Laser induced photocurrent transients in semiconductor liquid-junction solar cells employing n-WSe2 and n-MoSe2 photoanodes
The effects of perturbing a semiconductor liquid-junction solar cell from dark equilibrium conditions with a nanosecond (5 nsec FWHM) laser pulse of greater than bandgap energy were studied. It was found that n-WSe(,2) and n-MoSe(,2) photoanodes in either I('-)/I(,3)('-) or Fe('2+)/Fe('3+) redox couple containing electrolyte solutions exhibit biexponentially decaying photocurrent transients in response to photoexcitation. For a given redox couple, n-WSe(,2) was found to yield slightly higher peak photocurrents than n-MoSe(,2) due to be more favorable band bending under dark conditions. The redox couple I('-)/I(,3)('-) was found to be superior to Fe('2+)/Fe('3+) in spite of more positive redox potential of the Fe('2+)/Fe('3+) couple. The I('-)/I(,3)('-) apparently adsorbs onto the semiconductor surface, negating the effects of surface states which tend to limit the amount of equilibrium band bending present. Quantum yields of charge transfer were determined for both redox couples as a function of laser pulse energy and energy density. Quantum yields of charge transfer were also determined for variation of the I('-)/I(,3)('-) couple concentration for a fixed excitation energy. The resultant nonlinear dependency appears to follow the adsorption isotherm for I('-)/I(,3)('-);Decay time constants and quantum yields of charge transfer as a function of added load resistance were studied. Limiting values for cell series resistance, capacitance, and shunt resistance were determined using a simple electronic model. A more complex phenomenological model which can be used to determine rates of charge transfer and recombination, as well as values for the electronic constants of the cell, has also been applied to the interpretation of the biexponential decays;A flashlamp-pumped transverse-flow dye laser was utilized as the photoexcitation source for the photocurrent transient studies. An in-depth description of the design, construction, and characterization of this laser system is presented.