Read the text in order to answer the question:

Alternative solar cells

Solar cells are devices that convert the energy of sunlight into electricity for use in our daily lives. Although there are various materials from which a solar cell can be made, the most used is Silicon, since this element is a very abundant semiconductor and easy to process. Commercial silicon solar cells have a conversion efficiency of up to $24\%,$ that is$,$ they convert $24\%$ of the incident light into electrical energy.

A relatively new alternative are cells made of CdTe $($Cadmium Telluride$),$ which is also a semiconductor compound $($a material that can behave as an insulator or conductor depending on the conditions$).$ The elements Cadmium and Tellurium are less abundant than silicon, and forming the CdTe compound involves a series of physical and chemical processes that are generally more expensive and complex than what is involved in manufacturing silicon cells. Currently, the research being done on CdTe cells has led to the manufacture of solar cells with a record efficiency of up to $18\%($less than that of silicon$).$ And the reason why we are betting on this technology instead of continuing to produce silicon cells is because silicon cells have already reached their efficiency limit$,$ science and technology can no longer increase their efficiency above $24\%,$ while the theoretical limit of efficiency of CdTe cells is up to $30\%,$ in addition to the fact that CdTe takes advantage of certain sunlight that silicon cannot, such is the case of ultraviolet light, silicon lets a lot of light pass through ultraviolet while CdTe absorbs and transforms a good part of it into electricity. Although unfortunately both types of cells are always heated by the amount of light they absorb$,$ since not all of the absorbed light is converted into electricity, there is always a portion that is converted to heat, which is an undesirable effect that cannot be avoided. suppress.

Both silicon and CdTe are manufactured by physical means where crystals of the material are grown in cubic$-$type structures, however for silicon large ingots must be manufactured which are then cut into $1$ or $2$ mm thick wafers to be capable of absorbing enough sunlight, while different layers of material are deposited in the CdTe cells until they have a thickness of $3$ microns, which is enough to guarantee that up to $99\%$ of the incident light is absorbed$.$ The deposition method consists of heating the CdTe until it sublimes $($it goes from solid to vapor$),$ the CdTe vapor collides with a colder transparent substrate, and there the different layers of CdTe begin to crystallize, depending on the time it takes. The process is maintained will be the thickness of the material, but a face$-$centered structure known as Zinc$-$Blenda is always generated. Subsequently, in a similar process, metallic material is deposited to form the electrical contacts, since due to the type of manufacturing it is not possible to weld cables of any type to this crystalline structure.

Another difference is that silicon cells are polycrystalline, that is$,$ they are made up of crystals oriented in many directions, while CdTe cells tend to be crystals with a single arrangement in a single direction, and it is very important to identify said cirstalographic direction at the same time. time to manufacture them, otherwise the energy conversion efficiency drops greatly. For any type of solar cell, everything must be manufactured in clean rooms since contaminants floating in the air can spoil the cells or reduce their efficiency.

Finally, it should be noted that Tellurium is a relatively harmless metalloid, but Cadmium is an element with certain toxicity that can cause carcinogenic problems if ingested or inhaled. This aspect has slowed down research a bit due to the great care that must be taken when handling it$,$ and especially when disposing of it$,$ since it cannot simply be thrown away, in addition to the fact that the manufacture of CdTe cells generates a lot of waste that They must be stored and processed carefully, something that also generates costs.

Although there are many advantages that could exist in obtaining CdTe cells with greater efficiency than silicon cells, there are also many cons why researchers prefer not to continue with the development of this technology in solar cells.

Question:

$2.-$ The following options are keywords that have to do with the microstructure of solar cells, but they are mixed for both CdTe and Silicon cells. Check only those that correspond to the CdTe. $($Select $2$ correct answers$)$

a$)$ Zinc$-$Blenda type cubic system

b$)$ cubic system

c$)$ Unique crystallographic direction

d$)$ Polycrystallinity