Reading Exercise: Bright Prospects for Silicon

The age-old adage that 'the devil is in the details' aptly illustrates the persistent obstacles that hinder an innovative idea from maturing into a viable technology. It also often refers to the challenges that must be addressed to reduce costs of the resulting product to enhance its market appeal. Emanuel Sachs, from the Massachusetts Institute of Technology, has encountered many such challenges in his ongoing effort to develop affordable, high-efficiency solar cells.

In his latest endeavor, Sachs has discovered incremental methods to enhance the electricity output of conventional photovoltaic cells from sunlight without increasing costs. Specifically, he has improved the conversion efficiency of multi-crystalline silicon test cells from the typical 15.5 percent to nearly 20 percent—equivalent to more expensive single-crystal silicon cells. Such advancements could potentially reduce the cost of solar power from the current $1.90 to $2.10 per watt to $1.65 per watt.

With further refinements, Sachs aims to develop solar cells within four years that can generate electricity at one dollar per watt, a milestone that would make solar power competitive with electricity from coal-fired power plants.

B.

Most photovoltaic cells, such as those found on residential rooftops, utilize silicon to convert sunlight into electrical current. Metal contacts then conduct the electricity from the silicon to power devices or feed into the electrical grid. Engineers have predominantly favored single-crystal silicon as the active material since solar cells became practical and affordable three decades ago, according to Michael Rogol, managing director of Germany-based Photon Consulting.

Wafers of this material are typically sliced from an ingot made of a single large crystal pulled from molten silicon, resembling taffy. Initially, high-purity ingots were leftover from integrated circuit manufacturing, but later this process was adopted for producing photovoltaic cells themselves, as Rogol recounts.

While single-crystal cells offer high conversion efficiencies, they are costly to manufacture. Alternatively, multi-crystalline silicon cells, fabricated from lower-purity cast ingots composed of multiple smaller crystals, are cheaper to produce but unfortunately less efficient than single-crystal cells.

C.

Sachs, known for pioneering innovative methods to make silicon solar cells more affordable and efficient, has recently focused on refining the manufacturing of multi-crystalline silicon cells. His first improvement involves the silver contacts that collect electrical current from the surface of the silicon bulk,' he explains. In traditional manufacturing processes, cell makers use screen-printing methods ('similar to precise silk-screening of T-shirts,' Sachs notes) and silver ink containing silver particles to create these conductive wires.

However, standard silver wires tend to be wide and short, approximately 120 by 10 microns, with numerous non-conductive voids. Consequently, they obstruct significant sunlight and do not efficiently conduct as much current as required.

D.

At his startup company—based in Lexington, Massachusetts—1366 Technologies (the number refers to the flux of sunlight that strikes the Earth’s outer atmosphere: 1.366 watts per square meter), Sachs employs a unique wet process capable of producing thinner and taller wires measuring 20 by 20 microns.

The narrower bus wires use less expensive silver and can be positioned closer together, enabling them to draw more current from the adjacent active material, which limits the travel distance of free electrons. Simultaneously, these wires block less incoming light compared to standard ones.

E.

The second innovation modifies the broad, flat interconnect wires that gather current from the silver bus wires and connect neighboring cells electrically. The top interconnect wires can shade up to 5 percent of the cell’s surface area. “We apply textured mirror surfaces to these rolled wires,” Sachs explains.

These tiny mirrors reflect incoming light at a shallower angle—around 30 degrees—so that when the reflected rays hit the top glass layer, they remain within the silicon wafer through total internal reflection,” Sachs elaborates. (Divers and snorkelers frequently observe this optical phenomenon when viewing water surfaces from below.) The longer light remains inside, the greater the likelihood it will be absorbed and converted into electricity.

F.

Sachs anticipates that new anti-reflection coatings will further enhance the efficiency of multi-crystalline cells. One of his company’s future objectives involves transitioning from costly silver bus wires to more economical copper ones. Sachs has devised several strategies for successfully implementing this change. “Unlike silver, copper negatively impacts the performance of silicon PVs,” Sachs notes, “so it will be essential to incorporate a low-cost diffusion barrier to prevent direct contact between copper and silicon.” In this industry, attention to these small details is always critical.

G.

The price of silicon solar cells is expected to decrease as bulk silicon prices decline, as reported by the U.S. Energy Information Administration and Solarbuzz, an industry tracking firm. Recent years have seen a sharp increase in solar panel sales, leading to a global silicon shortage due to lagging production capacity for the active material. However, new silicon manufacturing facilities are now coming online. The reduced cost of materials and resulting lower system prices are poised to significantly stimulate demand for solar-electric technology, according to market analyst Michael Rogol from Photon Consulting.

Questions 1-5

Utilize the information in the passage to match the individuals or companies (listed as A-C) with the opinions or actions below. Write the appropriate letters A-C in boxes 1-5 on your answer sheet.

NB you may use any letter more than once

• A. Emanuel Sach
• B. Michael Rogol
• C. Solarbuzz

1. Gives a brief account of the history of the common practice to manufacture silicon batteries for a long time.

2. Made a joint prediction with another national agency.

3. Established an enterprise with a meaningful name.

4. Led forward in the solar-electric field by reducing the cost while raising the efficiency.

5. Expects to lower the cost of solar cells to a level that they could contend with the traditional way to generate electricity.

Questions 6-9

Do the statements below align with the information provided in the Reading Passage?

In boxes 6-9 on your answer sheet, write

TRUE if the statement is true

FALSE if the statement is false

NOT GIVEN if the information is not given in the passage

6. The Achille’s heel of single-crystal cells is the high cost.

7. The multi-crystalline silicon cells are ideal substitutions for single-crystal cells.

8. Emanuel Sachs has some determining dues about the way to block the immediate contact between an alternative metal for silver and the silicon.

9. In the last few years, there has been a sharp increase in the demand for solar panels.

Questions 10-14

Summarize the paragraphs in the Reading Passage using no more than three words per answer. Write your responses in boxes 10-14 on your answer sheet.

Emanuel Sachs made two major changes to the particulars of the manufacture 10 … . One is to take a 11 … in the production of finer wires which means more current could be attracted from the 12…. The other one is to set 13… above the interconnect silver bus wires to keep the incoming sunlight by 14… .

1. B (Đoạn B, “Wafers of the substance are typically sawed from an ingot consisting of one large crystal that has been pulled like taffy out of a vat of molten silicon.”)
2. C (Đoạn G, “Energy information Administration and the industry tracking firm Solarbuzz. A steep rise in solar panel sales in recent years had led to a global shortage of silicon.”)
3. A (Đoạn 
4. A (Đoạn G, “The reduced materials costs and resulting lower system prices will greatly boost demand for solar-electric technology”)
5. A (Đoạn 
6. TRUE (Đoạn B, “Although single-crystal cells offer high conversion efficiencies, they are expensive to make.”)
7. FALSE (Đoạn B, “The alternatives- multi-crystalline silicon cells, which factories fabricate from lower-purity, cast ingots composed of many smaller crystals—are cheaper to make, but unfortunately they are less efficient than single-crystal cells.”)
8. NOT GIVEN (Không có thông tin)
9. TRUE (Đoạn G, “A steep rise in solar panel sales in recent years had led to a global shortage of silicon because production capacity.”)
10. Multi-crystalline silicon cell (Đoạn C, “Sachs, who has pioneered several novel ways to make silicon solar cells less costly and more effective, recently turned his focus to the details of multi-crystalline silicon cell manufacture.”)
11. Proprietary wet process (Đoạn D, “Sachs is employing “a proprietary wet process that can produce thinner and taller” wires”) 
12. Neighboring active material (Đoạn D, “The slimmer bus wires use less costly silver and can be placed closer together so they can draw more current from the neighboring active material”)
13. Textured mirror surfaces (Đoạn E, “”We place textured mirror surfaces on the faces of these rolled wires…”)
14. Total internal reflection (Đoạn E, “…they stay within the silicon wafer by way of total internal reflection,” Sachs explains.”)