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CsPbBr3 inverse opal heterostructure perovskite solar cells
Journal of Power Sources 439 (2019) 227065
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Fluorescence resonance energy transfer effect enhanced high performance of Si quantum Dots/CsPbBr3 inverse opal heterostructure perovskite solar cells Shujie Zhou 1, Rui Tang 1, Hui Li, Lin Fu, Bo Li, Longwei Yin * Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan, 250061, PR China
H I G H L I G H T S
� Multi-dimensional Si QDs/CsPbBr3 IO perovskite solar cell has been fabricated. � FRET provides additional energy transfer path which benefits the light utilization. � Si QDs/CsPbBr3 heterojunction gives rise to an effective charge transfer process. � FRET and Si QDs/CsPbBr3 heterojunction devote to a competitive PCE up to 8.31%. A R T I C L E I N F O
A B S T R A C T
Keywords: CsPbBr3 Perovskite solar cells Fluorescence resonance energy transfer Heterostructure
CsPbBr3 based perovskite solar cells draw boosted investigation benefitting from their simplified preparation property and outstanding stability against moist and heat while the photo-electrical conversion efficiency (PCE) is still worth promotion. In addition, relatively wide band gap limits the light utilization ability of pristine CsPbBr3 which leads to insufficient photo-induced charge carrier population thereby a low photocurrent density. Herein, we for the first time demonstrate a strategy to combine crystalized Si quantum dots (QDs) with CsPbBr3 inverse opal (IO) which significantly enhances the solar energy utilization efficiency by virtue of providing an additional fluorescence resonance energy transfer (FRET) process from Si QDs to CsPbBr3 IO. Acting as donor, the emitted photoluminescence from Si QDs can be absorbed by CsPbBr3, which serves as acceptor, leading to an increased carrier population in the system. Meanwhile, the multi-dimensional heterojunction between Si QDs and CsPbBr3 IO effectively facilitates the system bulk charge transfer process. A greatly improved PCE up to 8.31% with an obviously enhanced photocurrent density up to 7.8 mA⋅cm 2 can be obtained with a competitive IPCE up to 81%. This strategy provides a new alternative method to develop high-performance perovskite solar cells and other photo-electronic devices.
1. Introduction In the past decade, organic-inorganic perovskite solar cells (PSCs) have gone through a huge leap on the photo-electrical conversion effi ciency (PCE), which has been increased to 23% [1–4], attracting more and more attention all around the world. However, the critical inherent defect of poor stability greatly hinders the industrial application of organic-inorganic PSCs, especially under humid environment or at high temperature [5,6]. For this, one of the most reliable methods is to replace the organic cation with other cations such as cesium, which
would not cause decomposition under high temperature, enabling an improved long-term stability of inorganic PSCs [7,8]. CsPbBr3 has been considered as a competitive candidate for inor ganic PSCs, benefiting from its fast carrier mobility [9,10], excellent long-term phase stability [11,12] and facile preparation methods [13]. Nevertheless, the PCE performance of pristine CsPbBr3 based PSCs re mains unsatisfying because of the following issues: poor light utilization ability originated from its relatively wide band gap [14,15] and high carrier recombination rate [16,17]. Therefore, great effort has been made to further improve the PCE performance of CsPbBr3 based PSCs,
* Corresponding author. E-mail address: [email protected] (L. Yin). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.227065 Received 15 April 2019; Received in revised form 26 June 2019; Accepted 23 August 2019 Available online 5 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Fabrication processes of Si QDs and Si QDs/CsPbBr3 IO.
among which, constructing heterostructures between CsPbBr3 with other narrow band matched semiconductors should be a potential strategy [18,19], resulting in a broadened light absorption range and a facilitated carrier transfer process [20]. Si quantum dots (QDs) have been considered as a widely used semiconductors which have tunable band gap, high carrier mobility and excellent photostability, as well as its abundant and non-toxic features [21–24]. In addition, it should be pointed out that Si QDs are also known as an effective fluorescence resonance energy transfer (FRET) donor material when they are hybrid with other acceptor semiconductors [25,26]. Namely, Si QDs are able to emit in visible frequencies after absorbing the solar spectrum originated from the quantum confinement effect; that is to say, the photo luminescence emitted by Si QDs can be reused via a donor-to-acceptor FRET system with less energy-transfer loss, compared with simple het erostructures [27,28]. For example, Thiyagu Subramani and coworkers reported that by means of employing nanocrystalline Si quantum dots and Si nanotips hybrids, the PCE performance of organic solar cells has been greatly improved resulting from the efficient FRET energy man agement [29]. Hence, constructing heterostructure between CsPbBr3 and Si QDs should not only facilitate the charge transfer process of pristine CsPbBr3 but also help to enhance the solar spectrum utilization efficiency of pristine CsPbBr3. However, to the best of our knowledge, prior literature has not reported any works related to the Si QDs coupled PSCs yet and seldom have any researches paid attention to the FRET process of Si QDs coupled perovskites system. In addition, rationally structural engineering can also help with the light utilization and charge transport. For this, inverse opals (IOs) are a kind of three-dimensional (3D) ordered photonic crystals consisting of
periodically close-packed arrays of voids that are surrounded by solid materials with different refractive index [30]. This periodical macro-porous structure on the one hand would contribute to the inci dent light scattering process [31,32], on the other hand, it provides direct charge transfer tunnels, which can effectively suppress the free carrier recombination, resulting in efficient charge transfer process [33]. In our previous work, we have proved that by introducing the CsPbBr3 IO structure, solar light absorption ability as well as the carrier extrac tion and injection efficiency can be obviously enhanced comparing with the conventional planar structures, which devoted to an obviously increased PCE of CsPbBr3 IO based PSCs [34]. Recently, it has also been reported that by QDs sensitizing, the profiled 3D/0D interface in troduces an electric field in the rear side of the device to promote carrier extraction, resulting in an increased photocurrent and open voltage [35]. Hence, it should be a promising strategy to fabricate 0D Si QDs coupled 3D CsPbBr3 IOs and investigate the energy transfer and charge transfer process in the multi-dimensional Si QDs/CsPbBr3 IO PSCs. Herein, we demonstrated a facile method to prepare Si QDs sensi tized CsPbBr3 IOs (Si QDs/CsPbBr3 IO) hybrid PSCs with a polystyrene (PS) template assisted, spin-coating method. As obtained Si QDs exhibit tunable photoluminescence properties, by virtue of constructing a donor-to-acceptor FRET system, the Si QDs emitted photoluminescence can be absorbed by CsPbBr3, which can induce additional energy transfer between Si QDs and CsPbBr3. It makes the CsPbBr3 secondarily excited and results in greatly enhanced solar spectrum utilization effi ciency. At the same time, benefiting from the uniquely matched band gap structure at the CsPbBr3/Si interface, facilitated charge extraction and injection process can be witnessed in the Si QDs/CsPbBr3 IOs PSCs, 2
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Journal of Power Sources 439 (2019) 227065
Fig. 1. SEM image of (a) PS template, (b) CsPbBr3 IO, (c) SiQDs/CsPbBr3 IO. (d) XRD patterns of CsPbBr3 IO and SiQDs/CsPbBr3 IO.
which gives rise to an increased photocurrent intensity up to 7.8 mA⋅cm 2 and an open voltage of 1.42 V, with a greatly improved PCE up to 8.31%. We believe that our work provides a new perspective of energy management for the design of inorganic PSCs, which can also be expanded and applied in other photovoltaic devices.
transport materials (HTM) (spiro-MeOTAD) are spin-coated on the Si QDs/CsPbBr3 IO, followed by evaporating Ag as the counter electrode. Fig. 1a–c exhibit the SEM images of PS template, CsPbBr3 IO and Si QDs/CsPbBr3 IO. It can be seen in Fig. 1a that the PS template shows a uniformly periodical arrangements, with a PS diameter around 300 nm. After CsPbBr3 filling and template removal, it can be seen in Fig. 1b that the PS templates can be thoroughly moved, leaving the threedimensional scaffolds with a diameter of about 280 nm. In Fig. 1c, it can be seen that the surface of CsPbBr3 IO becomes rough with obvious granular sensation after loaded with Si QDs. Fig. 1d demonstrates the XRD patterns of the as-obtained Si QDs/ CsPbBr3 IO samples. It can be seen that both the CsPbBr3 IO and Si QDs/ CsPbBr3 IO samples display good crystallizability. The highest peak located around 30� can be indexed to the (200) plane of cubic CsPbBr3 and the other five peaks can also be in good accordance with the (100), (110), (111), (210), (211), (220) planes of cubic CsPbBr3 (PDF 54–0752) [36]. After coupling with Si QDs, the Si QDs/CsPbBr3 IO sample shows additional peaks located at around 33� , 52� and 56� , which can be assigned to the (211), (220) and (400) planes (PDF 17–0901) of Si QDs [37]. By virtue of changing the etching time, Si QDs with different di ameters can be obtained, which can be seen in the TEM images in Figs. S1a–c. It can be seen that with the etching time increase, the QDs diameters are gradually decreased. As-obtained Si QDs exhibit distinct photoluminescence under ultraviolet lamp, as can be seen in the inset figures, which are denoted as B–Si QDs, G-Si QDs and R–Si QDs for Si QDs with different colors of blue, yellow and red. In addition, the photoluminescence spectra of these samples are tested, which can be seen in Fig. S2. It can be seen that the emission peaks of different QDs are centered at 435, 553 and 630 nm, respectively, which are in accordance with the blue, yellow and red emission light. In order to further investigate the chemical bonding state, the X-ray photo-electron spectroscopy (XPS) of the obtained Si QDs/CsPbBr3 IO sample is tested. In the survey spectra (Fig. S3a), the existence of ele ments Cs, Pb, Br, Si, O and C can be confirmed. The existence of C element should be attributed to the carbon-based containment. It can be
2. Results and discussion 2.1. Material characterization Scheme 1 demonstrates the fabrication process of Si QDs/CsPbBr3 IO and the detailed experimental section is provided in the supporting information. Generally, Si QDs are prepared via a chemical etching method. Si nanoparticles are used as the starting material, which are firstly oxidized by HNO3 to formSiO2 shell. Then, they are etched by HF under ultra sonic bath, which can remove the SiO2 shell and reduce the core size of Si nanoparticles and form hydride-terminated Si nanoparticles. After that, it will go through such repeated process and finally Si QDs can be obtained. By means of changing the etching time, Si QDs with different sizes can be prepared. The Si QDs/CsPbBr3 IO solar cells can be fabri cated via a template assisted spin-coating method. Typically, FTO sub strates are cleaned thoroughly in advance, following by a UV-ozone treatment for 20 min to ensure hydrophilia. Firstly, blocking layer TiO2 (bl-TiO2) and mesoporous TiO2 (meso-TiO2) are respectively spincoated on the FTO substrate, followed by an annealing process. Sec ondly, the PS templates are self-assembled on the FTO/bl-TiO2/mesoTiO2 substrate by virtue of a gas-liquid interface assembling process. Thirdly, CsPbBr3 precursor is prepared in advance and infiltrated into the voids of PS template and CsPbBr3 is then crystallized in the voids under an annealing treatment. Fourthly, as obtained Si QDs are dispersed in toluene in advance with different concentration, followed by immersing the CsPbBr3 infiltrated template into the dispersion for PS removal and Si QDs loading. The Si QDs/CsPbBr3 IO with different Si QDs loading amount is denoted as Si QDs-1/CsPbBr3 IO, Si QDs-2/ CsPbBr3 IO and Si QDs-3/CsPbBr3 IO, respectively. After that, hole 3
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Journal of Power Sources 439 (2019) 227065
Fig. 2. (a) UV–vis absorption spectrum and normalized photoluminescence spectrum of CsPbBr3 IO and Si QDs, respectively. (b) Steady state PL spectra of Si QDs, Si QDs/CsPbBr3 IO and CsPbBr3 IO. The excitation wavelength is 300 nm.
seen from Figs. S3b–d that the high-resolution core-level spectra show the peaks of Cs 3d (3d3/2: 737.9 eV and 3d5/2: 724.6 eV), Pb 4f (4d5/2: 143.1 eV and 4d7/2: 138.3 eV) and Br 2p (2p1/2: 189.1 eV and 2p3/2: 182.0 eV) respectively, in accordance with the previous reports of CsPbBr3 [38,39]. Also, the Pb 4d5/2 peak can be split into an additional peak at 144.9 eV, which should result from the oxygen-containing functional group such as Pb–O bond on the surface of CsPbBr3. It can be seen in Fig. S3e that the Si 2p peak can be split into two peaks which can be assigned Si–H bond and Si-Si bond, indicating the formation of H-terminated Si QDs. In addition, the peak located around 103.0 eV should be caused by the existence of Si-O bonding [40]. From Fig. S3f the high resolution XPS spectra of O 1s peak can be divided into two peaks which are centered at 530.1 eV and 529.5 eV, confirming to the location of Si–O bond and Pb–O bond respectively, which is consistent with Fig. S3c. It is also worth noticing that an additional peak located at
531.1 eV appears which can be assigned to the oxygen containing functional group, as is in accordance with the results in Fig. S3c. It can be revealed from the above results that Si QDs/CsPbBr3 composite is suc cessfully obtained while the Si QDs are uploaded onto the CsPbBr3 via forming a Pb–O bonding. 2.2. Optical performance The photoluminescence spectra of obtained Si QDs with different sizes are shown in Fig. S2. The colors of emitted wavelength evolve from red to blue with the etching time increasing. The mechanism underlying the change of emission wavelength with etching time increasing can be attributed to the size dependent bandgap of silicon owing to the quan tum confinement effect [41]. As FRET effect only happens when the emission region of the donor material overlaps with the absorption
Fig. 3. Time-resolved photoluminescence spectra of (a) pristine CsPbBr3 IO and pristine CsPbBr3 IO modified with Si QDs with Different Luminescent Colors, (b) pristine CsPbBr3 IO and pristine CsPbBr3 IO modified with different amount B–Si QDs. (c) Illustration of the fluorescence energy transfer and charge carrier sep aration process at the CsPbBr3/Si interface. (d-e) Transient absorption spectra of pristine CsPbBr3 IO and Si QDs/CsPbBr3 IO. (f) The corresponding kinetics plots. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 4
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Journal of Power Sources 439 (2019) 227065
the FRET process enables the fluorescence emitted by B–Si QDs to be transferred and absorbed by CsPbBr3 IO. The influences of Si QDs coupling on the system charge carrier transfer behavior are characterized via spectroscopy measurements such as time-resolved photoluminescence (TRPL) and transient absorption (TA). As shown in Fig. 3a, it can be seen that coupling CsPbBr3 IO with Si QDs of different emission wavelength, the carrier lifetime can be obvi ously increased, indicating a greatly enhanced carrier separation per formance achieved by Si QDs/CsPbBr3 system. This phenomenon should be attributed to the uniquely matched heterojunction at the Si QDs/ CsPbBr3 interface. The photoinduced electrons will transfer from the conductive band (CB) of Si QDs to the CB of CsPbBr3, leading to an effective carrier transfer process, which can increase the carrier popu lation of CsPbBr3 IO as well as prolonging the carrier lifetime (as shown in Fig. 3c). What’s more, as the FRET process existed within Si QDsCsPbBr3 IO donor-to-acceptor system, the carrier lifetime of B–Si QDs/ CsPbBr3 IO is the longest among the four samples. In order to investigate the energy transfer process in the Si coupled samples, TRPL of Si QDs/ CsPbBr3 IO with different Si QDs concentration is tested. It can be seen in Fig. 3b that the carrier lifetime of Si QDs/CsPbBr3 IO effectively pro longs after coupling with Si QDs, which should be caused by the increase of carrier concentration in the acceptor material because of FRET effect. It can be seen that the efficient energy transfer strongly reduces the Si QDs emission which thereby confirms that the FRET process exists in Si QDs/CsPbBr3 IO system. According to the work reported before, FRET theory predicts an increase in the excited-state lifetime of energy acceptor with the help of energy donor, as FRET introduces additional nonradiative decay path for the donor in the system [42]. However, excessive Si QDs could shorten the carrier lifetime instead, which should be caused by the increased concentration of recombination center arisen
Table 1 Carrier lifetime of CsPbBr3 IO, Si QDs-1/CsPbBr3 IO, Si QDs-2/CsPbBr3 IO and Si QDs-3/CsPbBr3 IO and the corresponding fitting parameters. samples
τ1 (ns)
A1mp (%)
τ2 (ns)
A2mp (%)
τavg
CsPbBr3 IO Si QDs-1/CsPbBr3 IO Si QDs-2/CsPbBr3 IO Si QDs-3/CsPbBr3 IO
2.26 1.33 1.6 1.61
1.8 87.5 74.6 71.8
0.1 4.62 5.18 5.01
98.2 12.5 26.4 28.2
0.73 2.42 3.51 3.49
region of acceptor material [42], QDs with photoluminescence centered at 435 nm (noted as B–Si QDs) are chosen as the target material to couple with CsPbBr3 in order to investigate the energy transfer process within the B–Si QDs-CsPbBr3 donor-to-acceptor FRET system. Fig. 2a demonstrates the absorption and photoluminescence (PL) spectra of the obtained CsPbBr3 and B–Si QDs, respectively. It can be seen that the B–Si QDs display an obvious PL peak centered around 435 nm. In addition, as for the CsPbBr3 sample, there exists an obvious absorption region from ~300 to 540 nm, which is in accordance with the intrinsic absorption region of CsPbBr3. According to the work reported before, the overlap between donor material (B–Si QDs) and acceptor material (CsPbBr3) can induce strong energy transfer between B–Si QDs and CsPbBr3 that is donor material can emit strong photoluminescence which could be absorbed by the acceptor material in the overlap region via FRET. As can be seen in Fig. 2b, the steady state PL spectra of B–Si QD, CsPbBr3 IO and B–Si QDs/CsPbBr3 IO samples are provided. It can be seen that the PL emission intensity of acceptor CsPbBr3 IO can be enhanced after coupling with B–Si QDs. At the same time, the PL emission intensity of B–Si QDs obviously decreases in the B–Si QDs/ CsPbBr3 IO sample. This result further confirms the FRET process be tween the B–Si QDs-CsPbBr3 donor-to-acceptor system, suggesting that
Fig. 4. (a) Photocurrent density-potential plots, (b) electrochemical impedance spectroscopy, (c) incident photon-to-electron conversion efficiency plots and (d) Mott-Schottky plots of pristine CsPbBr3 IO, SiQDs-1/CsPbBr3 IO, SiQDs-2/CsPbBr3 IO and SiQDs-3/CsPbBr3 IO. 5
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Journal of Power Sources 439 (2019) 227065
from Si QDs. In addition, the carrier lifetime can be calculated via a bi-exponential decay function and the fitting parameters are summarized in Table 1. The average carrier lifetime is calculated by the formula:
τavg ¼
based on CsPbBr3 IO, Si QDs-1/CsPbBr3 IO, Si QDs-2/CsPbBr3 IO and Si QDs-3/CsPbBr3 IO have been tested and shown in Fig. 4a, and the detailed PCE performance data is provided in Table S1. In addition, the cross-sectional SEM image of perovskite solar cells based on Si QDs/CsPbBr3 IO is shown in Fig. S5 Clear boundaries can be seen in the SEM image which can be assigned to bl-TiO2, meso-TiO2, perovskite layer and HTM layer respectively while it is difficult to figure out the IO structure in the cross sectional SEM image of the solar cells because the macrporous IO layer is filled with HTM. It can be seen that after coupling with Si QDs, the photocurrent density increased obvi ously, which is up to 7.8 mA/cm 2 for Si QDs-2/CsPbBr3 IO sample. When it comes to the reason why the PCE performance can be enhanced by introducing Si QDs, it should result from the unique energy transfer process and charge transfer process. Both processes can be optimized in this system, which on the one hand reduces the solar energy loss, on the other hand, results in effective charge injection and extraction. As FRET process provides additional nonradiative decay paths for the donor in the system, the utilization efficiency of incident light can be efficiently enhanced. As illustrated Fig. 2, the photoluminescence of Si QD can be absorbed by the CsPbBr3 IO, which would lead to greatly enhanced carrier concentration within the PSCs and enhanced open-circuit voltage (Voc) and photocurrent density. Also, it is worth noticing that for Si QDs1/CsPbBr3 IO and Si QDs-3/CsPbBr3 IO sample, both Voc and Jsc also display a boost comparing with pristine CsPbBr3 IO sample, which can be assigned to the formation of Si QDs/CsPbBr3 heterojunction. By means of energy band engineering between Si QDs and CsPbBr3, the photoinduced electron/hole pairs can be efficiently transferred to ETL and HTL, leading to an enhanced PCE performance. To confirm the reproducibility of the as-fabricated PSCs, the power conversion effi ciencies of all samples are tested with five different cells are provided in Fig. S8 statistically. It can be seen that the PCE performance of all samples have satisfying reproducibility, indicating the demonstrated strategy is reliable. Herein, a horizontal comparison among the latest reported CsPbBr3-based PSCs is provided in Table S2. It can be seen that as synthesized Si QD/CsPbBr3 PSCs possess comparable solar energy conversion efficiency. The electrochemical impedance spectroscopy (EIS) is performed to further investigate the charge transfer performance of the obtained solar cells. As can be seen in Fig. 4b, the Nyquist plots usually consist of two arcs, the first semicircle denotes the charge exchange process at the counter electrodes interface, and the second semicircle represents the charge recombination process at the perovskite/TiO2 interface [45]. It is suggested in the EIS spectra that Si QDs/CsPbBr3 IO sample possesses not only the most efficient charge exchange process but also most sup pressed charge recombination process, which indicates an effective charge transfer and extraction process within the Si QDs/CsPbBr3 sys tem. Also, for the Si-2 QDs/CsPbBr3 sample, the decreased charge recombination indicates that the donor-to-acceptor FRET process can provide additional energy transfer paths for Si QDs/CsPbBr3 system, which significantly suppresses the charge recombination as shown in Fig. 3c. The incident photon-to-electron conversion efficiency (IPCE) spectra of perovskite solar cells based on CsPbBr3 IO, Si QDs-1/CsPbBr3 IO, Si QDs-2/CsPbBr3 IO and Si QDs-3/CsPbBr3 IO is also tested and illustrated in Fig. 4c. All samples exhibit wide range IPCE-activity from 300 to 540 nm wavelength, which is consistent with the UV–vis absorption spectra. In addition, it can be seen that after coupling with Si QDs, IPCE increases obviously comparing with pristine CsPbBr3 IO samples, which should be attributed to the existence of Si/CsPbBr3 heterojunction enhancing the charge transfer and extraction process. Also, Si QDs-2/ CsPbBr3 IO displays the highest IPCE up to 81%, resulting from the efficient donor-to-acceptor FRET process which not only enhances the solar energy utilization efficiency but also provides additional energy transfer paths to initiate an efficient energy transfer and an increased charge concentration. It can be concluded from the discussion before that the coupling of Si QDs can indeed improve the PCE performance of
τ21 � A1 þ τ22 � A2 ; τ 1 � A1 þ τ 2 � A2
where A1, A2 are relative amplitudes of carrier lifetime, and τ1, τ2 are corresponding fluorescence lifetime. The average time constant (τavg) can reflect the excited-stated decay and free carrier recombination dy namics in the perovskite layers. It can be concluded that after coupling with Si QDs, the carrier lifetime increases dramatically from 0.73 ns of CsPbBr3 IO to over 2 ns, which should be ascribed to the formation of Si/CsPbBr3 heterojunction and the FERT process. In addition, it can be seen that with the increase of Si QDs concentration, the carrier lifetime increases at first but slowly reduces, which should be caused by the excessive Si QDs working as recombination centers. The prolonged carrier lifetime should ensure a greater probability for photoexcited electron-hole pairs to transfer from the perovskite layers to the electron transfer layer and hole transfer layer respectively. It is also worth noticing that τ1 represents the fast decay component, which should be attributed to the bulk recombination in the perovskite crystals, while τ2 stands for the slow decay component, which should result from the recombination of free carriers in the radiative channels [43]. It can be seen that, after coupling with Si QDs, τ2 in creases obviously while τ1 slightly decreases. Reasons for this phe nomenon might be ascribed to the effective increase of carrier population in Si QDs/CsPbBr3 IO, which increases the possibility of charge recombination within the bulk perovskite. However, with the effective energy transfer process between donor Si QDs and acceptor CsPbBr3, recombination of free carriers in the radiative channel can be suppressed and thereby increase the carrier lifetime. It can be indicated from the TRPL results, an effective energy transfer as well as carrier transfer process can be obtained in the Si QDs/CsPbBr3 IO systems, which in turn leads to an enhanced PCE performance. We further investigate the FRET process using TA spectroscopy. TA has been proved to be a critical method for studying excited-state in teractions, specifically for probing the energy and carrier transfer pro cesses in nanocrystals. An obvious ground-state bleaching can be seen in Fig. 3d centered around 533 nm, which can be assigned to the intrinsic absorption of CsPbBr3 and in good accordance with the UV–vis spectra in Fig. 2a. However, this shows up as a rapid decay in the bleaching of CsPbBr3 IO and a correspondingly rapid increase in its transient ab sorption after coupling with Si QDs, which can be seen in Fig. 3e. The sub-ps decay phase is assigned to ultrafast charge trapping/relaxation for the opposite reason of employing higher excitation density. The fast decay is mainly caused by diffusion and subsequent transfer of the photoexcited holes from the perovskite valence band (VB) to the one of the QDs, and the electrons from perovskite conductive band (CB) to the CsPbBr3/QDs interface [44]. FRET causes energy transfer from Si QDs to CsPbBr3 IO which depletes the ground state of CsPbBr3 IO. This accel erated bleaching decay should be caused by the FRET process between Si QDs and CsPbBr3 IO, which enables rapid carrier transfer from the donor to the acceptor. To monitor the charge carrier and transfer dynamics, transient kinetics at 533 nm for Si QD/CsPbBr3 IO and CsPbBr3 IO sample have been monitored and are shown in Fig. 3f. The bleach re covery kinetics at 533 nm for Si QD/CsPbBr3 IO is much faster comparing with CsPbBr3 IO. The faster recovery kinetics in the excitonic bleach position of Si QD/CsPbBr3 IO can be attributed to the effective FRET process between Si QDs and CsPbBr3 IO, indicating fast excited interactions between Si QDs as a donor and CsPbBr3 IO as an acceptor. 2.3. PCE performance The current density-voltage (J-V) curves of perovskite solar cells 6
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Journal of Power Sources 439 (2019) 227065
Fig. 5. (a) Diagram of Si QDs/CsPbBr3 IO PSC and (b) corresponding band structure.
pristine CsPbBr3 IO. The charge carrier density of the four samples is further analyzed by the Mott-Schottky plot (Fig. 4d). The donor density (Nd) can be calcu lated by the slope of the Mott-Schottky plots via the equation [46,47]: 1 2 ¼ � ðVbi C2 A2 qεε0 Nd
under illumination, FRET process will also happen within the Si QDs/ CsPbBr3 system, enabling part of solar energy to transfer from Si QDs to CsPbBr3, which then produces the photoinduced electrons in CsPbBr3. It can be concluded that both energy transfer process and charge transfer process will take place in Si QDs/CsPbBr3 system, which not only en hances the solar energy utilization efficiency via donor-to-acceptor FRET process but also facilitates the carrier transport of photo-induced electron/hole pairs by virtue of constructing a Si QDs/CsPbBr3 heterostructure.
VÞ
where A is electrode area, q is electronic charge, ε is dielectric constant of the sample, ε0 is permittivity of the vacuum, Nd is donor density, Vbi is built-in potential and V is applied voltage. The carrier concentration of Si QDs-2/CsPbBr3 IO is estimated to be 8.63 � 1018 cm 3, much higher than that of CsPbBr3 IO sample (2.94 � 1018 cm 3). The enhanced donor density in Si QDs/CsPbBr3 IO system should be attributed to the energy transfer process and charge transfer process brought by Si QDs, which on the one hand facilitates the bulk carrier separation, on the other hand, enhances the solar spectra utilization via providing additional energy transfer paths. In addition, the built-in potential can be estimated using the intercept of the linear regime with the x-axis of Mott-Schottky curves to about 1.40 V, which is accordance with VOC results in the J-V curves, suggesting that the photo-generated carriers can be separated efficiently by the presence of built-in field. The relatively higher built-in potential can not only suppress the back transfer of electrons from the ETL to the perovskite layer, but also benefit the charge collection and transfer of the photogenerated carrier. In addition, the long-term stability of perovskite solar cells must be one of the most important properties considering its future application. In that case, we tested the long-term stability properties against moist and heat. As can be seen in Fig. S9a, the PSC exhibits excellent long-term in ambient environment, which can remain more than 90% PCE in 10 days and more than 80% PCE in 65 days, making it a promising PSC for practical application. In addition, we tested the PCE under wet envi ronment with different humidity as is shown in Fig. S9b. It can be seen that the obtained Si QDs-2/CsPbBr3 IO perovskite solar cells can main tain up to 91% PCE under a relative humidity (RH) of 20% and about 70% residual efficiency under 70% RH. Despite an accelerating PCE decay can be witnessed with the increase of environment humidity, the stability of Si QDs-2/CsPbBr3 IO perovskite solar cells are still much better than the commonly reported organic PSCs such as MAPbI3 or MAPbBr3 based PSCs. In order to investigate the energy level relationship within the Si QDs/CsPbBr3 IO system, UPS spectra are used to figure out the valence band (VB) of Si QDs and CsPbBr3 (Fig. S10). Also, the band gap of CsPbBr3 and Si QDs can be calculated by the TAUC function-converted plots (Fig. S11), therefore the conductive band (CB) can be derived. As is illustrated in Fig. 5, the diagram of Si QDs/CsPbBr3 IO PSC is provided with the corresponding band structure of each layer. A matched band structure can be confirmed which provides driving force for photo-induced electrons to transfer from CB of Si QDs to CB of CsPbBr3 and then be extracted by ETL. Meanwhile, the photoinduced holes will transfer from the VB of CsPbBr3 to the VB of Si QDs and finally be injected to the HTM layer. At the same time, as shown in Fig. 3c,
3. Conclusion In summary, we for the first time designed and fabricated a novel Si QDs/CsPbBr3 IO hybrid for stable perovskite solar cells with competitive PCE performance. Si QDs with tunable photoluminescence properties can initiate effective donor-to-acceptor FRET process which gives rise to an additional non-radiative path for energy transfer, leading to an increased solar spectrum utilization efficiency as well as charge popu lation comparing with pristine CsPbBr3. In addition, due to the Si QDs/ CsPbBr3 heterojunction formed with uniquely matched band structure, the charge transfer process and carrier extraction can be further enhanced within the Si QDs/CsPbBr3 IO system. Hence, by introducing Si QDs to CsPbBr3 IO, both energy transfer process and charge transfer process can be optimized in this system, which not only reduces the solar energy loss, but also results in an effective photo-electrical conversion process. The PSCs based on Si QDs/CsPbBr3 IO exhibit greatly improved PCE of 8.31% with excellent stability in ambient environment. We believe that the present strategy could offer reference and draw lessons on rational design of other photoelectronic devices. Acknowledgements We acknowledge support from the project supported by the State Key Program of National Natural Science of China (No :51532005), the National Natural Science Foundation of China (No.:51472148, 51272137), and the Tai Shan Scholar Foundation of Shandong Province. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227065. References [1] D. Zhao, C. Wang, Z. Song, Y. Yu, C. Chen, X. Zhao, K. Zhu, Y. Yan, Four-terminal all-perovskite tandem solar cells achieving power conversion efficiencies exceeding 23%, ACS Energy Lett 3 (2018) 305–306. [2] E. Halvani Anaraki, A. Kermanpur, M.T. Mayer, L. Steier, T. Ahmed, S.-H. TurrenCruz, J. Seo, J. Luo, S.M. Zakeeruddin, W.R. Tress, Low-temperature Nb-Doped SnO2 electron-selective contact yields over 20% efficiency in planar perovskite solar cells, ACS Energy Lett 3 (2018) 773–778.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Fluorescence resonance energy transfer effect enhanced high performance of Si quantum Dots/CsPbBr3 inverse opal heterostructure perovskite solar cells Shujie Zhou 1, Rui Tang 1, Hui Li, Lin Fu, Bo Li, Longwei Yin * Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan, 250061, PR China
H I G H L I G H T S
� Multi-dimensional Si QDs/CsPbBr3 IO perovskite solar cell has been fabricated. � FRET provides additional energy transfer path which benefits the light utilization. � Si QDs/CsPbBr3 heterojunction gives rise to an effective charge transfer process. � FRET and Si QDs/CsPbBr3 heterojunction devote to a competitive PCE up to 8.31%. A R T I C L E I N F O
A B S T R A C T
Keywords: CsPbBr3 Perovskite solar cells Fluorescence resonance energy transfer Heterostructure
CsPbBr3 based perovskite solar cells draw boosted investigation benefitting from their simplified preparation property and outstanding stability against moist and heat while the photo-electrical conversion efficiency (PCE) is still worth promotion. In addition, relatively wide band gap limits the light utilization ability of pristine CsPbBr3 which leads to insufficient photo-induced charge carrier population thereby a low photocurrent density. Herein, we for the first time demonstrate a strategy to combine crystalized Si quantum dots (QDs) with CsPbBr3 inverse opal (IO) which significantly enhances the solar energy utilization efficiency by virtue of providing an additional fluorescence resonance energy transfer (FRET) process from Si QDs to CsPbBr3 IO. Acting as donor, the emitted photoluminescence from Si QDs can be absorbed by CsPbBr3, which serves as acceptor, leading to an increased carrier population in the system. Meanwhile, the multi-dimensional heterojunction between Si QDs and CsPbBr3 IO effectively facilitates the system bulk charge transfer process. A greatly improved PCE up to 8.31% with an obviously enhanced photocurrent density up to 7.8 mA⋅cm 2 can be obtained with a competitive IPCE up to 81%. This strategy provides a new alternative method to develop high-performance perovskite solar cells and other photo-electronic devices.
1. Introduction In the past decade, organic-inorganic perovskite solar cells (PSCs) have gone through a huge leap on the photo-electrical conversion effi ciency (PCE), which has been increased to 23% [1–4], attracting more and more attention all around the world. However, the critical inherent defect of poor stability greatly hinders the industrial application of organic-inorganic PSCs, especially under humid environment or at high temperature [5,6]. For this, one of the most reliable methods is to replace the organic cation with other cations such as cesium, which
would not cause decomposition under high temperature, enabling an improved long-term stability of inorganic PSCs [7,8]. CsPbBr3 has been considered as a competitive candidate for inor ganic PSCs, benefiting from its fast carrier mobility [9,10], excellent long-term phase stability [11,12] and facile preparation methods [13]. Nevertheless, the PCE performance of pristine CsPbBr3 based PSCs re mains unsatisfying because of the following issues: poor light utilization ability originated from its relatively wide band gap [14,15] and high carrier recombination rate [16,17]. Therefore, great effort has been made to further improve the PCE performance of CsPbBr3 based PSCs,
* Corresponding author. E-mail address: [email protected] (L. Yin). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.227065 Received 15 April 2019; Received in revised form 26 June 2019; Accepted 23 August 2019 Available online 5 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Fabrication processes of Si QDs and Si QDs/CsPbBr3 IO.
among which, constructing heterostructures between CsPbBr3 with other narrow band matched semiconductors should be a potential strategy [18,19], resulting in a broadened light absorption range and a facilitated carrier transfer process [20]. Si quantum dots (QDs) have been considered as a widely used semiconductors which have tunable band gap, high carrier mobility and excellent photostability, as well as its abundant and non-toxic features [21–24]. In addition, it should be pointed out that Si QDs are also known as an effective fluorescence resonance energy transfer (FRET) donor material when they are hybrid with other acceptor semiconductors [25,26]. Namely, Si QDs are able to emit in visible frequencies after absorbing the solar spectrum originated from the quantum confinement effect; that is to say, the photo luminescence emitted by Si QDs can be reused via a donor-to-acceptor FRET system with less energy-transfer loss, compared with simple het erostructures [27,28]. For example, Thiyagu Subramani and coworkers reported that by means of employing nanocrystalline Si quantum dots and Si nanotips hybrids, the PCE performance of organic solar cells has been greatly improved resulting from the efficient FRET energy man agement [29]. Hence, constructing heterostructure between CsPbBr3 and Si QDs should not only facilitate the charge transfer process of pristine CsPbBr3 but also help to enhance the solar spectrum utilization efficiency of pristine CsPbBr3. However, to the best of our knowledge, prior literature has not reported any works related to the Si QDs coupled PSCs yet and seldom have any researches paid attention to the FRET process of Si QDs coupled perovskites system. In addition, rationally structural engineering can also help with the light utilization and charge transport. For this, inverse opals (IOs) are a kind of three-dimensional (3D) ordered photonic crystals consisting of
periodically close-packed arrays of voids that are surrounded by solid materials with different refractive index [30]. This periodical macro-porous structure on the one hand would contribute to the inci dent light scattering process [31,32], on the other hand, it provides direct charge transfer tunnels, which can effectively suppress the free carrier recombination, resulting in efficient charge transfer process [33]. In our previous work, we have proved that by introducing the CsPbBr3 IO structure, solar light absorption ability as well as the carrier extrac tion and injection efficiency can be obviously enhanced comparing with the conventional planar structures, which devoted to an obviously increased PCE of CsPbBr3 IO based PSCs [34]. Recently, it has also been reported that by QDs sensitizing, the profiled 3D/0D interface in troduces an electric field in the rear side of the device to promote carrier extraction, resulting in an increased photocurrent and open voltage [35]. Hence, it should be a promising strategy to fabricate 0D Si QDs coupled 3D CsPbBr3 IOs and investigate the energy transfer and charge transfer process in the multi-dimensional Si QDs/CsPbBr3 IO PSCs. Herein, we demonstrated a facile method to prepare Si QDs sensi tized CsPbBr3 IOs (Si QDs/CsPbBr3 IO) hybrid PSCs with a polystyrene (PS) template assisted, spin-coating method. As obtained Si QDs exhibit tunable photoluminescence properties, by virtue of constructing a donor-to-acceptor FRET system, the Si QDs emitted photoluminescence can be absorbed by CsPbBr3, which can induce additional energy transfer between Si QDs and CsPbBr3. It makes the CsPbBr3 secondarily excited and results in greatly enhanced solar spectrum utilization effi ciency. At the same time, benefiting from the uniquely matched band gap structure at the CsPbBr3/Si interface, facilitated charge extraction and injection process can be witnessed in the Si QDs/CsPbBr3 IOs PSCs, 2
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Fig. 1. SEM image of (a) PS template, (b) CsPbBr3 IO, (c) SiQDs/CsPbBr3 IO. (d) XRD patterns of CsPbBr3 IO and SiQDs/CsPbBr3 IO.
which gives rise to an increased photocurrent intensity up to 7.8 mA⋅cm 2 and an open voltage of 1.42 V, with a greatly improved PCE up to 8.31%. We believe that our work provides a new perspective of energy management for the design of inorganic PSCs, which can also be expanded and applied in other photovoltaic devices.
transport materials (HTM) (spiro-MeOTAD) are spin-coated on the Si QDs/CsPbBr3 IO, followed by evaporating Ag as the counter electrode. Fig. 1a–c exhibit the SEM images of PS template, CsPbBr3 IO and Si QDs/CsPbBr3 IO. It can be seen in Fig. 1a that the PS template shows a uniformly periodical arrangements, with a PS diameter around 300 nm. After CsPbBr3 filling and template removal, it can be seen in Fig. 1b that the PS templates can be thoroughly moved, leaving the threedimensional scaffolds with a diameter of about 280 nm. In Fig. 1c, it can be seen that the surface of CsPbBr3 IO becomes rough with obvious granular sensation after loaded with Si QDs. Fig. 1d demonstrates the XRD patterns of the as-obtained Si QDs/ CsPbBr3 IO samples. It can be seen that both the CsPbBr3 IO and Si QDs/ CsPbBr3 IO samples display good crystallizability. The highest peak located around 30� can be indexed to the (200) plane of cubic CsPbBr3 and the other five peaks can also be in good accordance with the (100), (110), (111), (210), (211), (220) planes of cubic CsPbBr3 (PDF 54–0752) [36]. After coupling with Si QDs, the Si QDs/CsPbBr3 IO sample shows additional peaks located at around 33� , 52� and 56� , which can be assigned to the (211), (220) and (400) planes (PDF 17–0901) of Si QDs [37]. By virtue of changing the etching time, Si QDs with different di ameters can be obtained, which can be seen in the TEM images in Figs. S1a–c. It can be seen that with the etching time increase, the QDs diameters are gradually decreased. As-obtained Si QDs exhibit distinct photoluminescence under ultraviolet lamp, as can be seen in the inset figures, which are denoted as B–Si QDs, G-Si QDs and R–Si QDs for Si QDs with different colors of blue, yellow and red. In addition, the photoluminescence spectra of these samples are tested, which can be seen in Fig. S2. It can be seen that the emission peaks of different QDs are centered at 435, 553 and 630 nm, respectively, which are in accordance with the blue, yellow and red emission light. In order to further investigate the chemical bonding state, the X-ray photo-electron spectroscopy (XPS) of the obtained Si QDs/CsPbBr3 IO sample is tested. In the survey spectra (Fig. S3a), the existence of ele ments Cs, Pb, Br, Si, O and C can be confirmed. The existence of C element should be attributed to the carbon-based containment. It can be
2. Results and discussion 2.1. Material characterization Scheme 1 demonstrates the fabrication process of Si QDs/CsPbBr3 IO and the detailed experimental section is provided in the supporting information. Generally, Si QDs are prepared via a chemical etching method. Si nanoparticles are used as the starting material, which are firstly oxidized by HNO3 to formSiO2 shell. Then, they are etched by HF under ultra sonic bath, which can remove the SiO2 shell and reduce the core size of Si nanoparticles and form hydride-terminated Si nanoparticles. After that, it will go through such repeated process and finally Si QDs can be obtained. By means of changing the etching time, Si QDs with different sizes can be prepared. The Si QDs/CsPbBr3 IO solar cells can be fabri cated via a template assisted spin-coating method. Typically, FTO sub strates are cleaned thoroughly in advance, following by a UV-ozone treatment for 20 min to ensure hydrophilia. Firstly, blocking layer TiO2 (bl-TiO2) and mesoporous TiO2 (meso-TiO2) are respectively spincoated on the FTO substrate, followed by an annealing process. Sec ondly, the PS templates are self-assembled on the FTO/bl-TiO2/mesoTiO2 substrate by virtue of a gas-liquid interface assembling process. Thirdly, CsPbBr3 precursor is prepared in advance and infiltrated into the voids of PS template and CsPbBr3 is then crystallized in the voids under an annealing treatment. Fourthly, as obtained Si QDs are dispersed in toluene in advance with different concentration, followed by immersing the CsPbBr3 infiltrated template into the dispersion for PS removal and Si QDs loading. The Si QDs/CsPbBr3 IO with different Si QDs loading amount is denoted as Si QDs-1/CsPbBr3 IO, Si QDs-2/ CsPbBr3 IO and Si QDs-3/CsPbBr3 IO, respectively. After that, hole 3
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Fig. 2. (a) UV–vis absorption spectrum and normalized photoluminescence spectrum of CsPbBr3 IO and Si QDs, respectively. (b) Steady state PL spectra of Si QDs, Si QDs/CsPbBr3 IO and CsPbBr3 IO. The excitation wavelength is 300 nm.
seen from Figs. S3b–d that the high-resolution core-level spectra show the peaks of Cs 3d (3d3/2: 737.9 eV and 3d5/2: 724.6 eV), Pb 4f (4d5/2: 143.1 eV and 4d7/2: 138.3 eV) and Br 2p (2p1/2: 189.1 eV and 2p3/2: 182.0 eV) respectively, in accordance with the previous reports of CsPbBr3 [38,39]. Also, the Pb 4d5/2 peak can be split into an additional peak at 144.9 eV, which should result from the oxygen-containing functional group such as Pb–O bond on the surface of CsPbBr3. It can be seen in Fig. S3e that the Si 2p peak can be split into two peaks which can be assigned Si–H bond and Si-Si bond, indicating the formation of H-terminated Si QDs. In addition, the peak located around 103.0 eV should be caused by the existence of Si-O bonding [40]. From Fig. S3f the high resolution XPS spectra of O 1s peak can be divided into two peaks which are centered at 530.1 eV and 529.5 eV, confirming to the location of Si–O bond and Pb–O bond respectively, which is consistent with Fig. S3c. It is also worth noticing that an additional peak located at
531.1 eV appears which can be assigned to the oxygen containing functional group, as is in accordance with the results in Fig. S3c. It can be revealed from the above results that Si QDs/CsPbBr3 composite is suc cessfully obtained while the Si QDs are uploaded onto the CsPbBr3 via forming a Pb–O bonding. 2.2. Optical performance The photoluminescence spectra of obtained Si QDs with different sizes are shown in Fig. S2. The colors of emitted wavelength evolve from red to blue with the etching time increasing. The mechanism underlying the change of emission wavelength with etching time increasing can be attributed to the size dependent bandgap of silicon owing to the quan tum confinement effect [41]. As FRET effect only happens when the emission region of the donor material overlaps with the absorption
Fig. 3. Time-resolved photoluminescence spectra of (a) pristine CsPbBr3 IO and pristine CsPbBr3 IO modified with Si QDs with Different Luminescent Colors, (b) pristine CsPbBr3 IO and pristine CsPbBr3 IO modified with different amount B–Si QDs. (c) Illustration of the fluorescence energy transfer and charge carrier sep aration process at the CsPbBr3/Si interface. (d-e) Transient absorption spectra of pristine CsPbBr3 IO and Si QDs/CsPbBr3 IO. (f) The corresponding kinetics plots. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 4
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the FRET process enables the fluorescence emitted by B–Si QDs to be transferred and absorbed by CsPbBr3 IO. The influences of Si QDs coupling on the system charge carrier transfer behavior are characterized via spectroscopy measurements such as time-resolved photoluminescence (TRPL) and transient absorption (TA). As shown in Fig. 3a, it can be seen that coupling CsPbBr3 IO with Si QDs of different emission wavelength, the carrier lifetime can be obvi ously increased, indicating a greatly enhanced carrier separation per formance achieved by Si QDs/CsPbBr3 system. This phenomenon should be attributed to the uniquely matched heterojunction at the Si QDs/ CsPbBr3 interface. The photoinduced electrons will transfer from the conductive band (CB) of Si QDs to the CB of CsPbBr3, leading to an effective carrier transfer process, which can increase the carrier popu lation of CsPbBr3 IO as well as prolonging the carrier lifetime (as shown in Fig. 3c). What’s more, as the FRET process existed within Si QDsCsPbBr3 IO donor-to-acceptor system, the carrier lifetime of B–Si QDs/ CsPbBr3 IO is the longest among the four samples. In order to investigate the energy transfer process in the Si coupled samples, TRPL of Si QDs/ CsPbBr3 IO with different Si QDs concentration is tested. It can be seen in Fig. 3b that the carrier lifetime of Si QDs/CsPbBr3 IO effectively pro longs after coupling with Si QDs, which should be caused by the increase of carrier concentration in the acceptor material because of FRET effect. It can be seen that the efficient energy transfer strongly reduces the Si QDs emission which thereby confirms that the FRET process exists in Si QDs/CsPbBr3 IO system. According to the work reported before, FRET theory predicts an increase in the excited-state lifetime of energy acceptor with the help of energy donor, as FRET introduces additional nonradiative decay path for the donor in the system [42]. However, excessive Si QDs could shorten the carrier lifetime instead, which should be caused by the increased concentration of recombination center arisen
Table 1 Carrier lifetime of CsPbBr3 IO, Si QDs-1/CsPbBr3 IO, Si QDs-2/CsPbBr3 IO and Si QDs-3/CsPbBr3 IO and the corresponding fitting parameters. samples
τ1 (ns)
A1mp (%)
τ2 (ns)
A2mp (%)
τavg
CsPbBr3 IO Si QDs-1/CsPbBr3 IO Si QDs-2/CsPbBr3 IO Si QDs-3/CsPbBr3 IO
2.26 1.33 1.6 1.61
1.8 87.5 74.6 71.8
0.1 4.62 5.18 5.01
98.2 12.5 26.4 28.2
0.73 2.42 3.51 3.49
region of acceptor material [42], QDs with photoluminescence centered at 435 nm (noted as B–Si QDs) are chosen as the target material to couple with CsPbBr3 in order to investigate the energy transfer process within the B–Si QDs-CsPbBr3 donor-to-acceptor FRET system. Fig. 2a demonstrates the absorption and photoluminescence (PL) spectra of the obtained CsPbBr3 and B–Si QDs, respectively. It can be seen that the B–Si QDs display an obvious PL peak centered around 435 nm. In addition, as for the CsPbBr3 sample, there exists an obvious absorption region from ~300 to 540 nm, which is in accordance with the intrinsic absorption region of CsPbBr3. According to the work reported before, the overlap between donor material (B–Si QDs) and acceptor material (CsPbBr3) can induce strong energy transfer between B–Si QDs and CsPbBr3 that is donor material can emit strong photoluminescence which could be absorbed by the acceptor material in the overlap region via FRET. As can be seen in Fig. 2b, the steady state PL spectra of B–Si QD, CsPbBr3 IO and B–Si QDs/CsPbBr3 IO samples are provided. It can be seen that the PL emission intensity of acceptor CsPbBr3 IO can be enhanced after coupling with B–Si QDs. At the same time, the PL emission intensity of B–Si QDs obviously decreases in the B–Si QDs/ CsPbBr3 IO sample. This result further confirms the FRET process be tween the B–Si QDs-CsPbBr3 donor-to-acceptor system, suggesting that
Fig. 4. (a) Photocurrent density-potential plots, (b) electrochemical impedance spectroscopy, (c) incident photon-to-electron conversion efficiency plots and (d) Mott-Schottky plots of pristine CsPbBr3 IO, SiQDs-1/CsPbBr3 IO, SiQDs-2/CsPbBr3 IO and SiQDs-3/CsPbBr3 IO. 5
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from Si QDs. In addition, the carrier lifetime can be calculated via a bi-exponential decay function and the fitting parameters are summarized in Table 1. The average carrier lifetime is calculated by the formula:
τavg ¼
based on CsPbBr3 IO, Si QDs-1/CsPbBr3 IO, Si QDs-2/CsPbBr3 IO and Si QDs-3/CsPbBr3 IO have been tested and shown in Fig. 4a, and the detailed PCE performance data is provided in Table S1. In addition, the cross-sectional SEM image of perovskite solar cells based on Si QDs/CsPbBr3 IO is shown in Fig. S5 Clear boundaries can be seen in the SEM image which can be assigned to bl-TiO2, meso-TiO2, perovskite layer and HTM layer respectively while it is difficult to figure out the IO structure in the cross sectional SEM image of the solar cells because the macrporous IO layer is filled with HTM. It can be seen that after coupling with Si QDs, the photocurrent density increased obvi ously, which is up to 7.8 mA/cm 2 for Si QDs-2/CsPbBr3 IO sample. When it comes to the reason why the PCE performance can be enhanced by introducing Si QDs, it should result from the unique energy transfer process and charge transfer process. Both processes can be optimized in this system, which on the one hand reduces the solar energy loss, on the other hand, results in effective charge injection and extraction. As FRET process provides additional nonradiative decay paths for the donor in the system, the utilization efficiency of incident light can be efficiently enhanced. As illustrated Fig. 2, the photoluminescence of Si QD can be absorbed by the CsPbBr3 IO, which would lead to greatly enhanced carrier concentration within the PSCs and enhanced open-circuit voltage (Voc) and photocurrent density. Also, it is worth noticing that for Si QDs1/CsPbBr3 IO and Si QDs-3/CsPbBr3 IO sample, both Voc and Jsc also display a boost comparing with pristine CsPbBr3 IO sample, which can be assigned to the formation of Si QDs/CsPbBr3 heterojunction. By means of energy band engineering between Si QDs and CsPbBr3, the photoinduced electron/hole pairs can be efficiently transferred to ETL and HTL, leading to an enhanced PCE performance. To confirm the reproducibility of the as-fabricated PSCs, the power conversion effi ciencies of all samples are tested with five different cells are provided in Fig. S8 statistically. It can be seen that the PCE performance of all samples have satisfying reproducibility, indicating the demonstrated strategy is reliable. Herein, a horizontal comparison among the latest reported CsPbBr3-based PSCs is provided in Table S2. It can be seen that as synthesized Si QD/CsPbBr3 PSCs possess comparable solar energy conversion efficiency. The electrochemical impedance spectroscopy (EIS) is performed to further investigate the charge transfer performance of the obtained solar cells. As can be seen in Fig. 4b, the Nyquist plots usually consist of two arcs, the first semicircle denotes the charge exchange process at the counter electrodes interface, and the second semicircle represents the charge recombination process at the perovskite/TiO2 interface [45]. It is suggested in the EIS spectra that Si QDs/CsPbBr3 IO sample possesses not only the most efficient charge exchange process but also most sup pressed charge recombination process, which indicates an effective charge transfer and extraction process within the Si QDs/CsPbBr3 sys tem. Also, for the Si-2 QDs/CsPbBr3 sample, the decreased charge recombination indicates that the donor-to-acceptor FRET process can provide additional energy transfer paths for Si QDs/CsPbBr3 system, which significantly suppresses the charge recombination as shown in Fig. 3c. The incident photon-to-electron conversion efficiency (IPCE) spectra of perovskite solar cells based on CsPbBr3 IO, Si QDs-1/CsPbBr3 IO, Si QDs-2/CsPbBr3 IO and Si QDs-3/CsPbBr3 IO is also tested and illustrated in Fig. 4c. All samples exhibit wide range IPCE-activity from 300 to 540 nm wavelength, which is consistent with the UV–vis absorption spectra. In addition, it can be seen that after coupling with Si QDs, IPCE increases obviously comparing with pristine CsPbBr3 IO samples, which should be attributed to the existence of Si/CsPbBr3 heterojunction enhancing the charge transfer and extraction process. Also, Si QDs-2/ CsPbBr3 IO displays the highest IPCE up to 81%, resulting from the efficient donor-to-acceptor FRET process which not only enhances the solar energy utilization efficiency but also provides additional energy transfer paths to initiate an efficient energy transfer and an increased charge concentration. It can be concluded from the discussion before that the coupling of Si QDs can indeed improve the PCE performance of
τ21 � A1 þ τ22 � A2 ; τ 1 � A1 þ τ 2 � A2
where A1, A2 are relative amplitudes of carrier lifetime, and τ1, τ2 are corresponding fluorescence lifetime. The average time constant (τavg) can reflect the excited-stated decay and free carrier recombination dy namics in the perovskite layers. It can be concluded that after coupling with Si QDs, the carrier lifetime increases dramatically from 0.73 ns of CsPbBr3 IO to over 2 ns, which should be ascribed to the formation of Si/CsPbBr3 heterojunction and the FERT process. In addition, it can be seen that with the increase of Si QDs concentration, the carrier lifetime increases at first but slowly reduces, which should be caused by the excessive Si QDs working as recombination centers. The prolonged carrier lifetime should ensure a greater probability for photoexcited electron-hole pairs to transfer from the perovskite layers to the electron transfer layer and hole transfer layer respectively. It is also worth noticing that τ1 represents the fast decay component, which should be attributed to the bulk recombination in the perovskite crystals, while τ2 stands for the slow decay component, which should result from the recombination of free carriers in the radiative channels [43]. It can be seen that, after coupling with Si QDs, τ2 in creases obviously while τ1 slightly decreases. Reasons for this phe nomenon might be ascribed to the effective increase of carrier population in Si QDs/CsPbBr3 IO, which increases the possibility of charge recombination within the bulk perovskite. However, with the effective energy transfer process between donor Si QDs and acceptor CsPbBr3, recombination of free carriers in the radiative channel can be suppressed and thereby increase the carrier lifetime. It can be indicated from the TRPL results, an effective energy transfer as well as carrier transfer process can be obtained in the Si QDs/CsPbBr3 IO systems, which in turn leads to an enhanced PCE performance. We further investigate the FRET process using TA spectroscopy. TA has been proved to be a critical method for studying excited-state in teractions, specifically for probing the energy and carrier transfer pro cesses in nanocrystals. An obvious ground-state bleaching can be seen in Fig. 3d centered around 533 nm, which can be assigned to the intrinsic absorption of CsPbBr3 and in good accordance with the UV–vis spectra in Fig. 2a. However, this shows up as a rapid decay in the bleaching of CsPbBr3 IO and a correspondingly rapid increase in its transient ab sorption after coupling with Si QDs, which can be seen in Fig. 3e. The sub-ps decay phase is assigned to ultrafast charge trapping/relaxation for the opposite reason of employing higher excitation density. The fast decay is mainly caused by diffusion and subsequent transfer of the photoexcited holes from the perovskite valence band (VB) to the one of the QDs, and the electrons from perovskite conductive band (CB) to the CsPbBr3/QDs interface [44]. FRET causes energy transfer from Si QDs to CsPbBr3 IO which depletes the ground state of CsPbBr3 IO. This accel erated bleaching decay should be caused by the FRET process between Si QDs and CsPbBr3 IO, which enables rapid carrier transfer from the donor to the acceptor. To monitor the charge carrier and transfer dynamics, transient kinetics at 533 nm for Si QD/CsPbBr3 IO and CsPbBr3 IO sample have been monitored and are shown in Fig. 3f. The bleach re covery kinetics at 533 nm for Si QD/CsPbBr3 IO is much faster comparing with CsPbBr3 IO. The faster recovery kinetics in the excitonic bleach position of Si QD/CsPbBr3 IO can be attributed to the effective FRET process between Si QDs and CsPbBr3 IO, indicating fast excited interactions between Si QDs as a donor and CsPbBr3 IO as an acceptor. 2.3. PCE performance The current density-voltage (J-V) curves of perovskite solar cells 6
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Journal of Power Sources 439 (2019) 227065
Fig. 5. (a) Diagram of Si QDs/CsPbBr3 IO PSC and (b) corresponding band structure.
pristine CsPbBr3 IO. The charge carrier density of the four samples is further analyzed by the Mott-Schottky plot (Fig. 4d). The donor density (Nd) can be calcu lated by the slope of the Mott-Schottky plots via the equation [46,47]: 1 2 ¼ � ðVbi C2 A2 qεε0 Nd
under illumination, FRET process will also happen within the Si QDs/ CsPbBr3 system, enabling part of solar energy to transfer from Si QDs to CsPbBr3, which then produces the photoinduced electrons in CsPbBr3. It can be concluded that both energy transfer process and charge transfer process will take place in Si QDs/CsPbBr3 system, which not only en hances the solar energy utilization efficiency via donor-to-acceptor FRET process but also facilitates the carrier transport of photo-induced electron/hole pairs by virtue of constructing a Si QDs/CsPbBr3 heterostructure.
VÞ
where A is electrode area, q is electronic charge, ε is dielectric constant of the sample, ε0 is permittivity of the vacuum, Nd is donor density, Vbi is built-in potential and V is applied voltage. The carrier concentration of Si QDs-2/CsPbBr3 IO is estimated to be 8.63 � 1018 cm 3, much higher than that of CsPbBr3 IO sample (2.94 � 1018 cm 3). The enhanced donor density in Si QDs/CsPbBr3 IO system should be attributed to the energy transfer process and charge transfer process brought by Si QDs, which on the one hand facilitates the bulk carrier separation, on the other hand, enhances the solar spectra utilization via providing additional energy transfer paths. In addition, the built-in potential can be estimated using the intercept of the linear regime with the x-axis of Mott-Schottky curves to about 1.40 V, which is accordance with VOC results in the J-V curves, suggesting that the photo-generated carriers can be separated efficiently by the presence of built-in field. The relatively higher built-in potential can not only suppress the back transfer of electrons from the ETL to the perovskite layer, but also benefit the charge collection and transfer of the photogenerated carrier. In addition, the long-term stability of perovskite solar cells must be one of the most important properties considering its future application. In that case, we tested the long-term stability properties against moist and heat. As can be seen in Fig. S9a, the PSC exhibits excellent long-term in ambient environment, which can remain more than 90% PCE in 10 days and more than 80% PCE in 65 days, making it a promising PSC for practical application. In addition, we tested the PCE under wet envi ronment with different humidity as is shown in Fig. S9b. It can be seen that the obtained Si QDs-2/CsPbBr3 IO perovskite solar cells can main tain up to 91% PCE under a relative humidity (RH) of 20% and about 70% residual efficiency under 70% RH. Despite an accelerating PCE decay can be witnessed with the increase of environment humidity, the stability of Si QDs-2/CsPbBr3 IO perovskite solar cells are still much better than the commonly reported organic PSCs such as MAPbI3 or MAPbBr3 based PSCs. In order to investigate the energy level relationship within the Si QDs/CsPbBr3 IO system, UPS spectra are used to figure out the valence band (VB) of Si QDs and CsPbBr3 (Fig. S10). Also, the band gap of CsPbBr3 and Si QDs can be calculated by the TAUC function-converted plots (Fig. S11), therefore the conductive band (CB) can be derived. As is illustrated in Fig. 5, the diagram of Si QDs/CsPbBr3 IO PSC is provided with the corresponding band structure of each layer. A matched band structure can be confirmed which provides driving force for photo-induced electrons to transfer from CB of Si QDs to CB of CsPbBr3 and then be extracted by ETL. Meanwhile, the photoinduced holes will transfer from the VB of CsPbBr3 to the VB of Si QDs and finally be injected to the HTM layer. At the same time, as shown in Fig. 3c,
3. Conclusion In summary, we for the first time designed and fabricated a novel Si QDs/CsPbBr3 IO hybrid for stable perovskite solar cells with competitive PCE performance. Si QDs with tunable photoluminescence properties can initiate effective donor-to-acceptor FRET process which gives rise to an additional non-radiative path for energy transfer, leading to an increased solar spectrum utilization efficiency as well as charge popu lation comparing with pristine CsPbBr3. In addition, due to the Si QDs/ CsPbBr3 heterojunction formed with uniquely matched band structure, the charge transfer process and carrier extraction can be further enhanced within the Si QDs/CsPbBr3 IO system. Hence, by introducing Si QDs to CsPbBr3 IO, both energy transfer process and charge transfer process can be optimized in this system, which not only reduces the solar energy loss, but also results in an effective photo-electrical conversion process. The PSCs based on Si QDs/CsPbBr3 IO exhibit greatly improved PCE of 8.31% with excellent stability in ambient environment. We believe that the present strategy could offer reference and draw lessons on rational design of other photoelectronic devices. Acknowledgements We acknowledge support from the project supported by the State Key Program of National Natural Science of China (No :51532005), the National Natural Science Foundation of China (No.:51472148, 51272137), and the Tai Shan Scholar Foundation of Shandong Province. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227065. References [1] D. Zhao, C. Wang, Z. Song, Y. Yu, C. Chen, X. Zhao, K. Zhu, Y. Yan, Four-terminal all-perovskite tandem solar cells achieving power conversion efficiencies exceeding 23%, ACS Energy Lett 3 (2018) 305–306. [2] E. Halvani Anaraki, A. Kermanpur, M.T. Mayer, L. Steier, T. Ahmed, S.-H. TurrenCruz, J. Seo, J. Luo, S.M. Zakeeruddin, W.R. Tress, Low-temperature Nb-Doped SnO2 electron-selective contact yields over 20% efficiency in planar perovskite solar cells, ACS Energy Lett 3 (2018) 773–778.
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