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題目(和文)Cu(In,Ga)Se2太陽電池の高効率化に向けたヘテロ接合界面に関する研究Title(English)

Study of Heterojunction Interface for High-efficient Cu(In,Ga)Se2 solarcells

著者(和文)

西村 昂人

Author(English)

Takahito Nishimura

出典(和文)

学位:博士(工学),学位授与機関:東京工業大学,
報告番号:甲第10800号,授与年月日:2018年3月26日,
学位の種別:課程博士,
審査員:山田 明,中川 茂樹,間中 孝彰,宮島 晋介,PHAM NAM HAI,和田隆博

Citation(English)

Degree:Doctor (Engineering),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第10800号,
Conferred date:2018/3/26,
Degree Type:Course doctor,
Examiner:,,,,,

学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

論文要旨

Type(English)

Summary

Powered by T2R2 (Tokyo Institute Research Repository)

（博士課程）Doctoral Program論 文 要 旨ＴＨＥＳＩＳ ＳＵＭＭＡＲＹ

専攻：
Department of

電子物理工学

学生氏名：
Student’s Name

西村 昂人

専攻

申請学位（専攻分野）： 博士
Academic Degree Requested

指導教員（主）：Academic Supervisor(main)

Doctor of（山田工学明）

教授

指導教員（副）：Academic Supervisor(sub)要旨（英文 800 語程度）
Thesis Summary （approx.800 English Words ）Photovoltaic power generation is expected as one of clean renewable energy sources to solve environmental and energy
issues. Cu(In,Ga)Se2 (CIGS) is promising materials for absorber of solar cells because of its unique characteristics. CIGS
which has high absorption coefficient enable to be applied for absorber of flexible lightweight thin-film solar cells, andtherefore it is possible to realize reduction of mass production cost by roll-to-roll deposition in continuous high-speed
manufacturing process. The highest conversion efficiency of 22.9% has been achieved in CIGS solar cells. In this thesis, tofurther enhance conversion efficiency in CIGS solar cells, required condition for suppression of recombination at CdS/CIGS
hetero junction interface in CIGS solar cells was theoretically analyzed, and the effectiveness of Cu-deficient layer (CDL) on
CIGS was experimentally suggested, finally establishing the unique control methods of CDL.
A guideline for improvement of CdS/CIGS hetero interface in CIGS solar cells was suggested using one-dimensional
device simulation. Carrier concentration and energy bandgap in surface layer (SL) at the hetero interface were changed toreproduce effects of donor doping and valence band offset (∆EV), respectively, on CIGS surface region. High conversion
efficiency of 22.1% and 21.6% was obtained by suppressing interfacial recombination when SL had high donor concentration(ND) of 1.0 x 1018 cm-3 and high ∆EV of 0.2 eV, respectively, although the efficiency was 19.6% in SL with low ND of 1.0 x1015 cm-3 and with ∆EV of 0 eV. It was revealed that the required condition for SL to boost conversion efficiency is ND > |NA,CIGS| (|NA, CIGS|: absolute acceptor concentration in CIGS) or ∆EV ≥ 0.15 eV.
CIGS and Cu(In,Ga)3Se5 (Cu-deficient material with Cu/(Ga+In) of 0.33) films were compared to discuss the effects ofCDL formed on CIGS surface. The higher Cd content at the surface region in Cu(In,Ga) 3Se5 than CIGS was observed whenCdS layer was deposited on the top of the films. The bandgap of 1.51 eV in Cu(In,Ga)3Se5 was wider than that of 1.22 eV inCIGS. These results suggest a promotion of Cd-diffusion and a formation of ∆EV ~ 0.29 eV between CIGS andCu(In,Ga)3Se5 owing to Cu vacancies in Cu(In,Ga)3Se5. The same cross-section of a CIGS solar cell with an efficiency of
18.5% fabricated by three-stage process was compositionally, electrically, and structurally evaluated. CDL (Cu/(Ga+In) of0.31) with high Cd contents of 3.4at% unintentionally formed on the CIGS surface had lower carrier concentration of 4.8 x1010 cm-3 than that of 1014–1016 cm-3 in CIGS grain interior. This implies difficulty of Cd-doping into the CIGS surface tosatisfy the condition of ND > |NA, CIGS| owing to insufficient activation of Cd atoms. In contrast, the formation of ∆EV iseffective to satisfy the condition of ∆EV ≥ 0.15 eV owing to low Cu/(Ga+In) of 0.31.
Suppression effect of recombination at CdS/CIGS interface by introducing Cu(In,Ga)3Se5 was investigated in CIGS solarcells with energy bandgap of 1.2 and 1.4 eV. Open-circuit voltage (VOC) in both solar cells was clearly improved from 0.62 to0.64 V and from 0.66 to 0.75 V, respectively, with increasing Cu(In,Ga) 3Se5 thicknesses from 0 to 5 nm. This result suggests
the effectiveness of ΔEV at the CdS/CIGS interface by CDL formation to suppress interfacial recombination.
Growth mechanism of CDL during CIGS deposition by three-stage process was revealed, and Se irradiation process wasproposed for control of CDL. Se irradiation process leads to follow effects; (i) thin and homogeneous Cu 2Se layer wasformed in the second stage; (ii) Cu2Se changes to coexistence condition of Cu-deficient Cu2-xSe solid and Cu–Se liquid. A
uniform CDL with a thickness of 150–200 nm was formed via control of Cu2-xSe on CIGS with irradiation time of 5 min,whereas the CDL formed at an irradiation time of 10 min was rough and non-uniform.
The effect of CDL formation on CIGS solar cell performance was investigated. VOC and fill factor (FF) were successfullyincreased up to 0.672 V and 76.2% in a CIGS solar cell with an irradiation time of 5 min, and a maximum efficiency of
19.8% was achieved. These results suggest that the uniform CDL on CIGS leads to efficiency enhancement in CIGS solarcells. At an irradiation time of 10 min, VOC and FF decreased to 0.633 V and 72.9% owing to rough and non-uniform CDL.
Finally, the Se irradiation step is a simple, unique process that can boost the performance of CIGS solar cells.
備考 : 論文要旨は、和文 2000 字と英文 300 語を 1 部ずつ提出するか、もしくは英文 800 語を 1 部提出してください。
Note : Thesis Summary should be submitted in either a copy of 2000 Japanese Characters and 300 Words (Engｌish) or 1copy of 800
Words (English).
注意：論文要旨は、東工大リサーチリポジトリ（T2R2）にてインターネット公表されますので、公表可能な範囲の内容で作成してください。
Attention: Thesis Summary will be published on Tokyo Tech Research Repository Website (T2R2).