Final Report Summary - IQDOTPV (All-Inorganic Quantum Dot Films for Photovoltaic Applications)
The IQDotPV project was dedicated to the synthesis of colloidal semiconductor nanocrystals (NCs) designed for application in photovoltaics. Proposed goal was to develop synthetic approach and study colloidal solutions and thin films of NCs capped with inorganic ligands.
State-of-the-Art NCs can be produced in presence of organic ligands and dissolved in nonpolar media. Organic ligands provide perfect colloidal stabilization and surface traps passivation for NCs. However, those long-chain hydrocarbon ligands introduce insulating layers around each NC and suppress charge transport between NCs in NC solids. Small inorganic ligands supposed to minimize potential barrier for carriers tunnelling between NCs in NC solids. It has been shown that inorganically-capped NCs demonstrate improved charge transport and overall photovoltaic behaviour comparing to organically-capped ones. Project objectives can be summarized as following:
-Development of synthetic approach to colloidal NCs capped with various inorganic ligands
-Analysis of ligand nature effect on optical properties of colloidal nanocrystals (NCs) and charge transport in NC solids
-Examination of the order/disorder effect on the transport characteristics of NC solids.
Various target colloidal NCs were identified, representing typical NCs which are optically active in visible and near-infrared (NIR) parts of the spectrum: PbS, CdSe, CdS, HgTe, CdSe/CdS core/shell spherical NCs, CdS nanorods (NRs), CdSe and CdSe/CdS core/shell and core/crown nanoplatelets (NPLs). The optimal approaches to the syntheses of those colloidal NCs have been identified and optimized. On this step NCs have been synthesized in nonpolar media and capped with organic ligands.
On the next step of the project implementation we have chosen thioarsenates as suitable chalcogen-based inorganic ligands for colloidal NCs. We also recognized halometallates as a new but promising class of inorganic ligands. Possibility of using them has been successfully demonstrated for all studied NCs (metal and semiconductor ones). Optimal conditions for exchange of original organic ligands with metal chalcogenides, metal halides and halometallates have been identified. Solvent effect on ligand exchange has been studied for the first time. Furthermore, the possibility to use halide-based inorganic ligands for cation exchange in NCs has been revealed. This allows postsynthetic exchange of Cd-based NCs (which are optically active in visible part of spectrum) to NIR-active Pb-based NCs. Latter are more interesting for photovoltaics since they can absorb larger part of solar spectrum.
Effect of chalcogenide- and halide- based inorganic ligands on optical and electrical properties of colloidal NCs has been studied. Halide-based inorganic ligands ensures record-high PL QEs for inorganically-capped NCs but suffers from not enough electrical conductivity between NCs in NC solids. Chalcogenide-based inorganic ligands, in contrast, typically introduce some surface trap states but provide lower potential barrier between NCs in NC solids and thus support improved electrical transport. Mid-IR photodetectors with high light responsivity have been prepared on the basis of PbS NCs functionalized with various thioarsenides.
Randomly-packed and ordered all-inorganic NC solids have been successfully synthesized and studied. This study has shown that ordering of NCs leads to improved subthreshold behaviour of field-electron transistors.
Most significant results achieved during the project implementation include:
- development of the general methodology for surface functionalization of nanocrystals with metal halides
- achievement of the first inorganically-capped colloidal NCs with PL QE comparable to that of organically-capped NCs, namely: 20−30% for near-infrared emitting CH3NH3PbI3-capped PbS NCs and 50−65% for red-emitting CH3NH3CdBr3- and (NH4)2ZnCl4-capped CdSe/CdS nanocrystals
- an importance of Lewis acid−base properties of the solvents for ligand exchange and colloidal stability has been shown for the first time
- 80-nm large superlattices of PbS NCs with various inorganic ligands have been prepared
- NC ordering effect on electrical properties of NC solids has been demonstrated
During the project implementation lead halide perovskites have fallen into the focus of many researchers. Thus materials demonstrated defect-tolerant properties and outstanding performance in photovoltaics. Because of that the project has been partially re-focused on haloplumbates (as bulk material or as capping ligands for nanocrystals). Successful functionalization of PbS NCs with metal halides opened new class of inorganic capping ligands for colloidal NCs and corresponding report [J. Am. Chem. Soc., 2014, 136 (18), 6550] received enough citations to place it in the top 1% of its academic field according to Web of Science (January-July 2015). NCs functionalization with halide-based ligands recently allowed others to produce record-bright near-infrared NC-based solids [Nature, 2015, 523, 324] and solar cells with power conversion efficiency 8.95% [DOI: 10.1021/acs.nanolett.5b03271]. Furthermore, potential of bulk haloplumbates for X- and gamma- Rays detection and for light emission devices has been shown.
State-of-the-Art NCs can be produced in presence of organic ligands and dissolved in nonpolar media. Organic ligands provide perfect colloidal stabilization and surface traps passivation for NCs. However, those long-chain hydrocarbon ligands introduce insulating layers around each NC and suppress charge transport between NCs in NC solids. Small inorganic ligands supposed to minimize potential barrier for carriers tunnelling between NCs in NC solids. It has been shown that inorganically-capped NCs demonstrate improved charge transport and overall photovoltaic behaviour comparing to organically-capped ones. Project objectives can be summarized as following:
-Development of synthetic approach to colloidal NCs capped with various inorganic ligands
-Analysis of ligand nature effect on optical properties of colloidal nanocrystals (NCs) and charge transport in NC solids
-Examination of the order/disorder effect on the transport characteristics of NC solids.
Various target colloidal NCs were identified, representing typical NCs which are optically active in visible and near-infrared (NIR) parts of the spectrum: PbS, CdSe, CdS, HgTe, CdSe/CdS core/shell spherical NCs, CdS nanorods (NRs), CdSe and CdSe/CdS core/shell and core/crown nanoplatelets (NPLs). The optimal approaches to the syntheses of those colloidal NCs have been identified and optimized. On this step NCs have been synthesized in nonpolar media and capped with organic ligands.
On the next step of the project implementation we have chosen thioarsenates as suitable chalcogen-based inorganic ligands for colloidal NCs. We also recognized halometallates as a new but promising class of inorganic ligands. Possibility of using them has been successfully demonstrated for all studied NCs (metal and semiconductor ones). Optimal conditions for exchange of original organic ligands with metal chalcogenides, metal halides and halometallates have been identified. Solvent effect on ligand exchange has been studied for the first time. Furthermore, the possibility to use halide-based inorganic ligands for cation exchange in NCs has been revealed. This allows postsynthetic exchange of Cd-based NCs (which are optically active in visible part of spectrum) to NIR-active Pb-based NCs. Latter are more interesting for photovoltaics since they can absorb larger part of solar spectrum.
Effect of chalcogenide- and halide- based inorganic ligands on optical and electrical properties of colloidal NCs has been studied. Halide-based inorganic ligands ensures record-high PL QEs for inorganically-capped NCs but suffers from not enough electrical conductivity between NCs in NC solids. Chalcogenide-based inorganic ligands, in contrast, typically introduce some surface trap states but provide lower potential barrier between NCs in NC solids and thus support improved electrical transport. Mid-IR photodetectors with high light responsivity have been prepared on the basis of PbS NCs functionalized with various thioarsenides.
Randomly-packed and ordered all-inorganic NC solids have been successfully synthesized and studied. This study has shown that ordering of NCs leads to improved subthreshold behaviour of field-electron transistors.
Most significant results achieved during the project implementation include:
- development of the general methodology for surface functionalization of nanocrystals with metal halides
- achievement of the first inorganically-capped colloidal NCs with PL QE comparable to that of organically-capped NCs, namely: 20−30% for near-infrared emitting CH3NH3PbI3-capped PbS NCs and 50−65% for red-emitting CH3NH3CdBr3- and (NH4)2ZnCl4-capped CdSe/CdS nanocrystals
- an importance of Lewis acid−base properties of the solvents for ligand exchange and colloidal stability has been shown for the first time
- 80-nm large superlattices of PbS NCs with various inorganic ligands have been prepared
- NC ordering effect on electrical properties of NC solids has been demonstrated
During the project implementation lead halide perovskites have fallen into the focus of many researchers. Thus materials demonstrated defect-tolerant properties and outstanding performance in photovoltaics. Because of that the project has been partially re-focused on haloplumbates (as bulk material or as capping ligands for nanocrystals). Successful functionalization of PbS NCs with metal halides opened new class of inorganic capping ligands for colloidal NCs and corresponding report [J. Am. Chem. Soc., 2014, 136 (18), 6550] received enough citations to place it in the top 1% of its academic field according to Web of Science (January-July 2015). NCs functionalization with halide-based ligands recently allowed others to produce record-bright near-infrared NC-based solids [Nature, 2015, 523, 324] and solar cells with power conversion efficiency 8.95% [DOI: 10.1021/acs.nanolett.5b03271]. Furthermore, potential of bulk haloplumbates for X- and gamma- Rays detection and for light emission devices has been shown.