Influence of hole-transporting material addition on the performance of polymer solar cells

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Abstract
We report the effect of hole-transporting material (HTM) addition on the performance of polymer solar cells based on blends of poly(3-hexylthiophene) (P3HT) and soluble fullerene. N,N′-Diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-diphenyl]-4,4′-diamine (TPD) was chosen as a HTM because it is one of the well-established HTMs despite its drawback of wide band gap (3.1 eV) for solar cell applications. Two specialized measurement systems, synchrotron radiation grazing incidence angle X-ray diffraction (GIXD) and phase-mode atomic force microscopy (AFM), were employed to understand the correlation between device performance and nanostructures of blend films. Results showed that the addition of 3–7 wt% TPD improved the short circuit current density of unannealed devices due to the improved P3HT crystallization induced by the presence of TPD molecules. Although the short circuit current density of the binary blend device was recovered to the highest value after thermal annealing, the improved fill factor of TPD-added ternary blend devices at the 3–7 wt% TPD content led to the slightly enhanced power conversion efficiency at 3 wt% TPD in spite of reduced optical absorption in the ternary blend film.