Atomic Layer Deposition of TiO2 for a High

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Atomic Layer Deposition of TiO2 for High-efficiency Hole-blocking Layer in Hole-conductor-free Perovskite Solar Cells Processed in Ambient Air Hang Hu, Binghai Dong, Huating Hu, Fengxiang Chen, Mengqin Kong, Qiuping Zhang, Tianyue Luo, Li Zhao, Zhiguang Guo, Jing Li, Zuxun Xu, Shi-Min Wang, D. Eder, and Li Wan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02701 • Publication Date (Web): 24 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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ACS Applied Materials & Interfaces

Atomic

Layer

Deposition

of

TiO2

for

High-efficiency

Hole-blocking Layer in Hole-conductor-free Perovskite Solar Cells Processed in Ambient Air Hang Hua,b, Binghai Donga,b, Huating Huc, Fengxiang Chena,b, Mengqin Konga,b, Qiuping Zhanga,b, Tianyue Luoa,b, Li Zhaoa,b, Zhiguang Guoa, Jing Lia,b, Zuxun Xua,b,Shimin Wanga,b, Dominik Ederc, Li Wana,b* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China. c Institute of Physical Chemistry, University of Münster, Corrensstr. 28/30, 48149 Münster, Germany. a b

Corresponding author, Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China. E-mail addresses: [email protected] (B. Dong), [email protected] (L. Wan) 1 

ACS Paragon Plus Environment


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Atomic Layer Deposition of TiO2 for High-efficiency Hole-blocking Layer in Hole-conductor-free Perovskite Solar Cells Processed in Ambient Air

Hang Hua,b, Binghai Donga,b∗, Huating Huc, Fengxiang Chena,b, Mengqin Konga,b, Qiuping Zhanga,b, Tianyue Luoa,b, Li Zhaoa,b, Zhiguang Guoa, Jing Lia,b, Zuxun Xua,b,Shimin Wanga,b, Dominik Ederc, Li Wana,b* a

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials. Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China. c Institute of Physical Chemistry, University of Münster, Corrensstr. 28/30, 48149 Münster, Germany. b

ABSTRACT In this study we design and construct high-efficiency, low-cost, highly stable, hole-conductor-free, solid-state perovskite solar cells, with TiO2 as the electron transport layer (ETL) and carbon as the hole collection layer, in ambient air. Firstly, uniform, pinhole-free TiO2 films of various thicknesses were deposited on fluorine-doped tin oxide (FTO) electrodes by atomic layer deposition (ALD) technology. Based on these TiO2 films, a series of hole-conductor-free perovskite solar cells (PSCs) with carbon as the counter electrode was fabricated in ambient air and the effect of thickness of TiO2 compact film on the device performance was investigated in detail. It was found that the performance of PSCs depends on the thickness of the compact layer due to the difference in surface roughness, transmittance, charge transport resistance, electron-hole recombination rate, and the charge lifetime. The best-performance devices based on optimised TiO2 compact film (by 2000 cycles ALD) can achieve power conversion efficiencies (PCE) of as high as 7.82 %. Furthermore, they can maintain over 96 % of their initial PCE after ∗

Corresponding author, Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China. E-mail addresses: [email protected] (B. Dong), [email protected] (L. Wan)


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651 hours’ (about 1 month) storage in ambient air, thus exhibiting excellent long-term stability.

KEYWORDS atomic

layer deposition; hole-conductor-free perovskite solar

cells;

hole-blocking layer; carbon counter electrode; long-term stability

1. Introduction Since the introduction of organometal halide perovskite into liquid dye-sensitised solar cells (DSSC) in 20091, perovskite solar cells (PSCs) have boomed as a new-generation, promising photovoltaic device for clean energy conversion due to their high efficiency and low cost2. However, to realise commercial applications scientists still need to develop PSCs with higher efficiencies, enhanced stability and lower manufacturing cost. Thus, efforts have been made to improve the performance of PSCs, such as improvements of device architecture3-5, control of morphology and crystal growth6-9, optimisations of interface manipulation10,11. These efforts have improved PSC’s potential for use as cost-effective, high-efficiency solar cells. Generally, in most cases, typical PSCs are fabricated with very expensive organic/polymeric hole-transporting materials (HTM) such as Spiro-OMeTAD or poly(3-hexylthiophene) (P3HT), and noble metals (usually Au or Ag) acting as counter electrodes12,13. Besides, these PCSs require rigorous conditions (e.g. a high vacuum) for the deposition of Au or Ag to fabricate counter electrodes and for the assembly of PSC devices which significantly increase manufacturing costs. To reduce the overall cost of such devices, it is worth developing hole-conductor-free (HTM-free) PSCs assembled in normal conditions (ambient air) and replacing the noble metal with other available candidates. In hole-conductor-free PSCs, the use of an expensive hole-transporting layer (HTL) can be avoided14,15. Recently due to its low cost, considerable stability, and abundance, carbon materials have emerged as promising alternatives to noble metals (Au or Ag) as highly efficient counter



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electrodes (CE) in PSCs16-24. It’s has been confirmed25 that, in PSCs, the photo-generated charge carriers (electrons and holes) are both formed in the organometal halide perovskite layer which could also transport carriers with diffusion lengths exceeding 1 µm. It should be mentioned that electron-hole recombination within the device is one of the major known loss factors decreasing the open-circuit potential. In typical mesoporous-structured PSCs, the photoanode is comprised of a compact layer and a mesoporous layer. The compact layer (or hole-blocking layer) is able to transport, and collect, photo-generated electrons and can also prevent holes formed in the perovskite or HTL layer from reaching the fluorine-doped tin oxide (FTO) electrode to avoid a short-circuit of the cell. Therefore, the compact layer (hole-blocking layer) plays a crucial role in PSCs. TiO2 film with a thickness of dozens of nanometres in a compact layer has been used to inhibit electron-hole recombination and avoid direct contact between the perovskite layer and the FTO electrode26. In general the TiO2 compact layer was prepared by various conventional methods including: spin-coating27, spray pyrolysis28,29, electrochemical deposition30,

and

thermal

oxidation31,32.

Usually

in

these

methods,

high-temperature (> 450 ℃) thermal post-treatments are needed. Thus, on one hand, it greatly increases the fabrication cost and presents drawbacks in view of industrial scale roll-to-roll fabrication33 due to a lack of compatibility with flexible substrates; on another hand it can increase the occurrence of pin-holes in the TiO2 compact film34, which impaired the performance of PSCs35. In addition it is generally more challenging to precisely control the thickness of a compact layer film at the atomic to nanometric scale (e.g., from 10 nm down to 0.5 Å) when using the aforementioned methods. Compared with conventional techniques for preparation of the compact layer, the ALD technique has additional advantages of being a low-temperature process, offering precise thickness controllability, and excellent film conformity and uniformity36. Hence, ALD is capable of depositing homogeneous pin-hole free


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TiO2 thin-film on the photoanode of perovskite solar cells as it involves the chemical reactions of volatile, metal and oxidizing precursors at two separate stages where no sintering is needed after the deposition of blocking overlayer. In this work TiO2 thin films, of different thickness, were deposited on an FTO-coated glass substrate by ALD technique. Then we directly assembled them into a series of PSCs in ambient air with a mesoporous structure of Glass-FTO/Compact TiO2/Mesporous TiO2-CH3NH3PbI3/C and compared their photovoltaic performances. Based on these TiO2 compact films in the PSCs, a stable performance at over 96 % of the initial PCE was accomplished in the devices after 651 h. It was found that the ALD-TiO2 film was sufficient to serve as a high-efficiency hole-blocking layer and electron transport layer in the PSC leading to a PCE of 7.82 %.

2. Experimental work The ALD-TiO2 coating process was carried out using a Savannah S100 ALD reactor (Savannah System, Cambridge NanoTech. Inc., USA) equipped with a gas-flow system. TiO2 was deposited at 150 ºC using titanium (IV) isopropoxide (TTIP) and H2O as Ti and O precursors, respectively. High-purity nitrogen (N2, 99.999 %) was used as both the purge gas and carrier gas for both precursors. A steady flow of N2 at 50 sccm (standard cubic centimetres per minute) was used in the ALD process. Before ALD processing, the FTO/Glass substrates (sheet resistance 14 Ω cm-1) were cleaned by ultrasonication in acetone, ethanol, and deionised water, successively. TTIP was pulsed into the chamber for 0.2 s, after which water vapour was pulsed into the chamber for 0.05 s. For two half-deposition processes, the FTO/Glass substrates were exposed to vapour for 8 s, and the reactors were then purged with N2 for 35 s. The ALD cycling was repeated an appropriate number of times to obtain systematic variation in the coating thicknesses (as shown in Figure 1). On the top surface of ALD-TiO2 film, the mesoporous TiO2 layer was



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deposited by spin coating at 7000 r.p.m. for 60 s using a commercial TiO2 paste (average particle size, c. 20 nm) diluted in ethanol, and then sintered in air at 550 °C for 60 min. After cooling to room temperature, the perovskite layer was then deposited by spin coating at 4000 r.p.m. for 60 s using CH3NH3PbI3 liquid (dissolved in dimethyl formamide (DMF), at about 40% solid content, Shanghai MaterWin New Materials Co., Ltd). The monolithic cell fabrication is completed after a back-contact layer of carbon (JELCON CH-8 carbon ink, sheet resistance 10 Ω cm-1, JUJO Printing Supplies & Technology (Pinghu) Co., Ltd, China) forming the CE was fabricated by a blade-coating method on top of the device. The cross-section of the devices, and the top views of ALD-TiO2 films, were imaged by a field-emission scanning electron microscope (FE-SEM, Jeol JEM 6510LV) and a atomic force microscope (AFM, Nanoscope-IIIa). The transmission spectra were obtained by a UV-vis spectrophotometer (UV-3600, Shimadzu). The X-ray diffraction (XRD) data of the ALD-TiO2 film, after annealing, were collected with a Bruker-AXS D8 Advance. Current-voltage (I-V) characteristics of the devices were measured in the dark and under simulated AM 1.5G (100 mW cm-2 irradiance) respectively using a solar simulator (Oriel, model 91192-1000) and a source meter (Keithley 2400, USA). Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (Zennium, IM6, Germany) over the frequency range of 10 mHz to 2 MHz under simulated AM 1.5G (100 mW cm-2 irradiance).

3. Results and discussion Figures 2(a) and 2(b) show the surface morphology of an examplatory compact TiO2 layer fabricated on FTO substrates using ALD with, and without, annealing respectively. The film is highly compact and resembles the bumpyness of the underlying FTO substrate37,38. This can be explained by the nature of ALD, which is a surface sensitive process that creates self-limited layer-by-layer films. A similar morphological analysis for an atomic layer


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deposited TiO2 layer has been observed by Wu Y et al.37. Figures 2(a) and 2(b) reveal that the small TiO2 crystalline grains with size of tens of nanometres are closely conjoined: no nanoscale pinholes are observed. These signify that the surface coverage of ALD-TiO2 compact layer was particularly high. For comparison, the film was exposed to a typical annealing treatment; as shown in Figure 2(b), the morphology and uniformity of ALD-TiO2 film were not disrupted, indicating the high thermal stability of the ALD-TiO2 compact layer. An XRD pattern of an ALD-TiO2 film after annealing treatment is shown in Figure 2(c). The (010), (004), (200), (015), (211), and (204) planes are included, confirming the anatase phase of the TiO2. One focus of this work was to improve the cell efficiency and performance by finding the optimum thickness of the TiO2 compact films. Hence, we have used different number of ALD cycles (ranging from 200 to 4000) to tune the thickness of the ALD-TiO2 film and measured the corresponding cross-section of each FE-SEM image to determine the thickness of various ALD-TiO2 layers. As shown statistically in Figure 2(d), the thickness of the ALD-TiO2 film linearly increases with the number of cycles. The corresponding thicknesses of each film prepared by 200, 800, 1200, 1600, 2000, 4000 ALD cycles, were 16 nm, 68 nm, 106 nm, 145 nm, 181 nm, and 368 nm, respectively. We fitted an equation to the data in Figure 2(d) to show the relationship between the number of ALD cycles (X) and the thickness of ALD-TiO2 film (nm) (Y): Y = 0.091X – 2.41. From this equation, the thickness was calculated with a deposition rate of 0.9 Å per cycle. Figures 3(a) to 3(l) show typical AFM images of the pure FTO substrate and the ALD-TiO2 films for the different ALD cycles. The root mean square roughnesses (Rrms) of pure FTO substrate and each ALD-TiO2 film was calculated with 11.93 nm (FTO), 20.74 nm (800 cycles), 31.82 nm (1200 cycles), 24.80 nm (1600 cycles), 25.09 nm (2000 cycles), and 23.80 nm (4000 cycles), respectively. Dependence of Rrms of the ALD-TiO2 compact layer on different ALD cycles was shown in Figure 4(a). The surface roughnesses of all



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the ALD-TiO2 films were thus slightly bumpier than that of bare FTO substrate and increase with TiO2 film thickness, due to the growing TiO2 grains, thus creating continuously denser films. Theoretically speaking, the filling of bumpy surface by TiO2 nanoparticles on the last layer could affect the surface roughness of the next layer. The ALD-TiO2 films deposited at 1200 cycles (106 nm) reached a maximum value of Rrms. The illuminated light would be scattered strongly by the ALD-TiO2 films with rougher surface39, resulting in a great loss of light energy when passing through to the perovskite layer. Another criteria for the cell efficiency is the transparency of the TiO2 compact film. Figure 4(b) shows the transmittance spectra of the pure FTO and the ALD-TiO2 films with varying thickness. The pure FTO had over 98 % transmittance in the visible region (450 nm to 900 nm wavelength). For comparison, the transmittance decreased slightly with the introduction of the ALD-TiO2 films in direct correlation with the film thickness, yet still retained values above 92%. Only the thickest film reduced the transmittance to about 88 %. On one hand, for the device based on 1200 cycles ALD-TiO2 film, although the transmittance was high, the light scattering was also strong. On the other hand, the device based on 4000 cycles ALD-TiO2 film, had lowest transmittance. The above results indicated that the high transmittance and the weak light scattering of the ALD-TiO2 films (800 cycles, 1600 cycles, 2000 cycles) was sufficiently increase the utilization efficiency of illuminated light38, allowing for increased absorbance of light by the perovskite layer. Figure 5 shows the device structure and energy level diagram of the PSC. This PSC include a conductive Glass/FTO substrate, a compact layer, a perovskite-sensitised TiO2 mesoporous layer, and a carbon counter electrode. The actual images of ALD-TiO2 film on the top of a FTO/Glass substrate and the whole device are shown in Figures 6(b) and 6(c), respectively. After TiO2 was deposited by ALD on the bare FTO/Glass substrates, the as-formed TiO2/FTO/Glass films still show excellent transmission of light. Without any HTL in the device, the TiO2 compact film fabricated by ALD plays a crucial role


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in collecting, and transporting, the electrons, and effectively inhibiting the electron-hole recombination. The energy levels for the conduction band edges of TiO2, CH3NH3PbI3, are at -4.1, -3.9 eV, and as for the valence band edges they are at 7.3, -5.43 eV, respectively. Meanwhile the Fermi level of carbon is at -5.0 eV which is slightly higher than the energy levels of valence band edge of CH3NH3PbI3 (-5.43 eV). The good matching of band structure makes electron-hole separation available in the perovskite layer. Furthermore, to reduce costs, we assembled a group of PSCs based on free HTL and a carbon counter electrode, which will lead to a relatively lower fill factor (FF). The cross-sectional SEM image of the device is shown in Figure 6(a). The total thickness of the TiO2 mesoporous layer filled with the perovskite and capping layer is about 3 µm. The thicker top layer, with a thickness of about 10 µm, is the carbon counter electrode. Photovoltaic performance of the corresponding PSC devices were tested under AM 1.5G irradiation (100 mW cm-2) and in the dark, respectively. All of the devices were fabricated under ambient air at a humidity of about 55% and then stored in a drying oven (dry air environment, no sealing) for about 12 h. The J-V characteristics of these PSCs based on various TiO2 compact layers fabricated by different ALD cycles are shown in Figure 7(a), and the corresponding photovoltaic parameters are listed in Table 1. The devices based on the ALD-TiO2 films at low ALD cycles (e.g. at 200 cycles and 750 cycles) had low PCEs (3.21 %, 3.54 %, respectively, as shown in Table S1) The device at 800 cycles with the ALD-TiO2 compact layer had: Voc, Jsc, FF, respectively, of 856 mV, 12.74 mA cm-2, and 43.99 %, leading to a PCE of 4.85 %. With 1200 cycles applied to the ALD-TiO2 compact layer, the PCE of the device increased to 6.06 %. As the number of ALD cycles increased to 1600, PCE reached 7.03 %. The improvement mainly came from the increase in Voc (by around 100 mV, i.e., from 856 mV to 955 mV)) and Jsc (from 12.74 mA cm-2 to 18.30 mA cm-2). Furthermore, on increasing the number of ALD cycles to 2000, Voc and Jsc slightly increased by 10 mV and 1.23 mA cm-2,



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respectively, leading to a best-performance device with a PCE of as high as 7.82 %. However, as the number of ALD cycles increased to 4000, PCE decreased to 5.61 %, which was attributed to the decreases in Voc (by around 70 mV, i.e., from 960 mV to 895 mV) and Jsc (by around 5.09 mA cm-2, i.e., from 19.53 mA cm-2 to 14.44 mA cm-2) compared to the best-performance device. As shown in Figure 7(b), the onset of dark current emerges at about 600 mV in the J-V curve of PSC based on an 800 cycles ALD-TiO2 compact layer measured in the dark. On increasing the thickness, the onset shifts to over 650 mV and the current value decreases rapidly at the same applied bias in the dark, indicating a decrease in back electron-hole recombination from the conduction band of TiO2 and FTO/Glass to the HTL or perovskite layer. To authenticate the performance reproducibility of the devices, 12 separate devices (the same batch of samples) based on an optimised TiO2 compact film (2000 cycles ALD, 181 nm) were fabricated under the same experimental conditions. The distributions of photovoltaic parameters for them under simulated AM 1.5G (100 mW cm-2 irradiance) are shown in Figure 8 and Table S2. It can be found that all of them had a PCE of over 6.5 %, and 83.3 % of the devices had a PCE greater than 7.0 %, indicating their good reproducibility. Meanwhile, 83.3 % of the devices had a Voc of over 900 mV, and 83.3 % of them had a Jsc of above 18 mA cm-2, but their FF was poor, at less than 45%. Importantly, the PCE decay measurements further revealed that the ALD-TiO2 compact layer played a vital role in the stability of the devices assembled in ambient air. As shown in Figure 9, under the same testing conditions, the devices based on 1200 cycles of the ALD-TiO2 compact layer were measured immediately, and 651 h later, respectively. All the PSCs exhibited a PCE decay feature with parallel trends. To our surprise, Jsc and FF reduced slightly leading an approximately 3 % drop in the PCE of the device even after 651 h (the PCE still reached 5.87 %) indicating the excellent long-term stability of the as-fabricated PSCs.


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To investigate the charge transport in the as-fabricated hole-conductor-free PSCs devices based on ALD-TiO2 as the compact film, electrochemical impedance spectroscopy (EIS) measurements were performed over the frequency range of 10 mHz to 2 MHz at a 0.8 V bias under simulated AM 1.5G (100 mW cm-2 irradiance). In a typical mesoporous PSC device40, the device is constructed to an architecture of TiO2/perovskite/HTM/CE and thus there are three contact interfaces: TiO2/perovskite, perovskite/HTM, and HTM/CE, respectively. In our study, due to the absence of HTM layer, the devices comprised a simplified structure of TiO2/CH3NH3PbI3/C. As a result, two interfaces, i.e., TiO2/perovskite and perovskite/C, had to be taken into account. In this study perovskite CH3NH3PbI3 acts not only as a light absorber but also a hole conductor. We used a circuit model (the inset of Figure 10 (a)) consisting of two parallel resistive/capacitive (RC) elements connected in series with a series resistance (Rs) to obtain a good fit of the experimental data; the fitted parameters are listed in Table 2. It is well-known that the Rs value has a deleterious influence on FF of the device41. The devices based on 1200 cycles ALD-TiO2 layer had the lowest Rs of 49.46 Ω, in good agreement with the highest value of FF (47.67%), and other devices had much higher Rs with relatively lower FF (Figure 10 (c)). As shown in Figure 10 (a), the first arc at the high-frequency region represents the charge transfer resistance (Rtr) at the perovskite/C interface, while the second arc at the low-frequency region corresponds to the recombination resistance (Rrec) at the perovskite/TiO2 interface42. On increasing the number of ALD cycles from 800 to 4000 (the thickness of the TiO2 compact layer increased from 68 nm to 368 nm), the diameter of both arcs increased significantly, indicating that the value of Rtr at the perovskite/C interface, and that of Rrec at the perovskite/TiO2 interface, increased with the increase in thickness of the ALD-TiO2 compact layer. The devices with thinner ALD-TiO2 layer (from 800 cycles to 2000 cycles) had small values of Rtr from 199.92 Ω to 475.83 Ω. As the ALD-TiO2 film became thicker (4000 cycles), the corresponding values of Rtr increased to 1232.18 Ω.



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Probably, a thicker ALD-TiO2 film could weaken the attachment of the perovskite to the C counter electrode interface, thus inhibiting injection or extraction of photo-generated holes at the perovskite/C interface. Furthermore with the ALD-TiO2 compact layer being thicker (from 800 cycles to 2000 cycles), the increase in Rrec (from 161.64 Ω to 736.40 Ω) can reduce the electron-hole recombination probability at the perovskite/TiO2 interface, resulting in the increase in Voc of the PSCs from 856 mV to 965 mV (Figure 10 (d)). However, the device based on 4000 cycles ALD-TiO2 compact layer, had a large Rrec (3473.60 Ω) and a large Rtr (1232.18 Ω), which the Rtr was dominant

in

the

structure

of

TiO2/CH3NH3PbI3/C,

thus

resulting

in

comparatively lower Voc. Figure 10 (b) shows the corresponding Bode plots of the PSCs based on different thicknesses of ALD-TiO2 compact layer under simulated AM 1.5G (100 mW cm-2 irradiance). From these, the electron lifetimes τ are calculated (Table 2). It was found that, upon increasing the thickness of the ALD-TiO2 film (from 800 cycles to 2000 cycles), τ was incresed, but reduced at 4000 cycles. It was found that the device based on 2000 cycles of the ALD-TiO2 compact layer had the longest lifetime, i.e., as long as 782.60 ms which matched the J-V characteristics (Figure 10 (c)) where the PSCs based on 2000 cycles of the ALD-TiO2 compact layer had the highest PCE of 7.82 %. Integrating all the results, the best performance for the device based on 2000 cycles of the ALD-TiO2 compact layer, is mainly attributed to three reasons. First, the ALD-TiO2 film had better light utilization efficiency, owing to their lower surface roughness and high transmittance. Secondly, the device reached lower recombination rate due to the high Rrec. Finally, the device had the longest charge lifetime.

4. Conclusions In summary, we have developed HTM-free PSCs with high efficiency, low cost, and excellent long-term stability in an ambient atmosphere where


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thickness-controllable TiO2 forms the compact layer and cheap carbon forms the counter electrode. Detailed investigations with EIS, J-V, AFM, UV-Vis and other techniques revealed that the performance of PSCs depends on the thickness of the compact layer due to the difference in surface roughness, transmittance, charge transport resistance, electron-hole recombination rate, and the charge lifetime. The method developed here paves the way for the fabrication of low-cost, high-stability, PSCs for practical applications in the future. Acknowledgments This work was supported by the Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20114208110004), and the National Natural Science Foundation of China (Grant No. 21402045 and 51102087). This work was also financially supported by the Program for Middle-aged and Young Talents from Educational Commission of Hubei Province (Grant No. Q20120103), Natural Science Foundation of Hubei Province of China (Grant No. 2014CFB167 and 2015CFA118) and the Wuhan Science and Technology Bureau of Hubei Province of China (Grant No. 2013010501010140).



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Table 1 Summarized device parameters of PSCs based on 800 to 2000 cycles of the ALD-TiO2 compact layer. The corresponding parameters indicated are open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and power conversion efficiencies (PCE). Samples Voc(mV) Jsc(mA cm-2) FF(%) PCE(%) 800cycles TiO2 856 12.74 43.99 4.85 (68 nm) 1200cycles TiO2 895 14.19 47.67 6.06 (106 nm) 1600cycles TiO2 955 18.30 40.22 7.03 (145 nm) 2000cycles TiO2 965 19.53 41.47 7.82 (181 nm) 4000cycles TiO2 895 14.44 43.41 5.61 (368 nm)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2 Summary of electrochemical impedance spectroscopy parameters of PSCs based on 800 to 2000 cycles of the ALD-TiO2 compact layer. The corresponding parameters indicated are series resistance (Rs), charge transfer resistance (Rtr), recombination resistance (Rrec), and electron lifetime (τ). Samples Rs(Ω) Rtr(Ω) Rrec(Ω) τ(ms) 800cycles TiO2 93.62 199.92 161.64 685.66 (68 nm) 1200cycles TiO2 49.46 185.88 285.28 723.50 (106 nm) 1600cycles TiO2 154.53 374.23 386.91 732.53 (145 nm) 2000cycles TiO2 76.07 475.83 736.40 782.60 (181 nm) 4000cycles TiO2 79.32 1232.18 3473.60 372.72 (368 nm)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1

-OH + TTIP

CH(CH3)2OH

N2 purge CH(CH3)2OH

+ H2O

H C O

-OH + repeate TTIP

ALD Cycling

Ti CH(CH3)2OH

N2 purge

Figure 1. Schematic of a TiO2 ALD cycle on the top surface of an FTO/Glass substrate using TTIP and H2O precursors.



Figure 2 (b)

(a)

(204)

(200)

(004)

(105) (211)

(101)

Thickness of ALD-TiO 2 film (nm)

400

(C)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(d)

350 300 250 200 150 100 50 0

10

20

30

40

50

60

Two Theta (degree)

70

80

0

400 800 1200 1600 2000 2400 2800 3200 3600 4000

ALD cycle number

Figure 2. Top-view FE-SEM images of 1600 cycles ALD-TiO2 films fabricated on FTO electrodes by ALD (a) without annealing a nd (b) after annealing. (c) XRD pattern of ALD-TiO2 film after annealing. (d) Statistical analysis of the thickness of ALD-TiO2 films and changes in ALD cycle numbers. An equation was fitted to show the relationship between the ALD cycle number (X) and the thickness of ALD-TiO2 film (nm) (Y): Y = 0.091X – 2.41. The deposition rate was assumed to be 0.9 Å per cycle.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 (b)

(a)

800 cycles Rrms=20.74 nm

FTO Rrms=11.93 nm

(c)


(g)

(h)

(i)

(d)

(e)

(f)




(j)

(k)

(l)

Figure 3. (a) to (f) AFM 2D topography and (g) to (l) 3D views of the pure FTO substrate and the ALD-TiO2 films for the different ALD cycles.



Figure 4 35

100

(a) 30

(b)

90

Transmittance (%)

Rrms (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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25

20

80

FTO 800cycles TiO2 1200cycles TiO2

70

16000cycles TiO2 2000cycles TiO2

15

4000cycles TiO2

60 10 0

1000

2000

3000

ALD cycle number

4000

300

400

500

600

700

800

900

Wavelength (nm)

Figure 4. (a) Dependence of R rms of ALD-TiO2 compact layer on different ALD cycles. (b) Corresponding transmittance of various thickness of the ALD-TiO2 films on FTO/Glass substrates.


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Figure 5

(a)

(b) C electrode

-4.1eV

Perovskite

TiO2 mesoporous layer TiOCompact layer layer 2 compact

FTO

-3.93eV

CH3NH3PbI3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO2

C -5.0eV

-5.43eV

Glass -7.3eV

Figure 5. (a) The device structure and (b) energy level diagram of the perovskite solar cell.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6 (a)

(b)

C

(c) Mesoporous TiO2/CH3NH3PbI3 ALD-TiO2 FTO Glass

Figure 6. (a) Cross-sectional FE-SEM image of the PSC, (b) an actual image of a ALD-TiO2 film on the top of a FTO/Glass substrate and (c) an actual image of a PSC.


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Figure 7 (a) -2

Current Density (mA cm )

-2

15

10 800cycles TiO2 5

1200cycles TiO2 1600cycles TiO2 2000cycles TiO2

0 0.0

(b)

0.00

20


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.04 -0.08

-0.16 -0.20


0.4

-0.12

-0.16


-0.20

-0.24

-0.24

4000cycles TiO2 0.2

-0.08

800cycles TiO2 -0.12

-0.28 0.85

0.6

0.8

1.0

-0.28 0.0

0.2

0.4

0.86

0.87

0.88

0.6

0.89

0.90

0.8

1.0

Voltage (V)

Voltage (V)

Figure 7. J-V characteristics of PSCs based on different thickness of ALD-TiO2 compact layer under (a) simulated AM 1.5G (100 mW cm-2 irradiance) and (b) in the dark.



1000

6

800 600

4 400 200

0 40

0 20

30

16 12

20

8

10 0

4 0

2

4

6

8

10

12 0

2

4

6

8

10

12

-2

2

Voc (mV)

8

Jsc (mA cm )

PCE (%)

Figure 8

FF (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

Device number

Figure 8. Histograms of photovoltaic parameters for 12 devices (the same batch of samples) based on 2000 cycles compact TiO2 layer under simulated AM 1.5G (100 mW cm-2 irradiance).


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Figure 9 15 a-1200cycles TiO2 -2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b-1200cycles TiO2-after 651h 10

5 Jsc 0

Voc

FF

PCE

a 14.19 895 47.67 6.06 b 13.96 915 45.78 5.87

-5 0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 9. J-V characteristics of PSCs based on 1200 cycles of ALD-TiO2 compact layer under simulated AM 1.5G (100 mW cm-2 irradiance), which measured immediately and 651 h later, respectively.



Figure 10 -100

Z'' (ohm)

-80

800cycles TiO2

-50

1200cycles TiO2

-40

1600cycles TiO2

-30

-70

2000cycles TiO2

-20

-60

4000cycles TiO2

-10

(a)

1200cyclesTiO2 1600cyclesTiO2

-40

-50

0 0

200

400

600

800

1000

R tr

-40

(b)

800cyclesTiO2

-50

Phase (deg)

-90

1200

1400

R rec

Rs

-30 C tr

-20

2000cyclesTiO2 4000cyclesTiO2

-30

-20

C rec -10

-10 0 1000

2000

3000

4000

0 0.01

5000

Z' (ohm) 50

0.1

1

10

100

1000

10000 100000 1000000

Frequency (Hz)

(c)

400 1000

350

45

(d)

10

4500

1500

4000

900

300

3500

1200

8

150

30

Voc (mV)

200

Rs (ohm)

FF Rs

35

800

700

600

3000 2500

6

2000 4

Voc PCE τ Rrec

100 25 20 800

1200 1600 2000 2400 2800 3200 3600 4000

50

500

2

0

400

0

Rrec (ohm)

250

PCE (%)

40

900

1500 1000

600

500 0 800

1200

ALD cycle number

1600

2000

2400

2800

3200

3600

4000

ALD cycle number

Figure 10. (a) Nyquist curves, (b) Bode curves, (c) FF, Rs plots, and (d) PCE, Voc , Rrec , τ plots of the PSCs based on different thickness of ALD-TiO2 compact layers under simulated AM 1.5G (100 mW cm-2 irradiance) with a bias of 0.8 V.


300

τ (ms)

0

FF (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Atomic Layer Deposition of TiO2 for a High

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