Novel Combination of Efficient Perovskite Solar ... - ACS Publications

Subscriber access provided by UNIV LAVAL

Article

Novel Combination of Efficient Perovskite Solar Cells with Low Temperature Processed Compact TiO2 Layer Via Anodic Oxidation Yang Yang Du, Hongkun Cai, Hongbin Wen, Yuxiang Wu, Like Huang, Jian Ni, Juan Li, and Jianjun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02706 • Publication Date (Web): 06 May 2016 Downloaded from http://pubs.acs.org on May 9, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 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

ACS Applied Materials & Interfaces

Novel Combination of Efficient Perovskite Solar Cells with Low Temperature Processed Compact TiO2 Layer Via Anodic Oxidation Yangyang Du, Hongkun Cai, * Hongbin Wen, Yuxiang Wu, Like Huang, Jian Ni, Juan Li, Jianjun Zhang * College of Electronic Information and Optical Engineering, Nankai University, Tianjin, 300071, China

ABSTRACT: In this work, a facile and low temperature processed anodic oxidation approach is proposed for fabricating compact and homogeneous titanium dioxide film (AO-TiO2). In order to realize morphology and thickness control of AO-TiO2, the theory concerning anodic oxidation (AO) is unveiled and the influence of relevant parameters during the process of AO such as electrolyte ingredient and oxidation voltage on AO-TiO2 formation is observed as well. Meanwhile, we demonstrate that the planar perovskite solar cells (p-PSCs) fabricated in ambient air and utilizing optimized AO-TiO2 as electron transport layer (ETL) can deliver repeatable power conversion efficiency (PCE) over 13%, which possess superior open-circuit voltage (Voc) and higher fill factor (FF) compared to its counterpart utilizing conventional high temperature processed compact TiO2 (c-TiO2) as ETL. Through a further comparative study, it is indicated that the improvement of device performance should be attributed to more effective electron collection from perovskite layer to AO-TiO2 and the decrease of device series resistance. Furthermore, hysteresis effect about current density-voltage (J-V) curves in TiO2-based p-PSCs is also unveiled. KEYWORDS: Planar perovskite solar cells (p-PSCs), anodic oxidation (AO), AO-TiO2, low temperature, morphology. █

INTRODUCTION Perovskite solar cells (PSCs) have excited increasing research momentum and are considered to

1 ACS Paragon Plus Environment


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

Page 2 of 20

be a breath of fresh air in the emerging photovoltaic landscape due to their long carrier diffusion length (100-1000 nm),

1

high absorption coefficient,

2

tunable optical band gaps,

3

available

application for flexible solar cells, 4,5 as well as potential low manufacturing cost. 4 Till now, the studies of PSCs have witnessed tremendous breakthroughs in rapid efficiencies increase (from 6.5% in 2011 to 20.1% in 2015).6,7 Very recently, certified efficiency has reached 22.1% by NREL, approaching that of commercialized 2nd generation technologies such as polycrystalline silicon. Meanwhile, the PSCs architecture have been proven versatile that can be composed of either planar or planar/mesoporous hybrid architecture. And both device architectures can exhibit comparable efficiency over 18%.7-9 In those devices, a compact TiO2 (c-TiO2) acting as ETL plays a critical role for achieving optimized device performance, which can selectively collect electrons from perovskite layer and block holes at the same time to reduce carrier recombination. 8,10 Therefore, a plethora of preparation methods regarding c-TiO2 have been proposed in most literature reports, including spray pyrolysis, 11 sol-gel, 12 direct high temperature thermal oxidation, 13

sputtering, 14 and atomic layer deposition (ALD), 5 etc.. However, in most case, conventional

methods for c-TiO2 often require high temperature annealing above 500℃ that it not only increases fabrication cost but impedes the application on flexible substrates for PSCs.

11,12

Moreover, high temperature processed c-TiO2 often cause rich oxygen vacancy in film surface. This can lead to strong interfacial polarization or static dipole moment when c-TiO2 contact with perovskite layer, and thereby not facilitating electron collection. 15 Consequently, to address those problems and achieve higher module efficiency, interfacial modification is frequently needed for p-PSCs using TiO2 as ETL. 16 In this article, a facile and low temperature processed anodic oxidation compact titanium


Page 3 of 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


dioxide (AO-TiO2) is firstly combined with p-PSCs as the ETL to the best of our knowledge and the corresponding device architecture is presented in Figure 1a,b. The AO technology is proposed for preparing ETL due to the following reasons. First of all, it is well known that AO has been successfully used to fabricate metallic oxide nanotube by selecting the electrolyte containing fluoride ions, and a compact metallic oxide layer can be obtained in the absence of fluoride ions theoretically. 17,18,19 Secondly, AO is beneficial to the TiO2 fabrication even by low temperature processed below 120 ℃, thus facilitating the development of flexible p-PSCs. Thirdly, it is experimentally demonstrated that the TiO2 nanotube characteristics fabricated by AO, including conductivity, transmittance and morphology can be easily tuned by controlling the value of oxidation voltage as well as electrolyte ingredient, 20 which may also be suitable for AO-TiO2 film. Hence, the confluence of those merits with environmentally friendly make it an impressive approach for fabricating ETL in p-PSCs.

a

b

c Au Spiro-OMeTAD CH3NH3PbI3-xClx AO-TiO2 500 nm ITO

Figure 1.(a) Overall device structure illustration including ITO/AO-TiO2/CH3NH3PbI3-xClx/ Spiro-OMeTAD/Au; (b) The cross-section scanning electron microscope (SEM) image corresponding to p-PSCs construction; (c)AO structure diagram. █

EXPERIMENTAL METHODS Device fabrication. The neat ITO substrates were washed with ethanol and acetone (with 1：1

volume ratio) via sonication for 15 min, and then continuously washed with isopropanol for 15 min. Before the deposition of AO-TiO2, ITO substrates were treated in a UV-ozone clearer for 12 min. Subsequently, raw metallic titanium film (Ti) was sputtered on ITO glass by radio frequency



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

Page 4 of 20

(RF) magnetron sputtering from Ti target (99.9% purity). The RF power and sputtering pressure were 100 W and 0.3 Pa, respectively. Following this, Ti-coated ITO glass was fixed on the anode position as shown in Figure 1c. A compact TiOx film was gradually formed with the AO proceeding, and the resultant AO-TiO2 film was obtained by annealing TiOx at 120 ℃ for 40 min. Remarkably, AO-TiO2 thickness could be accurately controlled by changing raw Ti thickness, which would be further observed in results and discussion section. In addition, tuning oxidation voltage and electrolyte ingredient aims to control AO-TiO2 morphology and crystallization. As a result, ethylene was demonstrated a suitable electrolyte candidate to obtain favorable AO-TiO2 morphology, alone with the application of 30 V oxidation voltage. In contrast, conventional high temperature processed c-TiO2 (sol-gel) were fabricated according to the reference 12. Also, the CH3NH3PbI3-xClx films were obtained according to our previous report.

12

A Spiro-OMeTAD

solution was prepared by dissolving 80 mg Spiro-OMeTAD in 1 ml chlorobenzene, to which 18 ul of lithium bis (trifluoromethanesulfony) imide (LiTFSI) solution (280 mg/ml acetonitrile) and 10 ul of 4-tert-butylpyridine were added. The hole transport layer was prepared by spinning Spiro-OMeTAD solution at 3500 rpm for 40 s. Finally, a 150 nm thick gold counter electrode was deposited by thermal evaporation in high vacuum (under 10-4 Pa) atmosphere and the dot area of p-PSCs was 0.06 cm2.

Characterization. The film surface morphology was observed by scanning electron microscope (SEM) (Hitachi SU8010). The AO-TiO2 crystallization and phase identification was performed by X-ray diffraction (XRD) (Philips PANalytical X’Pert Pro, Cu Kα). The transmittance spectra were monitored by a UV-VIS spectrophotometer (Cary 5000). The electrical conductivity

was

performed

by

Photo-Dark

Conductivity

(PDC-40A).

Steady-state


Page 5 of 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


photoluminescence (PL) measurement was recorded using a steady-state fluorescence spectrometer (FL3-2-IHR221-NIR-TCSPC). The contact angle measurement was conducted by a KRUSS (DSA100). The device J-V characteristics were measured with Abet sun 2000 solar simulator under the illumination of an AM 1.5G (100 mW/cm2). The electrochemical impedance spectroscopy (EIS) (METEK) was employed to characterize the carrier transfer dynamics of p-PSCs. The external quantum efficiency (EQE) spectrum was tested to determine p-PSCs response to sunlight. The kelvin probe was measured to explore the AO-TiO2 work function. █

THEORY AND EQUATIONS The fundamental theory concerning fabricating AO-TiO2 is proposed with better comprehension

about the previous reports, 17-23 incorporating following processes：I) At the first stage，raw Ti films lose electrons and form Ti4+ ions when proper oxidation voltage is applied as shown in equation 1. Concurrently, O2- and OH- ions are generated in the cathode as shown in equation 2. Also, the generated ions can migrate by virtue of the electric field and electrolyte transport. II). As the process of AO, a portion of Ti4+ ions dissolve in the electrolyte，and others combine with O2- or OH- so as to form TiO2 or Ti(OH)4 at the interface between Ti/electrolyte as shown in equation 3,4. III) O2- and OH- ions continue transporting through the formed oxide and further react with Ti4+ ions at the interface between formed AO-TiO2/Ti film until the complete oxidation of raw Ti . IV) The resultant AO-TiO2 film is obtained by low temperature annealing to afore-prepared AO-TiOx as shown in equation 5.

Ti − 4e - → Ti4 + (1)

2OH - + 2e− → 2O2 - + H 2 ↑ (2)

Ti4 + + 2O2 - → TiO2 (3) 5 ACS Paragon Plus Environment


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

Page 6 of 20

Ti4 + + 4OH - → Ti(OH)4 (4) Ti(OH)4 ∆ → TiO2 + H 2O (5) RESULTS AND DISCUSSION

█

From above mentioned AO theory, it can be understood that AO-TiO2 formation is significantly depended on the reaction driving force determined by involved ions migration rate. However, the dependence of ions migration rate should be ascribed to equation 6, where v represents ions migration rate, constant A represents material parameter intimately related to electrolyte viscosity, and dE/dl represents electric field gradient along the ions migration direction. The lower electrolyte viscosity and the larger oxidation voltage, the higher ions migration rate. As a consequence, controlling AO-TiO2 formation can be realized by varying electrolyte ingredient and oxidation voltage, which have a significant influence on the variation of AO-TiO2 morphology and crystallization. 19-23

v = A

a

dE (6) dl

b

c

Figure 2. AO-TiO2 SEM images. (a) AO-TiO2 fabricated with water as electrolyte; (b) AO-TiO2 fabricated with glycerin as electrolyte; (c) AO-TiO2 fabricated with ethylene as electrolyte; Raw Ti thickness was 70 nm and 30 V oxidation voltage was applied. In order to verify our assumption, electrolytes with different viscosity such as deionized water, glycerol, and ethylene were respectively attempted (under identical other conditions). Figure 2 showed SEM images of AO-TiO2 fabricated with the aforementioned electrolytes, suggesting that AO-TiO2 prepared by different electrolytes indeed displayed various morphology characteristic.


Page 7 of 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


For comparison, raw Ti SEM image was also showed in Figure S1. When deionized water was served as electrolyte, numerous pin-holes were seen in Figure 2a. The presence of pin-holes was attributed to drastic ions migration accelerating reaction driving force, which resulted from the minimum viscosity of deionized water. On the other hand, although smooth AO-TiO2 film could be obtained with glycerin as electrolyte, it seemed that partial metallic Ti residue marked with red indicators could be noted (shown in Figure 2b). Likewise, the metallic Ti residue was supposed to arise from higher viscosity of glycerin hindering homogeneous ions transport, and thus leading to poor reaction driving force. Fortunately, as shown in Figure 2c and Figure S2, dense and quite uniform AO-TiO2 crystals could be obtained by using ethylene as electrolyte.

a

b

c

Figure 3. SEM images of perovskite films coated upon AO-TiO2 fabricated with different electrolytes. (a) Deionized water; (b) Glycerin; (c) Ethylene, respectively. Insets showed the corresponding amplification images. Apart from surface morphology, AO-TiO2 interfacial adhesion fabricated by aforementioned electrolytes was also detected by the contact angles measurement between perovskite ink and AO-TiO2 surface (shown in Figure S3), which had a significant influence on the subsequent perovskite morphology. The larger the contact angle, the poorer interfacial adhesion and the worse perovskite morphology. 24 Figure 3 showed corresponding SEM images of perovskite films coated upon AO-TiO2 fabricated with mentioned electrolytes. Obviously, it was implied that the perovskite layer interfaced with ethylene-based AO-TiO2 displayed better surface coverage compared to those interfaced with water or glycerol-based AO-TiO2 which agreed with the



measurement presented in Figure S3. This result together with above discussion consequently demonstrated that ethylene was beneficial for fabricating optimized AO-TiO2.

10 V

20 V

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

Page 8 of 20

30 V

40 V 50 V thermal oxidation

20

30

40

50

2θ(degree)

60

70

Figure 4. AO-TiO2 XRD evolution pattern as the function of oxidation voltage. In addition to the effect of electrolyte ingredient, the variation of oxidation voltage is another dominant contribution to affect AO-TiO2 formation and crystallization behavior. Thus, Figure 4 showed the AO-TiO2 XRD evolution pattern as the function of oxidation voltage with an invariant ethylene as electrolyte. The AO-TiO2 crystallization orientation presented a prominent transformation from amorphous phase to well crystallization with increasing oxidation voltage. When oxidation voltage was less than 30V, AO-TiO2 exhibited amorphous properties. However, 。

once the oxidation voltage exceeded 30 V, three preferential XRD diffraction peaks at 25.37 , 37.88

。

。

and 53.97 were detected, assigned to (101), (004), and (105) anatase crystal planes. This

finding manifested that complete oxidation reaction was impeded by a formed ultrathin oxide layer in the initial AO stage, which could be removed only by higher reaction driving force agreeing with the AO theory. Also, to eliminate the influence of annealing on AO-TiO2 crystallization, the XRD image of directly thermal annealing above 450 ℃ to raw Ti was incorporated in Figure 4. It showed unfavorable crystallization orientation unlike the AO-TiO2


Page 9 of 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


situation. In such way, those results implied that a certain threshold voltage was necessary for AO-TiO2 formation, coinciding well with the measurement of current function along with sweep voltage change (shown in Figure S4). On the other hand, because overlarge oxidation voltage caused drastic reaction driving force that deteriorated AO-TiO2 surface morphology determined by SEM images (shown in Figure S5), we justified that an optimal option for fabricating uniform, pinhole-free AO-TiO2 was that 30 V oxidation voltage was applied, alone with ethylene as electrolyte. Furthermore, the optimized AO-TiO2 energy dispersive spectroscopy (EDS) on silicon substrate was also measured to underpin AO-TiO2 formation (shown in Figure S6). Excess oxygen element proportion might be the consequence of integration between oxygen and silicon. Then, to investigate the effect of AO-TiO2 on p-PSCs performance, the device structure illustrated in Figure 1a was fabricated. Figure 5a showed the champion J-V curves for each device and corresponding photovoltaic parameters were summarized in Table 1. All the measurements for J-V curves were carried out at regular time intervals (0.02 V/s scan rate) and in air atmosphere. As can be observed, the device consisting of AO-TiO2 showed superior Voc and FF compared to that based on c-TiO2 (sol-gel) no matter at backward or forward scan direction despite of possessing weaker short circuit density (Jsc). Besides that, 30 devices for each construction were fabricated to monitor efficiency distribution and the result were shown in Figure 5b. The device performance based on AO-TiO2 presented a concentrated distribution compared to their counterparts. Furthermore, the average and standard deviation for 30 devices consisting of AO-TiO2 were 2.7% and 1.56%, and corresponding value for 30 devices consisting of c-TiO2 were 2.89% and 1.64%, showing the improved reproducibility by using AO-TiO2 as ETL.



a 7

b

c-TiO2(sol-gel) AO-TiO2

6

5 1

Counts

5

0 1 d r a d w r k a w c r d r a f o b a d w l ) l ) r k e a c e g g w r a - o l b l f o o - s s ( ( 2 2 O O i i 2 2 T i O i O T - T T O - O A c c A

5

4 3 2 1

0 . 1

) V

8 . 0

g a t l

( 6 . e 0

4 o . 0 V

2 . 0

0 . 00

) 2 m c / A m ( y t i s n e d t n e r r u C

0 2

0

7 8 9 10 11 12

8 9 10 11 12 13

Efficiency (%) 2

Current density (mA/cm )

c O i T O A ) l e g l o s ( 2 2 O i T c

) % ( E Q E

0 0 8

n (

0) 0 7m

n e l

0h 0t 6g

W

0e 0v 5a

0 0 4

0 0 0 0 0 0 0 0 0 03 7 6 5 4 3 2 1

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

Page 10 of 20

2

10

AO-TiO2 c-TiO2 via sol-gel

d

1

10

0

10

10

-1

10

-2

10

-3

10

-4

-0.5

0.0

0.5

1.0

Voltage (V)

Figure 5. Efficiency measurements. (a) J-V curves for p-PSCs without encapsulation with different scanning direction in light; (b) Histogram of efficiency distribution for 30 devices based on c-TiO2 (sol-gel) and AO-TiO2, respectively; (c) EQE spectrum for devices; (d) J-V curves for p-PSCs in dark.

Table 1. Photovoltaic parameters derived from J-V measurements corresponding to Figure 5a sample AO-TiO2-backward AO-TiO2-forward c-TiO2 (sol-gel) -backward c-TiO2 (sol-gel) -forward

Jsc(mA/cm2) 19.08 18.53 19.39 18.94

Voc(V) 1.00 0.97 0.94 0.90

FF (%) 70.6 59.4 66 56

PCE (%) 13.47 10.68 12.03 9.55

EQE spectra were performed (shown in figure 5c) to better grasp the varied parameters. The p-PSCs consisting of AO-TiO2 produced inferior response in the wavelength range between 470-800 nm, and the same trend was also found in transmittance spectra measurement (shown in Figure S7), which suggested that major light loss was responsible for inferior Jsc. Interestingly, distinguished response enhancement was observed in the wavelength range between 360-470 nm. Considering unfavorable AO-TiO2 transmittance, the elevated response was attributed to more effective electron collection of AO-TiO2.


Page 11 of 20

For another, the Voc dependence can be briefly written as equation 7, 15 in which J0 represents inverse saturated current density, K represents Boltzmann constant, T represents temperature and q represents electric charge. It is indicated that the lower J0 is attributed to the higher Voc. Therefore, corresponding J-V curves in dark were also shown in Figure 5d. When reverse voltage was applied, the current of AO-TiO2 based device was indeed lower than conventional device, in favor of device Voc improvement.

Voc = AKT/q × ln（JSC/J0）（7）

1200000

PL intensity (a.u.)

b

AO-TiO2 c-TiO2 （ sol-gel）

200

glass/AO-TiO2/perovskite

1000000 800000 600000 400000

150 100 50

200000 0

250

glass/perovskite glass/c-TiO2(sol-gel)/perovskite

a

Z'' (Ohm)

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


700

800

wavelength (nm)

900

0

0

100

200

300

400

500

600

700

800

Z' (Ohm)

Figure 6. (a) Steady-state photoluminescence (PL) demonstrating the charge collection properties; (b) Nyquist plots at V=1.0 v for devices based on AO-TiO2 and c-TiO2 (sol-gel), respectively. Furthermore, the improvement of FF is often attributed to the decrease of series resistance and the increase of recombination resistance. In our case, the difference between two specimens was only the fabrication about ETL. Hence, the higher FF was intimately related to more effective electrons collection between AO-TiO2 and perovskite. And steady-state photoluminescence (PL) measurement was shown in Figure 6a to verify our viewpoint. The pure CH3NH3PbI3-xClx film exhibited significant PL intensity around 770 nm. However, on contact with AO-TiO2 or c-TiO2 (sol-gel), PL quenching was clearly observed. The stronger quenching had an implication for effective electron collection. As expected, the PL quenching for AO-TiO2/perovskite was more effective than that of c-TiO2 (sol-gel)/perovskite, indicating the increased electron collection



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

Page 12 of 20

ability. The improved electron collection might benefit from the reduced work function of AO-TiO2 (4.7 versus 4.8 eV corresponding to c-TiO2), demonstrated by kelvin probe measurement. Moreover, electrochemical impedance spectroscopy (EIS) was employed to characterize p-PSCs charge transfer dynamics and corresponding nyquist plots were shown in Figure 6b. The first

semicircle

pointed

out

the

strong

interfacial

influence

consisting

of

ETL/CH3NH3PbI3-xClx/Spiro-OMeTAD. 16, 25 Hence, the reduced resistance at the first semicircle in AO-TiO2 based device (485 ohm versus 542 ohm in c-TiO2) would illustrate lower transport resistance at this region. In this regard, when AO-TiO2 was utilized as ETL, higher FF and Voc were justified.

Figure 7. Energy band schematic diagram. (a) Ideal perovskite film; (b) Ideal AO-TiO2/CH3NH3PbI3 interface; (c) Actual perovskite film; (d) Actual AO-TiO2/CH3NH3PbI3 interface. Unfortunately, as shown in Figure 6a, the p-PSCs were susceptible to present J-V hysteresis with regard to the scan direction no matter at AO-TiO2 or conventional c-TiO2 (sol-gel) situation. It was widely perceived that hysteresis phenomenon could be caused by following possibilities


Page 13 of 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


such as charge accumulation, dielectric polarization owning to perovskite materials ferroelectric properties and ions migration.

26-28

With understanding the previous reports, we considered that

efficient charge collection was a pivotal suppression for hysteresis. Therefore, we attempted to understand the physical mechanism at TiO2/Perovskite interface and corresponding energy band schematic diagram were shown in Figure 7. Theoretically, perovskite energy band should be flat without marginal curl as Figure 7a presented. In the interface between TiO2/Perovskite, the energy bend should be the situation like Figure 7b, which has a positive influence on electron collection. However, the disadvantageous perovskite/TiO2 interface barrier have been demonstrated.

29

Additionally, relative report has

revealed that CH3NH3+ cations could be partially lost during thermal annealing, resulting in electron deficiency in perovskite surface, and thereby leading to acceptor defects in perovskite surface, which had energy bend like Figure 7c. 29 On the other hand, oxygen vacancy is also rich in the TiO2 surface, which can be considered as donor defects. Consequently, instead of ideal physical model, the actual TiO2/perovskite interface energy band diagram was proposed as Figure 7d, in which electrons accumulation due to a negative potential barrier existed. Therefore, the hysteresis of TiO2-based p-PSCs cannot be eradicated unless interface engineering is carried out to reduce this potential barrier, which will be discussed in detail at other work. Finally, the dependence of device performance on AO-TiO2 thickness was further investigated. Also, AO-TiO2 transmittance spectrum and p-PSCs performance with the variation of AO-TiO2 thickness were shown in Figure S8 and Figure S9, respectively. In general, too thin ETL or HTL often lead to poor coverage to ITO substrate and direct contact between ITO and perovskite layer, which should result in high carrier recombination and inferior performance. However, series



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

Page 14 of 20

resistance would increase as AO-TiO2 thickness increased for its inferior conductivity determined by Figure S10 and Table S1, and at the same time there would be much sunlight losses (shown in Figure S8). Those in turn decreased p-PSCs performance. According to our experience, the optimized AO-TiO2 thickness should be controlled between 50-60 nm in favor of efficient p-PSCs. The thickness relationship between AO-TiO2 and raw Ti were recorded in Table S2. █

CONCLUSION In summary, we have demonstrated an efficient AO approach to prepare dense and uniform

AO-TiO2 as ETL, making the p-PSCs fabrication possible by fully low temperature processed. The influence of electrolyte ingredient and oxidation voltage on AO-TiO2 morphology and crystallization variation is observed at length according to the proposed AO theory instructions. By exploiting the optimized AO-TiO2 film in p-PSCs, a repeatable PCE value over 13% have been reached, alone with superior Voc and FF. Furthermore, it is experimentally demonstrated that the enhanced p-PSCs performance by employingAO-TiO2 is due to effective electron collection and meanwhile series resistance decrease. In addition, the J-V curves hysteresis effects for p-PSCs are unveiled and relative physical mechanism at TiO2/Perovskite interface is also proposed for its illustration. In a word, the findings presented here and the overall fabrication method represent a helpful new approach for fabricating repeatable and all low temperature processed efficient planar perovskite photovoltaic devices. █

ASSOCIATED CONTENT

Supporting information The raw Ti SEM images. The cross section SEM image of AO-TiO2 fabricated with ethylene as electrolyte. The measurement of current function along with sweep voltage when fabricating AO-TiO2 via ethylene. The AO-TiO2 SEM images with the oxidation voltage variation. The AO-TiO2 energy dispersive spectroscopy (EDS) on silicon substrate. The ITO/AO-TiO2 transmittance spectrum dependence on different thickness. The p-PSCs performance as variation


Page 15 of 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


of AO-TiO2 thickness. The p-PSCs electrochemical impedance spectroscopy (EIS) with various AO-TiO2 thickness. The thickness relationship between AO-TiO2 and raw Ti. █

AUTHOR INFORMATION

Corresponding Author *(H.C.) E-mail: [email protected] *(J.Z.) E-mail: [email protected]

Notes The authors declare no competing financial interest. █

ACKNOWLEDGEMENTS We appreciate financial support from the National Natural Science Foundation of China (Grant

No. 61504068). █

REFERENCES

(1) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. (2) De Wolf, S.; Holovsky, J.; Moon, S. J.; Löper, P.; Niesen, B.; Ledinsky, M.; Haug, F.; Yum, J.; Ballif, C. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5, 1035-1039. (3) Filip, M. R.; Eperon, G. E.; Snaith, H. J.; Giustino, F. Steric Engineering of Metal-Halide Perovskites with Tunable Optical Band Gaps. Nat. Commun. 2014, 5, 5757. (4) Roldán-Carmona, C.; Malinkiewicz, O.; Soriano, A.; Espallargas, G. M.; Garcia, A.; Reinecke, P.; Kroyer, T.; Dar, M. I.; Nazeeruddin, M. K.; Bolink, H. J. Flexible High Efficiency Perovskite Solar Cells. Energy Environ Sci. 2014, 7, 994-997. (5) Di, G. F.; Zardetto, V.; D'Epifanio, A.; Pescetelli, S.; Matteocci, F.; Razza, S.; Carlo, A. D.; Licoccia, S.; Kessels, W. M. M.; Creatore, M.; Brown, T. M. Flexible Perovskite Photovoltaic



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

Page 16 of 20

Modules and Solar Cells Based on Atomic Layer Deposited Compact Layers and UV‐ Irradiated TiO2 Scaffolds on Plastic Substrates. Adv. Energy Mater. 2015, 5, 140108. (6) Im, J. H.; Lee, C. R.; Lee, J. W.; Park, S. W.; Park, N. G. 6.5% Efficient Perovskite Quantum-Dot-Sensitized Solar Cell. Nanoscale. 2011, 3, 4088-4093. (7) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-Performance Photovoltaic Perovskite Layers Fabricated Through Intramolecular Exchange. Science 2015, 348, 1234-1237. (8) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.; Duan, H.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542-546. (9) Rakstys, K.; Abate, A.; Dar, M. I.; Gao, P.; Jankauskas, V.; Jacopin, G.; Kamarauskas, E.; Kazim, S.; Ahmad, S.; Gratzel, M.; Nazeeruddin, K. M.; Triazatruxene-Based Hole Transporting Materials for Highly Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 16172-16178. (10) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. (11) Jung, M. C.; Raga, S. R.; Ono, L. K.; Qi, Y. Substantial Improvement of Perovskite Solar Cells Stability by Pinhole-Free Hole Transport Layer with Doping Engineering. Sci. Rep. 2015, 5, 09863. (12) Du, Y.; Cai, H.; Ni, J.; Li, J.; Yu, H.; Sun, X.; Wu, Y.; Wen, H; Zhang, J. Air-Processed, Efficient CH3NH3PbI3−xClx Perovskite Solar Cells with Organic Polymer PTB7 as a Hole-Transport Layer. RSC Adv. 2015, 5, 66981-66987.


Page 17 of 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


(13) Ke, W.; Fang, G.; Wang, J.; Qin, P.; Tao, H.; Lei, H.; Liu, Q.; Dai, X.; Zhao, X. Perovskite Solar Cell with an Efficient TiO2 Compact Film. ACS Appl. Mater. Interfaces 2014, 6, 15959-15965. (14) Gao, Q.; Yang, S.; Lei, L.; Zhang, S.; Cao, Q.; Xie, J.; Li, J.; Liu, Y. An Effective TiO2 Blocking Layer for Perovskite Solar Cells with Enhanced Performance. Chem. Lett. 2015, 5, 624-626. (15) Shi, J.; Xu, X.; Li, D.; & Meng, Q. Interfaces in Perovskite Solar Cells. Small 2015, 11, 2472-2486. (16) Shih, Y. C.; Wang, L. Y.; Hsieh, H. C.; & Lin, K. F. Enhancing the Photocurrent of Perovskite Solar Cells Via Modification of the TiO2/CH3NH3PbI3 Heterojunction Interface with Amino Acid. J. Mater. Chem. A 2015, 3, 9133-9136. (17) Roy, P.; Berger, S.; Schmuki, P. TiO2 Nanotubes: Synthesis and Applications. Angew. Chem., Int. Ed. 2011, 50, 2904-2939. (18) Su, T. S.; Hsieh, T. Y.; Hong, C. Y.; & Wei, T. C. Electrodeposited Ultrathin TiO2 Blocking Layers for Efficient Perovskite Solar Cells. Sci. Rep. 2015, 5, 16098. (19) Ruan, C.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. Fabrication of Highly Ordered TiO2 Nanotube Arrays Using an Organic Electrolyte. J. Phys. Chem. B 2005, 109, 15754-15759. (20) Macak, J. M.; Hildebrand, H.; Marten-Jahns, U.; Schmuki, P. Mechanistic Aspects and Growth of Large Diameter Self-Organized TiO2 Nanotubes. J. Electroanal. Chem. 2008, 621, 254-266. (21) Tsuchiya, H.; Macak J. M.; Taveira L.; Balaur, E.; Ghicov, A.; Sirotna, K.; Schmuki, P.



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

Page 18 of 20

Self-Organized TiO2 Nanotubes Prepared in Ammonium Fluoride Containing Acetic Acid Electrolytes. Electrochem. Commun. 2005, 7, 576-580. (22) Ohtsuka, T.; Otsuki, T. The Aging of the Anodic Oxide of Titanium During Potentiostatic Condition by Ellipsometry. Corros. Sci. 2003, 45, 1793-1801. (23) Regonini, D.; Satka, A.; Jaroenworaluck, A.; Allsopp, D. W. E.; Bowen, C. R.; Stevens, R. Factors Influencing Surface Morphology of Anodized TiO2 Nanotubes. Electrochim. Acta 2012, 74, 244-253. (24) Yu, X.; Yu, X.; Zhang, J.; Zhao, G.; Ni, J.; Cai, H.; & Zhao, Y. Efficiency Boosting of Inverted Polymer Solar Cells with a Polyvinylpyrrolidone-Modified Al-Doped ZnO Electron Transport Layer. Sol. Energy Mater. Sol. Cells 2014, 128, 307-312. (25) Ma, Y.; Chung, Y. H.; Zheng, L.; Zhang, D.; Yu, X.; Xiao, L.; Chen, Z.; Wang, S.; Qu, B.; Gong, Q.; & Zou, D. Improved Hole-Transporting Property via HAT-CN for Perovskite Solar Cells without Lithium Salts. ACS Appl. Mater. Interfaces 2015, 7, 6406-6411. (26) Wu, B.; Fu, K.; Yantara, N.; Xing, G.; Sun, S.; Sum, T. C.; & Mathews, N. Charge Accumulation and Hysteresis in Perovskite-Based Solar Cells: An Electro-Optical Analysis. Adv. Energy Mater. 2015, 5. (27) Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; & Grätzel, M. Understanding the Rate-Dependent J–V Hysteresis, Slow Time Component, and Aging in CH3NH3PbI3 Perovskite Solar Cells: The Role of a Compensated Electric Field. Energy Environ. Sci. 2015, 8, 995-1004.

(28) Gottesman, R.; Haltzi, E.; Gouda, L.; Tirosh, S.; Bouhadana, Y.; & Zaban, A. Extremely Slow Photoconductivity Response of CH3NH3PbI3 Perovskites Suggesting Structural


Page 19 of 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


Changes Under Working Conditions. J. Phys. Chem. Lett. 2014, 5, 2662-2669.

(29) Xing, G.; Wu, B.; Chen, S.; Chua, J.; Yantara, N.; Mhaisalkar, S.; Mathews, N.; Sum, T. C. Interfacial Electron Transfer Barrier at Compact TiO2/CH3NH3PbI3 Heterojunction. Small 2015, 11, 3606-3613.



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

Page 20 of 20

Table Of Contents (TOC) graphic：：


Novel Combination of Efficient Perovskite Solar ... - ACS Publications

Recommend Documents