Solar cells harvest solar energy and convert them into electrical power. Solar energy arrives in the infrared, visible and ultraviolet spectral regions. For optimal power efficiency the band gap of the involved semiconductor is around 1-1.5 eV, making the solar cell absorb visible light and thus the cell is opaque. New applications are envisioned for transparent solar cells that harvest only the UV part, possibly also the infrared part, of the solar spectrum; they can be integrated on windows or the front pane of displays and could power local transparent electronic circuits from wide gap oxides or other connected devices such as sensors of WLAN repeaters.

Prof. Marius Grundmann from Universität Leipzig and his PhD student Robert Karsthof, MSc, and further coworkers have presented the first such transparent solar cell in a recent article in physica status solidi (a). It is made from the oxides of the abundant and cheap metals zinc and nickel. The absorption edge at 380 nm, just beyond the visible spectral range, is defined by the n-type ZnO. It makes the cell UV-sensitive and transparent in the visible. The generated electrons and holes are separated at a pn-junction made from ZnO/NiO using physical deposition methods. The cell has an external quantum efficiency of 55% and an efficiency of 3.1% for the absorbed light. Thus energy for powering transparent circuits can now be harvested with a transparent device. Further performance gains of the presented ZnO/NiO cells regarding larger transparency and higher efficiency for the harvest of the invisible UV part of the solar spectrum, offering about 36 W/m2, seem possible.

(text kindly provided by the authors)