Influence of Front Contact Layer on the Performance of Bismuth-Based Perovskite Solar Cells

Numerical analysis has been carried out using SCAPS-1D to investigate the power conversion efficiency of bismuth-based perovskite solar cells employing various Transparent Conductive Oxides (TCOs) such as Molybdenum Trioxide (MoO 3 ), Boron-doped Zinc Oxide (BZO) and Zinc Oxide (ZnO). For the initial simulation, the power conversion efficiencies obtained for MoO 3 , BZO and ZnO were 0.24 %, 0.17 % and 0.17 % respectively. The influence of thickness, donor concentration and working temperature of the TCOs were varied to study their impact on the device’s photovoltaic performance. By varying the thickness, doping concentration and operating temperature, the electrical parameters observed for the three selected TCOs exhibited insignificant impact on the device’s performance. However, the highest performance was achieved using MoO 3 at the thickness of 200 nm, donor concentration of 1 × 10 (cid:2869)(cid:2875) 𝑐𝑚 (cid:2879)(cid:2871) and the operating temperature of 300 K with the corresponding power conversion efficiency of 0.24 %, Jsc, Voc and FF of 0.2610 mA/cm 2 , 1.6509 V and 54.97 % respectively. The numerical simulation shows the potential of designing and fabricating an improved bismuth-based perovskite solar cell with MoO 3 as front contact as an alternative to Fluorine-doped Tin Oxide (FTO) and Indium-doped Tin Oxide (ITO).


I. INTRODUCTION
erovskite -based solar cells have shown an excellent performance due to their exceptional properties such as high absorption coefficient, high charge carrier mobility, low surface recombination rate, long diffusion length, direct and tunable band gap and relatively simple methods of processing [1 -3].The efficiency of these promising materials has rapidly increased from 3.8 % in 2009 [4] to 26.1 % in 2022 [5].Transparent conductive oxides (TCOs) have been commonly used as a front contact in perovskite-based solar cells.These TCOs are optically transparent and electrically conductive materials.For a TCO to work efficiently, its band gap must be greater than or equal to 3.1 eV [6].The TCO having high band gap transmits about 80 % of visible light [7 -9].The most commonly used TCOs are indium-doped tin oxide (ITO) and fluorine-doped tin oxide (FTO) owing to their high transparency and low resistivity [10 -17].Though, indium metal is reported as rare, toxic material, expensive and environmentally-unfriendly [18][19][20][21].Moreover, the low electrical conductivity, high leakage current and rigid to patterning by wet etching are the major drawbacks associated with FTO as reported by [22].To further discover an alternative TCO, this work has designed and simulated a bismuth-based perovskite solar cell to investigate the influence of Molybdenum trioxide (MoO3), boron-doped and Zinc Oxide (ZnO) used as front contact using SCAPS-1D.To attain the optimal device performance, the thickness, defect density, donor density and operating temperature of the TCOs were also studied for the three different TCOs.The device structure is presented in Fig. 1.
Fig. 1 The device structure of the bismuth-based perovskite solar cells.

A. Materials
The materials used are methyl-ammonium bismuth iodide (MBI) used as the absorber layer, TiO2 served as electron transporting layer (ETL), Spiro-OMeTAD used as hole transporting layer (HTL).The proposed TCOs are MoO3, BZO and ZnO acted as front contact while Gold (Au) as a counter electrode.The simulation input parameters of these layers were obtained from previous literature [23 -36] and were summarized in Table I.

B. Numerical Method
In this study, the design and simulation were conducted using SCAPS-1D (SCAPS 3.3.10version) simulating software with adopted AM1.5G solar illumination, incident power density of 1000 W/cm2, work point bias 0 V, frequency of 1.0 × 10 , electron and hole thermal velocities 10  and the input parameters itemized in Table I.The SCAPS-1D was designed and developed by the Department of Electronics and Information Systems (ELIS), University of Gent, Belgium [38].This simulating software can compute J-V characteristics of the perovskite solar cells by solving the Poisson equations, continuity equations for electrons and holes, and carrier transport [38,39].The simulation step by step procedure is shown in Fig. 2.

A. Effect of TCO on the (CH3NH3)3Bi2I9 PSC
A transparent conducive oxide layer plays a significant role in the performance of a solar cell since it is used as a front contact [41].Fig. 3 displays the simulated J-V characteristics of the devices with the various TCOs (MoO3, ZnO, and BZO).The photovoltaic parameters obtained were presented in Table II.From Table II, it can be observed that the device with the MoO3 exhibits the highest PCE while the devices with the ZnO and BZO acquire the same PCE.This shows that doping ZnO with boron metal might not increase the electrical conductivity of the TCO which is in line with the findings of [38].This is because boron atom acts as electron donor in ZnO, increasing the electrical conductivity and decreasing the transparency, hence, this decrease in transparency renders the electrical conductivity insignificant.The highest PCE was achieved using MoO3 with photovoltaic parameters; 1.6509 V, 0.2610mA/cm 2 , 54.97 % and 0.24 % for Voc, Jsc, FF, and PCE respectively.This indicates that high band-gap enables the transparent conductive oxide to transmit 80 % or more of visible light which has also been reported by [7 -9].The results obtained also show that MoO3 has high ability in transmitting ©DOP_KASU Publishing light into the device than the doped and undoped-zinc oxide.

Fig. 3 Influence of the three different TCOs on the J-V characteristics
Table II Photovoltaic parameters obtained using MoO3, BZO and ZnO as front contact.

B. Effect of TCO thickness on the performance MBI Perovskite solar cells
The influence of TCO layer thickness was investigated using the numerical simulation.The thickness was varied from 300 nm to 1200 nm for each of the chosen TCO and the remaining input parameters kept constant.Fig. 4 show the plotted J-V curves obtained for MoO3, BZO and ZnO based devices.Similarly, the photovoltaic parameters obtained for MoO3, BZO and ZnO were tabulated in Table III, IV and V respectively.As shown in the Table III, the Voc is relatively unchanged in contrast to Jsc and FF which slightly decreased from 0.2586 / -0.2505 / and 54.9 2% -54.65 % respectively.This shows that the decrease in Jsc might be responsible for the decrease in FF as reported by [40].On the other hand, no any significant change in PCE was observed.From Tables IV and V, rapid decrease in Jsc and FF was observed as the thicknesses of ZnO and BZO increased.Generally, the increase in TCO thickness results in increased optical absorption which can prolong the path of light and cause internal scattering as reported by [39].Hence, this decreases the quantity of photons being absorbed by the perovskite absorber layer and consequently the power conversion efficiency decreases.

C. Effect of TCO doping concentration on the performance MBI Perovskite solar cells
The doping concentrations of the TCOs were varied from 1.0 × 10  to 1.0 × 10  while the remaining input parameters kept constant.Fig. 5 show the J-V curves obtained using the three different TCOs.It could be seen from Tables VI, VII and VIII, all the photovoltaic parameters obtained remain constant as the doping concentration increased.This shows that deep and shallow donors cannot contribute to n-type conductivity due to presence of oxygen as revealed by [42].

D. Effect of working temperature
Influence of working temperature on the device performance was studied using the three different TCOs, the operating temperature was varied from 350 K to 500 K while the remaining input values remain constant.Fig. 6 presents the J-V characteristics of the devices.The parameters obtained are tabulated in Tables IX, X and XI.It can be seen from Tables IX, X and XI that there is increase in Voc and decrease in Jsc and FF while the power conversion efficiencies remain steady as the operating temperature increases.Generally, the working temperature has less effect on the cell performance.In this work, (CH3NH3)3Bi2I9 perovskite solar cells employing three different TCOs (MoO3, ZnO, and BZO) were studied using SCAPS Simulating software.The device performances were also studied against varied TCO thickness, ©DOP_KASU Publishing doping concentration and working temperature.The results show that controlling the thickness of the TCOs is the most important factor in enhancing the efficiency of the perovskitebased solar cells.However, MoO3 exhibited optimum efficiency of 0.24 % in contrast to ZnO and BZO with efficiency of 0.17 % each.It can be concluded that MoO3 is a possible alternative to FTO and ITO as a front contact layer in perovskite solar cells.

Fig. 4
Fig. 4 Effect of TCOs thickness on the J-V characteristics of the device.

Fig. 5
Fig. 5 Effect of TCOs doping concentration on the J-V characteristics

Fig. 6
Fig. 6 Effect of TCOs working temperature on the J-V characteristicsTableIXInfluence of temperature on the device using MoO3

Table III
Photovoltaic parameters for MoO3 at different thickness Table IV Photovoltaic parameters for ZnO at different thickness Table V Photovoltaic parameters for BZO at different thickness

Table X
Influence of temperature on the device using ZnOTable XI Influence of temperature on the device using BZO IV.CONCLUSION