論文 / 著書情報
Article / Book Information
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)

要約

Type(English)

OutlinePowered by T2R2 (Tokyo Institute Research Repository)

Study of Heterojunction Interface for High-efficient Cu(In,Ga)Se2 solar cells
Presented by Takahito Nishimura and Directed by Professor Akira Yamada
Department of Physical Electronics, Graduate School of Science and Engineering,Tokyo Institute of Technology

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 solarcells, and therefore 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, to further enhance conversion efficiency in CIGS solar cells, required condition for suppression ofrecombination at CdS/CIGS hetero junction interface in CIGS solar cells was theoretically analyzed, and theeffectiveness of Cu-deficient layer (CDL) on CIGS was experimentally suggested, finally establishing the uniquecontrol 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
to reproduce effects of donor doping and valence band offset (∆EV), respectively, on CIGS surface region. Highconversion efficiency of 22.1% and 21.6% was obtained by suppressing interfacial recombination when SL had highdonor 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 x 1015 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 effectsof CDL formed on CIGS surface. The higher Cd content at the surface region in Cu(In,Ga)3Se5 than CIGS was observed
when CdS layer was deposited on the top of the films. The bandgap of 1.51 eV in Cu(In,Ga)3Se5 was wider than thatof 1.22 eV in CIGS. These results suggest a promotion of Cd-diffusion and a formation of ∆EV ~ 0.29 eV between
CIGS and Cu(In,Ga)3Se5 owing to Cu vacancies in Cu(In,Ga)3Se5. The same cross-section of a CIGS solar cell with anefficiency of 18.5% fabricated by three-stage process was compositionally, electrically, and structurally evaluated. CDL(Cu/(Ga+In) of 0.31) with high Cd contents of 3.4at% unintentionally formed on the CIGS surface had lower carrier
concentration of 4.8 x 1010 cm-3 than that of 1014–1016 cm-3 in CIGS grain interior. This implies difficulty of Cd-doping
into the CIGS surface to satisfy the condition of ND > |NA, CIGS| owing to insufficient activation of Cd atoms. In contrast,the formation of ∆EV is effective to satisfy the condition of ∆EV ≥ 0.15 eV owing to low Cu/(Ga+In) ratio of 0.31.
Suppression effect of recombination at CdS/CIGS interface by introducing Cu(In,Ga) 3Se5 was investigated in CIGS
solar cells with energy bandgap of 1.2 and 1.4 eV. Open-circuit voltage (VOC) in both solar cells was clearly improved
from 0.62 to 0.64 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 interfacialrecombination.

Growth mechanism of CDL during CIGS deposition by three-stage process was revealed, and Se irradiation process
was proposed for control of CDL. Se irradiation process leads to follow effects; (i) thin and homogeneous Cu2Se layer
was formed in the second stage; (ii) Cu2Se changes to coexistence condition of Cu-deficient Cu2-xSe solid and Cu–Seliquid. 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 performance in CIGS solar cells was investigated. VOC and fill factor (FF) weresuccessfully increased 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 enhancementin CIGS solar cells. At an irradiation time of 10 min, VOC and FF decreased to 0.633 V and 72.9% owing to rough andnon-uniform CDL. Finally, the Se irradiation step is a simple, unique process that can boost the performance of CIGS
solar cells.