Chemical bath deposited rutile TiO2 compact layer toward efficient planar heterojunction perovskite solar cells

Accepted Manuscript Title: Chemical bath deposited rutile TiO2 compact layer toward efficient planar heterojunction perovskite solar cells Author: Chao Liang Zhenhua Wu Pengwei Li Jiajie Fan Yiqiang Zhang Guosheng Shao PII: DOI: Reference:

S0169-4332(16)31400-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.06.171 APSUSC 33541

To appear in:

APSUSC

Received date: Revised date: Accepted date:

20-5-2016 25-6-2016 27-6-2016

Please cite this article as: Chao Liang, Zhenhua Wu, Pengwei Li, Jiajie Fan, Yiqiang Zhang, Guosheng Shao, Chemical bath deposited rutile TiO2 compact layer toward efficient planar heterojunction perovskite solar cells, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.06.171 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Chemical bath deposited rutile TiO2 compact layer toward efficient planar heterojunction perovskite solar cells Chao Lianga, Zhenhua Wub, Pengwei Lia, Jiajie Fana, Yiqiang Zhanga*, Guosheng Shaoa* a State Centre for International Cooperation on Designer Low-Carbon and Environmental Material (SCICDLCEM), School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China b Henan Information Engineering School, Zhengzhou 450000, P. R. China *E-mail: [email protected]; [email protected]

Graphical abstract

Highlights: 1.

Rutile TiO2 thin film can be grown on FTO substrate below 100 oC.

2.

200 mM TiCl4 precursor solution results in the best PSC performance.

3.

UV/O3 treatment can reduce the carrier recombination effectively.

4.

Over 12% power conversion efficiency can be achieved for PSCs.

Abstract TiO2 is a best choice of electron transport layers in perovskite solar cells, due to its high electron mobility and stability. However, traditional TiO2 processing method requires rather high annealing temperature (>500 ℃), preventing it from application to flexible devices. Here, we show that TiO2 thin films can be synthesized via chemical bath deposition below 100 ℃. Typically, a compact layer of rutile TiO2 is deposited onto fluorine-doped tin oxide (FTO) coated substrates, in an aqueous TiCl4 solution at 70 ℃. Through the optimization of precursor concentration and ultraviolet-ozone surface modification, over 12% power conversion efficiency can be achieved for CH3NH3PbI3 based perovskite solar cells. These findings offer a potential low-temperature technical solution in using TiO2 thin film as an effective transport layer for flexible perovskite solar cells.

Keywords: Chemical bath deposition; Rutile TiO2; Perovskite solar cells; Ultraviolet-ozone treatment

Introduction Since Akihiro Kojima et al. firstly reported the utilization of organic-inorganic hybrid perovskite material in photovoltaic devices, the power conversion efficiency (PCE) of such perovskite based solar cells (PSCs) has reached a remarkable value of over 20% in a short span of six years[1-2]. Owing to the superiorities of high absorption coefficient, suitable direct band gap, small exciton binding energy, high carrier mobility, long diffusion length, and low-cost fabrication, this novel perovskite materials have attracted great attention in the field of optoelectronic thin-film materials and devices.[3-9] Organometal halide PSCs evolved from dye-sensitized solar cells (DSSCs), where conventional dye molecules and liquid electrolyte were replace

by

the

hybrid

CH3NH3PbI3

material

N-di-pmethoxyphenlamine)-9,9'-spiro-bifluorene]

and

[2,2',7,7'-tetrakis-(N, (spiro-OMeTAD),

respectively.[10-12] Typical PSCs were divided into two main structures: mesoporous structure and planar heterojunction one.[13] Planar heterojunction structure abandoned mesoporous TiO2 layer, consisting of five main layers: (1) transparent conductive oxide (TCO) electrode, (2) electron transport layer (ETL), (3) active perovskite layer, (4) hole transport layer (HTL) and (5) metal electrode.[14-15] In a planar PSC structure, ETL is necessary to transport electrons and block holes, preventing carriers from recombination. Metal oxides are usually employed as effective ETL, including TiO2, ZnO, SnO2, Nb2O5, WOx and WOx-TiOx etc.[16-23] Among them, TiO2 is the most commonly used ETL in PSCs, owning to its suitable band structure, high electron mobility, environmental friendliness and low

cost.[24-27] Several preparation approaches have been reported in order to obtain high-quality TiO2 ETL, such as spin-coating, sol-gel, and screen-printing.[28-31] While all these approaches require a very high annealing temperature of 500 ℃. This is unfavorable for their low-cost manufacture and application in flexible devices. Recently, Aswani Yella et al. reported a low-temperature chemical-bath deposition method for TiO2 growth.[32] Meanwhile Jin-Wook Lee et al. proved that rutile TiO2 ETL can help achieve higher short-circuit current and better PCE in PSCs, compared with anatase TiO2 one.[33] In this paper, a low-temperature chemical bath deposition (CBD) method was used to produce TiO2 ETL for PSCs. By optimizing TiCl4 precursor concentration, a PCE up to 11.18% was achieved via a planar heterojunction structure (FTO/TiO2/CH3NH3PbI3/spiro-OMeTAD/Au).

Interestingly,

it

is

found

that

ultraviolet-ozone treatment of TiO2 can effectively reduce the carrier recombination between TiO2 and perovskite. In conjunction with an improved short-circuit photocurrent density (Jsc) of 20.06 mA/cm2 and a good fill factor (FF) of 0.63, ultraviolet-ozone treated TiO2 generated a ~12.9% improvement in PCE, resulting in a PCE of 12.62% with respect to 11.18% for untreated TiO2. We also used photoluminescence (PL) spectroscopy and electrochemical impedance spectroscopy (EIS) to shed light on the underlying mechanisms for enhancement in efficiency.

Material and methods 1. Material The majority of chemicals was obtained from Alfa-Aesar without further purification. PbI2 was purchased from Xi'an Polymer Light Technology Corp. CH3NH3I was synthesized by the reaction of 24 mL methylamine (33 wt % in absolute ethanol, Alfa) and 30 mL of hydroiodic acid (57 wt % in water, Alfa) in a 250 mL round bottomed flask at 0 ℃ for 2 h with stirring.[34] The precipitate was recovered by evaporation at 50 ℃ for 1 h. The methyl ammonium iodide (CH3NH3I) was washed with diethylether by stirring the solution for 30 min, which was repeated three times. And then CH3NH3I was finally dried at 60 ℃ in vacuum oven for 24 h, stored in N2 atmosphere for future use.

2. Fabrication of perovskite solar cells The patterned F-doped SnO2 (FTO) substrates (8 ohm/sq) were washed with soap in water, followed by ultrasonic cleaning in de-ionized water, acetone, ethyl alcohol and de-ionized water for 10 min orderly. After drying in a nitrogen flow, the substrates were treated within a UV-ozone oven for 15 min to remove the organic residues. To grow TiO2 layer onto the FTO substrate, an aqueous stock solution of TiCl4 (99.9%) was diluted to precursor solutions with different TiCl4 concentrations (100 mM, 200 mM, 300 mM and 400 mM) at 0 ℃. The dissolution process lasted for a few minutes in ice bath until a homogeneous solution was formed. The FTO substrates were then immersed into the precursor solution and kept in an oven at 70 ℃ for 1 h. Afterwards the FTO substrates were washed with water and ethanol, and then

dried at 100 ℃ in air for an hour. In order to deposit the CH3NH3PbI3 active layer, CH3NH3I and PbI2 were mixed into dimethyl sulfoxide (DMSO) and γ-butyrolactone (GBL) (3:7, v/v), with vagrant stirring at 60 ˚C for 12 h in a N2-filled glovebox. The CH3NH3PbI3 precursor solution of CH3NH3I and PbI2 (1.25:1.25 molar ratio) was deposited onto FTO/TiO2 substrate (without O3/ultraviolet treatment or with O3/ultraviolet treatment) by a consecutive two-step spin-coating process at 1,000 r.p.m. and at 4,000 r.p.m. for 15 s and 25 s, respectively. Finally, toluene was dropped onto the substrate towards the end of spin coating. The resultant CH3NH3PbI3 layer was dried at 100 ℃ for 10 min. The HTL was then deposited by spin coating at 3,000 r.p.m. for 30 s. The spin-coating solution was prepared by dissolving 72.3 mg spiro-OMeTAD,

28.8

μl

4-tert-butylpyridine,

and

17.5

μl

lithium

bis(trifluoromethylsulphonyl)imide (LITFSI, 520 mg/ml) in acetonitrile in 1 ml chlorobenzene. Finally, Au electrode was deposited using a Trovato thermal evaporator through a shadow mask at a base pressure of 3  10-8 Torr. The active device area was 0.09 cm2. The preparation of the TiO2 compact layer was in atmosphere and other layers were all fabricated in an inert glovebox integrated with the Trovato thermal evaporator.

3. Measurements The crystal structure was characterized by a Rigaku (RINT-2500) X-ray diffractometer (Cu Ka radiation, λ=1.5418 Å). Scanning electron microscopy (SEM) was carried out using a field emission scanning electron microscope (JEM-7500 F). Ultraviolet–visible absorption spectra were recorded by a Shimadzu UV 3600

spectrophotometer at room temperature. The current density-voltage (J-V) curves were recorded with a Keithley 2400 source meter and a collimated Xenon lamp (300 W, Newport) calibrated with the light intensity of 100 mW cm-2 under the simulated AM 1.5 G solar irradiance. The J-V curves were measured by both reverse and forward scan in the range between 0 and 1.2 V. The incident photon-to-electron conversion efficiency (IPCE) was obtained on a computer-controlled IPCE system (Newport), including a Xenon lamp, a monochromator and a Keithley multimeter. Photoluminescence (PL) (excitation at 425 nm) experiments were performed using a FLSP920 spectrometer (Edinburgh Instruments LTD). Electrochemical impedance spectroscopy was carried out using an IM6e Electrochemical Workstation (ZAHENR, Germany) in the dark, by applying a 10 mV voltage perturbation over a constant forward bias (between 0 and 1 V) with the frequency ranging from 4 MHz to 0.01 Hz. The Z-View software (Scribner Associates Inc.) was used to fit the impedance spectra.

Results and discussion To prepare the TiO2 ETL by the chemical bath deposition method, TiCl4 was used as the oxidizer in an aqueous precursor solution under the ambient environment.[28] The precursor concentration was found to be a crucial factor in optimizing the quality of TiO2 layer and the related performance of PSCs. The morphologies of all TiO2 layers fabricated by the CBD method at 70 ℃ was characterized by scanning electron microscopy (SEM) (Fig. 1). As can be seen, the rough TiO2 film formed in the 100 mM TiCl4 precursor solution was not very

dense. This resulted in small grain size and small film thickness (~20 nm), failing to fully cover the FTO substrates. When the precursor concentration was increased to 200 mM, a dense TiO2 film was obtained with a smooth and pinhole-free surface. This benefited the subsequent deposition of the CH3NH3PbI3 layer. At a solution concentration of 300 mM, the as-prepared TiO2 film showed evident cracks, which might lead to current leakage through the PSCs. Raising the precursor concentration to 400 mM resulted in larger particles and even more cracks. Film thicknesses were measured using a stylus profiler. The results showed that the thickness of TiO2 films increased linearly against the TiCl4 concentration of the precursor solution, ranging from 20 nm to 310 nm (Fig. 2). Fig. 3a shows transmittance spectra of TiO2 films on FTO glass substrates, prepared at different TiCl4 concentrations in the precursor. With the concentration of TiCl4 precursor solution increasing from 100 mM to 400 mM, the transmittance in the visible range was significantly reduced from ~75% to ~50%. In particular, the transmittance for the film prepared in the 200 mM TiCl4 solution was more than 70%, which allows adequate photons through the TiO2 ETL for energy conversion in the CH3NH3PbI3 layer. At a high TiCl4 concentration of 400 mM, the transmission of the TiO2 film was only about 50%, which had an adverse impact on blocking too much light from the CH3NH3PbI3 active layer. Fig. 3b shows the X-ray diffraction (XRD) patterns of pure FTO glass and TiO2 films on the FTO coated glass substrates. No impurity peaks were observed, confirming the high purity of TiO2 and FTO. The broad peaks at 36.0° and 41.2° were well assigned to the (101) and (111) crystal

planes of the rutile TiO2 phase (JCPDS#21-1276), respectively. It is in line with report that the rutile TiO2 phase tended to form in an acidic solution containing Cl-.[35] Also, the rutile crystal structure of the FTO substrate could facilitate the nucleation of the rutile TiO2 phase.[36] The higher the TiCl4 concentration in the solution, the stronger the TiO2 diffraction peaks, indicating increased film thickness as evidenced by the profiler measurement. Fig. 4a shows our device configuration of PSCs, using a planar heterojunction structure: FTO/TiO2/CH3NH3PbI3/spiro-OMeTAD/Au. Initially, a TiO2 layer was grown directly onto the FTO substrate. The whole process was conducted under the ambient atmosphere at a low processing temperature (<100 ℃). A high-quality CH3NH3PbI3 layer was then spin coated on top of the TiO2 layer via a modified solvent method, which included a two-step spin coating, drops of toluene and rapid thermal annealing.[37] Subsequently, the spiro-OMeTAD-based hole transport layer was deposited by spin coating. Finally, the Au electrode layer was deposited by thermal evaporation. Fig. 4b is the corresponding energy level diagram that indicates the process for carrier dissociation, transport, and collection: electrons and holes were separated in the perovskite active layer through light absorption; electrons transported through the TiO2 ETL were collected by the FTO electrode; holes transported through the spiro-OMeTAD HTL were collected by the Au electrode. The TiCl4 concentration in the precursor on the TiO2 films was seen to have significant impact on the J-V curves (Fig. 4c) and data for solar cell performance (Table 1). For 100 mM TiCl4 solution, the short-circuit current density (Jsc) and fill

factor (FF) was relatively low because of increased recombination at the FTO surface and a correspondingly high series resistance. Increasing the concentration of TiCl 4 to 200 mM resulted in a substantially improved Jsc of 19.16 mA/cm2 and FF of 59%. This is attributed to complete coverage of the FTO surface by TiO2, leading to reduced current leakage and lower series resistance. However, further increase in the TiCl4 concentration to 300 mM, both Jsc and FF appeared to decrease. This is caused by two factors: 1) thicker TiO2 film led to significant reduction of light transmittance into the CH3NH3PbI3 active layer; 2) the formation of evident cracks led to interfacial roughness and potential current leakage through the device. This is consistent with the findings from electrochemical impedance spectroscopy, which attribute lower impedance to smaller recombination resistance of a solar cell (Fig. 5). For TiO2 film prepared at a TiCl4 concentration of 400 mM, due to severe cracks in the film and its low transmittance (~50%), the Jsc and FF decreased to 7.13 mA/cm2, and 49%, respectively. Since the open circuit voltage (Voc) of PSCs is mainly defined by the depletion region in the active absorber of the CH3NH3PbI3 layer, the Voc experienced little changes except for the sample of 400 mM, when phto-induced carrier concentration was too low. The champion device corresponded to the TiCl4 concentration of 200 mM, which yielded a Jsc of 19.16 mA/cm2, a Voc of 1.00 V, a FF of 59%, and a PCE of 11.18%. This is owing to adequate coverage of the FTO substrate with a reasonably thin layer of dense TiO2, which permitted significant light into the optical absorber of CH3NH3PbI3 layer. In order to probe the impact of TiCl4-derived TiO2 films as ETL on the PSCs,

steady-state photoluminescence (PL) measurements were used to study possible charge transfer and carrier recombination between TiO2 and perovskite. Fig. 4d depicts the PL emission of the pure CH3NH3PbI3 film and two-layer TiO2/CH3NH3PbI3 films derived from different TiCl4 concentration in the precursor solution. The TiO2 film grown from 200 mM TiCl4 demonstrated the highest PL quenching efficiency than those grown from other solutions (100 mM, 300 mM or 400 mM TiCl4), indicating its best charge-extraction capability. Besides, the quenching tendency is in perfect accordance with the corresponding photovoltaic characteristics of PSCs summarized in Table 1. It is interesting to note that ultraviolet-ozone treatment of the CBD-grown TiO2 films helps improve the performance of PSCs. It is plausible that such a treatment can not only remove organic contaminants effectively, but also induce a Ti-O dipole layer [38-39]. This leads to galvanization of the TiO2 surface and hence reduction of dangling bonds and associated defect states. In order to gain some further insight into the mechanism underlying the performance improvement resulted from the ultraviolet-ozone treatment, samples were examined by contact angle testing. As shown in Fig. 6, the static contact angles of CH3NH3PbI3 solution on the TiO2 surface decreased from 13.5° to 7.5° after the treatment, indicating that ultraviolet-ozone treatment can improve the wetting between the CH3NH3PbI3 layer and the TiO2 film, leading to enhanced interfacial quality between TiO2 and the CH3NH3PbI3 layer. The improved TiO2/CH3NH3PbI3 wetting can be further corroborated by the optical absorption and XRD characterization. Obviously, after the O3/ultraviolet

treatment, the light-absorbing property of the CH3NH3PbI3 thin films was further enhanced (Fig. 7a), suggesting thicker and/or denser CH3NH3PbI3 films can be obtained after the treatment. The crystallinity of CH3NH3PbI3 film with and without treatment was tested by XRD (Fig. 7b). As can be seen from the patterns, the main diffraction peaks, located at 14.12°, 28.44° and 31.87°, are correspondingly assigned to (110), (220), and (310) crystal planes of the halide perovskite structure. Evidently, under the same experimental condition, the sample with O3/ultraviolet treatment has stronger diffraction intensity, indicating more CH3NH3PbI3 material owing to improved wetting.[40] The J-V curves and incident photon-to-current conversion efficiency (IPCE) characteristics were shown in Fig. 7c-d. Table 2 summarizes photovoltaic parameters of the PSCs. After O3/ultraviolet treatment, the PSCs delivered a Jsc of 20.06 mA/cm2, a Voc of 1.01 V and a high FF of 63% (Fig. 7c). Compared with the one of untreated device (11.18%), the PCE of treated device is 12.62%, corresponding to an improvement of 12.9%. Also, the efficiency variations of the PSCs under ambient air at room temperature with humidity of 35% were recorded (Fig. 8). The O3/ultraviolet treated device demonstrated better stability with respect to the untreated one. The treated devices retained over 70% of the initial PCE (from 12.62% to 8.88%) after 16-day storage in air. In comparison, the PCE of untreated devices decayed rather fast, about 71% of their original efficiency lost (from 11.18% to 3.21%). Electrochemical impedance spectroscopy (EIS) was used to explore the performance-improvement mechanism of O3/ultraviolet treatment effect.[41-42] The

equivalent circuit for Z-View fitting shown in the Fig. 9a inset consisted of one series resistance Rs, one RC elements representing recombination resistance Rrec and a parallel chemical capacitance, and an additional RC element for interface between spiro-OMeTAD and Au contact. Fig. 9a depicts the Nyquist plots of PSCs with and without O3/ultraviolet treatment in the dark. In general, the lower frequency circle is associated

with

charge

recombination

at

the

interface

of

TiO2/perovskite/spiro-OMeTAD, and the higher frequency one is associated with charge transfer through the device.[43] Interestingly, only one semicircle was observed. The absence of the semicircle in the high-frequency range indicates that the contact resistance is very small. The device with O3/ultraviolet treatment exhibited a significantly higher recombination resistance (Rrec) than the one without O3/ultraviolet treatment. Since all device fabrication parameters were identical, except for the O3/ultraviolet treatment, the difference in Rrec is thus attributed to the TiO2 surface modification introduced by the O3/ultraviolet treatment. This implies that the O3/ultraviolet treatment may suppress the charge recombination and improve the charge separation accordingly. This difference in recombination rate between the devices with and without O3/ultraviolet treatment was further confirmed via the steady-state PL spectra test of the perovskite layer deposited on TiO2 film, as shown in Fig. 9b. Without the O3/ultraviolet treatment, the perovskite film exhibited a higher emission peak, compared with the one with O3/ultraviolet treatment. The difference in emission peak intensity indicated that O3/ultraviolet treatment could make the contact between the TiO2 and the perovskite active layer more intimate, facilitating the charge

transfer and effectively suppressing the carrier recombination.

Conclusion We have prepared thin films of rutile TiO2 nanocrystals on FTO substrates, using a simple chemical bath deposition method at a rather low temperature (<100 ℃). By tuning the concentration of the TiCl4 in the precursor solution, high-quality TiO2 films with a thickness about 60nm and over 70% visible-light transmittance were obtained from the 200 mM TiCl4 solution. The microstructure and thickness of the TiO2 films associated with TiCl4 concentration in the precursor solutions are key to their performance as an electron extraction layer in planar solar cells based on the architecture of FTO/TiO2/CH3NH3PbI3/spiro-OMeTAD/Au. The solar cell using the TiO2 film from the 200 mM TiCl4 solution exhibited a good power conversion efficiency of 11.18% under the simulated AM 1.5 G solar irradiance. Ultraviolet-ozone treatment of the surface of TiO2 film led to further improvement in the interface wettability between the TiO2 and perovskite films, leading to enhanced device performance (12.62% PCE).

Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant Nos. 21401167, 91233101, 11174256, 51202224, 51571182, 51402263, U1304514), the Fundamental Research Program from the Ministry of Science and Technology of the People's Republic of China (Nos. 2014CB931704), the China Postdoctoral Science Foundation (Grant Nos. 2013M540573), and the Fundamental and Advanced Technology Research Program from the Science and Technology Department of Henan Province (Grant Nos. 142300410031.0).

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Figures and tables

Fig. 1. Top-view SEM images of the TiO2 films on FTO substrates, deposited in aqueous solutions with different concentrations of TiCl4: (a) 100 mM, (b) 200 mM, (c) 300 mM, (d) 400 mM.

Fig. 2. The thickness of TiO2 films deposited using solutions of different concentrations of TiCl4 (100 mM, 200 mM, 300 mM and 400 mM).

Fig. 3. (a) Transmittance spectra and (b) The X-ray diffraction (XRD) patterns of the TiO2 films on FTO coated glass substrates, deposited at different concentrations of TiCl4 in precursor solutions (100 mM, 200 mM, 300 mM and 400 mM). All TiO2 peaks were indexed to rutile (JCPDS#21-1276).

Fig. 4. (a) Device structure of as-prepared perovskite solar cells. (b) Energy levels (with respect to vacuum) of the various device components. (c) J-V curves of planar heterojunction structures obtained under the AM 1.5 G solar illumination, with TiO2 films deposited using different concentrations of TiCl4 as the electron transport layer. (d) Photoluminescence spectra of CH3NH3PbI3 thin film based on TiO2 films from various TiCl4 precursor concentrations.

Fig. 5. The Nyquist plots of planar heterojunction structure PSCs based on 200 mM TiCl4 and 300 mM TiCl4 solution under dark condition.

Fig. 6. Optical images of CH3NH3PbI3 drops on the TiO2 (200 mM TiCl4): (a) without O3/ultraviolet treatment and (b) with O3/ultraviolet treatment.

Fig. 7. Absorption spectra (a) and X-ray diffraction (XRD) (b) of CH3NH3PbI3 on the TiO2 films without and with O3/ultraviolet treatment. The J-V curves (c) and the incident photon to current conversion efficiency (IPCE) (d) of the planar heterojunction structure PSCs based on 200 mM TiCl4 to produce the ETL.

Fig. 8. The PCE variation of PSCs placed in air at room temperature with the humidity of 35% during 16-day storage.

Fig. 9. (a) The Nyquist Plots of planar heterojunction PSCs based on 200 mM TiCl4 solution to deposit the ETL with or without O3/ultraviolet treatment under dark condition. (b) Photoluminescence (PL) spectra of the perovskite film on top of FTO, TiO2 without O3/ultraviolet treatment and TiO2 with O3/ultraviolet treatment.

Table 1. The photovoltaic parameter of PSCs based on TiO2 film as ETL derived from different concentrations of TiCl4 solution. TiCl4 (mM)

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

100

14.17

0.99

0.55

7.80

200

19.16

1.00

0.59

11.18

300

17.33

0.96

0.56

9.38

400

7.13

0.88

0.49

3.07

Table 2. The photovoltaic performance parameters of PSCs devices based on TiO2 from the 200 mM TiCl4 as the ETL, with or without O3/ultraviolet treatment. TiCl4 (mM)

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

200

19.16

1.00

0.59

11.18

200 (O3/UV)

20.06

1.01

0.63

12.62