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


Due to the enormous emissions of greenhouse gases such as CO2 which trap heat on earth and insulate it from the cold of space, the threat of climate change is imminent.

To reduce the CO2 emissions, various Carbon Capture and Utilization (CCU) technologies have been emerging. [1]. In CCU, the captured CO2 is reduced into products such as methanol that can be used in transportation fuels [2]. This promotes a carbon neutral economy by chemical reduction of captured CO2 from the atmosphere to value products using renewable energy sources and decreasing reliance on fossil fuels for their production.

The reduction of CO2 is thermodynamically and kinetically challenging as it consists of the breaking of two CNumerical device modeling for direct Z-scheme junctions using a solar cell simulator (1)O bonds that have a bond dissociation energy of 750 kJ/mol [3]. Catalysts can facilitate these reactions to synthesize high-value products such as dimethyl ether (DME), olefins and higher alcohols without relying on conventional energy sources [4].

Semiconductor catalysts are good candidates for this application. It has been reported that homogeneous catalysts for electro-, photo- and photoelectrocatalytic CO2 reduction reactions offer higher activities and selectivities than heterogeneous counterparts. However, the former is quite difficult to separate from the reaction mixture, thereby making them unsuitable for re-use, which is disadvantageous in terms of sustainability [3].

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].

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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+eRVredox

In Eq. (1), we take the case of a redox reaction of reactant “O” and product “R” at potential Vredox (in volts against the normal hydrogen electrode (NHE)). This can be measured in terms of energy levels Eredox (denoted in electron volts against vacuum) using the relationship as: Eredox=constanteVredox

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].

(Video) Solar Cells Lecture 3: Modeling and Simulation of Photovoltaic Devices and Systems

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 (VOC) 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.

(Video) Modelling and Simulation of Thin Film-based or Perovskite Solar Cell Using SCAPS Software

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.

Section snippets


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 (η) to indicate the effective conversion of sunlight to generated power.

  • current density at maximum power point (JMPP) 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.


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.


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.

© 2023 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.



What is Z scheme photocatalysts? ›

The artificial Z-scheme photocatalyst usually consists of two connected semiconductor photocatalysts: one is oxidation photocatalyst and another is reduction photocatalyst. As shown in Fig. 2 (all energy potentials are expressed vs.

What is the principle of solar cell? ›

Solar cell works on the principle of photovoltaic effect according to which free electrons are produced when light of certain wavelength is incident on semiconductors.

What is the Z-scheme simplified? ›

The Z scheme shows the pathway of electron transfer from water to NADP+. Using this pathway, plants transform light energy into "electrical" energy (electron flow) and hence into chemical energy as reduced NADPH and ATP.

What are the steps of Z-scheme? ›

  • Fidelity in tRNA Aminoacylation.
  • Initiation of Protein Synthesis.
  • Elongation.
  • Termination of Translation.
  • Transcription and Translation.

How do solar cells work for dummies? ›

When the sun shines onto a solar panel, energy from the sunlight is absorbed by the PV cells in the panel. This energy creates electrical charges that move in response to an internal electrical field in the cell, causing electricity to flow.

What is the theory for solar cell experiment? ›

The theory of solar cells explains the process by which light energy in photons is converted into electric current when the photons strike a suitable semiconductor device.

What is direct Z-scheme? ›

Direct Z-scheme photocatalysts are composite semiconductor photocatalysts in which the charge transfer between the component semiconductors follows a Z-scheme pathway without the aid of additional electron mediators.

What is Z-scheme and how it is formed? ›

The “Z‐scheme” describes the oxidation/reduction changes during the light reactions of photosynthesis. In the Z‐scheme, electrons are removed from water (to the left) and then donated to the lower (non‐excited) oxidized form of P680.

What is the end product of Z-scheme? ›

An ATP synthase enzyme uses that chemiosmotic potential to make ATP during photophosphorylation, whereas NADPH is a product of the terminal redox reaction in the Z-scheme. So the correct option is ' only ATP'.

What is the first step of Z-scheme? ›

In the Z‐scheme, electrons are removed from water (to the left) and then donated to the lower (non‐excited) oxidized form of P680. Absorption of a photon excites P680 to P680*, which “jumps” to a more actively reducing species. P680* donates its electron to the quinone‐cytochrome bf chain, with proton pumping.

What is the Z-scheme connected with? ›

Z-scheme of the light-dependent reactions happen in the thylakoid membrane of the chloroplasts and occur in the presence of sunlight. The sunlight is converted to chemical energy during these reactions. It is concerned with electron transfer along with the synthesis of ATP and NADPH.

What is the biggest disadvantage with solar cells for power generation? ›

High initial costs for material and installation and long ROI (however, with the reduction in the cost of solar over the last 10 years, solar is becoming more cost feasible every day) Needs lots of space as efficiency is not 100% yet. No solar power at night so there is a need for a large battery bank.

What would be a problem with solar cell use? ›

When the panel's energy cannot flow through to your inverter, it becomes overloaded and radiate excess heat, so they get 'hot'. It is one of the most common problems with solar panels world-wide. Hot spots can reduce your solar panel's performance and lifespan and, in some cases, can even make them irreparable.

What are three applications of solar cells? ›

Photovoltaic Applications
  • Solar Farms. Many acres of PV panels can provide utility-scale power—from tens of megawatts to more than a gigawatt of electricity. ...
  • Remote Locations. ...
  • Stand-Alone Power. ...
  • Power in Space. ...
  • Building-Related Needs. ...
  • Military Uses. ...
  • Transportation.

What are two limitations for using solar cells all over the world? ›

Limitations of Solar Energy: Can be harnessed only at those places which get plenty of sunlight. Cannot be harnessed beyond certain latitudes. Cannot be harnessed during night.

Which type of solar cells are currently the most efficient? ›

Monocrystalline solar panels are often considered the most efficient solar panel option. Therefore, they are typically installed for larger energy systems in commercial and residential properties.

What was the conclusion of the solar cell experiment? ›

In conclusion, our first experiment showed that the more far the light from the solar cell, less electricity is produced. The second experiment showed that different filters have different effects on voltage; for example, the blue one had less resistance to the light.

What are the two types of solar panel technology? ›

Two major technologies have been developed to harness it:
  • Photovoltaic solar technology, which directly converts sunlight into electricity using panels made of semiconductor cells.
  • Solar thermal technology, which captures the sun's heat. This heat is used directly or converted into. mechanical energy.
Feb 22, 2022

How solar cell converts solar energy to electrical energy? ›

The solar cell converts the solar energy directly into electrical energy with the help of photovoltaic effect. Note: The solar light when incident on the cell makes the electrons move to the excited state due to which they tend to move through the material and the electric current is therefore generated.

What is the Z-scheme of photosynthetic electron flow? ›

Z-scheme is a diagrammatic representation of the electron flow in non-cyclic phosphorylation, showing the change in energy potential of the electrons. The electrons lost by P680 (PS-II) are taken up by P700 (PS-I) and do not get back to P680 i.e., unidirectional and hence, it is called non- cyclic phosphorylation.

What is the difference between Z-scheme and S scheme photocatalyst? ›

As for the difference between S-scheme and Z-scheme photocatalyst, the S-scheme photocatalyst is usually composed of two n-type semiconductors, while the Z-scheme is usually composed of n-type and p-type semiconductors.

What is the Z-scheme of artificial photosynthesis? ›

Z-scheme photocatalytic system is one of the photocatalytic hydrogen evolution systems under visible light irradiation by introducing heterogeneous semiconductors, which applies a two-step excitation mechanism using two different photocatalysts.

How does the Z-scheme describe the light reactions of photosynthesis? ›

The “Z‐scheme” describes the oxidation/reduction changes during the light reactions of photosynthesis. In the Z‐scheme, electrons are removed from water (to the left) and then donated to the lower (non‐excited) oxidized form of P680.

What does the Z scheme for electron transport in photosynthesis reach NADP and reduce it to? ›

Electrons released from water finally reach NADP+ through PSII and PSI. Thus NADP+ is reduced to NADPH + H+.

What are the products of Z scheme photosynthesis? ›

An ATP synthase enzyme uses that chemiosmotic potential to make ATP during photophosphorylation, whereas NADPH is a product of the terminal redox reaction in the Z-scheme. So the correct option is ' only ATP'.

Which is the correct sequence of electron transport in Z scheme? ›

In z - scheme, PSII is excited first and releases an electron. This electron is accepted by an e− acceptor, plastoquinone and passed to the electron transport system (ETS) and then to plastocyanin which transfers electrons to PSI. PSI excites and releases electron which is finally accepted by NADP+ through ferredoxin.

What are the advantages of Z-scheme? ›

The Z-scheme heterojunction has a high separation efficiency of electron–hole pairs with strong redox ability and a wide light response range. The abovementioned advantages make the Z-scheme heterojunction provide a great opportunity for the conversion of CO2 to value-added chemicals.

What are the advantages of ZnO as photocatalyst? ›

ZnO as a photocatalyst afforded several advantages such as being photocatalytically more active than others (Jang et al. 2006) and having excellent chemical and photochemical stability, non-toxicity (Amna et al. 2015; Pung ET AL. 2012), low cost and large exciton binding energy (60 meV) for nanoparticles (Mollwo 1982).

Why is photocatalysis best? ›

Photocatalysis as a green technology is essential to clean up water and environmental detoxification via visible light-induced photocatalysis and has various applications, such as CO2 reduction, organic contaminant degradation, removal of toxic ions and heavy metal ions, water-splitting, antibacterial, self-cleaning, ...

Where does the Z-scheme occurs? ›

Z-scheme of the light-dependent reactions happen in the thylakoid membrane of the chloroplasts and occur in the presence of sunlight. The sunlight is converted to chemical energy during these reactions.

What is the significance of Z scheme of light reaction? ›

Z scheme describes the oxidation or reduction changes that take place in light reactions of photosynthesis. Here, both the photosystems are involved in the transfer of an electron, the transfer of electron resembles Z shape.

What is the equation of the light reaction of photosynthesis and explain where it takes place? ›

The chemical equation for photosynthesis is 6CO2+6H2O→C6H12O6+6O2. 6CO2+6H2O→C6H12O6+6O2. In plants, the process of photosynthesis takes place in the mesophyll of the leaves, inside the chloroplasts.

What is the basic function of the light reactions of photosynthesis the conversion of solar energy to chemical energy? ›

The overall purpose of the light-dependent reactions is to convert light energy into chemical energy. This chemical energy will be used by the Calvin cycle to fuel the assembly of sugar molecules. The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem.


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