Energy Materials

At the Laboratory for Energy Materials (LEM) we are interested in making and testing complex materials necessary for energy conversion and storage using low cost and innovative methods.

Currently our research is focused on the preparation and characterization of p-type semiconductor absorber layers for use in thin film solar cells. Its main scientific goal is to understand how to convert a simple precursor layer into single phase high quality semiconductor material.

High temperature electrodeposition allows the direct deposition of p-type semiconductors.

Precursor layers are converted into semiconductor thin films by the application of energy using either heat or light irradiation. The Laboratory for Energy Materials studies whether annealing is possible within 1 second.

A quality assessment of semiconductors using photo-electrochemistry allows the discriminiation of suitable layers for devices.

LEM researchers try to understand the growth processes in terms of structural, thermodynamic and kinetic parameters.

The Laboratory for Energy Materials is headed by Dr Phillip Dale.

RECENT NEWS

May 2017

On the fifth of May, the Laboratory for Energy Materials joined with the Laboratory of Photovoltaics to celebrate ten years of photovoltaic and semiconductor research at the University of Luxembourg. Over 120 guests attended a special public mini symposium covering both social and technical aspects of solar energy. For our invited guests we fabricated a special edition solar powered windmill, the cell of which is shown above. A special website for the event has been setup where you can find the history of LPV and LEM, and some pictures taken during the event.

Avril 2017

Dr Dale assisted at the Scienteens physics lab, where a class of 26 inquisitive young physicists from the Lycée des Garçons Luxembourg visited to experience life as scientific researchers. The day started with a briefing of the research problems involved with making and flowing mayonnaise. Mayonnaise must flow through pipes to be filled into jars, but contradictorily not fall off a pommes frite. The researchers then synthesized mayonnaise with different ratios of oil and egg to compare the resulting flow properties. They then split into two groups to investigate the difference between hand and machine mixing of the mayonnaise ingredients. To characterize the mayonnaise they examined the microstructure using an optical microscope, and the flow properties using a simple syringe apparatus and a more sophisticated rheometer. Finally the researchers gathered and analyzed their data together in order to present their findings.

March 2017, Dr Diego Colombara’s paper on gas-phase alkali doping of Cu(In,Ga)Se2
semiconductor for solar cells will appear in Nature Scientific Reports in 2017. We are delighted that one reviewer considered the work to be a “tour de force”. The paper illustrates how an important photovoltaic semiconductor is actually extrinsically doped from an unexpected source, namely its annealing atmosphere as it is being synthesized. This insight allows a new method of doping not only for Cu(In,Ga)Se2
but also for other important chalcogenide semiconductors.

Cu(In,Ga)Se2
(CIGS) solar cells are the most efficient thin film photovoltaic devices currently available. The excellent properties of CIGS are partially due to a beneficial effect induced by intentional alkali metal doping. Conventionally, this doping is achieved by diffusion from glass substrates or post deposition treatments with alkali fluorides, which are condensed state processes. In this paper we show that alkali doping occurs also from the gas-phase in most annealing apparatuses. More importantly, we show that deliberate alkali doping made from the gas-phase has a tremendous effect on the optoelectronic properties of the CIGS absorbers and solar cells, e.g. device efficiency can be improved from 2% to 8% by gas-phase sodium incorporation alone. This opens the possibility for many research groups that are not equipped for conventional post deposition treatments to study the effects of alkali doping on CIGS and other chalcogenide materials with applications in a range of technologies such as: photovoltaics, batteries, thermoelectricity and superconductivity.

We also try to unveil the mechanism of gas-phase alkali transport, which is currently unknown. We identify the most likely routes for gas-phase alkali transport based on new mass spectrometric studies and thermochemical computations. Mass spectrometric measurements performed under equilibrium conditions revealed the formation of atomic sodium and very small quantities of Na2
Se and Na2
Se2
in the gas-phase. We conclude that alkali gas-phase transport occurs through a plurality of routes and cannot be attributed to one single source.

A large-scale deployment of solar cells can generate terawatts of renewable electrical energy. To achieve this large-scale, earth abundant materials and fast fabrication are required. Here we present a breakthrough in the synthesis of Kesterite, Cu2
ZnSnSe4
, a solar cell semiconductor containing only earth abundant metals, formed from a copper-tin-zinc precursor. By electrodepositing copper-tin-zinc layers from liquid metal salts, the speed of metal deposition is increased by one order of magnitude, when compared to vacuum based sputtering deposition. The novelty of this work is the application of custom designed ionic liquids allowing high speed electrodeposition of thin metal layers. Specifically, we report on the design and synthesis of a new tin containing ionic liquid, and its application together with copper and zinc liquid metal salts in order to form Cu2
ZnSnSe4
.These salts are metal containing ionic liquids, where the cation of the liquid contains the metal to be electrodeposited, thus giving exceptionally concentrated solutions, where no detrimental solvent breakdown occurs and thus no limit to the cathodic electrochemical window exists. Previous electrochemical works always utilize a solvent and a metal salt. The significance of the work is that it enables higher deposition rates of metals than currently used thus saving time and energy. In turn this would enable faster solar cell production, important for large scale deployment. We highlight the quality of the thin film metal layers by converting them into semiconductor layers, subsequently used in working solar cells.

Most scientists today are keenly aware that their syntheses should be energy and materially efficient. The custom designed liquid metal salts allow the electrodeposition of technologically important metals at significantly lower energies and material losses compared to vacuum methods, whilst doing so at faster rates.

January 2017, Dr Jessica de Wild in collaboration with Dr Efterpi Kalesaki of the Theoretical Solid-State Physics group compare experimental measurements and theoretical calculations for the optical absorption spectra of Cu2
(Sn, Ge, Si)S3
. These emerging semiconductors are p-type with suitable properties to be used as absorber layers in thin film solar cells. Their work explains the observed features of the optical spectra and external quantum efficiency measurements that can be found in the literature. The work will appear in Physica Status Solidi (RRL) in 2017.

The ternary semiconductors Cu2
(Sn, Ge, Si)S3
have lately attracted interest as photovoltaic material and have multiple absorption onsets near the band edge. These onsets stem from the splitting of the valence band, due to their reduced symmetry, ie monoclinic phase. Theoretical values of the valence band splitting are compared with experimentally determined values from Tauc plots and inflection points and are shown to be in good agreement.

CuInSe2
is an important semiconductor for thin film solar cells. It is synthesized by heating a copper, indium, and selenium containing precursor in a gaseous selenium environment. Normally this heating is made with a resistive furnace or by using halogen light bulbs. These methods are relatively slow leading to annealing times of minutes to hours. Since thirty years various groups have attempted to speed up the heating process using high intensity lasers. In this work Dr Meadows demonstrates for the first time a working solar cell device built from a laser annealed CuInSe2
layer. There appear to be two important factors. Firstly the precursor should have suitable optical and morphological properties to absorb the incoming laser radiation and allow easy diffusion of the elements. Secondly, a selenium gas phase should be present above the solid precursor surface during heating to suppress selenium loss from its surface and promote grain growth. Once these factors are taken into account it is possible to produce CuInSe2
with the ability to generate power.

December 2016, Dr Jessica de Wild's paper on CTS phases to appear in Solar Energy Materials and Solar Cells journal.

Cu2SnS3 (CTS) is investigated as a potential solar absorber material, which is known to exist in several polymorphs. In the present work, the transition between the polymorphs is investigated. Raman spectra and XRD diffractograms show that cubic, mixed polymorphs and monoclinic polymorphs are synthesised with increasing temperature. Photoluminescence spectra of the mixed and cubic polymorphs show mainly defect emissions below the band-gap energy, while the monoclinic modification shows only one sharp peak, which is attributed to the conduction to valence band transition. It is concluded that monoclinic is the best for solar cells application and monoclinic absorber layers are made into devices. Unfortunately, the monoclinic polymorph grows in combination with a secondary phase containing sodium, NaxCuSnS3, with x between 0.5 and 1. This phase has semiconducting properties and a band gap of approximately 1.6 eV. Devices in which NaxCuSnS3 is either present or removed, gave similar power conversion efficiencies of above 1% indicating that the secondary phase is not limiting the efficiency. Further progress in increasing the efficiency of CTS solar cells is under investigation.

November 2016, Erika Robert's paper on Cu2
MX3
(M=Sn,Ge and X=S,Se) formation and equilibrium reaction chemistry will appear in the Journal of Alloys and Compounds. These compounds are under investigation as p-type semiconductor layers in thin film solar cells.

This work shows that copper group IV chalcogenide thin film semiconductors can be successfully produced by annealing a single copper film in the presence of the group IV chalcogenide (MX = SnS, SnSe, GeS). The relative vapor pressures of the gas phase MX species, control both generation and further diffusion of the gas to the sample. Critically the formation processes of the Cu2
SnS3
, Cu2
SnSe3
and Cu2
GeS3
(CGS) semiconductors are reversible at high temperature and the annealing environment is crucial for controlling their composition. Theoretical calculations of the kinetics of gas generation and diffusion processes were performed and explain the compositional gradients observed in Cu2
SnS3
and Cu2
SnSe3
. Low generation rates of the Sn(S,Se) gas phases are shown to be responsible for these gradients, whilst GeS vapor forms rapidly and insures the formation of a homogeneous Cu2
GeS3
thin film semiconductor. In this regard, the synthesis method can be applied and generalised to other copper chalcogenides films but particular care has to be taken in lowering the background pressure or heating the gas phase sources to maximize the equilibrium partial pressures above the thin film.

Caption for the picture: SEM top view images overlayed with composition maps of Cu (red) and Sn or Ge (blue) for Cu layers annealed in the presence of S and SnS at i-a) 525°C and ii-a) Cu2
SnS3
sample annealed at 525°C for 12 hours in the absence of S and SnS. The second row shows the same series for the Ge analog and the third row shows the series for SnSe/Se samples. The black color indicates that the Mo is uncovered, due to the open morphology of the sample. The green horizontal line indicates the position of the EDX line scan shown above each image.

October 2016, Dr Jessica de Wild's paper on air annealing and quantum efficiency modelling of Cu2SnS3 solar cells will appear in the IEEE Journal of Photovoltaics. Depending on whether the air annealing is done before or after buffer and window layer addition changes whether the diffusion length or depletion width changes. Oxygen is required to observe changes in diffusion length.

Air annealing of chalcopyrite solar cells at 200 °C or higher is often known to increase their power conversion efficiency. In our present work we investigate the nature of this effect for Cu2SnS3 (CTS) solar cells through modelling the experimental external quantum efficiency. We find that the cell efficiency increase stems from increased diffusion length and depletion width, and decreased interface recombination at the pn junction. The increased diffusion length is also reproduced when only the absorber layer is air annealed. When solar cells are annealed in N2, no increase in diffusion length was measured. We hence attribute the increase in diffusion length to passivation of the grain boundaries in the bulk by oxygen. The larger depletion width on air and N2 annealing in the devices is independent of the CdS buffer layer thickness and also occurs in its absence. We ascribe it to copper diffusion from the absorber layer to the n-type buffer and window layers. Interface recombination positively correlates with increasing buffer layer thickness. Based on our modeling we conclude that the CTS absorber layer is still too highly doped to obtain large depletion widths, and it is highly recombination active at the pn interface.

April 2016, Dr. Diego Colombara's paper on the electron transfer kinetics and surface stability during photoelectrochemical measurements of CIGS thin films, published in The Journal of Physical Chemistry C. (The article is a part of the Kohei Uosaki Festschrift special issue).

We report a comprehensive set of photoelectrochemical (PEC) measurements of CIGS and GaAs thin films, in order to assign the best conditions to predict the short circuit current of final devices based on these absorber layers. For routine applications, we found that the redox couple Eu3+/2+
is less reliable than the Fe(CN)63-/4-
, because of the slow electron transfer occurring between the absorber layer and the former electrolyte. With the addition of a thin layer of the wide bandgap CdS we are able to use the faster Fe(CN)63-/4-
redox couple, preventing at the same time hole injection into the p-type absorber layer of interest.

The synthesis of multinary compound films from layered precursors is only partially understood. Identical location microscopy resolves the multi-step synthesis of Cu2ZnSnSe4 from metallic stacks on the micron-scale. Large scale metal alloying and the seemingly illogical observation that ZnSe segregates preferentially on locations previously poor of zinc are revealed

In the framework of SCALENANO European project, our group developed the photo-electrochemical (PEC) testing of electrodeposited (ED) CIGS, for quality control and process monitoring at on-line level. In this review, we present a simple theoretical introduction to PEC characterization, a brief description of the apparatus, and some new data with statistical analysis. The photocurrent density (Jph) measured for CIGS of different quality shows to be a good proxy of the short-circuit current density (Jsc) of the photovoltaic device based on the same semiconductor. Moreover, dark current measurements in reverse configuration allowed us to draw correlations with the presence of pinholes in CZTS. Finally, a comprehensive summary of measured indicators and the corresponding process/device parameters analyzed is given, also for the other optical characterization techniques featured in this review, namely Raman scattering, spectral PL and PL/EL imaging.

November 2016, Dr. Ulrich Berner, Dr Diego Colombara and Dr Jessica de Wild published a paper on solution-processed CIGS solar cells, in Progress in Photovoltaics journal.

The effect on solar cell properties of three sodium sources is investigated: NaCl, NaHCO2, and NaSCN. It is shown by SIMS and XRD that the Ga distribution through the layer is affected by the anion of the Na, ie the Ga intermixing increased for HCOO, Cl and SCN anions respectively. This will affect solar cell properties like Jsc and Voc. The figure shows the Eg extracted from the EQE, with SCN having the highest band gap, and the photoluminescence maps after excitation of 514 nm laser light. The light penetrates up to 80 nm into the absorber layer, revealing the band gap of the absorber layer just below the CdS buffer layer. The results show that the absorber layer made with NaCl has a lower band gap at the top, ie it is pure CIS, while NaSCN has a higher band gap ie it contains Ga. This is consistent with what is found with SIMS and EQE. It is concluded that NaCl decreases the Ga intermixing, resulting in a CIS/CdS interface, while NaSCN increases the intermixing, increasing the Voc but also phase separation. Improving the NaCl annealing procedure, resulted in the highest reported power conversion efficiency of 13.3% for sulfur-free CIGSe solar cells by solution processing.

June 2015, Dr. Sara Tombolato paper on a novel chemical method to fabricate CZTSe thin films is out now on Solar Energy.

Cu2ZnSnSe4 films were prepared by selenization of metallic precursors obtained by a new wet process involving metal formates. Cu(HCOO)2 and Zn(HCOO)2 were used as copper and zinc sources respectively, while tin was introduced as a methoxide.

The elemental analysis of the resulting absorber layers revealed a very low carbon content (less than 0.2 wt%), which is believed to be a feature that chemical methods need to have in order to stand out as valuable alternatives to high-vacuum processes in this field. Solar cells with efficiencies of up to 2.39% with Voc of 207 mV, a Jsc of 31.2 mA/cm2 and a fill factor of 37.1% were achieved for these copper-poor CZTSe absorbers.