Interface Electrostatics of Solid-State Dye-Sensitized Solar Cells: A Joint Drift-Diffusion and Density Functional Theory Study

Abstract

Dye-sensitized solar cells (DSCs) have gained great attention in recent years due to their low-cost fabrication, flexibility, and high power conversion efficiency. In a DSC, due to interfaces between the dye and the charge-transport materials, the interface electrostatics becomes a key factor determining the overall performance of the cell. Liquid-electrolyte-based DSCs suffer from low stability, electrolyte leakage, and, in some cases, electrode corrosion. Replacing liquid electrolyte with a solid semiconducting material leads to poor interfacial contacts, hence the interface electrostatics becomes one of the limiting factors. In this work, we present a drift-diffusion and density functional theory (DFT) study of solid-state DSCs to investigate the electrostatics at the TiO2/organic dye/Spiro-OMeTAD interface and its impact on the adsorbed dye energy levels, its absorption spectrum, and the related charge injection. In our three-dimensional drift-diffusion model, we solve a set of drift-diffusion equations coupled to Poisson equation for electrons, holes, doping impurities, and interface traps simultaneously. After that, we use first-principles DFT modeling of dye-sensitized interfaces in the presence of the calculated electric fields. We find that interface traps located below the conduction band edge of mesoporous TiO2 influence the accumulation of photogenerated holes and built-in electric field near the interface. The built-in electric field leads to change in the energetics at the dye/TiO2 interface, leading to poor charge injection from excited dye into TiO2. The simulations were carried out for different electronic trap densities in TiO2 and different doping levels in the Spiro-OMeTAD hole-transport layer. This study helps to a better understanding of the interface electrostatics and its role in the charge injection mechanism of solid-state DSCs.

Recent advances in eco-friendly and cost-effective materials towards sustainable dye-sensitized solar cells

“…On the other hand, this solid-state compound led to poor interfacial contacts, causing interface electrostatics to become one of the limiting factors for reproducible, efficient and stable devices. 277 However, the most important issues regarding Spiro-OMeTAD are related to its sensitivity to both moisture and oxygen and its high production costs, mainly due to its lengthy synthesis protocol, which is low yield with a considerable impact in terms of solvents used for the separation and purification steps. 278,279 Furthermore, it necessitates the addition of additives based on lithium and/or cobalt (a well-known CRM) to boost its conductivity and hole mobility, thus making this system far from the sustainability targets of the DSSC scenario.…”

Application of carotenoids in sustainable energy and green electronics

“…They observed that due to the presence of carboxylic groups in the molecule, crocetin shows increased binding affinity to the porous TiO 2 surface. Increased TiO 2 dye binding results in increased pore-filling and efficient electron injection from crocetin molecules to TiO 2 91,92 . Short-circuit photocurrents of 2.84 mA and 0.45 mA were reported for the cells based on crocetin and crocin, respectively.…”

“…They obtained poor DSC efficiency (0.076%), which could be due to poor binding of βcarotene to the TiO 2 surface as β -carotene does not have carboxylic or hydroxyl groups. Poor pore filling and poor binding of dye molecules to the TiO 2 surface results in poor electron injection from the β -carotene dye to TiO 2 71,91,92 . In another study, Eka et al 70 extracted carotenoids from Citrus medica var Lemon and Musa aromatica and used them as photosensitizers for DSCs with TiO 2 Nanoparticle.…”

“…TiO 2 exhibit very fast eh + recombination (i.e., < 10 ns). Moreover, exponential trap tails near the surface of TiO 2 can lead to recombination of the charges injected by other layers 91,106,107 . In this case, bulk and surface modification of TiO 2 can help to improve charge injection, charge collection, and charge transport in general 109,110 .…”

Device simulation of all-perovskite four-terminal tandem solar cells: towards 33% efficiency

“…A finite-element drift-diffusion model is implemented in TiberCAD [46,47]. The model can perform 3-dimensional simulations, and has already been used to simulate organic solar cells [48], organic field-effect transistors [49], PSCs [14,50] and dye-sensitized solar cells [51]. Within the model, Poisson and drift-diffusion equations are solved simultaneously for electrons, holes, and trapping centers, considering the light absorption and the charge carrier generation-recombination processes.…”