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Coatings 2020, 10(1), 46; https://doi.org/10.3390/coatings10010046

Article
Enhancing Perovskite Solar Cell Performance through Surface Engineering of Metal Oxide Electron-Transporting Layer
1
Qinghai Huanghe Upstream Hydropower Development Co. Ltd. Photovoltaic Industry Technology Branch, Photovoltaic Technology Co., Ltd. of Huanghe Hydropower, Xining 810000, Qinghai, China
2
Faculty of Automation and Information Engineering, Xi’an University of Technology, Xi’an 710048, China
3
State Key Discipline Laboratory of Wide Band Gap Semiconductor Tecchnology, Shaanxi Joint Key Laboratory of Graphene, School of Microelectronics, Xidian University, Xi’an 710071, China
*
Author to whom correspondence should be addressed.
Received: 23 November 2019 / Accepted: 20 December 2019 / Published: 3 January 2020

Abstract

:
Perovskite solar cells have gained increasing interest in recent times owing to the rapidly enlarged device efficiency and tunable optoelectronic properties in various applications. In perovskite solar cells, interface engineering plays an important role in determining the final device efficiency and stability. In this study, we adopted TiCl4 treatment to reduce the surface roughness of the metal oxide layer and improve the perovskite film quality to obtain better device performance. After proper TiCl4 treatment, the efficiencies of TiCl4–TiO2- and TiCl4–ZnO-based devices were significantly enhanced up to 16.5% and 17.0%, respectively, compared with those based on pristine TiO2 and ZnO (13.2% and 10.2%, respectively).
Keywords:
perovskite solar cells; TiCl4 treatment; electron-transporting layer; interface engineering

1. Introduction

Perovskite solar cells (PSCs) have attracted continuously increased interest owing to their unique properties and high performance [1,2,3,4,5,6,7,8,9,10]. In order to further enhance device efficiency and stability, some additional interface and compositional engineering approaches are necessary [11,12,13,14,15,16,17,18]. Some interface engineering approaches have previously been investigated to optimize energy level alignment and relieve charge accumulation [19,20,21]. Generally, for interface transport layers, a high-quality film with properties including sufficient charge extraction capability, good film conductivity, and proper energy level should be guaranteed [19,22]. Hence, various interface transporting layers have been studied. Among them, metal oxides (e.g., ZnO, SnO2, and TiO2) have been widely investigated as electron-transporting layers (ETLs) in PSCs [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37], particularly TiO2 ETL. The surface and electronic properties of TiO2 play important roles in determining the final device performance, including power conversion efficiency (PCE), hysteresis behavior, and stability [20,38,39]. In order to enhance device performance and reduce hysteresis behaviors encountered in TiO2-based devices, various techniques have been used to optimize the TiO2 surface or electronic properties. This includes the addition of fullerene molecules, amino acids, or C60-SAM, which are commonly applied to optimize the charge transfer process [28,29,40,41]. Besides this, TiCl4 soaking treatment has been investigated in dye-sensitized solar cells to enhance device performance [42]. TiCl4 treatment has also been applied in PSC devices to fill the voids at the TiO2/perovskite layer interfaces and smooth the TiO2 surface [39,43,44].
In this study, to investigate the mechanism of TiCl4 and the role it plays with TiO2 and phenyl-C61-butyric acid methyl ester (PCBM) in PSCs, we investigated the effects of TiCl4 treatment on the quality of perovskite thin film and the corresponding device performance. Results showed that, after TiCl4 treatment, the perovskite film quality was significantly enhanced; moreover, the charge transfer and extraction became more efficient. Finally, compared to pristine TiO2- and ZnO-based devices (13.2% and 10.2%), the efficiencies of both TiCl4–TiO2- and TiCl4–ZnO-based devices were significantly improved up to 16.5% and 17.0%, respectively, with simultaneously enhanced open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF).

2. Results and Discussion

Surface morphologies of ETLs with and without TiCl4 treatment were investigated by tapping-mode atomic force microscopy (AFM) (Dimension Icon AFM, Bruker, Billerica, MA, USA). The surface roughness of the TiO2 surface decreased from 18.0 to 13.9 nm after TiCl4 treatment (Figure 1). After depositing the PCBM layer, the surface roughness was maintained without much change. This indicates that the surface roughness is mainly determined by TiCl4 treatments. These properties may affect perovskite crystallization and formation as well as the crystal size.
The energy levels of TiO2/PCBM electron-transporting layers with and without TiCl4 treatment were investigated by ultraviolet photoelectron spectroscopy (UPS) (Escalab 250Xi, Thermo Scientific, Waltham, MA, USA). In Figure 2, it can be seen that the TiO2/PCBM work function decreased from 4.18 to 4.09 eV after TiCl4 treatment. The decreased work function would result in better energy level alignment and more efficient electron transfer between the conduction band of perovskite and the Fermi level of ETL. Moreover, the potential difference between two electrode contacts, such as TiO2 and spiro-OMeTAD, was also enlarged due to the decreased work function, hence increasing the Voc of the device [33].
In order to illustrate the effect of TiCl4 treatment on the properties of perovskite thin film, the thin film morphologies of the perovskites upon ETLs with and without TiCl4 treatment were investigated. As shown in Figure 3, the perovskite crystal size was enhanced, indicating a favorable crystal growth process on the TiCl4-treated surface, which may have been caused by the low surface roughness value. The improved crystalline quality was also confirmed by the XRD patterns. As shown in Figure 4, strong diffraction intensity at around 14.6°, 28.9°, and 32.3° were observed and assigned to (110), (220), and (310) crystal planes of tetragonal perovskite phase, respectively. The diffraction pattern at 13.1° originated from PbI2 because of the excess PbI2 over CH3NH3I. These results are consistent with those previously reported [8,9]. Moreover, the diffraction intensity of the perovskite thin films with additional TiCl4 treatment exhibited slight enhancement. This indicates that the thin film crystallinity was increased, resulting in more efficient charge transport and collection.
The film absorption spectra of perovskite films deposited on TiO2 and TiCl4-treated ETLs were investigated by UV–Vis spectroscopy. Both films exhibited high absorption intensity over the entire UV–visible range, and the absorption band edge was around 1.61 eV (Figure 5). Compared with pristine TiO2 ETL, the absorption of the perovskite film based on TiCl4-treated TiO2 ETL was significantly enhanced, indicating enhanced crystallinity of the resulting thin film.
To further investigate the photophysical properties of the thin films, steady-state photoluminescence (PL) and time-resolved PL (TR-PL) measurements of perovskite thin films were further investigated. The perovskite film based on TiCl4-treated TiO2 ETL showed larger PL intensity decay compared to the pristine TiO2-based perovskite film (Figure 6), indicating a faster charge transfer process between TiO2 and perovskite thin films [30]. Furthermore, the PL lifetimes of perovskite films on glass were also measured (Figure 7). By fitting the TR-PL curves with biexponential function, two decay parts—fast decay and slow decay—could be obtained [14]. The average lifetimes derived from the fitting curves were around 25.7 and 29.8 ns for perovskite films with and without TiCl4 treatment, respectively. The decreased PL lifetime is related to the efficient charge carrier transfer induced quenching process [27]. This is essential for efficient charge extraction and collection of the device.
The effect of TiCl4 treatment on PSC device performance was studied based on a planar structure with a configuration of FTO/TiO2/PC60BM/CH3NH3PbI3−xClx/spiro-OMeTAD/Ag. Figure 8 exhibits the current density − voltage (JV) curves of PSC devices with and without TiCl4 treatment, and Table 1 summarizes the corresponding device parameters. In Figure 8, a PCE of 13.2% (Voc of 1.04 V, Jsc of 19.1 mA/cm2, and FF of 0.66) can be seen for the device with pristine TiO2. By comparison, all the parameters, i.e., Voc, Jsc, and FF, for the device with TiCl4 treatment were enhanced simultaneously, resulting in an improved average PCE of 16.5% (Voc of 1.08 V, Jsc of 22.4 mA/cm2, and FF of 0.68). The device series resistance (Rs) and shunt resistance (Rsh) were also calculated to further understand the improvement in performance. Compared to the device without TiCl4 treatment, the Rs value of the device with TiCl4 treatment decreased to 3.7 Ω·cm2, while the Rsh increased to 6.9 kΩ·cm2. The decreased Rs and increased Rsh are responsible for the Jsc and FF enhancement [7]. It needs to be mentioned that, when the high-temperature TiO2 was replaced with the low-temperature ZnO layer, the device also showed similar improvement. The ZnO with TiCl4 treatment revealed an improved PCE of 17.0% with a Voc of 1.08 V, Jsc of 23.2 mA/cm2, and FF of 0.67, which was much higher than those of the device with pristine ZnO.
The device hysteresis behavior is also a significant parameter to determine the final performance of the device. It can be affected by many factors, such as carrier trapping, ion motion, charge accumulation at interfaces, and so on [45,46,47,48]. The device hysteresis behaviors under different scan directions were measured (Figure 9). The device did not exhibit serious hysteresis behavior before and after TiCl4 treatment, indicating that TiCl4 has less effect than PCBM, which plays a major role in determining the final hysteresis behavior.
In order to further investigate the mechanism involved in performance enhancement, transient photocurrent and photovoltage measurements of perovskite solar cells were performed. Figure 10a shows the transient photocurrent decay of perovskite devices measured at short-circuit condition. After TiCl4 treatment, the devices exhibited faster decay with shorter lifetime (1.02 µs) compared to the pristine TiO2-based device (1.78 µs). This indicates that the device with TiCl4 treatment possessed more efficient charge transfer process. The transient photovoltage was used to determine the charge recombination process (Figure 10b). It can be clearly seen that the device with TiCl4 treatment exhibited much longer lifetime (2.32 ms) compared to the pristine TiO2-based device (1.66 ms). This indicates that the charge recombination process was efficiently suppressed. Hence, the Jsc and FF exhibited significant improvement.

3. Conclusions

In conclusion, we investigated the effect of TiCl4 treatment on perovskite thin film formation and PSC device performance. The results showed that TiCl4 treatment had a beneficial effect on the properties of perovskite thin film. It could enhance thin film crystallinity and improve charge transfer and extraction. Finally, the PCE was enhanced from 13.2% to 16.5% for devices based on TiO2 ETL and from 10.2% to 17.0% for devices based on ZnO ETL. This is important for understanding interfacial treatment and further improves device efficiency and stability.

Author Contributions

Conceptualization, Z.L.; methodology, G.L., X.W. and J.D.; validation, J.M.; formal analysis, G.L., M.Z. and J.M.; investigation, G.L., J.D. and Y.G.; resources, X.W., M.Z., and Y.L.; data curation, G.L., X.W. and J.M.; writing—original draft preparation, G.L.; writing—review and editing, Z.L., G.L. and J.M.; visualization, J.D., M.Z., Y.G. and Y.L.; supervision, Z.L.; project administration, Z.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China, grant number 61604119, 61704131, and 61804111, and the Fundamental Research Funds for the Central Universities.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Atomic force microscopy (AFM) images of (a) pristine TiO2 electron-transporting layer (ETL), (b) with TiCl4 treatment, and (c) with TiCl4 and PCBM treatment.
Figure 1. Atomic force microscopy (AFM) images of (a) pristine TiO2 electron-transporting layer (ETL), (b) with TiCl4 treatment, and (c) with TiCl4 and PCBM treatment.
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Figure 2. Ultraviolet photoelectron spectroscopy (UPS) spectra of ETL layers with and without TiCl4 treatment.
Figure 2. Ultraviolet photoelectron spectroscopy (UPS) spectra of ETL layers with and without TiCl4 treatment.
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Figure 3. SEM images of perovskite films deposited on TiO2 (a) and TiCl4-treated TiO2 ETLs (b).
Figure 3. SEM images of perovskite films deposited on TiO2 (a) and TiCl4-treated TiO2 ETLs (b).
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Figure 4. XRD patterns (a) and zoomed-in XRD patterns (b) of perovskite films deposited on TiO2 and TiCl4-treated TiO2 ETLs. “#” and “&” represent the characteristic peaks of fluorine-doped tin oxide (FTO) and TiO2, respectively.
Figure 4. XRD patterns (a) and zoomed-in XRD patterns (b) of perovskite films deposited on TiO2 and TiCl4-treated TiO2 ETLs. “#” and “&” represent the characteristic peaks of fluorine-doped tin oxide (FTO) and TiO2, respectively.
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Figure 5. UV–vis spectra of perovskite films deposited on TiO2 and TiCl4-treated TiO2 ETLs.
Figure 5. UV–vis spectra of perovskite films deposited on TiO2 and TiCl4-treated TiO2 ETLs.
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Figure 6. Photoluminescence (PL) spectra of perovskite films deposited on TiO2 and TiCl4-treated TiO2 ETLs.
Figure 6. Photoluminescence (PL) spectra of perovskite films deposited on TiO2 and TiCl4-treated TiO2 ETLs.
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Figure 7. Time-resolved PL (TR-PL) spectra of perovskite films deposited on TiO2 and TiCl4-treated TiO2 ETLs.
Figure 7. Time-resolved PL (TR-PL) spectra of perovskite films deposited on TiO2 and TiCl4-treated TiO2 ETLs.
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Figure 8. Current density − voltage (JV) characteristics of devices based on TiO2 (a) and ZnO (b) ETLs with and without TiCl4 treatment.
Figure 8. Current density − voltage (JV) characteristics of devices based on TiO2 (a) and ZnO (b) ETLs with and without TiCl4 treatment.
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Figure 9. Hysteresis behavior of devices based on TiO2 ETL without (a) and with (b) TiCl4 treatment.
Figure 9. Hysteresis behavior of devices based on TiO2 ETL without (a) and with (b) TiCl4 treatment.
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Figure 10. (a) Transient photocurrent and (b) photovoltage decay characteristics of perovskite solar cells based on TiO2 ETL with and without TiCl4 treatment.
Figure 10. (a) Transient photocurrent and (b) photovoltage decay characteristics of perovskite solar cells based on TiO2 ETL with and without TiCl4 treatment.
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Table 1. Device parameters of perovskite solar cells (PSCs) with and without TiCl4 treatment.
Table 1. Device parameters of perovskite solar cells (PSCs) with and without TiCl4 treatment.
InterlayerVoc (V)Jsc (mA/cm2)FFPCE (%)Rs (Ω cm2)Rsh (kΩ cm2)
TiO2/PCBM1.0419.10.6613.25.94.1
TiO2–TiCl4/PCBM1.0822.40.6816.53.76.9
ZnO/PCBM0.9320.70.5310.210.21.5
ZnO–TiCl4/PCBM1.0823.20.6717.05.12.8
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