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Two dimensional graphitic carbon nitride quantum dots modified perovskite solar cells and photodetectors with high performances
Journal of Power Sources 451 (2020) 227825
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Two dimensional graphitic carbon nitride quantum dots modified perovskite solar cells and photodetectors with high performances Pei Liu a, 1, Yue Sun a, 1, Shaofu Wang a, Huijie Zhang a, Youning Gong d, Fangjie Li a, Yunfan Shi a, Yunxiao Du a, Xiaofeng Li b, **, Shi-shang Guo a, ***, Qidong Tai c, ****, Changlei Wang b, *****, Xing-Zhong Zhao a, * a
Key Laboratory of Artificial Micro- and Nano- Structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan, 430072, China School of Optoelectronic Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province & Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou, 215006, China c The Institute of Technological Sciences, Wuhan University, Wuhan, 430072, China d Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen, 518060, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Two dimensional carbon nitride quan tum dots were introduced in PSCs and PDs. � G-CNQDs were employed as modifying reagent at the SnO2/perovskite interface. � The crystalline quality of perovskite absorber with low trap states as achieved. � Our fully air-processed PSC reached a high efficiency over 21%.
A R T I C L E I N F O
A B S T R A C T
Keywords: Interfacial engineering Two dimension polymeric material Graphitic carbon nitride dots Perovskite solar cell Air-process
Two dimensional materials have promising benefits in photoelectronic devices, including facilitating charge transport and reducing defects in perovskite solar cells (PSCs) and photodetectors (PDs) through interface en gineering. Herein, a two dimension polymeric material of graphitic carbon nitride quantum dots (g-CNQDs) interlayer is used to modify the SnO2/perovskite interface in both ambient PSCs and PDs to improve the overall performance. The introduction of g-CNQD layer improves the crystalline quality of sequential perovskite absorber with high phase purity, less grain boundaries, low trap states and suppressed carrier recombination due to the intrinsic cross-linkable feature and relatively smooth surface of g-CNQD. As a result, with optimal modification, fully air-processed PSC reached a champion power conversion efficiency of 21.23% with negligible
* Corresponding author. ** Corresponding author. *** Corresponding author. **** Corresponding author. ***** Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (S.-s. Guo), [email protected] (Q. Tai), [email protected] (C. Wang), [email protected] (X.-Z. Zhao). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2020.227825 Received 29 November 2019; Received in revised form 17 January 2020; Accepted 29 January 2020 Available online 1 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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Journal of Power Sources 451 (2020) 227825
hysteresis, and the PDs had remarkable performance enhancement. Moreover, our PSCs have good stability in ambient air, keeping over 90% of the initial efficiency after 30 days’ exposure.
1. Introduction
processed PHJ PSCs with g-CNQD interfacial modification approached a champion PCE of 21.23%, which is among the first class of totally ambient PSCs. In addition, the highest open circuit voltage (VOC) was achieved to be 1.175 V, giving a voltage loss as low as 0.355 V consid ering the bandgap of 1.53 eV. Taking the advantages of g-CNQD modification, high performance PDs were obtained with highly enhanced responsiveness. Moreover, our efficient PSCs have superior long-term and illumination stabilities in ambient air condition irre spective to high humidity.
Because of the prominent optoelectronic properties of organicinorganic perovskite materials, such as excellent light harvesting prop erties, long carrier diffusion distance, high charge mobility, and low fabrication cost, the corresponding optoelectronic devices such as perovskite solar cells (PSCs) and photodetectors (PDs) have been explored tremendously [1–6]. PSCs are believed to be the most potential competitor to silicon solar cells for commercial application [1–3,7,8]. In the past few years, PSCs have rapidly developed with the record certified power conversion efficiency (PCE) of 25.2% [9]. Generally, mesoporous scaffold and planar heterojunction (PHJ) structure are two representa tive device architectures in most state-of-the-art PSCs [10–13]. Now days, SnO2 was widely used as electronic transport material in PHJ PSCs due to its fast electronic collection rate, great optical properties and suitable energy level [14–16]. Recently, You’s group reported high performance surface passivated PSCs employing SnO2 as electronic transport layers (ETLs) with certified PCEs over 23% [13]. In another aspect, perovskite PDs have also been investigated with various archi tectures and response properties. The detecting wavelength region of PDs can be tuned from near ultraviolet to near infrared, and the material morphologies include single crystal and polycrystalline films [4–6]. Considering SnO2 based PSCs, many efforts have been explored to improve the performance and stability for further commercialization, such as architecture design, composition engineering, robust hole transport materials application and interfacial modification [3,10,12, 17–21]. In particular, interfacial engineering has been proved to be an effective route to improve devices photovoltaic properties by improving charge transport, reducing defect density, suppressing carrier recombi nation and facilitating the perovskite crystal growth [22–24]. Recently, a series of studies reported the interfacial passivation between SnO2 and perovskite to reduce the defect density and achieve better contact for high-performance PSCs [23–29]. Among them, two dimension (2D) materials and cross-linkable fullerene materials were widely investi gated as interface modulation reagent in efficient PSCs that could improve the film quality of following perovskite layer and promoting charge transfer kinetics as well [24,30,31]. However, the commonly used 2D interfacial materials are graphene and cross-linkable fullerenes, which are expensive and difficult to con trol the growth. Here, we find that graphitic carbon nitride is a metalfree, earth-abundant and nontoxic material with a polymeric 2D struc ture [32,33]. It has been widely used in pollutant degradation, water splitting and solar energy transfer due to its outstanding photocatalytic and optoelectronic properties [34–38]. Very recently, similar kinds of material have been incorporated into perovskite precursor to improve the crystal quality of perovskite films by slowing down the crystalliza tion rate and passivating the carrier recombination centers [39,40]. However, graphitic carbon nitride has rarely been employed in fully ambient-process planar PSCs and PDs, ignored its favorable charge transporting and defect passivating properties. In this work, fluorescent graphitic carbon nitride quantum dots (gCNQDs) with 2D cross-linkable framework were synthesized and intro duced at the SnO2/perovskite interface in fully air-processed PSCs and PDs, and it realized several favorable photoelectric properties. Firstly, the surface roughness of SnO2 ETL decreased by the g-CNQDs modifi cation, thus to be beneficial for the growth of perovskite absorber. Due to the intrinsic 2D cross-linkable structure and the smoother surface of ETL, high quality perovskite films were achieved with homogeneous surface and less grain boundaries. Furthermore, photogenerated charges could diffuse more fluently through the ETL/perovskite interface, benefited from the gradient energy level alignment. As a result, fully air-
2. Results and discussion Fluorescent g-CNQDs solution was prepared by filtering the resultant yellowish powders, which were synthesized by the mixture of urea and sodium citrate, in ethanol solution (experimental section) [32]. The size of obtained g-CNQDs was detected by transmission electron microscopy (TEM). As shown in Fig. 1a, the TEM image reveals that the g-CNQDs have a diameter in the range of 10–30 nm, and are well monodispersed. The ultraviolet–visible (UV–vis) absorption spectra and steady-state fluorescent spectra were performed to investigate the optical proper ties of the collected g-CNQDs. As depicted in Fig. 1b, the UV–vis ab sorption spectrum presents a characteristic absorption peak at 327 nm and 404 nm for g-CNQDs. Moreover, the emission peak at 545 nm for g-CNQDs was observed in the steady-state fluorescent spectra, which can be attributed to the optical selection of nanoparticles (quantum ef fect). The digital picture g-CNQDs dispersed in ethanol shows that the fluorescent g-CNQDs exhibit strong green fluorescent under 365 nm UV light. The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectrometer are further conducted to investigate the chemical structure and composition of the achieved g-CNQDs. As shown in Fig. 1c, XPS spectrum of g-CNQDs in C 1s region demonstrates two peaks at 285 eV and 288 eV, which can be ascribed to sp3 C–C bonds and – C–N bonds, respectively [32,41]. In particular, the energy peak sp2 N– – C–N bond demonstrates the basic sub at 288 eV related to sp2 N– structure for the information of the 2D polymeric material of g-CNQDs. Furthermore, Fig. 1d shows the N 1s spectrum of g-CNQDs, which can be – N–C bonds. Thus, deconvoluted into one peak at 400 eV assigned to C– the presence of those groups suggests that the heptazine heterocyclic ring unite is the basic substructure of g-CNQDs. These results are also supported by FT-IR spectra (Fig. 1e), where the characteristic band around 788 cm 1 is ascribed to the heptazine units and the bands at 1401 cm 1 and 1441 cm 1 correspond to the typical stretching modes of CN heterocycles [32]. To further demonstrate the influence of g-CNQD to SnO2 film, the surface scanning electron microscope (SEM) images of SnO2 and 1/3 gCNQD modified SnO2 (hereafter named as SnO2/g-CNQD) were shown in Figs. S1a and b. There is no big difference between them, which probably due to the ultra-thin thickness of g-CNQD layer that might not be easily detected by SEM. Thus, in order to confirm the information of g-CNQD on the surface of SnO2 layer, the energy dispersive spectrum (EDS) and distribution diagram images of element of g-CNQD modified SnO2 layer are conducted as shown in Fig. 2a. The binding energy peaks at 0.26, 0.52 and 3.45 Kev, corresponded to C, N and Sn elements, and the uniform distribution of elements at the substrate reveal that the gCNQDs were conformably coated on SnO2 layer. Furthermore, the atomic force microscope (AFM) is conducted to demonstrate the influ ence of g-CNQD to the surface of SnO2 layer, as provided in Fig. 2b. The root mean square roughness (Rq) value of ETL obviously decreased from 17.5 nm to 12.8 nm by the incorporation of g-CNQD, suggesting ETL with a more uniform and smooth surface was achieved, which is bene ficial for the growth of the following perovskite absorber [39]. The 2
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surface morphologies of perovskite deposited on pristine SnO2 and SnO2/g-CNQD ETL are presented in Fig. 2c to explore the influence of the g-CNQD modifying layer on the perovskite film. Obviously, high quality perovskite film with less grain boundaries is obtained for SnO2/g-CNQD ETL, which could be attributed to passivation effect of g-CNQDs facilitating the crystal growth [42–44].Furthermore, as shown in Fig. 2d, the X-ray diffraction (XRD) patterns of perovskite based on pristine and g-CNQD modified SnO2 ETL show that both films present a similar crystalline lattice of the cubic perovskite phase. Nevertheless, higher peak intensity of the main peak around 14� and 28� , corre sponded to the (110) and (220) crystal plane of cubic phase, can be observed for the perovskite film deposited on SnO2/g-CNQD ETL [13,16, 45]. This indicates a better crystallinity upon interface modulation, which might be ascribed to the preferred crystal orientation by the 2D reticulate structure of g-CNQD. Moreover, an impure PbI2 peak (2θ ¼ 12.7� ) can be founded in the SnO2/perovskite film, while undistin guished in the SnO2/g-CNQD/perovskite sample, implying that g-CNQDs could facilitate the growth of perovskite phase. The above results confirm that the uniform and smoother perovskite films with better crystallization and higher phase purity are achieved by the g-CNQD modification.
We expect a better performance of g-CNQD modified PSCs consid ering the higher quality perovskite absorber than the control film based on pristine SnO2 ETL. Thus, we fabricated planar PSCs to confirm the interface modification of g-CNQDs. The ultra-thin g-CNQD layer was deposited on SnO2 electron transport layer by spin coating. Devices with configuration of FTO/SnO2/g-CNQD/Perovskite/Spiro-OMeTAD/Au were fabricated in atmosphere environment, the scheme and corre sponding cross-sectional SEM image, of the g-CNQD modified device shown in Fig. 3a. We checked the performance of devices based on different thickness of g-CNQD modifying layer by controlling the con centration of g-CNQD in anhydrous ethanol, and found that the optimal concentration is 1/3 g-CNQD, which was diluted for 3 times from the saturation solution of g-CNQDs. Hereafter, the 1/2, 1/3 and 1/6 g-CNQD solutions were defined as diluting the saturated g-CNQDs solution with 2, 3 and 6 times of ethanol, respectively. The photovoltaic performance of PSCs based on bare and 1/3 g-CNQD modified SnO2 ETL are compared in Fig. 3. The current density-voltage (J-V) curves of those devices are presented in Fig. 3b, and their corresponding photovoltaic parameters of VOC, short circuit current density (JSC), fill factor (FF) and PCE are summarized in Table S1. The pristine SnO2 ETL based device exhibits a VOC of 1.111 V, a JSC of 22.68 mA cm 2 and a FF of 74.36%,
Fig. 1. Characterizations of the obtained g-CNQDs. a) TEM image b) UV–vis absorption (red line) and photoluminescence (black line) spectra of the g-CNQDs. The inset is a digital picture of g-CNQDs dispersed in ethanol. XPS spectra of g-CNQD in c) C 1s region and d) N 1s region. e) Fourier transform infrared characterization of the g-CNQDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3
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yielding a PCE of 18.74%. The device based on SnO2/g-CNQD ETL displays a relatively better performance: a PCE of 20.30% with a VOC of 1.128 V, a JSC of 23.41 mA cm 2 and a FF of 76.90%. The increased VOC and FF can be mainly attributed to the better contact and reduced trap density at the ETL/perovskite interface, resulting in the better quality of perovskite film. The enhanced JSC from 22.68 mA cm 2 to 23.41 mA cm 2 can be ascribed to the promoted electron extraction and trans portation caused by the reduced grain boundaries of perovskite film. Furthermore, the corresponding external quantum efficiency (EQE) plots and integrating current curves are displayed in Fig. 3c. The gCNQD modified device exhibits higher EQE spectrum than the device without modification over 500 nm wavelength, confirming the increased JSC by the g-CNQD interlayer that could facilitate photo generated charge carriers flow through the film. The reproducibility were verified by characterizing 50 PSCs based on pristine SnO2 and SnO2/g-CNQD ETL, as depicted in Fig. 3d. The higher PCE and better reproducibility for the devices with g-CNQD interlayer reveal that high performance PHJ PSCs with g-CNQD modified SnO2 ETL can be easily repeated even in ambient air with uncontrollable temperature and humidity. The carrier dynamics, recombination behavior and trap-state density in devices were further investigated to clarify the mechanism of the influence of g-CNQD modifying layer. Fig. 4a compares the dark current density of PSCs with bare and 1/3 g-CNQD modified SnO2 ETLs. The lower current density leakage can be observed for the modified device, suggesting the current leakage in the device is prevented to a certain extent and the charge recombination rate is reduced by the consequent
high quality film. Furthermore, to confirm the charge transport and recombination kinetics in the devices, electrochemical impedance spectroscopy (EIS) was carried out on the PSCs based on SnO2 and SnO2/ g-CNQD ETL under the one sun illumination condition at the given bias of corresponding open-circuit voltage. As shown in Fig. 4b, the Nyquist plots of PSCs display a two-semicircles features, the semicircle at the high frequency of 220 KHz can be ascribed to the transfer resistance (Rtr), while the semicircle at the low frequency of 1.5 Hz can be attributed to the recombination resistance (Rrec) [46,47]. Fitted with the simplified equivalent circuit model as shown in the inset, the corre sponding resistance parameters are obtained and summarized in Table S2. The SnO2 ETL based device shows a higher Rtr (92 Ω) than the SnO2/g-CNQD based one (Rtr ¼ 77 Ω), implying a faster charge transport in SnO2/g-CNQD modified device. Moreover, a higher Rrec is obtained of the PSC with g-CNQD modification, suggesting the suppressed carrier recombination. The lower Rtr and higher Rrec could be responsible for the better performance of SnO2/g-CNQD based PSCs with more favor able charge extraction properties and much suppressed carrier recom bination rate. The steady-state photoluminescence (PL) spectra of perovskite films on pristine and g-CNQD modified SnO2-coated FTO substrates are displayed in Fig. 4c. Compared to the SnO2-based perovskite film, a significantly decreased PL intensity is observed for the sample with g-CNQD interlayer, revealing the enhanced electron extraction ability by the modification of g-CNQDs, which is consistent with the results of dark current and EIS [48]. In addition, the Hall mobility μH of SnO2 (electron transport layer), SnO2/g-CNQD and spiro-OMeTAD (hole transport layer) films have been determined from
Fig. 2. The surface morphology of SnO2(/g-CNQD) and the crystallinity and morphology of perovskite films coated on them. a) EDS images of g-CNQDs modified SnO2 layer. b) AFM images of pristine SnO2 film and SnO2/g-CNQD film. c) SEM images of perovskite films based on SnO2 and SnO2/g-CNQD ETL. d) XRD spectra of perovskite films. 4
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Fig. 3. Architecture of the PSCs by g-CNQD modification and comparison between PSCs with/without g-CNQD modification. a) Schematic illustration and cross-section SEM image of the PSCs with g-CNQD modified SnO2 layers, the scale bare is 1 μm. b) The J-V curves, c) EQE spectra and integrated current density curves and d) histogram of efficiencies of PSCs with/without g-CNQD modification.
Hall measurements. As summarized in Table S3, the μH of SnO2 and SnO2/g-CNQD films are 9.62 � 10 2 cm2V 1s 1 and 9.33 � 10 1 cm2V 1s 1, respectively. It is notable that the mobility is significantly enhanced by the modification of g-CNQD, suggesting the improvement of carrier transport ability for ETL. Furthermore, compared to the perovskite film coated on SnO2 layer, a slight blue shifting of PL emis sion peak can also be found by the incorporation of g-CNQD, suggesting the reduced defects in SnO2/g-CNQD based film because of suppressing of the spontaneous radiative recombination between trap states. Because the spontaneous radiative recombination between trap states leads to a red-shifted emission peak compared with that from the band edge transition [49,50]. In addition, as depicted in Fig. 4d, the carries lifetime of 79 ns and 38 ns were evaluated by time-resolved PL (TRPL) for perovskite films coated on SnO2 and SnO2/g-CNQD ETL, respectively [8]. The shorter carrier lifetime for the g-CNQD modified sample further confirms the enhanced electron extraction and transportation proper ties, which could be responsible for the higher JSC of SnO2/g-CNQD based PSCs [51]. To assess the electron transport and defect density, electron-dominated devices (FTO/SnO2 (/g-CNQD)/per ovskite/PCBM/Ag) have been fabricated and measured; (here PCBM is phenyl-C61-butyric acid methyl ester). The current-voltage (I–V) curves of SnO2 and SnO2/g-CNQD based devices are displayed in Fig. 4e and f, respectively. The trap-state density (Nt) can be calculated by the trap-filled limit voltage (VTFL) method using the equation [52]:
CNQD with different g-CNQD concentration were systematically inves tigated by photovoltaic and photoelectronic tests. The statistics of VOC, JSC, FF and PCE of devices based on pristine SnO2 or g-CNQD modified SnO2 ETL are presented in Fig. 5a and listed in Table S4. The PSCs based on pristine SnO2 ETL show an averaged VOC of 1.074 � 0.018 V, JSC of 22.63 � 0.51 mA cm 2 and FF of 72.35 � 2.35%, yielding an averaged PCE of 17.58 � 0.84%. In contrast, the 1/3 g-CNQD modified SnO2 ETL based devices exhibit the best performance with an average PCE of 18.88 � 0.57%, resulted by an averaged VOC of 1.111 � 0.022 V, the highest JSC of 22.98 � 0.43 mA cm 2 and the highest FF of 74.00 � 1.43%. The XRD results demonstrate that the film quality first increase and then decrease as a function of the concentration of g-CNQD. Compared with the film on pristine SnO2 ETL, perovskite samples based on 1/6 g-CNQD and 1/3 g-CNQD exhibit better crystallization with preferred crystal orientation (Fig. S2). However, the peak intensity of the main peaks around 14� and 28� decreased by 1/2 g-CNQD modifi cation. The decrease of film quality might be caused by the difficulty of film formation on thick g-CNQD layer, which will become cluster of monodispersed QDs with high concentration at the surface of SnO2. Moreover, the transmittance spectra of FTO, pristine SnO2 and SnO2/gCNQD ETL coated on FTO substrate were shown in Fig. S3a. The transmittance spectra plots of pristine SnO2 is almost same as the gCNQD modified SnO2 one, indicating there is no other light loss occurred by the g-CNQD interlayer. Fig. S3b shows the UV–vis absorption spectra of perovskite films based on different substrate. Due to the influence of g-CNQD on the quality of perovskite film, the tendency of UV–vis ab sorption is consistent with the result of XRD patterns, showing a first increase and then decrease trend. In addition, the EQE spectra of devices are mainly related to the UV–vis absorption of perovskite films. There fore, higher EQE and current density of PSCs are obtained with 1/6 and 1/3 g-CNQD modification, as presented in Fig. 5b. Moreover, PL mea surements of perovskite coated on ETL with different g-CNQD concen tration were preformed to investigate the carrier dynamics and recombination behavior in devices with different g-CNQD concentra tion. As shown in Fig. S4, the emission peak intensity of perovskite film was decreased by 1/6 and 1/3 g-CNQD modification, and increased by
Nt ¼ 2εoεrVTFL/qL2 Where εo and εr are the vacuum permittivity and relative dielectric constant, respectively, q is the elemental charge of the electron, L is the corresponding thickness of perovskite film (~700 nm). The VTFL of samples based SnO2 and SnO2/g-CNQD layer are 0.14 V and 0.095 V, respectively. Therefore, the trap-state density is reduced from 1.01 � 1015 cm 3 to 6.67 � 1014 cm 3 after g-CNQD modification, revealing the quality of perovskite film was improved, which is beneficial to the performance of PSCs. To further confirm the influence of g-CNQD intercalated into the SnO2/perovskite interface, PSCs based on pristine SnO2 and SnO2/g5
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Fig. 4. Carrier transport and recombination of PSCs with/without g-CNQD modification. a) J-V curves and b) Nyquist plots for PSCs with or without g-CNQD modification were measured under the one sun illumination condition at the given bias of each open-circuit voltage. c) Steady-state PL and d) TRPL of perovskite films coated on SnO2 and SnO2/g-CNQD ETL. Trap-state density of perovskite based on e) SnO2 and f) SnO2/g-CNQD ETL.
1/2 g-CNQD modification. Furthermore, carriers in perovskite coated on 1/6 and 1/3 g-CNQD modified SnO2 films exhibit shorter lifetime values than the pristine sample, while the carries lifetime was increased by 1/2 g-CNQD modification, as depicted in Fig. 5c. Those results reveal that gCNQD layer with proper thickness enhances electron extraction ability and reduces carrier recombination, while the g-CNQD may form big clusters with high concentration, thus result in more carrier recombi nation and deteriorate electron extraction. The hysteresis behavior and stability of the optional devices based on 1/3 g-CNQD modified SnO2 ETL were further investigated. Fig. 5d shows the J-V curves of device measured under one sun illumination with a sweep speed of 100 mV s 1. This PSC exhibits a PCE of 21.23 (20.76)% with a VOC of 1.140 (1.135) V, a JSC of 23.39 (23.65) mA cm 2 and a FF of 79.62 (77.34)%, when measured under reverse (forward) voltage scans, indicating the negligible hysteresis for the g-CNQD modified de vices. As presented in Fig. 5e, the unencapsulated PSCs with SnO2/gCNQD ETL exhibits a steady-state PCE of 20.5% at maximum power point under continuous operation for 90 s, indicating the good illumi nation stability for g-CNQD modified PSCs. The long-term stability was displayed by the performance evolution of the unencapsulated devices stored in ambient air under room light with a humidity of ~30% upon 30 days’ exposure, as shown in Fig. 5f. And the J-V curves of this device
at different times are depicted in Fig. S5. The PCE of g-CNQD modified PSC keeps over 90% of initial performance after 30 days, suggesting the outstanding ambient stability against moisture and light. Taking the advantages of high quality perovskite film, we fabricated perovskite PDs with and without g-CNQD modification applying a simple structure as shown in the inset of Fig. 6a and b. A 532 nm laser was used as the light source, and the current-voltage (I–V) character izations of PDs were performed under different illumination power densities, ranging from 0.3 to 372.2 mW cm 2 with natural filters. The photocurrent has a linear relationship with voltage for both devices, as show in Fig. 6a and b. Stronger illumination provides a higher photo current for both PDs, which could be ascribed to the higher density of photogenerated carriers flow through the external circuit under a given bias. Compared to the pure SnO2 based perovskite detector, the g-CNQD modified one show relatively higher response irrespective to the illu mination power, implying that the later one has better responsiveness. As presented in Fig. 6c, the responsivity (R) of PD under different voltage and illumination were also calculated with the formula [4]: R¼ (Ilight-Idark)/(Pin � S) Where Ilight is the photogenerated current under light, Idark is the dark current, Pin is the power of incident light, and S is the working area (here 6
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Fig. 5. Photovoltaic and photoelectronic tests of PSCs with different concentration of g-CNQD, the hysteresis behavior and stability of devices based on SnO2/g-CNQD ETL. a) PV parameters of VOC, JSC, FF and PCE of PSCs with different concentration of g-CNQD. b) EQE spectra and integrated current density curves of devices. c) TRPL of perovskite films based on different concentration of g-CNQD. d) J–V curves measured under reverse and forward voltage scans with a scan rate of 100 mV s 1. e) Steady PCE of the best device at a maximum power point under one sun illumination. f) Long-term stability of the unencapsulated PSCs in ambient air (temperature�30 � C, humidity�30%).
defined as 0.027 cm2). R is an important parameter indicating the ability of detector responding to the optical signal. Since the glass substrate and metal electrode as well as the fabrication processes are identical, the difference can be only related to the g-CNQD modification. As we have discussed above, the high quality perovskite film was achieved by gCNQD modification, thus leading to the better photoconductivity of the film under illumination due to the lower defects. The transient photo current responses of PDs with and without g-CNQD modification measured under 372.2 mW cm 2 laser irradiation with a chopper of 0.5 s certain on/off interval are displayed in Fig. 6d. It can be obviously seen that the photocurrent was drastically enhanced due to the hindering of charge recombination at interface by the g-CNQD modification. The rise and decay times of the modified PD are calculated to be 81 and 41 m s, respectively, which are reasonable for our PDs fabricated in air with such a simple device structure.
surface of the g-CNQD layer that could facilitate the crystal growth and charge transportation. PSCs with good performance of increased JSC, VOC and FF are achieved by the favorable charge transport, low trap states and suppressed charge recombination upon proper g-CNQD modulation. Our best PSC exhibits a PCE up to 21.23% with negligible hysteresis and robust working stabilities against moisture and illumi nation out of glovebox. Taking the benefit of interface modification, efficient PDs were also obtained with higher current and faster response speed compared to the pristine ones. This work demonstrates a feasible route to achieve high-performance PSCs and PDs by interfacial engi neering of 2D polymeric material. 4. Experimental section Materials: PbI2(TCI, 99.99%), SnO2 colloid precursor (Alfa Aesar), N, N-dimethylformamide (DMF) (Aladdin, 99.8%) and dimethyl sulfoxide (DMSO) (Aladdin, 99.8%) were employed directly without any treat ment. Urea and sodium citrate were purchased from Sinopharm. For mamidinium iodide (FAI), methylammonium bromide (MABr), methylammonium chloride (MACl), Cesium Iodide (CsI) and 2,20 ,7,70 tetrakis(N,N-dip-methoxyphenylamine)-9,90 -spirobifluorene (spiroOMTAD) were all purchased from Xi’an Polymer Light Technology Corp and employed directly.
3. Conclusion In summary, fluorescent g-CNQDs are employed as modifying re agent at the SnO2/perovskite interface in fully air-process planar PSCs and PDs to enhance the photoelectric properties. High quality perovskite films with better surface morphology, less grain boundaries and purer phase were obtained by the intrinsic cross-linkable feature and smooth 7
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Fig. 6. Photoelectronic tests of PDs with/without g-CNQD modification. The current-voltage curves of PDs a) with and b) without g-CNQD modification under different illumination power densities. c) The responsivity of PDs with/without g-CNQD modification under different voltage and illumination. d) The transient photocurrent responses of devices with and without g-CNQD modification under 372.2 mW cm 2 laser irradiation with a chopper of 0.5 s certain on/off interval.
Synthesization of graphitic carbon nitride dots (g-CNQDs) solution: 1.68 mmol urea and 0.28 mmol sodium citrate were mixed in an agate mortar and grounded ~30 min into uniform powders. Then the mixture was put into railboat and heated at 180 � C for 60 min in a drying oven. The resultant yellowish powders were placed in an agate mortar and grounded ~30 min. After that, fluorescent g-CNQDs solution was ach ieved by dissolving the mixture in ethanol and filtering by 0.45 μm Nylon needle. Then, the 1/2, 1/3 and 1/6 g-CNQD solutions were defined as diluting the as-prepared saturated g-CNQDs solution with 2, 3 and 6 times of ethanol, respectively. Fabrication of the perovskite solar cells: The PSCs were fabricated with a structure of FTO/SnO2/g-CNQD/Perovskite/Spiro-OMAD/Au in ambient air with a relatively high humidity (~40%). The etched fluorine-doped tine oxide (FTO) glass was ultrasonically cleaned with acetone and ethanol for 20 min several times. Before using, the FTO glass was treated with UVO for 15 min. Then, a thin layer was deposited onto the substrate by spin-coating SnO2 nanoparticle solution at 3000 rpm for 30 s, and annealed at 150 � C for 30 min. After cooled down to room temperature, the g-CNQD solution with different concentration was spin coated onto the SnO2 film at 3000 rpm for 30 s, and a thermal annealing at 150 � C for 30 min. After that, 1.3 M of PbI2 in (DMF:DMSO ¼ 9.5:0.5) with amount of CsPbI3 additive was spin coated onto ETL at 1500 rpm for 30 s, and a thermal treatment at 70 � C for 1 min. After cooled down, the mixture solution of FAI:MABr:MACl (60 mg:6 mg:6 mg in 1 ml IPA) was deposited onto PbI2 at a speed of 1300 rpm for 30 s, followed by annealing at 150 � C for 15 min. The hole transport layer, spiro-OMTAD, was coated onto the perovskite layer by 4000 rpm for 30 s. At last, ~100 nm thick Au counter electrode was deposited by thermal evaporation. Fabrication of the perovskite photodetectors: Soda lime glass is used as the substrate, and cleaned with detergent, water, alcohol and acetone successively, assistant with ultrasonic batch. SnO2, g-CNQD and
perovskite layers were deposited the same as above. ~100 nm thick Au counter electrode was deposited by thermal evaporation with a mask, the width of two electrodes was chosen to be 90 μm. Measurements and Characterization: The SEM and EDS images were characterized on JEM 2100. The TEM image of g-CNQDs was measured by JEM-2010 FEF transmission electron microscope at an acceleration voltage of 200 KV. Transmittance and absorbance spectra were carried out on UV–vis spectrometer (Lambda 650s, PerkinElmer). The emission spectra of g-CNQDs was recorded on a fluorescence spectrophotometer (F-4600, Hitachi, Japan) with parameters involving an excitation wavelength of 360 nm, a scanning speed of 240 nm min 1, and a pho tomultiplier tube (PMT) voltage of 700 V. The XPS spectra was tested on ESCALAB 250XLwith Al Kα radiation of 1486.6 eV as the excitation source. Infrared spectra was obtained by Fourier transform infrared spectrometer (FT-IR, Nicolet iS10, Thermo Fisher, USA) with scanning range of 525–4000 cm 1. The surface roughness of electronic transport layers was detected by atomic force microscopy (AFM, Bruker Multi mode 8). The perovskite crystal structure was tested by an XRD spec trometer (D8 Advance, Bruker AXS, Germany) with Cu Kα radiation. The current density-voltage curves were observed by a sunlight simulator (Newport, 91192) under one sun illumination (AG 1.5G, 100 mV cm 2). Effective area of the solar cells was defined to be 0.09 cm2 using a metal mask. The external quantum efficiency spectra was performed from 300 nm to 900 nm with a 300 W xenon lamp (Newport 66984). For the performance characterizations of PDs, a 532 nm semiconductor laser is used as the illumination source with power density determined by PM100D. The performance of photodetectors is measured by a keysight B1500A semiconductor device analyzer system assisted with a probe station. The Steady-state and time-resolved PL (TRPL) was investigated on Delta Flex fluorescence spectrometer (HORIBA).
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Declaration of competing interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is in Wuhan University financially supported by the Sci ence and Technology Department of Hubei Province (No. 2019AAA020), the National Natural Science Foundation of China (No. 61974106), the Fundamental Research Funds for the Central Univer sities (2042019kf1003) and the National Key R&D Program (Grant 2016YFC1000701). The work in Soochow University is financially supported by Natural Science Foundation of Jiangsu Province (BK2019085) and Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (19KJB480003). We would like to acknowledge the Center for Electron Microscopy at Wuhan University for their substantial supports to SEM, TEM and XRD work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227825. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] M.A. Green, A. Ho-Baillie, H.J. Snaith, Nat. Photon. 8 (2014) 506–514. [3] M. Saliba, T. Matsui, K. Domanski, J. Seo, A. Ummadisingu, S.M. Zakeeruddin, J. P. Correa-Baena, W. Tress, A. Abate, A. Hagfeldt, M. Gr€ atzel, Science 354 (2016) 206–209. [4] L. Dou, Y.M. Yang, J. You, Z. Hong, W.H. Chang, G. Li, Y. Yang, Nat. Commun. 5 (2014) 5404. [5] Y. Fang, J. Huang, Adv. Mater. 27 (2015) 2804–2810. [6] M.I. Saidaminov, V. Adinolfi, R. Comin, A.L. Abdelhady, W. Peng, I. Dursun, M. Yuan, S. Hoogland, E.H. Sargent, O.M. Bakr, Nat. Commun. 6 (2015) 8724. [7] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Nature 517 (2015) 476–480. [8] Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao, J. Huang, Science 347 (2015) 967–970. [9] Best research-cell efficiencies. https://www.nrel.gov/pv/cell-efficiency.html. [10] M.M. Lee, J. Teuscher, t. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643–647. [11] M. Kim, G.H. Kim, T.K. Lee, I.W. Choi, H.W. Choi, Y. Jo, Y.J. Yoon, J.W. Kim, J. Lee, D. Huh, H. Lee, S.K. Kwak, J.Y. Kim, D.S. Kim, Joule 3 (2019) 2179–2192. [12] M. Liu, M.B. Johnston, H.J. Snaith, Nature 501 (2013) 395–398. [13] Q. Jiang, Y. Zhao, X. Zhang, X. Yang, Y. Chen, Z. Chu, Q. Ye, X. Li, Z. Yin, J. You, Nat. Photon. 13 (2019) 460–466. [14] C. Wang, D. Zhao, C.R. Grice, W. Liao, Y. Yu, A. Cimaroli, N. Shrestha, P.J. Roland, J. Chen, Z. Yu, P. Liu, N. Cheng, R.J. Ellingson, X. Zhao, Y. Yan, J. Mater. Chem. 4 (2016) 12080–12087. [15] G. Yang, C. Chen, F. Yao, Z. Chen, Q. Zhang, X. Zheng, J. Ma, H. Lei, P. Qin, L. Xiong, W. Ke, G. Li, Y. Yan, G. Fang, Adv. Mater. 30 (2018), e1706023. [16] Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Nat. Energy 2 (2016) 1–7. [17] Z. Yu, B. Chen, P. Liu, C. Wang, C. Bu, N. Cheng, S. Bai, Y. Yan, X. Zhao, Adv. Funct. Mater. 26 (2016) 4866–4873.
9
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Two dimensional graphitic carbon nitride quantum dots modified perovskite solar cells and photodetectors with high performances Pei Liu a, 1, Yue Sun a, 1, Shaofu Wang a, Huijie Zhang a, Youning Gong d, Fangjie Li a, Yunfan Shi a, Yunxiao Du a, Xiaofeng Li b, **, Shi-shang Guo a, ***, Qidong Tai c, ****, Changlei Wang b, *****, Xing-Zhong Zhao a, * a
Key Laboratory of Artificial Micro- and Nano- Structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan, 430072, China School of Optoelectronic Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province & Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou, 215006, China c The Institute of Technological Sciences, Wuhan University, Wuhan, 430072, China d Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen, 518060, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Two dimensional carbon nitride quan tum dots were introduced in PSCs and PDs. � G-CNQDs were employed as modifying reagent at the SnO2/perovskite interface. � The crystalline quality of perovskite absorber with low trap states as achieved. � Our fully air-processed PSC reached a high efficiency over 21%.
A R T I C L E I N F O
A B S T R A C T
Keywords: Interfacial engineering Two dimension polymeric material Graphitic carbon nitride dots Perovskite solar cell Air-process
Two dimensional materials have promising benefits in photoelectronic devices, including facilitating charge transport and reducing defects in perovskite solar cells (PSCs) and photodetectors (PDs) through interface en gineering. Herein, a two dimension polymeric material of graphitic carbon nitride quantum dots (g-CNQDs) interlayer is used to modify the SnO2/perovskite interface in both ambient PSCs and PDs to improve the overall performance. The introduction of g-CNQD layer improves the crystalline quality of sequential perovskite absorber with high phase purity, less grain boundaries, low trap states and suppressed carrier recombination due to the intrinsic cross-linkable feature and relatively smooth surface of g-CNQD. As a result, with optimal modification, fully air-processed PSC reached a champion power conversion efficiency of 21.23% with negligible
* Corresponding author. ** Corresponding author. *** Corresponding author. **** Corresponding author. ***** Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (S.-s. Guo), [email protected] (Q. Tai), [email protected] (C. Wang), [email protected] (X.-Z. Zhao). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2020.227825 Received 29 November 2019; Received in revised form 17 January 2020; Accepted 29 January 2020 Available online 1 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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hysteresis, and the PDs had remarkable performance enhancement. Moreover, our PSCs have good stability in ambient air, keeping over 90% of the initial efficiency after 30 days’ exposure.
1. Introduction
processed PHJ PSCs with g-CNQD interfacial modification approached a champion PCE of 21.23%, which is among the first class of totally ambient PSCs. In addition, the highest open circuit voltage (VOC) was achieved to be 1.175 V, giving a voltage loss as low as 0.355 V consid ering the bandgap of 1.53 eV. Taking the advantages of g-CNQD modification, high performance PDs were obtained with highly enhanced responsiveness. Moreover, our efficient PSCs have superior long-term and illumination stabilities in ambient air condition irre spective to high humidity.
Because of the prominent optoelectronic properties of organicinorganic perovskite materials, such as excellent light harvesting prop erties, long carrier diffusion distance, high charge mobility, and low fabrication cost, the corresponding optoelectronic devices such as perovskite solar cells (PSCs) and photodetectors (PDs) have been explored tremendously [1–6]. PSCs are believed to be the most potential competitor to silicon solar cells for commercial application [1–3,7,8]. In the past few years, PSCs have rapidly developed with the record certified power conversion efficiency (PCE) of 25.2% [9]. Generally, mesoporous scaffold and planar heterojunction (PHJ) structure are two representa tive device architectures in most state-of-the-art PSCs [10–13]. Now days, SnO2 was widely used as electronic transport material in PHJ PSCs due to its fast electronic collection rate, great optical properties and suitable energy level [14–16]. Recently, You’s group reported high performance surface passivated PSCs employing SnO2 as electronic transport layers (ETLs) with certified PCEs over 23% [13]. In another aspect, perovskite PDs have also been investigated with various archi tectures and response properties. The detecting wavelength region of PDs can be tuned from near ultraviolet to near infrared, and the material morphologies include single crystal and polycrystalline films [4–6]. Considering SnO2 based PSCs, many efforts have been explored to improve the performance and stability for further commercialization, such as architecture design, composition engineering, robust hole transport materials application and interfacial modification [3,10,12, 17–21]. In particular, interfacial engineering has been proved to be an effective route to improve devices photovoltaic properties by improving charge transport, reducing defect density, suppressing carrier recombi nation and facilitating the perovskite crystal growth [22–24]. Recently, a series of studies reported the interfacial passivation between SnO2 and perovskite to reduce the defect density and achieve better contact for high-performance PSCs [23–29]. Among them, two dimension (2D) materials and cross-linkable fullerene materials were widely investi gated as interface modulation reagent in efficient PSCs that could improve the film quality of following perovskite layer and promoting charge transfer kinetics as well [24,30,31]. However, the commonly used 2D interfacial materials are graphene and cross-linkable fullerenes, which are expensive and difficult to con trol the growth. Here, we find that graphitic carbon nitride is a metalfree, earth-abundant and nontoxic material with a polymeric 2D struc ture [32,33]. It has been widely used in pollutant degradation, water splitting and solar energy transfer due to its outstanding photocatalytic and optoelectronic properties [34–38]. Very recently, similar kinds of material have been incorporated into perovskite precursor to improve the crystal quality of perovskite films by slowing down the crystalliza tion rate and passivating the carrier recombination centers [39,40]. However, graphitic carbon nitride has rarely been employed in fully ambient-process planar PSCs and PDs, ignored its favorable charge transporting and defect passivating properties. In this work, fluorescent graphitic carbon nitride quantum dots (gCNQDs) with 2D cross-linkable framework were synthesized and intro duced at the SnO2/perovskite interface in fully air-processed PSCs and PDs, and it realized several favorable photoelectric properties. Firstly, the surface roughness of SnO2 ETL decreased by the g-CNQDs modifi cation, thus to be beneficial for the growth of perovskite absorber. Due to the intrinsic 2D cross-linkable structure and the smoother surface of ETL, high quality perovskite films were achieved with homogeneous surface and less grain boundaries. Furthermore, photogenerated charges could diffuse more fluently through the ETL/perovskite interface, benefited from the gradient energy level alignment. As a result, fully air-
2. Results and discussion Fluorescent g-CNQDs solution was prepared by filtering the resultant yellowish powders, which were synthesized by the mixture of urea and sodium citrate, in ethanol solution (experimental section) [32]. The size of obtained g-CNQDs was detected by transmission electron microscopy (TEM). As shown in Fig. 1a, the TEM image reveals that the g-CNQDs have a diameter in the range of 10–30 nm, and are well monodispersed. The ultraviolet–visible (UV–vis) absorption spectra and steady-state fluorescent spectra were performed to investigate the optical proper ties of the collected g-CNQDs. As depicted in Fig. 1b, the UV–vis ab sorption spectrum presents a characteristic absorption peak at 327 nm and 404 nm for g-CNQDs. Moreover, the emission peak at 545 nm for g-CNQDs was observed in the steady-state fluorescent spectra, which can be attributed to the optical selection of nanoparticles (quantum ef fect). The digital picture g-CNQDs dispersed in ethanol shows that the fluorescent g-CNQDs exhibit strong green fluorescent under 365 nm UV light. The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectrometer are further conducted to investigate the chemical structure and composition of the achieved g-CNQDs. As shown in Fig. 1c, XPS spectrum of g-CNQDs in C 1s region demonstrates two peaks at 285 eV and 288 eV, which can be ascribed to sp3 C–C bonds and – C–N bonds, respectively [32,41]. In particular, the energy peak sp2 N– – C–N bond demonstrates the basic sub at 288 eV related to sp2 N– structure for the information of the 2D polymeric material of g-CNQDs. Furthermore, Fig. 1d shows the N 1s spectrum of g-CNQDs, which can be – N–C bonds. Thus, deconvoluted into one peak at 400 eV assigned to C– the presence of those groups suggests that the heptazine heterocyclic ring unite is the basic substructure of g-CNQDs. These results are also supported by FT-IR spectra (Fig. 1e), where the characteristic band around 788 cm 1 is ascribed to the heptazine units and the bands at 1401 cm 1 and 1441 cm 1 correspond to the typical stretching modes of CN heterocycles [32]. To further demonstrate the influence of g-CNQD to SnO2 film, the surface scanning electron microscope (SEM) images of SnO2 and 1/3 gCNQD modified SnO2 (hereafter named as SnO2/g-CNQD) were shown in Figs. S1a and b. There is no big difference between them, which probably due to the ultra-thin thickness of g-CNQD layer that might not be easily detected by SEM. Thus, in order to confirm the information of g-CNQD on the surface of SnO2 layer, the energy dispersive spectrum (EDS) and distribution diagram images of element of g-CNQD modified SnO2 layer are conducted as shown in Fig. 2a. The binding energy peaks at 0.26, 0.52 and 3.45 Kev, corresponded to C, N and Sn elements, and the uniform distribution of elements at the substrate reveal that the gCNQDs were conformably coated on SnO2 layer. Furthermore, the atomic force microscope (AFM) is conducted to demonstrate the influ ence of g-CNQD to the surface of SnO2 layer, as provided in Fig. 2b. The root mean square roughness (Rq) value of ETL obviously decreased from 17.5 nm to 12.8 nm by the incorporation of g-CNQD, suggesting ETL with a more uniform and smooth surface was achieved, which is bene ficial for the growth of the following perovskite absorber [39]. The 2
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surface morphologies of perovskite deposited on pristine SnO2 and SnO2/g-CNQD ETL are presented in Fig. 2c to explore the influence of the g-CNQD modifying layer on the perovskite film. Obviously, high quality perovskite film with less grain boundaries is obtained for SnO2/g-CNQD ETL, which could be attributed to passivation effect of g-CNQDs facilitating the crystal growth [42–44].Furthermore, as shown in Fig. 2d, the X-ray diffraction (XRD) patterns of perovskite based on pristine and g-CNQD modified SnO2 ETL show that both films present a similar crystalline lattice of the cubic perovskite phase. Nevertheless, higher peak intensity of the main peak around 14� and 28� , corre sponded to the (110) and (220) crystal plane of cubic phase, can be observed for the perovskite film deposited on SnO2/g-CNQD ETL [13,16, 45]. This indicates a better crystallinity upon interface modulation, which might be ascribed to the preferred crystal orientation by the 2D reticulate structure of g-CNQD. Moreover, an impure PbI2 peak (2θ ¼ 12.7� ) can be founded in the SnO2/perovskite film, while undistin guished in the SnO2/g-CNQD/perovskite sample, implying that g-CNQDs could facilitate the growth of perovskite phase. The above results confirm that the uniform and smoother perovskite films with better crystallization and higher phase purity are achieved by the g-CNQD modification.
We expect a better performance of g-CNQD modified PSCs consid ering the higher quality perovskite absorber than the control film based on pristine SnO2 ETL. Thus, we fabricated planar PSCs to confirm the interface modification of g-CNQDs. The ultra-thin g-CNQD layer was deposited on SnO2 electron transport layer by spin coating. Devices with configuration of FTO/SnO2/g-CNQD/Perovskite/Spiro-OMeTAD/Au were fabricated in atmosphere environment, the scheme and corre sponding cross-sectional SEM image, of the g-CNQD modified device shown in Fig. 3a. We checked the performance of devices based on different thickness of g-CNQD modifying layer by controlling the con centration of g-CNQD in anhydrous ethanol, and found that the optimal concentration is 1/3 g-CNQD, which was diluted for 3 times from the saturation solution of g-CNQDs. Hereafter, the 1/2, 1/3 and 1/6 g-CNQD solutions were defined as diluting the saturated g-CNQDs solution with 2, 3 and 6 times of ethanol, respectively. The photovoltaic performance of PSCs based on bare and 1/3 g-CNQD modified SnO2 ETL are compared in Fig. 3. The current density-voltage (J-V) curves of those devices are presented in Fig. 3b, and their corresponding photovoltaic parameters of VOC, short circuit current density (JSC), fill factor (FF) and PCE are summarized in Table S1. The pristine SnO2 ETL based device exhibits a VOC of 1.111 V, a JSC of 22.68 mA cm 2 and a FF of 74.36%,
Fig. 1. Characterizations of the obtained g-CNQDs. a) TEM image b) UV–vis absorption (red line) and photoluminescence (black line) spectra of the g-CNQDs. The inset is a digital picture of g-CNQDs dispersed in ethanol. XPS spectra of g-CNQD in c) C 1s region and d) N 1s region. e) Fourier transform infrared characterization of the g-CNQDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3
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yielding a PCE of 18.74%. The device based on SnO2/g-CNQD ETL displays a relatively better performance: a PCE of 20.30% with a VOC of 1.128 V, a JSC of 23.41 mA cm 2 and a FF of 76.90%. The increased VOC and FF can be mainly attributed to the better contact and reduced trap density at the ETL/perovskite interface, resulting in the better quality of perovskite film. The enhanced JSC from 22.68 mA cm 2 to 23.41 mA cm 2 can be ascribed to the promoted electron extraction and trans portation caused by the reduced grain boundaries of perovskite film. Furthermore, the corresponding external quantum efficiency (EQE) plots and integrating current curves are displayed in Fig. 3c. The gCNQD modified device exhibits higher EQE spectrum than the device without modification over 500 nm wavelength, confirming the increased JSC by the g-CNQD interlayer that could facilitate photo generated charge carriers flow through the film. The reproducibility were verified by characterizing 50 PSCs based on pristine SnO2 and SnO2/g-CNQD ETL, as depicted in Fig. 3d. The higher PCE and better reproducibility for the devices with g-CNQD interlayer reveal that high performance PHJ PSCs with g-CNQD modified SnO2 ETL can be easily repeated even in ambient air with uncontrollable temperature and humidity. The carrier dynamics, recombination behavior and trap-state density in devices were further investigated to clarify the mechanism of the influence of g-CNQD modifying layer. Fig. 4a compares the dark current density of PSCs with bare and 1/3 g-CNQD modified SnO2 ETLs. The lower current density leakage can be observed for the modified device, suggesting the current leakage in the device is prevented to a certain extent and the charge recombination rate is reduced by the consequent
high quality film. Furthermore, to confirm the charge transport and recombination kinetics in the devices, electrochemical impedance spectroscopy (EIS) was carried out on the PSCs based on SnO2 and SnO2/ g-CNQD ETL under the one sun illumination condition at the given bias of corresponding open-circuit voltage. As shown in Fig. 4b, the Nyquist plots of PSCs display a two-semicircles features, the semicircle at the high frequency of 220 KHz can be ascribed to the transfer resistance (Rtr), while the semicircle at the low frequency of 1.5 Hz can be attributed to the recombination resistance (Rrec) [46,47]. Fitted with the simplified equivalent circuit model as shown in the inset, the corre sponding resistance parameters are obtained and summarized in Table S2. The SnO2 ETL based device shows a higher Rtr (92 Ω) than the SnO2/g-CNQD based one (Rtr ¼ 77 Ω), implying a faster charge transport in SnO2/g-CNQD modified device. Moreover, a higher Rrec is obtained of the PSC with g-CNQD modification, suggesting the suppressed carrier recombination. The lower Rtr and higher Rrec could be responsible for the better performance of SnO2/g-CNQD based PSCs with more favor able charge extraction properties and much suppressed carrier recom bination rate. The steady-state photoluminescence (PL) spectra of perovskite films on pristine and g-CNQD modified SnO2-coated FTO substrates are displayed in Fig. 4c. Compared to the SnO2-based perovskite film, a significantly decreased PL intensity is observed for the sample with g-CNQD interlayer, revealing the enhanced electron extraction ability by the modification of g-CNQDs, which is consistent with the results of dark current and EIS [48]. In addition, the Hall mobility μH of SnO2 (electron transport layer), SnO2/g-CNQD and spiro-OMeTAD (hole transport layer) films have been determined from
Fig. 2. The surface morphology of SnO2(/g-CNQD) and the crystallinity and morphology of perovskite films coated on them. a) EDS images of g-CNQDs modified SnO2 layer. b) AFM images of pristine SnO2 film and SnO2/g-CNQD film. c) SEM images of perovskite films based on SnO2 and SnO2/g-CNQD ETL. d) XRD spectra of perovskite films. 4
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Fig. 3. Architecture of the PSCs by g-CNQD modification and comparison between PSCs with/without g-CNQD modification. a) Schematic illustration and cross-section SEM image of the PSCs with g-CNQD modified SnO2 layers, the scale bare is 1 μm. b) The J-V curves, c) EQE spectra and integrated current density curves and d) histogram of efficiencies of PSCs with/without g-CNQD modification.
Hall measurements. As summarized in Table S3, the μH of SnO2 and SnO2/g-CNQD films are 9.62 � 10 2 cm2V 1s 1 and 9.33 � 10 1 cm2V 1s 1, respectively. It is notable that the mobility is significantly enhanced by the modification of g-CNQD, suggesting the improvement of carrier transport ability for ETL. Furthermore, compared to the perovskite film coated on SnO2 layer, a slight blue shifting of PL emis sion peak can also be found by the incorporation of g-CNQD, suggesting the reduced defects in SnO2/g-CNQD based film because of suppressing of the spontaneous radiative recombination between trap states. Because the spontaneous radiative recombination between trap states leads to a red-shifted emission peak compared with that from the band edge transition [49,50]. In addition, as depicted in Fig. 4d, the carries lifetime of 79 ns and 38 ns were evaluated by time-resolved PL (TRPL) for perovskite films coated on SnO2 and SnO2/g-CNQD ETL, respectively [8]. The shorter carrier lifetime for the g-CNQD modified sample further confirms the enhanced electron extraction and transportation proper ties, which could be responsible for the higher JSC of SnO2/g-CNQD based PSCs [51]. To assess the electron transport and defect density, electron-dominated devices (FTO/SnO2 (/g-CNQD)/per ovskite/PCBM/Ag) have been fabricated and measured; (here PCBM is phenyl-C61-butyric acid methyl ester). The current-voltage (I–V) curves of SnO2 and SnO2/g-CNQD based devices are displayed in Fig. 4e and f, respectively. The trap-state density (Nt) can be calculated by the trap-filled limit voltage (VTFL) method using the equation [52]:
CNQD with different g-CNQD concentration were systematically inves tigated by photovoltaic and photoelectronic tests. The statistics of VOC, JSC, FF and PCE of devices based on pristine SnO2 or g-CNQD modified SnO2 ETL are presented in Fig. 5a and listed in Table S4. The PSCs based on pristine SnO2 ETL show an averaged VOC of 1.074 � 0.018 V, JSC of 22.63 � 0.51 mA cm 2 and FF of 72.35 � 2.35%, yielding an averaged PCE of 17.58 � 0.84%. In contrast, the 1/3 g-CNQD modified SnO2 ETL based devices exhibit the best performance with an average PCE of 18.88 � 0.57%, resulted by an averaged VOC of 1.111 � 0.022 V, the highest JSC of 22.98 � 0.43 mA cm 2 and the highest FF of 74.00 � 1.43%. The XRD results demonstrate that the film quality first increase and then decrease as a function of the concentration of g-CNQD. Compared with the film on pristine SnO2 ETL, perovskite samples based on 1/6 g-CNQD and 1/3 g-CNQD exhibit better crystallization with preferred crystal orientation (Fig. S2). However, the peak intensity of the main peaks around 14� and 28� decreased by 1/2 g-CNQD modifi cation. The decrease of film quality might be caused by the difficulty of film formation on thick g-CNQD layer, which will become cluster of monodispersed QDs with high concentration at the surface of SnO2. Moreover, the transmittance spectra of FTO, pristine SnO2 and SnO2/gCNQD ETL coated on FTO substrate were shown in Fig. S3a. The transmittance spectra plots of pristine SnO2 is almost same as the gCNQD modified SnO2 one, indicating there is no other light loss occurred by the g-CNQD interlayer. Fig. S3b shows the UV–vis absorption spectra of perovskite films based on different substrate. Due to the influence of g-CNQD on the quality of perovskite film, the tendency of UV–vis ab sorption is consistent with the result of XRD patterns, showing a first increase and then decrease trend. In addition, the EQE spectra of devices are mainly related to the UV–vis absorption of perovskite films. There fore, higher EQE and current density of PSCs are obtained with 1/6 and 1/3 g-CNQD modification, as presented in Fig. 5b. Moreover, PL mea surements of perovskite coated on ETL with different g-CNQD concen tration were preformed to investigate the carrier dynamics and recombination behavior in devices with different g-CNQD concentra tion. As shown in Fig. S4, the emission peak intensity of perovskite film was decreased by 1/6 and 1/3 g-CNQD modification, and increased by
Nt ¼ 2εoεrVTFL/qL2 Where εo and εr are the vacuum permittivity and relative dielectric constant, respectively, q is the elemental charge of the electron, L is the corresponding thickness of perovskite film (~700 nm). The VTFL of samples based SnO2 and SnO2/g-CNQD layer are 0.14 V and 0.095 V, respectively. Therefore, the trap-state density is reduced from 1.01 � 1015 cm 3 to 6.67 � 1014 cm 3 after g-CNQD modification, revealing the quality of perovskite film was improved, which is beneficial to the performance of PSCs. To further confirm the influence of g-CNQD intercalated into the SnO2/perovskite interface, PSCs based on pristine SnO2 and SnO2/g5
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Fig. 4. Carrier transport and recombination of PSCs with/without g-CNQD modification. a) J-V curves and b) Nyquist plots for PSCs with or without g-CNQD modification were measured under the one sun illumination condition at the given bias of each open-circuit voltage. c) Steady-state PL and d) TRPL of perovskite films coated on SnO2 and SnO2/g-CNQD ETL. Trap-state density of perovskite based on e) SnO2 and f) SnO2/g-CNQD ETL.
1/2 g-CNQD modification. Furthermore, carriers in perovskite coated on 1/6 and 1/3 g-CNQD modified SnO2 films exhibit shorter lifetime values than the pristine sample, while the carries lifetime was increased by 1/2 g-CNQD modification, as depicted in Fig. 5c. Those results reveal that gCNQD layer with proper thickness enhances electron extraction ability and reduces carrier recombination, while the g-CNQD may form big clusters with high concentration, thus result in more carrier recombi nation and deteriorate electron extraction. The hysteresis behavior and stability of the optional devices based on 1/3 g-CNQD modified SnO2 ETL were further investigated. Fig. 5d shows the J-V curves of device measured under one sun illumination with a sweep speed of 100 mV s 1. This PSC exhibits a PCE of 21.23 (20.76)% with a VOC of 1.140 (1.135) V, a JSC of 23.39 (23.65) mA cm 2 and a FF of 79.62 (77.34)%, when measured under reverse (forward) voltage scans, indicating the negligible hysteresis for the g-CNQD modified de vices. As presented in Fig. 5e, the unencapsulated PSCs with SnO2/gCNQD ETL exhibits a steady-state PCE of 20.5% at maximum power point under continuous operation for 90 s, indicating the good illumi nation stability for g-CNQD modified PSCs. The long-term stability was displayed by the performance evolution of the unencapsulated devices stored in ambient air under room light with a humidity of ~30% upon 30 days’ exposure, as shown in Fig. 5f. And the J-V curves of this device
at different times are depicted in Fig. S5. The PCE of g-CNQD modified PSC keeps over 90% of initial performance after 30 days, suggesting the outstanding ambient stability against moisture and light. Taking the advantages of high quality perovskite film, we fabricated perovskite PDs with and without g-CNQD modification applying a simple structure as shown in the inset of Fig. 6a and b. A 532 nm laser was used as the light source, and the current-voltage (I–V) character izations of PDs were performed under different illumination power densities, ranging from 0.3 to 372.2 mW cm 2 with natural filters. The photocurrent has a linear relationship with voltage for both devices, as show in Fig. 6a and b. Stronger illumination provides a higher photo current for both PDs, which could be ascribed to the higher density of photogenerated carriers flow through the external circuit under a given bias. Compared to the pure SnO2 based perovskite detector, the g-CNQD modified one show relatively higher response irrespective to the illu mination power, implying that the later one has better responsiveness. As presented in Fig. 6c, the responsivity (R) of PD under different voltage and illumination were also calculated with the formula [4]: R¼ (Ilight-Idark)/(Pin � S) Where Ilight is the photogenerated current under light, Idark is the dark current, Pin is the power of incident light, and S is the working area (here 6
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Fig. 5. Photovoltaic and photoelectronic tests of PSCs with different concentration of g-CNQD, the hysteresis behavior and stability of devices based on SnO2/g-CNQD ETL. a) PV parameters of VOC, JSC, FF and PCE of PSCs with different concentration of g-CNQD. b) EQE spectra and integrated current density curves of devices. c) TRPL of perovskite films based on different concentration of g-CNQD. d) J–V curves measured under reverse and forward voltage scans with a scan rate of 100 mV s 1. e) Steady PCE of the best device at a maximum power point under one sun illumination. f) Long-term stability of the unencapsulated PSCs in ambient air (temperature�30 � C, humidity�30%).
defined as 0.027 cm2). R is an important parameter indicating the ability of detector responding to the optical signal. Since the glass substrate and metal electrode as well as the fabrication processes are identical, the difference can be only related to the g-CNQD modification. As we have discussed above, the high quality perovskite film was achieved by gCNQD modification, thus leading to the better photoconductivity of the film under illumination due to the lower defects. The transient photo current responses of PDs with and without g-CNQD modification measured under 372.2 mW cm 2 laser irradiation with a chopper of 0.5 s certain on/off interval are displayed in Fig. 6d. It can be obviously seen that the photocurrent was drastically enhanced due to the hindering of charge recombination at interface by the g-CNQD modification. The rise and decay times of the modified PD are calculated to be 81 and 41 m s, respectively, which are reasonable for our PDs fabricated in air with such a simple device structure.
surface of the g-CNQD layer that could facilitate the crystal growth and charge transportation. PSCs with good performance of increased JSC, VOC and FF are achieved by the favorable charge transport, low trap states and suppressed charge recombination upon proper g-CNQD modulation. Our best PSC exhibits a PCE up to 21.23% with negligible hysteresis and robust working stabilities against moisture and illumi nation out of glovebox. Taking the benefit of interface modification, efficient PDs were also obtained with higher current and faster response speed compared to the pristine ones. This work demonstrates a feasible route to achieve high-performance PSCs and PDs by interfacial engi neering of 2D polymeric material. 4. Experimental section Materials: PbI2(TCI, 99.99%), SnO2 colloid precursor (Alfa Aesar), N, N-dimethylformamide (DMF) (Aladdin, 99.8%) and dimethyl sulfoxide (DMSO) (Aladdin, 99.8%) were employed directly without any treat ment. Urea and sodium citrate were purchased from Sinopharm. For mamidinium iodide (FAI), methylammonium bromide (MABr), methylammonium chloride (MACl), Cesium Iodide (CsI) and 2,20 ,7,70 tetrakis(N,N-dip-methoxyphenylamine)-9,90 -spirobifluorene (spiroOMTAD) were all purchased from Xi’an Polymer Light Technology Corp and employed directly.
3. Conclusion In summary, fluorescent g-CNQDs are employed as modifying re agent at the SnO2/perovskite interface in fully air-process planar PSCs and PDs to enhance the photoelectric properties. High quality perovskite films with better surface morphology, less grain boundaries and purer phase were obtained by the intrinsic cross-linkable feature and smooth 7
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Fig. 6. Photoelectronic tests of PDs with/without g-CNQD modification. The current-voltage curves of PDs a) with and b) without g-CNQD modification under different illumination power densities. c) The responsivity of PDs with/without g-CNQD modification under different voltage and illumination. d) The transient photocurrent responses of devices with and without g-CNQD modification under 372.2 mW cm 2 laser irradiation with a chopper of 0.5 s certain on/off interval.
Synthesization of graphitic carbon nitride dots (g-CNQDs) solution: 1.68 mmol urea and 0.28 mmol sodium citrate were mixed in an agate mortar and grounded ~30 min into uniform powders. Then the mixture was put into railboat and heated at 180 � C for 60 min in a drying oven. The resultant yellowish powders were placed in an agate mortar and grounded ~30 min. After that, fluorescent g-CNQDs solution was ach ieved by dissolving the mixture in ethanol and filtering by 0.45 μm Nylon needle. Then, the 1/2, 1/3 and 1/6 g-CNQD solutions were defined as diluting the as-prepared saturated g-CNQDs solution with 2, 3 and 6 times of ethanol, respectively. Fabrication of the perovskite solar cells: The PSCs were fabricated with a structure of FTO/SnO2/g-CNQD/Perovskite/Spiro-OMAD/Au in ambient air with a relatively high humidity (~40%). The etched fluorine-doped tine oxide (FTO) glass was ultrasonically cleaned with acetone and ethanol for 20 min several times. Before using, the FTO glass was treated with UVO for 15 min. Then, a thin layer was deposited onto the substrate by spin-coating SnO2 nanoparticle solution at 3000 rpm for 30 s, and annealed at 150 � C for 30 min. After cooled down to room temperature, the g-CNQD solution with different concentration was spin coated onto the SnO2 film at 3000 rpm for 30 s, and a thermal annealing at 150 � C for 30 min. After that, 1.3 M of PbI2 in (DMF:DMSO ¼ 9.5:0.5) with amount of CsPbI3 additive was spin coated onto ETL at 1500 rpm for 30 s, and a thermal treatment at 70 � C for 1 min. After cooled down, the mixture solution of FAI:MABr:MACl (60 mg:6 mg:6 mg in 1 ml IPA) was deposited onto PbI2 at a speed of 1300 rpm for 30 s, followed by annealing at 150 � C for 15 min. The hole transport layer, spiro-OMTAD, was coated onto the perovskite layer by 4000 rpm for 30 s. At last, ~100 nm thick Au counter electrode was deposited by thermal evaporation. Fabrication of the perovskite photodetectors: Soda lime glass is used as the substrate, and cleaned with detergent, water, alcohol and acetone successively, assistant with ultrasonic batch. SnO2, g-CNQD and
perovskite layers were deposited the same as above. ~100 nm thick Au counter electrode was deposited by thermal evaporation with a mask, the width of two electrodes was chosen to be 90 μm. Measurements and Characterization: The SEM and EDS images were characterized on JEM 2100. The TEM image of g-CNQDs was measured by JEM-2010 FEF transmission electron microscope at an acceleration voltage of 200 KV. Transmittance and absorbance spectra were carried out on UV–vis spectrometer (Lambda 650s, PerkinElmer). The emission spectra of g-CNQDs was recorded on a fluorescence spectrophotometer (F-4600, Hitachi, Japan) with parameters involving an excitation wavelength of 360 nm, a scanning speed of 240 nm min 1, and a pho tomultiplier tube (PMT) voltage of 700 V. The XPS spectra was tested on ESCALAB 250XLwith Al Kα radiation of 1486.6 eV as the excitation source. Infrared spectra was obtained by Fourier transform infrared spectrometer (FT-IR, Nicolet iS10, Thermo Fisher, USA) with scanning range of 525–4000 cm 1. The surface roughness of electronic transport layers was detected by atomic force microscopy (AFM, Bruker Multi mode 8). The perovskite crystal structure was tested by an XRD spec trometer (D8 Advance, Bruker AXS, Germany) with Cu Kα radiation. The current density-voltage curves were observed by a sunlight simulator (Newport, 91192) under one sun illumination (AG 1.5G, 100 mV cm 2). Effective area of the solar cells was defined to be 0.09 cm2 using a metal mask. The external quantum efficiency spectra was performed from 300 nm to 900 nm with a 300 W xenon lamp (Newport 66984). For the performance characterizations of PDs, a 532 nm semiconductor laser is used as the illumination source with power density determined by PM100D. The performance of photodetectors is measured by a keysight B1500A semiconductor device analyzer system assisted with a probe station. The Steady-state and time-resolved PL (TRPL) was investigated on Delta Flex fluorescence spectrometer (HORIBA).
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Declaration of competing interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is in Wuhan University financially supported by the Sci ence and Technology Department of Hubei Province (No. 2019AAA020), the National Natural Science Foundation of China (No. 61974106), the Fundamental Research Funds for the Central Univer sities (2042019kf1003) and the National Key R&D Program (Grant 2016YFC1000701). The work in Soochow University is financially supported by Natural Science Foundation of Jiangsu Province (BK2019085) and Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (19KJB480003). We would like to acknowledge the Center for Electron Microscopy at Wuhan University for their substantial supports to SEM, TEM and XRD work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227825. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] M.A. Green, A. Ho-Baillie, H.J. Snaith, Nat. Photon. 8 (2014) 506–514. [3] M. Saliba, T. Matsui, K. Domanski, J. Seo, A. Ummadisingu, S.M. Zakeeruddin, J. P. Correa-Baena, W. Tress, A. Abate, A. Hagfeldt, M. Gr€ atzel, Science 354 (2016) 206–209. [4] L. Dou, Y.M. Yang, J. You, Z. Hong, W.H. Chang, G. Li, Y. Yang, Nat. Commun. 5 (2014) 5404. [5] Y. Fang, J. Huang, Adv. Mater. 27 (2015) 2804–2810. [6] M.I. Saidaminov, V. Adinolfi, R. Comin, A.L. Abdelhady, W. Peng, I. Dursun, M. Yuan, S. Hoogland, E.H. Sargent, O.M. Bakr, Nat. Commun. 6 (2015) 8724. [7] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Nature 517 (2015) 476–480. [8] Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao, J. Huang, Science 347 (2015) 967–970. [9] Best research-cell efficiencies. https://www.nrel.gov/pv/cell-efficiency.html. [10] M.M. Lee, J. Teuscher, t. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643–647. [11] M. Kim, G.H. Kim, T.K. Lee, I.W. Choi, H.W. Choi, Y. Jo, Y.J. Yoon, J.W. Kim, J. Lee, D. Huh, H. Lee, S.K. Kwak, J.Y. Kim, D.S. Kim, Joule 3 (2019) 2179–2192. [12] M. Liu, M.B. Johnston, H.J. Snaith, Nature 501 (2013) 395–398. [13] Q. Jiang, Y. Zhao, X. Zhang, X. Yang, Y. Chen, Z. Chu, Q. Ye, X. Li, Z. Yin, J. You, Nat. Photon. 13 (2019) 460–466. [14] C. Wang, D. Zhao, C.R. Grice, W. Liao, Y. Yu, A. Cimaroli, N. Shrestha, P.J. Roland, J. Chen, Z. Yu, P. Liu, N. Cheng, R.J. Ellingson, X. Zhao, Y. Yan, J. Mater. Chem. 4 (2016) 12080–12087. [15] G. Yang, C. Chen, F. Yao, Z. Chen, Q. Zhang, X. Zheng, J. Ma, H. Lei, P. Qin, L. Xiong, W. Ke, G. Li, Y. Yan, G. Fang, Adv. Mater. 30 (2018), e1706023. [16] Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Nat. Energy 2 (2016) 1–7. [17] Z. Yu, B. Chen, P. Liu, C. Wang, C. Bu, N. Cheng, S. Bai, Y. Yan, X. Zhao, Adv. Funct. Mater. 26 (2016) 4866–4873.
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