A Conversation with Art Nozik
T
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 embargo of 1973, and the initial motivation for SERI’s formation was energy securitythe 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 to be depleted by the mid-1980s. Now, because of advances in extraction technology for fossil resources, it is generally agreed that the planet has >100 years of fossil fuel in the ground. The vast majority of climate scientists have 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 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. 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’s renewable energy programs by Congress and since 1980 has led to large and unpredictable periodic swings in NREL’s annual budgets depending on the political philosophy of the elected Government Administration and Members of Congress. These were the political and funding challenges. EL. During your academic career, when and how did you became interested in renewable energy? Nozik: I first became interested in renewable energy, specifically both PVs and solar fuels (mainly solar H2O splitting for creating the so-called hydrogen economy), in the early 1970s. Regarding PVs, in 1972, I discovered a new defect semiconductor, Cd2SnO4, that had the best properties as a transparent conductor (very high transparency in the solar spectral region coupled with high conductivity) compared to any 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’s 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 DOEfunded facility. Also in 1972, following up my thesis research
oday we readily accept renewable energy as part of our energy supply chain. This was not the case 4 decades ago when fossil fuels provided a cheap form of energy. During the 1970s’ oil crisis, solar energy conversion gained prominence as a potential source of alternate energy. President Jimmy Carter initiated several renewable energy programs including the establishment of the Solar Energy Research Institute (now known as the National Renewable Energy Laboratory) in March 1977. Since then, many new initiatives have been taken to overcome our dependency on fossil fuels. Professor Art Nozik joined the efforts early on to pursue fundamental research in the area of light energy conversion using semiconductor materials. His outstanding contributions in the area of semiconductor photoelectrochemistry, tandem devices, semiconductor quantum dots (QDs), and multiple exciton generation have made a lasting impact in the field. Of particular interest is his ability to bring together physicists, chemists, and material scientists in discussing the scientific problems relevant to next-generation photovoltaics (PVs). Recently, a symposium honoring Professsor Nozik’s life-long contributions was held in Boulder Colorado (see accompanying Viewpoint (ACS Energy Lett. 2016, 1, 344−347)). I recently met Prof. Art Nozik (Figure 1) to discuss the progress made in fundamental research as well as challenges that lie ahead in tackling the clean energy challenge.
Photo courtesy of P. Kamat.
Figure 1. Prof. Art Nozik (right) with Prashant Kamat (left).
EL (ACS Energy Letters): You have been with NREL (formerly known as the Solar Energy Research Institute) since its early years. Could you please briefly describe some of the challenges you (and/ or NREL) tackled during the early years? Nozik: Before the National Renewable Energy Laboratory (NREL) (Figure 2) was officially designated as one of the 17 U.S. Department of Energy’s National Laboratories by © XXXX American Chemical Society
Received: July 13, 2016 Accepted: July 13, 2016 420
DOI: 10.1021/acsenergylett.6b00273 ACS Energy Lett. 2016, 1, 420−423
Viewpoint
http://pubs.acs.org/journal/aelccp
ACS Energy Letters
Viewpoint
Figure 2. Campus of National Renewable Energy Laboratory (NREL) at Golden, Colorado, which houses 1700 employees and over 1000 visiting scholars, postdocs and interns (Photo courtesy of NREL Photo Galleries. Source: http://www.nrel.gov/about/). Art Nozik who joined NREL in 1978 quickly became a prominent energy research leader at NREL.
conversion and exceed the well-known and widely accepted Shockley−Queisser (S−Q) conversion limit (∼32%) at 1 sun intensity. S−Q 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 about the same value shown by the S−Q analysis for a conventional (i.e., fully cooled carriers) multijunction PV cell containing >5 tandem p−n 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)−phonon scattering, and the latter process is generally much faster (ps to subps) than interfacial charge transfer from bulk semiconductors to molecular acceptors. This then led 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−many-phonon scattering events to dissipate the kinetic energy of the hot carriers; this slowed cooling process has been termed a phonon bottleneck. Subsequently (1982− 1983), hot electron transfer across semiconductor−molecule 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 (1986−1992) with III−V semiconductor superlattices, which consist of atomically flat films of multiple quantum well layers (having 1-D quantum confinement) separated by thin potential barrier layers (3 eV). Because the redox potential of the Fe3+/ Fe4+ couple is very positive (∼+1.6 V), it was clear that the positive holes photogenerated in TiO2 had very strong oxidation potentials. Then, in 1972, the famous Fujishima−Honda paper appeared in Nature, demonstrating that H2O could be photodecomposed to H2 and O2 in a photoelectrolysis cell containing a UV-illuminated TiO2 photoanode. This paper was consistent with my earlier finding of the strong oxidizing power of photoexcited TiO2. The oil embargo and energy crisis of 1973−1974 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−Honda paper, which clarified some issues with the use of TiO2 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 the industrial research laboratories to the new DOE SERI laboratory to establish and pursue both photoelectrochemical PV and solar fuels research in new DOE-funded programs; 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 and Biochemistry at the University of Colorado, Boulder. EL: What were some of the hurdles you faced during your research program and how did you overcome them? Nozik: A few years after the beginning of my research on the photoelectrolysis of H2O (1974), I became interested (together with my close and now deceased collaborator Professor Ferd Williams) in the possibility that photogenerated hot carriers could be harvested in photoelectrochemical cells to greatly enhance the conversion efficiency of solar photo421
DOI: 10.1021/acsenergylett.6b00273 ACS Energy Lett. 2016, 1, 420−423
ACS Energy Letters
Viewpoint
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 abovediscussed exciton dynamics, controversies, history, and potential MEG applications to solar photoconversion are reviewed and discussed in Annu. Rev. Phys. Chem 2001, 52, 193−231. EL: In your opinion, in what direction are semiconductor nanostructures for light energy conversion headed now and what are the unresolved issues? Could you please identif y one or two major issues? Nozik: Thermodynamic analyses show that the maximum theoretical conversion efficiency (ηmax) at 1 sun for QD PV solar cells exhibiting perfect MEG is 43−44% for a quantized band gap of 0.7−1.0 eV using a single photomaterial; to reach this efficiency, the threshold photon energy for MEG to begin is twice the band gap energy (Eg), and for photon energies above the MEG threshold, N excitons must be generated when the photon energy is N × Eg, where N is an integer (this is a staircase MEG characteristic) (see J. Appl. Phys. 2006, 100, 074510). Furthermore, when MEG is combined with solar concentration, ηmax increases dramatically and, for example, can reach 75% at a solar concentration of 500× using much smaller band gaps (0.18 eV) (see J. Phys. Chem. Lett. 2012, 3, 2857. Finally, nanocrystalline shape matters, for example, quantum rods, show better MEG characteristics than QDs because of enhanced Coulomb coupling in the rods. To date, the ideal MEG characteristics have not been achieved for solar photon conversion. Thus, future research on semiconductor nanostructures should (1) focus on achieving the MEG staircase characteristic with a MEG threshold of 2Eg and with the optimum band gaps for PV and solar fuels, (2) explore the effects of solar concentration combined with MEG in nanocrystals with small Eg, and (3) study the effects of nanocrystal shape on MEG performance in solar cells. Other critical issues to address with future research are to (4) study different nanocrystal materials, architectures, and nanocrystal surfaces to minimize surface recombination in solar cells (both above and below the MEG threshold photon energy), (5) ensure long-term photostability, and (6) continue investigating the promising new results on modifying nanocrystal surfaces with different ligands to control the nanocrystal band edge positions and enhance MEG quantum yields, in order to optimize PV cells and cells for solar fuel production. EL: The cost of silicon-based PVs has dropped exponentially over the last 40 years, thus creating a huge benef it to the development of new PV technology. In your opinion, f rom where do you see the next breakthrough in transformative research coming? Nozik: While there has indeed been great progress in reducing the cost of PV power, such that PV panels are now producing electricity at grid parity costs (∼$0.10/kWh nationally averaged compared to ∼$8/kWh in 1977 when SERI was created), the technological progress over the past 40 years in producing solar fuels at sufficiently low costs to make an impact in the global fuel economy has not been good (see, for example, J. Phys. Chem. Lett. 2015, 6, 1917−1918). While a
However, the largest advance in approaches for the utilization of hot carriers to enhance the performance of solar cells was initiated in 1984−1985 when carrier confinement in three dimensions and the associated 3-D size quantization, in the form of QDs (also termed nanocrystals), was conceived and demonstratednot 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−Q limit for PV cells together with lowering their areal cost is generally termed next- or future- or third-generation PVs. EL: Your research involving semiconductor PVs and QDs has signif icantly impacted a large portion of the foref ront energy research that is being conducted today. What led you to recognize the potential signif icance of these scientif ic concepts early in your career? Nozik: 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−one 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−one phonon scattering events. This process could repeat itself for all lowerenergy sub-bands until the hot carriers cooled to the lowestenergy 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−phonon 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−electron 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−Q limit. In addition to the potential for slowed cooling, QDs were also recognized as enhancing the possibility of creating multiple electron−hole 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 422
DOI: 10.1021/acsenergylett.6b00273 ACS Energy Lett. 2016, 1, 420−423
ACS Energy Letters
Viewpoint
Biography
vibrant PV industry is presently the fastest growing optoelectronic industry in the world, growing very rapidly at over 30%/yr and which will have produced ∼300 GW of peak power by the end of 2016 (equivalent to about 60−70 1 GWsized coal-fired or nuclear power plants), there is currently no commercial industry for solar fuels production (biofuels from biomass conversion is not considered here; only direct solar photon conversion using photoelectrochemical or photochemical cells is considered). This is a major issue for the large-scale implantation of renewable energy because about 60−70% of global energy consumption is in the form of liquid and gaseous fuel, electricity being 30−40%. Thus, future research and development in solar photoconversion should be
Arthur J. Nozik is Research Professor in the Department of Chemistry at the University of Colorado, Boulder and a Senior Research Fellow Emeritus at the National Renewable Energy Laboratory. Nozik received a B.Ch.E. from Cornell (1959) and a Ph.D. in Physical Chemistry from Yale (1967). His recent research interests include size quantization effects in semiconductor nanocrystals (including multiple exciton generation (MEG) from a single photon); advanced approaches to solar photoconversion to electricity and solar fuels; photogenerated carrier relaxation dynamics in semiconductor structures; and photoelectrochemical energy conversion at semiconductor−molecule interfaces. He has published over 250 papers and book chapters in these fields, written or edited 7 books, and delivered over 365 invited talks at universities and conferences. He has received many awards, including the Yale Cross Medal, Gerischer Award, Eni Award, and ECS Research Award. Dr. Nozik has been a Senior Editor of The Journal of Physical Chemistry for 12 years and served on editorial boards of many journals. A Festschrift Issue of The Journal of Physical Chemistry B honoring Dr. Nozik’s scientific career appeared in 2006. He is a Fellow of the APS, AAAS, and RSC; he is also a member of the ACS, ECS, and MRS. (Photo courtesy of NREL Photo Galleries. Source: http://www.nrel. gov/about/.)
focused on efficient and low-cost solar fuels; this area is where breakthroughs are required. EL. What is your advice to young scientists who are trying to make a major contribution in energy research today? Nozik: Despite the fast growing volume of literature in the area of PVs and solar fuels, it is very important for new and/or young researchers to read and become knowledgeable about the older literature in these areas that was produced starting in the late 1970s. I see many instances of new researchers “reinventing the wheel”. There are many older publications that contain concepts, fundamental understanding, and results related to advancing the basic science and applications of PVs and solar fuels that are unknown to a significant fraction of today’s researchers that are active in this area. The field of PV and solar fuels R&D is extremely important for the future welfare of our planet, and the scientific and technological challenges and rewards are great. It is also great that so many new and/or young scientists and engineers around the world are devoting their careers to the PV and solar fuels aspects of renewable energy, but to be most effective, they should devote time and energy to become fully educated about the older as well as more recent history of these fields. Finally, it must be noted that near-term breakthroughs are urgent because we are running out of time regarding our ability to ameliorate the severest consequences of climate change (see, for example, Science 2013, 339, 280−282).
Prashant V. Kamat, Editor-in-Chief, ACS Energy Letters University of Notre Dame, Notre Dame, Indiana 46556, United States
■
AUTHOR INFORMATION
Notes
Views expressed in this Viewpoint are those of the author and not necessarily the views of the ACS. The author declares no competing financial interest. 423
DOI: 10.1021/acsenergylett.6b00273 ACS Energy Lett. 2016, 1, 420−423
