▎ 摘　　要

Mn3O4 on reduced graphene oxide (r-GO) was easily synthesized by upcycling process of wasting manganese ions which were generated during oxidation reaction from graphite to GO. The yellow-brown GO suspension under acid media before neutralization immediately became black precipitates when the suspension was titrated into the concentrated NaOH solution. The method could convert the wasting manganese ions up to similar to 91 wt% to Mn3O4 to optimize work function in a hole transport layer (HTL) for organic solar cells. The hybrid materials exhibited an ideal electronic structure suitable for HTL, leading to the excellent power conversion efficiency of similar to 3.23%. (C) 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.