Mixed Cation Thiocyanate-Based Pseudohalide Perovskite Solar

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Mixed Cation Thiocyanate-Based Pseudohalide Perovskite Solar Cells with High Efficiency and Stability Yu-Hsien Chiang, Ming-Hsien Li, Hsin-Min Cheng, Po-Shen Shen, and Peter Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13206 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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ACS Applied Materials & Interfaces

Mixed Cation Thiocyanate-Based Pseudohalide Perovskite Solar Cells with High Efficiency and Stability Yu-Hsien Chiang,a Ming-Hsien Li,a Hsin-Min Cheng, a Po-Shen Shen,a and Peter Chen*a, b, c a

Department of Photonics, National Cheng Kung University, Tainan, 701, Taiwan

b

Research Center for Energy Technology and Strategy (RCETS), National Cheng Kung University, Tainan, 701, Taiwan

c

Advanced Optoelectronics Technology Center (AOCT), National Cheng Kung University, Tainan, 701, Taiwan

* email: [email protected]

KEYWORDS: pseudohalide, mixed cation, high stability, efficiency, dim-light, perovskite solar cells.

1 Environment ACS Paragon Plus


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ABSTRACT Novel organic–inorganic hybrid perovskite compounds composed of mixed A-site cation (Formamidinium and Cesium) and pseudohalides (SCN and I) ions are sucessfully synthesized. These new classes of hybrid perovskites photovoltaics exhibited remarkable power conversion efficiency of more than 16% with excellent stability against moisture in ambient environment and under low-light storage condition. The existence of SCN− ion inclusion is confirmed by secondary ion mass spectrometry and Fourier transform infrared spectroscopy. The SCN−-doped pseudohalide is advantageous for the formation of large perovskite grains, as well as the performance and stability of the device.

1. Introduction Hybrid organic–inorganic perovskites (HOIPs) are promising new materials for future photovoltaics.1 Although the conventional ABX3 model compound MAPbX3 (MA: methylammonium, X: halides) exhibits excellent power conversion efficiency, long-term stability remains challenging for real applications. Several new variant compounds have been proposed to address the stability issue. Recently, multiple cation (such as cesium (Cs) and formamidinium (FA) or MA) and pseudohalide perovskites have demonstrated favorable stability with high power conversion efficiencies (PCE).2-7 For example, FACsmixed perovskite solar cells deliver high efficiency (>16%) with improved stability.8-9 A triple-cation FAMACsPbX3 compound has exhibited extremely high efficiency and relatively good stability compared with the MAPbI3 counterpart.6 First-principle calculation revealed that the stabilization of photoactive alpha phase by mixing A-cations


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resulted from the increase in entropy and reduction of energy required for the formation of a solid solution.10 Meanwhile, partial replacement of halides with the SCN− ion has been shown both theoretically and experimentally to be benign toward structure stability against moisture.4, 11 The formation of CH3NH3Pb(SCN)xI3−x exhibits larger crystal sizes with decreased trap states.12 Jiang et al. reported the first SCN− ion-containing HOIPs;13 since then, several follow-up studies have reported several material and structural properties with improved photovoltaic performance and stabilities.4,

12, 14-17

For instance, 5% lead thiocyanate doping in the lead iodide precursor as

additive could significantly increase the device performance with PCE of 19.45% and minimize the hysteresis phenomena.5 The researchers observed the inclusion of a trace amount of S by secondary ion mass spectrometry (SIMS) (with sensitivity of ppm level) after annealing of their spin-coated perovskite. Motivated by the aforementioned progress, we reported in this article a new series of HOIP materials with a mixture of FA+ and Cs+ as A-site cations with a pseudohalide ion of SCN− in the X site. To the best of our knowledge, that these mixed cation thiocyanate-based perovskite solar cells are fabricated for the first time by various low temperature solution processes, such as sequential deposition and one-step spin-coating. Detailed material and photovoltaic characterizations are performed to investigate the properties of these new compounds. A notable PCE exceeding 16% was achieved with excellent stability under ambient and low-light condition.



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2. Experimental section 2.1 Materials The TiO2 paste for mesoporous TiO2 layer and FA iodide (FAI) were purchased from Dyesol Company. PbI2 (99.99%) and Pb(SCN)2 were purchased from Alfa and Sigma-Aldrich, respectively. The washing solvent for one-step spin-coating, DMF (Sigma-Aldrich, 99.8%), DMSO Sigma, >99.9%) and chlorobenzene were purchased from Sigma-Aldrich.

2.2 Device fabrication The FTO glass substrate (10 Ω/sq) was etched using zinc powder and HCl (2 N aqueous solution) before being cleaned using detergent, deionized water, acetone, and ethanol in an ultrasonic bath for 15 min each. The cleaned FTO substrate is further deposited with compact TiO2 layer by spraying titanium diisopropoxide bis(acetylacetonate), which is diluted with ethanol at a 1:39 volume ratio. The spraying compact layer process is conducted under 450 °C for 30 min. Then, for the mesoporous TiO2 layer, the diluted TiO2 paste (with 1:8 weight ratio of paste:ethanol) was spin-coated on the compact layer TiO2 at 4000 rpm for 30 s. The mesoporous TiO2 is dried on a hot plate at 100 °C for 10 min and then annealed at 450 °C for 30 min. The two-step sequential deposition method is described as follows: 1 M Pb(SCN)2 and x% (molar ratio with respect to Pb(SCN)2; x = 5, 10, and 15) of CsI were mixed in the dimethylformamide (DMF) solvent. The solution was spin-coated on the mesoporous TiO2 layer at 5000 rpm for 30 sec and annealing at 100 °C for 10 min. Then, the Pb(SCN)2 and x% of CsI film were dipped into FAI solution (10 mg/1 ml IPA) for 5 min and then annealed at 130 °C for 30 min. For one-step


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spin-coating, 154 mg of FAI, 26 mg of CsI, and 462 mg of PbI2 were mixed with 700 µl of DMF and 300 µl of dimethylsulfoxide (DMSO) as the pristine perovskite solution. For Pb(SCN)2 doping perovskite solution, 16 mg (5 at.% with respect to (w.r.t.) PbI2) and 32 mg (10 at.% w.r.t. PbI2) of Pb(SCN)2 powder were added into the pristine prepared perovskite solution. The perovskite film is fabricated by spin coating 50 µl of perovskite precursor on mesoporous TiO2 substrate at 1000 and 6000 rpm for 10 and 30 s, respectively. During the second step of spincoating, approximately 100 µl of chlorobenzene was poured on the substrate at 10 s before the end of spin-coating. The perovskite film was further annealed at 130 °C for 30 min to complete the reaction. After the perovskite film was cooled, the hole-transporting material SpiroOMeTAD was spin-coated on top of the perovskite layer at 4000 rpm for 30 sec. We dissolved 72.3 mg of Spiro-OMeTAD in 1ml chlorobenzene with 28.8 µl of 4-tert-Butylpyridine and 35 µl Li-TSFI (26 mg/100 ul ACN) as additive. The back electrode silver was thermally evaporated on the hole-transporting material (HTM) layer at the rate of 0.05 nm/s. For stability testing, the gold electrode was employed as electrode for the perovskite device.

2.3 Characterization The device performances of current density–voltage (J–V) measurements were measured in a solar simulator with AM 1.5 G (100 mW/cm2) spectra. Photovoltaic response was recorded by Keithley 2401. Light intensity was calibrated by certified standard silicon solar cells. We used a metal mask to define the device active area with aperture size of 0.2 cm2. For device stability measurement, the sample was stored at ambient environment within a relative humidity range of 40% to 60%. For dim-light soaking, the 5% Pb(SCN)2 doping of FA0.9Cs0.1PbI3 device was



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illuminated under a T5 lamp (Philips) with 200 lux (its spectrum is illustrated in Fig. S. 12), and the J–V curve was measured under 1 sun condition after various aging times. The xenon lamp (Newport, 300 W), monochromatic equipment (Newport Cornerstone 260), and Keithley 2401 were employed for incident photon-to-electron conversion efficiency (IPCE) measurement. The PL measurements were conducted by using diode laser with 532 nm light source and 1.1×2.2 mm2 beam size. We mounted the perovskite film on a translation stage equipped with an optical microscope to focus the images. We measured Fourier transform infrared (FTIR) spectra by a Thermo Nicolet spectrometer with 4 cm−1 resolution. Parameters of 25 keV energy of Ga beam for analysis and 1 keV energy of Cs beam for sputtering were employed for time-of-flight secondary ion mass spectroscopy. Scanning electron microscopy (SEM) images were obtained using field-emission SEM from ZEISS SUPRA™ 55.

3. Results and Discussion We employed two solution deposition methods, namely, sequential deposition and one-step spin-coating to fabricate mixed cation thiocyanate-based pseudohalide perovskite solar cells. In the sequential deposition method, we adjusted the Cs+ ion concentration in the perovskite film, whereas in the one-step spin coating, the SCN− ion doping level was varied to achieve the optimal efficiency. The absorption spectra for sequentially deposited FA1−xCsxPb(SCN)yI3−y coating on mesoporous TiO2 layer are shown in Fig. 1(a). The absorption onset for perovskite with the addition of cesium blueshifted, which is in good agreement with previous reports.6,

18

The normalized

photoluminescence (PL) spectra in Fig. 1(b) also indicate the same phenomena, that is,


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increasing the cesium ratio from 0% to 15% leads to higher radiative recombination energy level and a blueshift of PL peaks. The enlarged band-gap confirms the introduction of smaller ionic radii cesium ion (compared to FA) into the final perovskite films. The morphologies of different cesium ratios in FA1−xCsxPb(SCN)yI3−y are presented in Fig. S1, where the images of different cesium-doped FA1−xCsxPb(SCN)yI3−y exhibit similar grain sizes. However, all of these four samples showed non-uniform and rough surfaces, which are not beneficial for high-performance perovskite solar cells. The X-ray diffraction (XRD) patterns of different amounts of cesium, x = 0%, 5%, 10%, and 15%, are shown in Fig. 1(c). No obvious photo-inactive yellow phase FAPbI3 signal appears in the XRD result, which proves the successful structure transformation to the black photoactive alpha phase of FA1−xCsxPb(SCN)yI3−y perovskite films.7 We magnified the (110) XRD peak of FA1−xCsxPb(SCN)yI3−y (shown in Fig. S2) and observed a minor shift of the peak position toward higher degree, which imply the decrease in the lattice constant of the crystal structure. From XRD results, we found the signal of remaining PbI2 in the sequentially deposited FA1−xCsxPb(SCN)yI3−y, which may reduce the solar cell performance. Our J−V characteristic curves with different molar ratios of cesium in FA1−xCsxPb(SCN)yI3−y device are presented in Fig. 1(d).The best PCE of 11.1% is achieved in FA0.9Cs0.1Pb(SCN)yI3−y composition with an open-circuit voltage (Voc) of 0.87 V, a short-circuit current (Jsc) of 21.2 mA/cm2, and a fill factor (FF) of 60%. To evaluate the stability of mixed cation thiocyanate-based pseudohalide perovskite film, we performed a preliminary test on the FA1−xCsxPb(SCN)yI3−y device without the HTM, Spiro-OMeTAD, to exclude the impact of the HTM. The non-encapsulated sample was kept in the dark with 40%–60% relatively humidity. The photovoltaic action under 1 sun



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AM 1.5G spectra was traced. The notable result is presented in Fig. S3, in which the nonHTM device showed excellent stability with only 10% drop of the initial PCE after 300 hours. The data illustrate that the FA1−xCsxPb(SCN)yI3−y compound could be a promising material for further investigations. Thus, we changed the fabrication procedure to a wellrecognized one-step solvent engineering spin-coating method for enhancing our device performance. The doping levels of SCN− with respect to the iodide ion are 0%-10% molar concentration as additive in the precursor, with the ratio of FA:Cs kept constant at 9 to 1.

The UV-vis spectra for the three perovskite films (FA0.9Cs0.1PbI3 + x% Pb(SCN)2, where x = 0%, 5%, and 10%) are shown in Fig. S4. The absorption edge does not present an observable shift with the addition of SCN− ion doping, but absorbance decreases as the doping of SCN− ion increases. The existence of thiocyanate in the perovskite film is investigated by FTIR, as shown in Fig. S5. The pure Pb(SCN)2 film (red line) presents a characteristic peak at 2010 cm−1, which corresponds to the C≡N stretching mode of SCN.19 No thiocyanate signal presents in the FA0.9Cs0.1PbI3 film (green line) as expected. With 5% Pb(SCN)2 doping in the FA0.9Cs0.1PbI3 film, a small drop of transmission signal at 2070 cm−1 wavenumber can be assigned as the thiocyanate vibrational signal. This result agrees with the previous report, in which a shift of the C≡N vibrational signal is observed once the SCN− ion is incorporated into the perovskite film.16 However, the one-step spin-coating method produces an extremely limited amount of SCN− ion remaining in the final perovskite film. To further study the existence of the SCN− ion, we employed secondary ion mass spectra (SIMS) to confirm the distribution of SCN− ion inside the perovskite film (Fig. S6). We conducted the depth profile analysis for sulfur (S) and iodine (I) elements in the perovskite film. The intensity of sulfur is considerably weaker than the iodine


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signal across the whole perovskite film, especially in the surface region. The possible explanation of this phenomena may result from the ion exchange process. In our previous report, the lead thiocyanate transformed to lead iodide in MAI vapor environment.16 The thiocyanate ion could evaporate during the annealing process. We suggest that the evaporation of thiocyanate prevailed in the surface region than the bulk region which leads to a lower S signal on the surface of perovskites layer. A recent report about highly efficient SCN-doped perovskite also found the similar distribution of less S content on the surface.5 This result corresponds with our FTIR results, where thiocyanate absorption is tenuous. The PL spectra of the three perovskite films are presented in Fig. S7, where the center of normalized PL peaks present a minor blueshift with Pb(SCN)2 doping. The continuous absorption band feature indicates the perovskite behaves like a conventional bulk semiconductor. The blue-shifted absorption and PL spectra of the SCN doped perovskite are considered to be dominated by the change of electronic structure instead of the enlarged grain size. The morphologies of the one-step spin-coated perovskite film fabricated with different doping levels are shown in Fig. 2. We observed that the addition of the SCN− ion exerts a significant impact on grain size and film quality. The surface is relatively flat with large grains compared with that of sequentially deposited perovskites. The average grain size can be estimated with the ASTM protocol by counting the number of intercepts of numerous random straight lines with the grain boundary. The average grain sizes for 0%, 5%, and 10% doping of FA0.9Cs0.1PbI3 film are 215, 506, and 737 nm, respectively. We also calculated the grain size distributions (as illustrated in Fig. S8) of the three samples from the SEM images in Fig. 2. Both methods showed the enlargement of grain size with increased SCN− ion doping. Kim et al. studied the effect of including SCN− ion



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in the perovskite film formation.17 The team proposed that the Gibbs free energy for nucleation is significantly increased due to the inclusion of SCN− ion additive, thus resulting in slower nucleation rate and larger grain size. They found that with the addition of SCN− ion in CH3NH3PbI3, larger crystal grain sizes and prolonged carrier lifetime were observed. Possibly, the growth mechanism of SCN− ion doping for mixed cation perovskites is similar to that of CH3NH3PbI3 reported by Kim et al. The device performance based on pristine FA0.9Cs0.1PbI3 and 5% and 10% Pb(SCN)2-doped FA0.9Cs0.1PbI3 perovskite solar cells are shown in Fig. 3(a) with the J–V curve measurements. (The architecture of perovskite solar cells involved FTO/compact layer TiO2/mesoporous TiO2/perovskite absorber/Spiro–OMeTAD/metal electrode.) In Fig. S9, the cross-sectional SEM image shows that the thickness of mesoporous TiO2, perovskite capping layer, and HTM layer are approximately 150, 350, and 200 nm, respectively. Compared with FA0.9Cs0.1PbI3 device, the 5% Pb(SCN)2 doped device presents better open-circuit voltage (Voc) and fill factor (FF), resulting in a better overall PCE from 13.67% to 16.20%. With the addition of more Pb(SCN)2 at 10%, the device performance decreases significantly due to poor current density (Jsc) and FF. Notably, the hysteresis behavior is passivated by the addition of Pb(SCN)2 in the FA0.9Cs0.1PbI3 device (Fig. S10). In Fig. 3(b), the IPCE of the 0%, 5%, and 10% doped devices are displayed. The FA0.9Cs0.1PbI3 device presents the best IPCE compared with 5% and 10% Pb(SCN)2doped FA0.9Cs0.1PbI3 perovskite solar cells, corresponding with the highest Jsc among all of the doped devices. The detailed parameters of device performance are summarized in Table 1, in which FA0.9Cs0.1PbI3 and 5% and 10% Pb(SCN)2-doped FA0.9Cs0.1PbI3 perovskite solar cells can achieve PCEs with 13.97%, 16.20%, and 11.01%, respectively.


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Our PCEs outperformed the recent SCN-added FAPbI3 perovskite and demonstrated the marked success of using mixed A-site cations with pseudohalide SCN− ion doping for improved PCE and ambient stability.20

To illustrate the effect of Pb(SCN)2 doping on the variation in device performances, we employed XRD to scrutinize the crystallinity of FA0.9Cs0.1PbI3−x(SCN)x perovskites as presented in Fig. 4(a). Minor residuals of PbI2 could be observed as the signal at 12.7° remained in the FA0.9Cs0.1PbI3 structure. By increasing the Pb(SCN)2 concentration, the PbI2 signals are significantly enhanced without discernable Pb(SCN)2 traces. In our previous study, we observed the rapid transformation of Pb(SCN)2 to PbI2, which could lead to the excess of PbI2 in SCN-doped FA0.9Cs0.1PbI3.16 Roldan-Carmona et al. reported that a moderate excess of PbI2 in the perovskite active layer could enhance the device performance by increasing the crystalline size, decreasing the recombination trap state, passivation, and faster charge injection.21 Besides, the state-of-the-art perovskite devices with extremely high PCE exceeding 20% also showed small excess of PbI2.2, 6 The XRD patterns in Fig 4(a) and 4(b) are normalized to the intensity of TiO2 (101) signal for comparing their crystallinity. The intensity of (110) plane for perovskite at 2θ of approximately 14°(in Fig. 4(b)) illustrates that the 5% Pb(SCN)2 doped FA0.9Cs0.1PbI3 exhibits the higher intensity and narrow FWHM compared with pristine perovskite. This implies that the 5% Pb(SCN)2 doped FA0.9Cs0.1PbI3 has larger crystal size and better perovskite crystallinity. We also employed time-resolved photoluminescence (TRPL) measurement to investigate the carrier dynamic in the perovskite film which allows better understanding on the effect of crystal size on photovoltaic performance. In our TRPL results (Fig 5 (a)), two decay components can be



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observed and analyzed by a bi-exponential fitting curve to present different recombination processes. The fast decay after photoexcitation are mainly dominated by germinate (exciton) or trap-assisted recombination (exciton or free carrier) while the second decay is due to slow process from non-geminate bimolecular (free carrier) recombination.22 In Fig. 5 (a), the normalized TRPL spectra shows that the 5% Pb(SCN)2 doped FA0.9Cs0.1PbI3 has longer free carrier lifetime of 32.6 ns than that of non-doped perovskite with 16.38 ns. The longer free carrier lifetime indicates less non-radiative and slow non-geminate recombination pathway. This might be due to the better crystal quality or larger grain size after Pb(SCN)2 doping in perovskite film, in consistence with our XRD pattern and SEM top-view image. Therefore, the devices resulted in performance with higher FF and reduced hysteresis effect. For 10% of Pb(SCN)2 doping in FA0.9Cs0.1PbI3, the free carrier lifetime remains longer than other perovskite films with 43.3 ns which agrees with our SEM images having largest crystal size. However, the carrier dynamic at very early stage after photoexcitation, which is mainly dominated by geminate and trap-assisted recombination, showed very fast decay. The excess of Pb(SCN)2 doping in perovskite film enhances the germinate or trap-assisted recombination in perovskites film, resulting in poor free carrier generation. This serious recombination from defect sites or disorder in grain boundary damages the device performance of the 10% Pb(SCN)2 doped FA0.9Cs0.1PbI3. Thus, we consider that the prevailing excess of Pb(SCN)2 doping and residual PbI2 resulted in a detrimental effect on photovoltaic performance. After optimizing the perovskite film, we achieved the highest device performance with 16.9% of PCE by adding 3.75% of Pb(SCN)2 in FA0.9Cs0.1PbI3. (Fig. 5(b))


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The stability of device is of substantial importance for commercialization. We further tested the full device stability of our samples with storage in an ambient environment without encapsulation (40%–60% relative humidity). The FA0.9Cs0.1PbI3 sample (blue line) is accompanied as a reference with the 5% Pb(SCN)2 doping of FA0.9Cs0.1PbI3 (red line). In Fig. 6(a), the normalized PCE as a function of storage time for 5% Pb(SCN)2-doped FA0.9Cs0.1PbI3 cell displays highly stable performance without decay after five days. The enhanced device stability is attributed to the small amount of Pb(SCN)2 doping in the perovskite film with better film quality. Numerous groups observed the effect of adding thiocyanate on improving stability and higher moisture stability compared with materials without thiocyanate doping.13-14, 16, 23 The formation constant between Pb2+ and SCN− is markedly larger than that between Pb2+ and I−. Therefore, the incorporation of SCN into the PbX64− octahedral structure could enhance crystal stability.13 We further subjected the 5% Pb(SCN)2-doped FA0.9Cs0.1PbI3 device to slight soaking under 200 lux dim-light condition (green line in Fig. 6(a)). The device was encapsulated by UV epoxy resin in an N2-filled glove box. The indoor application or ambient environment energy harvesting could be an alternative niche market for emerging photovoltaics. Perovskite solar cells have demonstrated their considerable potential under low light condition, thus making the cells unique and competitive for commercialization toward such applications.24 The green line in Fig. 6(a) presents the photovoltaic performances under light soaking with 200 lux illumination. The device performance unexpectedly increased slightly in the first 200 hours. After 250 hours, the normalized PCE started to degrade and remained within 74% of initial PCE after 350 hours. These long-term stability measurements demonstrated that the Pb(SCN)2 doping in FA0.9Cs0.1PbI3 could be a promising material for real application



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under certain working conditions. However, the degradation mechanism under dim-light should be further investigated. We further tested the stability measurement of our best device where the 3.75% Pb(SCN)2 doped FA0.9Cs0.1PbI3 cell was aged under ambient with 30 % ±10 relative humidity and kept in the dark. The great ambient stability is presented in Fig. 6(b) where the device performance remains ~90% of initial PCE after 30 days. This result exhibits the impressive enhancement for perovskite stability with moderate Pb(SCN)2 incorporation. Recently, CuSCN has been applied as inorganic HTM for perovskite solar cells with high efficiencies.25-27 Ion migration and exchange was considered as a phenomena that could occur between CuSCN and perovskite.28-29 However, some results showed impressive thermal stability.25,

28

The effect of using CuSCN as HTM for perovskite

seems to be diverged. From the many positive results of SCN-based pseudohlide perovskite device, the ion exchange between I- (perovskite) and SCN- (from CuSCN) ion may not be the origin of instability. It is probable that the formation of CuI or other degradation mechanism (such as Cu migration) lead to the failure of the device. It would be interesting to use SCN doped perovskite together with CuSCN HTM to check whether the ion migration or exchange phenomena could be stabilized if we intentionally include the SCN pseudohalides ion in perovskite. The J-V curve of device performance and photovoltaic parameters for fresh and 30-day aged sample are illustrated in Fig. S11 and Table S2. The degradation of device performance are mainly resulting from lower Voc and Jsc from 1023 mV to 976 mV and 22.07 mA/cm2 to 20.74 mA/cm2, respectively. However, the color of Spiro-OMeTAD film on top of perovskite slayer has slightly faded after such long days. Therefore, we cannot exclude that the degradation originates from the


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chemical change of organic Spiro-OMeTAD film. For further stability test, the robust hole transporting material is necessary.

4. Conclusions A new class of HOIP compounds with mixed cations and pseudohalides were successfully synthesized. By adding Pb(SCN)2 into the perovskite structure, large grain size, longer carrier lifetime and high crystallinity structure are obtained. Remarkable photovoltaic performance exceeding 16% was achieved using these materials with 3.75% Pb(SCN)2 doping in FA0.9Cs0.1PbI3. The ambient stability and moisture resistance are significantly improved compared with the conventional MAPbX3 perovskite.



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Figures

Fig. 1. (a) Absorption spectra, (b) normalized photoluminescence spectra, (c) X-ray diffraction patterns, and (d) photovoltaic J–V characteristic responses for sequentially deposited FA1−xCsxPb(SCN)yI3−y perovskite film with different Cs+ ion ratios.


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Fig. 2. (a) Scanning electron microscopy images of one-step spin-coated FA0.9Cs0.1PbI3, with (a) 0%, (b)5%, and (c) 10% Pb(SCN)2 doping.

Fig. 3. (a) J−V curve with 0%−10% Pb(SCN)2 doping of FA0.9Cs0.1PbI3 perovskite solar cells. (b) IPCE for FA0.9Cs0.1PbI3 and 5% and 10% Pb(SCN)2-doped FA0.9Cs0.1PbI3 perovskite solar cells. J–V curve measurements were performed under AM 1.5G spectra by using 0.2 cm2 of mask to define the active area.



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Table 1. Photovoltaic characteristics of 0%- 10% Pb(SCN)2-doped FA0.9Cs0.1PbI3 device. Doping Level

Voc (mV)

Jsc (mA/cm2)

Fill Factor (%)

PCE (%)

0%

943.9

22.46

65.84

13.97

2.5%

998.4

21.26

73.66

15.64

5%

994.6

22.05

73.86

16.20

7.5%

984.7

20.06

69.47

13.72

10%

955.1

19.60

58.86

11.01

Fig. 4. (a) XRD patterns for different added amounts of Pb(SCN)2 in FA0.9Cs0.1PbI3 perovskite. (b) The magnified spectra for various SCN doping level perovskite crystal phase at (110) peak. XRD measurements were conducted for perovskite deposited on mesoporous TiO2 substrate. The label of # and * represent the PbI2 and the perovskite signal with the main peak at (001) and (110) phase, respectively. Spectra intensity are normalized to the intensity of TiO2 (101) signal.


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Fig. 5. (a) The normalized time-resolved PL signal for different Pb(SCN)2 doping into perovskite film. (b) The optimized perovskite solar cell with 3.75% of Pb(SCN)2 doping into perovskite film.

Fig. 6. (a) Device stability for FA0.9Cs0.1PbI3 and 5% Pb(SCN)2-doped FA0.9Cs0.1PbI3. Devices without encapsulation were stored in the ambient environment with relative humidity of 40%–60%. (red and blue lines) Encapsulated cell was kept under 200 lux dim light condition (green line). (b) The ambient stability test for our optimized perovskites 19 Environment ACS Paragon Plus


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solar cell kept at 30±10% of relative humidity without encapsulation. PCEs are measured under AM 1.5G illumination.

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Figures for XRD patterns, top-view SEM image, non-HTM device stability, UV-vis spectra, FTIR, SIMS, PL, grain size distribution, cross-sectional SEM image, hysteresis effect, J-V curve for stability device, T5 lamp spectra and table of device parameter.

Corresponding Author [email protected]

Note: A highly relevant article was published during the revision period: Y. Yu et.al., “Improving the Performance of Formamidinium and Cesium Lead Triiodide Perovskite Solar Cells using Lead Thiocyanate Additives”, ChemSusChem, 2016, 9, 3288 (DOI: 10.1002/cssc.201601027)

ACKNOWLEDGMENT The authors acknowledge the financial support from the Ministry of Science and Technology (MOST 105-2623-E-006-002-ET), (MOST 104-2119-M-006-004), and (MOST 103-2221-E-006-029-MY3). The authors are grateful for the funding from the Research Center for Energy Technology and Strategy (RCETS) and Advanced Optoelectronic Technology Center (AOTC), National Cheng Kung University. We thank


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the Top-Notch Project under the Headquarter of University Advancement at National Cheng Kung University for the financial support, which is sponsored by the Ministry of Education, Taiwan, ROC.

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