Nobuyuki Kuroda

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 Urbachs rule, and flat-band potentials (V fb s) 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 Urbachs 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.