Short Term Mobility 2016 Cefarin Nicola

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POROUS PEROVSKITE NANOCRYSTALS FOR

PHOTOVOLTAIC APPLICATION

Cefarin Nicola, PhD student

DIPARTIMENTO DI SCIENZE FISICHE E TECNOLOGIA DELLA MATERIA

CNR-IOM - Istituto Officina dei Materiali

Headquarters (Trieste, TASC)

c/o Area Science Park - Basovizza

Strada Statale 14 km 163,5 - 34149 Trieste

Final report

Thanks to the support of the Short Term Mobility (STM), I have performed my research activity in the

group of Francesca Toma at the Joint Center for Artificial Photosynthesis (JCAP),in the Chemical Sciences

Division at the Lawrence Berkeley National Laboratories (LBNL). My project has focused on the synthesis

and characterization of i) a new deposition process called Low Pressure Vapor Assisted Solution Process

(LP-VASP), previously developed at LBNL20, and ii) of porous polycrystalline perovskites synthesized at

our laboratories (TASC, Trieste).

Introduction:

Hybrid organic-inorganic lead halide perovskites (CH3NH3PbX3, X = I, Br, Cl) are a new class of

semiconductors that has emerged rapidly within the last few years. This material class shows excellent

semiconductor properties, such as high absorption coefficient1, tunable bandgap2, long charge carrier

diffusion length3, high defect tolerance4, and high photoluminescence quantum yield5,6. The unique

combination of these characteristics makes lead halide perovskites very attractive for application in

optoelectronic devices, such as single junction7,8 and multijunction photovoltaics9,10, lasers5,11, and

LEDs12.

CH3NH3PbX3 films can be fabricated by a variety of synthetic methods13, which aim at improving the

efficiency of this semiconducting material for energy applications14. However, optimization of

photovoltaic devices relies on the quality of the halide perovskite active layer, as well as its interfaces

with charge selective contacts (i.e. electron and hole transport layers), which facilitate photocarrier

collection in these devices. Specifically, continuous, pinhole-free active layers are necessary to minimize

shunt resistance, thereby improving device performance.

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Among the most widespread methods for fabricating halide perovskite thin films are solution-based and

vacuum-based processes. The most common solution process uses equimolar ratios of lead halide and

methylammonium halide dissolved in dimethylformamide (DMF), dimethylsulfoxide (DMSO), or γ-

butyrolactone (GBL), or mixtures of these solvents.2,15,16 Precursor molarity and solvent type, as well as

annealing temperature, time and atmosphere, must be precisely controlled to obtain continuous and

pinhole-free films.15 For example, to improve surface coverage, a solvent-engineering technique was

demonstrated to yield dense and extremely uniform films.16 In this technique, a non-solvent (toluene) is

dripped onto the perovskite layer during the spinning of the perovskite solution.16 These approaches are

usually well suited for mesoscopic heterojunctions, which employ mesoporous TiO2 as an electron

selective contact with increased contact area and reduced carrier transport length.

However, planar heterojunctions, which use selective contacts based on thin (usually TiO2) films, are

more desirable because they provide a simple and scalable configuration that can be more easily

adopted in solar cell technology. Therefore, the development of halide perovskite active layers that

show high efficiency and stability under operation for planar heterojunctions may lead to technological

advancements in this field. However, one of the main challenges to fabricate planar heterojunctions is

still represented by the homogeneity of the active layer. A few attempts, based on vacuum processes,

have been made to prepare uniform layers on thin TiO2 films. For example, Snaith and collaborators

have demonstrated a dual evaporation process, which yield highly homogenous perovskite layers with

high power conversion efficiencies for photovoltaic applications.17 While this work represents a

significant advancement in the field, the use of high vacuum systems and the lack of tunability of the

composition of the active layer limit the applicability of this method.

i) Synthesis and characterization of low pressure vapor-assisted solution process (LP-VASP)

Motivation: Interestingly, extremely high uniformity has been achieved with the vapor-assisted solution

process (VASP)18 and modified low pressure VASP (LP-VASP)6,19. While the VASP, proposed by Yang and

collaborators18, requires higher temperatures and the use of a glove box, the LP-VASP is based on the

annealing of a lead halide precursor layer in the presence of methyl ammonium halide vapor, at reduce

pressure and relatively low temperature in a fume hood. These specific conditions enable access mixed

perovskite compositions, and fabrication of pure MAPbI3, MAPbI3-xClx, MAPbI3-xBrx, and MAPbBr3 can be

easily achieved. Specifically, CH3NH3PbI3-xBrx films over the full composition space can be synthesized

with high optoelectronic quality and reproducibility6,19. In the LP-VASP, formation of CH3NH3PbX3 films

consists of a two-step procedure that comprises i) the spin-coating of the PbI2/PbBr2 (or PbI2, or

PbI2/PbCl2) precursor on glass substrate or fluorine-doped tin oxide (FTO) coated glass substrate with

planar TiO2, as electron transport layer, and ii) the low pressure vapor-assisted annealing in mixtures of

CH3NH3I and CH3NH3Br that can be finely adjusted depending on the desired optical band gap (1.6 eV

Eg 2.3 eV). Under these conditions, the methylammonium halide molecules present in the vapor phase

slowly diffuse into the lead halide thin film yielding continuous, pinhole-free halide perovskite films. This

process yields a two-fold volume expansion from the starting lead halide precursor layer to the

completed organic-inorganic lead halide perovskite. The standard thickness of the perovskite film is

about 400 nm. It is possible to vary this thickness between 100-500 nm by changing the speed of the

second spin coating step. The presented technique results in films of high optoelectronic quality, which

translates to photovoltaic devices with power conversion efficiencies of up to 19% using a Au/spiro-

OMeTAD /CH3NH3PbI3-xBrx/compact TiO2/FTO/glass solar cell architecture.19

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Methodology and results: In detail, before the perovskite deposition several conducting and

semiconducting materials need to be processed and deposited for the fabrication of a complete solar

cell. On cleaned glass coated with fluorine-doped tin oxide (FTO),a compact TiO2 layer is deposited by e-

beam evaporation while the substrate is kept at the temperature of 350°C.E-beam deposited TiO2 layers

have native defects due to the presence of an oxygen-deficient environment during the electron beam

evaporation. Therefore, the deep-level hole traps introduced in the TiO2 layer contribute to a high

power conversion efficiencies of and enhanced long-term stability.19

Once the TiO2 layer is deposited on the glass/FTO substrate it is possible to start with the LP-VASP

process which could be divided in two sequential steps:

1. Deposition of the lead halide (PbX2)precursor;

2. Conversion of the PbX2layer into perovskite with methylammonium halideCH3NH3X(also called

MAX).

The lead halide precursor (PbI2 or PbBr2 or a mix of both) dissolved in N,N-dimethylformamide (DMF) is

spin coated on the glass/FTO/TiO2substrateand dried on a hot plate for 15’ at 110ᵒC. The setup (fig 1a)

for the second step comprises a Schlenk line equipped with a vacuum pump and with Schlenk tubes. In

these tubes, we place the MAX powder on the bottom, and the glass/FTO/TiO2/PbX2 substrates slightly

above the MAX precursor powder. The tubes (fig 1b) are immersed in an oil bath at 120°C and kept at a

reduced pressure (0.185 torr) for 2 hours. The lead halide precursor thin film starts converting into the

perovskite material due to the MAX incorporation: at this temperature and pressure the MAX powder

sublimates and saturates the atmosphere inside the tube and slowly diffuses in the lead halide film.

Figure 1 a) LP-VASP setup

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Figure 1 b) Tube immersed in oil bath at 120ᵒC; c) Tube with sample during the process.

During this time, it is possible to observe the color change of the sample from yellow due to

PbI2/PbBr2layer to dark brown for CH3NH3PbI3 and again from yellow to orange

forCH3NH3PbBr3perovskite.After 2h the sample is carefully removed from the tube, rinsed in isopropyl

alcohol (IPA) to wash away the non-reacted MAX form the surface and finally dried with the N2 flow. By

using different PbX2 (PbI2 or PbBr2) precursors and MAX (CH3NH3I or CH3NH3Br) for the conversion, it is

possible to tune the final composition of the perovskite from pure CH3NH3PbI3 (through several

CH3NH3PbI3-XBrX mixed perovskites) to pure CH3NH3PbBr3. Accessing different perovskite composition is

key to tune the band gap of this material.

After this step ,the Spiro-OMeTAD hole transport material (HTM) and the gold back electrode are

sequentially deposited through spin coating and thermal evaporation, respectively. Figure 2shows an

example of a complete solar cell that I fabricated during my stay at LBNL.

Figure 2: LP-VASP perovskite based solar cells with different photoactive layers:

on leftCH3NH3PbBr3 while on the right CH3NH3PbI3.

Once the solar cell is complete, it is possible to characterize the device with JV curve measurements

(Plot1). In this type of measurement, the current density (J) is measured as a function of the applied

voltage (V). From this data, the relevant figures of merit of the solar cell (VOC, ISC and FF) can be extracted

and used to calculate the power conversion efficiency (η).

Substrate

Powder

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Plot 1: I-V curve of one of the solar cells with CH3NH3PbI3 as perovksite photoactive layer.

This device shows η= 9.02% that is a good value compared with the best efficiencies reached at the IOM-

CNR laboratories (around 6%). The characterization reported represents my first attempt of synthesizing

and characterizing a solar cell with this new technique. This measurement proves that LP-VASP is easy to

manage also for non-expert users. Once the same setup will be reproduced at the IOM-CNR

laboratories, I will acquire more experience with it and it will be possible to obtain high efficient devices.

ii) Synthesis and characterization of porous polycrystalline perovskites

Motivation: Through the LP-VASP, it has been demonstrated that thin film crystallographic texture (i.e.

preferential grain orientation) can have a significant impact on photovoltaic performance.6By probing

the photovoltaic properties at the nanoscale, it has been found that there exists facet-dependent defect

concentrations that lead to significant photovoltage inhomogeneity within.20 This research points to new

ways to engineer interfaces and synthesis conditions of halide perovskites to obtain higher efficiency.

Despite rapid progress in the field, a number of fundamental questions remain in order to understand

the optoelectronic and transport properties of halide perovskite-based photovoltaic devices and to

determine the role of local inhomogeneity on electronic properties correlated with specific

crystallographic orientations.

While in Trieste, I developed a synthetic methodology to obtain well-faceted perovskite cubes, which

are particularly suitable for understanding how electronic properties correlate with crystallographic

orientation through advance photoconductive atomic force microscopy characterization. The perovskite

cubes I can produce show defined orientation and facets and allow to more precisely correlate local

property variations with macroscopic characteristics. In addition, the achieved control over the structure

and crystallographic orientation allow us to better understand roles of grain boundaries, local

compositional disorder, and fundamental charge separation and transport mechanisms in this materials

system.

Methodology and results: Since the perovskite material is moisture sensitive it was necessary to prepare

the samples at the hosting facility (LBNL). There I had to adapt my methodology to the infrastructure

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and available equipment, as the perovskite crystal growth is heavily dependent on the conditions used

for the process. I was eventually able to reproduce the synthesis and to achieve a better understanding

of the process and on the parameters that determine the formation of the desired structure.

For this experiment, I have first synthesized theCH3NH3I and CH3NH3Br precursors. These molecules have

been characterized with 1H-NMR (Plot 2a and 2b)and 13C-NMR techniques.

Plot 2: a) 1H-NMR of synthesized CH3NH3I and b)

1H-NMR of synthesized CH3NH3Br.

The porous polycrystalline perovskite film is obtained in N2filled glove bag by spin coating the PbI2 (1.0

M in DMF) precursor and annealing it at 95ᵒC for 15 min. The sample is first dipped in isopropyl alcohol

(IPA) for 10” to pre-wet the layer, then immersed in MABr solution (5 mg/mL in IPA) for 45”, and finally

rinsed in IPA to wash the MABr excess from the top of the film. After this first step a layer of well shaped

cubic crystals is obtained (fig3a). The samples is then immersed in MAI solution (8 mg/mL in IPA) for 90”,

rinsed in IPA, and annealed at 95ᵒC for 15 min to yield the final porous material (fig 3b).In addition to

understand and correlate electronic properties with crystal facets, this unique morphology could be

exploited to increase the surface contact between the perovskite and the hole transporting material

(HTM) such as Spiro-OMeTAD in order to enhance the holes extraction rate from the photoactive layer.

Figure 3: a) SEM image of the polycrystalline perovskite after the first dipping in MABr/IPA solution;

b) SEM image of the porous perovskite layer after the second dipping in MAI/IPA solution.

Once I have obtained the layer, I have first characterized its morphology by SEM images.

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Then, I have characterized these films by X-ray diffraction(XRD), after the first (Plot 3a) and the second

dipping steps (Plot3b). In the XRD pattern in Plot3a, it is possible to notice that the peak at 2 Theta

12.68°,assigned to PbI2, is intense as well as the perovskite peak at 14.58°, thereby showing that the

perovskite formation is not still complete. In the second XRD pattern, while the perovskite peak

increases, the PbI2 peak decreases in intensity, there by indicating the conversion is almost complete.

Plot 3: a) XRD of the polycrystalline perovskite after the first dipping in MABr/IPA solution;

b)XRD of the porous perovskite layer after the second dipping in MAI/IPA solution.

In order to test the efficiency of this material, I have fabricated a solar cell with the porous

polycrystalline perovskite.100 nm of TiO2 were e-beam evaporated on cleaned glass-FTO coated

substrate. The perovskite layer was then deposited as described. To complete the device, Spiro-

OMeTAD HTL was spin coated, then a gold back contact thermally evaporated. Unfortunately the so

fabricated device was not working as expected, and more optimization is needed. From the J-V

characterization, it was possible to observe that both series resistance (1.8E+7 Ω) and shunt resistance

(8.1E+6Ω) are very high meaning that the device has shunt pathways due to the presence of pinholes.

The total coverage of the perovskite layer is necessary to have high efficient solar cells.

Plot 4: I-V curve porous polycrystalline perovksite based solar cell.

However, these perovskite structure are now going to be characterized by PC-AFM, as a result of this

collaboration.

0,00

200,00

400,00

600,00

800,00

10,00 12,00 14,00 16,00Inte

nsi

ty [

arb

un

its]

2 Theta [ᵒ]

PbI2/MABr

0,00

400,00

800,00

1200,00

10,00 12,00 14,00 16,00

Inte

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ty [

arb

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2 Theta [ᵒ]

PbI2/MABr/MAI

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Summary and outcome: To summarize it was possible to learn the LP-VASP process that I will reproduce

at the IOM-CNR laboratory. This process could be exploited to reach high efficient solar cell and to

combine our know-how on nano-fabrication with the more specific perovskite experience of the Toma’s

group.

A secondary aspect that will be taken in consideration is the deposition of the TiO2 layer through the e-

beam evaporation. It seems to be highly reproducible and allows to reach high VOC. At the IOM-CNR it is

possible to deposit the layer with this technique but an upgrade is needed to keep the sample at the

desired temperature (350ᵒ).

The porous polycrystalline perovskite even if promising needs a further optimization if it would be used

for a solar cell. The next steps will be focused on the improving of crystal size control and coverage

uniformity in order to obtain a suitable layer for the photovoltaic device.

This project has allowed me to start a new collaboration between my supervisor Dr. Massimo

Tormen(IOM-CNR) and Francesca Toma (JCAP-LBNL), in order to merge our knowledge and fabricate

high efficient solar cells.

References

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2. Noh, J. H., Im, S. H., Heo, J. H., Mandal, T. N. & Seok, S. I. Chemical Management for Colorful,

Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells. Nano Lett.13, 1764–1769

(2013).

3. Stranks, S. D. et al. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal

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