Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, China
bLaboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China
H I G H L I G H T S
•
Planar perovskite solar cells (PSCs) are fabricated with low-temperature process.•
Li2SiO3modified TiO2electron transport layer with low-temperature process.•
Enhanced electron mobility and reduced electron trap state in the modified TiO2.•
Promoted charge transport and reduced charge recombination in the modified PSCs.A R T I C L E I N F O Keywords:
Li2SiO3modified TiO2electron transport layer Low-temperature process
Planar perovskite solar cells Photoelectric properties
A B S T R A C T
Planar perovskite solar cells based on Li2SiO3modified TiO2electron transport layer are fabricated by one-step method. Here the Li2SiO3modified TiO2films are prepared by a simple soaking process at 65 °C. The effect of Li2SiO3modification on the photoelectric properties of TiO2films and devices has been systematically in- vestigated. Compared to the reference TiO2, the modified TiO2demonstrates better characteristics such as better surface wetting properties of perovskite precursor, enhanced electron mobility and reduced electron trap state density, which contribute to the favorable photovoltaic performance. At the optimized process, the device based on the modified TiO2film yields a champion power conversion efficiency of 18.67%, being much higher than 14.78% of the reference device. The improved performance is primarily attributed to the promoted charge transport and reduced charge recombination in the modified electron transport layer. This work provides a simple route to obtain efficient planar perovskite solar cells by low-temperature process, which will be com- patible withflexible plastic substrates in future practical applications.
1. Introduction
Recently, organic-inorganic hybrid perovskite solar cells (PSCs) have drawn tremendous attention due to their low production cost, simple fabrication process and remarkable photovoltaic performance [1–3]. The power conversion efficiency (PCE) of PSCs has increased rapidly from 3.8% to 22.7% in a few years [1,4–8]. Despite the ex- cellent photovoltaic performance, most of state-of-the-art high effi- ciency PSCs is composed of a complicated mesoscopic structure using some mesoporous semiconducting material as electron transport layer (ETL) such as TiO2[8], SnO2[9] or a scaffold (ZrO2[10]), which re- quire high-temperature annealing (> 450 °C). This process hampers the feasibility onflexible plastic substrates. Planar PSCs can be fabricated
by low-temperature process, which makes it possible to be applied to roll-to-roll commercial production.
In planar PSCs, the ETL plays an important role in extracting and transporting photogenerated electron from perovskite layer to F-doped SnO2(FTO) substrate and blocking the direct contact between FTO and hole transport layer [11]. Therefore, the exploration of novel materials as ETLs in PSCs is a crucial scientific issue. Many metal oxides such as SnO2[12,13], ZnO [14,15] and TiO2[16,17] have been widely used as ETLs in PSCs. Among them, TiO2is a good candidate due to the simple process, low cost and matched band energy level with perovskite ma- terials. Moreover, TiO2 ETL can be fabricated by low-temperature process. It is well-known that low-temperature processed TiO2ETL can be prepared by magnetron sputtering, atomic layer deposited method
https://doi.org/10.1016/j.jpowsour.2018.04.086
Received 28 January 2018; Received in revised form 10 April 2018; Accepted 25 April 2018
∗Corresponding author.
E-mail address:[email protected](S. Wu).
0378-7753/ © 2018 Elsevier B.V. All rights reserved.
(ALD) [18] or chemical bath deposited method (CBD) [19]. Among them, the CBD method is widely used to fabricate TiO2ETLs in PSCs due to the simple process. However, the TiO2ETLs prepared by CBD method suffer from low electron mobility and high trap state density [20,21]. These may have negative effects on electron transport in the TiO2/perovskite interface, which eventually influences the photovoltaic performance of PSCs.
To enhance the performance of PSCs, a lot of efforts have been made to improve the electric properties of solution-processed TiO2 ETLs.
Modifying the TiO2ETL by amino acid [22–24], self-assembled full- erene layers [25,26], methylammonium iodide [27], graphene quantum dots [28], thiols [29], carboxyl groups [30] and silane monolayers [31] have been employed to decrease the trap state density and promote the electron transport and collection. F. Giordano et al.
enhanced the efficiency of PSCs from 17% to over 19% by modifying the mesoporous TiO2ETL by lithium (Li) doping [32]. Similarly, im- proved PCE was obtained by treating the TiO2ETL with Li salts [33,34].
The enhanced photovoltaic performance can be ascribed to the passi- vated defects and increased conductivity of TiO2 ETL after Li salt treatment [35]. However, these Li salts treated TiO2films need to be sintered at high temperature, which is not compatible with flexible plastic substrate. It is necessary to explore simple low-temperature process to fabricate effective TiO2ETL for PSCs. In dye-sensitized solar cells, it was found that lithium silicate (Li2SiO3) modified TiO2elec- trode could improve the cell's performance [36]. However, there is no report that Li2SiO3modified TiO2(LS-TiO2) ETL by a simple low-tem- perature process has been used to prepare efficient planar PSCs.
Moreover, the effects of the Li2SiO3modification on surface wetting properties of perovskite precursor on ETL, the electric properties of TiO2ETL at nanoscale level, mobility and electron trap state density of TiO2ETL, and charge extraction and recombination in the modified PSCs have not been systematically investigated.
Based on these considerations, we fabricate LS-TiO2ETL by a simple soaking process in Li2SiO3aqueous solution. The planar PSCs with the structure of FTO/TiO2 or LS-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Ag have been prepared by low-temperature process. For convenience of presentation, these structures are abbreviated as “the referencefilm/
PSC” in which TiO2 without any modification is included, “LS-TiO2
films/PSCs”in which the TiO2films are treated with Li2SiO3. By opti- mizing the concentrations of Li2SiO3aqueous solutions and the soaking
time, the champion efficiency of 18.67% for LS-TiO2 PSC has been achieved, which is much higher than that of 14.78% for the reference PSC. The related mechanism has been systematically investigated. The photoelectric properties of ETLs have been studied by dark current- voltage (I-V) measurements, conductive atomic force microscopy (C- AFM) and Kelvin probe force microscopy (KPFM). These results reveal that LS-TiO2ETLs demonstrate the enhanced conductivity and electron mobility, and reduced electron trap state density compared with the TiO2ETL. The electron transport and recombination characteristics of LS-TiO2 ETL in PSC have been investigated by steady-state photo- luminescence (PL) and electrochemical impedance spectroscopy (EIS).
The results suggest that Li2SiO3 modification can promote electron extraction and transport, and suppress electron recombination at the TiO2/perovskite interface. Furthermore, all these processes are con- ducted at the low temperature (200 °C), which makes it compatible withflexible plastic substrates in future practical production.
2. Experimental section 2.1. Materials
CH3NH3I (MAI) was synthesized by the reported method [37]. PbI2
(99.9985%) Dimethyl sulfoxide (DMSO) (≥99.9%), 4-Hydroxybutyric acid lactone (GBL) (≥99.9%) and Li2SiO3were bought from Sigma- Aldrich, Aladdin and Alfa Aesar, respectively. 2, 2′, 7, 7′- Tetrakis (N,N- di-p-methoxyphenylamine)–9, 9′-spirobifluorene (Spiro-OMeTAD) was obtained from the company of Feiming in Shengzhen. FTO (15Ω/ square) substrate was purchased from Asahi Glass Company Limited of Japan.
2.2. Device fabrication
As shown inFig. 1a, the PSCs with the structure of FTO/TiO2or LS- TiO2/CH3NH3PbI3/Spiro-OMeTAD/Ag have been fabricated. Firstly, FTO substrates were ultrasonically cleaned with deionized water, acetone, isopropanol and ethanol successively, and dried with nitrogen (N2) flow. After oxygen UV treatments for 15 min, the cleaned FTO substrates were immersed in a TiCl4 solution (200 mM) in a closed vessel at 70 °C for 1 h. After washed with deionized water and ethanol, thefilms were dried with N2flow and annealed at 200 °C for 30 min in Fig. 1.(a) A schematic draw of the PSC structure, (b) XPS spectrum for whole survey scan in LS-TiO2films, XPS spectra for Si 2p (c) and Li 1s (d) in LS-TiO2films.
air. Then the TiO2/FTO electrodes were soaked in the Li2SiO3aqueous solutions (concentration varies from 0 to 5 × 10−1M) for different time (0–5min) to fabricate LS-TiO2 films. Next, the obtained films were rinsed with deionized water and ethanol, then dried at 65 °C for 10 min to produce modified LS-TiO2. For the reference sample, the TiO2films were not treated with Li2SiO3aqueous solution. CH3NH3I and PbI2with a molar ratio of 1:1 were dissolved in a mixed solution with DMSO and GBL (volume ratio of DMSO and GBL was 3:7) to obtain a 40% per- ovskite precursor solution in weight ratio. After the reference and LS- TiO2films were treated with oxygen UV for 10 min, perovskite pre- cursor solution was spin-coated onto them at a speed 4000 rpm for 40 s in the glove box. The chlorobenzene solution was dropped on the as- spun perovskitefilms during spin-coating at 20 s, respectively. Then, the perovskitefilms were annealed at 100 °C for 10 min immediately.
After annealing, a thin layer of Spiro-OMeTAD was deposited on the perovskite layer by spin-coating a chlorobenzene solution containing 80 mM Spiro-OMeTAD, 64 mM tert-butylpyridine (TBP) and 24 mM Li- bis(trifluoromethanesulfonyl)-imide (Li-TFSI) (520 mg/ml in acetoni- trile) at 5000 rpm for 30 s. These samples were left in dry air overnight in the dark. Ultimately, silver (Ag) electrode with thickness of
∼100 nm was evaporated on the sample surface through a shadow mask under a vacuum of 1 × 10−4Pa. The sample size was 0.045 cm2. 2.3. Characterization
The X-ray photoelectron spectroscopy (XPS) was measured by Al K- Alpha X-ray photoelectron spectrometer (Thermo Fisher Scientific, UK).
The amount of Li2SiO3introduced onto the TiO2layer was measured using inductively coupled plasma mass spectrometry (ICP/MS, SPECTRO ARCOS MV). The photovoltaic performance of PSCs was characterized using Keithley 2420 meter under an illumination of 100 mW/cm2(Newport 91160, 150 W solar simulator equipped with an AM 1.5 Gfilter). The radiation intensity was calibrated by a standard silicon solar cell (certified by NREL) as the reference. Contact angle was measures by video-based optical contact angle measurement instrument (Dataphysics OCA Pro 15, Germany). The morphology of the perovskite films was characterized by SEM (ZEISS ULTRA 55). Atomic Force Microscopy (AFM) (Asylum Research Cypher) was employed to in- vestigate the film surface morphology, conductivity and surface
potentials. The thickness of ETLs was performed by a profilometer (Dektak XT). The external quantum efficiency (EQE) was measured using EQE system (Newport 66902). The UV–vis absorption spectra of the perovskitefilms were measured by SHIMADZU UV-2550 spectro- photometer. The EIS was performed on the Zahner Zennium electro- chemical workstation under 100 W/m2 white LED light. For the EIS measurements, a 20 mV ac-sinusoidal signal source was employed over the constant bias with the frequency ranging from 1 Hz to 1 MHz. The PL spectra were measured by a fluorescence spectrophotometer (HITACHI F-5000) exited at 515 nm.
3. Results and discussion
Firstly, XPS measurements were conducted to investigate the che- mical composition of LS-TiO2ETLs.Fig. 1b displays the XPS spectrum of LS-TiO2in whole range survey scan. The binding energy of Si 2p is displayed inFig. 1c, suggesting the presence of Si in the LS-TiO2film, which coincides with the previous report [36]. A peak at 55.6 eV agreed with the binding energy of Li 1s is detected, which indicates the ex- istence of Li [32,38], as shown in Fig. 1d. To further confirm the amount of Li in the LS-TiO2film, the ICP/MS measurement was carried out. The amount of Li in the TiO2films is determined to be 0.46% at the optimum soaking process (concentration, 5 × 10−3M; time, 120 s).
This is consistent with the report [36].
To study the effect of Li2SiO3 modification on the photovoltaic performance, PSCs based on LS-TiO2ETL have been fabricated. The concentrations of Li2SiO3 solutions and the soaking time have been optimized.Fig. 2a shows the J-V curves of LS-TiO2PSCs modified by different Li2SiO3concentrations andFig. 2b demonstrates the PCE of LS-TiO2PSCs as a function of Li2SiO3concentrations. It is noted that the PCE increases with the increase of Li2SiO3 concentrations at the be- ginning. When the Li2SiO3concentration increases to 5 × 10−3M, the LS-TiO2PSC shows the best PCE of 18.67%. Then the device perfor- mance becomes worse with the increase of Li2SiO3concentrations once it is over 5 × 10−3M. This can be attributed to the fact that high Li2SiO3concentration may induce some defects on the surface of TiO2, which can act as the recombination center of electron and hole [33].
Fig. 2c displays the J-V curves of LS-TiO2PSCs with different soaking time in Li2SiO3solution of 5 × 10−3M andFig. 2d shows the PCE of LS- Fig. 2.J-V curves of LS-TiO2PSCs with different Li2SiO3concentrations (a) and different soaking time (c), PCE of LS-TiO2PSCs as a function of Li2SiO3con- centrations (b) and soaking time (d).
TiO2PSCs as a function of soaking time. It is found that the optimum soaking time is 120 s. Therefore, wefix the concentration of Li2SiO3
solution at 5 × 10−3M and the soaking time at 120 s. The results are shown fromFigs. 3–8andFigs. S1–S3.
Fig. 3a shows the measured J-V curves of reference and LS-TiO2
PSCs. The relevant performance parameters are listed inTable 1. It can be seen that short-circuit current density (Jsc) andfill factor (FF) of LS- TiO2PSC are much higher than those of the reference PSC. The open voltage (Voc) is enhanced to 1.042 V from 1.001 V after Li2SiO3treat- ment, demonstrating the less energy loss of electrons [34]. The Jscof
PSCs is significantly increased from 19.82 mA/cm2to 23.50 mA/cm2 due to the improvement of conductivity of electron [35]. The FF rises from 74.52% to 76.22%. The FF is related to the ratio of shunt re- sistance (Rsh) to series resistance (Rs), Rsh/Rs. The higher FF in LS-TiO2
PSC is ascribed to the larger Rsh/Rs[27,39]. As shown inFig. 3b–f, the distributions of PCE, Voc, Jsc, FF and Rsh/Rsfor reference and LS-TiO2
PSCs extracted from J-V curves for 30 devices are presented. It is clear to see that the dispersions of all parameter in LS-TiO2PSCs are nar- rower than that of the reference PSCs, which imply better photovoltaic performance and reproducibility of LS-TiO2 PSCs compared with the Fig. 3.(a) J-V curves of reference and LS-TiO2PSCs, (b) Efficiency statistics histograms of reference and LS-TiO2PSCs, (c) Voc, (d) Jsc, (e) FF and (f) Rsh/Rs
distribution of reference and LS-TiO2PSCs from 30 devices.
Fig. 4.(a) EQE and the integrated Jsccurves, and (b) Absorption spectra of reference and LS-TiO2PSCs, respectively.
Fig. 5.Contact angle for reference TiO2(a) and LS-TiO2(b)film. (c), (d) top-view SEM images of perovskitefilms deposited on TiO2and LS-TiO2films, respectively.
reference PSCs. To better understand the improved Jscin the LS-TiO2
PSCs, the EQE and the absorption spectra of the reference and LS-TiO2
PSCs were measured.Fig. 4shows the EQE, the integrated Jsccurves and absorption spectra of the reference PSC and LS-TiO2 PSCs. It is noted that EQE, absorption and integrated Jscfor LS-TiO2PSC in the range of 350–750 nm are higher than that of reference PSC, which is consistent with the Jscobtained in J-V curves discussed above.
The obtained positive effect stimulates us to study the surface characteristics of LS-TiO2film.Fig. 5a and b shows the contact angles of water directly dropped on the reference and LS-TiO2films. It can be seen that the contact angle is much smaller in the LS-TiO2film. The decreased contact angle suggests that surface wetting properties of perovskite precursor on TiO2 ETL are improved after the Li2SiO3
modification, which may facilitate to improve the coverage rate of perovskite materials on TiO2ETL [27]. The top-view SEM images of perovskitefilms deposited on TiO2and LS-TiO2are respectively pre- sented inFig. 5c and d.Fig. S1(Supporting Information) shows the grain size distribution histograms of perovskite layer obtained from Fig. 5c and d. The average grain size of perovskitefilm grown on TiO2
and LS-TiO2films is about 175 and 225 nm, respectively. Obviously, the perovskitefilm grown on LS-TiO2film has larger grain size than that of thefilm deposited on reference TiO2. For PSCs with good performance, large and regular grains are necessary [40,41].
To investigate the effect of Li2SiO3 modification on the electric properties of ETL at nanoscale level, the local electrical properties such as local current distribution and average contact potential were Fig. 6.AFM images (10 × 10μm) of TiO2(a) and LS-TiO2(d)films, respectively. C-AFM images of TiO2(b) and LS-TiO2(e)films, respectively. KPFM images of TiO2 (c) and LS-TiO2(f)films, respectively.
Fig. 7.(a) Dark I-V curves of TiO2 and LS-TiO2 device.
Measured data (square), which werefitted by the line (blue).
(b) Log(J)-log(V) plots for the reference and LS-TiO2films using the SCLC model with the device structure of FTO/TiO2
or LS-TiO2/PCBM/Ag. Measured data (square) werefitted by a line with a slope of 2 (blue) and the intercepts of thefitted line on log (J) were used to calculate the electron mobility.
(For interpretation of the references to colour in thisfigure legend, the reader is referred to the Web version of this ar- ticle.)
Fig. 8.(a) Nyquist plots of reference and LS-TiO2PSCs measured under light illumination. The open symbol represents the experimental data and the solid line is the fitting results. Insert: the equivalent circuit diagram which is used tofit the data of Nyquist plots. (b) Rrecobtained from thefitting results of Nyquist curves. (c) PL spectra of perovskitefilms deposited on reference and LS-TiO2films.
Table 1
Photovoltaic parameters of reference and LS-TiO2 PSCs.
Voc (V) Jsc(mA/cm2)c Jsc(mA/cm2)d FF(%) PCE(%)
Reference 1.001 19.82 18.59 74.52 14.78a(13.82)b
LS-PSC 1.042 23.50 22.31 76.22 18.67a(17.52)b
a Best PCE.
b Average PCE from 30 devices.
c Measured Jsc values from the solar simulator.
dIntegrated Jsc values from the EQE curves.
measurement [42]. It is well known that the KPFM is regarded to be an effective way to map the surface potential distribution on the local grain boundaries and grain bulk. Actually, the surface potential origi- nates from contact potential difference between the tip and the sample surface, which can reflect local polarization [45].Fig. 6c and f shows the KPFM images of TiO2and LS-TiO2films, respectively. It is noted that the average surface potential of LS-TiO2 film is reduced by
∼0.16 V, compared with the reference TiO2film. This suggests that the work function of TiO2 film is increased after Li2SiO3 modification, which indicates that electron can be extracted efficiently at higher energy level [43]. This is agreement with the enhanced Vocof LS-TiO2
PSC [42,46].
To explore the effect of Li2SiO3modification on the charge trans- port, electron trap state density and carrier mobility of TiO2and LS- TiO2 ETLs were characterized. The value of the electron trap state density can be obtained from the dark I-V analysis for electron-only devices with the structure of FTO/TiO2or LS-TiO2/Ag [35]. As shown inFig. 7a, the bluefitting line at the low bias voltage region indicates an ohmic response of electron-only device. In this ohmic contact area, the conductivity (σ0) can be estimated from the slope of I-V plot, I =σ0
(A/L)V, where A is the areas of sample (0.045 cm2), and L is the thickness of sample (≈65 nm, as shown inFig. S2).Fig. S3andTable S1 display the conductivity distributions of TiO2and LS-TiO2films from 40 devices. The calculated average conductivity of LS-TiO2
((7.04 ± 0.74) × 10−7S/cm) is higher than that of pure TiO2
((5.24 ± 0.8) × 10−7S/cm), suggesting the improved conductivity after Li2SiO3modification [35]. With the increase of current, the bias voltage exceeds the kink point, suggesting that trap states are com- pletely filled. The current increases significantly with much higher slope. The trap state density (Nt) can be estimated by the trap-filled limit voltage (VTFL) [47,48].
=
V eN L
εε
TFL 2t 2
0
where e is elementary charge (e = 1.6 × 10−19C), L is the thickness of TiO2layer and LS-TiO2(≈65 nm),εis the relative dielectric constant of the TiO2(≈50),ε0is the vacuum permittivity (ε0= 8.854 × 10−12F/
m). The VTFLof the devices based on TiO2(1.404 V) is higher than that of the device based on LS-TiO2 (0.785 V). Thus the calculated Nt of reference TiO2device (1.84 × 1015cm−1) is much higher than that of LS-TiO2 device (1.03 × 1015cm−1). This suggests that Li2SiO3 mod- ification can reduce the electron trap density, resulting in the promoted electron transport [49].
To measure the electron mobility of TiO2and LS-TiO2ETLs, we have fabricated the electron-only devices with the structure of FTO/TiO2or LS-TiO2/PCBM/Ag. The electron mobility of TiO2and LS-TiO2films can be estimated from the log(J)-log(V) curves in Fig. 7b by the Mott- Gurney equation as following [47]:
combination at the TiO2/CH3NH3PbI3/Spiro-OMeTAD interfaces and the recombination resistance (Rrec) can be obtained from thefitting pattern inserted inFig. 8a [50]. As shown inFig. 8b, the LS-TiO2PSC displays larger Rrec value than that of the reference PSC, which is beneficial to improve the performance of PSCs [51]. The steady PL spectra were used to investigate the electron extraction and transport [52,53].Fig. 8c shows the PL spectra of perovskitefilms deposited on reference and LS-TiO2films, respectively. As seen inFig. 8c, the re- ference film shows higher PL intensity, suggesting the larger charge recombination rates compared to the perovskite layer deposited on the LS-TiO2[27,39]. This is consistent with the EIS results [24]. These data confirm that the Li2SiO3modification on TiO2does reduce the charge recombination rates, contributing to the efficient electron extraction and transport, thus enhancing the photovoltaic performance [27,53].
4. Conclusions
In summary, we have fabricated efficient planar PSCs based on LS- TiO2ETL by low-temperature process. Compared with reference TiO2
ETL, the conductivity and mobility of LS-TiO2ETL are enhanced and the electron trap density is reduced. The LS-TiO2 PSC exhibits a champion efficiency of 18.67%, which is much higher than that of 14.78% of the reference PSC. The improved performance in LS-TiO2
PSC can be attributed to enhanced EQE, promoted charge transfer and reduced charge recombination rates. This work provides a simple and efficient strategy to fabricate efficient PSCs with low-temperature pro- cess.
Acknowledgements
We acknowledge thefinancial support of the National Key R & D Program of China (2016YFB0401502, 2016YFA0201002), the Natural Science Foundation of Guangdong Province (No. 2016A030313421), the Characteristic Innovation Project of Guangdong Provincial Department of Education (Science 2016, 22), the National Natural Science Foundation of China (Grant Nos. 51431006, 51472093, 61574065, 21403089), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016), the Science and Technology Planning Project of Guangdong Province (Grant No.2016B090906004, 2015B090927006), the Program for Changjiang Scholars and Innovative Research Team in University (IRT_17R70), and the Guangdong Innovative Research Team Program (No. 2011D039) and the MOE International Laboratory for Optical Information Technologies.
Appendix A. Supplementary data
Supplementary data related to this article can be found athttp://dx.
doi.org/10.1016/j.jpowsour.2018.04.086.
M. Graetzel, H. Han, Science 345 (2014) 295–298.
[11] B.-X. Chen, H.-S. Rao, W.-G. Li, Y.-F. Xu, H.-Y. Chen, D.-B. Kuang, C.-Y. Su, J. Math.
Chem. A 4 (2016) 5647–5653.
[12] Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Nat. Energy 2 (2016) 16177.
[13] Y. Zhou, Z. Zhang, Y. Cai, H. Liu, Q. Qin, Q. Tai, X. Lu, X. Gao, L. Shui, S. Wu, J.- M. Liu, Electrochim. Acta 260 (2018) 468–476.
[14] D. Liu, T.L. Kelly, Nat. Photon. 8 (2014) 133–138.
[15] W. Zhang, J. Xiong, L. Jiang, J. Wang, T. Mei, X. Wang, H. Gu, W.A. Daoud, J. Li, ACS Appl. Mater. Interfaces 9 (2017) 38467–38476.
[16] D. Bi, C. Yi, J. Luo, J.-D. Decoppet, F. Zhang, S.M. Zakeeruddin, X. Li, A. Hagfeldt, M. Gratzel, Nat. Energy 1 (2016) 16142.
[17] Y. Cai, Z. Zhang, Y. Zhou, H. Liu, Q. Qin, X. Lu, X. Gao, L. Shui, S. Wu, J. Liu, Electrochim. Acta (2017),http://dx.doi.org/10.1016/j.electacta.2017.12.135.
[18] F. Di Giacomo, V. Zardetto, A. D'Epifanio, S. Pescetelli, F. Matteocci, S. Razza, A. Di Carlo, S. Licoccia, W.M.M. Kessels, M. Creatore, T.M. Brown, Adv. Energy Mat. 5 (2015) 1401808.
[19] D. Yang, R. Yang, J. Zhang, Z. Yang, S. Liu, C. Li, Energy Environ. Sci. 8 (2015) 3208–3214.
[20] K. Wang, Y. Shi, Q. Dong, Y. Li, S. Wang, X. Yu, M. Wu, T. Ma, J. Phys, Chem. Lett. 6 (2015) 755–759.
[21] G. Yin, J. Ma, H. Jiang, J. Li, D. Yang, F. Gao, J. Zeng, Z. Liu, S.F. Liu, ACS Appl.
Mater. Interfaces 9 (2017) 10752–10758.
[22] Y. Ogomi, A. Morita, S. Tsukamoto, T. Saitho, Q. Shen, T. Toyoda, K. Yoshino, S.S. Pandey, T. Ma, S. Hayase, J. Phys. Chem. C 118 (2014) 16651–16659.
[23] Y.C. Shih, L.Y. Wang, H.C. Hsieh, K.F. Lin, J. Math. Chem. A 3 (2015) 9133–9136.
[24] L. Zuo, Z. Gu, T. Ye, W. Fu, G. Wu, H. Li, H. Chen, J. Am. Chem. Soc. 137 (2015) 2674–2679.
[25] K. Wojciechowski, S.D. Stranks, A. Abate, G. Sadoughi, A. Sadhanala, N. Kopidakis, G. Rumbles, C.-Z. Li, R.H. Friend, A.K.Y. Jen, H.J. Snaith, ACS Nano 8 (2014) 12701–12709.
[26] W. Ke, G. Fang, Q. Liu, L. Xiong, P. Qin, H. Tao, J. Wang, H. Lei, B. Li, J. Wan,
[38] M. Park, J.-Y. Kim, H.J. Son, C.-H. Lee, S.S. Jang, M.J. Ko, Nanomater. Energy 26 (2016) 208–215.
[39] Z. Zhang, Y. Zhou, Y. Cai, H. Liu, Q. Qin, X. Lu, X. Gao, L. Shui, S. Wu, J.-M. Liu, J.
Power Sources 377 (2018) 52–58.
[40] Y. Chen, B. Li, W. Huang, D. Gao, Z. Liang, Chem. Commun. 51 (2015) 11997–11999.
[41] Q. Tai, P. You, H. Sang, Z. Liu, C. Hu, H.L. Chan, F. Yan, Nat. Commun. 7 (2016) 11105.
[42] W. Chen, K. Li, Y. Wang, X. Feng, Z. Liao, Q. Su, X. Lin, Z. He, J. Phys. Chem. Lett. 8 (2017) 591–598.
[43] J. Li, B. Huang, E. Nasr Esfahani, L. Wei, J. Yao, J. Zhao, W. Chen, npj, Quantum Mater 2 (2017) 56.
[44] B.P. Nguyen, G.Y. Kim, W. Jo, B.J. Kim, H.S. Jung, Nanotechnology 28 (2017) 315402.
[45] T. Wu, L. Collins, J. Zhang, P.Y. Lin, M. Ahmadi, S. Jesse, B. Hu, ACS Nano 11 (2017) 11542–11549.
[46] W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, H. Chen, E. Bi, I. Ashraful, M. Graetzel, L. Han, Science 350 (2015) 944–948.
[47] R.H. Bube, J. Appl. Phys. 33 (1962) 1733–1737.
[48] D. Yang, R. Yang, X. Ren, X. Zhu, Z. Yang, C. Li, S.F. Liu, Adv. Mater. 28 (2016) 5206–5213.
[49] X. Ren, D. Yang, Z. Yang, J. Feng, X. Zhu, J. Niu, Y. Liu, W. Zhao, S.F. Liu, ACS Appl.
Mater. Interfaces 9 (2017) 2421–2429.
[50] A.R. Pascoe, N.W. Duffy, A.D. Scully, F. Huang, Y.-B. Cheng, J. Phys. Chem. C 119 (2015) 4444–4453.
[51] E. Guillén, F.J. Ramos, J.A. Anta, S. Ahmad, J. Phys. Chem. C 118 (2014) 22913–22922.
[52] B.A. Nejand, V. Ahmadi, S. Gharibzadeh, H.R. Shahverdi, ChemSusChem 9 (2016) 302–313.
[53] N. De Marco, H. Zhou, Q. Chen, P. Sun, Z. Liu, L. Meng, E.P. Yao, Y. Liu, A. Schiffer, Y. Yang, Nano Lett. 16 (2016) 1009–1016.
