Prospects of wide-gap chalcopyrites for thin film photovoltaic modules

The development of a standard process for the fabrication of Cu(In,Ga)Se₂ thin film solar cells by coevaporation leads to highly efficient devices with band-gaps in the range of 1–1.2 eV. The comparison of recent module efficiencies and the corresponding performance of small test cells demonstrates the high uniformity of the cell performance over large areas. In view of their higher open circuit voltage the use of absorbers with higher band-gaps is advantageous for the production of solar modules. However, to date the efficiencies of cells employing chalcopyrite absorbers with higher band-gaps lag behind. A consistent picture of the problems encountered in the case of wide-gap absorbers is drawn on the basis of results from photoelectron spectroscopy in conjunction with electrical measurements on the devices. This picture appoints the high efficiency of the cells with lower band gaps to the successful suppression of recombination at the heterointerface, resulting from a stabilized type inversion at the surface of the absorber. The formation of such a type-inverted surface appears to be a prerequisite for the achievement of high efficiency with wide band-gap absorbers.     

Large-area CIGS modules: Pilot line production and new developments

Thin-film solar modules based on Cu(In,Ga)Se₂ (CIGS) promise to become a lower-cost alternative to polycrystalline silicon. The ZSW and Wuerth Solar are developing and running industrial processes suitable for the mass production of CIGS modules. Yield and output in the Wuerth Solar pilot line are steadily improving, with average module efficiencies exceeding 10%. New developments at the ZSW include doubling the length of the linear evaporation source to increase the throughput and adjusting the processes to enable production of flexible modules. New calculations regarding module design for optimized performance are presented as well as results from outdoor testing of Wuerth Solar modules.     

Cu(In,Ga)Se2 thin-film solar cells with an efficiency of 18%

An efficiency of over 18% have been achieved in Cu(In,Ga)Se₂ (CIGS) thin-film solar cells. Solar cell parameters were estimated for the cells with efficiencies of more and less than 18%. A diode quality factor n and forward current (saturated current) J₀ of the cell with over 18% efficiency are lower than those with below 18% efficiency. This would be attributed to sufficient coverage of the CdS film with excellent uniformity as a buffer and/or window layer over the CIGS film because the process of CdS film formation was improved.     

CIGS solar cells on the way to mass production: Process statistics of a 30 cm×30 cm module line

All existing thin-film PV technologies, especially the Cu(In,Ga)Se₂ (CIGS)-based technology, have a high cost reduction potential at high production volumes. The challenge for the future is to combine high production volumes with high throughput, sufficient yield and superior quality. ZSW has developed in-line processes for all deposition steps for CIGS modules. In several batches average efficiencies of above 11% and a maximum of 12.7% have been achieved. For the next scaling up step Wurth Solar GmbH & Co. KG, a joint venture of ZSW, Wurth group, and EnBW AG, a German utility, was founded to commercialise CIGS technology.     

Improved performance of Cu(InGa)Se2 thin-film solar cells using evaporated Cd-free buffer layers

Polycrystalline Cu(InGa)Se₂ (CIGS) thin-film solar cells using evaporated In_xSe_y and ZnIn_xSe_y buffer layers are prepared. The purpose of this work is to replace the chemical bath deposited CdS buffer layer with a continuously evaporated buffer layer. In this study, a major effort is made to improve the performance of CIGS thin-film solar cells with these buffer layers. The relationship between the cell performance and the substrate temperature for these buffer layers is demonstrated. Even at the high substrate temperature of about 550°C for the buffer layer, efficiencies of more than 11% were obtained. Furthermore, the I−V characteristics of the cells using these buffer layers are compared with cells using CdS buffer layers fabricated by chemical bath deposition method. We have achieved relatively high efficiencies of over 15% using both the ZnIn_xSe_y and the CdS buffer layers.     

New perylene derivative dyes for dye-sensitized solar cells

We have studied the influence of the spacer alkyl chain length of perylenemonoimide (PMI) dyes on the device performance in dye-sensitized solar cells (DSSCs). We observed that the dyes with longer and brunched alkyl chains exhibit higher efficiencies in DSSCs. In line with these statements we now report the highest efficiency obtained under standard conditions for a perylene imide derivative with PMI-DA1 that performs 300 mV open circuit voltage, 9.79 mA/cm² short-circuit current and 1.61% overall conversion efficiency.     

Energy and exergy analysis of a new hydrogen-fueled power plant based on calcium looping process

A novel hydrogen-fueled power plant with inherent CO₂ capture based on calcium looping process is proposed in this paper. The analyzed system has been evaluated from the energy and exergy points of view, it enables determination of the contribution of main component to the total exergy loss. The results show that energy and exergy efficiencies of the system are 42.7% and 42.25% respectively, combustion chamber and regenerator are responsible for large exergy destructions, mainly due to irreversibilities associated with the combustion reactions, they have great potential for system efficiencies improvements. The effects of various air pressure ratios and gas turbine inlet temperatures on the system thermodynamic performance are also presented. The thermodynamic efficiencies increase with the increase in air pressure ratios and gas turbine inlet temperatures.     

Highly efficient Cu(In,Ga)Se2 mini-modules

In this contribution, we present results and the philosophy of our mini-module efforts. These efforts have achieved world record levels as well as a reproducible process. Various mini-module designs are tested using two different baseline Cu(In,Ga)Se₂ deposition recipes. Gridded mini-modules achieve highest efficiencies and are much less demanding on the ZnO:Al top contact than their conventional counterpart. For all of the designs tested, our experimental results are in the order of the expectations from our modeling. Gridded modules can achieve efficiency levels very close to those of the cells.     

The 3-dimensional dye-sensitized solar cell and module based on all titanium substrates

Here we report a 3-dimensional dye-sensitized solar cell (3D-DSSCs) and module simulating the fractal structure of the pine tree for capturing sunlight. Compared to traditional flat solar cells, this type of solar cell exhibits superiority of absorbing sunlight from all directions. The fabricated 3D-DSSC and module have achieved 3.36% and 3.19% efficiencies, respectively. The results show that the shade has little effect on the performance of 3D-DSSC and module. It is expected that this 3D-DSSC and module have strong potential for practical application due to their 3D light utilization.     

The effects of hydrothermal temperature and thickness of TiO2 film on the performance of a dye-sensitized solar cell

The effects of hydrothermal temperature on the preparation of TiO₂ colloids, and their film thickness on fluorine-doped tin oxide (FTO) glass, toward the performance of a dye-sensitized solar cell (DSSC) were investigated. Pore diameter and surface area of the TiO₂ are of paramount importance in determining the cell efficiency. With the increase of hydrothermal temperature, the pore diameter increases linearly; however, the surface area shows the reverse effect. It is found that the DSSC assembled with the TiO₂ films prepared under the hydrothermal temperature of 240 °C, and the film thickness larger than 10 μm gives optimal performance. The effect of film thickness of TiO₂ on the performance of the DSSC can be explained by the relative size of reactive species diffusing into the thin film and the lifetime of injected electrons. Electrochemical impedance spectroscopy (EIS) was also used to analyze the resistance of the cell, developed as a result of the change in the thickness of the TiO₂ thin film. The at-rest stability for over 200 days was monitored and the results show that the solar energy conversion efficiency was found to decrease from 5.0% of initial value to 3.0% at the end.     

Cu(In,Ga)Se2-based thin-film photovoltaic modules optimized for long-term performance

In this contribution we give an overview of the mechanisms behind degradation of Cu(In,Ga)Se₂-based modules. Based on the results from a detailed analysis of power losses in modules, prior to and after extended damp heat exposure, we discuss to what extent modules can be designed to achieve enhanced long-term performance. For conventional modules, we show that the stability can be improved by optimizing the interconnect and the front contact. Furthermore, we argue that gridded modules are better from a long-term performance point of view. A novel interconnect structure, specifically designed for long-term durability, is briefly discussed.     

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.     

Large area ZnO films optimized for graded band-gap Cu(InGa)Se2-based thin-film mini-modules

In this study, two deposition methods (i.e. MOCVD and sputtering methods) to prepare n-type ZnO window layers for CIGS-based thin-film solar cells are discussed. In order to make ZnO : Al transparent conductive oxide (TCO) films prepared by DC magnetron sputtering comparable to ZnO : B TCO prepared by MOCVD, a new ZnO sputtering process is proposed by introducing a multilayer structure. Using these films, CIGS thin-film solar cells with efficiencies of greater than 14% have been fabricated with an active area of 3.2 cm². This structure was adapted to fabricate CIGS thin-film mini-modules with efficiencies around 11% having aperture area of 50 cm².     

Porous SiO2 films prepared by remote plasma-enhanced chemical vapour deposition – a novel antireflection coating technology for photovoltaic modules

In this work an innovative antireflection coating technology for photovoltaic modules based on remote plasma-enhanced chemical vapour deposition of porous SiO₂ films is presented. We show that the proposed technology has the potential to significantly improve the performance of photovoltaic modules by effectively reducing the optical losses of the air/glass interface. As a result, the transmission of a glass pane measured at a single wavelength was increased from 91.7% to 100% by a single-layer porous SiO₂ antireflection coating on both sides of the glass pane. Furthermore, a double-layer porous SiO₂ antireflection coating on both sides of the glass pane increased the transmission weighted with the AM1.5G spectrum in the 400–1150 nm wavelength region from 91.6% to the remarkably high value of 99.4%.     

Influence of In2S3 film properties on the behavior of CuInS2/In2S3/ZnO type solar cells

Solar cells of CuInS₂/In₂S₃/ZnO type are studied as a function of the In₂S₃ buffer deposition conditions. In₂S₃ is deposited from an aqueous solution containing thioacetamide (TA), as sulfur precursor and In³⁺. In parallel, variable amounts of In₂O₃ are deposited that have an important influence on the buffer layer behavior. Starting from deposition conditions determined in a preliminary study, a set of parameters is chosen to be most determining for the buffer layer behavior, namely the solution temperature, the concentration of thioacetamide [TA], and the buffer thickness. The solar cell results are discussed in relation with these parameters. Higher efficiency is attained with buffer deposited at high temperature (70 °C) and [TA] (0.3 M). These conditions are characterized by short induction time, high deposition rate and low In₂O₃ content in the buffer. On the other hand, the film deposited at lower temperature has higher In₂O₃ content, and gives solar cell efficiency sharply decreasing with buffer thickness. This buffer type may attain higher conversion efficiencies if deposited on full covering very thin film.     

The influence of sodium on the properties of CuInS2 thin films and solar cells

The controlled incorporation of sodium into the absorber layer of CuInS₂ solar cells improved cell performance remarkably. Without toxic KCN treatment, conversion efficiencies of over 6% were achieved by sulfurization of sodium-containing precursors. We also investigated the characteristics of the sodium-incorporated CuInS₂ films by intentional addition and diffusion from a soda-lime glass. The ternary compound semiconductor of NaInS₂ was found to form mainly on the surface of each of the CuInS₂ films.     

Dye-sensitized solar cells based on anatase TiO2 hollow spheres/carbon nanotube composite films

Dye-sensitized solar cells (DSSCs) based on anatase TiO₂ hollow spheres (TiO2HS)/multi-walled carbon nanotubes (CNT) nanocomposite films are prepared by a directly mechanical mixing and doctor blade method. The prepared samples are characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-vis absorption spectroscopy and N₂ adsorption-desorption isotherms. The photoelectric conversion performances of the DSSCs based on TiO2HS/CNT composite film electrodes are also compared with commercial-grade Degussa P25 TiO₂ nanoparticles (P25)/CNT composite solar cells at the same film thickness. The results indicate that the photoelectric conversion efficiencies (η) of the TiO2HS/CNT composite DSSCs are dependent on CNT loading in the electrodes. A small amount of CNT clearly enhances DSSC efficiency, while excessive CNT loading significantly lowers their performance. The former is because CNT enhance the transport of electrons from the films to FTO substrates. The latter is due to high CNT loading shielding the visible light from being adsorbed by dyes.     

Unique TiO2 paste for high efficiency dye-sensitized solar cells

A novel titanium oxide paste based on Pechini sol-gel method and nanocrystalline titanium oxide powder have been successfully developed. Titanium oxide layers possess high inner surface area assuring high dye loading and well-connected nanocrystalline grains assuring good electron transport within the layer. The dye-sensitized layers have been used to assemble dye-sensitized solar cells with acetonitrile- and ionic liquid-based electrolyte. Overall conversion efficiencies of dye-sensitized solar cells (DSSCs) determined under standard test conditions (100 mW/cm², 25 °C and AM 1.5 G) are 10.2% for acetonitrile and 7.3% for ionic liquid-based electrolyte.     

Nano-grain SnO2 electrodes for high conversion efficiency SnO2-DSSC

The nano-grain ZnO/SnO₂ composite electrode was prepared by adding 5 w% of the 200-250 nm ZnO particles to the 5 nm SnO₂ colloid in the presence of hydroxypropylcellulose (M.W.=80,000). The nano-grain SnO₂ electrode was obtained by removing the ZnO particles from the composite electrode using acetic acid. The FE-SEM micrographs revealed that both electrodes consisted of interconnected nano-grains that were ca. 800 nm in size, and the large pores between the grains furnished the wide electrolyte diffusion channels within the electrodes. The photovoltaic properties of the nano-grain electrodes were investigated by measuring the I-V behaviors, the IPCE spectra and the ac-impedance spectra. The nano-grain electrodes exhibited remarkably improved conversion efficiencies of 3.96% for the composite and 2.98% for the SnO₂ electrode compared to the value of 1.66% for the usual nano-particle SnO₂ electrode. The improvement conversion efficiencies were mainly attributed to the formation of nano-grains, which facilitated the electron diffusion within the grains. The improved electrolyte diffusion as well as the light-scattering effects enhanced the photovoltaic performance of the SnO₂ electrode.     

Preparation of Zn doped Cu(In,Ga)Se2 thin films by physical vapor deposition for solar cells

Cu(In,Ga)Se₂ (CIGS) surface was modified with Zn doping using vacuum evaporation. Substrate temperatures and exposure times during the Zn evaporation were changed to control a distribution of Zn in the CIGS films. Diffusion of Zn in the CIGS films was observed at the substrate temperature of over 200°C. The diffusion depth of Zn increases with increasing the exposure time at the substrate temperature of 300°C. Solar cells were fabricated using the Zn doped CIGS films. A distribution of the efficiencies decreases with increasing the exposure time of Zn vapor. The doping of Zn at the film surface improved reproducibility of a high fill factor and efficiency. A solar cell fabricated using the CIGS film modified with Zn doping showed an efficiency of 14.8%.