Processing

Solution manufacturing of photovoltaics is key to the promise of ubiquitous solar-energy technologies, via high-throughput and large-area solution coating and printing of potentially low-cost and low-energy footprint modules based on emerging photovoltaic technologies. Emerging photovoltaics materials can be classified into two categories for the purpose of manufacturing, based on whether they can only be solution-manufactured or are also amenable to vacuum manufacturing. Small-molecule organic solar cells and hybrid organic-inorganic perovskite solar cells can be successfully manufactured using conventional vacuum deposition approaches and by solution manufacturing, whereas solution manufacturing is the only suitable production method for several material systems, including medium- and large-size conjugated molecules, dyes, conjugated polymers, and colloidal quantum dots, none of which are amenable to vacuum evaporation. To compete with a demonstrated manufacturing technology such as vacuum-processing, solution manufacturing is likely to have to achieve comparable or better performance, manufacturability, scalability, reliability, yield, and uniformity and also be noticeably more economical, as well as energetically and/or environmentally advantageous. The emergence of materials with unique capabilities (e.g., conjugated materials and colloidal quantum dots) and applications likely to be outside the reach of vacuum-based manufacturing techniques can also catalyze the adoption of solution manufacturing and related techniques. Overall, solution manufacturing has tremendous cross-platform appeal, both as a crucial enabler for emerging photovoltaic materials and as a potentially low-cost alternative for vacuum manufacturing of conventional thin-film photovoltaic technologies. Solution processed perovskite solar cells currently exhibit superior performance to vacuum evaporated ones; however, this is a rare instance that does not hold true in the case of conventional inorganic thin-film PV, where manufacturers prefer the vacuum-based approach thanks to its reliability and ability to yield the highest efficiency cells and modules. Solution manufacturing can be a very complex process, with a different set of challenges for every material system utilized. Developing a quantitative and holistic understanding of thin-film formation from solution which is the aim of this theme is therefore not only crucial to achieving good performance in the laboratory, but also to translating the laboratory successes to scalable and high-throughput manufacturing environments.

Fully-Printed Bulk Heterojunction Organic Solar Cells

Solution-processed bulk heterojunction (BHJ) solar cells have now achieved a certified power conversion efficiency (PCE) of 11.5% and are well on their way to achieving 15% in the coming years. Despite this progress, commercial implementation is hindered in part due to a significant performance gap between scalably printed solar cells and champion laboratory devices. This gap is substantially smaller in more mature vacuum-deposited organic optoelectronic and photovoltaic devices, where thin film deposition is well understood, easily controllable and upscalable. This project seeks to deploy scientific and engineering efforts toward scalably printing highly efficient organic solar cells and achieving performance on par with the best laboratory device developments. Our main approach will be to develop a scientific understanding and knowledgebase of the formation of the photoactive thin film, buffer layers, top electrode and interfaces in laboratory champion solar cell fabrication processes, from which we will propose rational approaches toward replicating the key layer and interfacial properties using scalable coating and printing approaches. The broader impact of this project is expected to be the development of methodologies by which the remarkable small-scale advances made in laboratories worldwide can be rationally, easily and quickly translated into fully-printed solar cells so as to closely track the best-in-class efficiencies as they increase steadily toward 15% and enable their successful prototyping and scale-up.

Printed Perovskite Thin-Film Solar Cells

Solution-processed hybrid organic-inorganic perovskite thin film solar cells have already demonstrated certified PCE in excess of 20%, making this nascent technology potentially competitive –on the basis of its efficiency alone– with more established thin-film photovoltaics platforms. Perovskite thin films have nevertheless been shown to be orders of magnitude inferior to single crystals in terms of trap density, carrier mobility, and carrier diffusion length. It is therefore highly desirable to develop scalable solution-processing methods and post-treatment approaches that promote the formation of high-quality polycrystalline thin films with large and well-connected domains, few grain boundary defects, as well as well textured microstructures and excellent electrical contact with the selective contacts and ultimately approach the performance theoretically achievable by single crystals. Practically speaking, the aim of this project is to demonstrate highly efficient printed perovskite solar cells with PCE > 20%. Our approach will consist first, of a scientific investigation of the solidification and crystallization of the perovskite formulations based on state-of-the-art laboratory recipes, with the aim of understanding the ink-to-perovskite transformation and its impact on optoelectronic properties of films. From this, ink-process-structure-property-performance feedback loops will be established which will help identify and systematically address the existing shortcomings of perovskite films with respect to their single crystal counterparts. This will be done for laboratory and more scalable manufacturing processes and leveraged toward more consistent fabrication of high quality polycrystalline perovskite films for photovoltaic applications via scalable routes.

Printed Hybrid-Tandem Solar Cells and Modules

The performance of single junction solar cells can be increased by several power points simply by integrating a spectrally complementary subcell at the front or the back. Emerging PV material systems, such as organics, quantum dots and perovskites are spectrally tunable and exhibit spectral complementarity. As such, they can be printed as front cells on top of conventional subcells (e.g., Si, CIGS, CdTe), or be integrated together across material platforms, forming highly efficient printed tandem or triple-junction devices. In this project, we aim to demonstrate printed hybrid tandem solar cells and modules based on monolithic integration of (i) two emerging PV platforms, namely organics with quantum dots, or organics with perovskites, and (ii) one emerging PV platform (perovskites) with a conventional Si subcell.

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