One-pot electrodeposition of compact layer and mesoporous scaffold

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One-pot electrodeposition of compact layer and mesoporous scaffold for perovskite solar cells Tzu-Sen Su, Yao-Shan Wu, Yung-Liang Tung, and Tzu-Chien Wei ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00566 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 2, 2018

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ACS Applied Energy Materials

One-pot electrodeposition of compact layer and mesoporous scaffold for perovskite solar cells Tzu-Sen Su†, Yao-Shan Wu‡, Yung-Liang Tung‡, Tzu-Chien Wei*†

†

Department of Chemical Engineering, National Tsing-Hua University, Taiwan

‡

Green Energy & Environment Research Laboratories, Industrial Technology Research Institute,

Hsinchu city, Taiwan.

Address: †

Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang

Fu Road, Hsin-Chu, Taiwan, 300, Republic of China ‡

Green Energy & Environment Research Laboratories, Industrial Technology Research

Institute,195, Section 4, Chung Hsing Road, Chutung, Hsinchu, Taiwan 31057, Republic of China

E-mail: [email protected] Tel: +886-35715131 ext.33669 Fax number: +886-35715408 *To whom correspondence should be addressed

1

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Abstract In this study, we report a facile method to sequentially electrodeposit (ED) TiO2 compact layer and mesoporous scaffold from a single solution. This bilayer TiO2 structure offers good controllability on the thickness and morphology by simply adjusting the depositing parameters. Currently, perovskite solar cell containing ED TiO2 bilayer exhibits similar power conversion efficiency when compared with the one using double spin-coating techniques, showing great potential to replace conventional tedious TiO2 film fabrication.

KEYWORDS: One-pot electrodeposition; Titanium dioxide; Compact layer; Mesoporous scaffold; Perovskite solar cell

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Introduction Since its pioneer work in 20091, perovskite solar cell (PSC) has developed into a promising category in emerging photovoltaics. Generally, the structure of a PSC can be classified into three types including planar type2-4, scaffold type5-7, and superstructured type8-10. Among them, scaffold type PSC holds the highest conversion efficiency of 22.1%11 to date. Moreover, scaffold type PSC also outperforms other structures in terms of stability12-13, reproducibility14-15 and hysteresis control16-17. A typical scaffold type PSC is built by a compact layer (CL), a mesoporous scaffold (MS) infiltrated with perovskite microcrystals, a perovskite capping layer, a hole transporting layer (HTL) and a metal electrode. TiO2 is reportedly a qualified material for both MS and CL, the former serves as the electron transporting layer (ETL), while the latter is for reducing the contact resistance and the interfacial charge recombination. A good ETL is a mesoporous film with favorable band position that allows electrons injecting from perovskite absorber in approximate 200-300 nm domain18-19, while an ideal CL should be a pinhole-free, fully-covered thin film within several tens of nanometers20-21. Because of different desired morphologies, the preparation methods of MS and CL are different in common. It has been reported a quality CL can be deposited using sputtering22-23, atomic layer deposition24, spray pyrolysis25-26, chemical bath deposition27 and electrodeposition (ED)28. On the other hand, the MS is almost fabricated without exception by spin-coating a binder-containing TiO2 nanoparticles paste, followed by high temperature sintering to form the mesoporous structure. To enhance the performance of the MS, a few post treatments like immersion in TiCl4 solution are 3



essential, which makes whole process tedious and hard to control. ED of TiO2 film has been demonstrated as a simple and efficient method to fabricate quality CL without the need of high temperature treatments or vacuum equipment28. More importantly, the morphology of ED film is tunable by simply altering the potential or current density. In this study, we take this advantage to realize fabricating CL and MS TiO2 layers from a single bath in a single process. In particular, by manipulating the deposition profile, a dense and thin layer was firstly deposited onto substrate at -0.6 V (versus Ag/AgCl), then the MS structure was deposited consecutively onto CL-coated FTO glass at -0.5 V (versus Ag/AgCl) from the same bath. Finally, ED CL and MS were co-sintered once to complete the fabrication of photo electrode. Figure 1 compares the processes of fabrication bilayer TiO2 using conventional spin-coating scenario (Figure 1a) and our ED scenario (Figure 1b), it can be seen that at least six the steps are involved in the conventional process, while there are only two steps for ED. Clearly, ED is a facile and energy-saving method to prepare bilayer TiO2.

Figure 1. Comparison of fabrication process of TiO2 CL and MS (a) conventional spin-coating scenario and (b) ED scenario. 4


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Results and Discussion Figure S1(a) shows the cyclic voltammgram (CV) of aqueous TiCl3 solution on a bare FTO substrate obtained at 25 mV/s from +0.2 V to -1.0 V (versus Ag/AgCl). As can be seen, the oxidation of Ti(III) starts from -0.67 V and reaches a peak potential at -0.35 V. Deposition potential exceeds -0.35 V enters diffusion controlled zone. The chronoamperometries of various deposition potentials were recorded in Figure S1(b). All curves (except -0.7 V) feature a sharp increase in current density when the reaction is initiated and a subsequent, mild decay to a steady current. This phenomenon is attributed to the overpotential (η) required for the nucleation is higher than the one for growth. The Tafel plot of ED shown in Figure 2a features three regions: At η < 0.1 V, the deposition is kinetic controlled with a linear slope of 22.5. At η > 0.2 V, the deposition is under diffusion controlled. The region between η = 0.1 to 0.2 V is the transition zone of above two regions. FESEM images of ED films deposited at -0.3V, -0.5V, -0.6V and -0.7V (versus Ag/AgCl) were also shown in Figure 2b. As can be seen, the ED film obtained from diffusion controlled zone (-0.3V) appeared a rough and dense thick film because the bottom FTO grains were fully vanished. By contrast, the thickness of ED film in kinetic controlled condition (-0.6V) is relatively thin because the bottom FTO grains is still recognizable. Accompanying by the mossy surface, the ED film obtained at -0.6 V has been demonstrated very suitable for use as the CL.28 5



Interestingly, the ED film obtained in the transition zone (-0.5V) appeared as a porous thick film; this result triggered us to apply it as the MS. Finally, the image taken from ED operation at -0.7V shows no deposits because of insufficient driving force for Ti(III) oxidation. The ED film obtained at -0.5V is of interest because if this porous structure can be used as the MS, the fabrication of CL and MS can be accomplished at once by a proper, stepwise ED profile. In addition, the chemical identity of ED film was confirmed by X-ray photoelectron spectroscopy (XPS). As shown in Figure S2, the stoichiometric ratio of Ti and O is 2, meaning the ED TiO2 has sufficient oxidation state.

Figure 2. (a) Tafel plot for the oxidation of Ti(III) to Ti(IV) oxide on FTO in various operated voltage vs. Ag/AgCl and (b) SEM images of ED TiO2 obtained at different voltages.

The deposition rates of ED-CL (at -0.6 V) and ED-MS (at -0.5 V) were calculated by dividing 6


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film thickness with depositing time. The ED-MS is thick enough so its thickness can be simply estimated from cross-sectional FESEM images (shown in Figure S3). On the other hand, the thickness of ED-CLs is estimated using a method involving optical transmittance loss from in our previous report (shown in Figure S4)28. The result shown in Figure 3 reveals a linear deposition rate of 2.85 nm/min and 27.85 nm/min for CL and MS, respectively. The resultant ED rate further enhances its competitiveness against other bottom-up deposition methods such as atomic layer deposition, which is reported approximately in several nanometers per minute29.

Figure 3. The deposition rate of ED-CL and ED-MS.

The optimal thickness of ED-MS was determined by fabricating ED-MS contained PSCs with

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different depositing time. It should be noted that the devices were purposely fabricated without CL in order to exclude the interference from CL. Shown in Figure S5 and Table 1 (Entry 1 to entry 4), PSCs fabricated using ED-MS for 2 minutes (ED-MS-2) exhibits best average short circuit current (JSC) of 18.68 mA/cm2, open circuit voltage (VOC) of 1.03 V, fill factor (FF) of 0.64 and a resultant power conversion efficiency (PCE) of 12.40%. ED-MS for 3 and 4 minutes (ED-MS-3, entry 3 and ED-MS-4, entry 4) show comparable JSC and VOC but suffer from considerable FF loss, which is believed to root from high series resistance (RS) of thick films. In entry 1, which used ED-MS-1, the FF also low and thus the PCE is only 11.81%. This result is explained by its significant different morphology, which is shown in Figure S6. ED-MS-1 appeared as a thick CL but not MS, we speculate this result relates to the nucleation on original FTO surface is faster than that on just-formed TiO2 surface, but this argument definitely warrants further study. Compare with the performance of ED-MS-2 and spin-coated scaffold (SC-MS, entry 5), the JSC, VOC and FF of ED-MS-2 employed PSC all outperformed those of PSC using SC-MS. The FESEM image of SC-MS shown in Figure S7 explains the deficiency, as it shows a porous structure constructed by fine TiO2 nanoparticles and abundant mesoporous channels. The charge extraction of SC-MS may be better than ED-MS-2 due to high contacting area, it is however in the absence of CL (like current situation), the HTL can easily penetrate and contact FTO to shunt 8


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the device. The PCE of SC-MS employed PSC is only 10.38 %. Table 1. Average IV parameters of PSCs fabricated using different combinations of CL and MS. Entry

CL

MS

JSC

VOC 2

FF

(V)

(mA/cm )

PCE (%)

1

n/a

ED-MS-1 18.76±0.29 1.033±0.015 0.602±0.023

11.81±0.65

2

n/a

ED-MS-2 18.68±0.37 1.034±0.007 0.635±0.015

12.40±0.40

3

n/a

ED-MS-3 18.62±0.18 1.041±0.060 0.629±0.020

12.19±0.47

4

n/a

ED-MS-4 18.29±0.14 1.023±0.017 0.616±0.017

11.41±0.52

5

n/a

SC-MS

17.75±0.38 0.994±0.015 0.584±0.027

10.38±0.31

6

ED-CL

SC-MS

19.50±0.28 1.056±0.012 0.724±0.017

14.94±0.35

7

ED-CL

ED-MS-2 18.79±0.13 1.044±0.010 0.724±0.018

14.20±0.37

8

SC-CL-4

SC-MS

18.89±0.22 1.033±0.016 0.641±0.030

12.80±0.67

An efficient CL in PSC can inhibit the charge recombination and lower that contact resistance at FTO surface

21, 30

. Herein, the ED-CL was obtained at -0.6 V (versus Ag/AgCl) and the

optimal depositing time is determined using CV as shown in Figure S8. The result suggests ED-CL for 250 seconds (ED-CL-250) revealed most suppressed peak current and most separate peak voltage among bare FTO, ED-CL-125 and ED-CL-500. Consequently, ED-CL-250 was chosen to integrate with ED-MS for later study.

The photovoltaic performance of PSCs fabricated in the presence of CL were also compared in Table 1 (Entry 6-8) and Figure 4. It is found ED-CL-250 improves PCE considerably no matter in the case of SC-MS (Entry 6) or ED-MS (Entry 7). Compared with the cases without CL (Entry 9



1-5), the improvement in FF is most profound, indicating CL is an inevitable component for efficient PSC. The cell performance of PSCs with SC-CL for 2000, 4000 and 6000 rpm (SC-CL-2, SC-CL-4, SC-CL-6) are shown in Figure S9. The PSCs with SC-CL-4 has appropriate thickness, resulting in higher efficiency than the others. Meanwhile the PSC equipped with ED-CL-250 (Entry 6, PCE=14.94%) outperforms the one equipped with the optimal SC-CL-4 (Entry 8, PCE=12.80%) in SC-MS employed PSCs, once again evidencing the importance of CL’s morphology. PSC fabricated using ED-CL-250 and ED-MS-2 (Entry 7) shows average performance of JSC of 18.79 mA/cm2, VOC of 1.04 V and FF of 0.724, resulting in average PCE of 14.20 % among 10 devices. Compared with PSC fabricated using the combination of common SC-CL and SC-MS (Entry 8, PCE=12.8%), the triumph of ED is obvious. It is interesting the comparison of Entry 6 (ED-CL-250 and SC-MS) and Entry 7 (ED-CL-250 and ED-MS-2) reveals a slight difference in PCE of 0.74%, which resulted from better JSC (19.50 mA/cm2) in SC-MS case while VOC and FF are almost identical. This result leads to two important conclusions. One is the individual functions of CL and MS are very clear; MS functions as the ETL for perovskite absorber, while CL functions as the blocking layer for interfacial charge recombination. Lack of CL or unqualified CL can result in low shunt resistance, which will reflect on FF in IV parameters. The second finding is the current morphology of ED-MS is not optimal, as the surface area of ED-MS needs to further increase for the purpose of 10


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extracting more current from perovskite absorber.

Figure 4. Box plots of I–V parameters of PSCs with different combinations of CL and MS.

To prove this finding, we additionally measured the steady-state photoluminescence (PL) spectra to examine the electron extraction capability. The PL was conducted on four different samples

including

FTO/ED-MS-2/perovskite,

FTO/ED-CL-250/ED-MS-2/perovskite,

FTO/ED-CL-250/SC-MS/perovskite and FTO/SC-CL-4/SC-MS/perovskite. The steady-state PL spectra shown in Figure 5 confirmed the result in IV parameters again as it shows all samples containing CL exhibit noticeable quenching on PL intensity. Moreover, ED-CL shows better PL

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intensity quenching than SC, indicating a good CL can offer more charge extraction capability. Most importantly, PL spectrum obtained from FTO/ED-CL-250/SC-MS/perovskite owns best PL intensity quenching, which is consistent with IV results depicted from best-performing devices of various TiO2 combinations (shown in Figure 6). For the hysteresis index (HI), the devices containing ED-MS have higher HI, which can be explained by lower porosity when compared with spin-coated scaffold and this result is consistent with PL results. Future investigations should optimize porosity of ED film to further improve the photovoltaic performance of PSCs.

Figure 5. Steady-state PL spectra of perovskite film on perovskite samples decorated with various CL and MS.

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Figure 6. The highest-performing PSCs with different combinations of CL and MS. The hysteresis index (HI) is defined as (Pmax(forward scan)/Pmax(backward scan))-1, which Pmax is the maximum power in I-V measurement.

Conclusions For the first time, we demonstrate that CL and MS of a PSC can be deposited from a single process using sequential ED profile in one pot of aqueous TiCl3 bath. Compared with common method, in which CL and MS are deposited separately and requiring sintering treatments in between, our finding provides a facile and controllable method to significantly shorten the fabrication process of photoelectrode, which can further strengthen the competitiveness of PSC over conventional silicon photovoltaics. This process can be improved and understood further by 13



optimizing ED potentials as well as adding additives in ED bath to increase the surface area of MS film.

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AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgments Funding for this work was provided by (Department of Industrial Technology) Ministry of Economic

Affairs,

Taiwan,

the

Ministry

of

Science

and

Technology,

Taiwan

(MOST103-2221-E-007-121-MY2) and the National Tsing-Hua University (104N2023E1).

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For Table of Contents Only

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Figure 1. Comparison of fabrication process of TiO2 CL and MS (a) conventional spin-coating scenario and (b) ED scenario. 253x83mm (150 x 150 DPI)


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Figure 2. (a) Tafel plot for the oxidation of Ti(III) to Ti(IV) oxide on FTO in various operated voltage vs. Ag/AgCl and (b) SEM images of ED TiO2 obtained at different voltages. 382x172mm (150 x 150 DPI)



Figure 3. The deposition rate of ED-CL and ED-MS. 213x149mm (150 x 150 DPI)


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Figure 4. Box plots of I–V parameters of PSCs with different combinations of CL and MS. 221x146mm (150 x 150 DPI)



Figure 5. Steady-state PL spectra of perovskite film on perovskite samples decorated with various CL and MS. 287x201mm (300 x 300 DPI)


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Figure 6. The highest-performing PSCs with different combinations of CL and MS. The hysteresis index (HI) is defined as (Pmax(forward scan)/Pmax(backward scan))-1, which Pmax is the maximum power in I-V measurement. 275x188mm (150 x 150 DPI)


One-pot electrodeposition of compact layer and mesoporous scaffold

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