Spiro-OMeTAD: Unique Redox Chemistry Driving The Hole Transport

Spiro-OMeTAD, an organic hole transport layer, has made its impact in designing efficient solar cells and light emitting devices. The four oxidation states, which maintain a redox equilibrium, facilitate the propagation of photogenerated holes to the collecting electrode. Spectroelectrochemical measurements characterized at oxidation potentials of 0.01, 0.13, 0.33, and 0.96 V vs Fc/Fc+ exhibit characteristic absorption at 525, 700, 900, and 565 nm, respectively. Titration of spiro-OMeTAD in solution with Ag+ induced sequential oxidation and independently confirmed the spectral identification of the oxidation states. Higher oxidation states (viz: +3 and +4) quickly revert to lower oxidation states (viz., +1 and +2), thus showing the reversibility of oxidation to attain a redox equilibrium. The hole transport attained through the cascade of oxidation states provide new insights into the operation of solar cells.