Impact of PEDOT: PSS Electrode Work Function on DPPEZnP-TBO Based Active Layer in Tandem Organic Solar Cells

Introduction

Population growth and the post-COVID industrial resurgence have led to a significant increase in energy demand. Researchers have suggested a variety of renewable energy sources to lessen reliance on fossil fuels, with solar energy emerging as the most favoured choice^1-4. In recent decades, organic photovoltaic technology has garnered growing interest. Organic solar cells (OSCs) serve as a cost-efficient substitute for standard inorganic solar cells^1,2. OSCs’ demand lies in their versatility, flexibility, lightweight design, and affordability^5-8. To produce low-cost organic solar cells, it is highly desirable to eliminate metal electrodes and indium-tin-oxide (ITO), due to the high cost of ITO and its limited mechanical flexibility^{6, 9}. Additionally, the deposition of metal electrodes requires high-vacuum environments, which are not conducive to cost-effective and large-scale production. Research on electrode work function is crucial as it governs charge extraction through proper energy level alignment between electrodes and active layers¹⁰. Proper alignment minimises interfacial barriers and enhances carrier collection in tandem cells. Organic solar cell devices are more stable and efficient when their work functions are optimised.  Comparative summary of solar cell devices employing ITO/PEDOT: PSS-based electrodes, highlighting their fill factors and power conversion efficiencies in Table 1^11-15. Due to their superior transparency, high conductivity (above 1000 S/cm), and compatibility with solution-based processing methods like spin coating, spraying, and printing, conductive polymers, such as poly(3,4-ethylenedioxythiophene)–poly (styrene sulfonate) (PEDOT: PSS), have emerged as promising substitutes for ITO for bottom electrodes. PEDOT: PSS has also been utilised as a top electrode in organic solar cells with inverted structures ^16–18. PEDOT: PSS is appropriate for hole collection in organic solar cells since it usually shows a working function of about 5.0 eV ¹⁹. Nevertheless, it is still difficult to obtain a low work function (LWF) PEDOT: PSS for electron collecting ^20,21. In earlier methods, a ZnO layer was deposited on PEDOT: PSS to help with electron collection. According to recent research, organic compounds such as polyethylenimine (PEI) and polyethylenimine ethoxylated (PEIE) can successfully reduce the work function of different conductors, such as PEDOT: PSS. Researchers have created ITO- and metal-electrode-free organic solar cells utilising LWF-PEDOT: PSS, however, the device efficiencies have remained comparatively low, at about 3%²². The low fill factor (around 52%) resulting from PEDOT: PSS restricted conductivity is the main cause of this low efficiency value. According to recent research, sulphuric acid post-treatment can significantly raise PEDOT: PSS conductivity to over 2000 S/cm ¹⁹. This improvement has the potential to increase the efficiency of organic photovoltaic cells that use PEDOT: PSS as the top and bottom electrodes. Furthermore, although spin coating has historically been used to apply PEI and PEIE modifications to reduce the work function of PEDOT: PSS, this approach is not optimal for large-area fabrication²³. Developing scalable methods to incorporate these materials will be crucial for advancing the performance and scalability of organic photovoltaic cells.

In this study, we simulate the performance of an organic solar cell device structure with ITO/PEDOT: PSS2/DPPEZnP-TBO2/ ZnO /PEDOT: PSS1/DPPEZnP-TBO1/Al layers at 300 K. The performance of the device is analysed using the Fluxim-SETFOS simulation software. We also study the open-circuit voltage (V_OC), short-circuit current density (J_SC), fill factor (FF), and power conversion efficiency (PCE) of OSCs to show how the electrode’s work function affects their performance.

Table 1. Comparison of PEDOT: PSS-based electrode solar cell devices with corresponding fill factor (FF) and power conversion efficiency (PCE).

Theory

In this work, we numerically model and investigate the electrical characteristics of organic solar cells using the commercially available program SETFOS (Fluxim), with an emphasis on how work function affects fill factor (FF) and power conversion efficiency (PCE) ²⁴. SETFOS provides a comprehensive investigation of charge creation, transport, and recombination mechanisms by simulating the electrical behaviour of solar systems using mathematical models. In organic photovoltaic cells, incident light creates excitons, which dissociate into free electrons and holes. The semiconductor continuity equations for electrons and holes, which are represented as follows ^25,26, describe how these charges flow.

In these equations, ∇ ⃑.J_n˜ is the divergence of electron current density, ∇ ⃑.J_p˜ is the divergence of hole current density, n corresponds to the densities of electrons, and p represents the densities of holes. The Langevin recombination rate is denoted by R_Langevin, while the corresponding recombination rates are indicated by R_nt and R_pt. The final terms in equations (1) and (2) account for optical charge generation, where g_np corresponds to the generation efficiency. These equations describe the behaviour of charge carriers and are coupled with Poisson’s equation.

Here, q is the elementary charge, ε is the relative permittivity, and ε₀ is the vacuum permittivity,  p and n are the free hole and electron concentration, p_t is the trapped hole concentration, n_t is the trapped electron concentration, E˜ represents the electric field, and A_doping and D_doping​ represent the acceptor and donor doping concentrations, respectively.

The Langevin recombination model describes the bimolecular recombination process in organic semiconductors (R_Langevin​), given by

The mobilities of electrons (μₙ) and holes (μₚ), as well as the concentrations of local charge carriers dictate the Langevin recombination rate. In this case, η is the Langevin recombination process efficiency and n_i is the intrinsic carrier concentration at thermal equilibrium.

The trap rate equation, which simulates the exchange of trapped carriers with energy levels in the organic semiconductor, describes the processes of trapping and de-trapping. For example, an electron trap interacts with the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO), the levels at rates ​ and , respectively, facilitating the exchange of electrons or holes²⁷.

This equation describes Shockley–Read–Hall (SRH) recombination, a process in which free electrons are captured by traps and then recombine with free holes.

These simulations highlight the significant impact of the electrode work function on the performance metrics of organic solar cells. The results highlight how proper energy level alignment at the electrode interfaces minimises charge extraction barriers and recombination losses, thereby enhancing the power conversion efficiency (PCE) and fill factor (FF). This emphasises the pivotal role of electrode work function engineering in optimising device performance and guiding the design of high-efficiency, scalable organic solar cells.

Material and Methods

The electrical parameters employed in our simulations for the ITO/PEDOT: PSS2/DPPEZnP-TBO2/ ZnO /PEDOT: PSS1/DPPEZnP-TBO1/Al structure are presented in Figure 1(a). We simulated the performance of an organic solar cell with the following layer structure: ITO, PEDOT: PSS, DPPEZnP-TBO, ZnO, PEDOT:  PSS, DPPEZnP-TBO, and aluminium (Al). In this configuration, ITO and PEDOT: PSS serve as the front electrode, while Al serves as the back electrode. The cell’s performance was evaluated using both flexible and rigid substrates. In this tandem OSC structure, ITO/PEDOT: PSS2/DPPEZnP-TBO2/ZnO forms the front cell, while PEDOT: PSS1/DPPEZnP-TBO1/Al constitutes the rear cell. The two sub-cells are connected through the ZnO/PEDOT: PSS1 interlayer, enabling efficient charge recombination and current matching. PEDOT: PSS layers act as charge transport and interface modification layers with position-dependent functions. PEDOT: PSS2 (near ITO) serves as a hole transport and anode buffer layer, enhancing ITO’s work function and facilitating hole extraction. PEDOT: PSS1 (between ZnO and DPPEZnP-TBO1) operates as an interfacial recombination layer in the tandem structure. It enables efficient charge recombination and ensures proper energy alignment, thereby improving device performance.

DPPEZnP-TBO2 and DPPEZnP-TBO1 are abbreviated as layer 2 and layer 1, respectively, which act as an active layer. Figure 1(b) illustrates the energy band alignment of the layers in the organic solar cell. It includes the energy band gap values for PEDOT: PSS, DPPEZnP-TBO, and ZnO. This alignment is crucial for understanding charge transport and efficiency in the device. DPPEZnP-TBO serves as the active layer, responsible for photon absorption, charge separation, and conducting charges to the electrodes. These include crucial measurements including fill factor (FF), power conversion efficiency (PCE), open-circuit voltage (VOC), and short-circuit current density (JSC). Table 2 displays the material characteristics used in the simulation of tandem organic solar cell architectures^28-30.

Figure 1: shows (a) the organic cell structure represented by a layer stack and (b) the energy band alignment of different layers in the OSC prior to contact. Click here to View Figure

Table 2: Simulation input parameters corresponding to the materials in tandem organic solar cell structures.

 Results and Discussions

Charge Dynamics

Figure 2(a) presents a schematic energy level diagram illustrating the energy levels of a semiconductor in contact with electrodes. The LUMO, HOMO, electron Fermi levels, and hole Fermi levels are represented by the colours red, blue, green, and violet, respectively. It draws attention to the Fermi energy levels for electrons (E_Fn) and holes (E_Fp), as well as the Lowest Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HOMO). When a semiconductor interacts with electrodes, its energy levels align relative to the electrodes’ Fermi levels³¹. This alignment governs charge carrier (electron and hole) movement and significantly impacts the semiconductor’s electrical properties, such as current conduction and junction characteristics. The diagram provides a clear visual representation of these energy levels and their interactions, offering insights into the semiconductor’s behaviour with electrodes. In organic semiconductors, charge transport primarily occurs through thermally assisted tunnelling, also known as hopping, between individual molecules³². This involves thermally activated tunnelling, where thermal energy helps carriers overcome energy barriers between adjacent molecules. Figure 2(a) also demonstrates how the HOMO, LUMO, and Fermi energy levels change layer by layer. As the barrier potential between the layers decreases, the contact type transitions from non-Ohmic to Ohmic. In organic solar cells, the interface between the electrode and the active layer plays a crucial role in charge transport³³. Ohmic contacts are formed when the electrode’s work function aligns closely with the HOMO (for hole transport) or the LUMO (for electron transport) of the organic material, enabling efficient charge flow and minimising energy loss. Conversely, non-Ohmic contacts result from significant energy barriers, which hinder charge transfer and reduce performance. Electrodes with low work functions (e.g., aluminium) are well-suited for electron transport through the LUMO. In contrast, high work function materials (e.g., gold or ITO with PEDOT: PSS) align better with the HOMO for hole transport³⁴. By carefully selecting electrode materials to match their work functions with the active layer’s energy levels, barriers can be minimised, leading to Ohmic contacts and enhanced efficiency in organic solar cells. The colour diagram, visually shown in figure 2 (b) the variation in energy levels, highlighting distinct phases of charge transport. The colour gradients in the diagram indicate transitions between HOMO, LUMO, E_Fn, and E_Fp, emphasising the alignment or misalignment of energy levels across layers.

Figure 2(a): Schematic energy level diagram of ITO/PEDOT: PSS2/DPPEZnP-TBO2/ ZnO /PEDOT: PSS1/DPPEZnP-TBO1/Al organic solar cell device structure. Click here to View Figure

Figure 2(b): Band diagram of (a) HOMO (b) LUMO, (c) EFN and (d) EFP Click here to View Figure

Figure 3: A representative layer-wise illustration of (a) charge generation and (b) charge density distribution in an ITO/PEDOT: PSS2/DPPEZnP-TBO2/ZnO/PEDOT: PSS1/DPPEZnP-TBO1/Al solar cell structure. Click here to View Figure

Figure 3 presents a layer-wise representation of (a) charge generation and (b) charge density distribution within the ITO/PEDOT: PSS2/DPPEZnP-TBO2/ ZnO /PEDOT: PSS1/DPPEZnP-TBO1/Al solar cell structure. The ITO layer acts as the transparent conductive electrode, enabling light to pass through to the active layers without significant charge generation. PEDOT: PSS2 Layer acts as the hole transport layer, as well as improves the work function matching between the ITO electrode and the DPPEZnP-TBO2 layer. Charge generation here is minimal due to its primary role in facilitating hole extraction rather than light absorption. The DPPEZnP-TBO (DPPEZnP2) layer is the first active layer where light absorption begins. Charge generation starts increasing and remains high. The ZnO Layer acts as the electron transport layer. Charge generation in this layer is very low, as it primarily facilitates electron mobility rather than light absorption. DPPEZnP-TBO (DPPEZnP1) Layer is the second active layer, where the significant light absorption occurs. Charge generation is moderate in this layer due to its optimised properties for absorbing photons and generating charge carriers. Charge generation decreases linearly from the DPPEZnP1 layer towards this layer, as fewer photons remain to be absorbed. The Aluminium Layer serves as the back electrode for charge collection.

In Figure 3(b), the charge density is higher in both active layers (DPPEZnP-TBO). The ZnO layer shows the highest electron density, while the PEDOT: PSS1 layer shows the highest hole density. Since the ZnO layer acts as the electron transport layer, it efficiently facilitates electron movement toward the electrode.

Figure 4: A representative layer-wise illustration of (a) recombination and (b) layer-wise colour map of recombination in ITO/PEDOT: PSS2/DPPEZnP-TBO2/ ZnO /PEDOT: PSS1/DPPEZnP-TBO1/Al solar cell structure. Click here to View Figure

Figure 4 provides a layer-wise illustration of (a) recombination and (b) layer-wise distribution of recombination using a colour map within the ITO/PEDOT: PSS2/DPPEZnP-TBO2/ ZnO /PEDOT: PSS1/DPPEZnP-TBO1/Al solar cell, highlighting regions of charge carrier loss. These results help to identify how the layers impact device performance. The recombination rate is significantly high in the DPPEZnP-TBO layers (active layers), where most charge generation occurs, leading to greater carrier interactions. In contrast, recombination is minimal in the ZnO and PEDOT: PSS layers due to their transport-dominated roles. Figure 4(b) shows that the highest recombination occurs in the range of thickness from 0 to 50 nm and 70 to 125 nm. When PEDOT: PSS serves as the electrode, carrier recombination significantly impacts key performance metrics.

Diffusion-driven carriers may recombine before reaching the contacts, resulting in a slight reduction in the short-circuit current density (JSC)³⁵. Under open-circuit conditions, carrier accumulation at the electrodes creates a potential difference defining the open-circuit voltage (V_OC)³⁶. Recombination narrows the separation between the electron and hole quasi-Fermi levels (E_Fn and E_Fp), lowering V_OC. The PEDOT: PSS work function strongly influences recombination rates and overall performance, as a stronger electric field across the absorber enhances charge separation and reduces recombination.

Figure 5: Current vs voltage characteristics of OSC with different work functions. Click here to View Figure

Current density (J) vs voltage (V) Analysis

Figure 5 presents the current density (J) vs voltage (V) characteristics of an ITO/PEDOT: PSS2/DPPEZnP-TBO2/ ZnO /PEDOT: PSS1/DPPEZnP-TBO1/Al organic solar cell.  The J-V characteristics of organic solar cells (OSC) are strongly influenced by the work function of the electrodes because the work function determines the energy alignment between the electrodes and the organic material layers³⁷. The work function is the energy required to move an electron from the electrode into the vacuum level, and this alignment plays a crucial role in charge injection and extraction. The work function of the electrode affects the ability of the electrode to inject charge carriers (electrons or holes) into the organic layers³⁸. For efficient charge injection, the electrode’s work function must align with the energy levels of the organic material’s conduction or valence bands (LUMO for electrons, HOMO for holes). If the electrode’s work function is too high or too low compared to the organic material’s energy levels, a significant energy barrier forms, hindering charge injection³⁹. After charge carriers are generated in the organic material by light absorption, they must be extracted by the electrodes. The electrode work function influences how easily charge carriers are extracted from the organic layer. If the electrode’s work function is well-matched with the energy levels of the organic layer, charge extraction is efficient, and the J-V characteristics improve.

As shown in Figure 6, the open-circuit voltage (V_OC) increases with the PEDOT: PSS work function after it exceeds 4.2 eV, reaching a stable value of approximately 0.56 V. In contrast, the short-circuit current density (J_SC) follows the same trend. The J_SC value stabilises up to a work function of 4.4 eV, but beyond this point, it increases rapidly. In organic solar cells, the electrical properties are highly dependent on the energy level alignment between the electrode materials and the organic layers. As the PEDOT: PSS work function increases, the alignment between the electrode and the active layer improves, leading to a higher V_OC due to better charge separation. J_SC increases with increasing work function because a higher work function improves charge extraction by reducing the energy barrier at the electrode–active layer interface³⁸.

Figure 6: Open circuit voltage (VOC), Short circuit current density (JSC) of OSC with different work functions.  Click here to View Figure

Figure 7: Fill factor (FF), Power conversion efficiency (PCE) of OSC with different work functions.   Click here to View table

Figure 7 shows that the fill Factor (FF) and power conversion efficiency (PCE) of an Organic Solar Cell (OSC) with varying anode work functions. Figure 7 shows that increasing the anode work function raises the hole density at the ITO/PEDOT: PSS interface (x = 0), thereby lowering the contact resistance. This inverse correlation between contact resistance and charge density at the electrode interface aligns with findings from previous studies⁴¹. Consequently, a reduction in series resistance is observed, leading to an improvement in the fill factor as the anode work function increases, thereby enhancing charge carrier extraction and overall device performance. The graph also shows the correlation between fill factor (FF) and power conversion efficiency (PCE) of the Organic Solar Cell (OSC) at different work function values. For work functions up to 4.4 eV, both the efficiency and fill factor remain stable, indicating consistent performance. Beyond 4.4 eV, both parameters increase linearly, suggesting that higher work functions further enhance charge carrier extraction and improve the device’s overall performance.

Conclusions

This study designs and analyses a planar ITO/PEDOT: PSS2/DPPEZnP-TBO2/ ZnO /PEDOT: PSS1/DPPEZnP-TBO1/Al organic solar cell using SETFOS to examine the effect of PEDOT: PSS electrode work function. Results show that optimising work function alignment between front and rear electrodes significantly impacts V_OC, J_SC, and FF, highlighting its crucial role in improving DPPEZnP-TBO tandem OSC performance. By varying the work function, both V_OC and J_SC were observed to change significantly with an increase in the work function. The best performance was achieved at a work function of 5.2 eV, resulting in a V_OC of 1.16 V, J_SC of 2.598 mA/cm², FF of 79.6%, and a power conversion efficiency (PCE) of 2.41%. These results suggest that optimising the electrode work function enhances charge mobility, minimises recombination losses, and improves overall device performance. This study offers key insights for enhancing the efficiency, stability, and scalability of DPPEZnP-TBO organic solar cells, supporting their potential for large-scale solar energy applications.

Acknowledgement

The authors would like to acknowledge Laxmi Narayan Dubey College, Motihari, and B.R. Ambedkar Bihar University, Muzaffarpur, for providing the simulation software, Fluxim-SETFOS.

Funding Sources

The author(s) received no financial support for the research, authorship, and/or publication of this article

Conflict of Interest

The authors do not have any conflict of interest

Data Availability Statement

This statement does not apply to this article

Ethics Statement

This research did not involve human participants, animal subjects, or any material that requires ethical approval

Informed Consent Statement

This study did not involve human participants, and therefore, informed consent was not required

Author Contributions

Pinaki Laha: Writing – review and editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Validation, Software.

Sanjay Narayan Barman: Investigation, Formal analysis, Data curation, Validation, Software.

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