倉田奨励金研究報告書 第 51 集｜2019 年度（第 51 回）助成
自然科学・工学研究部門：1 年助成■受領№1391

ペロブスカイト太陽電池向け非鉛材料の探索
代表研究者

カダカ

ビドゥラバ

物質・材料研究機構

研究員

Exploration of Pb-Free Halide Perovskite Materials for Solar
Cell Applications
Principal Researcher

DHRUBA B. KHADKA,
National Institute for Materials Science (NIMS), Researcher

鉛フリーペロブスカイトの候補として FASnI3 と BiI3 を検討した。スズ系ペロブスカイト（Sn-PSC）については Rb 元素を添加する手法を開発し、光電変換特性が 3.12％から 5.89％へ向上した。本手法により Sn-PSC の結晶性をある程度制御する事が可能となった。
Bi ベースの太陽電池については、さまざまな条件下で結晶化させる手法を開発して BiI3 膜の特性を検討し、倒立型デバイスにて約 1.02％の光電変換特性が得られた。本手法により様々なオプトエレクトロニクスアプリケーション向けに BiI3 材料をさらに最適化する事が可能となり得る。
Leading to our proposal for Pb-free perovskite, we have explored FASnI3 and BiI3 as an alternative
Pb-PSCs. For Sn-PSCs, the Rb incorporated FASnI3 leads the improvement in device with the PCE ofcontrol device (3.12%) to 5.89% for (Rb,FA)SnI3. This work paves the way for optimizations of Sn-HaP
film quality by modulating the crystal lattice for boosting the performance and stability device.
For Bi-based solar cells, I have investigated the crystallized BiI3 films under various ambient conditions.
The inverted device demonstrated the best PCE~1.02%. This work opens up multiple aspects for furtheroptimization of BiI3 material for other optoelectronic applications.

1．研究内容concentrations of RbX in DMSO solvent. As

A. Sn-halide perovskite solar cells:

depicted in the schematic of device fabrication
halide(Fig. 1), we prepared inverted Sn-PSCs withperovskites (Sn-HaP), a close cousin to Pb-HaP,

PEDOT: PSS (HTL), PC61BM (ETL) by spin

are promising alternatives. In this report, we havecoating, AZO (ESL) and Ag deposited by thermal

studied the role of Rb incorporation (i.e. smallerevaporation. Khadka et al. J. Mater. Chem. C, 8,cation in A-site) into the lattice structure of2307-2313, (2020).

FASnI3 for device efficiency and insight into

II. Results and Summary:device analysis.

The films with RbCl additive exhibit a significant

I. Device fabrication method: For the fabrication ofimprovement in film coverage by suppressing theSn-HaP films, the precursors were prepared bypinhole’s densities (Fig. 2a-d). We observed

dissolving FAI, SnI2, and SnF2 adding different

highly oriented crystallographic planes of (100)Amongthesealternatives,Sn-basedand (200) for Sn-HaP films with RbCl additive倉田奨励金研究報告書 第 51 集｜2019 年度（第 51 回）助成
自然科学・工学研究部門：1 年助成

whereas the pristine film grows with multiple
crystal orientations (Fig. 3e,f). The absorption
spectraof

thefilms(x=0-0.12)

(Fig.

3g)

demonstrate some effects with a higher content ofRbCl additive. Photoluminescence (PL) spectra ofFig.1 Schematics of fabrication of Sn-perovskite devices.

the corresponding films (Fig. 3h) display a slight
redshift with the higher RbCl content. It isobserved that the RbCl additive facilitates uniform
morphology with better crystal quality andsuppression of oxidation of Sn2+. This results in anenhancement in the device efficiency of 3.12% forpure Sn-HaP to 5.89% for Rb-incorporated(Rb0.08FA0.92SnI3) absorber layer (Fig. 3c) withhigherreproducibilityandsuperiorstability(Fig.3a). The device analysis corroborates that theincorporation of Rb in the FASnI3 structure plays acrucial role in ameliorating film growth, surface
chemistry, and mitigation of trap centers (Fig. 3b).
B. Bi-halide based solar cells:

Fig. 2. SEM images of Sn-HaP film with RbCl additive(a-d). XRD patterns (e,f), absorption spectra (g), andPL spectra (h).

Several Pb free photovoltaic absorbers; Sn, Ge, Bi,and Sb-based halide semiconductor materials havecentred the research focus for alternative materials.
Both Pb2+ and BiI+3 are highly polarizable cations
with large spin-orbit coupling effects. As anotherPb-free candidate, the BiI3 (Eg~1.75 eV) based
devices films were investigated and prepareddevice adopting inverted device structure. We foundthe BiI3 device is more stable than the Pb andSn-based HaP devices.
I. Device fabrication method:
The bismuth triiodide (BiI3) film was prepared byspin BiI3 precursor solution (100 mg/mL) in DMF.
The

schematicsoffilm

fabrication

andcrystallization by annealing under controlled
solvent vapour conditions are shown in Fig.4.
II. Results and Summary:
We report on the solution-processed BiI3 solar cells andtheir optoelectronic properties under different fabricationapproaches. The BiI3 films crystallized by annealing
under controlled solvent vapour conditions were foundto have dominant effects on film morphology as well as
the crystal growth as depicted in Fig. 5.

Fig. 3. J-V curves Sn-HaP based solar cell with Rb

additive(a). Defect density profile (b), device
stability under ambient condition (c).
We have fabricated an inverted device structure;
ITO/PTAA/BiI3/PCBM/BCP/Ag with BiI3 film
processed by annealing under solvent vapour
conditions as depicted in Fig. 6a, b. The J-V
characteristics of corresponding conditions areshown in Fig. 6c. The photovoltaic device with BiI3
film followed by annealing under chlorobenzene
vapour atmosphere demonstrated the best power倉田奨励金研究報告書 第 51 集｜2019 年度（第 51 回）助成
自然科学・工学研究部門：1 年助成

conversion efficiency of ~1.02% with an open
circuit voltage of 0.587 V. This is attributed to theimprovement in film quality comprising uniform
morphology, increase in grain size, and hence
mitigation of defect profile. The spectral responseof the best device is shown in Fig. 6d.

2．発表（研究成果の発表）
Fig. 4. Schematic of the fabrication approach ofthe BiI3 thin film. (a) Precursor solution of BiI3,(b) precursor deposition on the ITO/HTLsubstrate. The crystallization of BiI3 thin film by(c) thermal annealing (TA), (d) annealing theantisolvent (chlorobenzene (CB)) dripped film(AS), and (e) annealing under ambient solvent (CBor dimethylformamide (DMF)) vapor (CB orDMF-SVA).

“Research Paper”
1. Dhruba B. Khadka, * Y. Shirai, M. Yanagida, K.
Miyano, Attenuating the defect activities with arubidium additive for efficient and stable Sn-based
halide perovskite solar cells, J. Mater. Chem. C, 8,2307-2313, (2020).
2. Dhruba B. Khadka, * Y. Shirai, M. Yanagida, K.
Miyano, Ammoniated Aqueous Precursor Ink
Processed Copper Iodide as a Hole Transport Layer
for Inverted Planar Perovskite Solar Cells, Solar
Energy Materials and Solar Cells, 210 ,110486,(2020).
3. Dhruba B. Khadka, * Y. Shirai, M. Yanagida, K.
Miyano, Passivation of the Recombination Activities
with Rubidium incorporation for Efficient and StableFig. 5. SEM images of the BiI3 thin films crystallizedvia (a) TA, (b) CB-AS, (c) CB-SVA, (d) DMF-SVA.
The characteristics of respective films (e) XRD
patterns, and (f) Raman spectra.

Sn- HaP Solar Cells, IEEE proceeding, DOI:
10.1109/PVSC45281.2020.9300783.

0113-0116,(2020).
4. M. G. M. Pandian, Dhruba B. Khadka, * Y. Shirai,
S. Umedov, M. Yanagida, S. Subashchandran, A.
Grigorieva and K. Miyano. Effect of solvent vapour
annealing on bismuth triiodide film for photovoltaic
applications and its optoelectronic properties, J.
Mater. Chem. C, 8, 12173-12180, (2020).
5. S. Umedov, Dhruba B. Khadka, * M. Yanagida, S.
Subashchandran, A.

Grigorieva, Y. Shirai, A-site

Tailoring in the Vacancy-Ordered Double Perovskite
Fig. 6. Schematic of the energy band diagram (a),cross-sectional image of BiI3 based solar cells(b). J-V characteristics of BiI3-based solar cells(c). External quantum efficiency (EQE) spectra(d) of the corresponding device with BiI3 device
with CB-SVA.

Semiconductor Cs2SnI6 for Photovoltaic Application,
Solar Energy Materials and Solar Cells, Solar Energy
Materials & Solar Cells, 230,111180, (2021).

倉田奨励金研究報告書 第 51 集｜2019 年度（第 51 回）助成
自然科学・工学研究部門：1 年助成“Conference Presentation”
1. Dhruba B. Khadka, * Y. Shirai, M. Yanagida, K.
Miyano, Passivation of the Recombination Activities
with Rubidium incorporation for Efficient and Stable
Sn-HaP Solar Cell, IEEE Photovoltaic Specialists
Conference (IEEE PVSC-47), Chicago, Canada,
PVSC 47 Virtual Meeting-2020, Contributed Talk,
June 15th - August 21st, (Canada, 2020)
2. Dhruba B. Khadka, * Y. Shirai, M. Yanagida, K.
Miyano, Mitigation of the Recombination Activities
with Rubidium incorporation for Efficient and Stable
FASnI3 Solar Cells, International Conference onPerovskite, Organic Photovoltaics and Optoelectronics(IPEROP)-2020, Tsukuba, Japan, Poster Presentation,
20th-22th Jan, (Japan, 2020).