Epitaxial Silicon Solar Cells Research Articles

Improvement of Epitaxial Silicon Solar Cells on Upgraded Metallurgical Silicon by Heat Treatment.(Dept.E)

Improvement of Epitaxial Silicon Solar Cells on Upgraded Metallurgical Silicon by Heat Treatment.(Dept.E)

Theoretical and experimental study of Fermi Bragg reflector as application for thin silicon solar cells

The conception of efficient Bragg reflectors (BRs) based on dielectric layers is needed to improve the light trapping in thin film epitaxial silicon solar cells. Porous silicon multilayer structures (PSMS) used as BR show its potential to enhance optical confinement of low energy photons. In the present work, a novel design of PSMS will be studied in order to extend the reflectivity to long wavelength by adjusting BR thicknesses according to the Fermi profile law. Such new design named Fermi Bragg reflector (FBR) includes different configurations. Several FBR parameters are used in the simulation study, which is carried out by transfer matrix method. The optimal structures were realized using electrochemical etching dissolution of P+ type silicon substrate in hydrofluoric acid solution (HF) at two temperature conditions: room temperature (20 °C) and −20 °C. The measured optical response of FBR shows an enlargement in the width of high reflection range in comparing to the conventional BR. More than 80% of the photons were reflected in the spectral range 636–1155 nm for the FBR performed at ambient temperature and in the range 750–1353 nm to that carried out at −20 °C. This result proves that the FBR formed on crystalline silicon could enhance the light trapping in thin film silicon solar cells.

Comments on “Nanoscale Investigation of Carrier Lifetime on the Cross Section of Epitaxial Silicon Solar Cells Using Kelvin Probe Force Microscopy”

In recent years, there has been an increasing interest on the study of the complex interplay between the nanostructure and phototransport mechanism in many emerging photovoltaic technologies. Recently, Kelvin probe force microscopy has emerged as a powerful technique for probing photocarrier dynamics and gaining access to carrier lifetime at the nanoscale in a wide range of photovoltaic materials. In the paper entitled “Nanoscale investigation of carrier lifetime on the cross section of epitaxial silicon solar cells using Kelvin Probe Force Microscopy” by Narchi et al . ( IEEE J. Photovolt ., vol. 6, no. 6, pp. 1576–1580, Nov. 2016), an innovative method to asses minority carrier lifetime at the nanoscale using Kelvin probe force microscopy under frequency modulated electrical bias on the cross section of an epitaxial silicon solar cell was demonstrated. The purpose of this note is to complement the results obtained by Narchi et al ., by showing that two-dimensional images of the photocarrier dynamics of different photophysical processes can be obtained upon the implementation of a postacquisition data processing. In this note, we also discuss how this photocarrier dynamical images can be linked with the underlying physical processes that take place in the sample upon carrier injection.

Nanoscale Investigation of Carrier Lifetime on the Cross Section of Epitaxial Silicon Solar Cells Using Kelvin Probe Force Microscopy

Nanoscale Investigation of Carrier Lifetime on the Cross Section of Epitaxial Silicon Solar Cells Using Kelvin Probe Force Microscopy

Ultrathin PECVD epitaxial Si solar cells on glass via low-temperature transfer process

Fabrication of high-quality ultrathin monocrystalline silicon layers and their transfer to low-cost substrates are key steps for flexible electronics and photovoltaics. In this work, we demonstrate a low-temperature and low-cost process for ultrathin silicon solar cells. By using standard plasma-enhanced chemical vapor deposition (PECVD), we grow high-quality epitaxial silicon layers (epi-PECVD) from SiH4/H2 gas mixtures at 175 °C. Using secondary ion mass spectrometry and transmission electron microscopy, we show that the porosity of the epi-PECVD/crystalline silicon interface can be tuned by controlling the hydrogen accumulation there. Moreover, we demonstrate that 13–14% porosity is a threshold above which the interface becomes fragile and can easily be cleaved. Taking advantage of the H-rich interface fragility, we demonstrate the transfer of large areas (∽10 cm2) ultrathin epi-PECVD layers (0.5–5.5 µm) onto glass substrates by anodic bonding and moderate annealing (275–350 °C). The structural properties of transferred layers are assessed, and the first PECVD epitaxial silicon solar cells transferred on glass are characterized. Copyright © 2016 John Wiley & Sons, Ltd.

Cross-Sectional Investigations on Epitaxial Silicon Solar Cells by Kelvin and Conducting Probe Atomic Force Microscopy: Effect of Illumination.

Both surface photovoltage and photocurrent enable to assess the effect of visible light illumination on the electrical behavior of a solar cell. We report on photovoltage and photocurrent measurements with nanometer scale resolution performed on the cross section of an epitaxial crystalline silicon solar cell, using respectively Kelvin probe force microscopy and conducting probe atomic force microscopy. Even though two different setups are used, the scans were performed on locations within 100-μm distance in order to compare data from the same area and provide a consistent interpretation. In both measurements, modifications under illumination are observed in accordance with the theory of PIN junctions. Moreover, an unintentional doping during the deposition of the epitaxial silicon intrinsic layer in the solar cell is suggested from the comparison between photovoltage and photocurrent measurements.

Comparison of thin epitaxial film silicon photovoltaics fabricated on monocrystalline and polycrystalline seed layers on glass

AbstractWe fabricate thin epitaxial crystal silicon solar cells on display glass and fused silica substrates overcoated with a silicon seed layer. To confirm the quality of hot‐wire chemical vapor deposition epitaxy, we grow a 2‐µm‐thick absorber on a (100) monocrystalline Si layer transfer seed on display glass and achieve 6.5% efficiency with an open circuit voltage (VOC) of 586 mV without light‐trapping features. This device enables the evaluation of seed layers on display glass. Using polycrystalline seeds formed from amorphous silicon by laser‐induced mixed phase solidification (MPS) and electron beam crystallization, we demonstrate 2.9%, 476 mV (MPS) and 4.1%, 551 mV (electron beam crystallization) solar cells. Grain boundaries likely limit the solar cell grown on the MPS seed layer, and we establish an upper bound for the grain boundary recombination velocity (SGB) of 1.6x104 cm/s. Copyright © 2014 John Wiley & Sons, Ltd.

Gettering of transition metals by porous silicon in epitaxial silicon solar cells

AbstractEpitaxial silicon solar cells (“epicells”) are based on an epitaxial active layer (“epilayer”) grown on top of a low‐cost, inactive p+ silicon substrate. A key challenge is to mitigate transition metal out‐diffusion from the low‐purity substrate into the active layer. An embedded porous silicon (PSi) layer can be used to getter metals within the substrate. This was studied theoretically using density functional theory where large binding energies (∼1.9–2.2 eV) for metal segregation to PSi void surface were calculated for Fe and Cu. Incorporating this in a diffusion model yielded large gettering coefficients of ∼104 even at 1000 °C. To verify this experimentally, a test structure consisting of a 2‐µm thick epilayer grown on top of an 8.5 × 8.5 cm2 area of re‐organized PSi etched into the middle of an 8″ Cz, p+ wafer was used. These wafers were surface‐contaminated with metals (Fe, Ni, Cu) to ∼1014–1015 cm−2 and annealed at high temperatures (950–1000 °C) for up to 15 min. This allowed the metals to distribute throughout the wafer and getter to preferential sites. Direct total reflection X‐ray fluorescence mapping of Cu on the front side showed that the embedded PSi reduced the amount of Cu reaching the top surface by ∼103 times, compared to the areas without PSi. Moreover, SIMS depth profiling revealed large metal concentrations (1018–1019 cm−3) in the depth associated with PSi, while the metal concentrations were below detection limits in the surrounding area, suggesting a gettering coefficient of ∼103–104. A slow cooling rate and smaller pore radii were also found to be beneficial for gettering.

>16% thin‐film epitaxial silicon solar cells on 70‐cm2 area with 30‐µm active layer, porous silicon back reflector, and Cu‐based top‐contact metallization

ABSTRACTWe demonstrate the use of a copper‐based metallization scheme for the specific application of thin‐film epitaxial silicon wafer equivalent (EpiWE) solar cells with rear chemical vapor deposition emitter and conventional POCl3 emitter. Thin‐film epitaxial silicon wafer equivalent cells are consisting of high‐quality epitaxial active layer of only 30 µm, beneath which a highly reflective porous silicon multilayer stack is embedded. By combining Cu‐plating metallization and narrow finger lines with an epitaxial cell architecture including the porous silicon reflector, a Jsc exceeding 32 mA/cm2 was achieved. We report on reproducible cell efficiencies of >16% on >70‐cm2 cells with rear epitaxial chemical vapor deposition emitters and Cu contacts. Copyright © 2011 John Wiley & Sons, Ltd.

Nanoscale measurements of local junction breakdown in epitaxial film silicon solar cells

In this contribution, the authors report on near-field scanning optical microscopy measurements of the luminescence emitted from localized junction breakdown in epitaxial silicon solar cells. Our measurements suggest that the observed local reduction in breakdown voltage results from avalanche multiplication assisted by the reinforcing combination of (i) the local enhancement of the electrostatic field at the apex of inverted pyramid pits and (ii) the participation of defect states in the avalanche breakdown. Transmission electron microscopy reveals the microstructure of the defect responsible for the local junction breakdown.

Modelling of recombination velocity and doping influence in epitaxial silicon solar cells

This work presents a study of n +pp + type epitaxial solar cells developed on different wafers derived from an ingot UMG-Si recrystallized by HEM method with efficiencies of about 10%. More precisely it focuses on the development of software adapted to this type of cells, which expresses the variations of photovoltaic parameters as a function of epilayer thickness calculated for different values of recombination velocity and doping concentration, in order to optimize the epilayer thickness and efficiency of the cells. For this purpose, the one-dimensional (1D) model is evaluated through a simulation program which takes into account the interaction between several parameters as well as the restrictions between them. By modelling short circuit current density, open circuit voltage and efficiency, cells of different grain sizes have been studied. In cases of low recombination velocity calculated results have shown that the photocurrent density and conversion efficiency ( J sc ∼30 mA/cm 2, η∼13.8%) saturate when epilayer thickness values are higher than ∼65 μm and the gain is minimal, while they are heavily affected from epilayer thickness in cases of high recombination velocity. Calculated results also show that in cases of low doping concentration values photocurrent density and efficiency saturate for epilayer thickness values higher than 65 μm. However, for higher values of doping concentration higher photocurrent density and efficiency can be achieved by thinner epilayers. This study also suggests a second best value of epilayer thickness (≤50 μm), which is significantly lower and induces minor reductions in photocurrent density and efficiency values. A further comparison between simulated and experimental curves of quantum efficiency under 1000 W/m 2 illumination shows good agreement for wavelengths longer than 0.8 μm. However, near the blue part of the solar spectrum the measured quantum efficiency is significantly lower than the simulated one, due to the absence of surface passivation and the differences in the reflection coefficient between experimental and simulated devices.

Efficiency (>15%) for thin‐film epitaxial silicon solar cells on 70 cm2 area offspec silicon substrate using porous silicon segmented mirrors

AbstractWe report on the beneficial use of embedded segmented porous silicon broad‐band optical reflectors for thin‐film epitaxial silicon solar cells. These reflectors are formed by gradual increase of the spatial period between the layer segments, allowing for an enhanced absorption of low energy photons in the epitaxial layer. By combining these reflectors with well‐established solar cell processing by photolithography, a conversion efficiency of 15·2% was reached on 73 cm2 area, highly doped offspec multicrystalline silicon substrates. The corresponding photogenerated current densities (Jsc) were well above 31 mA/cm2 for an active layer of only 20 µm. Copyright © 2010 John Wiley & Sons, Ltd.

Epitaxially grown emitters for thin film silicon solar cells result in 16% efficiency

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/cm 2, approaching the Jsc of bulk silicon solar cells.

A porous silicon intermediate reflector in thin film epitaxial silicon solar cells as a gettering site of impurities

AbstractIn this work, we study the gettering properties of a porous silicon (Psi) intermediate reflector. This type of reflector is used currently in the thin film epitaxial silicon solar cell approach. The porous layers are electrochemically etched into low‐cost, highly contaminated UMG (Upgraded Metallurgical Grade) Si in order to investigate the influence of impurity diffusion to the active layer due to the high thermal budget during epitaxial growth. When implemented into a solar cell, a porous silicon (Psi) multilayer stack consisting of layers with alternating porosities can serve as a gettering site for impurities that originate from the substrate. This statement is confirmed in our present work by several techniques such as SIMS (Secondary Ion Mass Spectrometry) and TXRF (Total Reflection X‐Ray Fluorescence). Additionally to these experiments, solar cell results are presented, for which the active layer is grown with two different growth rates on top of porous silicon (Psi) multilayer stack and on two different types of multicrystalline silicon substrates: UMG (Upgraded Metallurgical Grade) and OFFSPEC (Out‐of spec silicon material treated as a waste from Integrated Circuits industry). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Short-Circuit Current Densities Exceeding 30 $ \hbox{mA/cm}^{2}$ by Use of Chirped Porous-Silicon Reflectors and Shallow Emitters in Thin-Film (20- $\mu\hbox{m}$) Epitaxial Silicon Solar Cells

We demonstrate the use of chirped porous-silicon broadband optical reflectors for thin-film epitaxial silicon solar cells. The benefits of chirped multilayer structures over conventional Bragg reflectors on the cell level are presented. By combining these chirped reflectors with shallow emitters, we show that both low- and high-energy photons are more effectively absorbed in the thin (20-mum ) epitaxial active layer of the cell. This resulted in a high conversion efficiency of 14.2% on screen-printed epitaxial solar cells made on highly doped multicrystalline silicon substrate. The corresponding photogenerated current densities are well above 30 mA/cm2 .

Chirped porous silicon reflectors for thin-film epitaxial silicon solar cells

The studies of porous silicon as a one-dimensional photonic crystal have led to solutions allowing the fabrication of broad photonic band gaps as large as several hundreds nanometers for various types of applications. In this work we demonstrate the use of the chirping process, i.e., the gradual increase in the spatial period of the structure, as it is used in image processing, to design porous silicon broad-band reflectors for thin-film silicon solar cells. Modeling of those layers is done using linear design method and a simulation software. Such chirped structures are fabricated by an anodization process. Samples are prepared on mono and multicrystalline silicon substrates with 15, 40, 60, and 80 layers. The reflectance spectra of such prepared porous reflectors are evaluated and the results show an increase in bandwidth of over 50% of the total width in comparison with the conventional, unchirped reflectors. We demonstrate clear advantages of introducing chirped multilayer structures over conventional ones. Although some understanding of chirping exists mainly in the field of image processing and photonics, detailed study of chirped porous structures as broad-band reflectors is needed for their implementation in thin-film epitaxial silicon solar cells. A record efficiency of almost 14% was reached for those types of reflectors on a large area 71 cm2 epitaxial Si solar cell, giving a boost in efficiency of 0.6% absolute in comparison with the solar cells containing conventional reflectors.

Simulation and implementation of a porous silicon reflector for epitaxial silicon solar cells

AbstractOne of the main challenges in the ongoing development of thin film crystalline silicon solar cells on a supporting silicon substrate is the implementation of a long‐wavelength reflector at the interface between the epitaxial layer and the substrate. IMEC has developed such a reflector based on electrochemical anodization of silicon to create a multi‐layer porous silicon stack with alternating high and low porosity layers. This innovation results in a 1–2% absolute increase in efficiency for screenprinted epitaxial cells with a record of 13·8%. To reach a better understanding of the reflector and to aid in its continued optimization, several extensive optical simulations have been performed using an in‐house‐developed optical software programme. This software is written as a Microsoft Excel workbook to make use of its user‐friendliness and modular structure. It can handle up to 15 individual dielectric layers and is used to determine the influence of the number and the sequence of the layers on the internal reflection. A sensitivity analysis is also presented. A study of the angle at which the light strikes the reflector shows separate regions in the physical working of the reflector which include a region where the Bragg effect is dominant as well as a region where total internal reflection plays the largest role. The existence of these regions is proved using reflection measurements. Based on these findings, an estimate is made for the achievable current gain with an ideal reflector and the potential of epitaxial silicon solar cells is determined. Copyright © 2008 John Wiley & Sons, Ltd.

Plasma Texturing and Porous Silicon Mirrors for Epitaxial Thin Film Crystalline Silicon Solar Cells

AbstractThin film silicon solar cells, consisting of an epitaxially grown active layer on a low quality highly doped silicon substrate, incorporate many attractive features usually associated with their sister cells based on bulk silicon. However, the efficiency of the current epitaxial semi-industrial screen printed cells is limited to 11-12% mainly due to optical shortcomings. This paper will give an overview of our work aimed at tackling the 2 most important problems: (i) Finding and implementing an adequate front surface texture and (ii) the simulation, fabrication and incorporation of an intermediate reflector.The former issue has been addressed by the development of plasma texturing based on halogen species. This method allows us to fulfil the sometimes contradictory requirements for the textured surface, i.e. a uniform and reduced reflection, a strong lambertian character to scatter the light and a limited removal of silicon. It will be shown that the scattering efficiency is dependent on both the wavelength of the impinging light and on the silicon removal during the texturing process.The second and main issue of this work is the limited absorption volume of the epitaxial layer. To resolve this drawback, an intermediate reflector is placed at the epi/substrate interface to enhance the path length of the low energy photons through the epi-layer. In practice, a multi-layer porous silicon stack is created by electrochemical anodization of the substrate. The reflection at the epi/reflector/substrate interface is a combination of several different effects including a Bragg mirror and Total Internal Reflection (TIR). Measurements of the external reflectance as well as extraction of the internal reflection parameters are used to clarify the issue. Advanced structures, including chirped porous silicon stacks, are introduced. Finally, the benefits of the reflector on the level of the epitaxial silicon solar cell are analysed. Efficiencies close to 14% are obtained for epitaxial cells incorporating an advanced porous Si reflector.

Porous silicon as an internal reflector in thin epitaxial solar cells

AbstractThin 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. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Large-area epitaxial silicon solar cells based on industrial screen-printing processes

Thin-film epitaxial silicon solar cells are an attractive future alternative for bulk silicon solar cells incorporating many of the process advantages of the latter, but on a potentially cheap substrate. Several challenges have to be tackled before this potential can be successfully exploited on a large scale. This paper describes the points of interest and how IMEC aims to solve them. It presents a new step forward towards our final objective: the development of an industrial cell process based on screen-printing for >15% efficient epitaxial silicon solar cells on a low-cost substrate. Included in the discussion are the substrates onto which the epitaxial deposition is done and how work is progressing in several research institutes and universities on the topic of a high-throughput epitaxial reactor. The industrial screen-printing process sequence developed at IMEC for these epitaxial silicon solar cells is presented, with emphasis on plasma texturing and improvement of the quality of the epitaxial layer. Efficiencies between 12 and 13% are presented for large-area (98cm 2 ) epitaxial layers on highly doped UMG-Si, off-spec and reclaim material. Finally, the need for an internal reflection scheme is explained. A realistically achievable internal reflection at the epi/substrate interface of 70% will result in a calculated increase of 3 mA/cm 2 in short-circuit current. An interfacial stack of porous silicon layers (Bragg reflectors) is chosen as a promising candidate and the challenges facing its incorporation between the epitaxial layer and the substrate are presented. Experimental work on this topic is reported and concentrates on the extraction of the internal reflection at the epi/substrate interface from reflectance measurements. Initial results show an internal reflectance between 30 and 60% with a four-layer porous silicon stack. Resistance measurements for majority carrier flow through these porous silicon stacks are also included and show that no resistance increase is measurable for stacks up to four layers.