if u used 2 types material it will be 1 junction and used 3 or 4 types material it will multi junction solar cells

Dear Akhil Raj Kumar Kalapala:

The number of junctions in a solar cell is determined by the number of p-n junctions in that cell. Single-, double-, and triple-junction solar cells have one, two, and three p-n junctions, respectively. A multi-junction (or tandem) solar cell is usually used to call the solar cell having more than two p-n junctions. If a solar cell consists of several layers of materials (known as a superlattice) but has only one p-n junction, it is called a single-junction multi-staked (or multi-layered) solar cell. You can use this concept to understand the solar cell structure from its name.

Hope it helps.

you can also follow this link. it will be helpful to know the benefits of multiple junctions solar cell over single junction solar cells.

http://www.sj-solar.com/technology/

Dear Akhil Raj Kumar Kalapala:

According to the question you sent about the solar cell efficiency, some suggestions are given below.

Roughly speaking, an increase in the number of junctions can generally increase the solar cell efficiency if each junction is made of different materials which provide different band gap energies such that the solar spectrum can be covered as much as possible. You can find more detail in the question which I answered [https://www.researchgate.net/post/Why_is_the_Jsc_of_tandem_solar_cells_are_typically_lower_than_Jsc_of_the_subcell_that_produces_lower_current]. Technically, there are many factors having to be considered for designing and making a multi-junction solar cell, e.g. current matching, carrier lifetime, fabrication process and induced defects.

With the use of the same material, increasing the number of junctions cannot boost the efficiency significantly because the probability of photon absorption increases a little bit.

A single junction would mean n-CdS/p-CdTe solar cell.

A multiple junction solar cell would mean having more than one junction: n-CdS/p-CdTe/n-In2S3/p-CuInS (2-junction solar cell). 

The whole idea behind using more than one junction is to stack up materials in a junction in a way that it increases the photon conversion efficiency i.e. creating more electron-hole pairs.

Very good answers and comments from Maetee and Maqsood ... You do not need more than these details Akhil Raj.

Standard solar cells use one active junction and the conversion efficiency is limited to about 30% in the best case. To increase the efficiency it is needful to extend the active part of the solar spectrum, i.e. generally the near infrared and the near UV. That could be done by th use of two or four junctions made with materials which the energy gap is suited. The junctions are frequently connected in series (so called monolithic tandem or multijunction cell), giving rise to a larger photovoltage, but the photocurrent is limited by the smallest current generate by one of the cells. Other possibilities result from the use of spectral splitting,techniques, orthe use of several terminals. To day th best result is about 45 % in conversion efficiency for very small devices devotes to high concentration of the solar energy.