Evergreen Solar (Evergreen), a Massachusetts silicon PV maker, recently announced that it filed for Chapter 11 bankruptcy and would try to sell its assets under the reorganization.

According to the Greentech Media report, those assets include “the String Ribbon technology that is at the core of Evergreen’s existence.”  Without String Ribbon, Michael Kanellos wrote, “Evergreen’s assets are more generic,” and if the technology can be sold off, “there won’t be a lot left to reorganize.”

So what is String Ribbon, and what are the technology assets that are up for sale?

According to Cleantech PatentEdge™, Evergreen is the assignee of 59 patents and published patent applications in the U.S. and abroad.

The String Ribbon technology is covered, at least in part, by a family of at least three patents – U.S. Patents Nos. 6,814,802, 7,022,180 and 7,507,291, each entitled “Method and apparatus for growing multiple crystalline ribbons from a single crucible” (collectively “Ribbon Patents”).

The basic ribbon technology, before the invention of the Ribbon Patents, is shown in FIG. 1A.  It involved continuous growth of a single ribbon of silicon sheet material (17) drawn through a single crucible (11). 

Inside the crucible (11) is a melt (12) of silicon and a pair of strings (15) extending through the crucible (11). 

The cooler liquid silicon crystallizes at the top of the meniscus (19), and the strings (15) become incorporated in and define the edge boundaries (18a, 18b) of the crystalline ribbon (17).

According to the Background section of the Ribbon Patents, the continuous growth of silicon ribbon obviates the need for slicing of bulk produced silicon to form wafers.

Instead of just one ribbon at a time, the invention of the Ribbon Patents enables continuous and concurrent growth of multiple semiconductor ribbons from a single crucible.

Figure 2A, for example, shows a continuous two-ribbon dual growth system (20) including a crucible (21) having a melt (22) of silicon and two pairs of strings (25a, 25b) extending through the crucible. 

Each of the pairs of string (25a, 25b) has a fixed distance between the strings.  Two crystalline ribbons (27a, 27b) of silicon are drawn from the melt (22) as the cooler liquid silicon crystallizes at the tops of the menisci (29a, 29b). 

The pairs of strings (25a, 25b) pass through holes in the bottom of the crucible and become incorporated in and define the edge boundaries of the crystalline ribbons (27a, 27b).

Additional embodiments, as shown in FIGS. 3A and 3B, include meniscus shapers (3a, 3b) placed around the two pairs of strings (35a, 35b) to partition the melt (32) to form subregions (3c, 3d) from which two ribbons (37a, 37b) grow.

The Ribbon Patents also teach a nine-ribbon growth system, shown in FIG. 6, which includes nine pairs of strings, nine meniscus shapers (6a-6i), and an afterheater (64).

According to the Ribbon Patents, the disclosed methods and apparatus allow for a substantially better rate of production and efficiency and reduce capital, material, and labor costs. 

But the Greentech Media piece points out that adopting the ribbon technology requires some changes to standard manufacturing processes, and the wafers and cells produced by the ribbon process are sized differently than those made by conventional manufacturing methods.

It will be interesting to see how much green will be offered for these green patents.

REhnu (pronounced “renew”) is a company based in Tucson, Arizona that makes and sells concentrated photovoltaic (CPV) solar energy systems.

The company believes that the greatest energy for the least cost may consist in first greatly concentrating solar energy (1200x), and then converting it to electricity utilizing highly efficient triple-junction photovoltaic cells.

The company’s technology is described in at least three pending patent applications:  U.S. Patent Application Publication Nos. 2009/0277440 (‘440 Application), 2009/0277498 (‘498 Application), and 2009/0277224 (‘224 Application).  Rehnu CEO and University of Arizona astronomy professor Dr. Roger P. Angel is a named inventor on each of the applications.

A solar tracker “module” has eight large dish reflectors (1) and 36-cell receivers pointed at the sun by a stiff, lightweight frame. Each of REhnu’s reflectors (1) is made from a single, back-silvered glass sheet, 3.1m per side.

The glass is produced in the factory as flat sheets, 3.3 m square according to processes described in the ’224 Application, allowing for use of economical float glass.

Fig. 1. Full-size two-axis solar tracker module disclosed in the ‘440 Application, entitled “Solar concentrator apparatus with large, multiple co-axial dish reflectors”. The reflectors are manufactured via a process disclosed in the ‘224 Application, entitled “Method of manufacturing large dish reflectors for solar concentrator apparatus”.

Then, each sheet is heated and molded into a paraboloid. After cooling, a reflective film of silver is applied to the back surface, like in ordinary mirrors. With rolled edges for stiffening, the resulting finished product is 3.1m x 3.1m.

Next comes the concentration. Each of the eight dishes reflects light into its own ball lens (5), suspended at the focus.  Each ball lens (5) concentrates the light (3) 400x before passing it on, evenly, to a concave array of 36 optical funnels (45).

Each optical funnel further concentrates the light (3) to 1200x while funneling the solar energy to its own triple-junction photovoltaic cell (15).

Triple-junction cells, unlike single-junction cells, have three different junctions, each of which separately captures solar energy at distinct wavelengths. This results in more than a doubling of conversion efficiency over single-junction cells, such as silicon.

Triple-junction cells are more costly than silicon cells but cost less per unit power generated when used with highly concentrated light.

According to REhnu’s receivers & cooling web page, “when sold in the quantity needed to generate several megawatts, the cost is projected to soon reach $5/cm², which at 1200x concentration and 30% module conversion efficiency, works out to $0.16/watt.”

This is roughly one-sixth of the cost per unit of power conversion compared to a single-junction photovoltaic cell used without concentration.    

The power produced by triple-junction cells is reduced by 1.35% for every 10°C rise in temperature, though, so cooling is very important. REhnu’s cells are cooled by liquid from a radiator and the radiator is cooled by fans.

Can concentrated solar energy from a ball lens coupled with increasingly efficient energy conversion from triple-junction photovoltaic cells produce cheap electricity from a small footprint on a commercial scale?

According to Rehnu’s web site, the University of Arizona has proven the technology in an 8-cell system, and larger systems continue to be tested to provide the answer to that question. 

Jonah Smith is President of the Notre Dame Intellectual Property Law Society and a J.D. candidate at Notre Dame Law School. Jonah is a contributor to the IP Lawyer Blog.

Alta Devices, a Santa Clara, California solar PV company, recently announced that its gallium arsenide solar cells have set a new efficiency record of 28.2%. 

The company’s press release quotes Alta co-founder Eli Yablonovitch, who says the trick to maximizing voltage is to generate more photons inside the solar cell.

According to Cleantech PatentEdge™, a couple of Alta’s patent families relate to a PV cell structure that increases voltage using a thin absorber layer to trap light.

U.S. Patent Application Publication Nos. 2010/0132780, entitled “Photovoltaice device” (’780 Application) and No. 2010/0126570, entitled “Thin absorber layer of a photovoltaic device” (’570 Application) are representative of these patent families. 

Both applications are directed to a PV unit (100) having several epitaxial layers including a buffer layer (102), a release layer (104), a window layer (106), a base layer (108), and an emitter layer (110).  The base layer (108) may comprise n-doped gallium arsenide semiconductor material, and the emitter layer (110) may comprise a p-doped aluminum gallium arsenide semiconductor material.

The combination of the base layer (108) and the emitter layer (110) may form an absorber layer for absorbing photons. 

According to the ’780 and ’570 Applications, this structure provides an absorber layer that is much thinner than those found in conventional solar cells (<500 nm versus up to several micrometers).

This is important because the thickness of the absorber layer is proportional to “dark current” levels in the cell, so a thinner absorber layer reduces dark current levels.  Dark current is the small electric current that flows through a PV cell even when no photons are entering the cell.

According to the ’780 and ’570 Applications, a lower dark current level means higher voltage and greater efficiency:

Because the open circuit voltage (V_oc) increases as the dark current is decreased in a photosensitive semiconductor device, a thinner absorber layer may most likely lead to a greater (V_oc) for a given light intensity and, thus, increased efficiency. As long as the absorber layer is able to trap light, the efficiency increases as the thickness of the absorber layer is decreased.

Alta’s web site says it is a development stage company.  The company certainly excels at development.

Array Converter (Array) is a Sunnvale, California, company that designs inverter-less solar power systems to directly convert the direct current (DC) electricity produced by solar modules into alternating current (AC) for the grid.

Array owns several patents relating to its converter technology, including U.S. Patent No. 7,884,500 and a patent family consisting of U.S. Patents Nos. 7,719,864, 7,929,324, and 7,929,326 (collectively “Converter Patents”).

The Converter Patents are directed to a DC to pulse amplitude modulated (PAM) current converter, which is referred to by its acronym, PAMCC, and to electrical power conversion systems comprising arrays of PAMCCs.

A PAMCC (400) receives direct current from a photovoltaic panel (401) through positive and negative input terminals (402, 403), each connected in series with coils L1 (406) and L2 (405), respectively.  These coils comprise a single transformer T1 (407).

A controller (412) controls several independent lines (419-422) of circuits (423-426).  Capacitors (438, 440) are across the input side of coils (430, 432), respectively and a neutral output terminal (432).

Control signals on lines 411 and 419-422 connect and disconnect the current provided by the PV panel (401) in sequence within the PAMCC (400) in such a way as to provide an output whose amplitude is a PAM signal approximating a sine wave.

Several claims of the Converter Patents are directed to a plurality of PAMCCs and recite the following key feature:

the current pulses of at least two converters are out of phase with respect to each other, thereby summing the current pulses of all of the converters such that a signal modulated onto the pulse output of the converters is demodulated

According to the Converter Patents, when an array of PAMCCs are connected in parallel such that the PAMCCs’ output pulses are out of phase with respect to each other, the result is an AC current waveform suitable for both local load use and connection to the utility grid:

An array of PAMCCs constructed in accordance with the present invention form a distributed multiphase inverter whose combined output is the demodulated sum of the current pulse amplitude modulated by each PAMCC. If the signal modulated onto the series of discontinuous or near discontinuous pulses produced by each PAMCC was an AC current sine wave, then a demodulated, continuous AC current waveform is produced by the array of PAMCCs. This AC current waveform is suitable for use by both the “load”, meaning the premises that is powered or partially power by the system, and suitable for connection to a grid.  For example, in some embodiments an array of a plurality of PV-plus-PAMCC modules are connected together to nominally provide split-phase, Edison system 60 cps 240 volt AC to a home.

Other advantages discussed by the Converter Patents include that the high voltage portion of the PAMCC is physically very small (a few square inches) and can be located on the back of a solar panel assembly, making insulation simple and lightweight.

Array’s web site touts the simplicity of the inverter-less technology and notes that it is “less than a quarter of the complexity of the simplest power inverter without the need of short-lived filled or plastic film components.”  

The bottom line, though, is cost:

Simplicity means every cost is reduced while Array Converter improves LCOE by both reducing each cost and increasing energy harvest.