The efficiency of solar cells is limited, because high energy photons are not efficiently used inside the solar cell. The energy of photons in excess of the band gap energy of the solar cell is wasted as heat. Furthermore the quantum efficiency for high energy photons is usually lower than one. The efficiency could therefore be increased by positioning a spectral conversion layer or quantum cutting layer on top of the solar cell. A spectral conversion layer could shift the high energy photons to a wavelength for which the spectral response of the solar cell is higher, while a quantum cutting layer could convert the high energy photons into two lower energy photons and in this way double the amount of energy used from these photons. Such a quantum cutting phosphor has been realized by co-doping Tb3+ and Yb3+ lanthanide ions. In that case the absorption of high energy photons by Tb3+ can be followed by f-f emission of two neighboring Yb3+ ions. The efficiency is Unfortunately limited due to weak Tb3+ f-f absorption, and therefore materials are required in which the UV and blue light is better absorbed.
Silicon nitride based materials are interesting hosts as quantum cutting materials for solar cells because they have shown efficient LED phosphors when doped with Ce3+ or Eu2+ ions. Besides, the luminescent properties of these materials can easily be tuned by for example changing the Si to N ratio or by the partly substitution of N by O, or Si by Al. Furthermore, a silicon nitride based quantum cutting layer could be integrated with the SiNx anti-reflection coating that is already used for solar cells.