Professor Nguyen is an internationally leading scientist, working in the field of organic photovoltaics (OPVs) – devices fabricated from carbon-based materials that convert sunlight into electricity. OPVs is a multidisciplinary field and has drawn scientists from various disciplines including physics, chemistry, materials science, engineering, and computation. Nguyen’s broad and deep understanding of chemistry, materials, engineering and physics is very rare and is extremely powerful particularly in the field of OPVs. Her PhD in Physical Chemistry focused on the development of processing protocols to control polymer conformation, film morphology, and photophysics of conjugated polymer. She employed a wide range of characterization techniques to help resolve the major controversies about the polymer photophysics and provide approaches to improve the performance of organic light-emitting diodes. For her postdoc at Columbia University, she switched to the field of molecular electronics, self-assemblies, transport charge transport in 1D systems, and field-effect transistors.
In 2004, she started her independent at the University of California, Santa Barbara. She decided to move away from the research topics that she did during her PhD and postdoc (photophysics, light-emitting diodes, and molecular electronics) to work on OPVs. OPVs have garnered significant attention over the last two decades due to their potential as lightweight, flexible, semitransparent and low-cost electricity generation technology for buildings, greenhouses, and portable power sources. She is recognized worldwide for her exceptional contributions to the development of efficient OPVs based on solution processed materials and advanced characterization methods. Prior to her work, research on the solution-processed OPVs has been dominated by the use of polymeric and fullerene materials. However, polymers suffer from a lack of synthetic reproducibility and difficult purification procedures, which hinder commercial viability and limit a reliable comparison of device architectures. In 2000, there were few reports on OPVs fabricated from small molecules with extremely low power conversion efficiency (~10-5 %). In 2006, recognizing the drawbacks of conjugated polymer based OPVs which led to device irreproducible, her group started develop molecular organic semiconductors for solution processed OPVs. In 2009, her group achieved a breakthrough with power conversion efficiency about 4.8%. The paper was selected for the inside cover of Advanced Functional Materials and was the most download paper for July and August 2009. It also helped increase the impact factor of the journal. This result marked the turning point for the OPV community and demonstrated that it was possible to achieve high performance with molecular organic semiconductors. In 2012, they could achieve 7% efficiency. In 2013, they reported the best OPV performance based on nonfullerene acceptor. This work demonstrated that it was possible to achieve efficient nonfullerene-based OPVs, which has led to rapid development of nonfullerene acceptors. Nowadays, the best OPVs are based on nonfullerene acceptors. The advantages of using these molecular materials are the ease of synthesis and purification and higher charge carrier mobilities than their polymeric analogues as a result of better molecular ordering. Most importantly, conjugated small molecules do not suffer from batch to batch variations, broad molecular weight distributions, end group contamination, and difficult purification methods, as is the situation for their polymeric counterparts. Since then, this approach has been utilized by the community to design the high performance solar cell materials achieving the power conversion efficiency of above 20%, exceeding the mark for commercialization.
Besides materials development, Nguyen also pioneered and advanced scanning probe microscopy and spectroscopy techniques to probe important processes and properties of multicomponent OPVs at multiple length scales. The evaluation of solar cell performance provides a composite picture of several, often interrelated variables. It is not always possible to extract information on the effect of molecular structure, processing conditions, fabrication protocols and thermal history on the intrinsic electronic properties of the individual components. Nguyen and her team developed an analytical method to visualize the buried interfaces and internal morphology of OPVs and other electronic devices, thereby obtaining a visual inspection of how processing and molecular components influence device structure and performance. They used a focused ion beam to cut a thin slice of the device and subsequently transfer it to a transmission electron microscopy (TEM) grid for examination or glass substrate for atomic force microscopy characterization. Nguyen and her team developed a state-of-the art instrument – conducting and photoconductive atomic force microscope – to probe nanoscale property of OPV devices. In 2008, this instrument was not commercially available. Her group acquired a scanning probe microscope, an inverted optical microscope, white light and laser sources, filter wheels, heating stage, sample holder with optical window, glovebox, etc. and integrated all the components. The tool is extremely powerful because it can provide information that cannot be obtained by other methods and help explain why some OPVs have high efficiency and some do not. She shared the instrument design with two companies to develop commercial instruments (Bruker and Asylum Research, Oxford Instruments). This tool now has been used widely by other fields including solid state battery, silicon PVs, perovskite PVs, semiconductor industry, catalyst industry, etc. In the past few years, Nguyen and her collaborators have combine solid state Nuclear Magnetic Resonance, X- ray scattering, and scanning probe microscopy, to probe OPV morphology at multiple length scales from atomic resolution to the bulk to gain insight into processes that govern OPV performance, thus, help advance the OPV field.
Nguyen and her team have also expanded the frontiers of materials processing. They are the first to use ternary blend to control the morphology and expand the absorption of OPVs to the near infrared region (Applied Physics Letters 2008, 93 (16), 389; 134 google scholar citations). Ternary blend approach has been used to achieve OPV efficiency over 18%. They were the first team to use Hansen solubility parameter to select the right solvent to achieve the best performance for OPV blend materials. Typically, for each OPV blend system, researchers must prepare devices in different solvents, thus, using Hansen solubility method has saved a lot of time in device optimization and has been used routinely in OPV industry. During her postdoc working with Louis Brus at Columbia University, Nguyen was exposed to the field of gold nanoparticles. At UCSB, Nguyen and her collaborators incorporated gold nanoparticles into OPVs with the goal of using surface plasmon to enhance the absorption of the OPV. They found the device efficiency was double. However, from the cross-sectional images by transmission electron microscopy, there were very few gold nanoparticles to yield such large improvement. As a control experiment, they added only the ligands used to encapsulate the gold nanoparticles, alkyl thiols, and found the efficiency improved even further. They spent several years to understand how additives could be used to control the film morphology and the OPV performance. Additive processing is now widely used by both academic researchers and industry to improve the OPV performance.
Nguyen and her group also developed advanced capacitance spectroscopy, current-voltage measurements, and device simulation in conjunction with ultrafast spectroscopy to quantify different loss processes in OPVs and ways to overcome these losses. Nguyen’s work has provided a deeper and more comprehensive understanding of the underlying mechanisms governing the device performance. Lastly, she and her collaborators have unraveled the loss mechanisms and provided a path forward to advanced solution-processed OPVs. Today, Nguyen’s group continues to be at the forefront of the OPVs.
Besides research, Professor Nguyen has a strong passion for teaching and training students. She is the 4th generation in her family to follow the teaching career. Her belief is that thoughtful mentoring of young students is one of the most important activities that a professor can undertake as it undoubtedly shapes the future of our society. She has supervised a large number of international research team of undergraduate, master and PhD students, postdoctoral scholars, high school students and teachers, and visiting scientists (155 in total from 22 countries). She helps students to develop certain skills such as critical thinking, problem solving and analysis, strategies for learning new subjects, team work, time management, multi- task, asking important scientific questions, communication, presentation, writing, etc. She has developed new courses to educate young people in the chemical and engineering challenges related to the climate crisis and the necessary transition in energy production and to prepare students for the real world. One of the courses helps students to get jobs every year.
Besides training her own group members, Nguyen also has mentored 25 young faculty and scientists from different countries (10 women). She provides guidance on faculty applications and interview, negotiation, proposal writing, mentoring students, and career-life balance. She has actively recruited and trained global scientists especially women and those from developing countries where science is not advanced (43 females and 20 from developing countries such as Turkey, Jordan, Nepal, Brazil, Morocco, Philippines, Mexico, Vietnam, Thailand, India, etc.).
Nguyen serves in several international advisory boards and program committees for major conferences in her field. She makes sure there is a gender equity especially in term of invited and plenary speakers and a balance of speakers from various career stages and countries. During the pandemic, she helped form a support-group of 21 science and engineering female professors from Brazil, US, UK, Germany, Taiwan, Sweden, Israel, the Netherlands, Spain, Japan, and Italy. They share information from teaching, research, mentoring students, balance family and work, dealing with bully by senior colleagues, diversity issue, publishing, gender bias, etc.
Her research impact in green energy together with her tremendous efforts in education at all levels, contribution to engineering societies, increasing diversity, and supporting women in science make her an exceptional candidate for the Wilhelm Exner Medal.