An inherent instability study using ab initio computational methods and experimental validation of Pb(SCN)2 based perovskites for solar cell applications

Perovskite materials with ABX3 chemistries are promising candidates for photovoltaic applications, owing to their suitable optoelectronic properties. However, they are highly hydrophilic and unstable in nature, limiting the commercialization of perovskite photovoltaics. Mixed halide ion-doped perovskites are reported to be more stable compared to simple ABX3 chemistries. This paper describes ab initio modeling, synthesis, and characterization of thiocyanate doped lead iodide CH3NH3PbI(3−x)(SCN)x perovskites. Several perovskite chemistries with an increasing concentration of (SCN)− at x = 0, 0.25, 0.49, 1.0, 1.45 were evaluated. Subsequently, ‘n-i-p’ and ‘p-i-n’ perovskite solar device architectures, corresponding to x = 0, 0.25, 0.49, 1.0 thiocyanate doped lead halide perovskite chemistry were fabricated. The study shows that among all the devices fabricated for different compositions of perovskites, p-i-n perovskite solar cell fabricated using CH3NH3PbI(3−x)(SCN)x perovskite at x = 1.0 exhibited the highest stability and device efficiency was retained until 450 h. Finally, a solar panel was fabricated and its stability was monitored.


Scientific RepoRtS
| (2020) 10:15241 | https://doi.org/10.1038/s41598-020-72210-4 www.nature.com/scientificreports/ as (CH 3 NH 3 ) 4 PbI 6 :2H 2 O when exposed to a relative humidity of 80%. Perovskite material easily absorbs moisture from the atmosphere and decomposes into precursor salts like (PbX 2 ) and CH 3 NH 3 X 5,6 . Due to moisture instability under ambient conditions, CH 3 NH 3 PbX 3 (MAPbX 3 ) films are required to be synthesized in an inert atmosphere inside a glove box to avoid long term exposure of the films to ambient conditions. However, the synthesis of perovskite material inside a glove box under inert conditions does not ensure long term stability when exposed to ambient conditions 4 . Moreover, the intrinsic material properties of organolead halide perovskites make them thermally unstable. The material undergoes significant decomposition when annealed at 85 o C under inert conditions. Furthermore, perovskite materials kept exposed to solar radiation for a longer duration, comes in contact with UV rays and undergoes photochemical degradation thereby resulting in distortion of the perovskite structure 7 . Degradation in PSC even occurs due to the elementary migration of ions in the perovskite material 8 . Thus these perovskite materials undergo structural deformation or alteration under thermal stress 9 . Various approaches have been reported to overcome the stability issues of perovskite material such as the use of water-resistive coating on top of the perovskite layer. However, despite using water-resistive coating, the intrinsic material property makes perovskite chemistries vulnerable to moisture 4,10-12 . Moreover, the use of hydrophobic encapsulation on top of the perovskite layer reduces its light absorption ability and formation of active charge carriers and thus, resulting in reduced photocurrent generation and diminished photovoltaic performance. Use of two-dimensional bilayer or multilayer perovskite architecture such as the incorporation of (C 6 H 5 (CH 2 ) 2 NH 3 + ) into the CH 3 NH 3 PbI 3 matrix improved moisture stability and allowed ambient fabrication of perovskite solar cells but photovoltaic efficiency and performance were reduced 4 . Similar studies were carried on mixed halide perovskites like CH 3 NH 3 PbI (3−x) Br x , CH 3 NH 3 PbI (3−x) Cl x 4 . Increased stability was reported for CH 3 NH 3 PbI (3−x) Br x perovskite chemistry where 10-15 mol % I − were replaced with Br − . The increase in stability was accredited to the stronger interaction between CH 3 NH 3 + and Br − . Furthermore, 2-Aminoethanethiol (2-AET) was reported to be used as a ligand to bridge the organic compound CH 3 NH 3 I (MAI) and inorganic compound PbI 2 which restricts the decomposition of MAPbI 3 perovskite structure and prevents fast growth of PbI 2 . The compact (PbI 2 )-2-AET-(MAI) molecule was reported to make the perovskite film intrinsically hydrophobic and the perovskite is reported to retain its crystal structure for > 10 min after immersion in water 10,13 .
Several studies have reported that perovskite film quality depends on the methods of synthesis and materials used for synthesis. The improved film morphology enhances photovoltaic performance and ensures long term stability of the perovskite films. For example, the use of non-halide lead precursors results in the formation of high-quality perovskites, which led to the synthesis of novel perovskite films with improved optoelectronic properties, photovoltaic performance and increased stability 4,10,11 . Similar studies suggested the use of iondoped or mixed halide perovskites to be an effective solution for improved stability of perovskite material under ambient conditions 4,5 . PSCs fabricated without encapsulation under relative humidity (RH) of 70% using Lead thiocyanate precursor Pb(SCN) 2 and Lead(II) tetrafluoroborate Pb(BF 4 ) 2 , have been reported to show improved moisture resistance as compared to normal PSCs prepared using PbI 2 precursor. The improved moisture resistance has been attributed to better intrinsic stability of the perovskite material due to the incorporation of SCN − or BF 4 − ions 4 . Furthermore, improved moisture stability was reported on an increasing percentage of (SCN) − and replacing two I − with (SCN) − in CH 3 NH 3 PbI 3 perovskite composition 4 . However, very limited studies have been performed on a systematic increase of (SCN) − percentage with the corresponding reduction in I − percentage in CH 3 NH 3 PbI 3 perovskite composition to understand its effect on moisture stability and photovoltaic performance of PSCs.
In this paper, we have applied ab initio modeling to study the optoelectronic properties of thiocyanate doped lead iodide CH 3 NH 3 PbI (3−x) (SCN) x perovskite for several compositions with increasing concentration of (SCN) − at x = 0, 0.25, 0.49, 1.0, 1.45 using Vienna Ab initio Simulation Package (VASP). Subsequently, the abovedefined compositions were synthesized by varying the concentration of lead thiocyanate (Pb(SCN) 2 ) precursor in methylammonium iodide (CH 3 NH 3 I) solution. The synthesized perovskite films with increasing concentration of (SCN) − were characterized by X-ray diffraction (XRD) to identify the perovskite crystal structure, and Raman Spectroscopy to identify the characteristic peaks of the perovskite composition. Photoluminescence spectroscopy and absorption spectroscopy were used to identify the intermediate defect states and bandgap of the material. Further, a comparative analysis was performed on these perovskite chemistries based on the ab initio analysis to rank them for suitability as a photovoltaic candidate and cross-validated the calculated optoelectronic properties with the experimental results. Furthermore, 'n-i-p' and 'p-i-n' perovskite solar device architectures corresponding to x = 0, 0.25, 0.49, 1.0 thiocyanate doped lead halide perovskite chemistry were fabricated and tested for feasibility, scalability, efficiency and stability for photovoltaic applications. It was found that, among all the fabricated devices with different perovskite chemistries, 'p-i-n' PSC device fabricated with CH 3 NH 3 PbI (3−x) (SCN) x , where x = 1.0 showed the highest stability. The device efficiency was retained until 450 h. Methods computational method. Density functional theory calculations were performed on the select CH 3 NH 3 PbI (3−x) (SCN) x perovskite stoichiometry with an increasing amount of (SCN) − in steps of x = 0, 0.25, 0.49, 1.0, 1.45. All the calculations were done using VASP software. The valence electronic states were expanded in a basis of plane-waves up to plane-wave kinetic energy of 500 eV. The core-valence interactions were described using the projector augmented wave (PAW) approach 14,15 . The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) functional 4 was used to describe the electron exchange-correlation interactions. The Brillouin zone was sampled using a gamma centered k-point mesh. The k-point grid spacing of 0.2 Å −1 was used for reciprocal space integration in structure optimization and spacing of 0.1 Å −1 in electronic structure calculations. Geometry relaxation for all the calculations was run until the relaxation forces on atoms were not > 0.01 eV Å −1 and the SCF convergence threshold was set to 10 -7 eV 16   Device fabrication. Two different types of PSC device architecture were realized namely, non-inverted (n-i-p) and inverted (p-i-n). The n-i-p device architecture follows a "bottom-up" approach and light traverses through the transparent FTO glass substrate followed by n-type ETL layer to the active perovskite layer whereas p-i-n device architecture follows a "top-down" approach and light traverses through FTO substrate followed by p-type HTL to active perovskite layer. The incident light as a stream of photons on the surface of the active perovskite material, with greater energy than the bandgap energy of the material, generates electron-hole pairs that are made to flow in opposite direction through ETL and HTL respectively across the external circuit to generate power. The fabricated n-i-p (Fig. 1a) and p-i-n ( www.nature.com/scientificreports/ Fluorine-doped tin oxide (FTO) plate. Patterned FTO plates were cleaned with neutral liquid detergent and de-ionized (DI) water, followed by bath sonication twice in ethanol for 30 s each and subsequently rinsed twice with DI water for 30 s each. The cleaned FTO substrates were dried under nitrogen and treated with UV-Ozone for 15 min inside a glove box to remove any other contaminants such as photoresists, human skin oil, cleaning solvent residues, etc. from the surface of the substrate. The UV ozone treated FTO substrates were spin-coated with two layers of TiO 2 synthesized earlier, a blocking TiO 2 layer followed by a mesoporous TiO 2 (mp-TiO 2 ) layer to form the complete ETL layer. The blocking TiO 2 layer was spin-coated at 4,000 r.p.m. for 30 s, followed by sintering at 450° C for 30 min. Subsequently, TiO 2 paste was spin-coated at 3,000 r.p.m. for 30 s followed by sintering at 500 °C for 30 min 4 . Post sintering, the mp-TiO 2 layer was treated with 40 mM TiCl 4 treatment in a beaker of ice-cold DI water at 70 °C for 30 min followed by sintering at 500° C for 30 min 4 . TiCl 4 treatment is known to substantially reduce surface traps, increase electron injection rate, retard electron-hole recombination and thus improving the overall performance of the photovoltaic device. Thiocyanate doped methylammonium lead iodide active perovskite layer with CH 3 NH 3 PbI (3−x) (SCN) x perovskite chemistry was fabricated on top of the mp-TiO 2 layer by two-step spin-coating procedure at varying Pb(SCN) 2 concentration. However, at x = 0, filtered PbI 2 solution was spin-coated on top of the mp-TiO 2 layer at 3,000 r.p.m. for 40 s and annealed at 90° C for 1 h 4 . Post formation of PbI 2 film, PbI 2 coated substrates were dipped in CH 3 NH 3 I solution for 60 s followed by spin coating at 3,000 r.p.m. for 20 s, followed by rinsing with isopropanol and annealing at 80° C for 20 min, to realize CH 3 NH 3 PbI 3 active perovskite film 4 . At x > 0, for increased variation of (SCN) − concentration in CH 3 NH 3 PbI (3−x) (SCN) x perovskite stoichiometry, perovskite films were realized similarly by two-step spin coating process using varying concentrations of filtered Pb(SCN) 2 solutions (500 mg, 550 mg, 650 mg Pb(SCN) 2 powder respectively in 1 ml DMF and CH 3 NH 3 I solution.

Material synthesis. Synthesis of electron transport materials (ETMs) and hole transport materials
Spiro-MeOTAD layer was spin-coated on top of perovskite film at 4,000 r.p.m. for 60 s 4 . The Spiro layer being highly moisture-sensitive was spin-coated inside the glove box under N 2 condition and was stored in the glove box for 48 h before Au deposition. Au electrode of 100 nm thickness was deposited by thermal evaporation on top of the Spiro layer.
Fabrication of p-i-n device architecture. Inverted PSCs were fabricated on 2.5 × 2.5 cm 2 FTO substrates 21,22,32 . FTOs were patterned, cleaned and treated with UV-ozone using a similar procedure as discussed above. The UV ozone treated FTO substrates were spin-coated with the filtered PEDOT:PSS solution at 4,000 r.p.m. for 1 min to form 40-50 nm thick film, followed by annealing at 140 °C for 10 min inside the glove box under N 2 environment. Fabrication of Thiocyanate doped lead iodide active perovskite layer with CH 3  The initial atomic structure for these systems was obtained from the optimized structure of (CH 3 NH 3 ) 4 Pb 4 I 12 by replacing appropriate numbers of I − ions with (SCN) − ions. One, two, four, and six I − ions were replaced with (SCN) − to maintain the point group symmetry to obtain the above-mentioned systems respectively. Different configurations were tried by changing the position of (SCN) − in these systems. Structure optimization and relaxation for the different systems followed the same convergence threshold criteria as defined in the Computational methods section. The optoelectronic properties for all the optimized systems were computed and the bandgaps for all perovskite stoichiometry resulted to be "direct" bandgaps. The computed bandgaps gradually increase with the increase of (SCN) − percentage from 0 to 50 in the perovskite structure (Fig. 2f). The computed bandgaps for perovskite chemistries at x = 0 and x = 0.25 are 1.52 eV and 1.55 eV respectively (Fig. 2a,b). For (CH 3 NH 3 ) 4 Pb 4 I 12 and (CH 3 NH 3 ) 4 Pb 4 I 11 SCN top of the valence band is dominated by 'p' orbitals of I − and the bottom of the conduction band is dominated by 'p' orbitals of (SCN) − (Fig. 2a,b). It is important to note that no intermediate trapped states are observed in the computed band structure of (CH 3 NH 3 ) 4 Pb 4 I 11 SCN due to the incorporation of (SCN) − , which reduces the chance of carrier recombination process in the perovskite material and thus ensuring longer carrier lifetime. The calculations further indicate strong ionic interactions between (SCN) − and adjacent Pb atoms which should correspond to the improved chemical stability of the perovskite material 4 .
However, an intermediate trapped state appears closer to the valence band at 0.12 eV due to the incorporation of two (SCN) − in the (CH 3 NH 3 ) 4 Pb 4 I 10 (SCN) 2 perovskite structure (Fig. 2c). The computed bandgaps for the corresponding perovskite chemistry at x = 0.49 is 1.587 eV and the energy gap between the intermediate trapped state and conduction band minimum is 1.467 eV (Fig. 2c) (Fig. 2c).
In the case of (CH 3 Fig. 2d. One peak appears at 1.81 eV, but it is the continuation of the conduction band and the same is shown by a dotted circle in Fig. 2d. The conduction band minimum was observed at 1.66 eV and the same is marked in Fig. 2d. It is evident from the inset picture of Fig. 2d, that there are occupied states of 'p' orbitals of 'Pb' , 'C' , and 'I' atoms in between the appearing peak and the conduction band. This proves that the peak appearing at 1.81 eV in Fig. 2d lies within the conduction band. Thus, there appear no intermediate defect states in PDOS of perovskite chemistry at x = 1.0 (Fig. 2d), similar to the previous cases at x = 0, 0.25 (Fig. 2b,c) and unlike the perovskite chemistry at x = 0.49.
Further, incorporation of six (SCN) − in the (CH 3 NH 3 ) 4 Pb 4 I 6 (SCN) 6 structure results in a bandgap of 2 eV as shown in Fig. 2e. A peak is also visible at 1.7 eV with low formation energy, closer to the conduction band and the same is depicted in Fig. 2e. However, the peak is not lying within the conduction band and the same is evident from the inset picture of Fig. 2e. This clearly shows that there are no occupiable states in between the appearing peak and the conduction band, thus it implies that the peak appearing at 1.7 eV is an intermediate trapped state. In this case, the top of the valence band shows a hybridization between I − 'p' and (SCN) − 'p' orbitals, and the bottom of the conduction band shows a hybridization between Pb 'p' and (SCN) − 'p' orbitals (Fig. 2e). The intermediate defect state is dominated by C 'p' orbitals of (SCN) − (Fig. 2e).
It is observed that the bandgap increases with an increase in % (SCN) − (Fig. 2f) and lies between 1.52 and 2 eV. Hence, the computed bandgap for perovskite chemistries at x = 0, 0.25, 0.49, 1.0 falls in the range of > 1.5 eV and < 2 eV which is considered to be the ideal bandgap for a solar material. However, the bandgap of perovskite chemistry at x = 1.45 is 2 eV which is not preferred for photovoltaic applications. Also, perovskite stoichiometry simulated at x = 0.49 (Fig. 2c) and 1.45 (Fig. 2e) have intermediate trapped states which can result in deterioration of photovoltaic performance and hence may not be a good choice for the fabrication of solar cells. Further, an increase in SCN percentage has been reported beneficial for improved film morphology, intrinsic stability, moisture resistance, and long term ambient stability 4,33 .
Therefore  UV absorption portrays absorption coefficients (α) at a different visible wavelength (λ in nm) and the optical bandgap of the material is calculated by plotting Tauc plot from UV absorption spectra results. The Tauc plot is plotted for (αhν) 2 versus (hν) where 'hν' corresponds to the bandgap energy of the material. The resulting Tauc plot has a distinct linear regime which denotes the onset of absorption. Thus, extrapolating this linear region to the abscissa yields the energy of the optical bandgap of the material as shown in Fig. 3a-e. According to the Tauc plot, the calculated optical bandgap gradually increases with an increase in the concentration of Pb(SCN) 2 in the CH 3 NH 3 PbI (3−x) (SCN) x perovskite structure (Fig. 3f). For the five different perovskite systems, we have synthesized, the calculated bandgap from the Tauc plot is very close to the DFT computed bandgaps and hence enables in establishing a correlation between material bandgap (Eg), Pb(SCN) 2 concentration, and (SCN) − percentage in the synthesized perovskite systems and the same is presented in Fig. 3f. Even though the bandgap computed via the PBE-GGA approach underestimated the experimental bandgap, the difference margin was less as spin orbit coupling (SOC) was not considered in our theoretical calculations. It has been reported that using GGA functional for computing bandgaps of methylammonium lead halide type perovskite systems, exclusion of SOC reduces the difference between experimental and DFT calculated bandgap 4,34 . As depicted by the Tauc plot in Fig. 3e, the bandgap (Eg) for the synthesized material with 750 mg/ml Pb(SCN) 2 concentration is greater than 2 eV (2.049 eV) and hence it may not be a viable solar material for fabrication of PSCs. For the other 4 perovskite chemistries bandgap falls in the range of ideal solar material and hence can be considered as an active material for PSC fabrication. with increased Pb(SCN) 2 concentration (mg/ml) in steps of 0, 500, 550, 650, respectively. The obtained PL spectra (Fig. 4a-e) can be well correlated to the computed PDOS via DFT calculations implying correct synthesis of the DFT computed materials (Fig. 2a-e). PL spectra portray the emission peaks corresponding to the localized defect states or shallow levels present in the electronic band structure of the synthesized material. Some of the PL spectra (Fig. 4a,b,d) show a single emission peak which implies there are no intermediate defect states and corresponds to the bandgap of the material and the results can be compared with the computed PDOS for perovskite chemistries at x = 0, 0.25, 1 respectively (Fig. 2a,b,d). PL spectra for perovskite chemistry at x = 0.49 and 1.45 show two emission peaks at 1.52, 1.64 and 1.749, 2.049 eV, respectively (Fig. 4c,e). 1.64 eV and 2.049 correspond to the bandgap of the material at x = 0.49, 1.45 respectively. However, the other emission peaks at 1.52 and 1.749 eV (Fig. 4c,e) correspond to the presence of intermediate defect states similar to computed PDOS (Fig. 4c,e) between the top of the valence band and bottom of the conduction band for the perovskite stoichiometry at x = 0.49,1 respectively.
Moreover, according to PL spectra for perovskite chemistry at x = 0.49, 1.45 the gap between the two emission peaks is 0.12 eV (Fig. 4c) and 0.3 eV (Fig. 4e). This gap is similar to the computed gap between the two peaks [1.587 eV, 1.467 eV] and [1.7 eV, 2 eV] respectively as projected in PDOS (Fig. 2c,e) implying correct synthesis of the DFT computed materials. The bandgap obtained from the PL for these 5 different perovskite chemistries (Fig. 4a-e) goes in accordance with the bandgap calculated from the Tauc plot (Fig. 3a-e). Hence, post material synthesis and characterization we could conclude the perovskite chemistry at x = 1.0 should be the best viable solar material for PSC fabrication in terms of favorable material bandgap, absence of intermediate defect states and 33.32% (SCN) − in the perovskite chemistry. However, n-i-p and p-i-n PSC devices were fabricated with all perovskite chemistries and were validated for the best solar material in terms of stability. characterization of perovskite solar cell. As mentioned in previous sections, n-i-p and p-i-n PSC devices using the varying composition of CH 3 NH 3 PbI (3−x) (SCN) x (at x = 0, 0.25, 0.49, 1) perovskite stoichiometry were fabricated on FTO substrates. However, we could not fabricate efficient PSC devices at x = 1.45 (i.e. 49.98% (SCN) − doping) in the perovskite chemistry as the Pb(SCN) 2 solution becomes highly viscous during material synthesis resulting in non-uniform film morphology and reducing the efficiency of the fabricated devices. The device characterizations were performed on perovskite solar cells kept in ambient conditions without encapsulation, for all n-i-p and p-i-n device architectures. However, it was found that among all the n-i-p and p-i-n devices fabricated, devices with CH 3 NH 3 PbI (3−x) (SCN) x perovskite at x = 1.0 showed the highest stability with moderate efficiency. However, n-i-p devices fabricated at x = 1.0 showed higher efficiency and lesser stability as compared to the fabricated p-i-n devices at x = 1.0. For fabricated p-i-n devices at x = 1.0, the device efficiency was retained until 450 h. The n-i-p and p-i-n devices were fabricated according to the architectures mentioned in Fig. 1a,b. The detailed characterization of the n-i-p and p-i-n device architectures fabricated with perovskite chemistry CH 3 NH 3 PbI (3−x) (SCN) x at x = 1.0 are mentioned below. The gradient and Y-intercept of the linear plot (Eq. 4) allow estimation of ideality factor and Io respectively. The calculated 'Vbi' , 'Io' , 'n' are 0.72 V, 3.51 × 10 -7 A , 2.54 (Fig. 5a,b) and 0.7 V, 4.56 × 10 -7 A , 2.26 (Fig. 5c,d) for n-i-p and p-i-n devices at respectively. Higher Vbi contributes to higher open-circuit voltage (Voc) and thus helps in the improved photovoltaic performance of the PSC. Deviations in the ideality factor from one indicate that, either there are unusual recombination mechanisms taking place or that the recombination is changing in magnitude. Higher is the recombination in the device, lower will be the short circuit current of the solar cell, and lower current results in improved perovskite film morphology.
Photovoltaic characteristics and device scalability. Photovoltaic characteristics under a light intensity of 1,000 W/m 2 n-i-p (Fig. 5e) and p-i-n (Fig. 5f) perovskite stoichiometry at x = 1.0 are presented in Fig. 5e,f. According to J-V characteristics n-i-p device produces an efficiency of 5.39% with a Fill Factor (FF) of 51, Voc of 0.77 V, and short circuit current density (Jsc) of 13.63 mA/cm 2 (Fig. 5e). However, efficiency produced by p-i-n device at a FF of 46 and Voc of 0.78 V is 3.39% which is less compared to the n-i-p devices due to lower Jsc of 9.34 mA/cm 2 (Fig. 5f). Device fabrication has been done varying the active device area to check on the scalable efficiency of the device. Unfortunately, with the increase in device area, Jsc decreases resulting in a decrease of PSC power conversion efficiency for both n-i-p and p-i-n device architecture (Fig. 5g,h) Dependency of solar parameters on light intensity. On increasing light intensity considerable changes were observed in the n-i-p and p-i-n solar cell characterization parameters such as Voc, Jsc, FF, and Efficiency ( Fig. 6a-h). An increase in light intensity from 1 solar, results in a gradual increase of Jsc in the case of n-i-p (Fig. 6a) and p-i-n devices (Fig. 6e) respectively. However, Jsc saturates beyond 1.06 kW/m 2 light intensity in the n-i-p device, unlike p-i-n device which implies the chosen material is capable of absorbing more solar energy. FF increases to its peak up to a certain light intensity and then gradually decreases for n-i-p device (Fig. 6b) and saturates for p-i-n device (Fig. 6f)  www.nature.com/scientificreports/ constant with the increase in light intensity for both n-i-p (Fig. 6c) and p-i-n (Fig. 6g) resulting in better device stability. Changes in Jsc, FF, Voc results in an increase of efficiency up to a certain light intensity and then decreases owing to the decrease in FF in n-i-p device and saturates owing to the saturation of FF in p-i-n device (Fig. 6e-h). The saturation of efficiency for p-i-n device indicates better stability of the p-i-n device as compared to the n-i-p device.  As shown in Figs. 7a and 8a, Jsc dropped down from 13.6 to 9.68 mA/cm 2 for n-i-p (Fig. 7a) and from 9.25 mA/cm 2 to 5.7 mA/cm 2 for p-i-n (Fig. 8a) device after 750 h. Jsc remains constant up to 450 and 525 h for n-i-p (Fig. 7b) and p-i-n (Fig. 8b) devices respectively beyond which Jsc drops down. However, unlike consistency in Jsc, FF drops after 150 h in the case of the n-i-p device (Fig. 7c) presumably due to the degradation of the top Au electrode. But the scenario is different in the case of p-i-n device and FF remains stable for 525 h (Fig. 8c) which contributes to better stability of p-i-n device. Voc was found to remain stable for 450 h both in the case of n-i-p and p-i-n device (Figs. 7d, 8d). Since the efficiency of a PSC device is directly proportional to parameters Jsc, FF, and Voc, despite stability in Jsc and Voc for 450 h, a fast decrease in FF causes a rapid decrease in efficiency of n-i-p device after 150 h (Fig. 7e). Hence in the case of n-i-p device, the variation of efficiency over time follows the same trend as FF. However, in the case of the fabricated p-i-n device, Jsc and FF remain stable up to 525 h and Voc up to 450 h and thus efficiency also remains stable for a longer time of 450 h (Fig. 8e). Hence in the case of the p-i-n device, the variation of efficiency over time follows the trend of Voc. fabrication and characterization of solar panel. A solar panel was made on acrylic with an array of solar cells connected as a combination of series and parallel. The panel consists of 12 FTO pieces as depicted in Supplementary Fig. S8. Each FTO piece consists of 3 p-i-n PSC devices fabricated with perovskite at x = 1.0 as an active light-harvesting material. The p-i-n solar cells provided longer stability with moderate efficiency and hence were used for the formation of a solar panel. Some of the PSCs were connected in series and some in parallel to get an increased Voc and Jsc respectively. The panel consists of three modules with four FTO pieces in each module connected in parallel and the three modules connected in series. On an average, each PSC generates a Jsc of 9 mA/cm 2 and a Voc of 0.75 V. The overall solar panel generated a Jsc of 108 mA/cm 2 and Voc of 2.25 V. The overall solar cell efficiency decreases to 3%. However, the efficiency was retained for 5,000 h.

Discussion
We have successfully computed the electronic properties such as band structure, bandgap, DOS, and PSDOS using DFT calculation for different perovskite CH 3   www.nature.com/scientificreports/ peaks found in the PL spectra of synthesized materials, thus confirming the correct synthesis of DFT computed materials. CH 3 NH 3 PbI (3−x) (SCN) x perovskite at x = 1.0 was considered as a best viable candidate for solar material for fabrication of PSC devices because of ideal optical bandgap suitable for solar applications, absence of intermediate defect states and a higher concentration of (SCN) − . However, PSC devices were fabricated and characterized with all five different perovskite stoichiometry for two different architectures namely n-i-p and p-i-n. Post fabrication and characterization, it was found that n-i-p PSC devices fabricated with CH 3 NH 3 PbI (3−x) (SCN) x perovskite at x = 1.0 showed the highest stability with moderate efficiency when exposed to ambient conditions, as compared to the PSC devices fabricated with other perovskite stoichiometry. Thus it can be concluded that the material ranked as the best solar material as per DFT simulations resulted in the fabrication of most moisturestable perovskite solar cells. Further, it was concluded that between n-i-p and p-i-n PSC devices fabricated with CH 3 NH 3 PbI (3−x) (SCN) x perovskite at x = 1.0, p-i-n PSC device showed the highest stability and efficiency up to 450 h under ambient conditions without encapsulation.