Photovoltaics International Papers

In principle solar cells are very simple: they convert sunlight to electricity and can be characterized by a single number – the solar cell efficiency. Manufacturers obviously want to achieve this efficiency at the lowest possible cost, so it is critical that the efficiency/cost ratio be optimized. To this end, knowledge of where the biggest gains can be achieved is key. This paper presents an in-depth loss analysis method developed at the Solar Energy Research Institute of Singapore (SERIS) and details how various losses in a silicon wafer solar cell can be quantified, which is not done in the case of a conventional solar cell measurement. Through a combination of high-precision measurements, it is shown that it is possible to fully quantify the various loss mechanisms which reduce short-circuit current, open-circuit voltage and fill factor. This extensive quantitative analysis, which is not limited to silicon wafer solar cells, provides solar cell researchers and production line engineers with a ‘health check’ for their solar cells–something that can be used to further improve the efficiency of their devices.

The potential for PV modules to fail before the end of their intended service life increases the perceived risk, and therefore the cost, of funding PV installations. While current IEC and UL certification testing standards for PV modules have helped to reduce the risk of early field (infant mortality) failures, they are a necessary, but not sufficient, part of determining PV module service life. The goal of the PV Durability Initiative is to establish a baseline PV durability assessment programme. PV modules are rated according to their likelihood of performing reliably over their expected service life. Modules are subjected to accelerated stress testing intended to reach the wear-out regime for a given set of environmental conditions. In parallel with the accelerated tests, modules are subjected to long-term outdoor exposure; the correlation between the accelerated tests and actual operation in the field is an ultimate goal of the programme. As understanding of PV module durability grows, the test protocols will be revised as necessary. The regular publication of durability ratings for leading PV modules will enable PV system developers and financiers to make informed deployment decisions.

Module assembly drives as much as a third of the total module cost and can have a significant impact on overall module performance in terms of efficiency and module lifetime. This paper reviews some of the newest moduling material trends, and the outlook for the module market.

Over the past two years the PV industry has been in disarray as massive global overcapacity has sent prices tumbling. In this context, technological innovation to reduce the costs of base materials and products has become increasingly important. The latest edition of the International Technology Roadmap for PV published in March offers insights into the latest developments as manufacturers continue to seek ways of cutting costs. This paper explains some of the key dynamics identified in the roadmap.

The rapid growth of the PV market during the last five to seven years entailed a considerable expansion of the encapsulation material market, which temporarily led to shortages in the supply chain. Simultaneously, module prices decreased significantly, which resulted in intense pressure on production costs and the cost of PV module components, inducing changes in the encapsulation material market towards new materials and suppliers. This pressure – together with the huge impact of the encapsulation material on module efficiency, stability and reliability – makes the selection of encapsulation technologies and materials a very important and critical decision in the module design process. This paper presents an overview of the different materials currently on the market, the general requirements of PV module encapsulation materials, and the interactions of these materials with other module components.

This is the second and concluding part of a study on the solar photovoltaic market. In the first part, photovoltaic energy was contrasted with other energy sources used to generate electricity, and cost points necessary to produce a sustainable photovoltaic market were identified. In this second part, learning rates required to attain those cost points are provided. The paper concludes by examining a scenario in which 15% of the world’s electricity in 2035 is generated using photovoltaic energy, and frames the challenge from both global investment and profitability perspectives.

This is the second part of a review article series about current topics in R&D concerning Cu(In,Ga)(Se,S)2 – or CIGS – solar cells. In the first part, which appeared in the previous edition of Photovoltaics International, the focus was on CIGS absorber layer formation. This second part will discuss another essential part of CIGS solar cells – the buffer layer – in conjunction with metastabilities in these types of cell.

A cost-effective and industrial version of the well-known passivated-emitter and rear cell (PERC) concept has been developed by imec. The imec i-PERC technology comprises a large-area p-type monocrystalline Si solar cell with, on its front, a homogeneous emitter, a thin thermal oxide layer and fine-line Ag screen-printed contacts; on its rear, the cell has a chemically polished surface, low-cost rear dielectric stack layers and local Al contacts. Yielding certified efficiencies of up to 20% and fill factors of 80%, these cells clearly outperform aluminium back-surface field (Al-BSF) cells. During the development stages, process complexity and additional tool investment were kept to a minimum. It is therefore believed that this technology can be picked up by companies in a straightforward way as the next-generation industrial solar cell technology.

A major cause of failure in PV modules is related to the penetration of the module by moisture and its retention within. The presence of moisture results in corrosion of metallic contacts or accelerates the molecular degradation of the encapsulant, causing a loss of transparency and in some cases the development of yellowing. The moisture penetration may be intrinsic to the resin itself, but most often it will occur at the interfaces. As a consequence, the adhesion of the resin to glass, metallization, cell and backsheet surfaces may be affected. Engineers involved in the assembly of PV modules used to link adhesion degradation issues to poor conditions for storing polymeric materials, especially the encapsulation resin and the backsheet. In this paper another cause, which has not yet been studied by specialists, is discussed. It is shown that the welding of copper strips can induce residues which prevent the satisfactory adhesion of the resin, resulting in elamination. This phenomenon is identified by ‘spots’ along the busbars after lamination. The study highlights the possible consequences of these defects for a module’s performance, after consecutive thermal cycling, damp-heat and humidity-freeze testing. Recommendations are also given for choosing a suitable solder flux and optimizing the soldering process, in order to maintain satisfactory control over potential delamination problems.

Emitter formation is one of the most critical processes in the fabrication of silicon wafer solar cells. The process for standard emitter formation adopted in the photovoltaic industry is tube-based diffusion, using phosphorus oxychloride as the dopant source. A potentially low-cost alternative that typically results in lower solar cell efficiencies is in-line diffusion, using phosphoric acid as the dopant source. The Solar Energy Research Institute of Singapore (SERIS) recently developed a technique called the ‘SERIS etch’, a non-acidic etch-back process technology that provides a controllable, uniform and substantially conformal etch-back suitable for solar cell processing. By using the SERIS etch, efficiencies of up to 18.7% have been demonstrated for omogeneous-emitter silicon wafer cells; a 0.4%abs efficiency improvement has also been achieved for a unique selective-emitter approach exploiting this novel etch. All work was carried out on industrial-grade p-type Cz wafers with conventional screen-printed metallization and a full-area aluminium back-surface field (Al-BSF). With Al local BSF (LBSF) homogeneous-emitter solar cells, efficiencies of 19.0% were achieved using in-line emitter diffusion and the SERIS etch, a 0.7%abs efficiency increase over the baseline efficiency at the time. To the authors’ knowledge, these are the highest solar cell efficiencies ever reported for in-line-diffused silicon solar cells. Moreover, the SERIS etch is a costeffective alternative to generating pyramid-textured surfaces without using conventional metal-assisted siliconetching processes.