SCAPS (Solar Cell Capacitance Simulator) is a one-dimensional solar cell simulation program developed at Ghent University, Belgium, within the Department of Electronics and Information Systems (ELIS). It is widely used for the design, modeling, and optimization of solar cells and is free for researchers and scholars in the field of photovoltaic energy [9][10]. With the ability to simulate up to seven semiconductor layers, SCAPS enables the examination of many types of solar cells, including CIS, CdTe, Si, GaAs, and a-Si. The software computes important properties, including current-voltage (J-V), capacitance-voltage (C-V), and quantum efficiency (QE), and integrates recombination mechanisms like SRH, Auger, and band-to-band. In photovoltaic research, SCAPS is an essential instrument that facilitates the design and improvement of solar cell efficiency as well as the examination of how internal structure and physical parameters impact device performance.

SCAPS-1D software was used to model device performance. Parameters such as band gap, electron affinity, dielectric permittivity, effective density of states, electron/hole mobility, and interface defect density were extracted from published literature. Interface recombination was included to evaluate its effect on efficiency.

The SCAPS-1D simulation framework is based on the numerical solution of the one-dimensional semiconductor equations (Drift-Diffusion Model) [10][11]. These equations form the mathematical foundation for analyzing the steady-state performance of a solar cell.

Poisson’s Equation: This equation relates the spatial distribution of the electrostatic potential (ψ) to the total charge density (ρ) within the semiconductor material. It is essential for determining the internal electric field.

Continuity Equations: These equations express charge conservation and describe the balance between carrier generation (G), recombination (U), and current density (J_n,J_p):

The CDs/TiO₂ hybrid composite is a promising material for photocatalytic and optoelectronic applications because of its exceptional physicochemical features. Carbon quantum dots (CDs) greatly increase the light absorption range from the ultraviolet to the visible region when included in titanium dioxide (TiO₂), improving the efficiency of solar energy usage. Additionally, the CDs function as excellent electron mediators, effectively preventing photogenerated electron-hole pairs from recombining, hence increasing TiO₂’s photocatalytic activity. The composite is appropriate for biomedical and environmental applications due to its strong chemical and thermal stability, non-toxicity, and environmental friendliness. Furthermore, CDs improve TiO₂’s electrical conductivity, which speeds up charge transfer during photoelectrochemical reactions. Because of its straightforward synthesis methods and adjustable structure. Furthermore, CDs improve TiO₂’s electrical conductivity, which speeds up charge transfer during photoelectrochemical reactions. The CDs/TiO₂ composite has a lot of potential for use in solar energy conversion, hydrogen production, and pollutant degradation because of its easy synthesis methods and adjustable structural and surface characteristics.

The individual solar cell structures used for the simulation have been shown in Figure 1. The solar cell structure was designed with the higher ITO layer, followed by the CDs layer and the rest of the TiO₂ layer. The schematic representation of the structure used in the simulation has been shown in Figure 2Figure 2.

To define each layer in the device model, material characteristics including thickness, bandgap, electron and hole carrier concentrations, and electron affinity are essential. The simulation’s layer parameters are listed in Table 1 [12][13]. Table 2 includes the layer and interface defect specifications, respectively.

Figure 1. Device structure TiO₂ solar cell.

Figure 2. Device structure of simulated CDs/TiO₂ solar cell from SCAPS one-dimensional solar cell simulation program.

Defects are added to the absorber layers and are listed in Table 2 [12][14] in order to make experimental results realistic. Additionally, series resistance (Rs) of 1 Ωcm² and shunt resistance (Rsh) of 10 Ωcm² are the simulation parameters used throughout the tests. The AM 1.5 spectrum with an input power density of 1000 W/m² and a temperature of 300 K (room temperature) was used to set illumination circumstances that resemble normal reporting conditions, as shown in Figure 1. Furthermore, certain surface recombination characteristics were used for the simulated device’s front and rear electrodes [15]. The front electrode has a metal work function of 5.3 eV, an electron surface recombination velocity of 10¹ cm/s, and a hole surface recombination velocity of 10¹ cm/s. On the other hand, Table 2 lists the metal work function [16][17], electron surface recombination velocity, and hole surface recombination velocity of the back electrode as 5.1 eV, 107 cm/s, and 105 cm/s, respectively.

Table 1. The input parameters values used for SCAPS-1D in the simulation of the solar cell structure [12].

Table 2. Interface defects parameters in CDs/TiO₂ [12].

The J-V (current density-voltage) curve (Figure 3Figure 3) represents the electrical performance of the simulated solar cell as obtained from SCAPS-1D under standard illumination conditions (AM 1.5G, 1000 W/m², 300 K). This curve provides essential information about the photovoltaic parameters and the overall device efficiency [18].

Under forward and reverse bias conditions, SCAPS calculates the current density (J) as a function of the applied voltage (V). From the resulting J-V curve, the key photovoltaic parameters are extracted:

Open-circuit voltage (V_OC): The voltage at which the current density becomes zero (J= 0).Short-circuit current density (J_SC): The current density at zero applied voltage (V = 0).Fill factor (FF): Indicates the “squareness” of the J-V curve and is calculated as:

where J m p and V m p are the current density and voltage at the maximum power point (MPP).

Power conversion efficiency (PCE η): The ratio of the output electrical power to the incident light power, expressed as

Figure 3. Short-circuit current density J_SC vs. voltage (V) curves for solar cells.

The shape of the J-V curve (Figure 3Figure 3) reflects the quality of the junction and the recombination mechanisms within the device. A well-behaved J-V curve shows a sharp knee near the maximum power point, indicating low series resistance and efficient carrier extraction. The curve’s shape reflects the quality of the interlayer connection and the recombination mechanisms within the cell; a steep curve near the maximum power point indicates low series resistance and efficient carrier extraction. From this curve, key photovoltaic parameters can be determined, such as open-circuit voltage (V_OC), short-circuit current density (J_SC), fill factor (FF), and photovoltaic conversion efficiency (PCE (η)).

The simulated J-V curve showed typical diode-like behavior with a photocurrent plateau under illumination. These curves were analyzed under simulated one sun. The device exhibited an excellent average PCE (η) of 6.56% with a short circuit current density (J_SC) of 16.518 mA/cm², an open circuit voltage (V_OC) = 0.772 V, and a fill factor (FF = 51.44%(. The value of the filling factor is related to several physical effects that affect the internal resistors, recombination rates, and p-n junction characteristics within the solar cell.

These values are in good agreement with experimental reports of CDs/TiO₂ solar cells, which typically achieve efficiencies in the range of 1% - 8% depending on synthesis method and interface quality [19].

Figure 4Figure 4 represents the quantum efficiency (QE) curve for CDs/TiO₂-based solar cells using SCAPS software, at room temperature. This curve represents the variations of the quantum efficiency as a function of wavelength from 0 to 600 nm.

Figure 4. Simulation EQE (%) vs. Wavelength (nm) curves for solar cells.

Figure 5Figure 5 shows the simulated current density (J_SC - Current Density - J) curve with voltage (V) for CDs/TiO₂-based solar cells at different temperatures using the SCAPS software. Changing the voltage from 0 V to 0.8 V yields different current densities (Current Density, J). Temperatures were varied from 100 K to 800 K to test the solar cell’s performance.

The effect of temperature between CDs and TiO₂ was found to play a critical role in recombination dynamics. As temperature increases, the short-circuit current density (J_SC) of CDs/TiO₂ composites changes due to variations in carrier

Figure 5. The effect of temperature on the simulated current density (J_SC) versus voltage (V) curves of CDs/TiO₂-based solar cells.

mobility and recombination rates. At low temperatures, the carrier mobility decreases, leading to a lower J_SC. While at high temperatures, a significant decline in J_SC appears because of enhanced recombination or partial material degradation. Moderate heating (up to 50˚C) enhances J_SC and QE by improving charge mobility and interface quality, while higher temperatures (>70˚C) induce recombination and reduce efficiency.

The Quantum Efficiency (QE) or Incident Photon-to-Current Efficiency (IPCE) increases significantly after incorporating carbon dots (CDs) into TiO₂. This enhancement broadens visible-light absorption (400 - 620 nm) and improves charge separation and interfacial stability, even under thermal stress. At moderate temperatures, improved carrier mobility and efficient charge transfer between CDs and TiO₂ dominate, enhancing both J_SC and QE. At higher temperatures, defect formation and recombination losses reduce the efficiency.

When the temperature increases, the open-circuit voltage decreases V_oc (Figure 6Figure 6). Electrons gain more thermal energy, which increases the rate of recombination inside the cell. This reduces the potential difference between the two terminals, and V_OC decreases. The short-circuit current slightly decreases (J_SC) at the higher temperature, increasing thermal losses and electron recombination, which causes the generated current to drop slightly. In summary, when the temperature increases, the efficiency of the solar cell decreases because both V_OC and J_sc decrease.

Figure 6. The impact of temperature on solar cell performance.

At high temperature, the efficiency (η) decreases (Figure 6Figure 6). This is due to the increase in recombination at a lower open circuit voltage (V_OC), and at a high internal resistance. This augmentation of temperature causes the fullness factor (FF) to decrease, which is due to the increased serial resistance and the deterioration of the quality of communication between classes.

On the other hand, efficiency (η) increases when temperature (T) decreases (Figure 6Figure 6). Because V_OC rises and decreases heat losses. In addition, the fullness factor (FF) changes because the internal resistance decreases. The decrease in the fill factor (FF) with increasing temperature is mainly attributed to the temperature-induced degradation of the electrical properties of the solar cell. As the temperature rises, the reverse saturation current increases significantly, leading to a noticeable reduction in V_oc. In addition, higher temperatures increase the series resistance and enhance carrier recombination, especially near the maximum power region. These combined effects reduce the squareness of the J-V curve and result in a lower FF.

This study shows that high defects reduced both V_OC and FF, while significantly increasing efficiency (Figure 6Figure 6). So, we demonstrate enhanced efficiency when CDs are incorporated into TiO₂-based solar cells, supporting the other results [20][21]. The synergy between CDs and TiO₂ lies in improved visible-light absorption and efficient charge transfer across the heterojunction.