Imprint-induced grain growth in perovskite layers
Andre Mayera, Neda Pourdavoudb, Tobias Haegerb, Ralf Heiderhoffb, Miriam Leifelsa,Johannes Ronda, Johannes Staabsa, Patrik Görrna, Thomas Riedlb, Hella-Christin ScheeraaChair of Large Area Optoelectronicsb
Chair of Electronic Devices
University of Wuppertal, 42119 Wuppertal, Germany
E-mail: amayer@uni-wuppertal.de

Recent work on methyl ammonium based halide perovskites has demonstrated their capability
as active materials for lasers [1] or solar cells [2]. However, solution-processed polycrystallinelayers, which are obtained by spin coating, are rough and may exhibit structural defects, suchas holes. It could be demonstrated that thermal nanoimprint is well suited to improve the qualityof these layers. The perovskite layers are flattened, holes are less prominent and especially thecrystal grains have grown. Most commonly, the quality of the layers is characterized for control
purposes and with respect to the application envisaged (e.g. by XRD, PL, ASE). The physics
underlying the crystal grain growth during the imprint process has not been studied yet [3].
The present investigation intends to close this apparent gap. Therefore we ‘simulate’ specificsituations during the imprint process as marked in Fig. 1, however, with an increased process
time to amplify the effect. The imprint is performed with flat stamps (planar hot pressing, PHP).
The stamp is substantially smaller than the sample, which allows us to characterize the impactof the temperature with and without pressure under exactly the same conditions (see insert). Ina further experiment we study the layers after annealing under a nitrogen atmosphere.
Previous experiments have shown that the perovskites change their morphology already during
the long heat-up time [3]. Since the ‘simulation’ asks for an experiment without a misleading
heat-up, the imprint system is not loaded with the sample before the imprint temperature isreached (hot loading, HL).
The simulation at room temperature (RT) demonstrates the effect of pressure, only (Fig. 2).
Obviously, the imprinted layer is compressed but no grain growth is observed. The simulation
at reduced temperature (Fig. 3) documents a grain growth for the imprinted part, only. Thismeans that grains under pressure are stimulated to grow at a temperature that would not be
sufficient for grain growth without pressure. The simulation at a typical imprint temperature(Fig. 4) results in a layer with similar grain sizes in the imprinted and the annealed part. Here,the impact of temperature is stronger than the one of pressure.
The simulation results allow to develop a perception of what happens during the imprint
process as sketched in Fig. 5. As soon as the imprint pressure is applied the perovskite layer iscompressed. With increasing temperature the static pressure induces a grain growth during theheat-up phase. However, the higher the temperature the more it dominates the grain growth.
To further substantiate this view we will discuss it with theoretical concepts in mind. The
necessary link is derived from the annealing experiment shown in Fig. 6, which shows auniform growth of the grains with time, however, at an elevated temperature, only. Such abehavior indicates a ‘normal’ grain growth [4]. We anticipate that the theory is able to provide
arguments for the dominance of pressure or temperature in the different temperature regimes.
[1] N. Pourdavoud et al, Adv. Mater. Technol. 3, 1700253 (2018); Adv. Mater. 29, 1605003 (2017).
[2] J. Xiao et al, J. Mater. Chem. A 3 , 5289 (2015).
[3] L. Dimesso et al, J. Mater. Sci. 54 , 2001 (2019); A. Mayer et al, JVST B 35, 06G803 (2017).
[4] H.V. Atkinson, Acta. Metall. 3, 469-491 (1988); M. Hillert, Acta. Metall. 13, 227-238 (1965)

p,Ta)b)

3sample2
imprinted
annealed

1tti

11 µm

Figure 1. Typical imprint procedure used for
PHP of MAPbBr3 – p = 100 bar, Ti = 150°C,
ti = 5 min, heat-up time 20 min. The labels referto specific situations characterized in Fig. 2 – 4.

Figure 2. Simulation of the impact of pressure –a) annealed part, b) imprinted part
(HL, 100 bar, RT, 60 min).

a)a)

b)

21 µm

Figure 3. Simulation of the impact of a reduced
temperature – a) annealed part, b) imprinted part
(HL, 100 bar, 75°C, 60 min).

1.

2.

p3

5 µm

Figure 4. Simulation of the impact of a typicalimprint temperature – a) annealed part,b) imprinted part (HL, 100 bar, 150°C, 5 min).

a)perovskiteb)b)stampsubstrate

3.

pT

4.

1 µm

Figure 5. Perception developed from thesimulation experiments – 1. Pristine layer,
2. compression, 3. grain growth, 4. final layer

Figure 6. Pure annealing experiment –75°C (top) / 125°C (bottom); a) 1 min,b) 20 min.(nitrogen atmosphere)