Abstract
Graphene quantum dots (GQDs), a newly emerging 0-dimensional graphene based material, have been widely exploited in optoelectronic devices due to their tunable optical and electronic properties depending on their functional groups. Moreover, the dispersibility of GQDs in common solvents depending on hydrophobicity or hydrophilicity can be controlled by chemical functionalization, which is particularly important for homogeneous incorporation into various polymer layers. Here we report that a surface-engineered GQD-incorporated polymer photovoltaic device shows enhanced power conversion efficiency (PCE), where the oxygen-related functionalization of GQDs enabled good dispersity in a PEDOT:PSS hole extraction layer, leading to significantly improved short circuit current density (Jsc) value. To maximize the PCE of the device, hydrophobic GQDs that are hydrothermally reduced (rGQD) were additionally incorporated in a bulk-heterojunction layer, which is found to promote a synergistic effect with the GQD-incorporated hole extraction layer.
Highlights
Donor-acceptor blended bulk heterojunction (BHJ) organic photovoltaic devices (OPVs) are considered promising generation solar cells due to their low-cost, light weight, flexibility, and solution processability[1,2,3]
Metal nanomaterials have lots of potential to induce monomolecular recombination, acting as trap sites and resulting in an inefficient hole extraction from the BHJ layers[20,21]. Insulator coated nanostructures such as SiO2 coated Au or Ag nanoparticles have been embedded in the BHJ or poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEODT:PSS) layers[22,23]. This process can alleviate the risk of short circuit and improve the power conversion efficiency (PCE), the particle size is out of proportion to the OPVs, because it is difficult to say the 80 nm particles are deeply embedded in the 80 nm thick BHJ layer or 40 nm thick PEDOT:PSS layer[20]
In order to investigate their positive effects in OPVs, the BHJ layer composed of thieno[3]-thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71 butyric acid methyl-ester (PC71BM) was spin coated on top of the graphene quantum dots (GQDs) incorporated PEDOT:PSS layer
Summary
We observed that the skein-like black wires, which presumably indicated grain boundaries, were untangled in Fig. 3D when compared with Fig. 3C (without GQDs) This results indicate that grain size of PEDOT:PSS film was increased because of the interaction between PEDOT and the incorporated GQDs. The grain size of both PEDOT:PSS films with or without GQDs were quantified using a line measurement with AFM images in Supplementary Fig. S4 online. We confirmed that the enhanced Jsc was dominantly contributed to the improvement of PCE of the GQDs-incorporated PEDOT:PSS device (GQDs in HTL) through the incident photon to current conversion efficiency (IPCE) measurement.
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