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Final Report Summary - SUNLIGHT (Solution-processed nanocrystal photovoltaics from environmentally benign and earth-abundant elements)

Summary of the project objectives:
This project aims to study colloidal nanocrystals of environmental benign compositions and their applications in solution-processed solar cells. Specifically, this project involves the following aspects:
1. Optimization of synthetic methods. Specifically, two nanocrystal systems of environmental benign composition will be chosen as the main focus of the project depending on the results of synthesis and feedback from device performance.
2. Functionalization of nanocrystal surfaces with difference ligands to facilitate their applications.
3. Investigation of the effects of nanocrystal size/morphology and ligands on charge transport.
4. Realization and optimization of solar cells based on nanocrystals or nanocrystal/organic hybrids. Correlate the effects of different synthetic and ligand conditions with the observed photovoltaic characteristics.

State-of-the-art:
In the search for next-generation cost-effective solar cells, colloidal inorganic semiconducting nanocrystal quantum dots (QDs) have received much interest due to their readily-tunable absorption across the visible/near-IR, their high absorption coefficients and photostability. They offer exciting opportunities for photovoltaics due to their size-tunable bandgap and the multiple excitation generation phenomenon, a mechanism by which the Shockley-Queisser limit can be potentially bypassed. Intensive investigations have been carried out on solar cells built from a variety of QDs of compositions such as CdS, CdSe, PbSe, and PbS in the quest for high-performance and low-cost photovoltaic devices. Different device configurations have been proposed, such as QD-sensitized, QD-organic bulk heterojunction, metal oxide/QD bi-layer heterojunction solar cells, and QD bulk nano-heterojunction. So far, metal oxide/QD bi-layer depleted heterojunction solar cell is among the most efficient systems allowing power conversion efficiency (PCE) as high as 10.6% for PbS QD solar cells (Lan et al, Nano Lett. 16, 4630-4634(2016)). Significant improvements on both materials aspects and device performance are however still necessary to promote these solar cells as a viable technology for our society. In particular, most state-of-the-art examples involve the use of highly toxic elements such as Pb or Cd. Toxicological issues may hinder their lab-to-market transition. In comparison, QD solar cells based on Zn-Cu-In-Se exhibit very promising PCE of 11.6% (Du et al, JACS, 138, 4201-4209 (2016)). However, these solar cells require the use of liquid electrolyte which is unfavorable for applications due to various electrolyte issues such as leakage, evaporation of solvent, high-temperature instability and flammability...etc. This project therefore focuses on QDs of environmental benign compositions and their incorporation into solid-state solar cells.

Work performed since the beginning of the project:
(1) Towards the first objective, relevant chemistry synthetic equipment (e.g. fume hood, schlenk lines, ovens...etc.) were installed in our laboratory. We have then explored the synthesis of environmental benign colloidal systems including FeS2, CuO, SnSe, CuInS2, and lanthanide-doped NaYF4 nanocrystals. In particular, synthetic optimization was performed on Zn-doped CuInS2 and lanthanide-doped NaYF4 two nanocrystal systems. Structural characterizations were performed on as-synthesized nanocrystals by techniques of transmission electron microscopy, scanning electron microscopy and powder X-ray diffraction. Optical characterizations were performed on these nanocrystals by UV-Vis absorbance and photoluminescence. (2) Towards the second objective, we experimented different methods including ligand-exchange in solution, and the "layer-by-layer spin-coating and ligand-exhange" method. To experiment ligand-exchange we first applied the relatively well-understood colloidal nanocrystals of lead sulfide (PbS). Despite the fact that PbS is toxic here it served as a model system to optimize ligand-exchange methods. To study the evidence of ligand-exchange, Fourier transform infrared spectroscopy was performed on nanocrystal films before and after ligand-exchange. Knowledge on ligand-exchange was used to correlate with solar cell and transistor performances. Ligand-exchange method was then further applied on Zn-doped CuInS2 and lanthanide-doped NaYF4 nanocrystals. (3) Towards the third objective, field-effect transistors (FETs) were fabricated based on a series of ligand-exchanged nanocrystals, including Zn-doped CuInS2 nanocrystals, PbS nanocrystals, and binary mixture composing both Zn-CuInS2 and PbS nanocrystals. These FETs were fabricated by applying the bottom-contact/bottom-gate device structure with SiO2 as the gate dielectrics and their field-effect mobilities were characterized. (4) Towards the fourth objective, solar cells were fabricated under different device structures by applying Zn-doped CuInS2 nanocrystals in the active absorber layer. Comparison studies were performed on solar cells applying only CuInS2 nanocrystals, PbS nanocrystals, and binary mixture composing both in the active absorber layer. On lanthanide-doped NaYF4 nanocrystals, hybrid solar cells combining organic-inorganic lead perovskites and upconversion lanthanide-doped NaYF4 nanocrystals were fabricated. Photovoltaic characterizations have been performed on all above-mentioned systems.

Main results achieved:
(1) Towards the first objective, we focused on Zn-doped CuInS2 and lanthanide-doped NaYF4 nanocrystals on which we can control well their morphology, dimension as well as their optical properties. In particular, revealed from optical characterizations, the incorporation of 10 mol% of Zn in the CuInS2 by adding of Zn in the synthesis can significantly enhance the photoluminescence properties due to the reduction of nanocrystal defects. On lanthanide-doped NaYF4 we have experimented different dopants, such as Yb3+/Er3+ co-doping and Er3+ alone, allowing different upconversion properties from 980 nm excitation or 1550 nm excitation to visible wavelength. (2) Towards the second objective, procedures exchanging the nanocrystal as-synthesized long-chain (insulating) ligands to short-chain ligands were successfully performed. In particular, the layer-by-layer spin-coating method allows a convenient and flexible control on the thickness of the ligand-exchanged nanocrystal thin films, which is important for device fabrication. Ligand-exchange was examined by Fourier transform infrared spectroscopy. PbS nanocrystals were served as test-beds to optimize the procedure and then the procedure was applied onto Zn-CuInS2 and NaYF4 nanocrystals. (3) Towards the third objective, FETs based on only PbS QDs exhibit p-type transport with a hole mobility of ~ 10^-4 – 10^-3 cm^2V^-1s^-1. By comparison, FETs based on Zn-CuInS2 QDs exhibit similar hole transport characteristics but with a lower hole mobility of ~10^-6 – 10^-5 cm^2V^-1s^-1. This low mobility values may be related to factors such as the amount of QD surfaces (acting as hopping barriers), electronic coupling between nanocrystals, charge trapping, and/or charge injections at the contacts of these Zn-CIS QD FETs. (4) Towards the fourth objective, comparison studies were performed on solar cells applying either Zn-CuInS2 NCs, or PbS NCs, or a binary mixture of both nanocrystals in the active absorber layer together with a TiO2 acceptor layer. We have also fabricated hybrid solar cells based on organo-metal halide perovskites on which upconversion lanthanide-doped NaYF4 nanocrystals are incorporated. Concerning the first topic, when the absorber layer contains only Zn-CuInS2 nanocrystals, the photovoltaic performance of the nanocrystal solar cell is rather limited exhibiting low photocurrent (about 0.02 mA/cm^2) and low power conversion efficiency (about 10^-3 %). Despite the limited solar cell performance when Zn-CuInS2 nanocrystals were applied alone, significant enhancement of photocurrent was achieved by incorporating Zn-CuInS2 nanocrystals into the PbS nanocrystal matrix in the solar cell absorber layer. In such a binary nanocrystal thin film charge transfer process was observed to occur within a few tens of picoseconds after photoexcitation by ultrafast optical spectroscopy. With 10% volume fraction of Zn-CuInS2 nanocrystals, the solar cell performance was significantly improved exhibiting a ~30% increase in the short-circuit current and a ~20% increase in the power-conversion efficiency (under 1-Sun illumination) compared to solar cells applying only PbS. In agreement with the charge transfer process identified through ultrafast pump/probe spectroscopy between these two QD components, transient photovoltage characteristics of single-component and binary QDs solar cells reveal longer carrier recombination time constants associated with the incorporation of Zn-CIS QDs. This work presents a straightforward, solution-processed method based on the incorporation of another QDs in the PbS QD matrix to control the carrier dynamics in colloidal QD materials and enhance solar cell performance. Concerning the second topic, functional solar cells based on organo-metal halide perovskites of the composition FA0.83Cs0.17Pb(I0.6Br0.4)3 with and without the insertion of upconversion lanthanide-doped NaYF4 nanocrystals have been fabricated. More than 10% increase of short-circuit current was observed in hybrid solar cells applying lanthanide-doped NaYF4 nanocrystals suggesting a boost of photovoltaic performance from the upconversion effect.

The main results were published in A. A. Bakulin et al. ACS Nano 7, 8771 - 8779 (2013) and Z. Sun et al. Scientific Reports, 5, 10625 (2015). The results on hybrid solar cells applying lanthanide-doped NaYF4 nanocrystals is currently under preparation to be submitted as soon as possible. Other publications supported by this project include: Z. Cao et al., Optical Materials Express, 4, 2525 - 2534 (2014); A. A. Bakulin et al., Proc. SPIE 9165, Physical Chemistry of Interfaces and Nanomaterials XIII, 91650U (2014), DOI:10.1117/12.2064068; A. A. Bakulin et al. The Journal of Physical Chemistry Letters, 6, 3663-3669 (2015); Z. Sun et al., Nanoscale, 8, 7377-7383 (2016); O. Selig et al., Journal of the American Chemical Society, 139, 4068-4074 (2017).

Prospects of the research career development of the fellow:
At the beginning of this project (in 2012) the fellow holded a permanent research scientist position (grade of CR2) from CNRS. In October 2014 the fellow was promoted to the grade of CR1. In September 2016, the fellow obtained the qualification of Habilitation à Diriger des Recherches (HDR), which allows her to further establish scientific independence in her career. In 2017 the fellow holds a research group containing 3 PhD students and 1 shared technician.

The expected final results and their potential impact and use:
In France, according to the The Energy Transition for Green Growth Act (la loi relative à la transition énergétique pour la croissance verte, LTECV) promulgated on the 17 of August, 2015, we must increase the proportion of renewable energy among our total consumption to 23% by 2020 and to 32% by 2030. This is in accordance with the objectives fixed by the European parliament (Directive 2009/28/EC). However, according to the data provided by the French ministry of the environment, energy and the sea in charge of international relations on climate, only 14.9% share of
renewable energy has been achieved in 2015 in France, lagging behind the planned objective (about 17%). There is therefore an urgent need to address this problem by reinforcing scientific efforts in the field of renewable energy.
The academic response to the above-described issue is based on the research and development of renewable energy harvest systems of improved merits (better performance, lower cost, improved lifetime, more adaptable to new needs...) than the current ones. The current project described here works on photovoltaic systems, which accounts for 8.1% and 24% of gross electricity production in metropolitan France and French overseas department, respectively. Specifically, we have achieved optimized colloidal synthesis on two colloidal nanocrystals (Zn-doped CuInS2 and lanthanide-doped NaYF4 nanocrystals) through colloidal synthesis and developed their application in solution-processed photovoltaics.
Not only providing fundamental knowledge towards future synthetic optimizations, the final results of this project provides a solution-processed “bottom-up” approach to achieve spatial charge separation and improved photovoltaic performance by incorporating Zn-CuInS2 nanocrystals into the PbS nanocrystal matrix. Results on the incorporation of lanthanide-doped NaYF4 nanocrystals into the FA0.83Cs0.17Pb(I0.6Br0.4)3 perovskite solar cells provide another solution-processed “bottom-up” approach to harvest NIR photons. These results provide both fundamental knowledge and new approaches at the laboratory level to boost the performance of solution-processed solar cells by applying environmentally benign colloidal nanocrystals.

Project website: Not applicable
Contact details: Dr. Zhuoying Chen, Email: zhuoying.chen@espci.fr

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CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
France
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