Complete Solar Cells Research Articles

Advanced Raman Spectroscopy of Methylammonium Lead Iodide: Development of a Non-destructive Characterisation Methodology.

In recent years, there has been an impressively fast technological progress in the development of highly efficient lead halide perovskite solar cells. However, the stability of perovskite films and respective solar cells is still an open point of concern and calls for advanced characterization methods. In this work, we identify appropriate measurement conditions for a meaningful analysis of spin-coated absorber-grade perovskite thin films based on methylammonium (MA) lead iodide (MAPbI3) by Raman spectroscopy. The material under investigation and its derivates is the most commonly used for high efficiency devices in the literatures and has yielded working solar cell devices with efficiencies around 10% in our laboratory. We report highly detailed Raman spectra obtained with excitation at 532 nm and 633 nm and their deconvolution taking advantage of the simultaneous fitting of spectra obtained with varying excitation wavelengths. Finally, we propose a fast and contactless methodology based on Raman to probe composition variations and/or degradation of these perovskite thin films and discuss the potential of the presented technique as quality control and degradation monitoring tool in other organic-inorganic perovskite materials and complete solar cell devices.

Recycling of Raw Materials, Silicon Wafers and Complete Solar Cells from Photovoltaic Modules

Photovoltaic modules (PVs) are an attractive way of generating electricity in reliable and maintenance-free systems with the use of solar energy. The average lifetime of photovoltaic modules is 25 to 30 years. To offset the negative impact of photovoltaic modules on the environment, it is necessary to introduce a long-term strategy that includes a complete lifecycle of all system components from the production phase through installation and operation to disposal. Recycling of waste products and worn-out systems is an important element of this strategy. Environmental benefits of recycling are related not only to the limited space of landfills, but also to energy saving, raw materials and emission reduction. An important argument for the recycling of photovoltaic modules is the reduction of energy consumption at their production stage through the reuse of existing purified materials. The paper presents selected methods of recycling of used or destroyed PV modules and photovoltaic cells and the results of practical experiments.

Thermal Stability of CuInSe2 solar Cell with Sputter-Zn(O,S) Buffer Layer As a Bottom Cell

The power conversion efficiency of solar cells with multiple sub-cells in a tandem device can be extended beyond the Shockley-Queisser limit for a single-junction device, shifting the absorption onset to longer wavelengths and reducing thermalization losses. Theoretical calculations have been reported that the optimum band-gaps of absorber materials for a series-connected two-junction are 1.72 eV for the top cell and 1.14 eV for the bottom cell, respectively. The CuInSe2 (CIS) solar cells are well suited to be used as bottom cell in tandem solar cells. In case of monolithic tandem solar cell, Cu(In,Ga)Se2 based-cells with CdS buffer layer are quite vulnerable to high temperatures above 200oC, destroying the p-n junction by the diffusion of Cd atoms. Thus, the thermal stability of buffer layer prepared on the absorber of bottom cell should be important factor in solar cell performance.In this study, we fabricated CIS solar cell with sputtered-Zn(O,S) buffer layer as a bottom cell because Zn-based buffer layer has a higher thermal stability as compared with CdS buffer layer. The completed solar cells were heated up to temperatures of 200 - 400oC for 60 min under vacuum in order to trace the degradation of cell performances. To investigate the interface quality and degradation mechanism after post-annealing process, the drive-level capacitance profiling and admittance spectroscopy have being applied as an analytic tool, which gives an information on bulk defect states, carrier density and defect energy at p-n junction. The temperature dependent current density-voltage measurements were also performed in the dark, red and white light from a solar simulator. These results clearly present that there is clear correlation between thermal stability and solar cell performance.

Effect of PbI2 deposition rate on two-step PVD/CVD all-vacuum prepared perovskite

In this work we fabricate all-vacuum processed methyl ammonium lead halide perovskite by a sequence of physical vapour deposition of PbI2 and chemical vapour deposition (CVD) of CH3NH3I under a static atmosphere. We demonstrate that for higher deposition rate the (001) planes of PbI2 film show a higher degree of alignment parallel to the sample's surface. From X-ray diffraction data of the resulted perovskite film we derive that the intercalation rate of CH3NH3I is fostered for PbI2 films with higher degree of (001) planes alignment. The stoichiometry of the produced perovskite film is also studied by Hard X-ray photoelectron spectroscopy measurements. Complete all-vacuum perovskite solar cells were fabricated on glass/ITO substrates coated by an ultra-thin (5nm) Zn-phthalocyanine film as hole selective layer. A dependence of residual PbI2 on the solar cells performance is displayed, while photovoltaic devices with efficiency up to η=11.6% were achieved.

Plasma-initiated rehydrogenation of amorphous silicon to increase the temperature processing window of silicon heterojunction solar cells

The dehydrogenation of intrinsic hydrogenated amorphous silicon (a-Si:H) at temperatures above approximately 300 °C degrades its ability to passivate silicon wafer surfaces. This limits the temperature of post-passivation processing steps during the fabrication of advanced silicon heterojunction or silicon-based tandem solar cells. We demonstrate that a hydrogen plasma can rehydrogenate intrinsic a-Si:H passivation layers that have been dehydrogenated by annealing. The hydrogen plasma treatment fully restores the effective carrier lifetime to several milliseconds in textured crystalline silicon wafers coated with 8-nm-thick intrinsic a-Si:H layers after annealing at temperatures of up to 450 °C. Plasma-initiated rehydrogenation also translates to complete solar cells: A silicon heterojunction solar cell subjected to annealing at 450 °C (following intrinsic a-Si:H deposition) had an open-circuit voltage of less than 600 mV, but an identical cell that received hydrogen plasma treatment reached a voltage of over 710 mV and an efficiency of over 19%.

Dispersive Non-Geminate Recombination in an Amorphous Polymer:Fullerene Blend.

Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer:fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions.

Characterizing microscale aluminum composite layer properties on silicon solar cells with hybrid 3D scanning force measurements.

This article presents a novel technique to estimate the mechanical properties of the aluminum composite layer on silicon solar cells by using a hybrid 3-dimensional laser scanning force measurement (3-D LSFM) system. The 3-D LSFM system measures the material properties of sub-layers constituting a solar cell. This measurement is critical for realizing high-efficient ultra-thin solar cells. The screen-printed aluminum layer, which significantly affects the bowing phenomenon, is separated from the complete solar cell by removing the silicon (Si) layer with deep reactive ion etching. An elastic modulus of ~15.1 GPa and a yield strength of ~35.0 MPa for the aluminum (Al) composite layer were obtained by the 3-D LSFM system. In experiments performed for 6-inch Si solar cells, the bowing distances decreased from 12.02 to 1.18 mm while the Si layer thicknesses increased from 90 to 190 μm. These results are in excellent agreement with the theoretical predictions for ultra-thin Si thickness (90 μm) based on the obtained Al composite layer properties.

Impact of atmospheric species on copper indium gallium selenide solar cell stability: an overview

An overview of the measurement techniques and results of studies on the stability of copper indium gallium selenide (CIGS) solar cells and their individual layers in the presence of atmospheric species is presented: in these studies, Cu(In,Ga)Se2 solar cells, their molybdenum back contact, and their ZnO:Al front contact were exposed to liquid water purged with gases from the atmosphere, like carbon dioxide (CO2), oxygen (O2), nitrogen (N2), and air. The samples were analyzed before, during, and after exposure in order to define their stability under these conditions. The complete CIGS solar cells as well as the ZnO:Al front contact degraded rapidly when exposed to H2O combined with CO2, while they were relatively stable in H2O purged with O2 or N2. This was caused by either degradation of the grain boundaries in the ZnO:Al film or by the dissolution of part of this film. Uncovered molybdenum films, on the other hand, oxidized rapidly in the presence of H2O and O2, while they were more stable in the presence of H2O with N2 and/or CO2.

Low temperature photoluminescence spectroscopy studies on sputter deposited CdS/CdTe junctions and solar cells

Abstract

Ion beam analysis of Cu(In,Ga)Se2 thin film solar cells

The present work investigates the potential of ion beam analysis (IBA) techniques such as the Rutherford backscattering spectrometry (RBS) and particle induced X-ray emission (PIXE) using helium ions to provide quantitative in-depth elemental analysis of various types of Cu(In,Ga)Se2 thin films. These films with a thickness of about 2μm are used as absorber layers in photovoltaic devices with continuously increasing the performance of this technology. The preparation process generally aims to obtain an in-depth gradient of In and Ga concentrations that optimizes the optoelectronic and electrical properties of the solar cell. The measurements were performed at directly accessible single or double layered CIGS absorbers and at buried absorbers in completed thin film solar cells. The IBA data were analyzed simultaneously in order to derive best fitted profiles that match all experimental RBS and PIXE spectra. For some samples elemental profiles deduced form synchrotron based, reference free grazing incidence X-ray fluorescence analysis were compared with the IBA results and an overall good agreement was observed within quoted uncertainties.

Amorphous silicon carbide passivating layers for crystalline-silicon-based heterojunction solar cells

Amorphous silicon enables the fabrication of very high-efficiency crystalline-silicon-based solar cells due to its combination of excellent passivation of the crystalline silicon surface and permeability to electrical charges. Yet, amongst other limitations, the passivation it provides degrades upon high-temperature processes, limiting possible post-deposition fabrication possibilities (e.g., forcing the use of low-temperature silver pastes). We investigate the potential use of intrinsic amorphous silicon carbide passivating layers to sidestep this issue. The passivation obtained using device-relevant stacks of intrinsic amorphous silicon carbide with various carbon contents and doped amorphous silicon are evaluated, and their stability upon annealing assessed, amorphous silicon carbide being shown to surpass amorphous silicon for temperatures above 300 °C. We demonstrate open-circuit voltage values over 700 mV for complete cells, and an improved temperature stability for the open-circuit voltage. Transport of electrons and holes across the hetero-interface is studied with complete cells having amorphous silicon carbide either on the hole-extracting side or on the electron-extracting side, and a better transport of holes than of electrons is shown. Also, due to slightly improved transparency, complete solar cells using an amorphous silicon carbide passivation layer on the hole-collecting side are demonstrated to show slightly better performances even prior to annealing than obtained with a standard amorphous silicon layer.

Characterization of Silicon Solar Cells and Module Materials Using Terahertz Radiation

In this paper, we present our investigations of silicon solar cells and module structures by means of terahertz (THz) imaging and Fourier Transform Infrared (FTIR) spectroscopy. Several types of structures were investigated: anisotropically etched surfaces of monocrystalline silicon wafers, complete commercial solar cells, interconnections of the cells, encapsulation materials, such as polydimethylsiloxane (PDMS) and glass. We demonstrated, how etching time and depth variations of the pyramid-shaped texture influence far-IR local reflectance spectra of the samples; how phase-sensitive imaging and time-domain data can be processed for full 3D characterization of cell and module structures, including layer thicknesses; positions of the surfaces and defects. In addition, applicability of sub-THz radiation to the investigation of module structures including glass is discussed.

Solar cells sensitized with near-infrared absorbing dye: Problems with sunlight conversion efficiency revealed in ultrafast laser spectroscopy studies

Complete solar cells sensitized with cyanine dye MK-245, having near-infrared absorption band optimized for tandem devices, are prepared, characterized and studied by time-resolved laser spectroscopy techniques. The photocurrent of the cell correlates with the amount of residual transient absorption signal, pointing to the poor electron injection as the main factor limiting the sunlight conversion efficiency. The measurements show that the lifetime of the excited state is drastically reduced from about 300 ps in solution to single picoseconds when the dyes are attached to titania nanoparticles. It is due to the self-quenching process that effectively competes with electron injection, even under the presence of co-adsorbent. In addition, high concentration of lithium cations in the electrolyte is necessary to improve electron injection quantum yield. The obtained results should be valid for a large class of similar compounds and reveal the main problems in obtaining sufficient performance of near-infrared absorbing dyes.

Preparation of Thylakoid/Polyaniline/Reduced Graphene Oxide/Glassy Carbon Integrated System and Photocurrent Enhancement

Solar energy is enormous, inexhaustible and environmentally friendly. Photosynthesis is a highly developed system that converts solar energy into chemical energy, taking place in thylakoid membrane of plants, algae and cyanobacteria. In the light reaction, solar energy is absorbed by light-harvesting complexes (LHC) containing antenna pigments. Absorbed solar energy is transferred to the reaction center, where the water photolysis into oxygen, protons and electrons occurs by oxygen evolving complex (OEC) attached to photosystem II (PSII). The generated electrons are transferred along the electron transfer pathway called Z-scheme and eventually stored in the form of NADPH. Since quantum yields of photosystem I (PSI) and photosystem II are very high, numerous attempts have been made to integrate these photosynthetic units (PSUs) into photoelectrochemical cells to directly convert light into electricity. Rather than using PSI or PSII, here we present our preliminary results on light energy conversion with thylakoid membrane that is installed on the electrode surface. Thylakoid membrane was isolated from spinach whose oxygen evolving activity was measured by a Clark-type oxygen sensor under the illumination. For the effective light energy conversion, we have developed an integrated system containing thylakoid multilayers on the glassy carbon (GC) electrode. First, 4-aminophenyl (4-AP)-modified GC surfaces were prepared. Isolated thylakoid was mixed with polyaniline (PANI) and graphene oxide (GO), and this composite material was applied onto the 4-AP-modified GC. Since GO has poor electric conductivity, we electrochemically reduced GO to highly conductive reduced graphene oxide (RGO) form. Thus prepared integrated system is denoted as Thyl/PANI/RGO/GC. This integrated system was proven effective in solar energy conversion. To achieve high photocurrent, we prepared the same system with different GO/chlorophyll ratios. After cycling potential to reduce GO to RGO, photocurrent was measured. The ratio of 3:1 (GO:chlorophyll) turned out to give highest photocurrent. The electron transfer pathways have been identified from the blocking experiments with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of QB site in PSII. Thylakoid suspension was treated with DCMU before immobilization and then applied to the electrode. Virtually no photocurrent enhancement was observed proving that the electron transfer was blocked. A complete solar cell composed of Thyl/PANI/RGO/GC anode and Pt/C air cathode was constructed. From the polarization curve, the maximum power density of 10.5 μW/cm2 with current density of 24.7 μA/cm2 under one sun illumination was obtained. The turnover frequency was calculated to be 5.8 water molecule oxidation per PSII unit per second, indicating that the immobilized thylakoid in the form of the multilayer films maintained its activity. This result shows that larger number of thylakoid units are needed for enhanced photocurrent and our method of integrating thylakoid using polyaniline and graphene oxide could be a useful way to this end.

Optical model for simulation and optimization of luminescent down-shifting layers filled with phosphor particles for photovoltaics

We developed an optical model for simulation and optimization of luminescent down-shifting (LDS) layers for photovoltaics. These layers consist of micron-sized phosphor particles embedded in a polymer binder. The model is based on ray tracing and employs an effective approach to scattering and photoluminescence modelling. Experimental verification of the model shows that the model accurately takes all the structural parameters and material properties of the LDS layers into account, including the layer thickness, phosphor particle volume concentration, and phosphor particle size distribution. Finally, using the verified model, complete organic solar cells on glass substrate covered with the LDS layers are simulated. Simulations reveal that an optimized LDS layer can result in more than 6% larger short-circuit current of the solar cell.

Adsorption Behavior of the 1,3-Dimethylimidazolium Thiocyanate and Tetracyanoborate Ionic Liquids at Anatase (101) Surface

The adsorption behavior of the ionic liquids 1,3-dimethylimidazolium thiocyanate and 1,3-dimethylimidazolium tetracyanoborate on the anatase (101) surface is studied through theoretical methods. This includes a comparison of the PBE and PBE0 functionals as well as the D3 and D3M+ dispersion correction schemes for the calculation of energetic and structural properties. The PBE functional was found to underestimate interaction energies, and the D3 dispersion correction was observed to overbind. While the surface titanium atoms predominantly form contact with anion’s nitrogen atoms, the oxygen atoms of anatase interact in hydrogen-bond-like structures with the acidic hydrogen atoms of the imidazolium cation in addition to ubiquitous electrostatic and dispersive interactions. Ionic liquid ion pairs remain stable after adsorption and closely match the geometry of the isolated ions. A band shift of the density of states of TiO2, possibly resulting in higher values of the open-circuit voltage in complete solar cells, is attributed to the adsorption, which might be interesting for application of these ionic liquids in real dye-sensitized solar cell devices.

Improved efficiency of hybrid organic photovoltaics by pulsed laser sintering of silver nanowire network transparent electrode.

In this Research Article, we demonstrate pulsed laser processing of a silver nanowire network transparent conductor on top of an otherwise complete solar cell. The macroscopic pulsed laser irradiation serves to sinter nanowire-nanowire junctions on the nanoscale, leading to a much more conductive electrode. We fabricate hybrid silicon/organic heterojunction photovoltaic devices, which have ITO-free, solution processed, and laser processed transparent electrodes. Furthermore, devices which have high resistive losses show up to a 35% increase in power conversion efficiency after laser processing. We perform this study over a range of laser fluences, and a range of nanowire area coverage to investigate the sintering mechanism of nanowires inside of a device stack. The increase in device performance is modeled using a simple photovoltaic diode approach and compares favorably to the experimental data.

Effective light trapping by hybrid nanostructure for crystalline silicon solar cells

Ag nanoparticles (NPs), as etching catalyst, are applied onto the pyramid textured surface of the semi-finished single crystalline silicon solar cell by spraying a solution containing AgNO3 and sodium citrate. Upon chemical etching, nanopits are formed with the Ag NPs staying inside of the nanopits. Complete solar cells with and without the Ag-assisted etching are fabricated to study the effects of the hybrid nanostructure. It is found that the optical reflection is effectively reduced by the nanostructure and the incident light is harvested more effectively for enhanced external quantum efficiency (EQE) by as much as 17% for the cells before the SiNx anti-reflection coating, and the EQE can be further enhanced by applying an electric bias 1V during the EQE measurement. It is believed that when the solar cell with the Ag NPs is illuminated, the local surface plasmon resonance (LSPR) is induced at the specific wavelength regions around 405 and 810nm. For ultraviolet light, the LSPR effect dominates the increases of the EQE while the effective medium effect is also believed to be responsible for the effective light trapping.

The electronic structure of metal oxide/organo metal halide perovskite junctions in perovskite based solar cells.

Cross-sections of a hole-conductor-free CH3NH3PbI3 perovskite solar cell were characterized with Kelvin probe force microscopy. A depletion region width of about 45 nm was determined from the measured potential profiles at the interface between CH3NH3PbI3 and nanocrystalline TiO2, whereas a negligible depletion was measured at the CH3NH3PbI3/Al2O3 interface. A complete solar cell can be realized with the CH3NH3PbI3 that functions both as light harvester and hole conductor in combination with a metal oxide. The band diagrams were estimated from the measured potential profile at the interfaces, and are critical findings for a better understanding and further improvement of perovskite based solar cells.

Transformation of the excited state and photovoltaic efficiency of CH3NH3PbI3 perovskite upon controlled exposure to humidified air.

Humidity has been an important factor, in both negative and positive ways, in the development of perovskite solar cells and will prove critical in the push to commercialize this exciting new photovoltaic technology. The interaction between CH(3)NH(3)PbI(3) and H(2)O vapor is investigated by characterizing the ground-state and excited-state optical absorption properties and probing morphology and crystal structure. These undertakings reveal that H(2)O exposure does not simply cause CH(3)NH(3)PbI(3) to revert to PbI(2). It is shown that, in the dark, H(2)O is able to complex with the perovskite, forming a hydrate product similar to (CH(3)NH(3))(4)PbI(6)·2H(2)O. This causes a decrease in absorption across the visible region of the spectrum and a distinct change in the crystal structure of the material. Femtosecond transient absorption spectroscopic measurements show the effect that humidity has on the ultrafast excited state dynamics of CH(3)NH(3)PbI(3). More importantly, the deleterious effects of humidity on complete solar cells, specifically on photovoltaic efficiency and stability, are explored in the light of these spectroscopic understandings.