GIWAXSA powerful tool for perovskite photovoltaics

RESEARCH HIGHLIGHTS

Journal of Semiconductors

(2021) 42, 060201

doi: 10.1088/1674-4926/42/6/060201

GIWAXS: A powerful tool for perovskite photovoltaics

Chenyue Wang

1

, Chuantian Zuo

2

, Qi Chen

1, †

, and Liming Ding

2, †

1

MIIT Key Laboratory for Low-dimensional Quantum Structure and Devices, Experimental Center of Advanced Materials, School of Materials

Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

2

Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for

Nanoscience and Technology, Beijing 100190, China

Citation: C Y Wang, C T Zuo, Q Chen, and L M Ding, GIWAXS: A powerful tool for perovskite photovoltaics[J]. J. Semicond., 2021,

42(6), 060201. /10.1088/1674-4926/42/6/060201

The power conversion efficiency (PCE) for perovskite sol-

ar cells (PSCs) now reaches 25.2%

[1]

. However, the perovskite

materials have complex compositions and variable phases,

calling for suitable characterization techniques to investigate

the underlying operation and degradation mechanism. Graz-

ing-incidence wide-angle X-ray scattering (GIWAXS) plays an

important role in studying perovskite materials. GIWAXS data

are generally two-dimensional diffractograms containing dif-

fraction rings of different crystal planes. Grazing-incidence

small-angle X-ray scattering (GISAXS) is similar to GIWAXS,

while it has a longer detection distance than that of GIWAXS

(Fig. 1(a))

[2]

. GISAXS enlarges the observable spatial range up

to 10–100 nm and reduces the measurement sensitivity of crys-

tallization, and it is mainly used to determine the morpho-

logy of bulk-heterojunction films in nanoscale

[3, 4]

. Compared

to GISAXS, GIWAXS is more popular in perovskite study. This

technique has several advantages as follows: (1) high signal-

to-noise ratio (SNR) and sensitive structural resolution; (2) no-

contact and nondestructive probing; (3) abundant structural in-

formation; (4) depth resolution; (5)

in-situ

observation. Here,

we discuss two applications of GIWAXS, i.e., the crystallograph-

ic information at steady state, and the

in-situ

measurement

to probe the temporal information. As an important structur-

al parameter of perovskite films, crystallographic orientation

affects the optoelectronic properties and materials stability.

The 2D GIWAXS diffractogram presents the Debye-Scherrer

ring for certain crystallographic plane, enabling characteriza-

tion of structural orientation of perovskite films. The orienta-

tion degree for crystal planes can be obtained quantitatively

according to the diffraction rings along the azimuth by using

Herman’s orientation function.

Quasi-2D perovskites receive attention due to their vari-

able structures, tunable composition, and relatively high stabil-

ity. The insulating organic long-chain cations in quasi-2D per-

ovskites can block carrier transport. Suitable crystal orienta-

tion can enhance the carrier transport in 2D perovskites, thus

improving device performance. GIWAXS measurements give in-

formation about crystal orientation, it can also tell the stack-

ing manner of grains at different depths, which is essential

for understanding the crystallization mechanism. For ex-

ample, by using GIWAXS, Choi

et al.

found that the nucle-

ation and crystallization of BA

2

MA

3

Pb

4

I

13

perovskite occurs at

the gas-liquid interface during annealing, which results in the

Correspondence to: Q Chen, ***********.cn; L M Ding, ***************

Received 22 MARCH 2021.

©2021 Chinese Institute of Electronics

vertical alignment of 2D perovskite crystals (Fig. 1(b))

[5]

. They

further regulated the solvent and cation to prepare highly ver-

tically orientated 2D perovskite films

[6]

. Rafael

et al.

found

that the intermediate solvent complexes provide building

blocks in the formation of 2D perovskites according to GI-

WAXS measurements

[7]

.

High-quality 3D perovskites tend to make strong orienta-

tion at certain azimuth angle. GIWAXS results can be used to

evaluate the crystallization quality of 3D perovskite thin films.

The results can also be used to guide the process optimiza-

tion, as well as to clarify the relationship between crystallo-

graphic orientation and device performance. Zheng

et al.

regu-

lated the preferential orientation of perovskite crystals and im-

proved the interfacial carriers transport in the corresponding

devices by substituting A-site alkali metal cations

[8]

.

Recently, residual strain was observed in perovskite films

due to the mismatch of the expansion coefficients for the sub-

strate and perovskites, which influences the operational stabil-

ity and efficiency of perovskite solar cells. Microscopically, the

residual stress within the film results from a biaxial stretch-

ing of the perovskite lattice in in-plane direction. The shift of

corresponding diffraction peaks at different azimuthal angles

reveals the lattice tilting and stretching. By depth-resolved GI-

WAXS, Zhu

et al.

observed a gradient strain in FA-MA per-

ovskite films (Fig. 1(c)). The performance of PSCs was im-

proved by reducing lattice mismatch of the crystals

[9]

. Wang

et al.

replaced A-site cations on the perovskite surface by us-

ing OAI post-treatment, forming a “bone-joint” configuration,

reducing surface residual stresses and thus improving humid-

ity and thermal stability of PSCs

[10]

.

In-situ

measurement is attractive in perovskite research.

It provides a rapid approach to track microstructural changes

in perovskite materials, including the crystallization and

aging processes. It is the key to unravel the kinetics process

of perovskite materials. The formation process of perovskite

crystals is not fully understood yet. The film formation pro-

cess includes liquid-film gelation stage and crystallization

stage. Many studies have shown that the orientation and

phase structure of perovskite are already established during

gelation stage. The quality of the perovskite precursor film

(gel) significantly affects the final perovskite film.

In-situ

GI-

WAXS provides information for the composition evolution

during spin-coating process. It also provides guidelines for pre-

paration conditions, such as spin speed and time, dripping

time of anti-solvent, etc. Amassian

et al.

have conducted a

series of

in-situ

GIWAXS studies on perovskite. They ob-

served the transition of perovskite precursors from liquid

2

Journal of Semiconductors doi: 10.1088/1674-4926/42/6/060201

(a)

GIWAXS

z

y

k

i

x

k

f

α

i

α

f

ψ

q

xz

GISAXS

α

f

χ

ψ

q

z

k

f

(b)

Oriented perovskite grown

on mp-TiO

2

substrate

1.6

1.2

0.8

0.4

0

00.5

1.01.52.0

Q

xy

(Å

−1

)

Nucleating from air−liquid

interface—preferential orientation

m

.

1

m

0

−

5

.

0

0

.

≈

≈

2

q

xy

Q

z

(

Å

−

1

)

q

(c)

31.695

2

θ

(

°

)

31.680

31.665

31.650

0

(e)

Carrier

gas

Tensile-strain

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