Surface and interface effects on the current-voltage characteristic curves of multiwall carbon nanotube-Si hybrid junctions selectively probed through exposure to HF vapors and ppm-NO2

The possibility to increase the efficiency of photovoltaic (PV) cells based on hybrid carbon nanotube (CNT)-Si heterojunctions is related to the ability to control the chemical properties of the CNT-Si interface and of the CNT bundle layer. In spite of the encouraging performances of PV cells based on multiwall (MW) CNT, so far few efforts have been made in the study of this device compared to single wall (SW) CNT-Si interfaces. Here, surface and interface effects on the current-voltage characteristic curves of MW CNT-Si hybrid junctions are investigated through exposure to HF vapors and to 10 ppm-NO2 and compared to the effects detected in SW CNT-Si junctions. Quite similar results in terms of open circuit voltage, short circuit current density, and efficiency are found for both cells, suggesting that exposure to HF vapors mostly affects the interface chemical properties, i.e., the silicon oxidation state, that in both junctions reach an optimal state about 50 h after etching. In turn, NO2 exposure has larger effects on the SW-based cell, consistently with the larger surface-to-volume ratio of SW with respect to MW. In both cases, the efficiency value reaches a maximum after 28 min, before dropping when the NO2 molecules desorb from the surface. A combined analysis of current-voltage curves and photoemission data collected along the different phases of gas exposures allowed us to relate changes in the electrical properties to the chemistry of Si at the interface.