Fine-Tuning Hole Collection Via Self-Assembled Monolayers for Advancing Versatile Platforms
Authors: Lee Min Jong , Shim Jae Won 
Published: December 2024

Recent advancements in transparent electronic devices have generated substantial research interest in transparent electrodes. Indium-tin-oxide (ITO) electrodes are widely utilized due to their high transmittance and low sheet resistance. However, their relatively low work function (WF) (< 5.0 eV) limits effective hole injection, necessitating methods to enhance the WF for improved energy alignment.

To overcome these challenges, self-assembled monolayer (SAM) treatment offers a promising solution by combining low-temperature deposition, precise adsorption, and film thickness control at the angstrom scale, enhancing the WF of electrode surfaces by adjusting the molecular polarity within the SAM. This method significantly improves interfaces in electronic devices, optimizing charge transport and reducing recombination.

This study investigates the effect of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) derivatives—MeO-2PACz, 2PACz, and Br-2PACz—on the WF of ITO electrodes. The derivatives adsorbed onto the ITO surface, forming interfacial dipoles through chemisorption, thereby increasing the WF. Theoretical calculations and experimental analyses confirmed these WF variations, demonstrating that Br-2PACz significantly increased the WF due to its higher dipole moment. These findings suggest potential applications of chemisorption and explore the feasibility of this approach across various domains.

The WF of ITO electrodes treated with 2PACz derivatives was measured using ultraviolet photoelectron spectroscopy and Kelvin probe, and validated through density functional theory calculations. The WF values for ITO, ITO/MeO-2PACz, ITO/2PACz, and ITO/Br-2PACz were 4.74, 4.81, 5.30, and 5.43 eV, respectively. This increase is attributed to the induced dipole moments of the derivatives. Atomic force microscopy showed that 2PACz derivatives treatment slightly reduced the surface roughness of ITO, indicating a more uniform surface. The root mean square roughness decreased from 1.056 nm for bare ITO to 0.912 nm for ITO/MeO-2PACz, 0.909 nm for ITO/2PACz, and 0.908 nm for ITO/Br-2PACz. This improvement in uniformity is crucial for enhancing device performance.

Among them, the treatment with Br-2PACz notably enhanced the properties of electrodes used in indoor organic photovoltaics (OPVs) and metal-insulator-metal (MIM) capacitors.

The indoor OPV device performance under artificial light was assessed with 2PACz derivatives-treated ITO electrodes. The devices used a polymer donor (PM6) and an NFA-based acceptor (Y6). Absorption spectra and energy band diagrams demonstrated effective photon absorption and energy level alignment. The Br-2PACz-based indoor OPV achieved a PCE of 31.0% and an excellent fill factor of 78.0% under LED 1000lx illumination, outperforming other variants due to improved hole injection and electron blocking from the higher WF.

The Br-2PACz-based MIM capacitors demonstrated significant improvements in capacitance and leakage current reduction. Br-2PACz-treated capacitors exhibited the highest capacitance density and lowest dissipation factor, indicating low series resistance. The leakage current at 2 V was 0.88 nA/cm² for Br-2PACz-treated devices, 19,000 times lower than TiN-based capacitors. The increased WF achieved through 2PACz derivatives treatment enhances the barrier height between the TiO2 dielectric and electrode, thereby reducing leakage current.

In conclusion, ultra-thin SAM treatment with 2PACz substituents effectively enhances electrode WF, thereby improving the performance of indoor OPVs and MIM capacitors. Br-2PACz, in particular, shows promise for high-efficiency, low-leakage applications, demonstrating significant potential for advancing electronic device performance through refined energy-level alignment. By manipulating the WF, this method presents a promising pathway to fine-tune the energy-level alignment in hybrid organic–inorganic electronic devices, thereby augmenting overall device performance