1,720,952 research outputs found
Modelling and Evaluation of Contact Resistance in High-Efficiency c-Si Solar Cells Featuring Carrier-Selective Passivating Contacts
No full textIn this thesis, contact resistivity for carrier-selective contacts (CSCs) is evaluated by using finite element simulations TCAD Sentaurus. First, the process of transmission line measurement (TLM) is modelled and validated based on current-voltage (I-V) data comparison between reference experiment and simulation results on polycrystalline silicon (poly-Si) based CSCs. Simulation and experimental data are in a good agreement, thus confirming that the modeling method accurately describes the main physical mechanism. Therefore, the simulation approach is used to evaluate the resistivity of complete contact stack for poly-Si and silicon heterojunction (SHJ) based CSCs. Simulation results reveals that the contact resistivity exhibits a clear dependence on tunneling mechanisms in terms of potential barrier size and band alignment. For poly-Si based CSCs, SiO2 thickness (potential barrier size) is the prevalent parameter impacting on the contact resistivity. Additionally, proper doping in poly-Si and buried region in c-Si can improve the band alignment, thus the contact resistivity becomes more resilient to the effect of the tunneling barrier. For SHJ based CSCs, low contact resistivity values are achieved with high carrier concentration in TCO and low activation energy in doped thin film silicon layer. In general, low activation energy reduces the potential barrier for carrier transport while high TCO carrier concentration allows a better band alignment. In particular, for p-type contact, high carrier concentration in TCO is crucial to ensure an efficient band alignment for band-to-band tunneling at TCO/doped-layer interface. Additionally, the contact resistance depends also on the bandgap and the thickness of the passivating intrinsic amorphous silicon (i-a-Si:H) as they impact on band alignment and also energy barrier size. Indeed, lower values of contact resistance are calculated for thinner i-a-Si:H and narrow bandgap because the reduction of the potential barrier opposing to hole collection. Finally, the presented simulation platform has the potential and flexibility of predicting the contact resistance for any type of CSC stack in terms of materials and number of layers.NextBase European projectElectrical Engineering | Sustainable Energy Technolog
Molybdenum Oxide in Hole-selective Contacts for Silicon-based Solar Cells
No full textAlthough front and back contact silicon heterojunction solar cells exhibit promising external parameters, they are limited by the front highly absorptive doped layers. Due to their opto-electronic properties, research and development groups have demonstrated that transition metal oxides (TMO) are potential alternatives for doped layers in solar cells. In particular, molybdenum oxide (MoOx) is a good candidate for the p-contact layers. This thesis presents an investigation of the introduction of MoOx in the front contact of solar cells. To this purpose, the evolution of the passivation is evaluated along the fabrication process together with temperature sensitivity. The application of the layer negatively influences the passivation quality. Moreover, the passivation quality decreases more after annealing and transparent conductive oxide (TCO) deposition. We mitigate such issues by treating the surface of the passivation layer before MoOx application. The optimized contact stack shows implied open-circuit voltage above 715 mV and lifetime 1.57 ms. Finally, the contact stack is implemented in solar cells demonstrators. The best solar cell exhibits a short circuit current density (JSC) equal to 35.64 mA/cm2, an open-circuit voltage (VOC) of 690 mV, a fill factor (FF) of 77.29% and a total efficiency (η) of 19.01%.Electrical Engineering | Electrical Power Engineerin
Numerical Simulations of IBC Solar Cells Based on Poly-Si Carrier-Selective Passivating Contacts
No full text This paper presents an analysis of physical mechanisms related to operation and optimization of interdigitated back contact (IBC) poly-silicon-based devices. Concepts of carrier selectivity and tunneling are used to identify the parameters that impact on the fill factor. Then, based on technology computer-aided design (TCAD) numerical simulations, we describe the device performance in terms of transport and passivation. A validation of the model is performed by matching measured and simulated R, T, and external quantum efficiency spectra and electrical parameters. As result of such process, the opto-electrical losses of the reference device are identified. Then, we execute a study of the impact of process parameters on the performance of the IBC device under analysis. Assuming a uniform SiO 2 layer, simulation results reveal that both n-type and p-type poly-Si contacts can be theoretically perfect (i.e., approx. lossless), if assuming no interface recombination but considering tunneling of both carrier types. In other words, there exists an optimum oxide thickness (1 nm) for which majority carriers tunneling works already very well, and minority tunneling is still low enough to not result in significant recombination. Moreover, SiO 2 thickness up to maximum 1.6 nm is crucial to achieve high efficiency. Regarding rear geometry analysis, the efficiency curve as a function of emitter width peaks at 70% of pitch coverage. Further, it is shown that diffused dopants inside crystalline silicon make the device resilient to passivation quality. Finally, the calibrated model is used to perform an optimization study aiming at calculating the performance limit. The estimated performance limit is 27.3% for a 100-μm-thick bulk, 20-nm-thick poly-silicon layers, silver as rear contact, and double ARC. Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Photovoltaic Materials and DevicesElectrical Sustainable Energ
Trap assisted tunneling modelling for aSi contact stacks in IBC-SHJ
No full textInterdigitated back contact silicon heterojunction (IBC-SHJ) solar cells have demonstrated 26.6\% world record efficiency by combining outstanding passivation of Si thin film layers with the absence of front shading contact in the crystalline silicon (c-Si) absorber bulk. Furthermore, this type of solar cell merges advantages of c-Si with Si thin film technology that allows for adjustment and tuning of bandgap and Fermi energy in deposited layers, and thus, modifying material properties that also impact the carrier collection. As heterointerfaces tailor to band offset and potential barriers for collecting carriers, the transport is described by thermionic emission and tunneling mechanisms. Moreover, Si thin film layers typically exhibit a defective matrix with characteristic traps and charge distribution that affect solar cell external parameters. Such phenomena involves the so called trap assisted tunneling (TAT) together with trap recombination mechanisms in a complex physical system.In this work, the effect of TAT mechanisms on IBC-SHJ is studied by means of numerical simulations based on TCAD Sentaurus. Firstly, the TAT model is implemented as non-local process. It was found that TAT exhibits a negligible effect on electron collection, but dominates transport mechanisms in case of hole contact. Such an effect depends on band alignment at the doped layer/transparent conductive oxide (TCO) interface, where band-to-band tunneling in combination with TAT describe the transport of holes. In particular, TCO carrier concentration in combination with activation energy of deposited layer allows the collection by either TAT or band-to-band tunneling. In more detail, the density of traps described typically as dangling bond improves the transport of collecting carriers. Thus, the collection of carriers is evaluated in terms of energy and trap concentration, demonstrating that traps with an energy level of 0.5 to 0.7eV over the valence band enhances the FF, and thus, the solar cell performance. Finally, the complete model is calibrated by comparing measured with simulated external parameters from a reference solar cell with and without TAT mechanisms. From this analysis, it is demonstrated that solar cell external parameters deploys more consistent values for Shockley-Read-Hall (SRH) recombination associated to bulk lifetime and surface recombination velocity using TAT model.Electrical Engineering | Sustainable Energy Technolog
Broadening the Thin-Film Horizon: Opto-electrical modelling of a monolithic Perovskite/CIGS tandem solar cell
No full textThe need for more efficient, cheaper, easily producible solar cells is growing as this combination of attributes can decrease the levelized cost of energy (LCOE). A twoterminal (2T) Perovskite and galliuminidiumgalliumselenide (Pvk/CIGS) tandem cell is an excellent candidate, as it can have a power conversion efficiency (PCE) of +30%, can be flexible (making it suitable for roll-to-roll production), andthe deposition of a Pvk solar cell (PSC) on top of established techniques like CIGS will only add one extra step to the production process, thereby improving the PCE significantlyThis thesis is part of LAFLEX-2T project which aims to design and engineer a highly efficient flexible 2T thin-film solar device based on the Pvk/CIGS tandem configuration. The aim of this thesis is to develop an opto-electrical model of the tandem configuration which replicates the inner physics of a reference solar cell. This can be used to give insights on the current losses occurring in the cell and on the limitations in the charge carrier transport towards the electrodes. Based on this analysis, a route of improvements is proposed with could result in more efficient solar cells.The simulation template uses optical and electrical simulations based on GENPRO4 and TCAD Sentaurus, respectively. An extensive model for Pvk/CIGS tandem cells is presented and validated using experimentally obtained J-V curve measurements. It was found that charge transport in the tunnel recombination junction (TRJ) depends on direct energy and indirect energy transfer, in terms of two tunneling mechanisms: band to band tunneling (B2BT) and trap assisted tunneling (TAT). For TAT, a non-local model facilitated by trap states is successfully implemented.Simulation results reveal that the transport in the TRJ of the reference solar cell is based on TAT. The loss analysis points out that reflectance losses are responsible for a loss of 10.92 mA/cm2. Similarly, losses due to parasitic absorption are equal to 5.32 mA/cm2. The CIGS bottom was identified as the current limiting layer. The dominant recombination mechanisms in the Pvk and CIGS absorber layers are Shockley-Read-Hall (SRH) and surface recombination calculated as 4.71 mA/cm2 and 1.59 mA/cm2 for top Pvk and bottom CIGS, respectively. An increase in losses in the absorber layers in maximum power point (MPP) compared to short circuit (SC) conditions exposed charge transport issues in the collecting path of charge carriers.After assessing the loss mechanisms, a roadmap for efficiency improvements is proposed. After implementation, an increase in the PCE of the reference tandem cell was observed from 10.63% to 26.69%.Electrical Engineering | Sustainable Energy Technolog
Numerical simulation of c-Si solar cells based on transition metal oxide as carrier selective contact: Drift diffusion and ab initio
No full textSilicon heterojunction solar cells employing transition metal oxides as carrier selective contact are of particular interest due to the potential of reducing parasitic absorption while featuring optimal electrical properties. Recently, a record efficiency of 23.5% was achieved by employing molybdenum oxide (MoOx) as carrier selective contact. MoOx exhibits advantageous properties with respect to the p-doped standard amorphous silicon contacts due to its lower parasitic absorption and better thermal stability. However, achieving an efficient carrier collection is challenging and not well understood yet. In this work, transport of charge is studied from drift diffusion and atomistic approach by means of numerical simulations. Two different state-of-art computational tools are employed: TCAD Sentaurus for drift diffusion simulations, and VASP for ab initio simulations. Through drift diffusion simulations, the contact formation of molybdenum oxide as carrier selective contact is consistently explored including quantum confinement and transport based in mid-gap energy states. The work function of MoOx is shown to be the core for an efficient charge collection. Thanks to experimental results, it is revealed relevant phenomenon at MoOx/intrinsic amorphous silicon (i-a-Si:H) interface which includes silicon oxide formation and charge accumulated. Therefore, a special focus at interface is here presented, in order to study the inner physics of the detrimental effects and how to avoid them. Altogether, drift diffusion simulations reveal that MoOx thickness is an essential parameter because it strongly determines the work function and hence the efficiency of the solar cell. All the knowledge acquired is used to provide guidelines on the fabrication of these type of solar cells.Looking at MoOx/a-Si:H interface, ab initio simulations are employed to study interface properties. Accordingly, such interface is analysed using both materials in their crystalline matrix. It is demonstrated that oxygen deficiency tunes the MoOx work function, a statement which is key for the proper contact formation and consistent with drift diffusion results. Finally, the charge arrangement at interface reveals the creation of an interface dipole together with silicon dioxide interlayer which is coherent with drift diffusion simulations analysis.Electrical Engineering | Sustainable Energy Technolog
Slow sub-gap energy states as the origin of hysteresis in perovskite solar cells: Device modeling using Sentaurus
No full textSolar cells can play a key role in the transition towards a sustainable future. This transition is one of the major challenges our society faces during the coming decades. Development of high-efficiency photovoltaic solutions at reasonable costs will help accelerating the transformation of our energy system. In this respect, perovskite solar cells are very promising due to their outstanding opto-electronical properties and low-cost fabrication. Obtaining a complete understanding of the device physics and charge transfer mechanisms inside perovskites is crucial for further device improvements. This thesis focuses on the notorious hysteresis in the current-voltage characteristics of perovskite solar cells. So far, this remarkable phenomenon has usually been explained using ion migration, despite the lack of clear experimental evidence. We implement a simulation platform of perovskite solar cells to analyse the charge transfer mechanisms among energy states including those with energy within the forbidden bandgap. We evaluate transient behaviour and identify the limiting physical mechanisms. To explain anomalous hysteresis in perovskite solar cells we use a novel approach in which charge accumulates near the material interfaces due to defects with relatively low capture cross-sections. Defects in lead halide perovskites create shallow sub-gap energy states, that act as charge carrier traps. Near the interfaces, this leads to accumulation of trapped charge carriers, effectively screening the electric field inside the perovskite layer. This reduces the device performance. A slow release of trapped charge due to low capture cross-sections results in hysteresis in the current-voltage curve at commonly used scan rates. TCAD Sentaurus is used as a platform to simulate J-V scans of a planar non-inverted architecture based on the archetypal perovskite MAPbI3, with TiO2 as electron transport layer and spiro-OMeTAD as hole transport layer. This thesis presents a systematic study of different trap distributions, both in the spatial and energetic domain. The capture cross-sections, densities, energy levels and locations of traps are varied and also the effect of scan rate is analysed. This work analyses both tail state defects and deep defects, based on reported values in literature. It is found that defects near the ETL/perovskite interface potentially cause anomalous hysteresis in the current-voltage curve. These defects have their transition energy around 0.25 eV and are possibly attributed to iodine interstitials.Applied Physic
Opto-electrical simulations of perovskite/c-Si tandem solar cells
No full textDue to the high-efficiency potential over 40%, the two-terminal (2T) perovskite/c-Si tandem solar cell becomes a desirable and promising candidate for solar cells. The photovoltaic materials and devices (PVMD) group in TU Delft has developed the c-Si solar cell using poly-SiOx as carrier-selective passivating contacts. The utilization of poly-SiOx as carrier selective passivating contacts is relatively new and very interesting to be analyzed as its bandgap can be varied by changing its oxygen content. This can be used in the solar cell to increase the Jsc. In this work, the 2T perovskite/c-Si tandem solar cells' optimization has been conducted by means of optical and electrical simulations. Optical simulations of the 2T, 3T and 4T perovskite/c-Si tandem solar cells in GenPro4 are presented. The main objective is to show the new type of bottom cell's optical performance in the 2T, 3T, and 4T tandem solar cell. Furthermore, the two-terminal (2T) perovskite/c-Si tandem solar cell's electrical modeling framework with the new type of the bottom cell has been developed. In this model, the optical generation profile from GenPro4 was successfully imported into the semiconductor simulations software called Sentaurus TCAD. The perovskite and c-Si solar cells have been optimized separately. The perovskite solar cell optimization was conducted by changing parameters such as contact resistance, surface recombination velocity and thickness of each layer. This leads to the increase in the perovskite solar cell efficiency from 15.83% to 21.50%. This top cell was combined with a c-Si bottom cell with an efficiency of 25% to form a two-terminal (2T) tandem solar cell. The two-terminal (2T) perovskite/c-Si was further optimized by optimizing the tunnel recombination junction. The optimization of TRJ was done by varying the doping concentration of indium tin oxide (ITO) and spiro-OMeTAD, respectively. As a result, the 33.70% efficiency of the two-terminal (2T) perovskite/c-Si tandem solar cell was obtained.Electrical Engineering | Sustainable Energy Technolog
Opto-Electrical Simulation of Perovskite/Silicon Tandem Solar Cell: ASA optimization of Pvk/c-Si tandem simulation
No full textEmerging PV technologies like Perovskite has lead to the development of Perovskite/Silicon (Pvk/c-Si) tandem device (Multi-junction device) and has now gained a lot of momentum and attention due to the fact that the tandem device can reach a Shockley-Quisser limit of about 44.1% efficiency. But to speed up its development, a lot of emphasis is made on conducting tandem solar cell device simulations to have get a good idea on the experiments that can be carried out. Solar cell simulations for tandem devices are carried out in advanced 2D/3D solar cell simulators like SETFOS or Synopsys TCAD Sentaurus. These softwares have large computational time and have complex user interface making simulations challenging. There are 1D solar cell simulators like ASA (Advanced Semiconductor Analysis) developed at TU Delft that has less computational time with an easy and intuitive interface. But ASA does not have the appropriate tunnelling models to simulate the tunnel recombination junction characteristics of perovskite silicon tandem devices. To make ASA a complete software suite for simulating perovskite silicon tandem device, tunnelling models proposed by Ieong et al. is used for both direct and band to band tunnelling. These tunnelling models use basic inputs like band energy data, electrostatic potential data and electric field data for a particular device and outputs the tunnelling generation rate that can simply be added to the continuity equations for electrons or holes during the simulation of the device. A fully fledged algorithm was designed for the automation of the direct tunnelling and the band to band tunnelling process with limited input from the user while using the models proposed by Ieong et al. In simple terms, the algorithm takes in the required input and scans through the device for every interface to check for tunnel contributions and performs calculations when necessary. All necessary conditions are included inside the algorithms to ensure that the simulation is accurately solved.The validation of the newly developed tunnelling algorithm was commenced for PN junction, Silicon hetero-junction and finally the Perovskite-Silicon tandem device. The JV curve for the above device simulations with and without incorporated tunnelling models was extracted and a comparison was made with literature results and simulation results from other software suites. The newly developed tunnelling algorithms was concluded to be accurate. The JV curve obtained from the ASA simulation showed good agreement with literature results and simulation results from other software suites with error percentages of less than 3% for open circuit voltage, short circuit current and fill factor. With proper tuning of the device layer thickness, texturing on multiple layers for the tandem device and enhancing the mobility of the non-absorber layers for the perovskite top cell, further improvement can be expected in the simulation. These methods can eventually lead to reaching the Shockley Quisser limit of about 35.7% efficiency for the perovskite silicon tandem solar cell even when general opto-electrical losses are taken into account.Electrical Engineering | Sustainable Energy Technolog
Simulations of shade tolerant solar cells with low breakdown voltages
No full textShading on photovoltaic modules is practically inevitable, especially in urban environments. The shadows cast by neighbouring objects on the solar panel force shaded solar cells to operate under reverse bias. In this case, instead of generating power, the shaded solar cell dissipates power, which is converted into heat and may induce the formation of hot-spots. Many attempts have been made to improve the shade tolerance photovoltaic modules. In this work, we focus on solar cells with low breakdown characteristics to build shade tolerant photovoltaic modules. These types of solar cells allow the current flow at low reverse bias voltages (around −4 V). The main design challenge is to maintain high conversion efficiencies while achieving low breakdown voltages.In order to design shade tolerant photovoltaic modules, the carrier transport mechanisms in the solar cell under reverse bias conditions are firstly investigated. A robust simulation template is created in Sentaurus TCAD to perform a parametric evaluation, including both the structural and operating parameters, of the device I-V characteristics. The silicon heterojunction interdigitated back contact solar cell with a silicon oxide passivation layer is among the most promising cell structures to achieve both the low breakdown voltage and the high efficiency. Band-to-band tunneling happens between the heavily doped p+ and n+ regions at the rear side, which allow charge carriers to recombine without entering the bulk of the solar cell. We analyze the effect of the tunneling mass, the gap distance and the dopants penetration length on the forward and reverse I-V curves. Simulations suggest that it is possible to design high efficiency solar cells with breakdown voltages as low as −1.2 V. In addition, device performances under different temperature and irradiance conditions are analysed for the purpose of further investigations on the system level.While this research study is mainly focused on the performance of solar cells, the results presented in this thesis facilitate comprehensive system level energy yield analyses of shade tolerant photovoltaic modules with low breakdown voltage solar cells.Electrical Engineering | Sustainable Energy Technolog
- …
