Low-Temperature TiOx Compact Layer for Planar Heterojunction

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Low-Temperature TiOx Compact Layer for Planar Heterojunction Perovskite Solar Cells Zonghao Liu, Qi Chen, Ziruo Hong, Huanping Zhou, Xiaobao Xu, Nicholas De Marco, Pengyu Sun, Zhixin Zhao, Yi-Bing Cheng, and Yang Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12123 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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ACS Applied Materials & Interfaces

Low-Temperature TiOx Compact Layer for Planar Heterojunction Perovskite Solar Cells Zonghao Liu, †,‡ Qi Chen,‡ Ziruo Hong, ‡ Huanping Zhou, ‡ Xiaobao Xu, †,‡ Nicholas De Marco, ‡ Pengyu Sun, ‡ Zhixin Zhao,* † Yi-Bing Cheng, †,§ and Yang Yang*‡ †

Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information,

Huazhong University of Science and Technology, Wuhan, Hubei 430074, P.R. China ‡

Department of Materials Science and Engineering, University of California, Los Angeles,

California 90095, USA §

Department of Materials Engineering, Monash University, Melbourne, Australia

KEYWORDS: Surface modification; Low-temperature; Pinhole-free; Charge transport layer; Perovskite solar cell.

ABSTRACT: Here, we demonstrate an effective low-temperature approach to fabricate a uniform and pinhole-free compact TiO2 layer for enhancing photovoltaic performance of perovskite solar cells. TiCl4 was used to modify TiO2 for efficient charge generation and significantly reduced recombination loss. We found that a TiO2 layer with an appropriate TiCl4 treatment possesses a smooth surface with full coverage of the conductive electrode. Further studies on charge carrier dynamics confirmed that the TiCl4 treatment improves the contact of

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the TiO2/perovskite interface, facilitating charge extraction and suppressing charge recombination. Based on the treatment, we improved the open circuit voltage from 1.01 V of the reference cell to 1.08 V, and achieved a power conversion efficiency of 16.4 %.

INTRODUCTION The solution-processable organic-inorganic hybrid perovskite solar cell (PVSCs) has attracted considerable interest because of their great promise for the development of low-cost thin film solar cells. 1,2The hybrid perovskite materials possess high absorption coefficients, suitable direct band gaps, small exciton binding energies, high carrier mobilities, long diffusion lengths, and superior defect tolerances. 3,4Since Miyasaka and co-workers first reported perovskite based dyesensitized solar cells (DSSCs), tremendous progress has been achieved to improve photovoltaic performance of PVSCs. 5-7To date, the power conversion efficiency (PCE) have reached 20.1 %.8 PVSCs evolved from solid state DSSCs, which employ mesoporous TiO2 layers to serve as the framework, which requires a high-temperature sintering process over 450 oC. 1In consideration of future commercialization, planar PVSCs with low-temperature (＜150 oC) processing has attracted particular attention due to the simple device structures and compatibility with flexible substrates.

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The planar heterojunction interface between the charge transport layer and

perovskite plays a vital role in further improving device photovoltaic performance. 9,12-15Ideally, a charge transport layer should possess a suitable energy level, sufficient conductivity, and good charge extraction capacity. In other words, the charge transport layer should have high film quality without pinholes, smooth surface morphology and intimate contact with perovskite. Suitable band align between perovskite and charge transport layer also play an important role in device performance.14,16-21 Low-temperature and solution processable metal oxide materials such


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as TiO2, ZnO and SnO2 are widely used as electron selection layers in both organic solar cells and PVSCs. usually

13,22-25

In planar PVSCs, the low-temperature TiO2 compact layer (ltc-TiO2) is

composed

of

anatase

TiO2

nanoparticles,

and

titanium

diisopropoxide

bis(acetylacetonate) added to enhance the interconnection of TiO2 nanoparticles and improve the film quality.

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However, pinholes in the ltc-TiO2 layer usually act as a sink where charge

recombination loss occurs. Therefore, it is important to fabricate a pinhole-free and uniform ltcTiO2 layer to enhance device performance. In addition, careful interface modifications of the TiO2 layer, such as self-assembled fullerene monolayers,

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graphene quantum dots,26 amino acids,27-30 silane monolayers,

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thiols32 and

carboxyl groups 33have been employed to enhance the charge selection and charge extraction at the TiO2/perovskite interface. Among these, chemical bath deposition of TiOx from TiCl4 is a common method to modify the surface of the mesoporous TiO2 layer in DSSCs, resulting in high quality TiO2 film via enhancing bonding between nanoparticles and retarding charge recombination.

34-36

Yella et al demonstrated a chemical bath deposited rutile nanocrystalline

TiO2 from TiCl4 solution at 70 oC as an efficient electron extraction layer for PVSC, and achieved better performance than TiO2 prepared by spray pyrolysis at 500 oC.

37

Cojocaru et al

also reported surface treatments of a high temperature TiO2 layer for PVSCs using a TiCl4 treatment.38 So far, previous reported works used TiCl4 treatments usually need another high temperature (500 oC) treatment, which is not compatible with plastics based flexible substrate and it is highly desirable to explore low-temperature (＜150 oC) processing of TiCl4 treated TiO2 for PVSCs. In this study, we employed a simple TiCl4 treatment to modify the ltc-TiO2 layer at low-temperature (＜150 oC) As a result, we obtained a uniform and pinhole-free compact TiO2 layer, which renders efficient hole-blocking and charge separation. We fabricated planar PVSCs


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using TiCl4 treated ltc-TiO2 films (TiCl-TiO2) and untreated TiO2 films (TiO2) for comparison. The TiCl-TiO2 based devices showed ～70 mV improvement in open circuit voltage, resulting in a PCE of 16.4% comparing to 15.6% from untreated cells. We also investigated the underlying mechanisms of the improvement originated from TiCl-TiO2. Our results suggest that careful surface treatments of charge collection layers provides a simple and effective route to facilitate charge transfer and suppress the charge recombination in PVSCs. More importantly, all the process is conducted under low temperature makes this method compatible with ITO or flexible substrate for the future practical application. RESULTS AND DISCUSSION First, a ltc-TiO2 layer with nanoparticle diameters of 5 nm was deposited onto ITO substrate. The film was then treated with 200 mM TiCl4 aqueous solution at 70 oC for different durations. The details are described in the experimental section. We studied the TiO2 layer with TiCl4 treatment for 0, 10, 20, and 30 min in the following discussion. Figure 1 shows the top-view scanning electron microscopy (SEM) images of the obtained TiO2 layer. It was found that the untreated TiO2 film had pinholes with sizes of 5-10 nm, and after treatment the pinholes were gradually filled, yielding a smoother surface. After 30 min of treatment, small cracks are observed from the SEM image in Figure 1d. The cracks should be unfavourable to the device performance due to the poor coverage of the conductive substrate, and longer treatments result in peeling off of the TiO2 film from the substrates. X-ray diffraction patterns (XRD) were used to investigate the structure of the TiCl4 treated TiO2. As shown in Figure S1, all the diffraction peaks of untreated TiO2 sample correlate to those of the pure anatase39, and the observed slightly widening of the diffraction peak for TiCl-TiO2 indicates that amorphous TiOx was formed after TiCl4 treatments.


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The TiO2/perovskite interface plays an important role in charge collection within PVSCs, where a pinhole free and uniform TiO2 layer is desired for effective charge extraction. The surface morphology of the obtained TiO2 layer was characterized by atomic force microscopy (AFM) and the results are shown in Figure 2. The roughness (RMS) of ltc-TiO2 reduced from 7.8 nm to 6.3 nm after 20 min of TiCl4 treatment. This is consistent with the pinhole size distribution in Figure S2 and the observed filling of pinholes via TiCl4 treatment and improvement of uniformity of the TiO2 layer morphology. In order to show this effect, we fabricated devices with a structure of ITO/ltc-TiO2/Au. Considering that cracks appeared after 30 min of TiCl4 treatment, we chose 20 min of TiCl4 treatment as optimized conditions. The current density-voltage (J-V) curve and schematic diagram of ITO/ltc-TiO2/Au devices are shown Figure 3. All devices show resistor behaviours. We noticed that the TiCl-TiO2 based devices exhibit lower current density, i.e. larger resistance, than that of the TiO2 devices (2.25 (TiCl-TiO2) vs. 1.18 (TiO2) ohm cm2). In the case of high quality films without any pinholes, assuming Ohmic contact on both sides, the resistance derived from the J-V characteristics should depend only on the conductivity of the TiO2 films. However, the dramatic difference in resistance we observed could be attributed to the pinhole effects, which result in direct contact between ITO and Au. For the TiCl-TiO2 layer, nanoscale pinholes were partially filled after TiCl4 treatments, which can effectively prevent the direct contact of TiO2 and Au.40 Less pinholes indicates that the TiCl-TiO2 should act as an effective blocking layer that physically separates the photovoltaic active layer and metallic electrode. According to our observation, the 20 min TiCl4 treated ltc-TiO2 film was chosen as optimal condition to fabricate photovoltaic cells, since it shows the best film properties and blocking effects. In this study, the perovskite film was prepared with a modified two-step solution process


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as previous report.

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We then incorporated ltc-TiO2 layer in a planar PVSCs having a device

configuration of ITO/TiO2/Perovskite/Spiro-OMeTAD/Au. The cross sectional image of the photovoltaic cell is shown in Figure S3. Device performance was characterized by J-V measurements under simulated AM 1.5G (100 mW cm-2) solar illumination with statistical distribution of photovoltaic parameters shown in Figure 4 and the maximal and average photovoltaic parameters summarized in Table 1. As shown in Figure 5, the devices based on TiO2 without TiCl4 treatment shows a short circuit current density (JSC) of 21.0 mA cm-2, an open circuit voltage (VOC) of 1.02 V, a fill factor (FF) of 72.9% and a PCE of 15.6 %. In comparison, the best device based on TiCl-TiO2 shows a slightly lower JSC of 19.7 mA cm-2, a VOC of 1.09 V, a higher FF of 75.9% and a higher PCE of 16.4%. It is well known that an anomalous hysteresis is frequently observed in perovskite solar cells, which has been attributed to the charge selective layers, ionic movement or ferroelectric effects. 42-46The hysteresis effects for both the ltc-TiO2 and TiCl-TiO2 devices are shown in Figure 5. The JSC was not significantly affected, however, the FF and VOC were significantly changed with respect to scan direction. The J-V curves of TiCl-TiO2 and TiO2 devices with different delay times and stabilized power outputs are shown in Figure S4 and Figure S5, respectively. It was found that both device show hysteresis phenomena, which have be speculated to originate from the in-efficient charge transfer at the perovskite/TiO2 interface23, the trapping/de-trapping of charge carrier at perovskite interface47,48, ionic displacement49 or ferroelectric effect50,51. It is important to measure the stabilized power output for perovskite solar cells.

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The stabilized power output confirms the

TiCl-TiO2 based devices (14.8%) have better device performance than those based on TiO2 (13.9%). The result shows that the TiCl4 treatment did not have distinct effect on the hysteresis. In addition, the Rsh and Rs values of the devices are listed in Table 1. The TiCl-TiO2 based


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devices showed slightly lower Rs values than those of TiO2 based devices, indicating the TiClTiO2 formed better electrical contact with perovskite. Meanwhile, the TiCl-TiO2 devices exhibited a much larger Rsh. It indicates that the TiCl-TiO2 has better hole blocking, leading to larger VOC. The dark current curve in Figure 5 also reflects the blocking effect of the two TiO2 layers. It was observed that the TiCl-TiO2 based devices have a larger onset voltage, which is consistent with the larger detected VOC. Therefore, it was suggested that the TiCl-TiO2 films have less pinholes and exhibit excellent hole blocking ability to suppress the charge recombination caused by direct contact of ITO with perovskite. To gain insight into the influence of TiCl4 treatment on the JSC of PVSCs, an external quantum efficiency (EQE) measurements was performed as shown in Figure 6. Both devices show photoresponse onset at 792 nm, consistent with the bandgap edge of CH3NH3PbI3-xClx.52 The EQE of TiCl-TiO2 based device shows obvious decrease in the range of 300 nm-600 nm relative to that of the TiO2 based device, which shall originate from the absorption of TiCl-TiO2. The transmittance of the TiCl4 treated ltc-TiO2 with different treatment times in Figure S6 indicates that after TiCl4 treatment the transmittance decreased. As shown in Figure S7 and Figure S8, the perovskite layer on TiCl-TiO2 and TiO2 showed nearly identical absorption, and the perovskite films showed nearly identical morphology with full surface coverage of the substrate. It can be safely deduced that the slight decrease of the JSC after TiCl4 treatment can be ascribed to the relatively lower transmittance of the TiCl4 treated ltc-TiO2 film than that of ltc-TiO2 film, which resulted in lower light harvesting efficiency. The hybrid perovskite materials have strong photoluminescence (PL) which is directly associated with the charge recombination.3,4 Thus, time resolved photoluminescence (TRPL) decay measurements is a good tool to study the charge dynamics. We performed the TRPL decay


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measurements to study the influence of TiCl4 treatments on the charge extraction process at the TiO2/CH3NH3PbI3-xClx interface. Figure 7 shows the TRPL decay curve for both samples of TiCl-TiO2/CH3NH3PbI3-xClx, and TiO2/CH3NH3PbI3-xClx, with CH3NH3PbI3-xClx on glass as a comparison. Both TiO2 layers show a quenching effect of the perovskite PL, indicating that charge transfer occurred at the TiO2/CH3NH3PbI3-xClx interface upon light excitation of the perovskite film. Fitted with a bi-exponential function,3,41,53 the bare CH3NH3PbI3-xClx sample shows a τ value of 75 ns. With insertion of TiO2 film, the τ value for TiO2 and TiCl-TiO2 samples were reduced to 27 ns and 10 ns, respectively. The significantly reduced τ value infers an efficient charge extraction process occurred at the interface. Moreover, it indicates that the TiCl-TiO2 is more favourable to create the interface with CH3NH3PbI3-xClx for charge extraction than the non-treated one, which is ascribed to both morphological and electrical property change in the TiO2 layer with TiCl4 treatment. From the morphological point of view, TiCl4 treatments have been previously reported to increase the wettability of the TiO2 surface, which may benefit the contact between perovskite layer and TiO2 layer and enhance charge collection.38The SEM and AFM measurements have shown reduced surface roughness that is likely to provide intimate contact with the perovskite layer. In the perspective of electrical properties, we find that the Fermi level of the TiO2 upshifted towards to the conduction band after TiCl4 treatment, as measured by ultraviolet photoelectron spectroscopy (UPS) shown in Figure S9. It means the TiO2 is heavily doped with a higher donor concentration after TiCl4 treatment, which is in good agreement with the absorption tail in the UV-vis spectrum. To further confirm this, the chemical states of TiO2 with TiCl4 treatment were measured by X-ray photoelectron spectroscopy (XPS). As shown in Figure S10 of the resolving Ti 2p XPS spectra of TiO2 films, TiO2 film with TiCl4 treatment and 150 oC sintering possesses a higher content of Ti3+ (in comparison with Ti4+), i.e.,


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the donor species in the TiO2 film, than that of TiO2 film without TiCl4 treatment and TiO2 film with TiCl4 treatment and 500 oC sintering. As a result, the higher donor concentration will facilitate electron extraction ability of TiCl-TiO2.

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determines the space depletion width (W), ࢃ = ට

In addition, according to the equation that

૛ࢿࢿ૙ ൫ࡱିࡱࢌ ൯ ࢋࡺ

, where N is the concentration of

electron donors, ε is the dielectric constant, ε0 is the vacuum permittivity, Ef is the Fermi level. 22 E-Ef is approximately the Fermi level difference between the work function of ITO and the Fermi level of ltc-TiO2. The TiCl-TiO2 should have narrow depletion width, which leads to enhanced carrier tunneling and a lower contact resistance at the interface, as well as higher bulk conductivity of TiO2 layer.54 As a result, TiCl-TiO2 based devices show a reduced series resistance of 3.32 ohm cm2 to that of the untreated ones. To further clarify the origin of the improved VOC and performance for the TiCl4-TiO2 based device, transient photovoltage decay measurements were employed to investigate the carrier dynamics. As shown in Figure 8 and Figure S11, both devices exhibit photovoltage decay constants on the scale of microseconds, which is consist with previous reports.

13,41

The charge

carrier lifetime for the TiCl-TiO2 based devices were measured to be nearly two times longer than that of the TiO2 based devices. This result indicates that the carrier recombination has been successfully suppressed via TiCl4 treatment. Considering the fact that the other layers were fabricated with the same procedure, the enhanced the VOC and FF should be attributed to the TiCl4 surface modification of the ltc-TiO2 film, where better coverage and superior charge selection capacity consequently reduces charge loss and facilitates charge separation. In spite of the high efficiency, one major concern is stability of perovskite solar cells.28,55,56 To study the influence of TiCl4 treatment on device stability, we performed an aging test of the devices in a nitrogen glove box at 80 oC without encapsulation. The normalized PCEs of TiO2


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and TiCl-TiO2 based devices as a function of storage time are summarized in Figure S12. Both device showed a fast decrease of the PCE after 7 hours aging test, which could be attributed to the migration of metal electrode through the hole transporting layer and eventually contact with the perovskite.57 After 37 hours exposure, TiO2 based devices showed 22.4% of initial PCE. At the meantime, the TiCl-TiO2 based device showed nearly identical stability properties under same condition with 26.8% of initial PCE, which suggest that the TiCl4 treatments did not have significant influence on the stability of the device. CONCLUSIONS In summary, we demonstrated a simple TiCl4 treatment to obtain uniform and pinhole free TiO2 compact layer via low-temperature processing for planar heterojunction perovskite solar cells. A modified interface after TiCl4 treatments facilitates charge extraction and suppress charge recombination. As a result, the device based on TiCl4 treated TiO2 compact layer showed significantly enhanced VOC and fill factor, achieving a PCE of 16.4%. EXPERIMENTAL Precursor synthesis. The TiO2 nanoparticles were synthesized by a non-hydrolytic sol–gel approach13, where the entire synthetic procedure was performed in ambient air. In a typical synthesis, 0.5 mL TiCl4 was added into 2 mL ethanol slowly with stirring, followed by adding 10 mL benzyl alcohol, leading to a yellow solution. The solution was heated at 80 °C for a period of 5 hours, forming a slightly milky suspension, which was then mixed with 200 mL diethyl ether and centrifuged to collect the precipitate. The as-obtained product was re-dissolved in 30 mL absolute ethanol and precipitated with the addition of 200 mL diethyl ether, and this step was repeated twice. The final TiO2 was collected and dispersed in ethanol to make a suspension with


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a concentration of 3-6 mg mL−1. To stabilize the as-obtained TiO2 solution, TiAcAc was added into the solution whose final concentration of 15 µL mL-1. Device fabrication and characterization. ITO substrate was ultrasonically cleaned with acetone, detergent solution, deionized water and isopropanol successively. After 3 minutes of oxygen plasma treatments, a 40 nm-thick layer of a TiO2 film was spin-coated on substrate with a TiAcAc stabilized TiO2 sol-gel solution, and annealed at 150 °C for 30 min in air. The TiO2 film was immersed in a TiCl4 solution (200 mM) in a closed vessel at 70 °C for desired time. After washed with deionized water and ethanol, the obtained film was dried with air flow and annealed at 150 °C for 30 min in air. A solution of PbI2 (dissolved in DMF, 450 mg mL-1) was then spin-coated on ITO/TiO2 substrate at 3000 rpm for 30 s in dry air. A mixture of CH3NH3I/CH3NH3Cl (dissolved in isopropanol, 50/5 mg mL-1) was spin-coated onto the dried PbI2 layer at room temperature at 3000 rpm for 30 s in dry air. Then, the obtained films were annealed in the air at 135 °C for 15 min. A solution of Spiro-OMeTAD was spin-coated on the perovskite film at 3000 rpm for 30s in dry air, where a Spiro-OMeTAD/chlorobenzene (80 mg mL-1) solution was employed with the addition of 35 µL Li-TFSI/acetonitrile (260 mg mL−1), and 30 µL 4-tert-butylpyridine. Finally, a 100 nm gold layer were deposited as counter electrode on the top of Spiro-OMeTAD layer through shadow masks via thermal evaporation under high vacuum (5 × 10−6 Torr). The current density–voltage (J-V) characteristics of photovoltaic devices were obtained using a Keithley 2400 source-measure unit. The photocurrent was measured under AM1.5G illumination at 100 mW cm−2 under a Newport Thermal Oriel 91192 1000 W solar simulator. Unless stated otherwise, the devices were masked and measured under the reverse voltage scan with different delay time. The light intensity was calibrated using a KG-5 Si diode. External quantum efficiencies were measured by an Enli Technology (Taiwan) EQE


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measurement system. The effective area of each cell was 0.108 cm2 defined by masks for all the photovoltaic devices discussed in this work. Time-resolved photoluminescence spectra and UV–Vis spectra. The time-resolved photoluminescence spectrum was acquired using the time-correlated single-photon counting technique (Pico harp 300), and the excitation light pulse was provided using a picosecond diode laser at a wavelength of 633 nm with a repetition frequency of 1 MHz (PDL 800B). The TiO2 layer and perovskite layer was deposited on to the quartz substrates as described above for timeresolved photoluminescence spectrum measurements. UV–Vis spectra were obtained with a Varian Cary 50 ultraviolet−visible spectrometer. The TiO2 layer and perovskite layer was deposited on to the quartz substrates as described above for UV–Vis spectra measurements. Transient photovoltage decay measurements. A white light bias was generated from an array of diodes (Molex 180081-4320). A nitrogen laser (LSI VSL-337ND-S, 337 nm) was used as the perturbation source, with a pulse width of 4 ns and a repetition frequency of 10 Hz. The intensity of the perturbation laser pulse was controlled to maintain the amplitude of transient VOC below 5 mV so that the perturbation assumption of excitation light holds. The voltages under open circuit and currents under short circuit conditions were measured over a 1 MΩ and a 50 Ω resistor, and were recorded on a digital oscilloscope (Tektronix DPO 4104B). SEM, AFM, XPS and UPS. A field-emission scanning electron microscope (FEI Nova 230 NanoSEM) was used to acquire SEM images. The instrument used an electron beam accelerated at 500 V to 30 kV, enabling operation at a variety of currents. The Image J software was used to analyze the pinhole size. Atomic force microscopy was performed using Bruker Dimension FastScan Scanning Probe Microscope (SPM) in ''tapping'' mode. XPS measurements were carried out on an XPS AXIS Ultra DLD (Kratos Analytical). An Al Kα(1,486.6 eV) X-ray was


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used as the excitation source. UPS measurements were carried out to determine the work function of the materials, and a He discharge lamp, emitting ultraviolet energy at 21.2 eV, was used for excitation. All UPS measurements of the onset of photoemission to determine the work function were performed using standard procedures with a −5.0 V bias applied to the samples. Supporting Information. XRD data, SEM cross-sectional image of device, current-voltage data for hysteresis, stabilized output data, transmittance data, SEM images of perovskite films, XPS and UPS data, photovoltage decay and long-term stability data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Z. Zhao ([email protected]); * Y. Yang ([email protected]). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Z. Liu. and Q. Chen. contributed equally to this work. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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This work was partially supported by the National Basic Research Program of China (973 program 2011CBA00703), the Fundamental Research Funds for the Central Universities (grant no. HUST: 2014TS016) and Director Fund of Wuhan National Laboratory for Optoelectronics (Z.Z.), Short-Term Program of Postgraduates Academic Training Abroad of Huazhong University of Science and Technology (Z.L.), Air Force Office of Scientific Research (AFOSR, Grant no. FA9550-15-1-0333) (Y.Y.), Office of Naval Research (ONR, Grant no. N00014-14-10648) (Y.Y.) and National Science Foundation (NSF ECCS-1509955) (Y.Y.). We thank the facility from Nano and Pico Characterization Core Lab at California Nanosystem Institute. We also thank the Analytical and Testing Center of Huazhong University of Science and Technology for XPS testing. We appreciate the insightful technical discussion with Yao-Tsung Hsieh. We sincerely acknowledge Lei Meng for his help with UPS characterization, Yongsheng Liu for his help with AFM characterization, Wenjun Zhang for his help for XPS characterization. REFERENCES (1)

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Figure 1. Top-view SEM image of TiO2 film with TiCl4 treatments for a) 0, b) 10, c) 20, d) 30 min.


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Figure 2. AFM image of TiO2 film with TiCl4 treatments for a) 0, b) 10, c) 20, and d) 30 min, respectively, three batch of samples were measured to obtain an averaged RMS value.


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Figure 3. The J-V curve of devices with ITO/TiO2/Au configurations based on TiCl-TiO2 layer (solid line) and TiO2 film (dashed line). Inset: the schematic diagram of ITO/TiO2/Au device.


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Figure 4. Statistical distribution of photovoltaic parameters, (a) VOC, (b) JSC, (c) FF and (d) PCE, extracted from current-voltage measurements of devices under simulated AM 1.5 (100 mW cm-2) illumination.


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Figure 5. Forward scan (FS, from -0.2 V to 1.2 V) and reverse scan (RS, from 1.2 V to -0.2 V) of current-voltage (J-V) curves for TiCl-TiO2 based devices and untreated ltc-TiO2 based devices under simulated AM 1.5 (100 mW cm-2) illumination with a step size of 20 mV and a delay time of 1ms. Dark current measurements were performed with a scan range from -1.5 V to 1.5 V and a scan rate of 0.05 V per step.


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Table 1 Device performance of PVSCs based on ltc-TiO2 films with and without TiCl4 treatment measured under simulated AM 1.5 (100 mW cm-2) illumination. Rsh and Rs represent shunt resistance and series resistance, respectively, derived from J-V curve. Device

JSC

VOC

FF

η

(mA cm-2)

(mV)

(%)

(%)

Rsh

Rs

(ohm cm2) (ohm cm2)

TiCl-

champion

19.7

1.09

75.9

16.4

2840

3.74

TiO2

average

19.9

1.08

71.9

15.4

1350.3

3.32

TiO2

champion

21.0

1.02

72.9

15.6

689.7

3.97

average

21.1

1.01

65.4

13.9

398.0

3.43


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Figure 6. External quantum efficiency (EQE) curve of the device based on TiCl4 treated ltcTiO2 film (TiCl-TiO2) and untreated ltc-TiO2 film (TiO2).


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Figure 7. a) Normalized TRPL decay of glass/CH3NH3PbI3-xClx sample, glass/TiO2/CH3NH3PbI3-xClx sample based on TiCl4 treated TiO2 film and untreated TiO2 film.


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Figure 8. Charge lifetime versus light intensity from transient photovoltage decay measurements of TiCl-TiO2 based devices (black square) and TiO2 based devices (red circle).


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graphic for manuscript (TOC)


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Low-Temperature TiOx Compact Layer for Planar Heterojunction

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