx) with high crystallinity. The peak at 12.65° corresponds to lead iodide (PbI2). Single halide perovskite precursor solution resulted in complete formation of perovskite film, as evidenced by the absence of the PbI2 peak. However, mixed halide perovskite precursor solution resulted in incomplete formation of perovskite film with a prominent PbI2 peak at 12.65°. Intensity of the PbI2 peak for mixed halide perovskite films was found to increase with increase in annealing temperature. This trend is attributed to incomplete conversion of PbI2 to perovskite at higher temperature resulting in some residual PbI2 phase. The presence of a PbI2 phase is attributable to phase decomposition of CH3NH3PbCl3 and the precursor reaction

23

. Thus, perovskite solar cells processed from

mixed halide precursor solution under the above mentioned experimental condition exhibit higher photovoltaic performance at lower temperatures. Annealing temperature had little effect on single halide perovskite film morphology. 10

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80 C 0 90 C 0 100 C 0 110 C 0 120 C

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Current density (mA/cm2)

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0 80 oC 90 oC 100 oC 110 oC 120 oC

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Figure 4. Current density-voltage characteristics in reverse scan (scan rate of 1V/s with step of 10mV) of (a) single halide (without PbCl2) and (b) mixed halide (with PbCl2) perovskite solar cells at different film annealing temperatures 80 °C, 90 °C, 100 °C, 110 °C and 120 °C processed from precursor solutions heated at 60 °C.


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Table 1. Photovoltaic parameters in reverse scan for single and mixed halide perovskite solar cells with films annealed at different temperatures 80 °C, 90 °C, 100 °C, 110 °C and 120 °C processed from precursor solutions heated at 60 °C. J-V characteristics were performed under the same scan rate (1V/s with step of 10mV) for single and mixed halide systems. Perovskite solution at 150oC Single halide (without PbCl2)

Mixed halide (with PbCl2)

Annealing temperature (°C) 80 °C 90 °C 100 °C 110 °C 120 °C 80 °C 90 °C 100 °C 110 °C 120 °C

Voc (Volt) 0.88±0.03 0.79±0.02 0.80±0.03 0.87±0.05 0.84±0.01 0.92±0.06 0.84±0.07 0.89±0.04 0.76±0.05 0.71±0.08

Jsc (mA/cm2) 17.56±0.10 17.61±0.11 21.08±0.10 18.65±0.11 19.79±0.08 15.46±0.12 10.94±0.10 15.94±0.15 10.56±0.16 11.93±0.17

Fill factor (FF) 0.76 0.62 0.67 0.70 0.73 0.61 0.70 0.40 0.71 0.69

Efficiency (η) 11.74±0.12 11.05±0.13 11.32±0.10 11.28±0.11 12.23±0.10 8.97±0.12 6.47±0.16 5.69±0.18 5.73±0.15 5.92±0.14

Figure 4a and b shows J-V characteristics for single and mixed halide perovskite solar cells respectively at different annealing temperatures of 80 °C, 90 °C, 100 °C, 110 °C and 120 °C, respectively. The J-V characteristics were collected at the same conditions with the same scan rate for all the cells. J-V curves with the forward and reverse scan for both single and mixed halide solar cells at different annealing temperature of perovskite films are shown in Figure S1 (supporting information). Single halide perovskite solar cells did not show much change in the JV curves between the forward and reverse scans at the same annealing temperature indicating negligible hysteresis (supporting information). However, the mixed halide processed devices show variations between the forward and reverse scans at the same annealing temperature indicating significant hysteresis. All the devices processed from single halide perovskite precursor exhibited good reproducibility. Efficiency for mixed halide perovskite solar cell decreased with increase in annealing temperature. In addition, perovskite solar cells made by mixed halide do not show consistent efficiency at higher temperature. Table 1 shows photovoltaic parameters for perovskite solar cells fabricated from single and mixed halide precursors at different film annealing temperatures of 80 °C, 90 °C, 100 °C, 110 °C and 120 °C, respectively. 9 ACS Paragon Plus Environment


(b) Mixed halide (with PbCl2)

110 °C annealed film



80°C annealed film

(a) Single halide (without PbCl2)


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Figure 5. AFM topography for (a) single halide (without PbCl2) and (b) mixed halide (with PbCl2) perovskite films at different annealing temperatures of 80 °C , 90 °C, 100 °C, 110 °C, and 120 °C processed with precursor solutions heated at 60 °C.


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AFM was conducted to investigate nanoscale interfacial morphology in the active layer of the perovskite solar cells. Figures 5a and b show the AFM topography images of single and mixed halide perovskite films respectively obtained using different annealing temperatures. For single halide perovskite films, the topography images exhibit almost no difference in grain size with variation in temperature. This uniformity reveals that annealing temperature has little effect in single halide perovskite films. However, the grain size of mixed halide perovskite films were observed to increase as temperature gets higher. The RMS roughness for single halide perovskite film at 80 oC, 90 oC, 100 oC, 110 oC and 120 oC are 18.2nm, 19.8nm, 23.4nm, 23.8nm, 29.4nm, respectively. Grain size for single halide perovskite films is in the range 200-300nm and forms agglomeration. The RMS roughness for mixed halide perovskite films at 80 oC , 90 oC, 100 oC, 110 oC, 120 oC are 13.8nm, 30.9nm, 44.2nm, 31.0nm,31nm, respectively, with grain size of 200300nm at 800C and increased up to 1µm with increase in temperature.

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Figure 6. XRD scans of perovskite films from (a) single halide (without PbCl2) and (b) mixed halide (with PbCl2) processed with precursor solutions heated at 150 °C. The films were annealed at 80 °C. 11 ACS Paragon Plus Environment


Perovskite solar cells fabricated from the heated perovskite precursor solution at 150 oC showed broad absorbance from 400 to 1100 nm (Figure S3 in supporting information). Figs. 6a and b show XRD scans of single and mixed halide perovskite films respectively with each annealed at 80 oC fabricated from perovskite precursor solutions heated at 150 oC. XRD data show an absence of a PbI2 peak indicating complete formation of perovskite structure. Sharp peaks of perovskite crystal are observed at 14.08°, 24.8°, 28.41°, 31.85°, and 43.19°, which are attributed to higher crystallinity. J-V characteristics for perovskite solar cells fabricated from perovskite precursor solutions heated at 150 oC are shown in Figure 7. Table 2 summarizes photovoltaic parameters for single and mixed halide perovskite solar cells under illumination. Perovskite films prepared from single and mixed halide precursor solutions were annealed at 80 oC. J-V curves show increase in fill factor up to 80% with device efficiency greater than 13% in comparison to perovskite solar cells prepared from perovskite precursor solution heated at 60 oC. J-V characteristics were performed under identical forward and reversed scan rate (1V/s with step of 10mV) for both single and mixed halide systems. Single halide perovskite solar cells showed higher efficiency than mixed halide due to larger short circuit current density. Higher fill factor was observed during reverse scan corresponding to comparable photocurrent between the forward and reversed scans (supporting information, table S2). This suggested that fill factor also depends on the scan direction. 10 5 2


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0 PbCl2 -5

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Figure 7. J-V characteristics in reverse scan (scan rate of 1V/s with step of 10mV) of perovskite solar cells for single halide (without PbCl2) and mixed halide (with PbCl2) processed with precursor solutions heated at 150 °C. The films were annealed at 80 °C. Table 2 Photovoltaic parameters in reverse scan (scan rate of 1V/s with step of 10mV) for perovskite solar cells fabricated from heated perovskite precursor (150 °C). Perovskite solution at 150oC Single halide (without PbCl2) Mixed halide (with PbCl2)

Film annealing temperature (oC)

Voc(Volt)

Jsc(mA/cm2)

Fill factor (FF)

Efficiency (η)

80 °C

0.90±0.01

18.65±0.01

0.78

13.14±0.01

80 °C

0.88±0.02

14.77±0.02

0.81

10.56±0.02

Figures 8a and b show AFM topography images from single halide (without PbCl2) and mixed halide (with PbCl2), respectively. These images reveal dense morphology with compact grain size. Thus, the films fabricated from the 150 °C heated, color-changed precursor solution (yellow to light brown) resulting in effective charge transport and increase in fill factor, which correlate with observed XRD and UV-Vis absorbance data. The RMS roughness for single and mixed halide perovskite films are 23.4 nm and 9.95 nm, respectively.

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Figure 8. AFM topography images of perovskite films (a) single halide (without PbCl2) and (b) mixed halide (with PbCl2) processed from precursor solutions heated at 150 °C. The films were annealed at 80 °C.



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Conclusions We have reported the effects of annealing temperatures on crystallinity and morphology formation of perovskite thin films using single and mixed halide precursors. Single halide processed perovskite solar cells do not show morphological changes and device characteristic difference with increasing perovskite film processing temperature. This type of perovskite solar cell can be fabricated across a broad range of annealing temperatures from 80 °C to 120 °C. However, mixed halide perovskite solar cells give higher device efficiency at lower film annealing temperatures, while higher annealing temperatures lead to decrease in device efficiency and morphologically increase in grain size with increasing temperatures. Our findings are supported by XRD measurements that show complete formation of perovskite structure for single halide perovskite solar cells. However, the peak intensity of PbI2 increases as the temperature gets higher in the mixed halide processed films. A new finding here is that the solar cell fill factor can be significantly improved by increasing temperature of the perovskite precursor solution prior to deposition.

Acknowledgements This research was benefited from the grants including NASA EPSCoR (NNX13AD31A), Pakistan-US Science and Technology Cooperation Program, and NSF MRI (grant no.1428992). This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility under Contract No. DE-AC02-06CH11357.

Supporting Information J-V characteristics and photovoltaic parameters for singe and mixed halide perovskite solar cells at different film annealing temperature 80 °C, 90 °C, 100 °C, 110 °C and 120 °C at both forward (/F) and reverse (/R) scan. J-V characteristics and photovoltaic parameters of perovskite solar cells processed with and without PbCl2 using heated solution (>150 °C) at both forward (/F) and reverse (/R) scan. UV-vis absorbance and EQE for perovskite solar cells fabricated from heated solution (150 °C). Photo of heated solution at 60 °C and 150 °C. This information is available free of charge via the Internet at http://pubs.acs.org 14 ACS Paragon Plus Environment

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Graphic abstract 10 5 2


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0 -5 -10 -15 PbCl2

-20 0.0

No PbCl2

0.2

0.4

0.6

0.8

Voltage (V)

Thermal annealing and precursor composition play critical roles in crystallinity control and morphology formation of perovskite thin films for achieving higher photovoltaic performance.


Efficient Perovskite Solar Cells by Temperature Control in Single and

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