Enhanced Performance and Photostability of Perovskite Solar Cells by

Apr 7, 2017 - Perovskite solar cells (PSCs) with high efficiency have recently received tremendous attention, but the stability under light irradiatio...
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Enhanced Performance and Photostability of Perovskite Solar Cells by Introduction of Fluorescent Carbon Dots Junjie Jin,† Cong Chen,† Hao Li,† Yu Cheng,† Lin Xu,† Biao Dong,† Hongwei Song,*,† and Qilin Dai*,‡ †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China ‡ Department of Physics, Atmospheric Sciences and Geoscience, Jackson State University, Jackson, Mississippi 39217, United States S Supporting Information *

ABSTRACT: Perovskite solar cells (PSCs) with high efficiency have recently received tremendous attention, but the stability under light irradiation, namely, photostability, of PSCs still represents a major obstacle that must be overcome before their practical applications can be used. The degeneration of perovskite under ultraviolet irradiation from sunlight is a major impacting factor. To solve this problem, in this work we introduce fluorescent carbon dots (CDs), which could effectively convert ultraviolet to blue light in the mesoporous TiO2 (m-TiO2) layer of the traditional PSCs. As a result, CD-based devices exhibit an improved power conversion efficiency (PCE) of 16.4% on average compared to 14.6% for bare devices, and the light stability of CD-based devices is highly enhanced. These devices can maintain nearly 70% of the initial efficiency after 12 h of full sunlight illumination, while the bare devices maintain only 20% of the initial efficiency. This work indicates that fluorescent down conversion based on CDs is a novel and effective approach to improve the performance and photostability of PSCs. KEYWORDS: perovskite solar cells, carbon dots, fluorescent down conversion, photostability, photovoltaic performance

1. INTRODUCTION Organic−inorganic hybrid perovskite materials, such as CH3NH3PbI3, have received intensive attention in the past few years.1−3 Solar cells based on perovskite materials have improved impressively in power conversion efficiency (PCE) from 3.8% in 2009 to 22.1% in 2016.4,5 This remarkable progress of PCE is attributed to the excellent properties of perovskite materials such as a high absorption coefficient, great charge carrier mobility, and long carrier diffusion length.6−11 Although current perovskite solar cell (PSC) devices exhibit good performance, there are some critical limitations that need to be overcome before their outdoor applications, such as the toxicity of lead, hysteresis in J−V measurements, and poor stability of PSCs. Among their major obstacles, the stability of PSCs is one of the major issues in current research. PSCs are known to be especially susceptible to moisture and UV light, showing drastic decreases in PCE upon exposure to these conditions.12−18 There are some reports about enhancing the moisture stability of PSCs by composition control of perovskite © 2017 American Chemical Society

materials, a hydrophobic hole-transport-material incorporation, surface passivation of perovskite materials, or hydrophobic encapsulation of the entire cell.19−22 However, the UV-light stability of PSCs is still challenging. It has been reported that the degradation of PSCs caused by UV light is dominantly due to the direct contact between the TiO2 layer with the perovskite film,23,24 and Snaith et al. reported that deep traps are induced by oxygen vacancies at the surface of TiO2 under UV-light illumination and lead to the recombination loss of photogenerated charges.25 Furthermore, the reaction between the strong oxidative photon-generated holes (in TiO2) and I ions (in CH3NH3PbI3) occurs, which leads to the degradation of the perovskite material and poor stability.26 In order to improve the UV-light instability of PSCs, we introduce fluorescent carbon dots (CDs) into PSCs for the first Received: February 15, 2017 Accepted: April 7, 2017 Published: April 7, 2017 14518

DOI: 10.1021/acsami.7b02242 ACS Appl. Mater. Interfaces 2017, 9, 14518−14524

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the energy levels in PSCs. (b) Cross-sectional view of the device. SEM images of (c) m-TiO2 and (d) mTiO2/CDs films in the device. AFM images of (e) m-TiO2 and (f) m-TiO2/CDs. spin-coated in a two-step method at 1000 and 4000 rpm for 10 and 30 s, respectively. During the second step, 100 μL of toluene was kept on the spinning substrate 15 s prior to the end of the spinning program. After the spin-coating, the films were annealed at 100 °C for 60 min in a glovebox. Then, the Spiro-OMeTAD {2,2′,7,7′-tetrakis[N,N-di(4methoxyphenyl)amino]-9,9′-spirobifluorene} solution [50 mg of Spiro-MeOTAD, 22.5 μL of 4-tert-butylpyridine, and 22.5 μL of acetonitrile solution containing 170 mg/mL of lithium bis(trifluoromethylsulfonyl)imide in 1 mL of chlorobenzene] was spincoated on the perovskite layer at 1500 rpm for 30 s. Finally, a 100 nm thick Au electrode was deposited by thermal evaporation on the top of the hole-transporting layer. 2.3. Device Characterization. The absorption spectra were recorded using a UV-1800 spectrometer. Fourier transform infrared (FTIR) spectra were obtained from KBr pellets on a Bruker Equinox 70 spectrometer in the 4000−500 cm−1 region. The PL characterization was carried out using a FluoroSense luminescence spectrometer. The morphology analysis of the CDs was performed on a Hitachi H-8100IV transmission electron microscope under an operating voltage of 200 kV. A SIRION field-emission scanning electron microscope was employed to evaluate the surface morphology of the films and cross-sectional view of the device. The roughness of the films was characterized by atomic force microscopy (AFM; 5500, Agilent, Santa Clara, CA). The J−V characteristics of the devices were measured under simulated 100 mW/cm2 AM 1.5G irradiation using an ABET Sun 2000 solar simulator calibrated with a reference silicon cell (RERA Solutions RR-1002) and Keithley model 2400 as a digital source meter. Incident photon conversion efficiency (IPCE) spectra were recorded using a SolarCellScan100 instrument. The timeresolved PL (TRPL) measurements were investigated at an excitation wavelength of 420 nm. Electrochemical impedance spectroscopy (EIS) was performed by a CHI630E electrochemical analyzer (ChenHua, China). The measurements were taken over a frequency range 0.1− 100 kHz, and the applied bias voltage was set as 0.80 V.

time. CDs are intriguing, recently discovered members of the carbon nanomaterial family alongside carbon nanotubes, fullerenes, and graphene.27−31 They are particularly attractive for photoelectronic-device applications because of their excellent properties such as good absorption in the UV region, wavelength-dependent emission, tunable optical and electronic properties, and excellent photochemical stability.32,33 Using this effective approach, we observe that both the light stability and PCE can be considerably improved. Accordingly, PSCs with CDs can maintain nearly 70% of the initial efficiency after 12 h of full sunlight illumination compared to the devices without CDs (20%), and meanwhile, the average PCE improves from 14.6% to 16.4% as CDs are incorporated into PSCs.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Carbon Dots (CDs). Carbon dots were prepared according to the literature method.34 In a typical synthesis, citric acid (1.05 g) and ethylenediamine (335 μL) were dissolved in deionized water (10 mL). Then, the solution was transferred into a poly(tetrafluoroethylene) (Teflon)-lined autoclave (30 mL) and kept at 200 °C for 5 h. 2.2. Device Fabrication. Fluorine-doped tin oxide (FTO) glass substrates were first etched with zinc powder and 2 M HCl diluted in water, and then ultrasonicated in deionized water, acetone, and ethanol successively. These steps were followed by an oxygen plasma treatment. A compact TiO2 (c-TiO2) was deposited onto the cleaned FTO glass by spin-coating 0.15 M titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropanol) in 1-butanol solution at 2000 rpm for 30 s and sintered at 500 °C for 30 min. After the material was cooled to room temperature, a mesoporous TiO2 (m-TiO2) film was spin-coated onto the FTO/c-TiO2 using the TiO2 paste (Dyesol DSL 18NR-T) with ethanol (1:4, mass ratio) solution at 4000 rpm for 30 s, which was followed by heating at 500 °C for 30 min. After being cooled down, the substrates (FTO/c-TiO2/m-TiO2) were immersed in the CD solution and kept in the dark for different times (0, 6, 12, 24, and 48 h). The perovskite films were deposited from a precursor solution containing methylammonium iodide (MAI) with PbCl2 at a molar ratio of 3:1 in DMF at 40 wt %. The perovskite solution was

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of CDs. CDs were synthesized by the modified hydrothermal method reported in the literature (see Experimental Section for details) 14519

DOI: 10.1021/acsami.7b02242 ACS Appl. Mater. Interfaces 2017, 9, 14518−14524

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ACS Applied Materials & Interfaces

Figure 2. (a) Absorption spectra of m-TiO2 with various concentrations of CDs. (b) PL spectra of m-TiO2 with various CD concentrations pumped by 360 nm light.

Figure 3. (a) J−V characteristics. (b) The corresponding histograms of PCE for devices without and with 1.0 mg/mL of CDs. (c) IPCE spectra of different concentrations of CDs. (d) Fitted curve of Nyquist plots (the circles represent the initial data) for the PSCs measured under light irradiation at a 0.8 V bias voltage.

bond vibrations (1390 cm−1), and CN stretching vibrations (1112 cm−1). The photoluminescence (PL) spectrum of CDs is considerably dependent on the excitation wavelength (Figure S4). As the excitation wavelength gradually increases from 300 to 440 nm, the PL peak wavelength shifts from 442 to 536 nm, which is a typical feature for CDs.35 Figure S5 shows the photostability of CDs; no obvious changes in PL intensity after 12 h of continuous UV excitation can be observed, which indicates that these CDs exhibit excellent photostability.36−38 The PL external quantum efficiencies (EQEs) of solid-state CDs and colloidal CDs are measured to be 12% and 70%, respectively.

and characterized using various spectroscopic methods (see Figures S1−S5 in Supporting Information). Transmission electron microscopy (TEM) images of CDs (Figure S1) indicate that the average diameter of the CDs is ∼4.8 nm. In the absorption spectrum (Figure S2), two typical bands of CDs are observed, located at 240 and 345 nm, corresponding to the ππ* transition of CC bonds and n−π* transition of CO bonds, respectively, and the excitation spectra (Figure S2) show two excitation peaks at 253 and 353 nm as the 440 nm wavelength is monitored.34 The FTIR spectrum (Figure S3) of the CDs indicates that the sample mainly contains OH stretching vibrations (3425 cm−1), CO bond vibrations (1632 cm−1), NH bending vibrations (1572 cm−1), CH 14520

DOI: 10.1021/acsami.7b02242 ACS Appl. Mater. Interfaces 2017, 9, 14518−14524

Research Article

ACS Applied Materials & Interfaces Table 1. Summary of Photovoltaic Parameters for PSCs with Various Concentrations of CDs CD conc (mg/mL) 0 0.5 1.0 2.0 4.0

JSC (mA/cm2) 20.89 22.01 22.64 21.89 21.12

± ± ± ± ±

0.23 0.25 0.20 0.19 0.21

VOC (V) 1.018 1.019 1.019 1.017 1.018

± ± ± ± ±

FF (%)

0.001 0.001 0.001 0.001 0.001

68.93 69.27 71.60 68.27 69.67

± ± ± ± ±

0.17 0.20 0.16 0.18 0.20

PCE (%) 14.61 15.48 16.40 15.08 14.83

± ± ± ± ±

0.35 0.33 0.36 0.28 0.29

R1 (Ω cm2) 278.9 190.4 154.6 211.5 255.7

± ± ± ± ±

6.5 7.8 5.8 4.7 5.3

Figure 4. (a) Stability of PSCs with various CD concentrations under light exposure in air for 12 h. (b) UV−vis absorption spectra of the PSCs with and without CDs before and after 12 h of 1 sun illumination.

3.2. Effect of CDs on Performance of PSCs. An energy band diagram of the devices with CDs is proposed in Figure 1a. Compact TiO2 (c-TiO2) and mesoporous TiO2 (m-TiO2) are prepared on FTO as the electron transport layer (ETL), and CDs are placed on top of m-TiO2. An MAPbClxI3−x perovskite film is coated on the ETL layer. Spiro-OMeTAD and gold are used as a hole-transport material (HTM) and back metal electrode, respectively. The corresponding film thicknesses of cTiO2, m-TiO2/CDs, MAPbClxI3−x, Spiro-OMeTAD, and Au are 60, 200, 320, 180, and 100 nm, respectively (Figure 1b). It is noted that a vacuum-level shift is caused by the CDs. This is attributed to the characteristics of the interface dipole created by CDs with a mass of amino groups on the surface.39,40 The SEM images of m-TiO2 and m-TiO2/CDs are as shown in Figure 1c,d. It is noticed that the surface feature of the two films looks identical due to the limitation of the SEM technique, but the difference between these two films can be identified by analyzing surface roughness via an atomic force microscope (AFM) (Figure 1e,f). The root-mean-square (RMS) roughness reduces from 17.1 to 15.2 nm as CDs are incorporated into mTiO2, indicating that surface roughness is improved by CDs, which can enhance device performance because of reduced recombination loss caused by an uneven surface.32 The optical properties of m-TiO2/CDs are studied using UV−vis absorption and PL spectra. As shown in Figure 2a, the absorbance of m-TiO2/CDs increases in the UV region compared to that of m-TiO2, indicating more UV-light absorption by m-TiO2/CDs than that by m-TiO2. Blue fluorescence can be observed from m-TiO2/CDs under 360 nm excitation (Figure S6), and the luminescence intensity increases as the CD concentration increases from 0 to 1.0 mg/ mL, and then decreases with further increasing of the CD concentration (Figure 2b). This can be attributed to the luminescence quenching due to aggregation of CDs as more CDs are introduced in the solution. The CD concentration’s effect on current density versus voltage (J−V) curves is investigated and shown in Figure 3a.

The average photovoltaic parameters for 40 devices with various concentrations of CDs are listed in Table 1. It can be observed that PCE increases as the CD concentration increases from 0 to 1.0 mg/mL. A further increase of CD concentration from 2.0 to 4.0 mg/mL leads to a decreased short-circuit current density (JSC) and PCE. The optimized CD concentration is found to be about 1.0 mg/mL, leading to a JSC of 22.85 mA cm−2, an open-circuit voltage (VOC) of 1.02 V, a fill factor (FF) of 71.80%, and a PCE of 16.8%. The statistical histogram of the PCEs from 40 pieces of PSCs without and with 1.0 mg/mL CDs is shown in Figure 3b. It shows that all the tested PSCs with CDs demonstrated a higher PCE than that of the bare device. On average, the PCE of the PSCs with CDs relatively increases by about 12.3% of that of the bare devices, from 14.6% to 16.4%. The hysteresis curves of both the control and CD-based devices are shown in Figure S7a at different scan directions. The hysteresis effect is suppressed significantly via incorporating CDs into PSCs. The reduced hysteresis is attributed to the effective modification of interfacial trap states and improvement in charge transfer at the interface of perovskite and TiO2.41 In addition, two optimized devices were tested with a bias voltage of 0.77 V for the control device and 0.82 V for the CD-based device under continuous AM 1.5G illumination for 200 s (Figure S7b). PCE values of the modified device and control device are 16.6% and 14.6%, respectively, which is consistent with the results in the J−V curves. The incident photo-to-current conversion efficiency (IPCE) spectra of the PSCs based on CDs with different concentrations are shown in Figure 3c. In a comparison to the bare device, the device with CDs shows outstanding increases of IPCE obtained at the wavelength range between 350 and 500 nm. This improvement is attributed to the combination effects of light harvesting in the UV region and interfacial modification by CDs. After CDs are incorporated into PSCs, more incident sunlight can be utilized via light conversion of UV light to visible light, leading to more light harvesting and more generated photoelectrons. Decreased interfacial impedance 14521

DOI: 10.1021/acsami.7b02242 ACS Appl. Mater. Interfaces 2017, 9, 14518−14524

Research Article

ACS Applied Materials & Interfaces from the EIS study (Figure 3d) and reduced PL intensity (Figure S8) result in reduced recombination loss in the devices and improved device performance. The PL lifetimes of CDs (a) in aqueous solution, (b) in m-TiO2 film, and (c) in m-TiO2 film covered with MAPbClxI3−x were analyzed using time-resolved photoluminescence spectroscopy (Figure S9). The average lifetime of CDs in m-TiO2 film is determined to be 0.83 ns, which is much shorter than that of CDs in aqueous solution (11.22 ns), indicating that photoinduced excitons are welldissociated at m-TiO2/CDs interfaces. The average lifetime of CDs decreases to 0.58 ns as MAPbClxI3−x is introduced, which indicates that electrons of CDs are probably transferred to MAPbClxI3−x by nonradiative decay. Electrochemical impedance spectroscopy (EIS) was used to investigate the charge recombination process of PSCs. Figure 3d presents the fitted curve of Nyquist plots for the devices with various CD concentrations conducted under 1 sun illumination at an applied bias of 0.8 V, which exhibits two distinct semicircles (one in the high-frequency range and the other in the lowfrequency range) measured in the frequency range 0.1 Hz to 100 kHz. The Nyquist plot was fitted using an equivalent circuit model, as illustrated in the inset of Figure 3d. RS is the series resistance of the TiO2 electrode; R1 is the interfacial resistance between TiO2 and perovskite, and R2 is the interfacial resistance between perovskite and HTM.42 After CDs are incorporated, R1 exhibits a significant decrease from 278.9 to 154.6 Ω cm2 (Table 1), which suggests that CDs improved electron transport between TiO2 and perovskite material, indicating a positive effect on electronics, resulting in increased JSC. The light stability of the solar cells is examined under full sunlight illumination without encapsulation in ambient atmosphere (Figure 4a). The devices with CDs all exhibit significantly improved stability. The normalized PCE remains more than 70% of the initial PCE after 12 h of light irradiation. However, the stability of PSCs without CDs is very poor, and PCE decreases to 20% of the initial PCE under the same testing conditions. It is noted that, under each condition, eight devices are averaged for the light stability testing, indicating high reliability (Table S1). The improvement of photostability of PSCs with CDs is attributed to the restrained decomposition of perovskite in contrast to that of PSCs without CDs. For verification of this point, the absorption spectra of m-TiO2/ MAPbClxI3−x films containing various amounts of CDs are measured after 12 h of irradiation from sunlight (Figure 4b). As can be seen, the absorption spectrum of m-TiO2/MAPbClxI3−x is similar to that of PbI2, indicating the serious decomposition of MAPbClxI3−x. Fortunately, the spectrum of m-TiO2/CDs/ MAPbClxI3−x changes little before and after irradiation, indicating that the presence of CDs can indeed prevent the decomposition of a perovskite film. Actually, the lightirradiation-induced decomposition of MAPbClxI3−x depends strongly on the wavelength of irradiation light. As shown in Figure 5, the normalized absorption intensities of perovskite film at 600 nm change under different light irradiation. The result indicates that the light-irradiation-induced decomposition of MAPbClxI3−x is wavelength-dependent. The decomposition decreases gradually with the increase of the irradiation wavelength. The absorption intensity under 360 nm UV-light irradiation decreases to 80% of its initial value while the absorption intensity keeps more than 90% of its initial value under other light irradiation. This means that device degradation is mainly caused by UV-light irradiation. It is possible that the strong oxidative photon-generated holes are

Figure 5. Normalized absorption at 600 nm for m-TiO2/perovskite film under irradiation with different wavelengths of light (the m-TiO2/ perovskite film samples were irradiated with a tunable pulsed laser with a pulse duration of 10 ns, repetition frequency of 10 Hz, and the same power density of 340 mW/cm2).

produced in the TiO2 layer under UV-light illumination, and they will react with the iodide anion in perovskite, resulting in the decomposition of perovskite film, leading to device degradation.26 The CDs introduced into mesoporous TiO2 can convert UV light of sunlight into light of the visible region, thus preventing the decomposition of MAPbClxI3−x. In addition, the introduction of CDs could result in a decrease in direct contact between the perovskite material and TiO2. Therefore, better light stability can be observed in PSCs with CDs. It should be highlighted that the long time stability of the PSCs with and without CDs was also compared in the dark in an ambient environment, and the results indicate that the PSCs with CDs display device stability similar to that of the PSCs without CDs (Figure S10). Hence, our present study offers a novel approach to improve device performance and photostability of PSCs, and we think this idea may be extended to perovskite LEDs.43,44

4. CONCLUSIONS In conclusion, we have demonstrated that the introduction of CDs in the m-TiO2 layer of PSCs can not only improve the photovoltaic performance, but also have a strong positive impact upon the light stability of PSCs. The introduction of CDs can facilitate transport of charge carriers and show effective photoluminescence, leading to enhanced PCE. In addition, CDs prevent the decomposition of perovskite film caused by UV light, leading to highly improved photostability. As a result, the optimized device exhibiting an average PCE of 16.4% is obtained, and maintains nearly 70% of its initial efficiency after 12 h of full sunlight illumination. Therefore, our present study offers a novel approach to improve the device performance and photostability of PSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02242. Synthesis methods, images, and spectra of CDs; table of relevant data for quantum yield calculations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 14522

DOI: 10.1021/acsami.7b02242 ACS Appl. Mater. Interfaces 2017, 9, 14518−14524

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Lin Xu: 0000-0001-5831-430X Hongwei Song: 0000-0003-3897-5789 Qilin Dai: 0000-0001-8680-4306 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program of China (973 Program) (2014CB643506), the National Natural Science Foundation of China (Grants 61674067, 11504131, 11374127, 81201738, 11304118), Graduate Innovation Fund of Jilin University (2015030), and The Jilin Province Natural Science Foundation of China (20150520090JH).

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DOI: 10.1021/acsami.7b02242 ACS Appl. Mater. Interfaces 2017, 9, 14518−14524