Work Function Modification via Combined Charge‐Based Through‐Space Interaction and Surface Interaction

Abstract

organic solar cells (OSCs) for several reasons: it is transparent in the visible region and it has not only high conductivity but also the capability to collect either electrons or holes through work function modification. [20] To illustrate, inverted OSCs, which have been extensively studied due to their superior long term ambient stability and printability relative to conventional OSCs, use ITO as a cathode by controlling its work function. [15,21,22] Thus, understanding the origin of the work function modification process is of critical importance for organic electronic devices. Currently, introducing a thin layer of interfacial materials between the electrode and the active layer is studied to modify the work function of the electrode through the formation of interfacial dipole interactions. For example, He et al. reported that the efficiency of OSCs can be raised to 9.2% by effective work function tuning with an alcohol/water-soluble conjugated polymer. [6] Lee et al. showed that high performance OSCs with 8.32% efficiency could be fabricated using a combination of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) and nonconjugated polyelectrolytes as recombination layers that lead to effective work function reduction at the ITO electrode. [15] Additionally, both anionic and cationic polyelectrolyte interlayers have been studied to achieve a proper energy level alignment for Work function modification of electrodes is an important factor to achieve high performance in organic electronics. However, a clear explanation of the origin of work function modification has remained elusive. Here, it is investigated how the work function of electrodes is affected by the chargebased through-space interaction with the well-known surface interaction. The studies reveal that the formation of a surface dipole leads to a work function shift, even when the work function modifying layer and substrate are separated. A work function shift is also demonstrated by electrophoretic deposition of ionic polyelectrolytes while the same polyelectrolytes do not cause any work function shift when they are spin cast. More noteworthy is that a neutral (nonionic) polymer which has no specific surface-interacting functional groups can induce work function shift of its substrate by a chargebased through-space interaction when deposited by electrospraying. These results provide a more comprehensive understanding of work function modification and motivate the design and selection of a wide range of effective work function modifying layers for organic electronics.

Dopant-Tunable Ultrathin Transparent Conductive Oxides for Efficient Energy Conversion Devices

“…Till date, the direct transition of n-type TCOs to p-type TCOs through surface modification by metal implantation has not been reported, except for surface modification using self-assembled molecules or co-deposition of metals [12][13][14]. To investigate the possibility of such a transition via EMi, we probed the WF of m-TCOs using an independent Kelvin probe (KP) microscope and performed a double check using UV photoelectron spectroscopy (Fig.…”

“…In addition, the work function (WF) of TCOs is important for balancing charge injection via the band alignment of devices. The WF of TCOs can be fine-tuned by adjusting the oxygen content [12] and modifying the surface using self-assembled molecules and polymers [13] or multilayer films [14]. However, applying these methods to practical devices is difficult owing to the strict requirements for high transparency, low R SH , and WF alignment.…”

Work Function-Tunable Amorphous Carbon–Silver Nanocomposite Hybrid Electrode for Optoelectronic Applications

“…To date, the tuning of WF for electrode materials such as transparent conducting oxides, conducting polymers, metals, and carbon nanostructures has been performed by chemical modification of their surface or introducing a WF modification layer at the interface between the semiconductor and the electrode. − Although these modifications optimize the WF to improve the extraction or injection of charge carriers, the principle behind this improvement is not fully understood in various optoelectronic devices. This ambiguity hinders the choice of materials and strategies to optimize the WF.…”