Texturing industrial multicrystalline silicon solar cells

Three potential techniques for texturing commercial multicrystalline silicon solar cells are compared on the basis of reflectance measurements. Wet acidic texturing, which would be the least costly to implement, produces a modest improvement in reflection before antireflection coating and encapsulation, whereas maskless reactive-ion etching texturing, and especially masked reactive-ion etched ‘pyramids’, generate a larger gain in absorption. After antireflection coating and encapsulation however, the differences between the methods are reduced. Short-circuit current measurements on wet acidic textured cells reveal that there is a significant additional current gain above that expected from the reduced reflection. This is attributed to both light-trapping and oblique coupling of incident light into the cell, resulting in generation closer to the junction.     

Rie-texturing of multicrystalline silicon solar cells

We developed a maskless plasma texturing technique for multicrystalline silicon cells using reactive ion etching that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to extremely low levels. Internal quantum efficiencies as high as those on planar cells have been obtained, boosting cell currents and efficiencies by up to 7% on evaporated metal and 4% on screen-printed cells.     

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.     

Production technology of large-area multicrystalline silicon solar cells

In 1996 a conversion efficiency of 17.1% had been obtained on 15 cm×15 cm mc-Si solar cell. In this paper, large-scale production technology of the high-efficiency processing will be discussed. Enlarging reactive ion etching (RIE) equipment size, technology of passivation, and fine contact grid with low resistance by screenprinted metallization, which is firing through PECVD SiN, have been investigated.     

High-efficiency multicrystalline silicon solar cells by liquid phase-epitaxy

Thin-film silicon cells produced on crystalline silicon substrates have the potential to achieve high cell efficiencies at low cost. We have used a modified liquid-phase epitaxy growth process to produce very smooth, high-quality silicon films on multicrystalline silicon substrates. Photoconductivity decay measurements indicate that the minority carrier lifetimes in these layers are at least 10 μs, sufficient to achieve cell efficiencies in excess of 16%. This efficiency potential is confirmed in small area cells, which have displayed efficiencies up to 15.4%. Further improvements up to 17% efficiency are possible in the short term, even without the introduction of any light-trapping schemes into the device structure.     

Characterization of PIII textured industrial multicrystalline silicon solar cells

Optimized textured structure is one of the most important elements for high efficiency multicrystalline silicon solar cells. In this paper, in order to incorporate low reflectance nanostructures into conventional industrial solar cells, structures with aspect ratios of about 1:1 and average reflectance of 8.0% have been generated using plasma immersion ion implantation. A sheet resistance of 56.9 Ω/sq has been obtained by adjusting the phosphorous diffusion conditions, while the thickness of the silicon nitride vary in 70–80 nm by extending the deposition time by 100 s. Under the conventional co-firing conditions, a solar cell with efficiency of 16.3% and short-circuit current density 34.23 mA/cm² has been fabricated.     

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.     

Hydrogen passivation of defects in multicrystalline silicon solar cells

The knowledge of how hydrogen interacts with defects and impurities in silicon is crucial for the understanding of device performance, especially for solar cells made from disordered silicon wafers. Hydrogen can be introduced in silicon by several techniques, but this paper will be focused on hydrogenation by means of plasma enhanced chemical vapor deposition of hydrogen-rich silicon nitride layer on the surface of the wafer. Passivation effects are observed after annealing and evaluated using minority carrier diffusion length measurements and light-beam-induced current scan maps.It was found that individual intragrain defects are well passivated, while deep levels are transformed into poorly recombining shallow levels at grain boundaries and dislocation clusters. In solar cells, the stability of the hydrogen passivation is much higher with this technique than with other hydrogenation techniques. This is probably due to an encapsulation of hydrogen by the frontwall silicon nitride coating layers and by the backside aluminum film.     

Local investigation of hot spot areas on multicrystalline silicon solar cells

The formation mechanism and the electrical consequences of hot spots have been investigated in multicrystalline solar cells. The hot spots were revealed by means of an IR camera when the cells are reverse biased in the dark. The minority carrier diffusion length (L_n), the open-circuit photovoltage (V_oc) and the short-circuit photocurrent (J_sc) were measured both in the hot spot area and far from this zone by masking off and etching small mesa diodes around suspect areas. Dark forward I–V curves were plotted in order to compute the values of ideality factor (M) and reverse saturation current (J_o).It is found that J_o and M are higher in the hot spot area, while J_sc, V_oc and, to a less extent, L_n are smaller. Large densities of dislocations and lineages structures are revealed in the abnormally heated region by chemical etching of the samples. The dislocations involved in the hot spot formation could act as shunting paths for leakage currents, in addition to their contribution to recombination-generation currents.     

Overview of solar cell technologies and results on high efficiency multicrystalline silicon substrates

Fabrication technologies for multicrystalline silicon (mc-Si) solar cells have advanced in recent years with efficiencies of mc-Si cells exceeding 18%. Intense efforts have been made at laboratory level to improve process technology, growth methods, and material improvement techniques to deliver better devices at lower cost. Deeper understanding of the physics and optics of the device led to improved device design. This provided a fruitful feedback to the industrial sector. Both screenprinting and buried-contact technologies yield cells of high performance. An increasingly large amount of research activity is also focussed on the fabrication of thin solar cells on cheap substrates such as glass, ceramic, or low quality silicon. Success of these efforts is expected to lead to high efficiency devices at much lower costs. Efforts are also being put on low thermal budget processing of solar cells based on rapid thermal annealing.     

Defect passivation of industrial multicrystalline solar cells based on PECVD silicon nitride

Low surface recombination velocity and significant improvements in bulk quality are key issues for efficiency improvements of solar cells based on a large variety of multicrystalline silicon materials. It has been proven that PECVD silicon nitride layers provide excellent surface and bulk passivation and their deposition processes can be executed with a high throughput as required by the PV industry. The paper discusses the various deposition techniques of PECVD silicon nitride layers and also gives results on material and device properties characterisation. Furthermore the paper focuses on the benefits achieved from the passivation properties of PECVD SiN_x layers on the multi-Si solar cells performance. This paper takes a closer look at the interaction between bulk passivation of multi-Si by PECVD SiN_x and the alloying process when forming an Al-BSF layer. Experiments on state-of-the-art multicrystalline silicon solar cells have shown an enhanced passivation effect if the creation of the alloy and the sintering of a silicon nitride layer (to free hydrogen from its bonds) happen simultaneously. The enhanced passivation is very beneficial for multicrystalline silicon, especially if the defect density is high, but it poses processing problems when considering thin (<200 μm) cells.     

Efficiency improvement by porous silicon coating of multicrystalline solar cells

Shallow junction multicrystalline Si solar cells have been processed by an anodical etching technique. More than 25% improvement in short-circuit current and photovoltaic energy conversion efficiency was demonstrated. It was shown that improved performance was caused by antireflection action of the porous silicon layer as well as by the cell surface and grain boundary passivation.     

Progress in the metallisation of n-type multicrystalline silicon solar cells

A study on the optimisation of front contacts of n-type multicrystalline silicon solar cells is presented. In this study the same cell processing was applied to two types of wafers: electronic grade (EG-Si) and metallurgic grade (MG-Si) silicon. The contact firing temperature was optimised, by measuring the contact resistivity of the front and back contacts for different firing temperatures. The front contacts were improved by deposing silver using an electrochemical process. The solar cells were characterised before and after the silver deposition. For all cells processed the line resistance was reduced by over 90% after the silver deposition. After the contact improvement, EG-Si cells showed an absolute efficiency improvement of 2.6%, but MG-Si cells suffered a reduction on the cell efficiency, an effect related to parasitic shunting existent in these cells.     

Texturisation of multicrystalline silicon solar cells by RIE and plasma etching

Texturing by reactive ion etching (RIE) is demonstrated as an attractive technical solution for lowering of reflectance of multicrystalline silicon solar cells. A suitable sequence of processes is developed to combine the advantage of reactive ion etching with “natural lithography” based on colloidal masks. The RIE single-wafer texturisation is driven to an industrial applicable batch process by plasma etching with a gain in efficiency of 0.3% absolute.     

UVCVD silicon nitride passivation and ARC layers for multicrystalline solar cells

This work intends to investigate the effectiveness of silicon nitride layers (SiN_x : H) deposited by photochemical vapor deposition (UVCVD) for antireflection and passivation purposes when applied to electromagnetically casted silicon solar cells (EMC). Effective reflectivity of 10.8% is achieved, as well as 66% increase of minority carrier lifetime.     

Formation and characterization of porous silicon layers for application in multicrystalline silicon solar cells

The electrochemical formation of porous silicon (PS) layers in the n⁺ emitter of silicon p–n⁺ homojunctions for solar energy conversion has been investigated. During the electrochemical process under constant polarization, a variation of the current density occurs. This effect is explained by considering the doping impurity gradient in the emitter and by TEM characterization of the PS layer structure. Optical transmission measurements indicate that modifications of the refractive index and absorption coefficient of PS are mainly related to the porosity value. Reflectivity measurements, spectral response and I–V characteristics show that PS acts as an efficient antireflection coating layer. However, beyond a critical layer thickness, i.e. when PS reaches the p–n⁺ interface, the junction properties are degraded.     

Absorptivity of silicon solar cells obtained from luminescence

Electroluminescence spectra were measured on textured and non-textured silicon solar cells at room temperature. From these spectra the absorptivity for band-to-band transitions A_BB(ω) of these solar cells could be extracted by application of the generalized Planck's law for the emission of luminescence via direct or indirect transitions.Our method is an alternative to the more conventional measurements of transmission and reflection and has the advantage, that only that part of the absorptivity leading to the generation of electron–hole pairs is determined, even if significant free-carrier absorption is present.     

Latest results on semitransparent POWER silicon solar cells

The paper presents the latest results of the polycrystalline wafer engineering result (POWER) silicon solar cell research (G. Willeke, P. Fath, The POWER silicon solar cell, Proceedings of the 12th EPVSEC, Amsterdam, 1994, pp. 766–768). Mono – as well as bifacially active semitransparent silicon solar cells have been created by forming perpendicularly overlapping grooves on the front and the rear side of a silicon wafer resulting in a regular pattern of holes. The developed very simple manufacturing process is fully compatible with an industrial production and uses POCl₃-tube diffusion, PECVD silicon nitride as single ARC and screen-printing metallization. Maximum efficiencies of η=11.2% for monofacial POWER cells on 0.4 Ω cm Cz material with a transparency of 18.2% and η=12.9% for bifacial cells on 1 Ω cm Cz material with a transparency of 16% have been obtained. Results for multicrystalline (mc) semitransparent mono- and bifacially active silicon solar cells are also presented.     

Impurities and defects in multicrystalline silicon for solar cells: low-temperature photoluminescence investigations

The low-temperature photoluminescence (PL) measurements (down to 4.2 K) were employed for the investigations of the defects and impurities in multicrystalline silicon (mc-Si) samples grown by block-casting method. The optical properties of as-grown, irradiated by gamma-rays, heat and hydrogen plasma treated samples were studied. It was found that carbon and oxygen as the residual impurity atoms are responsible for the formation of the zero-phonon PL lines with 0.9355 eV (T line) and 0.9652 eV (I line) after heat treatments at about 350–550°C. The appearance of PL lines with the energies of 0.9697 eV (A line) and 0.7894 eV (C line) after a gamma-rays irradiation can be attributed to the formation of carbon- and oxygen-related centers, respectively. The comparison of the PL properties of the mc-Si samples with the mono-crystalline one is performed. It is shown that the main peculiarities of the low-temperature PL spectra of mc-Si can be explained both by the influence of residual impurities and the residual strains in this material.     

Reduced light reflection of textured multicrystalline silicon via NPD for solar cells applications

Texturing by negative potential dissolution (NPD) process of p-type multicrystalline silicon for solar cells application is reported. The effect of the negative potential, KOH concentration, and texturing time of cast multicrystalline silicon was studied. Rapid texturing of multicrystalline silicon was achieved in a time-frame of 2 min with the application of negative potential of −30 V and the use of optimal alkaline concentration of 32 wt%. While texturing process in these optimal NPD conditions results in a step-free morphology, necessary in solar cells contacts printing, light reflection was reduced to minimal values, as well.