Comparison of p-n junction and inversion-layer silicon solar cells by means of experiment and simulation

Inversion-layer solar cells can be fabricated on crystalline silicon in a time- and energy-efficient way. In this article we experimentally investigate inversion layer cells of the type developed in the 1980s at the University of Erlangen. The best cell has an independently confirmed one-sun efficiency of 15.7%, the highest reported to date for this simple cell technology. In order to gain insight into the performance-limiting mechanisms, these cells are compared to p-n junction cells fabricated on identical substrates. Subsequently, the impact of the most important emitter parameters on the performance of both cell types is determined by means of two-dimensional numerical modelling. These simulations reveal that inversion-layer cells can principally produce the same efficiencies (> 23%) as p-n junction cells, provided the emitter parameters are properly adjusted and the front contact is of a sufficiently high quality. Therefore, a research project is presently under way at ISFH aiming at an improvement of inversion-layer cell efficiency above 18%. The basis for these new cells is the fact that silicon nitride films deposited at higher temperatures (∼400°C) demonstrate strongly improved passivation properties compared to the present 250° C silicon nitride films.     

Comparison of the open circuit voltage of simplified PERC cells passivated with PECVD silicon nitride and thermal silicon oxide

Plasma enhanced chemical vapor deposited silicon nitride films have been used to passivate both the front and rear surface of simplified PERC silicon solar cells (planar surface, single‐step emitter). An independently confirmed open circuit voltage (V_oc) of 667 mV was measured, proving the outstanding surface passivation provided by the silicon nitride films. The achieved V_oc represents a significant improvement for all‐SiN passivated silicon solar cells. A conversion efficiency of 17˙8% was obtained. For comparison, similar cells with different passivation schemes, including high quality, thermally grown TCA oxides and thin SiO₂/SiN double layers, were also investigated. Open circuit voltages up to 673 mV and conversion efficiencies up to 18˙3% were achieved. Copyright © 2000 John Wiley & Sons, Ltd.     

Recent progress in MIS solar cells

Metal–insulator–semiconductor (MIS)-type solar cells have an inherent cost advantage compared to p-n junction solar cells. First-generation MIS–inversion layer (MIS–IL) solar cells, already successfully produced in an industrial pilot line, are restricted to efficiencies of 15–16%. With the second-generation MIS–IL silicon solar cells, based on drastically improved surface passivation by plasma-enhanced chemical vapour-deposited silicon nitride, simple technology can be combined with very high efficiencies. The novel inversion layer emitters have the potential to outperform conventional phosphorus-diffused emitters of Si solar cells. A 17.1% efficiency could already be achieved with the novel point-contacted ‘truncated pyramid’ MIS–IL cell. A new surface-grooved line-contact MIS–IL device presently under development using unconventional processing steps applicable for large-scale fabrication is discussed. By the mechanical grooving technique, contact widths down to 2 μm can be achieved homogeneously over large wafer areas. Bifacial sensitivity is included in most of the MIS-type solar cells. For a bifacial 98 cm² Czochralski (Cz) Si MIS-contacted p-n junction solar cell with a random pyramid surface texture and Al as grid metal, efficiencies of 16.5% for front and 13.8% for rear side illumination are reported. A 19.5% efficiency has been obtained with a mechanically grooved MIS n⁺p solar cell. The MIS-type silicon solar cells are able to significantly lower the costs for solar electricity due to the simple technology and the potential for efficiencies well above 20%.©1997 John Wiley & Sons, Ltd.     

High‐quality surface passivation of silicon solar cells in an industrial‐type inline plasma silicon nitride deposition system

We have studied the surface passivation of silicon by deposition of silicon nitride (SiN) in an industrial‐type inline plasma‐enhanced chemical vapor deposition (PECVD) reactor designed for the continuous coating of silicon solar cells with high throughput. An optimization study for the passivation of low‐resistivity p‐type silicon has been performed exploring the dependence of the film quality on key deposition parameters of the system. With the optimized films, excellent passivation properties have been obtained, both on undiffused p‐type silicon and on phosphorus‐diffused n⁺ emitters. Using a simple design, solar cells with conversion efficiencies above 20% have been fabricated to prove the efficacy of the inline PECVD SiN. The passivation properties of the films are on a par with those of high‐quality films prepared in small‐area laboratory PECVD reactors. Copyright © 2004 John Wiley & Sons, Ltd.     

Textured silicon surface passivation by high‐rate expanding thermal plasma deposited SiN and thermal SiO2/SiN stacks for crystalline silicon solar cells

Expanding thermal plasma (ETP) deposited silicon nitride (SiN) with optical properties suited for the use as antireflection coating (ARC) on silicon solar cells has been used as passivation layer on textured monocrystalline silicon wafers. The surface passivation behavior of these high‐rate (>5 nm/s) deposited SiN films has been investigated for single layer passivation schemes and for thermal SiO₂/SiN stack systems before and after a thermal treatment that is normally used for contact‐firing. It is shown that as‐deposited ETP SiN used as a single passivation layer almost matches the performance of a thermal oxide. Furthermore, the SiN passivation behavior improves after a contact‐firing step, while the thermal oxide passivation degrades which makes ETP SiN a better alternative for single passivation layer schemes in combination with a contact‐firing step. Moreover, using the ETP SiN as a part of a thermal SiO₂/SiN stack proves to be the best alternative by realizing very low dark saturation current densities of <20 fA/cm² on textured solar‐grade FZ silicon wafers and this is further improved to <10 fA/cm² after the anneal step. Optical and electrical film characterizations have also been carried out on these SiN layers in order to study the behavior of the SiN before and after the thermal treatment. Copyright © 2008 John Wiley & Sons, Ltd.     

晶体硅太阳电池表面钝化技术

介绍了晶体硅太阳电池表面钝化技术的发展历程,表面钝化膜在晶体硅太阳电池中所起的作用,以及晶体硅太阳电池中各种钝化膜和表面钝化技术。阐述了国内和国际对晶体硅太阳电池表面钝化技术的最新研究动态,重点论述了SiO2,SiNx,SiCx和Al2O3,以及这些钝化膜的叠层钝化技术的优缺点。在此基础上进一步指出SiO2/SiNx叠层钝化膜将成为今后工业化生产的研究重点,Al2O3及其叠层钝化膜将成为今后实验室的研究重点,由于表面钝化是提高晶体硅太阳电池转换效率最有效的手段之一,今后晶体硅太阳电池表面钝化技术仍将是国内和国际研究的热点问题之一。     

Optimised antireflection coatings for planar silicon solar cells using remote PECVD silicon nitride and porous silicon dioxide

Silicon nitride (SiN) films fabricated by remote plasma‐enhanced chemical vapour deposition (RPECVD) have recently been shown to provide an excellent electronic passivation of silicon surfaces. This property, in combination with its large refractive index, makes RPECVD SiN an ideal candidate for a surface‐passivating antireflection coating on silicon solar cells. A major problem of these films, however, is the fact that the extinction coefficient increases with increasing refractive index. Hence, a careful optimisation of RPECVD SiN based antireflection coatings on silicon solar cells must consider the light absorption within the films. Optimal optical performance of silicon solar cells in air is obtained if the RPECVD SiN films are combined with a medium with a refractive index below 1·46, such as porous SiO₂. In this study, the dispersion of the refractive indices and the extinction coefficients of RPECVD SiN, porous SiO₂, and several other relevant materials (MgF₂, TiO_x, ZnS, B270 crown glass, soda lime glass, ethylene vinyl acetate and resin as used in commercial photovoltaic modules) are experimentally determined. Based on these data, the short‐circuit currents of planar silicon solar cells covered by RPECVD SiN and/or porous SiO₂ single‐ and multi‐layer antireflection coatings are numerically maximised for glass‐encapsulated as well as non‐encapsulated operating conditions. The porous SiO₂/RPECVD SiN‐based antireflection coatings optimised for these applications are shown to be universally suited for silicon solar cells, regardless of the internal blue or red response of the cells. Copyright © 1999 John Wiley & Sons, Ltd.     

硅微纳结构及其在新型太阳电池中的应用

综述了近年来各种硅微纳结构的特征和制备技术,介绍了其在新型太阳电池中的应用现状与前景.首先,阐述了硅微纳结构在传统p-n结、新型径向p-n结以及异质结太阳电池结构设计中的研究进展;其次,从光吸收增强、表面修饰及钝化的角度,分析了硅微纳结构太阳电池的增效措施;最后,提出了柔性硅微纳结构太阳电池开发的新思路.     

Surface passivation of crystalline silicon solar cells: a review

In the 1980s, advances in the passivation of both cell surfaces led to the first crystalline silicon solar cells with conversion efficiencies above 20%. With today's industry trend towards thinner wafers and higher cell efficiency, the passivation of the front and rear surfaces is now also becoming vitally important for commercial silicon cells. This paper presents a review of the surface passivation methods used since the 1970s, both on laboratory‐type as well as industrial cells. Given the trend towards lower‐cost (but also lower‐quality) Si materials such as block‐cast multicrystalline Si, ribbon Si or thin‐film polycrystalline Si, the most promising surface passivation methods identified to date are the fabrication of a p–n junction and the subsequent passivation of the resulting silicon surface with plasma silicon nitride as this material, besides reducing surface recombination and reflection losses, additionally provides a very efficient passivation of bulk defects. Copyright © 2000 John Wiley & Sons, Ltd.     

Experimental evidence of parasitic shunting in silicon nitride rear surface passivated solar cells

Many solar cells incorporating SiN_x films as a rear surface passivation scheme have not reached the same high level of cell performance as solar cells incorporating high‐temperature‐grown silicon dioxide films as a rear surface passivation. In this paper, it is shown by direct comparison of solar cells incorporating the two rear surface passivation schemes, that the performance loss is mainly due to a lower short‐circuit current while the open‐circuit voltage is equally high. With a solar cell test structure that features a separation of the rear metal contacts from the passivating SiN_x films, the loss in short‐circuit current can be reduced drastically. Besides a lower short‐ circuit current, dark I–V curves of SiN_x rear surface passivated solar cells exhibit distinct shoulders. The results are explained by parasitic shunting of the induced floating junction (FJ) underneath the SiN_x films with the rear metal contacts. The floating junction is caused by the high density of fixed positive charges in the SiN_x films. Other two‐dimensional effects arising from the injection level dependent SRV of the Si/SiN_x interfaces are discussed as well, but, are found to be of minor importance. Pinholes in the SiN_x films and optical effects due to a different internal rear surface reflectance can be excluded as a major cause for the performance loss of the SiN_x rear surface passivated cells. Copyright © 2002 John Wiley & Sons, Ltd.     

氧化铝钝化在晶体硅太阳电池中的应用

首先,回顾了氧化铝钝化技术的发展历程,对制备氧化铝钝化薄膜的手段进行了总结,并且详细描述了氧化铝的材料性质和钝化的机理。其次,指出氧化铝薄膜的优点在于优异的场效应钝化特性和良好的化学钝化性质,因此可以应用于低掺和高掺p型硅表面的钝化。此外,氧化铝薄膜及其叠层还具有良好的热稳定性,符合丝网印刷太阳电池的要求。最后,总结了氧化铝薄膜钝化技术在晶体硅太阳电池中的最新研究动态,指出氧化铝钝化薄膜用于工业生产中存在的问题,并针对这些问题提出了有效的解决方案。     

Front and Back‐Junction Carbon Nanotube‐Silicon Solar Cells with an Industrial Architecture

In the past, the application of carbon nanotube‐silicon solar cell technology to industry has been limited by the use of a metallic frame to define an active area in the middle of a silicon wafer. Here, industry standard device geometries are fabricated with a front and back‐junction design which allow for the entire wafer to be used as the active area. These are enabled by the use of an intermixed Nafion layer which simultaneously acts as a passivation, antireflective, and physical blocking layer as well as a nanotube dopant. This leads to the formation of a hybrid nanotube/Nafion passivated charge selective contact, and solar cells with active areas of 1–16 cm² are fabricated. Record maximum power conversion efficiencies of 15.2% and 18.9% are reported for front and back‐junction devices for 1 and 3 cm² active areas, respectively. By placing the nanotube film on the rear of the device in a back‐junction architecture, many of the design‐related challenges for carbon nanotube silicon solar cells are addressed and their future applications to industrialized processes are discussed.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Multicrystalline silicon solar cells exceeding 20% efficiency

This paper presents the first conversion efficiency above 20% for a multicrystalline silicon solar cell. The application of wet oxidation for rear surface passivation significantly reduces the process temperature and therefore prevents the degradation of minority‐carrier lifetime. The excellent optical properties of the dielectrically passivated rear surface in combination with a plasma textured front surface result in a superior light trapping and allow the use of substrates below 100 μm thickness. A simplified process scheme with laser‐fired rear contacts leads to conversion efficiencies of 20·3% for multicrystalline and 21·2% for monocrystalline silicon solar cells on small device areas (1 cm²). Copyright © 2004 John Wiley & Sons, Ltd.     

Comprehensive study of rapid, low-cost silicon surface passivationtechnologies

A comprehensive and systematic investigation of low-cost surface passivation technologies is presented for achieving high-performance silicon devices such as solar cells. Most commercial solar cells today lack adequate surface passivation, while laboratory cells use conventional furnace oxides (CFO) for high-quality surface passivation involving an expensive and lengthy high-temperature step. This investigation tries to bridge the gap between commercial and laboratory cells by providing fast, low-cost methods for effective surface passivation. This paper demonstrates for the first time, the efficacy of TiO₂, thin (<10 nm) rapid thermal oxide (RTO), and PECVD SiN individually and in combination for (phosphorus diffused) emitter and (undiffused) back surface passivation. The effects of emitter sheet resistance, surface texture, and three different SiN depositions (two direct PECVD systems and one remote plasma system) were investigated. The effects of post-growth/deposition treatments such as forming gas anneal (FGA) and firing of screen printed contacts were also examined. This study reveals that the optimum passivation scheme consisting of a thin RTO with a SiN cap followed by a very short 730°C anneal can 1) reduce the emitter saturation current density, J_0e, by a factor of >15 for a 90 Ω/sq. emitter, 2) reduce J_0e by a factor of >3 for a 40 Ω/sq, emitter, and 3) reduce S_back below 20 cm/s on 1.3 Ωcm p-Si. Furthermore, this double-layer RTO+SiN passivation is relatively independent of the deposition conditions (direct or remote) of the SiN film and is more stable under heat treatment than SiN or RTO alone. Model calculations are also performed to show that the RTO+SiN surface passivation scheme may lead to 17%-efficient thin screen-printed cells even with a low bulk lifetime of 20 μs     

The truncated-pyramid MIS inversion-layer solar cell: a comprehensive analysis

Metal–insulator–semiconductor inversion-layer (MISIL) solar cells are of significant interest because of their simple fabrication process. In this work a comprehensive analysis of the improved front surface design of truncated-pyramid MISIL silicon solar cells is presented. This analysis reveals the two most important effects that have led to an increase in the open-circuit voltage of more than 50 mV. Firstly, passivation of the non-grid area at the front surface is optimized to meet the special requirements of MISIL solar cells. Secondly, the MIS contact is investigated very thoroughly. This includes the recombination properties of the contact and the current transport via the MIS contact. The results of this investigation show that the contact area can be reduced without an increase in the series resistance of the MISIL solar cells and therefore the recombination losses at the metal contacts are reduced. As a result of these improvements, independently confirmed 1-sun efficiencies above 17% are achieved with truncated-pyramid MISIL solar cells. © 1998 John Wiley & Sons, Ltd.     

Silicon solar cells based on all‐laser‐transferred contacts

Crystalline silicon solar cells based on all‐laser‐transferred contacts (ALTC) have been fabricated with both front and rear metallization achieved through laser induced forward transferring. Both the front and rear contacts were laser‐transferred from a glass slide coated with a metal layer to the silicon substrate already processed with emitter formation, surface passivation, and antireflection coating. Ohmic contacts were achieved after this laser transferring. The ALTC solar cells were fabricated on chemically textured p‐type Cz silicon wafers. An initial conversion efficiency of over 15% was achieved on a simple cell structure with full‐area emitter. Further improvements are expected with optimized laser transferring conditions, front grid pattern design, and surface passivation. The ALTC process demonstrates the advantage of laser processing in simplifying the solar cell fabrication by a one‐step metal transferring and firing process. Copyright © 2013 John Wiley & Sons, Ltd.     

Front and rear silicon‐nitride‐passivated multicrystalline silicon solar cells with an efficiency of 18.1%

A solar cell process designed to utilise low‐temperature plasma‐enhanced chemical vapour deposited (PECVD) silicon nitride (SiN_x) films as front and rear surface passivation was applied to fabricate multicrystalline silicon (mc‐Si) solar cells. Despite the simple photolithography‐free processing sequence, an independently confirmed efficiency of 18.1% (cell area 2 × 2 cm²) was achieved. This excellent efficiency can be predominantly attributed to the superior quality of the rear surface passivation scheme consisting of an SiN_x film in combination with a local aluminium back‐surface field (LBSF). Thus, it is demonstrated that low‐temperature PECVD SiN_x films are well suited to achieve excellent rear surface passivation on mc‐Si. Copyright © 2002 John Wiley & Sons, Ltd.     

Rear side passivation of PERC‐type solar cells by wet oxides grown from purified steam

To considerably improve the conversion efficiency of industrial solar cells, the effective passivation of the rear surface is a prerequisite. Thermal grown silicon oxides provide an excellent level of surface passivation on lowly doped p‐type surfaces. However, dry thermal oxidation processes require relatively high temperatures (∼1000°C) and, due to the low growth rate, long process times. To decrease both oxidation temperature and process time, the dry oxidation process can be replaced by a wet oxidation. The most common way to introduce high purity water vapor into the oxidation tube is the pyrolytic generation from high purity gases (H₂ and O₂). A more easy and cost effective option for the supply of water vapor that was applied within this work is the direct purification of steam. The passivation quality of dry and wet oxides, the latter grown from pyrolytic generated water vapor and purified steam was compared within this work. The passivation quality obtained for the wet oxides grown from purified steam was found to be comparable to those oxides grown from pyrolytic generated water vapor. On laser fired contacts (LFC) solar cells conversion efficiencies well above 20% could be achieved independent of the oxide that was applied for the rear side passivation. Copyright © 2010 John Wiley & Sons, Ltd.     

Study of the hydrogenation mechanism by rapid thermal anneal of SiN:H in thin-film polycrystalline-silicon solar cells

A considerable cost reduction in photovoltaics could be achieved if efficient solar cells could be made from thin polycrystalline-silicon (pc-Si) films. Although hydrogen passivation of pc-Si films is crucial to obtain good solar cells, the exact mechanism of hydrogen diffusion through pc-Si layers is not yet understood. In this letter, the influence of the junction and the grain size are investigated. We find that the presence of a p-n junction acts as a barrier for hydrogen diffusion in thin-film polysilicon solar cells. Therefore, pc-Si solar cells should preferably be passivated before junction formation. Furthermore, pc-Si layers with large grains retain less hydrogen after passivation than layers with small grains. This indicates that hydrogen atoms get mainly trapped at the grain boundaries.     

A Polymer/Carbon‐Nanotube Ink as a Boron‐Dopant/Inorganic‐Passivation Free Carrier Selective Contact for Silicon Solar Cells with over 21% Efficiency

Traditional silicon solar cells extract holes and achieve interface passivation with the use of a boron dopant and dielectric thin films such as silicon oxide or hydrogenated amorphous silicon. Without these two key components, few technologies have realized power conversion efficiencies above 20%. Here, a carbon nanotube ink is spin coated directly onto a silicon wafer to serve simultaneously as a hole extraction layer, but also to passivate interfacial defects. This enables a low‐cost fabrication process that is absent of vacuum equipment and high‐temperatures. Power conversion efficiencies of 21.4% on an device area of 4.8 cm² and 20% on an industrial size (245.71 cm²) wafer are obtained. Additionally, the high quality of this passivated carrier selective contact affords a fill factor of 82%, which is a record for silicon solar cells with dopant‐free contacts. The combination of low‐dimensional materials with an organic passivation is a new strategy to high performance photovoltaics.