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CO Plasma-Treated TiO Film as an Effective Electron Transport Layer for High-Performance Planar Perovskite Solar Cells Kang Wang, Wenjing Zhao, Jia Liu, Jinzhi Niu, Yucheng Liu, Xiaodong Ren, Jiangshan Feng, Zhike Liu, Jie Sun, Dapeng Wang, and Shengzhong (Frank) Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11329 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 16, 2017
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CO2 Plasma-Treated TiO2 Film as an Effective Electron Transport Layer for High-Performance Planar Perovskite Solar Cells Kang Wang, Wenjing Zhao, Jia Liu, Jinzhi Niu, Yucheng Liu, Xiaodong Ren, Jiangshan Feng, Zhike Liu, Jie Sun, Dapeng Wang,* and Shengzhong (Frank) Liu* Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China
KEYWORDS: plasma-enhanced chemical vapor deposition, CO2 plasma treatment, TiO2 film, electron transport layer, perovskite, solar cells
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ABSTRACT Perovskite solar cells (PSCs) have received great attention due to their excellent photovoltaic properties especially for the comparable efficiency to silicon solar cells. The electron transport layer (ETL) is regarded as a crucial medium in transporting electrons and blocking holes for PSCs. In this study, CO2 plasma generated by plasma-enhanced chemical vapor deposition (PECVD) was introduced to modify the TiO2 ETL. The results indicated that CO2 plasma treated compact TiO2 layer exhibited better surface hydrophilicity, higher conductivity, and lower bulk defect state density in comparison with the pristine TiO2 film. The quality of the stoichiometric TiO2 structure was improved and the concentration of oxygen-deficiencies induced defect sites was reduced significantly after CO2 plasma treated for 90 s. The PSCs with TiO2 film treated by CO2 plasma for 90 s exhibited simultaneously the improved short-circuit current (Jsc) and fill factor (FF). As a result, PSC based TiO2 ETL with CO2 plasma treatment affords a power conversion efficiency (PCE) of 15.39%, outperforming that based on pristine TiO2 (13.54%). These results indicate that the plasma treatment by PECVD method is an effective approach to modify ETL for high performance planar PSCs.
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1. INTRODUCTION Perovskite solar cells (PSCs) have been recognized as a promising replacement for silicon solar cells, due to their high absorption coefficient, suitable direct band gap, high carrier mobility, and long diffusion length. In recent years, a certified efficiency of PSCs has been reached up to 22.1%,1 outperforming other third-generation solar cells, such as organic solar cells and dye-sensitized solar cells. PSCs have also shown other advantages, including low cost and easy to commercialize.2-8 The general formula of perovskites in PSCs is ABX3, where A is CH3NH3+, HC(NH2)2+, or Cs+, B is Pb2+ or Sn2+, and X is a halide.9-13 A typical configuration of PSC consisted of two electrodes, a hole transport layer (HTL), a perovskite layer, and an electron transport layer (ETL). The ETL plays an important role in blocking holes and transporting electrons.14 Although there are many types of materials used for ETL,15-18 most of ETL in the state-of-the-art PSCs are based on anatase-TiO2 owning to its suitable band structure, high electron mobility, environmental friendliness, and low cost.19 Up to now, the main synthetic methods for TiO2 consist of chemical-bath deposition,20 magnetron sputtering,21 and atomic layer deposition.22 Compared with the various deposition methods, the electron-beam evaporation (EBE) is an affordable technique which can be successfully employed to obtain the high crystalline, uniform, large area, and low cost high-quality TiO2 layers on various types of substrates.23 However, the TiO2-based planar heterojunction PSCs always suffer from the relatively high interface potential barrier and poor charge collection efficiency under steady-state forward voltage bias, which accused by the numerous deep traps and/or electronic defect states originated from oxygen vacancies within the TiO2 lattice.24 A number of studies reported that the TiO2 layer with surface modification can enhance the performance of the planar heterojunction solar cells.25 Snaith et al. reported that a fullerene C60 can modify the compact TiO2 and extract photogenerated charge more effectively.26 Petrozza et al. discovered that the PSCs based on TiOX/PC60BM layer can achieve a stabilized PCE of 17.6%, which is significantly higher than that of PSCs with only TiOX (14.4%).27 Liu et al. demonstrated that Li-doping of TiO2 can passivate the electron traps and increase the conductivity of TiO2 for improving the performance of PSCs.28 According to the reported results, it is an effective way to improve the performance of PSCs through improving the surface quality and
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reducing the defect densities of TiO2. Plasma treatment can contribute to a significant change in the surface energy, which plays a crucial role in membrane fouling and cleaning phenomena.29 Plasma-enhanced chemical vapor deposition (PECVD) treatment can promote the bulk defect passivation independently of surface effects.30 Gas discharge in CO2 by PECVD method is known to be characterized by intense vacuum-ultraviolet irradiation producing active oxygen species radicals. These radicals can effectively oxidize the surface and bulk of sample during the plasma treatment. In the CO2 plasma process, the gas phase contains numerous reactive species including ions, electrons, free radicals, and molecules with a variety of electronically excited states. The plasma can effectively oxidize the solid surface and that oxidizing plasma contributes to great improvement in interfacial adhesion. Simultaneously, the CO2 plasma impacts the surface species from the solid substrate, creating surface-active sites, which also removes the weak boundary layers and contaminants. Therefore, the CO2 plasma process offers a technique to tailor sample surface chemical modification without sacrificing bulk properties.31 In this study, we proposed a CO2 plasma method by PECVD to modify EBE deposited TiO2 layers. The CO2 plasma treated TiO2 films exhibited the better surface hydrophilicity, the higher conductivity, and the lower bulk defect density in comparison to the pristine TiO2 layer. The X-ray photospectroscopy (XPS) results revealed that 90 s CO2 plasma treatment on TiO2 can not only improve the quality of the stoichiometric TiO2 structure, but also reduce the concentration of oxygen-deficiency induced defect sites, resulting in the reduced electron recombination and the enhanced electron transport in PSCs. Therefore, the PSCs based on TiO2 film that treated by CO2 plasma for 90 s exhibited the improved short-circuit current (Jsc) and fill factor (FF) simultaneously. As a result, PSC based TiO2 ETL with CO2 plasma treatment affords a PCE of 15.39%, outperforming that based on pristine TiO2 (13.54%). 2. EXPERIMENTAL PROCEDURE 2.1 Preparation of the TiO2 layer TiO2 thin films were deposited on FTO substrate by the EBE. Firstly, the FTO glass substrate was washed by acetone, isopropanol, and ethanol in an ultrasonic processing for 30 min in sequence
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and then fixed on the substrate holder. The purchased TiO2 particles without any purification were placed in a crucible below the substrate holder and were evaporated by an incident electron beam (accelerating voltage of 10 kV) under vacuum. The deposition temperature was set at room temperature with a desired pressure of 4×10-4 Mbar. The electron gun voltage was fixed at 2.0 kV, the current was adjusted to 60 mA, and the substrate was rotated at 20 rpm. Finally, the uniform TiO2 film with a thickness of ~50 nm was obtained. The deposited TiO2 films were then treated with CO2 plasma by PECVD method for 90 s, 5 min, and 10 min at 200 °C, respectively. The main plasma treatment parameters, such as operating power of 60 W, CO2 gas flow rate of 50 sccm, and chamber pressure of 200 Pa, were finally optimized. Also, a TiO2 film heated by hot-plate at 200 °C for 20 min was chose as the pristine one for comparison. To investigate the impact of CO2 plasma treatment on the properties of the TiO2 film, all TiO2 films were not undergone the additional treatment. 2.2 Device fabrication The structure of PSC was FTO/TiO2/CH3NH3PbI3/HTL/Au. The PbI2 was purchased from Alfar Aesar (99.99%) and CH3NH3I (MAI) was synthesized according to our previous publication,32 1.106 g PbI2 and 0.38 g MAI were dissolved in 2 mL of γ-butyrolactone (GBL) at room temperature in glove box to form perovskite precursor solution. The solution was spin-coated on TiO2 layer at 4000 rpm for 30 s and then the substrate was annealed on a hot-plate at 120 °C for 10 min. Methylbenzene was used as anti-solvent during spin-coated process. 18 mg spiro-OMeTAD, 28.8 µL tert-butylpyridine, and 44 µL lithium bis imide acetonitrile solution (520 mg/mL) were dissolved in 2 ml chlorobenzene (CB) to form the HTL solution. The solution was spin-coated onto perovskite layer at 5000 rmp for 30 s. Au electrode with a thickness of 80 nm was then deposited by thermal evaporator. 2.3 Characterization The surface morphology and roughness images of the CO2 plasma treated and the pristine TiO2 films were characterized by field-emission scanning electron microscopy (SEM, HITACHI, SU 8020) and atomic force microscope (AFM, Brooke, Dimension ICON). X-ray photoelectron spectroscopy (XPS) was carried out using a photoelectron spectrometer (ESCALAB250Xi,
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Thermo Fisher Scientific). Photoluminescence (PL) (excitation at 532 nm) was obtained with an Edinburgh Instruments Ltd. FLS980 spectrometer. The absorption and transmittance spectra were measured using an UV-vis-near-IR spectrometer (PerkinElmer, Lambda 950). The current density versus voltage (J–V) characteristics of the PSCs were measured by using a Keithley 2400 source meter under the illumination of a AM 1.5 solar simulator with the light intensity of 100 mW/cm2 (AM 1.5G, SAN-EIELECTRIC XES-40S2-CE solar simulator), as calibrated by a NREL-traceable KG5 filtered silicon reference cell. The active area of all solar cells was defined by a 9 × 9 mm2 mask. All devices were scanned in the reverse and forward directions under the standard test procedure at a scan rate of 30 mV/s. Incident photon-to-current conversion efficiency (IPCE) spectra of the PSCs were measured by a Q Test Station 500TI system (Crowntech. Inc., USA). The monochromatic light intensity for IPCE was calibrated using a reference silicon detector. 3. RESULTS AND DISCUSSION Schematic diagram of the CO2 plasma treated TiO2 film by PECVD is shown in Figure 1. The obtained TiO2 films were treated by CO2 plasma for 90 s, 5 min, and 10 min, respectively. The thickness of all TiO2 films was identical independent of various treatment methods and duration. The top-view SEM images of the pristine and the CO2 plasma treated TiO2 films are shown in Figure S1. The morphology of the plasma treated TiO2 film was similar to that of the pristine TiO2 film, showing that the CO2 plasma treatment did not change the film morphology as expected. All TiO2 films had a uniform and flat surface with the grain size of about 100 nm. Figure 2 shows the AFM images of the pristine and the CO2 plasma treated TiO2 films. The root-mean-square (RMS) roughness of the pristine TiO2 film was decreased from 8.77 to 7.95 nm when the TiO2 film was treated by the CO2 plasma for 90 s. The smoother surface would be beneficial to improve the contact and reduce the resistance between the ETL and the perovskite film. However, when the CO2 plasma duration was increased to 5 and 10 min, the RMS roughness of the TiO2 films was slightly increased to 8.40 and 9.42 nm, respectively. The results indicated that the long duration CO2 plasma treatment will slightly adjust the surface morphology of TiO2 film which maybe due to the bombardment effect during the PECVD treatment.33
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The effect of the CO2 plasma and its treatment duration on the chemical properties and bonding states of the TiO2 films was investigated by XPS. Figure 3a shows the XPS spectra of the Ti 2p peaks of the pristine and the CO2 plasma treated TiO2 films. The Ti 2p1/2 and Ti 2p3/2 peaks of all TiO2 film exhibited the binding energies of about 463.5 and 457.7 eV,34 respectively, indicating that the CO2 plasma with various treatment durations had a negligible influence on the binding energy of Ti 2p. The O 1s XPS spectra of the pristine and the CO2 plasma treated TiO2 films are shown in Figure 3b. These O 1s spectra can be resolved into three nearly Gaussian distributions with peak position of 529.2, 530.7, and 532.7 eV. The peak at the binding energy of 529.2 eV (labeled as OM) was attributed to the O2- ions combined with Ti atom in a stoichiometric TiO2 structure. The peak at the binding energy of 530.7 (labeled as OV) and 532.7 (labeled as OH) were related to the phenomena of oxygen-deficiency and hydroxyl groups on the TiO2 film, respectively. The area ratio of the OV peak to the whole O 1s peak exhibited the relative quantity of oxygen-related defects in the film. The positions, areas, and area ratios of the O1s three peaks, for the pristine and the CO2 plasma treated TiO2 films with different treatment durations, were summarized in Table 1. In case of the pristine TiO2 film, the OM/(OM + OV + OH) and OV/(OM + OV + OH) area ratios of ~60.53% and ~35.78% were obtained, respectively. The results indicated that a large amount of oxygen-related defects, such as oxygen vacancy (VO), remained into the pristine TiO2 film due to the oxygen-deficiency during the fabrication of TiO2 film in EBE system, leading to the more defect states and high concentration of defect sites existed in the TiO2 film. Interestingly, when the TiO2 film treated by CO2 plasma for 90 s, the OV/(OM + OV + OH) area ratio decreased significantly to ~30.35%. Correspondingly, the OM/(OM + OV + OH) area ratio increased obviously to ~66.53%. These results stated that the CO2 plasma can effectively reduce the oxygen-related defects and improve the quality of TiO2 film, which were attributed to the generated oxygen radicals from CO2 plasma during the treatment in the PECVD system. When the CO2 plasma treatment duration extended to 5 and 10 min, the OM/(OM + OV + OH) area ratios slightly decreased to ~62.40% and ~61.62%, and the OV/(OM + OV + OH) area ratios slightly increased to ~34.13% and ~35.21%. The observed results suggested that the weakly rearranged Ti-O bonds after 90 s plasma treatment were cleaved again after a long duration bombardment during the PECVD system, contributing to an increase in the concentration of oxygen-related defects. Noticeably, the area ratio of OH/(OM + OV + OH) was the small value of ~3.69% in the
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pristine TiO2 film, which was due to the film was prepared in the hydrogen-free condition. After the CO2 plasma treatments even for various durations, the area ratio of OH/(OM + OV + OH) maintained the value of less than 3.46%, as listed in Table 1, implying that the EBE method as well as the CO2 plasma treatment are the promising approach to obtain metal oxide semiconductor with small amounts of hydroxyl groups. To investigate the effect of CO2 plasma treatment on the electron trap density in the TiO2 films, the current versus voltage (I–V) curves of TiO2 film with electrodes were measured, as shown in Figure 4. The trap filled limit transition point (VTFL) was related to trap density. An Ohmic response was emerged at the low bias. When the voltage increased and exceeded the kink-point voltage, the applied voltage at the kink-point voltage was defined as trap-filled limit voltage VTFL. The onset voltage VTFL is linearly proportional to the density of trap states ntrap.35
=
(1)
Where e is the elementary charge of the electron (e = 1.6×10−19 C), d is the TiO2 film thickness (~50 nm), ε is the relative dielectric constant of rutile TiO2 (ε = 48), ε0 is the vacuum permittivity (ε0 = 8.854×10−12 F/m), and ntrap is the trap state density of different treated films. The VTFL as a function of ntrap were summarized in Table S1. It was obviously observed that the VTFL was significantly decreased from 0.58 V in the pristine TiO2 film to 0.21, 0.23, and 0.30 V after the TiO2 films were treated by the CO2 plasma for 90 s, 5 min, and 10 min, respectively. On the basis of the relationship between the ntrap and VTFL, the calculated ntrap in the pristine TiO2 film was much higher than that in the CO2 plasma TiO2 films. Especially, for the 90 s CO2 plasma treated TiO2 film exhibited a remarkably lower ntrap of 0.45 × 1016 cm–3 compared to the pristine TiO2 film with ntrap of 1.23 × 1016 cm–3. However, the ntrap gradually increased with the increase in the plasma treatment duration, indicating that the TiO2 film suffered from a long duration CO2 plasma bombardment would be clearly destroyed, leading to the increase in the ntraps, which was consistent with the XPS results. Based on the structure shown in inset of Figure 4, the impact of the CO2 plasma treatment on the conductivity (σ) of the TiO2 film can be calculated. The σ was evaluated using the equation 2.36 =
(2)
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where A is the contact area and D is the thickness of the pristine or the CO2 plasma treated TiO2 film. Table S1 lists the calculated σ values. It is found that the conductivity of the 90 s CO2 plasma treated TiO2 film was over 6 times higher than that of the pristine TiO2. The results suggested that the CO2 plasma treatment in a proper duration was an effective method to enhance the conductivity of the TiO2 film. To further investigate the CO2 plasma treatment on the surface state of the TiO2 films, the contact angle measurement was carried out, as shown in Figure S2. Note that the pristine and the CO2 plasma treated TiO2 films were not undergone the additional UVO treatment. The static contact angles of water on the TiO2 surface with CO2 plasma treatment duration of 90 s, 5 min, and 10 min were 37°, 30°, and 26°, respectively, which were lower than that of the pristine film of 60°. This phenomenon indicated that the CO2 plasma treatment can improve the wetting between the CH3NH3PbI3 layer and the TiO2 film in comparison of the pristine one, which could effectively enhance the interfacial quality of the TiO2 and CH3NH3PbI3 layers. The optical transmission and absorbance spectra of the pristine and the CO2 plasma treated TiO2 films are shown in Figure S3. The transmittance for all the TiO2 films was over 80% in the visible region and there was no obvious change in the absorbance spectra, indicating that the CO2 plasma treatment had no influence on the optical properties of the TiO2 films. On the basis of the above-mentioned discussions, the TiO2 film with the excellent properties, such as smoother surface, lower bulk defect state density, and good quality in the stoichiometric structure, can be achieved after the optimized CO2 plasma treatment. To explore the influence of the CO2 plasma treated TiO2 film on the performance of PSC, a normal solar cell structure, consisting of FTO/pristine or CO2 plasma treated TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au, was designed. The thicknesses of TiO2, MAPbI3, Spiro-OMeTAD, and Au layers were confirmed to be 50, 350, 300, and 80 nm, respectively, as shown in cross-sectional SEM image of device (Figure S4). All the layers of the device have the uniform and compact structure. Figure 5a shows the J–V characteristic of the champion devices with the pristine or CO2 plasma treated TiO2 films as ETL layers, which were measured in the reverse scan direction under AM 1.5G irradiation with power density of 100 mW/cm2. The relevant photovoltaic parameters including open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and efficiency
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were summarized in Table 2. The device based on the pristine TiO2 ETL gives a PCE of 13.54% with a VOC of 1.04 V, a JSC of 20.66 mA/cm2, and a FF of 63.0%. When the TiO2 ETL was treated by the CO2 plasma for 90 s, the PCE was enhanced significantly to 15.39%, which was attributed to the improved JSC (21.41 mA/cm2) and FF (69.1%). The enhanced JSC and FF may originate from the low defect state density and the high electrical conductivity of the plasma treated TiO2 ETL. To further investigate the increase in the JSC, the external quantum efficiency (EQE) and the relevantly integrated current of the PSCs based on the pristine and the 90 s CO2 plasma treated TiO2 ETLs were evaluated, as shown in the Figure S6. It was obviously found that the EQE of the PSCs with the 90 s CO2 plasma treated TiO2 ETL was slightly higher than that of the PSCs with the pristine TiO2 ETL in the whole wavelength range, indicating improved charge separation and collection at the interface of TiO2/perovskite. The calculated EQE integrated current density increased from 20.09 mA/cm2 for the pristine TiO2 based PSCs to 20.87 mA/cm2 for the 90 s CO2 plasma treated TiO2 based PSCs, which was well consistent with the J–V measurement values, indicating that the carriers were separated and collected effectively in the PSCs with the 90 s CO2 plasma treated TiO2 ETL. On the other hand, the improved FF was attributed to the reduction of the series resistance (Rs) and the increase of the shunt resistance (Rsh), as listed in Table 2. Note that the photovoltaic parameters of the PSCs, especially for the FF, gradually degraded with the increase in the CO2 plasma treated TiO2 ETL duration. The degraded FF was mainly ascribed to the increase of Rs and the reduction of Rsh, as shown in the Figure 5a. The increase of Rs suggested the low conductivity and high resistivity for electrons transporting in the TiO2 ETL. The decrease of Rsh indicated that the carriers were extracted slowly and the high density defect of states aggravated the recombination of electrons with holes, which were due to the more defect sites and high concentration of defect states introduced into the TiO2 ETL after a long duration bombardment during the PECVD system. Hitherto, the planar PSCs with TiO2 ETL always suffered the notorious J–V hysteresis when measured under different scan directions.37 Figure 5b shows the J–V curves of the PSCs with the pristine and the 90 s CO2 plasma treated TiO2 ETLs measured under the reverse and forward scan directions. It was found that the device with the pristine TiO2 ETL exhibited a severe hysteresis. Surprisingly, the J–V curves under reverse and forward scan directions almost overlapped when
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the device chose the 90 s CO2 plasma treated TiO2 ETL. The relevant photovoltaic parameters were summarized in Table 3. Previous publications revealed that the J–V hysteresis in PSCs mainly originated from the unbalanced carrier transport within charge transport layer and the high density defect of states for electronic trapping at the perovskite/ETL interface.38 Therefore, the negligible hysteresis in PSC with the 90 s CO2 plasma treated TiO2 ETL could be attributed to the lower defect density and higher conductivity of the plasma treated ETL, contributing to the efficient extraction and transportation of carriers. To confirm the uniformity and reproducibility of the PSCs, the PCE histograms with the statistics analysis for the devices with the pristine and plasma treated TiO2 ETLs were shown in the Figure 5c and 5d, respectively. The average PCE of 10.56% and 15.14% was obtained based on the 20 individual devices with the pristine and the 90 s CO2 plasma treated TiO2 ETLs, respectively, suggesting that the CO2 plasma treatment can effectively improve the efficiency of PSC. To reveal the impact of CO2 plasma treatment on the electron extraction and transport mechanism from the perovskite absorber to the TiO2 ETLs, the steady-state PL and time-resolved PL (TRPL) were performed to investigate the charge transfer and carrier recombination. Figure 6a depicts the PL spectra of the FTO/perovskite, FTO/pristine TiO2/perovskite, and FTO/90 s CO2 plasma treated TiO2/perovskite samples. It was obviously found that the FTO/perovskite exhibited the highest PL intensity, resulting from the critical recombination of the excited charges in the perovskite layer. As expected, The FTO/90 s CO2 plasma treated TiO2/perovskite sample demonstrated the more effective electron extraction capability in comparison of FTO/pristine TiO2/perovskite. Figure 6b shows the TRPL spectra for the same samples in Figure 6a. The PL decay time and amplitudes can be fitted using the following exponential equation.39 = ∑ exp−⁄! +%
(3)
where Ai is the decay amplitude, τi is the decay time, and K is a constant for the baseline offset. Table 4 lists all fitting parameters. On the basis of the extracted parameters from the FTO/perovskite sample, the PL decay time τ1 and τ2 were 44.32 and 15.84 ns, and the corresponding amplitudes were 22.11% and 77.89%,
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respectively. When the perovskite was deposited on the pristine TiO2 film, both τ1 and τ2 decreased to 21.61 and 8.31 ns, and the corresponding amplitudes were 5.88% and 78.39%, respectively. In addition, when the perovskite was deposited on the 90 s CO2 plasma treated TiO2 film, the τ1 and τ2 further decreased to 11.68 and 3.13 ns, and the corresponding amplitudes transfer to 12.39% and 78.30%, respectively. The relevant average lifetime (τave) was evaluated using the following bi-exponential equation. !&' =
∑ () *) ∑ () *)
(4)
The τave of 25.88 ns was obtained in the FTO/perovskite sample and was significantly reduced to 15.07 ns when the pristine TiO2 ETL was introduced, and it was further dropped to 6.09 ns when the TiO2 ETL was treated by 90 s CO2 plasma, demonstrating that the transformation of electrons was faster from the perovskite film into the TiO2 ETL after the plasma treatment, which was well correlated with the result of the steady-state PL quenching. The faster electron injection rate from the perovskite layer to the ETL was beneficial to the charge separation and effectively suppressed charge recombination at the perovskite/ETL interface, contributing to the higher JSC and FF values, and in well agreement with the J–V and EQE measurements. 4. CONCLUSION We have introduced the CO2 plasma to modify the TiO2 films using PECVD system. The CO2 plasma method can not only improve the quality of the stoichiometric TiO2 structure, but also reduce the concentration of oxygen-deficiencies induced defect sites. The 90 s CO2 plasma treated TiO2 film exhibited the better surface hydrophilicity, the higher conductivity, and the lower bulk defect state density in comparison with the pristine TiO2 film. Compared to PSC based on pristine TiO2, the PSCs with CO2 plasma treated TiO2 shown the higher JSC and FF due to the reduced electron recombination and enhanced electron transport. Corresponding, the PCE was significantly enhanced from 13.54% to 15.39%. These results indicated that the plasma treatment by PECVD method is an effective approach to modify ETL for high performance planar heterojunction PSCs.
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TABLES Table 1 The position, areas, and area ratios of the deconvoluted O 1s peaks for the pristine and the CO2 plasma treated TiO2 films.
CO2 plasma treatment O 1s
OM
Pristine 90 s
5 min
10 min
Position (eV)
529.1
529.2
529.2
529.2
Area
116872.0
149307.0
125950.1
135876.3
Position (eV)
530.7
530.6
530.9
530.8
Area
69076.8
68129.7
68897.7
77642.8
Position (eV)
532.7
532.7
532.8
532.8
Area
7130.0
7000.0
6994.1
7000.0
OM/(OM+OV+OH) area ratio (%)
60.53
66.53
62.40
61.62
OV/(OM+OV+OH) area ratio (%)
35.78
30.35
34.13
35.21
OH/(OM+OV+OH) area ratio (%)
3.69
3.12
3.46
3.17
OV
OH
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Table 2 The key parameters of the champion PSCs based on the pristine and the CO2 plasma treated TiO2 ETLs. VOC (V) JSC (mA/cm2) FF (%) PCE (%) Rs (Ω·cm2) Rsh (kΩ·cm2)
TiO2 Pristine CO2 plasma treatment
1.04
20.66
63.0
13.54
56.25
0.65
90 s
1.04
21.41
69.1
15.39
21.52
1.58
5 min
1.04
21.26
68.4
15.12
51.98
1.11
10 min
1.04
21.36
63.3
14.32
52.03
1.04
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Table 3 The key parameters of the PSCs based on the pristine and the 90 s CO2 plasma treated TiO2 ETLs under reverse and forward scanning directions. TiO2 Pristine 90 s CO2 plasma
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
Reverse
1.04
20.66
63.0
13.54
Forward
0.86
21.12
46.8
8.49
Reverse
1.04
21.41
69.1
15.39
Forward
1.02
21.76
69.7
15.25
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Table 4 Parameters of the TRPL Spectroscopy based on the FTO/perovskite, FTO/pristine TiO2/perovskite, and FTO/90 s CO2 plasma treated TiO2/perovskite samples, respectively. Sample
τave (ns) τ1 (ns) Amplitude τ1 (%) τ2 (ns) Amplitude τ2 (%)
FTO/perovskite
25.88
44.32
22.11
15.84
77.89
FTO/pristine TiO2/perovskite
15.07
21.61
5.88
8.31
78.39
6.09
11.68
12.39
3.13
78.30
FTO/90 s CO2 plasma treated TiO2/perovskite
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FIGURES Figure 1. Schematic diagram of the procedure of the CO2 plasma treated TiO2 film by PECVD method and perovskite solar cell architecture.
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Figure 2. AFM images of (a) the pristine TiO2 film, and TiO2 films treated by CO2 plasma at 200 °C for (b) 90 s, (c) 5 min, and (d) 10 min, respectively.
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Figure 3. XPS spectra of (a) Ti 2p peaks and (b) O 1s peaks of the pristine and CO2 plasma treated TiO2 films.
Ti 2p3/2
(a)
OM
(b)
O 1s
OV 10 min CO2 plasma
OH 10 min CO2 plasma
Ti 2p1/2
Intensity (a.u.)
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470
5 min CO2 plasma
5 min CO2 plasma
465
460
90 s CO2 plasma
90 s CO2 plasma
Pristine TiO2
Pristine TiO2
455
Binding energy (eV)
450
534
530
532
528
Binding energy (eV)
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Figure 4. I–V curves for the FTO/pristine or CO2 plasma treated TiO2/Au. The inset shows the device structure.
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Figure 5. (a) The J–V curves of PSCs based on the pristine, and CO2 plasma treated TiO2 for 90 s, 5 min, and 10 min, respectively. (b) The J–V curves PSCs based on the pristine and the 90 s CO2 plasma treated TiO2 ETLs under reverse and forward scanning directions. Histograms of PSCs measured for 20 individual devices with (c) the pristine and (d) the 90 s CO2 plasma treated TiO2 ETLs
(b) 20
20
15
15
2) 2 J (mA/cm J (mA/cm )
2 2) Current Density (mA/Cm ) J (mA/cm
(a)
10 plasma 10 min min CO22 plasma 55 min CO22 plasma plasma
10
90 plasma 90 ss CO22 plasma Pristine Pristine TiO22
Pristine Pristine TiO TiO22 90 ss CO CO22 plasma
0
0.0
(c)
Reverse Forward
10
5
5
0 0.5 Volatge (V)
0.0
1.0
(d)
8
Pristine PristineTiO TiO 22
0.5 Voltage (V)
1.0
7
90 plasma 90 s CO22 plasma
6
7
5 Number of cells
6 Number of cells
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5 4 3
4 3 2
2
1
1 0
0
5
6
7
8
9
10 11 PCE(%)
12
13
14
15
16
10
11
12
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14 15 PCE (%)
16
17
18
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Figure 6. (a) Steady-state PL spectra of the FTO/perovskite, FTO/pristine TiO2/perovskite, and FTO/90 s CO2 plasma treated TiO2/perovskite samples. (b) TRPL of perovskite absorber
deposited on the various substrates.
(a)
(b)
150.0k
FTO/90 s CO2 plasma plasma treated treatedTiO TiO22/Perovskite /Perovskite FTO/pristine TiO2/Perovskite /Perovskite FTO/Perovskite
Normalized PL intensity (a.u.)
180.0k
PL intensity (a.u.)
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120.0k 90.0k 60.0k 30.0k 0.0 700
750
800
850
1
FTO/plasma TiO FTO/90 s FTO/plasma CO2 plasmatreated treated TiO treated TiO 2 /Perovskite 2/Perovskite 2/Perovskite FTO/pristine TiO2/Perovskite FTO/pristine TiO FTO/pristine TiO 2/Perovskite 2 /Perovskite FTO/Perovskite FTO/Perovskite FTO/Perovskite Fitting line Fitting line
0.1
0.01
1E-3
0
20
Wavelength (nm)
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60
Time (ns)
80
100
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ASSOCIATED CONTENT Supporting Information. Top-view SEM images of (a) the pristine TiO2 film, and CO2 plasma treated TiO2 films at 200 °C for (b) 90 s, (c) 5 min, and (d) 10 min, respectively; trap filled limit transition point; static contact angles of (a) the pristine TiO2 film, and TiO2 films treated by CO2 plasma for (b) 90 s, (c) 5 min, and (d) 10 min, respectively; transmittance and absorbance spectra of the TiO2 films with CO2 plasma treatments; cross-sectional SEM micrograph of planar perovskite solar cells; EQE of PSC devices based on the pristine and 90 s CO2 plasma treated TiO2 ETLs. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *Dr. Dapeng Wang: [email protected]; *Prof. Shengzhong (Frank) Liu: [email protected]
Author Contributions Kang Wang and Wenjing Zhao contributed equally.
Notes The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge all support from the National Key Research Program of China (2016YFA0202403), the National Nature Science Foundation of China (61674098), the Natural Science Foundation of Shaanxi Provincial Department of Education (2017KW-023), the Fundamental Research Funds for the Central Universities (GK201603053, GK201603054, GK201601010), the Changjiang Scholar and the Innovative Research Team (IRT_14R33) and the Chinese National 1000-talent-plan program. The authors would like to thank Idemitsu Kosan Co. Ltd., for their support throughout this work.
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CO Plasma-Treated TiO Film as an Effective Electron Transport Layer for High-Performance Planar Perovskite Solar Cells Kang Wang, Wenjing Zhao, Jia Liu, Jinzhi Niu, Yucheng Liu, Xiaodong Ren, Jiangshan Feng, Zhike Liu, Jie Sun, Dapeng Wang, and Shengzhong (Frank) Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11329 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 16, 2017
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CO2 Plasma-Treated TiO2 Film as an Effective Electron Transport Layer for High-Performance Planar Perovskite Solar Cells Kang Wang, Wenjing Zhao, Jia Liu, Jinzhi Niu, Yucheng Liu, Xiaodong Ren, Jiangshan Feng, Zhike Liu, Jie Sun, Dapeng Wang,* and Shengzhong (Frank) Liu* Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China
KEYWORDS: plasma-enhanced chemical vapor deposition, CO2 plasma treatment, TiO2 film, electron transport layer, perovskite, solar cells
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ABSTRACT Perovskite solar cells (PSCs) have received great attention due to their excellent photovoltaic properties especially for the comparable efficiency to silicon solar cells. The electron transport layer (ETL) is regarded as a crucial medium in transporting electrons and blocking holes for PSCs. In this study, CO2 plasma generated by plasma-enhanced chemical vapor deposition (PECVD) was introduced to modify the TiO2 ETL. The results indicated that CO2 plasma treated compact TiO2 layer exhibited better surface hydrophilicity, higher conductivity, and lower bulk defect state density in comparison with the pristine TiO2 film. The quality of the stoichiometric TiO2 structure was improved and the concentration of oxygen-deficiencies induced defect sites was reduced significantly after CO2 plasma treated for 90 s. The PSCs with TiO2 film treated by CO2 plasma for 90 s exhibited simultaneously the improved short-circuit current (Jsc) and fill factor (FF). As a result, PSC based TiO2 ETL with CO2 plasma treatment affords a power conversion efficiency (PCE) of 15.39%, outperforming that based on pristine TiO2 (13.54%). These results indicate that the plasma treatment by PECVD method is an effective approach to modify ETL for high performance planar PSCs.
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1. INTRODUCTION Perovskite solar cells (PSCs) have been recognized as a promising replacement for silicon solar cells, due to their high absorption coefficient, suitable direct band gap, high carrier mobility, and long diffusion length. In recent years, a certified efficiency of PSCs has been reached up to 22.1%,1 outperforming other third-generation solar cells, such as organic solar cells and dye-sensitized solar cells. PSCs have also shown other advantages, including low cost and easy to commercialize.2-8 The general formula of perovskites in PSCs is ABX3, where A is CH3NH3+, HC(NH2)2+, or Cs+, B is Pb2+ or Sn2+, and X is a halide.9-13 A typical configuration of PSC consisted of two electrodes, a hole transport layer (HTL), a perovskite layer, and an electron transport layer (ETL). The ETL plays an important role in blocking holes and transporting electrons.14 Although there are many types of materials used for ETL,15-18 most of ETL in the state-of-the-art PSCs are based on anatase-TiO2 owning to its suitable band structure, high electron mobility, environmental friendliness, and low cost.19 Up to now, the main synthetic methods for TiO2 consist of chemical-bath deposition,20 magnetron sputtering,21 and atomic layer deposition.22 Compared with the various deposition methods, the electron-beam evaporation (EBE) is an affordable technique which can be successfully employed to obtain the high crystalline, uniform, large area, and low cost high-quality TiO2 layers on various types of substrates.23 However, the TiO2-based planar heterojunction PSCs always suffer from the relatively high interface potential barrier and poor charge collection efficiency under steady-state forward voltage bias, which accused by the numerous deep traps and/or electronic defect states originated from oxygen vacancies within the TiO2 lattice.24 A number of studies reported that the TiO2 layer with surface modification can enhance the performance of the planar heterojunction solar cells.25 Snaith et al. reported that a fullerene C60 can modify the compact TiO2 and extract photogenerated charge more effectively.26 Petrozza et al. discovered that the PSCs based on TiOX/PC60BM layer can achieve a stabilized PCE of 17.6%, which is significantly higher than that of PSCs with only TiOX (14.4%).27 Liu et al. demonstrated that Li-doping of TiO2 can passivate the electron traps and increase the conductivity of TiO2 for improving the performance of PSCs.28 According to the reported results, it is an effective way to improve the performance of PSCs through improving the surface quality and
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reducing the defect densities of TiO2. Plasma treatment can contribute to a significant change in the surface energy, which plays a crucial role in membrane fouling and cleaning phenomena.29 Plasma-enhanced chemical vapor deposition (PECVD) treatment can promote the bulk defect passivation independently of surface effects.30 Gas discharge in CO2 by PECVD method is known to be characterized by intense vacuum-ultraviolet irradiation producing active oxygen species radicals. These radicals can effectively oxidize the surface and bulk of sample during the plasma treatment. In the CO2 plasma process, the gas phase contains numerous reactive species including ions, electrons, free radicals, and molecules with a variety of electronically excited states. The plasma can effectively oxidize the solid surface and that oxidizing plasma contributes to great improvement in interfacial adhesion. Simultaneously, the CO2 plasma impacts the surface species from the solid substrate, creating surface-active sites, which also removes the weak boundary layers and contaminants. Therefore, the CO2 plasma process offers a technique to tailor sample surface chemical modification without sacrificing bulk properties.31 In this study, we proposed a CO2 plasma method by PECVD to modify EBE deposited TiO2 layers. The CO2 plasma treated TiO2 films exhibited the better surface hydrophilicity, the higher conductivity, and the lower bulk defect density in comparison to the pristine TiO2 layer. The X-ray photospectroscopy (XPS) results revealed that 90 s CO2 plasma treatment on TiO2 can not only improve the quality of the stoichiometric TiO2 structure, but also reduce the concentration of oxygen-deficiency induced defect sites, resulting in the reduced electron recombination and the enhanced electron transport in PSCs. Therefore, the PSCs based on TiO2 film that treated by CO2 plasma for 90 s exhibited the improved short-circuit current (Jsc) and fill factor (FF) simultaneously. As a result, PSC based TiO2 ETL with CO2 plasma treatment affords a PCE of 15.39%, outperforming that based on pristine TiO2 (13.54%). 2. EXPERIMENTAL PROCEDURE 2.1 Preparation of the TiO2 layer TiO2 thin films were deposited on FTO substrate by the EBE. Firstly, the FTO glass substrate was washed by acetone, isopropanol, and ethanol in an ultrasonic processing for 30 min in sequence
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and then fixed on the substrate holder. The purchased TiO2 particles without any purification were placed in a crucible below the substrate holder and were evaporated by an incident electron beam (accelerating voltage of 10 kV) under vacuum. The deposition temperature was set at room temperature with a desired pressure of 4×10-4 Mbar. The electron gun voltage was fixed at 2.0 kV, the current was adjusted to 60 mA, and the substrate was rotated at 20 rpm. Finally, the uniform TiO2 film with a thickness of ~50 nm was obtained. The deposited TiO2 films were then treated with CO2 plasma by PECVD method for 90 s, 5 min, and 10 min at 200 °C, respectively. The main plasma treatment parameters, such as operating power of 60 W, CO2 gas flow rate of 50 sccm, and chamber pressure of 200 Pa, were finally optimized. Also, a TiO2 film heated by hot-plate at 200 °C for 20 min was chose as the pristine one for comparison. To investigate the impact of CO2 plasma treatment on the properties of the TiO2 film, all TiO2 films were not undergone the additional treatment. 2.2 Device fabrication The structure of PSC was FTO/TiO2/CH3NH3PbI3/HTL/Au. The PbI2 was purchased from Alfar Aesar (99.99%) and CH3NH3I (MAI) was synthesized according to our previous publication,32 1.106 g PbI2 and 0.38 g MAI were dissolved in 2 mL of γ-butyrolactone (GBL) at room temperature in glove box to form perovskite precursor solution. The solution was spin-coated on TiO2 layer at 4000 rpm for 30 s and then the substrate was annealed on a hot-plate at 120 °C for 10 min. Methylbenzene was used as anti-solvent during spin-coated process. 18 mg spiro-OMeTAD, 28.8 µL tert-butylpyridine, and 44 µL lithium bis imide acetonitrile solution (520 mg/mL) were dissolved in 2 ml chlorobenzene (CB) to form the HTL solution. The solution was spin-coated onto perovskite layer at 5000 rmp for 30 s. Au electrode with a thickness of 80 nm was then deposited by thermal evaporator. 2.3 Characterization The surface morphology and roughness images of the CO2 plasma treated and the pristine TiO2 films were characterized by field-emission scanning electron microscopy (SEM, HITACHI, SU 8020) and atomic force microscope (AFM, Brooke, Dimension ICON). X-ray photoelectron spectroscopy (XPS) was carried out using a photoelectron spectrometer (ESCALAB250Xi,
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Thermo Fisher Scientific). Photoluminescence (PL) (excitation at 532 nm) was obtained with an Edinburgh Instruments Ltd. FLS980 spectrometer. The absorption and transmittance spectra were measured using an UV-vis-near-IR spectrometer (PerkinElmer, Lambda 950). The current density versus voltage (J–V) characteristics of the PSCs were measured by using a Keithley 2400 source meter under the illumination of a AM 1.5 solar simulator with the light intensity of 100 mW/cm2 (AM 1.5G, SAN-EIELECTRIC XES-40S2-CE solar simulator), as calibrated by a NREL-traceable KG5 filtered silicon reference cell. The active area of all solar cells was defined by a 9 × 9 mm2 mask. All devices were scanned in the reverse and forward directions under the standard test procedure at a scan rate of 30 mV/s. Incident photon-to-current conversion efficiency (IPCE) spectra of the PSCs were measured by a Q Test Station 500TI system (Crowntech. Inc., USA). The monochromatic light intensity for IPCE was calibrated using a reference silicon detector. 3. RESULTS AND DISCUSSION Schematic diagram of the CO2 plasma treated TiO2 film by PECVD is shown in Figure 1. The obtained TiO2 films were treated by CO2 plasma for 90 s, 5 min, and 10 min, respectively. The thickness of all TiO2 films was identical independent of various treatment methods and duration. The top-view SEM images of the pristine and the CO2 plasma treated TiO2 films are shown in Figure S1. The morphology of the plasma treated TiO2 film was similar to that of the pristine TiO2 film, showing that the CO2 plasma treatment did not change the film morphology as expected. All TiO2 films had a uniform and flat surface with the grain size of about 100 nm. Figure 2 shows the AFM images of the pristine and the CO2 plasma treated TiO2 films. The root-mean-square (RMS) roughness of the pristine TiO2 film was decreased from 8.77 to 7.95 nm when the TiO2 film was treated by the CO2 plasma for 90 s. The smoother surface would be beneficial to improve the contact and reduce the resistance between the ETL and the perovskite film. However, when the CO2 plasma duration was increased to 5 and 10 min, the RMS roughness of the TiO2 films was slightly increased to 8.40 and 9.42 nm, respectively. The results indicated that the long duration CO2 plasma treatment will slightly adjust the surface morphology of TiO2 film which maybe due to the bombardment effect during the PECVD treatment.33
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The effect of the CO2 plasma and its treatment duration on the chemical properties and bonding states of the TiO2 films was investigated by XPS. Figure 3a shows the XPS spectra of the Ti 2p peaks of the pristine and the CO2 plasma treated TiO2 films. The Ti 2p1/2 and Ti 2p3/2 peaks of all TiO2 film exhibited the binding energies of about 463.5 and 457.7 eV,34 respectively, indicating that the CO2 plasma with various treatment durations had a negligible influence on the binding energy of Ti 2p. The O 1s XPS spectra of the pristine and the CO2 plasma treated TiO2 films are shown in Figure 3b. These O 1s spectra can be resolved into three nearly Gaussian distributions with peak position of 529.2, 530.7, and 532.7 eV. The peak at the binding energy of 529.2 eV (labeled as OM) was attributed to the O2- ions combined with Ti atom in a stoichiometric TiO2 structure. The peak at the binding energy of 530.7 (labeled as OV) and 532.7 (labeled as OH) were related to the phenomena of oxygen-deficiency and hydroxyl groups on the TiO2 film, respectively. The area ratio of the OV peak to the whole O 1s peak exhibited the relative quantity of oxygen-related defects in the film. The positions, areas, and area ratios of the O1s three peaks, for the pristine and the CO2 plasma treated TiO2 films with different treatment durations, were summarized in Table 1. In case of the pristine TiO2 film, the OM/(OM + OV + OH) and OV/(OM + OV + OH) area ratios of ~60.53% and ~35.78% were obtained, respectively. The results indicated that a large amount of oxygen-related defects, such as oxygen vacancy (VO), remained into the pristine TiO2 film due to the oxygen-deficiency during the fabrication of TiO2 film in EBE system, leading to the more defect states and high concentration of defect sites existed in the TiO2 film. Interestingly, when the TiO2 film treated by CO2 plasma for 90 s, the OV/(OM + OV + OH) area ratio decreased significantly to ~30.35%. Correspondingly, the OM/(OM + OV + OH) area ratio increased obviously to ~66.53%. These results stated that the CO2 plasma can effectively reduce the oxygen-related defects and improve the quality of TiO2 film, which were attributed to the generated oxygen radicals from CO2 plasma during the treatment in the PECVD system. When the CO2 plasma treatment duration extended to 5 and 10 min, the OM/(OM + OV + OH) area ratios slightly decreased to ~62.40% and ~61.62%, and the OV/(OM + OV + OH) area ratios slightly increased to ~34.13% and ~35.21%. The observed results suggested that the weakly rearranged Ti-O bonds after 90 s plasma treatment were cleaved again after a long duration bombardment during the PECVD system, contributing to an increase in the concentration of oxygen-related defects. Noticeably, the area ratio of OH/(OM + OV + OH) was the small value of ~3.69% in the
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pristine TiO2 film, which was due to the film was prepared in the hydrogen-free condition. After the CO2 plasma treatments even for various durations, the area ratio of OH/(OM + OV + OH) maintained the value of less than 3.46%, as listed in Table 1, implying that the EBE method as well as the CO2 plasma treatment are the promising approach to obtain metal oxide semiconductor with small amounts of hydroxyl groups. To investigate the effect of CO2 plasma treatment on the electron trap density in the TiO2 films, the current versus voltage (I–V) curves of TiO2 film with electrodes were measured, as shown in Figure 4. The trap filled limit transition point (VTFL) was related to trap density. An Ohmic response was emerged at the low bias. When the voltage increased and exceeded the kink-point voltage, the applied voltage at the kink-point voltage was defined as trap-filled limit voltage VTFL. The onset voltage VTFL is linearly proportional to the density of trap states ntrap.35
=
(1)
Where e is the elementary charge of the electron (e = 1.6×10−19 C), d is the TiO2 film thickness (~50 nm), ε is the relative dielectric constant of rutile TiO2 (ε = 48), ε0 is the vacuum permittivity (ε0 = 8.854×10−12 F/m), and ntrap is the trap state density of different treated films. The VTFL as a function of ntrap were summarized in Table S1. It was obviously observed that the VTFL was significantly decreased from 0.58 V in the pristine TiO2 film to 0.21, 0.23, and 0.30 V after the TiO2 films were treated by the CO2 plasma for 90 s, 5 min, and 10 min, respectively. On the basis of the relationship between the ntrap and VTFL, the calculated ntrap in the pristine TiO2 film was much higher than that in the CO2 plasma TiO2 films. Especially, for the 90 s CO2 plasma treated TiO2 film exhibited a remarkably lower ntrap of 0.45 × 1016 cm–3 compared to the pristine TiO2 film with ntrap of 1.23 × 1016 cm–3. However, the ntrap gradually increased with the increase in the plasma treatment duration, indicating that the TiO2 film suffered from a long duration CO2 plasma bombardment would be clearly destroyed, leading to the increase in the ntraps, which was consistent with the XPS results. Based on the structure shown in inset of Figure 4, the impact of the CO2 plasma treatment on the conductivity (σ) of the TiO2 film can be calculated. The σ was evaluated using the equation 2.36 =
(2)
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where A is the contact area and D is the thickness of the pristine or the CO2 plasma treated TiO2 film. Table S1 lists the calculated σ values. It is found that the conductivity of the 90 s CO2 plasma treated TiO2 film was over 6 times higher than that of the pristine TiO2. The results suggested that the CO2 plasma treatment in a proper duration was an effective method to enhance the conductivity of the TiO2 film. To further investigate the CO2 plasma treatment on the surface state of the TiO2 films, the contact angle measurement was carried out, as shown in Figure S2. Note that the pristine and the CO2 plasma treated TiO2 films were not undergone the additional UVO treatment. The static contact angles of water on the TiO2 surface with CO2 plasma treatment duration of 90 s, 5 min, and 10 min were 37°, 30°, and 26°, respectively, which were lower than that of the pristine film of 60°. This phenomenon indicated that the CO2 plasma treatment can improve the wetting between the CH3NH3PbI3 layer and the TiO2 film in comparison of the pristine one, which could effectively enhance the interfacial quality of the TiO2 and CH3NH3PbI3 layers. The optical transmission and absorbance spectra of the pristine and the CO2 plasma treated TiO2 films are shown in Figure S3. The transmittance for all the TiO2 films was over 80% in the visible region and there was no obvious change in the absorbance spectra, indicating that the CO2 plasma treatment had no influence on the optical properties of the TiO2 films. On the basis of the above-mentioned discussions, the TiO2 film with the excellent properties, such as smoother surface, lower bulk defect state density, and good quality in the stoichiometric structure, can be achieved after the optimized CO2 plasma treatment. To explore the influence of the CO2 plasma treated TiO2 film on the performance of PSC, a normal solar cell structure, consisting of FTO/pristine or CO2 plasma treated TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au, was designed. The thicknesses of TiO2, MAPbI3, Spiro-OMeTAD, and Au layers were confirmed to be 50, 350, 300, and 80 nm, respectively, as shown in cross-sectional SEM image of device (Figure S4). All the layers of the device have the uniform and compact structure. Figure 5a shows the J–V characteristic of the champion devices with the pristine or CO2 plasma treated TiO2 films as ETL layers, which were measured in the reverse scan direction under AM 1.5G irradiation with power density of 100 mW/cm2. The relevant photovoltaic parameters including open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and efficiency
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were summarized in Table 2. The device based on the pristine TiO2 ETL gives a PCE of 13.54% with a VOC of 1.04 V, a JSC of 20.66 mA/cm2, and a FF of 63.0%. When the TiO2 ETL was treated by the CO2 plasma for 90 s, the PCE was enhanced significantly to 15.39%, which was attributed to the improved JSC (21.41 mA/cm2) and FF (69.1%). The enhanced JSC and FF may originate from the low defect state density and the high electrical conductivity of the plasma treated TiO2 ETL. To further investigate the increase in the JSC, the external quantum efficiency (EQE) and the relevantly integrated current of the PSCs based on the pristine and the 90 s CO2 plasma treated TiO2 ETLs were evaluated, as shown in the Figure S6. It was obviously found that the EQE of the PSCs with the 90 s CO2 plasma treated TiO2 ETL was slightly higher than that of the PSCs with the pristine TiO2 ETL in the whole wavelength range, indicating improved charge separation and collection at the interface of TiO2/perovskite. The calculated EQE integrated current density increased from 20.09 mA/cm2 for the pristine TiO2 based PSCs to 20.87 mA/cm2 for the 90 s CO2 plasma treated TiO2 based PSCs, which was well consistent with the J–V measurement values, indicating that the carriers were separated and collected effectively in the PSCs with the 90 s CO2 plasma treated TiO2 ETL. On the other hand, the improved FF was attributed to the reduction of the series resistance (Rs) and the increase of the shunt resistance (Rsh), as listed in Table 2. Note that the photovoltaic parameters of the PSCs, especially for the FF, gradually degraded with the increase in the CO2 plasma treated TiO2 ETL duration. The degraded FF was mainly ascribed to the increase of Rs and the reduction of Rsh, as shown in the Figure 5a. The increase of Rs suggested the low conductivity and high resistivity for electrons transporting in the TiO2 ETL. The decrease of Rsh indicated that the carriers were extracted slowly and the high density defect of states aggravated the recombination of electrons with holes, which were due to the more defect sites and high concentration of defect states introduced into the TiO2 ETL after a long duration bombardment during the PECVD system. Hitherto, the planar PSCs with TiO2 ETL always suffered the notorious J–V hysteresis when measured under different scan directions.37 Figure 5b shows the J–V curves of the PSCs with the pristine and the 90 s CO2 plasma treated TiO2 ETLs measured under the reverse and forward scan directions. It was found that the device with the pristine TiO2 ETL exhibited a severe hysteresis. Surprisingly, the J–V curves under reverse and forward scan directions almost overlapped when
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the device chose the 90 s CO2 plasma treated TiO2 ETL. The relevant photovoltaic parameters were summarized in Table 3. Previous publications revealed that the J–V hysteresis in PSCs mainly originated from the unbalanced carrier transport within charge transport layer and the high density defect of states for electronic trapping at the perovskite/ETL interface.38 Therefore, the negligible hysteresis in PSC with the 90 s CO2 plasma treated TiO2 ETL could be attributed to the lower defect density and higher conductivity of the plasma treated ETL, contributing to the efficient extraction and transportation of carriers. To confirm the uniformity and reproducibility of the PSCs, the PCE histograms with the statistics analysis for the devices with the pristine and plasma treated TiO2 ETLs were shown in the Figure 5c and 5d, respectively. The average PCE of 10.56% and 15.14% was obtained based on the 20 individual devices with the pristine and the 90 s CO2 plasma treated TiO2 ETLs, respectively, suggesting that the CO2 plasma treatment can effectively improve the efficiency of PSC. To reveal the impact of CO2 plasma treatment on the electron extraction and transport mechanism from the perovskite absorber to the TiO2 ETLs, the steady-state PL and time-resolved PL (TRPL) were performed to investigate the charge transfer and carrier recombination. Figure 6a depicts the PL spectra of the FTO/perovskite, FTO/pristine TiO2/perovskite, and FTO/90 s CO2 plasma treated TiO2/perovskite samples. It was obviously found that the FTO/perovskite exhibited the highest PL intensity, resulting from the critical recombination of the excited charges in the perovskite layer. As expected, The FTO/90 s CO2 plasma treated TiO2/perovskite sample demonstrated the more effective electron extraction capability in comparison of FTO/pristine TiO2/perovskite. Figure 6b shows the TRPL spectra for the same samples in Figure 6a. The PL decay time and amplitudes can be fitted using the following exponential equation.39 = ∑ exp−⁄! +%
(3)
where Ai is the decay amplitude, τi is the decay time, and K is a constant for the baseline offset. Table 4 lists all fitting parameters. On the basis of the extracted parameters from the FTO/perovskite sample, the PL decay time τ1 and τ2 were 44.32 and 15.84 ns, and the corresponding amplitudes were 22.11% and 77.89%,
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respectively. When the perovskite was deposited on the pristine TiO2 film, both τ1 and τ2 decreased to 21.61 and 8.31 ns, and the corresponding amplitudes were 5.88% and 78.39%, respectively. In addition, when the perovskite was deposited on the 90 s CO2 plasma treated TiO2 film, the τ1 and τ2 further decreased to 11.68 and 3.13 ns, and the corresponding amplitudes transfer to 12.39% and 78.30%, respectively. The relevant average lifetime (τave) was evaluated using the following bi-exponential equation. !&' =
∑ () *) ∑ () *)
(4)
The τave of 25.88 ns was obtained in the FTO/perovskite sample and was significantly reduced to 15.07 ns when the pristine TiO2 ETL was introduced, and it was further dropped to 6.09 ns when the TiO2 ETL was treated by 90 s CO2 plasma, demonstrating that the transformation of electrons was faster from the perovskite film into the TiO2 ETL after the plasma treatment, which was well correlated with the result of the steady-state PL quenching. The faster electron injection rate from the perovskite layer to the ETL was beneficial to the charge separation and effectively suppressed charge recombination at the perovskite/ETL interface, contributing to the higher JSC and FF values, and in well agreement with the J–V and EQE measurements. 4. CONCLUSION We have introduced the CO2 plasma to modify the TiO2 films using PECVD system. The CO2 plasma method can not only improve the quality of the stoichiometric TiO2 structure, but also reduce the concentration of oxygen-deficiencies induced defect sites. The 90 s CO2 plasma treated TiO2 film exhibited the better surface hydrophilicity, the higher conductivity, and the lower bulk defect state density in comparison with the pristine TiO2 film. Compared to PSC based on pristine TiO2, the PSCs with CO2 plasma treated TiO2 shown the higher JSC and FF due to the reduced electron recombination and enhanced electron transport. Corresponding, the PCE was significantly enhanced from 13.54% to 15.39%. These results indicated that the plasma treatment by PECVD method is an effective approach to modify ETL for high performance planar heterojunction PSCs.
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TABLES Table 1 The position, areas, and area ratios of the deconvoluted O 1s peaks for the pristine and the CO2 plasma treated TiO2 films.
CO2 plasma treatment O 1s
OM
Pristine 90 s
5 min
10 min
Position (eV)
529.1
529.2
529.2
529.2
Area
116872.0
149307.0
125950.1
135876.3
Position (eV)
530.7
530.6
530.9
530.8
Area
69076.8
68129.7
68897.7
77642.8
Position (eV)
532.7
532.7
532.8
532.8
Area
7130.0
7000.0
6994.1
7000.0
OM/(OM+OV+OH) area ratio (%)
60.53
66.53
62.40
61.62
OV/(OM+OV+OH) area ratio (%)
35.78
30.35
34.13
35.21
OH/(OM+OV+OH) area ratio (%)
3.69
3.12
3.46
3.17
OV
OH
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Table 2 The key parameters of the champion PSCs based on the pristine and the CO2 plasma treated TiO2 ETLs. VOC (V) JSC (mA/cm2) FF (%) PCE (%) Rs (Ω·cm2) Rsh (kΩ·cm2)
TiO2 Pristine CO2 plasma treatment
1.04
20.66
63.0
13.54
56.25
0.65
90 s
1.04
21.41
69.1
15.39
21.52
1.58
5 min
1.04
21.26
68.4
15.12
51.98
1.11
10 min
1.04
21.36
63.3
14.32
52.03
1.04
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Table 3 The key parameters of the PSCs based on the pristine and the 90 s CO2 plasma treated TiO2 ETLs under reverse and forward scanning directions. TiO2 Pristine 90 s CO2 plasma
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
Reverse
1.04
20.66
63.0
13.54
Forward
0.86
21.12
46.8
8.49
Reverse
1.04
21.41
69.1
15.39
Forward
1.02
21.76
69.7
15.25
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Table 4 Parameters of the TRPL Spectroscopy based on the FTO/perovskite, FTO/pristine TiO2/perovskite, and FTO/90 s CO2 plasma treated TiO2/perovskite samples, respectively. Sample
τave (ns) τ1 (ns) Amplitude τ1 (%) τ2 (ns) Amplitude τ2 (%)
FTO/perovskite
25.88
44.32
22.11
15.84
77.89
FTO/pristine TiO2/perovskite
15.07
21.61
5.88
8.31
78.39
6.09
11.68
12.39
3.13
78.30
FTO/90 s CO2 plasma treated TiO2/perovskite
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FIGURES Figure 1. Schematic diagram of the procedure of the CO2 plasma treated TiO2 film by PECVD method and perovskite solar cell architecture.
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Figure 2. AFM images of (a) the pristine TiO2 film, and TiO2 films treated by CO2 plasma at 200 °C for (b) 90 s, (c) 5 min, and (d) 10 min, respectively.
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Figure 3. XPS spectra of (a) Ti 2p peaks and (b) O 1s peaks of the pristine and CO2 plasma treated TiO2 films.
Ti 2p3/2
(a)
OM
(b)
O 1s
OV 10 min CO2 plasma
OH 10 min CO2 plasma
Ti 2p1/2
Intensity (a.u.)
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Figure 4. I–V curves for the FTO/pristine or CO2 plasma treated TiO2/Au. The inset shows the device structure.
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Figure 5. (a) The J–V curves of PSCs based on the pristine, and CO2 plasma treated TiO2 for 90 s, 5 min, and 10 min, respectively. (b) The J–V curves PSCs based on the pristine and the 90 s CO2 plasma treated TiO2 ETLs under reverse and forward scanning directions. Histograms of PSCs measured for 20 individual devices with (c) the pristine and (d) the 90 s CO2 plasma treated TiO2 ETLs
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Figure 6. (a) Steady-state PL spectra of the FTO/perovskite, FTO/pristine TiO2/perovskite, and FTO/90 s CO2 plasma treated TiO2/perovskite samples. (b) TRPL of perovskite absorber
deposited on the various substrates.
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FTO/plasma TiO FTO/90 s FTO/plasma CO2 plasmatreated treated TiO treated TiO 2 /Perovskite 2/Perovskite 2/Perovskite FTO/pristine TiO2/Perovskite FTO/pristine TiO FTO/pristine TiO 2/Perovskite 2 /Perovskite FTO/Perovskite FTO/Perovskite FTO/Perovskite Fitting line Fitting line
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ASSOCIATED CONTENT Supporting Information. Top-view SEM images of (a) the pristine TiO2 film, and CO2 plasma treated TiO2 films at 200 °C for (b) 90 s, (c) 5 min, and (d) 10 min, respectively; trap filled limit transition point; static contact angles of (a) the pristine TiO2 film, and TiO2 films treated by CO2 plasma for (b) 90 s, (c) 5 min, and (d) 10 min, respectively; transmittance and absorbance spectra of the TiO2 films with CO2 plasma treatments; cross-sectional SEM micrograph of planar perovskite solar cells; EQE of PSC devices based on the pristine and 90 s CO2 plasma treated TiO2 ETLs. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *Dr. Dapeng Wang: [email protected]; *Prof. Shengzhong (Frank) Liu: [email protected]
Author Contributions Kang Wang and Wenjing Zhao contributed equally.
Notes The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge all support from the National Key Research Program of China (2016YFA0202403), the National Nature Science Foundation of China (61674098), the Natural Science Foundation of Shaanxi Provincial Department of Education (2017KW-023), the Fundamental Research Funds for the Central Universities (GK201603053, GK201603054, GK201601010), the Changjiang Scholar and the Innovative Research Team (IRT_14R33) and the Chinese National 1000-talent-plan program. The authors would like to thank Idemitsu Kosan Co. Ltd., for their support throughout this work.
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TABLE OF CONTENTS GRAPHIC
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