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

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 Fabrication of transparent ITO/Ga-doped ZnO coating as a front panel electrode toward efficient thin film solar cells M. Aleksandrova 1 , T. Tsanev 1 , T. Ivanova 2 , K. Gesheva 2 , V. Strijkova 3 , J. Singh 4 , A. K. Singh 5 1 Technical University-Sofia, Dept. Microelectronics, Bulgaria 2 Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, Bulgaria 3 Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Bulgaria 4 Dr. Harisingh Gour University Sagar, Depart.Physics, India 1 5 Government V.Y.T.PG.Autonomous College, Durg, India

2 nd 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. 2

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 State-of-the-art Currently, the most widely spread transparent conductive electrode (TCE) is made of indium tin oxide (ITO) or fluorine tin oxide (FTO). For energy level alignment with the perovskite photoconductors are used conductive polymers (PEDOT:PSS), or titanium dioxide (TiO 2 ). However, they are not effective optical filters and cannot reject neither Sci Rep 8, 1070 (2018) infrared, nor ultraviolet component of the sun spectrum Chem. Sus. Chem. 918 2592 (2016) 3

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 Experiment -DC (or ~RF) Thin films of ZnO doped by Ga (GZO), and ITO/GZO water cooling cathode system were prepared by RF sputtering of 3 inches- diameter targets positioned 12 cm above glass shield target substrates. The sputtering voltage was kept inert gas ejected Ar (Ar) ions particle constant for all sputtered combinations and it was - - Ar + - Ar + Ar electron 0.85 kV for the ITO films and 0.7 kV for the GZO Ar Ar plasma thin film films. The total sputtering pressure in the chamber (argon) was 1.10 -3 Torr for the sample of single substrate holder substrate (anode) layer GZO and bi-layer ITO/GZO without additional +DC (or ground) oxidation. For the ITO/GZO1 the total sputtering RF and DC sputtering of ITO/GZO pressure (argon + oxygen) was 1.10 -2 Torr due to and Al electrodes, respectively introduction of oxygen, which corresponds to 10% oxidation. 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. 4

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 Results 0.40 2.00 GZO2 0.35 refractive index 1.95 -2 , Ohm.cm 0.30 1.90 0.25 1.85 0.20 resistance x10 GZO1 0.15 1.80 0.10 1.75 GZO 0.05 1.70 0.00 1E-3 0.01 0.1 1E-3 0.01 0.1 total sputtering pressure, Torr total sputtering pressure, Torr Refractive index of GZO in the IR region, Electrical resistance of ITO/GZO at sputtered at different total sputtering different total sputtering pressures due to pressures due to additional target oxidation. 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 5 resulted in a resistance decrease to 0.03x10 -2 Ω.cm.

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 Results GZO: Ra = 13 nm ITO/GZO: Ra = 16 nm ITO/GZO1: Ra = 9.5 nm ITO/GZO2: Ra = 7.1nm AFM images of GZO film; bi-layer ITO/GZO without additional oxidation; ITO/GZO1 with 10 % and ITO/GZO2 with 20% of oxidation. Ra is average roughness. 6

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 Results Although great number of hills existed The smoother films are expected to in ITO/GZO films as compared to single result in the decrease of the optical GZO they were more uniformly losses during transmittance of the distributed and less sharp due to the visible light. To investigate this relation, ITO insertion. Further, the number and the transmission spectra in broad height of the sharp peaks decreased wavelength region between 190 nm with the oxygen content increase and 800 nm were measured as a during sputtering. function of the oxygen content. 7

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 Results 100 100 Transmittance, % ITO/GZO2 90 Reflectance, % 80 ITO/GZO1 80 ITO/GZO 70 GZO 60 60 50 40 GZO 40 ITO/GZO 20 30 ITO/GZO1 ITO/GZO2 20 0 10 200 300 400 500 600 700 0.8 1.2 1.6 2.0 2.4 Wavelength, nm Wavelength,  m Optical transmittance in the UV-VIS Optical reflection in the NIR range of range of single GZO layer and bi-layer single layer of GZO and bi-layer coatings coatings ITO/GZO without and with ITO/GZO without and with additional additional oxidation during sputtering. 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. 8

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 Results 1.0 E, eV ITO/GZO ITO/GZO1 0.8 3 Intensity, a.u. ITO/GZO2 3.8 GZO2 3.9 0.6 4 4.23 4.1 perovskite 4.3 CdS/ZnS QD Al 0.4 ITO 4.4 5 GZO 0.2 5.3 0.0 6 6 0 2 4 6 8 10 12 14 Binding energy, eV UPS spectra for determination of the Energy band diagram of CdS/ZnS core- shell quantum dots/perovskite solar work function of ITO/GZO at different cell with optimal ITO/GZO2 film as TCE. oxidation degree. 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. 9

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 Conclusions • 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 oxidation degree and the surface roughness of the GZO layer. • The optical 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 %. 10

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 • 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 of solar cells implementing ITO/GZO as a front filtering electrode and determination of the electrical losses at the bi-layer coating’s interface. 11

2 nd Coatings and Interfaces Web Conference (CIWC 2020) 15 – 31 May 2020 THANK YOU FOR YOUR ATTENTION! Corresponding author’s e -mail: m_aleksndrova@tu-sofia.bg Acknowledgements: The authors acknowledge the funding support from BNSF, grant KP06-India-6. 12