Zacharie Jehl Li KaoUniversitat Politècnica de Catalunya | UPC · Department of Electronic Engineering (EEL)
Zacharie Jehl Li Kao
PhD
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94
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September 2020 - August 2025
Publications
Publications (94)
Recent developments in Sb 2 Se 3 van der Waals material as an absorber candidate for thin film photovoltaic applications have demonstrated the importance of surface management for improving the conversion efficiency of this technology. Sb 2 Se 3 thin films' versatility in delivering good efficiencies in both superstrate and substrate configurations...
Low dimensional 2D materials such as graphene and carbon nano tubes have attracted significant attention from the solid-state device community and are considered as a potential candidate electrode to make functional contact with the Silicon (Si) and other industry compatible semiconductors. In this work we envisioned to utilize one such interesting...
Low dimensional 2D materials such as graphene and carbon nano tubes have attracted significant attention from the solid-state device community and are considered as a potential candidate electrode to make functional contact with the Silicon (Si) and other industry compatible semiconductors. In this work we envisioned to utilize one such interesting...
As single-junction solar cells are approaching theoretical limits, multijunction solar cells are becoming increasingly relevant, and low-cost wider bandgap light harvesters in tandem with silicon are the next frontier in thin film photovoltaic research. Cu-based chalcogenide compounds have achieved great success as standard absorbers, but performan...
The Kesterite solar cells research landscape is at a crossroad and despite a much improved understanding of the limitations of this class of materials, the current performance deficit contrasts with the several other thin film technologies reaching conversion efficiency values well above 20%. It is more important than ever for the Kesterite communi...
Quasi-1D chalcogenides have shown great promises in the development of emerging photovoltaic technologies. However, most quasi-1D semiconductors other than Sb2Se3 and Sb2S3 have been seldom investigated for energy generation applications. Indeed, cationic or anionic alloying strategies allow changing the bandgap of these materials, opening the door...
Kesterite materials are among the most promising emerging photovoltaic absorbers, despite the number of challenging issues this technology presents. The use of soft thermal post-deposition treatments (PDT) is key to improving the CdS/kesterite interface quality. Thermal treatments can result in a low-temperature phase transition which affects the o...
Small Atom Doping In article number 2200580, Alex Jimenez‐Arguijo, Edgardo Saucedo, and co‐workers implemented transient doping with H and Li during the synthesis of kesterite Cu2ZnSnSe4. This strategy allows engineering the defect structure of kesterite towards photovoltaic applications, as well as improving the thin film morphology, demonstrating...
Research on the kesterite (Cu2ZnSn(S,Se)4), CZT(S,Se)-based thin film solar cell has been substantially increasing throughout the past decade, reaching the forefront of the photovoltaic (PV) research community. Major advances have been reported at various levels, from the fundamental understanding of the material properties to improvements in the d...
Kesterite Cu2ZnSnSxSe4‐x is among the most promising inorganic earth‐abundant thin film photovoltaic technologies, although currently, the larger voltage deficit compared to more mature chalcogenide technologies is hampering solar‐to‐electricity conversion efficiency progress in these materials. Most of the latest reports agree on the CZTSSe defect...
This work reports a detailed study of the stability of different ITO‐based back contact configurations (including bare ITO contacts and contacts functionalized with nanometric Mo, MoSe2 and MoS2 layers) under the co‐evaporation processes developed for the synthesis of high efficiency Cu(In,Ga)Se2 (CIGSe) solar cells. The results show that bare ITO...
Sb2Se3 is a quasi-one-dimensional (1D) semiconductor, which has shown great promise in photovoltaics. However, its performance is currently limited by a high Voc deficit. Therefore, it is necessary to explore new strategies to minimize the formation of intrinsic defects and thus unlock the absorber's whole potential. It has been reported that tunin...
Accurate anionic control during the formation of chalcogenide solid solutions is fundamental for tuning the physicochemical properties of this class of materials. Compositional grading is the key aspect of band gap engineering and is especially valuable at the device interfaces for an optimum band alignment, for controlling interface defects and re...
In realizing wide-bandgap CuGaS2 (CGS) based solar cells, CGS thin film deposition methods and suitable n-type buffer layers need to be studied. Even though CGS belongs to the same chalcopyrite family as the more extensively-studied Cu(In,Ga)Se2, their different nature of defects and band profile require a different thin-film deposition approach as...
Sb2Se3 is an emerging earth-abundant material praised for its promising optoelectronic properties, although the presence of deep interfacial defects at the vicinity of the p-n junction currently limit its performance as photovoltaic absorber. Using a device modelling approach and a realistic set of material parameters, we unravel pathways to mitiga...
This work combines experimental results and modelling assessment of high Ga, wide bandgap CIGSe solar cells fabricated on a transparent glass/TCO substrate yielding efficiencies above 10% without AR coating. An alkali pre-deposition treatment is performed, and the material analysis of the devices by GDOES, XRD and Raman spectroscopy reveals a signi...
Germanium-based wide band gap kesterite semiconductor Cu2ZnGe(S,Se)4 (CZGeSSe) is considered a very promising absorber compound as top cell in tandem devices. Autonomy to tailor the band gap from ~1.47 eV (Cu2ZnGeSe4-CZGeSe) to ~2.2 eV (Cu2ZnGeS4-CZGeS), as well as non-toxic constituents makes this compound a strong candidate for further scientific...
The potential of tandem solar cells combining two chalcopyrite absorbers is evaluated using numerical modelling based on an exhaustive set of experimental parameters, offering a high degree of confidence in the numerical values reported in this communication. The simple yet reliable approach used in this work combines a transfer matrix‐based optica...
Fabrication on SLG/FTO transparent substrates gives
an important advance to fabrication of tandem solar cells. In
addition, a possible passivation using Al2O3 which can be
deposited with cheap systems can opens an increase in
development of CZTS solar cells. In this work, we report the
simultaneous use of FTO/Mo(20nm) as a transparent substrate
and...
The cover image is based on the Research Article Rear interface engineering of kesterite Cu2ZnSnSe4 solar cells by adding CuGaSe2 thin layers by Sergio Giraldo et al., https://doi.org/10.1002/pip.3366.
A method to probe the depth morphology, defect profile and possible secondary phases in a thin film semiconductor is presented, taking a standard Kesterite film as an example. Using a top-down approach based on a previously reported controlled Methanol-Br2 chemical etching, well-defined slabs of a state of the art Kesterite absorber are fabricated....
Supplementary information for Rear Band Gap Grading for Strategies on Sn−Ge-Alloyed Kesterite Solar Cells
Kesterite Cu2ZnSn(S,Se)4 thin film technology has been thoroughly investigated during the last decade as a promising solution in the field of low‐cost, sustainable, and environmental‐friendly photovoltaic technologies. However, despite efforts to boost kesterite solar cells performance by numerous strategies, the efficiencies remain stagnant around...
Kesterite solar cells are at a crossroads, and a significant breakthrough in performance is needed for this technology to stay relevant in the upcoming years. In this work, we propose to follow the proven strategy of band engineering to assist charge carrier collection taking inspiration from chalcopyrite solar cells. Using a process based on a com...
Transparent Photovoltaics
In article number 2000470, Alex J. Lopez‐Garcia and co‐workers report on the fabrication of transparent UV‐selective solar cells based on Zn(O,S) as the absorber. The use of Zn(O,S) as UV‐harvesting material is discussed, and a record device is presented showing a power conversion efficiency of 0.5% at an average visible t...
With the recent rise of new photovoltaic applications, it becomes necessary for thin film technologies such as Cu(In,Ga)Se2 to develop specific optoelectronic properties and take advantage of their high degree of tunability. The feasibility of efficient wide bandgap absorbers on transparent conductive oxide substrates is in that context of critical...
This work reports experimental evidence of a photovoltaic effect in transparent
UV-selective Zn(O,S)-based heterojunctions. Zn(O,S) has a strong interest for the
development of UV-selective solar cells with high transparency in the visible
region, required for the development of nonintrusive building-integrated photovoltaic
(BIPV) elements as trans...
Antimony selenide (Sb2Se3) based solar cell technology has experienced rapid development with demonstrated cell efficiency reaching ̴ 9.2% for devices in substrate configuration, hence motivating more intense research investigations. Though the effect of crystallographic orientation in this non-cubic material on device performance is now well under...
The lack of a systematic definition of intermittency in the power sector blurs the use of this term in the public debate: the same power source can be described as stable or intermittent, depending on the standpoint of the authors. This work tackles a quantitative definition of intermittency adapted to the power sector, linked to the nature of the...
Sb2Se3 Absorbers
In article number 2000141, Pedro Vidal‐Fuentes, Edgardo Saucedo, and co‐workers find the annealing temperature to be the main parameter controlling the composition of the Se‐rich quasi‐one‐dimensional Sb2Se3 absorbers for solar cell applications, synthesized by a sequential process using thermal evaporation of Sb, followed by react...
Tandem solar cells present a practical route toward low-cost cells with high-efficiency. CIGSe materials owing to their large absorption coefficient, tunable bandgap, low cost, and easy fabrication process, presents a perfect top-cell candidate in tandem device. To achieve this, fabricate of high efficiency CIGSe solar cells on transparent substrat...
Quasi‐one‐dimensional chalcogenides are under the spotlight due to their unique properties for several technological applications including computing, photonic, sensing and energy conversion. In particular, antimony chalcogenides have recently experienced incredible progresses as emerging photovoltaic materials. In this work, the fabrication by a s...
We report the effect of an ultra-thin Al2O3 layer (down to 3 nm) as interface passivation strategy for the improvement of the performance of Cu2ZnSnS4 /CdS based solar cells. After an initial optimization, the Al2O3 deposited by thermal evaporation is proved to improve the properties of the p-n junction. The fabricated devices showed an increment i...
Kesterite semiconductors, derived from the mineral Cu-2(Zn,Fe)SnS4, adopt superstructures of the zincblende archetype. This family of semiconductors is chemically flexible with the possibility to tune the physical properties over a large range by modifying the chemical composition, while preserving the same structural backbone. In the simplest case...
III-V materials have yet proved to be promising candidates for photovoltaic applications. Solar cell technologies based on III-V semiconductors are competitive in term of fabrication cost but also have the potential to reach the highest photovoltaic efficiencies. To overcome the 30% conversion limitation established for a single junction, the Shock...
Hot-carrier solar cells (HCSC) can potentially overcome the Shockley-Queisser limit, by having carriers at a higher temperature than the lattice. To this end, the carriers need to thermalize slower than power is generated by absorbing photons. In thin films, a hot-carrier distribution can only be achieved with very high incident power, by saturatin...
The latest progress and future perspectives of thin film photovoltaic kesterite technology are reviewed herein. Kesterite is currently the most promising emerging fully inorganic thin film photovoltaic technology based on critical raw‐material‐free and sustainable solutions. The positioning of kesterites in the frame of the emerging inorganic solar...
Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before...
We investigated on a way to demonstrate the selective extraction of hot photocarriers for ultra-high efficiency solar cells. We proposed a theoretical model studying the feedback of the extraction on the remaining population. The removal of high energy particles from the system was found to decrease its temperature, which can be used as a proof of...
Hot Carrier Solar Cells promise efficiencies way above Shockley-Queisser (SQ) limit, but are limited by the fast thermalization of the carriers. Ultra-thin solar cells can reduce this thermalization, but at the cost of a reduced absorption. We show light trapping can solve this issue, making the cell optically thicker, while electronically and phon...
Lagrange multipliers provide a powerful framework to devise the optimization of systems under constraints. It can be especially useful in the context of photovoltaics, where electrical or structural continuity relations impose connections between quantities, such as current matching between the different subcells of tandem devices. In this work, we...
Dip coating is a highly efficient solution deposition technique suitable for chemically-synthesized perovskite quantum dots (PQDs). In dip coating, withdrawal speed is a key factor that determines thickness of the films and may affect their quality. In this work, the optimum withdrawal speed that would result to optically smooth cesium lead halide...
Intermediate Band Solar Cell is an advanced concept for solar energy conversion in which two low-energy photons can promote an electron to the conduction band through a so-called intermediate band. To limit recom-bination and preserve the photo-generated voltage, generation to-and from the intermediate band should be matched. However, all practical...
Intermediate Band Solar Cell is an advanced concept for solar energy conversion in which two low-energy photons can promote an electron to the conduction band through a so-called intermediate band. To limit recombination and preserve the photo-generated voltage, generation to- and from the intermediate band should be matched. However, all practical...
The effect of energy-selective extraction on a hot carrier’s population is addressed in this study. Using an evaporative cooling model inspired by the field of cold atoms, we derive an analytical expression supported by numerical calculations to account for the removal of particles from the distribution and subsequent energy redistribution among th...
Energy selective electrical contacts have been proposed as a way to approach ultimate efficiencies both for thermoelectric and photovoltaic devices as they allow a reduction of the entropy production during the energy conversion process. A self-consistent numerical model based on the transfer matrix approach in the effective mass and envelope funct...
The hot carrier solar cell is a very promising clean energy technology, with the potential to achieve high conversion yields with constrained costs. Due to the hot carrier effect, the estimation of the achievable voltage needs some theoretical developments. The classical approach is to consider isentropic energy selective contacts, converting the e...
The hot carrier solar cell is a very promising clean energy technology, with the potential to achieve high conversion yields with constrained costs. Due to the hot carrier effect, the estimation of the achievable voltage needs some theoretical developments. The classical approach is to consider isentropic energy selective contacts, converting the e...
Abstract Heat-light soaking effects on Cu(In,Ga)Se2 (CIGS) solar cells with atomic layer deposition (ALD)-Zn(O,S) and chemical bath deposition (CBD)-ZnS(O,OH) buffer layers were investigated using temperature-dependent current–voltage and admittance spectroscopy measurements. Both CBD-ZnS(O,OH)/CIGS and ALD-Zn(O,S)/CIGS solar cells showed a signifi...
This paper reports a comparative study of Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cells with CBD-CdS, CBD-ZnS(O,OH) and ALD-Zn(O,S) buffer layers. Each buffer layer was deposited on CIGSSe absorber layers which were prepared by sulfurization after selenization (SAS) process by Solar Frontier K. K. Cell efficiencies of CBD-CdS/CIGSSe, CBD-ZnS(O,OH...
We address the evaluation of energy selective contacts for hot carrier solar cells through numerical modeling of heterostructures based on III-V materials with a resonant energy level that acts as the energy selective level. A self-consistent code is developed using the transfer matrix formalism and the effective mass approximation to model the beh...
We investigate on narrow bandgap CIGSe solar cells for tandem application in a spectral splitting devices. Numerical simulations using SCAPS are performed to optimize the Ga content throughout the absorber layer, depending on the illumination condition (AM 1.5 and tandem conditions using two different splitting filters; 620 nm and 880 nm). The simu...
Abstract The modeling of the effect of absorber surface sulfurization on state-of-the-art CIGSe-based solar cells is studied using the 1 dimensional modeling tool SCAPS 3.2. The evolution of the photovoltaic performance of the cell is modeled for different sulfur contents and sulfurization depth; these parameters are simultaneously varied to obtain...
Complex dielectric function of very thin ZnO films prepared with different oxygen concentration during deposi-tion has been studied in 220–1700 nm and 2–25 μm spectral range. Variation of oxygen concentration in depo-sition chamber is intended to stabilize film properties by compensating oxygen deficiency. The films obtained by magnetron sputtering...
In this work, the impacts of surface sulfurization of high-quality Cu(In1−x,Gax)Se2 (CIGS) thin films deposited by three-stage process on the film properties and the cell performance were investigated. The CIGS thin films were sulfurized at 550 °C for 30 min using H2S gas. The X-ray photoelectron spectroscopy analysis revealed that sulfur atoms dif...
Implementing photovoltaic devices based on high efficiency thin-film technologies on cheap, light-weight and flexible polymeric substrates is highly appealing to cut down costs in industrial production and to accelerate very large scale deployment of photovoltaics in the upcoming years. Lift-off processes, which allow separating active layers from...
The impacts of post-treatment on cell performance of ZnS(O,OH)/Cu(In,Ga)Se2 solar cells were investigated, when ZnS(O,OH) buffer layer was deposited using a thioacetamide (TAA)-ammonia based chemical solution. All parameters especially FF increased drastically after heat-light soaking treatment. Although the cell efficiency showed slight increase a...
Abstract A Zn(O,S)/ZnO double buffer layer deposited using an atomic layer deposition (ALD) system and boron-doped ZnO window layer deposited using a metal organic chemical vapor deposition (MOCVD) system were applied to Cd-free Cu(In,Ga)Se2(CIGS) thin film solar cells. The bandgap energy of the Zn(O,S) buffer layer was varied from 2.8 to 3.6 eV by...
Narrow-bandgap CuIn(Se1-xTex)(2)-based solar cells for bottom cell application in multijunction photovoltaic devices were developed and characterized. Compact and well crystallized 2 mu m films are obtained by single-stage coevaporation process on Mo-coated soda lime glass. Narrow bandgaps ranging from 0.98 eV to 0.88 eV were measured depending on...
We propose CuIn(Se1-x, Tex)2 as a quaternary chalcopyrite material for narrow-bandgap thin films solar cells, and focus on the purpose of realizing tandem photovoltaic devices by using these solar cells combined with a semi-transparent CIGSe solar cell. Numerical simulations are realized to evaluate the potential of such device, and the effect of t...
In the past years, reducing the thickness of the absorber layer in
CIGS-based solar cells has become a key issue to reduce the global
Indium consumption and thus increased its competitiveness. As the
absorber thickness is reduced, less photons are absorbed and
consequently the efficiency decreases. It is well known that scattering
light in the abso...
We investigate the optical absorption in a thin Cu(In,Ga)Se2 solar cell with a Lambertian white paint beneath a transparent back contact. Although this configuration has been proposed more than 30 years ago, it turns out that rigorous simulation of Maxwell’s equations demand powerful numerical calculations. This type of approach is time consuming a...
Improving the optical management is a key issue for ultrathin based solar cells performance. It can be accomplished either by trapping the light in the active layer or by decreasing the parasitic absorptions in the cell. We calculate the absorption of the different layers of Cu(In,Ga)Se2 (CIGSe) based solar cell and propose to increase the absorpti...
An innovative approach combining chemical etching and a “lift-off” process, which allows back contact processing after CIGSe deposition, permitted to use Au as a highly reflective back contact in ultrathin CIGSe solar cells. The Au back contact does not degrade the other parameters of the cell, as good ohmicity on CIGSe is achieved. An important ph...
The influence of Cu(In,Ga)Se{sub 2} (CIGSe) surface roughness on the photovoltaic parameters of state of the art devices is reported, highlighting the importance of the roughness of the as-grown CIGSe absorbers on solar cell efficiencies. As-grown CIGSe surface is progressively smoothed using a chemical etch, and characterized by SEM, AFM, XPS, {mu...
In this thesis, we investigate on the possibility to realize ultrathin absorber Copper Indium Gallium Di-Selenide (CIGSe) solar cells, by reducing the CIGSe thickness from 2500 nm down to 100 nm, while conserving a high conversion efficiency.Using numerical modeling, we first study the evolution of the photovoltaic parameters when reducing the abso...
This study addresses the potential of different approaches to improve
the generated current density in ultrathin Cu(In,Ga)Se2 (CIGSe) based
solar cells down to 0.1 μm. Advanced photon management, involving
both absorption enhancement and reflection reduction in the absorber, is
studied. In this contribution, the three main approaches used are: - Th...
Chemical and optical properties of the interface between a coevaporated
Cu(In,Ga)Se2 (CIGSe) absorber thin film and the Mo back
contact are investigated with the objective to reduce markedly the
thickness of CIGSe layers from two microns down to about 100 nm. First a
mechanical lift off technique allowed to separate Mo and CIGSe layers
and perform...
![Figure 8: PDMS stamp replicated from silicon master mold [31].](profile/Jean-Francois-Guillemoles/publication/273329576/figure/fig8/AS:391836016365583@1470432394571/PDMS-stamp-replicated-from-silicon-master-mold-31_Q320.jpg)
We propose new solutions for broadband light trapping in ultrathin (≤100 nm) CIGS solar cells. By introducing metallic nano-arrays embedded in the ZnO:Al front contact layer, we show that multi-resonant optical absorption of light can be achieved in solar cells with 100 nm-thick CIGS layer. We analyze the origin of this broadband light trapping by...
We propose new solutions for broadband light trapping in ultrathin (≤100 nm) CIGS solar cells. By introducing metallic nano-arrays embedded in the ZnO:Al front contact layer, we show that multi-resonant optical absorption of light can be achieved in solar cells with 100 nm-thick CIGS layer. We analyze the origin of this broadband light trapping by...
In this paper, the influence of reducing the thickness of the CIGSe absorber layer by bromine etching from 2.5μm to 0.5μm on electrical and optical solar cell properties is addressed. We observe a decrease in efficiency which is mainly caused by a reduced short circuit current, whereas the fill factor and the open circuit voltage are stable. Even w...
Within the UltraCIS project we have started to explore the possibility of reducing down the thickness of the Cu(In, Ga)Se2 (CIGSe) layer to the sub-micron level (0.1μm) while maintaining a high efficiency level of solar cells. The three main approaches we used are: - Reducing the CIGSe thickness by chemical etching combining the thickness reduction...
The electrodeposition of ZnO nanorods on ZnO:Al films with different orientations is reported. The influence of the total charge exchanged during electrodeposition on the nanorod's geometry (length, diameter, aspect ratio and surface density) and the optical transmission properties of the nanorod arrays is studied on a [0001]-oriented ZnO:Al substr...
The influence of CIGSe surface roughness over the photovoltaic parameters is reported. The as deposited rough CIGSe surface is smoothed using a chemical etch, and the bare CIGSe surface is characterized by SEM, AFM and Reflectivity. As the roughness decreases, the total reflectivity of the CIGSe surface increases, but the ratio of diffuse reflectiv...
Current versus voltage and temperature (IVT) characterizations have been performed on CIGS (Cu(In,Ga)Se2 solar cells previously submitted to a chemical etching of their absorber layer. This study aims to investigate the changes in recombination paths, if any, when the absorber layer thickness is reduced. Diode parameters saturation current, idealit...
Questions
Question (1)
Hello.
The question may seem trivial, but while I understand the basic principle, its application to full size solar panel remains somehow unclear to me.
Specifically, I do not understand why series resistance is not a bigger problem for monolithic integrated solar panels. Indeed, the current flows through a succession of TCO (often ITO or AZO) and metallic stripes (usually Mo) in the transverse direction, which can be seen (in a 1D representation) as a succession of wires connected in series with 2 different resistivities. (See Figure, from Scientific Reports volume5, Article number: 7690 (2015) )
As in any series connected circuit, the resistance of each element should be added to obtain the total resistance of the circuit. While this is not an issue for the metallic part, the succession of TCO stripes should add a considerable resistance to the whole circuit, resulting in poor photovoltaic performances. However, it is obviously not the case as solar panel manufacturer continue using monolithic integration rather than a simple planar design with a front metallic grid.
I know that I am missing an important point here, but I cannot see what exactly, so I am turning to the community to explain me where my mistake is.
Thank yo in advance.
