Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

Transcription

1 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics ICONN 214, Adelaide Dr. Enrico Della Gaspera CSIRO MATERIALS SCIENCE AND ENGINEERING / FUTURE MANUFACTURING FLAGSHIP

2 Introduction Different solution-based approaches... Metal Nano / Microcrystals (Cu, Ag, Ni ) Solution Processed Inorganic CSIRO Chemical Bath Deposition of Oxides / Sulphides (ZnO, ZnS, CdS...) Metal Chalcogenides Molecular Precursors (CdS, ZnS, CZTS, CIGS ) Metal Chalcogenide Nanocrystals (CdTe, ZnS, CZTS, CIGS ) Metal Oxide Nanocrystals / Sol-Gel (ZnO, TiO 2, CuO )...one goal: solution-processed inorganic solar cells FAST CHEAP + + LARGE SCALE PRODUCTION FLEXIBLE SUBSTRATES 2 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

3 Introduction Traditional CZTS solar cells architecture Charge collection grid: evaporated Al, Ag or Ni. Charge transport layer: sputtered ITO or doped-zno. Buffer layer: sputtered intrinsic ZnO. Window layer: chemical bath deposition of CdS. Absorbing layer: sputtered, evaporated or deposited from solution (molecular precursors, colloidal inks). TCO ZnO CdS CZTSSe Substrate: Molybdenum film sputtered on glass. Molybdenum Glass Even if the whole p-n junction can be deposited from solutions, scientists still rely on vacuum based depositions for the top layers and contacts for CTZS-CIGS devices. 3 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

4 Introduction Why is ZnO buffer important? Intrinsic ZnO acts as a shunt barrier between the CdS and the TCO. Improvement of the performances and the reliability of the device. ZnO film has to be continuous, dense and thin: NO ZnO: the shunt resistance of the device decreases, with the consequent detrimental effect on the performances (shorting). THICK ZnO: device too resistive (problem with charge collection). Also light is absorbed in the near UV (decrease in efficiency). Prog. Photovolt., 212, 2, 619. Thin Solid Films, 21, 387, 141. Energy Environ. Sci., 214 DOI: 1.139/C3EE42541J Sputtering of ZnO-based layers is a well established technique to prepare dense and homogeneous films. When deposited by sputtering, ZnO usually crystallizes with grains oriented along the c-axis of the hexagonal cell. MOTIVATION: Is it possible to obtain the morphology and the properties of sputtered films using a solution-process approach? 4 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

5 ZnO films from chemical bath deposition The idea is to adapt a Chemical Bath Deposition (CBD) method traditionally used to grow arrays of nanowires: a zinc salt is dissolved in water and ZnO forms in the presence of a base through heterogeneous nucleation on seeded surfaces: Several parameters can be changed to tune the ZnO morphology and properties: Zinc precursor Type of base ph Temperature Time Seeds Concentration... Publications on ZnO by CBD usually focus on long wires and on scattering layers. There are a few examples of dense coatings, but always very thick (microns). Even if precise control on nanowires length, density and alignment is achieved, the first few hundreds of nm are usually disordered. J. Mater. Chem., 24, 14, Nature Mater. 25, 4, 455. J. Am. Chem. Soc. 29, 131, J. Mater. Chem. C, 213,1, Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

6 ZnO films from chemical bath deposition Chemical Bath Seeds Deposition High T ( C) Low [OH ] Low [Zn2+] Substrate Oriented long wires Sol-Gel Low T ( C) High [OH ] High [Zn2+] Colloids 6 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics Oriented short rods: Dense film Not oriented Rods /wires

7 ZnO films by CBD Effect of Seeds S E E D S Colloids Not oriented, size <5 nm 1 nm Sol-Gel at 18 C Sol-Gel at 4 C Oriented, size <5 nm Oriented, size 1-3 nm 1 nm 1 nm C B D 2 nm 2 nm 2 nm Not aligned Thinner rods, higher density per area unit Unseeded substrates (even if crystalline) did not show any ZnO deposition Thicker rods, lower density per area unit 7 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

8 Frequency (%) Average size (nm) ZnO films by CBD Effect of ph and concentration [Zn]=[HMTA]=25mM [Zn]=[HMTA]=75mM ph 5 ph 7 ph 5 ph 7 2 nm mm, ph 5 25 mm, ph 7 75 mm, ph 5 75 mm, ph Higher Zn concentration causes enhanced nucleation and growth leading to denser coatings. The ph affects the lateral growth, because OH groups can absorb on (1) planes limiting vertical growth. Moreover seeds are more stable at neutral ph Size (nm) 25mM ph 5 25mM ph 7 75mM ph 5 75mM ph 7 J. Mater. Chem. 24, 14, 2575 J. Phys. Chem. C, 28, 112, 68 8 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

9 Thickness (nm) Frequency (%) Average size (nm) ZnO films by CBD Effect of time 15 min 65 nm 3 min 11 nm 6 min 14 nm 12 min 31 nm 4 nm Time (min) min 3 min 6 min 12 min Size (nm) Time (min) Increasing the reaction time causes a progressive increase in thickness, fairly linear up to 2 hours ( ~ 3 nm final thickness). Lateral size increases with time as well. 9 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

10 Absorbance Thickness (nm) Absorbance ZnO films by CBD Effect of temperature 6 C 65 C 7 C 75 C 8 C Increasing T 8 C 75 C 7 C 65 C 6 C Thickness Absorbance nm Wavelength (nm) Temperature ( C).4 Increasing the reaction temperature causes a progressive increase in thickness of ZnO films. ZnO growth is fairly slow below ~ 75 C. At higher temperatures it is much faster and harder to control. 1 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

11 ZnO films by CBD Sample facing UP Effect of sample positioning Precipitates can deposit on the surface of the sample causing irregular growth. 2 nm 2 nm Sample facing DOWN Precipitates deposit on the back of the substrate: ZnO growth is not affected. 2 nm 2 nm 11 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

12 Absorbance Intensity (a.u.) ZnO films by CBD Optimized conditions Optimized conditions: Seeds: sol-gel, 18 C Base: HMTA, 75mM Zinc: nitrate, 75 mm ph: 7 Temperature: 7 C Time: 3-6 min Position: facing down Substrate: any Sputtered SEM Sputtering CBD Sputtering CBD XRD Glass CBD (deg) 2 (deg) Sputtering Sputtering CBD Sputtering CBD CBD Wavelength 2 (deg) Wavelength (nm) (nm) UV-Vis CBD Sputtering Great reproducibility confirmed after repeated syntheses, also on different substrates (properly seeded): SiO 2, Si, CdS, Mo. These ZnO films prepared by CBD have very similar morphology and properties compared to sputtered layers! 12 Solution-processed Sputtering ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics CBD

13 Absorbance Absorbance Absorbance Absorbance ZnO films by CBD 3 3 Optical properties/ CdS CdS + discrete ZnO rods CdS + dense ZnO film CdS CdS + discrete ZnO rods CdS + dense ZnO film ZnO contribution Dense ZnO on glass Dense ZnO on glass/cds Dense ZnO on glass Dense ZnO on glass/cds Wavelength (nm) Wavelength (nm) Wavelength (nm) Wavelength (nm) At a set thickness, dense ZnO film has a much higher absorbance in the near UV. ZnO coatings are transparent in the visible and NIR range. Depositions with almost identical properties are obtained on glass and CdS. Provided the use of a seeded surface, this procedure can be applied on potentially any substrate, crystalline or amorphous. Moreover, due to the low deposition temperature, and the benign (aqueous at neutral ph) environment, it is applicable to plastic substrates. 13 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

14 Absorption (a.u.) Refractive Index, n ZnO films by CBD Optical properties/2 Sputtering Sol-Gel CBD NPs ZnO water and -OH CO 2 Organics Sputtering Sol-Gel CBD NPs Wavenumber (cm -1 ) FTIR shows very little amount of organic compounds and a sharp and intense Zn-O peak, very similar to the sputtered sample Wavelength (nm) The refractive index of ZnO films by CBD is only about 6% less than the bulk value*: very dense films (sputtered film is 2% less) Other solution processed films (NPs, sol-gel) show much larger porosity, presence of organic contaminants and are inherently less crystalline unless annealed at high temperatures * CRC Handbook of Chemistry and Physics, 27, Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

15 Intensity (a.u.) ZnO films by CBD Grain Alignment (2) P=2.97 (1) (11) A=98% P=2.89 P=2.74 P=2.5 A=94% A=87% A=75% Texture Coefficient P=1 A=% (deg) Alignment Factor P 1 A 1 n 1 c-axis 15 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

16 Current Density (ma/cm 2 ) ZnO films by CBD Solar Cells Champion cells: ITO ZnO CdS CZTSSe large grain CBD 1 CZTSSe fine grain Mo/MoSe 2 5 nm 1 nm Voltage (V) Sputtering Solar cells fabricated using ZnO from CBD show identical performances when compared to devices incorporating sputtered ZnO layers ZnO CBD ZnO Sputtering V OC (V) J SC (ma/cm 2 ) FF (%) Efficiency (%) van Embden et al. CZTSSe Solar Cells from Polar Nanocrystal Inks, submitted Della Gaspera et al. Mimicry of sputtered ZnO thin films through aqueous deposition, in preparation 16 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

17 Conclusions and Acknowledgements Summary High quality ZnO films have been prepared using a low temperature aqueous deposition. These ZnO films have very similar morphology and properties compared to sputtered ZnO. Sputtered CBD High efficiency (> 7%) solar cells have been obtained using both sputtered and CBD ZnO. Acknowledgements Dr. Jacek Jasieniak Dr. Joel van Embden Dr. Anthony Chesman Dr. Noel Duffy Flexible Electronics Theme Funding 17 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

18 Conclusions and Acknowledgements Summary High quality ZnO films have been prepared using a low temperature aqueous deposition. These ZnO films have very similar morphology and properties compared to sputtered ZnO. High efficiency (> 7%) solar cells have been obtained using both sputtered and CBD ZnO. Sputtered CBD If you want to know more about our solution-processed inorganic materials: Dr. Enrico Della Gaspera Today (Wed) 5-6 pm, Poster #154, Halls F&G Non injection, high concentration synthesis of Ga-doped ZnO colloidal nanocrystals Dr. Joel van Embden Today (Wed) 5-6 pm, Poster #82, Halls F&G CZTSSe thin films solar cells and Near infrared absorbing copper antimony sulphide nanocrystals Dr. Anthony Chesman Tomorrow (Thu), 11 am, Session TH 1.1, MR 4-5 One-pot, multigram synthesis of CZTS and CZGS nanocrystals using a dual precursor approach 18 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

19 Thank you Dr. Enrico Della Gaspera CMSE / Flexible Electronics / Solution Processed Inorganic Electronics Laboratory e enrico.dellagaspera@csiro.au t w CSIRO MATERIALS SCIENCE AND ENGINEERING / FUTURE MANUFACTURING FLAGSHIP

20 2 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

21 BACKUP SLIDES 21 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics

22 ZnO films by CBD Effect of amines DMAB, 6 C Dimethyl amino DETA, 9 C borane: CBD at low temperatures HMTA, 6 C (6 C) but uncontrolled growth of big crystals resulting in very rough films Diethylene triamine: Need much higher temperatures (9 C) and gives nanoparticulate coatings Hexamethylene tetraamine: CBD at low temperatures (6 C) and nice tuning of the rods growth according to reaction temperature 7 C 8 C 9 C 22 Solution-processed ZnO films as an alternative to sputtered buffer layers for inorganic photovoltaics