59th Annual Report on Research 2014

Narrative Progress Report

1. Introduction

            The research efforts in this project during this year (09/01/2013-8/31/2014) resulted in several important findings: (1) The shape of free-standing 3D graphene sheets can be tuned by changing particle sizes of Li₂O for a reaction between Li₂O and CO, (2) Configuration effect of TiO₂/graphene double layer photoelectrodes on performance of dye-sensitized solar cells was revealed, and (3) Several efficient composites materials for DSSCs were developed.

            So far, 8 papers have been published in reputed peer-reviewed journals.

2. Tuning shapes of free-standing 3D graphene sheets and their performance for dye-sensitized solar cells

Figure 1. FESEM images of graphene synthesized from the reaction of CO with different Li₂O particle sizes: (a) 0.4 μm. (b) 1 μm, (c) 3 μm, and (d) 10 μm,

            In this work, it was demonstrated that the shapes of 3D graphene sheets, which were formed via the reaction between Li₂O and CO, were strongly dependent on particle sizes of Li₂O. When Li₂O particle size was very small (about 0.4µm), the reaction generated carbon material with a granulated shape (Fig.1a). However, as Li₂O particle size increased, the generated carbon first changed to flower-structured graphene (Fig.1b and C) and then to honeycomb-structured graphene (Fig.1c). This happened because the particle sizes of Li₂O strongly affect its reaction kinetics with CO (Fig.2). Furthermore, the tuning shape of graphene from granulated, flower, and to honeycomb can increase its performance for dye-sensitized solar cells.

Fig.2 The pressure decrease of CO with reaction time.

3. Configuration effect of TiO₂/graphene double layer photoelectrodes on performance of dye-sensitized solar cells

         Two types of double-layer photoelectrodes for dye-sensitized solar cells (DSSCs) were designed to evaluate the contact resistance between FTO and semiconductor film (Fig.3): one with TiO₂/graphene composite as a top layer and TiO₂ as a bottom layer (structure A) and the other one with TiO₂ as a top layer and TiO₂/graphene composite as a bottom layer (structure B). Furthermore, performance evaluations showed that the DSSCs with structure B exhibited better power conversion efficiency (10.4%) than that of (8.14%) of the DSSC with structure A. The efficiency improvement was attributed to smaller contact resistance of the DSSC with structure B.

Fig.3 Schematic diagram of resistance in photoelectrodes with different layer structures.

4. Composite materials for DSSCs

         It is the first time to demonstrate that ZnO with poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) could be an efficient DSSC counter electrode with a maximum power conversion efficiency of 8.17%. However, the conversion efficiency of DSSCs with the ZnO/PEDOT:PSS counter electrode strongly depends on ZnO content. Furthermore, density functional theory (DFT) calculations, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) measurements revealed that the variation of the conversion efficiency with ZnO content is determined by fill factor (FF), which is dependent on combination between the catalytic activity of ZnO and the conductivity of PEDOT:PSS. The similar enhancement of power conversion efficiency was observed when NiO was introduced into the PEDOT counter electrode of a DSSC. However, the enhancement in NiO/PEDOT:PSS counter electrode is mainly due to the increase of current density instead of fill factor.

          Promoter and poison are two contradictory properties of materials. However, in this work, an unusual particle-size-induced promoter-to-poison transition of ZrN has been observed for the counter electrode of poly(3,4-ethylene-dioxythiophene): polystyrene sulfonate (PEDOT:PSS) in dye-sensitized solar cells (DSSCs). If the primary crystal particle size of ZrN was 37nm, it played as a promoting role for PEDOT:PSS counter electrodes, leading to a remarkable increase in DSSC power conversion efficiency. In contrast, when its primary crystal particle size was decreased to 21nm, ZrN becomes a serious poison for the PEDOT:PSS counter electrodes. This finding would be important for the development of highly efficient DSSC counter electrodes.