Photophysical Characterization of Photoelectrochemical Materials and Assemblies
LaboratoryLawrence Berkeley National Laboratory (LBNL)
Capability ExpertJason Cooper
Node Readiness Category2: Low-Temperature Electrolysis (LTE)
2: Photoelectrochemical (PEC)
Characterization Equipment and Expertise
A suite of complimentary optical spectroscopic methods that are specifically adapted for determining photophysical characteristics of photoelectrochemical materials and assemblies under operational conditions. The spectroscopic tools include in situ electrochemical spectroscopic ellipsometry (SE), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), photoluminescence spectroscopy (PL), and pump-probe transient absorption and reflection spectroscopy (TA and TR). The combination of these tools, along with the associated expertise for their application and analysis, provides direct information regarding optoelectronic and structural properties, (photo)chemical transformations under operational conditions, and efficiency-determining competitions between chemical reaction and photocarrier recombination over broad spectral and temporal ranges. Measurement cells have been specially developed to allow in situ and operando characterization under electrochemical and optical bias using all methods, and additional capabilities for ex situ measurements as a function of temperature and controlled ambient are also available. A brief overview of each capability is provided below:
- Spectroscopic ellipsometry: Under ex situ condition, this capability is suited for determining optical functions of newly discovered materials (semiconductors, catalysts, protective coatings, membranes), optical interactions of defects, and thicknesses of thin films, from the UV to the NIR spectral range. A heating and cooling stage (77-773 K) is available for tracking materials during thermal processing, thereby allowing in situ evaluation of the impacts of phase transformations, composition evolution, and interfacial reactions on optical characteristics and thicknesses. This capability can also be used as an environmental cell to characterize water uptake in membranes as a function of temperature and humidity and thermal reduction and oxidation reactions of solid state materials. A (photo)electrochemical cell provides opportunity to probe optical and physical property changes of electrified solid/electrolyte interfaces in quasi-darkness or under bias illumination, thereby allowing electrochromic, (photo)corrosion, and phase transformations to be characterized. Data collection times of ~1s mean that transient changes of materials as a function of bias and illumination can be established. Data from these measurements can aid in understanding of mechanisms associated with transduction of solar to chemical energy, as well as for identifying spectral features from other optical probes.
- Raman spectroscopy: A confocal Raman spectrometer is equipped with Teflon-coated immersion lenses and an electrochemical cell for probing chemical transformations of electrochemical and photoelectrochemical systems under operational conditions. Raman mapping under ex situ conditions is suited for analyzing phase segregation, as well as localized failure, of (photo)electrochemical systems.
- Fourier transform infrared spectroscopy: A time resolved infrared spectrometer, with accessories for attenuated total internal reflection, transmission, and variable angle reflection spectroscopies allows access to vibrational and free-carrier responses in materials on time scales ranging from ns to s. The spectrometer has detectors, sources, and beamsplitters for operation in the far- to mid-IR spectral range. Pulsed lasers can be synchronized with the spectrometer for tracking the evolution of photoexcited responses in time and tracking (photo)catalytic mechanisms.
- Photoluminescence spectroscopy: Steady state and time resolved photoluminescence, by TCSPC, measurements can be performed with photoexcitation over the visible range and detection from the UV to IR spectral range. Time resolution down to 100 ps is available. Measurements can be performed under operational photoelectrochemical conditions, allowing for characterization of radiative band-to-band and sub-gap defect recombination. Photoluminescence quenching due to charge extraction and interface recombination can be probed as a function of electrochemical bias or, in ex situ configuration, as a function of temperature from 10 K to 500 K.
- Pump-probe optical spectroscopy: Three pump-probe setups allow characterization of photocarrier dynamics continuously from sub-ps to s time scales and over UV-NIR spectral ranges and in transmission or reflection geometry. Measurement cells for operando spectroscopy are compatible with all setups and can be coupled to a dedicated potentiostat, thereby enabling direct probes of excited state spectra and their evolution to the ground state under electrochemical bias. In ex situ configuration, measurements can be performed as a function of temperature from 10 K to 500 K.
Spatial Collection Efficiency Extraction
Defined as the fraction of photogenerated charge carriers created at a specific point within the device that eventually contribute to the collected photocurrent, measurement of the spatial collection efficiency (SCE) can shed new light on different charge transport mechanisms and aid in device optimization. This non-destructive empirical method combines incident photon to current efficiency (IPCE) measurements with optical modeling of light absorption within the device. The result of the analysis is a z-axis depth profile of the photogenerated charge which contributed to collected current. To date, diffusion lengths of 10 nm have been resolved by this approach. This analysis is commonly performed as a function of applied potential to examine the modification of the space charge region as well as to distinguish between bulk and surface losses. Analysis of the SCE profile can reveal significant information on charge transport and loss mechanism in the material. With further development, the method will to enable us to study electrodes with various topography.
Systems can be broadly adapted and suitability must be determined on a case-by-case basis.
Capability is custom constructed for in situ and operando characterization of (photo)electrochemical materials and assemblies with high spectral and time resolution. Custom experimental setups and measurement cells have been developed for the express purpose of characterizing systems under relevant operational conditions. The combined suite of techniques provides insight into competitions between photocarrier recombination and reaction, charge transfer and trapping pathways, mechanisms of energy transduction.
Facility is in use for JCAP's program but can accommodate select additional projects. Use of custom, modified, and laser systems requires significant training by experienced personnel and careful attention to laser safety protocols. Given the considerable time investment by LBNL staff, salary support and/or long-term collaborative partnerships are required.
The capability allows characterization of key processes, including bulk and interfacial recombination, structural transformations, and (photo)chemical instabilities in functional photoelectrochemical assemblies. Vibrational spectroscopy allows characterization of phase transformations of catalysts and photoelectrochemical systems that occur between resting and active states. The combined suite of photophysical and vibrational spectroscopy tools provides opportunities for understanding mechanisms of energy transduction, identification of key factors contributing to efficiency loss, and determination of failure modes.
L.H. Hess, J.K. Cooper, A. Loiudice, C.-M. Jiang, R. Buonsanti, & I.D. Sharp, Probing interfacial energetics and charge transfer kinetics in semiconductor nanocomposites: New insights into heterostructured TiO2/BiVO4 photoanodes, submitted (2016).
J.K. Cooper, S.B. Scott, Y. Ling, J. Yang, S. Hao, Y. Li, F.M. Toma, M. Stutzmann, K.V. Lakshmi, & I.D. Sharp, Role of hydrogen in defining the n-type character of BiVO4, Chem. Mater. 28, 5761 (2016).
Y. Li, J.K. Cooper, W. Liu, C. Sutter-Fella, J.W. Beeman, A. Javey, J.W. Ager, F.M. Toma, & I.D. Sharp, Defective TiO2 with high photoconductive gain for efficient and stable planar heterojunction perovskite solar cells, Nature Comm. 7, 12446 (2016).
A. Loiudice, J. K. Cooper, L.H. Hess, T. Mattox, I.D. Sharp, & R. Buonsanti, Assembly and photocarrier dynamics of heterostructured nanocomposite photoanodes from multicomponent colloidal nanocrystals, Nano Lett. 15, 7347 (2015).
L. Chen, J. Yang, S. Klaus, L.J. Lee, R. Woods-Robinson, Y. Lum, J.K. Cooper, F.M. Toma, I.D. Sharp, A.T. Bell, & J.W. Ager, P-type transparent conducting oxide/n-type semiconductor heterojunctions for efficient and stable solar water oxidation, J. Am. Chem. Soc. 137, 9595 (2015).
J.K. Cooper, S. Gul, F.M. Toma, L. Chen, Y.-S. Liu, J. Guo, J.W. Ager, J. Yano, & I.D. Sharp, Indirect bandgap and optical properties of monoclinic bismuth vanadate, J. Phys. Chem. C, 119, 2969 (2015).
Y. Li, J.K. Cooper, R. Buonsanti, C. Giannini, Y. Liu, F.M. Toma, & I.D. Sharp, Fabrication of planar heterojunction perovskite solar cells by controlled low-pressure vapor annealing, J. Phys. Chem. Lett., 6, 493 (2015).
M.D. Sampson, J.D. Froehlich, J.M. Smieja, E.E. Benson, I.D. Sharp, & C.P. Kubiak, Direct observation of the reduction of carbon dioxide by rhenium bipyridine catalysts, Energy Environ. Sci., 6, 3748 (2013).
M. W. Louie and A. T. Bell, An investigation of thin-film N-Fe oxide catalysts for the electrochemical evolution of oxygen, J. Am. Chem. Soc.135, 12329 (2103).
S. Klaus, M. W.Louie, L. Trotochaud and A. T. Bell, Role of catalyst preparation on the electrocatalytic activity of Ni1-x FexOOH for the oxygen evolution reaction. J. Phys. Chem. C 119, 18303 (2015).
G. Segev, C.-M. Jiang, J. K. Cooper and I. D. Sharp, Quantification of the loss mechanisms in emerging water splitting photoanodes through empirical extraction of the spatial charge collection efficiency. Energy Environ. Sci., 10.1039/C7EE03486E (2018)
G. Segev, H. Dotan, D. Ellis, I. Piekner, D. Klotz, J. Beeman, J. K. Cooper, D. Grave, I. Sharp and A. Rothschild, The spatial collection efficiency of charge carriers in photovoltaic and photoelectrochemical cells. Joule, 2, 210 (2018).