XLI. Improvement of Performance of n-WSe Electrodes by Electrochemical Polymerization of o-Phenylenediamine at Surface Imperfections

The da rk e lect rochemical po lymer iza t ion of o -pheny lened iamine (OPD) a t n-WSe2 and n-MoSes single crys ta l e lectrodes (surface j_C axis exposed) w a s invest igated. The resul ts indicate that po lymer iza t ion occurs only at surface imperfec t ions (edges, steps) in the o therwise smooth van der Waal ' s surface. The photoelec t rochemical response, in aqueous solutions containing Fe(CN)6~-/ Fe(CN)64and I a / I , of e lectrodes subjec ted to this po lymer iza t ion is improved compared to the un t rea ted electrodes. The resul ts are consistent wi th a decrease in recombina t ion rate due to b locking of surface s tates by OPD polymeriza t ion . The character is t ics of severa l PEC cells before and af ter OPD polymer iza t ion are also described. Increases in the so la r to -e lec t r i ca l power efficiency, ~], range f rom 30-300% af ter surface p re t r ea tmen t . The na tu re of the surface of a semiconductor e lect rode in a photoelec t rochemical cell (PEC) p lays an impor t an t role in the efficiency of conversion of impinging rad ian t energy into net ex te rna l cur ren t (1). Much of the success in recent years in improvemen t of PEC per formance can be a t t r ibu ted to the discovery of etching procedures or chemical t rea tments which decrease the ex ten t of recombinat ion react ions at the semiconductor electrode. The l aye red semiconductor mater ia ls , e.g., MoSe2 and WSe2, while promis ing candidates as electrodes for PEC's, f r equen t ly show large energy losses via recombina t ion processes at c rys ta l imperfect ions in the photoact ive surface ( • axis) ( la , 2-7). These recombina t ion centers show meta l l i c l ike behavior in that da rk oxidat ion and reduct ion of solut ion species wi th redox potent ia ls corresponding to energies be tween the valence band and conduct ion edges is quite facile a t e lectrodes wi th exposed edges on the surface. Pa rk inson and co-workers (6, 7) have taken advan tage of the in terca la t ion p roper t ies of these l aye red mate r ia l s to a t tach bu lky molecular species at the edges to block these recombina t ion centers and thus p reven t e lect ron exchange wi th solut ion species. This method improved the overa l l energy conversion character is t ics of PEC's based on n-WSe2 and MoSe2 in aqueous I / I 3 solutions. We repor t here an a l t e rna t ive approach to blocking the recombina t ion associated wi th surface imper fections of n-WSes and n-MoSes electrodes by the da rk e lec t rochemica l ly in i t ia ted p o l y m e r i z a t i o n of an impervious coating at these sites. The s t ra tegy here is based on the fact tha t only the recombinat ion sites on n type mate r ia l s a l low apprec iab le da rk anodic currents. The oxidat ion of o r tho -pheny lened iamine (OPD) at metal l ic e lectrodes yields a po lymer ic produc t tha t effect ively blocks the e lect rode surface (8-10), and prevents e lect ron exchange wi th solut ion species. The use of this po lymer iza t ion react ion at the surface imperfect ions of l aye red crysta ls provides for the specific pass ivat ion of recombinat ion centers. A simplif ied represen ta t ion of the s t ra tegy involved here is shown in Fig. 1. In the absence of surface states or deep t raps electrons genera ted wi th in the space-charge region are dr iven to the bu lk of the elect rode where they are collected at the r ea r ohmic contact or recombine wi th minor i ty carr iers . When sur face states are present (Fig. l a ) that can t rap photo* Electrochemical Society Active Member.