Quantum dot sensitized p-type wide band gap semiconductors for photoelectrochemistry – QuePhelec

1. Chemical synthesis of QDs.
2. NiO sdeposition by doctor-blading and screen-printing
3. Electrochemical deposition of CuSCN
4. Charge transfer studies by transient photoluminescence spectroscopy
5. Optical modelisation
6. Composition studies by EDX and XPS
7. Studies of crystalline structures by XRD
8. Optical properties study by absoprtion and photoluminescence spectroscopies.
9. Morphological studies by SEM, STEM and HAADF.
10. Tests of the solar cell Under solar irradiation by solar simulator using I-V curves and IPCE spectroscopy.

1. Eco-friendly quantum dots (QDs) based on CuInS2 were fabricated and their optoelectronic properties were optimised. In parallel, PbS QDs were developed due to their absorption extending up to IR.
2. Dense mesoporous NiO layers were fabricated and passivated using methods of soft chemistry. In parallel, by collaborating with a manufacturer we optimize a commercial NiO pastes.
3. CuSCN nanowires as novel p-type materials were fabricated and optimized. Their 3D modelisation was performed.
4. Assemblies of QDs and p-type materials with various linkers were realized. Results of XPS, UPS, EDX and HAADF-STEM confirm the efficient deposition of the QDs on both NiO and CuSCN. Photophysical studies have allowed to find that the rate of hole transfer in the systems studied is high, in the order of 10^-8-10^-9 s-1. Ellipsometry and spectrophotometry were also performed on the samples with different deposition parameters. 1D numerical modelisations have allowed to validate optical indices (n et k) of the des QDs as well as to determine their size.
5. Elaboration of the solid components has been initiated. First diodes fabricated allowed to optimize the deposition conditions of the infiltration of the electron conductor (PCBM). First optical studies to characterize the charge transfer at the interface QD/fullerene have been started by transient fluorescence spectroscopy.
6. Several solar cells were fabricated. The best photovoltaic efficiency of 1.25% has been obtained for CuInSxSe2-x :Zn2+ deposited on mesoporous NiO. This is a record in the field of p-type QD sensitized solar cells. We have demonstrated that various treatments of NiO improve the photocurrent and potential observed in by the cells. The cells of NiO sensitized with PbS lead to the PCE of 0.06%.

According to the initial objectives of the project several points remain to be studied.
1. Studies of the assemblies by the electrochemical impedance will be performed on the interfaces of the p-type materials and QDs. The calculated injection resistance will allow to propose strategies to reduce charge recombination in the systems studied, to improve the charge injection and finally the cells efficiency.
2. Specific design of the photophysical experiments will allow to study simultaneously hole and electron transfer in the assemblies to provides for the balanced charge transfer, which is highly beneficial for the overall photovoltaic performance.
3. Mott-Schottky and UPS studies will be carried out to establish the band diagram of the assemblies.
4. Optical simulation will be continued to model and study the optoelectronic properties of the separate materials as well as the assemblies and entire cells.
5. Alternative low cost methods, such as SILAR or CBD, will be proposed to deposit QDs on the p-type materials.
6. Passivation of the surface of CuSCN nanowires will allow to improve the efficiency of the cells.
7. Development of the solid electron transporting materials will allow to fabricate for the first time solid inverted quantum dot sensitized solar cells with extended lifetime.

1. D. Aldakov, C. Chappaz-Gillot, R. Salazar, V. Delaye, K. A. Welsby, V. Ivanova, P. R. Dunstan, J. Phys. Chem. C, 2014, 118, 16095–16103: pubs.acs.org/doi/abs/10.1021/jp412499f

The objective of the QuePhelec project is to fabricate and study the assemblies of quantum dots (QDs) on wide band gap p-type semiconductors (SCs) for the use in photoelectrochemical (PEC) systems.
Sensitized nanostructured SCs are very promising materials for PEC systems, such as solar cells or photocatalytic systems. Today, most PEC systems use TiO2 sensitized with organometallic dyes and suffer from limited photostability and synthesis complexity. Organic dyes can be replaced by QDs, which can be better adapted for the sensitization of SCs. There are a lot of examples of assemblies of QDs on n-type SCs, however almost no assemblies of QDs on p-type SCs exist, generally due to not optimal optical and electronic properties of the latter. The role of the QD/p-SC assemblies is even more important taking into account their use in tandem PEC systems where sensitized p- and n-type SCs are combined thus allowing to improve the overall efficiency.
We propose to develop assemblies of non-toxic QDs on p-type materials. The QDs proposed will be synthesized in various sizes and have high absorption coefficients over a wide wavelength range. Their properties will be tuned by various organic ligands or inorganic shells.
As a SC we propose to use nanostructured NiO as it is the most studied p-type SC. Alternatively, SPrAM has recently developed an important p-type SC CuSCN in form of nanowires. Preliminary studies showed that it is well adapted for the use in assemblies with QDs. After the fabrication and characterisation of the components, they will be assembled together by using bifunctional linkers: first, the SC surface will be functionalised by the linkers, next the QDs will self-assemble onto the prepared surface from solution. As linkers we will use either commercial molecules or those developed in SPrAM.
It is important to determine properties of the assemblies that govern the interaction between the SCs and QDs. We propose to start with measuring the electronic levels of the components by Mott-Schottky technique. Charge transport of novel SC layers will be studied to find the charge mobilities by ToF techniques and conductivity. We then propose to study morphology of the assemblies by a variety of microscopy techniques: SEM, FIB and “slice-and-view”. Intrinsic band alignment at the interface QD/SC will be established by using XPS/ UPS. Transient PL studies by the XLIM partner will allow determining the mechanism and rates of charge transfer within the assembly and indicate the ways to improve the interaction between the components. Finally, electrical processes at the interfaces will be studied by electrochemical impedance to establish an equivalent circuit model of the assemblies and contribute to better understanding of the electronic interactions.
Optical properties of the QDs and p-type SCs will be then modelled by the IM2NP partner to provide information about optimal size, geometry and configuration of the assemblies. Optical properties of individual components and the assemblies will be studied by various techniques: ellipsometry, absorption at variable angles etc. All new assemblies developed by other partners will be modelled and characterized optically in IM2NP in order to optimise their light absorption.
Finally, in order to validate the best assemblies identified in the previous tasks, we will fabricate PEC systems. Solar cells with liquid or solid electrolyte will be prepared and studied by CEISAM and XLIM partners. To investigate charge transfer and transport dynamics, the best systems will be further studied by transient photoelectrical techniques.
In conclusion, the consortium proposed will benefit from complementarity to develop, deeply characterize novel QD/p-SC assemblies to establish their underlying fundamental interaction principles and at the end come up with new PEC systems.