A Comprehensive Review on Current Performance, Challenges and Progress in Thin-Film Solar Cells

Due to the recent surge in silicon demand for solar modules, thin-film photovoltaic (PV) modules have a potential to penetrate the market in significant numbers. As an alternate candidate, thin film technologies in PVs have the ability to achieve better performance. The competing thin-film PV technologies have the flexibility to adapt to any sort of curvature compared to rigid solar cells (SCs). Due to the peculiar characteristics of newer solar materials, stability issues, reflection losses, advancements in electrode materials and dopant materials with a photoactive layer are current challenges driving the industrial-academic voyage of development of solar materials for the betterment of Photo-conversion Efficiency (PCE). Based on the photoactive materials used over time, SC evolution was broadly classified into first, second and third generation SCs. In this review, the basic working mechanisms, various materials used, drawbacks and stability issues of different SCs are discussed extensively. Thin film SCs tend to absorb certain elastic deformations due to their flexible nature and to a certain extent. According to the NREL efficiency chart, multi-junctional SCs exhibit enhanced efficiency as compared to the other SCs. Among the third-generation SCs, the perovskite/Si tandem architecture shows a maximum efficiency of approximately 29%. Thin film flexible SCs find application in various sectors such as automobile, defense and/or energy storage device

Silicon thin films for mobile energy electronics

Consumer needs for mobile devices include the requirement for longer battery life, so that recharging can be performed less frequently or eliminated completely. To this end a key component of any mobile system is a high power and high energy density battery. An alternative to better batteries is for mobile devices to harvest some of their own energy. Solar energy is an accessible, free and environmentally friendly source of energy, making it ideal for powering mobile devices. In this work we present a low deposition temperature (150°C), thin-film solar power harvesting system. Low deposition temperature of thin film silicon and associated alloys allows for fabrication on plastic in order to realize lightweight and robust integrated systems. The system consists of a thin film transistor (TFT) circuit and thin film photovoltaic (PV) array. The circuit functions as a simple DC-DC regulator and maximum power point tracking unit (MPPT). Amorphous silicon (a-Si:H) is used as the primary thin-film material for the fabrication of the devices. One of the challenges when fabricating devices at low temperatures is the high defect density in a-Si:H due to hydrogen clustering. In here the He in addition to the SiH4 and H2 is used to minimise hydrogen clustering. Using the optimised films, TFT and PV devices are fabricated, and analysed. Low deposition temperatures influence TFT properties. Contact resistance and dynamic instability of TFTs are considered. New extraction methods and their effect on device mobility are presented. A power conditioning TFT circuit is proposed. A model is developed to analyse the circuit’s output stability as a function of stressing and light intensity. System efficiency and its dependence on circuit efficiency and solar cell utilisation are discussed. The PV array and the TFT circuit are fabricated using lithography techniques, with a maximum process temperature of 150°C. The circuit can provide a degree of output power stability over a wide range of light intensities and stressing times, making it suitable for use with SC. In this work peak system efficiency of 18% is achieved. Despite the circuit’s low efficiency, it has the advantage of fabrication on plastic substrates and better integrability within mobile devices

Non-Classical Crystallization of Thin Films and Nanostructures in CVD Process

Non-classical crystallization, where crystals grow by the building blocks of nanoparticles, has become a significant issue not only in solution but also in the gas phase synthesis such as chemical vapor deposition (CVD). Recently, non-classical crystallization was observed in solution in-situ by transmission electron microscope (TEM) using a liquid cell technique. In various CVD processes, the generation of charged nanoparticles (CNPs) in the gas phase has been persistently reported. Many evidences supporting these CNPs to be the building blocks of thin films and nanostructures were reported. According to non-classical crystallization, many thin films and nanostructures which had been believed to grow by individual atoms or molecules turned out to grow by the building blocks of CNPs. The purpose of this paper is to review the development and the main results of non-classical crystallization in the CVD process. The concept of non-classical crystallization is briefly described. Further, it will be shown that the puzzling phenomenon of simultaneous diamond deposition and graphite etching, which violates the second law of thermodynamics when approached by classical crystallization, can be approached successfully by non-classical crystallization. Then, various aspects of non-classical crystallization in the growth of thin films and nanostructures by CVD will be described

Hydrogenated amorphous silicon:impact of process conditions on material properties and solar cell efficiency

Thin-film silicon solar cells are one possible answer to the increasing energy demand of today. Hydrogenated amorphous silicon (a-Si:H) plays a crucial role therein - as absorber layers, but also as doped layers to build p-i -n junctions. This thesis is devoted to a-Si:H, with the main focus on thin-film silicon solar cells, but also with applications for opto-electronic devices, detectors, and other types of solar cells such as heterojunction solar cells. We discuss models of a-Si:H and develop further the representation of defects by amphoteric states. Using a simple model, we show - in agreement with layer-by-layer simulations and experimental results - that trapped electrons tend to dominate the electric field deformation in the initial state, whereas positively charged defects dominate in the degraded state. Experimentally, we define the deposition parameter space accessible by plasma-enhanced chemical vapor deposition (PECVD) and explore that space by varying the deposition temperature, pressure, excitation frequency, power, and H2/SiH4 ratio for intrinsic absorber layers. This leads to a catalog of a-Si:H absorber layers with tunable properties and we incorporate these materials into solar cells. For every pressure, we find an optimum hydrogen dilution where the light-induced degradation of solar cells is minimal and comparable for all pressures. Using narrow-bandgap absorbers, we demonstrate short-circuit current densities of Jsc=18.2 mA/cm2 with a 300-nm-thick absorber layer and extract more than 20 mA/cm2 from a cell with a 1000-nm-thick absorber layer. Using wide-bandgap absorbers, we achieve open-circuit voltages (Voc) of 1.04 V and Voc-fill factor products of 739mV. For such materials, we find an increased Voc dependence on substrate roughness. This is investigated by transmission electron microscopy and is attributed to porous a-Si:H material grown above peaks on the textured substrates. Depositing absorber layers in a triode reactor, we achieve efficiencies of 10.0% after light soaking. Further, we describe observations of a reversible, light-induced Voc increase of solar cells with thin p-type layers, and decrease with thick p-type layers, with a magnified effect on rough substrates. Based on layer measurements and simulations, we attribute the Voc increase to the degradation of the p-layer and the Voc decrease to the degradation of the absorber layer

Hydrogenated polymorphous silicon: establishing the link between hydrogen microstructure and irreversible solar cell kinetics during light soaking

This thesis is dedicated to hydrogenated polymorphous silicon (pm-Si:H) and solar cells based on this material. pm-Si:H is a nanostructured thin film deposited by conventional PECVD method. The effects of various deposition parameters (gas flow ratio, pressure, RF power, Ts) on material properties were investigated in order to optimize its quality. The strategy was to combine a wide range of diagnostics (spectroscopic ellipsometry, hydrogen exodiffusion, SIMS, FTIR, AFM, etc.). Due to the contribution of plasma synthesized silicon nanoparticles, the process condition of pm-Si:H shows the difference in contrary to a-Si:H deposition through ionized radicals. Studies on pm-Si:H deposition process allows to fabricate pm-Si:H PIN solar cells with a high initial efficiency of 9.22 % and fill factor of 74.1, but also demonstrate unusual light-induced effects, namely i) a rapid initial degradation, ii) an irreversible degradation, and iii) large macroscopic structural changes. Comprehensive investigation on the light-induced degradation kinetics of pm-Si:H PIN layer stacks reveals a pronounced hydrogen accumulation and delamination at the substrate/p-type layer interface under light-soaking, leading to macroscopic structural changes, e.g., peel-off and solar cell area loss. We have found that a PIN structure leads to facilitated delamination during lightsoaking, which we attribute to hydrogen accumulation at the substrate/p-layer interface, while use of a NIP structure prevents the hydrogen accumulation and delamination. This lead us to fabricate pm-Si:H NIP solar cells showing a high stabilized efficiency of 8.43 %, that shows a small (10 %) light-induced degradation after light-soaking for 500 hours.Cette thèse est consacrée au silicium polymorphe hydrogéné (pm-Si:H). Elle porte tout d'abord sur une étude du pm-Si :H puis sur une étude des cellules photovoltaïques fabriquées à partir de ce matériau. Le pm-Si:H est formé de couches minces nanostructurées et peut être déposé par PECVD conventionnelle. Les effets des différents paramètres de dépôt (mélanges gazeux, pression, puissance RF, température du substrat) sur les propriétés du matériau ont été étudiés pour optimiser sa qualité. La caractérisation des couches a été un enjeu primordial. Pour cela, nous avons choisi de combiner une palette très large de méthodes de caractérisation (ellipsomètrie spectroscopique, exodiffusion d'hydrogène, SIMS, FTIR, AFM, etc...). A cause de la contribution des nanoparticules de silicium dans le plasma, la nature du dépôt du pm-Si:H montre la différence contrairement au a-Si:H pour lequel le dépôt se fait par le biais de radicaux ionisés. L'étude des conditions du procédé nous a conduit à fabriquer des cellules solaires d'un rendement initial de 9.22 % avec un facteur de forme élevé (74.1), mais aussi de démontrer des effets de vieillissement inhabituels, tels que i) une dégradation initiale rapide, ii) une dégradation irréversible, et iii) de grands changements structuraux macroscopiques. Nous avons découvert que le principal problème se situe entre le substrat et la couche mince de silicium. L'hydrogène moléculaire diffuse et s'accumule à l'interface entre le substrat et la couche mince, ce qui introduit un délaminage local qui a pour conséquence une dégradation initiale rapide des performances des cellules. Nous avons trouvé que sous éclairement une structure PIN facilite l'accumulation d'hydrogène et le délaminage à l'interface entre le substrat et la couche dopée p. Cependant, l'utilisation d'une structure NIP empêche l'accumulation d'hydrogène et le délaminage. Cela nous a permis de fabriquer des cellules solaires pm-Si:H de structure NIP d'un rendement stable de 8.43 %, mais aussi de démontrer une degradation minimale (10 %) après un vieillissement de 500 heures

Low temperature (<150 °C) hydrogenated amorphous silicon grown by PECVD with source gas heating

Hydrogenated amorphous silicon (a-Si:H) is a semiconductor that is widely used in a variety of applications. A particularly important development has been the incorporation of this material into thin film transistor (TFT) arrays for the active matrix addressing of liquid crystal displays. Plasma Enhanced Chemical Vapour Deposition (PECVD) is one of the most successful techniques currently in use for the deposition of device quality a-Si:H. However, there is an increasing desire to improve process compatibility with low cost, plastic substrates. This entails trying to reduce the deposition temperature from approximately 250 - 300°C to below 150°C, whilst maintaining material quality. This thesis describes the design of a novel, low temperature PECVD system incorporating the facility to pre-heat the deposition source gases. The physical and electronic properties of a-Si:H deposited at <150°C are investigated, and the performance of TFT structures incorporating optimised material as the active layer is described. It is shown that the physical properties of a-Si:H produced at a substrate temperature of 125°C with the source gas line heated to 400 °C are commensurate with films deposited at 250-300 QC. The hydrogen content of the optimised film was found to be 10.5 %, with a Tauc bandgap of 1.66 e V. Pre-heating of the source gases also leads to an increase in the proportion of hydrogen bonded in the monohydride configuration. It is suggested that the diffusion of the film-forming gaseous species is enhanced by this technique, resulting in a reduction in the degree of disorder within the film and hydrogen elimination. Consequently, the concentration of hydrogen and the Tauc bandgap also decrease, leading to an increase in photoconductivity of one order of magnitude. TFTs exhibit a switching ratio of 1 Os, which is approximately an order of magnitude smaller than high temperature a-Si:H TFTs, but a comparable OFF current of approximately 10.12 A. However, the field effect mobility of these devices is very poor (10.3 cm2V·l s·I). This is thought to be due to a high interface state density at the boundary between the low temperature, gas-heated a-Si:H layer and the high temperature silicon nitride gate insulator.EPSRC. Philips Research Laboratorie

Vertical Thin Film Transistors for Large Area Electronics

The prospect of producing nanometer channel-length thin film transistors (TFTs) for active matrix addressed pixelated arrays opens up new high-performance applications in which the most amenable device topology is the vertical thin film transistor (VTFT) in view of its small area. The previous attempts at fabricating VTFTs have yielded devices with a high drain leakage current, a low ON/OFF current ratio, and no saturation behaviour in the output current at high drain voltages, all induced by short channel effects. To overcome these adversities, particularly dominant as the channel length approaches the nano-scale regime, the reduction of the gate dielectric thickness is essential. However, the problems with scaling the gate dielectric thickness are the high gate leakage current and early dielectric breakdown of the insulator, deteriorating the device performance and reliability. A novel ultra-thin SiNx film suitable for the application as the gate dielectric of short channel TFTs and VTFTs is developed. The deposition is performed in a standard 13.56MHz PECVD system with silane and ammonia precursor gasses diluted in nitrogen. The deposited 50nm SiNx films demonstrate excellent electrical characteristics in terms of a leakage current of 0.1 nA/cm² and a breakdown electric field of 5.6MV/cm. Subsequently, the state of the art performances of 0.5µm channel length VTFTs with 50 and 30nm thick SiNx gate dielectrics are presented in this thesis. The transistors exhibit ON/OFF current ratios over 10^9, the subthreshold slopes as sharp as 0.23 V/dec, and leakage currents in the fA range. More significantly, a high associated yield is obtained for the fabrication of these devices on 3-inch rigid substrates. Finally, to illustrate the tremendous potential of the VTFT for the large area electronics, a 2.2-inch QVGA AMOLD display with in-pixel VTFT-based driver circuits is designed and fabricated. An outstanding value of 56% compared to the 30% produced by conventional technology is achieved as the aperture ratio of the display. Moreover, the initial measurement results reveal an excellent uniformity of the circuit elements.1 yea

Hot-wire chemical vapour deposition of nanocrystalline silicon and silicon nitride : growth mechanisms and filament stability

Philosophiae Doctor - PhDNanocrystalline silicon (nc-Si:H) is an interesting type of silicon with superior electrical properties that are more stable compared to amorphous silicon (a-Si:H). Silicon nitride (SiNₓ) thin films are currently the dielectric widely applied in the microelectronics industry and are also effective antireflective and passivating layers for multicrystalline silicon solar cells. Research into the synthesis and characterization of nc-Si:H and SiNₓ thin films is vital from a renewable energy aspect. In this thesis we investigated the film growth mechanisms and the filament stability during the hot-wire chemical vapour deposition (HWCVD) of nc-Si:H and SiNₓ thin films. During the HWCVD of nc-Si:H, electron backscatter diffraction (EBSD) revealed that the tantalum (Ta) filament aged to consists of a recrystallized Ta-core with Ta-rich silicides at the hotter centre regions and Si-rich Ta-silicides at the cooler ends nearer to the electrical contacts. The growth of nc-Si:H by HWCVD is controlled by surface reactions before and beyond the transition from a-Si:H to nc-Si:H. During the transition, the diffusion of hydrogen (H) within the film is proposed to be the reaction controlling step. The deposition pressure influenced the structural, mechanical and optical properties of nc-Si:H films mostly when the film thickness is below 250 nm. The film stress, optical band gap, refractive index and crystalline volume fraction approached similar values at longer deposition times irrespective of the deposition pressure. Filament degradation occurred during the HWCVD of SiNₓ thin films from low total flow rate SiH₄ / ammonia (NH₃) / H₂ gas mixture. Similar to the HWCVD of nc-Si:H, the Ta-core recrystallized and silicides formed around the perimeter. However, Tanitrides formed within the filament bulk. The extent of nitride and silicide formation, porosity and cracks were all enhanced at the hotter centre regions, where filament failure eventually occurred. We also applied HWCVD to deposit transparent, low reflective and hydrogen containing SiNₓ thin films at total gas flow rates less than 31 sccm with NH₃ flow rates as low as 3 sccm. Fluctuations within the SiNₓ thin film growth rates were attributed to the depletion of growth species (Si, N, and H) from the ambient and their incorporation within the filament during its degradation

Enhanced crystallization of amorphous silicon thin films by nano-crystallite seeding

University of Minnesota Ph.D. dissertation. December 2013. Major: Mechanical Engineering. Advisor: Uwe Kortshagen. 1 computer file (PDF); x, 119 pages.Polycrystalline silicon (poly-Si) has become popular in recent years as a candidate for low cost, high efficiency thin film solar cells. The possibility to combine the stability against light degradation and electronic properties approaching melt-grown, wafer-based crystallline silicon, with the cost advantage of Silicon thin films is highly attractive. To fully realize this goal, efforts have been focused on maximizing grain size while reducing the thermal input involved in a critical ``annealing'' step. Of the variety of processes involved in this effort, studies have shown that poly-Si films obtained from solid-phase-crystallization (SPC) of hydrogenated amorphous silicon (a-Si:H), grown from non-thermal plasma-enhanced chemical vapor deposition (PECVD), exhibit the potential to achieve the highest quality grain structures. However, reproducible control of grain size has proven difficult, with larger grains typically requiring longer annealing times. In this work, a novel technique is demonstrated for more effectively controlling the final grain structure of SPC-processed films while simultaneously reducing annealing times. The process utilized involves SPC of a-Si:H thin films containing embedded nanocrystallites, intended to serve as predetermined grain-growth sites, or grain-growth ``seeds'', during the annealing process. Films were produced by PECVD with a system in which two plasmas were operated to produce crystallites and amorphous films separately. This approach allows crystallite synthesis conditions to be tuned independently from a-Si:H film synthesis conditions, providing a large parameter space available for process optimization, including the effects of particle size, shape, quantity, and location within the film. The work contained here-in outlines the effects of select parameters on the both grain size control and thermal budget. Reproducible control of both grain size and crystallization rate were demonstrated through varying initial seed crystal concentrations. Significant reductions in annealing times were demonstrated to occur in seeded films relative to unseeded films, with both seed crystal concentration and seed crystal geometry demonstrating significant effects on crystallization rate. Furthermore, the development of this technique has resulted in potentially new insights on the material system involved, with the observation of a potentially unique phase-transformation mechanism