Tell us a bit about your childhood and your path into the sciences
I was born in 1936 into a very poor family at the tail end of the Great Depression, the youngest sibling of 3 sisters and 2 brothers. 听My mother had emigrated in 1920 from Glusk , Russia (now in Belarus) to the U.S in the midst of the counter-revolutionary civil war between the White Russians and Cossacks against the Soviet Bolsheviks. Pogroms in Glusk were a common event, and after warnings by her neighbors that a pogrom was imminent the next day, my mother fled overnight, alone with 3 children, taking 2 years to cross Europe and the Atlantic Ocean arriving in Ellis Island in New York Harbor in early January, 1923. 听My father, who died at 46 in early 1938 when I was barely 2 years old, and was thus unknown to me, also mysteriously arrived with my mother at Ellis Island, and his death in 1938 left my immigrant mother with 6 children and no financial resources; she survived on welfare for some time during WW II and she worked at a weapons factory since the male work force (including my two brothers) was depleted by war service.
During my early youth several important events occurred that vividly remain in my memory: Pearl Harbor (1941), Wendell Wilkie鈥檚 failed presidential campaign against FDR (1940), 听Thomas Dewey鈥檚 failed campaign against FDR (1944), FDR鈥檚 sudden death (1945), and the splitting of the atom followed by the dropping of the atomic bomb on Hiroshima (1945). The latter profoundly stimulated my interest in science, though this emotion was moderated by the horror of the instantaneous death of tens of thousands of innocent civilians. Notwithstanding, I decided then, at 9, to become a scientist. 听听
I was enrolled in an orthodox religious school even before attending kindergarten. For a decade I attended the religious school for several hours every day after public school and on weekends. 听There, I was exposed to serious and devoted scholarly teachers who conveyed the passion, joy, and value of learning, of difficult study, of curiosity, and of asking and trying to answer complicated and difficult questions. This experience prepared me well for my scientific career, and inspired me to seek the highest level of advanced education possible. This journey into science led me away from the dogma and doctrines of organized religion.
I began working at age 14, first picking tobacco in the fields of nearby Connecticut; then as a paper boy, which led to a lifelong habit of reading the New York Times every day, which I continue to do, then in high school driving and selling ice-cream from an ice cream truck during the summer and also working every day after class during the school year for a drug distribution company.
Describe how you joined the energy research efforts at NREL and RASEI
Before the National Renewable Energy Laboratory (NREL) was officially designated as one of the 17 U.S. Department of Energy鈥檚 National Laboratories by President George H. W. Bush in 1990, it was initially designated as the Solar Energy Research Institute (SERI) and was created under the administration of President Jimmy Carter in March 1977. This was done in response to the global energy crisis created by the OPEC oil embargos of 1973 and 1978. The initial motivation for SERI鈥檚 formation was energy security (the U.S. was importing >50% of its petroleum needs in the years after the 1973 oil crisis, and the U.S. natural gas supply was predicted (erroneously) to be depleted by the mid-1980s). SERI grew from zero staff in 1977 to 1200 by 1979 (I joined in Oct, 1978).
During the late 1970s after SERI was established, there was a lot of skepticism in Congress and in the public more generally, that renewable energy, especially solar energy, was a viable energy option. For example, the cost of photovoltaic (PV) electricity in 1974 was $6-8/kWh (today it is less than $0.06 鈥� $0.08/kWh). In addition, there was a strong push-back on the political front from conservative senators and representatives, and their fossil fuel industry supporters, who didn鈥檛 like the idea of replacing fossil fuels with renewable energy; this resulted in many senators and representatives taking a look at the newly established SERI and holding congressional hearings (I participated in a few). The general feeling was that it was a haven for hippies of the 1960s, running around in Birkenstock sandals (I did wear a pair), not serious scientists. At universities, including CU, there was skepticism and aloofness about the new, unknown, and unproven group of scientists working at SERI. The skepticism gradually diminished, and in 1999 I became the first Professor Adjoint from NREL appointed to the Chemistry Faculty at CU.
But the early SERI research staff were good, dedicated, and innovative young scientists and began to make important advances in the science and engineering of renewable energy to bring its cost down along a Moore鈥檚 Law type of Learning Curve, resulting in renewable wind and PV energy costs being driven down to about 陆 of that from fossil and nuclear power plants by 2020. 听Furthermore, new funding from DOE鈥檚 Office of Basic Energy Sciences (BES) was obtained in 1980 to support a new SERI research Branch called the Solar Photoconversion Branch, focused on solar fuels and artificial photosynthesis, particularly hydrogen production through solar water splitting; I was the Branch Chief from 1980 to 1985, followed by my appointment in 1985 as one of the two initial NREL Senior Research Fellows in NREL鈥檚 new Fellows Program (there were a total of 3 initial Fellows appointed in 1985).
However, Ronald Reagan campaigned for President in 1980 on the promise to close the Dept of Energy because 鈥渢hey never produced a barrel of oil鈥�; Reagan鈥檚 campaign, together with the Iranian revolution in 1979 that removed the Shah of Iran plus the taking of hostages from the U.S. Embassy in Iran, led to the election of Ronald Reagan and the defeat of President Carter in the 1980 election. The situation at SERI after Reagan became President was tumultuous: 听although President Reagan never did close the DOE, the then Secretary of Energy James Edwards came to SERI in 1981 to consider closing it down, but he was persuaded by Golden Elder Joe Coors to not do so (that鈥檚 another story for another time). But 2/3 of the SERI staff was terminated, reducing it from 1200 to 400), and SERI funding was also reduced by two-thirds. 听But since significant renewable energy utilization was considered a very distant prospect, the basic science programs at SERI funded by BES survived, and flourishes today at greatly enhanced levels of staff and DOE funding, as well as excellent scientific and technological progress, reflected in part by its partnership with RASEI.
Beginning in the 1990s, advances in extraction technology for fossil resources, has led to general agreement that the planet has >100 years of fossil fuel in the ground. But the vast majority of climate scientists have also concluded that the present usage rate of fossil fuel cannot continue without drastic harmful consequences for the environmental health of the earth because of the associated emission of CO2听greenhouse gas into the atmosphere. Thus, in more recent years, climate change and the need for viable renewable energy alternatives to fossil energy to help sufficiently ameliorate climate change to avoid its worst consequences has become a critical mission for NREL. However, the climate change projections have engendered a strong political backlash, primarily to resist abandonment of the enormous fossil fuel supply and infrastructure at a huge near-term financial cost to the fossil fuel industry and its stakeholders. This has created a very strong political element in the support and funding of NREL鈥檚 renewable energy programs by Congress and since 1980 has led to large and unpredictable periodic swings in NREL鈥檚 annual budgets depending on the political philosophy of the elected Government Administration and Members of Congress. These are still the political and funding challenges and realities of NREL.
What do you enjoy doing outside of work?
My devoted and loving wife, Rhoda, and I both enjoy hiking, skiing (XC and downhill), biking, and travelling to the artic regions on National Geographic Expeditions.
What made you focus your research on harvesting solar energy?
I first became interested in renewable energy, specifically both photovoltaic (PV) and solar fuels (the latter mainly solar water splitting for creating the so-called hydrogen economy), in the early 1970s. Regarding PV, in 1972, I discovered a new defect semiconductor, Cadmium Stannate, or CTO, that had the best properties as a transparent conductor (very high transparency in the solar spectral region coupled with high conductivity) when compared to other known transparent conductor (ITO, FTO), and it held promise for use in PV solar cells; support for this application was provided by the NSF鈥檚 RANN (Research Applied to National Needs) program, which existed from 1971 to 1977. In August 1977, the RANN program was terminated and essentially transferred to ERDA (Energy Research and Development Agency), which in October 1977 was incorporated into the newly formed Department of Energy (DOE); SERI was included in this reorganization and became a DOE funded facility. Also in 1972, following up on my thesis research using M枚ssbauer spectroscopy to study the electronic properties of Iron-based systems, I discovered that Iron 3+ adsorbed on polycrystalline titanium dioxide surfaces could be oxidized to Iron 4+ when the titanium dioxide was photoexcited with supra-band-gap light (>3 eV). Because the redox potential of the Iron 3+ / Iron 4+ couple is very positive (鈭�+1.6 V), it was clear that the positive holes photogenerated in titanium dioxide had very strong oxidation potentials. Then, in 1972, the famous Fujishima鈭扝onda paper appeared in Nature, demonstrating that water could be photodecomposed to hydrogen and oxygen gas in a photoelectrolysis cell containing a near-UV-illuminated titanium dioxide photoanode. This paper was consistent with my earlier finding of the strong oxidizing power of photoexcited titanium dioxide.
The oil embargo and energy crisis of 1973鈭�1974 produced a sudden and intense flurry and level of support for research and development for alternatives to fossil energy; this included new R&D activities in industry and at universities. I joined Allied Chemical in 1974 to pursue research on photoelectrochemical energy conversion; this led to my paper in Nature in 1975 that was the first follow-up to the 1972 Fujishima鈭扝onda paper, which clarified some issues with the use of titanium dioxide as a photoanode in a photoelectrolysis solar fuel-producing cell. All of my early 1970s research described above and in the literature from 1975 to 1978 was carried out in the large basic corporate research laboratories of American Cyanamid and Allied Chemical. In 1978, I moved from Allied Chemical to the new DOE SERI laboratory to establish and pursue both photoelectrochemical PV and solar fuels research; I have continued this pursuit ever since at both NREL and beginning in 1999 as a Professor Adjoint and since 2012 as a Research Professor in the Department of Chemistry at CU, 麻豆官网 and since 2008 as a RASEI Fellow.
Could you describe a big obstacle you encountered in your research and how you overcame it?
Perseverance is necessary in tackling big problems. It often takes a lot of time, collaboration and effort to discover solutions to big challenges. , describes over 25 years of work on harvesting hot carriers.听
A few years after I initiated research on the photoelectrolysis of water (1974), I became interested (together with my close and now deceased collaborator Professor Ferd Williams, at the time Chair of Physics at the University of Delaware,) in the possibility that photogenerated hot carriers could be harvested in photoelectrochemical cells to greatly enhance the conversion efficiency of solar photoconversion and exceed the well-known and widely accepted detailed-balance Shockley鈭扱ueisser (S鈭扱) conversion efficiency limit (鈭�33 %) at 1 sun intensity. S鈭扱 type calculations assuming full utilization of hot carriers (zero energy loss from cooling) showed a maximum theoretical efficiency of about 66% for a single semiconductor photomaterial; this is the same value shown by the S鈭扱 analysis for a conventional (i.e., fully cooled carriers) multijunction PV cell containing >5 tandem p鈭抧 junctions of different compositions and band gaps. The realization of a hot carrier PV cell is very difficult because it requires that the rate of hot carrier interfacial charge transfer from the photoexcited semiconductor photoelectrode be faster than the rate of hot carrier cooling to the semiconductor band edges produced through electron (or hole)鈭抪honon scattering, and the latter process is generally much faster (ps to sub-ps) than interfacial charge transfer from bulk semiconductors to molecular acceptors. This then led us to the concept of slowed cooling rates of hot carriers through size quantization in quantum-confined semiconductor structures that creates relatively large separation between quantized electronic states, thus requiring simultaneous and improbable electron鈭抦any-phonon scattering events to dissipate the kinetic energy of the hot carriers; this slowed cooling process has been termed a phonon bottleneck. Subsequently (1982鈭� 1983), hot electron transfer across semiconductor鈭抦olecule interfaces was achieved with highly doped p-type photocathodes, wherein a thin (10 nm) space-charge layer is created with 1-D quantum confinement of electrons. Further work (1986鈭�1992) with III鈭扸 semiconductor superlattices, which consist of atomically flat films of multiple quantum well layers (having 1-D quantum confinement) separated by thin potential barrier layers (<4 nm) that allow interwell tunneling and miniband formation, demonstrated hot carrier cooling that was slower by 2 orders of magnitude (100s of ps) compared to bulk GaAs. The cooling was slowest when the photoexcitation intensity was very high, producing hot phonons; this effect was termed a hot phonon bottleneck.
The concept of hot carrier solar cells was extended to PV cells in the mid-1990s, and a p鈭抜鈭抧 PV cell with the i-region consisting of a superlattice also showed high electron temperature in the i-region due to slow hot carrier cooling.
However, the largest advance in approaches for the utilization of hot carriers to enhance the performance of solar cells was initiated in 1984鈭�1985 when carrier confinement in three dimensions and the associated 3-D size quantization, in the form of QDs (also termed nanocrystals), was conceived and demonstrated not only at SERI but independently and earlier in the 1980s by A. Ekimov and Al. L. Efros in Russia and by L. Brus at Bell Laboratories. The potential utility of QDs in solar cells for achieving more efficient solar photon conversion to electricity (PV) and solar fuels was fully appreciated by the end of the 1990s and has now become a quite large research field in its own right today. This and other approaches to beating the S鈭扱 limit for PV cells together with lowering their areal cost is generally termed next- or future- or third-generation PVs.
Although we found that a hot phonon bottleneck could slow hot electron cooling in a solar cell based on a semiconductor superlattice, the need for ultrahigh light intensity was a practical drawback. Furthermore, the fact that 1-D confinement in a superlattice produced electronic subbands with a dispersion in k space meant that hot carrier cooling from high quantum levels could proceed via an intersub-band transfer with a one electron鈭抩ne phonon scattering event; this produced a hot carrier in the next lowest sub-band that could cool further to the bottom of the subband via a cascade of single one electron鈭抩ne phonon scattering events. This process could repeat itself for all lower energy sub-bands until the hot carriers cooled to the lowest energy state of the system and thus were fully cooled. Slowed cooling in structures with 1-D quantum confinement and high light intensity arises because the formation of hot nonequilibrated, confined phonons coupled with the high photoexcitation intensity modifies the phonon characteristics to reduce the strength of electron鈭抪honon interactions compared to those for bulk semiconductors. However, in the 1990s, I and others recognized that 3-D confinement in QDs could produce a phonon bottleneck and slow hot carrier cooling without the need for high photoexcitation intensities (in QDs, the electron and hole charge carriers are coupled by Coulomb interactions and exist as neutral excitons); the quantized energy levels in 3-D confined structures produce no dispersion in k space, just pure discrete atomic-like levels, thus requiring hot carriers (excitons) to undergo simultaneous (and thus improbable) many-phonon鈭抏lectron interactions in order to cool. However, experimental verification of a phonon bottleneck in QDs has been controversial, with many publications showing either positive or negative support for a phonon bottleneck.
In the 1990s, it became clear to me that QDs incorporated into solar cells (for PVs or solar fuels) could greatly enhance solar conversion efficiencies and beat the S鈭扱 limit. In addition to the potential for slowed cooling, QDs were also recognized as enhancing the possibility of creating multiple electron鈭抙ole pairs (excitons in QDs) from a single absorbed photon due to a reverse Auger process driven by strong Coulomb coupling in the QDs; this predicted effect was first verified experimentally by Klimov and Schaller in 2004. The exciton multiplication process in QDs has been termed multiple exciton generation (MEG) or carrier multiplication (CM); it is a well-known process in bulk semiconductors, but there, it is termed impact ionization and involves free electrons and holes, not excitons. The forward Auger process, whereby a biexciton could undergo recombination of one exciton and transfer the recombination energy to the electron or hole of the remaining exciton, was proposed by Al. L. Efros in 1996 to explain photoluminescence blinking in QDs and is consistent with the inverse Auger process of MEG. All of the above discussed exciton dynamics, controversies, history, and potential MEG applications to solar photoconversion are reviewed and discussed in the review linked to the left.
What advice would you give to a junior researcher interested in entering the field of solar energy?
My advice to young researchers to achieve success in any field is to 鈥渢hink outside the box鈥� as much as possible. Maximize your creativity and learn to connect the dots of research progress in widely different fields that are outside of, or apparently tangential, to your own specialized area. 听This is done by stimulating your curiosity, exploring science outside of your narrow field, and reading, talking to, and listening to presentations of other scientists to the maximum extent allowed by your personal situation. Persistence in following your instincts and ideas despite setbacks is also critical - don鈥檛 allow discouragement to prevent ultimate success.
Why is it so important that we move to using renewable energy sources, in particular solar power?
The scientific evidence is now overwhelming that the present concentration of CO2听in the atmosphere (415 ppm and rising about 2 ppm/yr) 听compared to a stable value of 280 ppm at the start of the industrial revolution 100 years ago, is causing huge negative effects of climate change around the globe, and 听these effects will get progressively worse in the future unless this level of CO2听is reduced by using renewable energy to replace fossil fuel. 听Solar irradiance at the surface of the earth delivers as much energy in one hour as all the energy consumed on earth by humanity in one year (18 TW in 2020). The challenge is to implement this change from fossil energy to renewable energy at a reasonable financial cost and appropriate scale before the CO2听level reaches 450 ppm (the point of no return when the most drastic consequences of climate change are unavoidable and irreversible; 听the date of this point of no return is predicted to occur by 听about 2035 at the current and expected future increasing rate of CO2听emissions. 听
We are running out of time!