**5. Processing additive effect on solar cell performance**

The photophysical effects of 1,8-octanedithiol (ODT) additives on PCPDTBT and C71-PCBM composites and device performance were studied using photo-induced absorption spectroscopy.(Hwang et al., 2008) Reduced carrier loss due to recombination was found in BHJ films processed using the additive. From photobleaching recovery measurements reduced carrier losses were demonstrated. However, it was concluded that the amount of the reduction is not sufficient to explain the observed increase in the power conversion efficiency (by a factor of 2). Carrier mobility measurements in Field Effect Transistor (FET) configuration demonstrated that the electron mobility increased in the PCPDTBT:C71-PCBM when ODT is used as an additive, resulting in enhanced connectivity of C71-PCBM networks.(Cho et al., 2008) This work also showed that if the ODT was not completely removed from the BHJ films by placing them in high vacuum (> 10−<sup>6</sup> torr) the hole mobility actually decreased, implying that residual ODT may act as a hole trap. It was concluded that the improved electron mobility was the primary cause of the improved power conversion efficiency, while the hole mobility was found to be relatively insensitive to the additive.
