G. Padmakumar
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12 records found
1
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. ...
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.
A major challenge in multijunction devices is reduced light incoupling caused by interference fringes from optical microcavities. This paper reports a potential route to mitigate the interference effects with an effective front-window design. The concepts of interface scattering and grain scattering are implemented at the front side of superstrate tandem solar cells. A random texturing and periodic-hexagonal texturing approach on glass is used as interface scatterers. However, applying an interface scatterer alone is insufficient to eliminate the interference effects of optical cavities completely. Use of sputtered unintentionally doped zinc oxide (i-ZnO) or tin oxide (SnO) as grain scatterers stacked over random and periodic glass textures quenches the interference effects significantly. For a random textured glass substrate, a 1.5-μm thick i-ZnO layer could quench interference in the top cell, except for the effect of the optical cavity formed in the amorphous top cell. Hexagonal craters on glass, combined with a 0.9-μm thick i-ZnO layer, effectively mitigate fringes formed by all optical cavities in the device. This sample demonstrates the highest incoupled photon flux with 86% of photons entering the device. Use of a wide-bandgap grain scatterer, such as SnO, reduces parasitic absorption of high-energy photons while mitigating optical cavities. The design principles discussed in this work can be applied to any thin-film multijunction solar cells consisting of layers with contrasting refractive indices.
Techniques to facilitate excellent optical yield are required to manufacture high-performing solar cells. In thin-film solar cells, light scattering with the help of textured interfaces increases the absorption path length of photons and reduces the reflection of the photovoltaic active layer. These textures should also facilitate the growth of crack-free thin-film layers, ensuring high efficiency in multijunction devices. This work explores three texturing methods for glass that have the potential to be integrated into solar cells in a superstrate configuration. A detailed study of sacrificial texturing on glass using i-ZnO ((Formula presented.)) and indium-doped tin oxide ((Formula presented.)) is presented. The optical interaction of these textures is correlated to their root-mean-square (RMS) roughness ((Formula presented.)). It is demonstrated that high optical scattering can be achieved for both (Formula presented.) and (Formula presented.) but at different (Formula presented.) regimes. A novel texture with superimposed morphology, named superimposed sacrificial texturing ((Formula presented.)), is created by combining (Formula presented.) and (Formula presented.) through sequential wet etching. The (Formula presented.) exhibits exceptional transmission and light scattering properties. Nanocrystalline silicon (nc-Si:H) single-junction solar cells were fabricated in a superstrate configuration to investigate the impact of these textures on indirect bandgap thin-film solar cells. The efficiency of solar cells on (Formula presented.) is nearly 0.57% and 1.52% (absolute) more than (Formula presented.) and (Formula presented.) solar cells, respectively. By superimposing two textures, solar cells can combine the advantages of enhanced optical performance with high-quality nc-Si:H material growth.
This study investigates the transparent conductive oxides (TCOs) as front contact for thin-film solar cell applications by developing a bilayer design that decouples the optical and electrical functionalities. The bilayer front contact structure combines hydrogenated indium oxide (IOH) and non-intentionally doped zinc oxide (ZnO) materials. This design achieves enhanced optoelectrical properties with a mobility of 120 cm2/Vs and a carrier density of 1.97·1019 cm-3. Notably, the bilayer outperforms the expected average of its constituent layers in both transparency and conductivity, reflecting the benefits of optimized layer architecture. When integrated as the front electrode in a hydrogenated nanocrystalline silicon (nc-Si:H) solar cell, the IOH/ZnO bilayer yields a fill factor of 64.56 % and a power conversion efficiency of 7.85 %. When using an ITO front contact, the nc-Si:H solar cell reveals a fill factor of 56.27 % and an efficiency of 6.80 %. By successfully decoupling optical and electrical properties, the optimized IOH/ZnO bilayer offers a significant advancement over single-layer TCO configurations, presenting an innovative pathway for enhanced performance in thin-film solar cell technology.
Textured glass is used in a wide range of applications to improve optoelectrical performances, such as photovoltaics, biosensing, microfluidics, and photonics. Honeycomb textures have demonstrated an excellent performance in optical devices using crystalline silicon wafers as opaque substrates. As a pathway to translate these advantages to configurations implementing glass, hexagonal-shaped microsized craters (honeycombs) are made on glass in this study. We use photolithography combined with wet etching for this process. The relationship between photoresist mask design, glass–photoresist adhesion, wet-etching steps, and the mechanism of honeycomb formation is studied. It is demonstrated that the higher the isotropic nature of etching achieved, the deeper the hexagonal craters will be. The potential of hexagonal textures on glass to significantly reduce reflection to <8% over the entire spectral range is observed. Finally, hexagonal microsized textures with 5 μm periodicity and 1.01 μm depth that effectively diffuse 50% of the total transmitted light at near-infrared (1100 nm) wavelengths are developed.
Photovoltaic (PV) panel installations in buildings and transportation hubs pose additional safety challenges as the glare from the panels can impose adverse impacts like flash blindness in human eyes. This study substantiates that polymer encapsulated thin film modules offer significantly low glare levels that are essential for building integrated and transport hub installations. In this work, the glare hazard potential associated with matt ethylene tetrafluoroethylene (ETFE)-based polymer sheet used as the frontsheet for the production of flexible thin amorphous silicon (a-Si) PV modules is studied and compared with standard PV glass used in crystalline silicon (c-Si) PV panels. The specular reflectance extracted from the measured total and diffuse reflectance for an angle of incidence (AOI) of 8° and the angular intensity distribution (AID) of specular reflectance measured for AOI ranging from 10° to 80° are utilized for glare assessment of the frontsheets. The mean value of specular reflectance extracted from the measured total and diffused reflectance is as low as 0.5% for the polymer frontsheet and is 4% for glass. The AID measurements suggest that the reflection from the polymer frontsheet is highly diffusive in nature in contrast to glass and the measured specular reflectance is always close to a magnitude lower than that from glass for all AOI. With the increase in AOI, the specular AID reflectance increases exponentially for glass to become as high as 40%, which is almost 20 times less than that from the polymer frontsheet for an AOI of 80°. Further, the c-Si test structure with glass and thin a-Si PV module with matt ETFE-based polymer as frontsheet showed similar specular reflectance trends as that of glass and the polymer frontsheet, respectively.
Our study focuses on the optimization of front contact design by exploring a novel bilayer configuration that employs transparent conductive oxides (TCOs) to enhance the efficiency of thin-film silicon solar cells. The TCOs investigated include sputtered hydrogenated indium oxide (IOH), cerium-doped indium oxide (ICO), cerium and hydrogen co-doped indium oxide (ICOH), and intrinsic zinc oxide (i-ZnO). We highlight the suitability of these TCOs in a bilayer design, first analyzing their opto-electrical properties as monolayers and subsequently in bilayer configurations. The IOH/i-ZnO bilayer architecture, in particular, demonstrates promising opto-electrical properties on both flat glass and micro-textured glass substrates. IOH/i-ZnO on flat glass substrate demonstrates remarkable mobility (143.44 cm2/Vs) and a carrier concentration in the order of 1019cm-3. The mean of reflectance (R) trends consistently exceeds 80%, while the mean of transmittance (T) trends falls below 20% beyond 500 nm. The interference effects within the bilayers are minimized for designs on micro-textured glass, preserving values within a desirable range. These findings represent an innovative approach to front contact design for thin-film silicon solar cells, emphasizing the potential of bilayer configurations to advance solar cell technology.
Transparent conductive oxides (TCOs) are used as front electrode of thin film silicon (TF-Si) solar cells to increase power conversion efficiency. Metal oxides doped with different materials can be deployed as TCO. The preferred TCO is usually selected using a trade-off between transparency and conductivity. This work proposes a bi-layer front contact to address the limitation of this trade-off. IOH and i-ZnO are chosen as the best candidates for such architecture due to their good opto-electrical properties. A thin layer of IOH ensures good lateral conductivity and high transparency in the visible part of the solar spectrum. An additional i-ZnO layer provides minimized parasitic absorption losses along with low transverse resistivity. The best opto-electrical properties are achieved when deposition temperature and power density are set at 25°C and 1.5 W/cm2, 200°C and 2 W/cm2 for IOH and i-ZnO respectively.
This work focuses mainly on development of modulated surface texturing of Al substrate for thin-film, silicon-based, flexible solar cells. We compared the current roll-to-roll process at industrial level with a newly developed lab-scale texturing that offers better performance both with KOH and NaOH etching of Al foil. The kinetics of these etching chemicals, modelled with an Arrhenius equation, is evaluated in both methods, activation energy and pre-exponential factor are calculated depending on etching concentration. We also deployed a new roll-to-roll experiment that shows better optical properties than the baseline. Finally, we also show the first results of micromorph tandem devices on the baseline texturing.