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Interfacial modification using ultrasonic atomized graphene quantum dots for efficient perovskite solar cells
Organic Electronics 75 (2019) 105415
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Interfacial modification using ultrasonic atomized graphene quantum dots for efficient perovskite solar cells
T
Haoran Xiaa, Zhu Mab,∗, Zheng Xiaob, Weiya Zhoub, Hua Zhanga, Caichen Dua, Jia Zhuanga,∗∗, Xiaowei Chenga, Xingchong Liua, Yuelong Huangb a b
School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, 610500, PR China Institute of Photovoltaic, Southwest Petroleum University, Chengdu, 610500, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene quantum dots SnO2 Ultrasonic atomizing Interface modification Perovskite solar cells
Tin dioxide (SnO2) is a promising electron transport material to replace traditional titanium dioxide (TiO2) for fabricating efficient planar perovskite solar cells (PSCs). However, in order to realize process compatibility and larger scale device, low temperature solution processed SnO2 is normally used, which generates numerous trap states in ETL layer and directly affects the device performance. Here, an interfacial modification strategy proposed, depositing an ultrasonic atomized ultrathin graphene quantum dots (GQDs) layer between tin dioxide (SnO2) and perovskite layer. Ultrasonic atomized deposition can effectively prevent the damage of the surface chemical properties of SnO2 by aqueous solution. Additionally, we demonstrate that the GQDs change the surface property of SnO2 film, and optimized the charge transport capability in SnO2 and perovskite interface. Correspondingly, we obtained a significant power conversion efficiency (PCE) improvement for CH3NH3PbI3based PSCs from 13.61% to 16.54% and reached a highest steady-state PCE over 16%. We believe that the interfacial modification engineering by means of ultrasonic atomizing process is a promising tactic to obtain efficient perovskite solar cells.
1. Introduction Organometallic halide perovskite solar cells (PSCs) have attracted extensive attention because of the advantages of excellent photovoltaic properties, easily prepared process and low costs [1]. Based on the continuous efforts of researchers, the efficiency of PSCs has risen from 3.8% to 24.2% [2–4]. Well-known, conventional planar N-i-P perovskite solar cells normally use titanium dioxide (TiO2) as electron transport layer (ETL) to realize high efficiency PSCs [5]. However, compact TiO2 and mesoporous TiO2 usually need a high temperature sintering process (up to 500 °C), which makes manufacturing complex, meanwhile, leads to longer preparation time and higher energy consumption, and incompatible with indium tin oxide (ITO) and flexible conductive substrates [6]. Therefore, exploring a low temperature ETL becomes a critical issue for realizing efficient solar cells and its commercial application [7,8]. Recently, numerous efforts have focused on low-temperature annealing tin dioxide (SnO2) precursor and nanocrystalline, and attained a certified efficiency of > 21%, which is approach to the widely adopted commercial product of passivated emitter and rear silicon solar cells (PERC, ~21.5%) [9]. Meanwhile, SnO2 has ∗
moderate band-gap (3.6 eV) [10], high transparency, admirable electron mobility, and suitable conduction band edge (CBE), which works as an outstanding optical window and facilitates the efficient electron transfer from perovskite absorption layer to ETL [11–15]. However, the low temperature prepared SnO2 films inevitably generate numerous trap states, which originate from oxygen vacancies [16,17], easily capture electrons and affect the electrons extract process, finally resulting in serious hysteresis and efficiency drop in PSCs. Additionally, 0.57 eV energy barrier between SnO2/perovskite interface leads to insurmountable charge carrier accumulation and recombination [18]. Moreover, the surface defects of SnO2 film is detrimental to the contact of ETL and perovskite, which leading bad FF related to inferior series and parallel resistances of devices [19]. In order to alleviate or eliminate the above issues, great efforts have focused on interfacial modification engineering. Ma et al. had doped Ga into SnO2 layer to enhance physical, electrical contact with perovskite, obtained superior performance with much reduced hysteresis [20]. Fang et al. had demonstrated that the modifying layer of 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM) between SnO2 and perovskite can optimum energy matching, resulting improved
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z. Ma), [email protected] (J. Zhuang).
∗∗
https://doi.org/10.1016/j.orgel.2019.105415 Received 19 June 2019; Received in revised form 9 July 2019; Accepted 21 August 2019 Available online 29 August 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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in oven. Device Fabrication: The FTO coated glass substrates were sequentially ultrasonic cleaned with detergent, acetone, deionized water, ethanol for 15 min, respectively. A 15 min ultraviolet-ozone treatment carried out to remove the residual organics and improve the work function of FTO substrates. The SnO2 aqueous solution was diluted with deionized water (volume rate 1:6) before using, and then was deposited by spin-coating at 3000 rpm for 30 s as the electron transport layers on FTO substrates, and the SnO2 aqueous solution was filtered through a 0.22 μm filter before using. The SnO2 layer annealed at 150 °C for 30 min on a hotplate. 2 mg GQDs powder dissolved in 40 ml deionized water to obtained 0.05 mg/ml GQDs aqueous solution. The GQDs deposited on SnO2 layer by UAP, and heated at 100 °C for 10 min to evaporate moisture. The solution infuse rate is 0.35 ml/min, the working pressure is 1.5 psi, and controlled deposition variable by change working time. The UAP intelligently controlled by computer programs, no need manual operation. The advantages of UAP are high solution utilization rate, high automation, micron level atomization ability, scale-up manufacturing and open-air operation. Then the perovskite layer was deposited by one-step spin coating method in a glovebox. The 553.2 mg of PbI2, 190.8 mg of CH3NH3I (molar rate 1:1) were mixed in 1 mol of DMF solution. The precursor solution filtered through a 0.22 μm filter before using. The prepared precursor solution was dropped on the FTO/SnO2 or FTO/SnO2/GQDs substrates, and quickly spin-coated at 3000 rpm for 55 s. 80 μl chlorobenzene was drop casted quickly at 10 s before the 3000 rpm spin-coating ended. Then the substrates heated at 100 °C for 20 min on a hot plate. After the perovskite films were naturally cooled to room temperature, a hole-transport material was spin coated on the top of perovskite film at 3000 rpm for 30 s in a glove-box. The hole-transport materials solution containing Spiro-OMeTAD (72.3 mg), 4-tert-butylpyridine (tBP, 28.8 μL), Li-TFSI/ acetonitrile solution (17.5 μL, 520 mg/ml) and chlorobenzene (17.5 μL). Finally, Au back electrode deposited by magnetron sputtering at a pressure of 4.0 × 10−3 Pa. The active area of device controlled to 0.16 cm2. Measurement and characterization: The AFM image scanned by an atomic force microscope (AFM, Agilent 7500). The SEM image were scanned by a scanning electron microscopy (SEM, ZEISS EV0MA15), and the EDX maps obtained from energy dispersive X-ray spectroscopy (EDX) detector. The crystal structure of the perovskite films was characterized by X-ray diffraction (XRD; DX-2700, Dandong) with Cu Kα radiation (λ = 0.15406 nm) at a scanning rate of 5 deg/min. The absorption spectra of the films were measured using a ultraviolet–visible spectrophotometer (UV-2600, SHIMADZU). The steady and time-resolved photoluminescence (TRPL) spectroscopy measured at 760 nm with the 510 nm excitation at a pulse frequency of 1 MHz (FLS980, Edinburgh). The J-V characteristic of device was recorded using an electrochemical workstation under a simulated solar spectrum (AM1.5) provided by a solar simulator (Zolix SS150, Beijing, China). Electrochemical impedance spectroscopy (EIS) measured using CHI660D electrochemical analyzer (Chenhua, China). The monochromatic incident photon-to-current efficiency (IPCE) measured using an IPCE system (PVE300, Bentham, Inc.) from 300 to 800 nm.
performance of PSCs [21]. Yang and co-workers had introduced GQDs solution into SnO2 precursor solution to enhance conductivity and carrier transport performance of SnO2 layer achieved an optimized steady-state PCE > 20.0% [22]. Therein, GQDs is a novel carbon nanomaterial, which normally works as light absorber and electron acceptor in solar cells due to its superior optical properties, high electrical conductivity and tunable band gaps. The excellent photoelectric performance derives from the small size effect, quantum confinement, edge effects and vast surface chemical groups, making them different from traditional bulk materials [23]. Previous researchers have successfully applied it to perovskite solar cells and obtained remarkable results. Liu et al. had applied GD QDs to ETL, perovskite and HTL layer, improved stability and efficiency of PSCs [24]. Due to the similar solvent polarity of low temperature SnO2 and GQDs solution, it is very hard to adopt conventional spin-coating method to deposit GQDs on nano-crystal based SnO2 layer. Therefore, most works blend them together to fabricate a SnO2: GQDs mixed-ETL [22]. However, the component of GQDs in SnO2 is tiny, which further complicates the process, and excess GQDs would lead to dramatical drop of efficiency. Accordingly, exploring a solution-based, open-air and accurate controlled method is essential. Recently, high quality metal-oxide ETLs and perovskite layers have been prepared using ultrasonic atomizing method, which is solution based, scalable and open-air technology [25]. In this process, the material is dissolved in solvent and ultrasonically atomized into mist droplets with radius of several micrometer, which are carried out by inert gas flow and deposited onto substrates. Therefore, using ultrasonic atomizing method to prepare GQDs thin film as interfacial modification layer on SnO2 could effectively prevent the damage of the surface chemical properties of SnO2 by aqueous solution [26]. Furthermore, GQDs has suitable energy level, which perfectly matching with both SnO2 and CH3NH3PbI3 layer retarding the reversed electron leakage and mediating the electron transportation. Here, we applied ultrasonic atomizing process (UAP) to deposit graphene quantum dots ultrathin layer on SnO2 nano-crystalline film (ETL) and fabricate an efficient N-i-P planar PSCs. It was certified that the photo-generated electrons in GQDs can transport to SnO2 and offset the electron trap states in SnO2 interface, and improve the conductivity in SnO2. Moreover, the GQDs ultrathin layer optimized the interface contact and reduced energy barrier between SnO2 ETL and perovskite layer. As a result, the planar N-i-P PSCs based on FTO/SnO2/GQDs/ Perovskite/Au structure obtain a highest efficiency over 16.5%, with Voc of 1.03 V, Jsc of 23.36 mA/cm2, FF of 67.9%. 2. Experimental section Materials: PbI2 (99.99%), CH3NH3I (≥99.5%), 4-tertbutylpyridine (tBP, 96%) lithium bis-(trifluoromethanesulfonyl) imide (Li-TFSI, 99.9%), 2,2′,7,7′-Tetrakis (N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene(spiro-OMeTAD, 99.5%), were purchased from Xi'an Polymer Light Technology Corporation. Acetonitrile (99.9%) N, N-dimethylformamide (DMF, 99.9%), and chlorobenzene (99.9%) were achieved from Sigma-Aldrich. The SnO2 colloid precursor purchased from Alfa Aesar (tin (IV) oxide, 15% in H2O colloidal dispersion). Pyrene (97%) obtained from Aladdin. All the materials used without any further purification. Synthesis of GQDs: GQDs were synthesized use an alkali-mediated hydrothermal methods [23,27]. Firstly, 1.5 g 1, 3, 6-trinitropyrene was dispersed in 300 ml deionized water solution containing 0.4 M NaOH under ultrasonication for 2.5 h to acquire a homogeneous suspension, then transferred to 250 ml Teflon-lined stainless-steel autoclave and heated at 200 °C for 10 h in drying oven. Secondly, the GQDs solution was filtered with a 0.22 μm microporous membrane to removal of insoluble carbon produce, and then purified in a dialysis bag (retained molecular weight: 3500 Da) for 3 days, aiming to separate indissoluble small molecules and sodium salt. Finally, the GQD powders were obtained through rotary evaporation and drying at a temperature of 80 °C
3. Results and discussion Fig. 1 (a) shows the mechanism of the synthesis process of GQDs [27]. The edge-modified GQDs with functional group (-OH, –NH, –NH2 and-C-O-R), located at the surface of GQDs act as a passivation for the GQDs, facilitating the solubility of GQDs in water [28,29]. The FTIR spectrum of GQDs indicates that sample mainly contains –OH stretching vibrations (3426.9 cm−1), C–OH stretching vibrations (1276.9 cm−1) and C=C stretching vibrations (1587.5 cm−1) in Fig. 1 (b). Fig. 1 (c) shows the absorption spectrum of GQDs aqueous solution (0.05 mg/ ml), indicating that the optical band gap of the GQDs is about 2.45 eV, which is similar as previous reports [22]. The absorption spectrum 2
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Fig. 1. (a) Synthesis process of GQDs. (b) FTIR spectra of GQDs. (c) Ultraviolet absorption spectrum of GQDs. The inset image is the GQDs aqueous solution.
uniformly coated on the surface of the substrate by a certain amount of carrier gas to form a coating or a film. Ultrasonic spray has the following advantages compared traditional single or two-fluid spray methods, including high uniformity, high utilization rate of raw materials, less spatter, high control precision of coating thickness. The deposition process of GQDs could prohibit the damage of SnO2 layer. Moreover, all processes are accurately controlled by computer. Fig. 2(b and c) shows the SEM images of SnO2 films with and without GQDs. All the films exhibit uniform grain size and dense surface morphology. And the inset map in Fig. 2 (c) shows the uniform distribution of carbon
exhibits two pronounced excitonic absorption peaks of GQDs at about 350 nm and 490 nm, and with a broad absorption band from 300 to 600 nm. Inserted photograph is the solution of GQDs, showing the GQDs can well dispersed in water, which is transparent and yellow. Schematic diagram presents the fabrication process of SnO2 and GQDs layer in Fig. 2 (a). The SnO2 layer was prepared using spincoating method, and the GQDs deposited by UAP. Ultrasonic spraying is a spraying process using ultrasonic atomization technology. The material firstly prepared in a liquid state, and then the liquid paint is atomized into fine particles by an ultrasonic atomizing device, finally
Fig. 2. (a) Schematic diagram of the SnO2/GQDs film fabrication process. (b–c) SEM images inset, carbon element dispersion; (d–e) 3D structure (3 × 3 μm); (f–g) AFM images of SnO2 and SnO2/GQDs. 3
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Fig. 3. (a–c) Top-view SEM images and XRD of perovskite films with GQDs and without GQDs, respectively. (d–f) absorption spectrum and water contact angle of SnO2 and SnO2/GQDs film, respectively.
perovskite by GQDs. The special effect of GQDs on SnO2 layer was investigated using water contact angle measurements. Fig. 3(e and f) show the water contact angle of FTO/SnO2 film compared to SnO2/ GQDs films. The upside photograph exhibition the hydrophobic property of the SnO2 film with a contact angle over 55°. After depositing GQDs, the surface energy has decreased resulting in contact angle of below 35°. The decrease of surface energy and contact angle is contributed to the spreading of perovskite precursor solution on SnO2 film, resulting a good interface contact of SnO2 and perovskite film [31]. Fig. 4 (a) shows the model diagram of the planar-structure PSCs with the structure of FTO/SnO2/GQDs/MAPbI3/Spiro-OMeTAD/Au. Fig. 4 (b) show the SEM image of the cross sectional of the device. The functional layer is closely connected with contiguous layer, which is beneficial to the transfer of the charge and improvement of the series and parallel resistances of the device. It is obvious that there is a thin (about 80 nm) and uniform SnO2/GQDs layer between FTO and perovskite functional layer. The effect of GQDs on SnO2 layer was investigated by control atomizing times (0, 60, 120, 180, 210 s). The detailed data summarized in Table 1, corresponding box chat date illustrated in Fig. 4 (f). The PCE increases as the atomizing time increases from 0 to 120 s, which is attribute to the increase of Jsc and FF. A further increase of atomizing time from 120 to 210 s leads to a decreased on Jsc, FF and PCE. The optimized atomizing time is found to be 120 s, leading to highest Jsc of 23.36 mA/cm2, Voc of 1.03 V, FF of 67.9%, and PCE of 16.54%. It can be observed that the Voc of device have no significantly improvement after the GQDs deposited on SnO2 layer. In addition, the series and parallel resistances of the device also changed after the deposition of GQDs on SnO2 layer, that is will discussed in detail later. Fig. 4 (c) shows the highest current density-voltage (J-V) characteristics of PSCs based on SnO2 and SnO2/GQDs, and corresponding parameters are summarized in Table 2. The cell based on SnO2 exhibits an open-circuit voltage (Voc) of 1.01 V, a short circuit current density (Jsc) of 22.32 mA/cm2, a fill factor (FF) of 60.6%, and a PCE of 13.61%. Compared with reference cell, the device based on SnO2/GQDs has a similar value in Voc of 1.03 V, but the Jsc increased to 23.36 mA/cm2, the FF value increased 67.9%, resulting a PCE increase to 16.54%. The statistical histogram of the PCEs from 20 pieces of PSCs with and without GQDs shown in Fig. 4 (d). It shows that almost the tested device with GQDs demonstrated a higher PCE than that of the reference device. On average, the PCE of the PSCs with GQDs relatively increases by about 17.1% of that of the reference devices, from 12.6% to 15.2%.
elements in SnO2 surface, indicating the deposited GQDs on SnO2 surface by UAP is reliable, which has a special effect on the properties of SnO2 and performance of the PSCs. The surface morphology of SnO2 and SnO2/QGDs films observed by atomic force microscopy (AFM), Fig. 2(d and e) shows the 3D diagram of AFM of FTO/SnO2 films with and without GQDs. Using ultrasonic atomizing technology, the GQDs was uniformly deposited on the surface of SnO2, free of holes and crack. Meanwhile, the surface roughness (Root mean square) of SnO2 has slightly increased from 23.7 nm to 25.0 nm ascribe to deposited GQDs, Fig. 2(f and g). The ultrathin quantum dot layer acts as an analogous mesoporous layer, which is beneficial to the perovskite precursor adhere to SnO2 and optimized the interfacial contact of perovskite and SnO2. It well known that the surface energy and morphology of electron transport layer have a significant influence on perovskite deposition and crystal formation process. Both depend on several parameters such as surface roughness, chemical heterogeneities, and molecular rearrangement [30]. Firstly, the effect of GQDs on perovskite crystal formation was investigated using scanning electron microscope (SEM), Fig. 3(a and b). Compared with the perovskite film on SnO2, the film on SnO2/GQDs shows better surface topography, which have uniform density features and the number of larger particles has increased. In addition, the pinhole and grain boundary defects were obviously optimized, and all of this is due to the surface passivation of GQDs. Fig. 3 (c) shows the XRD measurement of perovskite film loaded on SnO2 and SnO2/GQDs ETLs. It is found that all samples show strong diffraction peaks at 14.08°, 28.4° and 31.8°, which corresponds to the lattice plane of (110), (220), (310), respectively, demonstrating a better crystallized of perovskite film. Further, we find two obvious differences in the perovskite films deposited on SnO2 and SnO2/GQDs ETL. Firstly, the peak intensity of (110) and (220) reflection of perovskite deposited on SnO2/GQDs ETL is higher than that on SnO2 ETL, indicating a higher crystallinity for perovskite film deposited on SnO2/GQDs ETL at the same conditions. Secondly, the peak width (FWHM) of perovskite deposited on SnO2/GQDs ETL is narrower than that on SnO2 ETL for (110) reflection, indicating larger crystallite size of the former. The light absorptions of perovskite films on FTO/SnO2 with and without GQDs illustrated in Fig. 3 (d). The absorption spectrum is in agreement with pure CH3NH3PbI3 perovskite. The absorption spectrum of perovskite films indicated no significant differences in 300–500 nm. However, the absorption spectrum shows obvious improvement in 500–800 nm, which is mainly attribute to the optimization crystals of 4
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Fig. 4. (a) Device model diagram. (b) Cross sectional SEM image of the PSCs with a structure of FTO/SnO2/GQDs/CH3NH3PbI3/Spiro-OMeTAD/Au. (c) J-V curves, (d) Statistic histograms of the PCE. (e) EQE spectra. (f) PCE tendency with different spraying time of GQDs.
−4.35 eV and −4.23 eV. In brief, the observed decrease in work function of SnO2 film by GQDs are mostly derived from the formation of dipoles on the interface [34]. Which leads to a shift of the vacuum level equally reduce the work function and the ohmic contact between perovskite and SnO2, hence “barrier-free” electron extraction from perovskite to SnO2 [31,35]. Optimized interface contact of SnO2 and perovskite layer by GQDs have beneficial effect on series and parallel resistances, resulting an improvement of device FF [18]. Fig. 5 (d) shows the energy band alignment of each component in the PSC devices. Here the energy level of GQDs is perfectly aligned between both of SnO2 and CH3NH3PbI3 indicating that GQDs may be not only retarding the reversed electron leakage but also mediating the electron transportation from perovskite to SnO2, playing a role like simplex electron channel from perovskite layer to ETL layer[36]. To further investigate the charge transport and recombination process in device, photoluminescence spectrum and time-resolved photoluminescence (TRPL) spectroscopy were used [37]. Fig. 6 (a) shows the steady photoluminescence (PL) spectrum of perovskite on different ETL layer. The spectrum show that devices based on SnO2/ GQDs possess faster emission quenching than that based on bare SnO2, indicating more efficient electron extraction and transport from perovskite to ETLs layer. The enhanced charge transferred from perovskite to the ETLs layer indicated that the energy barrier had been reduced, which could be caused by the high mobility of SnO2 attribute to the GQDs. Time-resolved photoluminescence (TRPL) results showed that the lifetime of the perovskite layer can be significantly reduced when depositing it on SnO2/GQDs ETLs, Fig. 6 (b). The average lifetime of charge carrier τave decreased from 29.93 ns to 19.96 ns, indicating efficient electron transfer from perovskite to SnO2. The more efficient electron transport from perovskite to the SnO2 ETLs indicates the suppressed electron recombination due to the perfect match in energy levels between the conduction band level of the perovskite and the SnO2 layer and the super-fast electron channel function of GQDs [38–40]. For further confirm the charge recombination process and interfacial properties of devices, electrochemical impedance spectroscopy (EIS) measurement was implement at 0.7 V bias voltage under dark condition. The EIS results of corresponding devices shown in Fig. 6 (c). As far as we know, the diameter of the Nyquist plots semicircle presents the charge transfer resistance (RCT) related to the current leakage and recombination of electrons in ETLs and light absorption layer. Larger
Table 1 Summary of Photovoltaic Parameters for PSCs with Various Contents of GQDs obtained by control atomizing time. Spraying time (s)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
T-0 T-60 T-120 T-180 T-210
Average (Max) 1.00 (1.01) 1.00 (1.01) 1.01 (1.03) 1.00 (1.02) 1.00 (1.02)
21.6 (22.3) 22.1 (22.9) 22.4 (23.3) 21.7 (22.8) 21.4 (22.6)
60.0 (62.1) 62.0 (64.2) 65.4 (67.9) 58.6 (60.6) 52.0 (55.0)
12.6 (13.6) 13.8 (14.7) 15.2 (16.5) 12.1 (13.4) 10.8 (12.9)
Table 2 Photovoltaic parameters of the PSCs with and without GQDs. Device
Voc (V) (V)
Jsc (mA/ cm2)
FF (%)
PCE(%)%)
Rs (Ω)
Rsh (Ω)
With GQDs Without GQDs
1.03 1.01
23.36 22.32
67.9 60.6
16.54 13.61
3.5 3.7
1413.8 559.9
Fig. 4 (e) shows the incident photon-to-current efficiency (IPCE) spectra of the PSCs with SnO2 and SnO2/GQDs ETLs, which is in good agreement with the J-V characteristics. In order to explore the mechanism of enhanced Jsc and FF, the surface potential characteristics and corresponding surface potential profiles of the SnO2 with and without GQDs measured by scanning kelvin probe microscopy (SKPM), Fig. 5(a and b). The SKPM images reveal a global 100 mV decrease after modified with GQDs in the Kelvin potential, implying a significant change of the surface energetics. Importantly, the change in surface potential runs through the entire surface. The contact potential difference (CPD) images are flattened and presented in the same color scale, while the histograms of the absolute CPD values of the two kinds of ETL films are plotted in Fig. 5 (c). This result indicated that photogenerated charge carriers could be injected from light absorber to SnO2/GQDs ETL easier [32]. The work function of the Pt/Ir-coated tip and sample can be calculated using the following equation [33]. CPD=(ϴtip-ϴsample)/e
(1)
The work function of SnO2 and SnO2/GQDs calculated to be 5
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Fig. 5. Topography KPFM images of SnO2 film, (a) without GQDs and (b) with GQDs. (c) Surface potential profiles of different SnO2 films. (d) Schematic illustration of the energy diagram and electron transfer process for GQDs modified ETLs [28].
GQDs lead to the dipoles formed along the interface, resulting in ohmic response and hence faster electron extraction from perovskite to SnO2. Fig. 6 (e) demonstrate the interfacial contact schematic diagram of SnO2/GQDs/perovskite. The functional group at the edge of GQDs will form hydrogen bonds with SnO2 and perovskite, which will optimize the interfacial contact of SnO2 and perovskite, meanwhile the GQDs can act as a super-fast electron channel for electron.
RCT means the less current leakage and electron recombined occurred at the interface [41,42]. As can be seen, the diameters of the semicircles of SnO2/GQDs based device increased obviously compare with reference device, which meant that after depositing GQDs on SnO2 films, the interfacial contact of ETLs and perovskite layer was improved dramatically resulting reduced the leakage and charge recombination at interface [43]. This is consistent with our previous data. The effects of GQDs on conductivity of SnO2 film was studied using a FTO/SnO2/(GQDs)/Au structure. The J-V curves measured under light conditions (AM 1.5G) shows that the conductivity of SnO2 film is significantly improved after depositing GQDs, Fig. 6 (d). The insert of
4. Conclusion In summary, low temperature, water soluble GQDs have been
Fig. 6. (a–b) Steady-state PL and TRPL spectrum of CH3NH3PbI3 with different ETLs. (c) Electrochemical impedance spectrum of PSCs with different ETLs (measured at 0.7 V bias voltage under dark condition). (d) J-V characteristics of the device FTO/SnO2/(GQDs)/Au measured under light. (e) Schematic illustration of the interfacial reaction between SnO2/GQDs and GQDs/perovskite. 6
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deposited by UAP as an effective interfacial modification layer to improve planar PSCs performance. Here, a maximum PCE of 16.54% was achieved benefiting from the increase in Jsc and FF by GQDs. The improved Jsc and FF is attributed to the enhanced conductivity, reduced recombination, optimized interfacial energy level alignment in perovskite solar cells. Our work showed the possibility of improved PSCs performance by interfacial modification with GQDs. Meanwhile, the utilized of ultrasonic spraying process also provide references for largescale commercial preparation of PSCs in the future.
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Acknowledgments The authors gratefully acknowledge the financial support from Sichuan Science and Technology Program (Grant No. 2018JY0015), Young scholar development fund of SWPU (Grant No. 201699010017), Scientific Research Starting Project of SWPU (Grant No. 2017QHZ021), and the National Key R&D Program of China (2016YFB0303600). References [1] M. Green, A. Ho-Baillie, H. Snaith, The emergence of perovskite solar cells, Nat. Photonics 8 (2014) 506–514. [2] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [3] H. Zhou, Q. Chen, G. Li, S. Luo, T. Song, H. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 345 (2014) 542–546. [4] https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies-190416.pdf. [5] Y. Lv, B. Cai, Y. Wu, S. Wang, Q. Jiang, Q. Ma, J. Liu, W. Zhang, High performance perovskite solar cells using TiO2 nanospindles as ultrathin mesoporous layer, J. Energy Chem. 27 (2018) 951–956. [6] Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Enhanced electron extraction using SnO2 for high-efficiency planar-structure HC(NH2)2PbI3-based perovskite solar cells, Nat. Energy 2 (2016) 16177. [7] Z. Ma, H. Lu, F. Zhao, Y. Xiang, J. Zhuang, H. Li, Low-temperature dynamic vacuum annealing of ZnO thin film for improved inverted polymer solar cells, RSC Adv. 7 (2017) 29357–29363. [8] Z. Wang, J. Luo, X. Zheng, W. Zhang, Y. Qin, Solution processed Nb2O5 electrodes for high efficient ultraviolet light stable planar perovskite solar cells, ACS Sustain. Chem. Eng. 7 (2019) 7421–7429. [9] Q. Jiang, Z. Chu, P. Wang, X. Yang, H. Liu, Y. Wang, Z. Yin, J. Wu, X. Zhang, J. You, Planar-structure perovskite solar cells with efficiency beyond 21%, Adv. Mater. 29 (2017) 1703852. [10] C. Alfonso, A. Charaï, A. Armigliato, D. Narducci, Transmission electron microscopy investigation of tin sub-oxide nucleation upon SnO2 deposition on silicon, Appl. Phys. Lett. 68 (1996) 1207–1208. [11] H. Snaith, C. Ducati, SnO2-based dye-sensitized hybrid solar cells exhibiting near unity absorbed photon-to-electron conversion efficiency, Nano Lett. 10 (2010) 1259–1265. [12] B. Park, S. Jain, X. Zhang, A. Hagfeldt, G. Boschloo, T. Edvinsson, Resonance Raman and excitation energy dependent charge transfer mechanism in halide-substituted hybrid perovskite solar cells, ACS Nano 9 (2015) 2088–2108. [13] C. Kilic, A. Zunger, Origins of coexistence of conductivity and transparency in SnO2, Phys. Rev. Lett. 88 (2002) 095501. [14] C. Diaz, M. Valenzuela, M. Segovia, K. Correa, R. Campa, A. Soto, Solution, Solidstate two step synthesis and optical properties of ZnO and SnO2 nanoparticles and their nanocomposites with SiO2, J. Clust. Sci. 29 (2018) 251–266. [15] D. Liu, Y. Wang, H. Xu, H. Zheng, T. Zhang, P. Zhang, F. Wang, J. Wu, Z. Wang, Z. Chen, S. Li, SnO2‐based perovskite solar cells: configuration design and performance improvement, Solar RRL 3 (2019) 1800292. [16] Y. Bai, Y. Fang, Y. Deng, Q. Wang, J. Zhao, X. Zheng, Y. Zhang, J. Huang, Low temperature solution-processed Sb:SnO2 nanocrystals for efficient planar perovskite solar cells, Chemsuschem 9 (2016) 2686–2691. [17] K. Godinho, A. Walsh, G. Watson, Energetic and electronic structure analysis of intrinsic defects in SnO2, J. Phys. Chem. C 113 (2009) 439–448. [18] J. JimenezLopez, W. Cambarau, L. Cabau, E. Palomares, Charge injection, carriers recombination and HOMO energy level relationship in perovskite solar cells, Sci. Rep. 7 (2017) 6106. [19] Y. Li, Molecular design of photovoltaic materials for polymer solar cells: toward suitable electronic energy levels and broad absorption, Acc. Chem. Res. 45 (2012)
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Interfacial modification using ultrasonic atomized graphene quantum dots for efficient perovskite solar cells
T
Haoran Xiaa, Zhu Mab,∗, Zheng Xiaob, Weiya Zhoub, Hua Zhanga, Caichen Dua, Jia Zhuanga,∗∗, Xiaowei Chenga, Xingchong Liua, Yuelong Huangb a b
School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, 610500, PR China Institute of Photovoltaic, Southwest Petroleum University, Chengdu, 610500, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene quantum dots SnO2 Ultrasonic atomizing Interface modification Perovskite solar cells
Tin dioxide (SnO2) is a promising electron transport material to replace traditional titanium dioxide (TiO2) for fabricating efficient planar perovskite solar cells (PSCs). However, in order to realize process compatibility and larger scale device, low temperature solution processed SnO2 is normally used, which generates numerous trap states in ETL layer and directly affects the device performance. Here, an interfacial modification strategy proposed, depositing an ultrasonic atomized ultrathin graphene quantum dots (GQDs) layer between tin dioxide (SnO2) and perovskite layer. Ultrasonic atomized deposition can effectively prevent the damage of the surface chemical properties of SnO2 by aqueous solution. Additionally, we demonstrate that the GQDs change the surface property of SnO2 film, and optimized the charge transport capability in SnO2 and perovskite interface. Correspondingly, we obtained a significant power conversion efficiency (PCE) improvement for CH3NH3PbI3based PSCs from 13.61% to 16.54% and reached a highest steady-state PCE over 16%. We believe that the interfacial modification engineering by means of ultrasonic atomizing process is a promising tactic to obtain efficient perovskite solar cells.
1. Introduction Organometallic halide perovskite solar cells (PSCs) have attracted extensive attention because of the advantages of excellent photovoltaic properties, easily prepared process and low costs [1]. Based on the continuous efforts of researchers, the efficiency of PSCs has risen from 3.8% to 24.2% [2–4]. Well-known, conventional planar N-i-P perovskite solar cells normally use titanium dioxide (TiO2) as electron transport layer (ETL) to realize high efficiency PSCs [5]. However, compact TiO2 and mesoporous TiO2 usually need a high temperature sintering process (up to 500 °C), which makes manufacturing complex, meanwhile, leads to longer preparation time and higher energy consumption, and incompatible with indium tin oxide (ITO) and flexible conductive substrates [6]. Therefore, exploring a low temperature ETL becomes a critical issue for realizing efficient solar cells and its commercial application [7,8]. Recently, numerous efforts have focused on low-temperature annealing tin dioxide (SnO2) precursor and nanocrystalline, and attained a certified efficiency of > 21%, which is approach to the widely adopted commercial product of passivated emitter and rear silicon solar cells (PERC, ~21.5%) [9]. Meanwhile, SnO2 has ∗
moderate band-gap (3.6 eV) [10], high transparency, admirable electron mobility, and suitable conduction band edge (CBE), which works as an outstanding optical window and facilitates the efficient electron transfer from perovskite absorption layer to ETL [11–15]. However, the low temperature prepared SnO2 films inevitably generate numerous trap states, which originate from oxygen vacancies [16,17], easily capture electrons and affect the electrons extract process, finally resulting in serious hysteresis and efficiency drop in PSCs. Additionally, 0.57 eV energy barrier between SnO2/perovskite interface leads to insurmountable charge carrier accumulation and recombination [18]. Moreover, the surface defects of SnO2 film is detrimental to the contact of ETL and perovskite, which leading bad FF related to inferior series and parallel resistances of devices [19]. In order to alleviate or eliminate the above issues, great efforts have focused on interfacial modification engineering. Ma et al. had doped Ga into SnO2 layer to enhance physical, electrical contact with perovskite, obtained superior performance with much reduced hysteresis [20]. Fang et al. had demonstrated that the modifying layer of 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM) between SnO2 and perovskite can optimum energy matching, resulting improved
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z. Ma), [email protected] (J. Zhuang).
∗∗
https://doi.org/10.1016/j.orgel.2019.105415 Received 19 June 2019; Received in revised form 9 July 2019; Accepted 21 August 2019 Available online 29 August 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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in oven. Device Fabrication: The FTO coated glass substrates were sequentially ultrasonic cleaned with detergent, acetone, deionized water, ethanol for 15 min, respectively. A 15 min ultraviolet-ozone treatment carried out to remove the residual organics and improve the work function of FTO substrates. The SnO2 aqueous solution was diluted with deionized water (volume rate 1:6) before using, and then was deposited by spin-coating at 3000 rpm for 30 s as the electron transport layers on FTO substrates, and the SnO2 aqueous solution was filtered through a 0.22 μm filter before using. The SnO2 layer annealed at 150 °C for 30 min on a hotplate. 2 mg GQDs powder dissolved in 40 ml deionized water to obtained 0.05 mg/ml GQDs aqueous solution. The GQDs deposited on SnO2 layer by UAP, and heated at 100 °C for 10 min to evaporate moisture. The solution infuse rate is 0.35 ml/min, the working pressure is 1.5 psi, and controlled deposition variable by change working time. The UAP intelligently controlled by computer programs, no need manual operation. The advantages of UAP are high solution utilization rate, high automation, micron level atomization ability, scale-up manufacturing and open-air operation. Then the perovskite layer was deposited by one-step spin coating method in a glovebox. The 553.2 mg of PbI2, 190.8 mg of CH3NH3I (molar rate 1:1) were mixed in 1 mol of DMF solution. The precursor solution filtered through a 0.22 μm filter before using. The prepared precursor solution was dropped on the FTO/SnO2 or FTO/SnO2/GQDs substrates, and quickly spin-coated at 3000 rpm for 55 s. 80 μl chlorobenzene was drop casted quickly at 10 s before the 3000 rpm spin-coating ended. Then the substrates heated at 100 °C for 20 min on a hot plate. After the perovskite films were naturally cooled to room temperature, a hole-transport material was spin coated on the top of perovskite film at 3000 rpm for 30 s in a glove-box. The hole-transport materials solution containing Spiro-OMeTAD (72.3 mg), 4-tert-butylpyridine (tBP, 28.8 μL), Li-TFSI/ acetonitrile solution (17.5 μL, 520 mg/ml) and chlorobenzene (17.5 μL). Finally, Au back electrode deposited by magnetron sputtering at a pressure of 4.0 × 10−3 Pa. The active area of device controlled to 0.16 cm2. Measurement and characterization: The AFM image scanned by an atomic force microscope (AFM, Agilent 7500). The SEM image were scanned by a scanning electron microscopy (SEM, ZEISS EV0MA15), and the EDX maps obtained from energy dispersive X-ray spectroscopy (EDX) detector. The crystal structure of the perovskite films was characterized by X-ray diffraction (XRD; DX-2700, Dandong) with Cu Kα radiation (λ = 0.15406 nm) at a scanning rate of 5 deg/min. The absorption spectra of the films were measured using a ultraviolet–visible spectrophotometer (UV-2600, SHIMADZU). The steady and time-resolved photoluminescence (TRPL) spectroscopy measured at 760 nm with the 510 nm excitation at a pulse frequency of 1 MHz (FLS980, Edinburgh). The J-V characteristic of device was recorded using an electrochemical workstation under a simulated solar spectrum (AM1.5) provided by a solar simulator (Zolix SS150, Beijing, China). Electrochemical impedance spectroscopy (EIS) measured using CHI660D electrochemical analyzer (Chenhua, China). The monochromatic incident photon-to-current efficiency (IPCE) measured using an IPCE system (PVE300, Bentham, Inc.) from 300 to 800 nm.
performance of PSCs [21]. Yang and co-workers had introduced GQDs solution into SnO2 precursor solution to enhance conductivity and carrier transport performance of SnO2 layer achieved an optimized steady-state PCE > 20.0% [22]. Therein, GQDs is a novel carbon nanomaterial, which normally works as light absorber and electron acceptor in solar cells due to its superior optical properties, high electrical conductivity and tunable band gaps. The excellent photoelectric performance derives from the small size effect, quantum confinement, edge effects and vast surface chemical groups, making them different from traditional bulk materials [23]. Previous researchers have successfully applied it to perovskite solar cells and obtained remarkable results. Liu et al. had applied GD QDs to ETL, perovskite and HTL layer, improved stability and efficiency of PSCs [24]. Due to the similar solvent polarity of low temperature SnO2 and GQDs solution, it is very hard to adopt conventional spin-coating method to deposit GQDs on nano-crystal based SnO2 layer. Therefore, most works blend them together to fabricate a SnO2: GQDs mixed-ETL [22]. However, the component of GQDs in SnO2 is tiny, which further complicates the process, and excess GQDs would lead to dramatical drop of efficiency. Accordingly, exploring a solution-based, open-air and accurate controlled method is essential. Recently, high quality metal-oxide ETLs and perovskite layers have been prepared using ultrasonic atomizing method, which is solution based, scalable and open-air technology [25]. In this process, the material is dissolved in solvent and ultrasonically atomized into mist droplets with radius of several micrometer, which are carried out by inert gas flow and deposited onto substrates. Therefore, using ultrasonic atomizing method to prepare GQDs thin film as interfacial modification layer on SnO2 could effectively prevent the damage of the surface chemical properties of SnO2 by aqueous solution [26]. Furthermore, GQDs has suitable energy level, which perfectly matching with both SnO2 and CH3NH3PbI3 layer retarding the reversed electron leakage and mediating the electron transportation. Here, we applied ultrasonic atomizing process (UAP) to deposit graphene quantum dots ultrathin layer on SnO2 nano-crystalline film (ETL) and fabricate an efficient N-i-P planar PSCs. It was certified that the photo-generated electrons in GQDs can transport to SnO2 and offset the electron trap states in SnO2 interface, and improve the conductivity in SnO2. Moreover, the GQDs ultrathin layer optimized the interface contact and reduced energy barrier between SnO2 ETL and perovskite layer. As a result, the planar N-i-P PSCs based on FTO/SnO2/GQDs/ Perovskite/Au structure obtain a highest efficiency over 16.5%, with Voc of 1.03 V, Jsc of 23.36 mA/cm2, FF of 67.9%. 2. Experimental section Materials: PbI2 (99.99%), CH3NH3I (≥99.5%), 4-tertbutylpyridine (tBP, 96%) lithium bis-(trifluoromethanesulfonyl) imide (Li-TFSI, 99.9%), 2,2′,7,7′-Tetrakis (N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene(spiro-OMeTAD, 99.5%), were purchased from Xi'an Polymer Light Technology Corporation. Acetonitrile (99.9%) N, N-dimethylformamide (DMF, 99.9%), and chlorobenzene (99.9%) were achieved from Sigma-Aldrich. The SnO2 colloid precursor purchased from Alfa Aesar (tin (IV) oxide, 15% in H2O colloidal dispersion). Pyrene (97%) obtained from Aladdin. All the materials used without any further purification. Synthesis of GQDs: GQDs were synthesized use an alkali-mediated hydrothermal methods [23,27]. Firstly, 1.5 g 1, 3, 6-trinitropyrene was dispersed in 300 ml deionized water solution containing 0.4 M NaOH under ultrasonication for 2.5 h to acquire a homogeneous suspension, then transferred to 250 ml Teflon-lined stainless-steel autoclave and heated at 200 °C for 10 h in drying oven. Secondly, the GQDs solution was filtered with a 0.22 μm microporous membrane to removal of insoluble carbon produce, and then purified in a dialysis bag (retained molecular weight: 3500 Da) for 3 days, aiming to separate indissoluble small molecules and sodium salt. Finally, the GQD powders were obtained through rotary evaporation and drying at a temperature of 80 °C
3. Results and discussion Fig. 1 (a) shows the mechanism of the synthesis process of GQDs [27]. The edge-modified GQDs with functional group (-OH, –NH, –NH2 and-C-O-R), located at the surface of GQDs act as a passivation for the GQDs, facilitating the solubility of GQDs in water [28,29]. The FTIR spectrum of GQDs indicates that sample mainly contains –OH stretching vibrations (3426.9 cm−1), C–OH stretching vibrations (1276.9 cm−1) and C=C stretching vibrations (1587.5 cm−1) in Fig. 1 (b). Fig. 1 (c) shows the absorption spectrum of GQDs aqueous solution (0.05 mg/ ml), indicating that the optical band gap of the GQDs is about 2.45 eV, which is similar as previous reports [22]. The absorption spectrum 2
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Fig. 1. (a) Synthesis process of GQDs. (b) FTIR spectra of GQDs. (c) Ultraviolet absorption spectrum of GQDs. The inset image is the GQDs aqueous solution.
uniformly coated on the surface of the substrate by a certain amount of carrier gas to form a coating or a film. Ultrasonic spray has the following advantages compared traditional single or two-fluid spray methods, including high uniformity, high utilization rate of raw materials, less spatter, high control precision of coating thickness. The deposition process of GQDs could prohibit the damage of SnO2 layer. Moreover, all processes are accurately controlled by computer. Fig. 2(b and c) shows the SEM images of SnO2 films with and without GQDs. All the films exhibit uniform grain size and dense surface morphology. And the inset map in Fig. 2 (c) shows the uniform distribution of carbon
exhibits two pronounced excitonic absorption peaks of GQDs at about 350 nm and 490 nm, and with a broad absorption band from 300 to 600 nm. Inserted photograph is the solution of GQDs, showing the GQDs can well dispersed in water, which is transparent and yellow. Schematic diagram presents the fabrication process of SnO2 and GQDs layer in Fig. 2 (a). The SnO2 layer was prepared using spincoating method, and the GQDs deposited by UAP. Ultrasonic spraying is a spraying process using ultrasonic atomization technology. The material firstly prepared in a liquid state, and then the liquid paint is atomized into fine particles by an ultrasonic atomizing device, finally
Fig. 2. (a) Schematic diagram of the SnO2/GQDs film fabrication process. (b–c) SEM images inset, carbon element dispersion; (d–e) 3D structure (3 × 3 μm); (f–g) AFM images of SnO2 and SnO2/GQDs. 3
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Fig. 3. (a–c) Top-view SEM images and XRD of perovskite films with GQDs and without GQDs, respectively. (d–f) absorption spectrum and water contact angle of SnO2 and SnO2/GQDs film, respectively.
perovskite by GQDs. The special effect of GQDs on SnO2 layer was investigated using water contact angle measurements. Fig. 3(e and f) show the water contact angle of FTO/SnO2 film compared to SnO2/ GQDs films. The upside photograph exhibition the hydrophobic property of the SnO2 film with a contact angle over 55°. After depositing GQDs, the surface energy has decreased resulting in contact angle of below 35°. The decrease of surface energy and contact angle is contributed to the spreading of perovskite precursor solution on SnO2 film, resulting a good interface contact of SnO2 and perovskite film [31]. Fig. 4 (a) shows the model diagram of the planar-structure PSCs with the structure of FTO/SnO2/GQDs/MAPbI3/Spiro-OMeTAD/Au. Fig. 4 (b) show the SEM image of the cross sectional of the device. The functional layer is closely connected with contiguous layer, which is beneficial to the transfer of the charge and improvement of the series and parallel resistances of the device. It is obvious that there is a thin (about 80 nm) and uniform SnO2/GQDs layer between FTO and perovskite functional layer. The effect of GQDs on SnO2 layer was investigated by control atomizing times (0, 60, 120, 180, 210 s). The detailed data summarized in Table 1, corresponding box chat date illustrated in Fig. 4 (f). The PCE increases as the atomizing time increases from 0 to 120 s, which is attribute to the increase of Jsc and FF. A further increase of atomizing time from 120 to 210 s leads to a decreased on Jsc, FF and PCE. The optimized atomizing time is found to be 120 s, leading to highest Jsc of 23.36 mA/cm2, Voc of 1.03 V, FF of 67.9%, and PCE of 16.54%. It can be observed that the Voc of device have no significantly improvement after the GQDs deposited on SnO2 layer. In addition, the series and parallel resistances of the device also changed after the deposition of GQDs on SnO2 layer, that is will discussed in detail later. Fig. 4 (c) shows the highest current density-voltage (J-V) characteristics of PSCs based on SnO2 and SnO2/GQDs, and corresponding parameters are summarized in Table 2. The cell based on SnO2 exhibits an open-circuit voltage (Voc) of 1.01 V, a short circuit current density (Jsc) of 22.32 mA/cm2, a fill factor (FF) of 60.6%, and a PCE of 13.61%. Compared with reference cell, the device based on SnO2/GQDs has a similar value in Voc of 1.03 V, but the Jsc increased to 23.36 mA/cm2, the FF value increased 67.9%, resulting a PCE increase to 16.54%. The statistical histogram of the PCEs from 20 pieces of PSCs with and without GQDs shown in Fig. 4 (d). It shows that almost the tested device with GQDs demonstrated a higher PCE than that of the reference device. On average, the PCE of the PSCs with GQDs relatively increases by about 17.1% of that of the reference devices, from 12.6% to 15.2%.
elements in SnO2 surface, indicating the deposited GQDs on SnO2 surface by UAP is reliable, which has a special effect on the properties of SnO2 and performance of the PSCs. The surface morphology of SnO2 and SnO2/QGDs films observed by atomic force microscopy (AFM), Fig. 2(d and e) shows the 3D diagram of AFM of FTO/SnO2 films with and without GQDs. Using ultrasonic atomizing technology, the GQDs was uniformly deposited on the surface of SnO2, free of holes and crack. Meanwhile, the surface roughness (Root mean square) of SnO2 has slightly increased from 23.7 nm to 25.0 nm ascribe to deposited GQDs, Fig. 2(f and g). The ultrathin quantum dot layer acts as an analogous mesoporous layer, which is beneficial to the perovskite precursor adhere to SnO2 and optimized the interfacial contact of perovskite and SnO2. It well known that the surface energy and morphology of electron transport layer have a significant influence on perovskite deposition and crystal formation process. Both depend on several parameters such as surface roughness, chemical heterogeneities, and molecular rearrangement [30]. Firstly, the effect of GQDs on perovskite crystal formation was investigated using scanning electron microscope (SEM), Fig. 3(a and b). Compared with the perovskite film on SnO2, the film on SnO2/GQDs shows better surface topography, which have uniform density features and the number of larger particles has increased. In addition, the pinhole and grain boundary defects were obviously optimized, and all of this is due to the surface passivation of GQDs. Fig. 3 (c) shows the XRD measurement of perovskite film loaded on SnO2 and SnO2/GQDs ETLs. It is found that all samples show strong diffraction peaks at 14.08°, 28.4° and 31.8°, which corresponds to the lattice plane of (110), (220), (310), respectively, demonstrating a better crystallized of perovskite film. Further, we find two obvious differences in the perovskite films deposited on SnO2 and SnO2/GQDs ETL. Firstly, the peak intensity of (110) and (220) reflection of perovskite deposited on SnO2/GQDs ETL is higher than that on SnO2 ETL, indicating a higher crystallinity for perovskite film deposited on SnO2/GQDs ETL at the same conditions. Secondly, the peak width (FWHM) of perovskite deposited on SnO2/GQDs ETL is narrower than that on SnO2 ETL for (110) reflection, indicating larger crystallite size of the former. The light absorptions of perovskite films on FTO/SnO2 with and without GQDs illustrated in Fig. 3 (d). The absorption spectrum is in agreement with pure CH3NH3PbI3 perovskite. The absorption spectrum of perovskite films indicated no significant differences in 300–500 nm. However, the absorption spectrum shows obvious improvement in 500–800 nm, which is mainly attribute to the optimization crystals of 4
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Fig. 4. (a) Device model diagram. (b) Cross sectional SEM image of the PSCs with a structure of FTO/SnO2/GQDs/CH3NH3PbI3/Spiro-OMeTAD/Au. (c) J-V curves, (d) Statistic histograms of the PCE. (e) EQE spectra. (f) PCE tendency with different spraying time of GQDs.
−4.35 eV and −4.23 eV. In brief, the observed decrease in work function of SnO2 film by GQDs are mostly derived from the formation of dipoles on the interface [34]. Which leads to a shift of the vacuum level equally reduce the work function and the ohmic contact between perovskite and SnO2, hence “barrier-free” electron extraction from perovskite to SnO2 [31,35]. Optimized interface contact of SnO2 and perovskite layer by GQDs have beneficial effect on series and parallel resistances, resulting an improvement of device FF [18]. Fig. 5 (d) shows the energy band alignment of each component in the PSC devices. Here the energy level of GQDs is perfectly aligned between both of SnO2 and CH3NH3PbI3 indicating that GQDs may be not only retarding the reversed electron leakage but also mediating the electron transportation from perovskite to SnO2, playing a role like simplex electron channel from perovskite layer to ETL layer[36]. To further investigate the charge transport and recombination process in device, photoluminescence spectrum and time-resolved photoluminescence (TRPL) spectroscopy were used [37]. Fig. 6 (a) shows the steady photoluminescence (PL) spectrum of perovskite on different ETL layer. The spectrum show that devices based on SnO2/ GQDs possess faster emission quenching than that based on bare SnO2, indicating more efficient electron extraction and transport from perovskite to ETLs layer. The enhanced charge transferred from perovskite to the ETLs layer indicated that the energy barrier had been reduced, which could be caused by the high mobility of SnO2 attribute to the GQDs. Time-resolved photoluminescence (TRPL) results showed that the lifetime of the perovskite layer can be significantly reduced when depositing it on SnO2/GQDs ETLs, Fig. 6 (b). The average lifetime of charge carrier τave decreased from 29.93 ns to 19.96 ns, indicating efficient electron transfer from perovskite to SnO2. The more efficient electron transport from perovskite to the SnO2 ETLs indicates the suppressed electron recombination due to the perfect match in energy levels between the conduction band level of the perovskite and the SnO2 layer and the super-fast electron channel function of GQDs [38–40]. For further confirm the charge recombination process and interfacial properties of devices, electrochemical impedance spectroscopy (EIS) measurement was implement at 0.7 V bias voltage under dark condition. The EIS results of corresponding devices shown in Fig. 6 (c). As far as we know, the diameter of the Nyquist plots semicircle presents the charge transfer resistance (RCT) related to the current leakage and recombination of electrons in ETLs and light absorption layer. Larger
Table 1 Summary of Photovoltaic Parameters for PSCs with Various Contents of GQDs obtained by control atomizing time. Spraying time (s)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
T-0 T-60 T-120 T-180 T-210
Average (Max) 1.00 (1.01) 1.00 (1.01) 1.01 (1.03) 1.00 (1.02) 1.00 (1.02)
21.6 (22.3) 22.1 (22.9) 22.4 (23.3) 21.7 (22.8) 21.4 (22.6)
60.0 (62.1) 62.0 (64.2) 65.4 (67.9) 58.6 (60.6) 52.0 (55.0)
12.6 (13.6) 13.8 (14.7) 15.2 (16.5) 12.1 (13.4) 10.8 (12.9)
Table 2 Photovoltaic parameters of the PSCs with and without GQDs. Device
Voc (V) (V)
Jsc (mA/ cm2)
FF (%)
PCE(%)%)
Rs (Ω)
Rsh (Ω)
With GQDs Without GQDs
1.03 1.01
23.36 22.32
67.9 60.6
16.54 13.61
3.5 3.7
1413.8 559.9
Fig. 4 (e) shows the incident photon-to-current efficiency (IPCE) spectra of the PSCs with SnO2 and SnO2/GQDs ETLs, which is in good agreement with the J-V characteristics. In order to explore the mechanism of enhanced Jsc and FF, the surface potential characteristics and corresponding surface potential profiles of the SnO2 with and without GQDs measured by scanning kelvin probe microscopy (SKPM), Fig. 5(a and b). The SKPM images reveal a global 100 mV decrease after modified with GQDs in the Kelvin potential, implying a significant change of the surface energetics. Importantly, the change in surface potential runs through the entire surface. The contact potential difference (CPD) images are flattened and presented in the same color scale, while the histograms of the absolute CPD values of the two kinds of ETL films are plotted in Fig. 5 (c). This result indicated that photogenerated charge carriers could be injected from light absorber to SnO2/GQDs ETL easier [32]. The work function of the Pt/Ir-coated tip and sample can be calculated using the following equation [33]. CPD=(ϴtip-ϴsample)/e
(1)
The work function of SnO2 and SnO2/GQDs calculated to be 5
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Fig. 5. Topography KPFM images of SnO2 film, (a) without GQDs and (b) with GQDs. (c) Surface potential profiles of different SnO2 films. (d) Schematic illustration of the energy diagram and electron transfer process for GQDs modified ETLs [28].
GQDs lead to the dipoles formed along the interface, resulting in ohmic response and hence faster electron extraction from perovskite to SnO2. Fig. 6 (e) demonstrate the interfacial contact schematic diagram of SnO2/GQDs/perovskite. The functional group at the edge of GQDs will form hydrogen bonds with SnO2 and perovskite, which will optimize the interfacial contact of SnO2 and perovskite, meanwhile the GQDs can act as a super-fast electron channel for electron.
RCT means the less current leakage and electron recombined occurred at the interface [41,42]. As can be seen, the diameters of the semicircles of SnO2/GQDs based device increased obviously compare with reference device, which meant that after depositing GQDs on SnO2 films, the interfacial contact of ETLs and perovskite layer was improved dramatically resulting reduced the leakage and charge recombination at interface [43]. This is consistent with our previous data. The effects of GQDs on conductivity of SnO2 film was studied using a FTO/SnO2/(GQDs)/Au structure. The J-V curves measured under light conditions (AM 1.5G) shows that the conductivity of SnO2 film is significantly improved after depositing GQDs, Fig. 6 (d). The insert of
4. Conclusion In summary, low temperature, water soluble GQDs have been
Fig. 6. (a–b) Steady-state PL and TRPL spectrum of CH3NH3PbI3 with different ETLs. (c) Electrochemical impedance spectrum of PSCs with different ETLs (measured at 0.7 V bias voltage under dark condition). (d) J-V characteristics of the device FTO/SnO2/(GQDs)/Au measured under light. (e) Schematic illustration of the interfacial reaction between SnO2/GQDs and GQDs/perovskite. 6
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deposited by UAP as an effective interfacial modification layer to improve planar PSCs performance. Here, a maximum PCE of 16.54% was achieved benefiting from the increase in Jsc and FF by GQDs. The improved Jsc and FF is attributed to the enhanced conductivity, reduced recombination, optimized interfacial energy level alignment in perovskite solar cells. Our work showed the possibility of improved PSCs performance by interfacial modification with GQDs. Meanwhile, the utilized of ultrasonic spraying process also provide references for largescale commercial preparation of PSCs in the future.
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