Solar Technologies.
Solar photovoltaics (PV) are the fastest-growing energy technology in the world and a leading candidate for terawatt-scale, carbon-free electricity generation by mid-century. Global PV deployment is dominated by crystalline silicon (c-Si) wafer-based technologies, which benefit from high power conversion efficiencies, abundant materials, and proven manufacturability. While PV module costs continue to decline rapidly, however, further system-level cost reductions will likely require lightweight and flexible module designs that are inaccessible with today's c-Si technologies.
Emerging thin-film PV technologies provide new functionality today and could reshape the solar landscape tomorrow. Emerging nanomaterials such as perovskites, organics, and QDs are structurally complex but simple to process. These materials open the door to new formats for deploying solar power. Unlike conventional technologies, organic solar cells can be made visibly transparent for ubiquitous deployment. Low-temperature processing allows lightweight substrates to be used, leading to high power-to-weight ratios and flexible cells that are easy to transport, store, and install. Flexible, monolithically integrated modules are inexpensive to manufacture and durable when deployed, with no wafers to break or solder joints to fail. In the long term these technologies could reach PV module and system cost floors unachievable with conventional silicon. They can also satisfy global energy needs without major constraints on material abundance, material production, or land use.
Our work in ONE Lab focuses on the basic photophysics and chemistry of emerging PV materials, from QDs to molecular organics to perovskites, as well as development of scalable device architectures and large-area processing methods for emerging thin-film PV technologies.
ONE Lab is a core contributor to the MIT GridEdge Solar research program, working toward scalable design and manufacturing of lightweight, flexible solar cells.
Team Members
Current Projects.
Processing Induced Distinct Charge Carrier Dynamics of Bulky Organic Halide Treated Perovskites.
State-of-the-art metal halide perovskite-based photovoltaics often employ organic ammonium salts, AX, as a surface passivator, where A is a large organic cation and X is a halide. These surface treatments passivate the perovskite by forming layered perovskites (e.g., A2PbX4) or by AX itself serving as a surface passivation agent on the perovskite photoactive film. It remains unclear whether layered perovskites or AX is the ideal passivator due to an incomplete understanding of the interfacial impact and resulting photoexcited carrier dynamics of AX treatment. In the present study, we use TRPL measurements to selectively probe the different interfaces of glass/perovskite/AX to demonstrate the vastly distinct interfacial photoexcited state dynamics with the presence of A2PbX4 or AX. Coupling the TRPL results with X-ray diffraction and nanoscale microscopy measurements, we find that the presence of AX not only passivates the traps at the surface and the grain boundaries, but also induces an alpha/delta-FAPbI3 phase mixing that alters the carrier dynamics near the glass/perovskite interface and enhances the photoluminescence quantum yield. In contrast, the passivation with A2PbI4 is mostly localized to the surface and grain boundaries near the top surface where the availability of PbI2 directly determines the formation of A2PbI4. Such distinct mechanisms significantly impact the corresponding solar cell performance, and we find AX passivation that has not been converted to a layered perovskite allows for a much larger processing window (e.g., larger allowed variance of AX concentration which is critical for improving the eventual manufacturing yield) and more reproducible condition to realize device performance improvements, while A2PbI4 as a passivator yields a much narrower processing window. We expect these results to enable a more rational route for developing AX for perovskite.
Dou B.D., deQuilettes D.W., Laitz M., Brenes R., Wang L., Wassweiler E.L., Swartwout R., Yoo J.J., Sponsellar M., Hartono N.T.P., Sun S., Bunoassisi T., Bawendi M., Bulović V., arXiv:2203.05904 (2022)
Predicting Low Toxicity and Scalable Solvent Systems for High-Speed Roll-to-Roll Perovskite Manufacturing.
Printed lead-based perovskite photovoltaics (PV) have gained interest due to their potential to be manufactured with scalable roll-to-roll techniques. In industrial scale-up, toxicity of inks can constrain roll-to-roll manufacturing due to the added cost of managing toxic effluents. Due to solvent toxicity, few perovskite solution chemistries in published works are scalable to gigawatt production capacity at low cost. Herein, it is shown that for scalable PV production, the use of aprotic polar solvents should be avoided due to their overall toxicity. Compliance with worldwide worker safety regulations for solvent exposure limits could require additional air handling requirements for some solvents, which in turn would affect cost-effectiveness. It is shown that costs associated with handling of hazardous substances can be significant and estimate an added cost of ¢3.7/W for dimethylformamide (DMF)-based inks. To solve this problem, a new perovskite ink solvent system is developed that is composed entirely of ether and alcohol, which has an effective exposure limit 14× higher than DMF, making it suitable for industrial coating processes. It is shown that the new ink solvent system is capable of fabricating high-efficiency perovskite solar cells processed in 1 min on a standard roll-to-roll system.
Photophysics.
Impact of Photon Recycling, Grain Boundaries, and Nonlinear Recombination on Energy Transport in Semiconductors.
A comprehensive framework for modeling energy carrier transport upon optical excitation in both excitonic and free carrier semiconductors is developed and applied. Using metal halide perovskite thin films as a model system, we demonstrate that processes such as nonlinear recombination and photon recycling can have a significant impact on the measured energy carrier profiles, especially for excitonic materials with short radiative lifetimes. Additionally, we find that film microstructure can lead to unique transport profiles that strongly depend on the material boundary behavior and the differences between the domain feature size and the energy carrier diffusion length. Our analysis provides a rigorous model of energy transport in semiconducting materials and a detailed assessment of the fundamental parameters needed for the design and optimization of electronic and optoelectronic devices.
Passivation Strategies in Emerging Photovoltaics.
Despite rapid advancements in power conversion efficiency in the last decade, perovskite solar cells still perform below their thermodynamic efficiency limits. Non-radiative recombination, in particular, has limited the external radiative efficiency and open circuit voltage in the highest performing devices. We review the historical progress in enhancing perovskite external radiative efficiency and determine key strategies for reaching high optoelectronic quality. Specifically, we focus on non-radiative recombination within the perovskite layer and highlight novel approaches to reduce energy losses at interfaces and through parasitic absorption. By strategically targeting defects, it is likely that the next set of record-performing devices with ultra-low voltage losses will be achieved.
Photon Recycling in High-Efficiency Photovoltaics.
Photon recycling is required for a solar cell to achieve an open-circuit voltage (VOC) and power conversion efficiency (PCE) approaching the Shockley-Queisser theoretical limit. The achievable performance gains from photon recycling in metal halide perovskite solar cells remain uncertain due to high variability in material quality and the nonradiative recombination rate. We quantify the enhancement due to photon recycling for state-of-the-art triple-cation perovskite films and corresponding solar cells. We show that, at the maximum power point (MPP), the absolute PCE can increase up to 2.0% in the radiative limit, primarily due to a 77 mV increase in (VMPP). For this photoactive layer, even with finite nonradiative recombination, benefits from photon recycling can be achieved when nonradiative lifetimes and external light-emitting diode (LED) electroluminescence efficiencies, QLEDe , exceed 2 μs and 10%, respectively. This analysis quantifies the significance of photon recycling in boosting the real-world performance of perovskite solar cells toward theoretical limits.
Brenes R.*, Laitz M.*, Jean J., deQuilettes D.W., Bulović V., Phys Rev Applied 12, 014017 (2019)
Scaling Solar Energy.
Vapor Transport Deposition of Next-Generation Solar Materials.
Current vapor-based deposition techniques for perovskite films are too slow to be cost-effective on a solar cell manufacturing line. We designed a manufacturing tool prototype that deposits perovskite films at a high rate and allows for tighter control over film formation. Using this tool, we research the fundamental film formation and device properties of vapor-deposited perovskite solar cells.
We are currently recruiting new graduate students for this project!
Roll-to-Roll Solar Energy with Safe Solvents.
Next generation thin-film solar has the potential to be incredibly low-cost, highly efficient, and reduce manufacturing CapEx – a current limitation to solar growth. In order to realize this potential, however, printing processes for solar materials must be fast (>10m/min) as well as utilize non-hazardous processes in order to be cost competitive. We have recently developed processes that allow us to easily print stable perovskite materials from low toxicity solvents at high speed. At small scale these methods have shown power conversion efficiency >20%. Improving upon these methods and fabricating mini-modules will be the key to the success of next generation solar technologies.