An Application of Cesium on the Restriction of Precursor

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An Application of Cesium on the Restriction of Precursor Crystallization for Highly Reproducible Perovskite Solar Cells Exceeding 20% Efficiency Gen Zhou, Jionghua Wu, Yanhong Zhao, Yiming Li, Jiangjian Shi, Yusheng Li, Huijue Wu, Dongmei Li, Yanhong Luo, and Qingbo Meng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01054 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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ACS Applied Materials & Interfaces

An Application of Cesium on the Restriction of Precursor Crystallization for Highly Reproducible Perovskite Solar Cells Exceeding 20% Efficiency

Gen Zhou,a† Jionghua Wu,ab† Yanhong Zhao,ab Yiming Li,ab Jiangjian Shi,ab Yusheng Li,ab Huijue Wu,a Dongmei Li,ab Yanhong Luo*ab and Qingbo Meng*ab

a

Key Laboratory for Renewable Energy Chinese Academy of Sciences (CAS), Beijing Key

Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS, Beijing 100190, China b

School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

† These authors contributed equally to this work. * Corresponding author. E-mail addresses: [email protected] (Q. Meng), [email protected] (Y. Luo).

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Abstract:

In this study we systematically explored the mixed cation perovskite Csx(MA0.4FA0.6)1-xPbI3 fabricated via sequential introduction of cations. The effects of Cs+ on the fabrication and performance of inorganic-organic mixed cation perovskite solar cells were examined in detail which is beyond the normal understanding of adjusting band gap. It is found that a combined intercalation of Cs+ and dimethyl sulfoxide (DMSO) in PbI2-DMSO precursor film formed a strong and steady coordinated intermediate phase to retard PbI2 crystallization, suppress yellow non-perovskite δ-phase and obtain a highly reproducible perovskite film with less defects and larger grains. The Cs-contained triple cation mixed perovskite Cs0.1(MA0.4FA0.6)0.9PbI3 devices yield over 20% reproducible efficiencies, superior stabilities and fill factors of around 0.8 with very narrow distribution.

Keywords: Cesium, Coordinated intermediate phase, PbI2 crystallization, High reproducibility, Perovskite solar cells


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1. Introduction During the past few years, hybrid inorganic-organic perovskite solar cells (PSCs) have emerged as one of the most promising candidate for the next-generation energy source with a rapid increase of power conversion efficiencies (PCE) from 3.8% in 2009 to the current record of 22.7%.1-9 Among the family of perovskite-type compounds, multiple cation lead halide with structure of APbX3, where A is a mixture of several organic or metal cations with proper radius as described in the Goldschmidt tolerance factor and X is a halide ion,10 has attracted most of the attentions due to its superior photovoltaic properties, adjustable bandgap, and excellent carrier transport characteristics.11 Originally, most of the researches focused on the organic monocation system, especially methylammonium lead iodide (CH3NH3PbI3 or MAPbI3).12-16 The advantages such as smooth morphology and superior phase stability of MAPbI3 perovskite film are obviously applicable to enhance the device performance. However, a structural transition from tetragonal to cubic phase at around 55 °C would potentially affect the quality of solar cells based on MAPbI3 absorber, and the materials bandgap was also expected to be further optimized.17 Researchers thus extended their attention to a mixed-cation system with both MA+ and formamidinium (HC(NH2)2+ or FA+) in a single perovskite layer.18,19 With a larger radius closing to the upper limit defined by Goldschmidt tolerance factor, FA+ expand the lattice and change the tilt of the PbI64- octahedra, which modifies the bandgap of MAPbI3 from ~1.6 eV to a more suitable value of ~1.5 eV.18-20 The MA/FA mixed-cation perovskite films efficiently overcome many drawbacks of monocation systems, such as the slightly larger bandgap of MAPbI3 and the humidity-sensitive property of FAPbI3.18,21 However, the MA/FA mixed-cation system still suffers from a series of stability problems such as the existence of yellow non-perovskite δ-phase and inadequate resistance to heat or illumination, which becomes obstacles on the way of


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commercialization of PSCs. Potential studies are focused on further introduction of inorganic cation, typically cesium ion (Cs+), into the perovskite structures for solar cells.20,22,23 As formerly reported, a superior efficiency and long-term stability has been shown in photovoltaic devices obtained by mixing inorganic Cs+ with organic MA/FA cations in perovskite absorber.20,23-26 Due to the excellent intrinsic thermo-stability and proper radius of Cs+, Grätzel and co-workers developed a triple Cs/MA/FA cation mixture through one-step deposition method. They reported that the existence of Cs would effectively suppress the yellow non-perovskite δ-phase during the film fabrication process and improve the devices stability.20 Park et al. also demonstrated the enhanced moisture stability and improved photovoltaic performances of PSCs based on CsxFA1-xPbI3 films.23 Very recently, You and co-workers extended the normal one-step method of Cs-doped perovskite film to a two-step sequential deposition method and obtained a PCE over 16% for planar PSCs.25 Zhou et al. then rapidly promoted the PCE of two-step prepared FA0.65MA0.25Cs0.1PbI3(Cl) perovskite devices to 19.8%.26 However, a further study is still needed to reveal the intrinsic impact of doping Cs on suppression of the yellow non-perovskite δ-phase and PbI2, as well as on the final performance of solar cells. Two-step sequential deposition method is undoubtedly the first choice for this investigation because of the more controllable and detectable intermediate states with separated introducing stage of Cs+, MA and FA cations. Meanwhile, it should be noticed that the crystallization of PbI2 film fabricated with two step method is a non-negligible obstacle to the formation of high-quality perovskite films, which can significantly influence the performance of PSCs.27-29 A general method of retarding PbI2 crystallization was developed as utilizing the strong coordination between Pb2+ and dimethyl sulfoxide (DMSO).27,30 However, DMSO is easily to escape from the precursor film at ambient conditions during the fabrication process of


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the second step. Thus we tried to find some new clues about the possible crystallization of PbI2 in a Cs-doped perovskite system. In this work, we developed an inorganic-organic multi-cation perovskite (Csx(FA0.4MA0.6)1xPbI3)

system by using a modified two-step method in order to investigate the effects of added

CsI on PbI2 precursor film and formation of perovskite layer. The reliable restriction of the yellow non-perovskite δ-phase and PbI2 crystallization in the Cs-contained precursor film was considered as one of the main reason that lead to high performance of Cs-contained perovskite devices. The lattice parameter of perovskite was also enlarged by the intercalation of Cs+, resulting in a highly uniform film with large grains and less defects. This enables more reproducible device performances reaching the highest PCE value of 20.3%.

2. Experimental section 2.1 Device fabrication The entire device fabrication process occurred under ambient conditions at a suitable humidity (~20%) and at room temperature (25±1 °C). First, a 20 nm-thick compact TiO2 layer was prepared by spin coating a slightly acidic solution layer of titanium isopropoxide on laserpatterned fluorine-doped tin oxide (FTO) glass, which was then sintered at 500 °C for 30 min. After that, the temperature of the compact layer was decreased to 150 °C. A 150 nm-thick mesoporous TiO2 layer was spin coated on the original film at 3000 rpm for 30 s from a solution containing 200 mg 30NRD Dyesol paste per 1 ml of EtOH. This layer was then sintered at 500 °C for 30 min. Next, the film was treated with an aqueous TiCl4 solution (25 mM) at 70 °C for 10 min, followed by subsequent washing and drying with deionized water, ethanol, and air flow. After that, an additional Li-doping layer of mesoporous TiO2 was added by spin coating a


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solution of Li-TFSI (100 mM) in acetonitrile on the obtained films at 3000 rpm for 30 s and then annealing at 500 °C for 30 min.31 The Csx(MA0.4FA0.6)1-xPbI2 perovskite layers were fabricated using a modified two-step method-sequential multi-drop coating method (SMDC).28 First, a solution of PbI2 (1.4 M) or PbI2 (1.4 M) with different molar percent CsI in dimethylformamide (DMF)/DMSO mixture (10/1, v/v) was spin coated onto a mesoporous TiO2 substrate at 3000 rpm for 30 s, followed by a vacuum processing procedure for a certain time to obtain PbI2-DMSO or PbI2-DMSO-(CsI)x precursor films (where x is the molar percentage of CsI/PbI2). FAI and MAI mixed solutions (MAI/FAI molar ratio of 4:6) in isopropanol (IPA) with concentration of 20 mg ml-1 was then dripped onto the continuously rotating precursor films.28 The solution was dripped every 2 s for each drop (approximately 10 µl) and the substrate was spun at 4000 rpm during the whole dripping process. The films changed to a red-brown color during spin coating, and then were annealed at 150 °C for 10 min and 120 °C for 40 min. A solution of Spiro-OMeTAD was spin coated on the perovskite film (3500 rpm, 30 s) as hole-transport material (HTM). Finally, 80 nm of gold was thermally evaporated on top of the device to form the back contact. The PSC with a structure of FTO/TiO2/perovskite/Spiro-OMeTAD/Au was accomplished.

2.2 Characterization In our experiments, X-ray diffraction (XRD) spectra were measured by an Empyrean series 2 XRD system with Cu Kα as the radiation source. Fourier transform infrared (FTIR) spectra were measured with a Bruker Spectrum TENSOR27 spectrometer. Scanning electron microscope (SEM) images were measured using a Hitachi S-4800 SEM, and the film thicknesses were determined by a surface profiler (KLA-Tencor, P-6). The steady-state photoluminescence (PL)


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spectra were measured using Edinburgh FLS 900 spectrometer (excitation at 445 nm). The detected emission wavelength of the steady-state PL is from 700 to 850 nm. UV-vis absorption spectra were measured by using Shimadzu UV-2550 spectrophotometer over a wavelength range of 350 to 850 nm. Impedance spectra (IS) of the devices were measured by the Zahner IM6e electrochemical workstation in the dark in the frequency ranging from 0.1 to 106 Hz at zero bias. The J–V characteristics were determined on a Keithley 2602 Source Meter under AM 1.5 irradiation (100 mW/cm2) from a New Port Solar Simulator 69911. The light intensity was calibrated with an NIM (National Institute of Metrology)-certified Si reference solar cell equipped with a KG3 colour filter before the J–V measurements. A mask with a black aperture was used to cover the solar cells to constrict their active area to 0.1 cm2. The J–V curves were measured under a forward scan from -0.05 to 1.1 V and a reverse scan from 1.1 to -0.05 V using a scan rate of 30 mV/s. Incident photon to electron conversion efficiency (IPCE) was measured from 300 nm to 900 nm with a 500 W xenon lamp.

3. Results and discussion

Figure 1. Schematic diagram of the SMDC method for fabrication of Csx(MA0.4FA0.6)1-xPbI3 perovskite films.


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Although one-step method is currently the most widely used in the preparation of Cs/MA/FA perovskite films,20,22 a controllable two-step sequential deposition method is generally considered as necessary for the purpose of investigating the effects of each composition on the fabrication process. As depicted schematically in Figure 1, a two-step SMDC method is used for perovskite film fabrication to controllably sequential introduction of cations.28 Briefly, precursor solution of PbI2 without and with different molar percent of CsI in DMF/DMSO was deposited on TiO2 mesoporous substrates, followed by a 30 min vacuum treatment at room temperature to remove the residual DMF. Slight yellow transparent films are formed just after spin coating for all the precursor solutions without or with CsI (Figure 2a). Then the MAI and FAI mixed solution in IPA were dripped sequentially on the rotating surface of the obtained precursor film under ambient conditions. Followed by a high temperature annealing process, perovskite films are obtained finally. However, the colour of PbI2 precursor films without or with CsI changes differently when stored in air as they have different absorption spectra seen in Figure 2a. The PbI2 film without CsI features an absorption peak at around 500 nm after 60 min storage, corresponding to the characteristic band-gap excitation of crystallized PbI2 semiconductors. While the PbI2 film with 10% CsI exhibit almost no such an absorption band, indicating uncrystallised PbI2.


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Figure 2. (a) UV-vis absorption spectra, (b) and (c) XRD patterns, and (d) peak intensity of PbI2 (12.7 °) of the precursor films without and with 10% CsI for different time storage in air. Insert in (a) is the photo of the precursor films. ∆ and * denote the identified diffraction peaks corresponding to PbI2(DMSO) and PbI2, respectively.

A series of XRD spectra have also been measured to study the crystallization for the precursor films during storage before the second step. Figure 2b and 2c are XRD patterns of the precursor films without and with 10% CsI for different time storage in air. In the general PbI2-DMSO precursor solution without CsI, a relative strong coordination of S=O with Pb2+ spontaneously forms and improve the solubility of PbI2.30 A broad diffraction peak at 2θ = 9.8 ° belonging to PbI2(DMSO) complex is clearly found in XRD patterns.30 Although, DMSO has been widely


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utilized to improve the performance of perovskites, the coordination bond is not long time stable and may be broken during storage or in the second step MAI/FAI/IPA dripping process.29 As shown in Figure 2b, PbI2 peak at 2θ = 12.7° for the PbI2-DMSO precursor film without Cs becomes much stronger after 60 min storage in air. When 10% CsI was added into the PbI2DMSO precursor solution, the diffraction peak of crystallized PbI2 was almost totally disappeared in the XRD patterns of films as-prepared and only a small peak of PbI2 appeared even after 120 min storage shown in Figure 2c. This result reveals the formation of a more steady and strong coordination phase among the possible components: Cs+, Pb2+ and DMSO. Figure 2d shows the relationship peak intensity of PbI2 (12.7 °) with different storing time for PbI2-DMSO precursor films without and with 10% CsI. We can clearly see that addition of CsI reduce the increasing rate of PbI2 peak intensity of the precursor films. Therefore, the introduction of CsI into precursor system may have a more strong impact on retarding PbI2 crystallization in the precursor films. Results from a serious of studies about the influence of CsI percentage and thermal annealing on the stability of the precursor films, shown in Figure S1, have further demonstrated that the introduction of CsI can help the films stay in amorphous state and benefit for perovskite transformation29 when FAI/MAI/IPA solvent was dropped in the second step.


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Figure 3. FTIR spectra of the PbI2-DMSO precursor films without and with 30% CsI asprepared and after storing 10 h in ambient condition.

The CsI influence on the PbI2-DMSO precursor films were further characterized by FTIR. Figure 3 displays the FTIR spectra without and with 30% CsI as-prepared and after storing 10 h in ambient condition. As reported before, the infrared peak for stretching vibration of S=O generally appeared at around 1045 cm-1 in the spectra of pure DMSO.32 When PbI2 was equally mixed with DMSO to form 1:1 adduct, ν(S=O) was shifted from 1045 cm-1 to 1022 cm-1 as depicted by the red line in Figure 3, which was in accordance with the O-bonded PbI2-DMSO complex. Comparing with FTIR spectra of PbI2-DMSO, a new interesting peak emerged at around 1108 cm-1 in the spectra of precursor film containing 30% CsI as shown by the green line in Figure 3, which suggests another coordination interaction has been formed by the introduction of CsI. In fact, the infrared absorption peak at around 1107 - 1120 cm-1 is usually a hallmark for


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a sulfoxide to coordinate via the S atom with a lone pair.33,34 Therefore, one can conclude that CsI has formed a stronger bond with S=O groups of a part of 1:1 PbI2-DMSO adducts via the S end, which raised the frequency of S=O stretching vibration to a higher wavenumber. It can speculate that PbI2 coordinates with DMSO via O end, while CsI formed a stronger bond with S=O groups via S end and a PbI2-DMSO-CsI intermediated phase forms. We also characterized the PbI2-DMSO and the PbI2-DMSO-CsI precursor film after 10 hrs’ storage in ambient conditions. The FTIR spectra clearly showed that the coordination peaks at 1022 cm-1 in CsIcontained precursor were still strong, while those peaks nearly disappeared in the pure PbI2DMSO precursor after 10 hrs’ storage. Thus this result is a further proof of the long-time stability of CsI-contained precursor film for perovskite fabrication.


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Figure 4. XRD patterns of perovskite films for different MAI/FAI/IPA dripping cycles. The perovskite films with 10% CsI (a) as-prepared and (b) after annealing. The perovskite films without CsI (c) as-prepared and (d) after annealing. α, δ, ∆ and * denote the identified diffraction peaks corresponding to α-FAPbI3 perovskite phase, δ-FAPbI3 phase, PbI2(DMSO), and PbI2, respectively.

A retarded crystallization of PbI2 is very significant to maintain the low crystallinity of the first step precursor film in a long time scale and thus extend the tolerable waiting time (“timewindow”) during a complex fabrication process of perovskite. In order to clarify the influence of CsI on perovskite transformation, detailed XRD patterns for the reaction process of the precursor films with MAI/FAI are recorded and shown in Figure 4. For the as-prepared films with 10% CsI (Figure 4a), XRD pattern show a continually growing peak at 14 ° with the MAI/FAI/IPA dripping cycles increased, implying a rapid formation of α-phase perovskite. On the contrary, for the as-prepared films without CsI (Figure 4c), peak at 11.7 ° for δ-FAPbI3 phase appear in the first few cycles. The peak for δ-FAPbI3 phase cannot be found in Figure 4a, indicating that PbI2DMSO-CsI intermediated phase can help the transformation of pure α phase perovskite. In addition, a peak position shift in around 14 ° is found in Figure 4a for the as-prepared perovskite films with CsI. According to Bragg's law, the decrease angle of the diffraction peak means an increasing of the lattice parameter. Thus our results have suggested that more FAI entered into perovskite lattice, which increases the lattice parameter because more FAI will add into reaction with the cycles increased.28 While the as-prepared perovskite films without CsI (Figure 4c) can’t find this obvious phenomenon. These suggest that PbI2-DMSO-CsI intermediated phase can help more FAI enter into the perovskite lattice, leading to a complete transformation of perovskite.


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The XRD patterns of annealing perovskite films with or without Cs are shown in Figure 4b and 4d. We can clearly see that perovskite films without Cs have a stronger PbI2 peak (12.7 °) intensity than perovskite films with Cs after annealing. The lower residual amount of PbI2 in perovskite films with Cs than those without Cs doping implies that PbI2-DMSO-CsI intermediated phase restrain the formation of PbI2 crystal and benefits to perovskite transformation.

Figure 5. Schematic of the reaction mechanism. (a) Effect of retarding PbI2 crystallization in CsI-contained precursor. (b) The cation exchange process during conversion of CsI-contained precursors to perovskite. (c) The structure of perovskite film after annealing. Herein, we propose a preliminary mechanism concerning the perovskite crystal growth by molecular exchange in the presence of PbI2-DMSO-CsI precursor film in view of microscopic dynamics. As shown in Figure 5a, PbI2 coordinates with DMSO via O end, while CsI formed a stronger bond with S=O groups via S end. The PbI2-DMSO-CsI intermediated phase effectively retards the crystallization process of PbI2 during the precursor film prepared step and storage stage. The MAI/FAI (4:6) mixed IPA solutions were then dripped successively on the rotating


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surface of the obtained precursor film under ambient conditions (Figure 5b). The PbI2-DMSOCsI intermediated phase helps FAI enter reaction more easily, which is beneficial for perovskite formation. After 150 annealing process, compact perovskite polycrystalline films are obtained as shown in Figure 5c.

Figure 6. (a) XRD patterns of the Csx(MA0.4FA0.6)1−xPbI3 films with various Cs content. (b) Shift of the XRD peak at around 14 o. (c) UV-visible absorption spectra and (d) Steady-state PL spectra of perovskite films. A series of measurements have been taken in order to further clarify the intrinsic properties of the perovskite films contained Cs. In Figure 6a and 6b, we show XRD data for perovskite films with 0-15% Cs. It is found that all compositions exhibit the typical α perovskite peak at ~14 °. Upon addition of small amounts of Cs from 0 to 15%, the diffraction angle peaks around 14°


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increases gradually from 14.01 ° to 14.06 °, rather than the appearance of two separate peaks with variable intensities, indicates that the three cations were all merged in the same lattice frame of a pure phase Csx(MA0.4FA0.6)1−xPbI3. The higher angle’s shift of the observed peak corresponds to the reduced lattice parameter of the Csx(MA0.4FA0.6)1−xPbI3 unit cell induced by partial substituting FA/MA cation with smaller Cs cation. Figure 6c and 6d present the UVvisible absorption and steady-state PL spectra of the perovskite films with different percentage of CsI. The optical bandgap is enlarged and PL emission peak shifted to blue side with an increasing amount of Cs, which is in excellent agreement with the peak shift in XRD patterns.

Figure 7. SEM top-view images of prepared Csx(MA0.4FA0.6)1−xPbI3 perovskite films with (a) 0% CsI, (b) 5% CsI, (c) 10% CsI and (d) 15% CsI, respectively, in the precursor solutions.


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The morphology of the perovskite film was investigated by SEM. Figure 7 depicts top-view SEM images of Csx(MA0.4FA0.6)1−xPbI3 films on mesoporous TiO2 coated FTO glass. All the films show a smooth pinhole-free surface. The grain size distributions of the perovskite films with different Cs doping percentage have been shown in Figure S2. It can be clearly seen in Figure 7 and Figure S2 that a small amount of Cs doping (5%) promoted the grain growth and uniformity of the perovskite films. However, some bright small grains have appeared with excess Cs (15%). These bright small grains are attributed to the apparent phase separation of the films which cannot be detected by XRD.35 Generally, CsI promoted the grain growth and uniformity of the perovskite films. As far as we have known, the reaction between MA/FA cations and DMSObounded PbI64- (non-crystallized phase of Pb-I in precursor) determined the nucleation process of perovskite films.27 The retarded crystallization of PbI2 in CsI-contained precursor corresponds a slow reaction rate of nucleation, indicating a stronger coordination among PbI64- and other solute molecules. According to the classic theory of nucleation and crystal growth:26

= 1.1 (1)

Where N is the nucleation rate, G the growth rate and Zs the number of grains per unit area. Obviously, the reduction of N leads to a lower Zs and a larger average size of the perovskite grains.36 On the one hand, the thickness of PbI2-DMSO precursor film was apparently increased by intercalation of CsI as shown in Table 1, leading to a sparser distribution of the nucleation sites for perovskite grains. On the other hand, the strong coordination bonds in the intermediate phase of PbI2-DMSO-CsI prevent high nucleation rates of perovskites. According to Equation 1, the number of grains per unit area will be consequently decreased, resulting in a larger average


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size of the perovskite grains. Furthermore, the SMDC method was also used to avoid the interference of inhomogeneous nucleation rates on the uniformity of films in the coexistence of different cations in the same growth period. Figure S3 provides the SEM side-view image of PSCs, depicting a perovskite film for 10% Cs doping with large grains in uniform size distribution compared with 0% Cs-contained device. These large grains throughout the layer of perovskite are obviously beneficial for the transport of carriers through devices.

Table 1. The thicknesses of PbI2-DMSO precursor films with different percentage of CsI

Molar percent of CsI (%)

0

5

10

15

30

Film thickness (nm)

352

402

479

556

726

Figure 8. (a) Nyquist plots (Inset shows an equivalent circuit of solar cells) and (b) The model of capacitance and frequency of the PSCs with 0%, 5% and 10% CsI in the dark at zero bias. (c) The density-of-states (DOS) distribution of defects in PSCs.


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In order to estimate the defects in the perovskite film, IS has been measured by placing the fabricated PSCs in a sinusoidal oscillator circuit. Figure 8a shows the Nyquist plots and the inset displays an equivalent circuit of solar cells. The following equation describes the relation among Z, Rs and Rct under high-frequency conditions, where Z' and Z" represent the real part and imaginary part of impedance, respectively; Rs is the series resistance and Rct is the recombination resistance.37 [

′

″ − +

] + ( ) = (2) 2 2

Obviously, this relationship is described as a circle function with variables of Z' and Z" and a radius of Rs + (1/2) Rct. A larger resistance of the recombination value indicates reduced recombination in devices and beneficial charge-transport properties. Therefore, the IS of the optimized PSCs should display a large radius related to a high level of recombination resistance, as the spectra of 10% CsI-contained device shown in Figure 8a. Figure 8b shows the relationships of capacitance and frequency. In the region of frequency lower than 100 Hz, all the devices showed a sharply descending trend of capacitances with the increasing of frequencies, indicating the presence of deep defect states. The statistics depicted in Figure 8c can be further combined with Equation 4 to quantitatively analyze the density-of-states (DOS) distribution of defects in PSCs:38 "() () = −

(3) !

VFB is the flat band voltage of the PSC, d is the widen of depletion region, which can be considered as the film thickness of the perovskite layer in PSCs,39 q is the elementary charge, kB is the Boltzmann constant, T is the measurement temperature. We sketch the results in Figure 8c. Here we consider$

%&'

()*' +

, as a constant and normalize the vertical coordinates. It is clearly seen


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that the density distributions of defects in devices with different percentage of Cs satisfy the following relationship in the low-frequency region (0.1– 100 Hz): DOS(10% Cs) < DOS (5% Cs) < DOS (0% Cs). Therefore, it is concluded that except for improving the film morphology, the introduction of Cs can efficiently suppress the defects and hence the recombination of carriers in fabricated perovskite films, in accordance with our expectation as retarding the PbI2 crystallization in precursors.

Figure 9. Statistical data and distributions of (a) Voc, (b) Jsc, (c) FF and(d) PCE for a batch of PSCs containing 10% Cs and a control group without Cs both measured for total 35 devices, respectively.


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The device performances with different Cs are shown in Figure S4. It has been seen that PSCs with 5, 10, 15% Cs enhanced PCEs in comparison to those with FAMAPbI3-based device, in which the highest PCE was obtained in the case of 10% Cs doping. We further conducted a large scale test with the MA/FA mixed cations (MA0.4FA0.6PbI3) and 10% Cs contained triple cation composition (Cs0.1(MA0.4FA0.6)0.9PbI3). Figure 9 shows the device statistics (open circuit voltage Voc, short circuit current density Jsc, fill factor (FF) and PCE) of these two types of devices. It is found that the introduction of Cs enhanced the overall Voc and FF values, and narrowed the distribution of Jsc and FF, especially, in a batch of devices, resulting in the higher PCE value and performance reproducibility. The J–V characteristics statistical results of PSCs prepared without or with 10% Cs are shown in Figure S5 with the precursor films being stored 2 h in the air before the second step deposition. All the device parameters for the PSCs without Cs are lower and with worse reproducibility than those with 10% Cs doping. The Jsc and FF dropped a lot for the devices without Cs doping since unmatched PbI2 was strongly formed in precursor films during storage, which affected the perovskite transformation, leading to the PSCs with lower performances and larger PCE distribution. The enhancement of Voc can partly be due to the larger band gap of Cs contained perovskite, which we discussed in Figure 6c. This property can also be explained by the following equation:40 - =

! 0 ln( + 1) (4) 0

In equation 5, js represents the reverse saturation current of device. As we investigated above, the suppression of defects by introducing Cs is favorable for inhibiting carrier recombination, leading to a lower js and correspondingly higher Voc. Then we considered the basic relationship between Voc and FF:40


21


22 = 1 −

Page 22 of 33

! 3 ! ln 1 +

− (5) ! - -

Where Vm is the voltage at the maximum power output point. Obviously, the value of FF will increase with the increasing of Voc. This is the reason for Cs contained group has higher FF. Furthermore, if we take other influence factor of the solar cells into account, the two-diode equation in steady state can be written as: 5 = 5 − 56 [exp( ) − 1] − 56 [exp ( ) − 1 ] (6) ! 2 !

J01 and J02 here are considered as the diode saturation current densities of diffusion current and recombination current, respectively. In order to make above equation simpler, we use the below equation to analysis, that is: 5; ∝ exp( ) − 1 (7) = !

Jf is the current densities considered the influence of the second and third term of equal (7); m is the ideal factor, which is between 1 and 2. When the diffusion current plays a dominant role in Jf, m is 1. When the recombination current play a dominant role, m is 2. As we know, the recombination current results from the defects recombination or other recombination, which has many negative effects for the performance of PSCs. And we will find that m>1 can seriously affect the FF and PCE if we simulate the J–V curves.41 The m has been calculated and shown in Figure S6. Compared with the J–V curves, the m value in Cs contained group is smaller (1.78) than that of control group (2.08). This means that recombination current plays a dominant role in the PSCs and perovskite without Cs has more defects than Cs contained perovskite films. Therefore, the inherent relationship about the fewer defects in Cs contained perovskite films with the high performance and reproducibility of PSCs is also clarified.


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Figure 10. (a) J–V curve and (b) IPCE spectrum and integrated Jsc of the champion device based on a Cs0.1(MA0.4FA0.6)0.9PbI3 perovskite absorber. (c) The stabilized power output of the champion device at 900 mV.

The J–V curve of the champion device for Cs0.1(MA0.4FA0.6)0.9PbI3 is presented in Figure 10a, which exhibits a PCE of 20.3% with a Jsc value of 23.3 mA/cm2, a Voc of 1086 mV and an FF of 0.80. Figure 10b presents the IPCE spectrum of the optimized PSC. The integrated current density value obtained from the IPCE data reached to 22.3 mA/cm2, which is well agreed with Jsc (23.3 mA/cm2) measured from the J–V curve. A stabilized power output of this champion device was also measured at the maximum power output point, as shown in Figure 10c, and a PCE of 19.4% with a stable current value of 21.6 mA/cm2 was steadily recorded over 120 seconds at 900 mV, which matches the J–V curve reasonably well.


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Figure 11. Stability test for MA0.4FA0.6PbI3 and Cs0.1(MA0.4FA0.6)0.9PbI3 devices (a) in an ambient atmosphere (dark, 25 oC, 20-35% relative humidity) and (b) under one sun illumination (AM 1.5, 100 mW/cm2) without encapsulation.

Furthermore, the long-term and light stability of these PSCs are measured and shown in Figure 11. In order to study the long-term stability, both the 10% Cs-contained PSCs and the control group were stored in ambient conditions in dark. After 720 h storage, the Cs-contained PSCs still maintained around 80% of the initial efficiency while the efficiency of control group decreased to 60% of the initial value. In addition, light stability test was also conducted by placing the devices under solar illumination in air condition without encapsulation. The cells were exposed to one sun illumination (AM 1.5, 100 mW/cm2) for a certain time, and then measured with a standard J–V characterization. For Cs contained PSCs, 84% of the initial performance was maintained after 150 min irradiation. Conversely, the performance of the control group declined to 63% of the original value in the same condition. This indicates that Cs contained perovskite devices have improved device stabilities. We also investigate the influence of Ru, due to its similarity with Cs with J-V characteristics as shown in Figure S7. Although the perovskite devices contained Ru show a good J–V performance and small hysteresis, its light stability is not


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very good.4,42 The mismatch of the lattice parameter may lead to this phenomenon.42 There is still more work to be done.

4. Conclusions In summary, a Cs-based triple cation mixed perovskite photovoltaic device was developed by introducing the CsI into the PbI2-DMSO precursors, followed by sequential introduction of MA/FA cations with a modified two-step method. The effects of Cs+ on the structure and properties of PbI2-DMSO precursor films have been well investigated, and a new triplecoordination intermediate phase among Pb2+, DMSO and Cs+ was formed. The presence of this coordination phase efficiently retarded the crystallization of PbI2 in precursor films and suppressed the yellow non-perovskite δ-phase, resulting in an obtained perovskite film with fewer defects, larger grains and a more uniform morphology. These superior properties improve the values of Voc and FF, and particularly narrow the distribution of FF among the obtained devices, thus ensuring high efficiency and reproducibility of Cs-contained PSCs. Finally, a PCE of 20.3% was achieved for the champion device based on Cs0.1(MA0.4FA0.6)0.9PbI3 perovskite absorber.

ASSOCIATED CONTENT Supporting Information The XRD patterns of PbI2-DMSO precursor films with different proportions of Cs. The SEM side-view images. Statistical results of grain size distribution of perovskite films. J-V characteristics for PSCs with different Cs content. Statistical results of J−V characteristics of


25


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PSCs prepared without or with 10% CsI precursors in the first step. The calculated process of ideal factors. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]; [email protected]

Notes The authors declare no completing financial interest.

Acknowledgements The authors would like to acknowledge financial support from the National Natural Science Foundation of China (Nos. 51572288, 11474333, 91433205, 51421002 and 51372272).

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Abstract Graphic


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An Application of Cesium on the Restriction of Precursor

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