Surface passivation and optical design of silicon heterojunction solar cells

Abstract

It has been predicted that due to the population growth the energy demand is increasing faster and faster. It has been well recognized that depleting fossil-fuel resources will not fulfill the energy need of the future world. Not to mention the problem of global warming caused by its combustion, and of energy security as a result of geopolitical developments. People have never stopped seeking for alternative energy sources. With the aid of rapid technology development renewable-energy sources with its unique characteristics of environmental compatibility and continual replenishment, become more and more appealing. Amongst all renewable energy sources solar energy is considered inexhaustible and very abundant and therefore much research is being carried out, aiming at making full use of it. A solar cell is a photovoltaic device that directly converts solar energy to electrical energy, a very versatile form of energy. Thanks to fast technologic progress and the price drop of some source materials, solar cells have been becoming cheaper. There are two ways to reduce the cost of electricity produced by solar cells: one is to increase the energy conversion efficiency mainly by means of reducing optical and electrical losses; the second is to lower the production cost by reducing the material cost and by simplifying the manufacturing process. So far crystalline-silicon (c-Si) based solar cells dominate the photovoltaic (PV) market, but there are two important challenges with conventional c-Si solar cells: (i) the relatively high material cost from the c-Si wafer and (ii) the high processing cost due to the relatively long processing time and high thermal budget. Worldwide researchers have been working on different solutions. For instance, the material cost can be reduced by the use of low-quality wafers and by slicing the ingot into thinner wafers. The use of laser doping, for instance, can replace the diffusion process in order to reduce thermal budget. Another solution, which led to the second generation of solar cells, is thin-film technology. Thin-film Si solar cells use only hundreds of nanometers or several micrometers thick Si layers and the processing temperature is only around 200 °C. However, the efficiency of thin-film silicon solar cells is relatively low especially after light-induced degradation. By combining advanced c-Si and thin-film Si technologies the silicon heterojunction (SHJ) solar cell has been developed. This type of solar cell consists of a c-Si wafer with layers of hydrogenated amorphous silicon (a Si:H) to passivate the surface and create emitter and back surface field (BSF) at front and back side respectively. These layers can be of intrinsic material (i-layer) or of p or n-type doped material (p-layer or n-layer). The SHJ solar cell has proved to be a high-efficiency concept and can be made with relatively simple and low-temperature processes. This thesis describes the development of the process to make SHJ solar cells in our facility. For this process first a reproducible wafer cleaning process has been developed. Different thin-film deposition technologies, e.g. plasma enhanced chemical vapor deposition (PECVD), sputtering and metal evaporation, have been used for fabricating the device. The properties of the layers have been characterized by techniques including transmittance and reflectance measurements, activation energy and dark conductivity measurements, and carrier lifetime measurements. For device characterization mainly current density versus voltage measurement, external quantum efficiency measurements, and reflectance measurements have been performed. In addition to experimental work, optical simulations have been carried out for optical analysis and design of the device. In order to simulate the optical effects of layers with thicknesses in the nanometer scale the interference of light was modelled using thin-film optics, while geometrical optics is used for modelling light scattering on the micrometer-scale surface morphology. The research in this thesis focusses on several aspects of SHJ solar cells including the deposition and optimization of the indium tin oxide (ITO) front contact, the intrinsic hydrogenated amorphous silicon (a-Si:H) passivating layer, and the emitter materials, and the optical design of the device structure. The influence of ITO sputtering conditions on the passivation of c-Si wafers by a-Si:H layers is studied in Chapter 3. A low sputtering power is favorable in order to maintain a high passivation of the c-Si/a-Si:H interface, as characterized by a long minority carrier lifetime. The degradation of the passivation caused by sputtering can be reduced by sputtering the ITO at a raised substrate temperature or by post-annealing after ITO deposition. However, the substrate temperature during sputtering can degrade the passivation if it is higher than a threshold value. Based on our results we suggest that the sputtering power for ITO deposition should be as low as possible, provided the plasma is stable and a reasonable deposition rate is maintained. The substrate temperature should be kept below a threshold value of 130 °C, avoiding any degradation of the passivation. One-hour post-annealing is an effective way to recover the degradation of the passivation caused by the sputtering process at the room temperature. However, any degradation caused by annealing during sputtering cannot be recovered by post-annealing probably due to irreversible hydrogen effusion. Since passivation of the c-Si by a-Si:H is sensitive to ITO sputtering, the device can perform better without an ITO layer on the rear side when metal fully covers the rear. In Chapter 4 we show that the intrinsic a-Si:H layer (i-layer) thickness has no significant influence on the optical performance of SHJ solar cells within the thickness range we investigated. However, the influence on the electrical performance is quite noticeable. There is an absolute fill factor (FF) decrease of 3% when the thickness of the i-layer increases from 3 to 7 nm in the emitter or from 1 to 5 nm in the back surface field (BSF). The influence of the i-layer thickness in the emitter on the passivation or open-circuit voltage (Voc) is different from in the BSF. In the BSF a thinner i-layer can be used or this layer can even be omitted without a noticeable effect on the Voc, while a gain in FF is obtained, because the n-type a-Si:H layer (n-layer) in the BSF can passivate the n-type c-Si surface well enough. In the emitter, a thicker i-layer has to be used to guarantee a good passivation quality. Surface recombination velocities of the wafer passivated by different a-Si:H layer stacks are also deduced from the carrier lifetime measurements, confirming the good passivating property of the n-layer and degradation of i-layer passivation after the p-type a-Si:H layer (p-layer) is applied. Annealing can improve the passivation of the n-layer and the i/n stack, but deteriorates the passivation of the i/p stack. Compared to homojunction c-Si solar cells, one main disadvantage of SHJ solar cells is the parasitic absorption loss caused by the ITO and the a-Si:H layers. The conventional method to design the anti-reflective (AR) coating, which is based on minimization of the reflectance, cannot be applied to this specific layer structure at the front of the SHJ solar cell. The reason is that a fraction of the light gained by applying an AR coating may be absorbed by the ITO and the a-Si:H layers. In Chapter 5 we show that for SHJ solar cells the current output can be estimated by examining the light absorption in the c-Si absorber using optical simulations. Using our advanced optical simulation program, a double-layer AR coating consisting of a SiOx layer and the ITO is designed for the SHJ solar cell and implemented in experimental SHJ solar cells. For a SHJ solar cell with a textured c-Si surface and utilizing this double-layer AR coating a short-circuit current density (Jsc) of 40.5 mA/cm2 is achieved. In order to reduce the parasitic absorption of the emitter in SHJ solar cells, one effective method is to use wide-gap silicon alloy materials. In Chapter 6 we demonstrate a SHJ solar cell with a p-type a-SiC:H emitter. Compared to the p-type a-Si:H emitter, the p-type a-SiC:H increases the current output of the solar cell by reducing not only parasitic absorption in the emitter, but also the reflection of the device. The reduction of reflection is due to the fact that the refractive index of p-type a-SiC:H is in between that of ITO and intrinsic a-Si:H. Since the electrical performance of the cell with a p-type a-SiC:H emitter is comparable to that with a p-type a-Si:H emitter, the optical benefit makes the p-type a-SiC:H a very promising alternative to p-type a-Si:H as the emitter material. Finally, a 4-cm2 SHJ solar cell employing the p-type a-SiC:H emitter was made, which shows a Jsc of 40.3 mA/cm2, a Voc of 682 mV, a FF of 75.5% and a conversion efficiency of 20.8%.