The Interdigitated Back Contact (IBC) cell with back electrodes is an alternative approach to producing high efficiency cells. Reducing the shadowing of the photovoltaic cell by the front electrode prevents the reduction of solar energy reaching the cell (the electrodes are located at the back of the cell).
The structure of an IBC type cell is shown in Fig. 1. Light passes through the stabilizing layer \( SiO_2 \) into the interior of the cell, which is constructed from an n-type monocrystalline wafer. Both contacts are located on the back surface of the cell. The resulting exciton breaks up into charges that diffuse into the p- and n-type areas directly above the contact areas (in these areas, the oxide has been removed from the back surface). Both the n-type and p-type junctions lie on the unlit side of the cell under the contact metallization. A schematic of the IBC-type cell is presented in Fig. 1a, where the contacts are shown as combs. Fig. 1b, on the other hand, shows the contacts collecting the charges accumulated in the n-type semiconductor and p-type of the cell.

The back contact cell design was perfected at Stanford University in the 1980s [1]. In the 1990s, a new company, SunPower, was formed and commercialized the technology, producing IBC photovoltaic panels with an initial \( 22.5\% \) efficiency. In early 2000, production was greatly simplified and expanded. Today, SunPower manufactures IBC cells that achieve \( 24.2\% \). The efficiency performance of both cells (at \( 24.2\% \)), and panels (at \( 22.4\% \)) has been confirmed by independent analysis [2]. In contrast, Trina Solar reported that its Main National Laboratory has set a new efficiency record of \( 25.04\% \) for an n-type monocrystalline silicon cell made with IBC technology. This result was independently confirmed by the Japan Technological Laboratory for Electrical and Environmental Safety (JET) [3].
A sketch of the current commercial cell structure is shown in Fig. 2 [4], [5]. Note that phosphor diffusion is used along the illuminated surface to control recombination along that surface [6].

An IBC photovoltaic cell has multiple localized junctions instead of one large p-n junction. Electron-hole pairs, generated by incident light that is absorbed on the front surface, can be collected at the back of the cell. The semiconductor-metal interfaces are as small as possible to reduce unwanted recombination. As shown in Fig. 2, the back of an IBC-type photovoltaic cell has two metal combs. One collects current from the n-type contact and the other collects current from the p-type contact. The front surface area is formed by heavily doped n-type silicon in order to reduce recombination on this surface. However, the doping intensity gradually decreases toward the back to act as a p-type semiconductor region. The front surface is textured and embedded with a two-layer anti-reflective coating. It acts as a passivator (silicon dioxide) on the front side of the cell.

Fig. 3 shows a photo of the cell from the front, whose color is uniformly dark, while the photo of the cell from the back clearly shows the alignment of the electrodes, which are charge-collecting combs. The collecting electrodes do not shade the surface through which the sun's rays can reach the inside of the cell and cause exciton formation.
The IBC photovoltaic cell is the most technologically complex but has the highest efficiency among mass-produced silicon cells. In general, the structure of a cell made with this technology has several advantages over the construction of conventional photovoltaic cells. The most obvious is the elimination of shading caused by the front electrode, which in the generated current can give a gain of \( 5-7\% \). The lower series resistance during current flow in the contacts is also important, due to the fact that the back contacts occupy almost the entire back surface, so the distance between them is small. Furthermore, it is a great advantage to separate the optical optimization, performed at the front, from the electrical optimization, performed at the rear.