Engineering advanced front textures for high-performance thin-film silicon photovoltaics
G. Padmakumar (TU Delft - Photovoltaic Materials and Devices)
A.H.M. Smets – Promotor (TU Delft - Photovoltaic Materials and Devices)
R.A.C.M.M. van Swaaij – Promotor (TU Delft - Photovoltaic Materials and Devices)
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Abstract
In this dissertation, light management techniques with front textures for thin-film silicon superstrate configuration solar cells are presented. Specifically, the textures are developed on glass to act as light scatterers at interfaces in thin-film devices. The thesis aims to study textures made on glass with a broader idea of transferring it to aluminium folie for fabricating pre-textured transparent conductive oxide (TCO)s in the roll-to-roll fabrication by LiFT PV B.V.
This thesis aims to answer a main question:
How to engineer a front glass texture with which high-performing thin-film silicon superstrate solar cells can be fabricated?
This main question is addressed through four different key questions in five different chapters. The optimisation of thin-film silicon deposition conditions is presented in Chapter 2. Amorphous silicon and nanocrystalline silicon layers are deposited at very high frequency conditions to obtain device-grade photoactive silicon. The processing conditions optimised in this chapter are used to fabricate silicon absorber layers in subsequent Chapters 3, 4, and 5 of the thesis to study light trapping by glass textures.
In Chapter 3, the glass surface is textured with randomly scattered texturing shapes (craters or protrusions), without an explicit placement rule. The textures are referred to as “random textures”. This chapter addresses the sub-question:
How does light interact with glass textures featuring nano-scale structures superimposed on micro-scale textures, and how does the resulting morphology influence nano-crystalline silicon growth?
This chapter demonstrates that a sequential wet etching technique can superimpose nano-sized craters on micro-sized craters on glass, with a higher optical performance and efficiency in solar cells when compared to both micro- and nano-textures individually.
Textures characterised by a repeating pattern of shapes at fixed, regular intervals are considered “periodic textures”. Chapter 4 specifically addresses the question:
How to design a periodic glass texture composed of micro-scale hexagonal craters to maximise light scattering efficiency?
To answer this, photolithography is used as a technique to make hexagonal-shaped micrometre-scale craters on the glass surface. The hexagonal textures increased the light scattering capability with deeper craters and higher periodicity value. Glass with hexagonal micro-textures demonstrated a diffusivity as high as 50% in the near infrared light.
Once the design of periodic textures is completed, the recipe to generate hexagonal shapes with different feature sizes is known. Chapter 5 answers the question:
How does a hexagonal periodic texture on glass influence light interaction, and how do its morphological characteristics affect the performance of thin-film silicon superstrate solar cells?
The hexagonal periodic textures made on glass are studied in detail for their light scattering and diffraction effects. Additionally, nanocrystalline silicon single-junction solar cells are fabricated on these textures with different feature sizes and studied for their electrical and optical performance. An optical performance of 28.60 mA/cm² was achieved for the single junction nc-Si:H solar cells without any external antireflective measures, indicating a high potential for hexagonal textures on glass in multijunction thin-film solar cell applications.
Chapter 6 explores light scattering of the developed textures when implemented on multijunction solar cells. The challenge of Fabry-Pérot interference in multilayers with contrasting refractive indices is identified in multijunctions, limiting their optical performance. This study addresses the sub-question:
What impact do interface and bulk scattering have on the optical performance of multijunction cells, and which strategies can effectively mitigate interference effects caused by optical micro-cavities?
For this, light scattering in bulk TCO grains, combined with random and periodic textures, is studied in detail. Hexagonal craters on glass, combined with a 0.9 micrometre thick i-ZnO layer, effectively mitigated fringes formed by all optical cavities in the device.
The design principles discussed in this work are not restricted to amorphous silicon/nanocrystalline silicon tandem devices but can be extended to any thin-film multijunction solar cell that constitutes layers with contrasting refractive indices.