Spiro-OMeTAD:\nUnique Redox Chemistry Driving The Hole\nTransport

Spiro-OMeTAD,\nan organic hole transport layer, has made\nits impact\nin designing efficient solar cells and light emitting devices. The\nfour oxidation states, which maintain a redox equilibrium, facilitate\nthe propagation of photogenerated holes to the collecting electrode.\nSpectroelectrochemical measurements characterized at oxidation potentials\nof 0.01, 0.13, 0.33, and 0.96 V vs Fc/Fc<sup>+</sup> exhibit characteristic\nabsorption at 525, 700, 900, and 565 nm, respectively. Titration of\nspiro-OMeTAD in solution with Ag<sup>+</sup> induced sequential oxidation\nand independently confirmed the spectral identification of the oxidation\nstates. Higher oxidation states (viz: +3 and +4) quickly revert to\nlower oxidation states (viz., +1 and +2), thus showing the reversibility\nof oxidation to attain a redox equilibrium. The hole transport attained\nthrough the cascade of oxidation states provide new insights into\nthe operation of solar cells.