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The 9th edition of the Workshop on Metallization and Interconnection for Crystalline Silicon Solar Cells was held as an online event but nevertheless reached the workshop goals of knowledge sharing and networking. The technology of screen-printed contacts of high temperature pastes continues its fast progress enabled by better understanding of the phenomena taking place during printing and firing, and progress in materials. Great improvements were also achieved in low temperature paste printing and plated metallization. In the field of interconnection, progress was reported on multiwire approaches, electrically conductive adhesives and on foil-based approaches. Common to many contributions at the workshop was the use of advanced laser processes to improve performance or throughput.

In this study we analyze the properties of silicone elastomers used in the fabrication of PV modules in the early 1980's, which were in operation outdoors in a semi-tropical climate for more than 20 years. We observe that the silicone materials have very similar properties to recent, freshly cured silicone. The information gathered gives evidence that silicone elastomers undergo only very limited degradation after years of exposure in the field in operating PV modules. A moderate decrease in elastic modulus was observed, but further studies are needed to determine whether this effect is real. The very limited degradation of silicone encapsulant is consistent with the low performance decrease reported in other studies.

This paper discusses some challenges that need to be tackled when designing a photovoltaic module using a shingled cells structure. We derive a simple analytical model to determine the conditions needed to avoid interconnection joint failure. It is found that interconnection materials with a low ratio of shear modulus G over shear strength τsh. str. is preferred for good interconnection joints reliability. As a result, solder joints appear inappropriate for the application, while electrically conductive adhesives (ECA) with low G/ τsh. str can better fulfill the requirements. An interconnection approach is also proposed which makes use of a combination of adjacent ECA and a non-conductive adhesive materials in a shingled configuration to help achieve string robustness and reliability.

The 6 th Metallization Workshop took place in Constance, Germany on 2 and 3 May 2016. At the workshop the latest progress in the understanding and application of metallization and interconnection was presented. Screen printed metallization continues to dominate. Material and application technologies are constantly further improved, with sub-40 μm fingers with high cell performance and low Ag consumption demonstrated. Cu plating technology is further perfected in anticipation of large scale industrial implementation, with improvements on adhesion and long term reliability. In interconnection, alternatives to the traditional ribbon soldering technology are proposed. Among them, the multi-wires interconnection schemes are shown to have a dramatic impact on metallization design and technology.

Here we demonstrate the influence of firing temperatures on the electronic properties of Atmospheric Pressure Plasma Liquid Deposition (APPLD) silicon dioxide films due to reformed material composition and its overall impact on surface passivation quality. Experimentally extracted electronic parameters using electrical capacitance-voltage-conductance (C-V-G) measurements on a Metal-Oxide-Semiconductor (MOS) structure reveal that films fired at 810 °C show a slightly higher concentration of negative fixed charges (− Qf) and interface trap charges (Dit) compared to films fired at 940 °C. Such a dependency on the firing temperature can be attributed to variation in the net concentrations of silanol and carbon groups within the films, subsequently influencing the type of passivation mechanism involved. We show that for films fired at 810 and 940 °C, the predominant passivation mechanisms are related to field effect induced by excess silanol groups and chemical passivation due to the absence of electrically active carbon related defects, respectively. Additionally, dielectric constant (K) extraction from C-V measurements at 1 kHz returns an almost 2-fold higher K-value for films fired at 810 °C (K ~ 21) compared to films fired at 940 °C (K ~ 12), due to excess silanol concentration in the former films.

This article gives a summary of the 8th Metallization and Interconnection workshop and attempts to place each contribution in the appropriate context. The field of metallization and interconnection continues to progress at a very fast pace. Several printing techniques can now achieve linewidths below 20 µm. Screen printing is more than ever the dominating metallization technology in the industry, with finger widths of 45 µm in routine mass production and values below 20 µm in the lab. Plating technology is also being improved, particularly through the development of lower cost patterning techniques. Interconnection technology is changing fast, with introduction in mass production of multiwire and shingled cells technologies. New models and characterization techniques are being introduced to study and understand in detail these new interconnection technologies.

This paper summarizes the trends in metallization and interconnection technology in the eyes of the participants of the 8th Metallization and Interconnection Workshop. Participants were asked in a questionnaire to share their view on the future development of metallization technology, the kind of metal used for front side metallization and the future development of interconnection technology. The continuous improvement of the screen-printing technology is reflected in the high expected percentage share decreasing from 88% in three years to still 70% in ten years. The dominating front side metal in the view of the participants will be silver with an expected percentage share of nearly 70% in 2029. Regarding interconnection technologies, the experts of the workshop expect new technologies to gain significant technology shares faster. Whereas in three years soldering on busbars is expected to dominate with a percentage share of 71% it will drop in ten years to 35% in the eyes of the participants. Multiwire and shingling technologies are seen to have the highest potential with expected percentage shares of 33% (multiwire) and 16% (shingling) in ten years.

We report the use of a screen-printable copper paste to form the front busbars on high efficiency photovoltaic cell structures in combination with copper fingers formed by light induced plating. Such a process route offers economic benefits relative to the fully plated front metallization due to the reduced requirement for laser ablation and increased cell performance. We demonstrate improved open circuit voltage and fill factor compared to cells that used plating for both the fingers and the busbars. These result from reduced contact between the semiconductor and the metal and from reduced shunting compared to the laser ablated and plated electrodes. p-type ‘PERC’ cells have been fabricated with copper plated fingers and screen-printed copper busbars with a median cell efficiency of 20.4%, compared to 20.3% for those with fully plated busbars. n-type ‘PERT’ cells reached a cell efficiency of 20.9% for both front metallization schemes. The screen-printed busbars gave a ∼5 mV advantage over the plated busbars in both cases.

Attaching interconnection ribbons to solar cells using electrically conductive adhesives is an attractive alternative to soldering, particularly if it can be achieved on cells without busbars. We report on ribbon bonding tests on busbarless PERC cells using a silicone-based electrically conductive adhesive, comparing the results with those of soldered control samples. The ribbon-bonded samples lead to somewhat lower performance, but promising results have nevertheless been obtained that indicate that equivalent performance could be reached with better process control. Proof-of-concepts of innovative structures combining electrically conductive adhesives and non-conductive adhesives are demonstrated, with the potential to yield high fill factors, moderate material consumption and easier application.

For the seventh-time experts and specialists from all over the world discussed the latest status, trends and new directions in the field of metallization and interconnection for crystalline silicon solar cells on October 23rd and 24th, 2017, in Constance, Germany. The first Workshop on Metallization for Crystalline Silicon Solar Cells was held in 2008 in Utrecht, The Netherlands to provide a forum for metallization specialists and was followed by Workshops in Constance, Germany, 2010; in Charleroi, Belgium, 2011; and again, in Constance in 2013, 2014 and 2016. In 2016, the Workshop was renamed as Workshop on Metallization and Interconnection for Crystalline Silicon Solar Cells to reflect the growing importance of interconnection. This year, about 151 participants from 17 countries all over the world came to discuss the results of 35 presentations and participate to panel discussions. The presentations are available on www.metallizationworkshop.info as pdf documents. These proceedings contain peer-reviewed papers relating to some of the workshop contributions. During the Workshop, the participants filled in a questionnaire about their views on the future of metallization and interconnection. In these proceedings, we also show the results of the 7th edition questionnaire plotted together with results from previous editions and we discuss general trends.

As in the previous workshop editions the participants of the 7th Metallization and Interconnection Workshop were asked to share their view on the future of metallization and interconnection. In this paper results of the survey are summarized. The trend of the past years is reflected. Established technologies like screen-printing or soldering on busbars are seen to dominate for the next years.

A scenario for future silicon photovoltaics is to use wafer-based solar cells with a thickness below 100 μm. A way to realize this scenario is the merging of cell and module processing, with thin wafers processed while attached to the superstrate glass. One of the challenges for this type of processing is the achievement of high performing surface passivation, i.e., with surface recombination velocities below 10 cm/s. In this paper, a detailed explanation for lifetime degradation on wafers bonded to glass by means of silicone is proposed. The degradation is due to cyclic silicone molecules outgassing during a-Si:H deposition from the adhesive used to bond the wafers. The cyclic molecules are incorporated in the a-Si:H layer and modify the amorphous silicon network. By the application of specific outgassing conditions before a-Si:H deposition, a significant amount of cyclic molecules is removed from the adhesive. Non-degraded a-Si:H layers and surface passivation comparable to standalone wafers are obtained, as shown with measures of lifetime and solar cell open circuit voltage.

A possible scenario for wafer-based silicon photovoltaics is the processing of solar modules starting from thin silicon wafers bonded to glass. However, interactions between the adhesive used for bonding and the solar cell processing can affect the surface passivation of the bonded wafer and decrease cell performances. A method that suppresses these interactions and leads to state-of-the-art a-Si:H surface passivation is presented in this Letter. The method is based on an increase of the surface cross-linking of a silicone adhesive by means of an O2 plasma and it is successfully tested on three different silicones. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A new process for deposition of silicon oxide films with excellent passivation properties was developed using an atmospheric pressure plasma reactor. This process consists of fast deposition at room temperature of a SiCxOyHz film followed by a rapid thermal anneal in air (similar treatment to a contact firing step) to convert it to a dense inorganic SiOx material. The material formed using this process shows improved passivation compared to low pressure PECVD films. The firing process and more particularly the firing temperature appears to play a critical role in passivation performance, and an optimum temperature was identified. Capacitance-Voltage measurements on a MOS structure show that the oxide layer has a very low Dit value with fixed negative charges, which has not been reported before for thick silicon oxide. This uniqueness is attributed to measured over-stoichiometry in oxygen in the dense film owing to the presence of bulk silanols. These films were successfully incorporated in PERC solar cells with cells showing efficiencies up to 19.7%.

In order to relax the mechanical constraints of processing thin crystalline Si wafers into highly efficient solar cells, we propose a process sequence, where a significant part of the process is done on module level. The device structure is an interdigitated-back-contact cell with an amorphous silicon back surface field. The record cell reaches an independently confirmed efficiency of 18.4%. Although the device deserves further optimization, the result shows the compatibility of processing on glass with efficiencies exceeding 18%, which opens the door to a high-efficiency solar cell process where the potentially thin wafer is attached to a foreign carrier during the full processing sequence.

The 4th Metallization Workshop held in May 2013 in Constance, Germany, enabled experts in metallization for crystalline silicon solar cells to obtain a clear view on the status of the technology, as well as to exchange and generate new ideas and insights. From the contributions on the workshop, it was clear that the traditional metallization technique of screenprinting Ag paste has been improved in a dramatic way over the last two years, accelerating the decrease of Ag consumption per cell while improving solar cell efficiency. This was achieved through enhanced understanding of screenprinted contacts, improving Ag pastes and evolutionary modifications to the screenprinting technique. Alternatives to screenprinting, including electroplating of Ni and Cu contacts, also continue to progress, though not quite at the same impressive rate of improvement as Ag printing.

A new silicone-based Electrically Conductive Adhesive (ECA) has been developed for the Metal Wrap-Through module technology. The ECA has a Ag filler content below 60% but maintains a conductivity after curing below 1mOhmcm. It is flexible over a wide range of temperatures, which enables stress-relief and is beneficial for durability. Four-cells mini-modules have been made using the newly developed silicone ECA and show excellent performance and durability. The mini-modules did not degrade after 1000hours in damp heat conditions, nor after 200 thermal cycles.

The participants of the 4th metallization workshop 2013 in Konstanz were asked to share their opinion on the future of metallization for crystalline silicon solar cells like during the previous three metallization workshop in Utrecht (2008), Konstanz (2010) and Charleroi (2011). The question “What is the future of metallization?” is one of the most important questions for the success of crystalline silicon solar cells in future. During the consecutive workshops between 71% and 75% of in average 170 participants have responded to the questionnaires. The participants represent the institutes/universities, material and equipment manufacturers and cell manufacturers. The results are presented in this document.

Solar modules made from thin-film crystalline-silicon layers of high quality on glass substrates could lower the price of photovoltaic electricity substantially. One way to create crystalline-silicon thin films on non-silicon substrates is to use the so-called “seed layer approach”, in which a thin crystalline-silicon seed layer is first created, followed by epitaxial thickening of this seed layer. In this paper, we present the first solar cell results obtained on 10-μm-thick monocrystalline-silicon (mono-Si) layers obtained by a seed layer approach on transparent glass-ceramic substrates. The seed layers were made using implant-induced separation and anodic bonding. These layers were then epitaxially thickened by thermal CVD. Simple solar cell structures without integrated light trapping features showed efficiencies of up to 7.5%. Compared to polycrystalline-silicon layers made by aluminum-induced crystallization of amorphous silicon and thermal CVD, the mono-Si layers have a much higher bulk diffusion lifetime.

We report on the beneficial use of embedded segmented porous silicon broad-band optical reflectors for thin-film epitaxial silicon solar cells. These reflectors are formed by gradual increase of the spatial period between the layer segments, allowing for an enhanced absorption of low energy photons in the epitaxial layer. By combining these reflectors with well-established solar cell processing by photolithography, a conversion efficiency of 15·2% was reached on 73 cm2 area, highly doped offspec multicrystalline silicon substrates. The corresponding photogenerated current densities (Jsc) were well above 31 mA/cm2 for an active layer of only 20 µm. Copyright © 2010 John Wiley & Sons, Ltd.

In this work, we study the gettering properties of a porous silicon (Psi) intermediate reflector. This type of reflector is used currently in the thin film epitaxial silicon solar cell approach. The porous layers are electrochemically etched into low-cost, highly contaminated UMG (Upgraded Metallurgical Grade) Si in order to investigate the influence of impurity diffusion to the active layer due to the high thermal budget during epitaxial growth. When implemented into a solar cell, a porous silicon (Psi) multilayer stack consisting of layers with alternating porosities can serve as a gettering site for impurities that originate from the substrate. This statement is confirmed in our present work by several techniques such as SIMS (Secondary Ion Mass Spectrometry) and TXRF (Total Reflection X-Ray Fluorescence). Additionally to these experiments, solar cell results are presented, for which the active layer is grown with two different growth rates on top of porous silicon (Psi) multilayer stack and on two different types of multicrystalline silicon substrates: UMG (Upgraded Metallurgical Grade) and OFFSPEC (Out-of spec silicon material treated as a waste from Integrated Circuits industry). (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

To answer the challenge of obtaining thinner silicon solar cells, we developed a technique based on the transfer of a thin layer of high-quality monocrystalline silicon to a foreign low-cost substrate. The transfer is enabled by the reorganisation of macroporous silicon at high temperature: driven by the minimisation of their surface energy, pores close and, in specific situations, merge to form a large single void, a few micrometers under the wafer surface. As demonstrated in this paper, the micron-thick film above it may be detached and used to process a solar cell. The difficulty of this approach lies in appropriately ordering the macropores, while its originality is to enable processing of a high-quality material directly from porous silicon. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

To answer the challenge of less expensive renewable electricity, the photovoltaics community is focusing on producing thinner silicon solar cells. A few years ago, in the field of silicon-on-nothing structures, micron-thick monocrystalline layers suspended over their parent wafer were produced by high-temperature annealing of specific arrays of macropores. Those macropores reorganise into one single void and leave a thin overlayer on top. Since this method may be an inexpensive way of fabricating high-quality silicon films, this paper investigates its potential for photovoltaic applications. In particular, we investigated if large surfaces can be produced and transferred to foreign substrates with this method. We fabricated basic solar cells, without rear-surface passivation, on 5 cm × 5 cm-large and 1-μm-thick films transferred to glass, that showed energy-conversion efficiencies up to 2.6%. These cells demonstrate the feasibility of the presented concept as a layer-transfer process for solar-cell application. After formation by annealing, the film is only barely attached to its parent wafer, but can still safely be handled provided that any abrupt gas flow or pumping to vacuum is avoided. After transfer and permanent bonding, the sample can be handled as any bulk wafer.

Thin-film polycrystalline-silicon (pc-Si) solar cells with a grain size in the order of 1–100 μm could substantially lower the price of photovoltaic energy if sufficiently high efficiencies are obtained on low-cost foreign substrates. A promising approach is the epitaxial thickening of seed layers made by aluminum-induced crystallization (AIC) of amorphous silicon. A clear difference in performance was found for AIC based solar cells grown by low- and high-temperature epitaxy. A large intragrain defect density was observed for this type of pc-Si solar cells that probably limits the cell performance. This paper studies intragrain defect formation in AIC based pc-Si layers by the investigation of state of the art absorber layers grown both by low- and high-temperature epitaxy on seed layers made on various substrates. The samples were characterized by defect etching in combination with scanning electron microscopy (SEM) and by cross-section transmission electron microscopy (TEM). The AIC seed layers themselves were found to be major sources of intragrain defects. Moreover, AIC seed layers prepared under different conditions can lead to different intragrain defect densities.

Thin layers of silicon for solar cells can be lifted-off from wafers by taking advantage of the property of macropores to reorganise at high temperature and, in specific conditions, to form a high-quality layer on top of an empty space. Those conditions need to be identified and this work focuses in particular on the annealing ambient composition. Its influence was investigated by scanning and transmission electron microscopy and by Fourier-transform infrared spectroscopy. Apart from hydrogen, which has been used in previous studies, argon was found to be also compatible with the reorganisation. This result points out that the presence of oxygen impurities may not be as deleterious as expected, but that those impurities complicate the set of reactions taking place during the high-temperature process.

Thin films of polysilicon are an attractive material to reduce the cost of photovoltaic energy. Among the different polysilicon techniques, fine-grained polysilicon deposited directly onto a foreign substrate is an interesting option, since no extra seed layer or crystallization step is needed. However, the fabrication of efficient fine-grained polysilicon solar cells is a real challenge due to the large number of defects and the small average grain size of only 0.2 µm. This paper reports on our recent progress with fine-grained polysilicon solar cells. Using a diffuse refracting top surface and a more efficient two-step hydrogenation, we obtained an efficiency of 5.0%.

Thin-film polycrystalline-silicon solar cells might become an alternative to bulk silicon solar cells if sufficiently high efficiencies can be obtained. In this work we made pc-Si layers using aluminium-induced crystallization and thermal CVD on alumina substrates. By using plasma texturing and optimizing the cell structure, we increased the current density of our cells and achieved a cell efficiency of 8.0%. At present, our cell efficiency seems to be mainly limited by the presence in our layers of a high density of electronically active intragrain defects. Intragrain quality improvement will therefore be very important to further increase our pc-Si cell efficiency.

Proceedings on Advanced Materials and Concepts for Photovoltaics EMRS 2007 Conference, Strasbourg, France; International audience; Among the methods for enlarging the grain size of polycrystalline silicon (poly-Si) thin films, aluminium induced crystallization (AIC) of amorphous silicon is considered to be a very promising approach. In the AIC process, a thin a-Si layer on top of an aluminium layer crystallizes at temperatures well below the eutectic temperature of the Al/Si system (Teu = 577 °C). By means of electron backscattering diffraction (EBSD), we have mainly studied the effect of the aluminium layer quality varying the deposition system on the grain size, the defects and the preferential crystallographic orientation. We have found a strong correlation between the mean grain size and the size distribution with the Al deposition system and the surface quality. Furthermore, we show for the first time that more than 50% of the surface of the AIC films grown on alumina substrates are (103) preferentially oriented, instead of the commonly observed (100) preferential orientation. This may have important consequences for epitaxial thickening of the AIC layer into polysilicon absorber layers for solar cells.

Polycrystalline silicon (grain size ~ 0.1–100 μm) solar cells on foreign substrates are a promising approach for the next generation silicon solar cells. Aluminum-induced crystallization AIC in combination with epitaxy is a possible way to obtain such absorber layers. It is believed that Si islands present on the surface of AIC seed layers have a negative effect on the epitaxy. The removal of these islands could therefore lead to an increased absorber layer quality and solar cell performance. In this paper, we present a selective island removal procedure based on the Al layer already present after AIC annealing. By selecting an etchant which removes Si at least as fast as Al (in this paper plasma etching using SF 6 ), the Al layer acts as a perfectly aligned etching mask for the fully developed islands.

One of the main challenges in the ongoing development of thin film crystalline silicon solar cells on a supporting silicon substrate is the implementation of a long-wavelength reflector at the interface between the epitaxial layer and the substrate. IMEC has developed such a reflector based on electrochemical anodization of silicon to create a multi-layer porous silicon stack with alternating high and low porosity layers. This innovation results in a 1–2% absolute increase in efficiency for screenprinted epitaxial cells with a record of 13� 8%. To reach a better understanding of the reflector and to aid in its continued optimization, several extensive optical simulations have been performed using an in-house-developed optical software programme. This software is written as a Microsoft Excel workbook to make use of its user-friendliness and modular structure. It can handle up to 15 individual dielectric layers and is used to determine the influence of the number and the sequence of the layers on the internal reflection. A sensitivity analysis is also presented. A study of the angle at which the light strikes the reflector shows separate regions in the physical working of the reflector which include a region where the Bragg effect is dominant as well as a region where total internal reflection plays the largest role. The existence of these regions is proved using reflection measurements. Based on these findings, an estimate is made for the achievable current gain with an ideal reflector and the potential of epitaxial silicon solar cells is determined. Copyright # 2008 John Wiley & Sons, Ltd.

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.

Material limitations of wafer-based silicon (Si) cell technology and efficiency limitations of thin film solar cell technologies need to be overcome in order to have affordable PV technology. New concepts that strive for better utilization of the sun's spectrum, hence better cell efficiency, are under development. These include multi-junction solar cells, intermediate band solar cells, cells utilizing impurity levels, quantum well and quantum dot solar cells, hot carrier cells and solar cell concepts incorporating spectrum conversion. In multi-junction solar cells, better spectrum utilization is obtained by stacking several solar cells. A record efficiency of over 39% has been achieved under 236 times the sun's light concentration. Analysis of impurity of photovoltaic and intermediate band solar cells has shown that theoretical cell efficiency is about 63.2% for single impurity level and single intermediate band. Energy-shifting of UV photons has been demonstrated by fabricating Si nanocrystals in SiO x and SiN x matrices. Proof of concept has been shown for quantum confinement in Si quantum dots and enhanced photon absorption using metal nanoparticles on the solar cell surface. Similarly, most of the advanced concepts are in the initial stage of experimentation. Significant breakthroughs are required before these concepts can contribute to mainstream PV production.

Thin film epitaxial silicon solar cells are considered a near future alternative to bulk silicon solar cells. However due to the limited thickness of the active layer they require efficient light trapping. Therefore we propose the development and implementation of such light confinement by means of a porous silicon (PS) intermediate reflector at the epi/substrate interface. The formation of the reflector is done by electrochemical etching of a highly doped Si substrate into a multilayer stack (Bragg-optical reflector), and is followed by epitaxial deposition of the active layer. The implementation of the PS reflector however requires detailed analysis of many problematic issues, foremost the optical optimisation of the stack for internal reflection at the Si/PS/Si interface. Other topics include the pore rearrangement during high-temperature CVD as well as the quality of the epitaxial layer grown on porous silicon. Another challenge is the resistance within the PS layers. For that purpose, SRP (Spreading Resistance Probe) and resistance measurements were performed to determine the conductive properties of rearranged PS. First cells with a 9-layer porous silicon reflector gave a very promising efficiency of 13.5% which is 1.5% higher compared to cells without internal reflector.

The growth of well-defined regular columnar macropores under front-side illumination on medium doped n-type silicon has been investigated. The various morphologies observed, with branched or columnar pores, differ from those reported so far on substrates of both higher and lower resistivities. Unlike stated by the macropore growth model, current density is found to influence the pore growth rate. These observations indicate that, under the conditions used in this work, the pore growth is at the border between meso- and macroporous mechanism. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thin-film solar cell technologies based on Si with a thickness of less than a few micrometers combine the low-cost potential of thin-film technologies with the advantages of Si as an abundantly available element in the earth's crust and a readily manufacturable material for photovoltaics (PVs). In recent years, several technologies have been developed that promise to take the performance of thin-film silicon PVs well beyond that of the currently established amorphous Si PV technology. Thin-film silicon, like no other thin-film material, is very effective in tandem and triple-junction solar cells. The research and development on thin crystalline silicon on foreign substrates can be divided into two different routes: a low-temperature route compatible with standard float glass or even plastic substrates, and a high-temperature route (>600°C). This article reviews the material properties and technological challenges of the different thin-film silicon PV materials.

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from polycrystalline-silicon (pc-Si) thin films on inexpensive substrates. We recently showed promising solar cell results using pc-Si layers obtained by aluminum-induced crystallization (AIC) of amorphous silicon in combination with thermal chemical vapor deposition (CVD). To obtain highly efficient pc-Si solar cells, however, the material quality has to be optimized and cell processes different from those applied for standard bulk-Si solar cells have to be developed. In this work, we present the different process steps that we recently developed to enhance the efficiency of pc-Si solar cells on alumina substrates made by AIC in combination with thermal CVD. Our present pc-Si solar cell process yields cells in substrate configuration with efficiencies so far of up to 8·0%. Spin-on oxides are used to smoothen the alumina substrate surface to enhance the electronic quality of the absorber layers. The cells have heterojunction emitters consisting of thin a-Si layers that yield much higher Voc values than classical diffused emitters. Base and emitter contacts are on top of the cell in interdigitated finger patterns, leading to fill factors above 70%. The front surface of the cells is plasma textured to increase the current density. Our present pc-Si solar cell efficiency of 8% together with the fast progression that we have made over the last few years indicate the large potential of pc-Si solar cells based on the AIC seed layer approach. Copyright © 2007 John Wiley & Sons, Ltd.

Rear side floating junction solar cells with localised contacts often show parasitic shunting losses. For p-type material, this is due to tunnelling that takes places between the passivating n + -type rear junction and the p + -type back surface field region underneath localised rear contacts. To avoid this, the rear metallisation and the passivation layer should electronically be separated. Alternatively, the (induced) doping concentration of these layers could be optimised. This paper discusses the constraints met upon incorporation of such lowly recombinative structures into a low-cost solar cell process on multi-crystalline Si (mc-Si) material.

Surface recombination velocities as low as 10 cm/s have been obtained by treated atomic layer deposition (ALD) of Al 2 O 3 layers on p-type CZ silicon wafers. Low surface recombination is achieved by means of field induced surface passivation due to a high density of negative charges stored at the interface. In comparison to a diffused back surface field, an external field source allows for higher band bending, that is, a better performance. While this process yields state of the art results, it is not suited for large-scale production. Preliminary results on an industrially viable, alternative process based on a pseudo-binary system containing Al 2 O 3 are presented, too. With this process, surface recombination velocities of 500–1000 cm/s have been attained on mc-Si wafers.

Optimization of plasma enhanced chemical vapor deposited hydrogenated silicon nitride (SiN x :H) towards bulk passivation of multi-crystalline silicon cells has been carried out for both low and high frequency (HF) plasma deposition. Experimental results showed that bulk passivation is not caused by hydrogen incorporation in the top silicon layer during deposition and subsequent diffusion towards the bulk during firing, but that it is released from the SiN x :H film. We demonstrate that the amount of passivation depends on the SiN x :H density and its resistance against etching in HF. Optimization of the density, varying deposition temperature and using hydrogen dilution resulted in an optimized passivation.

We present an improved process for free-standing monocrystalline silicon (FMS) thin film solar cells developed at IMEC. We demonstrate that a thick annealed porous layer, or the quasi-monocrystalline silicon (QMS) layer, incorporated into a two-side contacted thin film solar cell structure does not produce any considerable series resistance. The best cell so far exhibits a conversion efficiency of 12.6% with a fill factor of 75.7% for an active layer thickness of 20 μm. This cell process includes emitter formation by phosphorous diffusion, silicon nitride deposition for antireflection coating and front surface passivation, and 50-μm-thick annealed porous layer remained on the rear. Despite the expected small reduction in solar cell's efficiency, maintaining the porous layer in the device structure renders the processing and handling of the thin FMS film much easier, thus leading to a better yield and up-scalability. PC1D simulations show that a thick QMS layer can lessen the short circuit current density in some degree, depending on the active layer quality, thickness, optical confinement, etc., while the other cell parameters remain substantially unvaried; a 10% to 15% higher process yield is expected to be sufficient to reach a break-even between the two processes.

Thin-film polycrystalline-silicon solar cells offer a promising alternative to standard silicon solar cells. Until now, however, obtained efficiencies are too low to lead to a cost reduction in photovoltaics. To obtain cells with higher efficiencies, polycrystalline-silicon layers need large grains, good passivation and an optimized cell design. In this work we compare pc-Si solar cells with an amorphous silicon–crystalline silicon heterojunction emitter to cells with a diffused phosphorus emitter. In heterojunction cells, hydrogen passivation was carried out before emitter formation, which appears to make it more effective. Open-circuit voltages ( V oc ) were much higher for cells with a heterojunction emitter, reaching values up to 520 mV. A maximum efficiency of 5.3% was obtained on the heterojunction cells, while the homojunction cells led to substantially lower efficiencies. A cell concept based on a heterojunction emitter seems therefore most favorable to lead to highly efficient thin-film pc-Si solar cells. The high V oc and efficiency values obtained in this work form an important step towards cost-effective polycrystalline-silicon solar cells.

Thin-film polycrystalline-silicon (pc-Si) solar cells with a high efficiency could lower the price of photovoltaic electricity substantially. Efficient thin-film solar cells will not only lead to a cost reduction by the use of less silicon material, but will also reduce the module fabrication costs if a monolithic module process is used that integrates cell interconnection with cell contacting. Aluminium-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening recently proved to be a simple way to obtain large-grained pc-Si thin films with excellent properties for solar cells. However, cell processes different from those for bulk-Si cells have to be implemented to fully exploit the pc-Si material quality and to obtain working solar cells. In this work, we propose a simple monolithic module process for thin-film pc-Si solar cells, in which all contacts are on top of the cells in an interdigitated pattern. As a first step towards implementation of this process, we made single pc-Si solar cells with interdigitated top contacts, using pc-Si layers on ceramic substrates grown by AIC in combination with high-temperature epitaxy. Next, we made pc-Si modules using a simplified metallization scheme. The interdigitated pc-Si cells had much higher efficiencies than mesa cells with base contacts at the periphery of the cells due to a lower series resistance and a higher current density. The maximum obtained cell efficiency was 5.6%, which is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of Si was involved. First module results showed that good cell separation and isolation is crucial to obtain proper working modules. Our interdigitated cell results indicate that monolithic thin-film modules with interdigitated top contacts based on pc-Si layers made by AIC will most likely lead to high efficiencies.

Most types of thin-film solar cells imply a radical departure from the dominant bulk crystalline Si technology. This is not the case for epitaxial thin-film solar cells. In this technology, a high quality Si layer is deposited epitaxially on a low-cost Si substrate (e.g. cast Upgraded Metallurgical Grade silicon or high-throughput Si ribbons) and processed into a solar cell. This technology combines the well-known advantages of crystalline Si (high efficiency potential, stability and reliability) with a substantial cost reduction potential. Moreover, the similarity to the traditional Si technology lowers the threshold for adoption by the PV industry. This paper gives an overview of the field of epitaxial thin-film solar cells, covering substrates, deposition techniques and solar cell processing. Achievements reported in the literature are summarized and recent results are presented. Special attention is given to the crucial issue of achieving high currents by increasing absorbance in the active layer. This can be achieved by increased absorption through material engineering and through implementation of advanced light confinement schemes.

This study investigates the crystallographic properties of polycrystalline silicon films formed on doped (boron or phosphorus) or undoped silicate glass coated ceramic substrates using the chemical vapor deposition method at high temperatures (> 1000 °C). First, FT-IR analysis of the silicate oxide layers is presented in order to monitor the presence of the Si–O, B–O and/or P–O bonds in the layers prior to deposition. Then, the average grain size, crystalline orientation and boundary defects of these poly-Si films on silicate coated mullite and alumina were evaluated as a function of deposition parameters and boron/phosphorus content. The results show a significant increase of the grain size and narrower size distribution after CVD on BSG or PSG intermediate layers, whatever is the substrate. Large grains up to 7 μm are observed for 9 μm thick films. The enhancement in grain size is attributed to the nucleation change by defects or impurities rather due to the flowability of the silicate glass during the silicon deposition at high temperature. The open-circuit voltage measurements of these fine-grained poly-silicon films before hydrogenation shows a dependence on the grain size through the choice of the substrate. The open-circuit voltage increases substantially from 250 mV to 410 mV after 1 h plasma hydrogenation, despite partial etching of the emitter by the hydrogen atoms.

Thin-film silicon solar cells on low-cost foreign substrates could lead to a large cost reduction of photovoltaic electricity if sufficiently high efficiencies could be obtained. A possible approach is to make polycrystalline-silicon solar cells by epitaxial thickening of large-grained thin seed layers made by aluminium-induced crystallization (AIC) of silicon. Until now however, obtained efficiencies are too low to lead to the desired cost reduction. We report on the influence of the AIC seed layer morphology (grain size and presence/absence of secondary crystallites on top of the surface) on the epitaxial growth of absorber layers and on the resulting cell parameters. To increase the grain size of the seed layers, we investigated the use of a nitric acid treatment to oxidize the Al layers prior to the amorphous silicon deposition. We compared seed layers oxidized by nitric acid treatment to seed layers oxidized by a short exposure to the ambient air. The nitric acid treatment led to a larger grain size and strongly affected the structure of the secondary crystallites on most investigated samples. Removing these crystallites before epitaxial growth clearly led to larger grains and increased the open-circuit voltages (Voc) of our solar cells. Using the nitric acid treatment, absorber layers with grain diameters up to 50 μm were made, that reached efficiencies of 4.9%, with Voc values around 500 mV. However, seed layers that were made through oxidation by air exposure and had grain diameters below 12 μm led to similar Voc values and even higher efficiencies (5.3%). We believe that regions with very small grain size on the large-grained samples are responsible for the relative poor behaviour of the large-grained solar cells. Our results show that increasing the average grain size of AIC seed layers does not automatically lead to better solar cells.

Ever since the first publications by R.J. Schwartz in 1975, research into back-contact cells as an alternative to cells with a front and rear contact has remained a research topic. In the last decade, interest in back-contact cells has been growing and a gradual introduction to industrial applications is emerging. The goal of this review is to present a comprehensive summary of results obtained throughout the years. Back-contact cells are divided into three main classes: back-junction (BJ), emitter wrap-through (EWT) and metallisation wrap-through (MWT), each introduced as logical descendents from conventional solar cells. This deviation from the chronology of the developments is maintained during the discussion of technological results. In addition to progress on manufacturing these cells, aspects of cell modelling and module manufacturing are discussed and an outlook towards industrial implementation is presented. Copyright © 2005 John Wiley & Sons, Ltd.