Papers by Keyword: Photovoltaics

Abstract: To meet the requirements of second generation photovoltaics, spin coating and RF magnetron sputtering techniques have been utilized to fabricate zinc sulfide thin films for buffer layer optimization. During fabrication process, substrate temperatures for spin coating and RF magnetron sputtering processes are kept at room temperature and at 200 oC, respectively. Thin films are annealed at 500oC for 1 hour in an inert environment to acquire crystallinity and uniform surface morphology. XRD analysis reveals that thin films fabricated by spin coating and RF magnetron sputtering exhibit wurtzite and zinc blende crystal structures, respectively. SEM shows that the surface morphology of thin films fabricated by both techniques is uniform and homogeneous without voids and cracks. EDS results indicate that thin films fabricated via spin coating have equal stoichiometric ratio of zinc to sulfur (1:1). Whereas, an unequal stoichiometric ratio of zinc to sulfur is detected in RF magnetron sputtered thin films. According to optical studies, spin coated zinc sulfide thin films have 67% transmission with an energy band gap of 3.62 eV. While, RF magnetron sputtered thin films have 76% transmission with a wide energy band gap of 3.70 eV. Electrical properties depict that thin films fabricated by RF magnetron sputtering have higher carrier concentration, lower resistivity and higher conductivity than spin coated thin films. In comparison, RF magnetron sputtered zinc sulfide thin films exhibit best structural and optoelectronic properties for buffer layer in second generation solar cells.

Abstract: High-quality BFCO films with smooth morphology were deposited on (001)-oriented single crystal substrates by pulsed-laser deposition. The perfect ferroelectric nature of the BFCO films were probed by PFM and electrical P-E hysteresis measurements and the obtained saturated remeanent polarization value reaches up to 60 μC/cm². Besides, BFCO films show relatively low bandgap of 2.5 eV, can absorb the sunlight and convert the solar energy to electrically energy effectively. A photocurrent of 1.2 mA/cm² and an open-circuit voltage of about 0.53 V were obtained under AM1.5G illumination. The results mainfest the BFCO a potiential material for multifunctional devices, especially for photovoltaics.

Abstract: Thin crystalline silicon (c-Si) suffers from poor light absorption which hinders generation of high photocurrent in photovoltaic (PV) devices. To overcome this issue, efficient light trapping (LT) schemes need to be incorporated into the thin c-Si absorber. This paper presents ray tracing of LT schemes in thin c-Si to enhance broadband light absorption within 300-1200 nm wavelength region. For the ray tracing, mono c-Si wafer with 100 μm thickness is investigated and solar spectrum (AM1.5G) at normal incidence is used. Front and rear pyramid textures, silicon nitride (SiN_x) anti-reflective coating (ARC) and back surface reflector (BSR) are the LT schemes being studied in this work. With incremental LT schemes, optical properties of the thin c-Si are analyzed. From the absorption curve, maximum potential photocurrent density (J_max) is calculated, assuming unity carrier collection. The c-Si reference (without LT) exhibits J_max of 24.93 mA/cm². With incorporation of incremental LT schemes into the thin c-Si, the J_max increases, owing to enhanced light coupling and light scattering in the c-Si absorber. The J_max up to 42.12 mA/cm² is achieved when all the LT schemes are incorporated into the thin c-Si absorber. This represents 69% enhancement when compared to the J_max of the c-Si reference.

Abstract: Multiferroics, with two or more coexisting ferroic orders (ferroelectric, ferro (antiferro)-magnetic) in a single phase, display promising photovoltaic characteristics which can be utilised in solar energy harvesting. However, the efficacy is seriously challenged due to their wide band gap, far from the ideal value of ~1.52 eV for photovoltaic applications, resulting in overall unimpressive performance. In the present work, an approach towards imparting multiferroism in an otherwise non-ferroic system was adopted through strain engineering. Bulk SrMnO₃ (SMO) is antiferromagnetic-paraelectric. However, our previous first-principles studies predicted high-pressure phase transformation from bulk non-polar phase to a tetragonal polar phase. In light of the above, SMO was synthesised hydrothermally at 200°C for 96 h using water-soluble nitrate salts of strontium and manganese. FESEM study reveals the formation of hexagonal bipyramid shaped SMO crystals with elongated 1-D features. Powder x-ray diffraction studies and subsequent Rietveld refinement confirm the presence of hexagonal (P6₃/mmc) as well as tetragonal (P4mm) phases. Energy dispersive x-ray analysis (EDAX) confirms Sr/Mn ≈ 1, the stoichiometric ratio. UV-VIS spectroscopy was utilised to estimate the optical bandgap of the as-grown sample which was found to be in the range of 1.4-1.5 eV. Temperature-dependent magnetisation plot indicates the magnetic transition temperature, ~275K.

Abstract: Renewable energy sources, such as solar energy, could potentially provide an affordable alternative to conventionally generated electricity, especially in locations like the Caribbean which tend to have an abundant solar resource, but also high cost for electricity. Thin film and hybrid solar devices, including Dye-Sensitized Solar Cells (DSSCs), are especially promising energy solutions, due to the low cost of materials and equipment required for their fabrication. In this paper, we investigate the effect of doping titanium dioxide based DSSC photoanodes with lanthanum, cerium, and praseodymium species on the overall performance of the cell, along with results from optimization of the best performing cell formulation according to sintering time and sintering temperature, giving a maximum 39% increase in device efficiency.

Abstract: Organometal hybrid trihalide perovskite solar cells (PSCs) have emerged a new class of optoelectronic devices for various applications. PSCs have demonstrated unprecedented progress in efficiency reaching certified power conversion efficiency 22.1% after only several years of active research. In this paper, we demonstrate inverted planar mixed halide perovskite solar cells where perovskite layers are built by two-step modified interdifussion and one-step methods. We demonstrate how PSC parameters change by doping electron transport layer (ETL). We used N,N-dimethyl-N-octadecyl (3-aminopropyl) trimethoxysilyl chloride (DMOAP) as dopant for ETL [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM). The highest EQE and V_oc values were obtained for cell prepared by one-step method with fast crystallization and doped ETL but higher fill factor (FF) and shunt resistance (R_shunt) values were obtained for cells prepared by two-step method with undoped ETL.

Abstract: Sublimation-grown 3C-SiC crystals were implanted with B ions at elevated temperature (400 °C) using multiple energies (100 to 575 keV) with a total dose of 1.3×10¹⁷ atoms/cm² in order to form intermediate band (IB) in 3C-SiC. The samples were then annealed at 1400 °C for 60 min. An anomalous area in the center was observed in the PL emission pattern. The SIMS analysis indicated that the B concentration was the same both within and outside the anomalous area. The buried boron box-like concentration profile can reach ~3×10²¹ cm^-3 in the plateau region. In the anomalous area a broad emission band (possible IB) emerges at around ~1.7-1.8 eV, which may be associated with B-precipitates having a sufficiently high density.

Abstract: Copper indium gallium selenide/sulfide (CIGS) and copper zinc tin selenide/sulfide (CZTS) are two thin film photovoltaic materials with many similar properties. Therefore, three new processing steps – which are well-known to be beneficial for CIGS solar cell processing – are developed, optimized and implemented in CZTS solar cells. For all these novel processing steps an increase in minority carrier lifetime and cell conversion efficiency is measured, as compared to standard CZTS processing. The scientific explanation of these effects is very similar to its CIGS equivalent: the incorporation of alkali metals, ammonium sulfide surface cleaning, and Al₂O₃ surface passivation leads to electrical enhancement of the CZTS bulk, front surface and reduced front interface recombination, respectively.

Abstract: Donor–acceptor copolymer series containing 4,6-di (thien-2′-yl) thieno [3,4-c][1,2,5] thiadiazole or its derivatives serving as electron-acceptor units and various electron-donor units such as 9,9-bis (alkyl) fluorene, benzene, bithiophene or carbazole derivatives is reported. These copolymers possess narrow optical band gap in the range of 1.0 - 1.5 eV depending on the character of the donor units. They exhibit relatively high electron affinity. Absorption of copolymer thin films covers the whole visible spectral region extended up to NIR for some copolymers. The influence of side chain nature and molecular weight on their photophysical properties is shown. Selected copolymers are used in the blends with fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester ([60]PCBM) as active layers in bulk heterojunction photovoltaic devices. The results are discussed in relation to the copolymer structure, side chain nature and molecular weight.

Abstract: The widespread use of polymeric semiconductor compositions for creating flexible and inexpensive solar cells can be achieved by providing the higher values of the coefficient of efficiency. The cost-effective production of polymer solar cells is expected at the efficiency of them not less than 10 %, while now its real level does not exceed 4 %. Many laboratories work to develop semiconductor compositions of organic materials as donors and acceptors which are fullerene derivatives or nanosize particles of semiconductor inorganic compounds [1-6]. The prospect of polymer used depends on the photovoltaic materials and the polymer purity and to a greater extent on the structure of the films formed from the compositions under development. In the search for ways to achieve higher performance of solar cells it is essential to optimize the technology of polymeric composition preparation, of which the active layer is formed, as well as optimization of the layer formation. In order to get information about the relationship between the structure of formed layer and its photovoltaic characteristics it is suggested to analyze the structure of the active layer simultaneously with the monitoring of its current-voltage characteristics. The study of the material structure directly in the process of its evolution seems an urgent task, since the majority of modern methods of structure investigation (light and electron microscopy, X-ray analysis) is not able to detect structural changes occurring in a short period of time. The most useful tool for monitoring the structure of polymer active layer is high intensity X-ray diffraction.