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The charge transport properties of bulk heterojunction solar cells formed by blending poly-(3-hexylthiophene) (P3HT) and [6,6] phenyl C61 butyric acid methyl ester (PCBM) were improved by doping with single walled carbon nanotubes (SWNTs). The SWNTs used were not functionalized, and contained both metallic and semiconducting tubes. Their work function was found to be 4.89 eV. Unlike P3HT:PCBM interface, the P3HT:SWNT interface has been inefficient for charge generation. Using SWNTs at concentrations below 1 wt. %, the solar cell efficiency increased from 2.86% to 3.52% for 80 nm devices and from 2% to 3% in 125 nm devices at low light intensities. In both cases, the increment is because of higher fill factor with no change in short circuit current density and open circuit voltage. At higher light intensities, a 43% increase in fill factor and a 37% increase in short circuit current density were obtained, which doubled the efficiency. These improvements were primarily because of reduced recombination through improved charge extraction by SWNTs.

The effect of valence-band discontinuity at the collector base heterojunction on the current gain and base charge storage is modeled. It is shown that the onset of the Kirk effect is accompanied by a sharp drop in the current gain and ft due to the formation of a potential barrier. The variation of barrier height with collector current density is determined and its effect on current gain and base transit time described. The results discussed here are applicable to Si/SiGe double-heterojunction bipolar transistors.

Poly-(3-hexylthiophene) (P3HT)—single walled carbon nanotube (SWNT) solar cells were fabricated and compared with single layer P3HT devices. P3HT:SWNT devices were found to have higher efficiencies than P3HT only devices by at least a factor of two. Zero field mobility values of 1.20×10−7 cm2 V−1 s−1 and 5.97×10−7 cm2 V−1 s−1 were calculated from space charge regime for P3HT and P3HT:SWNT, respectively. The SWNTs were predominantly of metallic nature, as revealed by Raman spectroscopy. Morphology studies show that the SWNTs increase local ordering of P3HT nanocrystals which can improve hole transport. They also show that the cathode-organic surface roughness and volume increase in the presence of SWNTs which can have significant positive effect on charge generation and collection at this interface. Variation in short circuit current with incident light intensity shows higher superlinear slope with SWNTs which also indicates that SWNTs aid in charge extraction from the device.