MSE 131 Ceramic Materials

Razonabe, Junio, Sible

Synthesis and Characterization

of CsSnI3 Thin Films

(Shum, et al, 2010)

introduction1974– structural information of

CsSnI3 compound (Scaife,et al.)

few years later– yellow needlelike CsSnI3

synthesized (Mauersberger and Huber)

1991– black polymorph of CsSnI3 (Yamada et al.)

Yellow CsSnI3 raise temperature above 425K Black CsSnI3

introductionBlack CsSnI3

450K ideal cubic perovskite (B-α)

426K tetragonal perovskite (B-β) < 351K orthorhombic perovskite

(B-γ)

All three structures – direct band gap (Eg) at Z, R, and Γ points

Eg (B-α)< (B-β) < (B-γ)

CsSnI3 IS A LONG-OVERLOOKED SEMICONDUCTOR

objective of studyThis study:

Effective and inexpensive method of synthesizing high quality CsSnI3 thin films on large-area substrates (glass, ceramics and silicon)

Characterize the CsSnI3 thin filmsbandgap surface characteristics, domain

size– SEM and TEMcrystal structure– ED and XRDoptical property-- PL

bandgap

Two step synthesis: The synthesis of its high optical quality polycrystalline thin films is demonstrated

Alternate deposition of high purity SnI2 (or SnCl2) and CsI layers in vacuum (10−5 Torr) on glass, ceramics, and

silicon substrates by a thermal and e-beam evaporators

Rapid thermal annealing was followed in a dry N2 environment to

activate a self-limiting chemical reaction of CsI with SnI2

Photoluminescence

• CsI /SnI2 and CsI /SnCl2 layered thin film samples characteristic PL around 950nm.– Possible side products for the CsI /SnCl2 layered

samples:• CsSnCl3

• CsSnICl2,• CsSnI2Cl.

– first two have much larger band gaps than CsSnI3

– last one has a smaller band gap than CsSnI3.• CsI /SnCl2 layered sample

– very weak PL relative to 950 nm emission – PL peak positions are consistent with

calculated band gaps for CsSnCl3 and CsSnICl2.

• No PL or absorption around 1.5 µm observed (CsSnI2Cl band gap)– Hence these possible side products from CsI

/SnCl2 layered samples do not affect the intense band edge emission of CsSnI3 reported

synthesis

surface characteristicsCsSnI3 film on glass

substrate with triple domain region

PolycrystallineDomain size

~300nmDifferent Lattice

Spacing Different Crystal Orientation

electron diffractionRing-like pattern (Bottom Left inset)Large Domain Areas (Top Left Inset)

Single Crystal FeatureRed dots – Theoretical Crystal Structure

Perovskite Structure

crystal structureSn-I-Sn bond in(1) a-direction(2) b-direction(3) c-direction

XRD peaks match

optical property CsI /SnCl2 thin film layers on glass

substrate Typical annealing temperature: 190°C (with

time duration of 15s) Good polycrystalline film, having very

intense PL at 950nm

CsI /SnI2 film layers Annealing temperature is higher than 190°C

since the melting temperature of SnI2 at 320°C is higher than that of SnCl2 at 247°C.

optical property

optical property

optical property Absorption spectrum reflects nature of

inhomogeneity of film (composition and domain sizes)

Absorption coefficient steeply takes off after PL emission peak and is zero below it direct bandgap characteristic

Shoulder riding on the absorption curve 50meV away from PL peak CB2

More work is needed to fully understand nature of absorption in CsSnI3 thin films

conclusion