In our last article titled “Solar cells: Where is the world moving?” we introduced the various technologies of solar cell that are already commercialized. This article shall inculcate its readers about the technology which while still at labs, are projectedto dominate the market in the near future. It is hence important to understand such technologies, their offerings and their advantages.
As the name suggests, back contact solar cells have all or a part of their front contacts on its backside. This leads to reduced shading effect in the front side of the solar cell which in turn enhances the efficiency of the solar cell. Such type of cell topology is useful especially for concentrators or large area cells which demands higher current output. While there are many configurations possible, we would only talk about Interdigitated back contact solar cells here. The sole purpose of the front side of the cell is to allow the light to enter into the cell. With its reduced thickness, the light can easily reach the base of cell where the excited electrons can be collected via the interconnection (at rear side). Due to combination of all these affects, these cells have an enhanced power conversion efficiency of around 23%.
Figure 1: A typical architecture of Interdigitated Back contact solar cells (Source: PV Education)
A heterojunction in simple terms is defined as cell architecture where more than two junctions are formed within the cell. A typical heterojunction cell uses crystalline silicon sandwiched between two amorphous silicon of opposite polarity. They are interconnected via a thin intrinsic layer of amorphous silicon (Figure 2). They top and bottom contact points also use a transparent conducting oxide (TCO) to maximize the light transmittance as well as to enhance the current conductivity. The primary advantage of using such technology is that it would enable electron excitation from light of all intensity falling on it. This enables a high conversion efficiency which in conjunction with its reduced thickness of solar cell (100 ~120 m) enhances the overall efficiency of solar cell. The current reported record efficiency of solar cell was 25.6%, with a VOCof 0.74V, short circuit current of 41.8 mA/cm2 and fill factor of 82.7% (Masuko K, Shigematsu M, Hashiguchi T, Fujishima D, Kai M, Yoshimura N, et al.: Achievement of more than 25% conversion heterojunction solar cell. IEEE J Photovoltaics. 2014;4(6):1433-1435. DOI: 10.1109/JPHOTOV.2014.2352151). Available in both mono and multi crystalline, efforts are underway to reduce the fabrication cost of this technology for smooth commercialization and easy adaptation.
Figure 2: A typical architecture of Silicon heterojunction solar cell (Source: Waaree Energies)
Multijunction solar cells like its other cousins (of solar cells) despite being little old, is a very promising technology.As the name suggest these cells literally have multiple junction (Figure 3 on left) on solar cells interconnected to each other. The top most cell is usually of higher bang gap (band gap represents the minimum energy that is required to excite an electron up to a state in the conduction band where it can participate in conduction)and the subsequent junctions are of lower band gap. With such arrangement, the top most junction would absorb some light (with high energy) and pass therest (via the tunnel junction) to the lower junctions. The subsequent junctions would do the same until light reaches the end of the cell. The light absorption spectrum of this solar cell is evident from (right hand side) figure 3 where close to half of the incident light (grey light which represents standard spectrum of AM 1.5) is absorbed by the cell. The current reported efficiency of this technology is around 40.7% (1 sun concentration) and 44.7% (at 100 suns concentration) but from a very small solar cell as evident from figure 4. It would however be interesting to see how soon we can commercialize such high efficient technology.
Figure 3: A representation of 3 layered Multijunction solar cell (left) and its absorption spectra (right) (Source:N.V.Yastrebova (2007), High-efficiency multi-junction solar cells: current status and future potential)
Figure 4: Actual fabricated size of multijunction solar cell (Source: Fraunhofer)
We at Waaree Energies keep at both market and research trends. The time is not far when we would start using high efficient solar cells at commercially accepted rates. Additionally, we also expect a lot of variations and deviations in both cell architecture and materials which would enhance the efficiency and the power output of solar modules.
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HAMARA NATION
