Measurements of HOMO-LUMO levels of poly(3-hexylthiophene) thin films by a simple electrochemical method

Though poly(3-hexylthiophene) (P3HT) is one of the most commonly used polymers in organic solar cells, a broad range of values, derived from cyclic voltammetry (CV), has been reported for the lowest unoccupied molecular orbital (LUMO) position (from -3.53 to -2.70 eV); contrastingly, the highest occupied molecular orbital (HOMO) position is reported in a narrow range (from -4.92 to -5.20 eV). As a consequence of this wide distribution for the LUMO position, most researchers choose to use electrochemical techniques for determining only the HOMO position, and estimate the LUMO position by adding the experimental optical band-gap value. Here, three different electrochemical strategies (CV, potentiostatic EIS, and potentiodynamic EIS) for obtaining the HOMO and LUMO position for P3HT films formed under ambient conditions on transparent conductive substrates (indium tin oxide (ITO) glass) are compared. The results are discussed in the frame of limitations of each technique. The cyclic voltammetric response and the data derived from potentiostatic EIS using electric equivalent circuits include the response of all processes involved in the measurements, particularly masking the LUMO response due to the presence of energetic states. In contrast, potentiodynamic EIS measured in a wide frequency range results in a more reliable approach since it allows discerning between middle-frequency-dependent processes associated with energetic states in the gap of the P3HT/ITO films, and low-frequency-dependent processes associated with filling and emptying of LUMO and HOMO states, respectively. It is a powerful and simple method to analyze the electronic structure of semiconductor organic thin films.