Traditionally, photocurrent generation in a typical organic photovoltaic (OPV) device has been thought to occur predominantly by photoexcitation of the electron donor and subsequent electron transfer to an acceptor material with a higher electron affinity than the donor (the Channel I mechanism). More recently it has been acknowledged that photoexcitation of the electron acceptor can also play a significant role in photocurrent generation, particularly in low donor concentration devices. Photoinduced hole transfer or the so-called “Channel II” process occurs when the electron acceptor is photoexcited followed by oxidation of the electron donor. In order to probe the dynamics of the less-studied Channel II process it is desirable to have electron donors with low ionization potentials and large optical gaps with the absorption in the near-UV part of the spectrum.To this end, a series of triarylamine-based dendrimers have been synthesized to act as hole acceptors and transporters. Subtle variations to the surface substituents of these dendrimers led to changes in the thermal properties and neat film charge mobilities, but did not impact the absorption or electronic properties. The hole-mobilities of pristine films were affected by aggregation, with the surface substituents causing subtle differences. By increasing the length of the alkyl chain attached on the surface of the dendrimer from methyl to the n-butyl, the pristine film hole mobility increased. The dendrimers were fabricated into bulk heterojunction devices blended with PC70BM at different donor ratios ranging from 6 wt.% to 50 wt.%. Charge generation was mainly due to absorption of the incident light by the PC70BM. A low donor ratio led to a high incident photon-to-electron conversion efficiency (IPCE) and short-circuit current density (JSC), leading to a good power conversion efficiency (PCE). The fact that the dendrimer did not absorb appreciably and the increase in the PCE with decreasing dendrimer ratio in the blend indicated that these materials operated unambiguously via the Channel II photocurrent generation mechanism. The charge extraction efficiency was studied and the device performance was found to be closely relevant to the blend hole mobility rather than the pristine film hole mobility. The introduction of electronwithdrawing groups to the dendrimer changed the energy levels and led to a higher open-circuit voltage (VOC) in the dendrimer:PC70BM blend.Poly(dendrimer)s were prepared from two of the triarylamine-based dendrimers using ring opening metathesis polymerizations. With the three-methoxyl-group substituted dendrimer, three poly(dendrimer)s were synthesized having alkyl linkers of different lengths. The ionization potentials and the optical gaps of the poly(dendrimer)s were identical to those of their model dendrimer counterpart, while the pristine film hole mobilities of the poly(dendrimer)s were improved by two orders of magnitude. In bulk heterojunction blends with PC70BM, the decreasing donor ratio from 50 wt.% to 6 wt.% gave rise to enhanced IPCE and JSC. Compared to the model dendrimer:PC70BM composite, the poly(dendrimer):PC70BM blend enabled higher PCEs, mainly as a result of nearly the doubling of the fill factor (FF). Balanced hole and electron mobilities were observed in a poly(dendrimer):PC70BM blend with only 6 wt.% donor. The efficient charge extraction led to suppression of the bimolecular recombination by over 20% and thus increased the FF, compared to the blend containing the model dendrimer at the same concentration.Based on the three-methyl-group substituted dendrimer, two poly(dendrimer)s with and without alkyl linkers were developed from a redesigned synthetic route. Similar to the alkoxy containing poly(dendrimer)s, the absorption and electrochemical properties remained unchanged compared to those of the model dendrimer, while the device performance was improved in terms of an increased FF due to the substantially reduced bimolecular recombination.In conclusion, the study has provided important insights into the mechanism of Channel II charge generation. It was found that the high efficiency of exciton dissociation plays a crucial role in determining the best device performance for low donor solar cells, with results providing a platform for further study of how these devices work so well. The exceptional features including near-UV absorption and efficient hole extraction in blends add such poly(dendrimer)s to the list of promising electron donors in tandem organic solar cells with suitably chosen acceptors for complementary light harvesting.