Plasma hydrogenated,reactively sputtered aluminium oxide for silicon surface passivation

The intentional addition of hydrogen during reactive sputtering of AlO_x films has led to a dramatic improvement of the surface passivation of crystalline silicon wafers achieved with this technique. The 5 ms effective minority carrier lifetime measured on 1.5 Ω cm n‐type CZ silicon wafers is close to the 6 ms of a control wafer coated by atomic layer deposition (ALD) of AlO_x. Hydrogen‐sputtered films also provide excellent passivation of 1 Ω cm p‐type silicon, as demonstrated by an effective lifetime of 1.1 ms. It is likely that the improved passivation is related to the formation of an interfacial silicon oxide layer, as indicated by FTIR measurements. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface passivation schemes for high‐efficiency n‐type Si solar cells

An effective passivation on the front side boron emitter is essential to utilize the full potential of solar cells fabricated on n‐type silicon. However, recent investigations have shown that it is more difficult to achieve a low surface recombination velocity on highly doped p‐type silicon than on n‐type silicon. Thus, the approach presented in this paper is to overcompensate the surface of the deep boron emitter locally by a shallow phosphorus diffusion. This inversion from p‐type to n‐type surface allows the use of standard technologies which are used for passivation of highly doped n‐type surfaces. Emitter saturation current densities (J_0e) of 49 fA/cm² have been reached with this approach on SiO₂ passivated lifetime samples. On solar cells a certified conversion efficiency of 21.7% with an open‐circuit voltage (V_oc) of 676 mV was achieved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

19.4%‐efficient large‐area fully screen‐printed silicon solar cells

We demonstrate industrially feasible large‐area solar cells with passivated homogeneous emitter and rear achieving energy conversion efficiencies of up to 19.4% on 125 × 125 mm² p‐type 2–3 Ω cm boron‐doped Czochralski silicon wafers. Front and rear metal contacts are fabricated by screen‐printing of silver and aluminum paste and firing in a conventional belt furnace. We implement two different dielectric rear surface passivation stacks: (i) a thermally grown silicon dioxide/silicon nitride stack and (ii) an atomic‐layer‐deposited aluminum oxide/silicon nitride stack. The dielectrics at the rear result in a decreased surface recombination velocity of S_rear = 70 cm/s and 80 cm/s, and an increased internal IR reflectance of up to 91% corresponding to an improved J_sc of up to 38.9 mA/cm² and V_oc of up to 664 mV. We observe an increase in cell efficiency of 0.8% absolute for the cells compared to 18.6% efficient reference solar cells featuring a full‐area aluminum back surface field. To our knowledge, the energy conversion efficiency of 19.4% is the best value reported so far for large area screen‐printed solar cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface passivation of crystalline silicon by sputter deposited hydrogenated amorphous silicon

This letter shows that intrinsic hydrogenated amorphous silicon (a‐Si:H) films deposited by RF magnetron sputtering can provide outstanding passivation of crystalline silicon surfaces, similar to that achieved by plasma enhanced chemical vapour deposition (PECVD). By using a 2% hydrogen and 98% argon gas mixture as the plasma source, 1.5 Ω cm n‐type FZ silicon wafers coated with sputtered a‐Si:H films achieved an effective lifetime of 3.5 ms, comparable to the 3 ms achieved by PECVD (RF and microwave dual‐mode). This is despite the fact that Fourier transform infrared spectroscopy measurements show that sputtering and PECVD deposited films have very different chemical bonding configurations. We have found that film thickness and deposition temperature have a significant impact on the passivation results. Self‐annealing and hydrogen plasma treatment during deposition are likely driving forces for the observed changes in surface passivation. These experimental results open the way for the application of sputtered a‐Si:H to silicon heterojunction solar cells. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excellent passivation of thin silicon wafers by HF‐free hydrogen plasma etching using an industrial ICPECVD tool

In this work, hydrogen plasma etching of surface oxides was successfully accomplished on thin (～100 µm) planar n‐type Czochralski silicon wafers prior to intrinsic hydrogenated amorphous silicon [a‐Si:H(i)] deposition for heterojunction solar cells, using an industrial inductively coupled plasma‐enhanced chemical vapour deposition (ICPECVD) platform. The plasma etching process is intended as a dry alternative to the conventional wet‐chemical hydrofluoric acid (HF) dip for solar cell processing. After symmetrical deposition of an a‐Si:H(i) passivation layer, high effective carrier lifetimes of up to 3.7 ms are obtained, which are equivalent to effective surface recombination velocities of 1.3 cm s^–1 and an implied open‐circuit voltage (V_oc) of 741 mV. The passivation quality is excellent and comparable to other high quality a‐Si:H(i) passivation. High‐resolution transmission electron microscopy shows evidence of plasma‐silicon interactions and a sub‐nanometre interfacial layer. Using electron energy‐loss spectroscopy, this layer is further investigated and confirmed to be hydrogenated suboxide layers. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effective surface passivation of crystalline silicon using ultrathin Al2O3 films and Al2O3/SiNx stacks

We measure surface recombination velocities (SRVs) below 10 cm/s on p‐type crystalline silicon wafers passivated by atomic–layer–deposited (ALD) aluminium oxide (Al₂O₃) films of thickness ≥10 nm. For films thinner than 10 nm the SRV increases with decreasing Al₂O₃ thickness. For ultrathin Al₂O₃ layers of 3.6 nm we still attain a SRV < 22 cm/s on 1.5 Ω cm p‐Si and an exceptionally low SRV of 1.8 cm/s on high‐resistivity (200 Ω cm) p‐Si. Ultrathin Al₂O₃ films are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is extremely low compared to the frequently used plasma‐enhanced chemical vapour deposition of silicon nitride (SiN_x). Our experiments on silicon wafers passivated with stacks composed of ultrathin Al₂O₃ and SiN_x show that a substantially improved thermal stability during high‐temperature firing at 830 °C is obtained for the Al₂O₃/SiN_x stacks compared to the single‐layer Al₂O₃ passivation. Al₂O₃/SiN_x stacks are hence ideally suited for the implementation into industrial‐type silicon solar cells where the metal contacts are made by screen‐printing and high‐temperature firing of metal pastes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanism for crystalline Si surface passivation by the combination of SiO2 tunnel oxide and µc‐SiC:H thin film

This work demonstrates that the combination of a wet‐chemically grown SiO₂ tunnel oxide with a highly‐doped microcrystalline silicon carbide layer grown by hot‐wire chemical vapor deposition yields an excellent surface passivation for phosphorous‐doped crystalline silicon (c‐Si) wafers. We find effective minority carrier lifetimes of well above 6 ms by introducing this stack. We investigated its c‐Si surface passivation mechanism in a systematic study combined with the comparison to a phosphorous‐doped polycrystalline‐Si (pc‐Si)/SiO₂ stack. In both cases, field effect passivation by the n‐doping of either the µc‐SiC:H or the pc‐Si is effective. Hydrogen passivation during µc‐SiC:H growth plays an important role for the µc‐SiC:H/SiO₂ combination, whereas phosphorous in‐diffusion into the SiO₂ and the c‐Si is operative for the surface passivation via the Pc‐Si/SiO₂ stack. The high transparency and conductivity of the µc‐SiC:H layer, a low thermal budget and number of processes needed to form the stack, and the excellent c‐Si surface passivation quality are advantageous features of µc‐SiC:H/SiO₂ that can be beneficial for c‐Si solar cells. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Analysis of the current linearity at low illumination of high‐efficiency back‐junction back‐contact silicon solar cells

The relation between current and illumination intensity of three structures of high‐efficiency back‐junction back‐contact silicon solar cells was analyzed. Both, n‐type cells with non‐diffused front surface and p‐type cell with floating n‐emitter show a pronounced non‐linearity due to strong illumination dependence of the passivation quality of the non‐diffused surface and the floating junction respectively. Quantum efficiency (QE) of this cell type drops significantly for the illumination lower than 0.5 suns. In contrast the QE of n‐type cells with n⁺‐front surface field (FSF) is linear. Low illumination current characteristics of all three of the analyzed structures could be well described by physical models. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Improved control of the phosphorous surface concentration during in‐line diffusion of c‐Si solar cells by APCVD

Emitter formation for industrial crystalline silicon (c‐Si) solar cells is demonstrated by the deposition of phosphorous‐doped silicate glasses (PSG) on p‐type monocrystalline silicon wafers via in‐line atmospheric pressure chemical vapor deposition (APCVD) and subsequent thermal diffusion. Processed wafers with and without the PSG layers have been analysed by SIMS measurements to investigate the depth profiles of the resultant phosphorous emitters. Subsequently, complete solar cells were fabricated using the phosphorous emitters formed by doped silicate glasses to determine the impact of this high‐throughput doping method on cell performance. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

硅片及其太阳电池的光衰规律研究

采用氙灯模拟太阳光源,将光强调至1000 W/m2,研究常规太阳能级单晶硅片、多晶硅片和物理提纯硅片的原片、去损减薄片、热氧化钝化片、双面镀氮化硅(SiN x:H)膜钝化片、碘酒钝化片以及太阳电池的光衰规律.利用WT-2000少子寿命测试仪以及太阳电池I-V特性测试仪分别对硅片的少子寿命和太阳电池的I-V特性参数随光照时间的变化进行了测试.结果表明:所有硅片以及太阳电池在光照的最初60 min内衰减很快随后衰减变慢,180 min之后光衰速率变得很小,几乎趋于零.     

Comparison of batch and in‐line PECVD of a‐Si:H passivation layers for silicon heterojunction solar cells

We present PECVD deposition of i‐a‐Si:H in an in‐line configuration for the fabrication of silicon heterojunction solar cells. For industry, in‐line processing has the potential to increase production throughput and yield. We compared batch and in‐line fabrication of i‐a‐Si:H passivation samples with identical plasma conditions and observed that the a‐Si:H material properties do not significantly differ. In batch‐type production the substrate is in the plasma zone at the moment of ignition, whereas for in‐line deposition the substrate is introduced into the plasma zone when steady plasma conditions have been reached. Our preliminary results show that there are depositions conditions that result both for in‐line and batch‐type deposition in good i‐a‐Si:H passivation layers. Therefore both methods can equally well be considered for the production of silicon heterojunction solar cells. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Influence of precursor gas ratio and firing on silicon surface passivation by APCVD aluminium oxide

Using a high throughput, in‐line atmosphere chemical vapor deposition (APCVD) tool, we have synthesized amorphous aluminum oxide (AlO_x) films from precursors of trimethyl‐aluminum (TMA) and O₂, yielding a maximum deposition 150 nm min^–1 per wafer. For p‐type crystalline silicon (c‐Si) wafers, excellent surface passivation was achieved with the APCVD AlO_x films, with a best maximum effective surface recombination velocity (S_eff,max) of 8 cm/s following a standard industrial firing step. The findings could be attributed to the existence of large negative charge (Q_f ≈ –3 × 10¹² cm^–2) and low interface defect density (D_it ≈ 4 × 10¹¹ eV^–1 cm^–2) achieved by the films. This data demonstrates a high potential for APCVD AlO_x to be used in high efficiency, low cost industrial solar cells. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effective silicon surface passivation by atomic layer deposited Al2O3/TiO2 stacks

In this work atomic layer deposition of Al₂O₃ and TiO₂ has been used to obtain dielectric stacks for passivation of silicon surfaces. Our experiments on n‐ and p‐type silicon wafers deposited by thin Al₂O₃/TiO₂ stacks show that a considerably improved passivation is obtained compared to the Al₂O₃ single layer. For Al₂O₃ films thinner than 20 nm the emitter saturation current density decreases with increasing TiO₂ thickness. Especially the passivation of ultrathin (～5 nm) Al₂O₃ is very effectively enhanced by TiO₂ due to a decreased interface defect density as well as an increased fixed negative charge in the stacks. Hence, the thin Al₂O₃/TiO₂ stacks developed in this work can be used as a passivation coating for Si‐based solar cells. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

21.63% industrial screen‐printed multicrystalline Si solar cell

In this paper, we demonstrate industrially feasible large‐area solar cells achieving energy conversion efficiency up to 21.63% on p‐type boron doped multicrystalline Si wafers. Advanced light trapping, passivation and hydrogenation technology are used to achieve excellent light absorption with very low surface recombination velocity. The bulk lifetime of the multi‐crystalline Si wafers used for the fabrication exceeds 500 μs after optimized gettering and hydrogenation processes. The high bulk lifetime and excellent surface passivation enable V_oc to exceed 670 mV. The metallization process is carried out by screen printing and firing in a conventional belt furnace. Detailed performance parameters and quantum efficiency of the cells will be illustrated in the paper. In addition, free energy loss analysis and cell simulation are also performed using the control parameters measured during cell fabrication processes.     

硅异质结太阳电池的物理机制和优化设计

硅异质结太阳电池是一种由非晶硅薄膜层沉积于晶硅吸收层构成的高效低成本的光伏器件,是一种具有大面积规模化生产潜力的光伏产品.异质结界面钝化品质、发射极的掺杂浓度和厚度以及透明导电层的功函数是影响硅异质结太阳电池性能的主要因素.针对这些影响因素已经有大量的研究工作在全世界范围内展开,并且有诸多研究小组提出了器件效率限制因素背后的物理机制.洞悉物理机制可为今后优化设计高性能的器件提供准则.因此及时总结硅异质结太阳电池的物理机制和优化设计非常必要.本文主要讨论了晶硅表面钝化、发射极掺杂层和透明导电层之间的功函数失配以及由此形成的肖特基势垒;讨论了屏蔽由功函数失配引起的能带弯曲所需的特征长度,即屏蔽长度;介绍了硅异质结太阳电池优化设计的数值模拟和实践;总结了硅异质结太阳电池的研究现状和发展前景.     

Silicone oxidation for a‐Si:H passivation of wafers bonded to glass

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)     

Extremely low surface recombination in 1 Ω cm n‐type monocrystalline silicon

A key requirement in the recent development of highly efficient silicon solar cells is the outstanding passivation of their surfaces. In this work, plasma enhanced chemical vapour deposition of a triple layer dielectric consisting of amorphous silicon, silicon oxide and silicon nitride, charged extrinsically using corona, has been used to demonstrate extremely low surface recombination. Assuming Richter's parametrisation for bulk lifetime, an effective surface recombination velocity S_eff = 0.1 cm/s at Δn = 10¹⁵ cm^–3 has been obtained for planar, float zone, n ‐type, 1 Ω cm silicon. This equates to a saturation current density J_0s = 0.3 fA/cm², and a 1‐sun implied open‐circuit voltage of 738 mV. These surface recombination parameters are among the lowest reported for 1 Ω cm c‐Si. A combination of impedance spectroscopy and corona‐lifetime measurements shows that the outstanding chemical passivation is due to the small hole capture cross section for states at the interface between the Si and a‐Si layer which are hydrogenated during nitride deposition. (© 2016 The Authors. Phys. Status Solidi RRL published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temporary Surface Passivation for Characterisation of Bulk Defects in Silicon: A Review

Accurate measurements of the bulk minority carrier lifetime in high‐quality silicon materials is challenging due to the influence of surface recombination. Conventional surface passivation processes such as thermal oxidation or dielectric deposition often modify the bulk lifetime significantly before measurement. Temporary surface passivation processes at room or very low temperatures enable a more accurate measurement of the true bulk lifetime, as they limit thermal reconfiguration of bulk defects and minimize bulk hydrogenation. In this article we review the state‐of‐the‐art for temporary passivation schemes, including liquid immersion passivation based upon acids, halogen‐alcohols and benzyl‐alcohols, and thin film passivation usually based on organic substances. We highlight how exceptional surface passivation (surface recombination velocity below 1 cm s^?1) can be achieved by some types of temporary passivation. From an extensive review of available data in the literature, we find p‐type silicon can be best passivated by hydrofluoric acid containing solutions, with superacid‐based thin films showing a slight superiority in the n‐type case. We review the practical considerations associated with temporary passivation, including sample cleaning, passivation activation, and stability. We highlight examples of how temporary passivation can assist in the development of improved silicon materials for photovoltaic applications, and provide an outlook for the future of the field.

     

Wet-chemical passivation of atomically flat and structured silicon substrates for solar cell application

Special sequences of wet-chemical oxidation and etching steps were optimised with respect to the etching behaviour of differently oriented silicon to prepare very smooth silicon interfaces with excellent electronic properties on mono- and poly-crystalline substrates. Surface photovoltage (SPV) and photoluminescence (PL) measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were utilised to develop wet-chemical smoothing procedures for atomically flat and structured surfaces, respectively. Hydrogen-termination as well as passivation by wet-chemical oxides were used to inhibit surface contamination and native oxidation during the technological processing. Compared to conventional pre-treatments, significantly lower micro-roughness and densities of surface states were achieved on mono-crystalline Si(100), on evenly distributed atomic steps, such as on vicinal Si(111), on silicon wafers with randomly distributed upside pyramids, and on poly-crystalline EFG (Edge-defined Film-fed-Growth) silicon substrates.The recombination loss at a-Si:H/c-Si interfaces prepared on c-Si substrates with randomly distributed upside pyramids was markedly reduced by an optimised wet-chemical smoothing procedure, as determined by PL measurements. For amorphous-crystalline hetero-junction solar cells (ZnO/a-Si:H(n)/c-Si(p)/Al) with textured c-Si substrates the smoothening procedure results in a significant increase of short circuit current I_sc, fill factor and efficiency η. The scatter in the cell parameters for measurements on different cells is much narrower, as compared to conventional pre-treatments, indicating more well-defined and reproducible surface conditions prior to a-Si:H emitter deposition and/or a higher stability of the c-Si surface against variations in the a-Si:H deposition conditions.     

Study of hydrogenated AlN as an anti‐reflective coating and for the effective surface passivation of silicon

Stacks of aluminum oxide and silicon nitride are frequently used in silicon photovoltaics. In this Letter, we demonstrate that hydrogenated aluminum nitride can be an alternative to this dual‐layer stack. Deposited on 1 Ω cm p‐type FZ silicon, very low effective surface recombination velocities of 8 cm/s could be reached after firing at 820 °C. This excellent passivation is traced back to a high density of fixed charges at the interface of approximately –1 × 10¹² cm^–2 and a very low interface defect density below 5 × 10¹⁰ eV^–1 cm^–2. Furthermore, spectral ellipsometry measurements reveal that these aluminum nitride layers have ideal optical properties for use as anti‐reflective coatings. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)