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In this paper we propose a possible process simplification for pseudosquare 15.6×15.6 cm 2 Interdigitated Back Contact (IBC) CZ-Si solar cells. By using laser doping to create a selective BSF, one out of three diffusions and a wet etching step can be avoided, compared to a completely diffusion-based process flow. We show numerical optimization of this simplified cell structure in terms of: i) rear optics with additional SiN x capping layer, ii) contact fraction of the selective BSF. The results feature a top cell efficiency of 21.7% measured over the area of 239 cm 2 . Further characterization implies that cells were limited mainly by: i) FF losses attributed to lateral majority carrier transport in the BSF region and increased J 02 losses, ii) current loss which can be related to the BSF region.

In this work, the fabrication and properties of a Ge-based Kesterite Cu2ZnGeSe4 solar cell have been discussed. The substitution and the existence of the quaternary compound has been verified by physical methods. The device has a power conversion efficiency of 5.5% under AM1.5G illumination which is among the highest reported for pure Ge substitution. In depth electrical and optical analysis show that the Cu2ZnGeSe4 absorber has less bulk defects, less or no band tailing and no sub band gap emissions, which are all characteristic of Cu2ZnGeSe4 devices. These beneficial opto-electronic properties also result in a high open circuit voltage (V-oc) of 744 mV which is amongst the highest reported for Kesterite materials. Flemish government, Department Economy, Science and Innovation; European Union's Horizon 2020 research and innovation program [640868]

In this paper, we evaluate p-type passivated emitter and rear locally diffused (p-PERL) and n-type passivated emitter and rear totally diffused (n-PERT) large area silicon solar cells featuring nickel/copper/silver (Ni/Cu/Ag) plated front side contacts. By using front emitter p-PERL and rear emitter n-PERT, both cell structures can be produced with only a few adaptations in the entire process sequence because both feature the same front side design: homogeneous n+ diffused region with low surface concentration, SiO2/SiNx:H passivation, Ni/Cu/Ag plated contacts. Energy conversion efficiencies up to 20.5% (externally confirmed at FhG-ISE Callab) are presented for both cell structures on large area cells together with power-loss analysis and potential efficiency improvements based on PC1D simulations. We demonstrate that the use of a rear emitter n-PERT cell design with Ni/Cu/Ag plated front side contacts enables to reach open-circuit voltage values up to 676 mV on 1–2 Ω cm n-type CZ Si. We show that rear emitter n-PERT cells present the potential for energy conversion efficiencies above 21.5% together with a strong tolerance to wafer thickness and bulk resistivity. Copyright © 2014 John Wiley & Sons, Ltd.

In the last decades many techniques have been proposed to manufacture thin (Several parameters can be adjusted to change the morphology and, hence, the properties of the porous layer, both in the porous silicon formation and the succeeding thermal treatment. This work focuses on the effect of the parameters that control the porous silicon formation on the structure of the porous silicon layer after annealing and, more specifically, on the roughness of the top surface. The reported analysis shows how the roughness of the seed layer can be reduced to improve the quality of the epitaxial growth.

Surface texturing of crystalline silicon solar cells allows reduction of the reflection of sunlight and enhances photon generation. Plasma-based texturing offers significant advantages over the conventional wet approach, i.e., reduced Si consumption, thin wafer compatibility, increased throughput, and reduced chemical waste. However, it still suffers from insufficient electric yield, i.e., bad performance of plasma-processed solar cells. This paper describes advances made in plasma texturing by means of a linear microwave plasma source. The advantage of a two-step plasma-texturing process is demonstrated, leading to a smoother surface roughness, and resulting in an increased open-circuit voltage, together with a reduced processing time. In addition, the described technology allows a significant reduction of surface damage and could be implemented on a production line for crystalline silicon solar cell manufacturing.

This work focuses on the mechanisms of alkaline electroless Ni deposition on n-type Si substrates and silicide formation by rapid thermal treatment. The deposited Ni layers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS). The results indicate that Ni deposition occurs in two steps; a nucleation step, dominated by electrochemical processes, followed by autocatalytic deposition of Ni involving the reducing agent NaH2PO2. The oxygen content was found to be uniform in the Ni layer and was higher close to the Si/metal interface. The silicide formation from alkaline Ni deposits was characterized by 4 point probe measurements, in-situ x-ray dispersion (XRD) measurements, and TEM measurements. Results were compared to silicides formed from ‘pure’ sputtered Ni layers. It was observed that the silicide-formation temperature is higher for an electroless Ni layer than that for a sputtered Ni layer. The results suggest that the temperature ramping rate influences crystallographic phase formation.

In this work, an extensive characterisation of intrinsic amorphous silicon (a-Si) passivation layers deposited on n- and p-type silicon is reported. Low temperature capacitance-voltage measurements are utilised to enable parameter extraction from the c-Si/a-Si interface and a-Si bulk. Electron spin resonance enables atomic identification of defects present. Results reveal the presence of electrically active defects at the c-Si/intrinsic a-Si interface (∼1x1012 cm-2), and throughout the amorphous silicon layer bulk (∼8x1016 cm-3), which are atomically identified as Pb0 centres and D centres silicon dangling bond defects, respectively. The value of this work is the atomic identification of these defects in this stack, coupled with their electrical activity. That they can be detected by these techniques demonstrates the power of the methodology used to assess and quantify these defects. Therein lies the significance of this work: a methodology capable of fundamentally optimising amorphous silicon processing from an atomic perspective.

Excellent front surface passivation is one of the key issues for interdigitated back contact (IBC) silicon solar cells. To create a better understanding of passivation properties and find the most important issues when developing passivation schemes for various injection conditions, an advanced simulation model is needed. The software package Sentaurus TCAD is applied in this work. The main input parameters of the model are the interface state density, the amount of fixed charges, the capture cross sections and the activation energies which have been determined experimentally as reported previously. As an example, simulation results of the advanced model of the recombination at the Si-SiNx interface were presented and compared to the measured data.

To answer the challenge of less expensive renewable electricity, the photovoltaics community is focusing on producing thinner silicon solar cells. A few years ago, in the field of silicon-on-nothing structures, micron-thick monocrystalline layers suspended over their parent wafer were produced by high-temperature annealing of specific arrays of macropores. Those macropores reorganise into one single void and leave a thin overlayer on top. Since this method may be an inexpensive way of fabricating high-quality silicon films, this paper investigates its potential for photovoltaic applications. In particular, we investigated if large surfaces can be produced and transferred to foreign substrates with this method. We fabricated basic solar cells, without rear-surface passivation, on 5 cm × 5 cm-large and 1-μm-thick films transferred to glass, that showed energy-conversion efficiencies up to 2.6%. These cells demonstrate the feasibility of the presented concept as a layer-transfer process for solar-cell application. After formation by annealing, the film is only barely attached to its parent wafer, but can still safely be handled provided that any abrupt gas flow or pumping to vacuum is avoided. After transfer and permanent bonding, the sample can be handled as any bulk wafer.

Thin film epitaxial silicon solar cells are considered a near future alternative to bulk silicon solar cells. However due to the limited thickness of the active layer they require efficient light trapping. Therefore we propose the development and implementation of such light confinement by means of a porous silicon (PS) intermediate reflector at the epi/substrate interface. The formation of the reflector is done by electrochemical etching of a highly doped Si substrate into a multilayer stack (Bragg-optical reflector), and is followed by epitaxial deposition of the active layer. The implementation of the PS reflector however requires detailed analysis of many problematic issues, foremost the optical optimisation of the stack for internal reflection at the Si/PS/Si interface. Other topics include the pore rearrangement during high-temperature CVD as well as the quality of the epitaxial layer grown on porous silicon. Another challenge is the resistance within the PS layers. For that purpose, SRP (Spreading Resistance Probe) and resistance measurements were performed to determine the conductive properties of rearranged PS. First cells with a 9-layer porous silicon reflector gave a very promising efficiency of 13.5% which is 1.5% higher compared to cells without internal reflector.

We present an improved process for free-standing monocrystalline silicon (FMS) thin film solar cells developed at IMEC. We demonstrate that a thick annealed porous layer, or the quasi-monocrystalline silicon (QMS) layer, incorporated into a two-side contacted thin film solar cell structure does not produce any considerable series resistance. The best cell so far exhibits a conversion efficiency of 12.6% with a fill factor of 75.7% for an active layer thickness of 20 μm. This cell process includes emitter formation by phosphorous diffusion, silicon nitride deposition for antireflection coating and front surface passivation, and 50-μm-thick annealed porous layer remained on the rear. Despite the expected small reduction in solar cell's efficiency, maintaining the porous layer in the device structure renders the processing and handling of the thin FMS film much easier, thus leading to a better yield and up-scalability. PC1D simulations show that a thick QMS layer can lessen the short circuit current density in some degree, depending on the active layer quality, thickness, optical confinement, etc., while the other cell parameters remain substantially unvaried; a 10% to 15% higher process yield is expected to be sufficient to reach a break-even between the two processes.

Epitaxial thin film silicon solar cell technology is one of the most promising midterm alternatives for cost effective industrial solar cell manufacturing. Here, CVD is used to grow the active base layer. However, also the emitter can be grown by CVD, with doping profiles as desired. In this paper, solar cell processes are established integrating both a two-step CVD grown emitter, and state-of-the-art concepts for optical light trapping in epitaxial cells. In this way, the significant increase in Voc is combined with an improved short-circuit and leads to a record efficiency of 16.1% with a current density of 33.2 mA/cm2, approaching the Jsc of bulk silicon solar cells.

The new idea to fire the screenprinted contacts through porous silicon antireflection coating is tested for the first time from the viewpoint of PS eligibility towards a rapid high-temperature process. The morphological analysis (TEM and SE) reveals that the thickness of PS after firing is slightly increased simultaneously with increasing of its porosity. As a result no significant changes in reflectance characteristics are observed. The firing of the contacts through PS does not destroy antireflection properties and can be successfully used as an alternative way for preparation of PS ARC for silicon solar cells.

Epitaxial growth of thin crystalline layers on porous silicon can provide opportunities for silicon-on-insulator applications and silicon-based solar cells provided that the epitaxial layer has a sufficient crystallographic quality. Transmission electron microscopy (TEM) is used in this work to study the microstructural properties of porous silicon (PS) and of epitaxial Si layers grown on top of the PS. A more dense silicon layer exists in the upper part of the as-prepared porous silicon. The quality of the epitaxial layers is found to depend strongly on the morphology of the initial porous Si layers, and on the deposition temperature of the epitaxial silicon growth. The porous structure is completely destroyed after thermal CVD deposition of Si at too high temperature, resulting in a highly defective epitaxial layer. A high-quality epi-Si layer is obtained when depositing on a low porosity layer at 725°C. A stacked porous layer with a low porosity in the upper-part and a high porosity in the bulk can be formed by changing the conditions during the formation of the layer. On such a dual porous layer, an epitaxial silicon layer with a low defect density can be grown.

Porous silicon layers prepared on n + emitters are investigated by a combination of spectroscopic ellipsometry, transmission electron microscopy and sheet resistance measurements. The porous silicon is formed by an anodic surface treatment of uniformly doped emitters prepared by chemical vapour deposition and of diffused emitters. The results show a good correspondence between the thicknesses obtained with the different techniques. The porosity of the layers increases for increasing doping level and saturates at a doping above ∼5×10 19 /cm 3 . The formation rate shows an inverse doping dependence. The top surface layer of the porous silicon is in all cases more dense than the bulk, which is related to an initially faster formation rate.

Thin-film polysilicon solar cells are a promising low-cost alternative for bulk silicon solar cells due to their reduced material thickness. Recently, we showed that the use of an amorphous silicon/polycrystalline silicon heterojunction emitter instead of a diffused homojunction emitter led to a boost in the open-circuit voltage by 90 mV. Now, we present a full evidence that shows that this improvement is related to the absence of dopant smearing along the grain boundaries. By using scanning spreading resistance microscopy, we found an enlargement of the junction area by a factor of five in case of a homojunction. The tips of the dopant spikes represent lowly doped areas with an enhanced recombination.

Stacks of porous silicon layers have been successfully applied to maximize internal reflection at the interface between a silicon substrate and an epitaxially grown layer. The stack is consist of alternating porous layers of high and low porosity, defined by the quarter-wavelength rule. During the hydrogen bake prior to epitaxial growth of the epitaxial layer, the porous silicon stack crystallizes in the form of thin quasi-monocrystalline silicon layers incorporating large voids. Experimental data of the measured external reflectance have been linked to the internal reflectance. An optical-path-length enhancement factor of seven was calculated in the wavelength range of 900-1200 nm. Application on thin-film epitaxial solar cells showed a 12% increase in short-circuit current and efficiency

The decrease in wafer thickness seen as a route to cost reductions has raised a growing interest in techniques that allow the preparation of thin wafers without kerf loss. The Slim-cut process [1] is one of these new techniques and comprises mainly three stages: a stress layer deposition step on the top of a monocrystalline silicon sample, a heating step necessary to induce the stress on the silicon sample and detach a thin silicon layer, and a third step to clean the stress-inducing layer to obtain a silicon foil adapted to the fabrication of solar cells. One of the major problems of this technology consists in finding a stress layer that induces a sufficiently high contraction in order to achieve a rupture of the silicon without contamination of the foil. In this work we present a comparison between thin foils obtained by Slim-cut, using three different stress layers: i) a double screen printed Silver/Aluminum layer, ii) a dispensed epoxy paste, iii) an electrodeposited Nickel metallization. Results on lifetime measurements indicate that some of the stress layers, although capable of inducing large stress, severely degrade lifetime of the foil.

This work characterizes p-type Silicon surface passivation using a high-k material (Al 2 O 3 or HfO 2 ) combining capacitance voltage (CV) and lifetime measurements.

The degradation of strained n+-Si/p+-Si1−xGex epitaxial diodes, which are irradiated at room temperature with 1 MeV electrons in a high-voltage transmission electron microscope, is investigated. Special attention is given to the influence of the germanium content on the degradation of electrical characteristics and on the introduction of lattice defects into the epitaxial layer. The diode degradation and the deep level density for the x = 0.12 diodes are larger than for the x = 0.16 diodes. This can be explained by the fact that germanium atoms act as recombination centres for vacancies and interstitials thus decreasing the generation rate of stable radiation defects and by the fact that the energy absorbed in the epitaxial layer during irradiation decreases with increasing germanium content due to the smaller stopping power of germanium atoms.

The kinetics of the growth of epitaxial and polycrystalline undoped and heavily p-type-doped Si and Si 1− x Ge x layers have been studied over the pressure range from 4×10 −4 to 1.5 Torr at 625°C in an ultrahigh vacuum chemical vapour deposition reactor by growing layers for various times and measuring the resulting thicknesses. The pressure influence on the epitaxial as well as on the polycrystalline Si growth rate is discussed. We observed that nucleation of Si on Si as well as on SiO 2 is retarded. A model is proposed which explains this incubation time for the nucleation of Si on Si and the influence of the pressure on it. The addition of GeH 4 to the SiH 4 growth environment is found to retard the nucleation on SiO 2 even longer. On the other hand, a pressure increase and the addition of B 2 H 6 accelerate nucleation. Epitaxial layers can be grown selectively on Si by limiting the growth period to below the incubation time of polycrystalline material. Maximum selective thicknesses for various types and combinations of layers have been determined.

A next generation material for Si surface passivation is atomic layer deposited (ALD) Al 2 O 3 . However, conventional time-resolved ALD is limited by its low deposition rate. Initially, a high-deposition-rate prototype ALD reactor based on the spatially-separated ALD principle has been developed, with Al 2 O 3 deposition rates up to 1.2 nm/s. Later, the spatial ALD technique has been transferred to an actual in-line process development tool (PDT) for commercial high-throughput ALD of Al 2 O 3 , resulting in a deposition rate of 30 nm/min. The passivation quality and uniformity of the spatially-separated ALD Al 2 O 3 films are evaluated on p- and n-type Si, applying quasi-steady-state photo-conductance, carrier density imaging and infrared lifetime mapping. In all cases, a spatial ALD Al 2 O 3 layer of only 10 nm reached an excellent passivation quality and uniformity, comparable to reference wafers passivated by equivalent temporal plasma-assisted or thermal ALD Al 2 O 3 . Effective surface recombination velocities as low as 1.1 or 2.9 cm/s were obtained after annealing at 350 °C or firing, respectively. Using spatial ALD Al 2 O 3 passivated local Al back surface field p-type Si solar cells, the sufficient passivation of this high-throughput Al 2 O 3 layer is evaluated: an average gain in open circuit voltage as compared to SiO x rear passivated i-PERC cells is obtained.

This work proves that blistering is the partial delamination of a thick enough Al 2 O 3 layer caused by gaseous desorption in the Al 2 O 3 layer upon thermal treatments above a critical temperature: the Al 2 O 3 layer acts as a gas barrier and bubble formation occurs. First, using an atmospheric pressure rapid thermal processor with an atmospheric pressure ionization mass spectrometry, desorbing species upon heating of Si/Al 2 O 3 samples are identified: evident desorption peaks are observed around 400 °C for all spectra. The spectrum for m/e = 18, an indication of H 2 O, illustrates that gaseous desorption from Al 2 O 3 and from the Si substrate itself continues up to 600 °C and 700 °C, respectively. Also, it is shown that in the case of a 30 nm Al 2 O 3 layer, blistering starts at same annealing temperatures as gaseous desorption begins. In the case of a thin enough ( 2 O 3 film, blistering does not show. To complete the proof, elastic recoil detection measurements clearly show that after annealing a thick Al 2 O 3 film above 400 °C the H content is higher near the c-Si interface as compared to the near surface. Fortunately, effective lifetime and capacitance voltage measurements show that 5 to 10 nm Al 2 O 3 layers can still be adequate passivation layers after being annealed in N 2 environment at temperatures up to 500–700 °C: (i) interface trap densities (D it ) can remain below 1×1011 cm−2 and (ii) fixed charge densities (Q f ) stay negative and in the order of −3×1012 cm−2 Random local Al back surface field (BSF) solar cells, fabricated using a blistered film as rear surface passivation and no additional contact opening step, clearly show that random local BSFs are created upon firing of a blistered rear passivation layer covered by metal. Therefore, it is clear that blistering should be avoided, since it will reduce the overall rear surface passivation. The key to avoid blistering is using 5 to 10 nm Al 2 O 3 passivation layers and performing an annealing step prior to capping and co-firing. Al 2 O 3 /SiN x passivated local Al BSF p-type Si solar cells are made using an out-gassing step with temperatures up to 700 °C. For these cells, the reduction in blistering and hence improvement in rear surface passivation is clearly reflected in the gain in average Voc as a function of out-gassing temperature.

Interdigitated back-contact solar cells for high efficiency applications are usually manufactured in R&D laboratories by multiple subsequent lithography steps. Here the patterning by litho-free steps is proposed and implemented. The current status of the small area (2×2cm2 and 4×4cm2) n-type IBC solar cells integration flow is presented with exclusively laser-based patterning scheme: emitter, metal point contacts and electrode separation. Electrode separation is performed in 2 typical configurations: above emitter region or BSF region, where the 2nd location proofs to be beneficial. The best Suns V oc reaches up to 647 mV and p-FF up to 83%.

In this work we demonstrate the successful implementation of laser ablation of SiN x ARC to contact high ohmic emitters up to 120 Ω/sq with an advanced metallization on large area substrates. We propose Suns-Voc measurements as a fast and effective method to characterize the potential laser damage. We look at the laser ablation factors that can compromise the solar cell performance and we see how to limit the damage that can jeopardize the device performance. We define the laser ablation process window for emitters of different resistivity ranging from a typical 60 Ω/sq emitter to a emitter of 140 Ω/sq. A point-contact contacting scheme is proposed that leads to an improved V oc of the solar cell. Finally we present results of the electrical characterization of large area solar cells. Efficiencies up to 18.7% are obtained on large area, 160 µm thick CZ silicon substrates.

Interdigitated back contacted cells (IBC) constitute an excellent option for the achievement of high-efficiency on silicon material [1,2]. The effective implementation of an electrical field on the front side, also called Front Surface Field (FSF) is beneficial for this cell concept. It contributes to the reduction of the recombination and the enhancement of the lateral conductivity, improving the cell efficiency [3]. The ideal FSF would push the minority carriers away, while restricting the Auger-recombination-active region to a minimal depth. In addition, too high doping leads to increased absorption in a highly-recombinative region, due to bandgap narrowing. Therefore, a trade-off should be achieved in an n-type IBC solar cell by the combination of a highly-doped n+ surface with a shallow doping profile.

We show that the effect of heavy B doping modifies considerably the valence band offsets of the layers as well as the calculated cut-off wave-lengths of LWIR detectors.

Based on an analytical approach, developed by Jain and Roulston [1], the different contributions to the bandgap narrowing at T=0K are calculated for highly p-type doped Si and strained Si 1-x Ge x layers for Ge-concentrations between 0 and 30%. The different assumptions will be highlighted with special emphasis on the procedure we used to deal with the non-parabolic aspect of the valence band. This result will be used to calculate the apparent bandgap narrowing in these layers. These theoretical results will then be compared to the experimental results obtained on mesa-type Heterojunction Bipolar Transistors with strained Si 1-x Ge x -base and poly-emitter. The Ge-concentration in these layers was between 0 and 16% while the B-doping was varied between 5.1017/cm3 and 5.1018/cm3.

The behavior of polycrystalline p-n-junction solar cells with a grain size in the order of 1 /spl mu/m is investigated. The diffusion length appears larger than the average grain size at low doping levels but drastically decreases with increasing doping level, with moderate improvement through hydrogenation. The second diode currents are generally very large, leading to poor open-circuit voltages. We suggest that the zone of enhanced recombination, usually confined to the junction depletion region, extends into the base where very small grains are completely depleted due to carrier trapping at grain boundaries.

Epitaxial thin film silicon solar cell technology, consisting of a thin high quality epitaxial layer on top of a highly doped low-cost silicon substrate, is one of the most promising midterm alternatives for cost effective industrial solar cell manufacturing. Currently, CVD is used to grow the active layer. However, besides the growth of the base, also the emitter can be grown by CVD, which allows the growth of almost any doping profile, designed as desired to optimize the emitter and base properties. A two step emitter with a highly doped top layer acting as a front surface field and a moderate doped bulk layer was designed and tested. Very sharp transitions between the different epi-layers boost the open-circuit voltage drastically up to values around 650 mV. However, the Jsc of those cells were limited to values around 29 mA/cm2. To enhance the Jsc, light trapping methods are introduced to enhance the optical thickness of the 20 µm thick active base. In this paper, solar cell processes are established integrating both a CVD grown emitter, and state-of-the-art concepts for optical light trapping in epitaxial cells. In this way, the significant increase in Voc is combined with an improved short-circuit currents and leads to a record efficiency of 16.1% with a current density of 33.2 mA/cm2, approaching the Jsc of bulk silicon solar cells.

Interdigitated Back Junction (IBJ) solar cells are suited to be realized on thin wafers, since they need a combination of high diffusion lengths and thin substrates. Here we present an industrial type passivation stack at the rear surface of an IBJ solar cell, consisting of a low quality SiO 2 and a SiN x layer, which has proven its value in the i-PERC concept developed at IMEC. Quasi-Steady-State Photo Conductance (QSSPC) lifetime measurements indicate an improvement of lifetime from 10µs to 180µs after application and rapid thermal processing (RTP) of the industrial passivation stack. For the IBJ solar cell concept without back surface field (BSF); this results in an absolute increase in energy conversion efficiency of 2% absolute, going from 14% to 16% and with a BSF up to 18%. Out of the presented cell data we can conclude the excellent passivation quality of the applied industrial passivation stack.

When considering the cost of a photovoltaic module, still a large part consists of silicon wafer material. One solution is the usage of ribbon material, which provides moderate quality material at a low cost due to low kerf losses. Here we present the use of Ribbon on a Sacrificial Template (RST) wafers in a classic solar cell process. The quality of this material is determined by Quasi Steady State Photo Conductance (QSSPC) lifetime measurements indicating a minority carrier diffusion length up to 100µm. The classic solar cell processing combining various metallization techniques like screenprinting and evaporation leads to energy conversion efficiencies up to 11.5% on 11.44cm2 large ribbons. A big boost in efficiency is related to the use of plasma texturing, developed at IMEC, which proofs to be a good technique for texturing poly-crystalline material. The results presented in this paper show that RST material can become a cost competitive ribbon technology for solar cell applications.

A homogeneous emitter designed for a high efficiency Si solar cell needs to fulfill specific requirements: (1) a low surface concentration of dopants and (2) a good contact resistance. The emitter formation is studied in detail for an optimized POCl 3 diffusion scheme and for plasma doping.

This paper describes our progress achieved in thin-film free-standing monocrystalline silicon (FMS) solar cells by layer transfer process using porous silicon. The highest cell efficiency achieved for a 1times1 cm2 FMS solar cell is 13.5% with an open circuit voltage Voc=612 mV, current density Jsc =28.9 mA/cm2 and fill factor of FF=76.7%. In the effort of scaling-up of the cell area, the lift-off process was adapted to 10times10 cm2 substrates. The large-area epilayers from APCVD and LPCVD, respectively, were successfully processed into FMS solar cells, giving the best efficiency of 11.4% for the APCVD-FMS cell with an area of 11.1 cm2. Detailed study by quantum efficiency measurements, combined with SEM, revealed that the enhanced internal quantum efficiency and reflectance in the spectral range of lambda>825 nm for the APCVD-FMS cell can be attributed to the internal light reflection from the well-defined interface between the epilayer and the sintered porous, or quasi-monocrystalline silicon (QMS) layer

We have reported before on the use of (Al2O3)3(TiO2)1-x pseudo-binary alloys, deposited by spin coating and thermally treated, for field-induced surface passivation of p-type silicon. These layers show fixed negative charge densities in the order of 1012 cm -2 and yield low surface recombination velocities, but the passivating properties are degrading with time. It was found now that a treatment at high temperature (720degC-1000degC) in oxygen substantially reduces this degradation. The influence of temperature, oxygen flow and duration of the treatment on the effective lifetime is studied, as are effects occuring at the PBA/Si interface. A first optimization of this process leads to surface recombination velocities below 150 cm/s on low-resistivity FZ material and around 300 cm/s on CZ months after deposition. Experiments on the exact nature of the degradation mechanism are ongoing

QSSPC (quasi-steady-state photo-conductance) is a well-known technique for lifetime measurement, but up to now its use has been limited to silicon. In this paper we describe a simple method to extend this technique to germanium samples. The significant differences between silicon and germanium are separately analyzed. For each of these differences the correction to be made is described. With this procedure the distinctive features that made this technique so successful and widespread are maintained.

Thin-film polysilicon solar cells on foreign substrates are often considered as a promising low cost alternative to bulk silicon solar cells. Until now however, the obtained efficiencies and open-circuit voltages are far below those of other technologies. In this paper, we show how the open-circuit voltage can be enhanced significantly using an amorphous silicon ? crystalline silicon heterojunction emitter instead of a diffused homojunction emitter. Open-circuit voltages up to 536 mV were obtained for polysilicon layers with a heterojunction emitter. This is the highest open-circuit voltage obtained for polysilicon solar cells with a p-n structure on foreign substrates.

A model for the optical properties of closed-porosity silicon is developed as a tool for design and optimization of porous silicon stacks made of layers of alternate (high/low) porosity which are possible candidates for use as back reflectors in thin-film silicon solar cells on cheap substrates. The model takes into account the change in pore morphology during the epitaxial growth process of the silicon film on the stack, with the pores taking the form of relatively large closed cavities where retardation and interference effects play an important role. The calculated front-surface reflectance of structures consisting of a silicon epitaxial layer with a plasma-textured front surface grown over a reflecting porous silicon stack are compared with the corresponding experimental measurements, showing a good agreement between theory and experiment

Here we review the different methods used to form thin film polycrystalline silicon films on some commercially available ceramic substrates, namely alumina and mullite. The chemical vapor deposition (CVD) process on bare ceramics, the CVD on glassy layer (CVD-OGL) process, and the aluminum-induced crystallization (AIC) technique are reported. The latter is based on the overall layer exchange of adjacent Si and Al films during the formation of amorphous to polycrystalline Si. The structural quality of the different layers are reported and compared. The direct deposition of silicon on ceramics by high temperature CVD results on thick polycrystalline silicon layers but with small (

Epitaxial deposition of high-quality GaAs layers on Si substrates is very appealing in view of the lower price and weight of Si compared to GaAs and Ge, but the large lattice mismatch (4%) between both materials causes stresses and dislocations in the deposited layers. An elegant method to reduce these problems in the III-V layers is the realization of a Si wafer with a Ge cap layer, a so-called "virtual" Ge substrate. We present the realization of virtual Ge substrates by combination of two existing and cheap technologies: porosification of the Si substrate, and subsequent Ge deposition using CSVT (Close Spaced Vapor Transport). Direct CSVT deposition of Ge on porous Si substrates results in the growth of polycrystalline Ge; however, we have demonstrated that if a thin monocrystalline Ge seeding layer is first deposited on the porous Si substrate (by e.g. PECVD), subsequent CSVT deposition of Ge results in a thick mono-crystalline Ge layer of epitaxial quality equal to or better than the seeding layer (XRD FWHM = 100 arcsec).

At IMEC presently germanium photovoltaic cells are being developed for application in mechanically stacked solar cells. These stand‐alone germanium cells can also be applied in thermophotovoltaic (TPV) systems. Simulations using the spectrum of an Er2O3 selective emitter heated by a butane burner have been performed to find the most optimal cell parameters. This results in a maximum simulated cell efficiency of 21 percent. In the applied cell process, the emitter is realized by using a spin‐on diffusant and it is passivated using amorphous silicon, deposited by PECVD. Germanium photovoltaic cells have been realized with an AM1.5 efficiency of 6.7 percent, which is amongst the best germanium stand‐alone solar cell efficiencies reported in literature.

Epitaxial layers on heavily doped CZ substrates were grown in an APCVD epitaxial system with different epitaxial layer resistivities: 0.2, 0.5, 1.0 and 2.5 ohm.cm, in order to investigate the influence of the resistivity on the solar cell characteristics. Textured and untextured layers were compared. I-V characteristics under normalised AM 1.5 illumination, dark I-V and spectral response were measured. The efficiencies and the photogenerated current were found to increase with increasing layer resistivities in the range of their investigations.

Solar cells prepared in a thin (/spl ap/30 /spl mu/m) crystalline silicon membrane with a supporting frame allow an evaluation of the potential of c-Si thin film cells on cheap substrates. At the same time, lightweight and more radiation-hard solar cells may have direct applications in space. This paper studies the fabrication process of solar cells in /spl ap/30 /spl mu/m thick p-Si epitaxial layers, incorporating a p/sup ++/-Si etch-stop/back-surface field layer, using KOH etching. Wax, rubber and silicon nitride were tested as masking materials during the etching. It was found that both wax and silicon nitride could be used as materials for the masking of supporting frames for solar cell thinning up to 30 /spl mu/m. However, silicon nitride does not reliably protect the frontside structure.

An attempt is made to assess the accuracy of the simplifying assumption of total retransmission of light inside the "escape" or "loss" cone which is made in many models of optical confinement in thin-film silicon solar cells. A closed form expression is derived for the absorption enhancement factor as a function of the refractive index in the low-absorption limit for a thin-film cell with a flat front surface and a lambertian back reflector. Numerical calculations are carried out to investigate similar systems with antireflection coatings, and the investigation of cells with a textured front surface is achieved using a modified version of the existing ray tracing computer simulation program TEXTURE.

Thin-film solar cells have been produced in a pilot-line making use of industrial techniques. Typical laboratory process steps such as P/sub 2/O/sub 5/ solid-source diffusion, emitter etch-back, remote plasma hydrogenation, lithography, metal evaporation and lift-off, have been replaced by cost-effective techniques such as screen printing or POCl/sub 3/ diffusion, PECVD silicon nitride deposition and screen printing metallization and firing-through-nitride, respectively. The low-cost high-throughput techniques have been applied to epitaxial thin-film solar cells with success. Efficiencies of 12% have been obtained on 20 /spl mu/m thin-film solar cells made in epitaxial layers on highly doped multicrystalline silicon substrates without making use of optical path length enhancement. The hydrogenation induced by the firing-through-nitride process passivates the bulk of the epitaxial cell but not to the same extent as a remote plasma hydrogenation in the laboratory-scaled process.

In this paper the effects of direct and remote plasma nitrides on the performance of Cz-Si and SiGe bulk cells are analysed. The surface passivation properties of both kinds of nitrides are compared to a thin thermal oxide, grown at high temperatures. Internal quantum efficiency measurements prove that the surface recombination velocities are lowest in case of the remote plasma nitride layer. The extracted values of the surface recombination velocity are as low as 1.5/spl times/10/sup 3/ cm/s for the remote plasma nitride, whereas the value for the thermal oxide is twice as high, comparable to the value obtained for the direct plasma nitride. SiGe-cells show the same tendency, although the blue response is lower in absolute value. However, when using the appropriate SiGe-absorption coefficient for parameter extraction by PC-ID, the surface recombination velocity of the plasma nitrides on SiGe-emitters is comparable to what is found in case of Si. In addition, we also found that for both remote plasma and direct nitride layers, there is a significant enhancement of the red response of all types of cells, compared to the samples without nitride. A comparison was made of the behaviour of the cells, with starting lifetime of 10-30 /spl mu/s, showing that the enhancement of the red response by the use of a plasma nitride is comparable to the beneficial effects of a remote plasma hydrogenation. However, for starting lifetimes lower than 5 /spl mu/s, the separate hydrogenation step brings an additional improvement of the red response compared to the only-nitride case and the effect of the two treatments seems to be cumulative.

Utilizing the hydrogen-rich porous surface and aluminum ability to generate vacancies a new passivation scheme is proposed for mc-Si solar cells with porous silicon as an antireflection coating and aluminum as a back contact. This combination achieves a synergetic effect with aluminumn providing injection od vacancies through which hydrogen atoms from the porous silicon surface can diffuse into the mc-Si and effectively passivate the bulk.

Fast degradation is reported of p+ boron‐doped Si(1−x)Gex strained layers by 1‐MeV electron irradiations performed at room temperature. n+p+ diodes with x=0, 0.12, and 0.16 are fabricated on conventional p‐type Czochralski silicon substrates. Current/voltage, capacitance/voltage, and capacitance/temperature characteristics are studied before and after irradiation as a function of the electron fluence. A possible degradation mechanism is discussed.

Common features of thin film silicon (TFSi-) solar cells hydrogenated in a microwave induced remote plasma (MIRP) are studied. The thin films were epitaxially grown on ribbons and multicrystalline silicon (mc-Si). Optimal hydrogenation renditions are presented in relation to the sample conditions such as the surface passivation, the active carrier concentration in the epitaxial layer and the substrate material. The MIRP-hydrogenation is studied in the temperature range of 350-415/spl deg/C. A high passivation efficiency is observed at 400/spl deg/C for all substrates in spite of an unfavourable hydrogenation regime at 375/spl deg/C. In the absence of an oxide during the hydrogenation, an optimal hydrogenation time of 1 hour is observed irrespective of the substrate used. This optimal hydrogenation time is a consequence of the interaction between the incoming atomic hydrogen and the boron dopant in the epitaxial layer.

In this paper we report on modified "dry" chemical surface passivation schemes for contactless microwave-detected photoconductance decay measurements on Si (100) substrates. By varying the HF-concentration in the aqueous HF-solution and the preceeding chemical oxidation treatment it was found that the best passivation occurs after dipping in a 49%-HF-solution resulting in surface recombination velocities around 300 cm/s. Such a value is however still not sufficient for measuring bulk lifetimes >50 microseconds on wafers with typical thicknesses. To reduce further the surface recombination velocity, we investigate the influence of modifications of the normal HF-dip. Additions of HCl and isopropanol have been studied. Finally, a method based on stabilization of the surface passivation after the HF-dip by means of covering the wafer with a iodine-containing solid passivation layer is presented. The stability of the different passivation solutions are compared. The resulting hydride-configuration after the different surface treatments is studied by means of Fourier transform infra-red measurements with multiple internal reflection on double-sided mirror-polished substrates. This allows the identification of the detailed structure of the H-surface coverage (dihydrides, disturbed dihydrides, monohydrides).