Band edge disorder producing Urbach's tail and enhanced photochromic efficiency caused by density of states inside band gap in tungsten oxide films

The band gap of WO 3 and its disorder at the band edge were evaluated, assuming Urbach's rule, and flat-band potentials ( V fb s) were also measured on WO 3 films with high photochromic efficiency (HPE) and low photochromic efficiency (LPE). The main results are as follows: 1) the optical absorption of LPE- WO 3 sets on at higher photon energy than that of HPE- WO 3 , 2) HPE- WO 3 has the density of states deeper in the band gap than LPE- WO 3 does, and 3) V fb of HPE- WO 3 is higher than that of HPE- WO 3 . These results indicate that the band edge disorder of WO 3 , which is closely related to Urbach's tail, determines the Fermi level position, and therefore makes V fb high; they also imply that a high V fb dissociates optically excited electrons and holes to make colors center resulting in photochromism at a high efficiency.