Preparation and Characterization of Electron Transfer Layer for Perovskite Solar Cells

*Corresponding Author: Adam K. Kadhim as.18.70@grad.uotechnology.edu.iq Abstract In this paper, we present triple cation perovskites because it has excellent stability and PV performance. To characterize the triple-cation perovskite solar cells, X-ray diffraction, Field emission scanning electron microscope, and Ultraviolet-visible spectroscopy were used. The performance of perovskite solar cells was improved by reducing graphene oxide/bismuth oxide mixed mesoporous titanium dioxide as an effective electron transport layer. The perovskite layer deposited onto modified TiO2 layer showed a larger grain size with better crystalline nature. The optimum device has fabricated at room temperature without a glove box and obtained a power conversion efficacy of 17%.

generation solar cells based on nanostructures or nanostructured interfaces [22,23]. They present many innovative ways to convert solar energy into electricity or heat (in PV devices). Graphene is a carbon material with a hexagonal structure like a honeycomb lattice with 0.34 nm thickness [24]. Due to its amazing characteristics, such as high thermal and chemical durability, high electrical conductivity, high charge mobility, and low-cost manufacturing, graphene and graphene-based hybrid nanostructures have been utilized in PSCs [25]. Reduce graphene oxide (rGO) has excellent chemical, mechanical stability, high thermal conductivity, suitable optical properties, and easily varied chemical functionalization [26]. Because of its remarkable properties, it can be a promising dopant for the electron transport layer (ETL) to enhance the efficiency of PSCs [27]. Bismuth (Bi) is also a benign element, and its ionic radii comply with the tolerance factor rule, improving the stability of Bi-based perovskite materials [17,28]. Furthermore, Bi-based perovskite materials were discovered to have a greater absorption coefficient, making them an effective light-absorbing material for solar cell applications. Therefore, the performance of Bi-based perovskite structures has been encouraging and efficient charge extraction layers [29,30]. Bismuth oxide (Bi2O3) has been good electrochemical properties, stabilities, and relatively high power. Bi-based materials provide acceptable features for PVs, such as high absorption coefficients and appropriate bandgaps, as well as other appealing traits such as robustness and stability [31]. Most importantly, Bi2O3 prevents halide perovskites' corrosion and is a substantial stable metal for halide perovskites even under the most challenging circumstances [32].

Theoretical Part
A solar cell is an electrical device that receives solar energy and transforms it directly into electricity. When sunlight strikes a solar cell, it generates both current and voltage. As a result, solar energy may be used to create electricity. This technique necessitates the use of a substance that can absorb sunlight and produce electron-hole pairs. Before being gathered by the contact layers, these electron-hole pairs might spread and drift. These highenergy electrons and holes can create a photo-current when they flow via an external circuit from a solar cell. Finally, the higher-energy electrons and holes expend their energy into an external load before returning to the solar cell to recombine. Although various materials are viable for photovoltaic energy conversion, we most commonly employ PN junctions to convert solar energy to electricity in reality.

Materials Synthesis
A novel nanocomposite of rGO/Bi2O3 was synthesized via dispersing rGO (98.5% and conductivity ~ 600 S/m, Merck) and Bi2O3 (98.5%, 25-35 nm, Merck) separately in ethanol and stirring at room temperature. Later, 2% of Bi2O3 solution was added to rGO dispersion and mixed overnight at 70 o C. After that, rGO/Bi2O3 mixture was gathered via centrifugation and rinsed with deionized water four times. Finally, the resultant mixture was heated at 80 o C with a furnace overnight.

PSCs Fabrication
To pattern the FTO substrates, it was cleaned with distilled water, acetone, and IPA (99.8%, Sigma-Aldrich) in a sonication bath for 15 min. After that, the FTO glasses were heated at 50 o C for 10 min. The compact TiO2 (c-TiO2) films were then prepared by depositing an acid mixture of titanium isopropoxide (purity of 98%, EXIR) at 3000 rpm for 40 s. Eventually, the pure mp-TiO2 or rGO/Bi2O3 modified mp-TiO2 films were spinning-coated over c-TiO2 with a speed of 3000 rpm for 40 s. Next, to prepare modified mp-TiO2 precursors, 2% content of the rGO/Bi2O3 solution in chlorobenzene was inserted into the dispersed TiO2 and mixed for 120 min. Next, both c-TiO2 and mp-TiO2 films were annealed at 500 o C for 60 min. Next, the perovskite layer was prepared by stirring a mixture of PbI2 (600 mg) and lead bromide (15 mg) in DMF (950 µl) amount 10% of CsPbI3 and then was deposited on FTO/c-TiO2/mp-TiO2 substrates by spin-coating at 2000 rpm for 40 s, followed by annealing at 70 o C for 2 min. Next, a solution of formamidinium iodide, methylammonium bromide, and methylammonium chloride solution (60 mg: 6mg: 6mg) in IPA were poured on PbI2 layer, followed by a spin coating at 1300 rpm for 30 s and annealed at the 150 o C for 15 min. Next, the HTM layer was prepared by adding 17.5 μl bis (trifluoromethane) sulfonimide lithium salt (99.95%, Merck) in acetonitrile (520 mg/ml), and 28.8 μl 4tertbutylpyridine to 60 mg Spiro-OMeTAD in chlorobenzene. Next, the HTL mixture was fabricated at 3000 rpm for 40 s; it was deposited on top of the PVK film to develop the HTL film. A 70 nm gold electrode was deposited on the HTM to complete the PSC structure.

Characterization
The morphological properties of the perovskite layer were observed with a FESEM (TESCAN, Mira 3). X-ray diffraction spectra of the perovskites were measured by X-ray diffraction (Bruker, D8 advance). A UV-vis spectroscopy (Analytic Jena, Specord 250) was utilized to determine the optical merits of perovskites. The photoluminescence response of perovskites was characterized using a PL system (Teifsanje, FL-Ar-2015). The photocurrent-voltage (J-V) measurements of the PSCs were evaluated under a calibrated power density of 100 mW.cm -2 (one sun) using a Keithley Model 2400. The active area of 8 mm 2 was used.

Results and Discussion
SEM images were used to examine the morphologies of the perovskite films, as shown in Figure 1. Compared to pure mp-TiO2 based perovskite, a suitable PVS film for solar cell applications may be produced by adding rGO/Bi2O3. The perovskite's pinholes and grain boundaries (GBs) were enervated. Because they functioned as charge recombination sites, they were inappropriate for PSCs. This modification approach may decrease the density of trap states between the GBs, increasing the suppression of recombination processes and promoting the efficiency of the PSC, which is consistent with the findings of the PL and I-V tests.  recombination rates. The XRD patterns of perovskites deposited on mp-TiO2 ETMs are shown in Figure 3b. In X-ray spectra, the 2% rGO/Bi2O3 additive increased the primary (001) signal, improving the perovskite crystalline nature. PbI2 has a peak at 12.57°, which is overlooked by PSCs. This peak was significantly reduced when rGO/Bi2O3 was used, indicating resistance to humid degradation. The tests mentioned above showed that adding rGO/Bi2O3 improves ETM characteristics and perovskite quality.

Conclusions
We provide a new rGO/Bi2O3 additive for the fabrication of high-performance mesoporous PSCs. This method uses a nanocomposite structure of rGO/Bi2O3 integrated into mp-TiO2 as an electron acceptor/transport mediator ETLs. rGO/Bi2O3-based PSCs have superior charge injection and reduced recombination processes than control PSCs, resulting in greater Jsc and PCE. Our findings suggest that rGO/Bi2O3 might be a good additive in PSCs to improve the PSCs' PV parameters. Importantly, with the rGO/Bi2O3 additive, a champion PCE of 17% is achieved.