Origins of Efficient Perovskite Solar Cells with Low-Temperature

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Origins of Efficient Perovskite Solar Cells with LowTemperature Processed SnO Electron Transport Layer 2

Alan Jiwan Yun, Jinhyun Kim, Taehyun Hwang, and Byungwoo Park ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00293 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Energy Materials

Origins of Efficient Perovskite Solar Cells with Low-Temperature Processed SnO2 Electron Transport Layer Alan Jiwan Yun,+ Jinhyun Kim,+ Taehyun Hwang, and Byungwoo Park* Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Korea

Recently, SnO2 has been noticed as a promising material for electron-transport layer of planar perovskite solar cells.

SnO2 layer presents advantages of low-temperature processability and high

power-conversion efficiency, and understanding the correlations between the SnO2 properties and device performance will provide a key to realize more efficient perovskite solar cells.

Herein,

uniform electron-transport layer using SnO2 nanoparticles is fabricated, and the effect of annealing on the solar-cell performance is discussed.

Solar cells with low-temperature processed SnO2-

nanoparticle layer (below 120°C or even at room temperature) exhibit desirable short-circuit current, open-circuit voltage and fill factor with the highest efficiency of 19.0%.

Using atomic

force microscopy and ultraviolet photoelectron spectroscopy, both great surface uniformity and favorable band alignment of low-temperature processed SnO2 layer have been observed, which are responsible for the device performance.

Furthermore, deep electronic-trap states at the

SnO2/perovskite interface are investigated via impedance analysis.

Compared to the cells

processed over 160°C, low-temperature processed cells exhibit trap states shifting toward the bandedge and reduced trap density, verifying that controlling the interfacial trap states holds a dominance on the open-circuit voltage, and is a critical requisite to enable efficient perovskite solar cells.

These less-defective solar cells fabricated below 120°C show high thermal stability,

suggesting further commercial applications. Keywords: Perovskite Solar Cell, Low-Temperature Process, Tin Oxide, Electron Transport Layer, Electronic Traps *

E-mail: [email protected];

Phone: +82-2-880-8319;

+

Two authors contributed equally to this work.

Fax: +82-2-885-9671.

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Research Highlights ▶ Dependence of device performance on the annealing temperature of SnO2 layer. ▶ Effects of morphology and band diagram of SnO2-nanoparticle layer on the solar cell. ▶ Reduced traps and trap shift toward the bandedge for the low-temperature processed cells. ▶ Efficient and stable perovskite solar cells with negligible hysteresis fabricated below 120°C.


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Introduction Organometal halide perovskite (OHP) is regarded as one of the most promising candidate for next-generation photovoltaic materials in the aspects of both cost and efficiency. Intense researches on the applications of OHP to solar cell have achieved remarkable progress, increasing the power conversion efficiency (PCE) up to ~23% in just several years, as a result of accomplishments in the compositional engineering and morphology/crystallization control of OHP.1-3 Recent studies have improved both the photovoltaic property and stability of perovskite solar cells by modifying and optimizing the perovskite films.2-8

However, limitations still remain within the conventional

electron/hole transport layers (ETL/HTL) and interfaces, and so does demand for further advances.9 The perovskite solar cell (PSC) with current world-record efficiency adopts mesoscopic TiO2 as an electron-transport layer.

Though mesoscopic structure of TiO2 can boost electron extraction

from the perovskite layer to anode, it has a drawback requiring high-temperature process around 500°C that hinders the commercialization of PSC.10-13

On the other hand, planar PSC requires

simple and economic fabrication processes compared to the mesoscopic PSC, and thus has attracted huge attention.14-16

For planar PSC, SnO2 film has been recently noticed to be one of the most

promising ETL with high charge mobility and low photoactivity.17-22 Several researches have observed the effect of SnO2 annealing temperature on the solar cell performance.23-25 For example, Fang et al. fabricated SnO2-based ETL by spin-coating SnCl2 precursor solution at several temperatures (185°C - 500°C), and Docampo et al. used ALD-processed SnOx ETL (room temperature (RT) - 300°C) appointing the Fermi-level change to be an origin of the device properties.23,24 Nevertheless, the exact correlation between the SnO2-film characteristics and solar-cell performance are not yet fully understood, especially with nanoparticle-based SnO2 ETL. In this work, dependence of the device performance on the SnO2 annealing temperature is observed with n-i-p planar PSCs using nanoparticle-based SnO2 layer as an ETL.

In order to

understand the annealing effects on the SnO2 properties and declare the factors contributing to the photovoltaic performance, nanoscopic morphology and optical/electronic properties of SnO2 films are scrutinized with various annealing temperatures (RT - 200°C).

In addition, since the solar cell

performance is strongly correlated to various factors including the properties of perovskite, 3 ACS Paragon Plus Environment


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electron/hole transporting materials, and interfaces,9,26-34 the deep-trap states and band alignment are also discussed. With explication on how SnO2 annealing temperature affects the film property and sequentially the solar cell performance, insights to realize efficient perovskite solar cell with lowtemperature process are provided, expanding the perspective of its commercial applications.

Experimental Section SnO2 Film Deposition ITO substrate (Wooyang GMS) was etched by zinc powder (TCI Chemicals) and cleansed under sonication in acetone, ethanol and deionized water, respectively. The substrate was used after 15 min of UV-ozone treatment. In order to prevent agglomeration of nanoparticles, 15 wt. % SnO2 nanoparticle dispersion in H2O (Alfa-Aesar) was stirred and sonicated before dilution.

Then the

suspension was diluted to 3.4 wt. %, filtered by PTFE membrane (CHMLAB) and spin-coated on the ITO substrate at 3000 rpm for 30 s.

Subsequently, SnO2 layer was annealed at various

temperature (RT, 80°C, 120°C, 160°C, and 200°C) for 30 min on a hot plate, and UV-ozone treated for 15 min afterward. Solar Cell Fabrication For the Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskite (CsFAMA) film, 1.3 M (FA0.83MA0.17)Pb(I0.83Br0.17)3 perovskite precursor solution was prepared by dissolving formamidinium iodide (FAI; Great Solar Laboratory), methylammonium bromide (MABr; Great Solar Laboratory), lead iodide (PbI2; TCI Chemicals) and lead bromide (PbBr2; TCI Chemicals) in a mixture of n,n-dimethylformamide (DMF; Sigma-Aldrich) and dimethylsulfoxide (DMSO; SigmaAldrich) with the volume ratio of 4:1.

For Cs doping, 1.0 M CsI (TCI Chemicals) solution

dissolved in the same solvent was added. The perovskite solution was stirred for 2 h, and then spincoated on the as-prepared SnO2 layer in two steps of spinning program: 1000 rpm for 10 s for the first step and 5000 rpm for 20 s for the second step. 300 μL of chlorobenzene (Sigma-Aldrich) was dropped 3 s before the second spin-step ends, and the substrate was annealed at 100°C for 30 min on a hot plate.

Hole transporting material was prepared by dissolving 72.3 mg of spiro-MeOTAD

(Lumtec) in 1 mL of chlorobenzene (Sigma-Aldrich), and 28.8 μL of 4-tert-butylpyridine (Sigma4 ACS Paragon Plus Environment

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Aldrich) and 17.5 μL of bis(trifluoromethane)sulfonimide lithium salt (Sigma-Aldrich) in acetonitrile (520 mg/mL) were added as additives.

The HTM solution was spin-coated on the substrate, and

finally Au electrode was thermally evaporated. Characterization The surface topography of SnO2 layer was observed by using atomic force microscopy (NX - 10; Park Systems). 10 × 10 μm2.

Each measurement was conducted for the area of either 1 × 1 μm2 or

4-point probe was utilized to measure electric conductivity of SnO2 films

deposited on a glass substrate.

Work function and valence band maximum of each deposited

SnO2 layer were measured via multi-purpose x-ray photoelectron spectroscopy (Sigma Probe; Thermo VG Scientific) with argon sputtering. UPS measurement.

Light with energy hν = 21.22 eV was used in the

Optical properties of SnO2 films were measured with UV/Vis spectroscopy

(V-770; JASCO) with an integrating sphere. IPCE system (K3100; McScience).

IPCE measurements were performed by solar cell

Photocurrent density-voltage (J-V) curves were achieved via

solar cell measurement system (K-3000; McScience) for an active area of 0.09 cm2.

During the

measurement, the voltage was swept from the reverse to forward direction (from 1.2 to -0.1 V with a scan rate of 100 mV/s). mass 1.5 G).

One sun light was illuminated using a solar simulator (Xenon lamp, air

For impedance analysis, a potentiostat (Zive SP-1: WonATech) was adopted, and

the impedance of photovoltaic device was measured with 10 mV AC perturbation in the frequency range from 10 mHz to 100 kHz, under zero bias in dark condition.

Before this measurement, all

the devices had been relaxed in dark until voltage drops below 3 mV.35

To observe film

morphology of perovskite layers and cross-sectional image of devices, a field-emission scanning electron microscope (Merlin Compact: Zeiss) was used.

Crystal structures were examined by an

x-ray diffractometer (New D-8 Advance; Bruker).

Results and Discussion To investigate the effect of SnO2 annealing temperature on the solar cell performance, SnO2 films are annealed at various temperatures, with the resultant cell parameters including η, VOC, JSC, FF, and HI (Fig. 1).

Though the optimum annealing temperature of nanoparticle-based SnO2 ETL 5 ACS Paragon Plus Environment


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has been previously regarded to exceed 150°C,18,21 the most efficient solar cell is acquired at 120°C with the highest η of 19.0%. The η presents a trend of arch upward, and it is mainly attributed to the VOC and FF that show similar dependences with peaks at the 120°C condition as well (Figs. 1(a), (b) and (c)).

Interestingly, the low-temperature processed SnO2 annealed below 150°C or even at

RT outperforms the high-temperature processed SnO2 in the aspects of VOC, FF, and consequently η. Furthermore, as shown in Fig. 1(d), the JSC is measured to increase as SnO2 annealing temperature decreases.

This annealing effect on JSC is consistent with the IPCE measurement in Fig. S1, of

which the integrated JSC also increases with the lower-temperature processed SnO2.

On the other

hand, hysteresis is reduced for the SnO2 ETLs annealed at lower temperatures (Fig. 1(e)). The J-V curve of a champion cell for each annealing condition is shown in Figs. S2(a) and (b), with the corresponding cell parameters referred in Table S1. The upper layers including perovskite layer, HTL, and metal electrode are fabricated by the same procedures.

The differences in both crystallinity and microscopic morphology of perovskite

upper layers are also negligible (Figs. S4 and S5), and thereby we assume that these observed tendencies in Fig. 1 originate only from the features of SnO2 ETL and its interface. Further analyses on the SnO2 films and interfaces are conducted to understand the correlation between their properties and the solar cell performance, as well as to reveal the origins of outstanding performance by lowtemperature processed SnO2 ETL. First, the surface morphology of SnO2 film has been observed to understand how uniformity of the surface changes by the annealing condition and affects the solar cell performance. Considering the size of SnO2 nanoparticles (3-5 nm)18 and thickness of film (~20 nm in Fig. S6(a)), surface morphology has been observed in nanoscale using atomic force microscopy (AFM) rather than scanning electron microscopy (SEM) (Fig. S6(b)). SnO2 films are shown in Figs. 2(a–f).

Topographies of bare ITO and various

In general, the deposited SnO2 films demonstrate

considerably smoother surface than bare ITO, confirming that SnO2 nanoparticle layers are successfully formed regardless of the annealing temperature.

In addition, the surface roughness is

measured quantitatively in peak-to-valley average and root-mean-squared value (σrms), respectively (Fig. 2(g)), confirming that the SnO2 nanoparticle layer shapes more uniformly and smoothly when 6 ACS Paragon Plus Environment

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annealed at lower temperature below 150°C.

The σrms for SnO2 annealed at 160°C and 200°C are

larger than 0.8 nm in average, while those of low-temperature processed SnO2 are ~20% smaller. Similarly, the peak-to-valley roughnesses soar at 160°C and 200°C. The morphology of SnO2 nanoparticle layer comes out to depend on its annealing temperature, and the rate of solvent evaporation is presumed to be responsible.

Regarding that the thickness of

deposited SnO2 layer is observed to be ~20 nm, the measured roughnesses of SnO2 annealed at 160°C and 200°C are of notable scale to deteriorate the contact with perovskite upper layer by forming shunting paths such as pinholes. It is well-known that the charge collection efficiency and the shunt resistance of solar cells are deeply involved with nanoscopic morphology of ETL and the formation of shunting paths,20,22,36,37 and hence the uniformity of SnO2 ETL is expected to influence both JSC and FF of the perovskite solar cell. Accordingly, the rough surface of high-temperature processed SnO2 is reproached for its reduced JSC and FF.

Though the electrical conductivity is greater for

higher annealing temperature (Fig. 2(h)), it seems not critical for the charge transport within only ~20 nm thick ETL. To further scrutinize the correlations between the annealing temperature of SnO2 and cell parameters, work function and valence band maximum (VBM) of SnO2 are evaluated from the secondary electron edge and valence band spectrum (Figs. 3(a) and (b), respectively).

As SnO2

annealing temperature increases, the work function of SnO2 film first rises from 4.14 eV at RT to 4.39 eV at 120°C, and then drops to 4.19 eV at 200°C.

The VBM with respect to the Fermi level

(VBM = Eoffset – EF) is observed to increase as SnO2 annealing temperature decreases.

To compare

the band diagrams, the optical bandgap of each SnO2 film was measured (shown in Fig. 3(c)), exhibiting from 3.96 to 3.92 eV as the annealing temperature increases from RT to 200°C.

The

band diagrams of SnO2 films depicted in Fig. 3(d) (based on the values of Table S2) indicates that the high-temperature processed SnO2 has relatively high conduction band maximum (CBM) with the perovskite upper layer.

With the driving force of electron extraction from the perovskite layer

to ETL, the low-temperature processed SnO2 is likely to exhibit more preferred band alignment in short-circuit condition for efficient electron transport (Fig. 1(d)).



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Observing the shifts of band levels, we suspect that the defect formation, especially at the ETL/perovskite interface, also depends on the SnO2 annealing temperature.

To explore the

defects of SnO2-based solar cells, the impedance analysis is performed for capacitance as C ≡ 1/iωZ (Fig. 4(a)).

The capacitive feature of perovskite solar cell at high frequency is

determined by geometrical and depletion-layer capacitance of perovskite,38,39 while the lowfrequency capacitance is claimed to originate from the charging/discharging of electronic traps at the interfaces.40-43

In Fig. 4(a), the capacitances at high frequency over 100 Hz are almost

indistinguishable, but on the other hand, the low-frequency capacitances show clear differences depending on the SnO2 annealing temperature.

Considering that the fabrication procedures

except the SnO2-annealing temperatures are identical for the examined devices, the differences in the low-frequency capacitances reflects the contributions of only from the ETLs and/or ETL/perovskite interfaces, rather than other constituents such as perovskite layers and HTLs. This argument is also supported by the analyses including x-ray diffraction and SEM on the perovskite layers with negligible differences (Figs. S4 and S5). Deep-level traps at the interfaces contribute to the device capacitance under low-frequency perturbation by being charged and discharged repeatedly.40,44,45

From the capacitance-frequency

plot, the trap-density state nt vs. Eω (= E – Ebandedge) can be calculated.45,46

In Fig. 4(b), nt for

different SnO2 annealing temperatures are compared and the deep-trap distribution noticeably varies, indicating that the defect formation at the interface between the SnO2 ETL and perovskite layer is influenced by the SnO2-annealing temperature.

From the Gaussian fitting of trap-density

state, the trap density (Nt) and trap level (E0) have been evaluated quantitatively (Fig. 4(c) and Table S3).

As shown in Fig. 4(c), Nt draws an arch downward with the lowest trap density at

120°C, of which the trend is opposite to that of the observed VOC. These results confirm that the interfacial deep-traps play a significant role determining the VOC of the solar cell, and the schematics are in Fig. 4(d). When PSC is illuminated, the photo-generated electrons fill the electronic traps, and when an electron is trapped in a deep-level state, a nonradiative recombination process is likely to occur.

The recombination is believed to be the main factor


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undermining VOC of the solar cell, which is verified by these SnO2 devices with low Nt exhibiting high VOC, and vice versa (Fig. 4(c)). Various defects in the perovskite solar cells are the main reasons causing J-V hysteresis or irreversible degradation in the device performance. Plausibly, the low-temperature processed SnO2 devices demonstrate small hysteresis effects, and their reduced traps are thought to be one of the reason.

Yet, other factors including shallow-traps are also underlying, making tendency of

hysteresis slightly different with that of the interfacial deep-traps (Figs. 1(e) and 4(c)). Meanwhile, as we have tested the thermal stability of the as-fabricated devices, these less-defective solar cells with SnO2 annealed at 120°C sustain 90% of their initial PCEs after being kept in the 85°C/85% relative-humidity (RH) environment for over 300 h, as shown in Fig. S7. Deeper studies are to be performed on these stability results, including the roles of various traps at each layer and interfaces in the long-term degradation mechanisms.

Conclusions Uniform electron-transport layer using SnO2 nanoparticles is systematically investigated. By varying the annealing temperature from RT to 200°C, the best efficiency of 19.0% has been obtained from annealing at 120°C.

The morphological, optical and electronic properties of SnO2 layers are

investigated, and the interfacial trap states between SnO2 and perovskite layers are analyzed. Efficient perovskite solar cells with low-temperature processed SnO2 ETL are realized by uniform/smooth SnO2 surface, favorable band alignment, and reduced trap density plus shifting toward the bandedge.

These less-defective solar cells can sustain 90% of their initial PCEs from

the 85°C/85%-RH environment for over 300 h. Our straightforward work offers insights to realize more efficient and stable perovskite solar cells with lower cost, enlarging the potential of commercial applications.



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Supporting Information Details of materials (SEM, XRD, AFM) and photovoltaic performance (IPCE, J-V, steadystate current, 85°C/85% RH test).

The following files are available free of charge.

Corresponding Author * E-mail: [email protected]

Phone: +82-2-880-8319;

Fax: +82-2-885-9671.

Author Contributions A. J. Yun and J. Kim contributed equally to this work.

Acknowledgements This work is supported by the National Research Foundation of Korea (NRF: 2016R1A2B4012938) and Korea Institute of Energy Technology Evaluation and Planning (KETEP: 20183010014470).


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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Figure 1. Device performance vs. SnO2 annealing temperature. (a) Power-conversion efficiency (η), (b) open-circuit voltage (VOC), (c) fill factor (FF), (d) short-circuit current density (JSC), and (e) hysteresis index (HI = (ηrev – ηfor)/ηrev) of solar cells as a function of SnO2 annealing temperature. Each data point demonstrates the representative device of one experimental batch. (f) JV curves of champion cells at room temperature and 120°C, with the cell geometry by SEM (CsFAMA = Cs-doped (FA0.83MA0.17)Pb(I0.83Br0.17)3).


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Figure 2.

Morphological features and electrical properties of SnO2 films.

Surface topography of (a) bare ITO substrate, and

(b - f)SnO2 layer annealed at different temperatures. The AFM images are shown in the same color scale. (g) Roughnesses by peak-to-valley and root-mean-squared values as a function of annealing temperature, with bare ITO as a reference. (h) Electric conductivity of SnO2 thin films.



Figure 3. Energy level diagram of SnO2 electron-transport layer. (a) Secondary electron edge and (b) valence-band maximum of SnO2 layer at each annealing temperature (as measured by UPS). (c) Tauc plot of SnO2 films for the bandgap energy (Eg). The blue arrow indicates the shift of bandgap as the annealing temperature increases. (Detailed fitting parameters for (a), (b) and (c) are in Table S2.) (d) Schematic band diagram of each SnO2 layer with the perovskite as a reference.


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Figure 4. Effects of annealing temperature on the electronic trap states. (a) Capacitancefrequency plot and (b) trap distribution spectra of solar cells. (c) Trap density per volume (Nt) and VOC of each solar cell. Nt is obtained by fitting the trap density state in (b) using a Gaussian function. (d) Schematics for the role of interfacial traps in recombination.



Graphical Abstract (TOC Graphic)


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Origins of Efficient Perovskite Solar Cells with Low-Temperature

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