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Ultrathin single- and three-stage Cu(In,Ga)Se2absorber layers were analyzed with room temperature photoluminescence (PL) spectra. An anomalous blueshift was observed upon increasing carrier injection for both samples. This blueshift was attributed to the presence of bandgap fluctuations that are of the same order as the minority carrier diffusion length. From time resolved measurements, a diffusion length of a few 100 nms was deduced. The single-stage spectrum consists of two peaks, and the sample was, therefore, also analyzed by hyperspectral imaging, providing lateral PL and reflectance data with 1 μm resolution. Marginal variations were observed in the PL yield and spectra. This homogeneity could again be attributed to an intermediate scale of the bandgap fluctuation with an upper limit of 1 μm for the scale of the lateral bandgap fluctuations. The two peaks in the PL spectra of the single-stage sample could be attributed to interference, and correction methods were applied. The bandgap fluctuations were extracted for the three-stage and single-stage sample and were 45 meV and 72 ± 3 meV, respectively. It is suggested that this difference is attributed to the smaller grains and larger amount of grain boundaries in the single-stage sample.

To determine if Cu(In, Ga)Se2(CIGS) can be submitted to the constraints that will be imposed on the solar materials of the future by an ever growing demand and necessity, we produced ultrathin (500 nm) single-stage co-evaporated CIGS solar cells doped with varying amounts and types of alkali atoms. We subjected them to accelerated lifetime testing. In the present work, we observed the seeping of water into the grain boundaries which could be identified as one of the main culprits for performance degradation. In addition, this phenomenon could only be observed when alkali atoms were present in the absorber layer.

We use temperature-dependent contact resistivity (ρc) measurements to systematically assess the dominant electron transport mechanism in a large set of poly-Si passivating contacts, fabricated by varying (i) the annealing temperature (Tann), (ii) the oxide thickness (tox), (iii) the oxidation method, and (iv) the surface morphology of the Si substrate. The results show that for silicon oxide thicknesses of 1.3–1.5 nm, the dominant transport mechanism changes from tunneling to drift-diffusion via pinholes in the SiOx layer for increasing Tann. This transition occurs for Tann in the range of 850°C-950 °C for a 1.5 nm thick thermal oxide, and 700°C-750 °C for a 1.3 nm thick wet-chemical oxide, which suggests that pinholes appear in wet-chemical oxides after exposure to lower thermal budgets compared to thermal oxides. For SiOx with tox = 2 nm, grown either thermally or by plasma-enhanced atomic layer deposition, carrier transport is pinhole-dominant for Tann = 1050 °C, whereas no electric current through the SiOx layer could be detected for lower Tann. Remarkably, the dominant transport mechanism is not affected by the substrate surface morphology, although lower values of ρc were measured on textured wafers compared to planar surfaces. Lifetime measurements suggest that the best carrier selectivity can be achieved by choosing Tann right above the transition range, but not too high, in order to induce pinhole dominant transport while preserving a good passivation quality.

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.

To determine if Cu(In, Ga)Se-2 (CIGS) can be submitted to the constraints that will be imposed on the solar materials of the future by an ever growing demand and necessity, we produced ultrathin (500 nm) single-stage coevaporated CIGS solar cells doped with varying amounts and types of alkali atoms. We subjected them to accelerated lifetime testing. In the present work, we observed the seeping of water into the grain boundaries which could be identified as one of the main culprits for performance degradation. In addition, this phenomenon could only be observed when alkali atoms were present in the absorber layer. This work received funding from the European Union’s H2020 research and innovation program under grant agreement 715027. We acknowledge the use of a LEAP 5000 HR at APT Flanders, KU Leuven (Hercules Foundation under contract ZW/13/09)

In this work, an industrially viable and novel rear surface passivation approach for Copper Indium Gallium di-Selenide, Cu(In,Ga)Se2, CIGS, ultra-thin (500nm) solar cells is developed. The passivation layer was deposited by atomic layer deposition (ALD), and an alkali treatment was applied via spin coating. It was observed that selenization of the samples is required to create contact openings. The openings were visualized by SEM, and these results were supported by EDS. The impact of the oxide layer’s thickness, as well as the alkali solution’s molarity, was studied. Solar cells were produced for the optimal combination of these two parameters. As a result, with a relative 13% increase, the highest Vocof 623mV was achieved. Hence, the efficiency of the passivated solar cell was relatively increased by one-third, by using an industrially feasible, fast, and repeatable technique.

All-thin film perovskite/CIGS multijunction solar modules, combining a semi-transparent perovskite top solar module stacked on a CIGS bottom solar module, are a promising route to surpass the efficiency limits of single-junction thin-film solar modules. In this work, we present a scalable thin-film perovskite/CIGS photovoltaic module with an area of 3.76 cm2 and a power conversion efficiency of 17.8%. Our prototype outperforms both the record single-junction perovskite solar module of the same area as well as the reference CIGS solar module. The presented perovskite/CIGS thin-film multijunction solar module makes use of the “4-terminal architecture”, which stacks the perovskite solar module in superstrate configuration on top of the CIGS solar module in substrate configuration. Both submodules apply a scalable interconnection scheme that can accommodate scale-up towards square meter scale thin-film multijunction solar modules. In order to identify the future potential of the presented stacked perovskite/CIGS thin-film solar module, we quantify the various losses in the presented prototype and identify the key challenges of this technology towards very high power conversion efficiencies.

Commercially available phenyl-C61-butyric acid methyl ester (PCBM) is crosslinked with 1,6-diazidohexane (DAZH), resulting in films resistant to common solvents used in perovskite solar cell processing. By using crosslinked PCBM as an interlayer and (HC(NH2)2)0.66(CH3NH3)0.34PbI2.85Br0.15 as the active layer, we achieve small area devices and modules with a maximum steady-state power conversion efficiency of 18.1% and 14.9%, respectively.

In this work, we evaluate by means of simulations and measurements the effect of rear surface morphology on the performance of n-type rear-junction Passivated Emitter Rear Totally-diffused (PERT) solar cells with epitaxial emitter. A comparison of the electrical and optical performance between a rear polished and a rear textured surface was conducted on both test structures and solar cells. The effect of surface morphology on J 0e for both emitters formed by either diffusion or epitaxy was investigated. It was found that epitaxy on a textured surface provides J 0e values even better than boron diffusion and J 0e for both surfaces (textured rear and polished rear) using ALD-Al 2 O 3 /PECVD SiO x as dielectric passivation are much lower than the ones by wet oxidation. A small difference in V oc between both surfaces was also predicted in our electrical simulations. Overall, the measurements show that n-type PERT solar cells with textured rear epitaxial emitter can perform at almost the same efficiency level to the ones with polished rear. For both types of n-PERT cells (textured rear and polished rear), average efficiencies ( η ) and short-circuit current densities ( J sc ) exceeding 20.5% and 39 mA/cm 2 , respectively were achieved on 238.9 cm 2 cells. Moreover, from our optical simulations it was found that either a textured rear surface or a non-perfect polished rear surface are ideal for light trapping at standard illumination conditions, with light impinging perpendicular onto the cell. However, a perfectly polished rear surface or one with pyramids at angles of 45° should be avoided. Finally, from a cost of ownership (CoO) point of view it was found that the potential efficiency gain originating from a rear polished surface is not cost effective. For polishing to become cost effective, the cell in the module should have an efficiency of 21.05% which is 0.45% more than the textured case.

KF and NaF treatments were done for single-stage co-evaporated CuIn0.7Ga0.3Se2. The absorber layers were grown on a substrate with an alkali barrier layer and NaF was either added before or after absorber layer growth. No differences were found on the device performance amongst the procedures to add Na. This is expected if the single-stage process does not have a copper rich stage or a Ga gradient, which is likely since there was no change of the elemental fluxes during absorber layer growth and no Ga profile was measured. KF was added by post-deposition only. Current-voltage characteristics were measured and net doping concentrations were determined from capacitance-voltage measurements (CV). We see an improvement of the open-circuit voltage (Voc) with increasing KF amount, and a marginal increase of the fill factor. CV measurements showed increasing net acceptor concentration with increasing KF amount. Time resolved photoluminescence (PL) showed an increased decay time for KF treated cells and the PL peak shape changed. Without KF treatment the PL peak is symmetric, after KF treatment a further peak appears at higher energy in the PL spectrum. This higher energy peak increases in intensity with increasing KF concentration. The same effects were seen in a sample without Na, but here the Vocwas limited due to large tailing. Hence both Na and K are required for good cell efficiencies.

We have studied the crystallization reaction of polycrystalline Cu2ZnGeSe4 solar cell absorbers fabricated by H2Se selenization of sequentially deposited metal layer stacks. We have executed a stop experiment, stopping the crystallization reaction at different times during the process, then analyzing the subsequent X-ray diffraction patterns. We have found that mainly Cu3Ge and ZnSe phases form very rapidly at temperatures below 350°C. Depending on the order of the sequentially deposited metal layer stack, the formation reaction proceeds at different speeds. Putting the Ge layer in the bottom and the Cu layer on top leads to a very fast nucleation reaction of Cu2ZnGeSe4, leading to small grains that have obtained their final size already after 3 minutes of selenization at 460°C. The inverse stack with Ge on top and Cu in the bottom delays the nucleation of Cu2ZnGeSe4, leading to a somewhat slower formation reaction and larger Cu2ZnGeSe4 grains, which obtain their final grain size only after 15 minutes of selenization at 460°C.

Reducing the absorber layer thickness below 1 μm for a regular copperindiumgalliumdi-selenide (CIGS)solar celllowers the minimum quality requirements for the absorber layer due to shorter electron diffusion length. Additionally, it reduces material costs and production time. Yet, having such a thin absorber reduces the cell efficiency significantly. This is due to incompletelight absorptionand high Molybdenum/CIGS rear-surface recombination [1]. The aim of this research is to implement some innovative rearsurface modificationson a 430 nm thick CIGS absorber layer to reduce both these affects: analuminium oxidepassivationlayer to reduce the back-surface recombination and point contact openings usingnano-particlesfor electrical contact. The impact of the implementation of all these rear-surface modifications on the opto-electrical properties of the CIGS solar cell will be discussed and analyzed in this paper.

Cu 2 ZnGeSe 4 (CZGeSe) can be considered as a potential alternative for wide band gap thin film devices. In this work, CZGeSe thin films were deposited on Mo-coated soda lime glass substrates by sequential deposition of sputtered Cu, Zn and e-beam evaporated Ge layers from elemental targets followed by annealing at high temperature using H 2 Se gas. We report on the effect of the precursor stack order and composition and the impact of the annealing temperature on the physical properties of CZGeSe thin films. The optimal layer morphology was obtained when using a Mo/Cu/Zn/Ge precursor stack annealed at 460 °C. We have observed that the formation of secondary phases such as ZnSe can be prevented by tuning the initial composition of the stack, the stack order and the annealing conditions. This synthesis process allows synthesizing CZGeSe absorber with an optical band gap of 1.5 eV.

In this work, the addition of molar concentrations of Na+ and/or K+ in the form of fluorides during the processing of Cu2ZnSnSe4 absorber layers was investigated. The beneficial effect of alkali salts on the performance of CZTSe solar cells has been demonstrated through physical and electrical characterization. A maximum power conversion efficiency of 8.3% has been obtained through controlled addition of Na+ and K+ in the CZTSe absorber as opposed to 6.7% from uncontrolled addition of both. The open circuit voltage in both cases was very much the same but the devices with controlled addition showed lower p-type doping, a low density of charged defects and less band tailing compared to the rest. This research is partially funded by the Flemish government, Department Economy, Science and innovation. This research has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 640868.

The fabrication and properties of a Ge-based Kesterite Cu2ZnGeSe4(CZGSe) solar cell are discussed. The existence of the quaternary compound has been verified by physical methods such as X Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). The Cu2ZnGeSe4solar cell has a power conversion efficiency (PCE) of 5.5% under AM1.5G illumination which is among the highest reported for pure Ge substitution. Detailed low temperature current-voltage and time-resolved photoluminescence measurements show that the Cu2ZnGeSe4absorber has less bulk defects and less band tailing in contrast to the typical characteristics of Cu2ZnSnSe4devices. These beneficial opto-electronic properties also resulted in a high open circuit voltage (Voc) of 744mV which is amongst the highest values reported for Kesterite materials.

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.

Copper indium gallium selenide/sulfide (CIGS) and copper zinc tin selenide/sulfide (CZTS) are two thin film photovoltaic materials with many similar properties. Therefore, three new processing steps – which are well-known to be beneficial for CIGS solar cell processing – are developed, optimized and implemented in CZTS solar cells. For all these novel processing steps an increase in minority carrier lifetime and cell conversion efficiency is measured, as compared to standard CZTS processing. The scientific explanation of these effects is very similar to its CIGS equivalent: the incorporation of alkali metals, ammonium sulfide surface cleaning, and Al2O3 surface passivation leads to electrical enhancement of the CZTS bulk, front surface and reduced front interface recombination, respectively.

This work studies the performance of commercially available Al pastes for the rear side contact formation with an industrially feasible process. The pastes are specifically designed for local Al-BSF/PERC (Passivated emitter and rear cell) cells. With this type of metallization, fill factors (FF) up to 80% are demonstrated, to our knowledge, the highest FF ever reported on fully screen printed PERC cells with industrial type emitter. Additionally, the use of an AlSi eutectic (12.7% Si) target for PVD sputtering is evaluated in comparison to a reference process using a pure Al target with the same kind of deposition technique. It was found that the thickness of the Al p+ BSF layer realized with the 12.7% Si containing Al target is systematically thicker than the reference due to a smaller metal Si interface. With this method, solar cells have been fabricated showing a potential efficiency above 20%, with standard Ag front screen printing on 60 Ω/□ emitter.

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.

In this work, we report on a self-aligned nickel silicide formation technique based on excimer laser annealing (ELA). We evaluate this process for the front contact formation of industrial PERC type solar cells on random pyramid textured Si surfaces where damage to surface texture, emitter passivation, or to the shallow junction should be avoided or minimized. PERC type solar cells obtained by POCl3 diffusion were processed on large area (12.5x12.5 cm2) CZ-Si. Self-aligned litho-free Ni/Cu contacts defined by ps-laser ablation of the SiO2/SiNx anti-reflective coating (ARC) and subsequent ELA of the Ni layer were compared to conventional Ag screen printed contacts. The novel ELA process results in an absolute gain in Jsc of 0.8 mA/cm2 as well as a drop of 0.3 Ω.cm2 in series resistance (Rs) compared to SP Ag contacts due to reduced shading and resistance losses. This leads to 0.5% absolute increase in efficiency from 19.3% to 19.7% since other characteristics (Voc, pFF) could be maintained to the same level. In this work, the best performing cell with the ELA process reached an outstanding 20.0% energy conversion efficiency with Jsc=39.3 mA/cm2, Voc=649.8 mV, and FF=78.3%.

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.

This paper describes the realisation of cost-efficient thermophotovoltaic (TPV) cells based on germanium substrates. Because the majority of the photons incident on the TPV cell in a typical TPV system will have a long wavelength it is important to apply optical confinement in the TPV cell. In this paper this has been done by using a highly reflective rear contact. Electrical contact at the rear has been created with a laser (laser fired contact, LFC) such that the metal is locally heated and contact is formed. The optimal TPV cell is based on a low doped ( 1 × 10 15 cm - 3 ) p -type germanium substrate having a 500 nm thick n-type emitter ( 1 × 10 19 cm - 3 ) , where front and rear are both passivated with hydrogen rich PECVD amorphous silicon. An AM1.5G record efficiency has been measured for a germanium TPV cell with an LFC rear contact. The application of optical confinement leads to a clear improvement of the spectral response in the wavelength region which is dominant in TPV systems and results in a potential 20% increase of current density compared to the use of a classical germanium photovoltaic cell.

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 polycrystalline-silicon (pc-Si) solar cells with a grain size in the order of 1–100 μm could substantially lower the price of photovoltaic energy if sufficiently high efficiencies are obtained on low-cost foreign substrates. A promising approach is the epitaxial thickening of seed layers made by aluminum-induced crystallization (AIC) of amorphous silicon. A clear difference in performance was found for AIC based solar cells grown by low- and high-temperature epitaxy. A large intragrain defect density was observed for this type of pc-Si solar cells that probably limits the cell performance. This paper studies intragrain defect formation in AIC based pc-Si layers by the investigation of state of the art absorber layers grown both by low- and high-temperature epitaxy on seed layers made on various substrates. The samples were characterized by defect etching in combination with scanning electron microscopy (SEM) and by cross-section transmission electron microscopy (TEM). The AIC seed layers themselves were found to be major sources of intragrain defects. Moreover, AIC seed layers prepared under different conditions can lead to different intragrain defect densities.

© The Royal Society of Chemistry 2016. Transitioning perovskite photovoltaics from the rapid progress in lab-scale devices to industrially viable large area modules is a key challenge for the economic breakthrough of the technology. In this work, we demonstrate ultrasonic spray coating as a scalable and versatile linear deposition technique for high efficiency perovskite photovoltaics. We show the versatility of concurrently pumped ultrasonic spray coating by rapidly and precisely optimizing precursor ratios based on PbCl2, Pb(CH3CO2)2·3H2O, PbBr2, CH3NH3Br, and CH3NH3I to achieve highly crystalline and pinhole-free layers. Initial power conversion efficiencies of 15.7% for small scale devices and 11.7% for 3.8 cm2modules were achieved with current-voltage sweeps and tracked to 13.4% for devices and 10.4% for modules under continuous illumination and bias at the maximum power point. Process versatility is further demonstrated with the in situ bandgap control in CH3NH3PbIxBr3-xlayers. crosscheck: This document is CrossCheck deposited related_data: Supplementary Information copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal history: Received 25 January 2016; Accepted 4 February 2016; Accepted Manuscript published 4 February 2016; Advance Article published 15 February 2016; Version of Record published 1 March 2016 ispartof: Journal of Materials Chemistry A vol:4 issue:10 pages:3792-3797 status: published

32nd European Photovoltaic Solar Energy Conference and Exhibition; 1268-1271, Epitaxial silicon foils (

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.

Standalone germanium solar cells are under development for application in high-efficiency mechanically stacked multijunction solar cells and thermophotovoltaic systems. To realize a suitable device, a more fundamental research has been done on germanium doping, surface passivation, and contact formation. In this paper, emitter formation and contact realization are discussed in detail. Emitter formation is done by phosphorous diffusion from a spin-on dopant (SOD) source. Critical parameters are the diffusion time, diffusion temperature, and phosphorous content in the SOD. Front contact formation is done by an innovative method, where the contacting metal is diffused through the amorphous silicon passivation layer. The specific properties of the diffusing metal, diffusion temperature, and diffusion time are important. Using the developed process, a stand-alone germanium solar cell has been realized with a world-class AM1.5G energy conversion efficiency of 7.8%

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.

Passivation of a germanium surface has proved to be challenging. Various materials have been examined for this purpose, like for example silicon nitride and amorphous silicon. In this work the optimisation of PECVD amorphous silicon and the influence of the preliminary surface treatment for passivation purposes are described. Furthermore, experiments done to extract the surface recombination velocity and the bulk lifetime of a germanium substrate will be presented. Optimisation of the deposition parameters, in combination with a pre-deposition in situ hydrogen plasma, results in a surface recombination velocity of 17 cm/s on a lowly doped germanium substrate.

We propose a novel approach to thin-film silicon solar cells, namely the freestanding monocrystalline silicon layer transfer process with heterojunction emitter (FMS-HJ). High crystallographic quality mono-Si films were deposited on freestanding porous silicon (PS) films by chemical vapor deposition (CVD). These free-standing mono-Si (FMS) films were processed into solar cells by creating a-a-Si/c-Si heterojunction. In our preliminary experiments a thin-film FMS-HJ solar cell with 9.6% efficiency was realized in a 20-μμm-thin active layer. Copyright © 2005 John Wiley & Sons, Ltd.

The space charge limited currents J in conducting organic materials often show J∼Vm (where m>2) dependence on voltage V. Two models have been used extensively to understand this behavior. In one model, the mobility μ is assumed to be dependent on electric field F. In the other model, an exponential distribution of traps is assumed and the mobility is taken to be independent of field. In this model the influence of the high electric fields present in organic polymers has not been taken into account. We present a model to calculate the J(V) characteristics for single carrier organic polymers taking into account the change in the occupancy of the traps due to the high electrical fields present in these materials. We show that the field dependent trap occupancy (FDTO) model can effectively explain the current–voltage characteristics reported by several different groups. If the FDTO model is used, it is not necessary to use the two different models mentioned above.

Epitaxial boron-doped emitters by CVD can provide a valuable alternative to the use of aluminum or diffused boron for the creation of p-type emitters. Compared to the traditional boron-diffused emitters for high-efficiency cells, epitaxial emitters need shorter process time, no boron skin removal step and have more opportunities to optimize the emitter doping profile. Our work proves that epitaxial emitters can be a good alternative for the p-type emitter. Very promising cell results with the highest cell efficiency of 17.0% on FZ material with LPCVD emitter and 16.7% on CZ material with APCVD emitter under 1 Sun have been achieved. Good fill factors have been obtained, which indicate good metal contacts are obtained on the epitaxial emitter. Cell results on n-type material are very encouraging and indicate a high potential of such epitaxial emitters for rear junction n-type solar cells.

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.

One possible method to improve the efficiency of crystalline silicon (Si) solar cells is by alloying with germanium (Ge). Although the improved absorption of the alloy leads to a gain in the current, the reduction of the alloy bandgap causes a loss in voltage, which overrides the increased current of the SiGe-alloy solar cell. There has been a number of theoretical studies to circumvent this behavior. However, to date there has been no detailed study, which discusses the technological implementation of these concepts in solar cells. In this paper, the design issues of crystalline Si-SiGe heterostructure will be dealt with in an attempt to reduce the effect of the increased dark current of the alloyed cells, while at the same time sustaining the enhancement in the current. The enhanced back surface field at the back p/sup +/-Si/p-SiGe interface reduces the base component of the recombination current of the heterostructure cell if recombination caused by dislocations is neglected. A higher infrared (IR) response which results in a higher short-circuit current (2 mA/cm/sup 2/ higher than a reference Si cell) has been recorded for the Si-Si/sub 0.9/Ge/sub 0.1/-thin-film structure of 15 /spl mu/m thickness. The reduction in dark saturation current which has been predicted based on the theoretical calculations could not be realized in the heterostructure SiGe/Si cell due to the degradation effect of the misfit dislocations that decreases the bulk lifetime, and increases the interface recombination velocity. In a structure which contains a p/sup +/-SiGe buffer layer, an efficiency of 12.5% is achieved for a SiGe cell with 15 /spl mu/m thickness without texturing or optical confinement, which is about the same as the Si reference cell with equal active thickness, but with a higher short-circuit current. These results, for the first time, experimentally prove that alloying with Ge offers a higher current and might have a room for improving the efficiency of the multijunction solar cells or dual bandgap cells when SiGe is used to convert the IR-part of the spectrum.

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

Current-voltage analysis using different optical band pass filters has been performed on Cu2ZnSnSe4 and Cu2ZnSn(S, Se)(4) thin-film solar cells. When using red or orange light (i.e. wavelengths above 600 nm), a distortion appears in the I-V curve of the Cu2ZnSnSe4 solar cell, indicating an additional potential barrier to the current flow in the device for these conditions of illumination. This barrier is reduced when using a Cu2ZnSn(S, Se)(4) absorber. Numerical simulations demonstrate that the barrier visible under red light could be explained by a positive conduction band offset at the front interface coupled with compensating defects in the buffer layer.