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Effect of annealing temperature on internal absorption, charge recombination and internal quantum efficiency of HC(NH2)2PbI3 perovskite solar cells
Journal Pre-proof Effect of annealing temperature on internal absorption, charge recombination and internal quantum efficiency of HC(NH2)2PbI3 perovskite solar cells Yang Liu, Hao Zhang, Bin Xu, Leijing Liu, Chan Im, Wenjing Tian PII:
S1566-1199(19)30535-X
DOI:
https://doi.org/10.1016/j.orgel.2019.105508
Reference:
ORGELE 105508
To appear in:
Organic Electronics
Received Date: 5 August 2019 Revised Date:
14 October 2019
Accepted Date: 14 October 2019
Please cite this article as: Y. Liu, H. Zhang, B. Xu, L. Liu, C. Im, W. Tian, Effect of annealing temperature on internal absorption, charge recombination and internal quantum efficiency of HC(NH2)2PbI3 perovskite solar cells, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105508. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
The effect of annealing temperature for the perovskite layer on the internal absorption(Q), charge recombination and internal quantum efficiency (IQE) of HC(NH2)2PbI3 (FAPbI3) perovskite solar cells (PeSCs) was investigated. The PeSC with FAPbI3 film annealed at 145 °C has relatively better internal absorption, lower charge recombination and higher charge collection, leading to the higher device IQE.
Effect of annealing temperature on internal absorption, charge recombination and internal quantum efficiency of HC(NH2)2PbI3 perovskite solar cells Yang Liua, Hao Zhanga, Bin Xua, Leijing Liua, **, Chan Imb and Wenjing Tiana,* a
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China. b
Key Department of Chemistry, Konkuk University, Seoul 05029, Korea.
Abstract
Keywords: Perovskite solar cell Internal absorption Charge recombination Internal quantum efficiency
Despite the rapid development of perovskite solar cells (PeSCs) in photovoltaic field, clear understanding about fundamental mechanisms of PeSCs is still sparse. For PeSCs, the thermal annealing temperature of perovskite layer has a great influence on the physical processes of PeSCs. Here, the effect of annealing temperature at 100°C, 120°C, 145°C and 150 °C for the perovskite layer on the internal absorption, charge recombination and internal quantum efficiency of formamidinium lead triiodide (HC(NH2 )2PbI3, or FAPbI3) PeSCs was investigated by transfer matrix method (TMM), photo-induced charge extraction by linearly increasing voltage (photo-CELIV) and photocurrent density (Jph) versus effective voltage (Veff ) (Jph-Veff ) measurements. We obtained the internal absorption (Q) spectra of PeSCs by TMM, and estimated the internal quantum efficiency (IQE) spectrum of PeSC devices from Q spectra and external quantum efficiency (EQE) spectrum. It was found that the PeSC with FAPbI3 film annealed at 145 °C has relatively higher power conversion efficiency (PCE) and IQE. Furthermore, the analysis of photo-CELIV and Jph-Veff characteristics demonstrated that the PeSC with FAPbI3 film annealed at 145 °C possesses relatively low charge recombination and high charge collection. In a word, there is less power loss of PeSCs when the annealing temperature for the FAPbI3 layer is 145 °C because of the better internal absorption, suppressed charge recombination and higher charge collection.
1. Introduction Organic–inorganic hybrid perovskites as a light harvester in solar cells have attracted much attention due to their high light absorption in the visible region, low exciton binding energy, bipolar transporting ability, superb charge carrier mobility, long-range balanced charge transport lengths and tunable band gap energy [1-4]. In spite of the impressive progresses regarding perovskite solar cells (PeSCs), clear understanding about underlying mechanisms therein is still sparse. Usually, the operation of PeSC can be analyzed in terms of a sequential * Corresponding author. ** Corresponding author. E-mail addresses: [email protected], [email protected]
set of processes. The first process is light absorption that converts the absorbed photon of perovskite layer into an exciton [5]. Upon photoexcitation, the excitons are generated and then separated into free charges. After generation, the free charges transport in the perovskite layer and then transfer into the charge transport layers (CTLs). Subsequently, the charges are collected by the respective electrodes [6], and the charge collection process is in competition with charge recombination process within PeSCs. The initial light absorption stage is critically important on the transformation of light into electricity for PeSCs, thus it is crucial to estimate the quantitative number of photons being captured in the perovskite layer. Often the precise absorption degree of incident light only in the active layer of organic solar cell (OSC) is discussed in term of internal absorptance spectrum (Q) calculated by using transfer matrix method
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(TMM) [7]. Furthermore, the internal quantum efficiency (IQE) spectrum of solar cells can be estimated from a conventional incident photon to current efficiency (IPCE) spectrum (the overall PCE and its spectrally resolved version of IPCE is often denoted as external quantum efficiency (EQE)) with an accurate Q. Meanwhile, the IQE of solar cells is determined by the combination of internal absorption, charge transport and charge collection processes. There are many studies have shown that Q and IQE of OSCs can be accurately obtained by TMM, and in conjunction with the experimental EQE spectra [8-11]. Recently, many research groups have applied to the TMM framework on various studies of PeSCs [12-15]. Formamidinium lead triiodide (HC(NH2)2PbI3, FAPbI3) has arisen as one of the most-studied perovskite absorbers with superior properties, such as broad light absorption, high phase transition temperature and good thermal stability [16-19]. The solution processability of FAPbI3 perovskite layer is one of the simplest methods of deposition, and only a heating step can convert the deposited solution to the crystalline perovskite form [20-22]. The property of FAPbI3 perovskite film is greatly affected by the thermal annealing process. The study by G. Kanatzidis et al [23]. showed that FAPbI3 films has two polymorphs at ambient temperature: black trigonal perovskite phase (α-phase) with a direct bandgap of ~1.45eV and yellow hexagonal non-perovskite phase (δ-phase) with an indirect bandgap of ~2.48 eV. While, according to other reports [21, 24, 25], the FAPbI3 films can have only α-phase at a suitable annealing temperature. The work of Loi et al. revealed that photoluminescence spectra of FAPbI3 films show monoexponential decay at room temperature and an ultrafast decay is observed when the temperature decreased [28]. Furthermore, the annealing temperature of the as-deposited FAPbI3 film was critical to PeSCs performance. The PeSCs based on porous TiO2 layer showed the highest PCE with FAPbI3 film annealed at 150 °C [24], while the PeSCs based on compact TiO2 layer present very similar performance with FAPbI3 films annealed at temperature from 150 °C to 210 °C [27]. In our previous report [21], the PeSCs based on compact ZnO layer showed the highest power conversion efficiency with FAPbI3 film formed at 145 °C. Though many studies have been done to investigate the properties of FAPbI3 films or improve the photovoltaic device performance, the underlying optoelectronic processes of PeSCs affected by thermal annealing of perovskite layer, especially the FAPbI3 absorber films annealed with different temperature by means of TMM are rarely reported. And the comprehensive study of the internal absorption, charge recombination and IQE within PeSCs controlled by annealing temperature of FAPbI3 films will give a further understanding of the essential properties of perovskite materials and devices. Herein, we fabricate planar FAPbI3 PeCSs by varying the annealing temperature of FAPbI3 films at 100°C, 120°C, 145°C and 150 °C. We obtained the Q spectra of PeSCs by means of TMM, and estimated the spectrally resolved IQE of PeSCs from Q spectra and EQE. The results show that the PeSC with FAPbI3 film annealed at 145 °C has a relatively high IQE and PCE, which implies that more power is lost within PeSCs when
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the annealing temperature of FAPbI3 film is either lower or higher than 145 °C. Furthermore, the results of photoinduced charge extraction by linearly increasing voltage (photo-CELIV) and photocurrent density (Jph) versus effective voltage (Veff) measurements indicate that FAPbI3 PeSCs with perovskite films annealed at 145 °C possess lower charge recombination and higher charge collection, leading to the significant improvement of IQE. The current study gains more insight into the fundamental physics of perovskite solar cells.
2. Experiment section 2.1 Materials TheZnO nanoparticles were prepared according to the work of Janssen et al [29]. Lithium salts (>99.0%) were purchased from Aldrich. Lead iodide (beads, -10 mesh, 99.999% trace metals basis), HC(NH2)2I (>98.0%) and Spiro-OMeTAD (2,20,7,70tetrakis-(N,Ndi- 4-methoxyphenylamino)-9,90 -spirobifuorene, >99.0%) was purchased from Xi'an Polymer Light Technology Corp. (PLT). We will denote hereafter this spiro-OMeTAD as Spiro. Ultradry solvents N,N-dimethyl formamide (DMF, >99.9%) and isopropanol (>99.9%) were obtained from J&K and Acros, respectively. All the chemicals and solvents were kept in a glove-box before starting our experiment. Prepatterned indium tin oxide (ITO)-covered glass substrates were cleaned for about 20 min in detergent followed by ultrasonic cleaning in deionized water, acetone, and 2-propanol in sequence for 20 min each. Then the ITO substrates were treated with plasma for about 5 min. The ZnO solution was spin-coated on ITO substrates at 1500 rpm for 50 s, and followed by baking at 150 °C for 15 min. A 460 mg/mL solution of PbI2 in DMF with TBP (120 µL TBP in 1ml DMF) was then spin-coated on the top of the ZnO layer at 4000 rpm for 30 s in the glove-box. The PbI2-coated ZnO thin film was followed by annealing at 70 °C for 10 min. The films were preheated at 100°C for 3 min before dipping into a FAI 2-propanol solution (15 mg/mL) for 20 s followed by annealing at 100 °C, 120 °C, 145 °C and 150 °C for 15 min to obtain the desired crystallite formation. As hole-transporting material (HTM), a Spiro solution (80 mg of Spiro, 10.5 µL of 4-tert-butylpyridine(tBP), and 46.5 µL of a lithium-bis(tri-fluoromethanesulfonyl) imide(Li-TFSI) solution (170 mg Li-TFSI/ 1 mL acetonitrile) in 1 mL chlorobenzene) was spin-coated at 4000 rpm for 30 s on the HC(NH2)2PbI3. The substrates with HTM were left overnight in the dry air in dark at room temperature. Finally, 80 nm thick Ag was thermally evaporated through a shadow mask defining active area of 0.04 cm2 on the top of HTL to produce a completed PeSC device. 2.3 Device characterization The current–voltage characteristics of the solar cells were measured by using a computer-controlled Keithley 2400 source meter measurement system with an AM1.5G filter at a calibrated intensity of 100 mW cm-2 illumination, as determined by a standard silicon reference cell (91150V Oriel Instruments). EQE spectra were measured in air under shortcircuit conditions using a commercial EQE setup (Crowntech
QTest Station 1000AD), which was equipped with a 100 W Xe arc lamp, filter wheel, and monochromator. Monochromated light was chopped at a frequency of 80 Hz and photocurrents measured using a lock-in amplifier. The setup was calibrated against a certified silicon reference diode. The CELIV set up consists of a pulsed laser (Continuum Minilete TM Nd:YAG), a synthesized function generator (Stanford Research System DS345), a digital delay generator (Stanford Research System DG645), and an oscilloscope (Tektronix MSO 4054) for signal observation and recording. Photo-CELIV measurements are carried out under ambient conditions, and samples are irradiated through the ITO side by one 10 ns, 532 nm laser flash. The linearly increasing voltage with an offset voltage Uoffset = 0.6 V and Umax =3.4 V is applied to the device, while the Ag electrode is grounded. The chosen Uoffset exactly compensates for the build-in potential (Vbi) of the devices, thereby nulling the internal electric field before the charge carrier extraction. 2.4 Thin film characterization The thicknesses of ITO, ZnO, FAPbI3, Spiro and Ag layer were estimated from a Veeco Dektak 150 surface profilometer. The Ellipsometry measurements were carried out with a Horiba Jobin Yvon UVISEL 2 ellipsometer under incident angles of 70° for photon energies between 0.6 and 6 eV with 30 meV increment. The ZnO and Spiro films were prepared following the aforementioned processes except using the clean Si wafers as the substrate. It should be noted that the FAPbI3 films were prepared on the substrate of Si/ZnO, in order to keep the FAPbI3 films same as that in the devices. Modelling of the data was performed using Deltapsi2 software by HORIBA Scientific Company, shown in Fig. S6. Before the films were measured, the bare silicon oxide layer was measured, and resulted in a thickness of 2 nm. All measurements were done at room temperature (~298 K). Subsequent extraction of optical constants refractive index ( n ) and extinction coefficient ( k ) were performed by using Deltapsi2 software (HORIBA Scientific Co.). The obtained n and k values of the FAPbI3 perovskite layers are shown in the Fig. S7, and the n, k values of other films were reported in our previous study [15]. Steady-state fluorescence spectra were measured on a Shimadzu RF-5301PC spectrophotometer. Time-resolved photoluminescence (TrPL) spectra were obtained on a PL spectrometer (Edinburgh Instruments, FLS 980), excited with a picosecond pulsed diode laser (EPL-375). A Hamamatsu C5680–04 streak camera was used for TPL. The X-ray diffraction (XRD) patterns were recorded on Rigaku SmartLab X-ray diffractometer with Cu Kα radiation (λ= 1.5418 °A) at 25 °C. The data were collected with a 0.01° step size (2θ). A field emission scanning electron microscope (Hitachi SU8020) was used to acquire SEM images. UV-Vis-NIR spectra were obtained using a Shimadzu UV-2550 spectrophotometer. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) were performed using a multi-technique surface analysis XPS/UPS system (VG Scienta R3000).
3. Results and discussion 3.1 Characterizations of FAPbI3 films All perovskite solar cells (PeSCs) used in this study were fabricated with the same device architecture (shown in Fig. S1). The only changed parameter is the annealing temperature of FAPbI3 films (at 100 °C, 120 °C, 145 °C and 150 °C), thus the effect of annealing temperature on the performance of PeSCs should be addressed mostly as the change of FAPbI3 films properties. To clarify the effect of thermal annealing temperature on FaPbI3 films, X-ray diffraction (XRD) and scanning electron microscopy (SEM) of FAPbI3 films prepared at different annealing temperatures were investigated. The XRD reflections (Fig. S2) of FAPbI3 films at 13.92°, 24.26°, 28.04° and 31.42° indexed to (110), (202), (220) and (222) reflections, respectively, corresponding to the black polymorph of FAPbI3. The yellow polymorph coexists with the black polymorph for the FAPbI3 films annealed at 100 °C, but the yellow polymorph disappears with further elevating the annealing temperature to 120–150°C [21,24]. In addition, the increased intensity of the XRD peak of PbI2 at 2θ = 12.68° reflects the decomposition of FAPbI3 films at higher annealing temperature from 120°C to 150 °C. Fig. 1 gives the top-view SEM images of the FAPbI3 films fabricated at different annealing temperatures. The perovskite film annealed at 100 °C appears relatively loose morphology with small grain size and wide grain boundaries. As the annealing temperature increases to 145°C, the crystalline grains become larger and pack more compactly, which is beneficial to reducing the charge recombination and energy loss during the photophysical processes of PeSCs, as demonstrated by previous studies [30-31]. However, further increasing annealing temperature to 150°C, the grain size is growing larger, but there are some much smaller crystalline grains among the bigger ones, which can increase the number of grain boundaries, leading to higher charge recombination [14].
Fig. 1 Top-view SEM images of the perovskite films annealed at different temperatures: 100℃, 120℃, 145℃ and 150℃ (a–e).
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Fig. 2 Energy bands in glass/perovskite films annealed at different temperatures; (a) UPS binding energy spectra showing secondary cut-off (Ecut-off) and work function (WF), (b) UPS binding energy spectra showing (EF-EV), and (c) Energy level diagram for perovskite films annealed at different temperatures showing VBM, CBM, Fermi energy (EF), and bandgap (Eg). To obtain more insights into the impact of annealing temperature on the quality of FAPbI 3 films, ultraviolet photoelectron spectroscopy/X-ray photoemission spectroscopy (UPS/XPS) was measured for FAPbI3 films deposited on ZnO coated ITO glass substrates. The binding energies in UPS spectra are referenced to the Fermi level (EF) at 0V [32]. The work function (WF) of perovskite films which is the difference between EF and vacuum level (Evac) in an energy band diagram can be obtained by calculating the difference between the cutoff of UPS (Fig. 2a) and incident photon energy (21.22 ev), as shown in Fig. 2c. Fig. 2b shows the valence band edge from the Fermi level (EF-EV). Thus, we can obtain the valence band minimum (VBM) of different FAPbI3 films (Fig. 2c) [33], The UPS spectra of Figure 2b was also plotted on a logarithmic intensity scale to see more clearly the valence band edge of different perovskite films [34,35], as shown in Figure S4. The value of the optical bandgap as the fundamental gap (bandgap, E g ) considered by absorption measurements (Fig. S3) is about 1.52 eV, 1.49 eV, 1.49 eV and 1.50 eV of FAPbI 3 samples with annealing temperature at 100 ℃, 120 ℃, 145 ℃ and 150℃, respectively. Therefore, the conduction band maximum (CBM) of FAPbI3 films were estimated as 4.31 eV, 4.13 eV, 4.04 eV and 4.04 eV. According to previous report [36], the conduction band (Ec) of ZnO nanoparticles is 4.20 eV and the HOMO of Spiro-MeOTAD HTM is 5.20 eV. Thus, it’s clear that FAPbI3 films annealed at higher temperature can extract both the electrons and holes more effectively [37]. In order to make a clear insight into the chemical modifications occurring during the annealing process, the XPS spectra were recorded to confirm the chemical bonding states in the samples. Fig. S5 shows the XPS data of Pb 4f, I 3d, C 1s and N 1s core levels of
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Fig. 3 (a) Current density–voltage curves and (b) EQE spectra of four different FAPbI3 PeSCs. FAPbI3 films with the different annealing temperatures. The obtained atomic ratios of [Pb]:[I]:[C]:[N] were shown in Table S1. Obviously, the FAPbI3 films were composed of some PbI2, which were also proved by XRD result. The main Pb4f peak split into 4f 5/2 and 4f 7/2 doublet, and I3d core level is a doublet consisting of 3d 5/2 and I 3d 3/2 components, which is in agreement with the report of literature [38]. In addition, the signals of Pb 4f, I 3d, C 1s, and N 1s gradually shifted towards higher BE values as a function of annealing temperature, indicating that the charge redistribution of glass/ITO/ZnO/FAPbI3 samples is sensitive to perovskite morphology (Fig. 1), structural and composition (Fig. S2) variations induced by the annealing temperature. As control, charging effects were negligible as the monitored Zn 2p signal showed smaller variations in the binding energy. The shift from Pb and I support that the FAPbI3 film has become more n-doped [37], because the FAPbI3 film become Pb rich. 3.2 Device properties of PeSCs Fig. 3a presents the current–voltage characteristics of PeSCs with perovskite layer annealed at different temperature. We will hereafter denote the PeSCs with FAPbI3 layers annealed at 100 °C, 120 °C, 145 °C and 150 °C as 100-PeSC, 120-PeSC, 145PeSC and 150-PeS, respectively. The obtained solar cell device parameters, open circuit voltage (VOC), short circuit currentdensity (JSC), fill factor (FF) and power conversion efficiency (PCE) are listed in Table 1. The best performance was obtained for 145-PeSC, mainly due to the highest JSC and FF. The external quantum efficiency (EQE) spectra demonstrate that 145-PeSC exhibit a higher photo-to-current conversion efficiency than other PeSCs (see Fig. 3b). The integrated JSC obtained from EQE spectrum for the four PeSCs is 14.41, 17.35, 19.98, 18.52 mA/cm2, respectively. The slight mismatch between the integrated JSC of EQE and the values obtained from J-V curves is also frequently observed elsewhere,
Fig.4 Measured photocurrent output and PCE at the maximum power point of the devices versus time at ambient air (RH of 40%–45%, temperature of 22℃–25℃).
Table 1 Summary of the photovoltaic performance observed for four different FAPbI3 PeSCs.
which was ascribed to the different testing settings [39]. To gain some understanding of the stabilized power output under the operational conditions [40-42], the photocurrent and PCE versus time of the four type devices close to the maximum power point was measured as shown in Figure 4. The photocurrent of all devices rose quickly to their maximum values after the light was turned on, and then stabilized with little change over the time period of 120s. In addition, the PCE of all four type devices decreased quickly within two 2h without encapsulation at ambient air, but the 145-PeSC experienced a slightly slower degradation. For the four PeSCs, the significant different PCE is mainly governed by JSC and FF. Often, JSC is regarded as a measure of charge photogeneration degree when the internal electric field can be described with a simple flat band model [11]. Therefore, the different JSC can be regarded as a matter of difference between the amount of absorbed light, hence number of photons, and the number of extracted charges at a maximum power point for a given PeSC. Since the only changed parameter here is the annealing temperature of perovskite layer, the appearance of different FF should be addressed mostly as a bulk-induced changes. Thus, the matter discussed here can be expressed as the intrinsic bulk-induced property. To discuss it further, JSC should be carefully testified by comparing with the maximum possibility of photon absorption which can be obtained as a form of spatially and spectrally resolved
Fig. 5 Spatially and spectrally resolved mappings of simulated numbers of absorbed photons for four different FAPbI3 PeSCs.
internal absorptance (Q) spectra by transfer matrix method (TMM). Comparing the Q spectra with EQE will give internal quantum efficiency (IQE), which can provide a quantitative assessment on photocurrent forming facility of different PeSCs. 3.3 Internal absorption and internal quantum efficiency of PeSCs The spectrally and spatially resolved internal absorption spectra within the perovskite layers of devices obtained by TMM are shown in Fig. 5. At relatively short wavelength, the optical field intensity is rapid decaying of four samples, while at longer wavelength, the pronounced interference effect range is clearly to be recognized. Subsequently, those maps are spatially integrated to build internal absorptance (Q) spectra, as shown in Fig. 6a. The Q spectra are quite similar at wavelength < 600 nm in 120-PeSC, 145-PeSC, 150-PeSC, but different at wavelength > 600 nm, due to the optical interferences effect [39]. Obviously, the amplitude of the Q spectra for 100-PeSC is lower compared with that for other three PeSCs, which indicates that the perovskite layer can absorb less photons in 100-PeSC. As shown in Fig. 6b, the exciton generation rate of 100PeSC is lower than that of other PeSCs when assuming all the absorbed photons in the FAPbI3 layer can convert to excitons. Meanwhile, we simulated Jsc as a function of perovskite thickness (assuming 100% IQE) for these four PeSCs, as shown in Fig. 6c. It should be noted the simulated JSC values represent the upper limits of photocurrent and do not take into account charge recombination and other electrical losses that are experienced in practical PeSCs [43]. The simulated JSC rapidly increases with respect to the perovskite thickness increasing from 0 to 300 nm and then tends to saturate when further increasing the thickness. However, the higher Jsc was achieved in 145-PeSC, owing to the better internal absorption of FAPbI3 perovskite. EQE is defined as the ratio of the number of extracted electrons converted from the current to the number of incident photons on the PeSC [11], and IQE defined as the ratio of the number of extracted electrons converted from the current to the number of photons
Fig. 6 (a) The internal absorption (Q), (b) the exciton generation rate various perovskite thickness, (c) simulated Jsc
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Fig. 7 (a) Time-resolved PL decay recorded for the glass/ZnO/Perovskite films annealed at different temperatures, (b) the charge mobility versus delay time for four diffeent FAPbI3 PeSCs. (assuming IQE is 100%) various perovskite thickness and (d) calculated IQE spectra for four different FAPbI3 PeSCs. absorbed in a perovskite layer of PeSC. IQE (Fig. 6d) provides information about how efficiently the solar cell is able to generate an electron−hole pair, separate and subsequently extract these charges at respective selective contacts. IQE can be calculated from EQE and Q spectra by equation: IQE =
(1)
For the four PeSCs with the same 275 nm FAPbI3 films, the simulated JSC is 18.04, 20.37, 21.27, 20.94 mA/cm2, respectively. In this simulation, we assume that IQE is 100%, which means all photons absorbed in the FAPbI3 films can convert to charges collected by electrodes. Comparing the simulated JTMM with experimental JEQE shown in Table S1, we can see that the simulated JTMM was 21.13%, 17.05%, 10.36%, 12.56% which is higher than JEQE, respectively. Obviously, the difference between the simulated JTMM and experimental JEQE for the 145-PeSC was lower than that of other PeSCs, indicating that there is a enhance in IQE of 145-PeSC (Fig. 6d), due to the decrease of charge recombination and increase of charge collection.
3.4 Charge recombination and collection in PeSCs IQE of PeSCs is mainly controlled by the charge transport and charge collection properties, thus we had recourse to the timeresolved photoluminescence (TrPL) spectroscopy to characterize the dynamics of charge transport based on ITO/ZnO/FAPbI3 samples, as shown in Fig. 7a. The thickness of the four perovskite films was the same, so the charge transport route to ZnO layers should not be different, which indicates that the decay process of TrPL is determined by the crystal quality and grain size of the perovskite films. The TrPL data were fitted with a bi-exponential decay function [44], and the fitting parameters are summarized in Table S3. The smaller time constants τ1 origins from the initially photogenerated excitons starting to migrate toward defects [44], while the larger time constant τ2 reflects the exciton lifetime of FAPbI3 layers. The order of τ2 is: τ2 (145℃) < τ2 (120℃) < τ2 (150 ℃) < τ2 (100℃), thus the FAPbI3 film annealed at 145℃ induces the fastest PL decay, indicating that 145-PeSC presents best charge separation ability among the four PeSCs [45,46]. Furthermore,
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Fig. 8 (a) The time-dependent charge carrier density versus delay time and (b) double logarithmic plot of Jph −Veff characteristics for four different FAPbI3 PeSCs. a more detailed investigation of the charge mobility for the four PeSCs has been accomplished using photo-CELIV [47]. Fig. S8 displays the photo-CELIV curves collected of the four PeSCs with various delay time. Fig. 7b exhibits the time dependence of the charge mobility of the four PeSCs. As a first observation, one can see that the increase in tdelay may correspond to a decreasing charge mobility for the four PeSCs, typically attributed to energy relaxation of the charge packet towards the tail of the density of states distribution [48]. In addition, at the delay time of 2 µs, the mobility of the four PeSCs is within the range of 7~9×10-3cm2V-1s-1, while the charge mobility of 145-PeSC was the highest, which is coincident with previously report [47]. The CELIV technique was also carried out to complementarily investigate the effect of annealing temperature on charge recombination of PeSCs [49-51]. The density of extracted charges versus the delay time was obtained, as shown in Fig. 8a. The density of extracted charges decreases with increasing delay time because of charge recombination loss inside the PeSC devices. For145-PeSC, the charge density exhibits a slower decay as the delay time increasing. The initial density n0 and the recombination lifetime τ were obtained from the fitting to the decay data (Table S4), and the solid lines show a satisfactory fit to our data, as shown in Fig. S9. The higher n(0) and prolonged recombination lifetime τ of 145-PeSC suggest that the PeSC device can generate more free charges which can survive much longer without recombination. Moreover, the charge collection properties of the four PeSCs were analyzed, which is also important in determining the device performance and IQE. Fig. 8b shows the double logarithmic plot of photocurrent Jph as a function of the effective bias Veff (Jph = JL-JD, where JL is the current density measured under irradiation and JD is the dark current, Veff = V0 - Vb, where V0 is the built-in potential measured at Jph = 0, and Vb is the applied bias) [48]. At high Veff (>1.0 V), the saturated photocurrents Jph,sat depends only on the charge generation [52], giving rise to almost 100 % collection of the photo-generated charges at the electrodes. But 145-PeSC possesses a slightly higher charge collection efficiency ηcc (97 %) than that of other devices (93%, 96%, 94%, respectively, shown in Table S3). As Veff decreases to 0.34 V at the maximum power output point and 0.17 V at Voc point, ηcc of 145-PeSC was also much higher than that of other PeSCs. At low Veff region, the number of photo-generated charges that can be extracted increases due to the suppression of the charge recombination, leading to a better charge collection [53].
Consequently, 145-PeSC presents better Q spectra, higher charge mobility, longer charge lifetime and higher charge collection among all PeSC devices, which is the origin of the observed higher IQE and PCE of 145-PeSC.
4 Conclusions The planar FAPbI3 PeSCs with ZnO ETL were fabricated by varying the annealing temperatures of FAPbI3 layers at 100°C, 120°C, 145°C and 150 °C. We estimate EQE, Q and spectrally resolved IQE of PeSCs with different temperature annealed FAPbI3 layers by means of TMM. The results show that the FAPbI3 PeSCs with FAPbI3 layer annealed at 145 ℃ has a relatively high IQE and PCE, due to lower charge recombination and higher charge collection which are demonstrated by photo-CELIV and Jph-Veff measurements. As a result, the performence of device with FAPbI3 film annealed at 145 ℃ is enhanced in comparison with the other three type PeSC devices. This work can give a further understanding about the effect of annealing temperature on physical processes for perovskite solar cells.
Acknowledgements This work was supported by the NSFC-NRF Scientific Cooperation Program (No. 21811540393), Program for Chang Jiang Scholars, Innovative Research Team in University (No. IRT101713018) and Program for Changbaishan Scholars of Jilin Province.
References [1] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E. Moser, M. Gratzel, N.-G. Park, Scientific Report 2 (2012) 591. [2] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 338 (2012) 643. [3] J.-P. Correa-Baena, A. Abate, M. Saliba, W. Tress, T. J. Jacobsson, M. Grätzel, A. Hagfeldt, Energy Environ. Sci. 10 (2017) 710-727. [4] J. Shi, Y. Li, Y. Li, D. Li, Y. Luo, H. Wu, Q. Meng, Joule 2 (2018) 123. [5] J. M. Ball, S. D. Stranks, M. T. Hörantner, S. Hüttner, W. Zhang, E. J. W. Crossland, I. Ramirez, M. Riede, M. B. Johnston, R. H. Friend, H. J. Snaith, Energy Environ. Sci. 8 (2015) 602-609. [6] C. Chen, S. Hsiao, C. Chen, H. Kang, Z. Huang, H. Lin, J. Mater. Chem. A 3 (2015) 9152-9159. [7] G. F. Burkhard, E. T. Hoke, M. D. McGehee, Adv. Mater. 22 (2010) 3293-3297. [8] H. Hoppe, N. Arnold, N. S. Sariciftci, D. Meissner, Solar Energy Materials & Solar Cells 80 (2003) 105-113. [9] H. Hoppe, N. Arnold, D. Meissner, N. S. Sariciftci, Thin Solid Films 451–452 (2004) 589-592. [10] G. F. Burkhard, E. T. Hoke, S. R. Scully, M. D. McGehee, Nano Lett. 9 (2009) 4037-4041.
[11] H. Park, J. An, J. Song, M. Lee, H. Ahn, M. Jahnel, C Im, Solar Energy Materials & Solar Cells 143 (2015) 242-249. [12] J. M. Ball, S. D. Stranks, M. T. Horantner, S. Hüttner, W. Zhang, E. J. W. Crossland, I. Ramirez, M. Riede, M. B. Johnston, R. H. Friend, H. J. Snaith, Energy Environ. Sci. 8 (2015) 602-609. [13] J. Correa-Baena, M. Anaya, G. Lozano, W. Tress, K. Domanski, M. Saliba, T. Matsui, T. J. Jacobsson, M. E. Calvo, A. Abate, M. Grätzel, H. Míguez, A. Hagfeldt, Adv. Mater. 28 (2016) 5031-5037. [14] Y. Liu, H. Zhang, B. Xu, L. Liu, G. Chen, C. Im, W. Tian, J. Mater. Chem. A 6 (2018) 7922-7932. [15] Y. Liu, B. Jin, H. Zhang, Y. Zhang, Y. Kim, C. Wang, S. Wen, B. Xu, C. Im, W. Tian, ACS Applied Materials & Interfaces 11 (2019) 14810-14820. [16] G. E. Eperon, S. D. Stranks, C. Menelaou, M. B. Johnston, L. M. Herz, H. J. Snaith, Energy Environ. Sci. 7 (2014) 982-988. [17] Z. Wang, Y. Zhou, S. Pang, Z. Xiao, J. Zhang, W. Chai, H. Xu, Z. Liu, N. P. Padture, G. Cui, Chem. Mater. 27 (2015) 7149-7155. [18] S. Wozny, M. Yang, A. M. Nardes, C. C. Mercado, S. Ferrere, M. O. Reese, W. Zhou, K. Zhu, Chem. Mater.27 (2015) 4814-4820. [19] F. Ma, J. Li, W. Li, N. Lin, L. Wang, J. Qiao, Chem. Sci. 8 (2017) 800-805. [20] A. Binek, F. C. Hanusch, P. Docampo, T. Bein, J. Phys. Chem. Lett. 6 (2015) 1249-1253. [21] J. Song, W. Hu, X. Wang, G. Chen, W. Tian, T. Miyasaka, J. Mater. Chem. A 4 (2016) 8435-8443. [22] D. Yuan, A. Gorka, M. Xu, Z. Wang, L. Liao, Phys. Chem. Chem. Phys. 17 (2015) 19745-19750. [23] C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem. 52 (2013) 9019-9038. [24] J. Lee, D. Seol, A. Cho, N. Park, Adv. Mater. 26 (2014) 4991-4998. [25] C. Wu, X. Zheng, Q. Yang, Y. Yan, M. Sanghadasa, S. Priya, J. Phys. Chem. C 120 (2016) 26710-26719. [26] S. Aharon, A. Dymshits, A. Rotem, L. Etgar, J. Mater. Chem. A 3 (2015) 9171-9178. [27] V. L. Pool, B. Dou, D. G. V. Campen, T. R. Klein-Stockert, F. S. Barnes, S. E. Shaheen, M. I. Ahmad, M. F. A. M. van Hest, M. F. Toney, Nature Communication 8 (2017) 14075(1)- 14075(8). [28] H. Fang, F. Wang, S. Adjokatse, N. Zhao, J. Even, M. A. Loi, Light: Science & Applications, 5 (2015) e16056(1)- e16056(7). [29] W. J. E. Beek, M. M. Wienk, M. Kemerink, X. Yang, R. A. J. Janssen, J. Phys. Chem. B 109 (2015) 9505-9516. [30] J. Song, L. Liu, X. Wang, G. Chen, W. Tian, T. Miyasaka, J. Mater. Chem. A 5 (2015) 13439-13447. [31] (a) Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Nature Energy 2 (2016) 16177(1)16177(7). (b) L. Wang, C. McCleese, A. Kovalsky, Y. Zhao, C. Burda, J. Am. Chem. Soc.136 (2014) 12205-12208. (c) Q. Chen, H. Zhou, T. Song, S. Luo, Z. Hong, H. Duan, L. Dou, Y. Liu, Y. Yang, Nano Lett. 14 (2014) 4158-4163. [32] S. Prathapani, P. Bhargava, S. Mallick, Appl. Phys. Lett. 112 (2018) 092104(1)- 092104(4). [33] S. Tao, I. Schmidt, G. Brocks, J. Jiang, I. Tranca, K. Meerholz, and S. Olthof, Nat. Commun. 10 (2019) 2560(1)- 2560(10). [34] F. Zu, P. Amsalem, D. A. Egger, R. Wang, C. M. Wolff, H. Fang, M. A. Loi, D. Neher, L. Kronik, S. Duhm, and N. Koch, J. Phys. Chem. Lett. 10 (2019) 601-609.
7
[35] J. Endres, D. A. Egger, M. Kulbak, R.A. Kerner, L. Zhao, S.H. Silver, G. Hodes, B.P. Rand, D. Cahen, L.Kronik, and A. Kahn. J. Phys. Chem. Lett. 7 (2016) 2722-2729. [36] Y. Liu, J. Qian, H. Zhang, B. Xu, Y. Zhang, L. Liu, G. Chen, W. Tian, Organic Electronics 62 (2018) 269-276. [37] Y. Li, X. Xu, C. Wang, B. Ecker, J. Yang, J. Huang, Y. Gao, J. Phys. Chem. C 121 (2017) 3904-3910. [38] C. Li, Q. Guo, H. Zhang, Y. Bai, F. Wang, L. Liu, T. Hayat, A. Alsaedi, Z. Tan, Nano Energy 40 (2017) 248-257. [39] V. Shrotriya, G. Li, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Adv. Funct. Mater.16 (2006) 2016-2023. [40] L. K. Pno, Y. Qi, S. (Frank) Liu. Joule 2 (2018) 1961-1990. [41] K. Domanski , E. A. Alharbi, A. Hagfeldt, M. Grätzel, and W. Tress. Nature Energy 3 (2018) 61-67. [42] M. Saliba. Science 359 (2018) 388-389. [43] Q. Lin, A. Armin, R. C. R. Nagiri, P. L. Burn, P. Meredith, Nature Photonics 9 (2015) 106-112. [44] Z. Zhu, Y. Bai, H. K. H. Lee, C. Mu, T. Zhang, L. Zhang, J. Wang, H. Yan, S. K. So, S. Yang, Adv. Funct. Mater. 24 (2014) 7357-7365. [45] K. Lu, C. Zhao, L. Luan, J. Duan, Y. Xie, M. Shao, B. Hu, J. Mater. Chem. C 6 (2018) 5055-5062. [46] M. Azam, S. Yue, K. Liu, Y. Sun, J. Liu, K. Ren, Z. Wang, S. Qu, Z. Wang, Journal of Alloys and Compounds 731 (2018) 375-380. [47] Y. Chen, J. Peng, D. Su, X. Chen, Z. Liang, ACS Appl. Mater. Interfaces 7 (2015) 4471-4475. [48] T. M. Clarke, J. Peet, A. Nattesta, N. Drolet, G. Dennler, C. Lungenschmied, M. Leclerc, A. J. Mozer, Organic Electronics 13 (2012) 2639-2646. [49] A. J. Mozer, G. Dennler, N. S. Sariciftci, M. Westerling, A. Pivrikas, R. Österbacka, G. Juška, Phys. Rev. B 72 (2005) 035217(1)035217(10). [50] Y. Gao, R. C. I. MacKenzie, Y. Liu, B. Xu, P. H. M. van Loosdrecht, W. Tian, Adv. Mater. Interfaces 2 (2015) 1400555(1) 1400555(7). [51] Y. Gao, A. Pivrikas, B. Xu, Y. Liu, W. Xu, P. H.M. van Loosdrecht, W. Tian, Synthetic Metals 203 (2015) 187-191. [52] Z. Wu, B. Wu, H. L. Tam, F. Zhu, Organic Electronics 31 (2016) 266-272. [53] B Wu, Z. Wu, Q. Yang, F. Zhu, T.-W. Ng, C.-S. Lee, S.-H. Cheung, S.-K. So, ACS Appl. Mater. Interfaces 8 (2018) 14717-14724.
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Highlights The effect of thermal annealing on the internal absorption, charge recombination and internal quantum efficiency of FAPbI3 PeSCs is presented. The effect of thermal annealing on charge recombination within PeSCs were analyzed by photo-CELIV method. The effect of thermal annealing on charge collection within PeSCs were analyzed by Jph-Veff characteristic.
We declare that we have no conflict of interest.
S1566-1199(19)30535-X
DOI:
https://doi.org/10.1016/j.orgel.2019.105508
Reference:
ORGELE 105508
To appear in:
Organic Electronics
Received Date: 5 August 2019 Revised Date:
14 October 2019
Accepted Date: 14 October 2019
Please cite this article as: Y. Liu, H. Zhang, B. Xu, L. Liu, C. Im, W. Tian, Effect of annealing temperature on internal absorption, charge recombination and internal quantum efficiency of HC(NH2)2PbI3 perovskite solar cells, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105508. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
The effect of annealing temperature for the perovskite layer on the internal absorption(Q), charge recombination and internal quantum efficiency (IQE) of HC(NH2)2PbI3 (FAPbI3) perovskite solar cells (PeSCs) was investigated. The PeSC with FAPbI3 film annealed at 145 °C has relatively better internal absorption, lower charge recombination and higher charge collection, leading to the higher device IQE.
Effect of annealing temperature on internal absorption, charge recombination and internal quantum efficiency of HC(NH2)2PbI3 perovskite solar cells Yang Liua, Hao Zhanga, Bin Xua, Leijing Liua, **, Chan Imb and Wenjing Tiana,* a
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China. b
Key Department of Chemistry, Konkuk University, Seoul 05029, Korea.
Abstract
Keywords: Perovskite solar cell Internal absorption Charge recombination Internal quantum efficiency
Despite the rapid development of perovskite solar cells (PeSCs) in photovoltaic field, clear understanding about fundamental mechanisms of PeSCs is still sparse. For PeSCs, the thermal annealing temperature of perovskite layer has a great influence on the physical processes of PeSCs. Here, the effect of annealing temperature at 100°C, 120°C, 145°C and 150 °C for the perovskite layer on the internal absorption, charge recombination and internal quantum efficiency of formamidinium lead triiodide (HC(NH2 )2PbI3, or FAPbI3) PeSCs was investigated by transfer matrix method (TMM), photo-induced charge extraction by linearly increasing voltage (photo-CELIV) and photocurrent density (Jph) versus effective voltage (Veff ) (Jph-Veff ) measurements. We obtained the internal absorption (Q) spectra of PeSCs by TMM, and estimated the internal quantum efficiency (IQE) spectrum of PeSC devices from Q spectra and external quantum efficiency (EQE) spectrum. It was found that the PeSC with FAPbI3 film annealed at 145 °C has relatively higher power conversion efficiency (PCE) and IQE. Furthermore, the analysis of photo-CELIV and Jph-Veff characteristics demonstrated that the PeSC with FAPbI3 film annealed at 145 °C possesses relatively low charge recombination and high charge collection. In a word, there is less power loss of PeSCs when the annealing temperature for the FAPbI3 layer is 145 °C because of the better internal absorption, suppressed charge recombination and higher charge collection.
1. Introduction Organic–inorganic hybrid perovskites as a light harvester in solar cells have attracted much attention due to their high light absorption in the visible region, low exciton binding energy, bipolar transporting ability, superb charge carrier mobility, long-range balanced charge transport lengths and tunable band gap energy [1-4]. In spite of the impressive progresses regarding perovskite solar cells (PeSCs), clear understanding about underlying mechanisms therein is still sparse. Usually, the operation of PeSC can be analyzed in terms of a sequential * Corresponding author. ** Corresponding author. E-mail addresses: [email protected], [email protected]
set of processes. The first process is light absorption that converts the absorbed photon of perovskite layer into an exciton [5]. Upon photoexcitation, the excitons are generated and then separated into free charges. After generation, the free charges transport in the perovskite layer and then transfer into the charge transport layers (CTLs). Subsequently, the charges are collected by the respective electrodes [6], and the charge collection process is in competition with charge recombination process within PeSCs. The initial light absorption stage is critically important on the transformation of light into electricity for PeSCs, thus it is crucial to estimate the quantitative number of photons being captured in the perovskite layer. Often the precise absorption degree of incident light only in the active layer of organic solar cell (OSC) is discussed in term of internal absorptance spectrum (Q) calculated by using transfer matrix method
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(TMM) [7]. Furthermore, the internal quantum efficiency (IQE) spectrum of solar cells can be estimated from a conventional incident photon to current efficiency (IPCE) spectrum (the overall PCE and its spectrally resolved version of IPCE is often denoted as external quantum efficiency (EQE)) with an accurate Q. Meanwhile, the IQE of solar cells is determined by the combination of internal absorption, charge transport and charge collection processes. There are many studies have shown that Q and IQE of OSCs can be accurately obtained by TMM, and in conjunction with the experimental EQE spectra [8-11]. Recently, many research groups have applied to the TMM framework on various studies of PeSCs [12-15]. Formamidinium lead triiodide (HC(NH2)2PbI3, FAPbI3) has arisen as one of the most-studied perovskite absorbers with superior properties, such as broad light absorption, high phase transition temperature and good thermal stability [16-19]. The solution processability of FAPbI3 perovskite layer is one of the simplest methods of deposition, and only a heating step can convert the deposited solution to the crystalline perovskite form [20-22]. The property of FAPbI3 perovskite film is greatly affected by the thermal annealing process. The study by G. Kanatzidis et al [23]. showed that FAPbI3 films has two polymorphs at ambient temperature: black trigonal perovskite phase (α-phase) with a direct bandgap of ~1.45eV and yellow hexagonal non-perovskite phase (δ-phase) with an indirect bandgap of ~2.48 eV. While, according to other reports [21, 24, 25], the FAPbI3 films can have only α-phase at a suitable annealing temperature. The work of Loi et al. revealed that photoluminescence spectra of FAPbI3 films show monoexponential decay at room temperature and an ultrafast decay is observed when the temperature decreased [28]. Furthermore, the annealing temperature of the as-deposited FAPbI3 film was critical to PeSCs performance. The PeSCs based on porous TiO2 layer showed the highest PCE with FAPbI3 film annealed at 150 °C [24], while the PeSCs based on compact TiO2 layer present very similar performance with FAPbI3 films annealed at temperature from 150 °C to 210 °C [27]. In our previous report [21], the PeSCs based on compact ZnO layer showed the highest power conversion efficiency with FAPbI3 film formed at 145 °C. Though many studies have been done to investigate the properties of FAPbI3 films or improve the photovoltaic device performance, the underlying optoelectronic processes of PeSCs affected by thermal annealing of perovskite layer, especially the FAPbI3 absorber films annealed with different temperature by means of TMM are rarely reported. And the comprehensive study of the internal absorption, charge recombination and IQE within PeSCs controlled by annealing temperature of FAPbI3 films will give a further understanding of the essential properties of perovskite materials and devices. Herein, we fabricate planar FAPbI3 PeCSs by varying the annealing temperature of FAPbI3 films at 100°C, 120°C, 145°C and 150 °C. We obtained the Q spectra of PeSCs by means of TMM, and estimated the spectrally resolved IQE of PeSCs from Q spectra and EQE. The results show that the PeSC with FAPbI3 film annealed at 145 °C has a relatively high IQE and PCE, which implies that more power is lost within PeSCs when
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the annealing temperature of FAPbI3 film is either lower or higher than 145 °C. Furthermore, the results of photoinduced charge extraction by linearly increasing voltage (photo-CELIV) and photocurrent density (Jph) versus effective voltage (Veff) measurements indicate that FAPbI3 PeSCs with perovskite films annealed at 145 °C possess lower charge recombination and higher charge collection, leading to the significant improvement of IQE. The current study gains more insight into the fundamental physics of perovskite solar cells.
2. Experiment section 2.1 Materials TheZnO nanoparticles were prepared according to the work of Janssen et al [29]. Lithium salts (>99.0%) were purchased from Aldrich. Lead iodide (beads, -10 mesh, 99.999% trace metals basis), HC(NH2)2I (>98.0%) and Spiro-OMeTAD (2,20,7,70tetrakis-(N,Ndi- 4-methoxyphenylamino)-9,90 -spirobifuorene, >99.0%) was purchased from Xi'an Polymer Light Technology Corp. (PLT). We will denote hereafter this spiro-OMeTAD as Spiro. Ultradry solvents N,N-dimethyl formamide (DMF, >99.9%) and isopropanol (>99.9%) were obtained from J&K and Acros, respectively. All the chemicals and solvents were kept in a glove-box before starting our experiment. Prepatterned indium tin oxide (ITO)-covered glass substrates were cleaned for about 20 min in detergent followed by ultrasonic cleaning in deionized water, acetone, and 2-propanol in sequence for 20 min each. Then the ITO substrates were treated with plasma for about 5 min. The ZnO solution was spin-coated on ITO substrates at 1500 rpm for 50 s, and followed by baking at 150 °C for 15 min. A 460 mg/mL solution of PbI2 in DMF with TBP (120 µL TBP in 1ml DMF) was then spin-coated on the top of the ZnO layer at 4000 rpm for 30 s in the glove-box. The PbI2-coated ZnO thin film was followed by annealing at 70 °C for 10 min. The films were preheated at 100°C for 3 min before dipping into a FAI 2-propanol solution (15 mg/mL) for 20 s followed by annealing at 100 °C, 120 °C, 145 °C and 150 °C for 15 min to obtain the desired crystallite formation. As hole-transporting material (HTM), a Spiro solution (80 mg of Spiro, 10.5 µL of 4-tert-butylpyridine(tBP), and 46.5 µL of a lithium-bis(tri-fluoromethanesulfonyl) imide(Li-TFSI) solution (170 mg Li-TFSI/ 1 mL acetonitrile) in 1 mL chlorobenzene) was spin-coated at 4000 rpm for 30 s on the HC(NH2)2PbI3. The substrates with HTM were left overnight in the dry air in dark at room temperature. Finally, 80 nm thick Ag was thermally evaporated through a shadow mask defining active area of 0.04 cm2 on the top of HTL to produce a completed PeSC device. 2.3 Device characterization The current–voltage characteristics of the solar cells were measured by using a computer-controlled Keithley 2400 source meter measurement system with an AM1.5G filter at a calibrated intensity of 100 mW cm-2 illumination, as determined by a standard silicon reference cell (91150V Oriel Instruments). EQE spectra were measured in air under shortcircuit conditions using a commercial EQE setup (Crowntech
QTest Station 1000AD), which was equipped with a 100 W Xe arc lamp, filter wheel, and monochromator. Monochromated light was chopped at a frequency of 80 Hz and photocurrents measured using a lock-in amplifier. The setup was calibrated against a certified silicon reference diode. The CELIV set up consists of a pulsed laser (Continuum Minilete TM Nd:YAG), a synthesized function generator (Stanford Research System DS345), a digital delay generator (Stanford Research System DG645), and an oscilloscope (Tektronix MSO 4054) for signal observation and recording. Photo-CELIV measurements are carried out under ambient conditions, and samples are irradiated through the ITO side by one 10 ns, 532 nm laser flash. The linearly increasing voltage with an offset voltage Uoffset = 0.6 V and Umax =3.4 V is applied to the device, while the Ag electrode is grounded. The chosen Uoffset exactly compensates for the build-in potential (Vbi) of the devices, thereby nulling the internal electric field before the charge carrier extraction. 2.4 Thin film characterization The thicknesses of ITO, ZnO, FAPbI3, Spiro and Ag layer were estimated from a Veeco Dektak 150 surface profilometer. The Ellipsometry measurements were carried out with a Horiba Jobin Yvon UVISEL 2 ellipsometer under incident angles of 70° for photon energies between 0.6 and 6 eV with 30 meV increment. The ZnO and Spiro films were prepared following the aforementioned processes except using the clean Si wafers as the substrate. It should be noted that the FAPbI3 films were prepared on the substrate of Si/ZnO, in order to keep the FAPbI3 films same as that in the devices. Modelling of the data was performed using Deltapsi2 software by HORIBA Scientific Company, shown in Fig. S6. Before the films were measured, the bare silicon oxide layer was measured, and resulted in a thickness of 2 nm. All measurements were done at room temperature (~298 K). Subsequent extraction of optical constants refractive index ( n ) and extinction coefficient ( k ) were performed by using Deltapsi2 software (HORIBA Scientific Co.). The obtained n and k values of the FAPbI3 perovskite layers are shown in the Fig. S7, and the n, k values of other films were reported in our previous study [15]. Steady-state fluorescence spectra were measured on a Shimadzu RF-5301PC spectrophotometer. Time-resolved photoluminescence (TrPL) spectra were obtained on a PL spectrometer (Edinburgh Instruments, FLS 980), excited with a picosecond pulsed diode laser (EPL-375). A Hamamatsu C5680–04 streak camera was used for TPL. The X-ray diffraction (XRD) patterns were recorded on Rigaku SmartLab X-ray diffractometer with Cu Kα radiation (λ= 1.5418 °A) at 25 °C. The data were collected with a 0.01° step size (2θ). A field emission scanning electron microscope (Hitachi SU8020) was used to acquire SEM images. UV-Vis-NIR spectra were obtained using a Shimadzu UV-2550 spectrophotometer. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) were performed using a multi-technique surface analysis XPS/UPS system (VG Scienta R3000).
3. Results and discussion 3.1 Characterizations of FAPbI3 films All perovskite solar cells (PeSCs) used in this study were fabricated with the same device architecture (shown in Fig. S1). The only changed parameter is the annealing temperature of FAPbI3 films (at 100 °C, 120 °C, 145 °C and 150 °C), thus the effect of annealing temperature on the performance of PeSCs should be addressed mostly as the change of FAPbI3 films properties. To clarify the effect of thermal annealing temperature on FaPbI3 films, X-ray diffraction (XRD) and scanning electron microscopy (SEM) of FAPbI3 films prepared at different annealing temperatures were investigated. The XRD reflections (Fig. S2) of FAPbI3 films at 13.92°, 24.26°, 28.04° and 31.42° indexed to (110), (202), (220) and (222) reflections, respectively, corresponding to the black polymorph of FAPbI3. The yellow polymorph coexists with the black polymorph for the FAPbI3 films annealed at 100 °C, but the yellow polymorph disappears with further elevating the annealing temperature to 120–150°C [21,24]. In addition, the increased intensity of the XRD peak of PbI2 at 2θ = 12.68° reflects the decomposition of FAPbI3 films at higher annealing temperature from 120°C to 150 °C. Fig. 1 gives the top-view SEM images of the FAPbI3 films fabricated at different annealing temperatures. The perovskite film annealed at 100 °C appears relatively loose morphology with small grain size and wide grain boundaries. As the annealing temperature increases to 145°C, the crystalline grains become larger and pack more compactly, which is beneficial to reducing the charge recombination and energy loss during the photophysical processes of PeSCs, as demonstrated by previous studies [30-31]. However, further increasing annealing temperature to 150°C, the grain size is growing larger, but there are some much smaller crystalline grains among the bigger ones, which can increase the number of grain boundaries, leading to higher charge recombination [14].
Fig. 1 Top-view SEM images of the perovskite films annealed at different temperatures: 100℃, 120℃, 145℃ and 150℃ (a–e).
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Fig. 2 Energy bands in glass/perovskite films annealed at different temperatures; (a) UPS binding energy spectra showing secondary cut-off (Ecut-off) and work function (WF), (b) UPS binding energy spectra showing (EF-EV), and (c) Energy level diagram for perovskite films annealed at different temperatures showing VBM, CBM, Fermi energy (EF), and bandgap (Eg). To obtain more insights into the impact of annealing temperature on the quality of FAPbI 3 films, ultraviolet photoelectron spectroscopy/X-ray photoemission spectroscopy (UPS/XPS) was measured for FAPbI3 films deposited on ZnO coated ITO glass substrates. The binding energies in UPS spectra are referenced to the Fermi level (EF) at 0V [32]. The work function (WF) of perovskite films which is the difference between EF and vacuum level (Evac) in an energy band diagram can be obtained by calculating the difference between the cutoff of UPS (Fig. 2a) and incident photon energy (21.22 ev), as shown in Fig. 2c. Fig. 2b shows the valence band edge from the Fermi level (EF-EV). Thus, we can obtain the valence band minimum (VBM) of different FAPbI3 films (Fig. 2c) [33], The UPS spectra of Figure 2b was also plotted on a logarithmic intensity scale to see more clearly the valence band edge of different perovskite films [34,35], as shown in Figure S4. The value of the optical bandgap as the fundamental gap (bandgap, E g ) considered by absorption measurements (Fig. S3) is about 1.52 eV, 1.49 eV, 1.49 eV and 1.50 eV of FAPbI 3 samples with annealing temperature at 100 ℃, 120 ℃, 145 ℃ and 150℃, respectively. Therefore, the conduction band maximum (CBM) of FAPbI3 films were estimated as 4.31 eV, 4.13 eV, 4.04 eV and 4.04 eV. According to previous report [36], the conduction band (Ec) of ZnO nanoparticles is 4.20 eV and the HOMO of Spiro-MeOTAD HTM is 5.20 eV. Thus, it’s clear that FAPbI3 films annealed at higher temperature can extract both the electrons and holes more effectively [37]. In order to make a clear insight into the chemical modifications occurring during the annealing process, the XPS spectra were recorded to confirm the chemical bonding states in the samples. Fig. S5 shows the XPS data of Pb 4f, I 3d, C 1s and N 1s core levels of
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Fig. 3 (a) Current density–voltage curves and (b) EQE spectra of four different FAPbI3 PeSCs. FAPbI3 films with the different annealing temperatures. The obtained atomic ratios of [Pb]:[I]:[C]:[N] were shown in Table S1. Obviously, the FAPbI3 films were composed of some PbI2, which were also proved by XRD result. The main Pb4f peak split into 4f 5/2 and 4f 7/2 doublet, and I3d core level is a doublet consisting of 3d 5/2 and I 3d 3/2 components, which is in agreement with the report of literature [38]. In addition, the signals of Pb 4f, I 3d, C 1s, and N 1s gradually shifted towards higher BE values as a function of annealing temperature, indicating that the charge redistribution of glass/ITO/ZnO/FAPbI3 samples is sensitive to perovskite morphology (Fig. 1), structural and composition (Fig. S2) variations induced by the annealing temperature. As control, charging effects were negligible as the monitored Zn 2p signal showed smaller variations in the binding energy. The shift from Pb and I support that the FAPbI3 film has become more n-doped [37], because the FAPbI3 film become Pb rich. 3.2 Device properties of PeSCs Fig. 3a presents the current–voltage characteristics of PeSCs with perovskite layer annealed at different temperature. We will hereafter denote the PeSCs with FAPbI3 layers annealed at 100 °C, 120 °C, 145 °C and 150 °C as 100-PeSC, 120-PeSC, 145PeSC and 150-PeS, respectively. The obtained solar cell device parameters, open circuit voltage (VOC), short circuit currentdensity (JSC), fill factor (FF) and power conversion efficiency (PCE) are listed in Table 1. The best performance was obtained for 145-PeSC, mainly due to the highest JSC and FF. The external quantum efficiency (EQE) spectra demonstrate that 145-PeSC exhibit a higher photo-to-current conversion efficiency than other PeSCs (see Fig. 3b). The integrated JSC obtained from EQE spectrum for the four PeSCs is 14.41, 17.35, 19.98, 18.52 mA/cm2, respectively. The slight mismatch between the integrated JSC of EQE and the values obtained from J-V curves is also frequently observed elsewhere,
Fig.4 Measured photocurrent output and PCE at the maximum power point of the devices versus time at ambient air (RH of 40%–45%, temperature of 22℃–25℃).
Table 1 Summary of the photovoltaic performance observed for four different FAPbI3 PeSCs.
which was ascribed to the different testing settings [39]. To gain some understanding of the stabilized power output under the operational conditions [40-42], the photocurrent and PCE versus time of the four type devices close to the maximum power point was measured as shown in Figure 4. The photocurrent of all devices rose quickly to their maximum values after the light was turned on, and then stabilized with little change over the time period of 120s. In addition, the PCE of all four type devices decreased quickly within two 2h without encapsulation at ambient air, but the 145-PeSC experienced a slightly slower degradation. For the four PeSCs, the significant different PCE is mainly governed by JSC and FF. Often, JSC is regarded as a measure of charge photogeneration degree when the internal electric field can be described with a simple flat band model [11]. Therefore, the different JSC can be regarded as a matter of difference between the amount of absorbed light, hence number of photons, and the number of extracted charges at a maximum power point for a given PeSC. Since the only changed parameter here is the annealing temperature of perovskite layer, the appearance of different FF should be addressed mostly as a bulk-induced changes. Thus, the matter discussed here can be expressed as the intrinsic bulk-induced property. To discuss it further, JSC should be carefully testified by comparing with the maximum possibility of photon absorption which can be obtained as a form of spatially and spectrally resolved
Fig. 5 Spatially and spectrally resolved mappings of simulated numbers of absorbed photons for four different FAPbI3 PeSCs.
internal absorptance (Q) spectra by transfer matrix method (TMM). Comparing the Q spectra with EQE will give internal quantum efficiency (IQE), which can provide a quantitative assessment on photocurrent forming facility of different PeSCs. 3.3 Internal absorption and internal quantum efficiency of PeSCs The spectrally and spatially resolved internal absorption spectra within the perovskite layers of devices obtained by TMM are shown in Fig. 5. At relatively short wavelength, the optical field intensity is rapid decaying of four samples, while at longer wavelength, the pronounced interference effect range is clearly to be recognized. Subsequently, those maps are spatially integrated to build internal absorptance (Q) spectra, as shown in Fig. 6a. The Q spectra are quite similar at wavelength < 600 nm in 120-PeSC, 145-PeSC, 150-PeSC, but different at wavelength > 600 nm, due to the optical interferences effect [39]. Obviously, the amplitude of the Q spectra for 100-PeSC is lower compared with that for other three PeSCs, which indicates that the perovskite layer can absorb less photons in 100-PeSC. As shown in Fig. 6b, the exciton generation rate of 100PeSC is lower than that of other PeSCs when assuming all the absorbed photons in the FAPbI3 layer can convert to excitons. Meanwhile, we simulated Jsc as a function of perovskite thickness (assuming 100% IQE) for these four PeSCs, as shown in Fig. 6c. It should be noted the simulated JSC values represent the upper limits of photocurrent and do not take into account charge recombination and other electrical losses that are experienced in practical PeSCs [43]. The simulated JSC rapidly increases with respect to the perovskite thickness increasing from 0 to 300 nm and then tends to saturate when further increasing the thickness. However, the higher Jsc was achieved in 145-PeSC, owing to the better internal absorption of FAPbI3 perovskite. EQE is defined as the ratio of the number of extracted electrons converted from the current to the number of incident photons on the PeSC [11], and IQE defined as the ratio of the number of extracted electrons converted from the current to the number of photons
Fig. 6 (a) The internal absorption (Q), (b) the exciton generation rate various perovskite thickness, (c) simulated Jsc
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Fig. 7 (a) Time-resolved PL decay recorded for the glass/ZnO/Perovskite films annealed at different temperatures, (b) the charge mobility versus delay time for four diffeent FAPbI3 PeSCs. (assuming IQE is 100%) various perovskite thickness and (d) calculated IQE spectra for four different FAPbI3 PeSCs. absorbed in a perovskite layer of PeSC. IQE (Fig. 6d) provides information about how efficiently the solar cell is able to generate an electron−hole pair, separate and subsequently extract these charges at respective selective contacts. IQE can be calculated from EQE and Q spectra by equation: IQE =
(1)
For the four PeSCs with the same 275 nm FAPbI3 films, the simulated JSC is 18.04, 20.37, 21.27, 20.94 mA/cm2, respectively. In this simulation, we assume that IQE is 100%, which means all photons absorbed in the FAPbI3 films can convert to charges collected by electrodes. Comparing the simulated JTMM with experimental JEQE shown in Table S1, we can see that the simulated JTMM was 21.13%, 17.05%, 10.36%, 12.56% which is higher than JEQE, respectively. Obviously, the difference between the simulated JTMM and experimental JEQE for the 145-PeSC was lower than that of other PeSCs, indicating that there is a enhance in IQE of 145-PeSC (Fig. 6d), due to the decrease of charge recombination and increase of charge collection.
3.4 Charge recombination and collection in PeSCs IQE of PeSCs is mainly controlled by the charge transport and charge collection properties, thus we had recourse to the timeresolved photoluminescence (TrPL) spectroscopy to characterize the dynamics of charge transport based on ITO/ZnO/FAPbI3 samples, as shown in Fig. 7a. The thickness of the four perovskite films was the same, so the charge transport route to ZnO layers should not be different, which indicates that the decay process of TrPL is determined by the crystal quality and grain size of the perovskite films. The TrPL data were fitted with a bi-exponential decay function [44], and the fitting parameters are summarized in Table S3. The smaller time constants τ1 origins from the initially photogenerated excitons starting to migrate toward defects [44], while the larger time constant τ2 reflects the exciton lifetime of FAPbI3 layers. The order of τ2 is: τ2 (145℃) < τ2 (120℃) < τ2 (150 ℃) < τ2 (100℃), thus the FAPbI3 film annealed at 145℃ induces the fastest PL decay, indicating that 145-PeSC presents best charge separation ability among the four PeSCs [45,46]. Furthermore,
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Fig. 8 (a) The time-dependent charge carrier density versus delay time and (b) double logarithmic plot of Jph −Veff characteristics for four different FAPbI3 PeSCs. a more detailed investigation of the charge mobility for the four PeSCs has been accomplished using photo-CELIV [47]. Fig. S8 displays the photo-CELIV curves collected of the four PeSCs with various delay time. Fig. 7b exhibits the time dependence of the charge mobility of the four PeSCs. As a first observation, one can see that the increase in tdelay may correspond to a decreasing charge mobility for the four PeSCs, typically attributed to energy relaxation of the charge packet towards the tail of the density of states distribution [48]. In addition, at the delay time of 2 µs, the mobility of the four PeSCs is within the range of 7~9×10-3cm2V-1s-1, while the charge mobility of 145-PeSC was the highest, which is coincident with previously report [47]. The CELIV technique was also carried out to complementarily investigate the effect of annealing temperature on charge recombination of PeSCs [49-51]. The density of extracted charges versus the delay time was obtained, as shown in Fig. 8a. The density of extracted charges decreases with increasing delay time because of charge recombination loss inside the PeSC devices. For145-PeSC, the charge density exhibits a slower decay as the delay time increasing. The initial density n0 and the recombination lifetime τ were obtained from the fitting to the decay data (Table S4), and the solid lines show a satisfactory fit to our data, as shown in Fig. S9. The higher n(0) and prolonged recombination lifetime τ of 145-PeSC suggest that the PeSC device can generate more free charges which can survive much longer without recombination. Moreover, the charge collection properties of the four PeSCs were analyzed, which is also important in determining the device performance and IQE. Fig. 8b shows the double logarithmic plot of photocurrent Jph as a function of the effective bias Veff (Jph = JL-JD, where JL is the current density measured under irradiation and JD is the dark current, Veff = V0 - Vb, where V0 is the built-in potential measured at Jph = 0, and Vb is the applied bias) [48]. At high Veff (>1.0 V), the saturated photocurrents Jph,sat depends only on the charge generation [52], giving rise to almost 100 % collection of the photo-generated charges at the electrodes. But 145-PeSC possesses a slightly higher charge collection efficiency ηcc (97 %) than that of other devices (93%, 96%, 94%, respectively, shown in Table S3). As Veff decreases to 0.34 V at the maximum power output point and 0.17 V at Voc point, ηcc of 145-PeSC was also much higher than that of other PeSCs. At low Veff region, the number of photo-generated charges that can be extracted increases due to the suppression of the charge recombination, leading to a better charge collection [53].
Consequently, 145-PeSC presents better Q spectra, higher charge mobility, longer charge lifetime and higher charge collection among all PeSC devices, which is the origin of the observed higher IQE and PCE of 145-PeSC.
4 Conclusions The planar FAPbI3 PeSCs with ZnO ETL were fabricated by varying the annealing temperatures of FAPbI3 layers at 100°C, 120°C, 145°C and 150 °C. We estimate EQE, Q and spectrally resolved IQE of PeSCs with different temperature annealed FAPbI3 layers by means of TMM. The results show that the FAPbI3 PeSCs with FAPbI3 layer annealed at 145 ℃ has a relatively high IQE and PCE, due to lower charge recombination and higher charge collection which are demonstrated by photo-CELIV and Jph-Veff measurements. As a result, the performence of device with FAPbI3 film annealed at 145 ℃ is enhanced in comparison with the other three type PeSC devices. This work can give a further understanding about the effect of annealing temperature on physical processes for perovskite solar cells.
Acknowledgements This work was supported by the NSFC-NRF Scientific Cooperation Program (No. 21811540393), Program for Chang Jiang Scholars, Innovative Research Team in University (No. IRT101713018) and Program for Changbaishan Scholars of Jilin Province.
References [1] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E. Moser, M. Gratzel, N.-G. Park, Scientific Report 2 (2012) 591. [2] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 338 (2012) 643. [3] J.-P. Correa-Baena, A. Abate, M. Saliba, W. Tress, T. J. Jacobsson, M. Grätzel, A. Hagfeldt, Energy Environ. Sci. 10 (2017) 710-727. [4] J. Shi, Y. Li, Y. Li, D. Li, Y. Luo, H. Wu, Q. Meng, Joule 2 (2018) 123. [5] J. M. Ball, S. D. Stranks, M. T. Hörantner, S. Hüttner, W. Zhang, E. J. W. Crossland, I. Ramirez, M. Riede, M. B. Johnston, R. H. Friend, H. J. Snaith, Energy Environ. Sci. 8 (2015) 602-609. [6] C. Chen, S. Hsiao, C. Chen, H. Kang, Z. Huang, H. Lin, J. Mater. Chem. A 3 (2015) 9152-9159. [7] G. F. Burkhard, E. T. Hoke, M. D. McGehee, Adv. Mater. 22 (2010) 3293-3297. [8] H. Hoppe, N. Arnold, N. S. Sariciftci, D. Meissner, Solar Energy Materials & Solar Cells 80 (2003) 105-113. [9] H. Hoppe, N. Arnold, D. Meissner, N. S. Sariciftci, Thin Solid Films 451–452 (2004) 589-592. [10] G. F. Burkhard, E. T. Hoke, S. R. Scully, M. D. McGehee, Nano Lett. 9 (2009) 4037-4041.
[11] H. Park, J. An, J. Song, M. Lee, H. Ahn, M. Jahnel, C Im, Solar Energy Materials & Solar Cells 143 (2015) 242-249. [12] J. M. Ball, S. D. Stranks, M. T. Horantner, S. Hüttner, W. Zhang, E. J. W. Crossland, I. Ramirez, M. Riede, M. B. Johnston, R. H. Friend, H. J. Snaith, Energy Environ. Sci. 8 (2015) 602-609. [13] J. Correa-Baena, M. Anaya, G. Lozano, W. Tress, K. Domanski, M. Saliba, T. Matsui, T. J. Jacobsson, M. E. Calvo, A. Abate, M. Grätzel, H. Míguez, A. Hagfeldt, Adv. Mater. 28 (2016) 5031-5037. [14] Y. Liu, H. Zhang, B. Xu, L. Liu, G. Chen, C. Im, W. Tian, J. Mater. Chem. A 6 (2018) 7922-7932. [15] Y. Liu, B. Jin, H. Zhang, Y. Zhang, Y. Kim, C. Wang, S. Wen, B. Xu, C. Im, W. Tian, ACS Applied Materials & Interfaces 11 (2019) 14810-14820. [16] G. E. Eperon, S. D. Stranks, C. Menelaou, M. B. Johnston, L. M. Herz, H. J. Snaith, Energy Environ. Sci. 7 (2014) 982-988. [17] Z. Wang, Y. Zhou, S. Pang, Z. Xiao, J. Zhang, W. Chai, H. Xu, Z. Liu, N. P. Padture, G. Cui, Chem. Mater. 27 (2015) 7149-7155. [18] S. Wozny, M. Yang, A. M. Nardes, C. C. Mercado, S. Ferrere, M. O. Reese, W. Zhou, K. Zhu, Chem. Mater.27 (2015) 4814-4820. [19] F. Ma, J. Li, W. Li, N. Lin, L. Wang, J. Qiao, Chem. Sci. 8 (2017) 800-805. [20] A. Binek, F. C. Hanusch, P. Docampo, T. Bein, J. Phys. Chem. Lett. 6 (2015) 1249-1253. [21] J. Song, W. Hu, X. Wang, G. Chen, W. Tian, T. Miyasaka, J. Mater. Chem. A 4 (2016) 8435-8443. [22] D. Yuan, A. Gorka, M. Xu, Z. Wang, L. Liao, Phys. Chem. Chem. Phys. 17 (2015) 19745-19750. [23] C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem. 52 (2013) 9019-9038. [24] J. Lee, D. Seol, A. Cho, N. Park, Adv. Mater. 26 (2014) 4991-4998. [25] C. Wu, X. Zheng, Q. Yang, Y. Yan, M. Sanghadasa, S. Priya, J. Phys. Chem. C 120 (2016) 26710-26719. [26] S. Aharon, A. Dymshits, A. Rotem, L. Etgar, J. Mater. Chem. A 3 (2015) 9171-9178. [27] V. L. Pool, B. Dou, D. G. V. Campen, T. R. Klein-Stockert, F. S. Barnes, S. E. Shaheen, M. I. Ahmad, M. F. A. M. van Hest, M. F. Toney, Nature Communication 8 (2017) 14075(1)- 14075(8). [28] H. Fang, F. Wang, S. Adjokatse, N. Zhao, J. Even, M. A. Loi, Light: Science & Applications, 5 (2015) e16056(1)- e16056(7). [29] W. J. E. Beek, M. M. Wienk, M. Kemerink, X. Yang, R. A. J. Janssen, J. Phys. Chem. B 109 (2015) 9505-9516. [30] J. Song, L. Liu, X. Wang, G. Chen, W. Tian, T. Miyasaka, J. Mater. Chem. A 5 (2015) 13439-13447. [31] (a) Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Nature Energy 2 (2016) 16177(1)16177(7). (b) L. Wang, C. McCleese, A. Kovalsky, Y. Zhao, C. Burda, J. Am. Chem. Soc.136 (2014) 12205-12208. (c) Q. Chen, H. Zhou, T. Song, S. Luo, Z. Hong, H. Duan, L. Dou, Y. Liu, Y. Yang, Nano Lett. 14 (2014) 4158-4163. [32] S. Prathapani, P. Bhargava, S. Mallick, Appl. Phys. Lett. 112 (2018) 092104(1)- 092104(4). [33] S. Tao, I. Schmidt, G. Brocks, J. Jiang, I. Tranca, K. Meerholz, and S. Olthof, Nat. Commun. 10 (2019) 2560(1)- 2560(10). [34] F. Zu, P. Amsalem, D. A. Egger, R. Wang, C. M. Wolff, H. Fang, M. A. Loi, D. Neher, L. Kronik, S. Duhm, and N. Koch, J. Phys. Chem. Lett. 10 (2019) 601-609.
7
[35] J. Endres, D. A. Egger, M. Kulbak, R.A. Kerner, L. Zhao, S.H. Silver, G. Hodes, B.P. Rand, D. Cahen, L.Kronik, and A. Kahn. J. Phys. Chem. Lett. 7 (2016) 2722-2729. [36] Y. Liu, J. Qian, H. Zhang, B. Xu, Y. Zhang, L. Liu, G. Chen, W. Tian, Organic Electronics 62 (2018) 269-276. [37] Y. Li, X. Xu, C. Wang, B. Ecker, J. Yang, J. Huang, Y. Gao, J. Phys. Chem. C 121 (2017) 3904-3910. [38] C. Li, Q. Guo, H. Zhang, Y. Bai, F. Wang, L. Liu, T. Hayat, A. Alsaedi, Z. Tan, Nano Energy 40 (2017) 248-257. [39] V. Shrotriya, G. Li, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Adv. Funct. Mater.16 (2006) 2016-2023. [40] L. K. Pno, Y. Qi, S. (Frank) Liu. Joule 2 (2018) 1961-1990. [41] K. Domanski , E. A. Alharbi, A. Hagfeldt, M. Grätzel, and W. Tress. Nature Energy 3 (2018) 61-67. [42] M. Saliba. Science 359 (2018) 388-389. [43] Q. Lin, A. Armin, R. C. R. Nagiri, P. L. Burn, P. Meredith, Nature Photonics 9 (2015) 106-112. [44] Z. Zhu, Y. Bai, H. K. H. Lee, C. Mu, T. Zhang, L. Zhang, J. Wang, H. Yan, S. K. So, S. Yang, Adv. Funct. Mater. 24 (2014) 7357-7365. [45] K. Lu, C. Zhao, L. Luan, J. Duan, Y. Xie, M. Shao, B. Hu, J. Mater. Chem. C 6 (2018) 5055-5062. [46] M. Azam, S. Yue, K. Liu, Y. Sun, J. Liu, K. Ren, Z. Wang, S. Qu, Z. Wang, Journal of Alloys and Compounds 731 (2018) 375-380. [47] Y. Chen, J. Peng, D. Su, X. Chen, Z. Liang, ACS Appl. Mater. Interfaces 7 (2015) 4471-4475. [48] T. M. Clarke, J. Peet, A. Nattesta, N. Drolet, G. Dennler, C. Lungenschmied, M. Leclerc, A. J. Mozer, Organic Electronics 13 (2012) 2639-2646. [49] A. J. Mozer, G. Dennler, N. S. Sariciftci, M. Westerling, A. Pivrikas, R. Österbacka, G. Juška, Phys. Rev. B 72 (2005) 035217(1)035217(10). [50] Y. Gao, R. C. I. MacKenzie, Y. Liu, B. Xu, P. H. M. van Loosdrecht, W. Tian, Adv. Mater. Interfaces 2 (2015) 1400555(1) 1400555(7). [51] Y. Gao, A. Pivrikas, B. Xu, Y. Liu, W. Xu, P. H.M. van Loosdrecht, W. Tian, Synthetic Metals 203 (2015) 187-191. [52] Z. Wu, B. Wu, H. L. Tam, F. Zhu, Organic Electronics 31 (2016) 266-272. [53] B Wu, Z. Wu, Q. Yang, F. Zhu, T.-W. Ng, C.-S. Lee, S.-H. Cheung, S.-K. So, ACS Appl. Mater. Interfaces 8 (2018) 14717-14724.
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Highlights The effect of thermal annealing on the internal absorption, charge recombination and internal quantum efficiency of FAPbI3 PeSCs is presented. The effect of thermal annealing on charge recombination within PeSCs were analyzed by photo-CELIV method. The effect of thermal annealing on charge collection within PeSCs were analyzed by Jph-Veff characteristic.
We declare that we have no conflict of interest.