**2. Evolution of solar cells**

The 'photovoltaic effect' literally means generation of a voltage upon exposure to light. The phenomenon was first observed by the French physicist Edmund Becquerel on an electrochemical cell in 1839, while it was observed by British scientists W.G. Adams and R.E. Day on a solid-state device made of selenium in 1876 [3]. From the 1950s onwards, there was rapid progress in the performance of commercial solar cells from <1% to >23% [2] and silicon has been the 'work-horse' of the photovoltaic industry since then. The evolution of silicon solar cells is shown in **Figure 1**.

The first silicon solar cells demonstrated by Russell Ohl of Bell Laboratories during 1940s were based on natural junctions formed from impurity segregation during the recrystallization process [3]. The cells had an efficiency of <1% due to lack of control over the junction location and the quality of the silicon material. The nomenclature for naming the regions (p-type: side which is illumination and n-type: other side) given by Ohl are since then being used for the solar cell naming conventions.

During the 1950s, there was rapid development in the high-temperature diffusion process for dopants in silicon. Person, Fuller and Chaplin of Bell Laboratories demonstrated a 4.5% efficient solar cell with lithium-based doping, which improved to 6% with boron diffusion. The solar cell had a 'wrap-up' around structure (**Figure 1(b)**) with both contacts on back side to avoid shading losses, but led to higher resistive losses due to the wrap-around structure. By 1960, the cell structure evolved to as shown in **Figure 1(c)**. Since the application was for space explorations, high resistivity substrate of 10 Ω cm was used to have maximum radiation resistance. Vacuum evaporated contacts were used on both sides, while a silicon monoxide coating was used as an antireflective coating (ARC) on the front-side (FS) [3].

In early 1970s it was found that having sintered aluminum on the rear-side improved the cell performance by forming a heavily doped interface known as the 'back-surface field (Al-BSF)' and gettering of the impurities [3]. The Al-BSF reduces recombination of the carriers on the rear-side and hence improves the voltage and the long-wavelength spectral response. Implementation of finer and closely

#### **Figure 1.**

*Evolution of silicon solar cells. (a) 1941: Solar cell reported with grown-in junction, (b) 1954: Solar cell p-n junction formed with dopant diffusion, (c) 1970: Violet cell with Aluminum back-surface field, (d) 1974: Black cell with chemically textured surface [3].*

#### *Industrial Silicon Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.84817*

spaced fingers reduced the requirement on the junction doping and eliminated the dead layer. An ARC of titanium dioxide (TiOx) was used and its thickness was selected to reduce the reflection for shorter wavelengths and gave a violet appearance to the solar cells. Further improvement was made by texturing the wafers using anisotropic etching of (100) wafers to expose the (111) surfaces. The texturing led to improved light-trapping and gave the cells a dark velvet appearance. The improved cell architecture is shown in **Figure 1(d)**. In 1976, Rittner and Arndt demonstrated terrestrial solar cells with efficiencies approaching 17% [3].

The passivated emitter solar cell (PESC) achieved a milestone of 20% efficiency in 1984–1986. The metal/silicon contact area was only 0.3% in PESC cells, while a double layer ARC of ZnS/MgF2 was used in both cell structures. In 1994, passivated emitter rear locally diffused (PERL) cell with an efficiency of 24% were demonstrated [3]. As compared to the PESC cell, the PERL cell had inverted pyramids on FS for better light-trapping and oxide-based passivation on both sides. Oxide passivation layer on the rear-side also improved the internal reflectance of the long wavelength and hence the spectrum response.

In addition to the evolving solar cell architectures, there has also been continuous development in the manufacturing domain in terms of increased throughput, improved process-steps and reduced costs. A brief over-view of the manufacturing of Si substrates and various types of solar cells is given in the next section.
