Amorphous hydrogenated silicon-nitride films for applications in solar cells

The application of anti-reflective coatings (ARC) is a good method to improve the solar cell construction. The authors developed the RF plasma enhanced chemical vapour deposition method for preparation of amorphous silicon-nitrogen (a-Si:N:H) films for potential optoelectronic applications. The films have been obtained on borosilicate glass and monocrystalline silicon (1 0 0) (Cz-Si) in a process with optimised technological parameters such as a content of gaseous mixture of silane (SiH₄) and ammonia (NH₃). The properties of samples have been investigated by optical spectroscopy (PERKIN-ELMER Lambda 19) and scanning electron microscopy (SEM). The correlation between film properties and process parameters has been found. The results of optical investigations show that these materials are characterised by a variable optical gap dependent on the nitrogen content. After deposition of a-Si:N:H, a decrease in the total reflectivity, as compared to that of monocrystalline Si, was observed. The simulation of multicrystalline silicon solar cells performance with and without the ARC was done with the use of PC1D programme. The influence of the ARC on solar cell efficiency was observed. The obtained results indicate that a-Si:N:H films are suitable for application as antireflective and protective coatings for solar cells.     

Influence of stain etching on low minority carrier lifetime areas of multicrystalline silicon for solar cells

In this work the use of HF/HNO₃ solutions for texturing silicon-based solar cell substrates by stain etching and the influence of texturing on minority carrier lifetimes are studied. Stain etching is currently used to decrease the reflectance and, subsequently improve the photogenerated current of the cells, but also produces nanostructures on the silicon surface. In the textured samples it has been observed that an improvement on the minority carrier lifetime with respect to the samples treated with a conventional saw damage etching process is produced on grain boundaries and defects, and the origin of this effect has been discussed.     

Silicon solar cells for terrestrial applications

The ways in which silicon solar cells can be designed to give maximum efficiency are reviewed. It is suggested that, although the reduction of processing costs is an extremely important objective of the development programme aimed at widespread application in highly developed countries, the improvement of design might have sufficient cost benefit even if based on current processing technology for the application of solar cells in remote localities which are not easily served with electricity via transmission lines.     

Surface texturing of GaAs using a nanosphere lithography technique for solar cell applications

In this study, we present novel methods to texture the surface of GaAs substrates using the nanosphere lithography (NSL) technique that is based on arrays of SiO₂ nanospheres. Closed-packed arrays of SiO₂ nanospheres were formed on a benzocyclobutene (BCB) layer, followed by embedding SiO₂ nanospheres into the BCB layer. To texture the GaAs surface, three patterns were fabricated by nanosphere lithography. First, a convex pattern from the shape of the nanospheres was produced on the surface of GaAs. Second, a concave shape was produced on the surface of GaAs by additional wet etching to remove SiO₂ nanospheres. These two methods were found to be effective in reducing the reflectance to a range of 400-800 nm. Finally, the arrays of SiO₂ nanospheres were transferred onto the GaAs by dry-etching using a mixture of Cl₂ and BCl₃ gases, resulting in arrays of GaAs nanorods. The dry-etched surface structure showed the lowest reflectance.     

Optimization of KOH etching process to obtain textured substrates suitable for heterojunction solar cells fabricated by HWCVD

In this work, we have studied the texturization process of (100) c-Si wafers using a low concentration potassium hydroxide solution in order to obtain good quality textured wafers. The optimization of the etching conditions have led to random but uniform pyramidal structures with good optical properties. Then, symmetric heterojunctions were deposited by Hot-Wire CVD onto these substrates and the Quasi-Steady-State PhotoConductance technique was used to measure passivation quality. Little degradation in the effective lifetime and implicit open circuit voltage of these devices (< 20 mV) was observed in all cases. It is especially remarkable that for big uniform pyramids, the open-circuit voltage is comparable to the values obtained on flat substrates.     

Black SiC formation induced by Si overlayer deposition and subsequent plasma etching

Black SiC formation by plasma etching with SF₆/O₂ chemistry is reported. Black SiC was produced by depositing Si overlayer on SiC and then etching the Si/SiC stack sequentially, thus replicating the black Si morphology to SiC. Black SiC is obtained with almost zero reflectance over the wavelengths from 300 nm to 1050 nm. Thicker Si film was advantageous, and it was important to optimize the etch condition considering both the black Si morphology and the flattening effect of SiC.     

Modeling solar cells: A method for improving their efficiency

After a brief discussion on the theoretical basis for simulating solar cells and the available programs for doing this we proceed to discuss two examples that show the importance of doing numerical simulation of solar cells. We shall concentrate in silicon Heterojunction Intrinsic Thin film aSi/cSi (HIT) and CdS/CuInGaSe₂ (CIGS) solar cells. In the first case, we will show that numerical simulation indicates that there is an optimum transparent conducting oxide (TCO) to be used in contact with the p-type aSi:H emitter layer although many experimental researchers might think that the results can be similar without regard of the TCO film used. In this case, it is shown that high work function TCO materials such as ZnO:Al are much better than smaller work function films such as ITO. HIT solar cells made with small work function TCO layers (<4.8 eV) will never be able to reach the high efficiencies already reported experimentally. It will also be discussed that simulations of CIGS solar cells by different groups predict efficiencies around 18–19% or even less, i.e. below the record efficiency reported experimentally (20.3%). In addition, the experimental band-gap which is optimum in this case is around 1.2 eV while several theoretical results predict a higher optimum band-gap (1.4–1.5 eV). This means that there are other effects not included in most of the simulation models developed until today. One of them is the possible presence of an interfacial (inversion) layer between CdS and CIGS. It is shown that this inversion layer might explain the smaller observed optimum band-gap, but some efficiency is lost. It is discussed that another possible explanation for the higher experimental efficiency is the possible variation of Ga concentration in the CIGS film causing a gradual variation of the band-gap. This band-gap grading might help improve the open-circuit voltage and, if it is appropriately done, it can also cause the enhancement of the photo-current density.     

Novel aspects in thin film silicon solar cells-amorphous, microcrystalline and nanocrystalline silicon

The improvement of photodegradation of a-Si:H has been studied on the basis of controlling the subsurface reaction and gaseous phase reaction. We found that higher deposition temperature, hydrogen dilution and triode method are effective to reduce the SiH₂ density in the film and to suppress the photodegradation of solar cells. These results are explained in terms of the hydrogen elimination reaction in the subsurface region and the contribution of the higher silane radicals to the film growth. The high-rate deposition of μc-Si:H was obtained by means of a high-pressure method and further improvement in deposition rate and the film quality was achieved in combination with the locally high-density plasma, which enables effective dissociation of source gases without thermal damage. It was also found that the deposition pressure is crucial to improve the film quality for device. This technique was successfully applied to the solar cells and an efficiency of 7.9% was obtained at a deposition rate of 3.1 nm/s. The potential application of nanocrystalline silicon is also discussed.     

Towards ultra-thin CdTe solar cells using MOCVD

A study was made on very thin CdTe absorber < 1 µm layers to investigate limitations in CdTe collection efficiency. Metal organic chemical vapour deposition (MOCVD) was used to deposit cadmium sulfide (CdS), cadmium zinc sulfide (Cd_0.9Zn_0.1S) and cadmium telluride (CdTe). Improvements in photon collection in the blue, where the absorption length is shorter, have been achieved using a wider band gap Cd_0.9Zn_0.1S ternary alloy to replace CdS as the window layer. Solar cell capacitance simulator (SCAPS) modelling software [M. Burgelman, P. Nollet, S. Degrave, Thin Solid Films, 361-362 (2000) 527-532] has been used to calculate device parameters as a function of the absorber layer thickness (controlled by in situ using laser reflectometry). One feature of the MOCVD grown devices is the apparent absence of pin-holes, demonstrated by growth of an ultra-thin absorber (200 nm) with conversion efficiency of nearly 4%.     

An effective method of Cu incorporation in CdTe solar cells for improved stability

Thin film CdTe solar cells of the superstrate configuration have been fabricated in order to study the effect of Cu on device stability. The study focused on two distinct sets of solar cells: in one set of devices Cu was introduced during the formation of the back contact, by sputtering a small thickness of Cu onto the CdTe surface prior to the application of a graphite electrode; for the second set of devices Cu was introduced in CdS by briefly immersing the CdS films in a CuCl solution prior to the deposition of CdTe with the back contact electrode being sputtered Mo. The solar cells were light soaked under approximately AM1.5 conditions for nearly 700 h during 4 h ON/4 h OFF cycles. Device degradation correlated well with the amount of Cu for the devices with Cu in the back contact. Cells with larger amounts of Cu exhibited larger degradation, suggesting that the amount of Cu utilized during the back contact formation must be minimized. On the other hand, a number of devices fabricated without any Cu in the back contact, but with Cu in the CdS, exhibited nearly no degradation during the light soaking process suggesting that in addition to the amount of Cu used for the fabrication of CdTe cells, the method of incorporating this element is also critical in achieving long term device stability.     

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.     

Pathways to thin absorbers in CdTe solar cells

We present approaches to reduce the absorber thickness of CdTe solar cells. The investigations were done with CdTe absorber films deposited by the close-space sublimation (CSS) technique. Using these CdTe films, complete solar cells were produced in our own laboratory. The absorber thickness as the crucial parameter was varied between 1 and 11 µm in these experiments. It is analyzed how process steps following the CdTe layer deposition influence the structure of the absorber films as well as the solar cell properties. Three ways of back contact formation are compared. These include (i) the wet chemical etching of the CdTe surface, (ii) a plasma etching step, and (iii) the vacuum deposition of a thin intermediate copper layer. In the latter case, voids and shunts related to preferential etching at grain boundaries are avoided admitting the use of thinner absorber films. Thus, the solar-cell efficiencies were increased from below 9% to more than 10% while the CdTe film thickness was reduced from 11 µm to less than 4 µm.     

N silicon solar cells emitters realized using phosphoric acid as doping source in a spray process

The spray technique is used to realize the n⁺ emitter from phosphoric acid H₃PO₄ as a doping source. Emulsions have been prepared using several organic solvents. It was found that H₃PO₄:2-butanol mixture provides the most uniform deposited layer. The sheet resistance and the n⁺ profile were measured with a four point probe and the Hall profiling, respectively. The variety of emitters obtained are characterized by a sheet resistance ranging from 10 to 86 Ω/□ and a junction depth of about 0.2 to 0.7 μm which can be adequate for emitters in a polycrystalline silicon solar cell process.     

Room temperature synthesis of nanocrystalline silicon by aluminium induced crystallization for solar cell applications

The aim of this study is to synthesis large area, plastic compatible of p-type nanocrystalline silicon through conventional sputter system. The growth of and p-type doping of nanocrystalline silicon onto plastic substrates using D.C. magnetron sputtering was investigated. The film properties were examined by Raman spectroscopy, X-ray Diffraction, scanning electron microscopy and energy dispersive spectrometry. Nanocrystalline silicon was achieved with careful control of ion bombardment energy. Through a narrow experimental, window room temperature, nanocrystalline silicon can be synthesised on aluminium. It is believed the aluminium reduces the required energy for crystallite nucleation. PN junction was formed through sputtering of Al/Al-Si/n-type Si/AZO structure. The I-V characteristic showed good rectifying behaviour and confirms p-type doping via aluminium induced crystallization.     

Thin-film solar cells

The rapid progress that is being made with inorganic thin-film photovoltaic (PV) technologies, both in the laboratory and in industry, is reviewed. While amorphous silicon based PV modules have been around for more than 20 years, recent industrial developments include the first polycrystalline silicon thin-film solar cells on glass and the first tandem solar cells based on stacks of amorphous and microcrystalline silicon films (“micromorph cells”). Significant thin-film PV production levels are also being set up for cadmium telluride and copper indium diselenide.     

Thin-film Si:H-based solar cells

Recent developments in the photovoltaic (PV) industry, driven by a shortage of solar grade Si feedstock to grow Si wafers or ribbons, have stimulated a strong renewed interest in thin-film technologies and in particular in solar cells based on protocrystalline hydrogenated amorphous silicon (a-Si:H) or nanocrystalline/microcrystalline (nc/μc)-Si:H. There are a number of institutions around the world developing protocrystalline thin-film Si:H technologies as well as those based on tandem and triple junction cells consisting of a-Si:H, a-Si:Ge:H and nc/μc-Si:H. There are also several large commercial companies actively marketing large production-scale plasma-enhanced chemical vapor deposition (PECVD) deposition equipment for the production of such modules. Reduction in the cost of the modules can be achieved by increasing their stabilized efficiencies and the deposition rates of the Si:H materials. In this paper, recent results are presented which provide insights into the nature of protocrystalline Si:H materials, optimization of cell structures and their light-induced degradation that are helpful in addressing these issues. The activities in these areas that are being carried out in the United States are also briefly reviewed.     

Preparing thick, defect-free films of anatase titania for dye-sensitized solar cells

Anatase titania (TiO₂) nanoparticle films were prepared on fluorine-doped tin oxide (FTO) and tin-doped indium oxide (ITO) substrates. The films were characterized by X-ray diffraction, scanning electron microscopy, profilometry, Raman spectroscopy, and optical microscopy. The results show that defects are initiated during the sintering step and continue to propagate once the film is cooled. The sintering and annealing steps were controlled by reducing the pressure and the rate of temperature change. These steps reduced the stresses generated during film preparation, allowing thick titania films on both FTO and ITO substrates to be prepared with minimal defects. Using the optimized conditions for film preparation, 20 μm thick films of titania on FTO and ITO substrates were obtained with calculated defect densities of 2.5 and 7.8%. Films as thick as 25 μm were prepared on FTO substrates with a defect density of only 6.0%. Dye-sensitized solar cells (DSSCs) were fabricated using the titania films prepared by both standard and vacuum sintering methods. DSSCs made with 20 μm titania films sintered at intermediate pressures show improvements to short-circuit current, open-circuit voltage, and device efficiency.     

CdTe photoelectrochemical solar cells—chemical modification of surfaces

Chemical modification of bothn andp type CdTe has been found to improve the performance and stability of PEC solar cells. The surfaces, modified by Ru³⁺, have been examined by a variety of techniques. Modification results in enhanced barrier height at the surface due to the formation of a passivating oxide layer.     

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.