title: Balancing light absorptivity and carrier conductivity of graphene quantum dots for high-efficiency bulk heterojunction solar cells.
abstract: Graphene quantum dots (GQDs) have been considered as a novel material because their electronic and optoelectronic properties can be tuned by controlling the size and the functional groups of GQDs. Here we report the synthesis of reduction-controlled GQDs and their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency (PCE). Three different types of GQDs--graphene oxide quantum dots (GOQDs), 5 h reduced GQDs, and 10 h reduced GQDs--were tested in BHJ solar cells, and the results indicate that GQDs play an important role in increasing optical absorptivity and charge carrier extraction of the BHJ solar cells. The enhanced optical absorptivity by rich functional groups in GOQDs increases short-circuit current, while the improved conductivity of reduced GQDs leads to the increase of fill factors. Thus, the reduction level of GQDs needs to be intermediate to balance the absorptivity and conductivity. Indeed, the partially reduced GQDs yielded the outstandingly improved PCE of 7.60% in BHJ devices compared to a reference device without GQDs (6.70%).
authors.affiliation: SKKU Advanced Institute of Nano Technology and School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea.
authors.forename: Jung Kyu
authors.initials: JK
authors.lastname: Kim
authors.forename: Myung Jin
authors.initials: MJ
authors.lastname: Park
authors.forename: Sang Jin
authors.initials: SJ
authors.lastname: Kim
authors.forename: Dong Hwan
authors.initials: DH
authors.lastname: Wang
authors.forename: Sung Pyo
authors.initials: SP
authors.lastname: Cho
authors.forename: Sukang
authors.initials: S
authors.lastname: Bae
authors.forename: Jong Hyeok
authors.initials: JH
authors.lastname: Park
authors.forename: Byung Hee
authors.initials: BH
authors.lastname: Hong