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 WO₃ and its disorder at the band edge were evaluated, assuming Urbach's rule, and flat-band potentials (V_fbs) were also measured on WO₃ films with high photochromic efficiency (HPE) and low photochromic efficiency (LPE). The main results are as follows: 1) the optical absorption of LPE-WO₃ sets on at higher photon energy than that of HPE-WO₃, 2) HPE-WO₃ has the density of states deeper in the band gap than LPE-WO₃ does, and 3) V_fb of HPE-WO₃ is higher than that of HPE-WO₃. These results indicate that the band edge disorder of WO₃, 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.