Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin film metal oxide photoanodes for solar water splitting

Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, xi lamba , represents the wavelength dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract xi lamba through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo oxidation, and observed that it is self consistent for different illumination conditions and applied potentials. We found agreement between the extracted xi lamba spectrum and the photoconductivity spectrum measured by time resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite s absorption spectrum. Low energy non contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO2 and BiVO4 photoanodes, demonstrating the broader utility of the method for determining xi lamb

Author Correction Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin film metal oxide photoanodes for solar water splitting

Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, xi lamba , represents the wavelength dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract xi lamba through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo oxidation, and observed that it is self consistent for different illumination conditions and applied potentials. We found agreement between the extracted xi lamba spectrum and the photoconductivity spectrum measured by time resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite s absorption spectrum. Low energy non contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO2 and BiVO4 photoanodes, demonstrating the broader utility of the method for determining xi lamb

External Quantum Efficiency Spectra of BiVO4 Thin Film Photoanodes under Bias Illumination

External quantum efficiency EQE of bismuth vanadate thin film photoanodes, measured in a pH 7 potassium phosphate buffer solution with sodium sulfite hole scavenger, was observed to substantially decrease when measured under white light bias LB . While the EQE exhibited a fast initial decrease across its full spectral range, a amp; 8764;3.5 eV 350 nm feature under front illumination conditions became disproportionally suppressed after being under LB strongest when it is also incident on the front side of the sample for several tens of minutes, in spite of this wavelength being outside the spectral range encompassed by the LB source. Applied potential does not have a strong effect on the qualitative behavior. From its different decay time, the wavelength specific decrease of the 3.5 eV feature, and its responsible mechanism, is distinct from the initial, spectrally uniform decrease of EQE, which happens at a faster timescale and is similar for all illumination conditions. To more closely examine the suppression of the 3.5 eV feature, we compare calculated depth dependent optical generation profiles and behaviors under different illumination conditions, which imply the involvement of in gap states and long lived states deeper into the conduction or alternatively, valence band. Possible mechanisms are discusse

The Spatial Collection Efficiency of Charge Carriers in Photovoltaic and Photoelectrochemical Cells

The spatial collection efficiency portrays the driving forces and loss mechanisms in photovoltaic and photoelectrochemical devices. It is defined as the fraction of photogenerated charge carriers created at a specific point within the device that contribute to the photocurrent. In stratified planar structures, the spatial collection efficiency can be extracted out of photocurrent action spectra measurements empirically, with few a priori assumptions. Although this method was applied to photovoltaic cells made of well-understood materials, it has never been used to study unconventional materials such as metal-oxide semiconductors that are often employed in photoelectrochemical cells. This perspective shows the opportunities that this method has to offer for investigating new materials and devices with unknown properties. The relative simplicity of the method, and its applicability to operando performance characterization, makes it an important tool for analysis and design of new photovoltaic and photoelectrochemical materials and devices. Understanding the optoelectronic and transport properties of semiconductors is essential for producing high-efficiency photovoltaic and photoelectrochemical cells. To this end, empirical extraction of the spatial collection efficiency (i.e., the fraction of photogenerated charge carriers created at a specific point within the device that contribute to the photocurrent) is a useful, nondestructive, analytical tool to study new materials, junctions, and devices. This perspective describes how the spatial collection efficiency can be extracted by combining photocurrent action spectra with optical absorption profiles. The result is high-resolution depth profiles of device functionality with very few assumptions, which paves the way to operando semiconductor tomography. The challenges and opportunities that this method offers for analysis of complex materials are discussed. Since the method is based on widely used spectral response measurements, it can be an important addition to the toolbox of analytical methods for material research for future solar energy conversion systems. Quantifying transport and loss mechanisms is a key step in producing high-efficiency solar energy conversion and storage devices. Empirical extraction of the spatial collection efficiency under real operating conditions is a useful analytical tool for studying such processes. We describe how the spatial collection efficiency can be extracted and apply the method to a α-Fe2O3 water splitting photoanode. The extracted spatial collection efficiency profiles provide significant insights on driving forces and the advanced optoelectronic properties of α-Fe2O3

The Spatial Collection Efficiency of Charge Carriers in Photovoltaic and Photoelectrochemical Cells

The spatial collection efficiency portrays the driving forces and loss mechanisms in photovoltaic and photoelectrochemical devices. It is defined as the fraction of photogenerated charge carriers created at a specific point within the device that contribute to the photocurrent. In stratified planar structures, the spatial collection efficiency can be extracted out of photocurrent action spectra measurements empirically, with few a priori assumptions. Although this method was applied to photovoltaic cells made of well-understood materials, it has never been used to study unconventional materials such as metal-oxide semiconductors that are often employed in photoelectrochemical cells. This perspective shows the opportunities that this method has to offer for investigating new materials and devices with unknown properties. The relative simplicity of the method, and its applicability to operando performance characterization, makes it an important tool for analysis and design of new photovoltaic and photoelectrochemical materials and devices. Understanding the optoelectronic and transport properties of semiconductors is essential for producing high-efficiency photovoltaic and photoelectrochemical cells. To this end, empirical extraction of the spatial collection efficiency (i.e., the fraction of photogenerated charge carriers created at a specific point within the device that contribute to the photocurrent) is a useful, nondestructive, analytical tool to study new materials, junctions, and devices. This perspective describes how the spatial collection efficiency can be extracted by combining photocurrent action spectra with optical absorption profiles. The result is high-resolution depth profiles of device functionality with very few assumptions, which paves the way to operando semiconductor tomography. The challenges and opportunities that this method offers for analysis of complex materials are discussed. Since the method is based on widely used spectral response measurements, it can be an important addition to the toolbox of analytical methods for material research for future solar energy conversion systems. Quantifying transport and loss mechanisms is a key step in producing high-efficiency solar energy conversion and storage devices. Empirical extraction of the spatial collection efficiency under real operating conditions is a useful analytical tool for studying such processes. We describe how the spatial collection efficiency can be extracted and apply the method to a α-Fe2O3 water splitting photoanode. The extracted spatial collection efficiency profiles provide significant insights on driving forces and the advanced optoelectronic properties of α-Fe2O3

Electronic excitations of alpha Fe2O3 heteroepitaxial films measured by resonant inelastic x ray scattering at the Fe L edge

Resonant inelastic x ray scattering RIXS spectra of hematite amp; 945; Fe2O3 were measured at the Fe L3 edge for heteroepitaxial thin films which were undoped and doped with 1 Ti, Sn, or Zn, in the energy loss range in excess of 1 eV to study electronic transitions. The spectra were measured for several momentum transfers q, conducted at both low temperature T 14 K and room temperature. While we cannot rule out dispersive features possibly owing to propagating excitations, the coarse envelopes of the general spectra did not appreciably change shape with q, implying that the bulk of the observed L edge RIXS intensity originates from mostly nondispersive ligand field excitations. Summing the RIXS spectra over q and comparing the results at T 14 K to those at T 300 K revealed pronounced temperature effects, including an intensity change and energy shift of the amp; 8776;1.4 eV peak, a broadband intensity increase of the 3 4 eV range, and higher energy features. The q summed spectra and their temperature dependencies are virtually identical for nearly all of the samples with different dopants, save for the temperature dependence of the Ti doped sample s spectrum, which we attribute to being affected by a large number of free charge carriers. Comparing with magnetization measurements for different temperatures and dopings likewise did not show a clear correlation between the RIXS spectra and the magnetic ordering states. To clarify the excited states, we performed spin multiplet calculations which were in excellent agreement with the RIXS spectra over a wide energy range and provide detailed electronic descriptions of the excited states. The implications of these findings to the photoconversion efficiency of hematite photoanodes is discusse

Electronic excitations of α- Fe2 O3 heteroepitaxial films measured by resonant inelastic x-ray scattering at the Fe L edge

Resonant inelastic x-ray scattering (RIXS) spectra of hematite (α-Fe2O3) were measured at the Fe L3 edge for heteroepitaxial thin films which were undoped and doped with 1% Ti, Sn, or Zn, in the energy-loss range in excess of 1 eV to study electronic transitions. The spectra were measured for several momentum transfers q, conducted at both low temperature (T=14 K) and room temperature. While we cannot rule out dispersive features possibly owing to propagating excitations, the coarse envelopes of the general spectra did not appreciably change shape with q, implying that the bulk of the observed L-edge RIXS intensity originates from (mostly) nondispersive ligand field excitations. Summing the RIXS spectra over q and comparing the results at T=14 K to those at T=300 K revealed pronounced temperature effects, including an intensity change and energy shift of the ≈1.4 eV peak, a broadband intensity increase of the 3-4 eV range, and higher energy features. The q-summed spectra and their temperature dependencies are virtually identical for nearly all of the samples with different dopants, save for the temperature dependence of the Ti-doped sample's spectrum, which we attribute to being affected by a large number of free charge carriers. Comparing with magnetization measurements for different temperatures and dopings likewise did not show a clear correlation between the RIXS spectra and the magnetic ordering states. To clarify the excited states, we performed spin multiplet calculations which were in excellent agreement with the RIXS spectra over a wide energy range and provide detailed electronic descriptions of the excited states. The implications of these findings to the photoconversion efficiency of hematite photoanodes is discussed