Linda Cattin

Development of a reproducible technique to grow large area single crystalline perovskite wafers is an open research gap in the field of single crystalline perovskite solar cells. A graphite-based localized heating technique for growing large area methylammonium lead bromide (CH₃NH₃PbBr₃; MAPBr) single crystalline thin film (SCTF) on different buffer layers, such as glass/indium doped tin oxide (ITO), glass/ITO/poly(triaryl amine) (PTAA), and glancing angle deposition (GLAD) coated glass/ITO/TiO₂ substrates is reported, and their charge transport properties are discussed. It is observed that the localized heating technique can confine the supersaturation of the precursor mainly to the center of the substrate, leading to a restricted number of nucleations within a specific area on the substrate. Here, such 2-3 seed crystals obtained initially are allowed to grow to a larger size of up to 65 mm². The X-ray diffraction (XRD) analysis indicated that the large area SCTF is an actual single crystal and not a heterogeneous group of small crystals merged together with a crystallinity index (CI) of 92.60 ± 0.11% which was comparable to that of the bulk single crystal (97.74 ± 0.47%). The atomic force microscopy (AFM) image depicted a smooth SCTF surface (R _a = 4.37 ± 0.01 nm), and the wave-like pattern is attributed to the substrate morphology, implying that the topography of the substrate plays a crucial role in obtaining a planar SCTF. The XRD, UV-visible, photoluminescence (PL), Raman, and FTIR spectra analyses revealed that the large area SCTF is phase pure and free of residual impurities. The charge injection characteristics of the SCTFs grown on different buffer layers were investigated using PL emission (PLE) and PL decay analyses. The decrease in the PLE intensity for the SCTFs grown on PTAA and TiO₂ substrates implied exciton quenching behavior, indicating the injection of the photogenerated charge carriers into the charge transfer layers (CTLs). The decrease of the fast decay component from τ₁ = 4.77 ± 0.18 ns for glass to τ₁ = 3.32 ± 0.07 ns for TiO₂ and τ₁ = 3.15 ± 0.33 ns for PTAA is ascribed to the interfacial recombination of the charges accumulated at the CTL/perovskite interface. These results propose that the localized heating technique can be employed for growing large area single crystalline perovskite wafers for optoelectronic and photovoltaic device applications.

In the present study, MoO₃:MoS₂ hybrid thin layers have been synthesized through partial oxidation of MoS₂. We have demonstrated that the reaction requires darkness conditions to decrease the oxidation rate, thus obtaining the hybrid, MoO₃:MoS₂. A simple liquid-phase exfoliation (LPE) is carried out to achieve homogenous MoS₂ nanoflakes and high reproducibility of the results after MoS₂ oxidation. XPS analyses reveal the presence of MoO₃, MoS₂, and MoOxSy in the hybrid layer. These results are also confirmed by X-ray diffraction and high-resolution TEM. Optical absorbance reveals that the absorption peaks of the MoO₃:MoS₂ hybrid are slightly redshifted with the appearance of absorption peaks in the near-infrared region due to the defects created after the oxidation reaction. The composition and atomic percentages of each component in the hybrid layer as a function of reaction time have also been reported to give perspective guides for improving electronic and optoelectronic devices based on 2D-MoS₂.

Ternary planar heterojunction organic solar cells (PHJ-OPVs) were fabricated using three organic small molecules, alpha-sexithiophene (α-6T), aluminum phthalocyanine chloride (AlPcCl) and fullerene (C₆₀). These molecules can be easily sublimated under a vacuum; they have complementary optical absorption spectra and their energy band structure alignment is favorable for electronic charge transfers. Moreover, α-6T and AlPcCl have almost the same HOMO, which is desirable to avoid any decrease in open circuit voltage. The AlPcCl intercalated layer bridges the energy levels of the electron donor, α-6T, and the electron acceptor, C₆₀, which facilitates charge transport through the energy cascade effect. Moreover, the charge carrier mobility measurements of AlPcCl, using the space charge limited current method, demonstrated that it iss ambipolar. All these properties combine to improve the power conversion efficiency (PCE) of PHJ-OPVs by moving from binary structures (α-6T/C₆₀, α-6T/AlPcCl and AlPcCl/C₆₀) to ternary ones (α-6T/AlPcCl/C₆₀). We show, in this study, that both interfaces of the ternary PHJ-OPVs are efficient for carrier separation. After optimization of the different layer thickness, we show that, by comparing the optimum efficiencies of the binary PHJ-OPVs, the realization of ternary PHJ-OPVs, based on the active layers α-6T/AlPcCl/C₆₀, using the following optimized method, allows us to achieve a PCE of 4.33%.

In order to grow semi-transparent organic photovoltaic cells (OPVs), multilayer dielectric/metal/dielectric (D/M/D) structures are used as a transparent top electrode in inverted OPVs. Two different electrodes are probed, MoO₃/Ag/MoO₃ and MoO₃/Ag/Cu:Ag/ZnS. Both of them exhibit high transmission in visible and small sheet resistance. Semi-transparent inverted OPVs using these electrodes as the top anode are probed. The active organic layers consist in the SubPc/C₆₀ couple. The dependence of the OPV performances on the top electrode was investigated. The results show that far better results are achieved when the top anode MoO₃/Ag/MoO₃ is used. The OPV efficiency obtained was only 20% smaller in comparison with the opaque OPV, but with a transparency of nearly 50% in a broad range of the visible light (400–600 nm). In the case of MoO₃/Ag/Cu:Ag/ZnS top anode, the small efficiency obtained is due to the presence of some Cu diffusion in the MoO₃ layer, which degrades the contact anode/organic material.

In this work, nanostructured TiO₂ and poly-3,4-ethylenedioxythiophene (PEDOT) layers were electrochemically prepared over transparent electrodes. Morphological characterization evidenced the presence of nanostructures as planed with 50-nm-wide TiO₂ rod formations followed by 30-nm-wide PEDOT wires. Different characterizations were made to the deposits, establishing their composition and optic properties of the deposits. Finally, photovoltaic cells were prepared using this modified electrode, proving that the presence of PEDOT nanowires in the cell achieves almost double the efficiency of its bulk analogue.

Studying the electrochemical characteristics is an important step for determining interactions between molecules and the chemical environment. Moreover, the electrochemical evaluation of dyes is highly needed to establish the behavior of electro-active chemical species inside dye-sensitized solar cells (DSSCs). Four compounds, M8-1, M8-2, M8-O1, and M8-O2 (with a common organic structure (E)-2-cyano-3-(5-((E)-2-(9,9-diethyl-7-(phenylamino)-9H-fluoren-2-yl)vinyl)thiophen-2-yl)acrylic acid), are studied in two solvents, tetrahydrofuran (THF) and dimethylsulfoxide (DMSO). Among the studied compounds, M8-1 has highlighted characteristics compared with the others: its ground and excited states oxidation potential are the highest (1.14 and −1.22 V, respectively). Also, it shows the lowest energy gap between the excited state oxidation potential and the TiO₂ conduction band. Relating to the substituent effect, the shorter the length, the higher the energetic difference in the electronic transition (M8-1 and 2). Comparing characteristics through quantum chemistry, the values obtained in DMSO are the most predictable. The injection energies signal that M8-1 is the best injector. The performances in solar cells are measured in three TiO₂ materials: Degussa (D-TiO₂), active opaque (A-TiO₂), and transparent (T-TiO₂). The IPCE results show the A > T > D average tendency, and the family of substituted alkyl has higher values than the alcoxyl one. Furthermore, in the first family the methyl substituent has a higher value than the ethyl one. M8-1 has the highest IPCE value, on average. In terms of efficiency, the alkyl substituted family again has higher values than the alcoxyl family. On average, the methyl substituent has a higher value than the ethyl one in both families. M8-1 has the highest efficiency value.

Ternary and binary planar heterojunctions (PHJs) have been realized and characterized. The outer layers of the active organic layers are pentathiophene (5T) and fullerene (C₆₀), while the intercalated layer is AlPcCl. The binary, 5T/C₆₀, 5T/AlPcCl and ternary 5T/AlPcCl/C₆₀ PHJs were characterized by J-V and EQE measurements. The morphology of the organic layers was studied by scanning electron microscopy and atomic force microscopy, while the band structure of 5T was estimated by cyclic voltammetry. The study shows that it is possible to overcome the difficulties linked to the need for the good band matching of the three successive organic layers by using as a first electron donor layer, a layer whose morphology allows the ternary structure to behave as it was two diodes in parallel. Actually, due to this specific morphology the intercalated layer is discontinuous, which allows achieving parallel mechanism in planar ternary photovoltaic cells. This parallel mechanism in the 5T/AlPcCl/C₆₀ organic photovoltaic cells (OPVCs) allows achieving efficiency of 1.25%, which represents a 65 % increase by comparison with the best binary 5T/C₆₀ OPVC. It means that the morphology of the 5T layer, with its many protrusions and holes, allows ternary OPVCs to behave like parallel-linkage diodes. This behaviour allows the ternary OPVCs to achieve efficiencies higher than those obtained with the binary 5T/C60 OPVCs.