Laser irradiation of ZnO:Al/Ag/ZnO:Al multilayers for electrical isolation in thin film photovoltaics
© Crupi et al.; licensee Springer. 2013
Received: 9 August 2013
Accepted: 6 September 2013
Published: 23 September 2013
Laser irradiation of ZnO:Al/Ag/ZnO:Al transparent contacts is investigated for segmentation purposes. The quality of the irradiated areas has been experimentally evaluated by separation resistance measurements, and the results are complemented with a thermal model used for numerical simulations of the laser process. The presence of the Ag interlayer plays two key effects on the laser scribing process by increasing the maximum temperature reached in the structure and accelerating the cool down process. These evidences can promote the use of ultra-thin ZnO:Al/Ag/ZnO:Al electrode in large-area products, such as for solar modules.
In this letter, we demonstrate how the energy density threshold for the scribing of the transparent contacts can be significantly reduced by replacing the standard thick AZO single layer with a 10 times thinner AZO/Ag/AZO multilayer structure with better electrical and optical properties. More specifically, for the lowest used pulse energy, we measure a separation resistance for the AZO/Ag/AZO structure 8 orders of magnitude higher compared to much thicker AZO, currently used in thin film solar cells. The experimental results and the numerical simulations provide clear evidences of the key role played by the silver interlayer to steep temperature increase at the DMD/glass interface, leading to a more efficient P1 scribing through a reduction of the fluence in a single laser pulse. These results could open great opportunities for the implementation of thin AZO/Ag/AZO electrodes on large-area modules liable to segmentation, such as for α-Si:H solar panels.
AZO/Ag/AZO multilayers were sequentially deposited on conventional soda lime glass substrates by RF magnetron sputtering at room temperature in argon atmosphere with a working pressure of 1 Pa. A ceramic AZO target containing 2 wt.% Al2O3 and a pure Ag target were employed as source materials. The sputtering powers were 225 and 30 W for AZO and Ag, respectively. The deposition times were set in order to obtain 40 nm for both top and bottom AZO films and an optimum thickness of 10 nm for the Ag interlayer. This value was selected to fabricate a DMD structure that has high optical transparency in the visible range and good electrical conductivity . The thicknesses of the films were verified by Rutherford backscattering spectrometry (RBS; 2.0-MeV He+ beam) measurements in normal detection mode. Laser treatments were performed in air by a single pulsed (12 ns) Nd:YAG laser operating with an infrared (λ = 1,064 nm), Gaussian-shaped (FWHM = 1 mm) beam. The laser power was varied to obtain fluences in the range from 1.15 to 4.6 J/cm2. The morphologies of the AZO/Ag/AZO multilayer after the laser irradiation process were investigated by field emission scanning electron microscopy (SEM) using a Zeiss Supra 25 microscope (Oberkochen, Germany). Electrical sheet resistance (Rsh) of about 8 Ω/sq was measured on the as-deposited DMD electrode using a four-point terminal method by employing an HL5560 system (Bio-Rad, Hercules, CA, USA), while the change of the conductivity due to laser ablation process has been mapped by lateral current–voltage characteristics acquired with a Keithley 4200 semiconductor characterization system (Cleveland, OH, USA). Additionally, to simulate the laser process in our materials, a finite element method based on COMSOL Multiphysics software was employed.
Results and discussion
Specific heat, Cp (J kg−1 K−1)
Density, ρ (g cm−3)
Thermal conductivity, κ (W m−1 K−1)
Absorption coefficient, α (cm−1) (at 1,064 nm)
1.03 × 105
4 × 103
Reflection coefficient, R (at 1,064 nm)
A single nanosecond laser pulse has been used to investigate the scribing process of an ultra-thin DMD electrode (AZO/Ag/AZO structure). Given a reduced pulse energy of 1.15 J/cm2, the separation resistance of AZO/Ag/AZO is enhanced by 8 orders of magnitude compared to thicker AZO, currently used in thin film solar cells. The thermal behaviour, simulated using a finite element approach, shows that the silver interlayer plays two key effects on the scribing process by increasing the maximum temperature reached in the structure and fastening the cool down process. It is worth noting that although only a partial ablation of the DMD occurs at low laser fluences, the presence of the rip at the edge of the spot ensures an excellent electrical isolation, while such a morphology in standard TCO upon laser processing has never been reported to our knowledge. The presence of Ag has two main effects on the laser process: (1) higher temperature gradients and (2) different expansion and contraction of each layer during and after the irradiation, respectively. The latter point is a consequence not only of the first one (high thermal gradient between glass and film) but also of the difference in the thermal expansion coefficients of the materials: 18.9 × 10−6, 4.75 × 10−6 and 8.9 × 10−6 K−1 for Ag, AZO and soda lime, respectively. The substrate and coatings will expand differently upon the temperature change during the laser irradiation. As a result, thermally induced stresses are expected to arise. Because of the lower thermal expansion coefficient, AZO layers will suffer a reduced expansion with respect to the inner Ag film, and a compressive stress is then exerted by the inner layer on the outer layers which, after the thermal quenching, gives birth to the observed laceration. Our results, in combination with its excellent electro-optical properties, make the AZO/Ag/AZO electrode a suitable candidate for use in large-area modules, liable to segmentation, such as for α-Si:H solar panels.
The authors would like to thank C. Percolla and S. Tatì (CNR-IMM MATIS) for their expert technical assistance. This work has been partially funded by the MIUR project PON01_01725.
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