Evaluation of a New Acid Solution for Texturization of Multicrystalline Silicon Solar Cells

Surface texturing methods using an alkaline solution for monocrystalline Si (c-Si) solar cells have been widely accepted to improve cell performance. However, multicrystalline Si (mc-Si) cells are difficult to be texturized by alkaline etching, because the grains in the substrates are randomly oriented. In this study, we considered a HF/HNO3/H2SO4 acid solution for texturing the mc-Si cells. We evaluated the morphology of the textured surfaces and the reflectance spectra from the surfaces. The deep dimple textured structures are formed on the surfaces for only 30 seconds of the acid texturing process. This behavior results from the effect of H2SO4 in the solution. This process obtains up to 14.7% conversion efficiencies of the acid textured cells. These conversion efficiencies are up to 1.3 times larger than those of the mirror-etched cells

Optimizing the fabrication process for next generation nano-textured solar cells with high conversion efficiency using industrially viable solar cell processes

The need for photovoltaic (PV) cells with high conversion efficiency and low surface reflectance has been on the rise. With this in mind, the fabrication process of PV cells has been investigated with interest in the surface etching phase in order to reduce surface reflectance using nanotexturing. The purpose of this thesis is to investigate how to improve the etching phase by optimising the key parameters for an atmospheric dry etch (ADE) with different processes to determine which can aid in reducing surface reflectance. To obtain a process which provides a low surface reflectance, parameters in the etching phase are changed, including multicrystalline versus monocrystalline , the etch time and the use of different cleaning techniques. Once the Si wafers have been put through the etching process, they are tested using a spectrometer to determine surface reflectance. A scanning electron microscope (SEM) and an atomic force microscope (AFM) were then used in order to observe how the etching process has affected the surface morphology. Different parameters have been shown to have a varying effect on the surface reflectance of the etched Si surface, in particular cleaning processes before etching and etch time. Etch time has shown to have the greatest effect on surface reflectance. It can be seen that reflectance will vary with etch time as a result of the chemical reaction between the Si found on the surface of the Si wafer and the etching gas. Certain conditions influence the Si etch rate and surface reflectance better than others. Further investigation into the use of multicrystalline silicon (mc-Si) using monocrystalline Si samples as a reference to assist in determining optimum etching phase. Etched mc-si samples can be processed using the same parameters and cleaning processes as the monocrystalline Si, resulting in identifiable crystal orientations. Application of the same test conditions in order to determine the surface reflectance and morphology will provide a further insight to what degree the etching phase needs to be improved upon in order to obtain low surface reflectance

A new texturing technique for silicon solar cells using gas phase etching

Solar energy is vital to combatting climate change. However, not all incident photons can transmit into a typical solar cell for electricity generation, a portion of the photons are lost to front surface reflectance. By changing the front surface texture of typical solar cells from microscale to nanoscale, the solar cell front surface reflectance (at wavelengths of interest), can be reduced to almost 0%. Such a change would increase the number of photons available in the solar cell for electricity generation. Despite the excellent optical properties nano-textures can offer, all current industrially relevant nanoscale texturing methods have drawbacks that prevent them from gaining a significant market share. This thesis explores texturing in the gas-phase to overcome the drawbacks of other nanoscale texturing techniques, whilst still providing exceptional optical properties to improve solar cell efficiency. Texturing Si in the gas-phase using ozone and hydrofluoric acid vapours at atmospheric pressure and low temperature was investigated. The premise of the texturing mechanism was to use the increased activity of ozone to oxidise the Si surface, at significantly lower temperatures than for O2 oxidation, then etch the oxide away using HF vapour, causing an ultra-low reflectance surface texture to form. A texturing tool was designed, built, and refined to study gas-phase texturing in this project. With this new tool, texturing reproducibility and uniformity were found to be dependent upon the surface chemistry of the Si wafers. By intentionally changing the surface chemistry via a precursor containing colloidal silica and IPA, reproducible and uniform texturing could be performed on sample sizes up to full-sized industrial Si wafers, with an average reflectance as low as 1.8% +/- 0.2%in 2 mins texturing. The mechanism behind the precursor was attributed to the Na+ counter-ions presentin the colloidal silica and a full texturing mechanism was proposed for the first time, accounting for the different morphologies gas-phase texturing could produce. Upon implementing gas-phase textured Si into a solar cell for the first time, a median cell efficiency of 17.24% was achieved, without any optimisation of the solar cell fabrication methods for the nano-texture

On Electrical Properties of Black Silicon for Photovoltaic Applications

Silicon solar cells are the leading force in the photovoltaics market due to their low mass-production costs and wide range of application scenarios. Enhancement of optical generation and reduction of recombination loss are two important aspects of high-efficiency solar cell development. Black silicon (b-Si) texturing, one of the most effective light-trapping techniques, has received considerable attention for solar cell applications. However, the development of high-efficiency b-Si solar cells is significantly hindered by a lack of in-depth understanding of the electrical properties of b-Si textures. It is widely observed that the inferior electrical performance of a b-Si emitter outweighs the gain in optical performance, resulting in lower efficiencies. However, it is also found that the surface recombination loss of a passivated b-Si texture can be unexpectedly low, which could contribute to high efficiency for some solar cell architectures. This thesis aims to determine (1) how b-Si surface morphology should be optimized to achieve high performance solar cells and (2) whether b-Si textures are indeed better than conventional textures. This thesis first provides a literature review of solar cell surface texturing techniques with an emphasis on b-Si texturing. The primary research approaches of this thesis are then shown. A systematic investigation of b-Si field-effect passivation enhancement is presented, exploring the root cause of the low surface recombination loss for undiffused b-Si surfaces and determining the optimal combinations of surface passivation schemes and b-Si morphologies. Significant field-effect passivation enhancement is found when the surface charge density is moderate, and the enhancement strength increases as the distance between the opposite surfaces decreases. Next, a fundamental study of POCl3-diffused b-Si emitters with various textures is presented, covering dopant distribution characteristics, emitter lateral conductance behaviour, and recombination loss mechanisms. Optimization strategies for b-Si emitters are also proposed based on the findings. It is shown that, in general, b-Si emitters with shallow nanofeatures can achieve higher electrical performance than those with deep nanofeatures. Finally, the weighted average reflectance (WAR) is proven as an effective surface morphology metric for a wide range of surface textures that can forecast the efficiency at the early stage of b-Si solar cell fabrication. By correlating solar cell performance reported in the literature to WAR, it is shown that multi-crystalline silicon solar cell efficiency can be improved with b-Si texturing, and this is predominately attributed to an increase in short-circuit current density via the blue response improvement. It is also found that some b-Si textures can improve the performance of mono-crystalline silicon solar cells. Device simulations show that the electrical performance of hierarchical (combination of microtexture and nanotexture) and inverted-pyramidal b-Si textures can be comparable to or even better than random pyramids. As such, these textures show great potential for mono-crystalline silicon solar cells