Back contact (BC) solar cell, is a type of Si solar cell technology, where all the electrical contacts are located at the rear side (back side) of the device. In contrast, other Si solar cell technology, such as PERC, PERT, TOPCon and HJT, have their electrical contacts located at both front and rear sides. These electrical contacts have different polarities (i.e. positive and negative electrodes). These contacts allow photo-generated current to flow through the solar device, where external circuits can collect to generate power.
Back contact solar technology was first studied by scientists such as Richard Swanson from Stanford University [1]. Swanson went on to found Sunpower Corporation in the 1980s to mass produce the technology. Sunpower trademarked the technology as Interdigitated Back Contacts (IBC).
BC solar technology until today is still the highest efficiency solar technology. In May 2024, Longi claims a third party certified world record Si solar cells efficiency based on BC structures, at 27.3% [2]. This is reaching the theoretical limit of Si solar cells at about 29.4% [3]
Advantages
BC solar devices have all the electrical contacts located on the rear side. As a result, the front side (sun facing side) is not shaded by any electrodes. Therefore, the device can absorb more sunlight to produce more power. Thus enabling higher efficiency solar devices.
Additionally, BC devices usually look more aesthetically pleasing. In the absence of the reflective electrical contacts, the front side of the device have a uniformly dark blue colour. Thus, BC solar panels are popular choice for residential roof top solar installations
Disadvantages
As all the electrodes are located at the rear side, making a BC solar cell is more complicated.
In particular, this architecture requires special pattern designs for electrical positive and negative areas on the rear side of the device. These include the p-n junctions, metal electrodes of different polarities. Hence, this involves more processing steps, including additional lasers, diffusion masks and chemical etching masks. The additional processes also affect manufacturing yields, which increase the fabrication cost.
Thus, BC solar cells are currently more expensive to make than the other technology. Moreover, since all the contacts are located on the rear side of the device, the cells are more susceptible to bowing issues. This may cause issues during cell encapsulation.
Furthermore, the ability of the BC solar cells to absorb reflected light on the rear side, i.e. the so-called bifaciality of solar devices, is also lower than the other technology. Consequently, this affects the overall energy yield of BC devices during real world operation.
The diagram below shows a typical IBC solar cell, in comparison to PERC, PERT, TOPCon and HJT solar cells.
[1] https://www.sciencedirect.com/science/article/abs/pii/037967878690061X
[2] https://www.longi.com/en/news/longi-hi-mo9-bc-world-record/