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)     

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)     

Theoretical investigation on the passivation layer with linearly graded bandgap for the amorphous/crystalline silicon heterojunction solar cell

Passivation layer with linearly graded bandgap (LGB) was proposed to improve the performance of amorphous/crystalline silicon heterojunction (SHJ) solar cell by eliminating the large abrupt energy band uncontinuity at the a‐Si:H/c‐Si interface. Theoretical investigation on the a‐Si:H(p)/the LGB passivation layer(i)/c‐Si(n)/a‐Si:H(i)/a‐Si:H(n⁺) solar cell via AFORS‐HET simulation show that such LGB passivation layer could improve the solar cell efficiency (η) by enhancing the fill factor (FF) greatly, especially when the a‐Si:H(p) emitter was not efficiently doped and the passivation layer was relatively thick. But gap defects in the LGB passivation layer could make the improvement discounted due to the open‐circuit voltage (V_OC) decrease induced by recombination. To overcome this, it was quite effective to keep the gap defects away from the middle of the bandgap by widening the minimum bandgap of the LGB passivation layer to be a little larger than that of the c‐Si base. The underlying mechanisms were analysed in detail. How to achieve the LGB passivation layer experimentally was also discussed. (© 2016 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)     

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)     

Role of a‐Si:H bulk in surface passivation of c‐Si wafers

The low thermal stability of hydrogenated amorphous silicon (a‐Si:H) thin films limits their widespread use for surface passivation of c‐Si wafers on the rear side of solar cells. We show that the thermal stability of a‐Si:H surface passivation is increased significantly by a hydrogen rich a‐Si:H bulk, which acts as a hydrogen reservoir for the a‐Si:H/c‐Si interface. Based on this mechanism, an excellent lifetime of 5.1 ms (at injection level of 10¹⁵ cm^–3) is achieved after annealing at 450 °C for 10 min. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Noise in boron doped amorphous/microcrystallization silicon films

Hydrogenated silicon (Si:H) film was grown by radio frequency plasma enhanced chemical vapor deposition (PECVD) method. The transition between hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) was characterized by X-ray diffraction analysis. A semiconductor system was used to measure low frequency noise (1/f noise) and random telegraph switching noise of Si:H films. The results show that the 1/f noise of μc-Si:H is 4 orders of magnitude lower than that of a-Si:H and no RTS noise was found in both films. It also shows that using μc-Si:H instead of a-Si:H film as a sensing layer will enable the development of high performance uncooled microbolometer.     

Microcrystalline silicon‐carbon alloys as anti‐reflection window layers in high efficiency thin film silicon solar cells

Microcrystalline silicon‐carbide (μc‐SiC:H) films were prepared using hot wire chemical vapor deposition at low substrate temperature. The μc‐SiC:H films were employed as window layers in microcrystalline silicon (μc‐Si:H) solar cells. The short‐circuit current density (J_SC) in these n‐side illuminated n–i–p cells increases with increasing the deposition time t_W of the μc‐SiC:H window layer from 5 min to 60 min. The enhanced J_SC is attributed to both the high transparency and an anti‐reflection effect of the μc‐SiC:H window layer. Using these favourable optical properties of the μc‐SiC:H window layer in μc‐Si:H solar cells, a J_SC value of 23.8 mA/cm² and cell efficiencies above 8.0% were achieved with an absorber layer thickness of 1 μm and a Ag back reflector. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of SiNx:H film properties according to gas mixture ratios for crystalline silicon solar cells

The Hydrogenated silicon nitride (SiN_x:H) using plasma enhanced chemical vapor deposition is widely used in photovoltaic industry as an antireflection coating and passivation layer. In the high temperature firing process, the SiN_x:H film should not change the properties for its use as high quality surface layer in crystalline silicon solar cells. For optimizing surface layer in crystalline silicon solar cells, by varying gas mixture ratios (SiH₄ + NH₃ + N₂, SiH₄ + NH₃, SiH₄ + N₂), the hydrogenated silicon nitride films were analyzed for its antireflection and surface passivation (electrical and chemical) properties. The film deposited with the gas mixture of SiH₄ + NH₃ + N₂ showed the best properties in before and after firing process conditions.The single crystalline silicon solar cells fabricated according to optimized gas mixture condition (SiH₄ + NH₃ + N₂) on large area substrate of size 156 mm × 156 mm (Pseudo square) was found to have the conversion efficiency as high as 17.2%. The reason for the high efficiency using SiH₄ + NH₃ + N₂ is because of the good optical transmittance and passivation properties. Optimized hydrogenated silicon nitride surface layer and high efficiency crystalline silicon solar cells fabrication sequence has also been explained in this study.     

Raman characterization of the structural evolution in amorphous and partially nanocrystalline hydrogenated silicon thin films prepared by PECVD

Hydrogenated silicon (Si:H) thin films were obtained by plasma‐enhanced chemical vapor deposition (PECVD). Raman spectroscopy was used to investigate the structural evolution in phosphor‐doped n‐type amorphous hydrogenated silicon thin films, which were prepared under different substrate temperatures and gas pressures. Meanwhile, the effect of nitrogen doping on the structure of P‐doped thin films was also investigated by Raman spectroscopy. Moreover, the transition from the amorphous state to the nanocrystalline state of undoped Si:H films deposited through low argon dilution was studied by Raman spectroscopy, X‐ray diffraction, and transmission electron microscopy. The results show that Raman spectroscopy can sensitively detect the structural evolution in hydrogenated silicon thin films deposited under different conditions in a PECVD system. Copyright © 2010 John Wiley & Sons, Ltd.     

High critical oxygen concentration in microcrystalline silicon solar cells

For microcrystalline silicon based p–i–n solar cells the effect of deposition conditions on the critical oxygen concentration was investigated. All solar cells were prepared by 13.56 MHz plasma‐enhanced chemical vapour deposition. The critical oxygen concentration defines the lowest oxygen concentration in the intrinsic absorber layer causing a deterioration of the solar cell performance. For intentional contamination of ～1.2–1.3 µm thick i‐layers, the oxygen was inserted by a controllable leak at the process gases supply line, i.e. by a gas pipe leak. For µc‐Si:H deposited at a discharge power of 0.53 W/cm² we find a critical oxygen concentration of 1–2 × 10¹⁹ cm^–3 in agreement with values commonly reported in literature. However, changing the deposition conditions, we find that the critical oxygen concentration in µc‐Si:H cells is not fixed. At reduced power of 0.20 W/cm² a much higher value for the critical oxygen concentration of 1 × 10²⁰ cm^–3 is observed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

钙钛矿/硅叠层太阳电池中平面a-Si:H/c-Si异质结底电池的钝化优化及性能提高

最近,旋涂法制备的钙钛矿/平面硅异质结高效叠层太阳电池引起人们广泛关注,主要原因是相比于绒面硅衬底制备的钙钛矿/硅叠层太阳电池,其制备工艺简单、制备成本低且效率高.对于平面a-Si:H/c-Si异质结电池, a-Si:H/c-Si界面的良好钝化是获得高转换效率的关键,进而决定了钙钛矿/硅异质结叠层太阳电池的性能.本文主要从硅片表面处理、a-Si:H钝化层和P型发射极等方面展开研究,通过对硅片表面的氢氟酸(HF)浸泡时间和氢等离子体预处理气体流量、a-Si:H钝化层沉积参数、钝化层与P型发射极(I/P)界面富氢等离子体处理的综合调控,获得了相应的优化工艺参数.对比研究了p-a-Si:H和p-nc-Si:H两种缓冲层材料对I/P界面的影响,其中高电导、宽带隙的p-nc-Si:H缓冲层既能够降低I/P界面的缺陷态,又可以增强P型发射层的暗电导率,提高了前表面场效应钝化效果.通过上述优化,制备出最佳的P-type emitter layer/aSi:H(i)/c-Si/a-Si:H(i)/N-type layer (inip)结构样品的少子寿命与implied-Voc分别达到2855μs和709 mV,表现出良好的钝化效果.应用于平面a-Si:H/c-Si异质结太阳电池,转换效率达到18.76%,其中开路电压达到681.5 mV,相对于未优化的电池提升了34.3 mV.将上述平面a-Si:H/c-Si异质结太阳电池作为底电池,对应的钙钛矿/硅异质结叠层太阳电池的开路电压达到1780 mV,转换效率达到21.24%,证明了上述工艺优化能够有效地改善叠层太阳电池中的硅异质结底电池的钝化及电池性能.     

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.     

Low‐temperature atomic layer deposition of MoOx for silicon heterojunction solar cells

The preparation of high‐quality molybdenum oxide (MoO_x) is demonstrated by plasma‐enhanced atomic layer deposition (ALD) at substrate temperatures down to 50 °C. The films are amorphous, slightly substoichiometric with respect to MoO₃, and free of other elements apart from hydrogen (&11 at%). The films have a high transparency in the visible region and their compatibility with a‐Si:H passivation schemes is demonstrated. It is discussed that these aspects, in conjunction with the low processing temperature and the ability to deposit very thin conformal films, make this ALD process promising for the future application of MoO_x in hole‐selective contacts for silicon heterojunction solar cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The p recombination layer in tunnel junctions for micromorph tandem solar cells

A new tunnel recombination junction is fabricated for n–i–p type micromorph tandem solar cells. We insert a thin heavily doped hydrogenated amorphous silicon (a-Si:H) p + recombination layer between the n a-Si:H and the p hydrogenated nanocrystalline silicon (nc-Si:H) layers to improve the performance of the n–i–p tandem solar cells. The effects of the boron doping gas ratio and the deposition time of the p-a-Si:H recombination layer on the tunnel recombination junctions have been investigated. The current-voltage characteristic of the tunnel recombination junction shows a nearly ohmic characteristic, and the resistance of the tunnel recombination junction can be as low as 1.5 ·cm 2 by using the optimized p-a-Si:H recombination layer. We obtain tandem solar cells with open circuit voltage V oc = 1.4 V, which is nearly the sum of the V oc s of the two corresponding single cells, indicating no V oc losses at the tunnel recombination junction.     

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.     

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)     

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.     

Silicon surface passivation by aluminium oxide studied with electron energy loss spectroscopy

The origin behind crystalline silicon surface passivation by Al₂O₃ films is studied in detail by means of spatially‐resolved electron energy loss spectroscopy. The bonding configurations of Al and O are studied in as‐deposited and annealed Al₂O₃ films grown on c‐Si substrates by plasma‐assisted and thermal atomic layer deposition. The results confirm the presence of an interfacial SiO₂‐like film and demonstrate changes in the ratio between tetrahedrally and octahedrally coordinated Al in the films after annealing. These observations reveal the underlying origin of c‐Si surface passivation by Al₂O₃. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)