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Minimizing energy losses in perovskite solar cells using plasma-treated transparent conducting layers
Thin Solid Films 593 (2015) 10–16
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Minimizing energy losses in perovskite solar cells using plasma-treated transparent conducting layers Van-Duong Dao a, Liudmila L. Larina a,b, Ho-Suk Choi a,⁎ a b
Department of Chemical Engineering, Chungnam National University, Yuseong-Gu, Daejeon 305-764, Korea Department of Solar Photovoltaics, Institute of Biochemical Physics, Russian Academy of Sciences, 119334 Moscow, Russia
a r t i c l e
i n f o
Article history: Received 1 July 2015 Received in revised form 31 August 2015 Accepted 17 September 2015 Available online 25 September 2015 Keywords: Plasma treatment Blocking layer Electron recombination Perovskite solar cells
a b s t r a c t This study reports for increasing the efficiency of perovskite solar cells (PSCs) by modifying the surface of a fluorine-doped indium tin oxide (FTO) substrate using an atmospheric pressure plasma treatment. Surface modification of the FTO film involved several challenges, such as control of the blocking layer uniformity, removal of pinholes, and deposition of a dense layer. This strategy allows the suppression of charge recombination at the interface between the FTO substrate and hole conductor. Electrochemical impedance spectroscopy analysis showed that the plasma treatment increased the charge transfer resistance between the FTO and hole conductor from 95.1 to 351.1 Ω, indicating enhanced resistance to the electron back reaction. Analyses of the open-circuit photovoltage decay revealed that modification of the surface of the FTO substrate by plasma treatment increased time constant from 6.44 ms to 13.15 ms. The effect is ascribed to suppression of the electron recombination rate. PSCs based on the newly developed electrode had 39% higher efficiency than reference devices. The obtained results provide direct evidence in favor of the developed strategy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Perovskite solar cells (PSCs) have attracted considerable interest as next-generation solar energy conversion devices because of their simple fabrication and high energy conversion efficiencies [1–5]. The theoretical maximum efficiency of PSCs, estimated to be around 33%, exceeds the appropriate parameter in conventional dye-sensitized solar cells (DSCs) [6]. To date, the best reported power conversion efficiencies are 15% for PSCs based on a CH3NH3PbI3 sensitizer and 19.3% for cells based on a CH3NH3PbI3−xClx sensitizer [2–5,7]. It has been shown that one of the losses of photo-injected electrons in DSCs and PSCs is the loss due to electron–hole recombination at the interface of the FTO substrate and redox electrolyte or at the FTO/hole transport material (HTM) interface. Indeed, the underlying FTO substrate is in contact with the electrolyte or HTM instead of an uncoated surface. Because the typical doping level (Nd = 1020 cm− 3) of FTO yields a space charge layer width of around a few nanometers [8], the probability that electrons will tunnel across this thin barrier is very high, even if a built-in potential exists at the substrate surface. The probability of electron transfer from the FTO substrate is especially high under open-circuit conditions, because the gradient of free energy (the quasi-Fermi levels) for electrons in FTO and holes in the HTM is very large owing to an upward
⁎ Corresponding author. E-mail address: [email protected] (H.-S. Choi).
http://dx.doi.org/10.1016/j.tsf.2015.09.035 0040-6090/© 2015 Elsevier B.V. All rights reserved.
shift of the quasi-Fermi level in FTO under illumination. The depletion region in FTO was derived from the Mott–Schottky plot, and “metallic” electron-transfer behavior was confirmed by electrochemical impedance spectroscopy (EIS) analysis [9]. Thus, PSC device performance can be further improved by significant suppression of electron transfer via the FTO substrate [10–13]. For this purpose, a 50–100-nm-thick TiO2 blocking layer (BL) has been introduced by atomic layer deposition, spin-coating, spray pyrolysis, or thermal oxidation of a sputtered titanium film [14–16]. However, the proposed technologies suggest coverage of the FTO substrate with thick BLs. Expensive vacuum equipment and targets are drawbacks for the development of an economical and continuous process for device fabrication. Furthermore, for a high BL thickness, the photocurrent is reduced because of the formation of a region insufficient for electron transport from the bulk TiO2 layer to FTO via the BL [17]. Therefore, the growth of a thin, uniform BL is still an issue for PSC technology. Here, we present a novel strategy for increasing the efficiency of PSCs by using FTO transparent conducting layers modified by atmospheric pressure plasma treatment on glass substrates. Surface modification of the FTO film involves several challenges, such as control of the BL uniformity, removal of pinholes, and deposition of a dense layer. The developed technology allows the suppression of charge recombination at both the FTO substrate/hole conductor interface and the FTO/working electrode (WE) interface. PSCs based on the plasmamodified FTO layers demonstrate a power conversion efficiency of 2.09%, which is 105% higher than that of PSCs with conventional untreated FTO layers and without a mesoporous TiO2 layer. Accordingly,
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the observed efficiency in PSC samples using a mesoporous TiO2 layer was 39% higher. 2. Experimental section 2.1. Materials synthesis The organometal halide perovskite sensitizer CH3NH3PbI3 was synthesized using a previously reported method [1]. The precursor CH3NH3I was prepared by stirring equimolar amounts of methylamine (40% in methanol) and hydroiodic acid (57 wt.% in water) at 0 °C for 2 h. The precursor solution was evaporated on a rotary evaporator at 40 °C; then the precipitated CH3NH3I was washed three times with diethyl ether and dried under vacuum. Finally, CH3NH3PbI3 was synthesized by stirring equimolar amounts of CH3NH3I and PbI2 in γ-butyrolactone at 60 °C overnight. To prepare the HTM, we used chlorobenzene and acetonitrile with a volume ratio of 1:0.1 as a solvent. The concentrations of 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD), lithium bis(trifluoromethane) sulfonamide salt, and 4-tert-butylpyridine in the mixed solvent were 170, 64, and 198 mM, respectively. Methylamine (40% in methanol) was purchased from Tokyo Chemical Industry Co., and spiro-MeOTAD was provided by Merck. All other reagents and chemicals were purchased from Sigma–Aldrich unless otherwise stated and were ACS grade or higher. 2.2. Solar cell fabrication and characterization FTO glass substrates (Pilkington, USA) were cut to dimensions of 1.6 × 1.6 cm2 and patterned by etching with Zn powder and 2 M HCl. The etched substrates were cleaned with water, acetone, and ethanol in ultrasonic cleaner. Four types of WEs were prepared and compared. For WE 1, a TiO2 BL/pristine FTO substrate was fabricated using a dipcoating method. The substrate was immersed in 40 mM TiCl4 aqueous solution for 30 min and then rinsed with water and ethanol. Finally, the sample was sintered at 500 °C for 30 min. For WE 2, we used a plasma-treated FTO glass substrate. After cleaning, the FTO substrates were treated by atmospheric plasma under the following operating conditions: power of 150 W, Ar gas flow rate of 5 lpm, plasma treatment time of 1 min, and substrate moving speed of 5 mm/s. Finally, a TiO2 BL was deposited on the plasma-treated FTO glass by the dip-coating method described above. WE 3 was prepared by depositing 1-μmthick mesoporous TiO2 on the TiO2 BL/pristine FTO sample. TiO2 was deposited by spin-coating at 4000 rpm for 30 s. For this purpose, TiO2 paste (Dyesol 18NR-T) was diluted in anhydrous ethanol with a weight ratio of 1:3. Finally, the film was sintered in air at 500 °C for 30 min. The sintered TiO2 film was immersed in 40 mM aqueous TiCl4 solution at 70 °C for 30 min. After cleaning, the sample was annealed at 500 °C for 30 min. WE 4 consists of a 1-μm-thick mesoporous TiO2 film deposited on a TiO2 BL/plasma-treated FTO substrate. The mesoporous TiO2 layer was fabricated as described above. The prepared electrodes were coated with a perovskite precursor solution and heated at 100 °C for 15 min. Subsequently, all the electrodes under study were coated with the HTM solution using spin-coating at 4000 rpm for 30 s. The samples were left in the dark in air before sputtering of 60 nm Au counter electrodes to complete the solar cells. 2.3. Measurements The surface morphologies of the FTO and BLs were characterized using a high-resolution scanning electron microscope (HRSEM) (Jeol JSM 7000F). The water contact angles were measured using a drop shape analyzer (DSA 100, KRUSS GmbH). The photocurrent–voltage (J–V) characteristics were assessed using an IviumStat device under 1000 W/m2 illumination intensity using a sun. 3000 solar simulator
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consisting of a 1000 W mercury-based Xe arc lamp and AM 1.5 G filters. The incident-photon-to-current efficiencies (IPCEs) were measured in the spectral range of 300 to 800 nm using an IPCE system for DSCs (Ivium Technologies.). Calibration was conducted using a PECSI02calibrated silicon photodiode as a standard. A diode laser with variable power and modulation control (Coherent Labs, 10 mW, 623 nm) was used as the light source. Illumination was always incident on the WE side of the solar cell. The intensity was measured using a calibrated Si photodiode. The impedance spectrum of the PSCs was measured under constant light illumination (100 mW cm−2) biased under opencircuit conditions and under dark with different bias voltages at frequencies of 100 kHz to 100 mHz with a perturbation amplitude of 10 mV. 3. Results and discussion The aim of this study was to fabricate a uniform, dense, and pinholefree TiO2 BL to improve the efficiency of PSCs by surface modification of FTO transparent conducting substrates through an atmospheric pressure plasma treatment. A TiO2 BL was fabricated on the FTO glass substrate by a sequence of processes, as shown in Fig. 1. Fig. 1a–d shows HRSEM images of the pristine FTO, plasma-treated FTO, TiO2 BL on the pristine FTO, and TiO2 BL on the plasma-treated FTO, respectively. No visible differences were observed in the surface morphology of the FTO layer before and after plasma treatment. We observed, however, that the surface became superhydrophilic after plasma treatment [18]. Fig. 2a, b shows images of the water contact angles for the pristine and plasma-treated FTO samples, respectively. The obtained data show that plasma treatment of the surface changes the water contact angle from 83.4° to 0°, which indicates superhydrophilicity. The superhydrophilic surface provides favorable conditions for uniform and pinhole-free coverage from the aqueous TiCl4 solution. Therefore, we treated the FTO surface with plasma to address those issues. Fig. 1c, d shows HRSEM images of TiO2 BLs formed on the pristine and plasma-treated FTO glasses, respectively. The surface of the BL on the pristine FTO is not uniform and exhibits many microcracks and pinholes (yellow dotted loops) as presented in Fig. 1c, which formed because of the hydrophobicity of pristine FTO surface. The existence of naked FTO sites is well known to increase the dark current, which negatively affects device efficiency [19]. Shunting via the substrate significantly decreases the open-circuit voltage (Voc) [8]. However, the change in the surface properties of the FTO substrate under plasma treatment yields uniform pinhole-free coverage of the substrate with tightly connected small seeds of TiO2 BL. A pinhole-free, uniform, and tight BL is important for optimal solar cell operation because it enhances the suppression of charge recombination at the interface. In addition, such a BL increases the effective surface area for perovskite adsorption, enhancing the light harvesting efficiency. The transmittances of the samples at wavelengths of 350–800 nm were estimated. Transmittance spectra are shown in Fig. 3. The transmittances of the pristine FTO, plasma-treated FTO, pristine FTO/BL, and plasma-treated FTO/BL at 550 nm were estimated to be 65.61%, 65.98%, 68.72%, and 70.98%, respectively. These numbers demonstrate that plasma treatment of the FTO surface enhances the transmittance of the FTO/BL structure compared to the pristine FTO/BL. The increase in the transmittance of the FTO/BL structure can be explained by effective scattering enhancement in the TiO2 nanoparticle structure, which can be assigned to Rayleigh scattering [20]. The Rayleigh scattering is the elastic scattering of electromagnetic waves induced by particles with sizes much smaller than the wavelength (particle sizeb 1/10 wavelength). We estimated the transmittances of samples at the wavelengths of 350–800 nm, and, in particular, at the wavelength of 550 nm. The particle size was estimated as ~ 25 nm from SEM image. This value satisfies the requirement of Rayleigh scattering. Since the Rayleigh scattering intensity is inversely proportional to the fourth power of wavelength (~ λ− 4) and the short wavelength is scattered
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Fig. 1. Schematic illustration of fabrication TiO2 BL with plasma treatment of FTO glass substrate. HRSEM images of (a) pristine FTO, (b) plasma-treated FTO, (c) TiO2 BL on pristine FTO, and (d) TiO2 BL on plasma-treated FTO. Scale bar is 500 nm.
more than the long wavelength in the Rayleigh limit, we can expect an enhancement in the transmittance at short wavelength range. Our result is in good agreement with previous studies [17,21]. The increase in transmittance induced by plasma treatment can enhance the light harvesting efficiency of devices. To investigate how the fundamental photovoltaic (PV) parameters are affected by plasma treatment, we fabricated PSCs with a device architecture similar to the commonly used planar configuration. The cells employed two types of WEs. One set of thin-layer cells was fabricated on FTO substrates covered only by a TiO2 BL with the thickness of 40 nm as shown in inserted Fig. 4a, and the second set was fabricated on FTO substrates covered with a double layer consisting of a TiO 2 BL/mesoporous TiO2 layer as described above. To clarify the effect of plasma treatment on the devices' PV parameters, we studied thin-layer cells employing only the TiO2 BL. The structure of the devices is shown in Fig. 4a. The device consists of an FTO glass substrate, thin film of TiO2 BL, perovskite layer with a thickness of 734 nm, spiro-MeOTAD layer with a thickness of 288 nm, and Au counter electrode with a thickness of 60 nm. Even the thin layer cells employed only TiO2 BL cannot deliver a high efficiency, but such structure provides a high sensitivity of PV parameters to the change of BL properties, and the plasma treatment effect can be derived. We compared the device fabricated on a pristine FTO conducting layer with a cell based on FTO modified under atmospheric pressure plasma treatment. The J–V characteristics of the solar cells studied in the dark and under illumination are presented in Fig. 4b. The PV parameters are listed in Table 1. The cell fabricated on pristine FTO exhibited a short-circuit photocurrent density (Jsc) of 8.90 mA ⋅ cm− 2, Voc of 416.67 mV, and fill factor (FF) of 27.10%. In the solar cell based on plasma-treated FTO, Voc increased by 330.07 mV, and Jsc increased from 8.90 to 11.43 mA ⋅ cm−2. These changes in the PV parameters of the device
induced by plasma treatment can be explained by modification of the surface properties of the FTO substrate. Indeed, the hydrophilic surface of the FTO allows the formation of a pinhole-free uniform TiO2 BL and thus the formation of a uniform perovskite layer in close contact with the TiO2 BL. This layer blocks the pathways between the spiroMeOTAD and underlying highly doped FTO conducting layer, suppressing charge recombination at the FTO/HTM interface. Consequently, the charge collection efficiency increases, suggesting an increase in Jsc. Because the Voc value is determined by the difference between the quasi-Fermi level in TiO2 and the redox level in the HTM, we can expect an increase in Voc. Note that enhanced light harvesting due to increased transmittance, as well as perovskite loading due to a larger surface area of plasma-treated TiO2 BL, also affect the increase in Jsc. Thus, the efficiency is increased by 105% compared to that of the cell based on conventional untreated FTO layers. Note that the PSC shows a low efficiency because the mesoporous TiO2 layer was not included in the cell structure. IPCE measurements (Fig. 4c) confirmed these results. The cell with plasma-treated FTO had a considerably higher photocurrent than the reference cell in the entire absorption range of perovskite. For the cell with plasma-treated FTO at λ = 500 nm, the photocurrent was almost doubled. The value of Jsc matches well the photocurrent obtained by integration of the IPCE function at wavelengths of 350–800 nm for both cells. We also found that the dark current of the reference cell was larger than that of the cell fabricated on the plasma-treated FTO (Fig. 4b), indicating suppressed recombination at the FTO/HTM interface and the FTO/perovskite absorber interface. Fig. 5a shows time-resolved photocurrent responses in the second time scale for the reference cell and the cell fabricated on the plasmatreated FTO substrate under 1 sun illumination. For the reference cell, the magnitude of the photocurrent rises over a period of 20 s.
Fig. 2. Water contact angles measured at the surface of (a) pristine and (b) plasma-treated FTO.
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Fig. 3. Transmittance spectra of pristine FTO, plasma-treated FTO, TiO2 blocking layer on pristine FTO, and TiO2 blocking layer on plasma-treated FTO.
The photocurrent response curves demonstrate a temporal rise indicating a superposition of at least two different components with fast and slow time constants. The kinetics can be attributed to charge accumulation at the bulk and in interface traps localized at the TiO2 BL/ perovskite interface [22,23]. For the device fabricated on plasmatreated FTO, the magnitude of the photocurrent rises to a steady-state value during a much shorter time of around 3 s, indicating a decrease in the bulk-state defects and interfacial band gap traps at the TiO 2 BL/perovskite interface. The observed difference in the timeresolved photocurrent responses reflects the difference between the surface morphology of the plasma-treated and pristine FTO substrates. Indeed, the superhydrophilic surface of the plasma-treated FTO substrate provides conditions for uniform and pinhole-free coverage with TiO2 BL (Fig. 1d) and thus a reduction in the bulk and localized interfacial traps. In contrast, the surface of the BL formed on the pristine FTO is not uniform and exhibits many microcracks and pinholes, which formed because of the hydrophobicity of the FTO surface. To examine the role of the BL in the suppression of charge recombination, we provide comparative EIS analyses of both cells. Fig. 5b shows Nyquist plots measured under the open-circuit condition at frequencies of 100 kHz to 100 mHz with a perturbation amplitude of 10 mV. To get essential parameters to characterize the device under 1 sun illumination, we used the equivalent circuit as shown in Fig. 5b. This equivalent circuit has been proposed in previous publications [24,25]. The equivalent circuit is consisted of three parts. The first part, determined at high frequency intercept with the real axis, is the ohmic internal resistance (Rh), which is assigned to the resistance of external circuit and FTO glass substrate. The second part is corresponding to a semicircle in the high frequency range, which is attributed to the diffusion of holes through the HTM (Rhtm). To fit this part, the R–C circuit is employed. Finally, the third part is the impedance at the frequency range corresponding to right incomplete semicircle, which is attributed to recombination resistance (Rrec). To fit this one, the R–C is also used. We found that the charge transfer resistance (Rrec) between FTO and the hole conductor (the same value can be assigned to the resistance between FTO and the perovskite absorber, and between the BL and the hole conductor, BL–perovskite) was 95.07 Ω for the reference cell and 351.1 Ω for the cell fabricated on plasma-treated FTO. Note that Rrec is inversely proportional to the electron recombination rate [26]. Therefore, plasma treatment of the FTO substrate results in suppression of electron recombination at the TiO2 BL/HTM and FTO/HTM interfaces as well as at the FTO/HTM and FTO/perovskite interfaces. The main loss in charge collection is known to be recombination of electrons from the TiO2 film. Here, it is the loss from the TiO2 BL to the hole
Fig. 4. (a) HRSEM cross-sectional image of the device. Layers from the bottom are: FTO glass, TiO2 BL, perovskite, spiro-MeOTAD, and Au. (b) J–V characteristics of devices with pristine and plasma-treated FTO in the dark and under 1 sun illumination. (c) IPCE curves of FTO/BL/CH3NH3PbI3/Spiro-OMeTAD/Au and plasma-treated FTO/BL/CH3NH3PbI3/ Spiro-OMeTAD/Au.
conductor [27]. The obtained EIS results suggest an increase in both Jsc and Voc in the cell based on plasma-treated FTO. Indeed, the decreased recombination rate suggests an upward shift of the Fermi level under illumination, resulting in the increased Voc. Our PV data (Table 1) are consistent with the EIS analysis. We provide a comparative study of solar cells with a conventional structure of FTO/BL/1-μm-thick-TiO2/CH3NH3PbI3/spiro-MeOTAD/Au
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Table 1 PV parameters of PSCs shown in Figs. 4b and 6a. PSC
Jsc (mA⋅cm−2)
PSC1 PSC2 PSC3 PSC4
8.90 ± 1.51 11.43 ± 0.97 12.66 ± 0.84 15.22 ± 0.79
Voc (mV)
FF (%)
η (%)
416.67 ± 10.27 747.14 ± 16.25 722.21 ± 21.35 766.53 ± 7.07
27.10 ± 7.53 23.79 ± 3.59 39.95 ± 3.85 43.26 ± 2.13
1.02 ± 0.25 2.09 ± 0.21 3.63 ± 0.19 5.04 ± 0.17
PSC1: Pristine FTO/BL/perovskite/HTM/Au. PSC2: Plasma-treated FTO/BL/perovskite/HTM/Au. PSC3: Pristine FTO/BL/TiO2/perovskite/HTM/Au. PSC4: Plasma-treated FTO/BL/TiO2/perovskite/HTM/Au.
fabricated on pristine and plasma-treated FTO substrates. The structure of the perovskite solar cell is shown in the inset in Fig. 6a. The J–V characteristics of both devices under 1 sun illumination and under dark are presented in Fig. 6a. The experimental PV parameters of the solar cells are listed in Table 1. The device using the plasma-treated FTO showed an energy conversion efficiency of 5.04 ± 0.17%, which was higher than that of the PSC fabricated on pristine FTO, 3.63 ± 0.19%. The Voc and Jsc values of the device fabricated on pristine FTO were lower than those of the PSC fabricated on plasma-treated FTO. The IPCE spectra for the two cells are compared in Fig. 6b. A strong increase in the photocurrent was observed in the PSC fabricated on plasma-treated FTO at wavelengths of 350 to 750 nm; twice the photocurrent was recorded in the entire spectral range under study. The value
Fig. 6. (a) J–V characteristics and (b) IPCE curves of devices fabricated on pristine and plasma-treated FTO with TiO2 mesoporous layer in the dark and under 1 sun illumination. Inset in (a) shows cross-sectional image of PSC with mesoporous TiO2 layer.
Fig. 5. (a) Time-resolved photocurrent responses in the second time scale for reference cell and cell fabricated on plasma-treated FTO conducting substrate under 1 sun illumination. (b) Nyquist plots for PSC fabricated on pristine and plasma-treated FTO under open-circuit condition.
of Jsc matches well the photocurrent obtained by integration of the IPCE function at wavelengths of 350–800 nm for both cells. At the same time, the treatment suppresses the dark current, shifting the onset by 200 mV to a higher potential. These data indicate the suppression of the back reaction of electrons with the HTM via the FTO. To further understand the effect of blocking electron back reactions on the PV parameters of the solar cell, we conducted EIS analyses in the dark of devices fabricated with a TiO2 mesoporous layer on pristine and plasma-treated FTO. The inset in Fig. 7a shows Nyquist plots measured at a bias voltage of 0.5 V. The impedance responses of both devices exhibit two distinct semicircles. The first, at high frequencies, is attributed to the charge transfer resistance at the gold back contact and the second semicircle, at low frequencies, is associated with the charge transfer resistance Rrec between TiO2/FTO and the hole conductor or perovskite absorber [26]. It has been proven that the main route for electron transfer to the hole conductor is via the blocking TiO2 film [27]. Because Rrec is inversely proportional to the electron recombination rate, the increase in Rrec causes suppression of the electron back reaction [26]. The potential dependence of Rrec, derived from the Nyquist plots for the two cells, is presented in Fig. 7a. We found that for both solar cells, Rrec decreases with increasing applied voltage, indicating an increased rate of recombination due to an upward shift of the quasi-Fermi level in TiO2. The trend is in good agreement with the data for a conventional solid-state DSC [12]. Fig. 7a shows that the plasma treatment causes an increase in
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Fig. 7. (a) Potential dependence of charge transfer resistance (Rrec) estimated from the Nyquist plots for electron recombination from TiO2 or FTO to the perovskite absorber or hole conductor. Data are presented for devices based on pristine and plasma-treated FTO substrates. Inset shows Nyquist plots of devices at an applied voltage of 0.5 V derived from EIS analysis. (b) Transient photovoltage decays of PSCs fabricated on pristine and plasma-treated FTO (point lines). Solid lines represent fits using two exponential functions.
Rrec over the potential range of 0.5 to 0.8 V. These results indicate enhanced resistance to the electron back reaction and are consistent with the shift in the dark current onset shown in Fig. 6a. The blocking behavior of the plasma-treated FTO/BL derived from EIS analysis can explain the increase in both Jsc and Voc. Under open-circuit conditions, back reactions of electrons with holes via both FTO and TiO2 contribute to the establishment of the photostationary state. The decreased recombination rates at the TiO2/HTM and FTO/HTM interfaces suggest an increase in the electron density in the conduction band of TiO2 under steady illumination, yielding an upward shift of the quasi-Fermi level under illumination. Therefore, the EIS experimental data suggest enhanced charge collection efficiency in the cell fabricated on plasma-treated FTO. The photovoltage is given by [8] qU photo ¼ kB T ln
nc n0c
ð1Þ
where nc is the electron density in the conduction band under steady illumination, n0c is the electron density in the conduction band in the dark, kB is the Boltzmann constant, and q is the elementary charge.
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In accordance with eq 1, the observed decrease in the recombination rates leads to the increase in Voc. The higher charge collection efficiency and light collection efficiency due to the increase in the transmittance of the plasma-treated FTO/BL structure also contributes to the enhancement of Jsc. Thus, our PV data (Table 1) are consistent with the EIS analysis. Now, we test the effect of plasma treatment of FTO on the properties of the BL. Fig. 7b shows the time-resolved open-circuit photovoltage responses in the millisecond time scale for solar cells with a TiO2 mesoporous layer fabricated on pristine and plasma-treated FTO substrates. Because the photovoltage is determined by the concentration of electrons in the TiO2 conduction band under steady illumination (see Eq (1)), after termination of the light, the kinetics of the voltage is governed by relaxation of the charge carriers in the TiO2 conduction band. The main pathways are electron recombination with holes in the HTM via TiO2 and the conducting FTO substrate. The surface defects, which create energy levels in the forbidden gap and act as electron or hole traps, can also contribute to the recombination process [28]. Indeed, when the light is switched off, we observe a decay of the experimental photovoltage transients to zero (Fig. 7b). We found that the shunting in the solar cell fabricated on pristine FTO causes the voltage to decay more rapidly than that in the cell based on plasma-treated substrate. These results reveal that surface modification of the FTO substrate by plasma treatment enhances the effectiveness of the BL [29,30]. The voltage decay curves were fitted with function: V(t) = Vo + A*e−t/τ, where A is constant, and τ is time constant. The results of fitting experimental voltage decays are shown in Fig. 7b. The corresponding fitting parameters are listed in Table 2. The decay in the solar cell fabricated on pristine FTO is fitted well with the time constant of τ = 6.44 ms, while the decay in the cell using the plasma-treated FTO substrate is well with the time constant of τ = 13.15 ms. This reveals that the surface modification of FTO substrate through plasma treatment strongly affects the time constant of voltage decay with increasing time constant. The similar behavior of voltage decay was observed when the effect of blocking the back reaction was provided by the surface passivation of metal oxide in quantum dot solar cells [31]. Therefore, the transfer of electrons related to the lowering of the quasi-Fermi level is suppressed in the cell that uses the plasmatreated FTO substrate, indicating the blocking effect of modification of the FTO surface at the FTO/HTM interface. Using time-resolved photovoltage measurements, we have shown that the variation in the shape of the transient photovoltage decay, which is affected by changes in the charge transfer kinetic parameters, is controlled by the quality of the FTO surface underlying the TiO2 BL. We found from EIS analysis that the plasma treatment increases Rrec over the potential range of 0.5 to 0.8 V (Fig. 7a), suggesting enhanced resistance to the electron back reaction. Therefore, the aforementioned results are in complete agreement with the EIS analysis and J–V characteristics of the devices in the dark. 4. Conclusion The efficiency of a PSC increased significantly when the BL was deposited on an FTO glass substrate modified by atmospheric pressure plasma treatment. The water contact angle of the FTO became zero under plasma treatment for 1 min at a power of 150 W, Ar gas flow rate of 5 lpm, and substrate moving speed of 5 mm/s. The obtained conversion efficiency of 5.04% is higher than that of a PSC fabricated on pristine FTO glass substrate, 3.63%. The improved efficiency is attributed to Table 2 Fitting parameters obtained from the open-circuit photovoltage decay curves. Cells
Vo (V)
A
τ (s)
Pristine FTO Plasma-treated FTO
0.062 0.17
3.8 x 1012 9.1 x 105
0.00644 0.01315
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suppression of electron recombination at the TiO2/HTM and FTO/HTM interfaces due to the high-density, uniform, and good distribution of the TiO2 BL on the superhydrophilic surface of the plasma-treated FTO. Enhanced light collection efficiency due to an increase in the perovskite loading and increased transmittance of the plasma-treated FTO/BL contribute to the improvement in the efficiency of the PSC. The obtained results provide direct evidence in favor of the developed strategy for increasing the efficiency of PSCs by surface modification of the FTO transparent conducting layers by atmospheric pressure plasma treatment. Acknowledgments This research was supported by a National Research Foundation (NRF) grant funded by the Korean government (MSIP) (NRF2014R1A2A2A01006994). This work was also supported by a Korea CCS R&D Center (KCRC) grant funded by the Korean government (MSIP) (2014M1A8A1049345), by the NRF-RFBR Joint Research Program through the National Research Foundation (NRF-2013K2A1A7076282), by the National Research Foundation Postdoctoral Fellowship Program 2014 (NRF-2014K2A4A1034681), and by the Korea Brain Pool Program 2013 (131S-6-3-0538). References [1] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J.E. Moser, M. Gratzel, N.-G. Park, Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Sci. Report 2 (2012) 591. [2] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643. [3] J.H. Heo, S.H. Im, J.H. Noh, T.N. Mandal, C.-S. Lim, J.A. Chang, Y.H. Lee, H. Kim, A. Sarkar, M.K. Nazeeruddin, M. Gratzel, S.I. Seok, Efficient inorganic–organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors, Nat. Photonics 7 (2013) 486. [4] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gratzel, Sequential deposition as a route to high-performance perovskitesensitized solar cells, Nature 499 (2013) 316. [5] J.T. Wang, J.M. Ball, E.M. Barea, A. Abate, J.A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H.J. Snaith, R.J. Nicholas, Low-temperature processed electron collection layers of graphene/TiO2 nanocomposites in thin film perovskite solar cells, Nano Lett. 14 (2014) 724. [6] T. Stergiopoulos, P. Falaras, Minimizing energy losses in dye-sensitized solar cells using coordination compounds as alternative redox mediators coupled with appropriate organic dyes, Adv. Energy Mater. 2 (2012) 616. [7] H. Zhou, Q. Chen, G. Li, S. Luo, T.B. Song, H.S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 354 (2014) 542. [8] L.M. Peter, Dye-sensitized nanocrystalline solar cells, Phys. Chem. Chem. Phys. 9 (2007) 2630. [9] P.J. Cameron, L.M. Peter, Characterization of titanium dioxide blocking layers in dye-sensitized nanocrystalline solar cells, J. Phys. Chem. B 107 (2003) 14394. [10] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells, Chem. Rev. 110 (2010) 6595.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Minimizing energy losses in perovskite solar cells using plasma-treated transparent conducting layers Van-Duong Dao a, Liudmila L. Larina a,b, Ho-Suk Choi a,⁎ a b
Department of Chemical Engineering, Chungnam National University, Yuseong-Gu, Daejeon 305-764, Korea Department of Solar Photovoltaics, Institute of Biochemical Physics, Russian Academy of Sciences, 119334 Moscow, Russia
a r t i c l e
i n f o
Article history: Received 1 July 2015 Received in revised form 31 August 2015 Accepted 17 September 2015 Available online 25 September 2015 Keywords: Plasma treatment Blocking layer Electron recombination Perovskite solar cells
a b s t r a c t This study reports for increasing the efficiency of perovskite solar cells (PSCs) by modifying the surface of a fluorine-doped indium tin oxide (FTO) substrate using an atmospheric pressure plasma treatment. Surface modification of the FTO film involved several challenges, such as control of the blocking layer uniformity, removal of pinholes, and deposition of a dense layer. This strategy allows the suppression of charge recombination at the interface between the FTO substrate and hole conductor. Electrochemical impedance spectroscopy analysis showed that the plasma treatment increased the charge transfer resistance between the FTO and hole conductor from 95.1 to 351.1 Ω, indicating enhanced resistance to the electron back reaction. Analyses of the open-circuit photovoltage decay revealed that modification of the surface of the FTO substrate by plasma treatment increased time constant from 6.44 ms to 13.15 ms. The effect is ascribed to suppression of the electron recombination rate. PSCs based on the newly developed electrode had 39% higher efficiency than reference devices. The obtained results provide direct evidence in favor of the developed strategy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Perovskite solar cells (PSCs) have attracted considerable interest as next-generation solar energy conversion devices because of their simple fabrication and high energy conversion efficiencies [1–5]. The theoretical maximum efficiency of PSCs, estimated to be around 33%, exceeds the appropriate parameter in conventional dye-sensitized solar cells (DSCs) [6]. To date, the best reported power conversion efficiencies are 15% for PSCs based on a CH3NH3PbI3 sensitizer and 19.3% for cells based on a CH3NH3PbI3−xClx sensitizer [2–5,7]. It has been shown that one of the losses of photo-injected electrons in DSCs and PSCs is the loss due to electron–hole recombination at the interface of the FTO substrate and redox electrolyte or at the FTO/hole transport material (HTM) interface. Indeed, the underlying FTO substrate is in contact with the electrolyte or HTM instead of an uncoated surface. Because the typical doping level (Nd = 1020 cm− 3) of FTO yields a space charge layer width of around a few nanometers [8], the probability that electrons will tunnel across this thin barrier is very high, even if a built-in potential exists at the substrate surface. The probability of electron transfer from the FTO substrate is especially high under open-circuit conditions, because the gradient of free energy (the quasi-Fermi levels) for electrons in FTO and holes in the HTM is very large owing to an upward
⁎ Corresponding author. E-mail address: [email protected] (H.-S. Choi).
http://dx.doi.org/10.1016/j.tsf.2015.09.035 0040-6090/© 2015 Elsevier B.V. All rights reserved.
shift of the quasi-Fermi level in FTO under illumination. The depletion region in FTO was derived from the Mott–Schottky plot, and “metallic” electron-transfer behavior was confirmed by electrochemical impedance spectroscopy (EIS) analysis [9]. Thus, PSC device performance can be further improved by significant suppression of electron transfer via the FTO substrate [10–13]. For this purpose, a 50–100-nm-thick TiO2 blocking layer (BL) has been introduced by atomic layer deposition, spin-coating, spray pyrolysis, or thermal oxidation of a sputtered titanium film [14–16]. However, the proposed technologies suggest coverage of the FTO substrate with thick BLs. Expensive vacuum equipment and targets are drawbacks for the development of an economical and continuous process for device fabrication. Furthermore, for a high BL thickness, the photocurrent is reduced because of the formation of a region insufficient for electron transport from the bulk TiO2 layer to FTO via the BL [17]. Therefore, the growth of a thin, uniform BL is still an issue for PSC technology. Here, we present a novel strategy for increasing the efficiency of PSCs by using FTO transparent conducting layers modified by atmospheric pressure plasma treatment on glass substrates. Surface modification of the FTO film involves several challenges, such as control of the BL uniformity, removal of pinholes, and deposition of a dense layer. The developed technology allows the suppression of charge recombination at both the FTO substrate/hole conductor interface and the FTO/working electrode (WE) interface. PSCs based on the plasmamodified FTO layers demonstrate a power conversion efficiency of 2.09%, which is 105% higher than that of PSCs with conventional untreated FTO layers and without a mesoporous TiO2 layer. Accordingly,
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the observed efficiency in PSC samples using a mesoporous TiO2 layer was 39% higher. 2. Experimental section 2.1. Materials synthesis The organometal halide perovskite sensitizer CH3NH3PbI3 was synthesized using a previously reported method [1]. The precursor CH3NH3I was prepared by stirring equimolar amounts of methylamine (40% in methanol) and hydroiodic acid (57 wt.% in water) at 0 °C for 2 h. The precursor solution was evaporated on a rotary evaporator at 40 °C; then the precipitated CH3NH3I was washed three times with diethyl ether and dried under vacuum. Finally, CH3NH3PbI3 was synthesized by stirring equimolar amounts of CH3NH3I and PbI2 in γ-butyrolactone at 60 °C overnight. To prepare the HTM, we used chlorobenzene and acetonitrile with a volume ratio of 1:0.1 as a solvent. The concentrations of 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD), lithium bis(trifluoromethane) sulfonamide salt, and 4-tert-butylpyridine in the mixed solvent were 170, 64, and 198 mM, respectively. Methylamine (40% in methanol) was purchased from Tokyo Chemical Industry Co., and spiro-MeOTAD was provided by Merck. All other reagents and chemicals were purchased from Sigma–Aldrich unless otherwise stated and were ACS grade or higher. 2.2. Solar cell fabrication and characterization FTO glass substrates (Pilkington, USA) were cut to dimensions of 1.6 × 1.6 cm2 and patterned by etching with Zn powder and 2 M HCl. The etched substrates were cleaned with water, acetone, and ethanol in ultrasonic cleaner. Four types of WEs were prepared and compared. For WE 1, a TiO2 BL/pristine FTO substrate was fabricated using a dipcoating method. The substrate was immersed in 40 mM TiCl4 aqueous solution for 30 min and then rinsed with water and ethanol. Finally, the sample was sintered at 500 °C for 30 min. For WE 2, we used a plasma-treated FTO glass substrate. After cleaning, the FTO substrates were treated by atmospheric plasma under the following operating conditions: power of 150 W, Ar gas flow rate of 5 lpm, plasma treatment time of 1 min, and substrate moving speed of 5 mm/s. Finally, a TiO2 BL was deposited on the plasma-treated FTO glass by the dip-coating method described above. WE 3 was prepared by depositing 1-μmthick mesoporous TiO2 on the TiO2 BL/pristine FTO sample. TiO2 was deposited by spin-coating at 4000 rpm for 30 s. For this purpose, TiO2 paste (Dyesol 18NR-T) was diluted in anhydrous ethanol with a weight ratio of 1:3. Finally, the film was sintered in air at 500 °C for 30 min. The sintered TiO2 film was immersed in 40 mM aqueous TiCl4 solution at 70 °C for 30 min. After cleaning, the sample was annealed at 500 °C for 30 min. WE 4 consists of a 1-μm-thick mesoporous TiO2 film deposited on a TiO2 BL/plasma-treated FTO substrate. The mesoporous TiO2 layer was fabricated as described above. The prepared electrodes were coated with a perovskite precursor solution and heated at 100 °C for 15 min. Subsequently, all the electrodes under study were coated with the HTM solution using spin-coating at 4000 rpm for 30 s. The samples were left in the dark in air before sputtering of 60 nm Au counter electrodes to complete the solar cells. 2.3. Measurements The surface morphologies of the FTO and BLs were characterized using a high-resolution scanning electron microscope (HRSEM) (Jeol JSM 7000F). The water contact angles were measured using a drop shape analyzer (DSA 100, KRUSS GmbH). The photocurrent–voltage (J–V) characteristics were assessed using an IviumStat device under 1000 W/m2 illumination intensity using a sun. 3000 solar simulator
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consisting of a 1000 W mercury-based Xe arc lamp and AM 1.5 G filters. The incident-photon-to-current efficiencies (IPCEs) were measured in the spectral range of 300 to 800 nm using an IPCE system for DSCs (Ivium Technologies.). Calibration was conducted using a PECSI02calibrated silicon photodiode as a standard. A diode laser with variable power and modulation control (Coherent Labs, 10 mW, 623 nm) was used as the light source. Illumination was always incident on the WE side of the solar cell. The intensity was measured using a calibrated Si photodiode. The impedance spectrum of the PSCs was measured under constant light illumination (100 mW cm−2) biased under opencircuit conditions and under dark with different bias voltages at frequencies of 100 kHz to 100 mHz with a perturbation amplitude of 10 mV. 3. Results and discussion The aim of this study was to fabricate a uniform, dense, and pinholefree TiO2 BL to improve the efficiency of PSCs by surface modification of FTO transparent conducting substrates through an atmospheric pressure plasma treatment. A TiO2 BL was fabricated on the FTO glass substrate by a sequence of processes, as shown in Fig. 1. Fig. 1a–d shows HRSEM images of the pristine FTO, plasma-treated FTO, TiO2 BL on the pristine FTO, and TiO2 BL on the plasma-treated FTO, respectively. No visible differences were observed in the surface morphology of the FTO layer before and after plasma treatment. We observed, however, that the surface became superhydrophilic after plasma treatment [18]. Fig. 2a, b shows images of the water contact angles for the pristine and plasma-treated FTO samples, respectively. The obtained data show that plasma treatment of the surface changes the water contact angle from 83.4° to 0°, which indicates superhydrophilicity. The superhydrophilic surface provides favorable conditions for uniform and pinhole-free coverage from the aqueous TiCl4 solution. Therefore, we treated the FTO surface with plasma to address those issues. Fig. 1c, d shows HRSEM images of TiO2 BLs formed on the pristine and plasma-treated FTO glasses, respectively. The surface of the BL on the pristine FTO is not uniform and exhibits many microcracks and pinholes (yellow dotted loops) as presented in Fig. 1c, which formed because of the hydrophobicity of pristine FTO surface. The existence of naked FTO sites is well known to increase the dark current, which negatively affects device efficiency [19]. Shunting via the substrate significantly decreases the open-circuit voltage (Voc) [8]. However, the change in the surface properties of the FTO substrate under plasma treatment yields uniform pinhole-free coverage of the substrate with tightly connected small seeds of TiO2 BL. A pinhole-free, uniform, and tight BL is important for optimal solar cell operation because it enhances the suppression of charge recombination at the interface. In addition, such a BL increases the effective surface area for perovskite adsorption, enhancing the light harvesting efficiency. The transmittances of the samples at wavelengths of 350–800 nm were estimated. Transmittance spectra are shown in Fig. 3. The transmittances of the pristine FTO, plasma-treated FTO, pristine FTO/BL, and plasma-treated FTO/BL at 550 nm were estimated to be 65.61%, 65.98%, 68.72%, and 70.98%, respectively. These numbers demonstrate that plasma treatment of the FTO surface enhances the transmittance of the FTO/BL structure compared to the pristine FTO/BL. The increase in the transmittance of the FTO/BL structure can be explained by effective scattering enhancement in the TiO2 nanoparticle structure, which can be assigned to Rayleigh scattering [20]. The Rayleigh scattering is the elastic scattering of electromagnetic waves induced by particles with sizes much smaller than the wavelength (particle sizeb 1/10 wavelength). We estimated the transmittances of samples at the wavelengths of 350–800 nm, and, in particular, at the wavelength of 550 nm. The particle size was estimated as ~ 25 nm from SEM image. This value satisfies the requirement of Rayleigh scattering. Since the Rayleigh scattering intensity is inversely proportional to the fourth power of wavelength (~ λ− 4) and the short wavelength is scattered
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Fig. 1. Schematic illustration of fabrication TiO2 BL with plasma treatment of FTO glass substrate. HRSEM images of (a) pristine FTO, (b) plasma-treated FTO, (c) TiO2 BL on pristine FTO, and (d) TiO2 BL on plasma-treated FTO. Scale bar is 500 nm.
more than the long wavelength in the Rayleigh limit, we can expect an enhancement in the transmittance at short wavelength range. Our result is in good agreement with previous studies [17,21]. The increase in transmittance induced by plasma treatment can enhance the light harvesting efficiency of devices. To investigate how the fundamental photovoltaic (PV) parameters are affected by plasma treatment, we fabricated PSCs with a device architecture similar to the commonly used planar configuration. The cells employed two types of WEs. One set of thin-layer cells was fabricated on FTO substrates covered only by a TiO2 BL with the thickness of 40 nm as shown in inserted Fig. 4a, and the second set was fabricated on FTO substrates covered with a double layer consisting of a TiO 2 BL/mesoporous TiO2 layer as described above. To clarify the effect of plasma treatment on the devices' PV parameters, we studied thin-layer cells employing only the TiO2 BL. The structure of the devices is shown in Fig. 4a. The device consists of an FTO glass substrate, thin film of TiO2 BL, perovskite layer with a thickness of 734 nm, spiro-MeOTAD layer with a thickness of 288 nm, and Au counter electrode with a thickness of 60 nm. Even the thin layer cells employed only TiO2 BL cannot deliver a high efficiency, but such structure provides a high sensitivity of PV parameters to the change of BL properties, and the plasma treatment effect can be derived. We compared the device fabricated on a pristine FTO conducting layer with a cell based on FTO modified under atmospheric pressure plasma treatment. The J–V characteristics of the solar cells studied in the dark and under illumination are presented in Fig. 4b. The PV parameters are listed in Table 1. The cell fabricated on pristine FTO exhibited a short-circuit photocurrent density (Jsc) of 8.90 mA ⋅ cm− 2, Voc of 416.67 mV, and fill factor (FF) of 27.10%. In the solar cell based on plasma-treated FTO, Voc increased by 330.07 mV, and Jsc increased from 8.90 to 11.43 mA ⋅ cm−2. These changes in the PV parameters of the device
induced by plasma treatment can be explained by modification of the surface properties of the FTO substrate. Indeed, the hydrophilic surface of the FTO allows the formation of a pinhole-free uniform TiO2 BL and thus the formation of a uniform perovskite layer in close contact with the TiO2 BL. This layer blocks the pathways between the spiroMeOTAD and underlying highly doped FTO conducting layer, suppressing charge recombination at the FTO/HTM interface. Consequently, the charge collection efficiency increases, suggesting an increase in Jsc. Because the Voc value is determined by the difference between the quasi-Fermi level in TiO2 and the redox level in the HTM, we can expect an increase in Voc. Note that enhanced light harvesting due to increased transmittance, as well as perovskite loading due to a larger surface area of plasma-treated TiO2 BL, also affect the increase in Jsc. Thus, the efficiency is increased by 105% compared to that of the cell based on conventional untreated FTO layers. Note that the PSC shows a low efficiency because the mesoporous TiO2 layer was not included in the cell structure. IPCE measurements (Fig. 4c) confirmed these results. The cell with plasma-treated FTO had a considerably higher photocurrent than the reference cell in the entire absorption range of perovskite. For the cell with plasma-treated FTO at λ = 500 nm, the photocurrent was almost doubled. The value of Jsc matches well the photocurrent obtained by integration of the IPCE function at wavelengths of 350–800 nm for both cells. We also found that the dark current of the reference cell was larger than that of the cell fabricated on the plasma-treated FTO (Fig. 4b), indicating suppressed recombination at the FTO/HTM interface and the FTO/perovskite absorber interface. Fig. 5a shows time-resolved photocurrent responses in the second time scale for the reference cell and the cell fabricated on the plasmatreated FTO substrate under 1 sun illumination. For the reference cell, the magnitude of the photocurrent rises over a period of 20 s.
Fig. 2. Water contact angles measured at the surface of (a) pristine and (b) plasma-treated FTO.
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Fig. 3. Transmittance spectra of pristine FTO, plasma-treated FTO, TiO2 blocking layer on pristine FTO, and TiO2 blocking layer on plasma-treated FTO.
The photocurrent response curves demonstrate a temporal rise indicating a superposition of at least two different components with fast and slow time constants. The kinetics can be attributed to charge accumulation at the bulk and in interface traps localized at the TiO2 BL/ perovskite interface [22,23]. For the device fabricated on plasmatreated FTO, the magnitude of the photocurrent rises to a steady-state value during a much shorter time of around 3 s, indicating a decrease in the bulk-state defects and interfacial band gap traps at the TiO 2 BL/perovskite interface. The observed difference in the timeresolved photocurrent responses reflects the difference between the surface morphology of the plasma-treated and pristine FTO substrates. Indeed, the superhydrophilic surface of the plasma-treated FTO substrate provides conditions for uniform and pinhole-free coverage with TiO2 BL (Fig. 1d) and thus a reduction in the bulk and localized interfacial traps. In contrast, the surface of the BL formed on the pristine FTO is not uniform and exhibits many microcracks and pinholes, which formed because of the hydrophobicity of the FTO surface. To examine the role of the BL in the suppression of charge recombination, we provide comparative EIS analyses of both cells. Fig. 5b shows Nyquist plots measured under the open-circuit condition at frequencies of 100 kHz to 100 mHz with a perturbation amplitude of 10 mV. To get essential parameters to characterize the device under 1 sun illumination, we used the equivalent circuit as shown in Fig. 5b. This equivalent circuit has been proposed in previous publications [24,25]. The equivalent circuit is consisted of three parts. The first part, determined at high frequency intercept with the real axis, is the ohmic internal resistance (Rh), which is assigned to the resistance of external circuit and FTO glass substrate. The second part is corresponding to a semicircle in the high frequency range, which is attributed to the diffusion of holes through the HTM (Rhtm). To fit this part, the R–C circuit is employed. Finally, the third part is the impedance at the frequency range corresponding to right incomplete semicircle, which is attributed to recombination resistance (Rrec). To fit this one, the R–C is also used. We found that the charge transfer resistance (Rrec) between FTO and the hole conductor (the same value can be assigned to the resistance between FTO and the perovskite absorber, and between the BL and the hole conductor, BL–perovskite) was 95.07 Ω for the reference cell and 351.1 Ω for the cell fabricated on plasma-treated FTO. Note that Rrec is inversely proportional to the electron recombination rate [26]. Therefore, plasma treatment of the FTO substrate results in suppression of electron recombination at the TiO2 BL/HTM and FTO/HTM interfaces as well as at the FTO/HTM and FTO/perovskite interfaces. The main loss in charge collection is known to be recombination of electrons from the TiO2 film. Here, it is the loss from the TiO2 BL to the hole
Fig. 4. (a) HRSEM cross-sectional image of the device. Layers from the bottom are: FTO glass, TiO2 BL, perovskite, spiro-MeOTAD, and Au. (b) J–V characteristics of devices with pristine and plasma-treated FTO in the dark and under 1 sun illumination. (c) IPCE curves of FTO/BL/CH3NH3PbI3/Spiro-OMeTAD/Au and plasma-treated FTO/BL/CH3NH3PbI3/ Spiro-OMeTAD/Au.
conductor [27]. The obtained EIS results suggest an increase in both Jsc and Voc in the cell based on plasma-treated FTO. Indeed, the decreased recombination rate suggests an upward shift of the Fermi level under illumination, resulting in the increased Voc. Our PV data (Table 1) are consistent with the EIS analysis. We provide a comparative study of solar cells with a conventional structure of FTO/BL/1-μm-thick-TiO2/CH3NH3PbI3/spiro-MeOTAD/Au
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Table 1 PV parameters of PSCs shown in Figs. 4b and 6a. PSC
Jsc (mA⋅cm−2)
PSC1 PSC2 PSC3 PSC4
8.90 ± 1.51 11.43 ± 0.97 12.66 ± 0.84 15.22 ± 0.79
Voc (mV)
FF (%)
η (%)
416.67 ± 10.27 747.14 ± 16.25 722.21 ± 21.35 766.53 ± 7.07
27.10 ± 7.53 23.79 ± 3.59 39.95 ± 3.85 43.26 ± 2.13
1.02 ± 0.25 2.09 ± 0.21 3.63 ± 0.19 5.04 ± 0.17
PSC1: Pristine FTO/BL/perovskite/HTM/Au. PSC2: Plasma-treated FTO/BL/perovskite/HTM/Au. PSC3: Pristine FTO/BL/TiO2/perovskite/HTM/Au. PSC4: Plasma-treated FTO/BL/TiO2/perovskite/HTM/Au.
fabricated on pristine and plasma-treated FTO substrates. The structure of the perovskite solar cell is shown in the inset in Fig. 6a. The J–V characteristics of both devices under 1 sun illumination and under dark are presented in Fig. 6a. The experimental PV parameters of the solar cells are listed in Table 1. The device using the plasma-treated FTO showed an energy conversion efficiency of 5.04 ± 0.17%, which was higher than that of the PSC fabricated on pristine FTO, 3.63 ± 0.19%. The Voc and Jsc values of the device fabricated on pristine FTO were lower than those of the PSC fabricated on plasma-treated FTO. The IPCE spectra for the two cells are compared in Fig. 6b. A strong increase in the photocurrent was observed in the PSC fabricated on plasma-treated FTO at wavelengths of 350 to 750 nm; twice the photocurrent was recorded in the entire spectral range under study. The value
Fig. 6. (a) J–V characteristics and (b) IPCE curves of devices fabricated on pristine and plasma-treated FTO with TiO2 mesoporous layer in the dark and under 1 sun illumination. Inset in (a) shows cross-sectional image of PSC with mesoporous TiO2 layer.
Fig. 5. (a) Time-resolved photocurrent responses in the second time scale for reference cell and cell fabricated on plasma-treated FTO conducting substrate under 1 sun illumination. (b) Nyquist plots for PSC fabricated on pristine and plasma-treated FTO under open-circuit condition.
of Jsc matches well the photocurrent obtained by integration of the IPCE function at wavelengths of 350–800 nm for both cells. At the same time, the treatment suppresses the dark current, shifting the onset by 200 mV to a higher potential. These data indicate the suppression of the back reaction of electrons with the HTM via the FTO. To further understand the effect of blocking electron back reactions on the PV parameters of the solar cell, we conducted EIS analyses in the dark of devices fabricated with a TiO2 mesoporous layer on pristine and plasma-treated FTO. The inset in Fig. 7a shows Nyquist plots measured at a bias voltage of 0.5 V. The impedance responses of both devices exhibit two distinct semicircles. The first, at high frequencies, is attributed to the charge transfer resistance at the gold back contact and the second semicircle, at low frequencies, is associated with the charge transfer resistance Rrec between TiO2/FTO and the hole conductor or perovskite absorber [26]. It has been proven that the main route for electron transfer to the hole conductor is via the blocking TiO2 film [27]. Because Rrec is inversely proportional to the electron recombination rate, the increase in Rrec causes suppression of the electron back reaction [26]. The potential dependence of Rrec, derived from the Nyquist plots for the two cells, is presented in Fig. 7a. We found that for both solar cells, Rrec decreases with increasing applied voltage, indicating an increased rate of recombination due to an upward shift of the quasi-Fermi level in TiO2. The trend is in good agreement with the data for a conventional solid-state DSC [12]. Fig. 7a shows that the plasma treatment causes an increase in
V.-D. Dao et al. / Thin Solid Films 593 (2015) 10–16
Fig. 7. (a) Potential dependence of charge transfer resistance (Rrec) estimated from the Nyquist plots for electron recombination from TiO2 or FTO to the perovskite absorber or hole conductor. Data are presented for devices based on pristine and plasma-treated FTO substrates. Inset shows Nyquist plots of devices at an applied voltage of 0.5 V derived from EIS analysis. (b) Transient photovoltage decays of PSCs fabricated on pristine and plasma-treated FTO (point lines). Solid lines represent fits using two exponential functions.
Rrec over the potential range of 0.5 to 0.8 V. These results indicate enhanced resistance to the electron back reaction and are consistent with the shift in the dark current onset shown in Fig. 6a. The blocking behavior of the plasma-treated FTO/BL derived from EIS analysis can explain the increase in both Jsc and Voc. Under open-circuit conditions, back reactions of electrons with holes via both FTO and TiO2 contribute to the establishment of the photostationary state. The decreased recombination rates at the TiO2/HTM and FTO/HTM interfaces suggest an increase in the electron density in the conduction band of TiO2 under steady illumination, yielding an upward shift of the quasi-Fermi level under illumination. Therefore, the EIS experimental data suggest enhanced charge collection efficiency in the cell fabricated on plasma-treated FTO. The photovoltage is given by [8] qU photo ¼ kB T ln
nc n0c
ð1Þ
where nc is the electron density in the conduction band under steady illumination, n0c is the electron density in the conduction band in the dark, kB is the Boltzmann constant, and q is the elementary charge.
15
In accordance with eq 1, the observed decrease in the recombination rates leads to the increase in Voc. The higher charge collection efficiency and light collection efficiency due to the increase in the transmittance of the plasma-treated FTO/BL structure also contributes to the enhancement of Jsc. Thus, our PV data (Table 1) are consistent with the EIS analysis. Now, we test the effect of plasma treatment of FTO on the properties of the BL. Fig. 7b shows the time-resolved open-circuit photovoltage responses in the millisecond time scale for solar cells with a TiO2 mesoporous layer fabricated on pristine and plasma-treated FTO substrates. Because the photovoltage is determined by the concentration of electrons in the TiO2 conduction band under steady illumination (see Eq (1)), after termination of the light, the kinetics of the voltage is governed by relaxation of the charge carriers in the TiO2 conduction band. The main pathways are electron recombination with holes in the HTM via TiO2 and the conducting FTO substrate. The surface defects, which create energy levels in the forbidden gap and act as electron or hole traps, can also contribute to the recombination process [28]. Indeed, when the light is switched off, we observe a decay of the experimental photovoltage transients to zero (Fig. 7b). We found that the shunting in the solar cell fabricated on pristine FTO causes the voltage to decay more rapidly than that in the cell based on plasma-treated substrate. These results reveal that surface modification of the FTO substrate by plasma treatment enhances the effectiveness of the BL [29,30]. The voltage decay curves were fitted with function: V(t) = Vo + A*e−t/τ, where A is constant, and τ is time constant. The results of fitting experimental voltage decays are shown in Fig. 7b. The corresponding fitting parameters are listed in Table 2. The decay in the solar cell fabricated on pristine FTO is fitted well with the time constant of τ = 6.44 ms, while the decay in the cell using the plasma-treated FTO substrate is well with the time constant of τ = 13.15 ms. This reveals that the surface modification of FTO substrate through plasma treatment strongly affects the time constant of voltage decay with increasing time constant. The similar behavior of voltage decay was observed when the effect of blocking the back reaction was provided by the surface passivation of metal oxide in quantum dot solar cells [31]. Therefore, the transfer of electrons related to the lowering of the quasi-Fermi level is suppressed in the cell that uses the plasmatreated FTO substrate, indicating the blocking effect of modification of the FTO surface at the FTO/HTM interface. Using time-resolved photovoltage measurements, we have shown that the variation in the shape of the transient photovoltage decay, which is affected by changes in the charge transfer kinetic parameters, is controlled by the quality of the FTO surface underlying the TiO2 BL. We found from EIS analysis that the plasma treatment increases Rrec over the potential range of 0.5 to 0.8 V (Fig. 7a), suggesting enhanced resistance to the electron back reaction. Therefore, the aforementioned results are in complete agreement with the EIS analysis and J–V characteristics of the devices in the dark. 4. Conclusion The efficiency of a PSC increased significantly when the BL was deposited on an FTO glass substrate modified by atmospheric pressure plasma treatment. The water contact angle of the FTO became zero under plasma treatment for 1 min at a power of 150 W, Ar gas flow rate of 5 lpm, and substrate moving speed of 5 mm/s. The obtained conversion efficiency of 5.04% is higher than that of a PSC fabricated on pristine FTO glass substrate, 3.63%. The improved efficiency is attributed to Table 2 Fitting parameters obtained from the open-circuit photovoltage decay curves. Cells
Vo (V)
A
τ (s)
Pristine FTO Plasma-treated FTO
0.062 0.17
3.8 x 1012 9.1 x 105
0.00644 0.01315
16
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suppression of electron recombination at the TiO2/HTM and FTO/HTM interfaces due to the high-density, uniform, and good distribution of the TiO2 BL on the superhydrophilic surface of the plasma-treated FTO. Enhanced light collection efficiency due to an increase in the perovskite loading and increased transmittance of the plasma-treated FTO/BL contribute to the improvement in the efficiency of the PSC. The obtained results provide direct evidence in favor of the developed strategy for increasing the efficiency of PSCs by surface modification of the FTO transparent conducting layers by atmospheric pressure plasma treatment. Acknowledgments This research was supported by a National Research Foundation (NRF) grant funded by the Korean government (MSIP) (NRF2014R1A2A2A01006994). This work was also supported by a Korea CCS R&D Center (KCRC) grant funded by the Korean government (MSIP) (2014M1A8A1049345), by the NRF-RFBR Joint Research Program through the National Research Foundation (NRF-2013K2A1A7076282), by the National Research Foundation Postdoctoral Fellowship Program 2014 (NRF-2014K2A4A1034681), and by the Korea Brain Pool Program 2013 (131S-6-3-0538). References [1] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J.E. Moser, M. Gratzel, N.-G. Park, Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Sci. Report 2 (2012) 591. [2] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643. [3] J.H. Heo, S.H. Im, J.H. Noh, T.N. Mandal, C.-S. Lim, J.A. Chang, Y.H. Lee, H. Kim, A. Sarkar, M.K. Nazeeruddin, M. Gratzel, S.I. Seok, Efficient inorganic–organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors, Nat. Photonics 7 (2013) 486. [4] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gratzel, Sequential deposition as a route to high-performance perovskitesensitized solar cells, Nature 499 (2013) 316. [5] J.T. Wang, J.M. Ball, E.M. Barea, A. Abate, J.A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H.J. Snaith, R.J. Nicholas, Low-temperature processed electron collection layers of graphene/TiO2 nanocomposites in thin film perovskite solar cells, Nano Lett. 14 (2014) 724. [6] T. Stergiopoulos, P. Falaras, Minimizing energy losses in dye-sensitized solar cells using coordination compounds as alternative redox mediators coupled with appropriate organic dyes, Adv. Energy Mater. 2 (2012) 616. [7] H. Zhou, Q. Chen, G. Li, S. Luo, T.B. Song, H.S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 354 (2014) 542. [8] L.M. Peter, Dye-sensitized nanocrystalline solar cells, Phys. Chem. Chem. Phys. 9 (2007) 2630. [9] P.J. Cameron, L.M. Peter, Characterization of titanium dioxide blocking layers in dye-sensitized nanocrystalline solar cells, J. Phys. Chem. B 107 (2003) 14394. [10] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells, Chem. Rev. 110 (2010) 6595.
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