Numerical modeling of SiH4 discharge for Si thin film deposition for thin film transistor and solar cells

Amorphous and microcrystalline silicon thin films are used in solar cells as a multi-junction photovoltaic device. Plasma enhanced chemical vapor deposition is used and high deposition rate of a few nm/s is required while keeping film quality. SiH₄ is used as a precursor diluted with H₂. Electron impact processes give complex interdependent plasma chemical reactions. Many researchers suggest keeping high H/SiH_x ratio is important. Numerical modeling of this process for capacitively coupled plasma and inductively coupled plasma is done to investigate which process parameters are playing key roles in determining it. A full set of 67 volume reactions and reduced set are used. Under most of conditions, CCP shows 100 times higher H/SiH₃ ratio over ICP case due to its spatially localized two electron temperature distribution. Multi hollow cathode type CCP is also modeled as a 2 × 2 hole array. For Ar, the discharge is well localized at the neck of the hole at a few Torr of gas pressure. H₂ and SiH₄ + H₂ needed higher gas pressure and power density to get a multi hole localized density profile. H/SiH₃ was calculated to be about 1/10.     

Development of CZTS-based thin film solar cells

The low cost, environmental harmless Cu₂ZnSnS₄ (CZTS)-based thin film solar cells are fabricated by using abundant materials. The CZTS film possesses promising characteristic optical properties; band-gap energy of about 1.5 eV and large absorption coefficient in the order of 10⁴ cm^− 1. All constituents of this CZTS film, which are abundant in the crust of the earth, are non-toxic. Therefore, if we can use CZTS film practically as the absorber of solar cells, we will be free from both of the resource saving problem and the environmental pollution.In our CZTS project, CZTS absorber films were prepared by two independent techniques. One is three rf sources co-sputtering followed by annealing in sulfurized atmosphere. The latest conversion efficiency of over 6.7% was achieved by this technique. The other is co-evaporation technique. CZTS films were grown on Si (100) by vacuum co-evaporation using elemental Cu, Sn, S and binary ZnS as sources. XRD patterns indicated that the polycrystalline growth was suppressed and the orientational growth was relatively induced in a film grown at higher temperatures.In this presentation, the development of CZTS-based thin film solar cells will be surveyed.     

Growth regimes of CdTe deposited by close-spaced sublimation for application in thin film solar cells

We have systematically investigated the growth of CdTe thin films by Close Spaced Sublimation (CSS). Thin films of CdTe were deposited onto CdS substrates held at temperatures in the range 250 to 550 °C. The effect of substrate temperature and evaporation rate on structure and surface morphology of CdTe films were investigated. Up to 450 °C substrate temperature the growth rate was almost constant and decreased exponentially for higher temperatures. The structures of the CdTe films were determined by XRD and a strong (111) orientation was observed within the temperature range 250 °C-470 °C. Above 470 °C the texture changed to mostly (311) and (220) orientations. Surface morphology and grain size of CdTe growth was determined with AFM and SEM. The morphology of the layers showed three major modes: Columnar grains with a diameter of 0.2 μm and a length of 6 μm for temperatures from 250 °C to 350 °C, pyramidal grains with a diameter of 0.5-1.5 μm up to 470°C and irregular shaped grains with a diameter of 5-10 μm for temperatures up to 550 °C. The roughness increased linearly from 15 nm to 220 nm within the substrate temperature range.     

Modelling multivalent defects in thin film solar cells

Multivalent defects, e.g. double donors/acceptors or amphoteric defects, are important in materials used in solar cell production in general and in chalcopyrite materials in particular. We extended our thin film solar cell simulation software scaps to enable the simulation of multivalent defects with up to five different charge states; the algorithms presented are however able to simulate an arbitrary number of possible charge states. The presented solution method avoids numerical inaccuracies caused by the subtraction of two almost equal numbers.This new modelling facility is afterwards used to investigate the consequences of the multivalent character of defects for the simulation of chalcopyrite based solar cells.     

Cu2ZnSnS4-type thin film solar cells using abundant materials

The development of environment friendly type thin film solar cell Cu₂ZnSnS₄ (say CZTS-type) fabricated using abundant materials is introduced in this paper. Three RF sources co-sputtering continued with vapor phase sulfurization method (inline-type vacuum apparatus) is utilized. Those processes were sequentially done in different apparatus in our previous work. In this study, an inline-type vacuum apparatus is firstly introduced to acquire higher quality of CZTS films. Inductively Coupled Plasma Spectroscopy (ICPS) is used to analyze the minute material composition of CZTS thin film solar cell. By optimizing the material composition, 5.74% of conversion efficiency is obtained. It is the best data for CZTS-type thin film solar cells at present.     

Lead antimony sulfides as potential solar absorbers for thin film solar cells

In an effort to prepare thin films of novel semiconductor materials that contain only cost effective, abundant, and relatively less-toxic materials, lead antimony sulfides films have been prepared. Herein, we report the thin film preparation of semseyite (Pb₉Sb₈S₂₁) via annealing of precursor films under sulfur vapor. Pb/Sb alloy precursor films suffered substantial changes in stoichiometry and produced rough films, whereas precursor films composed of multilayers of PbS and amorphous (Sb,S) produced smooth and compact phase-pure films composed of fine grains. Optical measurements indicated a direct band gap of 1.93 eV and a strong absorption coefficient of 1.0 × 10⁵ cm^− 1.     

Synthesis and characterization of AgInSe2 for application in thin film solar cells

AgInSe₂ (AIS) films were grown on n-type Si substrates by the ultra-high-vacuum pulsed laser deposition technique from the AIS target synthesized from high-purity materials. The X-ray diffraction and microscopic studies of the films show that films are textured having terrace-like surface morphology. The optical studies of the films show that the optical band gap is about 1.24 eV. The electrical conductivity of AgInSe₂/Si films shows excellent diode characteristics. The photoconductivity of the AgInSe₂/Si device shows photocurrent of 2.8 mA at a bias-voltage of − 1 V with an open circuit voltage of 0.15 V. This shows that AIS films are very good absorber material for solar cell technology.     

TCAD simulations for thin film solar cells with nanoplate structures

The novel thin film solar cell with a nanoplate structure that can solve the conflict between the light absorption and the carrier transport in amorphous silicon thin film solar cell was investigated by TCAD simulations. This new structure has n-type amorphous silicon nanoplate array on the substrate, and p-type amorphous silicon-carbon as window layer and intrinsic amorphous silicon as absorption layer are sequentially grown along the surface of each n-type amorphous silicon nanoplate. Under AM 1.5 G sunlight illumination, the light is absorbed along the vertical direction of nanoplate while the carrier transport is along the horizontal direction. Therefore, nanoplate with the larger height can absorb most of the sunlight. The advantage of this novel structure is that the thickness of the solar cell can be used as thin as possible for effective transport of photo-generated carriers in comparison with the planer one.     

High mobility transparent conducting oxides for thin film solar cells

A special class of transparent conducting oxides (TCO) with high mobility of > 65 cm² V^− 1 s^− 1 allows film resistivity in the low 10^− 4 Ω cm range and a high transparency of > 80% over a wide spectrum, from 300 nm to beyond 1500 nm. This exceptional coincidence of desirable optical and electrical properties provides opportunities to improve the performance of opto-electronic devices and opens possibilities for new applications. Strategies to attain high mobility (HM) TCO materials as well as the current status of such materials based on indium and cadmium containing oxides are presented. Various concepts used to understand the underlying mechanisms for high mobility in HMTCO films are discussed. Examples of HMTCO layers used as transparent electrodes in thin film solar cells are used to illustrate possible improvements in solar cell performance. Finally, challenges and prospects for further development of HMTCO materials are discussed.     

Surface and thin film analysis of CdTe layers for solar cells

A solid state diffusion source with appropriate concentrations of donor and acceptor impurities has been used for studying the processes which occur during the thermal treatment of CdTe layers. The surface of the CdTe is investigated by XPS before and after annealing. The element ratios on the CdTe layer surface are determined in at%. Mathematical processing of the XP spectra is used to establish the ratios between the elements bound on the CdTe layer surface. The chemistry of the annealing-caused changes in CdTe layer properties are discussed.     

Electrical,morphological and structural properties of RF magnetron sputtered Mo thin films for application in thin film photovoltaic solar cells

Molybdenum (Mo) thin films were deposited using radio frequency magnetron sputtering, for application as a metal back contact material in “substrate configuration” thin film solar cells. The variations of the electrical, morphological, and structural properties of the deposited films with sputtering pressure, sputtering power and post-deposition annealing were determined. The electrical conductivity of the Mo films was found to increase with decreasing sputtering pressure and increasing sputtering power. X-ray diffraction data showed that all the films had a (110) preferred orientation that became less pronounced at higher sputtering power while being relatively insensitive to process pressure. The lattice stress within the films changed from tensile to compressive with increasing sputtering power and the tensile stress increased with increasing sputtering pressure. The surface morphology of the films changed from pyramids to cigar-shaped grains for a sputtering power between 100 and 200 W, remaining largely unchanged at higher power. These grains were also observed to decrease in size with increasing sputtering pressure. Annealing the films was found to affect the resistivity and stress of the films. The resistivity increased due to the presence of residual oxygen and the stress changed from tensile to compressive. The annealing step was not found to affect the crystallisation and grain growth of the Mo films.     

Advanced APCVD-processes for high-temperature grown crystalline silicon thin film solar cells

Crystalline silicon thin film (cSiTF) solar cells based on the epitaxial wafer-equivalent (EpiWE) concept combine advantages of wafer-based and thin film silicon solar cells. In this paper two processes beyond the standard process sequence for cSiTF cell fabrication are described. The first provides an alternative to wet chemical saw damage removal by chemical vapor etching (CVE) with hydrogen chloride in-situ prior to epitaxial deposition. This application decreases the number of process and handling steps. Solar cells fabricated with different etching processes achieved efficiencies up to 14.7%. 1300 degrees C etching temperature led to better cell results than 1200 degrees C. The second investigated process aims for an improvement of cell efficiency by implementation of a reflecting interlayer between substrate and active solar cell. Some characteristics of epitaxial lateral overgrowth (ELO) of a patterned silicon dioxide film in a lab-type reactor constructed at Fraunhofer ISE are described and first solar cell results are presented.     

Combination of surface texturing and nanostructure coating for reduction of light reflection in ZnO/Si heterojunction thin film solar cell

In this study, we report a single heterojunction solar cell based on n-type zinc oxide/p-type silicon. Three different solar cells were fabricated based on ZnO thin film on Si substrate, ZnO nanorods on Si substrate, and ZnO nanorods on micro-pyramidal structure of Si substrate. The comparison between these three kinds of solar cells was studied. Pyramidal structure of silicon was fabricated using chemical etching technique of p-type Si (100). The chemical solution consists of NaOH, isopropyl alcohol and hydrazine hydrate. The results showed that Si micro-pyramids can enhance optical absorption of Si substrates by increasing surface area and entrapping of incident light. For fabrication of uniform ZnO nanorods, a seed layer of ZnO was deposited on Si substrates via radio frequency magnetron sputtering technique. This layer can be used as an active n-type material in heterojunction solar cells as well. ZnO nanostructures can increase light absorption due to their high specific surface area. The combination of ZnO nanorods and Si micro-pyramids can enhance light trapping effect and increase the efficiency of solar cells. The structural and morphology of samples were studied using field-emission scanning electron microscopy, atomic force microscopy and X-ray diffractometry while the optical properties were investigated using photoluminescence and reflectance spectrometry. The efficiency and fill factor of solar cells were obtained from current–voltage characteristics using a solar simulator and a source-meter. The results showed that the efficiency of solar cell based on nanostructures of ZnO/micropyramids of Si is highly increased due to high anti-reflective behavior of this sample.     

Doping profiles in CdTe/CdS thin film solar cells

CdS/CdTe thin film solar cells showing comparable properties as commercial cells have been prepared by close space sublimation (CSS) in our own laboratory. We characterised the cells by capacitance-voltage profiling (C-V), thermal admittance spectroscopy (TAS), and thermally stimulated capacitance measurements (TSCAP). The doping profiles of the CdTe layer obtained by C-V measurements confirm the well known rise in dopant concentration with increasing depth if the usual evaluation procedure is employed. However, the TAS and TSCAP measurements reveal deep acceptors in the CdTe layer with a large concentration exceeding that of the shallow dopants. Under these conditions, C-V measurements are shown to yield an apparently rising dopant concentration even for homogeneous doping. A combined simulation of doping profiles measured at different temperatures using a fixed and uniform shallow and deep doping fits well to measured doping concentration. These results indicate how to get reliable information on the shallow dopant concentration.     

Recent advances in very high frequency plasma enhanced CVD process for the fabrication of thin film silicon solar cells

We have deposited amorphous silicon (a-Si) and nanocrystalline silicon (nc-Si) materials and the total p-i-n configurations for solar cells in a high vacuum multichamber system ASTER using very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) process. The deposition process is monitored and controlled by in-situ diagnostic tools to maintain reproducibility of the material quality. In this paper we show our recent results on single junction (amorphous silicon) and tandem (a-Si/nc-Si) cells on plastic foil using the Helianthos concept. The tandem cell efficiency on Asahi U-type SnO₂:F coated glass is ~ 12% and this is achieved by employing nc-Si deposited at high pressure (p) conditions of 5 mbar and a small inter-electrode distance (d) of 5 mm. The deposition scheme of this cell on glass was adapted for the SnO₂:F coated Al foil substrates from Helianthos b.v., especially taking into account the expansion of the foil during deposition. The inter-electrode distance d was one of the variables for this optimisation process. Depositions at four inter-electrode distances of 6 mm, 8 mm, 10 mm and 12 mm (keeping the pressure-distance product constant) revealed that the deposition rate increases at higher distances, reaching 0.6 nm/s at a d of 10 mm and pressure p of 3.0 mbar. The Raman crystalline ratio showed a monotonic increase with the combination of higher d and lower p. Tandem cells with an area of 2.5 cm² on plastic foil fabricated by the Helianthos concept and employing the above mentioned nc-Si made at 0.6 nm/s in the bottom cell and a-Si in the top cell, showed an efficiency of 8.12%, with a short circuit current density of 10 mA/cm². The combined deposition time of the photoactive silicon layers of the top and bottom cells amounted to only 85 min.     

具有2D-EDAPbI4薄覆盖层的环境稳定的FAPbI3基钙钛矿太阳能电池（英文）

二维(2D)卤化铅钙钛矿材料是钙钛矿太阳能电池(PSC)中最有前途的吸光材料之一,具有优异的稳定性和缺陷钝化作用.然而,这些稳定的二维PSC的转换效率仍远远落后于三维钙钛矿电池.在本文中我们通过原位生长的方法将2D EDAPbI4层成功制备在3D FAPbI3层表面。这种合理设计的2D-3D钙钛矿薄膜分层结构可以明显提高电池的效率.另外,由于EDAPbI4层的高抗湿性和抑制迁移, 2D-3D电池器件显示出明显增强的长期稳定性,在200 h内一直保持初始转换效率,甚至在500 h后仍能保持其初始转化效率的90%.