Effect of oxygen precipitation on the performance of Czochralski silicon solar cells

We have demonstrated the effect of oxygen precipitation on the performance of Czochralski (CZ) silicon solar cells. The oxygen precipitates in silicon substrates were formed by a low–high two-step annealing. With the increase of oxygen precipitation, the minority carrier diffusion length of CZ silicon solar cells decreases and, meanwhile, the leakage currents due to the carrier recombination at the defect states get increased. The external quantum efficiency (EQE) measurement shows that the decrease of the solar cell efficiency due to oxygen precipitates mainly takes place in the long wavelength range of light. The short-circuit current and open-circuit voltage of solar cells both become smaller, while the fill factor does not significantly change. The efficiency of solar cells is reduced to 12.7% from an original value of 17.5%. All these results suggest that the oxygen precipitation is a limitation factor for the improvement of solar cell efficiency, which should be strictly controlled during the crystal growth and cell fabrication.     

Germanium-doped Czochralski silicon for photovoltaic applications

Germanium (Ge)-doped Czochralski (GCZ) silicon has been grown for photovoltaic (PV) applications. It is found that Ge doping improves the mechanical strength of CZ silicon, resulting in the reduction of breakage during wafer cutting, cell fabrication and module assembly. Boron-oxygen (B-O) defects that lead to the light-induced degradation (LID) of carrier lifetime are effectively suppressed by Ge doping. The decrease in the maximum concentration of B-O defects increases with an increase of Ge concentration. The efficiency of GCZ silicon solar cells and the power output of corresponding PV modules both exhibit smaller loss under sunlight illumination. The current work suggests that GCZ silicon should be potentially a novel substrate for thin solar cells with low LID effect.     

Degradation of carrier lifetime in Cz silicon solar cells

The lifetime degradation induced by light illumination or carrier injection which is observed in Czochralski-grown silicon (Cz-Si) leads to a significant decrease of solar cell efficiency. Thus, the reduction of this effect has a high potential for the improvement of Cz-Si solar cells. In the present work both, the analysis of the underlying defect and its technological reduction are discussed. A clear correlation of the Cz-specific metastable defect with the oxygen and boron concentration in Cz-Si has been observed. Especially, recently performed lifetime measurements on oxygen-free boron-doped p-type MCz silicon and gallium-doped oxygen-contaminated Cz-silicon, both of which show no degradation, confirm this hypothesis. While the quantitative correlation between the defect concentration and boron is linear, the increase of the defect concentration induced by the interstitial oxygen concentration is superlinear, i.e. it follows a potential law of power approximately 5. Beyond the defect analysis, two different ways to reduce the metastable defect concentration are discussed. A proper material choice by substituting or reducing one of the major components of the metastable defect can completely avoid the degradation effect. The excellent performance of oxygen-free MCz-Si and gallium-doped Cz-Si is reflected in the achieved record efficiencies of 22.7% and 22.5%, respectively. In standard boron-doped oxygen-contaminated p-type Cz-Si a strong reduction of the metastable defect concentration can be achieved by a high-temperature process step resulting in an improvement of the stable bulk lifetime by a factor of 2–4.     

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.     

Surface and bulk-passivated large area multicrystalline silicon solar cells

We have investigated the surface and bulk passivation technique on large-area multicrystalline silicon solar cells, a large open-circuit voltage has been obtained for cells oxidized to passivate the surface and hydrogen annealed after deposition of silicon nitride film on both surfaces by plasma CVD method (P---SiN) to passivate the bulk. The texture surface like pyramid structure on multicrystalline silicon surface has been obtained uniformly using reactive ion etching (RIE) method. Combining these RIE method and passivation schemes, the conversion efficiency of 17.1% is obtained on 15 cm × 15 cm multicrystalline silicon solar cell. Phosphorus diffusion, BSF formation, passivation technique and contact metallization for low-cost process sequence are also described in this paper.     

Degradation and annealing of amorphous silicon solar cells by current injection: experiment and modeling

In this paper we report in detail on the effect of current injection in amorphous silicon solar cells. A set of devices has been degraded and then annealed at different current intensities. Device performances during the whole experiment have been monitored by current–voltage characteristics and quantum efficiency curves. It has been found that annealing rate increases with current intensity, while stabilized photovoltaic parameters decrease. Time evolution of efficiency and short-circuit current during degradation has been reproduced by a numerical device modeling, resulting in a pronounced increase of defects near the p–i interface. The model also demonstrated that annealing results are not well reproduced if current-induced annealing is not energy selective.     

Study of electrical properties of oxidized porous silicon for back surface passivation of silicon solar cells

Back surface passivation becomes a key issue for the silicon solar cells made with thin wafers. The high surface recombination due to the metal contacts can be lowered by reducing the back contact area and forming local back surface field (LBSF) in conjunction with the passivation with dielectric layer. About 3×10^-7 m thick porous silicon (PS) layer with pore diameter mostly of 1×10^-8–5×10^-8 m was formed by chemical etching of silicon using the acidic solution containing hydrofluoric acid (HF), nitric acid (HNO₃) and De-ionized water in the volume ratio 1:3:5 at 298 K for which etching time was kept constant for 360 s. Electrical properties of oxidized PS was studied through the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of the metal–insulator–semiconductor (MIS) device in which the oxidized PS was used as an insulating layer and the results were further analyzed. The C–V curves of all the studies MIS devices showed the negative flatband voltage varying from -2 to , confirming that the oxidized layer of PS has fixed positive charge.     

Porous silicon layer transfer processes for solar cells

A promising cost-effective way of converting sun light into electricity could be a solar cell realized in a thin monocrystalline silicon film, due to its potential to achieve cell efficiencies of more than 20% in a 20 μm thick film. A porous silicon layer transfer technique provides an opportunity to get monocrystalline films on low-cost substrates such as glass. This paper reviews various processes, which are being developed for the layer transfer using porous silicon as a sacrificial layer while reusing of starting silicon substrate. The four basic steps—porous silicon formation, active layer deposition, layer separation and transfer, and device fabrication—have been identified in layer transfer process. The processes have been categorized and compared on the basis of the sequence of steps used in individual processes.     

Development of low cost production technologies for polycrystalline silicon solar cells

To develop a technology of forming grooves for low cost cell production, a multi-blade wheel grinding method was investigated. The process time of groove formation on the surface of 10 × 10 cm² polycrystalline silicon substrate was reduced to 30 s by a newly developed high-speed groove formation machine. Simultaneous formation of junction and anti-reflection coating by atmospheric pressure chemical vapor deposition (APCVD) technique was also investigated. For electrodes formation process, single firing method for both side electrodes made possible to simplify the firing process and to speed up from a conventional speed of 400 mm/min to 5000 mm/min.     

Multicrystalline silicon solar cells with porous silicon emitter

A review of the application of porous silicon (PS) in multicrystalline silicon solar cell processes is given. The different PS formation processes, structural and optical properties of PS are discussed from the viewpoint of photovoltaics. Special attention is given to the use of PS as an antireflection coating in simplified processing schemes and for simple selective emitter processes as well as to its light trapping and surface passivating capabilities. The optimization of a PS selective emitter formation results in a 14.1% efficiency mc-Si cell processed without texturization, surface passivation or additional ARC deposition. The implementation of a PS selective emitter into an industrially compatible screenprinted solar cell process is made by both the chemical and electrochemical method of PS formation. Different kinds of multicrystalline silicon materials and solar cell processes are used. An efficiency of 13.2% is achieved on a 25 cm² mc-Si solar cell using the electrochemical technique while the efficiencies in between 12% and 13% are reached for very large (100–164 cm²) commercial mc-Si cells with a PS emitter formed by chemical method.     

High-efficiency low-cost integral screen-printing multicrystalline silicon solar cells

This paper describes how the efficiency and throughput of industrial screen-printed multi-Si solar cells can be increased far beyond the state-of-the-art production cells. Implementation of novel processes of isotropic texturing, shallow emitter or single diffusion selective emitter, combined with screen-printed metallization fired through a PECVD SiN_x ARC layer, have been described. Novel dedicated fabrication equipment for emitter diffusion and a PECVD SiN_x deposition system are developed and implemented thereby removing the processing bottlenecks linked to the diffusion and bulk passivation processes. Several types of back-contacted solar cells with improved visual appeal required for building integrated photovoltaic (BIPV) application have been developed.     

Fabrication and characterisation of parallel multijunction thin film silicon solar cells

The parallel multijunction solar cell design offers the exciting possibility of high efficiency at low cost. To date, there has been no detailed report on the experimental characteristics of these devices. This paper reports on the beginnings of a detailed experimental investigation of the parallel multijunction solar cell. Progress is reported on the fabrication of parallel multijunction thin-film silicon solar cells (on inert single-crystal silicon substrates), specifically designed and fabricated to serve as experimental test-beds for the detailed study of cell performance limiting mechanisms. Of particular interest is the importance of junction space-charge-region recombination in heavily defected parallel multijunction cells.     

The application of grating couplers in thin-film silicon solar cells

As an alternative to randomly textured transparent conductive oxides as front contact for thin-film silicon solar cells, the application of periodic light grating couplers was studied. The periods and groove depths of transparent gratings made of zinc oxide were tuned independently from each other and varied between 1 and 4 μm and 100 and 600 nm, respectively. The one-dimensional grating couplers were realized using photolithography. We have analysed the optical properties of the gratings and the properties of amorphous and microcrystalline silicon solar cells incorporating these grating couplers. The achieved results are discussed with respect to the performance of cells deposited on flat and randomly textured substrates.     

A simple fabrication process for 20% efficient silicon solar cells

Recently, a substantially simplified PERC silicon solar cell has been developed at ISFH with independently confirmed 1-sun efficiencies of up to 20.0%. This paper describes the details of the relatively simple cell fabrication process and experimentally characterizes the new cells. The simplified design involves reflection control by means of random pyramids, the direct evaporation of the front metal grid onto the random pyramids, elimination of the need for nontextured areas underneath the contact grid, and the use of a single phosphorous diffusion (1-step emitter).     

High-efficiency silicon space solar cells

SHARP's activities on Si solar cells developments and features of Si solar cells for space use in comparison with GaAs solar cells are presented. Two types of high-efficiency silicon solar cells and the same kinds of high-efficiency solar cells with integrated bypass function (IBF cells) were developed and qualified for space applications. The NRS/LBSF cells and NRS/BSF cells showed an average of 18% and 17% efficiencies, respectively, at AMO and 28°C conditions. The IBF cells have P⁺N⁺ diodes on the front surface to protect itself from reverse voltage due to shadowing. The designs and features of these solar cells are presented. The radiation tests results of these solar cells are also presented. The NRS/BSF cells showed lower degradation rate compared to conventional BSFR cells with the same thickness (100 μm). But the NRS/LBSF cells showed a higher degradation rate than the BSFR cells. The IBF cells showed almost the same radiation characteristics as the same kinds of cells without IBF. The results of radiation tests on these high-efficiency solar cells and the discussions about the radiation characteristics of them are presented. In the last section, the future silicon solar cell development plan is discussed.     

Modeling of light-induced degradation of amorphous silicon solar cells

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.     

Amorphous silicon solar cells deposited with non-constant silane concentration

The performance and light-soaking behavior of hydrogenated amorphous silicon (a-Si:H) solar cells with absorber layers deposited under non-constant silane concentration (SC) - a measure of silane dilution in hydrogen - using plasma enhanced chemical vapor deposition (PECVD) are investigated. Constant SC values during deposition close to the amorphous to microcrystalline phase transition lead to the formation of crystallites after a certain thickness. To prevent this transition, SC is adjusted during growth to produce an amorphous material that is close to the microcrystalline phase transition without the inclusion of a detectable microcrystalline phase. By adjusting SC during deposition it was possible to achieve an increased open-circuit voltage that is up to 40 mV higher than that for a conventional amorphous silicon solar cell at initial efficiencies above 9%. The best solar cells produced with non-constant SC show improved stability against light induced degradation, which leads to a relative loss in fill factor of only 11.4%, resulting in a stabilized fill factor of 62.5%.     

Thin film polycrystalline silicon solar cells on mullite ceramics

In this work, we present the structural quality of polycrystalline silicon films formed by high-temperature chemical vapor deposition (CVD) on mullite ceramics coated with spin-on flowable oxides (FO_x) serving as intermediate layers (ILs). The average grain size and the size distribution were investigated by optical microscopy. It is found that more than 65% of the surface of polysilicon films grown on boron-doped FO_x is covered by large grains of 5–10 μm. The intra-grain and inner-grain defects as well as the grain orientation were analyzed with the electron backscattering diffraction (EBSD) technique. Twin-type defects such as Σ3 and Σ9 are frequently present in these silicon layers, which are slightly (1 1 0) preferentially oriented. Finally, we present the photovoltaic data on test solar cells made on these CVD polysilicon films. An efficiency of about 3.3% is reported. The limiting factors, as well as possible improvements, are discussed.     

Amorphous silicon solar cells

The perfectioning of the deposition techniques of amorphous silicon over large areas, in particular film homogeneity and the reproducibility of the electro-optical characteristics, has allowed a more accurate study of the most intriguing bane of this material: the degradation under sun-light illumination. Optical band-gap and film thickness engineering have enabled device efficiency to stabilize with only a 10–15% loss in the as-deposited device efficiency. More sophisticated computer simulations of the device have also strongly contributed to achieve the highest stable efficiencies in the case of multijunction devices. Novel use of nanocrystalline thin films offers new possibilities of high efficiency and stability. Short term goals of great economical impact can be achieved by the amorphous silicon/crystalline silicon heterojunction. A review is made of the most innovative achievements in amorphous silicon solar cell design and material engineering.     

Reversible increase of photocurrents in excimer laser-crystallized silicon solar cells

Excimer laser-crystallized silicon solar cells fabricated show a steady increment of the current densities with exposure to simulated sunlight, over a 30 min period. The current density of the amorphous silicon cell under identical conditions remains steady, with no significant change. The process was observed to be reversible upon cooling, and the performance increase is attributed to the energy barrier introduced by the enhanced bandgap of a nanocrystalline silicon middle layer, created as a result of the crystallization. It is suggested that the thermal energy due to prolonged illumination allows carriers to cross the barrier increasing output currents.