Their remarkable light absorption and energy transport characteristics make single-wall carbon nanotubes (SWNTs) intriguing candidates for photovoltaic applications. Here, we study the photophysical properties of polymer wrapped SWNT films with varying thickness, so as to help identify processes which may affect device performance and that may impose limitations on the use of SWNT thin films as active layers in photovoltaic devices.

Non-radiative exciton decay is of particular interest in this context and we have studied its dependence on the thickness of monochiral SWNT films. The films were made of PFOBPy wrapped and  highly (6,5) enriched SWNT suspensions using a modified version of the filtration wet transfer technique reported earlier.¹ Non radiative decay was probed by means of photoluminescence (PL) spectroscopy and by comparison of thin film data with data obtained from bulk suspensions. For thicker films we observe a pronounced decrease of the PL quantum yield (QY) for emission from the first subband S₁ exciton state. However, the total PL QY is much less strongly affected by interactions within the thin film network because of a considerable shift of PL intensity to lower lying states. Surprisingly, some of the lower energy emission resembles PL phonon-sidebands of the first subband exciton which was previously attributed to the coupling between K-point phonons and dark excitons by Murakami et. al.²

1. Wu, Z.. Science 305, Nr. 5688 (2004): 1273–76.
2. Murakami, Y., Benjamin L., Said K., Nobutsugu M., Tatsuya O., Shigeo M. Phys. Rev. B 79, Nr. 19 (2009): 195407.