Volume 2, Issue 4 - Photovoltaics

Photovoltaic solar energy technology (PV) may reach the terawatt deployment scale within a decade. This is enabled by rapid cost reduction through which PV is able to compete with conventional power in an increasing fraction of electricity markets globally. Although further cost reduction is essential for very large scale deployment and requires dedicated research and development efforts, it is not sufficient. Availability of fully sustainable technology and (electrical & physical) integration are other necessary ingredients for multi-terawatt-scale use. This paper quantifies and discusses the development challenges of PV and reviews some of the recent publications addressing the future of PV.

Crystalline silicon solar cells dominate today's global photovoltaic (PV) market. This paper presents the status and trends of the most important industrial silicon wafer solar cells, ranging from standard p-type homojunction cells to heterojunction cells on n-type wafers. Owing to ongoing technological innovations such as improved surface passivation and the use of increasingly thinner wafers, the trend towards higher cell efficiencies and lower $/watt costs is expected to continue during the next 10 years, making silicon wafer based PV modules a moving target for any competing PV technology.

Production of photovoltaics is growing worldwide on a gigawatt scale. Among the thin film technologies, Cu(In,Ga)S,Se 2 (CIS or CIGS) based solar cells have been the focus of more and more attention. This paper aims to analyze the success of CIGS based solar cells and the potential of this technology for future photovoltaics large-scale production. Specific material properties make CIS unique and allow the preparation of the material with a wide range of processing options. The huge potential lies in the possibility to take advantage of modern thin film processing equipment and combine it with very high efficiencies beyond 20% already achieved on the laboratory scale. A sustainable development of this technology could be realized by modifying the materials and replacing indium by abundant elements.

Solar cell processing into modules is mostly responsible for the product's reliability, has a severe impact on product costs and controls 10–15% of its efficiency. This article gives an overview on current module technology and highlights innovative approaches to reduce material cost and increase module efficiency. High potential approaches like back contact technology are those that simultaneously address cell and module technology to offer efficiency gains in the range of 10%. Module reliability expectations of 25 years or more require quality assurance beyond the common type approval standards. In its second part, the article addresses stress factors for PV modules. To ensure fast innovation cycles, accelerated aging tests are used to reproduce these stress factors in the laboratory. Results from certification testing are discussed as well as new approaches for improved testing. More realistic test results can be obtained by simulating combinations of stress factors.

This work presents large-area metal wrap through (MWT) solar cells with passivated surfaces made from Czochralski grown silicon (Cz-Si). Besides further improvement in conversion efficiency, the paper addresses industrial applicability of the fabrication process and technology. Three different base materials are compared – conventional boron-doped Cz-Si, gallium-doped Cz-Si and boron-doped magnetically cast Cz-Si (mCz-Si). Stencil printing is applied for front contact formation resulting in increased aspect ratio and finger homogeneity and thus low series resistance losses. Regarding material consumption, the presented cells allow for a reduction of silver usage by 30%. The peak efficiency for cells made from mCz-Si with an edge length of 156 mm is 20.2% (confirmed) in the annealed state.

Photon management is a key component in the development of efficient solar cells. Especially light-trapping concepts have a high potential to realize enhanced efficiencies. Here, we give an overview over several light trapping concepts for photon management in solar cells. These include basic as well as advanced light-trapping concepts. The theoretical limits of light path enhancement of the different concepts are given and experimental work on these topics is presented. The potential of 3D photonic crystals is discussed in the context of the corresponding approaches as well.

This paper explores the large scale transmission of solar electricity to Southeast Asia from Australia. Despite the expense and losses incurred in long distance transmission of Australian solar electricity, it appears to be competitive with locally produced solar electricity because of high insolation levels in Australia. Supplementation of locally produced electricity (both from renewable and conventional sources) with power from Australia, together with substantial integrated energy storage, would allow a high solar electricity fraction to be achieved in Southeast Asia.