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Polymer:fullerene nanoparticles (NPs) offer two key advantages over bulk heterojunction (BHJ) films for organic photovoltaics (OPVs), water-processability and potentially superior morphological control. Once an optimal active layer morphology is reached, maintaining this morphology at OPV operating temperatures is key to the lifetime of a device. Here we study the morphology of the PDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}):PC71BM ([6,6]-phenyl C71 butyric acid methyl ester) NP system and then compare the thermal stability of NP and BHJ films to the common poly(3-hexylthiophene) (P3HT): phenyl C61 butyric acid methyl ester (PC61BM) system. We find that material T g plays a key role in the superior thermal stability of the PDPP-TNT:PC71BM system; whereas for the P3HT:PC61BM system, domain structure is critical. Refereed/Peer-reviewed

Sequential deposition of monolayers, composed of nanoparticles with varied donor-acceptor concentration ratios, has allowed the fabrication of organic photovoltaic (OPV) active layers with engineered vertical morphology. The performance of polymer-polymer poly(9,9-dioctylfluorene-co-bis-N,N-phenyl-1,4-phenylenediamine):poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (PFB:F8BT) and polymer-fullerene poly(3-hexylthiophene):phenyl C61 butyric acid methyl ester (P3HT:PCBM) nanoparticulate (NP), graded nanoparticulate (GNP) and bulk heterojunction (BHJ) OPV devices have been studied. For both material systems the highest device VOC is observed for the graded structure. Furthermore, thermal treatments can be used to alleviate parasitic series resistance in the GNP devices, thus improving device JSC and efficiency. Overall, this work shows that the nanoparticle approach provides a new experimental lever for morphology control in OPV devices. Refereed/Peer-reviewed

Water-based polymer nanoparticle dispersions offer the prospect of addressing two of the main challenges associated with printing large area organic photovoltaic (OPV) devices; namely how to control the nanoscale architecture of the active layer and eliminate the need for hazardous organic solvents during device fabrication. However, to date, the efficiencies of nanoparticulate-based devices have been inferior to that of the corresponding bulk-heterojunction devices. Here we present an approach for producing optimised OPV devices from polymer nanoparticles via the fabrication of multilayered device architectures. We show that by controlling both nanoparticle morphology and inter-particle interactions it is now possible to build polyfluorene OPV devices from aqueous dispersions of nanoparticles that are more efficient than the corresponding bulk heterojunction devices. In particular we show that: (1) the polyfluorene nanoparticle morphology is suited to effective charge separation, (2) thermal treatment of the deposited layers results in improved interparticle connectivity and effective charge transport, and (3) the optimal device thickness is a delicate balance between the repair of layer defects and the creation of stress cracking in the nanoparticulate film. As such, this work offers insights for the development of printable photovoltaic devices based on water-dispersed nanoparticulate formulations. Refereed/Peer-reviewed

Structure and morphology play a critical role in determining the performance of organic photovoltaic devices. In this paper, variation of the postannealing cooling rate is used to create a series of "snapshots" of the vertical and interfacial reorganization processes that occur upon annealing. The data show that slower cooling rates result in significantly enhanced device efficiencies primarily driven by increased short circuit current and fill factor. UV-vis spectroscopy, X-ray diffraction (XRD), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), atomic force microscopy (AFM), and contact angle measurements are used to probe the origin of these improvements. Our results show evidence for a distinct and changing vertical stratification and interfacial structure in the device throughout the annealing process, with both composition and crystallinity varying through the active layer. The implications of these changes are discussed in terms of device properties. Refereed/Peer-reviewed

Water-based nanoparticulate (NP) and bulk heterojunction (BHJ) organic photovoltaic (OPV) devices based on blends of poly(9,9-dioctylfluorene-co-N,N-bis(4-butylphenyl)-N,Ndiphenyl-1, 4-phenylenediamine) (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole (F8BT) have been fabricated with aluminium and calcium/aluminium cathodes. The NP devices exhibit power conversion efficiencies (PCEs) that are double that of the corresponding BHJ device. Moreover, the addition of calcium into the cathode structure results in a dramatic increase in open circuit voltage and PCEs approaching 1% for water-based polyfluorene OPV devices. Refereed/Peer-reviewed

Water-processable nanoparticle dispersions of semiconducting polymers offer an attractive approach to the fabrication of organic electronic devices since they offer: (1) control of nanoscale morphology and (2) environmentally friendly fabrication. Although the nature of phase segregation in these polymer nanoparticles is critical to device performance, to date there have been no techniques available to directly determine their intra-particle structure, which consequently has been poorly understood. Here, we present scanning transmission x-ray microscopy (STXM) compositional maps for nanoparticles fabricated from poly(9,9-dioctyl-fluorene-2,7-diyl-co-bis-N, N'-(4-butylphenyl)-bis-N, N'-phenyl-1,4-phenylenedi-amine) (PFB) and poly(9,9-dioctylfluorene-2,7-diyl-co-benzothiadiazole) (F8BT) 1:1 blend mixtures. The images show distinct phase segregation within the nanoparticles. The compositional data reveals that, within these nanoparticles, PFB and F8BT segregate into a core–shell morphology, with an F8BT-rich core and a PFB-rich shell. Structural modelling demonstrates that the STXM technique is capable of quantifying morphological features on a sub-10 nm length scale; below the spot size of the incident focused x-ray beam. These results have important implications for the development of water-based 'solar paints' fabricated from microemulsions of semiconducting polymers. Refereed/Peer-reviewed