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.     

Laser‐fired contact for n‐type crystalline Si solar cells

Laser‐fired contacts to n‐type crystalline silicon were developed by investigating novel metal stacks containing Antimony (Sb). Lasing conditions and the structure of metals stacks were optimized for lowest contact resistance and minimum surface damage. Specific contact resistance for firing different metal stacks through either silicon nitride or p‐type amorphous silicon was determined using two different models and test structures. Specific contact resistance values of 2–7 mΩcm² have been achieved. Recombination loss due to laser damage was consistent with an extracted local surface recombination velocity of ~20 000 cm/s, which is similar to values for laser‐fired base contact for p‐type crystalline silicon. Interdigitated back contact silicon heterojunction cells were fabricated with laser‐fired base contact and proof‐of‐concept efficiencies of 16.9% were achieved. This localized base contact technique will enable low cost back contact patterning and innovative designs for n‐type crystalline solar cell. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Stack system of PECVD amorphous silicon and PECVD silicon oxide for silicon solar cell rear side passivation

A stack of hydrogenated amorphous silicon (a‐Si) and PECVD‐silicon oxide (SiO_x) has been used as surface passivation layer for silicon wafer surfaces. Very good surface passivation could be reached leading to a surface recombination velocity (SRV) below 10 cm/s on 1 Ω cm p‐type Si wafers. By using the passivation layer system at a solar cell's rear side and applying the laser‐fired contacts (LFC) process, pointwise local rear contacts have been formed and an energy conversion efficiency of 21·7% has been obtained on p‐type FZ substrates (0·5 Ω cm). Simulations show that the effective rear SRV is in the range of 180 cm/s for the combination of metallised and passivated areas, 120 ± 30 cm/s were calculated for the passivated areas. Rear reflectivity is comparable to thermally grown silicon dioxide (SiO₂). a‐Si rear passivation appears more stable under different bias light intensities compared to thermally grown SiO₂. Copyright © 2008 John Wiley & Sons, Ltd.     

Industrially feasible,dopant‐free,carrier‐selective contacts for high‐efficiency silicon solar cells

Dopant‐free, carrier‐selective contacts (CSCs) on high efficiency silicon solar cells combine ease of deposition with potential optical benefits. Electron‐selective titanium dioxide (TiO₂) contacts, one of the most promising dopant‐free CSC technologies, have been successfully implemented into silicon solar cells with an efficiency over 21%. Here, we report further progress of TiO₂ contacts for silicon solar cells and present an assessment of their industrial feasibility. With improved TiO₂ contact quality and cell processing, a remarkable efficiency of 22.1% has been achieved using an n‐type silicon solar cell featuring a full‐area TiO₂ contact. Next, we demonstrate the compatibility of TiO₂ contacts with an industrial contact‐firing process, its low performance sensitivity to the wafer resistivity, its applicability to ultrathin substrates as well as its long‐term stability. Our findings underscore the great appeal of TiO₂ contacts for industrial implementation with their combination of high efficiency with robust fabrication at low cost. Copyright © 2017 John Wiley & Sons, Ltd.     

Metal aerosol jet printing for solar cell metallization

Metal aerosol jet printing is a new non‐contact direct‐write technique for the front side metallization of highly efficient industrial silicon solar cells. With this technique the first layer of a two‐layer contact structure is created. It features a low contact resistance and good mechanical adhesion to the silicon surface. The second layer is formed by light‐induced silver plating (LIP) to increase the line conductivity. To form the first layer a metal‐containing aerosol is created in the printer and focused via a second surrounding gas stream through a nozzle and deposited onto the substrate. The focussing gas avoids the contact between the aerosol and the nozzle tip. In addition, line widths significantly smaller than the outlet diameter of the nozzle tip can be reached. Fine and continuous lines with a width of 14 µm were printed using a metal organic ink. As the adhesion of these layers was not sufficient, a commercially available screen‐printing paste for solar cell metallization was modified and tested. Monocrystalline silicon solar cells of 12·5 cm × 12·5 cm with an aluminum back surface field were processed, achieving energy conversion efficiencies up to 17·8%. Copyright © 2007 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.     

Electrical properties of fine line printed and light‐induced plated contacts on silicon solar cells

The properties of fine‐line printed contacts on silicon solar cells, in combination with light‐induced plating (LIP), are presented. The seed layers are printed using an aerosol system and a new metallization ink called SISC developed at Fraunhofer ISE. The influence of multiple layer printing on the contact geometry is studied as well as the influence of the contact height on the line resistivity and on the contact resistance. The dependence between contact resistance and contact height is measured using the transfer length model (TLM). Further on, it is explained by taking SEM images of the metal–semiconductor interface, that a contact height of less than 1 µm or a minimum ink amount of only 4–6 mg is sufficient to contact a large area (15·6 cm × 15·6 cm) silicon solar cell on the front side and results in a contact resistance R_c × W < 0·5 Ω cm. As the line resistivity of fine‐line printed fingers needs to be reduced by LIP, three different plating solutions are tested on solar cells. The observed differences in line resistivity between ρ_f = 5 × 10⁻⁸ and 2 × 10⁻⁸ Ω m are explained by taking SEM pictures of the grown LIP‐silver. Finally, the optimum LIP height for different line resistivities is calculated and experimentally confirmed by processing solar cells with an increasing amount of LIP silver. Copyright © 2010 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.     

20.1%‐efficient crystalline silicon solar cell with amorphous silicon rear‐surface passivation

We have developed a crystalline silicon solar cell with amorphous silicon (a‐Si:H) rear‐surface passivation based on a simple process. The a‐Si:H layer is deposited at 225°C by plasma‐enhanced chemical vapor deposition. An aluminum grid is evaporated onto the a‐Si:H‐passivated rear. The base contacts are formed by COSIMA (contact formation to a‐Si:H passivated wafers by means of annealing) when subsequently depositing the front silicon nitride layer at 325°C. The a‐Si:H underneath the aluminum fingers dissolves completely within the aluminum and an ohmic contact to the base is formed. This contacting scheme results in a very low contact resistance of 3.5 ±0.2 mΩ cm² on low‐resistivity (0.5 Ω cm) p‐type silicon, which is below that obtained for conventional Al/Si contacts. We achieve an independently confirmed energy conversion efficiency of 20.1% under one‐sun standard testing conditions for a 4 cm² large cell. Measurements of the internal quantum efficiency show an improved rear surface passivation compared with reference cells with a silicon nitride rear passivation. Copyright © 2005 John Wiley & Sons, Ltd.     

All‐screen‐printed back‐contact back‐junction silicon solar cells with aluminum‐alloyed emitter and demonstration of interconnection of point‐shaped metalized contacts

In the following, high‐efficiency back‐contact back‐junction silicon solar cells with aluminum‐alloyed emitter are described. First, the theoretical background for the cell concept is explained. To that purpose, the bulk lifetime and the front surface field characteristics are considered. Three different process sequences for the phosphorus‐diffused profiles on the front and back surfaces are depicted: One exhibits a shallow field, and two sequences have deeper, driven‐in profiles. For realizing high efficiencies, such cell structures must meet several prerequisites, such as firing‐stable front and rear passivations, and functional small screen‐printed Al structures. Furthermore, it must be possible to create contacts on the Si surfaces using the driven‐in P‐profiles. With such a structure, cell efficiencies of 20.0% are reached. An analysis of the series resistance and area‐weighted recombination is performed. The results are compared with the measured cell parameters. Two‐dimensional simulations show the efficiency potential when decreasing the width of the backside field and when a cell structure, which would inhibit a passivated aluminum‐alloyed p⁺‐emitter, is created. Also, an advanced concept is demonstrated where a point array of both polarities on the cell backside is interconnected externally on module level. To that purpose, the cell is soldered to a printed wiring circuit board by using a reflow soldering process. Copyright © 2013 John Wiley & Sons, Ltd.     

Impact of excess phosphorus doping and Si crystalline defects on Ag crystallite nucleation and growth in silver screen‐printed Si solar cells

Good quality contacts between metal and silicon emitter are crucial for high crystalline solar cell efficiencies. We investigate the impact of defects originating from electrically inactive phosphorus on contact formation within silver thick film metallized silicon solar cells. For this purpose, emitters with varying sheet resistance, depth, and dead layer were metallized with silver pastes from different generations. Macroscopic contact resistivity measurements were compared with the microscopic contact configurations studied by scanning electron microscopy. The density of direct contacts between Ag crystallites grown into Si and the Ag finger bulk is essential for low contact resistivity. The presence of glass‐free regions needed for such direct contacts depends on the paste composition and on the surface texture, and does not vary with the Si emitter properties. Indeed, the decrease in contact resistivity correlates with increasing density of Ag crystallites embedded in the Si surface. Furthermore, the density of Si surface‐embedded Ag crystallites scales proportional to the electrically inactive P and is independent of the sheet resistance. Using the newest silver paste, the Ag crystallite density is independent of the emitter doping, but the Ag crystallite size increases as a function of the thickness of the dead layer. Transmission electron microscopy characterization of the excess P‐doped Si crystal lattice shows that significant strain and Si bond weakening may play a major role for both Ag crystallite nucleation and growth. Finally, we studied Si crystal defects by metallizing nanocracks, dislocations, and grain boundaries and found that Ag crystallite nucleation is defect‐property dependent. Copyright © 2013 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.     

Modeling the potential of screen printed front junction CZ silicon solar cell with tunnel oxide passivated back contact

Carrier selective passivated contacts composed of thin oxide, n + polycrystalline Si and metal on top of a n‐Si absorber can significantly lower the recombination current density (J_orear ≤8 fA/cm²) under the contact while providing excellent specific contact resistance (5–10 mΩ‐cm²); 25.1% efficient small area cells with photolithography front contacts on boron doped selective emitter and Fz wafers have been achieved by Fraunhofer ISE using their tunnel oxide passivated contact (TOPCon) approach. This paper shows a methodology to model such passivated contact cells using Sentaurus device model, which involves replacing the TOPCon region by carrier selective electron and hole recombination velocities to match the measured J_orear of the TOPCon region as well as all the light IV values of the cell. We first validated the methodology by modeling a 24.9% reference cell. The model was then extended to assess the efficiency potential of large area TOPCon cells on commercial grade n‐type Cz material with screen‐printed front contacts. To use realistic input parameters, a 21% n‐type PERT cell was fabricated on Cz wafer (5 Ω‐cm, 1.5‐ms lifetime). Modeling showed that the cell efficiency will improve to only 21.6% if the back of this cell is replaced by the above TOPCon, and the performance is limited by the homogenous emitter. Efficiency was then modeled to improve to 22.6% with the implementation of selective emitter (150/20 Ω/sq). Finally, it is shown that screen printing of 40‐µm‐wide lines and improved bulk material (10 Ω‐cm, 3‐ms lifetime) can raise the single side TOPCon Cz cell efficiency to 23.2%. Copyright © 2016 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.     

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.     

Experimental and simulated analysis of front versus all‐back‐contact silicon heterojunction solar cells: effect of interface and doped a‐Si:H layer defects

Front silicon heterojunction and interdigitated all‐back‐contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high efficiency and low cost because of their good surface passivation, heterojunction contacts, and low temperature fabrication processes. The performance of both heterojunction device structures depends on the interface between the crystalline silicon (c‐Si) and intrinsic amorphous silicon [(i)a‐Si:H] layer, and the defects in doped a‐Si:H emitter or base contact layers. In this paper, effective minority carrier lifetimes of c‐Si using symmetric passivation structures were measured and analyzed using an extended Shockley–Read–Hall formalism to determine the input interface parameters needed for a successful 2D simulation of fabricated baseline solar cells. Subsequently, the performance of front silicon heterojunction and IBC‐SHJ devices was simulated to determine the influence of defects at the (i)a‐Si:H/c‐Si interface and in the doped a‐Si:H layers. For the baseline device parameters, the difference between the two device configurations is caused by the emitter/base contact gap recombination and the back surface geometry of IBC‐SHJ solar cell. This work provides a guide to the optimization of both types of SHJ device performance, predicting an IBC‐SHJ solar cell efficiency of 25% for realistic material parameters. Copyright © 2013 John Wiley & Sons, Ltd.     

Buried contact solar cells with innovative rear localised contacts

A new rear contacting scheme using low‐temperature processes to form localised contacts without the use of photolithography has been developed. It uses randomly nucleated, aluminium‐induced, localised regions of solid phase epitaxial growth of p ⁺ silicon onto the rear surface of a wafer through a thick rear surface passivating oxide. This rear contacting technique has been applied on solar cells with front buried contacts and results have shown that a suitable ohmic contact to the rear can be formed through oxide as thick as 3000 Å and using only low temperature sintering below the eutectic temperature of silicon and aluminium. This low‐temperature sintering avoids the destruction of the interfacial oxide which has been shown to provide good surface passivation for the rear of the solar cells. Microscopic images indicate the possibility of forming p ⁺ rear contacts with aluminium‐induced crystallisation, but without requiring any additional deposition of silicon. The source of silicon for the latter appears to be from the reduction of the silicon dioxide by the aluminium. Copyright © 2004 John Wiley & Sons, Ltd.     

High‐Speed Organic Single‐Crystal Transistor Responding to Very High Frequency Band

Although high carrier mobility organic field‐effect transistors (OFETs) are required for high‐speed device applications, improving the carrier mobility alone does not lead to high‐speed operation. Because the cut‐off frequency is determined predominantly by the total resistance and parasitic capacitance of a transistor, it is necessary to miniaturize OFETs while reducing these factors. Depositing a dopant layer only at the metal/semiconductor interface is an effective technique to reduce the contact resistance. However, fine‐patterning techniques for a dopant layer are still challenging especially for a top‐contact solution‐processed OFET geometry because organic semiconductors are vulnerable to chemical damage by solvents. In this work, high‐resolution, damage‐free patterning of a dopant layer is developed to fabricate short‐channel OFETs with a dopant interlayer inserted at the contacts. The fabricated OFETs exhibit high mobility exceeding 10 cm² V^?1 s^?1 together with a reasonably low contact resistance, allowing for high frequency operation at 38 MHz. In addition, a diode‐connected OFET shows a rectifying capability of up to 78 MHz at an applied voltage of 5 V. This shows that an OFET can respond to the very high frequency band, which is beneficial for long‐distance wireless communication.     

Metallo‐Hydrogel‐Assisted Synthesis and Direct Writing of Transition Metal Dichalcogenides

Two dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted interest for their compelling nanoscale new properties and numerous potential applications including fast optoelectronic devices, ultrathin photovoltaics, and high‐performance catalysts. Large‐scale growth of uniform TMDC materials is essential for investigating their physics and for their integration into devices. However, the wafer scale deposition of TMDCs on arbitrary nonselective substrates is still beyond the current state‐of‐the‐art. In this article, a method to synthesize layered TMDCs (MoS₂ and WS₂) at the wafer‐scale by sulfurization of transition metal ions (Mo⁵⁺ and W⁶⁺) in a gelatin template (metallo‐hydrogel) is reported. This process is adaptable to versatile substrates, including amorphous silicon oxide, high‐temperature quartz, and silicon. Although the products are nominally few layer materials, direct band photoluminescent (≈1.8 eV), similar to single‐ or decoupled multilayer MoS₂ is observed. Finally, the solution‐based deposition enables contact printing of TMDC channels to be useable for device applications including thin film transistors with printed silver contacts using the same process.