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Sunday, October 21, 2018

#393 High-end solar cells

As indicated earlier, silicon serves very well "mass photovoltaics". However, in order to meaningfully increase efficiency of solar cells, multi-junction, compound semiconductors-based cells known as tandem solar cells are being employed.


The goal is to improve the use the solar spectrum by stacking up semiconductors featuring different energy gaps in multi-junction cells arranged such that energy gap width increases from the bottom to the top of the stack. This arrangement assures absorption of the fairly broad range of wavelengths included in the solar spectrum with shortest being absorbed by the top layers and longer penetrating deeper into the stack where they are absorbed by the narrower bandgap materials. The result is high-cost class solar cells, but also the cells featuring the highest efficiencies approaching 50%.

Posted by Jerzy Ruzyllo at 12:34 PM | Semiconductors | Link

Sunday, September 16, 2018

#392 Silicon rules photovoltaics

In reference to the previous entry... Vast majority of solar cells is manufactured using silicon (in the order of decreasing efficiency either single-crystal, or multicrystalline, or amorphous depending on the needs) which is on one hand by far the most common and highly manufacturable semiconductor material, and on the other features energy gap which sufficiently well matches energy spectrum of the sunlight.


All in all, silicon very well serves the needs of photovoltaic industry and we are very lucky to have this outstanding element so readily available (sand!).

Posted by Jerzy Ruzyllo at 09:15 AM | Semiconductors | Link

Sunday, August 26, 2018

#391 Which solar cells where?

 As mentioned in previous entry, solar cells come in the variety of shapes. A question  is what type of cell is used where?


The choice is based on various criteria. For instance, what is the area available and how it defines the cost of the cells, and thus, materials/methods used to manufacture them? Is it a several square miles big part of the desert or few square meters wide satellite panel? Or maybe less than centimeter square cell powering a wrist watch or calculator?  In the first case cost is an issue because such a solar farm is meant to produce energy at the cost comparable with the energy obtained from other sources.  In the second case cost is not an issue because at whatever cost the highest efficiency solar cells must be used. In the third case the cost of the cell is marginal and is not even a consideration.

Posted by Jerzy Ruzyllo at 11:55 AM | Semiconductors | Link

Sunday, August 19, 2018

#390 Solar cells - efficiency

 Solar cells come in the broad range of material-device structure combinations. From elemental semiconductor  (silicon) to compound semiconductor based, from inorganic to organic semiconductors, from thin-film amorphous to single-crystal cells, from flexible to rigid, from low-cost to costly, ect. etc., performance of various classes of solar cells varies widly.


Parameter commonly used to describe performance of the solar cell  is its efficiency defined as a ratio of maximum power generated by the cell and the input power of the solar energy.  Efficiency of the solar cell is a function of material from which it is constructed and complexity of its structure, and thus, complexity of the processing steps employed in its manufacture. It can be as low as few % for the low-cost cells to close to 50% for the complex, multilayer cells. A universally valid rule is that the efficiency of solar cell is proportional to its cost which encompasses cost of the material used and cost of the manufacturing process.


Posted by Jerzy Ruzyllo at 03:13 PM | Semiconductors | Link

Sunday, July 15, 2018

#389 MOS potential barrier

In blogs # 382 and # 384 formation of the p-n junction and Schottky diodes were discussed as  two ways to form  potential barriers upon which operation of semiconductor devices is based. Actually, there is a third way, and pretty important one, although, it works in the device somewhat differently that p-n junction and Schottky contact.

The third way of potential barrier formation involves bringing semiconductor to contact with an insulator which results in the alteration of potential distribution in semiconductor in the region immediately adjacent to its interface with insulator. To convert such structure into a current controlling device metal contact (referred to as a gate) needs to be formed on the surface of an insulator making it into what is known as Metal-Insulator-Semiconductor, or MIS configuration; more commonly used is synonymous term Metal-Oxide-Semiconductor, or MOS.  Rings the bell? I am sure it does.


The difference between p-n junction and Schottky contact structures on one end and MOS devices on the other is in the direction of the current flow. In the former case current flows in the direction normal to the surface and potential barrier heights control the flow. In the case of the former, current flowing in the direction parallel to the surface can be controlled by altering distribution of the space-charge associated with the potential barrier.

Posted by Jerzy Ruzyllo at 11:22 AM | Semiconductors | Link

Sunday, June 10, 2018

#388 Solar cells and LEDs, cont

 While both devices are "playing" with light, LEDs and solar cells feature very different needs regarding semicondsuctor materials used to manufacture them as well as different device configurations, or in other words different designs.


As far as materials are concnerned, the former require direct-bandgap semiconductors so that energy released as a result of the electron-hole recombination is released in the form of light (photon) as opposed to the "packet" of the energy of the lattice vibrational wave call phonon (well, to keep it simple call it heat).  In the case of the latter the main concnern regarding material selection is a trade-off between cost and efficiency of the cell.


Regarding designs, in the case of the LED the goal is to let generated light out of the device in an as unobstructed fashion as possible. In the case of the solar cells the focus is on letting as much of thesunlight into the device as possible.  On the surface it may look like not much of the difference - light out vs. light in - but in reality differences between device configurations of LEDs and solar cells are rather serious (may be more on this one later).



Posted by Jerzy Ruzyllo at 01:46 PM | Semiconductors | Link

Sunday, May 20, 2018

#387 Solar cells and LEDs

 Some more about light and semiconductor diodes….


In terms of interactions with light, semiconductor diodes represent two distinct, very different classes of devices. Some of them are converting light into electricity and those are obviously solar cells working of the principles of the photovoltaic effect and photodiodes working as light detectors.


The devices in the second group work in the exactly opposite direction and convert electricity into light with light emitting diodes, or LEDs in short, being a prime example of light emitting device. Here, the effect of photoluminescence is a foundation of the device operation.


Posted by Jerzy Ruzyllo at 07:58 AM | Semiconductors | Link

Sunday, April 22, 2018

#386 Photovoltaic, photoelectric

Two well established physical effects occuring in solids are photovoltaic and photoelectric effects. As they are quite often confused here is a quick explanation of what is what.

In the case of photovoltaic effect, free charge carriers generated as a result of absorption of energy from sunlight remain within the solid and contribute to the current flowing across it. The most common way to exploit this effect is by constructing semiconductor diodes which are well known as solar cells. In contrast, in the case of the photoelectric effect electrons generated by absorption of light are emitted to the outside of the solid. The prime use of this effect is in the detection of light.



Posted by Jerzy Ruzyllo at 11:23 AM | Semiconductors | Link

Sunday, March 11, 2018

#385 Diodes

As mentioned in the previous blog presence of the potential barrier makes a piece semiconductor display non-symatrical (rectifying) diode-like current-voltage characteristics and the type of the potential barrier determines type of diode. And again, a p-n junction barrier  forms  p-n junction diode, while metal-semiconductor originating potential barrier forms Schottky diode. New message is that this is it as far as types of semiconductor diodes are concerned.


Posted by Jerzy Ruzyllo at 08:15 PM | Semiconductors | Link

Sunday, February 18, 2018

#384 Potential barrier at M-S contact

In reference to blog #382....


An alternative to p-n junction way of potential barrier formation is by bringing into contact semiconductor and metal featuring different work functions and to form the metal-semiconductor contact also known as Schottky contact (after German physicist Walter Schottky, 1886 -1976). In this case difference in work functions of materials in contact alters potential distribution across the junction which results in formation of a potential barrier. 


Similarly to p-n junction, a reverse bias voltage applied to the Schottky junction increases height of the potential barrier and prevents the flow of majority carriers from semiconductor to metal.  A forward bias, lowers the potential barrier at the contact  and allows large current to flow from semiconductor to metal. Similarly to  p-n junction, the result is a non-symmetrical, rectifying current-voltage characteristic making metal-semiconductor contact a diode known as Schottky diode.

Posted by Jerzy Ruzyllo at 03:38 PM | Semiconductors | Link

‹‹ ›› is the personal blog of Jerzy Ruzyllo. With over 35 years of experience in academic research and teaching in the area of semiconductor engineering (currently holding position of a Distinguished Professor of Electrical Engineering and Professor of Materials Science and Engineering at Penn State University), he has a unique perspective on the developments in this progress driving technical domain and enjoys blogging about it.

With over 2000 terms defined and explained, Semiconductor Glossary is the most complete reference in the field of semiconductors on the market today.

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