An improvement in un-Encapsulated perovskite solar cell’s environmental stability via introduction of an electrode interface layer

ABSTRACT Perovskite solar cells (PSCs) have shown power conversion efficiency (PCE) up to 25.5%, but they are still struggling with their poor environmental stability. We have improved the ecological stability of un-encapsulated PSCs via interface modification of the top metal electrode. We used CH3NH3PbI3-xClx perovskite as the light absorber and Ag metal as the top electrode. The interface of Ag electrode was modified by introducing a thin layer of MoOx/Al before Ag. We observed that MoOx/Al interlayer before Ag imparts better stability to the devices than conventional Ag electrode only. We found that MoOx/Al interlayer impedes the diffusion of oxygen and metal electrode into the perovskite film and considerably halts the device degradation upon being kept for about 350 h under laboratory ambient conditions (room temperature 25±2oC and humidity 40±5%). This work mainly highlights the role of interfaces in PSCs degradation, and will help to address the stability issues in PSCs. Graphical Abstract


Introduction
Perovskite semiconductors have shown excellent electrical and optical properties [1][2][3], which make them suitable for a variety of applications in solar cells [4], light-emitting diodes [5], detectors [6], etc. Perovskite solar cells (PSCs) have shown an enormous possibility for the cost-effective conversion of solar energy into electrical energy. The first PSC was fabricated in 2009, which had an efficiency of ~ 3.8%, and since then, there has been a remarkable improvement in its performance. Now, the efficiency has gone beyond 25% [1], approaching that of the conventional Si solar cells [7,8]. The materials and the processing cost of PSCs are very low compared to other solar cell technologies [9]. But the long-term stability is a critical issue, and these devices degrade very fast in ambient conditions; for the successful commercialisation of PSCs, the issue of poor stability needs to be sorted out. PSCs are thin film solar cells, and the thin films of perovskite semiconductors are mainly prepared by spin coating technique; for scalable large area fabrication, other coating techniques like spray coating [10], doctor bled coating [11], slot die coating [12], screen printing [13] and thermal deposition [14,15] are also well established. PSCs incorporate multiple thin layers of different materials playing different roles in the solar cells, such as light-absorbing perovskite layer (active layer) like methylammonium lead iodide (CH 3 NH 3 PbI 3 ), methylammonium lead iodide chloride (CH 3 NH 3 PbI 3-x Cl x ), formamidinium lead iodide (CH(NH 2 ) 2 PbI 3 ) etc., electron transport layer (ETL) like titanium oxide (TiO 2 ) [16], tin oxide (SnO 2 ) [17,18], phenyl C 61 butyric acid methyl ester (PCBM) [19] etc. and hole transport layer (HTL) like poly(3-hexylthiophene) (P3HT) [20], 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-MeOTAD) [21], nickel oxide (NiO x ) [22] etc. along with top and bottom electrodes. One of the electrodes is optically transparent, and for that purpose, transparent conducting oxides (TCOs) such as fluorine-doped tin oxide (FTO) and indium tin oxide (ITO) are used, whereas for other electrode metals like Al, Ag, Au, Cu, etc. are used.
The performance of PSCs depends upon the material purities, processing methods, and environmental growth conditions. Small variations in these parameters lead to a significant change in the performance of PSCs [23]. The ageing of the perovskite precursor is also an important parameter that needs to be considered for maximum solar cell performance. Su et al. investigated the effect of perovskite precursor ageing on the PSCs performance and found that an ageing time of 6 h was optimal for maximum performance of the solar cells [24]. The solar cells fabricated with freshly made precursor solution and excessively aged precursor solution resulted in the formation of inferior perovskite films with poor absorption and increased recombination sites. The solar cells prepared with 6 h aged precursor solution led to the formation of solar cells with greater efficiency, stability, and reproducibility. In addition to these factors, the defects and the non-radiative losses in the perovskite layers have also been observed to result in inferior device performance. Therefore, efforts are also being made to passivate the defects in perovskite films. Cai et al. used potassium oleate (KOA) as an interface modifier at the perovskite-HTL interface and achieved over 23% efficiency and quite a high opencircuit voltage (V oc ) of 1.16 V [25]. It was observed that KOA plays a multi-functional role, where K + ions penetrate the perovskite layer and fill the +ve charge vacancy, and the COO − group coordinates with Pb +2 ions and passivates the defects, the hydrophobic longchain olefins cover the perovskite and imparts environmental stability to the devices. PSCs exhibit degradation and show considerable loss of photovoltaic efficiency when exposed to oxygen, moisture, UV light, and high temperature [26][27][28][29][30]. The degradation dominantly occurs within the light-absorbing active layer [31]. Still, it is greatly affected by the performance and thicknesses of the HTL, ETL, and the electrode materials used in solar cells [32]. Degradation occurs because of chemical reactions taking place in solar cells, and the chemical reactions occur not only between the interlayers, such as the perovskite layer, and the ETL or HTL, but the chemical reactions also take place when the metal electrode diffuses into the perovskite layer and reacts there to cause degradation in device performance [29][30][31][32][33][34][35]. However, adding the metal oxide buffer layers between the perovskite layer and the metal electrode has been observed to reduce the direct diffusion of the metal electrode into the perovskite layers [35,36,37].
We have investigated and report here the environmental stability of the PSCs via adding a metal oxide buffer layer such as MoO x between the HTL and the metal electrode. Usually, Ag is used as an anode in conjunction with HTL for hole extraction, but Ag has shown to react rapidly with the perovskite layer [29]. Because of the Ag reaction with the perovskite layer, both Ag electrode and perovskite light absorber lose their electrical properties, and solar cell efficiency decays very rapidly. Here, in addition to MoO x , we have also used a thin layer of Al before Ag to impede the diffusion of both the oxygen and Ag into the perovskite layer. The performance of un-encapsulated solar cells with Ag and MoO x /Al/Ag electrodes was studied from time to time for degradation in their efficiency. We have observed that the devices with MoO x /Al/Ag electrode were more stable than the conventional devices with Ag electrodes only.

Growth and characterisation of perovskite films
For the solar cell fabrication, we used FTO-coated glass substrates and deposited a thin layer of SnO 2 before perovskite film, which serves as the compact hole blocking and ETL in the solar cells [38]. The growth of the perovskite layer depends very strongly on the surface energy of the underlying layer/film and the substrate [39]. To avoid any possible morphological changes and disparity in other properties of the perovskite films due to variation in the surface energy of the underlying substrate, for individual film characterisation, all the perovskite films were grown on SnO 2 coated FTO substrates only. This assured that the properties of the perovskite active layer in the solar cells were the same as studied here. Proceeding to any deposition, the FTO substrates were cleaned sequentially with soap solution, distilled water, acetone, and isopropyl alcohol, and treated with air plasma for 20 minutes in a plasma chamber. After that, 15% colloidal dispersion of SnO 2 in water was spin-coated on the FTO substrates at 2000 rpm for 60 sec. and annealed at 120°C for 60 min. in the air to form a compact SnO 2 film. After the samples were cooled down to room temperature, they were subjected to air plasma treatment for 5 min. The air plasma treatment increases the surface energy and reduces the contact angle of SnO 2 . For the contact angle measurement, a drop of distilled water was put on the SnO 2 film before and after the plasma treatment, and the contact angles were measured. The contact angle of the SnO 2 film without plasma treatment was measured to be ~43°, and after the air plasma treatment, the contact angle was reduced to ~4°. The photographs of the contact angle measurements of both cases are shown in Fig. S1 of the supplementary information. Further, for the growth of the perovskite films, the samples with plasma-treated SnO 2 film were transferred to a glove box filled with high purity dry nitrogen (O 2 < 1 ppm, H 2 O < 1 ppm). The perovskite precursor was prepared by mixing MAI, PbI 2 , and PbCl 2 in 4:1:1 molar ratio in anhydrous DMF and spin-coated over SnO 2 coated FTO substrates at 3000 rpm for 60 sec. and annealed at 100°C for 60 min. This resulted in the conversion of pale yellow film into dark brown film, which indicated the formation of CH 3 NH 3 PbI 3-x Cl x perovskite crystals. The fabrication of perovskite films is schematically shown in Figure 1. Further, these samples were used for the essential characterisation like light absorption, crystal structure, and surface morphology of the perovskite films. For light absorption, crystal structure, and surface morphology, the perovskite films were subjected to UV-vis absorption spectroscopy using a UV-vis absorption spectrometer (UV-2401 PC) from Shimadzu, Japan, X-ray diffraction (XRD) using X-ray diffractometer (Cu Kα radiation, 1.54 Å) from Rigaku, Japan, and Field Emission Scanning Electron Microscopy using FE-SEM from ZEISS, USA.

Perovskite solar cells fabrication
PSCs were prepared on 2 × 2 cm 2 FTO-coated glass substrates, and the substrates were patterned to remove unwanted FTO from the substrate for device fabrication. Prior to any deposition, the patterned substrates were cleaned and air plasma treated for 15 min. The schematic structure of the PSCs with Ag electrode only is shown in Figure 2(a), whereas Figure 2(b) shows the photograph of the solar cell.
Here FTO works as the cathode, and Ag works as the anode. We used SnO 2 as an electron transport layer (ETL) because SnO 2 is an oxygen-deficient n-type semiconductor and the light absorbing layer was a mixed halide perovskite film of CH 3 NH 3 PbI 3-x Cl x . The patterned FTO substrates were coated with SnO 2 first and then with CH 3 NH 3 PbI 3-x Cl x perovskite on the SnO 2 layer. The methods of growth of both layers are discussed in the previous section. For the HTL, we used Spiro-MeOTAD as a p-type organic semiconductor. Spiro-MeOTAD films were prepared by spin coating of its 75 mg/ml solution in anhydrous chloroform, doped with 28 µl of TBP and 17.5 µl of 520 mg/ml solution of LiTFSI in acetonitrile, at 2000 rpm for 60 sec. To complete the solar cells, 100 nm of Ag was thermally deposited at 1 Å/s onto Spiro-MeOTAD through a shadow mask in a vacuum chamber at the base pressure of 2 × 10 −6 mbar, which resulted in the device active area of ~ 0.09 cm 2 .
For the PSCs with modified structure with MoO x /Al interlayer in between the Spiro-MeOTAD and Ag electrode, all the fabrication steps up to Spiro-MeOTAD HTL were identical as both type of devices were prepared together. It is worth mentioning here that in most efficient PSCs, the HTL is usually a thin layer of Spiro-MeOTAD doped with tertbutylpyridine (TBP) and Lithium bis-(trifluoromethanesulfonyl)imide (Li-TFSI) [22]. However, the hygroscopic nature of Li-TFSI leads to rapid degradation in cell performance in a moist environment. For a more robust and stable HTL, we deposited a thin layer of MoO x for protection from environmental degradation. 20 nm of MoO x was thermally deposited on Spiro-MeOTAD in a vacuum chamber at 0.5 Å/s, at the base pressure of 2 × 10 −6 mbar. Then, quite a thin layer of 30 nm of Al was deposited by thermal evaporation through a shadow mask, followed by thermal deposition of 80 nm of Ag at 1 Å/s through the same mask, at the base pressure of  2 × 10 −6 mbar. The structure of the modified devices is shown schematically in Figure 3. As a whole, two types of solar cells were prepared FTO/SnO 2 /Perovskite/ Spiro-MeOTAD/Ag and FTO/SnO 2 /Perovskite/Spiro-MeOTAD/MoO x /Al/Ag and studied for their air stability.

Solar cell characterisation
HT To determine the photovoltaic parameters and the PCE of PSCs, the solar cells were exposed to 100 mW/ cm 2 illumination of an AM1.5 G class AAA solar simulator from Photo Emission Tech (PET), USA, and tested for their current density-voltage (J-V) characteristics under illumination using a Keithley 2420 SourceMeter unit (SMU) interfaced with a computer. The photovoltaic parameters were extracted from the illuminated J-V characteristics that led us to calculate the PCE. To obtain the degradation profiles of solar cells, they were stored and tested in ambient air for their PCE at regular time intervals. The solar cells were not encapsulated or protected externally by any means for these studies. Figure 4 shows the XRD patterns of the FTO substrate and CH 3 NH 3 PbI 3-x Cl x film deposited on the SnO 2coated FTO glass substrate. In the XRD pattern of the FTO/SnO 2 /CH 3 NH 3 PbI 3-x Cl x sample, the diffraction peaks occurred at 2θ values of 14.2°, 28.6°, 43.2°, and 58.8°, which corresponded to the (110), (220), (330) and (440) planes of CH 3 NH 3 PbI 3-x Cl x perovskite and it indicated the formation of highly crystalline tetragonal phase of CH 3 NH 3 PbI 3-x Cl x perovskite [40][41][42][43][44]. The intensity of XRD peaks corresponding to (110) and (220) planes of the perovskite was very much dominating compared to the peaks at (330) and (440), which indicates that the growth of the perovskite crystals was oriented towards (110) and (220) planes. The XRD peaks of FTO and SnO 2 were not visible in the XRD pattern of the FTO/SnO 2 /CH 3 NH 3 PbI 3-x Cl x sample because the intensity of XRD peaks of perovskite film was so dominating that the XRD peaks for FTO and SnO 2 were not visible. Figure 5 shows the UV-vis absorption spectrum of CH 3 NH 3 PbI 3-x Cl x perovskite film on the FTO/SnO 2 substrate. Since SnO 2 film was quite thin (~ 40 nm) and it is a wide band gap material (3.8 eV), the absorption contribution of SnO 2 in the absorption graph was not visible. The absorption graph shown in Figure 5 corresponds to absorption in the perovskite film only. The perovskite film exhibited broad light absorption spanning from 300 to 850 nm. From the extrapolation of the linear part of the absorption spectrum at the absorption edge to zero absorption and using the Tauc's relationship, the perovskite film's bandgap was found to be 1.55 eV. Figure 6 shows the FE-SEM image of the CH 3 NH 3 PbI 3-x Cl x perovskite film on the FTO/SnO 2 substrate. The perovskite film was quite good; we did not see any holes or irregularities in the film, and it covered the substrate completely. The size of perovskite grains was quite large, up to a few microns. Large grain size results in reduced recombination of photo-generated charge carriers and, as a result, higher PCEs.  3%. The average PCE of the three devices with MoO x /Al interlayer and Ag electrode was 9.7 ± 0.6%. The reason behind the marginal decrease in the PCE with MoO x /Al interlayer electrode was that adding buffer layer MoO x possibly introduces some series resistance for charge extraction and enhances the charge recombination at the interface between the perovskite and the MoO x layer, or MoO x acts like an energy barrier limiting its hole extraction efficiency [45]. Direct deposition of Ag metal electrode on the MoO x layer leads to the lowering of the device's PCE [45]. Moreover, when Al is deposited on the MoO x layer, the perovskite solar cells show a marginal decrease in PCE; however, further deposition of Ag metal electrode on the top of the MoO x /Al layer renders the perovskite solar cells environmentally stable. The top layer combination of MoO x /Al/Ag prevented the further diffusion of oxygen and moisture into the PSCs' active layer, hence stable PCE.

Results and discussion
Both types of devices were stored together in air with the temperature being 25 ± 2°C and relative humidity being 40 ± 5% without encapsulation and tested for degradation in their PCE from time to time. Figure 9   (b). The profile of PSCs with Ag electrode only suggests the occurrence of rapid degradation due to which the PCE decreased from 12% to less than 1% within 150 h; however, no such rapid degradation was found to happen with MoO x /Al interlayer before Ag, as it retained up to 90% of the initial PCE for up to 350 h, i.e. marginal degradation in PCE from an initial value of 10.3% to 9.3% after 350 h was observed. This can be attributed to the fact that the MoO x layer prevents further diffusion of the metal electrode into the active layer, and the combination of Al made solar cells more environmentally stable. The role of the MoO x /Al interface layer can be better understood from the model shown in Figure 10. A thin layer of MoO x along with Al provides greater resistance to the oxygen and moisture diffusion into the device. Here, the degradation of CH 3 NH 3 PbI 3-x Cl x happens exactly in the same way as that of CH 3 NH 3 PbI 3 in air. The oxygen and moisture diffusion into the devices without the MoO x layer leads to the decomposition of perovskite into CH 3 NH 3 I, PbI 2, and PbCl 2 . CH 3 NH 3 I decomposes to CH 3 NH 2 and HI. HI releases I 2 in the presence of oxygen. The I 2 releases through HTL and reacts with the Ag electrode to form AgI [46]. The remaining product left in the perovskite film is PbI 2 , which is yellow. The chemical reactions in the PSCs are shown in Fig. S2 of the supplementary information, which is in accordance with other reports [47][48][49]. The chemical reactions in the solar cells result in severe losses in their optical and electrical properties. Qi et al. reported that when perovskite solar cells were exposed to the air, Spiro-MeOTAD enhanced the    degradation in the conventional perovskite solar cells structure [50], with Li-TFSI migrating from the bottom to the top across the Spiro-MeOTAD film. Here, Li-TFSI slowly accumulated at the top surface of HTL and induced rapid film degradation. However, the introduction of the MoO x layer at the top of the Spiro-MeOTAD film forms an oxide layer (upon the reaction of MoO x with oxygen), which acts as an oxygen barrier and prevents the further diffusion of oxygen into the perovskite layer and thus, the stability of the device is retained. These results suggest that the MoO x /Al interlayer along with the Ag electrode imparts better stability to PSCs than the conventional solar cells structure with an Ag electrode only. Here, the MoO x and Al/ Ag electrode plays an important role in preventing the diffusion of oxygen and moisture into the active perovskite absorber layer and retaining the effective PCE. It is important to mention that here, a very thin layer of MoO x (~ 20 nm) may aid in pinning the work function at the spiro-OMeTAD interface with concurrent nucleation of a stable and passivating oxide layer, presumably Al 2 O 3 at the (MoO x /Al) interface leading to higher stability of the device. A detailed investigation is underway to elucidate this mechanism.

Conclusions
The PSCs were prepared using a mixed halide CH 3 NH 3 PbI 3-x Cl x perovskite as an active light absorber with SnO 2 as ETL and Spiro-MeOTAD as HTL. The perovskite films were subsequently uniform and composed of large crystal grains. The PSCs with Ag electrodes degraded rapidly due to the diffusion of oxygen and moisture into the devices and their reaction with device constituents. The degradation of PSCs gets reduced with the introduction of the MoO x /Al interface layer before the Ag electrode. The introduction of the MoO x /Al interlayer decreases the diffusion of oxygen and moisture into the perovskite layer and prevents deterioration in the solar cell efficiency. Thus, the electrode interface layer plays a vital role in dictating the stability and efficiency of the PSCs. From the above perovskite device stability studies, it can be conjectured that both MoO x interlayer and MoO x /metal (Al, Ag) combination is imperative for the long-term stability of PSCs.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
The work was internally supported by CSIR-National Physical Laboratory.