Critical Role of the Sorting Polymer in Carbon Nanotube-Based Minority Carrier Devices

A prerequisite for carbon nanotube-based optoelectronic devices is the ability to sort them into a pure semiconductor phase. One of the most common sorting routes is enabled through using specific wrapping polymers. Here we show that subtle changes in the polymer structure can have a dramatic influence on the figures of merit of a carbon nanotube-based photovoltaic device. By comparing two commonly used polyfluorenes (PFO and PFO-BPy) for wrapping (7,5) and (6,5) chirality SWCNTs, we demonstrate that they have contrasting effects on the device efficiency. We attribute this to the differences in their ability to efficiently transfer charge. Although PFO may act as an efficient interfacial layer at the anode, PFO-BPy, having the additional pyridine side groups, forms a high resistance layer degrading the device efficiency. By comparing PFO|C₆₀ and C₆₀-only devices, we found that presence of a PFO layer at low optical densities resulted in the increase of all three solar cell parameters, giving nearly an order of magnitude higher efficiency over that of C₆₀-only devices. In addition, with a relatively higher contribution to photocurrent from the PFO-C₆₀ interface, an open circuit voltage of 0.55 V was obtained for PFO-(7,5)-C₆₀ devices. On the other hand, PFO-BPy does not affect the open circuit voltage but drastically reduces the short circuit current density. These results indicate that the charge transport properties and energy levels of the sorting polymers have to be taken into account to fully understand their effect on carbon nanotube-based solar cells.