別紙41 (課程博士(英文))Dateofsubmission (month day, ycar)

Deparlmenl
MeC11anica1 五ngineerjng

Studen11D Number

D 145108January 07,2019
Masanobu lzaki

Supervisors
APP11Canl's name

Seiji Yokoyama

Pei Loon Khoo

Abstract (Doctor)
Title ofThesis

Study on the characteristics ofElectrochemica11y prepared copper oxide
PhotovoltaicDevices(電気化学的に形成した酸化銅光電変換素子の特性に関する研究)

Approx.800 、¥OTds

Solar ce11S, or photovoltaics (PV),、Nhich converts light into electricity using se111iconductormaterials, are proving to be essential and indispensable for a sustainable society. N山nerous
kinds ofsolar ce11S have been the focus ofresearch interest of late, and thin solid films oflnetal

Oxide (MO) semiconductors, such as copper oxide based photovoltaics (PV) has always been
Considered a material suitable for the realization oflo、和・cost solar ce11S. cuprous oxide (CU20)and cupric oxide (CUO) are stable forms ofcopper oxides, atthe same time being intrinsica11y
P・type semiconductors with 2.1 ev and l.4 ev respectively. Attractive points of these oxides
Oriσinate from their non・toxicity, abundance, and lo、N cost. AISO, one main aspect of interest
to be noted is the preparation process,、Nhich can be achieved by electrochemical preparation
ElectrochemicaⅡy・prepared zno/CU20 pv had been reported to achieve 128%, ho、Never, thethe引'etica11imit is approximately 20少'o according to the shockley・Queisser Efficiency Limit,
based on the bandgap energy. The main objective ofthis research is to further investigate theIimiting factors of the power conversion efficiency (PCE) of copper oxide based
electrochemicaⅡy‑prepared photovoltaics and methods to further increase, based on close
Studies on the properties of its solid state physics
In this thesis, in chapter l, the introduction, background, revie、N, and purpose of this study
are discussed. Latest climate researches are brieay concluded and the urgency of addressing
global warming by switching to Nnewable energy, and the importance of solar energy isdiscussed. Metal oxide (MO) pvs has the criteria of a high・potential candidate as cheap, andWidespread. Among these MOS, copper oxides possess unique preferable propedies such as
ideal bandgap energies,毓 the same time being able to be fabricated by simple methods Hke
the electrochemical preparation.1t is then necessary to investigate the electrochemicaⅡy・Prepared copper oxides from its solid state physics. The definition of po、Ner conversion
efficiency (PCE) is defined with proportionalin relationship 、vith the short・circuit current
density (jsc) and open・circuit voltage (POC) of a pv. HO、Never, the independent variables ofPreparations are 、vide and complex, and insufficient information ca11S for the detailed study ofdeter111ining the affecting factor during preparations. AISO, a strategy using multiple bandgaps(using both copper oxide) as the light・absorbing layer is discussed.
In chapter 2,、ve report the fabrication of basic cU20 PV, and the successfulincrease in /、SCalong 、¥ith external quantum efficiency (EQE) and carrier mobⅡity, by annealing in vacuum
Using a rapid thermal annealing. The optimum annealing temperature was found to be 423 K

The e仟ect of annealing during preparation is further studied in detail, which includes its
microstructural, optical and electrical propedies. The mechanism is discussed, and theelucidation of the mechanism concludes a contributing factor of an increased carrier mobility
due to decreased scattering defects. AISO, the relationship bet、へleen the interface structure andン0c is discussed

In chapter 3, we reportthe photovoltaic properties ofbasic cuo pv successfUⅡy fabl'icatedby electrodeposition from a basic aqueous solution containing copper acetate hydrate andammonia 、vith highly resistive polysiloxane layer inS引'tion as the buffer layer. This study
indicates the abHity of cuo to function as a pv,、Nhich validates the strategy of using both
CU20 and cuo as the light absorbing layer to increase the liσht・absorption 、¥idth.
In chapter 4,、ve report the fabrication of directly stacked cU20/Cuo pv prepared byelectrodeposition and annealing. A decrease in EQE due to the formation of nanopores andVoids was observed. HO、Never, the EQE 、Nas studied in detail and showed an expansion ofabsorption width 、vhich originateS 介om the charge c011ection ofboth the cU20 and cuo layer,、¥ith a t、VO‑way carrier transport.、Nhen a low reverse biased voltage 、Nas applied, the charge
C0Ⅱection corresponding to the absorption edge originating 介om the cuo layer could be clearly
Observed due to improved carrier contr01. HO、Never, there is a necessity for an in・depth
investigation into the energy states and the band alignment ofthe cu・oxide pv
Chapter 5 reports on the study of chemical and energy states, along 、vith the band alignment
Study of the copper oxides. X‑ray photoelectron spectroscopy evaluation 、彫as carried out indetail. Methods to accurately ascertain the type of oxides were explored 、Nhich includes theAuger parameter and the 、Nagner plot. The band alignlnents 、¥ere calculated based on the xps
results, and correspond to the resuHs of the study in chapter 4, explaining the t、NO‑、刃ay now
Of carrier transport. An improved band alignment is proposed to further increase the pcE ofthe directly stacked cU20/CUO PV
In chapter 6, the results obtained in this study are summarized. Limiting factors, such as
Scattering defects like grain boundaries and nanopores, the condition of heterointerfaces, andSemiconductor qualities were identified. However, the viability of multi・bandgap copper・based pv is proved based on these results as a sound strategy to further increase the pcE anda proposalto realize further improvementis discussed. The scientific kno、¥1edge obtained here
WiⅡ Support further improvements in the electroch引ηicaⅡy・prepared copper oxide based
Photoactive layers