Effect of low segregation coefficient on Ga‐doped multicrystalline silicon solar cell performance

High‐quality Ga‐doped ingots are grown in different casting furnaces at optimized growth parameters; 3·5 kg ingots exhibit normal distribution of diffusion lengths along their height with very high diffusion lengths at the center of the ingot. Effective lifetimes as high as 1·1 ms are realized in 10 Ω cm Ga‐doped wafers after proper P‐diffusion and hydrogen passivation. Average effective lifetimes above 400 µs are also realized after P‐diffusion and hydrogen passivation for Ga‐doped wafers cut from 75 kg ingot where the response to P‐diffusion and hydrogen passivation is pronounced. High effective lifetimes are realized over the whole ingot with minimum values of 20 µs at the top of the ingot, indicating the possible use of about 85% of the ingot for solar cell production. Conversion efficiencies above 15·5% were realized in utilizing more than 80% of the ingot. High efficiencies of about 16% were realized in wafers with resistivities higher than 5 Ω cm p ‐type multicrystalline silicon wafers. Copyright © 2005 John Wiley & Sons, Ltd.     

19% efficient n‐type Czochralski silicon solar cells with screen‐printed aluminium‐alloyed rear emitter

High and stable lifetimes recently reported for n‐type silicon materials are an important and promising prerequisite for innovative solar cells. To exploit the advantages of the excellent electrical properties of n‐type Si wafers for manufacturing simple and industrially feasible high‐efficiency solar cells, we focus on back junction n⁺np⁺ solar cells featuring an easy‐to‐fabricate full‐area screen‐printed aluminium‐alloyed rear p⁺ emitter. Independently confirmed record‐high efficiencies have been achieved on n‐type phosphorus‐doped Czochralski‐grown silicon material: 18·9% for laboratory‐type n⁺np⁺ solar cells (4 cm²) with shadow‐mask evaporated front contact grid and 17·0% for front and rear screen‐printed industrial‐type cells (100 cm²). The electrical cell parameters were found to be perfectly stable under illumination. Copyright © 2006 John Wiley & Sons, Ltd.     

n‐Type multicrystalline silicon wafers prepared from plasma torch refined upgraded metallurgical feedstock

n‐Type silicon wafers present some definite advantages for the photovoltaic industry, mainly due to the low capture cross sections of minority carriers for most metallic impurities. This peculiarity is beneficial for multicrystalline silicon (mc‐Si) wafers in which the interaction between crystallographic defects and impurities is the main source of recombination centres. Most importantly, this peculiarity could be of a great interest when mc‐Si ingots are produced directly from upgraded and purified metallurgical silicon feedstock. It is of a paramount importance to verify if the advantages of conventional n‐type silicon also characterizes n‐type wafers provided by a direct metallurgical route. It is found, in raw wafers, that minority carrier diffusion lengths are three times higher in n‐type than in p‐type wafers, when the wafers are cut from the same ingot, where the bottom is p‐type and the top is n‐type, due to the difference in the segregation coefficients of doping elements (boron and phosphorus). After different processing steps and gettering treatments the minority carrier diffusion lengths are always neatly larger in n‐type than in p‐type wafers The results confirm the interest for n‐type silicon. Copyright © 2009 John Wiley & Sons, Ltd.     

N‐type solar‐grade silicon purified via the metallurgical route: characterisation and fabrication of solar cells

This study focuses on the characterisation and the fabrication of solar cells using n‐type multicrystalline silicon purified via the metallurgical route. Electrical and chemical analyses were performed on wafers taken from several positions along the crystallised ingot. The impact of the fabrication processing steps was investigated via effective carrier lifetime measurements. Solar cells were processed, and their efficiencies were found to be dependent on the position of the wafer along the ingot height, that is, the wafer's resistivity. A maximum conversion efficiency of 15.0% was obtained on cells from the bottom part of the ingot. In this study, the minimum resistivity value of 0.4 Ω cm resistivity is given in order to reach adequate cell efficiency. Light‐soaking experiments were then performed on the fabricated cells. No significant variations of the cell performances were observed even after 110 h at 60 °C, meaning that the fabricated cells are stable under illumination. Copyright © 2012 John Wiley & Sons, Ltd.     

High‐efficiency solar cells on phosphorus gettered multicrystalline silicon substrates

Measurements of the dislocation density are compared with locally resolved measurements of carrier lifetime for p‐type multicrystalline silicon. A correlation between dislocation density and carrier recombination was found: high carrier lifetimes (>100 µs) were only measured in areas with low dislocation density (<10⁵ cm⁻²), in areas of high dislocation density (>10⁶ cm⁻²) relatively low lifetimes (<20 µs) were observed. In order to remove mobile impurities from the silicon, a phosphorus diffusion gettering process was applied. An increase of the carrier lifetime by about a factor of three was observed in lowly dislocated regions whereas in highly dislocated areas no gettering efficiency was observed. To test the effectiveness of the gettering in a solar cell manufacturing process, five different multicrystalline silicon materials from four manufacturers were phosphorus gettered. Base resistivity varied between 0·5 and 5 Ω cm for the boron‐ and gallium‐doped p‐type wafers which were used in this study. The high‐efficiency solar cell structure, which has led to the highest conversion efficiencies of multicrystalline silicon solar cells to date, was used to fabricate numerous solar cells with aperture areas of 1 and 4 cm². Efficiencies in the 20% range were achieved for all materials with an average value of 18%. Best efficiencies for 1 cm² (20·3%) and 4 cm² (19·8%) cells were achieved on 0·6 and 1·5 Ω cm, respectively. This proves that multicrystalline silicon of very different material specification can yield very high efficiencies if an appropriate cell process is applied. Copyright © 2006 John Wiley & Sons, Ltd.     

Very low surface recombination velocity of crystalline silicon passivated by phosphorus‐doped a‐SicxNy:H(n) alloys

Hydrogenated and phosphorus‐doped amorphous silicon carbonitride films (a‐SiC_xN_y:H(n)) were deposited by plasma‐enhanced chemical vapor deposition (PECVD) on crystalline silicon surface in order to explore surface passivation properties. Very silicon‐rich films yielded effective surface recombination velocities at 1 sun‐illumination as low as 3 cm s⁻¹ and 2 cm s⁻¹ on 1 Ω cm p‐ and n‐type crystalline silicon substrates, respectively. In order to use them as anti‐reflection coating, we increased alternatively either the carbon or nitrogen content of these films. Also, a combination of passivation and antireflective films was analyzed. Finally, we explored the passivation stability under high‐temperature steps. Copyright © 2007 John Wiley & Sons, Ltd.     

Fundamental boron–oxygen‐related carrier lifetime limit in mono‐ and multicrystalline silicon

Boron‐doped crystalline silicon is the most relevant material in today's solar cell production. Following the trend towards higher efficiencies, silicon substrate materials with high carrier lifetimes are becoming more and more important. In silicon with sufficiently low metal impurity concentrations, the carrier lifetime is ultimately limited by a metastable boron–oxygen‐related defect, which forms under minority‐carrier injection. We have analysed 49 different Czochralski‐grown silicon materials of numerous suppliers with various boron and oxygen concentrations. On the basis of our measured lifetime data, we have derived a universal empirical parameterisation predicting the stable carrier lifetime from the boron and oxygen content in the crystalline silicon material. For multicrystalline silicon it is shown that the predicted carrier lifetime can be regarded as a fundamental upper limit. Copyright © 2005 John Wiley & Sons, Ltd.     

On the mechanical strength of monocrystalline,multicrystalline and quasi‐monocrystalline silicon wafers: a four‐line bending test study

Quasi‐monocrystalline silicon wafers have appeared as a critical innovation in the PV industry, joining the most favorable characteristics of the conventional substrates: the higher solar cell efficiencies of monocrystalline Czochralski‐Si (Cz‐Si) wafers and the lower cost and the full square‐shape of the multicrystalline ones. However, the quasi‐monocrystalline ingot growth can lead to a different defect structure than the typical Cz‐Si process. Thus, the properties of the brand new quasi‐monocrystalline wafers, based on low and high crystal defect densities, have been for the first time studied from a mechanical point of view, comparing their strength with that of both Cz‐Si monocrystalline and typical multicrystalline materials. The study has been carried out employing the four line bending test and simulating them by means of FE models. For the analysis, failure stresses were fitted to a three‐parameter Weibull distribution. High mechanical strength was found in all the cases. However, the quasi‐monocrystalline wafers characterized by large density of bulk defects, due to the noticeable density of extended defects, showed lower fracture tensions. Copyright © 2013 John Wiley & Sons, Ltd.     

About the origin of low wafer performance and crystal defect generation on seed‐cast growth of industrial mono‐like silicon ingots

The era of the seed‐cast grown monocrystalline‐based silicon ingots is coming. Mono‐like, pseudomono or quasimono wafers are product labels that can be nowadays found in the market, as a critical innovation for the photovoltaic industry. They integrate some of the most favorable features of the conventional silicon substrates for solar cells, so far, such as the high solar cell efficiency offered by the monocrystalline Czochralski‐Si (Cz‐Si) wafers and the lower cost, high productivity and full square‐shape that characterize the well‐known multicrystalline casting growth method. Nevertheless, this innovative crystal growth approach still faces a number of mass scale problems that need to be resolved, in order to gain a deep, 100% reliable and worldwide market: (i) extended defects formation during the growth process; (ii) optimization of the seed recycling; and (iii) parts of the ingots giving low solar cells performance, which directly affect the production costs and yield of this approach. Therefore, this paper presents a series of casting crystal growth experiments and characterization studies from ingots, wafers and cells manufactured in an industrial approach, showing the main sources of crystal defect formation, impurity enrichment and potential consequences at solar cell level. The previously mentioned technological drawbacks are directly addressed, proposing industrial actions to pave the way of this new wafer technology to high efficiency solar cells. Copyright © 2012 John Wiley & Sons, Ltd.     

Improved micromorph solar cells by means of mixed‐phase n‐doped silicon oxide layers

A good light trapping scheme is necessary to improve the performance of amorphous/microcrystalline silicon tandem cells. This is generally achieved by using a highly reflective transparent conducting oxide/metal back contact plus an intermediate reflector between the component cells. In this work, the use of doped silicon oxide as alternative n‐layer in micromorph solar cells is proposed as a means to obtain high current values using a simple Ag back contact and no extra reflector between the component cells n‐doped silicon oxide layers with a wide range of optical and electrical properties have been prepared. The influence of different deposition regimes on the material properties has been studied. The main findings are the following: (i) when carbon dioxide is added to the gas mixture, sufficiently high hydrogen dilution is necessary to widen the transition region from highly conductive microcrystalline‐like films to amorphous material characterized by low electrical conductivity; (ii) lower refractive index values are found with lower deposition pressure. Optimal n‐doped silicon oxide layers have been used in both component cells of micromorph devices, adopting a simple Ag back contact. Higher current values for both cells are obtained in comparison with the values obtained using standard n‐doped microcrystalline silicon, whereas similar values of fill factor and open circuit voltage are measured. The current enhancement is particularly evident for the bottom cell, as revealed by the increased spectral response in the red/infrared region. The results prove the high potential of n‐doped silicon oxide as ideal reflector for thin‐film silicon solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Towards implementation of floating cast method for growing large‐scale high‐quality multicrystalline silicon ingot using designed double crucibles

We report on our attempt to scale up the floating cast method to grow high‐quality multicrystalline silicon ingot using specially designed double crucibles. The cross section of the grown ingot showed the large crystal grain size originating from initially formed dendrite crystals around the top of the melt. By using double crucibles, the residual melt was spontaneously removed from the inner crucible to the outer one at the final stage of the crystal growth. Performance of small‐scale solar cells revealed no significant changes between the top and the bottom part of the ingot except the edge of the bottom due to the non‐uniform removal of the melt. This suggests that strong contact of the ingot with the crucible was successfully avoided by the double crucibles. Therefore, the floating cast method combined with specially designed double crucibles is concluded to be feasible, which could be implemented to realize high‐quality multicrystalline silicon ingot for practical size wafers. Copyright © 2013 John Wiley & Sons, Ltd.     

Optical and electrical properties of laser texturing for high‐efficiency solar cells

A process using laser‐ablated pits to texture the front surface of monocrystalline and multicrystalline silicon solar cells is described. Optical and electrical characterization demonstrates that the technique performs as well as upright random pyramid texturing and causes no laser‐induced defects or laser shunting. Double‐sided buried contact solar cells fabricated with laser texture performed as well as those fabricated with upright random pyramid textures on 1 Ω cm, p type float‐zoned wafers. Copyright © 2006 John Wiley & Sons, Ltd.     

Low‐temperature formation of local Al contacts to a‐Si:H‐passivated Si wafers

We have passivated boron‐doped, low‐resistivity crystalline silicon wafers on both sides by a layer of intrinsic, amorphous silicon (a‐Si:H). Local aluminum contacts were subsequently evaporated through a shadow mask. Annealing at 210°C in air dissolved the a‐Si:H underneath the Al layer and reduces the contact resistivity from above 1 Ω cm² to 14·9 m Ω cm². The average surface recombination velocity is 124 cm/s for the annealed samples with 6% metallization fraction. In contrast to the metallized regions, no structural change is observed in the non‐metallized regions of the annealed a‐Si:H film, which has a recombination velocity of 48 cm/s before and after annealing. Copyright © 2004 John Wiley & Sons, Ltd.     

Screen‐printed epitaxial silicon thin‐film solar cells with 13·8% efficiency

Amidst the different silicon thin‐film systems, the epitaxial thin‐film solar cell represents an approach with interesting potential. Consisting of a thin active c‐Si layer grown epitaxially on top of a low‐quality c‐Si substrate, it can be implemented into solar cell production lines without major changes in the current industrial process sequences. Within this work, ∼30‐μm‐thick epitaxial layers on non‐textured and highly doped monocrystalline Czochralski (Cz) and multicrystalline (mc) Si substrates have been prepared by CVD. Confirmed efficiencies of 13·8% on Cz and 12·3% on mc‐Si substrates have been achieved by applying an industrial process scheme based on tube and in‐line phosphorus diffusion, as well as screen‐printed front and back contacts fired through a SiN_x anti‐reflection coating. An extensive solar cell characterisation, including infrared lock‐in thermography and spectral response measurements is presented. Copyright © 2003 John Wiley & Sons, Ltd.     

Realistic efficiency potential of next‐generation industrial Czochralski‐grown silicon solar cells after deactivation of the boron–oxygen‐related defect center

We measure carrier lifetimes of different Czochralski‐grown silicon (Cz‐Si) materials of various boron and oxygen concentrations and determine the maximum achievable lifetime after an optimized thermal treatment. We obtain very high and stable bulk lifetimes of several milliseconds, virtually eliminating the boron–oxygen (BO) defect complex, which previously limited the carrier lifetime in boron‐doped Cz‐Si materials after prolonged illumination. Based on these experimental results, we introduce a new parameterization of the bulk lifetime of B‐doped Cz‐Si after permanent deactivation of the BO center. Notably, we measure lifetimes up to 4 ms on 2‐Ωcm Cz‐Si wafers at an injection level of 1/10 of the doping concentration. Importantly, these high lifetime values can be reached within 10 and 20 s of BO deactivation treatment. A detailed analysis of the injection‐dependent lifetimes reveals that the lifetimes after permanent deactivation of the BO center can be well described by a single‐level recombination center characterized by an electron‐to‐hole capture cross‐section ratio of 12 and located in the middle of the silicon band gap. We implement the novel parameterization into a two‐dimensional device simulation of a passivated emitter and rear solar cell using technologically realistic cell parameters. The simulation reveals that based on current state‐of‐the‐art solar cell production technology, efficiencies reaching 22.1% are realistically achievable in the near future after complete deactivation of the BO center. Copyright © 2016 John Wiley & Sons, Ltd.     

Efficiency (>15%) for thin‐film epitaxial silicon solar cells on 70 cm2 area offspec silicon substrate using porous silicon segmented mirrors

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 (J_sc) were well above 31 mA/cm² for an active layer of only 20 µm. Copyright © 2010 John Wiley & Sons, Ltd.     

Boron‐doped selective silicon epitaxy: high efficiency and process simplification in interdigitated back contact cells

The present research and development activities in crystalline silicon photovoltaics include the exploration of doping technologies alternative to the mainstream diffusion process. The goal is to identify those technologies with potential to increase the solar cell efficiency and reduce the cost per watt peak. In that respect, this work presents the selective epitaxial growth of silicon as a candidate for boron doping; showing the results of the evaluation of boron‐doped silicon epitaxial emitters on slurry and diamond‐coated wire‐sliced Czochralski material, their integration in interdigitated back contact solar cells, and the development of a novel process sequence to create the interdigitated rear junction of these devices using selective epitaxial growth. Boron‐doped silicon epitaxy is demonstrated to perform in the high efficiency range (>22%), and the use of selective epitaxial growth is proposed as a route for the simplification of the interdigitated back contact solar cell flow. Copyright © 2013 John Wiley & Sons, Ltd.     

Material requirements for the adoption of unconventional silicon crystal and wafer growth techniques for high‐efficiency solar cells

Silicon wafers comprise approximately 40% of crystalline silicon module cost and represent an area of great technological innovation potential. Paradoxically, unconventional wafer‐growth techniques have thus far failed to displace multicrystalline and Czochralski silicon, despite four decades of innovation. One of the shortcomings of most unconventional materials has been a persistent carrier lifetime deficit in comparison to established wafer technologies, which limits the device efficiency potential. In this perspective article, we review a defect‐management framework that has proven successful in enabling millisecond lifetimes in kerfless and cast materials. Control of dislocations and slowly diffusing metal point defects during growth, coupled to effective control of fast‐diffusing species during cell processing, is critical to enable high cell efficiencies. To accelerate the pace of novel wafer development, we discuss approaches to rapidly evaluate the device efficiency potential of unconventional wafers from injection‐dependent lifetime measurements. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of high‐efficiency large‐area screen‐printed solar cells on direct kerfless epitaxially grown monocrystalline Si wafer and structure

This paper demonstrates the potential of epitaxially grown Si wafers with doped layers for high‐efficiency solar cells. Boron‐doped 239 cm² 180–200 µm thick 2 Ω‐cm wafers were grown with and without 15 µm thick p⁺ layer, with a doping in the range of 10¹⁷~10¹⁸ cm⁻³. A layer transfer process involving porous Si layer to lift off epi‐Si wafers from the reusable substrate was used. The pp⁺ wafers were converted into n⁺pp⁺ passivated emitter rear totally diffused (PERT) cells by forming an oxide‐passivated POCl₃‐diffused n⁺ emitter at the front, and oxide/nitride‐passivated epitaxially grown p⁺ BSF at the entire back, with local screen‐printed contacts. To demonstrate and quantify the benefit of the epi‐grown p⁺ layer, standard passivated emitter and rear cells (PERCs) with local BSF and contacts were also fabricated on p‐type epi‐Si wafers as well on commercial‐grade Cz wafers. Sentaurus 2D device model was used to assess the impact of the epi‐grown p⁺ layer, which showed an efficiency gain of ~0.5% for this PERT structure over the traditional PERC. This was validated by the cell results, which showed an efficiency of ~20.1% for the PERC, and ~20.3% for the PERT cell using epi‐Si wafers. Experimental data showed higher FF in PERT cells, largely because of the decrease in lateral resistance on the rear side. Efficiency gain, a result of higher FF, was greater than the recombination loss in the p⁺ layer because of the lightly doped thick p⁺ epi‐grown region used in this study. Copyright © 2016 John Wiley & Sons, Ltd.     

Resistivity and lifetime variation along commercially grown Ga‐ and B‐Doped Czochralski Si ingots and its effect on light‐induced degradation and performance of solar cells

A systematic study of the variation in resistivity and lifetime on cell performance, before and after light‐induced degradation (LID), was performed along ∼900‐mm‐long commercially grown B‐ and Ga‐doped Czochralski (Cz) ingots. Manufacturable screen‐printed solar cells were fabricated and analyzed from different locations on the ingots. Despite the large variation in resistivity (0·57–2·5 Ω cm) and lifetime (100–1000 µ s) in the Ga‐doped Cz ingot, the efficiency variation was found to be ≤ 0·5% with an average efficiency of ∼17·1%. No LID was observed in these cells. In contrast to the Ga‐doped ingot, the B‐doped ingot showed a relatively tight resistivity range (0·87–1·22 Ω cm), resulting in smaller spread in lifetime (60–400 µ s) and efficiency (16·5–16·7%) along the ingot. However, the LID reduced the efficiency of these B‐doped cells by about 1·1% absolute. Additionally, the use of thinner substrate and higher resistivity (4·3 Ω cm) B‐doped Cz was found to reduce the LID significantly, resulting in an efficiency reduction of 0·5–0·6%, as opposed to >1·0% in ∼1 Ω cm ∼17% efficient screen‐printed cells. As a result, Ga‐doped Cz cells gave 1·5 and 0·7% higher stabilized efficiency relative to 1 and 4·3 Ω cm B‐doped Cz Si cells, respectively. Copyright © 2005 John Wiley & Sons, Ltd.