Showing papers by "Miro Zeman published in 2017"

GenPro4 Optical Model for Solar Cell Simulation and Its Application to Multijunction Solar Cells

Abstract: We present a new version of our optical model for solar cell simulation: GenPro4 . Its working principles are briefly explained. The model is suitable for quickly and accurately simulating a wide range of wafer-based and thin-film solar cells. Especially adjusting layer thicknesses to match the currents in multijunction devices can be done with a minimum of computational cost. To illustrate this, a triple junction thin-film silicon solar cell is simulated. The simulation results show very good agreement with external quantum efficiency measurements. The application of an MgF2 antireflective coating or an antireflective foil with pyramid texture is considered. Their effects on the implied photocurrents of top, middle, and bottom cells are investigated in detail.

Opto‐electrical modelling and optimization study of a novel IBC c‐Si solar cell

Abstract: Interdigitated back contact (IBC) crystalline silicon (c-Si) solar cells are attracting a lot of attention because of their capability to reach world record conversion efficiency. Because of the relatively complex contact pattern, their design and optimization typically require advanced numerical simulation tools. In this work, a TCAD-based simulation platform has been developed to account accurately and in detail the optical and passivation mechanisms of front texturization. Its validation has been carried out with respect to a novel homo-junction IBC c-Si solar cell based on ion implantation and epitaxial growth, comparing measured and simulated reflectance, transmittance, internal quantum efficiency, external quantum efficiency spectra, and current density–voltage characteristics. As a result of the calibration process, the opto-electrical losses of the investigated device have been identified quantitatively and qualitatively. Then, an optimization study about the optimal front surface field (FSF) doping, front-side texturing morphology, and rear side geometry has been performed. The proposed simulation platform can be potentially deployed to model other solar cell architectures than homo-junction IBC devices (e.g., passivated emitter rear cell, passivated emitter rear locally diffused cell, hetero-IBC cell). Simulation results show that a not-smoothed pyramid-textured front interface and an optimal FSF doping are mandatory to minimize both the optical and the recombination losses in the considered IBC cell and, consequently, to maximize the conversion efficiency. Similarly, it has been showed that recombination losses are affected more by the doping profile rather than the surface smoothing. Moreover, the performed investigation reveals that the optimal FSF doping is almost independent from the front texturing morphology and FSF passivation quality. According to this result, it has been demonstrated that an IBC cell featuring an optimal FSF doping does not exhibit a significant efficiency improvement when the FSF passivation quality strongly improves, proving that IBC cell designs based on low-doped FSF require a very outstanding passivation quality to be competitive. Deploying an optimization algorithm, the adoption of an optimized rear side geometry can potentially lead to an efficiency improvement of about 1%abs as compared with the reference IBC solar cell. Further, by improving both emitter and c-Si bulk quality, a 22.84% efficient solar cell for 280-μm thick c-Si bulk was simulated. Copyright © 2017 John Wiley & Sons, Ltd.

Silicon Solar Cell Architecture with Front Selective and Rear Full Area Ion‐Implanted Passivating Contacts

Abstract: In this work, the application of carrier-selective passivating contacts based on tunneling silicon-dioxide and ion-implanted poly-Si in front and rear contacted Si solar cells is presented. This paper addresses the need to minimize the contact recombination while still keeping high short circuit current. We aim to solve such trade-off with a novel solar cell architecture called Passivated Rear and Front ConTacts (PeRFeCT). Such design employs a selective passivating contact combined with standard homojunction on the front side in order to minimize contact recombination, while achieving high optical transparency and a full area passivating contact on the rear side. The opto-electrical modeling of this front/rear contacted architecture indicates a potential efficiency above 26%. As technology demonstration, we also report on the optimization of front surface field and processing of 2.8 × 2.8 cm2 wide solar cells leading to a 20.1% conversion efficiency.

Quantification of Shading Tolerability for Photovoltaic Modules

Abstract: Despite several decades of research in the field of photovoltaic (PV) systems, shading tolerance has still not been properly addressed. PV modules are influenced by shading concerning many factors, such as number and configuration of cells in the module, electrical and thermal characteristics of the cells, number and type of bypass circuits, electrical characteristics of bypass elements, and shading profile features. Along with the random nature of shading profile over the lifetime of a PV system, it is difficult to choose the best module for a location which is most of the time sunny, partly cloudy, or cloudy. This paper suggests a measurable parameter, the so-called shading tolerability (ST), to classify PV modules regarding the ability to oppose shading effects. Based on mathematical and probability analysis, the ST parameter is extracted and then measured using a large area steady state solar simulator. Finally, the results of on-field experiments are presented as a proof for the shading quantification method and its significant contribution to performance ratio improvement.

Back-contacted BaSi 2 solar cells: an optical study

Abstract: We present the optical investigation of a novel back-contacted architecture for solar cells based on a thin barium (di)silicide (BaSi2) absorber. First, through the analysis of absorption limits of different semiconducting materials, we show the potential of BaSi2 for photovoltaic applications. Then, the proposed back contacted BaSi2 solar cell design is investigated and optimized. An implied photocurrent density of 40.3 mA/cm2 in a 1-μm thick absorber was achieved, paving the way for novel BaSi2-based thin-film solar cells.

Determination of the temperature dependency of the electrical parameters of CIGS solar cells

Abstract: Two types of Cu(In,Ga)Se2 (CIGS) solar cells, both designed for implementation in CIGS modules, were subjected to temperatures between 25 oC and 105 oC. Simultaneous exposure to AM1.5 illumination allowed the measurement of their electrical parameters at these temperatures. These two types of solar cells, produced with different deposition routes on soda lime glass (SLG) and polyimide (PI) substrates, showed large variations in the temperature dependency of their electrical parameters. It was shown that the temperature dependency of the open circuit voltage (Voc) was dependent on its room temperature value: a high Voc at 25 °C led to a slower loss of Voc when the temperature was increased. For the Voc, the normalised temperature dependency varied between −0.28%/°C and −0.47%/°C, which is in agreement with the literature. The temperature dependency of the short circuit current density (Jsc) showed more surprising results: while the PI samples had the expected positive temperature dependency (0.03 to 0.32...

Understanding the present and the future electricity needs: Consequences for design of future Solar Home Systems for off-grid rural electrification

Abstract: Solar Home Systems (SHSs) can fulfil the basic energy needs of the globally unelectrified population With costs as one of the biggest barriers for SHS uptake, optimizing the system size with energy needs is crucial Where most solutions focus only on the present needs, this work also addresses the future energy needs The methodology includes extensive mapping of the current electricity needs in rural Cambodia through data analysis on existing SHSs in the field Additionally, a 2-month field research was carried out in Cambodia to assess the qualitative state of electricity usage and investigate the future (2021) energy needs A data analysis was performed on 111 SHSs (100 Wp, 1200 Wh) SHS users were found to have a mean energy consumption of 310 Wh/day, with σ = 159 Wh Most energy was consumed at night The field research showed a clear demand for more energy and more appliances The appliances attached to SHS in the future will be more diverse in power consumption and usage duration, resulting in a wide variety of energy consumption and high power peaks, causing fast and deep battery discharges Three load profiles are presented Solutions are discussed that can be applied to ensure the SHSs fit with future energy needs

Surface passivation of n-type doped black silicon by atomic-layer-deposited SiO2/Al2O3 stacks

Abstract: Black silicon (b-Si) nanotextures can significantly enhance the light absorption of crystalline silicon solar cells. Nevertheless, for a successful application of b-Si textures in industrially relevant solar cell architectures, it is imperative that charge-carrier recombination at particularly highly n-type doped black Si surfaces is further suppressed. In this work, this issue is addressed through systematically studying lowly and highly doped b-Si surfaces, which are passivated by atomic-layer-deposited Al2O3 films or SiO2/Al2O3 stacks. In lowly doped b-Si textures, a very low surface recombination prefactor of 16 fA/cm2 was found after surface passivation by Al2O3. The excellent passivation was achieved after a dedicated wet-chemical treatment prior to surface passivation, which removed structural defects which resided below the b-Si surface. On highly n-type doped b-Si, the SiO2/Al2O3 stacks result in a considerable improvement in surface passivation compared to the Al2O3 single layers. The atomic-layer-deposited SiO2/Al2O3 stacks therefore provide a low-temperature, industrially viable passivation method, enabling the application of highly n- type doped b-Si nanotextures in industrial silicon solar cells.

Too Many Junctions? : A Case Study of Multijunction Thin-Film Silicon Solar Cells

Abstract: The benefit of two-terminal multijunction solar cells in regard to the number of junctions (subcells) is critically evaluated. The optical and electrical losses inherent in the construction of multijunction cells are analyzed using information from thin-film silicon photovoltaics as a representative case. Although the multijunction approach generally reduces the thermalization and nonabsorption losses, several types of losses rise with the number of subcells. Optical reflection and parasitic absorption are slightly increased by adding supporting layers and interfaces. The output voltages decline because of the tunnel recombination junctions, and more importantly of the illumination filtered and reduced by the top subcell(s). The loss mechanisms consume the potential gains in efficiency of multijunction cells. For thin-film silicon, the triple-junction is confirmed to be the best performing structure. More generally, only when each component subcell shows a high ratio between the output voltage and the bandgap of the absorber material, a multijunction cell with a large number of subcells can be beneficial. Finally, the high voltage and low current density of multijunction cells with a large number of subcells make them difficult to optimize and manufacture, vulnerable to any changes in the solar spectrum, and thus less practical for the ordinary terrestrial applications.

Passivation mechanism in silicon heterojunction solar cells with intrinsic hydrogenated amorphous silicon oxide layers

Abstract: In this work, we use intrinsic hydrogenated amorphous silicon oxide layers (a-SiOx:H) with varying oxygen content (cO) but similar hydrogen content to passivate the crystalline silicon wafers. Using our deposition conditions, we obtain an effective lifetime (τeff) above 5 ms for cO ≤ 6 at. % for passivation layers with a thickness of 36 ± 2 nm. We subsequently reduce the thickness of the layers using an accurate wet etching method to ∼7 nm and deposit p- and n-type doped layers fabricating a device structure. After the deposition of the doped layers, τeff appears to be predominantly determined by the doped layers themselves and is less dependent on the cO of the a-SiOx:H layers. The results suggest that τeff is determined by the field-effect rather than by chemical passivation.

Migration of Open Volume Deficiencies in Hydrogenated Amorphous Silicon During Annealing

Abstract: The nanostructure of hydrogenated amorphous silicon (a-Si:H) is studied by means of doppler broadening positron annihilation spectroscopy (DB-PAS) and Fourier transform infrared (FTIR) spectroscopy. The evolution of open volume deficiencies is monitored during annealing, demonstrating that small vacancies and other small vacancy clusters that are initially present in the a-Si:H nanostructure agglomerate into larger vacancy clusters. The migration of open volume deficiencies is less pronounced for a-Si:H deposited at higher hydrogen-to-silane gas flow rate ratio, R . FTIR spectroscopy reveals the presence of a peculiar peak in the refractive index in the infrared—and hence the calculated mass density—which occurs just before H effusion from the films starts. The combined results of DB-PAS and FTIR spectroscopy indicate that a stress buildup caused by the accumulation of H2 in agglomerating vacancies during annealing can explain the sudden mass density increase. At higher temperatures, stress is released with the onset of H effusion. The H effusion consists of a two-stage process involving small open volume deficiencies and nanosized voids, contrasting earlier interpretations. The reduced amount of hydrogen migration and enhanced hydrogen passivation degree are suggested as key factors to the reduced light-induced degradation associated with increased R values.

A simple methodology for estimating battery lifetimes in Solar Home System design

Abstract: The proliferation of Solar Home Systems (SHS) in recent times hopes to provide an alleviating solution to the global problem of energy poverty. Battery is usually the most expensive but important part of an SHS; it is also normally the first part to fail. Estimating the battery lifetime can help make informed system design choices, and it is therefore an important exercise for an SHS designer. Battery lifetime modelling is often a complex task requiring empirical data or reliance on modelling cell level electrochemical phenomena. This paper presents a simple battery lifetime estimation method specific to the application and candidate battery choices at hand. An SHS application specific simulation is carried out for a year and the effect of microcycles on the battery activity is analyzed. The concept of active Depth-of-Discharge (DOD) is introduced. Cyclic ageing of the battery is thus quantified and relative cycle lives of 2 battery technologies are compared. A delicate trade-off is demonstrated between battery sizing and lifetime. The described methodology is also compared with an empirical model and the lifetime results are found to be within 3.85%. The methodology described in this paper can potentially help SHS designers in making quick, reasonable estimations of battery lifetimes based on the intended application and battery manufacturer's data.

Periodic and Random Substrate Textures for Liquid-Phase Crystallized Silicon Thin-Film Solar Cells

Abstract: A major limitation in current liquid-phase crystallized (LPC) silicon thin-film record solar cells is optical losses caused by their planar glass–silicon interface. In this study, silicon is grown on nanoimprinted periodically, as well as randomly textured glass substrates, and successfully implemented into state-of-the-art LPC silicon thin-film solar cells. Compared with an optimized planar reference device, both textures enhance absorption of light. Interlayer and process optimization allowed achieving a material quality comparable with the planar reference device. On the random texture, an open-circuit voltage above 630 mV was obtained, as well as an external quantum efficiency exceeding the planar reference device by +3 mA/cm2.

Quadruple‐Junction Thin‐Film Silicon Solar Cells Using Four Different Absorber Materials

Abstract: We fabricated and studied quadruple-junction wide-gap a-Si:H/narrow-gap a-Si:H/a-SiGex:H/nc-Si:H thin-film silicon solar cells. It is among the first attempts in thin-film photovoltaics to make a two-terminal solar cell with four different absorber materials. Several tunnel recombination junctions were tested, and the n-SiOx:H/p-SiOx:H structure was proven to be a generic solution for the three pairs of neighboring subcells. The proposed combination of absorbers led to a more reasonable spectral utilization than the counterpart containing two nc-Si:H subcells. Besides, the use of high-mobility transparent conductive oxide and modulated surface texture significantly enhances the total light absorption in the absorber layers. This work paved the way toward high-efficiency quadruple-junction cells, and a practical estimation of the achievable efficiency was given.

Light-Induced Effects on the a-Si:H/c-Si Heterointerface

Abstract: Light-induced effects on the minority carrier lifetime of silicon heterojunction structures are studied through multiple-exposure photoconductance decay (MEPCD). MEPCD monitors the effect of the measurement flash from a photoconductance decay setup on a sample over thousands of measurements. Varying the microstructure of the intrinsic hydrogenated amorphous silicon (a-Si:H) used for passivation of n-type crystalline silicon (c-Si) showed that passivating films rich in voids produce light-induced improvement, while denser films result in samples that are susceptible to light-induced degradation. Light-induced degradation is linked to an increase in dangling bond density at the a-Si:H/c-Si interface, while light-induced improvements are linked to charging at the a-Si:H/c-Si interface. Furthermore, doped a-Si:H is added to make samples with an emitter and back surface field (BSF). These doped layers have a significant effect on the light-induced kinetics on minority carrier lifetime. Emitter samples exhibit consistent light-induced improvement, while BSF samples exhibit light-induced degradation. This is explained through negative charging at the BSF and positive charging at the emitter. Full precursors with a BSF and emitter exhibit different kinetics based on which side is being illuminated. This suggests that the light-induced charging at the a-Si:H/c-Si interface can only occur when a-Si:H has sufficient generation.

Artifact Interpretation of Spectral Response Measurements on Two-Terminal Multijunction Solar Cells

Abstract: Multijunction solar cells promise higher power-conversion efficiency than the single-junction. With respect to two-terminal devices, an accurate measurement of the spectral response requires a delicate adjustment of the light- and voltage-biasing; otherwise it can result in artifacts in the data and thus misinterpretation of the cell properties. In this paper, the formation of measurement artifacts is analyzed by modeling the measurement process, that is, how the current–voltage characteristics of the component subcells evolve with the photoresponse to the incident spectrum. This enables the examination on the operation conditions of the subcells, offering additional information for the study of artifacts. In particular, the influence of shunt resistance, bias-light intensity, and bias voltage on the measurement is examined. Having observed the dynamics and vulnerability of the measurement, the proper ways to configure and interpret a measurement are discussed in depth. As a practical example, simulations of the measurements on a quadruple-junction thin-film silicon solar cell demonstrate that the modeling can be used to interpret eventual irregularities in the measured spectral response. The application of such tool is especially meaningful taking account of the diverse and rapid development of novel hybrid multijunction solar cells, in which the role of reliable characterizations is essential.

Hydrogenated amorphous silicon oxide (a-SiOx:H) single junction solar cell with 8.8% initial efficiency by reducing parasitic absorptions

Abstract: Hydrogenated amorphous silicon oxide (a-SiOx:H) solar cells have been successfully implemented to multi-junction thin film silicon solar cells. The efficiency of these solar cells, however, has still been below that of state-of-the-art solar cells mainly due to the low Jsc of the a-SiOx:H solar cells and the unbalanced current matching between sub-cells. In this study, we carry out optical simulations to find the main optical losses for the a-SiOx:H solar cell, which so far was mainly optimized for Voc and fill-factor (FF). It is observed that a large portion of the incident light is absorbed parasitically by the p-a-SiOx:H and n-a-SiOx:H layers, although the use of these layers leads to the highest Voc × FF product. When a more transparent and conductive p-nc-SiOx:H layer is substituted for the p-a-SiOx:H layer, the parasitic absorption loss at short wavelengths is notably reduced, leading to higher Jsc. However, this gain in Jsc by the use of the p-nc-SiOx:H compromises the Voc. When replacing the n-a-S...

Daylighting simulation and analysis of buildings with dynamic photovoltaic window shading elements

Abstract: Photovoltaic (PV) windows with shading devices benefit buildings in terms of power generation, daylighting control, glare protection, etc. In order to harvest the maximum solar energy from the window area, solar tracking is applied to the PV shading elements. The complicated movement trajectories and dynamic daylighting are difficult to simulate by conventional methods. In this work, we introduce an optical simulation method of daylighting in complex building environment using DIVA and Grasshopper, which are plugins of Rhinoceros, a commercial 3D computer graphics and computer-aided design (CAD) software. An algorithmic model of the PV blinds is built by Grasshopper based on amodified model of a reference office. Then DIVA is applied to evaluate the daylighting performance of the dynamic and static PV blinds. Simulation results show that the working plane reveals higher illuminance level and lower glare level with the dynamic PV blinds than that with their static counter parts. It is demonstrated that the proposed simulation method can deal with the daylighting evaluation of complex and dynamic building environment.

Electron tomography analysis of 3D interfacial nanostructures appearing in annealed Si rich SiC films

Abstract: The optical and electrical properties of Si rich SiC (SRSC) solar cell absorber layers will strongly depend on interfacial layers between the Si and the SiC matrix and in this work, we analyze hitherto undiscovered interfacial layers. The SRSC thin films were deposited using a plasma-enhanced chemical vapor deposition (PECVD) technique and annealed in a nitrogen environment at 1100 °C. The thermal treatment leads to metastable SRSC films spinodally decomposed into a Si–SiC nanocomposite. After the thermal treatment, the coexistence of crystalline Si and SiC nanostructures was analysed by high resolution transmission electron microscopy (HRTEM) and electron diffraction. From the quantitative extraction of the different plasmon signals from electron energy-loss spectra, an additional structure, amorphous SiC (a-SiC) was found. Quantitative spectroscopic electron tomography was developed to obtain three dimensional (3D) plasmonic maps. In these 3D spectroscopic maps, the Si regions appear as network structures inside the SiC matrix where the a-SiC appears as an interfacial layer separating the matrix and Si network. The presence of the a-SiC interface can be explained in the framework of the nucleation and growth model.

Performance Optimization of Semi-Transparent Thin-Film Amorphous Silicon Solar Cells

Abstract: Semi-transparent solar cells possess tremendous potential in glass-based PV applications. Using an optical model, GenPro4, we provide with a simulation method to optimize the configuration of such solar cells. For a single-junction amorphous silicon solar cell, the optimized thickness of the absorber layer is obtained at 170 nm to realize a target average transmittance of 20% in the visible range of the sunlight spectrum. A sample cell was fabricated accordingly to verify the proposed method. Measurement results show that an average transmittance of 20.04% is achieved with the conversion efficiency of 6.94%. The presented optimization method can also be applied to the estimation of color appearance of semi-transparent solar cells.

Ultra-thin LiF Layer As The Electron Collector For a-Si:H Based Photovoltaic Cell

Abstract: An ultra-thin LiF layer in conjunction with an Al layer is employed as the electron collector for the a-Si:H based single-junction thin film photovoltaic cell. The cell has the structure of boron doped μ-SiOx (hole collector) - intrinsic a-Si:H (photoactive layer) - LiF / Al (electron collector and back electrode). The substrate used is U type Asahi glass, which is also acting as the transparent front electrode. For the cell with the 1.5 nm thick LiF layer, annealed at 120oC, the open current voltage (VOC) of 0.936 V, the short current density (JSC) of 13.598 mA/cm2, and the fill factor (FF) of 0.690 are achieved. The JSC and VOC values are comparable to the values measured for the a-Si:H based p-i-n reference cell, but the FF value is found to be lower, which is attributed to the losses due to recombination at the intrinsic a-Si:H / LiF / Al junction. The current versus voltage measurements are carried out under the standard test conditions. The JSC values are corrected according to the external quantum efficiency measurements of the cells in the AM1.5 spectrum region between 270 nm and 800 nm.

Treatment of organic pollutants using a solar energy driven photo-oxidation device.

Abstract: Two of the main problems of society in the near future are the access to clean water and energy. In particular, organic pollutants can be a major health threat. Within available methods, a trade-off can be established between the pollutant treatment price and the final pollutant concentration that can be achieved. In this paper, a water treatment device is proposed in order to decouple these two variables. It consists of a BiVO4 photoanode combined with a thin film silicon solar cell. BiVO4 is an earth-abundant material with a bandgap energy of 2.4 eV. Here, its good catalytic properties are shown for the degradation of phenol and chloroform when combined with an external bias voltage of 1 V versus Ag/AgCl. In addition, to cover the voltage needs, an a-Si:H/nc-Si:H solar cell has been coupled with the BiVO4 photoanode. This solar cell has been specifically designed to work under the transmitted spectrum of BiVO4, with thicknesses of 300 and 2000 nm for the top and bottom cell, respectively. This device has successfully been fabricated, and tested for removal of organic contaminants from an aqueous solution, performing even better than the BiVO4 photoanode alone with a similar external bias voltage applied.

Spectroscopic electron tomography analysis of 3D interfacial nanostructures appearing in annealed Si rich SiC films

Abstract: Spectroscopic electron tomography analysis of 3D interfacial nanostructures appearing in annealed Si rich SiC films

New insights into the nanostructure of innovative thin film solar cells gained by positron annihilation spectroscopy

Abstract: Recent studies showed that positron annihilation methods can provide key insights into the nanostructure and electronic structure of thin film solar cells. In this study, positron annihilation lifetime spectroscopy (PALS) is applied to investigate CdSe quantum dot (QD) light absorbing layers, providing evidence of positron trapping at the surfaces of the QDs. This enables one to monitor their surface composition and electronic structure. Further, 2D-Angular Correlation of Annihilation Radiation (2D-ACAR) is used to investigate the nanostructure of divacancies in photovoltaic-high-quality a-Si:H films. The collected momentum distributions were converted by Fourier transformation to the direct space representation of the electron-positron autocorrelation function. The evolution of the size of the divacancies as a function of hydrogen dilution during deposition of a-Si:H thin films was examined. Finally, we present a first positron Doppler Broadening of Annihilation Radiation (DBAR) study of the emerging class of highly efficient thin film solar cells based on perovskites.

Thin-film amorphous silicon nanopillar solar cells: An investigation of the optical potential

Abstract: We present an optical investigation of nanopillar thin-film solar cells based on amorphous silicon, showing implied photocurrent density values > 14 mA/cm2 for a volumetric equivalent thicknesses of 85 nm.