Overview of wet chemical texturing processes for cast-mono silicon materials

Transcription

1 Overview of wet chemical texturing processes for cast-mono silicon materials Sebastian Patzig-Klein 1, Eckard Wefringhaus 2, Cornelia Klein 1, Wilfried Benko 1, Franck Delahaye 1, Steffen Queißer 1, Jürgen Schweckendiek 1, Hartmut Nussbaumer 1 1 : GmbH, Hoehenweg 1, Gütenbach, Germany 2 : SC Konstanz e.v., Rudolf-Diesel-Straße 15, Konstanz, Germany Corresponding author, Phone: , Fax: , Sebastian.Patzig-Klein@rena.com ABSTRACT: The presented study provides a detailed insight for wet chemical texturing procedures of cast-mono or mono-like silicon substrates. Fundamental investigations establish the demand for adapted etching techniques in order to address the exceptional cast-mono surface properties. Fundamental evaluation parameters are texturing homogeneity, magnitude of effective surface reflectance and quality of produced texturing structures. The impact on solar cell characteristics, relevant processing steps and industrial application is discussed and interpreted with respect to material classifications and cost-effective manufacturing sequences. Keywords: Texturization, Etching, Silicon 1 NTRODUCTON Wet chemical etching procedures are of enormous relevance to silicon solar cell processing. n addition to the removal of parasitical saw damage and impurities the significant reflectance reduction of shiny wafer surfaces is a well-established key production step (1). Lately a new type of crystalline silicon material is entering the solar cell and module market (2). The so-called monolike or cast-mono substrates exhibit multicrystalline surface regions and prominent percentages of quasimono areas, showing no visible grain boundaries (3). The anticipated efficiency advantage for mono-like silicon materials gives reason to an expected increase of its absolute market share up to 50 % until 2020 (4). The hybrid cast-mono crystal nature is challenging well-established wet chemical texturing techniques (2), (5). Our investigations are focussed on benchmarking state-of-the art and adapted texturing procedures in respect to most homogeneous texturing results and lowest possible surface reflections. The impact of produced silicon surface characteristics on processing steps and solar cell performance is examined and interpreted. Encapsulation effects are discussed for textured and processed cast-mono silicon wafers. 2 EXPERMENTAL All experiments were conducted at the nternational Solar Energy Center e.v. (SC), Konstanz (Germany) and at the Solar Technology Center (STC), Freiburg (Germany). The cast-mono silicon wafers of representative suppliers were pre-characterized by analysis of scanning images and PL measurements. Substrate classification was conducted according to the determined quasimono area share. Standard solar cell processing steps included wet chemical texturing, standard POCl 3 diffusion (68 Ohm/sq), wet chemical junction isolation, standard PECVD-SiNx deposition, standard front and backside metallization including printing of pads and contact firing, respectively. and junction isolation processes were conducted by npilot wet chemical texturing equipment. n order to investigate encapsulation effects chosen solar cells were processed to form mini-modules. Contact soldering was followed by EVA lamination, structured glass front side and TEDLAR sheet backside encapsulation steps. Solar cell and solar module characterization was carried out by Reflectance, V- and Spectral Response measurements. 3 RESULTS AND DSCUSSON 3.1 Cast-mono Si characteristics and classification The seed-layer induced directed solidification of cast-mono silicon ingots is directly reflected by its material characteristics. Vertical block position, i.e. the height distance from the seed, is determining the effective metal segregation and oxygen concentration. Lateral block position, i.e. the distance from the crucible side wall, is crucial for the amount of SiC precipitation and induced re-crystallization (6). n order to preserve the high-prized seed layer and to enable advanced interphase control cycle time needs to be carefully adapted (5), (6). The classification of produced cast-mono Si materials is based on the effective quasi-mono area share (5). Typical results for Gen 5 (5x5 blocks) furnaces upgraded for cast-mono Si crystallization are summarized in Table. Table : Classification and representative percentage ranges of a Gen 5 cast-mono Si crystallization ingot. cast-mono quasi-mono area crystallization Si class share percentage > 90 % % % % < 60 % % The lowest and highest cast-mono Si qualities show material properties close or similar to conventional multicrystalline (mc-si) and monocrystalline (cz-si) silicon wafers, respectively.

2 3.2 Processing sequence of cast-mono Si wafers Plummeting photovoltaic module prices and severe technological barriers hamper the predicted success of cast-mono Si materials. n fact, cost-effective production is only possible for nearly 100 % quasimono crystallization percentages (7). Solar cell processing steps have to be adapted to the exceptional cast-mono Si material properties. n particularly and silicon wafers are in strong need for special surface treatments in order to return the estimated efficiency advantage compared to mc-si materials (4). wet chemical etching equipment provides highly cost-effective techniques for saw-damage removal, back-side polishing, reduction of surface reflectance and junction isolation processing (1). Systematic investigations of wet chemical texturing for cast-mono Si include material classification and comparison of standard and cast-mono texturing strategies. Standard solar cell processing steps, wafer surface and solar cell characterization complete the investigated processing sequence, Figure 1. Standard "cast-mono" Si wafer classification "Low-grade" "Hybrid" "Premium" Wet chemical texturing Standard "cast-mono" 3.3 Wet chemical texturing of cast-mono Si wafers Standard acidic ( ) texturing is insensitive to effective quasi-mono area shares. Homogeneously distributed oval etching pits are exhibited for all three cast-mono Si wafer classes. These well-known features result in surface reflectance close to iso-textured mc-si substrates, shown in Figure 2 and summarized in Table. Figure 2: Laser Scanning Microscopy images (130 x 129 µm²) for (left) and (right) cm-si wafers textured by. Standard alkaline ( ) texturing results in surface properties highly dependent on quasimono area shares. For increasing tilting of crystal orientations and particularly for crystals differing from the original seed layer shingle-like structures arise and gradually replace texturing pyramids characteristic for solutions, Figure 3. Surface characterization Reflectance measurement Laser Scanning Microscopy Standard diffusion Front-side: Ag 68 Ohm/Sq POCl 3 Standard junction isolation noxside Standard passivation PECVD SiNx Standard metallization Standard contact formation Cofiring front- and back-side Solar cell characterization V-Measurement Back-side: Full Al-BSF + pads Figure 1: Solar cell processing sequence for cast-mono Si wafers. Figure 3: Laser Scanning Microscopy images (130 x 129 µm²) for (left) and (right) cm-si wafers textured by. cm-si wafers with well aligned crystal orientations offer superior low surface reflectance. Decreasing shares of quasi-mono areas result in inhomogeneous standard alkaline surface texturing, Table. Table : Averaged surface reflectance (AMG 1.5: nm) for standard textured cm-si wafers % 26.5 % 27.1 % (0.6 %) (0.8 %) (0.3 %) 25.1 % 17.8 % 12.1 % (3.5 %) (7.0 %) (1.2 %)

3 Neither standard acidic nor standard alkaline texturing techniques meet all aspects of cast-mono silicon materials. n particular and silicon surfaces are susceptible to local alkaline induced crystal polishing (Refl. > 31 %). These highly reflective surface areas, representative for inhomogeneous texturing as well as unacceptable high averaged surface reflectance have to be addressed by novel wet chemical etching solutions. cast-mono texturing techniques & mitigate the discussed ambivalent crystal surface situation. n Table a summary of effective surface reflectance results for the two cast-mono texturing generations is given. Table : Averaged surface reflectance (AMG 1.5: nm) for cast-mono & textured cm-si wafers % 16.9 % 12.2 % (2.8 %) (6.6 %) (1.6 %) 25.7 % 17.9 % 15.3 % (1.7 %) (4.8 %) (0.9 %) The averaged surface reflectance is preserved while variation of overall surface texturing is decreased. cast-mono texturing techniques & enhance the formation of texturing pyramids thereby impeding the production of highly reflective shingle-like structures to a large extent, Figure 4. The standard solar cell processing sequence was controlled by a cz-si reference group textured by solutions. Relevant solar cell data are given in Table V. Table V: Averaged solar cell data for cz-si wafers textured by. Standard deviations are given in brackets (0.1) (0.1) (0.5) (0.3) Studied solar cell efficiencies generally respond to the cast-mono silicon material classification. Highest values are obtained for cm-si wafers, while Hybride and groups exhibit a negative gap of %. These crystallization induced solar cell performance differences can be controlled by adaption of wet chemical texturing processes, Table V. Table V: Averaged solar cell efficiency () compared for wet chemical textured cm-si wafers (0.1) (0.3) (0.2) (0.1) (0.4) (0.2) (0.1) (0.4) (0.3) Figure 4: Laser Scanning Microscopy images (130 x 129 µm²) for cm-si wafers textured by cast-mono (left) and (right). cast-mono texturing solutions & balance the exceptional surface nature of and Hybrid mono-like silicon materials. Homogeneous distribution of texturing pyramids is ensured for cm-si wafers. 3.4 Solar cell data for cast-mono Si wafers The impact of standard and cast-mono texturing qualities on solar cell performance is investigated according to Figure 1. Within each silicon group intrinsic material effects are equivalently distributed due to statistical substrate mixing (0.1) (0.4) (0.2) cast-mono texturing solutions enable an absolute efficiency gain of up to 0.2 % compared to standard alkaline texturing ( ). This result is valid for all material classes under investigation. The relative efficiency advantage compared to standard acidic texturing ( ) is highest for cm-si wafers (+ 0.5 %). For cm-si materials is decreased to % due to effective surface texturing for increasing crystal misalignment. Light harvesting qualities of crystalline silicon surfaces are enhanced by wet chemical texturing processes. Short circuit currents () for cm-si wafers closely follow the specific reflectance characteristics for each texturing solution under investigation. Crystallization related material qualities have a minor influence on values as can be shown from the results for standard acidic texturing solutions, Table V.

4 Table V: Averaged short circuit curent () compared for wet chemical textured cm-si wafers. Table V: Averaged open current voltage () compared for wet chemical textured cm-si wafers (0.2) (0.3) (0.2) (0.7) (5.1) (0.7) (0.1) (0.5) (0.2) (0.7) (3.9) (1.7) (0.2) (0.5) (0.2) (0.6) (4.1) (1.5) (0.1) (0.4) (0.2) (0.6) (4.5) (1.0) The advantageous level of cast-mono & texturing compared to standard etching solutions is particularly valid for decreasing quasi-mono area shares, i.e. for and cm-si substrates. For Premium cast-mono Si materials the level of superior solution is matched by cast-mono texturing technique. Recombination of minority charge carriers in the material bulk, the space charge region and at the surface of solar cells is characterized by open current voltage (). Surface passivation by PECVD-SiNx was adapted according to the best known method for each produced texturing structure. The and Low-Grade cm-si solar cells follow the expected material classification. On the other hand cm-si wafers exhibit exceptional high losses assignable to silicon feedstock and crystallization effects. Recently the density of crystal dislocations, e.g. sub-grain boundaries, and ineffective impurity segregation was shown to be crucial quality parameters for cast-mono silicon materials (8). Highly homogeneous standard acidic texturing indicates superior high values for all cast-mono Si substrate classes. This is assigned to the established smoothness of isotropic acidic etching features. cast-mono texturing results show comparable high values for and Lowgrade cm-si materials. n contrast standard alkaline texturing solutions exhibit lowest relative levels independent from cm- Si wafer classification, Table V. This result is attributed to the increased surface area of pyramide texturing features. The Fill Factor () is characteristic for the electrical related performance loss of solar cells including parallel, serial and contact resistance. Metallization and surface contacting factors were adapted according to the best known method for each produced texturing structure. For standard acidic texturing the impact of Si material classification can be neglected. solutions produce state-of-the-art smoothened surfaces beneficial in terms of standard solar cell contacting. On the other hand for and cast-mono texturing solutions values are systematically reduced for increasing quasi-mono area shares. n contrast cast-mono texturing is able to balance the ambivalent surface features thereby preventing electrical performance losses, Table V. Table V: Averaged Fill Factor () compared for wet chemical textured cm-si wafers. Standard deviations are given in brackets (0.2) (0.2) (0.5) (0.2) (0.4) (0.5) (0.1) (0.4) (0.6) (0.3) (0.3) (0.5)

5 3.5 Encapsulation of cast-mono Si solar cells n addition to solar cell data the encapsulation effects are highly relevant for photovoltaic applications. Esthetical requirements for solar modules are addressed to a large extent by the improved surface homogeneity of cast-mono texturing & techniques. The characterization and encapsulation of cast-mono Si solar cells was conducted according to Figure 5. Solar cell characterization V-Measurement Contact formation Spectral Response 4 SUMMARY For the first time systematic studies of novel cast-mono Si materials are presented in respect to their specific wet chemical texturing characteristics. The interpretation and discussion includes standard solar cell processing sequences and obtained data for solar cells and solar modules. Relevant Cost-of-Ownership (CoO) calculations reveal a significant advantage for cast-mono texturing & techniques compared to standard alkaline solutions. However, in terms of low-cost manufacturing provides the state-of-the-art silicon surface structuring step for standard solar cell manufacturing. Hence, the potential for significant cost reductions in wet chemical processing is indicated, Table X. Table X: Calculated CoO compared for wet chemical textured cm-si wafers. Front-side soldering Back-side solderding EVA encapsulation Front-side: structured glass CoO [$/Wp] CoO [$/Wp] CoO [$/Wp] Back-side: TEDLAR sheet The disclosed results contribute to the prospective proliferation of cast-mono technologies in silicon based photovoltaics. 4.1 Conclusions on cast-mono Si crystallization Solar "mini-module" characterization V-Measurement Spectral Response Figure 5: Solar mini-module processing sequence for cast-mono Si solar cells. Solar cell encapsulation effects increase electrical resistances leading to a deterioration of values by - 3 rel.%. Reflectance and EQE measurements indicate a significant impact of surface texturing qualities. n particular surface areas textured by standard alkaline solutions exhibit increased reflectance at low wavelengths ( nm) after encapsulation. On the other hand solar modules textured by or cast-mono texturing techniques point to decreased surface reflectance for low and high wavelength ranges ( nm, nm). EQE values are consequently improved for and cast-mono techniques, while QE is dominated by material and processing characteristics. The positive impacts of encapsulation effects are reflected by increased solar module data for these two texturing strategies: : rel.%, : rel.% and : rel.%. The breaching of market entry barriers like the constant pressure for reduction of manufacturing costs and satisfaction of high quality demands is crucial for the success of new technologies. n this respect cast-mono or mono-like silicon feedstock crystallization needs to continue its promising evolution. n particular reduction of cycle time, reuse of the seed-layer, effective impurity segregation and advanced control of crystal/liquid interphases need to be addressed. n addition to the important increase of absolute quasimono share per ingot stress relaxation to prevent subgrain formation is strongly necessary (9), (10). 4.2 Conclusions on cast-mono Si solar cell processing Wet chemical texturing techniques meet the demand for high throughput, superior stable and certified processing qualities. cast-mono texturing solutions provide costeffective techniques for saw-damage removal and surface reflectance control. The ambivalent texturing characteristics are moderated resulting in improved surface homogeneity and enhanced light harvesting conditions. Requests of standard solar cell processing are fulfilled leading to an absolute efficiency advantage of 0.2 % or 0.5 %.

6 This is in comparison to standard alkaline or standard acidic texturing, respectively. The success of cast-mono or mono-like silicon materials can be driven by adaptable processing sequences. The quality to address the multitude of different crystal orientations is a key factor and therefore a necessary characteristic for versatile wet chemical texturing techniques. 5 REFERENCES (1) D. Richard, Photon nt. Ed. December (2011) (2) S.K. Chunduri, Photon nt. Ed. June (2011) (3) H. Zhang et al., 21 st Workshop on Crystalline Silicon Solar Cell & Modules (2011) Breckenridge (USA). (4) (5) S.K. Chunduri, Photon nt. Ed. June (2012) (6) Z. Zheng, 22 nd Workshop on Crystalline Silicon Solar Cells & Modules (2012) Vail (USA). (7) L. Zhou et al., Future Photovoltaics August (2012) (8) B. Marie et al., Proceedings of the 26 th EU-PVSEC (2011) (9) A. Jouini et al., 4 th nternational Workshop on Crystalline Silicon Solar Cells (2010) Taipei (Taiwan). (10) C.W. Lang et al., 22 nd Workshop on Crystalline Silicon Solar Cells & Modules (2012) Vail (USA).