Synthesis Breakout. Overarching Issues 1. What are fundamental structural and electronic factors limiting Jsc, Voc, and FF in typical polymer bulk-heterojunction cells? Rational P- and N-type materials design and synthesis Multi-scale structural control; master self-assembly, phase separation New processing strategies for film formation, device fabrication 2. Mutual reinforcement of research and education What is the best way to educate a trained workforce? How to develop unique skills needed for OPV and related industries?
Disconnect between the design of new materials and the anticipated performance of devices. Need for theory, computation, device modeling In polymer BHJ solar cells, it is unclear to what extent the crystallinity of the donor domains is important. For example, P3HT is crystalline, whereas Leclerc s PCDTBT system is much less so. Current computational modeling helps at a preliminary level, i.e. weeding out clearly poor structures. It therefore provides a first level of screening. Current computational modeling does not provide a realistic picture of what we encounter in the solid state, and worse for blends. In other words, the precision we find in molecular crystals and one dimensional systems breaks down when we have to consider multichromophore 2D and 3D interactions. Finding computational modeling capabilities that can help will in many cases need powerful hardware as well as software that gives mobility/band gap/homo-lumo properties that can be well verified by experiment. This probably requires connectivity to DOD and DOE centers.
Characterization Techniques What are important characterization and measurement methods whose throughput and dependability could be improved for use of synthetic design (with faster insight for improved design). Some specific rapid feedback characterization and property measurement techniques are particularly desirable for tuning design. There is major need for dependable mobility measurement along and perpendicular to a surface. Thin film transistor (OFET) results do not necessarily give good insight into mobility properties in OPVs. Photovoltaic behavior obtained readily for faster design alterations. Energy levels and energy offsets how to determine them in blends and as a function of processing, in particular in measurements using the device configuration. Certification from other sources than NREL, Newport?
How do HOMO/LUMO energies affect OPV processes? Can we gain structural control over losses, by comparison to theoretical maximum electrical and PCE performance metrics? And also control molecularly and morphologically related processes And also the charge transport and excited state processes
Materials Types Is there a clear choice for concentrating on small molecules, polymeric materials, systems that have characteristics of both? These include: Interfacial layer materials Nanoparticle surface engineering Active layer materials Processing strategies, or post-processing strategies Synthetic possibilities limit/control choices of strategies. Fullerenes How can we replace expensive fullerene acceptors? Are they the ultimate acceptors in their present form? Are there ways to accelerate their synthesis and evaluation? Fullerenes versus other choices Syntheses and purifications Other Carbon Nanomaterials Separated CNTs, graphene forms (e.g., nanoribbons)
General strategic approaches for synthetic pursuit. An important opportunity resides in controlling the precision of the polymerization sequence, as per P3HT. That has not been achieved with narrow bandgap D-A backbone structures. Such materials would allow us to understand the importance of average molecular weight and dispersity on power conversion efficiencies, kinetics of thin film formation, final morphology, etc. Polymers with backbones with small molecules of high efficiency linked together with short aliphatic linkers offer possibilities to combine small molecule electronics with polymer solution-based processibility. For example condensation polymerization via ADMET chemistry might address this. But, such alternating chromphore/soft block copolymers may not give organized chromophore morphologies that arise from the small molecule themselves. Are there small molecular systems with structural attributes that minimize delamination during roll to roll processes? Examples: Brian Gregg tried small molecule+polymer. PTCDI+polymer linker. (Long enough molecule+polymer) Perhaps small molecules with templates are good targets?
Inorganic Opportunities Are fullerene traits especially important to give their effective behavior as n- carrier materials in OPVs, e.g.: (a) 3D low aspect ratio shape (b) do not aggregate that much (c) isotropic transport, (d) high mobility, (e) multiple reduction states (f) low reorganization energies. However, negative characteristics are that they do not absorb light at useful wavelengths, and remain expensive to use in substantial amounts. Can other organic analogues be devised (especially with vis-nir absorption capability)? What about inorganic based n-carriers? quantum dots? n-type QD, p-type polymer? p-type QD + n-type polymer? Surfactant on the interface of QD? Hyperbranched QD for better transport? Controlled polymn from surface? High loading to over the percolation threshold. 30% or higher, the processing is difficult. Graft the polymer to the QD. Amphiphilic diblock, converting a core block into n- type or p-type.. CdSe nanosheet. Interface modified, electron compatibility. Inter-mixing. Earth-abundant, n-type semiconductor; mixing the n-type polymer to p-type QD. Modified versions of fullerene+p3ht works, too Size of fullerene (0.8 nm) especially important by comparison to other possibilities?
Interfacial Layers What are the synthetic design rules for interfacial layers to act as selective contacts? What properties do they need to replace PEDOT:PSS? What will be their role in inverted bulk-heterojunction cells? Stabilizing physical cohesion at interfaces Orienting active layer components Measurement techniques Interfacial modifiers are important for controlling electrode work function, minimize surface trap states, tuning morphology (especially at interfaces), improving adhesion and avoiding delamination. This in turn influences possibilities for manufacturing and processing. There are established interfacial modifiers already. Can we modify their functionality for other new uses and retain electronic characteristics? Is there a preference for SAMs, inorganic interfacial layers, polymer layers? Can we compute the behaviors of such modifiers, to decide what might work to improve electric behavior? How best to identify what impurities are in existing or new materials that influence electrical behavior? (e.g., an inorganic case of ZnO? Oxygen deficiency?) Can these be used to avoid delamination at interfaces?
What is the future of small-molecule materials? Vacuum-deposited small molecules versus solution-processed small molecules (Some European groups feel that vapor deposited small molecule cells will be the winner, as with OLEDs) Design rules to increase OPV performance Small molecules: Heliatak was successful, but for a general strategy not so clear about its future for manufacturing. They did two layers the same to get 10.6%? Low cost is a key for future marketplace success. Solution processing small molecules is potentially much lower cost. A system that combines the purity /order of small molecules and processibility of polymers would be desirable. Is there a sweet spot of structure type between small molecules and polymers that will do this?
High Throughput Experimentation Challenges What are the best strategies to apply high-throughput experimentation to accelerate OPV development in materials synthesis and device testing? How can we best take the step from small spin-coated cells to roll-to-roll processing? Inter-disciplinary collaboration Multi-university shared facilities
National facilities for Processing Development UMAss NSEC: R2R manufacturing UMinnesota: coating center, MURI on printed electronics (Dan Frisbie) NIST (Dean): doctor blading? NREL has spray system, not much for OPV Other Processing Challenges/Opportunities Understand solvent annealing effects (in situ and ex situ) Understand effects of processing additives Understand thermal annealing effects In situ film deposition and crosslinking and/or fusion to substrate
Materials Design Aided by Theory How can computation and simulation best be used to guide synthetic studies? Jointly-funded closely collaborating groups Multi-user facilities Available softwares