Organic solar cells with 2-Thenylmercaptan/AU self-assembly film as buffer layer

The interface between an electrode and the organic active layer is an important factor in organic solar cells (OSCs) that influences the power conversion efficiency (PCE). In this report, a buffer layer of 2-thenylmercaptan/Au self-assembly film is introduced into OSCs as a substitute for the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) layer. The electrode/active layer interface is meliorated by Au-S coordinate bond of self assembly after applying this buffer layer. The series resistance reduces from 20 Ω cm² in a device based on PEDOT:PSS to 10.2 Ω cm². Correspondingly, the fill factor (FF) increases from 0.50 to 0.64. Moreover, due to the dipole of this self-assembled layer, the open circuit voltage (V_oc) also increases slightly from 0.54 V to 0.56 V and the PCE reaches 2.5%.     

Organic solar cells incorporating buffer layers from indium doped zinc oxide nanoparticles

Monodisperse, indium doped zinc oxide (IZO) nanoparticles were prepared via the polyol-mediated synthesis and incorporated into regular and inverted poly-(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C₆₁-butyric acid methyl ester organic photovoltaic devices as buffer layers between the active layer and the cathode. Efficient hole blocking at the particle buffer layers leads to an enhanced open-circuit voltage of the solar cells. This effect is even more pronounced for inverted device architectures. Device degradation studies revealed a solar cell performance reduction upon sample exposition to ambient atmosphere. However, this degradation is fully reversible under UV illumination. In addition, the n-doped IZO particles form suitable charge carrier transport layers for an efficient recombination in an intermediate recombination zone in tandem solar cells. Accordingly we have fabricated fully solution-processed tandem solar cells and investigated their optoelectronic properties.     

铜铟硒多晶薄膜太阳电池的制备技术

简述了铜铟硒多晶薄膜太阳电池的研究现状，讨论了其制备技术及其特点，给出了进一步研究和要解决的现存问题。     

Development and manufacturing of CIS thin film solar modules

Thin film modules based on CIS-technology with power outputs ranging between 5 and 40 W and corresponding circuit aperture area efficiencies between 9.6% and 11% have been introduced recently by Siemens Solar. Current status of production yield and performance is presented demonstrating significantly higher performance than alternative thin film technologies. Further developments have resulted in new champion efficiencies of 12.1% for a large commercial size modules and 14.7% for a small laboratory module.     

Present status and future prospects of CIGSS thin film solar cells

Efficiencies of CuIn_1−xGa_xSe_2−yS_y (CIGSS) modules are comparable to those of lower end crystalline-Si modules. CIGSS layers are prepared by reactive co-evaporation, selenization/sulfurization of metallic or compound precursors, reactive co-sputtering and non-vacuum techniques. CuIn_1−xGa_xS₂ (CIGS2) layers are prepared by sulfurization of Cu-rich metallic precursors and etching of excess Cu_2−xS. Usually heterojunction partner CdS and transparent-conducting bilayer ZnO/ZnO:Al layers are deposited by chemical bath deposition (CBD) or RF magnetron sputtering. CIGSS solar cell efficiencies have been improved by optimizing Cu, Ga and S proportions and providing a minute amount of Na. This paper reviews preparation and efficiency improvement techniques for CIGSS solar cells.     

Optimized chemical bath deposited CdS layers for the improvement of CdTe solar cells

CdS layers grown by chemical bath deposition (CBD) are treated in different ways to improve the performance of CdS/CdTe solar cells. It has been found that the open circuit voltage of the CdS/CdTe solar cell increases when the CBD CdS is annealed with CdCl₂ before the deposition of CdTe by close spaced sublimation (CSS). A thin CBD CdS (∼80 nm) with bi-layer structure can significantly improve the short circuit current of the CdS/CdTe solar cells.     

Spray pyrolysis of barrier layers for flexible thin film solar cells on steel

Thin film chalcopyrite solar cells grown on light-weight, flexible substrates are an appealing product. An insulating barrier layer is a requisite for flexible steel substrates to protect the chalcopyrite absorber layer from in-diffusion of iron and also to isolate the solar module, electrically, from the metal substrate. Spray pyrolysis is presented here as a means to deposit an aluminium oxide barrier layer. Optimised spray deposition conditions are investigated and subsequent solar cell results are presented. Resistivity measurements in conjunction with thermography allow assessment of the barrier layer’s insulating properties and occurrence of pin-holes in the layer. Resulting Cu(In,Ga)Se₂ cells, with a barrier layer, reach an efficiency of 14.4%.     

2D-numerical analysis and optimum design of thin film silicon solar cells

Device modeling for p–i–n junction μc-Si basis thin film polycrystalline Si solar cells has been examined with a simple model of columnar grain structure and its boundary condition utilizing two-dimensional device simulator. As the simulation results of solar cell characteristics show, open-circuit voltage (V_oc) and curve fill factor (FF) considerably depend on those structural parameters, while short-circuit current density (J_sc) is comparatively stable by courtesy of homogeneous built-in electric field in the i layer. It has also been found that conversion efficiency over 12% could be expected with 1 μm grain size and well-passivated condition with 3 μm thick i-layer.     

Second generation CIS solar modules

Multinary Cu(In,Ga)(Se,S)₂ absorbers (abrev. CIGSSe) are promising candidates for reducing the cost of photovoltaics well below the cost of crystalline silicon. Shell Solar has pioneered production of this new thin film technology and is now with the first generation at a volume of well over 1 MW/year. In a separate pilot line for second generation products we have further improved the performance of CIGSSE based solar modules. We developed a novel CIGSSE formation technique called stacked elemental layer rapid thermal processing (SEL-RTP). This process has recently been scaled up from initial laboratory sized mini-modules (10 × 10 cm²) to full sized power modules of 60 × 90 cm². The present paper concentrates on in situ and ex situ characterization techniques that were developed to control and further improve large area CIGSSE processing. The crystalline thin film formation process has been analyzed with in situ thin film calorimetry and in situ X-ray diffraction (XRD). That work has added fundamental insights and accelerates the optimization process. The depth distribution of gallium and sulfur has been determined by secondary ion mass spectroscopy (SIMS) for different selenization and sulfurization processes. Appropriate profiles of these elements allow for a deliberate bandgap profiling within the Cu(In,Ga)(S,Se)₂ absorber. In addition further quality control tools like X-ray fluorescence analysis and Raman spectroscopy for stoichiometry monitoring, photoluminescence lifetime mapping and thermographic imaging have been developed in order to improve large area uniformity and reproducibility.

Some first full sized modules from the new pilot line look very promising: Aperture area efficiencies of up to 13.1% for monolithic thin film circuits on 0.54 m² and a module power of 65 W represent an international champion value for large are thin film solar modules.     

Experimental study of variations of the solar spectrum of relevance to thin film solar cells

The influence of variations in the incident solar spectrum on solar cells is often neglected. This paper investigates the magnitude of this variation and its potential influence on the performance of thin film solar cells in a maritime climate. The investigation centres on the analysis of a large number of measurements carried out in Loughborough, UK, at 10 min intervals over a period of 30 months. The magnitude of the spectral variation is presented both on a daily and a seasonal basis. Of the different thin film materials studied, amorphous silicon is shown to be the most susceptible to changes in the spectral distribution, with the “useful fraction” of the light varying in the range +6% to −9% of the annual average, with the maximum occurring in summer time.     

Broadband energy harvesting with nano-composite PbS-Nanocrystal/Excimer laser crystallized thin film silicon hybrid solar cells

Photovoltaics between hydrogenated amorphous silicon (a-Si:H) and PbS-nanocrystals (NCs) are demonstrated. Enhanced spectral response was observed below 375 and beyond 800 nm for a-Si:H/PbS-NCs compared to a-Si:H devices using external quantum efficiency measurements (EQE). Further improvement of EQE was observed from hybrid devices fabricated using a-Si:H subjected to excimer laser crystallization (ELC). This improvement is attributed to the reduction in the a-Si:H band gap through ELC, resulting in a lower barrier for hole extraction from the PbS-NCs. Techniques discussed in this paper would be useful for the fabrication of highly efficient a-Si:H and nanocrystal based hybrid photovoltaics in future.     

Substrates for thin crystalline silicon solar cells

Choice of substrate for thin crystalline silicon solar cells requires a compromise between cost and quality. There are three generic substrate types, namely a transparent substrate (such as glass), an opaque substrate (such as a ceramic or metal) and low-cost multicrystalline silicon. Glass has the advantage of eliminating absorption within the substrate. However, the larger effective diffusion length, the improved surface passivation and the increased process flexibility obtainable with an opaque substrate, particularly low-cost multicrystalline silicon, may considerably outweigh the modest optical benefits of a transparent substrate. In this paper it is shown that the advantage in effective diffusion length that is required of a cell grown on an opaque substrate in order to offset the light-trapping advantages of a glass substrate is about a factor of two.     

CVD of CuGaSe2 for thin film solar cells with various transport agents

Chemical vapor deposition (CVD) in an open tube system was employed to deposit single-phase CuGaSe₂ thin films on plain and Mo-coated glass substrates. The use of HCl and ternary CuGaSe₂ source material resulted in non-stoichiometric volatilization of the source material. The use of binary source materials – Cu₂Se and Ga₂Se₃ – in combination with I₂ and HCl as the respective transport agents yielded single-phase CuGaSe₂ thin films while the source materials were volatilized stoichiometrically. Mo/CuGaSe₂/CdS/ZnO devices were fabricated from these samples exhibiting an open-circuit voltages up to V_oc=853 mV.     

薄膜太阳电池的研究进展及应用前景

阐述了非晶硅薄膜电池、多晶硅薄膜电池、锑化镉薄膜电池、铜铟镓硒薄膜太阳电池和染料敏化TiO2太阳电池的研究现状,简要介绍了我国薄膜太阳电池研究的进展,指出了太阳电池在我国的应用前景。     

Interface engineering in chalcopyrite thin film solar devices

Successful interface engineering requires compositional and electronic material characterization as a prerequisite for understanding and intentionally generating interfaces in photovoltaic devices. The paper gives an overview with several examples, all referring to Cu(In,Ga)(S,Se)₂ (“CIGSSe”)-based solar cells, with an emphasis on characterization using highly specialized methods, such as elastic recoil detection analysis, X-ray emission spectroscopy and photoelectron spectroscopy using synchrotron and ultraviolet light for excitation, inverse photoemission spectroscopy and Kelvin probe force microscopy. First, the determination of the depth profile of the band gap energy E_g in the absorber layer is demonstrated. The modification of E_g towards both interfaces is discussed in terms of beneficial electronic effects. Next, the interface between absorber and buffer layers with alternative and promising non-toxic materials is considered. Between CIGSSe and a ZnSe buffer deposited by the metalorganic chemical vapor deposition (MOCVD) method a buried ZnS interface was found. For a Zn(O,OH) buffer processed with an ion layer gas reaction (ILGAR) the correlation of surface composition, valence band maximum and efficiency of the resulting solar cell is shown. In addition, another approach is considered where a ZnMgO window layer is sputtered directly on the absorber omitting any buffer layer. The determination of the potential distribution at the ZnMgO/CIGSSe interface supports the understanding of this new and simpler way to get good cell performances even without any buffer. Finally, monolithically integrated solar modules without encapsulation were investigated before and after accelerated aging tests and changes at the interconnects were identified.     

The role of rear surface in thin silicon solar cells

The aim of this work is the study of the light-trapping ability of rear surface relief for thin silicon solar cells. With a simple model, the conditions needed to achieve efficient thin cells are discussed first. It is shown how the combined effect of back surface passivation and the confinement of light impacts the cell performance. If the illuminated face is not textured, light-trapping must be accomplished by the rear surface. A rear saw-tooth relief grating is proposed as a good back reflector producing also the tilt of reflected rays. Since technological limitations lead relief features to have sizes near the range of the optical wavelengths, the behavior of such structures is analyzed using a rigorous electromagnetic approach. The influence of the depth-to-period ratio of the grating in the internal reflectivity is analyzed. Finally, it is calculated that the back internal reflectivity is much higher when the exit medium is air than when it is aluminum.     

A technological evolution from bulk crystalline age to multilayers thin film age in solar photovoltaics

Recent advances in solar cell device technologies are surveyed, and a new trend underlying is predicted by a term “technological evolution from the bulk crystalline age to the multilayered thin film age”. In the paper, firstly, recent progress of thin film fabrication technologies for active materials of photovoltaic device are reviewed, and their significancies such as wide area, low temperature growth etc., are pointed out from currently developed live technologies. Secondly, some R & D efforts to develop the next generation type solar cells utilized by full use of multi-layers thin film growth technology are introduced together with some newly developed integrated process technology for the thin film solar cells. Then, some topics in the high cost performance multi-layers thin film solar cells are also introduced. In the final part of this paper, the current state of the art in the field of thin film solar cells and their industrialization are overviewed and the market expansion toward the 21st century is forecast, and discussed.     

Status of thin film solar cells in research, production and the market

Photovoltaics is one of the fastest growing industries at present. Over the last five years, the production of photovoltaic solar cells has steadily increased at an annual average of 40%, driven not only by the progress in materials and processing technology, but by market introduction programmes in many countries around the world. This growth is mainly being attained by an increase in manufacturing capacities based on the technology of crystalline, single junction devices. Consistent with the time needed for any major change in energy infrastructure, another 20–30 years of sustained and aggressive growth will be required for photovoltaics to substitute a significant share of conventional energy sources. The question is whether a switch will be possible with the current technologies alone or whether this growth will only be possible with the continuous introduction of new technologies. It leads us to the search for new developments with respect to material use and consumption, device design and production technologies as well as new concepts to increase overall efficiency. This paper analyses the current status of thin film solar cells and their outlook for future developments.     

Processing options for CdTe thin film solar cells

Processing options for addressing critical issues associated with the fabrication of thin film CdTe solar cells are presented, including window and buffer layer processing, post-deposition treatment, and formation of stable low resistance contacts. The paper contains fundamental data, engineering relationships and device results. Chemical surface deposited CdS and Cd_1−xZn_xS films are employed as the n-type heteropartner window layers. Maintaining junction quality with ultra-thin window layers is facilitated by use of a high resistance oxide buffer layer, such as SnO₂, In₂O₃ or Ga₂O₃, between the heteropartner and the transparent conductive oxide. Thermal annealing of the CdTe/CdS heterostructure in the presence of CdCl₂ and O₂ shifts the chemical equilibrium on the surface of the absorber layer, which influences the bulk electrical properties. Aspects of back contacting CdTe/CdS devices, including etching, Cu application, contact annealing, back contact chemistry and secondary contacts, are discussed. Two commonly employed etches used to produce a Te-rich layer, nitric acid/phosphoric acid mixtures and Br₂/methanol are compared, including the nature and stability of the final treated CdTe surface. The diagnostic abilities of the surface sensitive VASE and GIXRD techniques are highlighted. Various methods of Cu delivery are discussed with consideration to; reaction with Te, processing simplicity, processing time and possible industrial scale-up. Some aspects of back contact stability are presented, including discussion of apparent robust back contacts, which contain a thick Te component.     

Surface plasmon enhanced GaAs thin film solar cells

As a new method to improve the light trapping in solar cells, surface plasmon resonance (SPR) has attracted considerable attention because of its unique characteristics. Several studies have been reported on the photocurrent improvement of Si solar cells by surface plasmons, while little research has been done on III-V solar cells. In this work, we performed a systematic study of SPR on GaAs thin film solar cells with different sizes of Ag nanoparticles on the surface. The nanoparticles were fabricated by annealing E-beam evaporated Ag films in a N₂ atmosphere. It was found that the surface plasmon resonance wavelength does not undergo a simple red-shift with increasing metal thickness. It depends on the shape of the metal nanoparticles and the interparticle spacing. It is necessary to optimize the particle size to obtain an optimum enhancement throughout the visible spectrum for solar cells. We found that the optimum thickness of the Ag film was 6 nm under our experimental conditions. Furthermore, from the calculation based on the external quantum efficiency data, the short circuit current density of a GaAs solar cell with 6 nm Ag film after annealing was increased by 14.2% over that of the untreated solar cell.