Recent patent issues on intermediate reflectors for high efficiency thin-film silicon photovoltaic devices

Abstract

The recent surge of unveiled US patents on the intermediate reflectors for thin-film silicon (Si) photovoltaic (PV) devices reflects the paramount importance of light trapping to improve the conversion efficiency. Here, the recent patent issues on the intermediate reflectors of thin-film Si PV devices are reviewed. Highly transparent and conductive metal oxide intermediate reflectors have the advantage of the higher efficiency for the fabricated multi-junction solar cells compared to the Si alloy intermediate reflectors. However, their high lateral electrical conductivity leads to the lateral shunting during the monolithic series integration of segments. To avoid the lateral shunt creations, an additional laser scribe or a coating process that induces a high production cost is necessary. In addition, a low conversion efficiency for hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline Si (μc-Si:H) double-junction PV modules employing a metal oxide intermediate reflector stems from the decrease in the active area as a result of the additional process. Meanwhile, double-junction PV modules employing an n-type hydrogenated microcrystalline Si oxide (n-μc-SiO_x:H) intermediate reflector provide a higher conversion efficiency. Since the Si alloy intermediate reflector can avoid the lateral shunting, it may be a promising option for cost-effective mass production of large-area thin-film Si multi-junction PV modules. Although the developed intermediate reflectors have the high potential, the current status is limited at the research and development (R&D) level. Therefore, the up-scaling with the low cost, high throughput, and high yield is a key technological mission for mass production.

Introduction

It is known that recent global warming caused by excess emission of CO₂ warns that the climate change and nuclear power plants are no longer stable and cheap energy source after Fukushima nuclear meltdowns. Sufficient supplies of clean energy are intimately linked with global stability, economic prosperity, and quality of life. Accordingly, clean renewable energy including solar, wind, and hydrogen energy becomes a prime issue. A photovoltaic (PV) module using solar light is a promising candidate among the renewable energy sources because the sun is our primary source of clean, abundant energy. Actually, there has been an explosive, worldwide increase in the PV module market during the last two decades. However, the oversupply of bulk crystalline silicon (c-Si) PV modules that currently shares 80% of products and the decrease in government subsidies due to the recent global economic crisis threat the PV business by causing the rapid drop in the module price. The thin-film silicon (Si) PV modules using hydrogenated amorphous Si (a-Si:H) and/or hydrogenated microcrystalline Si (µc-Si:H) absorbers have been considered as promising alternatives to the bulk c-Si PV modules due to the various advantages of remarkably low consumption of the raw Si material (<1% of consumption of bulk c-Si PV modules), large-area deposition, low-temperature production, and low temperature coefficient. The thin-film Si PV technology also profits from the wide experience base of display industries [1]. However, the recent sharp drop in the module price gives rise to a need for a new breakthrough in the conversion efficiency (η) as well as the cost of the thin-film Si PV modules.

However, the so-called “Staebler–Wronski effect (SWE)” in a-Si:H-based films remains as a major technical challenge for the commercialization of the thin-film Si PV modules [2], [3]. SWE is the light-induced degradation arising from the photocreation of dangling bonds accomplished by nonradiative recombination of photogenerated electron–hole pairs [4]. To reduce SWE in a-Si:H absorbers that leads to the degradation of thin-film Si solar cells, there have been extensive investigations during the past 30 years. As a result, stabilized η (η_sta) about 10% has been reported for a-Si:H single-junction solar cells [5]. However, reported highest η_sta of research and development (R&D)-level a-Si:H single-junction PV modules is 8.7% [6] and that of single-junction PV module products is 6–7% [5]. Double-junction solar cells composed of a-Si:H top and the µc-Si:H bottom cells stacked in series have been developed to achieve a high value of η_sta by guiding incident light to the appropriate absorbers [7], [8], [9], [10]. Because the µc-Si:H bottom cell is very stable against red light irradiation [11], the reduced thickness of the a-Si:H absorber in the top cell compared to that of a-Si:H absorbers in conventional single-junction solar cells provides a good stability against light soaking. To date, η_sta of 10–11% has been reported for R&D-level a-Si:H/μc-Si:H double-junction PV modules, while η_sta of 9–10% has been achieved for a-Si:H/μc-Si:H double-junction PV module products [12]. The stability of the a-Si:H/μc-Si:H double-junction solar cells mainly depends on the light-induced degradation as well as the thickness of the absorber in the top cell. Thus, improved light trapping in the a-Si:H top cell using an intermediate reflector becomes the center of R&D interest.

A lower refractive index (n) of an intermediate reflector compared to n of Si layers (~4.0) is essential in enhancing the internal reflection [13]. It was reported that highly conductive and transparent zinc oxide (ZnO) intermediate reflectors having n of ~2.0 significantly increased η_sta of superstrate-type a-Si:H/μc-Si:H double-junction solar cells [14], [15]. Despite the improvement of the cell performances, however, the metal oxide intermediate reflector including ZnO is not suitable for mass production of superstrate type thin-film Si PV modules due to the lateral shunting [16] caused by the leakage current path generation between the intermediate reflector and metal back contact. To prevent a poor fill factor (FF) as a result of the monolithic series integration, at least an additional step is required for the isolation of the exposed intermediate reflector after the laser scribe of Si layers from the subsequently coated metal back contact. This additional step gives rise to a high production cost. In the case of substrate-type a-Si:H/μc-Si:H double-junction solar cells, improved light trapping by employing a textured ZnO intermediate reflector was also reported [17]. However, the substrate-type a-Si:H/μc-Si:H double-junction PV modules employing the textured ZnO intermediate reflector also suffer from a similar lateral shunting. A leakage current path occurs between the highly conductive ZnO intermediate reflector and subsequently prepared transparent front electrode like indium tin oxide (ITO) and ZnO.

Alternatively, hydrogenated n-type amorphous silicon-oxide (n-a-SiO_x:H) and hydrogenated n-type microcrystalline (or nanocrystalline) silicon-oxide (n-μc-SiO_x:H) intermediate reflectors with a constant n value were developed [18], [19], [20], [21], [22]. The SiO_x:H intermediate reflectors can considerably reduce the lateral shunting due to the lower lateral conductivity than the ZnO intermediate reflectors. Also, the SiO_x:H intermediate reflectors can be removed simultaneously with adjacent Si layers via the laser scribe of Si layers. Hence, no additional step for the monolithic integration of a-Si:H/μc-Si:H double-junction PV modules is necessary. Moreover, the in situ preparation of SiO_x:H intermediate reflectors using plasma enhanced chemical vapor deposition (PECVD) is possible. Due to the key technological issue of improved light trapping for thin-film Si PV devices, considerable inventions focused on the intermediate reflector are timely published as patents. In this work, the author will review the recent trends of patents on the intermediate reflectors of thin-film Si PV modules.