Periodic hexagonal microtexture arrays (also known as honeycombs) are successfully implemented for the first time in a superstrate glass configuration. Hexagonal textures on glass demonstrate an anti-reflective effect when compared to flat glass. It is shown that light scattering increases at the honeycomb interfaces with an increase in texture height and periodicity. The performance of the textures is demonstrated using thin-film single-junction PV devices based on an indirect bandgap semiconductor material, nanocrystalline silicon (nc-Si:H), which requires light trapping in the infrared region of the spectrum. Inspecting the nc-Si:H bulk absorber suggests a conformal, crack-free growth of crystals on the hexagonal arrays. Short-circuit current density (J_SC) increases with an increase in the aspect ratio of the superstrate, without compromising voltage and fill factor. The J_SC enhancement is attributed to a combined benefit of (i) the anti-reflective nature of developed textures, (ii) trapping light within the absorbing layer through multiple order diffraction at the front and (iii) reflection from a back reflector with adapted hexagonal morphology. With the above observations, a J_SC of 28.6 mA/cm² (photovoltaic conversion efficiency of 9.3 %) is achieved for a 5μm periodic texture with a height of 1μm (aspect ratio = 0.21). This is the highest reported J_SC for a single-junction nc-Si:H solar cell in a superstrate configuration without an external anti-reflection coating. ... Read more

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. ... Read more

Photovoltaic solar energy is one of the most powerful renewable source and is a promising solution for one of the major challenges that our current generation faces: the transition of the global energy sector from fossil-based to zero-carbon emissions. In order to reach this goal, the second generation of photovoltaic technology has been based on thin films.
This thesis study, as a part of Flamingo PV project in collaboration with HyET company, focuses on development of a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cell on aluminium substrate. The objective is to achieve a stabilised efficiency higher than 14% and an open-circuit voltage in the range of 1.7 and 1.9 V. The implementation of a performing solar device will ultimately move to Roll-to-Roll fabrication on large scale and provide a new competitive product in terms of flexibility, light-weight and cost-effectiveness.
The investigation has been largely built on optical modelling implemented with GENPRO4 soft- ware. Initially, the light model and the applied texturing have been validated for the triple junc- tion architecture. An algorithm has been developed to find the thickness of active layers in current matching condition and enhance the spectral utilisation. The design of the solar cell has been anal- ysed by changing layers thickness or materials towards an optimised optical performance.
By conducting a sensitivity analysis on the intrinsic layers thickness, it was found that an al- teration of ±5% affects more the photo-generation in the middle sub-cell. Moreover, the analysis carried out on back reflector has shown the silver metal to be more favourable than aluminium. It enhances the reflectivity properties, especially for the bottom sub-cell which is the current limiting junction. Further improvements have been found through the inclusion of an encapsulant at the front side. The embedding of the triple-junction solar cell with a stack of anti-reflective coatings increases the light trapping above 4% on average, improving the photo-generated current density by 0.5 mA/cm2 in each of the intrinsic layers.
Several thicknesses combinations have been modelled for current matched active layers, but only some of them resulted to be feasible. This has emerged that the spectral utilisation may achieve 27.3 mA/cm2 and 28.5 mA/cm2 with the encapsulant integration. The FTO is currently deposited as front reflector in thin films on Al substrate due to its good opto-electrical properties. However, the front reflector performance may be improved by developing a series of different transparent conductive materials. Alternatively, the FTO material may be replaced by IOH with better transparency property and still high conductivity in the near infra-red region of the solar spectrum.
The optical modelling has been an essential tool to forecast the multi-junction operation in ex- perimental section of this work. The fabrication of thin-film solar cells has been divided into two main research areas: Si-based triple-junction devices on Al substrate and GeSn:H thin-films. For the former, two deposition series have been processed to analyse the enhancement of electrical parameters when the absorber layers are current matched. Furthermore, other elements of IV group besides silicon have been examined to achieve a low energy bandgap alloy and increase the light absorption at long wavelength of light spectrum. The first attempts have been investigated for amorphous and nanocrystalline materials based on germanium and tin with an achievable bandgap energy of 0.9 eV.
To conclude, a comprehensive optical modelling has been developed for Si-based triple-junction device on Al substrate. This aims to provide a potential pathway towards the fabrication of a new and efficient flexible and lightweight solar cell technology. ... Read more