Enhanced oxidation power in photoelectrocatalysis based on a micrometer-localized positive potential in a terrace hetero p\u2013n junction

Mohd Fairus Ahmad,Toshiyuki Abe,Motoya Suzuki,Keiji Nagai

doi:10.1038/s41427-018-0058-x

Abstract

Generally, p–n junction-based solar energy conversion has the disadvantage of a loss in potential gain in comparison with the photon energy. In this study, we found a more positive potential for a lateral domain interface of p–n junction than for a conventional p–n junction. A terrace bilayer (TB) p–n junction of phthalocyanine (H2Pc) and 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) was studied using scanning Kelvin probe microscopy (SKPM), and its electronic properties were analyzed using the contact potential difference (VCPD) data. The analysis of VCPD in the single layer region and the bilayer region (BLR) indicated a vacuum level shift through the electron transfer from PTCBI into indium tin oxide (ITO), from H2Pc into ITO and from H2Pc into PTCBI. Furthermore, the comparison of these VCPD data indicated a micrometer-localized positive potential in the boundary region (BDR) of the terrace bilayer structure of p-type on n-type. The gain difference of the VCPD reached +0.1 V in comparison with the BLR. The phenomena can be explained as a lateral dipole at the p–n junction. Similar phenomena were observed in TB-H2Pc/C60/ITO and TB-H2Pc/PTCBI/Au. The gain was extracted as oxidation power in photoelectrochemistry; i.e., at −0.2 V vs. Ag/AgCl a greater anodic current was observed for a patterned terrace bilayer electrode. Additionally, as a photocatalyst film (i.e., a H2Pc (dot)/PTCBI/PTFE membrane filter), the p–n dot terrace structure showed a higher quantum efficiency (5.1%) than that of the bilayer (3.2%) for the decomposition of acetic acid. The present design and method were utilized to obtain an efficient photocatalyst, especially through the mitigation of potential loss from the photon energy to redox powers without changing the molecular component.

Highlights

We focus on studying p–n junctions using two terraced-bilayer (TB) structures consisting of a wellknown and typical combination as the photovoltaic[39] materials, namely, PTCBI and H2Pc on an ITO substrate [PTCBI = 3,4,9,10-perylenetetracarboxylic-bisbenzimidazole (n-type); H2Pc = 29H,31H-phthalocyanine (p-type); indium tin oxide (ITO)]
This paper described the lateral VCPD characteristic at the boundary region (BDR) for the terrace bilayer electrode of n-type PTCBI and p-type H2Pc and a comparison of the single layer region (SLR) and bilayer region (BLR)
The analysis of the VCPD values of the SLRs and BLRs was consistent with the electron transfer and vacuum level shift at the interfaces through the Fermi level alignment that occurs from the Organic semiconductor materials (OSMs) into the ITO

Summary

Introduction

Organic semiconductor materials (OSMs) have been studied for many photovoltaic applications, such as solar cells[1,2,3,4,5,6,7,8,9,10,11,12], through the design and synthesis of new molecules, morphology control[1,2,3,4,5,6,7,8,9,10,11,12], and extension to photocatalysts[13,14,15,16,17,18,19]. The Fermi level (EF) of an OSM is useful for analyzing its characteristics, such as band bending and transport performance[20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. Scanning Kelvin probe microscopy (SKPM) as a surface potential analysis method is a technique for analyzing the Fermi level of an OSM and strongly affects its photogenerated carriers and open circuit voltage (VOC). The OSM heterojunction structure has been commonly used to maximize the carrier generation performance during photoirradiation. A disadvantage of p–n junctions is the drastic loss of the exciton energy compared to the energy difference between the edges of the n-type conduction band and the

Methods

Results

Conclusion