David Duché

Slow Bloch modes for enhancing the absorption of light in thin films for photovoltaic cells

This paper deals with the improvement of “light harvesting” in photovoltaic cells by using photonic nanostructures. We theoretically study a poly-3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) thin film periodically nanostructured in order to increase its absorption. The periodic nanostructuration allows “slow Bloch modes” (group velocity close to zero) to be coupled inside the material. The P3HT/PCBM photonic crystal parameters are adjusted to maximize the density of Bloch modes and obtain flat dispersion curves. The light-matter interaction is thus strongly enhanced, which results in a 35.6% increase of absorption in the 600–700nm spectral range.

Optical-electrical simulation of organic solar cells: excitonic modeling parameter influence on electrical characteristics

This paper deals with Organic Solar Cells (OSCs) simulation using finite element method. Optical modeling is performed via Finite Difference Time Domain method whereas the continuity and Poisson’s equations are solved to obtain electrical characteristics of the OSC. In this work, simulation results point out the influence of physical parameters such as the exciton diffusion coefficient or the exciton lifetime on OSC performances. The comparison of modeling results and experimental measurement allows the exciton recombination, dissociation rate and lifetime to be determinated.

Optical-electrical simulation of organic solar cells: Influence of light trapping by photonic crystal and ZnO spacer on electrical characteristics

This paper deals with organic solar cells (OSC) simulation using finite element method. Optical modeling is performed via finite difference time domain method whilst the continuity and Poisson’s equations are solved to obtain electrical characteristics of the OSC. In this work, simulation results point out the OSC structure influence on its performances, either by the interface grating or by the ZnO optical spacer introduced between the active layer (P3HT:PCBM layer) and the metallic electrode. The comparison of modeling results and experimental measurement allows us to confirm and forecast the enhancement of the photovoltaic properties such as the power conversion efficiency.

Time evolution studies of dithieno[3,2-b:2′,3′-d]pyrrole-based A–D–A oligothiophene bulk heterojunctions during solvent vapor annealing towards optimization of photocurrent generation

Solvent vapor annealing (SVA) is one of the main techniques to improve the morphology of bulk heterojunction solar cells using oligomeric donors. In this report, we study time evolution of nanoscale morphological changes in bulk heterojunctions based on a well-studied dithienopyrrole-based A–D–A oligothiophene (dithieno[3,2-b:2 0 ,3 0-d]pyrrole named here 1) blended with [6,6]-phenyl-C 71-butyric acid methyl ester (PC 71 BM) to increase photocurrent density by combining scanning transmission electron microscopy and low-energy-loss spectroscopy. Our results show that SVA transforms the morphology of 1 : PC 71 BM blends by a three-stage mechanism: highly intermixed phases evolve into nanostructured bilayers that correspond to an optimal blend morphology. Additional SVA leads to completely phase-separated micrometer-sized domains. Optical spacers were used to increase light absorption inside optimized 1 : PC 71 BM blends leading to solar cells of 7.74% efficiency but a moderate photocurrent density of 12.3 mA cm A2. Quantum efficiency analyses reveal that photocurrent density is mainly limited by losses inside the donor phase. Indeed, optimized 1 : PC 71 BM blends consist of large donor-enriched domains not optimal for exciton to photocurrent conversion. Shorter SVA times lead to smaller domains; however they are embedded in large mixed phases suggesting that introduction of stronger molecular packing may help us to better balance phase separation and domain size enabling more efficient bulk heterojunction solar cells.

The influence of branched alkyl side chains in A–D–A oligothiophenes on the photovoltaic performance and morphology of solution-processed bulk-heterojunction solar cells

Besides providing sufficient solubility, branched alkyl chains also affect the film-forming and packing properties of organic semiconductors. In order to avoid steric hindrance as it is present in wide-spread alkyl chains comprising a branching point position at the C2-position, i.e., 2-ethylhexyl, the branching point can be moved away from the π-conjugated backbone. In this report, we study the influence of the modification of the branching point position from the C2-position in 2-hexyldecylamine (1) to the C4-position in 4-hexyldecylamine (2) connected to the central dithieno[3,2-b:2′,3′-d]pyrrole (DTP) moiety in a well-studied A–D–A oligothiophene on the optoelectronic properties and photovoltaic performance in solution-processed bulk heterojunction solar cells (BHJSCs) with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor material. Post-treatment of the photoactive layers is performed via solvent vapor annealing (SVA) in order to improve the film microstructure of the bulk heterojunction. The time evolution of nanoscale morphological changes is followed by combining scanning transmission electron microscopy with low-energy-loss spectroscopic imaging (STEM-SI), solid-state absorption spectroscopy, and two-dimensional grazing incidence X-ray diffraction (2D-GIXRD). Our results show an improvement of the photovoltaic performance that is dependent on the branching point position in the donor oligomer. Optical spacers are utilized to increase light absorption inside the co-oligomer 2-based BHJSCs leading to increased power conversion efficiencies (PCEs) of 8.2% when compared to the corresponding co-oligomer 1-based devices. A STEM-SI analysis of the respective device cross-sections of active layers containing 1 and 2 as donor materials indeed reveals significant differences in their respective active layer morphologies.

Modeling the Back Contact of Cu 2 ZnSnSe 4 Solar Cells

In this paper, we analyze the theoretical impact of the Molybdenum (Mo) back contact and the MoSe2 interfacial layer on the performances of a Cu2 ZnSnSe4 (CZTSe)-based solar cell. MoSe2 layers are formed spontaneously in the Mo/CZTSe interface during the annealing of the absorber, but disagreeing interpretations about their actual role in affecting the device figures of merit (VOC , JSC , FF, and η) have been proposed in the literature. In our approach, we have simulated three structures presenting different conditions at the back contact: ideal-contact/CZTSe (flat-band), Mo/CZTSe, and Mo/MoSe2/CZTSe. For these three layers, an accurate explanation of the selection of critical material parameters is given. The numerical simulations, performed with SCAPS 3.2.01, show that the low values of Mo work function (≤4.95 eV) would have a strong detrimental effect on the VOC and FF of the cell if no interfacial layers were present at the Mo/CZTSe interface. On the other hand, a beneficial effect of the MoSe2 layer on the VOC of the device is demonstrated when this layer is included in the structure. This trend is confirmed by experimental measurements. The expected band diagram of the full ZnO/CdS/CZTSe/MoSe2/Mo structure is provided.

Photonic crystals for improving light absorption in organic solar cells

We theoretically and experimentally study the structuration of organic solar cells in the shape of photonic crystal slabs. Using a Finite Difference Time Domain (FDTD) method, we investigate the double structuration of the PEDOT:PSS layer and the metallic electrode. By taking advantage of the optical properties of photonic crystals slabs, we show the possibility to couple Bloch modes with very low group velocities in the active layer of the cells. Such Bloch modes, also called slow Bloch modes (SBMs), allow increasing the lifetime of photons within the active layer. We show that an absorption gain ranging between 4% and 11% is possible according to the band gap of the organic material. Finally, we present experimental demonstration performed using nanoimprint to directly pattern the standard organic semiconductor P3HT :PCBM blend in thin film form in the shape of a photonic crystal able to couple SBMs.

Model of self assembled monolayer based molecular diodes made of ferrocenyl-alkanethiols

There has been significant work investigating the use of self assembled monolayers (SAMs) made of ferrocenyl terminated alkanethiols for realizing molecular diodes, leading to remarkably large forward-to-reverse current rectification ratios. In this study, we use a multiband barrier tunneling model to examine the electrical properties of SAM-based molecular diodes made of HSC 9 Fc, HSC 11 Fc, and HSC i FcC 13Ai (0 i 13). Using our simple physical model, we reproduce the experimental data of charge transport across various ferrocenyl substituted alkanethiols performed by Nijhuis, Reus, and Whitesides [J. Am. Chem. Soc. 132, 18386–184016 (2010)] and Yuan et al. [Nat. Commun. 6, 6324 (2015)]. Especially, the model allows predicting the rectification direction in HSC i FcC 13Ai (0 i 13) based molecular diodes depending on the position of the ferrocenyl (Fc) moiety within the molecules. We show that the asymmetry of the barrier length at both sides of the Highest Occupied Molecular Orbital of the ferrocenyl moiety strongly contributes to the rectifying properties of ferrocenyl-alkanethiol based molecular junctions. Furthermore, our results reveal that bound and quasi-bound states play an important role in the charge transport.

Photonic crystals for light trapping within organic solar cells

We propose a methodology allowing the design of the active layer of organic solar cells in the shape of a photonic crystal. An optimised photonic crystal allows trapping the light in a layer at specific wavelengths thanks to a coupling of a low group velocity mode called slow Bloch mode. This method is used to design two structures which allow to improve absorption of light in organic solar cells for wavelengths close to the band gap of an active layer composed of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butiryc acid methyl ester (PCBM). Nevertheless, while the first structure does not allow an efficient charges harvesting by the electrodes, the second structure can be beneficial for both the optical and the electrical properties of the cell thanks to the structuring of the Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer.

1D and 2D numerical simulations of Cu2ZnSnSe4 solar cells

1D and 2D numerical simulations can be employed to perform optimizations of thin film solar cells and analysis of the physical mechanisms affecting the performances. In this work we discuss optical optimizations of a CZTSe solar cell predicted by a 1D model and a 2D model implementing grain boundaries as p-type phases with low band gap. The results of the simulations point out the significant impact that these GBs can have on the Voc loss and possible correlations with other experimental results obtained by K-AFM and C-AFM.