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Introduction
In order to increase efficiency in flexible thin-film solar cells, it is crucial to employ advanced light trapping schemes [1]. Such light-trapping schemes must be coupled with high quality absorber layers to ensure high open-circuit voltages in multi-junction solar cells [2][3][4]. The challenge is that the introduction of textured interfaces that facilitate enhanced light trapping competes with the ability to process high quality PV materials on top of it. To cope with these limitations, modulated surface texturing (MST) approach has been employed [5]. It consists of superimposing the micro-sized craters induced on the substrate with naturally nano-texturing of a TCO [6]. The embodiment of this approach has lead very high efficiency for tandem micro-morph solar cells on glass substrates [7].
In this work, we show the development of such MST approach on temporary Al foil substrates. We first investigate and characterize different texturing techniques, then we grow nc-Si:H absorber layer to investigate its quality.
Experimental details
Bare Al foils are divided in two different categories; i) direct etching and ii) sacrificial layer etching. The first samples are etched directly in KOH diluted in H2O at T > 30 °C. The second category samples instead undergo a sputtering deposition of ITO/AZO as sacrificial layers. This layer is subsequently etched in HF:H2O2:H2O or KOH diluted in H2O at T > 30 °C. The samples are characterized by AFM, SEM and angular intensity diffraction spectrophotometer.
Results and discussion
Aluminum samples etched in KOH result in a relatively deep craters with an aspect ratio of 10% (RRMS = 322 nm, LC = 3.02 μm). Al samples etched via sacrificial layer have a similar aspect ratio to the one of direct etching (~10%). The difference between ITO and AZO sacrificial is the craters’ size and depth. The different etching solutions (HF:H2O2:H2O or KOH/H2O) have also an impact on craters’ distribution in Al textured samples. The physical mechanism of this etching is that the porous sacrificial layers deposited allow an anisotropic etching that will induce craters in the Al substrate once the etching is completed. A further etching with diluted H3PO4 leads a cleaning of solid precipitates after texturing process. All these process conditions set, it is possible to grow high quality, >2 μm-thick nc-Si absorber layers in combination with excellent light trapping for micro-morph tandem application.
References
[1] A. Shah et. al., Prog. Photovolt: Res. Appl. 2004; 12:113–142 (DOI: 10.1002/pip.533).
[2] H. Sai et. al., Appl. Phys. Lett 101, 173901 (2012).
[3] M. Kambe et al., doi: 10.1109/PVSC.2009.5411411.
[4] J. Bailat et al., JAP 2003; 93: 5727–5732.
[5] H. Tan, et al, Appl Phys. Lett. 103, 173905 (2013).
[6] J. Müller et. al., Solar Energy, doi.org/10.1016/j.solener.2004.03.015.
[7] H.Tan et. al., doi: 10.1002/pip.
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