UNIVERSITY OF MASSACHUSETTS

ERG 1 researchers will synthesize structures applicable to light harvesting and photocurrent generation, with a global objective of preparing fundamentally new electronically active materials with optimizable photophysical properties, and integrating them into the active layer of photovoltaic devices. A unifying objective of the synthetic strategy is to provide a range of polymer and nanocomposite materials geared towards morphological and photovoltaic device studies undertaken in ERGs 2 and 3. Synthetic efforts will anticipate features of self-assembly, morphology, processibility, and charge transport, providing EFRC researchers with materials that contain tunable photophysical features, and that lead to efficient photovoltaic devices. The primary research thrusts of ERG 1 fall into two main areas. The first is the synthesis of conjugated homopolymers, block copolymers, segmented structures and materials with well-defined structural architectures, with control over component energy levels using electron withdrawing and donating functionality. The second is the preparation of p-type conjugated polymers with end-groups suitable for functionalization of n-type nanoparticles and nanorods, leading to effective dispersion of the particles and rods, and the preparation of hierarchically oriented nanomaterial assemblies for devices. ERG 2 The design and fabrication of a high-efficiency photovoltaic device requires precise control over the nanoscale morphology, molecular ordering, and interfacial properties of all components comprising the device. While obvious in statement, in practice, these tasks are quite challenging; most research on polymer-based photovoltaic devices to date has systematically considered at most one of these details. Researchers in ERG 2 integrate expertise in the synthesis, theory and engineering of polymers with established track records in controlling the self-assembly of polymers and the manipulation of polymer morphologies and individual chains in thin films. This expertise will be used to develop routes to control the morphology and structure of polymer-based and polymer/inorganic-based hybrid photovoltaic devices so as to optimize efficiency. The knowledge gained from these studies will then be used for device fabrication. ERG 3 In an organic solar cell, the conversion of light energy to electric current involves five fundamental processes, (1) the absorption of light to create excitons, (2) the diffusion of excitons to a region of high electric potential mismatch where charge separation can occur, (3) charge separation, (4) charge transport of the holes and electrons to their respective electrodes, and (5) transport of the carriers across the respective organic-electrode interfaces . All of these processes have to be understood individually to be able to optimize them for high efficiency solar cells. Detrimental effects, including charge carrier recombination and trapping, also need to be assessed to build structures that inhibit these effects. Associated with many of those processes are characteristic times scales that can be extremely fast (down to femtoseconds), particularly in nanoscale systems like the ones proposed in ERG 1 and ERG 2, since the distances involved are so small. Using time-resolved optical techniques it is possible to distinguish and quantify exciton lifetimes, charge separation dynamics, and electron and hole lifetimes that affect processes. The primary goal of ERG 3 will be to elucidate photophysical details of charge and energy transport within nanostructured PV composite films. Single-molecule spectroscopy and ultrafast spectroscopy will provide valuable mechanistic insight and feedback to the synthetic components of our center as to the role of molecular architecture and confinement geometry on primary processes in PV systems. Using this information, ERG 3 will also provide working PV cell characterization using device absorption and efficiency measurements.

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