Electrode materials play a critical role in the performance of organic photovoltaics (OPVs), influencing charge extraction, device efficiency, and stability. The choice of electrode must balance conductivity, transparency, and compatibility with the organic active layer. Key materials include transparent conductive oxides (TCOs), metal grids, and alternative conductors such as PEDOT:PSS. Each class of materials presents unique advantages and challenges, particularly concerning work function alignment and interfacial resistance.

Transparent conductive oxides are widely used as electrodes in OPVs due to their high transparency and conductivity. Indium tin oxide (ITO) is the most common TCO, offering a sheet resistance of around 10–20 Ω/sq and optical transparency exceeding 85% in the visible spectrum. However, ITO has limitations, including brittleness, high cost due to indium scarcity, and susceptibility to diffusion into organic layers. Alternatives such as fluorine-doped tin oxide (FTO) and aluminum-doped zinc oxide (AZO) have been explored, though their conductivities are generally lower than ITO. AZO, for instance, typically exhibits a sheet resistance of 30–50 Ω/sq but benefits from better stability and lower material costs. Work function matching is crucial for TCOs to minimize energy barriers at the electrode-active layer interface. ITO has a work function of approximately 4.7 eV, which aligns reasonably well with common organic semiconductor highest occupied molecular orbital (HOMO) levels. However, modifications such as oxygen plasma treatment or UV-ozone exposure can tune the work function to improve charge extraction.

Metal grids offer a solution to the trade-off between conductivity and transparency. Thin metallic grids, often made of silver or gold, provide high conductivity while maintaining transparency through sparse patterning. Silver grids can achieve sheet resistances below 1 Ω/sq with transparency above 80%, depending on grid design parameters such as line width and spacing. However, metal grids introduce optical losses due to diffraction and shadowing effects, which can reduce photocurrent generation. Additionally, the work function of metals like silver (4.3–4.7 eV) and gold (5.1–5.3 eV) must be carefully matched to the organic semiconductor to avoid large energy offsets. Interfacial resistance can arise from poor adhesion between the metal grid and the underlying substrate, necessitating adhesion layers like chromium or titanium, which may complicate fabrication.

Conductive polymers, particularly poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), serve as promising alternatives to TCOs and metal grids. PEDOT:PSS combines reasonable conductivity (up to 4000 S/cm for highly conductive formulations) with excellent mechanical flexibility and solution processability. Its work function (~5.0–5.2 eV) is well-suited for hole extraction in many OPV systems. However, PEDOT:PSS suffers from acidity and hygroscopicity, which can degrade device stability over time. Interfacial resistance can also be problematic due to inhomogeneous charge transport at the PEDOT:PSS-organic layer boundary. Additives like dimethyl sulfoxide (DMSO) or ethylene glycol are often incorporated to enhance conductivity, but these may affect film morphology and interfacial properties.

Other emerging electrode materials include carbon-based conductors like graphene and carbon nanotubes (CNTs). Graphene offers high transparency (>90%) and tunable work function (4.3–4.8 eV), but its sheet resistance remains higher than ITO (100–500 Ω/sq for single-layer films). CNT networks can achieve sheet resistances of 50–100 Ω/sq with 80–90% transparency but require doping to improve charge transport. Metal nanowire networks, particularly silver nanowires, provide low sheet resistance (<20 Ω/sq) and high transparency (>90%), but concerns persist regarding long-term stability and junction resistance between nanowires.

Work function engineering is essential to minimize energy losses at the electrode-organic layer interface. For hole-collecting electrodes, materials with work functions close to the HOMO level of the donor material are preferred, while electron-collecting electrodes should align with the acceptor’s lowest unoccupied molecular orbital (LUMO). Interfacial layers, such as metal oxides or conjugated polyelectrolytes, are often employed to bridge work function mismatches, though this topic extends into interface engineering and is not discussed here. Interfacial resistance arises from poor contact quality, chemical reactions, or morphological mismatches, all of which can impede charge extraction. Techniques such as thermal annealing, solvent treatment, or surface modification can mitigate these effects.

The mechanical properties of electrode materials also influence OPV performance, particularly for flexible devices. TCOs are brittle and prone to cracking under bending stress, whereas PEDOT:PSS and metal nanowires exhibit superior flexibility. Durability under environmental stressors like humidity and UV exposure is another consideration, with TCOs generally outperforming organic conductors in stability tests.

In summary, electrode selection for OPVs involves trade-offs between conductivity, transparency, work function alignment, and stability. Transparent conductive oxides like ITO remain benchmarks but face challenges related to cost and brittleness. Metal grids offer high conductivity but introduce optical losses, while PEDOT:PSS provides flexibility and solution processability at the expense of environmental stability. Emerging materials like graphene and metal nanowires show promise but require further development to match the performance of conventional options. Optimizing electrode materials necessitates careful consideration of interfacial resistance and energy level alignment to maximize charge extraction efficiency in organic photovoltaics.