Numerical device modeling for direct Z-scheme junctions using a solar cell simulator (2023)

In 1972, Fujishima and Honda reported that the semiconductor TiO₂ can be used as a photocatalyst in water to produce Hydrogen [5]. This led to extensive research and development in the area of semiconductor catalysts to improve their photocatalytic efficiency and deployment in various applications especially in the sustainability sector [6].

These catalysts are used in various applications. It is reported that the material has usage in applications such as wastewater treatment [7], [8], [9], hydrogen production [10], [11], [12], CO₂ reduction [13], [14], [15], [16] and air purification [17], [18], [19]. There are various ways in which an established photocatalyst can be improved using dye anchoring, metal deposition, heterogeneous composites, doping, surface adsorbates and hybrids with nano-materials [6].

Although TiO₂ can facilitate water splitting into H₂ and O₂, its large bandgap of 3.2 eV allows it to utilize the UV part of the spectrum, which results in a low spectral utilization. Hence, novel semiconductor materials with lower band gap and efficient charge separation need to be explored as candidates to improve this spectral utilization [20]. $O+e\rightleftarrows R{V}_{redox}^{\circ }$

In Eq. (1), we take the case of a redox reaction of reactant “O” and product “R” at potential $V_{redox}^{\circ }$ (in volts against the normal hydrogen electrode (NHE)). This can be measured in terms of energy levels $E_{redox}$ (denoted in electron volts against vacuum) using the relationship as: $E_{redox}=constant-e{V}_{redox}^{\circ }$

Where the $constant$ value is between −4.48 eV and $e$ is the unit charge [21].

The semiconductor catalyst must supply charge carriers that are (i) electrons, at a higher energy level than their reduction level and (ii) holes are supplied lower than the reaction oxidation energy level. Moreover, extra energy is needed to overcome the reaction overpotentials. Hence, using these semiconductor catalysts we can conduct reactions in a non-spontaneous direction by adding irradiant energy and/or electrical energy that can be converted to chemical energy as fuel [21]. This is ideal for the current demand for emission free production methods of chemical synthesis.

Z-scheme junctions are composite semiconductors that use a two-photon excitation method to achieve higher charge separation similar to photosynthesis in plants. It addresses the issue with single component catalysts like TiO₂ which requires a large bandgap for high charge separation but at the expense of low spectral utilization. These features are mutually exclusive and Z-scheme junctions can in principle overcome this. However, further studies must be carried out to improve their stability, light harvesting, charge separation and transportation to reach economic viability and physical/chemical understanding [22].

Direct Z-scheme semiconductor materials or photosystem (PS-PS) systems are composite semiconductors. This structure is intended to be similar to a tandem solar cell which utilizes junctions of semiconductor materials of different energy band gaps and electron affinities to enhance the solar cell’s spectral utilization and produce high voltage outputs i.e. better charge separation. However, in this report, the working of a dual n-type Z-scheme junction is elaborated and is quite different from that of a tandem solar cell.

This paper demonstrates the use of solar cell simulators to model the effectiveness of charge carrier separation in a photocatalyst. The charge carriers are used in the reduction and oxidation sites of the semiconductor catalyst while in a traditional solar cell these carriers contribute directly to electric power generation. Consequently solving the semiconductor equations to describe the carrier kinetics in both systems is equivalent. Moreover, the performance parameters of the photocatalyst as a solar cell will reveal the expectations of the structure under different bias and illumination conditions. This study proposes the use of solar cell simulator SCAPS to simulate direct Z-scheme junctions. For such a Z-scheme heterojunction direct recombination at the interface from both sides needs to be included [23].

It is crucial to engineer photocatalysts that can prevent bulk charge recombination and as such improve the light-harvesting efficiency of the system. Equivalent to thin-film photovoltaic technology, many options to improve water splitting and CO₂ reduction catalysts have been proposed: bandgap engineering, crystal facet engineering and surface heterojunction optimization [24].

Although characterization of Z-scheme junctions using experimental methods such as photo reduction testing, radical species trapping, metal loading and X-ray photoelectron spectroscopy (XPS) are present [25]. These techniques do not establish a direct link between the experimental parameters and the final device efficiency of converting irradiation to chemical energy. Simulations can address these issues by using the experimental data from characterization and estimating the impact on efficiency by evaluating the charge carrier migration in the photo-catalyst [25].

In this paper, a method to solve the semiconductor equations for a Z-scheme heterojunction is presented. This is done by including defect traps at the interface to allow the photo-generated low energy carriers to recombine, thereby achieving high open circuit voltage $\left({\mathrm{V}}_{\mathrm{OC}}\right)$ or charge separation compared to the oxidation or reduction potentials of the desired product.

Solar cell capacitance simulator (SCAPS 1-D) was developed at Ghent University and is available for the research community [26]. It is used to simulate the performance of thin film solar cells and can simulate conventional characterization techniques. The program was developed initially for structures such as CuInSe₂ and CdTe cells, but due to the generality of the semiconductor equations that are solved other structures can be modeled.

The simulator has user-friendly and a graphical interface that gives output data that is useful for the catalyst research and development community. A key reason why SCAPS is used over other solar simulators is because of its ability to include a recombination model based on defects with different properties that are desired in the working of this model and is elaborated further in Section 2.2.

Methodology

The primary objective of this study is to evaluate the direct Z-scheme device performance. This is indicated by the efficient conversion of sunlight to electric power or charge carriers that can be used for redox reactions. The three main solar cell parameters used to describe the desired characteristics are:

• •

  the efficiency ($\mathrm{\eta }$) to indicate the effective conversion of sunlight to generated power.

• •

  current density at maximum power point (J_MPP) to indicate the number of carriers to oxidation and

  (Video) Solar Cell Simulation: Absorption, Drift-Diffusion and Advanced Optics with Setfos

Results and discussion

SCAPS models the Z-scheme junction at a given irradiation and gives the corresponding current density-voltage (JV) curve. The solar cell parameters are estimated using this JV curve. The energy bands can be simulated at a given voltage. It is useful in establishing whether the junction modeled is indeed a Z-scheme junction.

Conclusion

Z-scheme semiconductor materials have desirable optoelectrical properties that make them viable chemical catalysts for reduction and oxidation reactions. As there are various combinations of semiconductor layers possible for this composite material, the development of a screening model was imperative to evaluate the feasibility of the material and the desired application. It can be used as a primary study tool to evaluate various semiconductor candidates that can be used for charge carrier

CRediT authorship contribution statement

Nithin Thomas Jacob: Developed and modeled the Z-scheme junction, Writing of the manuscript. Jeroen Lauwaert: Provided expert insights, Proofreading of the manuscript. Bart Vermang: Provided expert insights, Proofreading of the manuscript, Secured funding for this project. Johan Lauwaert: Conceived the original idea, Developed and modeled the Z-scheme junction, Writing of the manuscript, Provided expert insights, Proofreading of the manuscript, Secured funding for this project.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to acknowledge Catalisti VLAIO (Vlaanderen Agentschap Innoveren & Ondernemen) for their funding through the Moonshot SYN-CAT project (HBC.2020.2614). The funder played no role in the study design, data collection, analysis and interpretation of data, or the writing of this manuscript. The cooperation of the consortium partners is acknowledged and appreciated. All authors read and approved the final manuscript.