Simultaneous Determination of Quantum Efficiency and Energy Efficiency of Semiconductor Photoelectrochemical Cells by Photothermal Spectroscopy

Dur ing a photoe lec t rochemical reac t ion only a por t ion of the l igh t energy absorbed by the semiconductor (CdS or TiO2 single crys ta l ) is u t i l ized in the e lec t rode reaction. The unused por t ion of energy is expended th rough var ious mechanisms as heat. Therefore by moni tor ing t empera tu re changes wi th in the photoanode as a funct ion of e lec t rode potent ia l and l ight intensi ty, in fo rmat ion concerning the efficiency of the process can be obtained. Expe r imen ta l resul ts a re presented and in t e rp re t ed using a model for the energy balance wi th in the system. This pe rmi t s the de te rmina t ion of the quan tum and energy efficiencies s imul taneous ly wi thout the need to ca l ibra te the l ight source. A number of photoe lec t rochemical cells based on semiconductor e lect rodes for photoelect rosynthes is (e.g,, the photodecomposi t ion of wa te r ) and the convers ion of solar energy to e lec t r ic i ty have ,been r e por ted ( ! -15) . The ma jo r factors de te rmin ing the efficacy of these semiconductor e lec t rode ceils a re the quan tum efficiency for e lec t ron flow (i.e., number of electrons f lowing /number of photons absorbed) , and the power conversion efficiency (i.e., chemical or e lect r ica l power o u t p u t / i n p u t r ad ian t power ) . I t is not uncommon to find near un i ty quan tum yields for e lect ron flow at a sufficiently high bias (posit ive for the case of n t y p e semiconductors) dur ing i r rad ia t ion wi th g rea te r than bandgap energy l ight for m a n y semiconductors. However , even when high quan tum efficiencies have been obtained, the power efficiencies were much lower (10-15). Fo r example , for photooxida t ion at a CdS single c rys ta l anode, Wrighton et al. (15) r epor ted tha t w i th an Se 2solut ion the m a x i m u m monochromat ic power efficiency obta ined was 3.4% wi th a m a x i m u m quan tum efficiency of 49%. Such findings demons t ra te tha t the ma jo r pa r t of the l ight energy absorbed by the semiconductor is not used for the photo-ass i s ted oxida t ion bu t r a the r is conver ted to heat energy p robab ly via radia t ionless t rans i t ions wi th in the conduction band of the semiconductor (i.e., when the photon energy is in excess of the bandgap energy) or e lec t ron-hole recombina t ion processes. Consequently, t he rmal measurements of the semiconductor e lec t rode dur ing etectroIysis can aid in the de te rmina t ion of the efficiencies and perhaps in the e lucidat ion of the mechanism of the cell p rocesses. We recent ly descr ibed a new spectroscopic method cal led Pho to the rmal Spect roscopy (PTS) (16). This technique involves p lac ing a thermis tor on or in close p rox imi ty to a sample and measur ing t empera tu re changes (i.e., the rmis tor res is tance changes) dur ing sample i r r ad ia t ion wi th monochromat ic light. This technique can be eas i ly adap ted to cases when the sample is a semiconductor e lec t rode (17). Cahen (18) has also shown tha t s imi la r measurements wi th photoacoustic spectroscopy can be used to de te rmine the efficiency of so l id-s ta te photovol ta ic devices. We re por t in this pape r measurements of the conversion of l ight energy to chemical a n d / o r e lec t r ica l energy at * Electrochemical Society Act ive Member. ~ Electrochemical Society Student Member. K e y words: photoelectrochemical , semiconductor e lec t rode , quantum efficiencyj energy efficiency, temperature changes. CdS and Ti02 photoanodes by d i rec t de te rmina t ion of the t e m p e r a t u r e changes of semiconductor electrodes. The resul ts were in t e rp re t ed using a theore t ica l equation for the energy ba lance wi th in the system. In this manner the quan tum efficiency and power efficiency could be de te rmined wi thout ca l ibra t ion of the i r rad ia t ion source. Experimental Procedure.--The basic pho to the rmal expe r imen t was essent ia l ly car r ied out as prev ious ly descr ibed wi th s l ight modifications (16, 17). In these exper iments the photoanodes s tudied were CdS and TiOz single c rystals. The resul ts repor ted here are for monochromat ic rad ia t ion at wavelengths corresponding to energies grea te r than the bandgap energy (for example , for CdS, 490 nm) . The pho to the rmal responses were obta ined for the photoanodes dur ing both anodic po la r i zat ion and under open-c i rcu i t condit ions in the e lect ro lyte solutions. The corresponding cur ren t and t empe ra tu re changes were then p lo t ted as a funct ion of potential . Exper imen t s were car r ied out in two l abora tories wi th two different exper imen ta l setups ( re fe r red to as A and B) . InstrumentaL--Block diagrams of these are shown in Fig. 1 and 2. The appara tus (A) in Fig. 1 employed a 500W high pressure me rc u ry l amp (Ushio Electr ic Company Limi ted) housed in an Ushio Model UI 501C housing. A lens was used to focus the l ight beam and in ter ference filters (Koshin Kogaku Company Limi ted) were employed to select the wave leng th of exci t ing light. A shut te r wi th two timers, made by special o rder for this expe r imen t ( I sh ikawa Seisakusho Company Limi ted) was used to fix prec ise ly the i r rad ia t ion period. Sys tem B is shown in Fig. 2. The l ight source was a 2500W shor t arc xenon l amp housed in an Oriel Model LH-152N housing. The ou tput of the l amp was first chopped mechan ica l ly (on 20 sec; off 40 sec) and then focused using f/1 quar tz optics on the entrance slit of a J a r r e l l A s h Monochromator (Model 82-410). The resul t ing monochromat ic l ight (10 nm bandpass) was then f i l tered of second o rde r and finally focused on the photoanode wi th in the e lec t rochemical cell. Al l cells were equipped wi th p l a t inum counter e lectrodes and sa tu ra t ed calomel re ference electrodes (SCE). I r r ad ia t ion was accomplished th rough opt ica l ly flat windows both in the wa te r baths and in the cells. The working electrodes were p laced a sufficient distance f rom the cell window (,-,2 cm) to avoid any