Flexible thin films on textiles for solar cells

In recent years, there has been an increase in studies about developing photovoltaic fabrics which can be used in different textile and clothing applications. The flexibility of the solar cells could be useful in many applications, for example providing power for small portable electronic devices such as personal digital assistants or on a larger scale for sunshades and canopies. In this work, we have taken the direct approach to deposit amorphous silicon cells directly onto fabrics. To achieve that, we have studied approaches to obtaining flexible conductive surfaces on polyester fabrics by using a double layer of metal and commercially available conductive polymer. Then both single and stacked metal contact layers and thin amorphous silicon films were built on glass and flexible substrates for optical and electric characterisation. It was shown by bending tests that the conductive fabrics retain both flexibility and electrical conductivity. Finally, complete n-i-p single junction a-Si:H cells were fabricated on different types of substrates such as glasses, polyester fabric and polytetrafluoroethylene fabric (PTFE). Several challenging aspects related to the fabrication and characterisation of solar cells on fabrics are highlighted. Cells on woven fabrics were shown to be active photovoltaic devices though with lower response than equivalent cells on rigid glass substrates

(aSiGe/ncSi) Superlattice thin film silicon solar cell

Nanocrystalline silicon solar cell has come up as a potential material in the photovoltaic industry. There are various benefits of nanocrystalline silicon which makes it superior to its counterparts such as amorphous silicon and amorphous silicon germanium. Narrow band gap of 1.12eV helps to generate more current by utilizing the red and infrared region of the solar spectrum. High current producing ability makes it suitable material for the bottom cell of a tandem solar cell. The growth of nanocrystalline silicon is not as simple as the growth of amorphous silicon. The material grows in a conical fashion which results to large grain boundary formation if not controlled properly. The large grain boundaries hamper the electronic properties of the material. To prevent the formation of the large grain boundaries several design of nanocrystalline silicon solar cell has been used. Hydrogen profile, Power profile and Superlattice structures help to control the crystallinity of the material as it grows. There are other deposition parameters such as deposition temperature, pressure and frequency if changed may alter the morphology of the material by changing the grain size of crystals. For high mobility and more absorption of photons we need large grains sizes of \u3c220\u3e which can be achieved by high temperature and high pressure deposition conditions. We are going to discuss about the superlattice structure and ways to improve the quality of the solar cell. Amorphous silicon germanium has superior absorption coefficient as compared to amorphous silicon therefore in this report we would discuss how we can incorporate amorphous silicon germanium with nanocrystalline silicon to enhance the current of the solar cell. The grain structure and superior electronic property of nanocrystalline silicon will be utilized to collect the carriers from the thin amorphous silicon germanium tissue. We will also show the increase in current can be achieved by incorporating a back-reflector in a solar cell

Deposition & characterisation of silicon and conductive layers on woven polyester

Textile and semiconductor processes were combined to produce a flexible solar panel by depositing silicon thin film onto woven polyester. Semiconductor processes such as evaporation, Plasma Enhanced Chemical Vapour Deposition (PECVD) and RFsputtering were done under vacuum conditions. Microwave PECVD proved difficult for textile substrates as the weave was damaged using parameter settings commonly used for conventional substrates such as silicon wafers and glass. PECVD parameters such as temperature, gas flow-rate, mixture, pressure and power were adjusted to allow the textile to be processed and a good quality silicon thin film to be deposited. An extra conductive layer was introduced between the textile and metal back-contact to support the cell. The silicon film structure changed from amorphous to mixed crystal growth in an amorphous matrix, as revealed by Raman spectroscopy and light transmission. The silicon Raman spectrum often had three peaks with the middle one, a fingerprint for nanocrystal growth with a hexagonal wurtzite structure in between the amorphous and crystalline peaks. Process conditions for pure amorphous and microcrystalline structures were also established, requiring two peaks to fit the Raman spectrum. Different structures have different band-gap energies and these were determined by measuring the variation in light transmission. An amorphous structure has a band-gap energy of 1.8eV while a crystalline silicon structure has a band-gap of 1eV and a mixed nanocrystalline content has an intermediate value which depends on the crystal size. A microcrystalline structure has a band-gap of 1.6eV

Study of transport properties and defect density profile in nanocrystalline silicon germanium devices

Nanocrystalline Silicon-Germanium (nc-SiGe:H) is a useful material for photovoltaic devices and photo-detectors. Its bandgap can be tuned between Si (~1.1 eV) and Ge (~0.7 eV) by changing the alloy composition during growth. The material exhibits a good absorption extending to the infrared region even with thin layers. However previous work has shown that devices with higher Ge content have poor device performance. Also, very little work has been done previously to measure and understand the defect spectrum of nanocrystalline (Si,Ge). Defects control recombination, and hence, the performance of solar cell devices. This work deals with studying the fundamental device physics of nc-SiGe:H including defect density, lifetime and mobility and their relationship with impurities, grain size and Hydrogen bonding. Capacitance-Frequency measurements at different temperatures are used to estimate the trap density profile within the bandgap of nc-SiGe:H. We also study device performance and how to maintain uniform crystallinity in intrinsic layers of devices so as to obtain the best device performance. We show that one can use hydrogen grading or power grading to produce films with uniform crystallinity. We will report on a systematic study of the varying Germanium content in nc-SiGe:H the relationship between Ge content and transport properties. It is found that upon adding Ge to Si during growth, the intrinsic layer changes from n-type to p-type. This can be reversed back by using ppm levels of phosphorus doping, and devices of reasonable quality can then be obtained. Measurement of defect densities showed that adding ppm levels of phosphine reduced the midgap defect densitie

Photonic and plasmonic structures for enhancing efficiency of thin film silicon solar cells

Crystalline silicon solar cells use high cost processing techniques as well as thick materials that are ~ 200µm thick to convert solar energy into electricity. From a cost viewpoint, it is highly advantageous to use thin film solar cells which are generally made in the range of 0.1-3µm in thickness. Due to this low thickness, the quantity of material is greatly reduced and so is the number and complexity of steps involved to complete a device, thereby allowing a continuous processing capability improving the throughput and hence greatly decreasing the cost. This also leads to faster payback time for the end user of the photovoltaic panel. In addition, due to the low thickness and the possibility of deposition on flexible foils, the photovoltaic (PV) modules can be flexible. Such flexible PV modules are well suited for building-integrated applications and for portable, foldable, PV power products. For economical applications of solar cells, high efficiency is an important consideration. Since Si is an indirect bandgap material, a thin film of Si needs efficient light trapping to achieve high optical absorption. The previous work in this field has been mostly based on randomly textured back reflectors. In this work, we have used a novel approach, a periodic photonic and plasmonic structure, to optimize current density of the devices by absorbing longer wavelengths without hampering other properties. The two dimensional diffraction effect generated by a periodic structure with the plasmonic light concentration achieved by silver cones to efficiently propagate light in the plane at the back surface of a solar cell, achieves a significant increase in optical absorption. Using such structures, we achieved a 50%+ increase in short circuit current in a nano-crystalline (nc-Si) solar cell relative to stainless steel. In addition to nc-Si solar cells on stainless steel, we have also used the periodic photonic structure to enhance optical absorption in amorphous cells and tandem junction amorphous/nano-crystalline cells. These structures have been fabricated on flexible plastic substrates. We will describe the use of periodic structures to achieve increased light absorption and enhanced photocurrents in thin film solar cells, and also compare them systematically with other textured substrates. We discuss the various technological aspects and obstacles faced before successful fabrication of such structure, and during the fabrication of solar cells on these structures. The ideas of periodic texturing and random texturing will be compared and an implementation of them together will be discussed

Thin Film Solar Cells on Transparent Plastic Foils

The focus of this thesis is on the optimization and fabrication of p-i-n amorphous silicon (a-Si:H) solar cells both on glass and transparent plastic substrates. These solar cells are specifically fabricated on transparent substrates to facilitate the integration of thin film batteries with these solar cells. To comply with plastic substrates, different silicon layers are optimized at the low processing temperature of 135 C. In the first part of the optimization process, the structural, electronic, and optical properties of boron- and phosphorous-doped, hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) at the substrate temperature of 135 C are elaborated. Additionally, in this part, the deposition of protocrystalline silicon (pc-Si) films on glass substrates are investigated. In the device integration and fabrication part of this thesis, the optimization process is continued by fabricating single junction devices with different hydrogen dilution ratios for the cell absorber layer. The optimum device performance is achieved with an absorber layer right at the transition from amorphous to microcrystalline silicon. To further improve the performance of the fabricated solar cells, amorphous silicon carbide buffer layers are introduced between the nc-Si p-layer and the undoped pc-Si absorber layer. Single junction p-p'-i-n solar cells are fabricated and characterized both on glass and plastic substrates. Our measurements show conversion efficiencies of 7.0% and 6.07% for the cells fabricated on glass and plastic substrates, respectively. In the last part of this research, the light trapping enhancement in amorphous silicon solar cells using Distributed Bragg Reflectors (DBRs) are experimentally demonstrated. Reflectance characteristics of DBR test structures, consisting of amorphous silicon (a-Si) / amorphous silicon nitride (SiN) film stacks are analysed and compared with those of conventional ZnO/Al back reflectors. DBR optical measurements show that the average total reflectance over the wavelength region of 600-800 nm is improved by 28% for DBR back structures. Accordingly, single junction amorphous silicon solar cells with DBR and Al back reflectors are fabricated both on glass and plastic substrates. Our results show that the short-circuit current density and consequently the conversion efficiency is enhanced by 10% for the cells fabricated on textured transparent conductive oxide substrates. In addition, these DBR back structures are designed and employed to improve the efficiency of semi-transparent solar cells. In this application, the optimized DBR structures are designed to be optically transparent for the part of the visible range and highly reflective for the red and infra-red part of the spectrum. Using these DBR structures, the efficiency of the optimum semi-transparent solar cell is enhanced by 5%

Characterization of doped amorphous silicon thin films through the investigation of dopant elements by glow discharge spectrometry: A correlation of conductivity and bandgap energy measurements

16 páginas, 4 figuras, 4 tablas.-- et al.The determination of optical parameters, such as absorption and extinction coefficients, refractive index and the bandgap energy, is crucial to understand the behavior and final efficiency of thin film solar cells based on hydrogenated amorphous silicon (a-Si:H). The influence of small variations of the gas flow rates used for the preparation of the p-a-SiC:H layer on the bandgap energy, as well as on the dopant elements concentration, thickness and conductivity of the p-layer, is investigated in this work using several complementary techniques. UV-NIR spectrophotometry and ellipsometry were used for the determination of bandgap energies of four p-a-SiC:H thin films, prepared by using different B2H6 and SiH4 fluxes (B2H6 from 12 sccm to 20 sccm and SiH4 from 6 sccm to 10 sccm). Moreover, radiofrequency glow discharge optical emission spectrometry technique was used for depth profiling characterization of p-a-SiC:H thin films and valuable information about dopant elements concentration and distribution throughout the coating was found. Finally, a direct relationship between the conductivity of p-a-SiC:H thin films and the dopant elements concentration, particularly boron and carbon, was observed for the four selected samples.Financial support from “Plan Nacional de I+D+I” (Spanish Ministry of Science and Innovation, and FEDER Program) through the project MAT2010-20921-02-01 is gratefully acknowledged. Also, B. Fernández and A. Menéndez acknowledge financial support from “Juan de la Cierva” and “Torres Quevedo” Research Programs of the Ministry of Science and Innovation of Spain, respectively (both co-financed by the European Social Fund).Peer reviewe

Characterization of Doped Amorphous Silicon Thin Films through the Investigation of Dopant Elements by Glow Discharge Spectrometry. A Correlation of Conductivity and Bandgap Energy Measurements

The determination of optical parameters, such as absorption and extinction coefficients, refractive index and the bandgap energy, is crucial to understand the behavior and final efficiency of thin film solar cells based on hydrogenated amorphous silicon (a-Si:H). The influence of small variations of the gas flow rates used for the preparation of the p-a-SiC:H layer on the bandgap energy, as well as on the dopant elements concentration, thickness and conductivity of the p-layer, is investigated in this work using several complementary techniques. UV-NIR spectrophotometry and ellipsometry were used for the determination of bandgap energies of four p-a-SiC:H thin films, prepared by using different B2H6 and SiH4 fluxes (B2H6 from 12 sccm to 20 sccm and SiH4 from 6 sccm to 10 sccm). Moreover, radiofrequency glow discharge optical emission spectrometry technique was used for depth profiling characterization of p-a-SiC:H thin films and valuable information about dopant elements concentration and distribution throughout the coating was found. Finally, a direct relationship between the conductivity of p-a-SiC:H thin films and the dopant elements concentration, particularly boron and carbon, was observed for the four selected samples

Synthesis and characterisation of doped silicon nanoparticles by hot wire thermal catalytic and spark pyrolysis

Includes abstract.Includes bibliographical references (leaves 94-109).Doped silicon nanoparticles with clean surfaces and which are suitable forelectronic applications, have successfully been produced. The doped silicon nanoparticles produced in this work will find application in the emerging field of flexible electronics. Two bottom-up production processes were utilised: hot wire thermal catalytic pyrolysis and spark pyrolysis. The latter is a new process,developed as part of this research for silicon nanoparticle synthesis. In each method silicon nanoparticles were synthesised using mixtures of silane and phosphine ordiborane, to achieve n and p-type doping respectively

Hybrid Silicon-Organic Heterojunction Structures for Photovoltaic Applications

The concept for inorganic-organic device is an attractive technology to develop devices with better characteristics and functionality due to the complementary advantages of inorganic and organic materials. This chapter provides an overview of the principal requirements for organic and inorganic semiconductor properties and their fabrication processes and focus on the compatibility between low temperature plasma enhanced chemical vapor deposition (PECVD) and polymer organic materials deposition. The concept for inorganic-organic device was validated with the fabrication of three hybrid thin film photovoltaic structures, based on hydrogenated silicon (Si:H), organic poly(3-hexythiophene): methano-fullerenephenyl-C61-butyric-acid-methyl-ester (P3HT:PCBM), and poly(3,4ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) films. Optoelectronic characteristics, performance characteristics, and interfaces of the different configurations aspects are discussed. Hybrid ITO/PEDOT:PSS/(i)Si:H/(n)Si:H structure results in a remarkably high short circuit current density as large as 17.74 mA/cm2, which is higher than the values in organic or inorganic reference samples. Although some hybrid structures demonstrated substantial improvement of performance, other hybrid structures showed poor performance, further R&D efforts seem to be promising, and should be focused on deeper study of organic materials and related interface properties