Binary synergetic ions reduce defect density in ambient air processed perovskite solar cells

Binary synergetic ions reduce defect density in ambient air processed perovskite solar cells

Solar Energy 198 (2020) 335–342 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Binary syn...

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Solar Energy 198 (2020) 335–342

Contents lists available at ScienceDirect

Solar Energy journal homepage: www.elsevier.com/locate/solener

Binary synergetic ions reduce defect density in ambient air processed perovskite solar cells ⁎

Hongyu Liua, Peng Zhanga, Fei Wangb, , Chong Jiaa, Yiqing Chena, a b

T

⁎⁎

School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui 230009, People’s Republic of China School of Electronic Science and Applied Physics, Hefei University of Technology, Hefei, Anhui 230009, People’s Republic of China

A R T I C LE I N FO

A B S T R A C T

Keywords: Perovskite solar cell Binary synergetic ions Defect density Performance and stability Ambient air process

At present, significant research efforts are being concentrated on enhancing the performance and stability of perovskite solar cells (PSC) by lowering defect traps. In this study, NH4+ and SCN- binary ions as additive were incorporated into perovskite precursor to control the crystal growth. Our best performance based on the devices fabricated under fully open air condition was improved from 15.67 to 18.75%, boosting by 20%. The stability results display that devices containing additive maintained ~90% of the initial efficiency for 400 h in ambient air with a humidity of 30%. We first study the fundamentals of defect properties and carrier recombination kinetics behind the multifaceted role of mixed NH4+ and SCN- ions. Compared to using a single active specie as additive, mixed-ions devices exhibit effective bifacial trap passivation as well as lowered defect density in not only perovskite bulk material but also interfaces significantly, leading to facilitated electron transport. Our work can manifest a simple ambient air based approach by the mixed binary ions as additives in order to potentially promote the commercial prospects of PSCs.

1. Introduction The organometallic halide perovskite solar cells (PSCs) have attracted considerable attention and the remarkable power conversion efficiency (PCE) has risen from 3.8% to over 20% within 6 years (Kojima et al., 2009; Kim et al., 2012; Burschka et al., 2013; Lee et al., 2012; Yang et al., 2015; Yang et al., 2019).Up to now, more and more investigation has been focus on the morphology control and compositional tuning of perovskite absorb layer (Azam et al., 2018; Bi et al., 2016; Bi et al., 2017); interface modification as well as novel design of device architecture (Zhou et al., 2014; Jiang et al., 2016; Jeon et al., 2015). Unfortunately, the performance enhancement of the devices based on the pristine perovskite materials has run into the bottleneck, because the perovskite films fabricated by one step or two step solution method usually produce inhomogeneous surface with many grain

boundaries and trap states, which are assigned to the nonradiative recombination centers (Ball et al., 2013; Xing et al., 2014; Fang et al., 2015). Moreover, the relatively low temperature or humidity stability of perovskite layer is usually caused by ionic migration of organic components (Hou et al., 2017; Jiang et al., 2018). It has been still a challenge to control the crystal growth and lower the defect density of perovskite films. The defect or trap state is usually originated from the surface dangling bonds located at the grain boundary, leading to trapping free carrier and subsequently performance degenerating (Shao et al., 2014). Therefore, it is crucial to improve the surface morphology and reduce defect density for enhancing performance and stability of PSCs. Varied types of additives, such as nanoparticles (Zhang et al., 2019), metal halide salts (Boopathi et al., 2016), fullerene derivatives (Xu et al., 2015; Zhang et al., 2017), inorganic acid (Zhang et al., 2015; Heo

Abbreviations: PSC, perovskite solar cells; PCE, power conversion efficiency; IPFB, Iodopentafluorobenzene; SCN-, thiocyanate anions; NH4+, ammonium cations; RH), relative humidity; MAI, CH3NH3I; DMF, dimethylformamide; DMSO, Dimethyl sulfoxide; DMF, N, N-dimethylformamide; FTO, F-doped SnO2; HTL, hole transport layer; PL, photoluminescence; IPCE, incident-photon-to-current conversion efficiency; EIS, electrochemical impedance; FE-SEM, field emission scanning electron microscope; XPS, X-Ray photoemission spectroscopy; J-V, current density and voltage; SCLC, space-charge-limited current; Ndefect, defect density; TFL, defect-filled limit; VTFL, defect-filled limit voltage; Rct, recombination resistance; Rs, series resistance ⁎ Corresponding author at: School of Electronic Science and Applied Physics, Hefei University of Technology, No. 193 tunxi Rd., Hefei City, Anhui Province 230009, People’s Republic of China. ⁎⁎ Corresponding author at: School of Materials Science and Engineering, Hefei University of Technology, No. 193 tunxi Rd., Hefei City, Anhui Province 230009, People’s Republic of China. E-mail addresses: [email protected] (F. Wang), [email protected] (Y. Chen). https://doi.org/10.1016/j.solener.2020.01.069 Received 5 November 2019; Received in revised form 12 January 2020; Accepted 24 January 2020 0038-092X/ © 2020 Published by Elsevier Ltd on behalf of International Solar Energy Society.

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2.1. Preparation of perovskite layer

et al., 2015), organic halide salt (Docampo et al., 2014) and polymer (Cai et al., 2018), have been used as an effective way to control the nucleation of perovskite crystal, improve the morphology and passivate the trap state in perovskite film. Actually, the active agents in most additives are cations or anions, which can function as improving the morphology of perovskite films. In order to passivate the point defects in perovskite crystal, it is the best to choose small or even no size mismatch of cation/anion as the additive ions. Thiocyanate anions (SCN-) and ammonium cations (NH4+) are two representative examples. Due to a similar ionic radius of SCN- (~0.217 nm) to I- anion (~0.220 nm), the incorporation of SCN- ions does not exert influence on the crystal structure of perovskite (Halder et al., 2015). Kim et al. reported that SCN- anions as additive can retard the nucleation for perovskite crystal, resulting in the grain size of perovskite crystal found to be enlarged significantly from the nanometer to the micrometer scale (Kim et al., 2016). Tai and other groups demonstrated that Pb(SCN)2 as an additive could significantly improve the stability of the device (Tai et al., 2016; Zhang et al., 2018; Sun et al., 2017). Liang and several other groups reported the incorporation of SCN- anions can passivate the defects both in bulk perovskite and its interface, leading to reduction of non-radiative recombination in perovskite material, consequently resulting in performance enhancement of the devices (Chen et al., 2015; Ke et al., 2016). In addition, Mhaisalkar proved that NH4+ as an additive used in optimum amount can avoid formation of nonperovskite phase in perovskite crystal (Han et al., 2018). Kang et al. applied NH4I additive into the precursor solution, which can retard the crystal growth by the formation of NH4PbI3 intermediate phase, resulting in enlarged grain size and decreased grain boundary (Si et al., 2017). Rong et al. employed NH4Cl additive to assist the crystallization of perovskite, wherein the formation and transition of intermediate phase CH3NH3X·NH4PbX3(H2O)2 (X = I or Cl) can produce perovskite film with high-quality, leading to improvement of air stability of the devices (Rong et al., 2017). However, the enhanced photovoltaic performance previously gained has been mostly based on the devices with inverted p-i-n architectures, which were fabricated under controlled experiment environment (i.e., glove box) (Sun et al., 2017; Ke et al., 2016; Si et al., 2017), one of the major barriers limiting the application prospects of PSCs Herein, binary ions including NH4+ and SCN- ions additive was applied to improve the efficiency and stability of PSCs. The n-i-p configured devices were fabricated in ambient air without glove box. It demonstrates that an addition of NH4SCN can simultaneously improve the crystallinity and grain boundaries of the perovskite layer, consequently resulting in promoting the steady-state efficiency of the devices from 15.67 to 18.75%, increased by 20%. The multifaceted role of NH4+ and SCN- binary ions on defect properties as well as carrier recombination dynamics in perovskite film have been explored elaborately. On one hand, the combination of NH4+ and SCN- ions can slow down the nucleation of perovskite grains and generate larger grain size. On the other hand, NH4+ and SCN- ions both can properly be filled into the MA or I- vacancies, resulting in lowering the trap density and prolonging carrier effective lifetime. The results demonstrate that the synergistic additive consisting of two or more active ions into perovskite precursor is a viable strategy for reducing defect trap and improving the photovoltaic performance as well as intrinsic stability of perovskite solar cells in order to fabricate the devices under fully open air condition.

In this report, the as-prepared perovskite film was fabricated via a one-step solution method and the pure CH3NH3PbI3 perovskite is assigned to the control sample. CH3NH3I (MAI) was prepared by using the similar procedures according to the previous literature (Im et al., 2011). The reference precursor solution was prepared by the dissolving CH3NH3I (synthesized), PbI2 (Xi’an Polymer) in dimethylformamide (DMF) with a 1:1:1 molar ratio. In brief, 0.159 g CH3NH3I (MAI) and 0.461 g PbI2 was blended in the mixture of 72 µL Dimethyl sulfoxide (DMSO) and 635 µL anhydrous N,N-dimethylformamide (DMF, Aldrich). For preparation of perovskite precursor added with NH4SCN additive, NH4SCN (Aldrich) was added into the MAPbI3 precursor with the ratios ranging from 12.5 to 17.5 mol%. (For example, the as-prepared sample with 15 mol% NH4SCN was prepared by adding 11.4 mg of NH4SCN into 707 µL MAPbI3 precursor solution). Then, the precursor solution was stirred for 2 h at room temperature. 2.2. n-i-p configured solar cell fabrication The perovskite solar cells with n-i-p planar structure are composed of the c-TiO2 /pristine or NH4SCN-added CH3NH3PbI3/spiro-OMeTAD layers. The laser etched F-doped SnO2 (FTO) substrate (15 Ω sq−1, 1.5 cm × 2 cm)) was cleaned with detergent, deionized water, acetone and iso-propanol successively by ultra-sonication for 20 min beforehand. 0.15 M precursor solution (titanium diisopropoxide bis (acetylacetonate) solution (Aldrich) dissolved in butyl alcohol) was spincoated onto a FTO substrate firstly at 500 r.p.m for 3 s and subsequently 2000 r.p.m for 30 s, and then sintered at 500 °C for 30 min to gain a dense c-TiO2 layer of 30 nm. The perovskite precursor solution was spin-coated onto c-TiO2 layer by spin-coating at 4000 r.p.m for 30 s. During the spin-coating, the substrate was treated with ethyl acetate drop-casting. Then, the perovskite film was annealed at 100 °C for 10 min. For the hole transport layer (HTL), 72.3 mg spiro-OMeTAD was dissolved in 1 mL chlorobenzene, containing 17.5 µL additives (520 mg Li-TFSI in 1 mL acetonitrile) and 28.8 µL 4-tert-butylpyridine. The Spiro-OMeTAD solution was spin-coated onto the perovskite layer at 3000 rpm for 30 s. Finally, 100 nm of Ag was deposited on the HTL through a shadow mask and the active area is around 0.12 cm2. 2.3. Measurements and characterization The J-V curves were recorded at the scan rate of 0.2 V s−1 and delay time of 100 ms both in the forward and reverse scan mode, respectively. Steady-state photoluminescence (PL) was carried out on F-4500 Fluorescence Spectrophotometer, excited with a 521 nm laser. The incident-photon-to-current conversion efficiency (IPCE) spectra and the electrochemical impedance (EIS) spectra were performed on a Zahner Zennium electrochemical workstation. In EIS measurement, the perturbation voltage was 20 mV and the biased voltage was ranged from 0 V to 0.7 V over the frequency range 500 mHz–1 MHz in dark conditions. The EIS experimental data were fitted using Zview software. The film morphology was characterized by field emission scanning electron microscope (FE-SEM, SU8020). X-ray diffraction pattern was collected by D/MAX2500V, scanning at a speed of 5°/m. Ultraviolet–visible absorbance spectra were examined with CARY 5000 (Agilent, Australia). X-Ray photoemission spectroscopy (XPS) data were obtained by a VG ESCALAB250 Xi (Thermo, USA) surface analysis system equipped with a monochromatic Al Ka X-ray (1486.6 eV) source. The energy state of the spectroscopy was calibrated by inorganic carbon at 284.80 eV as a reference to take sample charging into consideration (Riha et al., 2011). Photovoltaic J-V (current density–voltage) characteristics of PSCs was measured using Keithley 4200 source meter under AM 1.5 irradiation equipped with a solar simulator (XES-301S), which has been calibrated by a Si-reference cell certified by NREL under open air conditions.

2. Experimental It is worth noting that material synthesis, device fabrication, and characterization were all carried out in ambient air with the relative humidity (RH) ranging from 30%~50%.

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Fig. 1. (a)–(d) Top view scanning electron microscope (SEM) of the as-prepared perovskite films with different addition levels of NH4SCN:(a), (b) 0 mol%; (c), (d): 15 mol%; (e) X-ray diffraction patterns analysis of perovskite films with different concentrations of NH4SCN additive; (f) UV–vis absorption spectra, (Inset) A plot of [hν × α]2 against hν used to determine the band gap of the perovskite layer with different concentrations of NH4SCN additive.

3. Results and discussion

to form a NH4PbI3 intermediate phase in the nucleation process, resulting in slowing the nucleation rate, boosting the surface coverage as well as increasing of the grain size of the perovskite film, subsequently leading to the decrease of grain boundary and facilitating carrier transport in perovskite interfaces (Si et al., 2017). As shown in Fig. S1(a) (ESI), the cross-sectional SEM image of film with 15 mol% NH4SCN additive manifest the perovskite layer(~360 nm) is very dense, uniform and smooth, which can restrain direct contact between c-TiO2 layer and HTM layer. Therefore, the synthetically using of SCNand NH4+ ions as additive can play a significant role in the regulation of nucleation and crystallization process of the perovskite films, generating the improved perovskite layer.

Fig. 1(a–d) exhibits the surface morphology and grain size of perovskite film with and without NH4SCN additive. As shown in Fig. 1c and d, there is a distinct enlargement of grain size compare to the pristine film. The grain size of the perovskite crystal with 15 mol% NH4SCN additive grows to more than 500 nm, significantly increased compare to the pristine film with ~250 nm in grain size (Fig. 1a and b). Some previous reports proved that SCN- anion can promote the growth of perovskite crystal (Kim et al., 2016; Chen et al., 2015). In consideration of the smaller ionic radius of the NH4+ ion (1.61 A) than that of the MA+ (1.80 A) ion, NH4+ ion may preferentially react with PbI2 337

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introduction of excessive NH4SCN in the perovskite crystal results in decreased absorption intensity of photoactive layer and hindered carrier transport across the perovskite boundary, leading to performance worsen. The hysteresis behavior in the J-V curve is a very important issue for PSCs (Jeon et al., 2014; Wang et al., 2017). The hysteresis index (HI) can be calculated according to the following formula:

We explored the crystal structure changes of perovskite crystal with the growing adding level of NH4SCN additive. Fig. 1e displays XRD pattern of the samples with 0 mol%~17.5 mol% additive, with typical diffraction peaks near 14.1°, 28.5°, and 31.8°, indexed to the (1 1 0), (2 2 0), and (3 1 0) of crystal planes. Due to the decomposition or volatilization of SCN- in the annealing process as demonstrated in previous literature (Chen et al., 2015), there is no residue of SCN- ion in perovskite crystal. Compare to the control film, all the added samples exhibit a stronger major (1 1 0) characteristic peaks, implying an improved crystallinity of perovskite film. In order to confirm the elemental composition of perovskite film, XPS spectra of as-prepared film with 15 mol% NH4SCN additive was given in Fig. S1(b)–(f) (ESI). The S 2p spectrum in Figs. S1(b) and S1(f) indicates the peak located around 161.3 eV at the low binding energy region, verifying the existence of SCN- anions in perovskite samples. UV–vis absorption spectra was carried out to investigate the influence of different concentrations of NH4SCN additive on the optical properties of the perovskite film. As shown in Fig. 1f, when the addition level of NH4+ and SCN- does not exceed 15 mol%, the UV–vis absorption intensities of perovskite film are stronger than that of the control sample, but the absorption intensities abruptly go down when addition level is more than 15 mol%. The sample added with 15 mol% NH4SCN additive manifest the highest intensity, resulting from improved crystal grain size and surface coverage (Lindblad et al., 2014), which is consistent with the SEM and XRD results presented in Fig. 1(a–e). Therefore, the incorporation of an optimal concentration of NH4SCN additive is beneficial to enhancement of optical absorption, whereas the excess of NH4+ ions in perovskite crystal will lead to the decrease of absorption intensities. Generally speaking, the increased absorption intensity of photoactive layer can lead to more photo-induced carrier and the increased Jsc. The band gapsEg of the pristine and 15 mol% NH4SCN-added films were evaluated by Tauc plots, presented in the inset of Fig. 1f. A slight red-shifted band gapsEg of 1.598 eV and 1.593 eV are extracted for the control and sample with 15 mol% NH4SCN, respectively. Morphological and optical property analysis indicates a good production of photo-induced carrier and defect-free surface, resulting in an enhancement in device photovoltaic performances. To investigate the role of NH4+-SCN- mixed ions on the device photovoltaic parameters, the detailed J-V curve of the champion devices with varying additive contents was displayed in Fig. 2a, with the corresponding champion and average parameters summarized in Table 1. With the change of NH4SCN additives ratio, the obtained PCE displays a non-monotonic variation, probably attributed to the interaction between the grain size (poorer at lower ratios) and contact resistance (larger at higher ratios). We obtained an optimized adding level of 15 mol% for NH4SCN at which the PCE arrives a best value of 18.75%, with Voc of 1.057 V, Jsc of 22.64 mA cm−2 and FF of 78.37%, attributed to the passivation effect of NH4+ and SCN- ions. The incorporation of 15 mol% NH4SCN additive improve the efficiency to around 20%. Fig. 2b and c display that the best FF and Voc of the PSCs by introducing NH4+ and SCN- ions is increased by 7.37% and 9.1%, demonstrating the great merits of NH4+ and SCN- in performance enhancement. As the introduction of NH4+ and SCN- ions has negligible effect on the bandgap of perovskite films. Thus, the obvious increase of FF and Voc of the NH4SCN-added devices should have other mechanisms. The reproducibility of the 30 devices with different NH4SCN concentrations was also characterized, as shown in Fig. 2d and Table 1. Fig. 2d shows the efficiency histograms fitted with Gaussian distributions of the pristine and devices with 15 mol% NH4SCN additive. NH4SCN based PSCs gained a better average PCE of 17.97%, significantly increasing from 14.99% for pristine devices, as shown in Table 1. The results verify that the incorporation of NH4+ and SCN- ions can effectively improve the photovoltaic performance of PSCs with high repeatability. Whereas, there is a clear trend of PCE improvement for the solar cells as the addition level is no more than 15 mol%. The

Hysteresis factor =

PCEreverse − PCEfroward PCEreverse

(1)

Fig. 3a displays J-V curves in forward and reverse scan based on the best performance devices without and with NH4SCN additive. As shown in Table S1 (ESI), compare to the pristine devices with the average HI being 0.46 ± 0.01, the NH4SCN-added device manifests a smaller HI being 0.28 ± 0.05. This reduction of HI can directly be attributed to the suppression of carrier trapped by surface defect or the limited ion migration involving I- ions in the perovskite film (Liu et al., 2016; Yu et al., 2016; Kwon et al., 2016). Fig. 3b shows the IPCE spectra of the champion devices with varying addition levels. We observe an evident increase in IPCE with NH4SCN in nearly the whole absorption spectrum of perovskite, implying that the gains of photo-induced carrier occur not only at the electrode but also in the bulk perovskite material. As shown in Fig. 3c, the estimated Jsc values by integration of IPCE spectra are 20.75 and 22.64 mA cm−2 for the best performance devices based on the pristine perovskite film and the film with 15 mol% of NH4SCN additive, respectively. These integrated Jsc values agree with those measured from J-V curves in Fig. 2a. To explore the long-term stability, we monitored the changing PCE of the devices based on pristine and 15 mol% NH4SCN-added devices without encapsulation in ambient air with 30% humidity. Fig. 3d displays the normalized PCE versus the storage time and Fig. S2 (ESI) further displays the average stability of 20 control devices. As shown in Fig. 3d, it is confirmed that the NH4SCN based devices maintain ~90% initial PCE even after 400 h. However, the pristine devices quickly deteriorate after 400 h stored in air, probably resulting from the degradation and limited grain size of perovskite film. Generally, on one hand, grain boundaries can be assigned to a pathway for permeation of moisture (Wang et al., 2017). On the other hand, due to the fact that SCN- anion can coordinate with MA+ cations by formation of Hbonding, the device stability can be improved significantly (Sun et al., 2017). To investigate the photophysical origin of the improved PCE by using NH4+ and SCN- binary ions, we carried out steady state photoluminescence (PL) spectroscopy. Fig. 4a presents PL spectra of the perovskite films with different addition ratios. It is worth noting that the perovskite films were directly deposited on FTO glass. All as-prepared samples display a PL peaked at 782 nm, implying that the NH4SCN additive does not change the band gap of the perovskite material. But the pristine sample display very weak intensity, whereas the samples processed with NH4SCN additive display relatively stronger PL, approximately 4.5, 6.0, 6.6 and 6.7 times for the samples with the addition of 10, 12.5, 15 and 17.5 mol% of NH4SCN additive, as compared to the pristine perovskite film. Such increase of PL intensities verify that reduction of the nonradioactive loss occurs within the bulk material (Zuo et al., 2017) processed with the addition of NH4SCN additive, which indicates suppressed charge carrier recombination, leading to the observed growth of Jsc. To further investigate thoroughly the impact of NH4+ and SCNactive species on the defect density (Ndefect) of the perovskite layer, space-charge-limited current (SCLC) method were employed. The electron-only devices based on a planar structure comprising FTO substrate/c-TiO2/perovskite/Au were fabricated. The corresponding JV characteristics was presented in log-log coordinate system under dark conditions, as shown in Fig. 4b. On the basis of the different values of the exponent n according to the relation of J ∝ Vn, the J-V curves can be 338

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Fig. 2. Devices performance characteristics with different addition levels of NH4SCN under simulated AM 1.5, 100 mW/cm2 irradiance in ambient air. (a) J-V characteristics (in the reverse scan mode) of the best performance devices. (b) and (c) The best photovoltaic parameters change with addition levels : (b) PCE and FF (c) Voc and Jsc. (d) Histogram data of PCEs for 30 solar cells using MAPbI3 with and without 15 mol% NH4SCN additive.

1.97 × 1016 to 9.05 × 1015 cm−3. The defect density of the added perovskite layer is 2 times lower than that of the control sample, indicating that NH4SCN additive can passivate the defect state at the perovskite interface. The binary effects of NH4+ and SCN- additive on the reduction of defect density can be attributed to: Firstly, it has been reported that due to NH4+ and SCN- ions as additives, the retarded nucleation process of perovskite crystal as well as extra heterogeneous nucleation sites can be realized, resulting in more uniform perovskite film with enlarged crystalline grain size and less pinhole (Chen et al., 2015; Si et al., 2017). Secondly, generally speaking, point defects such as vacancies and interstitials are dominant defects in perovskite film (Buin et al., 2014). Because NH4+ cations have smaller ionic sizes than those of MA in the perovskite crystal, then, NH4+ cations could be properly filled into the vacancies of MA, or enter into the interstitial places of MA to passivate defect states. The SCN- anions can fill I- vacancies, which might also reduce trap states. Lastly, due to the increased formation energy of these point defects, the addition of NH4+ cations can passivate the cation defects, thus reduce the defect density. With the effective carrier extraction, we expect that the change of recombination properties of the perovskites interfaces. Therefore, we

appropriately represented as three characteristic segments (Azam et al., 2018; Li et al., 2017). The current is observed to grow linearly with the increase of the voltage at the low bias due to an Ohmic contact. However, the current is observed to grow non-linearly as the voltage increases to the high bias, referred to as the defect-filled limit (TFL) regime, where the injected charge could fill all the defect states attainable (Shi et al., 2015). The value at the cross-linked point of the two fitting curves where the current suddenly surges can be defined as the defect-filled limit voltage (VTFL) (Liu et al., 2017). The Ndefect close to the valence band edge can be calculated according to formula (2) (Liu et al., 2015):

vTFL =

eNdefect L2 (2)

2εε0

where e is the elementary charge, Ndefect is the trap density, L is the thickness of the film between the two electrodes, ε0 and ε are the vacuum permittivity and the relative dielectric constant of perovskite material, respectively. The VTFL of the devices without and with 15 mol % NH4SCN additive are fitted as 1.35 and 0.62 V, respectively, which are comparable to the recent reports (Wang et al., 2017; Gao et al., 2019). As a result, the corresponding trap-state density decreases from

Table 1 Photovoltaic parameters of perovskite solar cells with varied concentrations of NH4SCN additive. NH4SCN

Best values

contens

Jsc (mA.cm

0 mol% 10 mol% 12.5 mol% 15 mol% 17.5 mol%

1.017 1.054 1.037 1.057 1.060

Average values −2

)

Voc (V)

FF (%)

PCE (%)

Jsc (mA.cm−2)

Voc (V)

FF (%)

PCE (%)

20.75 21.16 22.21 22.64 21.07

72.99 71.74 74.47 78.37 77.47

15.67 16.75 17.16 18.75 17.31

1.005 1.035 1.031 1.06 1.034

20.04 21.89 21.84 21.99 21.10

71.9 72.9 74.84 76.85 75.87

14.99 16.40 17.00 17.97 16.79

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Fig. 3. (a) J-V hysteresis of solar cells under reverse and forward voltage scanning; (b) Incident photon-to-electron conversion efficiency (IPCE) spectrum of devices with varying addition levels and (c) IPCE spectra of the best performance PSCs without and with 15 mol% NH4SCN additive (d) Normalized PCEs of the devices without and with 15 mol% NH4SCN additive as a function of aging time. The devices were stored in ambient conditions with about 30% humidity and 25 °C.

to 0.7 V. As shown in the detailed fitting results of Table 2, a slight difference is observed in the series resistance (Rs), which is generally influenced by contact properties (Juarez-Perez et al., 2014; Pockett et al., 2015; Guillén et al., 2014). Furthermore, with the growing of the NH4SCN concentration from 0 to 17.5 mol%, we observe the increase first and then decrease of the diameter of the semicircle, which implies the Rct gradually grows and then decreases. The device 15 mol% NH4SCN exhibits the largest Rct (98412 Ω), indicating the most efficient recombination blocking effect. The carrier effective lifetime can be calculated according to τn = Rct × Cμ, where Cμ is the chemical capacitance (Liu et al., 2015). Under dark conditions, τn is mainly decided by the Rct. As shown in

probed carrier recombination dynamics by means of EIS measurement in the dark. In the dark, the perovskite solar cells could be regarded as a leakage capacitor. Fig. 5a displays the Nyquist plots of the devices fabricated with different concentrations of NH4SCN under 0.7 V applied bias accompanied by fitting with equivalent circuit in the inset. The semi-circle assigned to the recombination resistance (Rct) at the interface of perovskite/c-TiO2 or HTL layer (Juarez-Perez et al., 2014; Pockett et al., 2015). The chemical capacitance CPE is assigned to the chemical capacitance at the perovskite/TiO2 interface. CPE and Rct respectively can reflect the carrier recombination dynamics across the perovskite interface in the devices. Fig. S3 (ESI) make a comparison of the according Nyquist plots measured at applied bias ranging from 0 V

Fig. 4. (a) PL spectra of the PSCs with various concentrations of NH4SCN additive; (b) Dark J-V characteristics of electron-only devices based on perovskite films with different concentrations of NH4SCN additive. 340

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Fig. 5. (a) EIS of PSCs with and without NH4SCN additive measured in the dark, Nyquist plots of devices with various addition levels at 0.7 V applied bias. The solid line indicates the fitting curve and the inset shows the equivalent circuit. (b) The electron lifetime changes with the concentration of NH4SCN additive.

Rs(Ω)

Rct(Ω)

CPE-T(F)

CPE-P

effective lifetimes, displaying a lower recombination order. Therefore, our work enriches our fundamental understanding on the defect properties of PSCs, with which further improvement of the device performance and stability can be achieved by using synergistic binary ions as additives.

26.1

32,917

2.1089E-8

0.996

Declaration of Competing Interest

48.56 17.45 20.84 29.89

44,261 70,478 98,412 52,253

1.7962E-8 1.8136E-8 1.8725E-8 1.6749E-8

0.991 0.998 0.983 0.992

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.

Table 2 Fitting parameters of the electrochemical impedance spectra (EIS) for the devices with various concentrations of NH4SCN additive at 0.7 V applied bias in the dark.

w/o NH4SCN w/NH4SCN 10 mol% 12.5 mol% 15 mol% 17.5 mol%

Acknowledgments

Fig. 5b, the τn calculated from the sample with 15 mol% NH4SCN additive is 1.65 ms, much larger than that of the pristine sample (0.67 ms). However, when the content of NH4SCN increases to 17.5 mol %, the value of Rct greatly reduces, which might relate to the formation of non-perovskite phase. As shown in Fig. S4 (ESI), τn decrease with the growth of the bias voltage. It is notable that carrier lifetime in the device with NH4SCN becomes evidently longer across a wide range of bias voltages. This trend indicates the general picture of suppressed recombination in the devices where the carriers can live longer, which could be originated from decreased carrier trapping in perovskite. Usually charge carrier lifetime is strongly related to the defect/trap densities inside the perovskite layer, especially non-radiation recombination. Therefore, compared to the single NH4+ or SCN- as additives, binary NH4+ and SCN- active species can do passivate the halide or cation-induced deep defect state in both bulk perovskite material and the interface, leading to decrease of Shockley-Read-Hall recombination loss and facilitation of charge transport. To this end, the observation in EIS consists with the increase of photovoltaic parameters, i.e. Voc and FF, and device stability.

This work was partially supported by the Fundamental Research Funds for the Central Universities (no. JZ2017HGBZ0921) and the National Natural Science Foundation of China (no. 51571080). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2020.01.069. References Azam, M., Yue, S., Rui, X.U., Kong, L., Ren, K., Sun, Y., Liu, J., Wang, Z., Qu, S., Lei, Y., 2018. Highly efficient solar cells based on Cl incorporated tri-cation perovskite materials. J. Mater. Chem. A. 6, 13725–13734. Ball, J.M., Lee, M.M., Hey, A., Snaith, H.J., 2013. Low-temperature processed meso-superstructured to thin-film perovskite solar cells. Energy Environ. Sci. 6, 1739–1743. Bi, D., Tress, W., Dar, M.I., Peng, G., Luo, J., Renevier, C., Schenk, K., Abate, A., Giordano, F., Baena, J.P.C., 2016. Efficient luminescent solar cells based on tailored mixedcation perovskites. Sci. Adv. 2, 1501170. Bi, D., Luo, J., Zhang, F., Magrez, A., Athanasopoulou, E.N., Hagfeldt, A., Grätzel, M., 2017. Morphology engineering: a route to highly reproducible and high efficiency perovskite solar cells. ChemSusChem 10, 1624–1630. Boopathi, K.M., Mohan, R., Huang, T., Budiawan, W., Lin, M.Y., Lee, C.H., Ho, K.C., Chu, C.W., 2016. Synergistic improvements in stability and performance of lead iodide perovskite solar cells incorporating salt additives. J. Mater. Chem. A. 4, 1591–1597. Buin, A., Pietsch, P., Xu, J., Voznyy, O., Ip, A.H., Comin, R., Sargent, E.H., 2014. Materials processing routes to trap-free halide perovskites. Nano Lett. (14), 6281–6286. Burschka, J., Pellet, N., Moon, S.-J., Humphry-Baker, R., Gao, P., Nazeeruddin, M.K., Grätzel, M., 2013. Sequential deposition as a route to high-performance perovskitesensitized solar cells. Nature 499, 316–319. Cai, Y., Zhang, Z., Zhou, Y., Liu, H., Qin, Q., Lu, X., Gao, X., Shui, L., Wu, S., Liu, J., 2018. Enhancing the efficiency of low-temperature planar perovskite solar cells by modifying the interface between perovskite and hole transport layer with polymers. Electrochim. Acta 261, 445–453. Chen, Y., Li, B., Huang, W., Gao, D., Liang, Z., 2015. Efficient and reproducible CH3NH3PbI3-x(SCN)x perovskite based planar solar cells. Chem. Commun. 51, 11997–11999. Docampo, P., Hanusch, F.-C., Stranks, S.D., Döblinger, M., Feckl, J.-M., Ehrensperger, M., Minar, N.-K., Johnston, M.-B., Snaith, H.J., Bein, T., 2014. Solution deposition-

4. Conclusion To summarize, we show that a binary NH4+ or SCN- ions can work effectively to passivate the surface defect in PSCs. The crystallization of perovskite with lowered defect density has been improved in the presence of NH4SCN additive. Under optimized conditions, the PCE of devices with NH4SCN fabricated in ambient air enhances from 15.67% to 18.75%, owing to the suppression of non-radiation recombination at the perovskite interface. Due to NH4+ and SCN- do passivate defect trap in perovskite film, the deterioration of PSCs significantly slows down, leading to maintaining 90% of the initial efficiency after 400 h. We further investigate recombination dynamics to unravel the multifaceted role of binary NH4+ and SCN- ions and the origin of the enhanced performance. The NH4SCN added devices are concerned with longer 341

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