Semi-transparent solar cells have attracted massive industry interest as they offer tri-functionality by forming the skin of the building, enabling natural light illumination, and generating power. Cu(In,Ga)Se₂ (CIGSe) photovoltaics are a promising technology option for such applications. There are two different approaches to achieve transparent CIGSe solar cells. One involves the fabrication of ultra-thin CIGSe absorbers on see-through transparent back contacts. This method, however, is limited by the formation of GaO_x parasitic layers at the back contact interface, which leads to efficiency losses. An alternative approach involves spatially segmented full-thickness solar cells with transparent parts separating them to give the semi-transparent characteristic.

As proof of concept, we present a top-down micro-structuring approach, where an opaque CIGSe solar cell is spatially segmented into micro-sized line-shaped solar cells. By varying the lines’ width and spacing, it is possible to control the window’s average visual transparency (AVT), making it suitable for different applications. The fabrication process begins with a photolithography step, where the selected pattern of micro-lines is designed on top of a complete CIGSe solar cell stack. An aqueous bromine solution etches the developed solar cell areas, while the photoresist protects the solar cell lines [1]. The bromine solution results in a slight over-etch of the window layer by about 20-30 µm. This over-etch reduces the active device area, leading to a performance loss, which is particularly pronounced in narrower lines (narrower than 200 µm). After the bromine etching, the sample was dipped in sodium hypochlorite (commercial bleach) to remove the exposed molybdenum, thereby making the areas in between the solar cell lines fully transparent. To ensure proper charge carrier collection, we performed simulations to design a metallic front contact grid that minimizes shadowing. Narrow, tapered aluminum electrical contacts were evaporated on top of the full length of the line solar cells, with a width between 30 mm and 1 mm. The top-down, materials inefficient process resulted in a semi-transparent CIGSe solar cell with a total area efficiency of 6.1 % and an AVT of 49 %.

Developing a more sustainable and materials-efficient fabrication approach, we use sputter-deposition and lift-off processes to achieve selective deposition of the CIGSe micro-striped solar cells. Here, Cu-In-Ga is deposited onto a patterned photoresist, followed by a lift-off process. The Cu-In-Ga lines are subsequently transformed into CIGSe lines by a selenization process. Solar cell devices are then completed by depositing a buffer layer in a chemical bath and by sputtering a i-ZnO/ZnO:Al window layer. With this approach, we have achieved a power conversion efficiency of approximately 6% for individual lines.