The rapid growth of the world population has increased the global need for energy, which has become undoubtedly quite strong. To date, the energy requirements have been mostly fulfilled by conventional sources of energy, which include a major portion of fossil fuels. But nowadays, environmentally clean energy sources are moving forward. During the past few decades, various clean energy forms have been introduced, which include wind energy and hydropower, as well as geothermal, tidal, and solar energy. Renewable energy sources are unlimited and can be constantly replenished. In the coming years, renewable energy sources will contribute to decarbonizing energy systems. Solar energy safeguards both human health and a healthy environment . Akmam and Karapinar fabricated a dye-sensitized solar cell (DSSC) with selenium@activated carbon (Se@AC) composites as an alternative to the Pt counter electrode (CE) via chemical activation. The fabricated DSSC showed a power conversion efficiency (PCE) of 5.67%, an open-circuit voltage (VOC) of 0.648 V, a short-circuit current density (JSC) of 13.26 mA/cm2, and a fill factor (FF) of 66%. The PCE is close to that of the Pt-based counter electrode (PCE = 6.86%). Akman used hydrothermal methods to synthesize the photoanodes with different doping sources to further improve the stability of DSSCs. For 1.0 mol % Mn doping and an Eu compact layer, an efficiency of 4.20% was obtained.

Currently, perovskite solar cells (PSCs) are attracting the attention of research communities worldwide because of their outstanding and unique properties. PSCs possess desirable characteristics such as cost-effectiveness, extended carrier diffusion lengths, and adjustable direct bandgaps. Also, there are well-established fabrication techniques that have positioned PSCs as a solution-processable photovoltaic technology . Over the past few years, a significant improvement in the PCE of the PSCs was reported, from 3.8% in 2009 to 26.1% in 2023 . PSCs consist of an absorber layer sandwiched between charge transport layers (CTLs), that is, the hole transport layer (HTL) and the electron transport layer (ETL). Light generates excitons, which further dissociate into electrons and holes. The electrons and holes are transported to ETL and HTL, respectively, without recombining . Ozturk et al. addressed the role of a passivation agent at grain boundaries and the surface of perovskite films, namely, quinary kesterite nanocrystals Cu2NiSn(S,Se)4 (CNTSSe) obtained through a facile hot-casting method. Through passivation, efficiencies of 20.8% for Cs0.05(FA0.90MA0.10)0.95Pb(I0.90Br0.10)3, 18.9% for MAPbI3, and 18.7% for FAPbI3 perovskite layers were observed under ambient conditions and illumination for over 900 h. Mohammed et al. optimized triple-cation PSCs that maintained 83% of the efficiency after 1600 h under ambient conditions with humidity levels of 35–40%. Here, 3,4-dihydroxyphenethylamine hydrochloride (3,4-DpACl) was used as an additive during perovskite fabrication.

Despite significant research efforts, there are stability issues when working under critical environmental conditions, which is an essential issue for practical applications in the future. The structure of PSCs is ABX3, where A and B are the cationic sites and X is the anionic site. In double perovskite solar cells (DPSCs), the unit cell is twice that of the perovskite, that is, A2BB′O6. It has two cations at the sites B and B′ with corner-sharing BO6 and B′O6 units featuring a rock salt-like arrangement . The commercialization of PSCs is impeded because of toxicity and long-term instability. DPSCs turned out to be better than PSCs because of better tunability, higher environmental stability, and higher efficiency. In DPSCs, the double perovskite layer is sandwiched between the CTLs. In 2021, Kumar et al., reported a PCE of 9.68% for a La2NiMnO6 (LNMO)-based device structure after the bandgap had been optimized using the SCAPS-1D software . In 2022, Porwal et al. reported a PCE of 23.64% with Cs2SnI6 as double perovskite layer calculated via SCAPS-1D . In 2023, Alla et al. simulated, using SCAPS-1D, a Cs2CuSbX6-based DPSCs with an efficiency exceeding 29% .

In 2023, Singh et al. optimized a lead-free DPSC with La2NiMnO6 as absorber layer, Cu2O as HTL, and ZnOS as an ETL. They obtained an efficiency of 25.44% with VOC = 1.1027 V, JSC = 27.89 mA/cm2, and FF = 82.69%, suggesting the suitability of La2NiMnO6. In 2024, Singh et al. proposed a planar DPSC with La2NiMnO6 as absorber layer, Cu2O as HTL, and WS2 as ETL, and several parameters of the absorber layer including thickness, defect density, series and shunt resistance, interfacial defect density, and various metal electrodes were studied. An efficiency of 18.89% with VOC = 0.7919 V, JSC = 27.89 mA/cm2, and FF = 85.52% was reported for the device structure FTO/WS2/La2NiMnO6/Cu2O/Au . In 2023, the highest optimized efficiency of 24.08% was reported for the device configuration FTO/WS2/LNMO/Cu2O/Au, representing La2NiMnO6 as an eco-friendly and non-toxic oxide material usable for further applications .

In literature, DPSCs with inorganic Cu2O have been studied, but in this manuscript we also consider organic materials. The optimized PSC device displays a higher efficiency of 27.84% with Cu2O and 27.38% with PEDOT:PSS for the planar n-i-p FTO/WS2/LNMO/HTL/Au device structure. However, highly efficient organic HTLs have a few disadvantages over inorganic HTLs, including multistep synthesis requiring additional doping, leading to device instability . In this manuscript, La2NiMnO6 (LNMO) is used as a double perovskite light absorbing layer for the device structure FTO/WS2/LNMO/HTL/Au, where WS2 is used as ETL. We optimize the La2NiMnO6 double perovskite with respect to organic (PEDOT:PSS) and inorganic (Cu2O) HTLs. This work mainly explains the impact of HTLs on the double perovskite material because, until now, the efficiency is low in this type of solar cell. The results highlight the potential of these HTLs for enhanced device performance in DPSCs. Also, the optimized parameters from these studies indicate pathways for experimental work regarding better performance.

Figure 1: (a) Methodology of SCAPS-1D program, (b) energy band diagram of the planar n-i-p DPSC, and (c) planar n-i-p device structure of DPSC.

The device behavior can be studied and solved using SACPS-1D by solving 1D Poisson and continuity equations. The Poisson equation is as follows :

where e is the electronic charge, ϕ is the electric potential, ε0 is the vacuum permittivity, εr is the relative permittivity, p(x) and n(x) are, respectively, hole and electron position dependence, ND is the shallow donor density, NA is the acceptor donor density, and ρp and ρn are, respectively, the free density distributions of holes and electrons. The Poisson equation explains the change in electric field with respect to charge densities. The continuity equations are :

where Jn and Jp are electron and hole density, respectively, G is the generation rate of free e− and h+, and R is the recombination rate of e− and h+ per unit volume.

Table 1: Parameters for the optoelectronic simulation of planar device.

In 2024, a lead-free DPSC was both designed and fabricated. The included LNMO material was synthesized using the sol–gel method. The experimental and simulated J–V curves showed PCEs of 4.5 % and 10%, respectively. For the simulation, TiO2 was used as ETL, and NiO was used as HTL, with La2NiMnO6 as absorber . The DPSC showed promising characteristics. Applications of double perovskite compounds include fuel cells, UV sensors, electrochemical sensors, indoor photovoltaics, and light-emitting diodes . Double perovskite LNMO nanoparticles and nanorods were synthesized via a hydrothermal process, and it was found that they had a larger saturation magnetization . Simulation results yielded PCEs of over 26%, but there are still challenges associated with the application of DPSCs .

In this simulation study, Cu2O and PEDOT:PSS are the two HTLs that are analyzed concerning the double perovskite material LNMO. The HTL needs better conductivity, better electron blocking, and more hole mobility for better carrier transportation at the perovskite/HTL interface. It is hydrophobic with a wider bandgap and does not easily deteriorate. Inorganic HTLs proved to perform better. Some examples of inorganic HTLs are CuI, Cu2O, and CuSCN. Organic HTLs consist of polymers or complex molecules, which affect the photovoltaic properties of the device in terms of light absorption and carrier mobility. Some examples of organic materials are PEDOT:PSS, P3HT, Spiro-OMeTAD, and PTAA. Our simulations were performed with Cu2O and PEDOT:PSS HTLs .

The absorber layer in a PSC is sandwiched between the CTLs such that upon illumination the formed electrons and holes get captured by the corresponding CTL . The thickness of the absorber layer plays a crucial role regarding the device performance. In this simulation, the thickness of the absorber layer was varied from 250 to 650 nm for different HTLs, and the resulting PCE was calculated. Along with the absorber layer, the HTL is also an important part of a PSC. It blocks electrons, and only holes are captured to make carrier transportation feasible across the perovskite/HTL interface. Here, the impact of both inorganic Cu2O and organic PEDOT:PSS as HTLs is studied.

Table 2: Obtained device parameters at different absorber layer thicknesses with Cu2O and PEDOT:PSS as HTLs.

Figure 2: Optimization of the absorber layer with (a) Cu2O and (b) PEDOT:PSS.

Table 3: Obtained device parameters with Cu2O and PEDOT:PSS as HTLs at different temperatures.

Figure 3: Optimization of device temperature with (a) Cu2O and (b) PEDOT:PSS.

Table 4: Obtained device parameters for various defect densities of the absorber layer with Cu2O and PEDOT:PSS as HTLs.

Figure 4: Optimization of total defect density of absorber layer with (a) Cu2O and (b) PEDOT:PSS.

Table 5: Literature-based device and optimized parameters of the planar device FTO/WS2/LNMO/HTL/Au with both inorganic and organic HTL materials.