It has been recognized for some time that the spontaneous emission by atoms is not necessarily a fixed and immutable property of the coupling between matter and space, but that it can be controlled by modification of the properties of the radiation field. This is equally true in the solid state, where spontaneous emission plays a fundamental role in limiting the performance of semiconductor lasers, heterojunction bipolar transistors, and solar cells. If a three-dimensionally periodic dielectric structure has an electromagnetic band gap which overlaps the electronic band edge, then spontaneous emission can be rigorously forbidden.

https://link.aps.org/pdf/10.1103/PhysRevLett.58.2059

Inhibited Spontaneous Emission in Solid-State Physics and Electronics

Learn the basic properties and designs of modern VLSI devices, as well as the factors affecting performance, with this thoroughly updated second edition. The first edition has been widely adopted as a standard textbook in microelectronics in many major US universities and worldwide. The internationally-renowned authors highlight the intricate interdependencies and subtle tradeoffs between various practically important device parameters, and also provide an in-depth discussion of device scaling and scaling limits of CMOS and bipolar devices. Equations and parameters provided are checked continuously against the reality of silicon data, making the book equally useful in practical transistor design and in the classroom. Every chapter has been updated to include the latest developments, such as MOSFET scale length theory, high-field transport model, and SiGe-base bipolar devices.

/pdf/fundamentals-of-modern-vlsi-devices-471xd7l9pf.pdf

Fundamentals of Modern VLSI Devices

The efficiency of silicon solar cells has a large influence on the cost of most photovoltaics panels. Here, researchers from Kaneka present a silicon heterojunction with interdigitated back contacts reaching an efficiency of 26.3% and provide a detailed loss analysis to guide further developments.

Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26

The light trapping properties of textured optical sheets have become of recent interest in photovoltaic energy conversion since light trapping allows a significant reduction in the thickness of active solar cell material. Previous analyses have concentrated on sheets with randomly textured (Lambertian) surfaces. The texturing of crystalline silicon substrates with anisotropic etches to give surfaces covered by square based pyramids defined by intersecting (111) crystallographic planes is a widely used technique for reflection control in silicon solar cells. This paper analyzes the light trapping properties of substrates with such pyramidally textured surfaces. Important differences are found from the case of Lambertian surfaces with practical consequences for the design of high efficiency silicon solar cells.

Light trapping properties of pyramidally textured surfaces

With a global market share of about 90%, crystalline silicon is by far the most important photovoltaic technology today. This article reviews the dynamic field of crystalline silicon photovoltaics from a device-engineering perspective. First, it discusses key factors responsible for the success of the classic dopant-diffused silicon homojunction solar cell. Next it analyzes two archetypal high-efficiency device architectures – the interdigitated back-contact silicon cell and the silicon heterojunction cell – both of which have demonstrated power conversion efficiencies greater than 25%. Last, it gives an up-to-date summary of promising recent pathways for further efficiency improvements and cost reduction employing novel carrier-selective passivating contact schemes, as well as tandem multi-junction architectures, in particular those that combine silicon absorbers with organic–inorganic perovskite materials.

/pdf/high-efficiency-crystalline-silicon-solar-cells-status-and-47y1w8zg6w.pdf

High-efficiency crystalline silicon solar cells: status and perspectives

Recent advances in silicon solar cells using the backside point-contact configuration have been extended resulting in 27.5-percent efficiencies at 10 W/cm2(100 suns, 24°C), making these the most efficient solar cells reported to date. The one-sun efficiencies under an AM1.5 spectrum normalized to 100 mW/cm2are 22 percent at 24°C based on the design area of the concentrator cell. The improvements reported here are largely due to the incorporation of optical light trapping to enhance the absorption of weakly absorbed near bandgap light. These results approach the projected efficiencies for a mature technology which are 23-24 percent at one sun and 29 percent in the 100-350-sun (10-35 W/ cm2) range.

27.5-percent silicon concentrator solar cells

A new type of silicon photovoltaic cell designed for high-concentration applications is presented. The device is called the point-contact-cell and shows potential for achieving energy conversion efficiencies in the neighborhood of 28 percent at the design operating point of 500× geometric concentration and 60°C cell temperature. This cell has alternating n and p regions that form a polkadot array on the bottom surface. A two-layermetallization on the bottom provides contact. Initial experimental results have yielded a cell with 20-percent efficiency at a concentration of 88.

/pdf/point-contact-silicon-solar-cells-1gmbmgyfff.pdf

Point-contact silicon solar cells

The parameters that control the transport of minority carriers in heavily doped Si:B have been measured by a combination of steady state electrical and transient optical techniques. Electron diffusion length and electron lifetime measurements have been conducted on doped-as-grown wafers to extract the minority carrier electron mobility as a function of acceptor doping density. Effective band-gap narrowing in p+epitaxial layers has been characterized using bipolar test structures. Significant findings: 1) the electron mobility is about 2.5 times larger in heavily doped p-type Si than in n-type. 2) bandgap narrowing exceeds 120 meV at N _{A} = 2 \times 10^{20} cm-3.3) minority electron lifetime in processed p+Si is not well modeled over a large doping range by an "Auger" coefficient.

Measurement of electron lifetime, electron mobility and band-gap narrowing in heavily doped p-type silicon

Experimental results are presented for thin high resistivity concentrator silicon solar cells which use a back-side point-contact geometry. Cells of 130 and 233 µm thickness were fabricated and characterized. The thin cells were found to have efficiencies greater than 22 percent for incident solar intensities of 3 to 30 W/cm2(30-300 "suns"). Efficiency peaked at 23 percent at 11 W/cm2measured at 22-25°C. Strategies for obtaining higher efficiencies with this solar cell design are discussed.

Silicon point contact concentrator solar cells

Using SIPOS films (SiO x , x oe , by at least a factor of fifty when compared to a conventional emitter structure with a metallic contact. Previous attempts to explain this J oe reduction have focused on the bulk properties of the SIPOS films. Experimental results of this study which attained J oe values of 2 × 10-14A/cm2, suggest that an interfacial layer between the SIPOS and underlying silicon may play an important if not dominant, role with the implication that the high oxygen concentrations previously used may not be essential for J oe reduction. This would permit the use of lower oxygen content SIPOS films whose lower bulk resistivities would allow utilization of these films in structures such as scaled VLSI bipolar devices and high concentration point contact solar cells where series resistance is detrimental to device performance.

Y.H. Kwark

Papers

27.5-percent silicon concentrator solar cells

Point-contact silicon solar cells

Measurement of electron lifetime, electron mobility and band-gap narrowing in heavily doped p-type silicon

Silicon point contact concentrator solar cells

SIPOS Heterojunction contacts to silicon