TiO2 Coated Graphene/ Mesoporous TiO2 Composite Layer for Perovskite Solar Cells Produced By Supramolecular Protein
Authors: Hashima Yuki , Raifuku Itaru , Ishikawa Yasuaki , Okamoto Naofumi , Uenuma Mutsunori , Yamashita Ichiro , Uraoka Yukiharu 
Published: February 2020

[Introduction]
CH3NH3PbI3 have attracted much attention as a light absorbing material for perovskite solar cells (PSCs). Owing to the compounds has better characteristics for solar cells and can be fabricated by solution process, high efficiency and inexpensive solar cells are available. Recently, PSCs have been widely researched to improve the photoelectric conversion efficiency (PCE). One of such research reported the method for mixing graphene in mesoporous TiO2 (meso-TiO2) layer. Graphene exhibits high carrier mobility and lower LUMO levels than TiO2. Therefore, the electron can be transported easily from TiO2 to graphene. However, PSCs using only graphene in electron transport layer have reported that PCE decreased because the LUMO level difference between the graphene and the CH3NH3PbI3 is large. Therefore, we propose a method to improve PCE by reducing the contact area between perovskite and graphene using a TiO2 coated graphene. In this report, CDT-1, which is a supramolecular protein composed of Dps with displaying carbon nano-horn binding peptide (NHBP-1) and Ti binding peptide (TBP-1), was used to coat TiO2 on graphene. We demonstrated the fabrication of TiO2 coated graphene using CDT-1 and the characterized the change upon PSCs using TiO2 coated graphene in meso-TiO2 layer.
[Methods]
The structure of CDT-1 is shown in Fig. 1. Both twelve NHBP-1s and twelve minTBP-1s are displayed on the outer surface of assembled CDT-1. Multiple NHBP-1 on the outer surface adsorbed to graphene and made graphene / CDT-1 bio-conjugates. Graphene / CDT-1 were then dipped in Titanium precursor solution. Multiple Ti binding peptides was effective to make the titanium bio-mineralization faster. Finally, precipitation of TiO2 coated graphene was obtained by ultracentrifugation. The structure of PSCs in this research is shown in Fig. 2. The compact layer of TiO2 (CL-TiO2) was prepared by spin coating on transparent conductive substrate. The meso-TiO2 layer was spin coated on the CL-TiO2 using the TiO2 coated graphene / meso-TiO2 composite paste (0, 0.1, 0.5, 1.0 wt%) and annealed at 450°C in air for 60 min.
The perovskite layer was deposited onto the meso-TiO2 layer by spin-coating of PbI2 CH3NH3I precursor DMF solution, and perovskite layer was baked with 2 steps (65oC, 1 min to 100oC, 2 min) in a nitrogen filled glovebox. The hole transport layer (HTL) was spin coated on the perovskite layer using Spiro-OMeTAD, and the gold electrode was formed by thermal evaporation on HTL. The change of the photoelectric conversion characteristics on the concentration of TiO2 coated graphene were evaluated by current-voltage (I-V) measurements.
[Results and discussions]
The SEM images are shown in Fig. 3. The white dots in Fig. 3 are the Fe-Oxide core in CDT-1 with 4.5 nm diameter adsorbed on graphene. As a result, CDT-1 was confirmed to be adsorbed high density by NHBP-1. We analyzed TiO2 on graphene by EELS-TEM mapping. The Fig. 4 (a) shows TEM images of TiO2 coated graphene. The Fig. 4 (b) shows Ti mapping TEM-EELS images. We fabricated TiO2 coated graphene resulted from TEM-EELS images. It was successfully.The Fig. 5 (a) compares the representative I-V curves of a TiO2 coated graphene (in variable concentration)/meso-TiO2 based PSCs. The Fig. 5 (b) shows changing the PCE and Fill Factor (FF) by the TiO2 coated graphene at various concentrations in meso-TiO2. With the addition of the a TiO2 coated graphene in the meso-TiO2, PCE increased from 8.4% to 12.5% in the 0.5 wt% TiO2 coated graphene/meso-TiO2, The FF increased from 53% to 68% in the 0.5 wt% TiO2 coated graphene/meso-TiO2.These results suggest the internal resistance is reduced by using TiO2 coated graphene.
[Conclusion]
We successfully synthesized TiO2 coated graphene using supramolecular protein. The TiO2 coated graphene/meso-TiO2 composite based PSCs exhibited an increase in PCE and FF as compared to meso-TiO2 based PSCs.

Figure 1