Photoluminescence emission with energies smaller than the ZnO band gap can be related to a number of intrinsic effects of the ZnO crystal including oxygen and zinc vacancies (V O , V Zn ), interstitial atoms (O i , Zn i ) or anti-site defects (O Zn , Zn O ), as well as various surface states. A schematic of mechanisms for possible visible light emission sources in ZnO thin films is illustrated in figure in the SI. In the measured spectra, the broad peak (FWHM around 0.4 eV) centered around 525 nm giving green emission is very pronounced with its tails reaching the blue region. The green emission can be attributed to the transition between the band-edge and local defect-originating energy levels within the band-gap. It is commonly acknowledged that the transitions responsible for this green emission are between conduction band minimum and oxygen vacancies (V O ). A second candidate would be intra-gap transitions between extended interstitial zinc levels (ex -Zn i i) and zinc vacancies (V Zn ). Although both Zn i and V Zn are widely recognized as major defects within the material. we do not see any emission related to neither Zn i -V B, Zn i -V Zn nor CB -V Zn transitions, which should be observed in the violet and blue ranges. This allows us to conclude that the defect responsible for the green emission in our material is the V O . The broad peak of red luminescence (FWHM of 0.45-0.55 eV), centered at around 700 nm is related to energy levels almost exactly in the middle of the energy band of ZnO -1.77 eV, which is typically attributed to surface-related mid-gap states (SS) of unknown charge. It can be strongly influenced by the processing of the material. The reason for two broad visible bands is that ZnO films have a large surface to volume ratio and high porosity leading to a relatively broad distribution of physical properties of the nanocrystallites. Since the F X and D 0 X emissions are strongly related to the energy gap of the material and the crystalline quality, they tend to differ very little between the samples. On the other hand, the distribution of defects and surface states is strongly depended on the material morphology and processing conditions. The latter is also true for the SBX, which appear after annealing at 800 °C.