Low-cost air-stable perovskite solar cells by incorporating inorganic materials

Herein, we demonstrate a new fabrication strategy for low-cost and stable-operating perovskite solar cells (PSCs) suitable for commercialization. This is performed by fabrication of the device under ambient conditions using Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(Br 0.17 I 0.83 ) 3 formulation as the mixed cation and halide (MCH) perovskite and CuInS 2 (CIS) as the inorganic hole transporting layer. The deposited MCH perovskite with uniform, compact and smooth microstructure containing equiaxed large grains showed excellent thermal and structural stability under intense conditions at 85 °C for 2 h in humid air (i.e., 45% relative humidity). Moreover, it had higher phosphorescence emission, absorption and bandgap energy than the conventional MAPbI 3 due to efficient conversion of PbI 2 to the perovskite compound, as further confirmed by XRD analysis. Photovoltaic measurements under operational conditions revealed that MCH-based PSC showed a 27% higher photoconversion efficiency compared with the control device composed of MAPbI 3 raised by less charge recombination. CIS-based MCH device aged for 30 days at 25 °C and 40% humidity retained 91% of the maximum efficiency with low standard deviation of all photovoltaic parameters, indicating excellent potential for industrialization.


Introduction
During the last decade, organometal halide perovskites have attracted significant attention due to their prominent optoelectronic properties such as direct bandgap, large absorption coefficients and long diffusion length. Solar cell devices based on these perovskites are a pioneer technology in the solar energy conversion field, exhibiting an outstanding power conversion efficiency (PCE) of 22% and 25% in form of tandem devices. 1,2 In their most common version, perovskite solar cells (PSCs) are composed of a layer structure formed on a transparent electrode made of a thin layer of nanoparticulate titania as electron transport layer (ETL), an organometal halide perovskite layer (i.e., MAPbI3 and FAPbI3 as the conventional perovskites), an organic hole transporting layer (HTL) (i.e., Spiro-OMeTAD as the standard one) and a top gold electrode. However, there are concerns related to the degradation of the components upon association with moisture, heat, oxygen, UV light and other potential factors, 3,4 limiting their application in operation. Diverse strategies have been proposed to improve the stability of the components. 5,6,7,8, 9 Amending the chemical composition of the perovskite by modifying or combining its anions and cations is an effective mechanism for improving its electronic properties and stability simultaneously. 10 , 11 , 12 Due to instability and volatility of the organic components of MAPbI3 and FAPbI3, their partial or complete replacement by more stable inroganic ones such as Cs has been reported. 13,14, 15 The later has an ionic radius of 1.81 Å, which is considerably smaller than that of MA (2.70 Å) or FA (2.79 Å). Not only the inorganic stable CsPbI3 perovskite crystallizes at the high temeparatute of 310 °C, but also its corresponding cell shows lower efficiency than the conventional PSCs. 16,17 In addition, by decreasing the temperature and exposure to ambient air after several minutes, CsPbI3 transforms to the non-perovskite yellow phase. 18  Therefore, the quantity of cesium is controlled in the range 0.1-0.2 to block the cesium iodide phase development and decrease in trap density, resulting in an increase in PCE. 21 In addition, the partial replacement of FA by Cs gained better moisture stability compared to FAPbI3 perovskite. 22 It is also known that adopting different halides into the perovskite structure gives rise to a shift in the bandgap, the smaller ionic radius of the halogen, the higher bandgap of the perovskite. 23  Generally, PSCs with high efficiency were taken utilizing organic materials, such as Spiro-OMeTAD, as HTLs. This material is expensive to synthesize or to acquire, it has a limited conductivity and necessitates the utilization of additives and its stability is questioned. 25,26 For this reason, other possible inorganic HTLs have been recently intensively studied due to their low-cost and higher hole mobility and stability. 27 CuInS2 (CIS) has recently been employed as a promising inorganic HTL due to its suitable bandgap for the solar spectrum, low toxicity and a high extinction coefficient, although the PCEs of their cells are lower than those of Spiro-OMeTAD based devices. 28 Such favorable characteristics of CIS enabled us to construct low-cost and stable PSCs in this work. All cells were fabricated and characterized under ambient conditions of 40% relative humidity to further reduce the device cost and to evaluate their long-term stability.

Materials
Fluorine doped tin oxide (FTO) was purchased from Solaronix (Resistance 7 Ω/square). Sigma-Aldrich and all of the reagents were used without further purification except CB, which was purified. MAPbI3 perovskite film, as a control cell, was deposited by a sequential two-step method.

Fabrication of PSCs
First, 70 μL of 1.5 M PbI2 solution in a mixed solvent of DMF and DMSO (4: 1) were spun onto the TiO2 mesoporous film, which was spin coated at 6500 rpm for 5 s, followed by annealing at 70 °C for 30 min. In the second step, the perovskite material was formed on

Results and discussion
We     (Figures 5c and 5d). The inset of Figure 5d shows EDS mapping of Cs, indicating uniform distribution of cesium within MCH perovskite.    days in air atmosphere held at room temperature and 40% relative humidity.

Conclusions
In summary, we report a facile method for construction of stable PSC device under ambient conditions using the mixed