Fabrication of transparent ITO/Ga-doped ZnO coating as a front - - PowerPoint PPT Presentation

Fabrication of transparent ITO/Ga-doped ZnO coating as a front panel electrode toward efficient thin film solar cells

• M. Aleksandrova1, T. Tsanev1, T. Ivanova2,
• K. Gesheva2, V. Strijkova3, J. Singh4, A. K. Singh5

1Technical University-Sofia, Dept. Microelectronics, Bulgaria 2Central Laboratory of Solar Energy and New Energy Sources,

Bulgarian Academy of Sciences, Bulgaria

3Institute of Optical Materials and Technologies, Bulgarian

Academy of Sciences, Bulgaria

• 4Dr. Harisingh Gour University Sagar, Depart.Physics, India

5Government V.Y.T.PG.Autonomous College, Durg, India

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2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

Aim/Novelty

• Aim: To prepare electrode for solar cell that is highly conductive, highly

transparent for the visible light, highly reflective for the infrared range and smooth. This is a precondition for reduced optical and electrical losses in the solar cells, resulting in an efficiency increase.

• Novelty: According to the literature, the effect of the oxygen partial

pressure during sputtering on the ZnO doped by Ga (GZO) film’s morphology and the optical properties has not been yet investigated. In this work we tried to fill this gap by preparing GZO/ITO system by RF sputtering of GZO films at different oxygen pressures for application in CdS/ZnS core-shell quantum dots/perovskite low-cost solar cell.

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2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

State-of-the-art

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2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

Sci Rep 8, 1070 (2018)

• Chem. Sus. Chem. 918

2592 (2016) Currently, the most widely spread transparent conductive electrode (TCE) is made of indium tin

• xide

(ITO)

• r

fluorine tin oxide (FTO). For energy level alignment with the perovskite photoconductors are used conductive polymers (PEDOT:PSS),

• r

titanium dioxide (TiO2). However, they are not effective optical filters and cannot reject neither infrared, nor ultraviolet component of the sun spectrum

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Experiment

2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

Thin films of ZnO doped by Ga (GZO), and ITO/GZO were prepared by RF sputtering of 3 inches- diameter targets positioned 12 cm above glass

• substrates. The sputtering voltage was kept

constant for all sputtered combinations and it was 0.85 kV for the ITO films and 0.7 kV for the GZO

• films. The total sputtering pressure in the chamber

(argon) was 1.10-3 Torr for the sample of single layer GZO and bi-layer ITO/GZO without additional

• xidation. For the ITO/GZO1 the total sputtering

pressure (argon + oxygen) was 1.10-2 Torr due to introduction of oxygen, which corresponds to 10%

• xidation. For ITO/GZO2 the total sputtering

pressure (argon + oxygen) was 1.10-1 Torr due to introduction of more oxygen, which corresponds to 20% oxidation. Simple solar cell was produced by spin coating of the functional layers.

• DC (or ~RF)

+DC (or ground) substrate holder (anode) substrate thin film plasma cathode water cooling system shield

Ar+ Ar+

inert gas (Ar) ions ejected particle target Ar Ar Ar Ar

• electron

RF and DC sputtering of ITO/GZO and Al electrodes, respectively

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Results

2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

1E-3 0.01 0.1 1.70 1.75 1.80 1.85 1.90 1.95 2.00

GZO2 GZO1 GZO refractive index total sputtering pressure, Torr

Refractive index of GZO in the IR region, sputtered at different total sputtering pressures due to additional target oxidation.

1E-3 0.01 0.1 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

resistance x10

• 2, Ohm.cm

total sputtering pressure, Torr

Electrical resistance of ITO/GZO at different total sputtering pressures due to additional target oxidation. It can be noted an increase of the refractive index of GZO from 1.72 without additional oxidation of the target, to 1.98 after 10-1 Torr introduction of oxygen. The GZO single layer showed a resistivity of 1.67x10-2 Ω.cm. Insertion of ITO resulted in a resistance decrease to 0.03x10-2 Ω.cm.

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ITO/GZO: Ra = 16 nm

Results

2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

GZO: Ra = 13 nm ITO/GZO1: Ra = 9.5 nm ITO/GZO2: Ra = 7.1nm AFM images of GZO film; bi-layer ITO/GZO without additional

• xidation; ITO/GZO1 with 10 % and ITO/GZO2 with 20% of oxidation.

Ra is average roughness.

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Results

2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

Although great number of hills existed in ITO/GZO films as compared to single GZO they were more uniformly distributed and less sharp due to the ITO insertion. Further, the number and height of the sharp peaks decreased with the oxygen content increase during sputtering. The smoother films are expected to result in the decrease of the optical losses during transmittance of the visible light. To investigate this relation, the transmission spectra in broad wavelength region between 190 nm and 800 nm were measured as a function of the oxygen content.

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Results

2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

200 300 400 500 600 700 20 40 60 80 100

GZO ITO/GZO ITO/GZO1 ITO/GZO2

Transmittance, % Wavelength, nm

0.8 1.2 1.6 2.0 2.4 10 20 30 40 50 60 70 80 90 100 ITO/GZO2 ITO/GZO1 ITO/GZO GZO

Reflectance, % Wavelength, m

Optical reflection in the NIR range of single layer of GZO and bi-layer coatings ITO/GZO without and with additional

• xidation during sputtering.

Optical transmittance in the UV-VIS range of single GZO layer and bi-layer coatings ITO/GZO without and with additional oxidation during sputtering. The mean optical transmittance of the ITO/GZO2 film in the visible region was 91.3% and the transparency for the UV component was 6%. IR wavelengths reflection and respectively heat rejection was greater than 65 % for ITO/GZO2.

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2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

Results

2 4 6 8 10 12 14 0.0 0.2 0.4 0.6 0.8 1.0 ITO/GZO ITO/GZO1 ITO/GZO2

Intensity, a.u. Binding energy, eV

E, eV 3 4 5 6 4.3 ITO 4.4 4.23 GZO GZO2 CdS/ZnS QD perovskite 3.9 6 4.1 Al 3.8 5.3

UPS spectra for determination of the work function of ITO/GZO at different

• xidation degree.

Energy band diagram of CdS/ZnS core- shell quantum dots/perovskite solar cell with optimal ITO/GZO2 film as TCE. With an increase of the oxidation degree, the work function of GZO2 decreased to 4.23, thus forming breakdown of the interface barrier height from 0.4 eV into two partitions – 0.33 eV and 0.07 eV. Therefore, the solar cells will have enhanced electrons extraction at the interface and increased performance is expected.

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Conclusions

2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

• ITO/GZO

by-layered coatings were deposited on glass substrates by RF sputtering at various oxygen contents. The electrical and optical properties of the bi- layer system were strongly affected by the

• xidation degree and the surface roughness
• f the GZO layer.
• The
• ptical

data showed that the additionally oxidized GZO films with ITO underlayer exhibited slightly differed mean visible transmittance over 90%, but the difference of the reflection spectra in the IR range is more significant – the films IR rejection ability differs with almost 40 %.

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• GZO2 is characterized with a lower work

function than GZO and intermediate work function between the CdS/ZnS core-shell quantum dots used in solar cells and the ITO

• film. This suggests facilitation of the electron

extraction from the absorber to the cathode.

• In summary, ITO/GZO2 could serve as a

transparent conductive electrode and sunshade coating. Additionally, UV filtration is achieved, which is expected to slow down the aging processes in the cells.

• The future work will be related to impedance

spectroscopic study for detailed estimation of the contact properties

• f

solar cells implementing ITO/GZO as a front filtering electrode and determination of the electrical losses at the bi-layer coating’s interface.

2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

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THANK YOU FOR YOUR ATTENTION!

Acknowledgements: The authors acknowledge the funding support from BNSF, grant KP06-India-6.

2nd Coatings and Interfaces Web Conference (CIWC 2020) 15–31 May 2020

Corresponding author’s e-mail: m_aleksndrova@tu-sofia.bg