Shivananda Pukhrem

Since the first solar cell was produced by Bell Labs in the 1950s, solar photovoltaic (PV) technology has been gradually evolving. The work resulted in the development of a compound which is formed of semiconductor elements found in the periodic table and the synthesis of an organic solar cell. Broadly, photovoltaic technologies are now classified as: crystalline silicon solar cells, thin-film solar cells, and organic solar cells. In the following paragraphs, an overview of various concepts in photovoltaic technology based on crystalline silicon wafers are briefly described. Such concepts were used from the early 1990s to deliver relatively high-efficiency solar modules for the market. As the $/watt of a solar panel is dropping, the evolution in photovoltaic technology is also progressing.

High-efficiency concepts of crystalline silicon (c-Si) wafer based solar cells

Many researchers are working on c-Si solar cell solutions aimed at overcoming the limitations faced using the traditional method of photovoltaic technology production. The prime approach to increase the efficiency of c-Si solar cell would reduce both surface and bulk recombination within the cell. For this reason, most of the high-efficiency c-Si solar cells is based on monocrystalline wafers. Notably, in c-Si PV technology, there are three important concepts: PERL, IBC, and Hetero-junction types of solar cells.

1. PERL: PERL is an abbreviation for Passivated Emitter Rear Locally diffused. This concept was first developed by Prof. Martin Green’s group at the University of New South Wales (UNSW) in the late 1980s and early 1990s. Many of us who know him have dubbed him as a “father of photovoltaics.” Through this concept, a collaboration between Suntech and UNSW achieved a 20.3% efficiency record for a production solar cell in March 2012. This concept has been an example for various PV technologies developed afterwards. Figure 1, below, shows the PERL solar cell concept, which uses p-type Float zone silicon wafers.

Fig 1 : PERL solar cell.

In the PERL concept, the front contact area, the emitter layer, and the rear contact are together able to achieve higher efficiency.

• Now, the optical losses in the front contact area are reduced by implementing a textured inverted pyramid structure coated with an anti-reflector. The contact area at the front side has been made as small as possible. This enhances the total amount of light coupled into the solar cell by allowing collection of reflected light for the second time with less shadowing losses.
• The emitter is heavily doped underneath the contacts. In PERL, this is achieved by phosphorous-diffused regions. The rest of the emitter is moderately doped to preserve the “blue response.” A silicon oxide is passivated on the top of the emitter to suppress the surface recombination velocity.
• In the rear surface of the solar cell, point contacts are used in combination with the thermal oxide passivation layers to reduce the unwelcome surface recombination at the uncontacted area. Heavily doped boron acts as a local back surface to limit the recombination of the minority electrons at the metal back.

2. IBC: IBC is an abbreviation for an Interdigited Back Contact solar cell. Back-contacted solar cells, in contrast to PERL, use n-type Float zone monocrystalline wafers. SunPower commercialized IBC solar cell modules with an initial achievement of 22.5% of efficiency. Now SunPower has achieved an efficiency of 24.2% from a monocrystalline silicon IBC solar cell.

Fig 2: IBC solar cell.

The IBC solar cell has many localized junctions instead of a single large p-n junction. The electron-hole pairs generated by the incident light that is absorbed at the front surface can still be collected at the rear of the cell. The semiconductor-metal interfaces are kept as small as possible to reduce the unwelcome recombination at this defect-rich interface. Such a small cross-section of metal fingers also reduces the resistive losses of the contacts. As depicted in Figure 2, the back of the IBC solar cell has two metal grids. One collects the current from the n-type contact and the other contact collects the current from the p-type contact. The front surface field is created by being heavily n-doped at the front of the cell to reduce surface recombination. However, the doping intensity decreases gradually towards the back to act as a p-doped region. Finally, it behaves like a p-n junction. The front surface acts as a passivation (silicon dioxide) of the defects at the front interface. As in case of PERL, the top front surface is textured and deposited with double layered anti-reflection coating.

3. Hetero-junction: So far, both PERL and IBC solar cells are homo-junction solar cells, or a p-n junction with a depletion zone (i.e., these junctions are fabricated by different doping types within the same semiconductor material). This means that the band gap in the p- and n-doped material is the same. However, in hetero-junction solar cells, the junction is made from two different semiconductor materials. One semiconductor material is p-doped and the another type of semiconductor is n-doped. The crystalline silicon wafer–based hetero-junction solar cell concept was invented by the Japanese company Sanyo, which is currently part of Panasonic. It is also called a HIT solar cell, which stands for heterostructure with intrinsic thin film, and has achieved an efficiency of 24.7%.

Fig 3: Hetero-Junction solar cell.

In c-Si wafer–based hetero-junction solar cells, one semiconductor material is from an n-type float zone monocrystalline silicon wafer and the other material from hydrogenated amorphous silicon. As depicted in Figure 3, there are two junctions — front and rear — in the solar cell. The front junction is formed by a thin layer of intrinsic amorphous silicon with deposition of a thin layer of p-doped amorphous silicon on top of it. In Figure 3, i/p a-Si stands for intrinsic p-doped amorphous silicon. Similarly, the rear junction (i/n a-Si) is composed of a thin layer of intrinsic amorphous with deposition of n-doped amorphous silicon on top. HIT allows the introduction of the n-type backside contact scheme as seen in IBC, thus allowing it to use a bi-facial configuration (i.e., it can collect light from the front as well as scattered and diffuse light falling on the back of the solar cell).

Remarks

A comparison remark of the above three concepts is presented here. From this remark, a customer can decide which is the right PV technology for their needs. The c-Si wafer–based HIT solar cell from Panasonic achieved an efficiency of 24.7% on a wafer size of 102 square centimeters, making it a favorable PV technology in the commercial market. However, it is still important to acknowledge the below remarks.

1. PERL vs IBC

Often, the PERL concept requires a more expensive process in fabrication than IBC or HIT. The IBC solar cell doesn’t suffer from shading losses of a front metal contact grid. And due to the use of n-type float zone c-Si wafers, the IBC solar cell doesn’t suffer from light-induced degradation. Another important note is that the IBC n-type silicon wafer is not sensitive to impurities like iron impurities. However, in PERL p-type float zone c-Si wafers, boron doping is homogeneously distributed over the IBC n-type float zone c-Si wafer. This means that within one n-type wafer the electrical properties can vary, resulting in a lower energy yield of solar cell production from n-type float zone c-Si wafers like IBC.

2. PERL & IBC  vs HIT

In HIT solar cells, the use of amorphous silicon in the contact area makes it a good passivation material which enables a longer lifetime of the charge carriers, thereby increasing the yield. In PERL and IBC, diffusion to the contacts takes place in the emitter layer through a metal finger spacing, but in HIT, it occurs through a transparent conductive oxide metal ITO which shortens the diffusion length as compared with PERL and IBC.