Thermal Manipulation of Deactivation Processes in Luminescent Photoelectrochemical Cells Employing Tellurium‐Doped Cadmium Sulfide Photoelectrodes

Abstract

Temperature significantly modifies the efficiencies of luminescence and photocurrent in an n‐type, single crystal, normalCdS:normalTe‐normalbased photoelectrochemical cell (PEC) employing aqueous polyselenide electrolyte. Between 20° and 100°C photocurrent (quantum yield Φx ) from ultrabandgap 501.7 nm excitation increases modestly by ≲ 20%, whereas photocurrent from bandgap edge 514.5 nm excitation increases by about an order of magnitude, reaching 50–100% of the 20°C 501.7 nm photocurrent. Undoped normalCdS exhibits a similar photocurrent‐temperature profile. Higher temperatures thus extend the wavelength response of normalCdS ‐ and normalCdS:normalTe‐normalbased PEC's. In contrast to the increase in photocurrent, emissive efficiency (quantum yield Φr ) of normalCdS:normalTe drops by a factor of ∼10–20 over the same temperature range; this decline is relatively insensitive to potential and excitation wavelength (501.7 or 514.5 nm). Little change in the spectral distribution of emission false(λmax∼600 normalnmfalse) is observed with temperature. A dependence of emission intensity on potential is observed for the first time with 514.5 nm excitation in the high temperature regime. These results are interpreted in terms of band bending, optical penetration depth, the known red‐shift of the normalCdS bandgap edge with temperature, and the competitive nature of emission and photocurrent. Relationships among Φx , Φr , and Φr0/Φr (out‐of‐circuit to in‐circuit emission intensity) are discussed.

Effects of Temperature on Excited-State Descriptions of Luminescent Photoelectrochemical Cells Employing Tellurium-Doped Cadmium Sulfide Electrodes

“…Over the 20-100C excursion emission intensity is seen to drop by factors of ".8 and 30 for 501.7 and 514.5 nm e-xitation, respectively. These factors are consistent with the 10-to 20-fold decline observed in polyselenide electrolyte; in those experiments there also appeared to be little potential dependence of the results (9).…”

“…In fact, at 100C the 514.5 nm photocurrent has reached^\60% of the ultraband gap photocurrent. Similar effects were observed in polyselenide electrolyte for both CdS:Te and undoped CdS (9). The photocurrent enhancement for 514.5 nm excitation is a predictable consequence of an absorption edge which red shifts with te.=perature: As the absorptivity for 514.5 nm light increases with temperature, progressively larger fractions of light will be absorbed in the depletion region.…”

Excited-state processes of relevance to photoelectrochemistry

“…(1) was also obtained for PECs based on CdS^Sei-x (0 < x < 1) and CdS:Te (9, 10). For CdS:Te which exhibits both edge emission (2.4 eV; \max ~510 nm) and PL due to a Te-bound exciton (~2.0 eV; Xmax ~600 nm), rough agreement was also observed at several different excitation wavelengths for which optical penetration depths and 4>t0 values were different, and at several temperatures where <£r" and the spectral distribution of PL were different (9,14,15). It.…”

Luminescent Properties of Semiconductor Electrodes.

“…13 In concluding this section we note that all of the single-crystal n-Cd;XSel.X samples examined exhibit quenching of their PL intensity when used as PEC electrodes, as illustrated in Fig. 14; the extent of quenching and corresponding properties related to optical energy conversion are summarized in We examined both n-CdS:Te- (46,47,75) and n-CdSxSelX-based PEC's (52,53) for agreement with eq. [7].…”

“…The optical band gap temperature coefficient, dEg/dT, for undoped CdS of -5.2xi0 -4 eV/K (45) suggested that CdS:Te might absorb a larger fraction of 514.5-nm light in the depletion region if temperature were to be increased. These expectations were realized using a cell modified for variable-temperature work (46,47).…”