Periodic Reporting for period 5 - PAIDEIA (PlAsmon InduceD hot Electron extraction with doped semiconductors for Infrared solAr energy)
Reporting period: 2025-05-01 to 2025-09-30
The PAIDEIA project stands for 'plasmon-induced hot electron extraction with semiconductors doped for infrared solar energy'.
The project's assumption is that almost half of the radiation from the Sun is not in the visible region, but in the infrared. This part of the solar irradiance is not absorbed by most commercial photovoltaic cells, with some very expensive exceptions. The idea behind PAIDEIA is to use non-toxic nanomaterials based on elements abundant on Earth, capable of effectively absorbing the infrared part of solar irradiation. We call these materials "doped semiconductor nanocrystals". If we create a junction between these nanomaterials and semiconductors that are also based on elements abundant on Earth, such as titanium dioxide or tin oxide, we can extract charges from doped semiconductor nanocrystals photogenerated by solar irradiation (hot electrons) and these charges can be collected by the electrodes and produce a photocurrent. This type of photovoltaic cell is based on a completely different physical phenomenon than conventional photovoltaic cells.
Within the framework of the PAIDEIA project, the principal investigator and the research team have achieved highly promising results. For the first time, the team observed the transfer of hot electrons from indium tin oxide (ITO) nanocrystals to molybdenum disulfide (MoS2). This ITO/MoS2 heterostructure represents an unprecedented configuration compared to conventional hot-electron junctions involving classical metals and semiconductors. Its uniqueness lies in its ability to absorb infrared radiation, thereby harvesting a portion of the solar spectrum that is not captured by traditional photovoltaic materials or metals such as silver and gold, which primarily absorb visible light.
A particularly noteworthy finding is the absorption by indium tin oxide nanocrystals at 1750 nm—a region well within the infrared spectrum, which begins above 800 nm. This spectral range corresponds to a portion of solar radiation where very few materials exhibit absorption, and those that do are typically expensive and/or toxic. Current photocurrent measurements, despite the complexity of the heterostructure (only a few nanometers thick), are yielding encouraging results. Consequently, the project has delivered significant advances in the fabrication of nanocomposite films and their spectroscopic and optoelectronic characterization.
An important development within PAIDEIA was the innovative preparation of films based on ITO nanocrystals. Through a annealing-based procedure applied after film deposition, the electrical resistance of these films was modulated by three orders of magnitude. This control was achieved through a systematic study of the annealing time and temperature of the nanostructured ITO films. Furthermore, parametric amplifiers extending into the infrared range were assembled for the optical characterization of PAIDEIA materials—an achievement unprecedented in the scientific community. These amplifiers enable the generation of ultrashort light pulses, in the order of tens of femtoseconds, with wavelengths spanning from approximately one micrometer to beyond ten micrometers. This range encompasses nearly all vibrational fingerprints of organic compounds and biologically relevant materials.
From a condensed matter physics perspective, the project also facilitated the study of electron-hole pairs between monolayers of various transition metal dichalcogenides. Given the complexity of the ITO/MoS2 heterostructure, a prototype solar cell is currently under development to assess both photocurrent and current–voltage characteristics. Many experimental findings have been corroborated using state-of-the-art computational tools. While some theoretical contributions were provided through international collaborations, the research team frequently developed proprietary algorithms to predict and model the optical and optoelectronic behavior of the studied structures.
PAIDEIA has fostered collaboration among young scientists from diverse countries and disciplines—including physics, chemistry, and materials science—enabling them to investigate innovative materials and their junctions from both theoretical and experimental perspectives. The project has also catalyzed outstanding initiatives led by early-career researchers, resulting in prestigious funding awards such as ERC Starting Grants and MSCA Global Fellowships.
The project's assumption is that almost half of the radiation from the Sun is not in the visible region, but in the infrared. This part of the solar irradiance is not absorbed by most commercial photovoltaic cells, with some very expensive exceptions. The idea behind PAIDEIA is to use non-toxic nanomaterials based on elements abundant on Earth, capable of effectively absorbing the infrared part of solar irradiation. We call these materials "doped semiconductor nanocrystals". If we create a junction between these nanomaterials and semiconductors that are also based on elements abundant on Earth, such as titanium dioxide or tin oxide, we can extract charges from doped semiconductor nanocrystals photogenerated by solar irradiation (hot electrons) and these charges can be collected by the electrodes and produce a photocurrent. This type of photovoltaic cell is based on a completely different physical phenomenon than conventional photovoltaic cells.
Within the framework of the PAIDEIA project, the principal investigator and the research team have achieved highly promising results. For the first time, the team observed the transfer of hot electrons from indium tin oxide (ITO) nanocrystals to molybdenum disulfide (MoS2). This ITO/MoS2 heterostructure represents an unprecedented configuration compared to conventional hot-electron junctions involving classical metals and semiconductors. Its uniqueness lies in its ability to absorb infrared radiation, thereby harvesting a portion of the solar spectrum that is not captured by traditional photovoltaic materials or metals such as silver and gold, which primarily absorb visible light.
A particularly noteworthy finding is the absorption by indium tin oxide nanocrystals at 1750 nm—a region well within the infrared spectrum, which begins above 800 nm. This spectral range corresponds to a portion of solar radiation where very few materials exhibit absorption, and those that do are typically expensive and/or toxic. Current photocurrent measurements, despite the complexity of the heterostructure (only a few nanometers thick), are yielding encouraging results. Consequently, the project has delivered significant advances in the fabrication of nanocomposite films and their spectroscopic and optoelectronic characterization.
An important development within PAIDEIA was the innovative preparation of films based on ITO nanocrystals. Through a annealing-based procedure applied after film deposition, the electrical resistance of these films was modulated by three orders of magnitude. This control was achieved through a systematic study of the annealing time and temperature of the nanostructured ITO films. Furthermore, parametric amplifiers extending into the infrared range were assembled for the optical characterization of PAIDEIA materials—an achievement unprecedented in the scientific community. These amplifiers enable the generation of ultrashort light pulses, in the order of tens of femtoseconds, with wavelengths spanning from approximately one micrometer to beyond ten micrometers. This range encompasses nearly all vibrational fingerprints of organic compounds and biologically relevant materials.
From a condensed matter physics perspective, the project also facilitated the study of electron-hole pairs between monolayers of various transition metal dichalcogenides. Given the complexity of the ITO/MoS2 heterostructure, a prototype solar cell is currently under development to assess both photocurrent and current–voltage characteristics. Many experimental findings have been corroborated using state-of-the-art computational tools. While some theoretical contributions were provided through international collaborations, the research team frequently developed proprietary algorithms to predict and model the optical and optoelectronic behavior of the studied structures.
PAIDEIA has fostered collaboration among young scientists from diverse countries and disciplines—including physics, chemistry, and materials science—enabling them to investigate innovative materials and their junctions from both theoretical and experimental perspectives. The project has also catalyzed outstanding initiatives led by early-career researchers, resulting in prestigious funding awards such as ERC Starting Grants and MSCA Global Fellowships.
The research team carried out several activities within the framework of the PAIDEIA project:
i) We developed various types of junctions composed of doped semiconductor nanocrystals and wide-bandgap semiconductors. Notable examples include junctions between indium tin oxide (ITO) nanocrystals and titanium dioxide, as well as between ITO nanocrystals and molybdenum disulfide monolayers.
ii) Using advanced spectroscopic techniques, we investigated the photo-generation of electric charges and electron–hole pairs in doped semiconductor nanocrystals. The ultrafast properties of these materials are particularly significant, as the lifetime of photogenerated charges is typically well below one picosecond (one trillionth of a second). Our primary focus was on ITO nanocrystals, given their widespread use in hot-electron extraction in the infrared region. However, the limited availability of indium presents a critical challenge. Consequently, identifying alternative doped semiconductors is a key objective of the PAIDEIA project. For instance, we explored copper sulfide nanoflakes, which offer the advantage of being composed of copper and sulfur—elements that are abundant on Earth.
iii) We examined junctions formed by ITO nanocrystals and various semiconductors, including titanium dioxide, tin oxide, and molybdenum disulfide monolayers. These studies encompassed ultrafast optical characterization alongside detailed morphological analyses.
iv) Based on the aforementioned junctions, we fabricated photovoltaic devices. Preliminary results are promising, particularly for systems incorporating ITO nanocrystals and molybdenum disulfide, where measurable photocurrent generation has been achieved.
i) We developed various types of junctions composed of doped semiconductor nanocrystals and wide-bandgap semiconductors. Notable examples include junctions between indium tin oxide (ITO) nanocrystals and titanium dioxide, as well as between ITO nanocrystals and molybdenum disulfide monolayers.
ii) Using advanced spectroscopic techniques, we investigated the photo-generation of electric charges and electron–hole pairs in doped semiconductor nanocrystals. The ultrafast properties of these materials are particularly significant, as the lifetime of photogenerated charges is typically well below one picosecond (one trillionth of a second). Our primary focus was on ITO nanocrystals, given their widespread use in hot-electron extraction in the infrared region. However, the limited availability of indium presents a critical challenge. Consequently, identifying alternative doped semiconductors is a key objective of the PAIDEIA project. For instance, we explored copper sulfide nanoflakes, which offer the advantage of being composed of copper and sulfur—elements that are abundant on Earth.
iii) We examined junctions formed by ITO nanocrystals and various semiconductors, including titanium dioxide, tin oxide, and molybdenum disulfide monolayers. These studies encompassed ultrafast optical characterization alongside detailed morphological analyses.
iv) Based on the aforementioned junctions, we fabricated photovoltaic devices. Preliminary results are promising, particularly for systems incorporating ITO nanocrystals and molybdenum disulfide, where measurable photocurrent generation has been achieved.
The results obtained on junctions between indium tin oxide (ITO) nanocrystals and doped semiconductors surpass the current state of the art. Traditionally, photovoltaic cells exploiting plasmon-induced hot electron extraction are fabricated using noble metals such as gold or silver, rather than doped semiconductors. Our work represents the first report on heterojunctions combining ITO nanocrystals with molybdenum disulfide monolayers, and among the earliest demonstrations of infrared hot electron extraction. Furthermore, the spectroscopic characterization of these materials enabled the development of one of the first optical parametric amplifiers capable of generating femtosecond pulses at wavelengths up to 10 micrometers. In addition, the fabrication of films based on ITO nanoparticles allowed precise engineering of their electrical resistance over three orders of magnitude.