Band Edge Disorder Producing Urbach's Tail and Enhanced Photochromic Efficiency Caused by Density of States inside Band Gap in Tungsten Oxide Films

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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.
社団法人応用物理学会の論文
1996-10-15