Selective laser ablation of SiNx layers on textured surfaces for low temperature front side metallizations

For an alternative front side metallization process without screen printing of metal paste the selective opening of the front surface anti‐reflection coating could be realized by laser ablation. A successful implementation of this scheme requires direct absorption of the laser light within the anti‐reflection coating, since the emitter underneath must not be damaged severely. Additionally, the ablation must be feasible on textured surfaces. In this paper, we show that laser light with a wavelength of 355 nm and a pulse length of approximately 30 ns is absorbed directly by a typical silicon nitride anti‐reflection coating. Based on lifetime measurements on ablated samples it is shown that a damage free laser ablation of SiN_x layers on planar surfaces is possible. The characteristic ablation structure on textured surfaces is explained and quantified by rigorous coupled wave analysis (RCWA) simulations. Finally, high efficiency solar cells with a standard emitter (R_sh approx. 50 Ω/sq) have been processed using laser ablation of the silicon nitride anti‐reflection coating. These cells show efficiencies of up to 19·1%, comparable to the reference solar cells using photolithographically opened contact areas. Copyright © 2008 John Wiley & Sons, Ltd.     

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

In this work, we report on ion‐implanted, high‐efficiency n‐type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser‐patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiN_x, followed by screen‐printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO₂/SiN_x passivation stack on the back to form the pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO₂/SiN_x passivation layers, without inducing appreciable damage or defects on the surface of the n⁺ back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n‐type passivated emitter, rear totally diffused cell. Copyright © 2014 John Wiley & Sons, Ltd.     

SiOyNx/SiNx stack: a promising surface passivation layer for high‐efficiency and potential‐induced degradation resistant mc‐silicon solar cells

A thin SiO_yN_x film was inserted below a conventional SiN_x antireflection coating used in c‐Si solar cells in order to improve the surface passivation and the solar cell's resistance to potential‐induced degradation (PID). The effect of varying the flow ratio of the N₂O and SiH₄ precursors and the deposition temperature for the SiO_yN_x thin film upon material properties were systematically investigated. An excellent surface passivation was obtained on FZ p‐type polished silicon wafers, with the best results obtained with a SiO_yN_x film deposited at a very low temperature of 130 °C and with an optical refractive index of 1.8. In the SiO_yN_x/SiN_x stack structure, a SiO_yN_x film with ~6 nm thickness is sufficient to provide excellent surface passivation with an effective surface recombination velocity S_eff < 2 cm/s. Furthermore, we applied the optimized SiO_yN_x/SiN_x stack on multicrystalline Si solar cells as a surface passivation and antireflection coating, resulting in a 0.5% absolute average conversion efficiency gain compared with that of reference cells with conventional SiN_x coating. Moreover, the cells with the SiO_yN_x/SiN_x stack layers show a significant increase in their resistance to PID. Nearly zero degradation in shunt resistance was obtained after 24 h in a PID test, while a single SiN_x‐coated silicon solar cell showed almost 50% degradation after 24 h. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparison of TiO2 and other dielectric coatings for buried‐contact solar cells: a review

This paper compares the optical, electronic, physical and chemical properties of dielectric thin films that are commonly used to enhance the performance of bulk silicon photovoltaic devices. The standard buried‐contact (BC) solar cell presents a particularly challenging set of criteria, requiring the dielectric film to act as: (i) an anti‐reflection (AR) coating; (ii) a film compatible with surface passivation; (iii) a mask for an electroless metal plating step; (iv) a diffusion barrier for achieving a selective emitter; (v) a film with excellent chemical resistance; (vi) a stable layer during high‐temperature processing. The dielectric coatings reviewed here include thermally grown silicon dioxide (SiO₂), silicon nitride deposited by plasma‐enhanced chemical vapour deposition (a‐ SiN_x :H) and low‐pressure chemical vapour deposition (Si₃N₄), silicon oxynitride (SiON), cerium dioxide (CeO₂), zinc sulphide (ZnS), and titanium dioxide (TiO₂). While TiO₂ dielectric coatings exhibit the best optical performance and a simple post‐deposition surface passivation sequence has been developed, they require an additional sacrificial diffusion barrier to survive the heavy groove diffusion step. A‐ SiN_x :H affords passivation through its high fixed positive charge density and large hydrogen concentration; however, it is difficult to retain these electronic benefits during lengthy high‐temperature processing. Therefore, for the BC solar cell, Si₃N₄ films would appear to be the best choice of dielectric films common in industrial use. Copyright © 2004 John Wiley & Sons, Ltd.     

Industrial high‐rate (∼5 nm/s) deposited silicon nitride yielding high‐quality bulk and surface passivation under optimum anti‐reflection coating conditions

High‐quality surface and bulk passivation of crystalline silicon solar cells has been obtained under optimum anti‐reflection coating properties by silicon nitride (a‐SiN_x:H) deposited at very high deposition rates of ∼5 nm/s. These a‐SiN_x:H films were deposited using the expanding thermal plasma (ETP) technology under regular processing conditions in an inline industrial‐type reactor with a nominal throughput of 960 solar cells/hour. The low surface recombination velocities (50–70 cm/s) were obtained on p‐type silicon substrates (8·4 Ω cm resistivity) for as‐deposited and annealed films within the broad refractive index range of 1·9–2·4, which covers the optimum bulk passivation and anti‐reflection coating performance reached at a refractive index of ∼2·1. Copyright © 2005 John Wiley & Sons, Ltd.     

Approach for Al2O3 rear surface passivation of industrial p‐type Si PERC above 19%

Atomic layer deposition (ALD) of thin Al₂O₃ (≤10 nm) films is used to improve the rear surface passivation of large‐area screen‐printed p‐type Si passivated emitter and rear cells (PERC). A blister‐free stack of Al₂O₃/SiO_x/SiN_x is developed, leading to an improved back reflection and a rear recombination current (J_0,rear) of 92 ± 6 fA/cm². The Al₂O₃/SiO_x/SiN_x stack is blister‐free if a 700°C anneal in N₂ is performed after the Al₂O₃ deposition and prior to the SiO_x/SiN_x capping. A clear relationship between blistering density and lower open‐circuit voltage (V_OC) due to increased rear contacting area is shown. In case of the blister‐free Al₂O₃/SiO_x/SiN_x rear surface passivation stack, an average cell efficiency of 19.0% is reached and independently confirmed by FhG‐ISE CalLab. Compared with SiO_x/SiN_x‐passivated PERC, there is an obvious gain in V_OC and short‐circuit current (J_SC) of 5 mV and 0.2 mA/cm², respectively, thanks to improved rear surface passivation and rear internal reflection. Copyright © 2012 John Wiley & Sons, Ltd.     

20.0% Efficiency Si Nano/Microstructures Based Solar Cells with Excellent Broadband Spectral Response

Spectral response of solar cells determines the output performance of the devices. In this work, a 20.0% efficient silicon (Si) nano/microstructures (N/M‐Strus) based solar cell with a standard solar wafer size of 156 × 156 mm² (pseudo‐square) has been successfully fabricated, by employing the simultaneous stack SiO₂/SiN_x passivation for the front N/M‐Strus based n⁺‐emitter and the rear surface. The key to success lies in the excellent broadband spectral responses combining the improved short‐wavelength response of the stack SiO₂/SiN_x passivated Si N/M‐Strus based n⁺‐emitter with the extraordinary long‐wavelength response of the stack SiO₂/SiN_x passivated rear reflector. Benefiting from the broadband spectral response, the highest open‐circuit voltage (V_oc) and short‐circuit current density (J_sc) reach up to 0.653 V and 39.0 mA cm^?2, respectively. This high‐performance screen‐printed Si N/M‐Strus based solar cell has shown a very promising way to the commercial mass production of the Si based high‐efficient solar cells.     

Theory and experiments on the back side reflectance of silicon wafer solar cells

New passivation layers for the back side of silicon solar cells have to show high performance in terms of electrical passivation as well as high internal reflectivity. This optical performance is often shown as values for the back side reflectance R_b which describes the rear internal reflection. In this paper, we investigate in detail the meaning of this single‐value parameter, its correct determination and the use in one‐dimensional simulations with PC1D. The free‐carrier‐absorption (FCA) as non‐carrier‐generating absorption channel is analyzed for solar cells with varying thickness. We apply the optical analysis to samples with different thickness, silicon oxide layer thickness, rear side topography as well as passivation layers (SiO₂, SiN_x, SiC and stack systems). Additionally, the optical influence of the laser‐fired contacts (LFC) process is experimentally investigated. Finally, we show that with correct parameters, the one‐dimensional simulation of very thin silicon solar cells can successfully be performed. Copyright © 2007 John Wiley & Sons, Ltd.     

Performance improvement of c-Si solar cell by a combination of SiNx/SiOx passivation and double P-diffusion gettering treatment

SiN_x/SiO_x passivation and double side P-diffusion gettering treatment have been used for the fabrication of c-Si solar cells. The solar cells fabricated have high open circuit voltage and short circuit current after the double P-diffusion treatment. In addition to better surface passivation effect, SiN_x/SiO_x layer has lower reflectivity in long wavelength range than conventional SiN_x film. As a consequence, such solar cells exhibit higher conversion efficiency and better internal quantum efficiency, compared with conventional c-Si solar cells.     

Enhanced Omnidirectional Photovoltaic Performance of Solar Cells Using Multiple‐Discrete‐Layer Tailored‐ and Low‐Refractive Index Anti‐Reflection Coatings

An optimized four‐layer tailored‐ and low‐refractive index anti‐reflection (AR) coating on an inverted metamorphic (IMM) triple‐junction solar cell device is demonstrated. Due to an excellent refractive index matching with the ambient air by using tailored‐ and low‐refractive index nanoporous SiO₂ layers and owing to a multiple‐discrete‐layer design of the AR coating optimized by a genetic algorithm, such a four‐layer AR coating shows excellent broadband and omnidirectional AR characteristics and significantly enhances the omnidirectional photovoltaic performance of IMM solar cell devices. Comparing the photovoltaic performance of an IMM solar cell device with the four‐layer AR coating and an IMM solar cell with the conventional SiO₂/TiO₂ double layer AR coating, the four‐layer AR coating achieves an angle‐of‐incidence (AOI) averaged short‐circuit current density, J_SC, enhancement of 34.4%, whereas the conventional double layer AR coating only achieves an AOI‐averaged J_SC enhancement of 25.3%. The measured reflectance reduction and omnidirectional photovoltaic performance enhancement of the four‐layer AR coating are to our knowledge, the largest ever reported in the literature of solar cell devices.     

Multi‐crystalline Si solar cells with very fast deposited (180 nm/min) passivating hot‐wire CVD silicon nitride as antireflection coating

Hot‐wire chemical vapor deposition (HWCVD) is a promising technique for very fast deposition of high quality thin films. We developed processing conditions for device‐ quality silicon nitride (a‐SiN_x:H) anti‐reflection coating (ARC) at high deposition rates of 3 nm/s. The HWCVD SiN_x layers were deposited on multicrystalline silicon (mc‐Si) solar cells provided by IMEC and ECN Solar Energy. Reference cells were provided with optimized parallel plate PECVD SiN_x and microwave PECVD SiN_x respectively. The application of HWCVD SiN_x on IMEC mc‐Si solar cells led to effective passivation, evidenced by a V_oc of 606 mV and consistent IQE curves. For further optimization, series were made with HW SiN_x (with different x) on mc‐Si solar cells from ECN Solar Energy. The best cell efficiencies were obtained for samples with a N/Si ratio of 1·2 and a high mass density of >2·9 g/cm³. The best solar cells reached an efficiency of 15·7%, which is similar to the best reference cell, made from neighboring wafers, with microwave PECVD SiN_x. The IQE measurements and high V_oc values for these cells with HW SiN_x demonstrate good bulk passivation. PC1D simulations confirm the excellent bulk‐ and surface‐passivation for HW SiN_x coatings. Interesting is the significantly higher blue response for the cells with HWCVD SiN_x when compared to the PECVD SiN_x reference cells. This difference in blue response is caused by lower light absorption of the HWCVD layers (compared to microwave CVD; ECN) and better surface passivation (compared to parallel plate PECVD; IMEC). The application of HW SiN_x as a passivating antireflection layer on mc‐Si solar cells leads to efficiencies comparable to those with optimized PECVD SiN_x coatings, although HWCVD is performed at a much higher deposition rate. Copyright © 2007 John Wiley & Sons, Ltd.     

High efficiency screen‐printed EFG Si solar cells through rapid thermal processing‐induced bulk lifetime enhancement

This paper shows that one second (1 s) firing of Si solar cells with screen‐printed Al on the back and SiN _x anti‐reflection coating on the front can produce a high quality Al‐doped back‐surface‐field (Al‐BSF) and significantly enhance SiN _x ‐induced defect hydrogenation in the bulk Si. Open‐circuit voltage, internal quantum efficiency measurements, and cross‐sectional scanning electron microscopy pictures on float‐zone silicon cells revealed that 1 s firing in rapid thermal processing at 750°C produces just as good a BSF as 60 s firing, indicating that the quality of Al‐BSF region is not a strong function of RTP firing time at 750°C. Analysis of edge‐defined film‐fed grown (EFG) Si cells showed that short‐term firing is much more effective in improving the hydrogen passivation of bulk defects in EFG Si. Average minority‐carrier lifetime in EFG wafers improved from ∼3 to ∼33 μs by 60 s firing but reached as high as 95μs with 1 s firing, resulting in 15·6% efficient screen‐printed cells on EFG Si. Copyright © 2004 John Wiley & Sons, Ltd.     

Laser‐fired contact optimization in c‐Si solar cells

In this work we study the optimization of laser‐fired contact (LFC) processing parameters, namely laser power and number of pulses, based on the electrical resistance measurement of an aluminum single LFC point. LFC process has been made through four passivation layers that are typically used in c‐Si and mc‐Si solar cell fabrication: thermally grown silicon oxide (SiO₂), deposited phosphorus‐doped amorphous silicon carbide (a‐SiC_x/H(n)), aluminum oxide (Al₂O₃) and silicon nitride (SiN_x/H) films. Values for the LFC resistance normalized by the laser spot area in the range of 0.65–3 mΩ cm² have been obtained. Copyright © 2011 John Wiley & Sons, Ltd.     

Moth‐eye antireflection coating fabricated by nanoimprint lithography on 1 eV dilute nitride solar cell

We report on the performance of biomimicked antireflection coating applied to dilute nitride solar cell. The coating consists of nanostructures replicating the moth‐eye geometry and has been fabricated by nanoimprint lithography directly within the window layer covering the dilute nitride absorbing junction. The mean reflectivity within the spectral range of 320–1800 nm remains under 5% for incident angles up to 45°. The effect of the coating on the cell performance was assessed by measuring the current–voltage characteristics under simulated solar illumination. A clear performance increase was identified when comparing a solar cell with the moth‐eye coating with a solar cell having a standard SiN_x/SiO₂ coating. Copyright © 2012 John Wiley & Sons, Ltd.     

N型表面钝化晶体硅太阳电池Al2O3/SiNx减反射叠层的优化研究

减反射特性是进一步提高N型太阳电池能量转换效率的重要因素之一。研究采用Al2O3/SiNx叠层优化了N型太阳电池的减反射特性,并通过理论模拟和实验测量系统地探讨了叠层中SiNx厚度对表面反射性能的影响。研究证实在Al2O3层上增加一层SiNx,可以有效地优化表面减反射性质,从而提高N型太阳电池的光伏性质。     

Preparation and measurement of highly efficient a‐Si:H single junction solar cells and the advantages of μc‐SiOx:H n‐layers

Reducing the optical losses and increasing the reflection while maintaining the function of doped layers at the back contact in solar cells are important issues for many photovoltaic applications. One approach is to use doped microcrystalline silicon oxide (μc‐SiO_x:H) with lower optical absorption in the spectral range of interest (300 nm to 1100 nm). To investigate the advantages, we applied the μc‐SiO_x:H n‐layers to a‐Si:H single junction solar cells. We report on the comparison between amorphous silicon (a‐Si:H) single junction solar cells with either μc‐SiO_x:H n‐layers or non‐alloyed silicon n‐layers. The origin of the improved performance of a‐Si:H single junction solar cells with the μc‐SiO_x:H n‐layer is identified by distinguishing the contributions because of the increased transparency and the reduced refractive index of the μc‐SiO_x:H material. The solar cell parameters of a‐Si:H solar cells with both types of n‐layers were compared in the initial state and after 1000 h of light soaking in a series of solar cells with various absorber layer thicknesses. The measurement procedure for the determination of the solar cell performance is described in detail, and the measurement accuracy is evaluated and discussed. For an a‐Si:H single junction solar cell with a μc‐SiO_x:H n‐layer, a stabilized efficiency of 10.3% after 1000 h light soaking is demonstrated. Copyright © 2015 John Wiley & Sons, Ltd.     

Laser ablation of SiO2 for locally contacted Si solar cells with ultra‐short pulses

We apply ultra‐short pulse laser ablation to create local contact openings in thermally grown passivating SiO₂ layers. This technique can be used for locally contacting oxide passivated Si solar cells. We use an industrially feasible laser with a pulse duration of τ_pulse ∼ 10 ps. The specific contact resistance that we reach with evaporated aluminium on a 100 Ω/sq and P‐diffused emitter is in the range of 0·3–1 mΩ cm². Ultra‐short pulse laser ablation is sufficiently damage free to abandon wet chemical etching after ablation. We measure an emitter saturation current density of J_0e = (6·2 ± 1·6) × 10⁻¹³ A/cm² on the laser‐treated areas after a selective emitter diffusion with R_sheet ∼ 20 Ω/sq into the ablated area; a value that is as low as that of reference samples that have the SiO₂ layer removed by HF‐etching. Thus, laser ablation of dielectrics with pulse durations of about 10 ps is well suited to fabricate high‐efficiency Si solar cells. Copyright © 2007 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.     

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Atomic‐layer‐deposited aluminium oxide (Al₂O₃) is applied as rear‐surface‐passivating dielectric layer to passivated emitter and rear cell (PERC)‐type crystalline silicon (c‐Si) solar cells. The excellent passivation of low‐resistivity p‐type silicon by the negative‐charge‐dielectric Al₂O₃ is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al₂O₃ film covered by a 200 nm plasma‐enhanced‐chemical‐vapour‐deposited silicon oxide (SiO_x) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single‐layer of Al₂O₃, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.     

Broadband triple-layer SiOx/SiOxNy/SiNx antireflective coatings in textured crystalline silicon solar cells

The purpose of this study is to reduce textured crystalline silicon (TCS) substrate surface-reflectivity over a wide spectral range (300–1100 nm), to improve the step coverage of the textured structure, and to shift the minimal value of reflection from the unabsorbed region to the absorbed region. The TCS solar-cell interface between air and silicon was added to a SiO_x/SiO_xN_y/SiN_x triple-layer anti-reflective coatings (TLARCs) structure using the plasma-enhanced chemical vapor deposition (PECVD) growth method. This paper presents theoretical and practical discussions, as well as the experimental results of fabricating the films and devices. The average reflection of the SiO_x/SiO_xN_y/SiN_x TLARs reduced to 2.01% (300–1100 nm). The minimal value of reflection was shifted from 1370 nm (unabsorbed region) to 968 nm (absorbed region). The SEM images show effective step coverage. In comparison to the untreated TCS solar cells, applying the experimental SiO_x/SiO_xN_y/SiN_x TLARCs to conventional TCS solar cells improved the short-circuit current density (J_sc) by 7.78%, and solar-cell efficiency by 10.95%. This study demonstrates that the SiO_x/SiO_xN_y/SiN_x TLARCs structure provides antireflective properties over a broad range of visible and near-infrared light wavelengths. An effective step coverage and minimal value of reflection from unabsorbed region shift to the absorbed region is demonstrated.