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Efficient CH3NH3PbI3-x(SeCN)x perovskite solar cells with improved crystallization and defect passivation
Journal Pre-proof Efficient CH3NH3PbI3-x(SeCN)x perovskite solar cells with improved crystallization and defect passivation Zhu Ma, Zheng Xiao, Weiya Zhou, Lifen Jin, Dejun Huang, Huifeng Jiang, Tian Yang, Yuchuan Liu, Yuelong Huang PII:
S0925-8388(19)34785-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.153539
Reference:
JALCOM 153539
To appear in:
Journal of Alloys and Compounds
Received Date: 6 November 2019 Revised Date:
20 December 2019
Accepted Date: 24 December 2019
Please cite this article as: Z. Ma, Z. Xiao, W. Zhou, L. Jin, D. Huang, H. Jiang, T. Yang, Y. Liu, Y. Huang, Efficient CH3NH3PbI3-x(SeCN)x perovskite solar cells with improved crystallization and defect passivation, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2019.153539. 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.
Credit Author Statement Zhu Ma has initiated the idea of this work. Zhu Ma, Zheng Xiao, Weiya Zhou, Lifen Jin, Dejun Huang carried out the experiment. Zhu Ma, Zheng Xiao, Huifeng Jiang, Tian Yang, Yuchuan Liu, Yuelong Huang performed the analysis and writing up the manuscript. In addition, the authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Efficient CH3NH3PbI3-x(SeCN)x perovskite solar cells with improved crystallization and defect passivation Zhu Ma1*, Zheng Xiao1, Weiya Zhou1, Lifen Jin1, Dejun Huang1, Huifeng Jiang2, Tian Yang2, Yuchuan Liu2, Yuelong Huang*1 1. Institute of Photovoltaic, Southwest Petroleum University, Chengdu 610500. P.R. China 2. School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500. P. R. China
Corresponding author: Zhu Ma (Tel: +86 13880863057; Fax: +86 02883037195; E-mail: [email protected])
Abstract The presence of grain boundaries and defects in perovskite films can be detrimental to performance of perovskite solar cells (PSCs), such as leading to nonradiative recombination and environmental instability. Additive engineering is a key tactic to control crystal growth and passivate defects of perovskite film. In this work, lead-free alkali metal additive of potassium selenocyanate (KSeCN) was firstly added in precursor solution to prepare a novel perovskite of CH3NH3PbI3-x(SeCN)x. The SeCN- is substitute for I- to enhance structure stability, simultaneously facilitate nucleation, meanwhile, effectively increase crystalline grain size and passivate the defects. Finally, the power conversion efficiency of planar perovskite solar cells boosts from 14.34% to 18.41%, and the hysteresis reduces from 21% to 1.5%, nearly negligible. Our findings provide an avenue for crystalline regulation and defects passivation to further improve the performance of perovskite solar cells and propose a promising CH3NH3PbI3-x(SeCN)x-based perovskite. Keywords: CH3NH3PbI3-x(SeCN)x, crystallization
perovskite
solar
cells,
KSeCN
additive,
stability,
Introduction Over the past few years, perovskite solar cells (PSCs) have attracted extensive research attention in the photovoltaic field. The power conversion efficiency (PCE) has been boosted from 3.8% to a maximum of 25.2% currently, which can compete with the traditional rivals, such as CIGS and CdTe, even the silicon solar cells [1]. Such tremendous enhancement is largely due to the optimization of perovskite composition, design of device architecture, interface engineering and process development [2, 3]. The preparation process of one-step deposition is one of the most widely used technology, because of its simple fabrication and great reproducibility [4]. However, the rapid crystallization process tends to produce relatively uncontrollable morphology with small grains, excessive grain boundaries, amount of shallow and intrinsic defects when using one-step deposition [5], which finally lead to obvious carrier recombination and environmental instability of perovskite film [6, 7]. As known, CH3NH3PbI3 is easily decomposed due to its inherent instability of crystal structure to moisture, and the loss of device original PCE could be easily triggered with the presence of a trace amount of water [8, 9]. In previous study, water molecule is found to be trapped between the organic cation CH3NH3+ and the inorganic anion [PbI3]− due to the Coulomb attraction [10]. Moreover, H2O as a “catalyst” to remove I− from [PbI3]−, which in turn combines with H+ of the organic cation, resulting in the formation of HI and PbI2 [11]. Therefore, it is significant to prohibit the interaction of water molecules and [PbX3]-, and enhance the bonding between Pb and X. Normally, the hydrophobic protective layer and encapsulation technology were used to prevent the penetration of moisture [12]. Meanwhile, exploring suitable X site anion to enhance the coordinate bonding with Pb is another avenue to improve crystal structure stability of CH3NH3PbI3 [13, 14]. Based on previous studies, the larger size of [PbX3]−, the lower bonding ionicity and the smaller vertical detachment energy (VDE), means better stability of perovskite structure [15]. The VDE of [PbX3]− is determined by the VDE of X−, therefore, it is important to explore better matched ionic or cluster ion to optimize the
VDE of [PbX3]− and enhance perovskite stability against moisture. As homologues to iodine, chlorine and bromine have similar properties and suitable atomic radius for incorporation into perovskites. Sargent et al. successfully fabricated CH3NH3PbBr3 and CH3NH3PbCl3 perovskites which had increased band gaps but greatly improved stability [16]. Similar observations were also reported for mixed halide perovskites CH3NH3PbI3-xBrx, when I− was substituted by Br−. The enhanced stability was attributed to the stronger interaction between Br− and CH3NH3+ [17, 18]. Mathews et al could show that the addition of chloride by a sequential deposition method enhanced the photovoltaic performance [19]. These studies suggest that ion doping might be a possible way to improve the inherent stability of perovskite films. The Pb(SCN)2 additive was proposed to prepare perovskite, which was carried out at ambient environment with 70% relative humidity [20, 21]. High quality CH3NH3PbI3-x(SCN)x perovskite film and device efficiency up to 15% were obtained with optimizing components [22]. It is can be seen that the substitute of thiocyanate SCN- moiety with I- can also greatly improve the structure stability of perovskite to moisture [23]. However, some reports have also shown that the addition of Pb(SCN)2 affects the full reaction of PbI2 with methylamine iodine, which easily leads to excessive PbI2 in the modified perovskite film, which hinders the further improvement of the performance of the PSCs [24, 25]. Recently, a novel pesudohalide ion was found to replace I-, which is selenite ion (SeCN-) with a VDE of 3.39 eV, similar as SCN- ion (3.52 eV) and I- ion (3.30 eV). According to the Goldschmidt tolerance factor and VDE theory, the novel CH3NH3PbI3-x(SeCN)x could exhibit excellent stability. However, the 3D structure of CH3NH3PbI3-x(SeCN)x has not been experimentally prepared till now. Moreover, in previous study, the Pb(SCN)2 additive based perovskite exhibits outstanding stability. However, the excess PbI2 residue still limits the device efficiency [26]. In order to avoid excess PbI2, it is of great significance to explore pseudo-halogenated additives without metal lead on the perovskite film. It has recently been reported that alkali metal halide additives can improve the morphology and crystallinity of CH3NH3PbI3 films, thereby promoting charge generation and separation [27].
Here, we prepared a highly efficient PSCs by adding a lead-free alkali metal additive of KSeCN to one-step precursor solution. We adopted PSCs with the structure of the FTO/TiO2/CH3NH3PbI3-x(SeCN)x/Spiro-oMeTAD/Ag. The results show that the perovskite film prepared by adding 3% KSeCN has large crystal grains and uniform morphology. With this approach, the PCE of CH3NH3PbI3-base device improved to 18.41%. More importantly, the device remained at 85.8% efficiency after being left unsealed in glove box for 500 h, and the hysteresis effect of the obtained device was significantly reduced to negligible.
Experimental Section Materials Potassium selenocyanate (KSeCN 99.9%) was obtained from Aladdin. Titanium isopropoxide (Ti(OCH(CH3)2)4, 99.9999%), N,N-dimethylformamide (DMF) and ethanol (C2H6O) were purchased from Sigma-Aldrich. Lead (II) iodide (PbI2, 99.99%), methanaminium
iodide
2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)
(CH3NH3I,
99.5%),
amino]-9,9'-spirobifluorene
(Spiro-oMeTAD, 99.8%), tert-butylpyridine (tBP), lithium bis(trifluoromethane sulfonimide) (Li-TFSI) and acetonitrile (C2H3N) were purchased from Xi’an Polymer Light Technology Corp. All above materials were purchased without any purification.
Device fabrication The FTO substrates were sequentially ultrasonic cleaned with detergent, deionized (DI) water, acetone, and anhydrous ethanol for 15 min, respectively. Before using, the substrates were dried with nitrogen stream and treated with UV-ozone for 15 min. For the planar TiO2 electron transporting layer, the precursor solution of titanium isopropoxide was spin-coated on glass/FTO substrate at 3000 rpm for 30 s. After pre-annealed at 150
for 20 min, the film was then annealed 500
for 30 min.
553 mg PbI2, 191 mg CH3NH3I and different amount of KSeCN were dissolved in 1 ml DMF to prepare perovskite precursor solution with stirring at 60 °C overnight, and the KSeCN and CH3NH3PbI3 molar ratio ranging from 0% to 6%. The perovskite precursor solution was spin-coated onto the TiO2 layer at 3000 rpm for 55 s, followed
by chlorobenzene (85 mL) drip during the spinning process. The prepared film was annealed on hotplate at 100
for 20 min. The hole transporting layer was spin-coated
on perovskite absorption layer at 3000 rpm for 30 s with a solution comprising 1 mL chlorobenzene, 72.3 mg of Spiro-oMeTAD, 28.8 µL of tBP and 17.5 µL of Li-TFSI solution (520 mg Li-TFSI in 1 mL acetonitrile). Subsequently, the samples were aged in a dry box for 3 hours to fully oxidize the Spiro-oMeTAD. Finally, 100 nm of silver were consecutively deposited by physical vapor deposition at 4×10-3 Pa to complete the device fabrication. The active area of solar cells was controlled to 0.16 cm2.
Materials and Devices Characterization The scanning electron microscope (SEM) images were investigated by using ZEISS system (EV0MA15), and the system was connected to an energy dispersive X-ray spectroscopy (EDS) detector. The X-ray diffraction (XRD) patterns of the perovskite thin films (2θ scans) were performed on a DX-2700 system using Cu-Kα radiation (λ=0.15406 nm) as the X-ray source. Scans were taken with 0.5 mm wide source and detector slits, and the X-ray generator settings at 40 kV and 30 mA. X-ray photoelectron spectroscopy (XPS) measurements were carried out by an X-ray photoelectron spectrometer (KRATOS, AXIS Ultra DLD) with the monochromated Al Kα X-ray source (hν=1486.6 eV, 200 W). Absorption spectra were carried out by an UV-vis spectrometer (UV-2600, SHIMADZU). FTIR spectra of thin films were then acquired on a Thermo Scientific Nicolet 6700 FTIR spectrophotometer. Steady-state photoluminescence (PL) and time-resolved PL (TRPL) were measured by FLS-980 Edinburgh instruments. The current density-voltage (J-V) characteristics of PSCs were measured with an electrochemical workstation (CHI660D) under a simulated solar spectroscopy (AM 1.5) produced by a solar simulator (Zolix SS150). The intensity of the solar simulator was calibrated to 100 mW/cm2 using a silicon reference photodiode. The devices were measured both in reverse scan (1.2 V→0 V, step 0.01 V) and forward scan (0 V→1.2 V, step 0.01 V). The devices were taken out for external quantum efficiency (EQE) measurements using a PVE 300 (Bentham, Inc.) EQE measurement system.
Results and discussion
Figure 1 Illustration of the one step process to prepare perovskite films.
The preparation and crystallization processes of perovskite films are shown in Fig. 1. For CH3NH3PbI3, PbI2: CH3NH3I: DMF precursor was prepared, spin-coated onto ETL, followed by chlorobenzene drip, and annealed at 100
for 20 min. In
conventional process, the crystal growth process is quick and hard to control, easily generates small grains and amounts of grain boundaries. For CH3NH3PbI3-x(SeCN)x, KSeCN: PbI2: CH3NH3I: DMF precursor was prepared, other steps are similar. In this process, the addition of SeCN- replaces a part of I- and a strong electrostatic interaction is generated between Se2- and CH3NH3+, which is beneficial to improve the stability of perovskite[28]. Moreover, the SeCN− is a kind of pseudohalide, which has a similar chemical property as halide. However, the interaction between Pb2+ and SeCN- is much stronger than that of between Pb2+ and I-, due to SeCN- is a strong coordination bond bridging group. Furthermore, the coordination of SeCN- ligands could reduce the activation energy of crystal grains, similar as SCN-[29]. Hence, the additive of SeCN- coordinated with Pb2+ could retard perovskite crystal nucleation and result in nucleation of large-sized colloidal clusters in the precursor solution, finally enlarge the grain size of the perovskite film during crystallization. The change of perovskite morphology by adding SeCN- can be observed by SEM. Surface topography of perovskite films prepared with different amounts of KSeCN were analyzed by SEM in Fig. 2(a-d). Interestingly, the grain size of
perovskite film was directly related to the amount of KSeCN incorporated, the overall grain size trend is increasing with the increase of KSeCN concentration. The average grain size obtained without KSeCN additive is approximately 220 nm. Only adding 1.5% KSeCN, the grain size dramatically increases to 311 nm. The average grain size continuously increases to 460 nm and 479 nm with 3% and 6% KSeCN, respectively. The statistics data of grain size was extracted from SEM images with 1 µm scale bar. From Fig. 2(e), the perovskite film with 3% KSeCN exhibits bigger grain, more uniform grain size and fewer grain boundaries, facilitate to achieve effective PSCs. Although the grain size of perovskite film with 6% of KSeCN is the largest, they vary in size. Moreover, the films also exhibit amounts of cracks and pinholes, which are deleterious for the device performance.
Figure 2 Top-surface SEM images (a-d) and histogram of grain size distribution (e) of perovskite films prepared with 0% KSeCN, 1.5% KSeCN, 3% KSeCN, 6% KSeCN.
Figure 3 (a) Schematic depiction of the hypothesized CH3NH3PbI3-x(SeCN)x-based perovskite structure. (b) FTIR spectrum, (c) XRD patterns and (d) Reversal FWHM of the different concentration of KSeCN.
Fig. 3(a) shows the schematic depiction of the hypothesized KSeCN perovskite structure. SeCN- was introduced to replace partially iodine, and reduce the interaction between the water molecule and [PbX3]-, so that the water molecule is unlikely to be trapped between the cation and the anion. Second, enhanced bonding between Pb and X in [PbX3]− can effectively controls the migration of X− and prohibits the formation of AgX [30]. The backbones between CH3NH3+and Se2- is able to enhanced electrostatic interaction [28]. The role of the chemical bond in the perovskite without added KSeCN is only the force of I and Pb. This force is relatively weak and tends to cause decomposition when the perovskite is affected by the external environment. After adding KSeCN, the interaction between Pb2+ and CN- is stronger, making the framework structure of CH3NH3PbI3 more stable and difficult to decompose [23]. The energy dispersive X-ray analysis in Fig. S1 confirmed that Se, Pb and I were almost stoichiometric in the film samples. The statistics of the elemental analysis summarized in the table S1. With the increase of KSeCN additive, the Se element dramatically increases. To explore the role of KSeCN additive, XPS measurement was carried out. Fig. S2 shows the XPS spectra of the KSeCN added and unadded perovskite film samples. The Pb 4f spectrum exhibited two contributions, 4f7/2 and
4f5/2 located at 137.7 and 142.4 eV for the film without KSeCN and 137.9 and 142.7 eV with KSeCN, respectively [31]. The shift of the Pb peaks toward higher binding energies is evidence for the formation of stronger ionic bonding between Pb2+ and SeCN- ions. In order to verify the presence of C=N in the perovskite film with KSeCN, films were characterized by flourier transform infrared spectroscopy (FTIR). Fig. 3(b) illustrates normalized FTIR spectrum of perovskites and pure KSeCN. The characteristic peaks of pure CH3NH3PbI3 are mainly concentrated at 910 cm-1, 1467 cm-1, 1629 cm-1. The characteristic peaks of pure KSeCN are mainly concentrated at 831 cm-1, 1579 cm-1, 2074 cm-1. After adding different concentration of KSeCN in CH3NH3PbI3, the perovskite films exhibited a new peak around 1579 cm−1 , which derives from pure KSeCN corresponding to C=N stretch. The influence of perovskite crystal formed by adding alkali metal additive KSeCN were further observed. As shown in Fig. 3(c), the XRD patterns of the perovskite crystals prepared without KSeCN appear diffraction peaks at the (110), (220) and (310), and the corresponding diffraction peaks are 14.1°, 28.5° and 31.8°, respectively. All above diffraction peaks are originated from pure CH3NH3PbI3 perovskite [31]. With the increasing of KSeCN amount, the intensities of all diffraction peaks increase and the positions of diffraction peaks do not change, which indicates that the KSeCN additive can assists the crystal growth and improves the crystal quality. Moreover, the XRD characteristics of perovskites are consistent with the morphologies. The film crystallinities obtained from different KSeCN contents were compared by plotting the reciprocal full width at half maximum (FWHM) from the XRD spectra as shown in Fig. 3(d). As the intensity of the diffraction peak increases, the reciprocal FWHM increases and the FWHM of the diffraction peak becomes narrower, indicating that the addition of KSeCN effectively improves the crystallization characteristics of perovskite film.
Figure 4 (a) Cross-sectional SEM image of the device. J-V curve (b), Histograms of PCE distribution statistics for devices (c) and external quantum efficiency curves (d) of the PSCs with different KSeCN concentration. (e) J-V curves of the PSCs under forward and reverse sweep voltage scanning. (f) Stability measurement of PSCs with and without KSeCN.
In order to confirm the effect of KSeCN on perovskite, the perovskite solar cells with a structure FTO/TiO2/CH3NH3PbI3-x(SeCN)x/Spiro-oMeTAD/Ag was fabricated, as described in the cross-sectional SEM image of a completed PSCs (Fig. 4(a)). The Fig. 4(b) shows the J-V curves measured by PSCs with different amount of KSeCN additive, the key parameters are summarized in Table 1. The control device without KSeCN additive exhibited a short-circuit current density (JSC) of 22.02 mA/cm2, open-circuit voltage (VOC) of 1.06 V, fill factor (FF) of 61.20%, and a maximum power conversion efficiency (PCE) of 14.34%. In contrast, the PSCs based on 3% KSeCN additive exhibit significant improvements in all key parameters. The devices present a JSC of 23.35 mA/cm2, VOC of 1.07 V, FF of 73.6%, and a maximum PCE of 18.41%. The PCEmax dramatically increases 28.3%, compare to control devices. From average value of Table 1 and Fig. S3, it is clearly showed that all parameters increased first and then decreased with the increase of KSeCN concentration, and the results are reliable and reproducible. This result is consistent with the morphology results observed in the SEM of Fig 2. It can be seen that with the increase of KSeCN
concentration, the grain size dramatically increases, the surfaces become smoother and pinholes are suppressed, which are helpful for performance enhancement. When excessive 6% KSeCN is added, the crystal growth process is not easy to regulate and amounts of cracks and pinholes are generated, which leads to leak current, carrier recombination and performance reduction. In order to further verify the improved reproducibility, 65 separate devices each based on different KSeCN content were prepared and reverse-scanned under AM 1.5G irradiation. As shown in Fig. 4(c), in comparison with intrinsic CH3NH3PbI3 based devices, PCE value of CH3NH3PbI3-x(SeCN)x-based device displays narrower distribution with smaller standard deviation. These results indicate that KSeCN is a promising perovskite additive for the construction of efficient and reproducible PSCs. Fig. 4(d) shows the external quantum efficiency (EQE) spectra of the cells using different content KSeCN additive. It can be confirmed that the improvement of the CH3NH3PbI3-x(SeCN)x layer plays an important role in further generation and transport of photocurrent. Meanwhile, the EQE tendency is consistent with JSC in Table 1 [33]. The hysteresis phenomena of PSCs were investigated by comparing the J-V characteristic of reverse can and forward scan, which is shown in Fig. 4(e). For the device without KSeCN additive, the PCE of reverse scan and forward scan is 14.34% and 11.28%, respectively. Under the best concentration of KSeCN, PCE of 18.41% from reverse scan and 18.13% from forward scan for are obtained. Here, we use the hysteresis index (HI) to quantify the hysteresis level [33]: HI =
(1)
PCERS and PCEFS are represented PCE for reverse and forward scan, respectively. It can be easily calculated that the HI of PSCs without KSeCN is 21.3%, however, the HI of PSCs with 3% KSeCN is only 1.5%. A rapid reduction of hysteresis level was obtained after adding KSeCN, which derives from the stronger bonding between SeCN- and Pb2+, passivation of surface trap states and the improvement of interface properties.
At the same time, the long-term stability of PSCs was evaluated, as shown in Fig. 4(f). The unencapsulated devices are placed in nitrogen-filled glove box and the performance of the device is tested every 24 hours. After continuous testing for 500 h, the PCE of devices with 3% KSeCN can still be maintained above 85.3%, but the PCE of the device without KSeCN is severely attenuate to 40.4% after 500 h measurement.
After
systematically
CH3NH3PbI3-x(SeCN)x-based
perovskite
measured solar
the cells,
performance the
of
intrinsic
CH3NH3PbI3-x(SeCN)x exhibits excellent PCE, negligible hysteresis and good stability. It can be predicted that with the optimization of crystal growth processes, interface engineering, device structure and carrier transporting materials, the comprehensive performance can be further improved. In order to describe the mechanism of enhanced device performance after adding KSeCN, the optical properties of CH3NH3PbI3-x(SeCN)x-based perovskite film were investigated. As depicted in Fig. 5(a), it can be seen that the perovskite exhibited obviously enhanced absorption in the ranges of 304~418 nm and 507~762 nm with 3% KSeCN,
compare
to
intrinsic
CH3NH3PbI3.
Especially,
the
CH3NH3PbI3-x(SeCN)x-based perovskite could effectively use the ultraviolet light, purple light and blue light. This perovskite structure also solves the problem of low utilization efficiency at shortwave region compared to normal solar materials [34]. However, after adding excess KSeCN with 6%, the UV-vis absorption intensity is even lower than pristine perovskite. The improved absorption of perovskite layer can be benefit for the generation of photocurrent. As shown in table 1, it can be clearly seen that the absorbance tendency of all perovskite film is good coincidence with the current density of corresponding PSCs. Table 1 Device performance parameters based on different concentrations of KSeCN additive. KSeCN
Maximum (average) Voc(V)
Jsc(mA/cm2)
FF(%)
PCE(%)
0%
1.06 (1.03)
22.02 (20.32)
61.20 (61.09)
14.34 (14.00)
1.5%
1.05 (1.04)
22.62 (21.20)
67.10 (67.78)
15.94 (15.00)
3%
1.07 (1.05)
23.35 (22.54)
73.60 (72.80)
18.41 (17.32)
6%
1.03 (1.00)
19.34 (19.05)
60.30 (55.26)
12.04 (10.50)
Figure 5 UV-visible absorption spectra (a) of samples based on glass/perovskite, PL spectra (b) and TRPL spectra (c) of samples based on Glass/FTO/perovskite with different concentrations of KSeCN additive.
For deep analyze the carrier recombination of the perovskite films with different concentrations of KSeCN, the steady-state and time-resolved PL spectroscopy was used [35]. As shown in Fig. 5(b), the perovskite films exhibit emissive peaks at 774 nm, 770 nm, 770 nm and 768 nm for 0%, 1.5%, 3%, 6% additive, respectively. The obvious blue-shift of CH3NH3PbI3-x(SeCN)x-based perovskites lead to passivation of surface trap states [36], which is caused by the doping of SeCN- ion. As previous depicted, the bonding between SeCN- and Pb2+ is more stable than I- and Pb2+. Meanwhile, the steady-state PL intensity of FTO/perovskite samples increases firstly with the increasing of KSeCN content and reaches the maximum at 3%, and then decrease at 6%. The results of enhanced PL intensity also indicate that the addition of an appropriate amount of KSeCN can passivate the defect state on the surface of the perovskite or the grain boundary, finally reduce the nonradiative recombination. Fig. 5(c) shows the TRPL decay of samples based on Glass/FTO/perovskite with different concentrations of KSeCN additive. As shown in Table 2, for pure CH3NH3PbI3 sample, τ1=41 ns, τ2=120 ns, resulting in an amplitude average lifetime of 118 ns. For 3% KSeCN based perovskite, τ1 and τ2 are increased to 80 ns and 264 ns, with an amplitude average lifetime of 262 ns. The average PL lifetime is determined by the equation: τ=
(2)
where A1 and A2 are the relative amplitude fraction for each decay component and τ1 and τ2 are the fast and slow decay PL species. The TRPL results showed that the carrier lifetime of CH3NH3PbI3-x(SeCN)x-based perovskites can be significantly
increased when adding appropriate KSeCN additive, indicating efficient charge carrier transportation and collection, which is also facilitate to suppress J-V hysteresis. Table 2 Characteristic parameters of PL lifetime by fitting the TRPL spectra for perovskite. KSeCN Content(%)
τ1 (ns)
τ2 (ns)
τ (ns)
0
41
120
118
1.5
58
189
188
3
80
264
262
6
4.9
80
79
Conclusions In summary, it has been proved that adding appropriate alkali metal halide additive KSeCN into perovskite can effectively improve the device comprehensive performance of CH3NH3PbI3-x(SeCN)x-based PSCs. Here, a maximum steady-state PCE of 18.41%, 1.5% hysteresis index and 85.3% efficiency remain after 500 hours measurement were achieved after optimizing the KSeCN ration in perovskite precursor. The enhanced device performance is attributed to stable perovskite structure, enlarged grain size and passivated defects. Our study indicates that the effective CH3NH3PbI3-x(SeCN)x-based perovskite is a promising candidate to substitute traditional perovskite, further improvement could be obtained by regulating crystallization process, optimizing interface and using effective transport material.
Acknowledgements The authors gratefully acknowledge the financial support from Sichuan Science and Technology Program (Grant No. 2018JY0015), Young Scholars Development Fund of SWPU (Grant No. 201699010017), and Scientific Research Starting Project of SWPU (Grant No. 2017QHZ021). References [1] https://www.nrel.gov/pv/cell-efficiency.html. [2] Z. Ma, W. Y. Zhou, Z. Xiao, H. Zhang, Z. Li, J. Zhuang, C. T. Peng, Y. L. Huang, Negligible hysteresis planar perovskite solar cells using Ga-doped SnO2 nanocrystal as electron transport layers, Organic Electronics, 71 (2019) 98-105. [3] J. P. C. Baena, M. Saliba, T. Buonassisi, M. Graetzel, A. Abate, W. Tress, A. Hagfeldt. Promises and challenges of perovskite solar cells, Science, 358 (2017) 739-744.
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Highlights: 1. KSeCN was introduced to improve crystallization and defect passivation. 2. CH3NH3PbI3-x(SeCN)x is a promising and stable perovskite. 3. Planar perovskite solar cells got a PCE of 18.41% and negligible hysteresis.
Declaration of interest statement The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Sincerely yours Zhu Ma
S0925-8388(19)34785-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.153539
Reference:
JALCOM 153539
To appear in:
Journal of Alloys and Compounds
Received Date: 6 November 2019 Revised Date:
20 December 2019
Accepted Date: 24 December 2019
Please cite this article as: Z. Ma, Z. Xiao, W. Zhou, L. Jin, D. Huang, H. Jiang, T. Yang, Y. Liu, Y. Huang, Efficient CH3NH3PbI3-x(SeCN)x perovskite solar cells with improved crystallization and defect passivation, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2019.153539. 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.
Credit Author Statement Zhu Ma has initiated the idea of this work. Zhu Ma, Zheng Xiao, Weiya Zhou, Lifen Jin, Dejun Huang carried out the experiment. Zhu Ma, Zheng Xiao, Huifeng Jiang, Tian Yang, Yuchuan Liu, Yuelong Huang performed the analysis and writing up the manuscript. In addition, the authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Efficient CH3NH3PbI3-x(SeCN)x perovskite solar cells with improved crystallization and defect passivation Zhu Ma1*, Zheng Xiao1, Weiya Zhou1, Lifen Jin1, Dejun Huang1, Huifeng Jiang2, Tian Yang2, Yuchuan Liu2, Yuelong Huang*1 1. Institute of Photovoltaic, Southwest Petroleum University, Chengdu 610500. P.R. China 2. School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500. P. R. China
Corresponding author: Zhu Ma (Tel: +86 13880863057; Fax: +86 02883037195; E-mail: [email protected])
Abstract The presence of grain boundaries and defects in perovskite films can be detrimental to performance of perovskite solar cells (PSCs), such as leading to nonradiative recombination and environmental instability. Additive engineering is a key tactic to control crystal growth and passivate defects of perovskite film. In this work, lead-free alkali metal additive of potassium selenocyanate (KSeCN) was firstly added in precursor solution to prepare a novel perovskite of CH3NH3PbI3-x(SeCN)x. The SeCN- is substitute for I- to enhance structure stability, simultaneously facilitate nucleation, meanwhile, effectively increase crystalline grain size and passivate the defects. Finally, the power conversion efficiency of planar perovskite solar cells boosts from 14.34% to 18.41%, and the hysteresis reduces from 21% to 1.5%, nearly negligible. Our findings provide an avenue for crystalline regulation and defects passivation to further improve the performance of perovskite solar cells and propose a promising CH3NH3PbI3-x(SeCN)x-based perovskite. Keywords: CH3NH3PbI3-x(SeCN)x, crystallization
perovskite
solar
cells,
KSeCN
additive,
stability,
Introduction Over the past few years, perovskite solar cells (PSCs) have attracted extensive research attention in the photovoltaic field. The power conversion efficiency (PCE) has been boosted from 3.8% to a maximum of 25.2% currently, which can compete with the traditional rivals, such as CIGS and CdTe, even the silicon solar cells [1]. Such tremendous enhancement is largely due to the optimization of perovskite composition, design of device architecture, interface engineering and process development [2, 3]. The preparation process of one-step deposition is one of the most widely used technology, because of its simple fabrication and great reproducibility [4]. However, the rapid crystallization process tends to produce relatively uncontrollable morphology with small grains, excessive grain boundaries, amount of shallow and intrinsic defects when using one-step deposition [5], which finally lead to obvious carrier recombination and environmental instability of perovskite film [6, 7]. As known, CH3NH3PbI3 is easily decomposed due to its inherent instability of crystal structure to moisture, and the loss of device original PCE could be easily triggered with the presence of a trace amount of water [8, 9]. In previous study, water molecule is found to be trapped between the organic cation CH3NH3+ and the inorganic anion [PbI3]− due to the Coulomb attraction [10]. Moreover, H2O as a “catalyst” to remove I− from [PbI3]−, which in turn combines with H+ of the organic cation, resulting in the formation of HI and PbI2 [11]. Therefore, it is significant to prohibit the interaction of water molecules and [PbX3]-, and enhance the bonding between Pb and X. Normally, the hydrophobic protective layer and encapsulation technology were used to prevent the penetration of moisture [12]. Meanwhile, exploring suitable X site anion to enhance the coordinate bonding with Pb is another avenue to improve crystal structure stability of CH3NH3PbI3 [13, 14]. Based on previous studies, the larger size of [PbX3]−, the lower bonding ionicity and the smaller vertical detachment energy (VDE), means better stability of perovskite structure [15]. The VDE of [PbX3]− is determined by the VDE of X−, therefore, it is important to explore better matched ionic or cluster ion to optimize the
VDE of [PbX3]− and enhance perovskite stability against moisture. As homologues to iodine, chlorine and bromine have similar properties and suitable atomic radius for incorporation into perovskites. Sargent et al. successfully fabricated CH3NH3PbBr3 and CH3NH3PbCl3 perovskites which had increased band gaps but greatly improved stability [16]. Similar observations were also reported for mixed halide perovskites CH3NH3PbI3-xBrx, when I− was substituted by Br−. The enhanced stability was attributed to the stronger interaction between Br− and CH3NH3+ [17, 18]. Mathews et al could show that the addition of chloride by a sequential deposition method enhanced the photovoltaic performance [19]. These studies suggest that ion doping might be a possible way to improve the inherent stability of perovskite films. The Pb(SCN)2 additive was proposed to prepare perovskite, which was carried out at ambient environment with 70% relative humidity [20, 21]. High quality CH3NH3PbI3-x(SCN)x perovskite film and device efficiency up to 15% were obtained with optimizing components [22]. It is can be seen that the substitute of thiocyanate SCN- moiety with I- can also greatly improve the structure stability of perovskite to moisture [23]. However, some reports have also shown that the addition of Pb(SCN)2 affects the full reaction of PbI2 with methylamine iodine, which easily leads to excessive PbI2 in the modified perovskite film, which hinders the further improvement of the performance of the PSCs [24, 25]. Recently, a novel pesudohalide ion was found to replace I-, which is selenite ion (SeCN-) with a VDE of 3.39 eV, similar as SCN- ion (3.52 eV) and I- ion (3.30 eV). According to the Goldschmidt tolerance factor and VDE theory, the novel CH3NH3PbI3-x(SeCN)x could exhibit excellent stability. However, the 3D structure of CH3NH3PbI3-x(SeCN)x has not been experimentally prepared till now. Moreover, in previous study, the Pb(SCN)2 additive based perovskite exhibits outstanding stability. However, the excess PbI2 residue still limits the device efficiency [26]. In order to avoid excess PbI2, it is of great significance to explore pseudo-halogenated additives without metal lead on the perovskite film. It has recently been reported that alkali metal halide additives can improve the morphology and crystallinity of CH3NH3PbI3 films, thereby promoting charge generation and separation [27].
Here, we prepared a highly efficient PSCs by adding a lead-free alkali metal additive of KSeCN to one-step precursor solution. We adopted PSCs with the structure of the FTO/TiO2/CH3NH3PbI3-x(SeCN)x/Spiro-oMeTAD/Ag. The results show that the perovskite film prepared by adding 3% KSeCN has large crystal grains and uniform morphology. With this approach, the PCE of CH3NH3PbI3-base device improved to 18.41%. More importantly, the device remained at 85.8% efficiency after being left unsealed in glove box for 500 h, and the hysteresis effect of the obtained device was significantly reduced to negligible.
Experimental Section Materials Potassium selenocyanate (KSeCN 99.9%) was obtained from Aladdin. Titanium isopropoxide (Ti(OCH(CH3)2)4, 99.9999%), N,N-dimethylformamide (DMF) and ethanol (C2H6O) were purchased from Sigma-Aldrich. Lead (II) iodide (PbI2, 99.99%), methanaminium
iodide
2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)
(CH3NH3I,
99.5%),
amino]-9,9'-spirobifluorene
(Spiro-oMeTAD, 99.8%), tert-butylpyridine (tBP), lithium bis(trifluoromethane sulfonimide) (Li-TFSI) and acetonitrile (C2H3N) were purchased from Xi’an Polymer Light Technology Corp. All above materials were purchased without any purification.
Device fabrication The FTO substrates were sequentially ultrasonic cleaned with detergent, deionized (DI) water, acetone, and anhydrous ethanol for 15 min, respectively. Before using, the substrates were dried with nitrogen stream and treated with UV-ozone for 15 min. For the planar TiO2 electron transporting layer, the precursor solution of titanium isopropoxide was spin-coated on glass/FTO substrate at 3000 rpm for 30 s. After pre-annealed at 150
for 20 min, the film was then annealed 500
for 30 min.
553 mg PbI2, 191 mg CH3NH3I and different amount of KSeCN were dissolved in 1 ml DMF to prepare perovskite precursor solution with stirring at 60 °C overnight, and the KSeCN and CH3NH3PbI3 molar ratio ranging from 0% to 6%. The perovskite precursor solution was spin-coated onto the TiO2 layer at 3000 rpm for 55 s, followed
by chlorobenzene (85 mL) drip during the spinning process. The prepared film was annealed on hotplate at 100
for 20 min. The hole transporting layer was spin-coated
on perovskite absorption layer at 3000 rpm for 30 s with a solution comprising 1 mL chlorobenzene, 72.3 mg of Spiro-oMeTAD, 28.8 µL of tBP and 17.5 µL of Li-TFSI solution (520 mg Li-TFSI in 1 mL acetonitrile). Subsequently, the samples were aged in a dry box for 3 hours to fully oxidize the Spiro-oMeTAD. Finally, 100 nm of silver were consecutively deposited by physical vapor deposition at 4×10-3 Pa to complete the device fabrication. The active area of solar cells was controlled to 0.16 cm2.
Materials and Devices Characterization The scanning electron microscope (SEM) images were investigated by using ZEISS system (EV0MA15), and the system was connected to an energy dispersive X-ray spectroscopy (EDS) detector. The X-ray diffraction (XRD) patterns of the perovskite thin films (2θ scans) were performed on a DX-2700 system using Cu-Kα radiation (λ=0.15406 nm) as the X-ray source. Scans were taken with 0.5 mm wide source and detector slits, and the X-ray generator settings at 40 kV and 30 mA. X-ray photoelectron spectroscopy (XPS) measurements were carried out by an X-ray photoelectron spectrometer (KRATOS, AXIS Ultra DLD) with the monochromated Al Kα X-ray source (hν=1486.6 eV, 200 W). Absorption spectra were carried out by an UV-vis spectrometer (UV-2600, SHIMADZU). FTIR spectra of thin films were then acquired on a Thermo Scientific Nicolet 6700 FTIR spectrophotometer. Steady-state photoluminescence (PL) and time-resolved PL (TRPL) were measured by FLS-980 Edinburgh instruments. The current density-voltage (J-V) characteristics of PSCs were measured with an electrochemical workstation (CHI660D) under a simulated solar spectroscopy (AM 1.5) produced by a solar simulator (Zolix SS150). The intensity of the solar simulator was calibrated to 100 mW/cm2 using a silicon reference photodiode. The devices were measured both in reverse scan (1.2 V→0 V, step 0.01 V) and forward scan (0 V→1.2 V, step 0.01 V). The devices were taken out for external quantum efficiency (EQE) measurements using a PVE 300 (Bentham, Inc.) EQE measurement system.
Results and discussion
Figure 1 Illustration of the one step process to prepare perovskite films.
The preparation and crystallization processes of perovskite films are shown in Fig. 1. For CH3NH3PbI3, PbI2: CH3NH3I: DMF precursor was prepared, spin-coated onto ETL, followed by chlorobenzene drip, and annealed at 100
for 20 min. In
conventional process, the crystal growth process is quick and hard to control, easily generates small grains and amounts of grain boundaries. For CH3NH3PbI3-x(SeCN)x, KSeCN: PbI2: CH3NH3I: DMF precursor was prepared, other steps are similar. In this process, the addition of SeCN- replaces a part of I- and a strong electrostatic interaction is generated between Se2- and CH3NH3+, which is beneficial to improve the stability of perovskite[28]. Moreover, the SeCN− is a kind of pseudohalide, which has a similar chemical property as halide. However, the interaction between Pb2+ and SeCN- is much stronger than that of between Pb2+ and I-, due to SeCN- is a strong coordination bond bridging group. Furthermore, the coordination of SeCN- ligands could reduce the activation energy of crystal grains, similar as SCN-[29]. Hence, the additive of SeCN- coordinated with Pb2+ could retard perovskite crystal nucleation and result in nucleation of large-sized colloidal clusters in the precursor solution, finally enlarge the grain size of the perovskite film during crystallization. The change of perovskite morphology by adding SeCN- can be observed by SEM. Surface topography of perovskite films prepared with different amounts of KSeCN were analyzed by SEM in Fig. 2(a-d). Interestingly, the grain size of
perovskite film was directly related to the amount of KSeCN incorporated, the overall grain size trend is increasing with the increase of KSeCN concentration. The average grain size obtained without KSeCN additive is approximately 220 nm. Only adding 1.5% KSeCN, the grain size dramatically increases to 311 nm. The average grain size continuously increases to 460 nm and 479 nm with 3% and 6% KSeCN, respectively. The statistics data of grain size was extracted from SEM images with 1 µm scale bar. From Fig. 2(e), the perovskite film with 3% KSeCN exhibits bigger grain, more uniform grain size and fewer grain boundaries, facilitate to achieve effective PSCs. Although the grain size of perovskite film with 6% of KSeCN is the largest, they vary in size. Moreover, the films also exhibit amounts of cracks and pinholes, which are deleterious for the device performance.
Figure 2 Top-surface SEM images (a-d) and histogram of grain size distribution (e) of perovskite films prepared with 0% KSeCN, 1.5% KSeCN, 3% KSeCN, 6% KSeCN.
Figure 3 (a) Schematic depiction of the hypothesized CH3NH3PbI3-x(SeCN)x-based perovskite structure. (b) FTIR spectrum, (c) XRD patterns and (d) Reversal FWHM of the different concentration of KSeCN.
Fig. 3(a) shows the schematic depiction of the hypothesized KSeCN perovskite structure. SeCN- was introduced to replace partially iodine, and reduce the interaction between the water molecule and [PbX3]-, so that the water molecule is unlikely to be trapped between the cation and the anion. Second, enhanced bonding between Pb and X in [PbX3]− can effectively controls the migration of X− and prohibits the formation of AgX [30]. The backbones between CH3NH3+and Se2- is able to enhanced electrostatic interaction [28]. The role of the chemical bond in the perovskite without added KSeCN is only the force of I and Pb. This force is relatively weak and tends to cause decomposition when the perovskite is affected by the external environment. After adding KSeCN, the interaction between Pb2+ and CN- is stronger, making the framework structure of CH3NH3PbI3 more stable and difficult to decompose [23]. The energy dispersive X-ray analysis in Fig. S1 confirmed that Se, Pb and I were almost stoichiometric in the film samples. The statistics of the elemental analysis summarized in the table S1. With the increase of KSeCN additive, the Se element dramatically increases. To explore the role of KSeCN additive, XPS measurement was carried out. Fig. S2 shows the XPS spectra of the KSeCN added and unadded perovskite film samples. The Pb 4f spectrum exhibited two contributions, 4f7/2 and
4f5/2 located at 137.7 and 142.4 eV for the film without KSeCN and 137.9 and 142.7 eV with KSeCN, respectively [31]. The shift of the Pb peaks toward higher binding energies is evidence for the formation of stronger ionic bonding between Pb2+ and SeCN- ions. In order to verify the presence of C=N in the perovskite film with KSeCN, films were characterized by flourier transform infrared spectroscopy (FTIR). Fig. 3(b) illustrates normalized FTIR spectrum of perovskites and pure KSeCN. The characteristic peaks of pure CH3NH3PbI3 are mainly concentrated at 910 cm-1, 1467 cm-1, 1629 cm-1. The characteristic peaks of pure KSeCN are mainly concentrated at 831 cm-1, 1579 cm-1, 2074 cm-1. After adding different concentration of KSeCN in CH3NH3PbI3, the perovskite films exhibited a new peak around 1579 cm−1 , which derives from pure KSeCN corresponding to C=N stretch. The influence of perovskite crystal formed by adding alkali metal additive KSeCN were further observed. As shown in Fig. 3(c), the XRD patterns of the perovskite crystals prepared without KSeCN appear diffraction peaks at the (110), (220) and (310), and the corresponding diffraction peaks are 14.1°, 28.5° and 31.8°, respectively. All above diffraction peaks are originated from pure CH3NH3PbI3 perovskite [31]. With the increasing of KSeCN amount, the intensities of all diffraction peaks increase and the positions of diffraction peaks do not change, which indicates that the KSeCN additive can assists the crystal growth and improves the crystal quality. Moreover, the XRD characteristics of perovskites are consistent with the morphologies. The film crystallinities obtained from different KSeCN contents were compared by plotting the reciprocal full width at half maximum (FWHM) from the XRD spectra as shown in Fig. 3(d). As the intensity of the diffraction peak increases, the reciprocal FWHM increases and the FWHM of the diffraction peak becomes narrower, indicating that the addition of KSeCN effectively improves the crystallization characteristics of perovskite film.
Figure 4 (a) Cross-sectional SEM image of the device. J-V curve (b), Histograms of PCE distribution statistics for devices (c) and external quantum efficiency curves (d) of the PSCs with different KSeCN concentration. (e) J-V curves of the PSCs under forward and reverse sweep voltage scanning. (f) Stability measurement of PSCs with and without KSeCN.
In order to confirm the effect of KSeCN on perovskite, the perovskite solar cells with a structure FTO/TiO2/CH3NH3PbI3-x(SeCN)x/Spiro-oMeTAD/Ag was fabricated, as described in the cross-sectional SEM image of a completed PSCs (Fig. 4(a)). The Fig. 4(b) shows the J-V curves measured by PSCs with different amount of KSeCN additive, the key parameters are summarized in Table 1. The control device without KSeCN additive exhibited a short-circuit current density (JSC) of 22.02 mA/cm2, open-circuit voltage (VOC) of 1.06 V, fill factor (FF) of 61.20%, and a maximum power conversion efficiency (PCE) of 14.34%. In contrast, the PSCs based on 3% KSeCN additive exhibit significant improvements in all key parameters. The devices present a JSC of 23.35 mA/cm2, VOC of 1.07 V, FF of 73.6%, and a maximum PCE of 18.41%. The PCEmax dramatically increases 28.3%, compare to control devices. From average value of Table 1 and Fig. S3, it is clearly showed that all parameters increased first and then decreased with the increase of KSeCN concentration, and the results are reliable and reproducible. This result is consistent with the morphology results observed in the SEM of Fig 2. It can be seen that with the increase of KSeCN
concentration, the grain size dramatically increases, the surfaces become smoother and pinholes are suppressed, which are helpful for performance enhancement. When excessive 6% KSeCN is added, the crystal growth process is not easy to regulate and amounts of cracks and pinholes are generated, which leads to leak current, carrier recombination and performance reduction. In order to further verify the improved reproducibility, 65 separate devices each based on different KSeCN content were prepared and reverse-scanned under AM 1.5G irradiation. As shown in Fig. 4(c), in comparison with intrinsic CH3NH3PbI3 based devices, PCE value of CH3NH3PbI3-x(SeCN)x-based device displays narrower distribution with smaller standard deviation. These results indicate that KSeCN is a promising perovskite additive for the construction of efficient and reproducible PSCs. Fig. 4(d) shows the external quantum efficiency (EQE) spectra of the cells using different content KSeCN additive. It can be confirmed that the improvement of the CH3NH3PbI3-x(SeCN)x layer plays an important role in further generation and transport of photocurrent. Meanwhile, the EQE tendency is consistent with JSC in Table 1 [33]. The hysteresis phenomena of PSCs were investigated by comparing the J-V characteristic of reverse can and forward scan, which is shown in Fig. 4(e). For the device without KSeCN additive, the PCE of reverse scan and forward scan is 14.34% and 11.28%, respectively. Under the best concentration of KSeCN, PCE of 18.41% from reverse scan and 18.13% from forward scan for are obtained. Here, we use the hysteresis index (HI) to quantify the hysteresis level [33]: HI =
(1)
PCERS and PCEFS are represented PCE for reverse and forward scan, respectively. It can be easily calculated that the HI of PSCs without KSeCN is 21.3%, however, the HI of PSCs with 3% KSeCN is only 1.5%. A rapid reduction of hysteresis level was obtained after adding KSeCN, which derives from the stronger bonding between SeCN- and Pb2+, passivation of surface trap states and the improvement of interface properties.
At the same time, the long-term stability of PSCs was evaluated, as shown in Fig. 4(f). The unencapsulated devices are placed in nitrogen-filled glove box and the performance of the device is tested every 24 hours. After continuous testing for 500 h, the PCE of devices with 3% KSeCN can still be maintained above 85.3%, but the PCE of the device without KSeCN is severely attenuate to 40.4% after 500 h measurement.
After
systematically
CH3NH3PbI3-x(SeCN)x-based
perovskite
measured solar
the cells,
performance the
of
intrinsic
CH3NH3PbI3-x(SeCN)x exhibits excellent PCE, negligible hysteresis and good stability. It can be predicted that with the optimization of crystal growth processes, interface engineering, device structure and carrier transporting materials, the comprehensive performance can be further improved. In order to describe the mechanism of enhanced device performance after adding KSeCN, the optical properties of CH3NH3PbI3-x(SeCN)x-based perovskite film were investigated. As depicted in Fig. 5(a), it can be seen that the perovskite exhibited obviously enhanced absorption in the ranges of 304~418 nm and 507~762 nm with 3% KSeCN,
compare
to
intrinsic
CH3NH3PbI3.
Especially,
the
CH3NH3PbI3-x(SeCN)x-based perovskite could effectively use the ultraviolet light, purple light and blue light. This perovskite structure also solves the problem of low utilization efficiency at shortwave region compared to normal solar materials [34]. However, after adding excess KSeCN with 6%, the UV-vis absorption intensity is even lower than pristine perovskite. The improved absorption of perovskite layer can be benefit for the generation of photocurrent. As shown in table 1, it can be clearly seen that the absorbance tendency of all perovskite film is good coincidence with the current density of corresponding PSCs. Table 1 Device performance parameters based on different concentrations of KSeCN additive. KSeCN
Maximum (average) Voc(V)
Jsc(mA/cm2)
FF(%)
PCE(%)
0%
1.06 (1.03)
22.02 (20.32)
61.20 (61.09)
14.34 (14.00)
1.5%
1.05 (1.04)
22.62 (21.20)
67.10 (67.78)
15.94 (15.00)
3%
1.07 (1.05)
23.35 (22.54)
73.60 (72.80)
18.41 (17.32)
6%
1.03 (1.00)
19.34 (19.05)
60.30 (55.26)
12.04 (10.50)
Figure 5 UV-visible absorption spectra (a) of samples based on glass/perovskite, PL spectra (b) and TRPL spectra (c) of samples based on Glass/FTO/perovskite with different concentrations of KSeCN additive.
For deep analyze the carrier recombination of the perovskite films with different concentrations of KSeCN, the steady-state and time-resolved PL spectroscopy was used [35]. As shown in Fig. 5(b), the perovskite films exhibit emissive peaks at 774 nm, 770 nm, 770 nm and 768 nm for 0%, 1.5%, 3%, 6% additive, respectively. The obvious blue-shift of CH3NH3PbI3-x(SeCN)x-based perovskites lead to passivation of surface trap states [36], which is caused by the doping of SeCN- ion. As previous depicted, the bonding between SeCN- and Pb2+ is more stable than I- and Pb2+. Meanwhile, the steady-state PL intensity of FTO/perovskite samples increases firstly with the increasing of KSeCN content and reaches the maximum at 3%, and then decrease at 6%. The results of enhanced PL intensity also indicate that the addition of an appropriate amount of KSeCN can passivate the defect state on the surface of the perovskite or the grain boundary, finally reduce the nonradiative recombination. Fig. 5(c) shows the TRPL decay of samples based on Glass/FTO/perovskite with different concentrations of KSeCN additive. As shown in Table 2, for pure CH3NH3PbI3 sample, τ1=41 ns, τ2=120 ns, resulting in an amplitude average lifetime of 118 ns. For 3% KSeCN based perovskite, τ1 and τ2 are increased to 80 ns and 264 ns, with an amplitude average lifetime of 262 ns. The average PL lifetime is determined by the equation: τ=
(2)
where A1 and A2 are the relative amplitude fraction for each decay component and τ1 and τ2 are the fast and slow decay PL species. The TRPL results showed that the carrier lifetime of CH3NH3PbI3-x(SeCN)x-based perovskites can be significantly
increased when adding appropriate KSeCN additive, indicating efficient charge carrier transportation and collection, which is also facilitate to suppress J-V hysteresis. Table 2 Characteristic parameters of PL lifetime by fitting the TRPL spectra for perovskite. KSeCN Content(%)
τ1 (ns)
τ2 (ns)
τ (ns)
0
41
120
118
1.5
58
189
188
3
80
264
262
6
4.9
80
79
Conclusions In summary, it has been proved that adding appropriate alkali metal halide additive KSeCN into perovskite can effectively improve the device comprehensive performance of CH3NH3PbI3-x(SeCN)x-based PSCs. Here, a maximum steady-state PCE of 18.41%, 1.5% hysteresis index and 85.3% efficiency remain after 500 hours measurement were achieved after optimizing the KSeCN ration in perovskite precursor. The enhanced device performance is attributed to stable perovskite structure, enlarged grain size and passivated defects. Our study indicates that the effective CH3NH3PbI3-x(SeCN)x-based perovskite is a promising candidate to substitute traditional perovskite, further improvement could be obtained by regulating crystallization process, optimizing interface and using effective transport material.
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Highlights: 1. KSeCN was introduced to improve crystallization and defect passivation. 2. CH3NH3PbI3-x(SeCN)x is a promising and stable perovskite. 3. Planar perovskite solar cells got a PCE of 18.41% and negligible hysteresis.
Declaration of interest statement The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Sincerely yours Zhu Ma