Final Report Summary - SOLAR FUEL BY III-VS (Direct photoelectrochemical generation of solar fuels using dilute nitride III-V compound semiconductor heterostructures on silicon: epitaxy, electrochemistry, and interface characterization)
The research programme of the Marie Curie International Outgoing Fellowship ‘Solar Fuel by III-Vs’ (Project No.: 300971) addresses crucial issues of fundamental and technological importance in the field of solar fuel generation. The key success factor is a strong multidisciplinary approach combining top-level electrochemistry, high-end epitaxial III-V device preparation and cutting-edge surface science analytics. At the core of the project are the objectives of:
► obtaining record solar to hydrogen efficiencies and lifetimes with III-V(/Si) tandems.
► enhancing III-V device stability in contact with electrolyte using nitrogen.
► advancing the scientific understanding of the decomposition of III-Vs.
After exploring heteroepitaxial integration of dilute nitride III-V top absorber material on active Si substrates as a principally attractive tandem combination for photoelectrochemical (PEC) water splitting, the project broadened and switched its focus to all III-V tandem designs as well as their PEC performance and durability. Systematic investigation of nitrogen ion bombardment and metal co-catalyst sputter deposition led to significant device lifetime extensions. Important modelling work recognized the major influence of parasitic sunlight absorption during illumination through the PEC electrolyte as a key factor determining limiting efficiencies as well as optimum band gap combinations. An analysis of the spectral mismatch between typical laboratory light sources and solar irradiance resulted in the insight of insufficient accuracy of any previous tandem PEC work most probably causing major overestimations in solar to hydrogen (STH) conversion efficiencies in existing studies up to this date. For the realization of optimum tandem configurations the project generated major progress by establishing inverted metamorphic device growth and processing schemes for solar fuel generation.
The research results achieved within the project duration include:
• First heteroepitaxial dilute nitride GaPN/Si(100) device structures.
• Parameter space for a combined nitrogen ion bombardment and metal co-catalyst sputtering GaInP surface protection scheme enhancing water splitting device durability.
• Realistic detailed balance limiting efficiency and optimum band gap combinations under consideration of sunlight absorption in PEC electrolytes.
• Discovery of the technologically important relation between electrolyte film thickness and cumulated overvoltage loss limiting the solar to hydrogen conversion efficiency.
• Impact of the spectral mismatch of laboratory illumination source with solar irradiance on PEC performance characterization.
• First accurate tandem PEC solar to hydrogen conversion efficiency characterization scheme.
• Development of inverted metamorphic device growth and processing schemes for solar fuel generation.
• First STH conversion efficiency optimized PEC tandem design.
• World-record STH conversion efficiency with inverted metamorphic tandem devices.
Among all achievements the following two aspects highlight the significant scientific and technological impact of the results which may turn into major economic and societal milestones when solar fuel technology is consequently advanced towards the application level.
Accurate tandem STH efficiency characterization: The fellow recognized the crucial impact of the illumination source spectral distribution on photoelectrochemical (PEC) device characterization results, inducing:
(i) strong sub-absorber current misdistribution not at all compatible with the solar spectrum;
(ii) misjudgment of relative performances inducing incorrect conclusion for further device development; and
(iii) strong trends towards overrating tandem device solar to hydrogen (STH) conversion efficiency spread throughout the PEC community and reported literature results of the past.
A high-impact manuscript describing the issue, demonstrating its magnitude, and proposing corrective actions and advanced accuracy characterization protocols is about to be submitted.
World-record IMM PEC devices: The new line of research on inverted metamorphic (IMM) tandem PEC devices initiated by the fellow enabled a disruptive approach and resulted in significant advances in STH conversion efficiency. While we were able to show that classical upright GaInP/GaAs tandem designs will hardly be able to achieve 10% (despite a probably overrated world record of 12.4%), the inverted design inherently improves bottom cell performance by providing an effective Au back reflector (ca. +2%). Advanced light distribution by top cell thinning will enable an additional performance gain of about +2%, but disruptive advances are achieved with metamorphic reduction of the bottom cell band gap (ca. 5%) alone, on the long run a slight decrease of the top absorber band gap (accompanied with further bottom cell band gap reduction) will enable even higher efficiencies.
In summary, the described findings above have major scientific impact on the photoelectrochemistry community accelerating the development of solar fuel generation for a future renewable energy economy and sustainable society. The project derived realistic limiting efficiencies and associated tandem band gap combinations to aspire to; defined a novel, accurate scheme for solar to hydrogen efficiency characterization providing means for a fair comparison of achieved results; and developed inverted metamorphic III-V tandem and growth processing for the actual realization of high quality tandem structures – immediately exceeding the long-lasting PEC efficiency world-record figure by far.
► obtaining record solar to hydrogen efficiencies and lifetimes with III-V(/Si) tandems.
► enhancing III-V device stability in contact with electrolyte using nitrogen.
► advancing the scientific understanding of the decomposition of III-Vs.
After exploring heteroepitaxial integration of dilute nitride III-V top absorber material on active Si substrates as a principally attractive tandem combination for photoelectrochemical (PEC) water splitting, the project broadened and switched its focus to all III-V tandem designs as well as their PEC performance and durability. Systematic investigation of nitrogen ion bombardment and metal co-catalyst sputter deposition led to significant device lifetime extensions. Important modelling work recognized the major influence of parasitic sunlight absorption during illumination through the PEC electrolyte as a key factor determining limiting efficiencies as well as optimum band gap combinations. An analysis of the spectral mismatch between typical laboratory light sources and solar irradiance resulted in the insight of insufficient accuracy of any previous tandem PEC work most probably causing major overestimations in solar to hydrogen (STH) conversion efficiencies in existing studies up to this date. For the realization of optimum tandem configurations the project generated major progress by establishing inverted metamorphic device growth and processing schemes for solar fuel generation.
The research results achieved within the project duration include:
• First heteroepitaxial dilute nitride GaPN/Si(100) device structures.
• Parameter space for a combined nitrogen ion bombardment and metal co-catalyst sputtering GaInP surface protection scheme enhancing water splitting device durability.
• Realistic detailed balance limiting efficiency and optimum band gap combinations under consideration of sunlight absorption in PEC electrolytes.
• Discovery of the technologically important relation between electrolyte film thickness and cumulated overvoltage loss limiting the solar to hydrogen conversion efficiency.
• Impact of the spectral mismatch of laboratory illumination source with solar irradiance on PEC performance characterization.
• First accurate tandem PEC solar to hydrogen conversion efficiency characterization scheme.
• Development of inverted metamorphic device growth and processing schemes for solar fuel generation.
• First STH conversion efficiency optimized PEC tandem design.
• World-record STH conversion efficiency with inverted metamorphic tandem devices.
Among all achievements the following two aspects highlight the significant scientific and technological impact of the results which may turn into major economic and societal milestones when solar fuel technology is consequently advanced towards the application level.
Accurate tandem STH efficiency characterization: The fellow recognized the crucial impact of the illumination source spectral distribution on photoelectrochemical (PEC) device characterization results, inducing:
(i) strong sub-absorber current misdistribution not at all compatible with the solar spectrum;
(ii) misjudgment of relative performances inducing incorrect conclusion for further device development; and
(iii) strong trends towards overrating tandem device solar to hydrogen (STH) conversion efficiency spread throughout the PEC community and reported literature results of the past.
A high-impact manuscript describing the issue, demonstrating its magnitude, and proposing corrective actions and advanced accuracy characterization protocols is about to be submitted.
World-record IMM PEC devices: The new line of research on inverted metamorphic (IMM) tandem PEC devices initiated by the fellow enabled a disruptive approach and resulted in significant advances in STH conversion efficiency. While we were able to show that classical upright GaInP/GaAs tandem designs will hardly be able to achieve 10% (despite a probably overrated world record of 12.4%), the inverted design inherently improves bottom cell performance by providing an effective Au back reflector (ca. +2%). Advanced light distribution by top cell thinning will enable an additional performance gain of about +2%, but disruptive advances are achieved with metamorphic reduction of the bottom cell band gap (ca. 5%) alone, on the long run a slight decrease of the top absorber band gap (accompanied with further bottom cell band gap reduction) will enable even higher efficiencies.
In summary, the described findings above have major scientific impact on the photoelectrochemistry community accelerating the development of solar fuel generation for a future renewable energy economy and sustainable society. The project derived realistic limiting efficiencies and associated tandem band gap combinations to aspire to; defined a novel, accurate scheme for solar to hydrogen efficiency characterization providing means for a fair comparison of achieved results; and developed inverted metamorphic III-V tandem and growth processing for the actual realization of high quality tandem structures – immediately exceeding the long-lasting PEC efficiency world-record figure by far.