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Improving the optoelectronic properties of mesoporous TiO2 by cobalt doping for high-performance hysteresis-free perovskite solar cells Siraj Sidhik, Andrea Cerdan-Pasaran, Diego Esparza, Tzarara Lopez-Luke, Ramon Carriles, and Elder De La Rosa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16312 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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ACS Applied Materials & Interfaces

Improving the optoelectronic properties of mesoporous TiO2 by cobalt doping for highperformance hysteresis-free perovskite solar cells Siraj Sidhik1, Andrea Cerdan Pasarán1, Diego Esparza2, Tzarara López Luke1 , Ramón Carriles1, Elder De la Rosa1,* 1 2

Centro de Investigaciones en Optica, A.P. 1-948, Leon, Gto., 37150, Mexico. Universidad Autónoma de Zacatecas. Av. Ramón López Velarde #801, Zacatecas, C.P. 98000,

Mexico.

ABSTRACT We for the first time report the incorporation of cobalt into a mesoporous TiO2 electrode for application in perovskite solar cells (PSCs).The Co-doped PSC exhibits excellent optoelectronic properties; we explain the improvements by passivation of electronic trap or sub-band gap states arising due to the oxygen vacancies in pristine-TiO2, enabling faster electron transport and collection. A simple post-annealing treatment is used to prepare the cobalt-doped mesoporous electrode;

UV-vis,

XPS,

SCLC,

photoluminescence

and

electrochemical

impedance

measurements confirm the incorporation of cobalt, enhanced conductivity and the passivation effect induced in the TiO2. An optimized doping concentration of 0.3 mol% results in maximum power conversion efficiency of 18.16%, 21.7% higher than a similar cell with a undoped TiO2 electrode. Also, the device shows negligible hysteresis and higher stability, retaining 80.54% of the initial efficiency after 200 h.

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Keywords: Perovskite; doping; cobalt; meso-TiO2; passivation; conductivity; hysteresis; stability.

1. Introduction Perovskite solar cells (PSCs) have made a rapid stride in the past few years with maximum power conversion efficiency (PCE) reaching more than 22%1. Although, further improvement is still expected2, this rapid increase was unprecedented for any photovoltaic (PV) material. As for silicon, GaAs, CdTe and CIGS have taken several decades to realize their full potential as a PV material3. The commercialization of a PV device to replace the singlecrystalline silicon solar cell mainly depends on three factors i.e., cost, performance and lifetime. These PSCs are highly promising in terms of first two factors, whereas the long-term stability still remains a problem. Perovskite usually represents a large class of crystalline materials, of which the ones used in solar cells are having a general structure of ABX3, where A represents an organic cation such as methylammonium (MA+) or formamidinium (FA+)4-5, B is a divalent metal6-12 like Pb2+ or Sn2+ and X is a halide such as I-, Cl- and Br-13-18. This hybrid perovskite material can be processed using different deposition techniques which include spin coating, vapor deposition19-20, dipcoating21-22, two-step interdiffusion23, blading24-25, ink-jet printing26, spray pyrolysis27 and thermal evaporation19. The enhanced PV performance of PSC can be attributed to the excellent optoelectronic properties like high absorption in the visible region, long diffusion length in the range of micrometer indicating sharp band edges and tunable bandgap with different cations, metals and halides28-30.


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Recently, Sang II Seok’s group (KRICT/UNIST, South Korea) achieved the highest certified PCE of 22.1%1. The highest efficiency PSCs are having a mesoporous TiO2 layer infiltrated with perovskite precursor solution, to form a solid perovskite after subsequent annealing. This mesoporous TiO2 has been widely employed as high surface-area electrodes, particularly in dye-sensitized solar cells (DSSCs)31-32, Quantum Dot Sensitized Solar Cells (QDSSCs)33-34 and PSCs30, 35 due to an appropriate band alignment, long electron lifetimes, large bandgap and lower fabrication cost36-39. However, the presence of large density of deep electronic traps below the conduction band (CB) of TiO2 proves to be disadvantageous for solar cell applications40-42. Indeed, the trap states induce charge recombination and hinders the charge carrier transport within the solar cell, affecting the open-circuit voltage and short-circuit current36, 43.These electronic trap states occur due to non-stoichiometry induced defects in TiO2, which has an influence on the two key parameters of a solar cell; the efficiency and photostability36,

39, 41-42

. One of the strategies to improve the power conversion efficiency of

PSCs is by enhancing the charge transfer ability of TiO2 by using substitutional dopants44-46. However, very few studies of doping mesoporous TiO2with different elements, in PSCs have been reported. Some of the dopants have reduced the charge recombination and increased the charge extraction in PSCs by minimizing the trap states below the CB30. Furthermore, the introduction of some dopants modifies the CB of TiO2 by either shifting it upward or downward, affecting the electron injection and the open-circuit voltage (Voc) within the device47. Heo et al.,48 reported the surface incorporation of Li+ ions by sintering bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) on the mesoporous TiO2resulting in a substantial improvement in PCE and reduced hysteresis. Recently, Fabrizio et al.30, utilized a similar lithium salt surface treatment for n-doping the mesoporous TiO2 achieving a higher performance PSCs with negligible hysteresis



behavior. The doping mechanism influences the properties of TiO2 in different ways, by modifying the electronic properties (recombination, charge transport and band shift) and morphological variation and it is really difficult to disentangle the effect of each of the variation on the performance of solar cells or optoelectronic devices. In this work, the mesoporous TiO2 for the first time was doped with cobalt cations in a simple and effective way, by a cobalt salt (Co-TFSI, FK209) surface treatment with a postannealing process for application in PSCs. The Co-doped TiO2 showed improved conductivity, lower charge-transfer resistance, reduced deep-level trap states and a properly matched work function which enabled efficient transportation of electrons through the mesoporous TiO2, leading to a substantial increment in average PCE compared with the undoped electrodes from 14.92% to 18.16%, with negligible hysteresis and good stability.

2. Experimental Section 2.1 Materials Titanium (IV) isopropoxide (99.999%), anhydrous N,N-dimethylformamide (DMF, 99.8%), acetonitrile (99.8%), 4-tert-butyl pyridine (TBP, 96%), chlorobenzene (95%), anhydrous ethanol (99.99%), Dimethyl Sulfoxide (DMSO, 99.9%) and lithium bis(trifluoromethyl sulphonyl)imide (Li-TFSI,95%) were purchased from Sigma Aldrich. Methylammonium iodide (MAI) and TiO2 paste (18 NR-T) were purchased from Dyesol. 2,2’,7,7’-tetrakis[N,N-di(4methoxyphenyl)amino]-9,90-spirobifluorene (Spiro-OMeTAD) was purchased from 1-Material. Lead iodide (PbI2, 99.99%) and Tris(2-(1Hpyrazol-1-yl)-4-tert-butylpyridine) cobalt (III) bis(trifluoromethylsulphonyl)imide (FK209-Co-TFSI, 99%) was purchased from Lumtec Corp. All the purchased materials were used without any further purification.


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Fluorine-doped tin oxide (FTO, MTI-TEC 15) coated glasses were etched using Zinc powder and 2M HCl diluted in de-ionized water to obtain the desired pattern. The perovskite (CH3NH3PbI3) precursor solution was prepared by dissolving equimolar ratio of MAI and PbI2 in DMF:DMSO (4:1, v/v) by stirring overnight at 70o C. To prepare the Spiro-MeOTADsolution, 80 mg of spiro-MeOTAD, 30 µl of TBP, 15 µl of Li-TFSI solution (520 mg Li-TFSI in 1 ml of acetonitrile) were dissolved in 1 ml of chlorobenzene. 2.2 Device fabrication 2.2.1 Substrate preparation The patterned FTO glasses were cleaned using detergent and de-ionized water followed by sonication with absolute ethanol, isopropanol, and absolute ethanol-acetone (50:50) for 15 min. The samples were rinsed with absolute ethanol and dried under nitrogen; followed by UVozone treatment for 30 min. A blocking layer of TiO2 (bl-TiO2, ~30 nm) was deposited on the FTO substrates by spray pyrolysis using a solution mixture of titanium (IV) isopropoxide, acetylacetone, and anhydrous ethanol. A mesoporous layer of TiO2 (meso-TiO2, 250 nm) was formed by spin-coating the TiO2 paste diluted in anhydrous ethanol (1:5, weight ratio) at 2000 rpm for 10 s. The samples were dried at 100oC for 10 min immediately after the spin coating process; afterward they were annealed at 500oC for 1 h under dry air. 2.2.2 FK209 complex treatment of meso-TiO2 layer The meso-TiO2 layer was treated with different concentration of FK209 complex dissolved in acetonitrile, by spin-coating at 3000 rpm for 10 s. It is further annealed at 450oC for 30 min under dry air inside an oven. Prior to their use, the substrates were stored in an oven kept at 50oC and humidity less than 20%. 2.2.3 Perovskite solar cell fabrication 5 ACS Paragon Plus Environment


To prepare the perovskite film, 100 µl of the perovskite solution were spin coated on mesoporous TiO2 at 5000 rpm for 40 s. During the spin coating process, 1 ml of ethoxyethane (95%, Sigma-Aldrich) was dropped instantaneously at the center of substrate, 10 s after the spinning starts as reported in our previous publication49. The whole process was executed under the intake of an air-extractor hood whose operation started a few seconds before the start of the spin coating process and was kept on until the end of the deposition process50. A dark brown perovskite film obtained was transferred to a hotplate for annealing at 100oC for 3 min forming an almost dark perovskite film. Further, 100 µl of the spiro-MeOTAD solution was spin-coated on perovskite film at 3500 rpm for 30 s. The solid hole transporting material was doped with 1.3% of FK209 complex prepared by diluting 400 mg of the Co complex in acetonitrile49. The obtained device was left overnight in a desiccator for oxidation of spiro-MeOTAD. Finally, a 70 nm thick silver top electrode was evaporated using a thermal evaporator under a vacuum of 10-6 Torr, at an evaporation rate of 0.05 nm/s to form the complete device. The entire process was carried out in ambient conditions. 2.2.4 Device measurement and characterization Scanning electron microscopy (SEM) images were obtained using a JSM-7800F microscope with an energy range of 1-5 KeV and X-Ray energy dispersive spectrometer equipped with an EDAX detector. The roughness profile of perovskite film was obtained using atomic force microscopy (AFM, Solver P47H-PRO, NT-MDT, Russia). The crystallinity studies were performed by X-Ray diffraction (XRD) with Cu Kα radiation. The absorption spectra were recorded by diffuse reflectance in the range from 400 to 900 nm using an Agilent Cary series UV-vis-NIR spectrophotometer (Cary 5000) and a 60 mm integrating sphere. Current density (JV) curves were measured using a reference 600 Gamrypotentiostat scanning from 0 to 1 V at the 6 ACS Paragon Plus Environment

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rate of 100 mV/ s. The samples were illuminated with an Oriel Sol 3A simulator for measuring the current from the device. The intensity of light was adjusted using a KG-2 filter against a NREL calibrated Si-solar cell to obtain one-sun light intensity (100 mW/cm2). Incident photonto-current conversion efficiency (IPCE) was measured with a Newport monochromator model 74125. The photoluminescence (PL) spectra of the OHP films were recorded by an ACTON Spectrapro2300iand a photomultiplier tube. The PL spectra were recorded at room temperature on a fluorescence spectrophotometer with an excited wavelength of 300 nm and 515 nm for TiO2 samples and TiO2/perovskite samples. Electrochemical impedance spectroscopy (EIS) measurements were carried out at open circuit voltage at frequencies from 100 MHz to 0.1 Hz for an irradiation intensity of one-sun light intensity (100 mW/cm2) using an Oriel Sol 3A simulator and a reference 600 Gamry potentiostat. The EIS measurements were fitted using Zview software.

3. Results and discussion



Figure 1. Surface topographic SEM and AFM images of the fabricated films: (a) and (e) for glass/FTO/meso-TiO2, (b) and (f) for glass/FTO/Co-TFSI treated-meso-TiO2, (c) and (g) for 8 ACS Paragon Plus Environment

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glass/FTO/meso-TiO2/perovskite, (d) and (h) for glass/FTO/Co-TFSI treated mesoTiO2/perovskite. The rectangular boxes indicate the region analyzed for measuring the RMS roughness of the films. The Co ions are introduced into the meso-TiO2 by spin-coating and further sintering of tris(2-(1Hpyrazol-1-yl)-4-tert-butylpyridine) cobalt (III) bis(trifluoromethylsulphonyl)imide (FK209-Co-TFSI) at 450oC for 30 min. Figure S1 shows the absorption spectrum and the molecular structure of the FK209 salt. The UV-visible absorption spectra of FK209 salt shows an almost negligible absorption band at 450-550 nm, with further increment in absorption after 350 nm which is clearly outside the solar spectrum. Also, it does not coincide with light harvesting region of perovskite making it suitable for incorporation into the TiO2, thus avoiding the competition in photogeneration ability of perovskite51. As the physical nature of component layer plays an important role in the photovoltaic performance of perovskite devices52, surface topographic SEM and AFM images were studied. Figure 1a and b represent the top-view SEM images of bare meso-TiO2 and Co-TFSI treated meso-TiO2. The mesoporous TiO2 with treatment appears to be more uniform than the bare TiO2, which seems to have irregular lumps of substances distributed all over the surface. Moreover, the treatment made the meso-TiO2 surface smoother as indicated by the root mean square (RMS) roughness of 9.514 nm compared to the RMS roughness of 14.141 nm for bare TiO2 which would act as a better foundation for growing the perovskite film (see Figure 1e and f). Also Figure 1c and d depicts the SEM image of perovskite film fabricated on top of bare meso-TiO2 and Co-TFSI treated meso-TiO2. The treated meso-TiO2 shows uniform perovskite film with a slightly larger grain size of ~50-300 nm compared to the perovskite film fabricated on bare TiO2 having a grain size of ~50-200 nm. Also, the AFM images provided in Figure 1g and h shows a reduction in RMS roughness from 15.967 nm to 13.838 nm. The compact nature of perovskite film with slightly reduced roughness



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prevents the shunting path formed due to direct contact between the hole transporting and an electron transporting material.

Figure 2. XRD patterns of a) mesoporous TiO2, b) perovskite films deposited on mesoporous TiO2with (red curve) and without (black curve) Co-TFSI interface modification. To study the crystalline nature of fabricated films, the XRD spectra of bare meso-TiO2, Co treated meso-TiO2 and the perovskite layer formed on each of the meso-TiO2 based substrates were recorded and shown in Figure 2. The spectra related to meso-TiO2 exhibits only diffraction peaks corresponding to anatase phase53. It can be observed that the intensity and width of anatase phase remain the same in both cases and no distinctive peaks of Cobalt have been identified, due to a lower concentration of the same54. The XRD pattern of perovskite film confirms the ο

tetragonal crystalline structure of perovskite and lattice parameters of a=b= 8.87 A and c= 12.65 ο

A

has been verified55.The perovskite films fabricated on top of Co-TFSI treated meso-TiO2

shows stronger peaks at 14.1o, 23.5o, 28.4o, 31.8o, 34.8o, 40.5o and 43.2o corresponding to (110), (211), (220), (310), (312), (224) and (314) when compared to perovskite films fabricated on top of bare meso-TiO2, depicting the high crystalline nature of perovskite layer. Figure S2 shows the variation of diffraction peak intensity for the diffraction planes (110), (220) and (310) of the 10 ACS Paragon Plus Environment

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perovskite. It indicates that, the perovskite sample with cobalt-treated TiO2 was having slightly higher diffraction peak intensity depicting improved crystallinity.

Figure 3. High-resolution XPS spectra showing a) Co 2p, b) O 1s and c) Ti 2ppeaks for mesoporous bare-TiO2and Co-doped TiO2.

The incorporation of cobalt ions in the meso-TiO2 was verified using X-ray photoemission spectroscopy (XPS). Figure S3. represents the XPS survey of the untreated TiO2 and Co-TFSI treated TiO2. The Co-treated TiO2 shows an additional peak of Co 2p, other than the O 1s and Ti 2p peaks. No traces of fluorine or sulfur from the Co-TFSI precursor salt were detected in the doped TiO2 samples and the C 1s peaks were used for referencing the Co-treated and the untreated samples. Figure 3 depicts the XPS spectra of Co 2P, O 1s and Ti 2p for Cotreated and untreated samples. The Co 2p3/2 and Co 2p1/2peaks were observed at 781.73 and 797.48 eV, with two satellite peaks towards higher binding energies for the cobalt-treated TiO2 (see Figure 3a)54, 56. Figure 3b shows the O 1s spectra for Co-treated TiO2 and untreated TiO2. The Co-treated TiO2 shows a slightly pronounced shoulder at a higher energy to the main peak 11 ACS Paragon Plus Environment


when compared to the untreated TiO2. The deconvolution of the obtained signal shows a small peak at 531.2 eV for the Co-treated samples, that can be assigned to the interaction of cobalt with oxygen 36, 57. The Ti 2p spectra depicted in Figure 3c, shows no difference between the treated and untreated samples. However, for a higher doping concentration of Co, the Ti 2p peak was shifted towards the lower binding energy by 0.5 eV (see Figure S4). This shift by the Co doping can be attributed to the charge transfer effect, indicating that the Co ions are intercalated within the TiO2 lattice and it does not form a separate phase58. Table S1 and S2 present the XPS summary of pristine and Co-doped TiO2. The atomic weight percentage of Ti and O in pristine TiO2 was calculated to be 31.58% and 62.49%, which indicates the presence of oxygen vacancies due to a decrease of 0.67% in the content of O according to the stoichiometric ratio of Ti and O. In Co-doped TiO2, the cobalt atom which is 2.57% replaces the Ti atoms in the lattice leaving it to 28.91% from 31.58% in pristine TiO2. This could lead to passivation of oxygen defects by replacing the Ti4+ ions with the Co3+ ions from the Co(III)-TFSI salt.


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Figure 4. (a) Cross-section SEM image of the complete device architecture, (b). Current-Voltage characteristics under AM1.5G irradiation at 100 mWcm-2and (c) IPCE and integrated current density curve of perovskite solar cells with pristine meso-TiO2 and Co-TFSI treated meso-TiO2, (d) J-V characteristic under both forward and reverse scan directions.

Further, perovskite devices with pristine and Co-doped TiO2 were fabricated to study the influence of cobalt ions on the photovoltaic performance of the devices. Figure 4a presents the cross-section scanning electron microscope (SEM) image of the complete device architecture (FTO/bl-TiO2/meso-TiO2/Co-TFSI/CH3NH3PbI3/spiro-OMeTAD/Ag. The optimized perovskite device comprises of 30 nm thick compact TiO2 layer, 250 nm thick mesoporous TiO2 layer, 270 nm thick perovskite capping layer, 150 nm thick spiro-MeOTAD layer and 60 nm Ag electrode.



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Different concentration of FK209 salt (dissolved in acetonitrile) labeled as 0 mM, 1.5 mM, 3mM, and 5 mM had been utilized to optimize the photovoltaic performance of perovskite device. Figure 4b and 4c depict the J-V and external quantum efficiency (EQE) curve for different concentration of FK209 salt used for passivating mesoporous TiO2 film. The key photovoltaic parameters for different batches are summarized in Table. 1. Table. 1 The average photovoltaic parameters of 15 devices for different concentration of FK209 treated electron transport layer. Jsc (mA cm-2)

Jsc (EQE) (mA cm-2)

0 mol%

18.92±0.5

18.78

0.97±0.03

68.02±0.05 12.47±0.43

0.1 mol%

22.01±0.3

22.4

1.015±0.01

72.53±0.02 16.20±0.28

0.3 mol%

22.89±0.4

22.9

1.020±0.01

74.12±0.02 17.30±0.33

0.5 mol%

21.67±0.6

21.7

1.002±0.03

69.3±0.04

FK209

Voc (V)

FF (%)

PCEavg (%)

(mM)

15.03±0.35

The perovskite device with pristine meso-TiO2 (without FK209 treatment) provides an average photo conversion efficiency (PCE) of 12.47% with an average current density (Jsc) of 18.92 mAcm-2, open-circuit voltage (Voc) of 0.97 V, and fill factor (FF) of 68.02. It can be observed that, FK209 treatment of meso-TiO2 dramatically increases the photovoltaic performance of perovskite devices up to 0.3 mol% after which the performance starts decreasing. Although we could observe improvements in the cell performance even with low concentration (~0.1 mol% ) of FK209, the best results were obtained with ~0.3 mol% concentration FK209 salt. An average PCE of 17.30% corresponding to an average Jsc of 22.89 mA cm-2, Voc of 14 ACS Paragon Plus Environment

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1.020 V, and FF of 74.12% was obtained. A substantial increment in Jsc and FF was observed, followed by a slight increment in Voc. The increment in Jsc, FF and Voc can be attributed to suppressed charge recombination, due to the passivation of oxygen defects. As there is no drastic change observed in the morphology of perovskite film, many of the changes from change in trap density to change in charge transfer resistance can be mainly attributed to the improvement of TiO2. If the concentration of FK209 salt is increased beyond 0.3 mol%, photovoltaic performance of the device starts degrading. A concentration of 0.5 mol% delivers a PCE of 15.03%, with a corresponding Jsc of 22.04 mA/cm2, Voc of 1.015 V and FF of 69.5. The average PCE (PCEavg) of 15 devices follows a similar trend as the PCEmax with the pristine-TiO2 giving a PCEavg of 12.47±0.43%, and the FK209 treated TiO2 samples with 16.20±0.28% (0.1 mol%), 17.30±0.33% (0.3 mol%) and 15.03±0.35% (0.5 mol%) (see Table. 1). The statistical analysis of photovoltaic parameters corresponding to perovskite devices with 0 mol% and 3 mol% doping are presented in Figure S5. The increment in PCE corresponds to an increase in Jsc, Voc, and FF. Figure 4c represents the integrated current density (Jsc(EQE)) curve as a function of wavelength, estimated by integrating the product of solar spectral curve with the measured IPCE obtained under low illumination. The integrated current density is lower than the Jsc obtained from the J-V curves for all the samples. This mismatch can be attributed to the reflection of light on the surface at lower illumination during the measurement of IPCE, while the intensity of illumination for measurement of J-V curves is considerably higher34. Fig. 4d represents the J-V curve of the best cell measured in forward and reverse scan directions with the key photovoltaic parameters given in Table. 2. The perovskite device with bare-TiO2 shows clear hysteresis, whereas the one treated with Co barely shows any hysteresis. The device based on pristine mesoTiO2 gives a PCE of 14.92% for a corresponding Jsc of 21.16 mA/cm2, Voc of 0.99 V, and FF of



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70.63% in the forward scan direction. For a reverse scan direction, the PCE changes to 13.31% with a Jsc of 20.95 mA/cm2, Voc of 0.96 V and FF of 66.20%. When Co-treated TiO2 was used, the J-V curves almost overlapped each other indicating reduced hysteresis with a PCE of 18.16% for forward scan and 17.94% for the reverse scan. A significant reduction in the hysteresis phenomenon can be attributed to the reduced trap density or space charge within the Co-treated TiO2. Table. 2 The key J-V parameters for perovskite devices with bare meso-TiO2 and meso-TiO2 treated with Co-TFSI.

TiO2 TiO2/ Co-TFSI

Scan direction

Jsc (mA/cm2)

Voc (V)

F.F (%)

PCE (%)

Forward

21.16

0.99

70.63

14.92

Reverse

20.95

0.96

66.20

13.31

Forward Reverse

23.07 23.08

1.04 1.03

75.7 75.5

18.16 17.94


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Figure 5. (a) Optical absorption spectra and (b) steady-state PL spectra of meso-TiO2 film and Co treated meso-TiO2. (c) Optical absorption spectra and (d) steady-state PL spectra of the mesoTiO2/perovskite film and Co treated meso-TiO2/perovskite film, (e) The energy-band diagram representing FTO/bl-TiO2/ Co-treated meso-TiO2/ perovskite/HTM/Ag PSC, (f) Schematic representation showing the recombination mechanism due to the presence of deep traps in undoped TiO2. To verify the increment in Jsc, Voc and FF, optical spectroscopic measurements were carried out. Fig. 5a depicts the UV-Vis absorption spectra of bare meso-TiO2 and Co-TFSI (0.3 mol%) treated meso-TiO2. It can be observed that, the treated TiO2 is having no observable absorption in the region of photovoltaic interest similar to the bare meso-TiO2. The optical absorption of treated meso-TiO2 slightly shifts towards the shorter wavelength leading to a better transmission in the shorter wavelength region. This blue shift in the optical absorption is an indication of wider energy or optical bandgap. The optical bandgap extracted from the tauc plot (αhv vs. eV) shown in Fig. S6 and S7 provide a bandgap of bare meso-TiO2 and Co-treated TiO2 as 3.61 and 3.63 eV. A significant quenching in the steady-state photoluminescence (PL) intensity of Co-treated TiO2 film indicates that, this film is more effective in the extraction of electrons due to reduced surface trap states or recombination centers59 (Fig. 5b). Further studies, (Fig. 5c) show an enhanced absorption in the range of 500-750 nm for Co-treated meso-TiO2/perovskite than the bare meso-TiO2/perovskite. However, a slight increment in absorption along the range 400-500 nm has been observed for perovskite samples with pristine TiO2 due to the presence of un-reacted PbI250.

This could be one of the reasons for the increment in Jsc. The Tauc plot measured from the absorption spectra presents no variation in the bandgap of perovskite material (~1.58 eV) (see Figure S5). Fig. 5d depicts the PL spectra of perovskite film fabricated on bare TiO2 and CoTFSI treated the substrate. The bare TiO2/perovskite film had higher PL intensity, indicating the presence of strong recombination in the sample. However, the PL intensity of treated TiO2/perovskite sample was significantly suppressed demonstrating faster charge extraction, 18 ACS Paragon Plus Environment

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before their recombination at the interface60. Hence, this Co-TFSI treated TiO2 improves the charge extraction from active material and suppresses carrier recombination at the interface due to the passivation of oxygen defects, leading to enhanced Jsc and FF. Figure 5e represents the expected energy-band diagram of the entire device: FTO/Co-doped TiO2/perovskite/spiroMeOTAD/Ag. A slight increment in the bandgap of Co-dopedTiO2 electrode was verified by the UV-vis absorption spectra and Tauc plot shown in Figures 5a and S6. Hence, the CB of the doped TiO2 electrode was believed to move closer to that of perovskite absorber, which facilitates a more efficient electron transport and collection. It also results in an increment in Voc as observed in the PV characteristics (see Table. 1). Based on the above discussion, a schematic diagram depicting the positive role of cobalt doping in mesoporous TiO2 is presented in Figure 5f; the electronic trap states due to the presence of oxygen vacancies in pristine -TiO2, results in deep energy levels below the CB of the TiO2. These energy levels act as a cascading path for the recombination of injected electrons with the holes present in the VB of the perovskite absorber and spiro-MeOTAD. The introduction of cobalt in the TiO2 lattice passivates these oxygen defects by replacing the Ti (VI) ions with the Co (III) ions, thus avoiding the recombination process leading to an enhanced PV performance with negligible hysteresis.



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Figure 6. (a) Current-Voltage curves for non-treated and Co-treated mesoporous TiO2 thin films. (b) Nyquist plot for perovskite devices with bare and Co-TFSI treated meso-TiO2with inset showing the equivalent circuit for fitting the Nyquist plot. To verify the charge transfer and extraction ability of perovskite device containing pristine and Co-doped TiO2 (0.3 mol%), SCLC and electrochemical impedance measurements were performed. The electron transport mobility of untreated and Co-treated TiO2 was investigated using space charge limited current (SCLC) measurements36,

48

. This method is

helpful in elucidating and comparing the bulk charge transport behavior of the two TiO2 layers. For the SCLC measurement, the electron-only diode devices; comprising of meso-TiO2 with and without Co-treatment coated on FTO substrates were used, to enable electron injection. The respective films were prepared by spin-coating technique followed by annealing at 500oC for 1 h. Finally, Ag was evaporated as the counter electrode on the prepared TiO2 films. Figure 6a shows the J-V curves for both the fabricated samples. Two regimes can be identified from the JV plot, the ohmic regime at lower voltages and SCLC regime at higher voltages61. The ohmic regime shows a linear dependence of current with voltage. Further increase in voltage leads to a trap-limited SCLC regime. The un-doped TiO2 shows the quadratic dependence of current with the voltage at the SCLC regime indicating the presence of traps or space charges, for e.g. Ti


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interstitials, Ti(III), oxygen vacancies, etc36, 62. Whereas, low concentration doping of cobalt (0.3 mol%) in TiO2 shows a linear dependence of current-voltage curve, indicating suppression of traps or space charges arising due to the oxygen vacancies. Also, a substantial increment in conductivity had been observed for the doped TiO2 when compared to the un-doped one. Electrochemical impedance spectroscopy (EIS) measurements were carried out to analyze the series and charge transfer resistance in both the devices. Figure 6b shows the Nyquist plot for perovskite devices with untreated TiO2 and Co-treated TiO2 under one sun illumination. The fitted EIS data for PSCs with untreated and Co-treated TiO2 are presented in Table. S3. The equivalent circuit shown in the inset of Figure 6b; the series resistance Rs mainly depends on the FTO substrate and the external wire contact, Rct represents the charge transfer resistance of the FTO/TiO2/perovskite/HTM interfaces, and C is the capacitance. In this case, only the interface between TiO2 and perovskite is modified, and hence Rct will depend on the variation of resistance at this interface. Therefore, the only semi-circle resolved from EIS spectra (shown in Figure 6b) corresponds to the charge transfer behavior between the perovskite and TiO2 interface. According to the literature, the contact resistance (Rs) can be extracted from the intercept in the x-axis of the high-frequency curve63. The values of Rs for both these devices were almost similar, indicating the negligible effect of cobalt doping on the contact resistance. However, the PSC with Co-treated TiO2 presents a lower value of Rct indicating better film coverage and improved charge transfer ability. This results in an improved charge collection efficiency of the TiO2 electrode with the cobalt doping, resulting in an enhanced Jsc, FF and PCE. Hence, it can be perceived that the Jsc increases with the increase in absorption of perovskite and improved conductivity of cobalt doped TiO2, Voc with the increase in carrier generation, proper



alignment of fermi level and reduction in recombination process and the FF with the improved charge transfer and injection characteristics. Despite the high PV efficiency provided by PSCs, the durability in terrestrial application still remains a major concern64. To assess the stability of the fabricated PSCs with cobalt-doped TiO2 we monitored them for a period of 8 days. The devices were encapsulated using a UVcurable polymer and placed in an ambient environment at 27oC and 40-50% humidity. The temporal evolution of the photovoltaic parameters is presented in Figure. S8. The PSC shows good stability by retaining 80.54% of the primary efficiency.

4. Conclusion A doping technique for preparing mesoporous TiO2 with enhanced optoelectronic properties has been demonstrated. The cobalt ions were incorporated into the lattice of mesoporous TiO2. The doping mechanism was carried out by a simple post-annealing treatment of TiO2 with cobalt salt. SEM, AFM, XRD, UV-vis, PL, XPS, SCLC and electrochemical impedance measurements were carried out to compare the performance of Co-doped and undoped TiO2 perovskite devices. This doping mechanism modified the electronic properties of TiO2 film. It also helped in passivating the electronic defect levels or space regions due to the presence of oxygen vacancies within the TiO2 leading to improved charge transportation and collection efficiency. A low level of cobalt doping (0.3 mol%) in TiO2 gave the maximum power conversion efficiency improvement (PCE) from 14.92% to 18.16%, with negligible hysteresis. This increment in the PCE corresponds to an overall increment in the current density, open-circuit voltage and fill factor. It also showed an enhanced stability by maintaining 80.54% of the initial efficiency for a span of 200 h. ASSOCIATED CONTENTS 22 ACS Paragon Plus Environment

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Supporting Information Absorption spectra and molecular structure of the FK209 salt with the solar spectrum, XPS survey of Co-doped and undoped TiO2, XPS Ti 2p spectra of Co-doped and undoped TiO2 electrodes, XPS summary of pristine and cobalt-doped TiO2, Tauc plot for band gap extraction of undoped, Co-doped TiO2 and perovskite on both the electrodes, Device performances of the polymer-encapsulated perovskite device fabricated with Co-doped TiO2 as a function of storage time, The fitted EIS data of perovskite solar cells with un-doped TiO2 and Co-doped TiO2 electrodes. AUTHOR INFORMATION Corresponding Author *

Email: [email protected], Tel: +52 (477) 441 42 00

ACKNOWLEDGMENTS We acknowledge financial support from CONACYT through grant 259192 and the CEMIE-Solar (207450) consortium projects P27 and P28. One of the authors (Siraj Sidhik) acknowledges the Doctoral fellowship from SENER-CONACYT. We also thank Dr. Sergio Calixto Carrera, Christian Albor and Carlos Juarez for technical support.

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