We have tailored transient absorption (TA) techniques to illuminate each step of polymer solar cell operation – from light absorption to charge collection. We present TA spectra with over nine decades of time resolution, allowing us to globally deconvolute a series of participating excitonic and polaronic states and the rates coupling them. Polarization resolution is used to extract nm-scale spatial dynamics, notably of long range charge separation, and electric-fields are applied to discriminate between trapped and mobile charges and extract in-situ mobilities.

We apply these techniques to a range of polymer:polymer and polymer:fullerene bulk heterojunction devices with well-defined morphologies, in conjunction with photovoltaic performance measurements. In F8BT/PFB blends (F8BT = poly(9,9΄-dioctylfluorene-co-benzothiadiazole; PFB = poly(9,9´-dioctylfluorene-co-bis-N,N´-(4,butylphenyl)-bis-N,N´-phenyl-1,4-phenylene-diamine), diffusion-limited charge generation is resolved on a picosecond timescale and related to nm-scale phase separation, but device performance is thwarted by inefficient long range charge separation and subsequent collapse to a triplet exciton state. Devices comprised of combinations of P3HT, F8TBT and PCBM (P3HT = poly(3-hexylthiophene); F8TBT = poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2’,2’’-diyl); PCBM = (6,6)-phenyl C61-butyric acid methyl ester) exhibit ultrafast charge generation, substantially improved charge separation yields and accordingly better external quantum efficiencies. We relate these measurements to local morphological/mobility effects near the heterojunction. Overall, we find that the efficiency of geminate charge pair dissociation is the key determinant of device efficiency, and our work draws from a range of materials and morphologies to generalize strategies for improvement.