Optimizing annealing steps for crystalline silicon solar cells with screen printed front side metallization and an oxide‐passivated rear surface with local contacts

Silicon solar cells that feature screen printed front contacts and a passivated rear surface with local contacts allow higher efficiencies compared to present industrial solar cells that exhibit a full area rear side metallization. If thermal oxidation is used for the rear surface passivation, the final annealing step in the processing sequence is crucial. On the one hand, this post‐metallization annealing (PMA) step is required for decreasing the surface recombination velocity (SRV) at the aluminum‐coated oxide‐passivated rear surface. On the other hand, PMA can negatively affect the screen printed front side metallization leading to a lower fill factor. This work separately analyzes the impact of PMA on both, the screen printed front metallization and the oxide‐passivated rear surface. Measuring dark and illuminated IV‐curves of standard industrial aluminum back surface field (Al‐BSF) silicon solar cells reveals the impact of PMA on the front metallization, while measuring the effective minority carrier lifetime of symmetric lifetime samples provides information about the rear side SRV. One‐dimensional simulations are used for predicting the cell performance according to the contributions from both, the front metallization and the rear oxide‐passivation for different PMA temperatures and durations. The simulation also includes recombination at the local rear contacts. An optimized PMA process is presented according to the simulations and is experimentally verified. The optimized process is applied to silicon solar cells with a screen printed front side metallization and an oxide‐passivated rear surface. Efficiencies up to 18.1% are achieved on 148.8 cm² Czochralski (Cz) silicon wafers. Copyright © 2009 John Wiley & Sons, Ltd.     

High potential of thin (<1 µm) a‐Si: H/µc‐Si:H tandem solar cells

Silicon based thin tandem solar cells were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a 30 × 30 cm² reactor. The layer thicknesses of the amorphous top cells and the microcrystalline bottom cells were significantly reduced compared to standard tandem cells that are optimized for high efficiency (typically with a total absorber layer thickness from 1.5 to 3 µm). The individual absorber layer thicknesses of the top and bottom cells were chosen so that the generated current densities are similar to each other. With such thin cells, having a total absorber layer thickness varying from 0.5 to 1.5 µm, initial efficiencies of 8.6–10.7% were achieved. The effects of thickness variations of both absorber layers on the device properties have been separately investigated. With the help of quantum efficiency (QE) measurements, we could demonstrate that by reducing the bottom cell thickness the top cell current density increased which is addressed to back‐reflected light. Due to a very thin a‐Si:H top cell, the thin tandem cells show a much lower degradation rate under continuous illumination at open circuit conditions compared to standard tandem and a‐Si:H single junction cells. We demonstrate that thin tandem cells of around 550 nm show better stabilized efficiencies than a‐Si:H and µc‐Si:H single junction cells of comparable thickness. The results show the high potential of thin a‐Si/µc‐Si tandem cells for cost‐effective photovoltaics. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

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.     

Laser processing of Al2O3/a‐SiCx:H stacks: a feasible solution for the rear surface of high‐efficiency p‐type c‐Si solar cells

We explore the potential of laser processing aluminium oxide (Al₂O₃)/amorphous silicon carbide (a‐SiC_x:H) stacks to be used at the rear surface of p‐type crystalline silicon (c‐Si) solar cells. For this stack, excellent quality surface passivation is measured with effective surface recombination velocities as low as 2 cm/s. By means of an infrared laser, the dielectric film is locally opened. Simultaneously, part of the aluminium in the Al₂O₃ film is introduced into the c‐Si, creating p+ regions that allow ohmic contacts with low‐surface recombination velocities. At optimum pitch, high‐efficiency solar cells are achievable for substrates of 0.5–2.5 Ω cm. Copyright © 2012 John Wiley & Sons, Ltd.     

Current transport studies of amorphous n/p junctions and its application in a‐Si:H/HIT‐type tandem cells

This paper presents an understanding of the fundamental carrier transport mechanism in hydrogenated amorphous silicon (a‐Si:H)‐based n/p junctions. These n/p junctions are, then, used as tunneling and recombination junctions (TRJ) in tandem solar cells, which were constructed by stacking the a‐Si:H‐based solar cell on the heterojunction with intrinsic thin layer (HIT) cell. First, the effect of activation energy (E_a) and Urbach parameter (E_u) of n‐type hydrogenated amorphous silicon (a‐Si:H(n)) on current transport in an a‐Si:H‐based n/p TRJ has been investigated. The photoluminescence spectra and temperature‐dependent current–voltage characteristics in dark condition indicates that the tunneling is the dominant carrier transport mechanism in our a‐Si:H‐based n/p‐type TRJ. The fabrication of a tandem cell structure consists of an a‐Si:H‐based top cell and an HIT‐type bottom cell with the a‐Si:H‐based n/p junction developed as a TRJ in between. The development of a‐Si:H‐based n/p junction as a TRJ leads to an improved a‐Si:H/HIT‐type tandem cell with a better open circuit voltage (V_oc), fill factor (FF), and efficiency. The improvements in the cell performance was attributed to the wider band‐tail states in the a‐Si:H(n) layer that helps to an enhanced tunneling and recombination process in the TRJ. The best photovoltage parameters of the tandem cell were found to be V_oc = 1430 mV, short circuit current density = 10.51 mA/cm², FF = 0.65, and efficiency = 9.75%. Copyright © 2015 John Wiley & Sons, Ltd.     

Very low surface recombination velocity of crystalline silicon passivated by phosphorus‐doped a‐SicxNy:H(n) alloys

Hydrogenated and phosphorus‐doped amorphous silicon carbonitride films (a‐SiC_xN_y:H(n)) were deposited by plasma‐enhanced chemical vapor deposition (PECVD) on crystalline silicon surface in order to explore surface passivation properties. Very silicon‐rich films yielded effective surface recombination velocities at 1 sun‐illumination as low as 3 cm s⁻¹ and 2 cm s⁻¹ on 1 Ω cm p‐ and n‐type crystalline silicon substrates, respectively. In order to use them as anti‐reflection coating, we increased alternatively either the carbon or nitrogen content of these films. Also, a combination of passivation and antireflective films was analyzed. Finally, we explored the passivation stability under high‐temperature steps. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

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.     

Comprehensive analysis of advanced solar cell contacts consisting of printed fine‐line seed layers thickened by silver plating

This work presents a detailed analysis of a new two‐layer process to contact industrial solar cells. However, most of the results seem to be transferable to standard screen print paste contacts. The seed layer was created by a pad or screen printer and thickened by light‐induced plating (LIP) of silver. These contact structures were investigated microscopically to gain a better understanding of the observed electrical parameters. A review of the present microscopic contact formation model for flat surfaces is presented. This model was extended and applied to surfaces textured with random pyramids. This analysis has revealed two new types of silver crystallites which can be described by a crystallographic model. The dependence of the silver crystallite density on the surface doping concentration was investigated. Next, the dependence of the contact resistance on the width of the seed layer was measured showing that the contact resistivity increases with a reduction of the seed layer width. These results have been further approved by an analysis of SEM images of wet‐chemically etched contacts examining the density of crystallites and the fraction of removed SiN_x layer. Contact resistance R_C measurements before and after LIP of silver showed surprisingly a positive influence of the plating process on R_C. A detailed microscopical analysis revealed four new possible current flow paths due to the LIP of a conventional contact or a seed layer. The results led to an extension of the existing model for a screen‐printed contact. Copyright © 2008 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.     

High‐rate deposition of a‐Si:H thin layers for high‐performance silicon heterojunction solar cells

In this paper, we describe a technique for high‐quality interface passivation of n‐type crystalline silicon wafers through the growth of hydrogenated amorphous Si (a‐Si:H) thin layers using conventional plasma‐enhanced chemical vapor deposition. We investigated the onset of crystallization of the a‐Si:H layers at various deposition rates and its effect on the surface passivation properties. Epitaxial growth occurred, even at a low substrate temperature of 90 °C, when the deposition rate was as low as 0·5 Å/s; amorphous growth occurred at temperatures up to 150 °C at a higher deposition rate of 4·2 Å/s. After optimizing the intrinsic a‐Si:H layer deposition conditions and then subjecting the sample to post‐annealing treatment, we achieved a very low surface recombination velocity (7·6 cm/s) for a double‐sided intrinsic a‐Si:H coating on an n‐type crystalline silicon wafer. Under the optimized conditions, we achieved an untextured heterojunction cell efficiency of 16·7%, with a high open‐circuit voltage (694 mV) on an n‐type float‐zone Si substrate. On a textured wafer, the cell efficiency was further enhanced to 19·6%. Copyright © 2011 John Wiley & Sons, Ltd.     

Diffusion‐based model of local Al back surface field formation for industrial passivated emitter and rear cell solar cells

In this work, the back surface field (BSF) formation of locally alloyed Al‐paste contacts employed in recent industrial passivated emitter and rear cell solar cell designs is discussed. A predictive model for resulting local BSF thickness and doping profile is proposed that is based on the time‐dependent Si distribution in the molten Al paste during the firing step. Diffusion of Si in liquid Al away from the contact points is identified as the main differentiator to a full‐area Al‐BSF; therefore, a diffusion‐based solution to the involved differential equation is pursued. Data on the Si distribution in the Al and the resulting BSF structures are experimentally obtained by firing samples with different metal contact geometries, peak temperature times and pastes as well as by investigating them by means of scanning electron microscopy and energy dispersive X‐ray spectroscopy. The Si diffusivity in the Al paste is then calculated from these results. It is found that the diffusivity is strongly dependent on the paste composition. Furthermore, the local BSF doping profiles and thicknesses resulting from different contact geometries and paste parameters are calculated from the Si concentration at the contact sites, the diffusivity and solubility data. These profiles are then used in a finite element device simulator to evaluate their performance on solar cell level. With this approach, a beneficial paste composition for any given rear contact geometry can be determined. Two line widths are investigated, and the effects of the different paste properties are discussed in the light of the solar cell results obtained by simulation. Copyright © 2013 John Wiley & Sons, Ltd.     

Improved conversion efficiency of a‐Si:H/µc‐Si:H thin‐film solar cells by using annealed Al‐doped zinc oxide as front electrode material

In recent years, zinc oxide has been investigated as a front electrode material in hydrogenated amorphous silicon/hydrogenated microcrystalline silicon (a‐Si:H/µc‐Si:H) tandem solar cells. Such as for other transparent conducting oxide materials and applications, a proper balancing of transparency and conductivity is necessary. The latter is directly related to the density and the mobility of charge carriers. A high density of charge carriers increases conductivity but leads to a higher absorption of light in the near‐infrared part of the spectrum due to increased free‐carrier absorption. Hence, the only way to achieve high conductivity while keeping the transparency as high as possible relies on an increase of carrier mobility. The carrier density and the mobility of sputtered Al‐doped zinc oxide (ZnO:Al) can be tailored by a sequence of different annealing steps. In this work, we implemented such annealed ZnO:Al films as a front electrode in a‐Si:H/µc‐Si:H tandem solar cells and compared the results with those of reference cells grown on as‐deposited ZnO:Al. We observed an improvement of short‐circuit current density as well as open‐circuit voltage and fill factor. The gain in current density could be attributed to a reduction of both sub‐band‐gap absorption and free‐carrier absorption in the ZnO:Al. The higher open‐circuit voltage and fill factor are indicators of a better device quality of the silicon for cells grown on annealed ZnO:Al. Altogether, the annealing led to an improved initial conversion efficiency of 12.1%, which was a gain of +0.7% in absolute terms. Copyright © 2013 John Wiley & Sons, Ltd.     

Dynamic Pixel Models for a‐Si TFT‐LCD and Their Implementation in SPICE

A dynamic analysis of an amorphous silicon (a‐Si) thin film transistor liquid crystal display (TFT‐LCD) pixel is presented using new a‐Si TFT and liquid crystal (LC) capacitance models for a Simulation Program with Integrated Circuit Emphasis (SPICE) simulator. This dynamic analysis will be useful when predicting the performance of LCDs. The a‐Si TFT model is developed to accurately estimate a‐Si TFT characteristics of a bias‐dependent gate to source and gate to drain capacitance. Moreover, the LC capacitance model is developed using a simplified diode circuit model. It is possible to accurately predict TFT‐LCD characteristics such as flicker phenomena when implementing the proposed simulation model.     

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.     

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.     

DC‐sputtered ZnO:Al as transparent conductive oxide for silicon heterojunction solar cells with µc‐Si:H emitter

Silicon heterojunction (SHJ) solar cells are highly interesting, because of their high efficiency and low cost fabrication. So far, the most applied transparent conductive oxide (TCO) is indium tin oxide (ITO). The replacement of ITO with cheaper, more abundant and environmental friendly material with texturing capability is a promising way to reduce the production cost of the future SHJ solar cells. Here, we report on the fabrication of the SHJ solar cells with direct current‐sputtered aluminum‐doped zinc oxide (ZnO:Al) as an alternative TCO. Furthermore, we address several important differences between ITO and the ZnO:Al layers including a high Schottky barrier at the emitter/ZnO:Al interface and a high intrinsic resistivity of the ZnO:Al layers. To overcome the high Schottky barrier, we suggest employing micro‐crystalline silicon (µc‐Si:H) emitter, which also improves temperature threshold and passivation of the solar cell precursor. In addition, we report on the extensive studies of the effect of the ZnO:Al deposition parameters including layer thickness, oxygen flow, power density and temperature on the electrical properties of the fabricated SHJ solar cells. Finally, the results of our study indicate that the ZnO:Al deposition parameters significantly affect the electrical properties of the obtained solar cell. By understanding and fine‐tuning all these parameters, a high conversion efficiency of 19.2% on flat wafer (small area (5 × 5 mm²) and without any front metal grid) is achieved. Copyright © 2014 John Wiley & Sons, Ltd.     

Improved micromorph solar cells by means of mixed‐phase n‐doped silicon oxide layers

A good light trapping scheme is necessary to improve the performance of amorphous/microcrystalline silicon tandem cells. This is generally achieved by using a highly reflective transparent conducting oxide/metal back contact plus an intermediate reflector between the component cells. In this work, the use of doped silicon oxide as alternative n‐layer in micromorph solar cells is proposed as a means to obtain high current values using a simple Ag back contact and no extra reflector between the component cells n‐doped silicon oxide layers with a wide range of optical and electrical properties have been prepared. The influence of different deposition regimes on the material properties has been studied. The main findings are the following: (i) when carbon dioxide is added to the gas mixture, sufficiently high hydrogen dilution is necessary to widen the transition region from highly conductive microcrystalline‐like films to amorphous material characterized by low electrical conductivity; (ii) lower refractive index values are found with lower deposition pressure. Optimal n‐doped silicon oxide layers have been used in both component cells of micromorph devices, adopting a simple Ag back contact. Higher current values for both cells are obtained in comparison with the values obtained using standard n‐doped microcrystalline silicon, whereas similar values of fill factor and open circuit voltage are measured. The current enhancement is particularly evident for the bottom cell, as revealed by the increased spectral response in the red/infrared region. The results prove the high potential of n‐doped silicon oxide as ideal reflector for thin‐film silicon solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Analysis of industrial c‐Si solar cell's front metallization by advanced numerical simulation

The influence of the front metallization on the performance of industrially fabricated c‐Si solar cells is quantified by means of a combination of optical, semiconductor, and circuit modeling. Special attention is given to the front metallization of conventionally processed emitters. First, we verify our model by reproducing the measured I–V curves of industrially fabricated cells. Based on this, the potential changes in performance are predicted by variations in finger spacing, finger resistivity, busbar numbers, and contact resistivity. We also assess the benefits derived from the higher aspect ratios of the fingers: if the fingers are 120 (or 60 µm) wide, increasing the finger height from 15 to 25 µm increases their efficiency by 0.06% (or 0.7%) absolute if a conventional emitter is used. This implies that, if the fingers cannot be made as narrow as 60 µm in industrial mass production, the economic gains from higher aspect ratios may be rather limited, because there is a trade‐off between improved cell efficiency and the increasing cost of silver. Simulations show that, for the light‐induced plating approach, the seed layer must be made as narrow as 50 µm in order to achieve an efficiency gain of 1% absolute with a plating of 10 µm silver. Copyright © 2011 John Wiley & Sons, Ltd.