Comparison between normal and reverse thin crystalline silicon solar cells

The newly developed ingot growing techniques, as the three-grain and the columnar multigrain ingot processes, are now offering the possibility of slicing thinner wafers (≤ 100 μm). In this paper we present the results obtained on p type large area (≥ 100 cm²) and 100 μm thick wafers by using both conventional and reverse cell manufacturing technologies.The conventional cells are provided with aluminium or boron BSF plus screen-printed silver mirror or a silver-aluminium net; the reverse cells have a FSF and the deep back junction completely covered by a screen-printed or CVD silver layer.The constructing parameters have been chosen on the base of one and two dimensions modeling and both raw material and devices have been completely characterized.This work shows that very thin wafers do not introduce serious problems for the conventional manufacturing of solar cells. The efficiencies of the normal and of the reverse cells are found to be comparable and are of the same order than those of thicker cells, however at a significant lower cost. The main obtained result has to be related to the demonstration of a cell manufacturing feasibility starting from very thin wafers.     

Silicon solar cells with antireflection diamond-like carbon and silicon carbide films

Optical properties of diamond-like carbon and silicon carbide (SiC) films in dependence on deposition conditions were investigated. It was established that the films having refractive index from 1.6 to 2.3 may be obtained. The film optical bandgap and hardness may be changed from 1.5 to 4 eV and from 1 to 20 GPa, correspondingly. The films were deposited onto the front side of silicon solar cells (SCs). It has been shown that deposition of single- or two-layer diamond-like carbon antireflection (AR) coatings enables the SCs efficiency to be improved 1.35–1.5 times. The improvement is connected with decreasing of reflection losses and passivation of recombination active centers. SiC AR coatings improve the solar cell efficiency up to 1.3 times.     

Early stage growth mechanisms of CdTe thin films deposited by close space sublimation for solar cells

The early stage growth mechanisms of sublimation-grown thin-film polycrystalline CdTe are evaluated by growth interrupts and ex-situ SEM/AFM analysis for growth under 100 Torr of inert gas. Development of island size, density and coverage demonstrates that growth proceeds by island nucleation, island growth and density increase, followed by coalescence, channel formation and secondary nucleation. Addition of material to the islands occurs partly by the ‘step-flow’ mechanism. Grains in the completed films are considered to arise from individual nuclei. Nucleation and coalescence models are used to explain the correlation between increased substrate temperature and increased CdTe grain size in sublimation deposited CdTe films.     

Determination of photo-active region in organic thin film solar cells with an organic heterojunction

The photo-active region in the solar cells consisting of Cu-phthalocyanine (CuPc) and perylene-derivative (PV) layers was determined by using exciton blocking layers (EBLs) inserted in these layers. The photocurrent density was low when the EBL was placed near the CuPc/PV interface. With the increase of the distance between the EBL and the CuPc/PV interface, the photocurrent increased. However, when the distance reached a certain value, it leveled off owing to the limited diffusion length of excitons. From the analysis of the relationship between the position of EBL and the photocurrent density, the photo-active regions in the CuPc and PV layers were estimated to be 8 and 12 nm thick from the interface, respectively.     

Microstructure evolution of CuInSe2 thin films prepared by single-bath electrodeposition

The composition and the microstructure evolutions of CuInSe₂ thin films under single-bath electrodeposition processes were investigated. It was found that the film composition was mainly determined by the [Se⁴⁺]/[Cu²⁺] ratios in solution, but the film microstructure is strongly dependent on the initial concentrations of Se⁴⁺, Cu²⁺, and In³⁺ precursors. Higher initial concentrations of Cu²⁺ and In³⁺ in solution are beneficial for the fabrication of compact CuInSe₂ thin films with highly crystallized and large grain sized chalcopyrite phase. The microstructure evolution suggests that prior adsorption and reduction of Cu²⁺ ions and the formation of Cu₂Se compound on the substrate can promote the nucleation, growth, and coarsening of CuInSe₂ crystal to form a high quality thin film during the electrodeposition processes.     

Optoelectronic properties of chlorine- and oxygen-doped CdTe thin films

The influence of oxygen codopant upon the optoelectronic properties of chlorine-doped CdTe films is being investigated. It is shown that a small quantity of oxygen decreases the resistivity of films, whereas at higher concentrations oxygen codopant increases the resistivity of films up to 6 orders of magnitude. A subsequent annealing in tellurium vapor pressure decreases the resistivity of films. It is supposed that an anomalous resistivity drop around 0.22 kPa is caused by shallow acceptor complexes that oxygen forms with group I impurities like copper and silver. At higher concentrations oxygen forms isoelectronic complexes with cadmium vacancies, which cause a high resistivity of films. Te annealing extracts oxygen from the films as Te forms with dissolved oxygen tellurium oxide TeO₂ which easily sublimates. Photoconductivity of the oxygen and chlorine-doped CdTe films is poor, or is not detected.     

Recent progress on CdTe/CdS thin film solar cells

The CdTe/CdS thin film solar cell is the most suitable to be fabricated on the form of thin films. The processes used to make all the films, which compose the cell, are quite simple and fast. An efficiency of 16.5% has been reached on laboratory scale and modules of 0.6 × 1.2 m² with efficiency larger than 8% are now fabricated and commercialized. A strong contribution to the development of this type of solar cells has been given by the Parma University group with the discovery of a new ohmic back contact for CdTe which is very stable in respect to any other ohmic contact used for CdTe, and by the development of a new all dry process to make the cell. An efficiency of 15.8% has been recently obtained on a 10⁻⁴ m² soda-lime glass without using any copper or any other metal of the first group of the periodic table of the elements at the back contact.     

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.     

Preparation of ZnO nanoparticles and nanosheets and their application to dye-sensitized solar cells

ZnO nanoparticles which have diameter of 15 nm were prepared by hydrothermal method. ZnO nanosheets were obtained by re-hydrothermal treatment of ZnO nanoparticles. The phase and morphology were investigated by using transmission electron microscope, scanning electron microscope and X-ray diffraction. Also, absorbance spectra were measured by using a UV–vis spectrophotometer. A fill factor of 0.55, short-circuit current of 2.059 mA/cm², open-circuit voltage of 0.593 V and an overall light to electricity conversion efficiency of 1.55% for the solar cell based on ZnO nanosheets were obtained.     

a-C:H absorber layer for solar cells matched to solar spectrum

An a-C:H-based absorber layer for photovoltaic application was fabricated by a DC PECVD. The stepped voltage biasing of the deposition process makes it possible to tailor the bandgap of the manufactured layers and match them to the solar spectrum. Such system can be used as intrinsic layer in p–i–n solar cells as well as in converter solar cells.     

Back electrode formation for poly-Si thin film solar cells on glass having AIC-grown seeding layer

Various conductive materials (Al, Mo and TiN) were deposited onto glass substrates to evaluate whether poly-Si seed layers can be formed on such substrates by means of Al-induced crystallisation (AIC) of a-Si at low temperature around 450°C. The material located between the glass and the poly-Si film serves as the back electrode of a substrate-type thin-film solar cell configuration. The outcome of the investigation is that Mo is found to be not compatible with the AIC process. In contrast, Al and TiN showed moderate to good compatibility. TiN is the only viable choice for high-temperature applications (>540°C). Al has satisfactory back electrode properties whereas TiN has a medium high resistivity (120 μΩ cm) and an estimated low back reflectance at the near-infrared wavelengths critical for light trapping.     

An effective use of nanocrystalline CdO thin films in dye-sensitized solar cells

Thin films of cadmium oxide (CdO) were synthesized by layer-by-layer deposition method on indium doped tin oxide (ITO) substrates. Post-deposition annealing at 250 °C for 24 h produced pure phase CdO films by removal of trace amount of cadmium hydroxide, as confirmed from X-ray diffractogram. First time employment of CdO in place of TiO₂ in dye-sensitized solar cells is reported to check feasibility and cell performance. A dye-sensitized nanocrystalline CdO photo-electrode was obtained by adsorbing cis-dithiocyanato (4,4′-dicarboxylic acid-2,2′-bipyridide) ruthenium (II) (N3) dye by keeping at 45 °C for 20 h. The efficiency of dye-sensitized nanocrystalline CdO thin film solar cell was increased from 0.24% to 2.95% due to dye adsorption. This must be highest reported conversion efficiency for other metal oxides than TiO₂based dye-sensitized solar cells.     

New ultra thin CIGS structure solar cells using SCAPS simulation program

The present contribution reports on the performances of ultra thin chalcopyrite Cu (In,Ga) Se (CIGS) solar cells. An alternative ZnO/CdS/CIGS/Si structure has been proposed using solar cell capacitance simulator (SCAPS). The main idea behind this analysis is the improvement of the device efficiency using materials cheaper than conventional CIGS. For that purpose, a 1 μm of a new layer p-Si has been added. Various thicknesses of CIGS absorber layer ranging from 0.1 to 1 μm have been used. Our findings showed that the increase of the absorber layer thickness leads to the improvement of the performance of the new CIGS solar cells. It was found that the best structure must have a window layer ZnO, a buffer layer (CdS), an absorbent layer (CIGS) and a Si layer with thicknesses of 0.02, 0.05, 1 and 1 μm, respectively. Cells with these features give conversion efficiency of 21.3%. The present results showed that the new ultra thin CIGS solar cells structure has performance parameters that are comparable to those of the conventional ones with reduced cost.     

Chemically and electrochemically deposited thin films for solar energy materials

Direct energy gap materials, e.g. CdTe, CuInSe₂, CuInGaSe₂, CdSe, ZnP₂ and Zn₃P₂, are the most interesting for thin-film solar cell applications. Among the various methods of preparation of these films, chemical bath deposition and electrodeposition deserve special attention because they have been shown to be inexpensive, low-temperature and non-polluting methods. Based on Pourbaix diagrams of CdS, CdTe, CuInSe₂, CdSe, etc., drawn from basic considerations, the best parameters for their electrodeposition are deduced. Theoretical considerations on the chemical-bath deposition of CdS, CdSe and Sb₂S₃ are also indicated. In particular, the role of the complexing agent and of the ligands in chemical bath deposition quality is discussed, as are the uniformity and stability of the films. The photoelectrochemical, Schottky barrier and heterojunction solar cell properties based on chemically and electrochemically deposited thin films with heteropolyacids are shown. Future trends for chemically and electrochemically deposited polycrystalline thin films are addressed. Results from very recent work done in the improvement of chemically and electrochemically deposited thin films are presented. Significant results obtained on advanced CdS/CdTe, CdS/CIS and CdS/CIGS solar cells developed by industry and by laboratory groups worldwide are indicated. Emerging low cost materials or/and less environmental hazards materials which may introduce solar cells into worldwide market are considered in the conclusion.     

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.     

Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD

Polycrystalline silicon (poly-Si) films ( 10 μm) were grown from dichlorosilane by a rapid thermal chemical vapor deposition (RTCVD) technique, with a growth rate up to 100 Å/s at the substrate temperature (T_s) of 1030°C. The average grain size and carrier mobility of the films were found to be dependent on the substrate temperature and material. By using the poly-Si films, the first model pn⁺ junction solar cell without anti-reflecting (AR) coating has been prepared on an unpolished heavily phosphorus-doped Si wafer, with an energy conversion efficiency of 4.54% (AM 1.5, 100 mW/cm², 1 cm²).     

Low temperature polycrystalline silicon: a review on deposition, physical properties and solar cell applications

This review article gives a comprehensive compilation of recent developments in low temperature deposited poly Si films, also known as microcrystalline silicon. Important aspects such as the effect of ions and the frequency of the plasma ignition are discussed in relation to a high deposition rate and the desired crystallinity and structure. The development of various ion energy suppression techniques for plasma enhanced chemical vapour deposition and ion-less depositions such as HWCVD and expanding thermal plasma, and their effect on the material and solar cell efficiencies are described. The recent understanding of several important physical properties, such as the type of electronic defects, structural effects on enhanced optical absorption, electronic transport and impurity incorporation are discussed. For optimum solar cell efficiency, structural considerations and predictions using computer modelling are analysed. A correlation between efficiency and the two most important process parameters, i.e., growth rate and process temperature is carried out. Finally, the application of these poly Si cells in multijunction cell structures and the best efficiencies worldwide by various deposition techniques are discussed.     

Schottky solar cells with amorphous carbon nitride thin films prepared by ion beam sputtering technique

This paper reports on the successful deposition of amorphous carbon nitride thin films (a-CN_x) and fabrication of ITO/a-CN_x/Al Schottky thin-film solar cells by using the technique of ion beam sputtering. XPS and Raman spectra are used to characterize the deposited thin films. Nitrogen atoms are incorporated into the films in the form of carbon–nitrogen multiple bands. Their optical properties are also investigated using a spectroscopic ellipsometer and UV/VIS/NIR spectrophotometer. The refraction of the carbon nitride thin films deposited lies in the range of 1.7–2.1. The Tauc optical band gap is about 0.6 eV. The photovoltaic values of the device, short-circuit current and open-circuit voltage are 1.56 μA/cm² and 250 mV, respectively, when exposed to AM1.5 illumination (100 mW/cm², 25°C).     

Effects of grain boundaries on cell performance of poly-silicon thin film solar cells by 2-D simulation

The effect of grain boundaries on the performance of poly-Si thin film solar cells was studied theoretically using a 2-D simulation assuming the presence of either rectangular-shaped or graded width grain boundaries in the i-layer of p/i/n structure of solar cells. The grain boundary had an adverse effect mainly on V_oc. J_sc gradually increased and saturated with increasing solar cell thickness in cells without grain boundaries, whereas it reached a maximum for an i-layer thickness of 5 μm in polycrystalline silicon cells. The calculation using the graded width model showed that the efficiency of the p⁺/p⁻/n⁺ structure was better than that of the p⁺/n⁻/n⁺ structure. A slight p-type doping of the i-layer was found to be effective in improving cell performance.     

MIS-stimulated back-surface passivation of interdigitated back-contacts solar cells

The possibility of surface recombination losses reduction on the rear side of interdigitated back-contact solar cells by field-effect passivation is investigated. To provide field-effect passivation, an additional biased metal/insulator/semiconductor (MIS) structure is formed between n⁺⁺ and p⁺-doped regions. The source of the bias is the potential that appears in the solar cell under illumination. Two-dimensional (2D) numerical simulations were performed to determine the best passivation conditions. In particular, the influence of symmetric and asymmetric capture cross sections for electrons and holes at the rear side of the cell is simulated and the advantage of symmetric capture cross section for this type of passivation is discussed. Experimental and calculated data are compared for Si/SiO₂ interface.