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Organic photovoltaic devices (OPVs) based on conjugated polymers and fullerene blends offer an attractive method to produce renewable energy. It has been demonstrated that they can be manufactured on fl exible substrates and at low cost over large areas using the high volume technique of roll-to-roll printing. [1][2][3] Whilst much work has been devoted to the donoracceptor system P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl-C 61 -butyric acid methyl ester), attention is now turning to the use of donor materials which have a reduced energy gap (permitting more of the sun's spectral emission to be harvested) and an increased ionization potential (leading to an increased open-circuit voltage and thus greater power conversion effi ciency [ 4 , 5 ] ).One promising new donor polymer for OPV applications is PCDTBT (poly [N-9 ′ -heptadecanyl-2,7-carbazole-alt-5,5-(4 ′ ,7 ′ -di-2-thienyl-2 ′ ,1 ′ ,3 ′ -benzothiadiazole)] whose chemical structure is shown in Figure 1 . [ 4 ] When PCDTBT is blended with the fullerene acceptor PC 70 BM, OPV devices have been created having a power conversion effi ciency of ∼ 6%, [ 6 ] one of the highest OPV effi ciencies to date. A marked difference in the fabrication process of these devices compared to the benchmark P3HT:PCBM system is the requirement for thermal annealing. It has been established that thermal annealing at ∼ 150 ° C [ 7 ] (between the glass transition and melting temperatures of P3HT) is required to drive the crystallization of the two components to form nanoscale, phase-separated domains. [8][9][10] Without such a thermal anneal process, the power conversion effi ciency (PCE) of P3HT:PCBM is limited to around 1%-2% [ 11 ] -a value that is improved to between 4 and 5% in an annealed device. [ 7 , 11 ] Recently we have established that this thermal anneal also modifi es the vertical structure of P3HT:PCBM thin fi lms. [ 12 ] In as-cast P3HT:PCBM fi lm, the surface is relatively depleted in PCBM. However by annealing the fi lm, PCBM is driven to the fi lm surface, an effect that we believe improves the electronic functionality of the device. In PCDTBT:PCBM OPVs however, a high temperature annealing process has been reported to be detrimental to device effi ciency. [ 6 ] Rather a low-temperature anneal/ drying stage at 70 ° C appears suffi cient to fully optimize device effi ciency although the benefi t of this stage has not been explicitly quantifi ed. Devices incorporating a TiO x optical spacing layer are also subsequently annealed at 80 ° C. [ 6 ] In this paper, we use neutron refl ectivity (NR) and grazing incidence wide-angle X-ray scattering (GI-WAXS) to explore the nanoscale structures formed in freshly cast PCDTBT:PCBM thin fi lms and those that have been annealed at a relatively low temperature. We make two main fi ndings. Firstly, we fi nd that the surface of freshly cast PCDTBT:PCBM fi lms is relatively enriched in PCBM with a negative concentration gradient existing away from the fi lm surface. Secondly, thermal annealing at 70 ° C does not signifi cantly m...

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