Counterion Engineering toward High-Performance and pH-Neutral Polyoxometalates-Based Hole-Transporting Materials for Efficient Organic Optoelectronic Devices

Although protonated polyoxometalates (POMs) are promising hole-transporting layer (HTL) materials for optoelectronic devices owing to their excellent hole collection/injection property, pH neutrality, and noncorrosiveness, POMs are seldom used as high-performance HTL materials. Herein, we designed and synthesized a series of mixed-additive POMs with pH-neutral counterions (NH₄⁺, K⁺, and Na⁺) as HTL materials. X-ray photoelectron spectroscopy and single-crystal X-ray analyses indicated that the use of the lacunary heteropolyanion [P₂W₁₅O₅₆]^12- as an intermediate ensured successful incorporation of the counterions into the mixed-addenda POMs without causing deterioration of the POM frameworks. The hole-transporting layer performance of POM-NH₄, which was characterized by a high work function and good conductivity and could be prepared using a low-cost method surpassed those of its protonated counterpart POM-4 and many classic HTL materials. An organic solar cell (OSC) modified with POM-NH₄ delivered a power conversion efficiency of 18.0%, which was the highest photovoltaic efficiency achieved by POM-based OSCs to date. Moreover, an HTL material based on POM-NH₄ reduced the turn-on voltage of an organic light-emitting diode from 4.2 to 3.2 V. The results of this study suggest that POMs are promising alternatives to the classic HTL materials owing to their excellent hole-collection ability, low costs, neutral nature, and high-chemical stability.