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Replacing toxic solvents with nonhazardous solvents is one of the key challenges for industrial scale commercialization of thin film perovskite photovoltaics. Here, nonhazardous solvent/alcohol/acid systems are presented for the single-step deposition of pinhole-free perovskite layers with combined lead halide precursors of Pb(CH3CO2)(2), 3H(2)O, PbCl2, and CH3NH3I. Comparable performance to standard hazardous inks is achieved: devices with 15.1% power conversion efficiency are demonstrated and maintain 13.5% tracked for 5 min at maximum power point. Blade coated 4 cm 2 solar modules fabricated with highest performing device ink attain 11.9% in power conversion efficiency. K.L.G. and J.G.T. contributed equally to this work. The work of K.L.G. was supported by the Roger Van Overstraeten Society. The work of J.G.T. was partially supported by the Natural Science and Engineering Research Council of Canada. The work of U.W.P. was supported by the German Academic Exchange Service. This work has been partially supported by Solliance, a partnership of R&D organizations from Belgium, the Netherlands, and Germany working in thin film photovoltaic solar energy.

We perform optical loss analyses and energy yield modelling of perovskite/silicon multijunction solar cells to elucidate the superiority of different architectures, e.g., 4-terminal solar cells and 2-terminal solar cell.

A simple analytical method is presented to perform a power-loss analysis of high-efficiency silicon Interdigitated Back Contact (IBC) solar cells. The method assumes one-dimensional current flows for both minority and majority carriers, as well as a constant gradient (or linear profile) of minority carrier concentration from the front to the back of the solar cell. The power-loss analysis method is applied to a real IBC silicon solar cell with a conversion efficiency of 23.3%.

The role of hydrogen in promoting thin film splitting from crystalline silicon wafers with pores or trenches during high temperature annealing is investigated. During the treatment, trenches are transformed into spherical voids that may laterally channel and split off the substrate. It is shown that the conditions necessary for hydrogen to contribute to the establishment of high stress levels around transformed voids or of pressure inside the voids are usually not satisfied. Hence promoting void coalescence by substantial void volume growth resulting from stress enhanced vacancy diffusion and/or exfoliation of separated voids are unlikely to occur. Also, there are no experimental evidence that confirms the role of hydrogen in triggering premature void collapse by Griffith fracture at relatively lower stress levels in conjunction with reduced surface energy. Therefore, it is concluded that splitting occurs during high temperature annealing only when neighboring voids are close enough to systematically coalesce. In that case, hydrogen may react at high temperature with the internal silicon surface of the voids (walls) and contribute to breaking the thin straps separating the voids which promotes channelling and film splitting (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Epitaxial 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.

Thermal atomic layer deposition (ALD) of Al2O3 provides an adequate level of surface passivation for both p-type and n-type Si solar cells. To obtain the most qualitative and uniform surface passivation advanced cleaning development is required. The studied pre-deposition treatments include an HF (Si-H) or oxidizing (Si-OH) last step and finish with simple hot-air drying or more sophisticated Marangoni drying. To examine the quality and uniformity of surface passivation - after cleaning and Al2O3 deposition - carrier density imaging (CDI) and quasi-steady-state photo-conductance (QSSPC) are applied. A hydrophilic surface clean that leads to improved surface passivation level is found. Si-H starting surfaces lead to equivalent passivation quality but worse passivation uniformity. The hydrophilic surface clean is preferred because it is thermodynamically stable, enables higher and more uniform ALD growth and consequently exhibits better surface passivation uniformity.

Texturing and polishing crystalline silicon in the PV industry are both time consuming and inefficient, the need for a streamlined solution is prevalent. To obtain high conversion efficiency surface decoupling is needed, one side0 of the wafer is ideally textured while the other is completely polished. The standard industrial process results in a total silicon loss of 40μm and a complete surface decoupling cycle time in the order of an hour from an as cut until diffusion ready. For an industrial wafer thickness of 180μm more than 20% of the silicon is lost. In addition to silicon loss, there are multiple wet processing steps needed for decoupling which are time consuming and hinder the high throughput manufacturing needed to achieve the ultimate goal of a low per watt peak price. The technique is proposed in this paper decouples the front and rear surfaces and utilizes in only one wet process step for both texturing and polishing. This process simplification leads to significant savings by decreasing costly cycle time, saving consumables and reducing the amount of silicon loss.

Crystalline Si (c-Si) technology is dominating the photovoltaics market. These modules are nonetheless still relatively expensive, in particular because of the costly silicon wafers, which require large thickness mostly to ease handling. Thin-film technologies, on the other hand, use much less active material, exhibit a much lower production cost per unit area, but achieve an efficiency still limited on module level, which increases the total system costs. A meet-in-the-middle is possible and is the object of this paper. The development of c-Si thin-foil modules is presented: first, the fabrication of the active material on a glass module and then the processing of the Si foils into solar cells, directly on module level. The activity of IMEC in this area is put into perspective with regard to worldwide research results. It appears that great opportunities are offered to this cell concept, although some challenges still need to be tackled before cost-effective and reliable industrial production can be launched. Copyright © 2012 John Wiley & Sons, Ltd.

One of the main driving forces to lower the silicon solar cell modules overall cost is to decrease the thickness of the silicon substrate and, at same time, to reduce the material losses caused by the sawing of the silicon ingot. For this reason, a considerable number of methods have been proposed in the last decade to manufacture thin silicon foils which aim to replace the commonly used sawing technique. One of these methods is based on peeling off a thin layer from a silicon substrate by means of the residual stress induced either through the only deposition of a layer on a silicon substrate or through a succeeding thermal cycle of such a bilayer. Up to now, three different materials have been successfully employed in this technique: Nickel, Nickel/Chromium alloy and a Silver/Aluminium system. In this work we demonstrate the possibility to induce the lift-off of a thin silicon foil by means of curing an epoxy layer on top of a silicon wafer. This new method has the advantages of reducing metal contamination in the silicon and lowers the operating temperatures below 150°C.

Atomic layer deposition (ALD) of thin Al2O3 (

The potential of contact passivation for increasing cell performance is indicated by several results reported in the literature. However, scant characterization of the tunneling layers used for that purpose has been reported. In this paper, contact passivation is investigated by insertion of an ultra-thin AlOx layer between an n-type emitter and a Ti/Pd/Ag contact. By using a 1.5nm thick layer, an increase of the minority carrier lifetime by a factor of 2.7 is achieved. Since current-voltage measurements indicate that an ohmic behavior is conserved for AlOx layers as thick as 1.5nm, a 1.5nm AlOx layer is found to be a candidate of choice for contact passivation.

Recently, the interest for the implementation of local ion implantation for emitter formation has been renewed, as it results in high cell efficiencies [1] , while the amount of process steps is reduced. After implantation, a thermal diffusion (annealing) is necessary for the removal of the ion-induced damage, the activation of dopants and the formation of the desired profile shape. The goal is to change the implanted retrograde near surface (10-100 nm) inactive profile, into a damage free, box-like shaped, and fully active emitter (or BSF) with a low saturation current density (J 0e ). Here, we study different post-implantation annealing schemes, to achieve high performance boron emitters for n-type IBC solar cells. We show experimentally, that the use of oxygen gas during the boron anneal has an impact on the electrical performance (J0e) of the emitter. On the other hand, the obtained J0e values are higher than those obtained by the standard diffusion approach. Hence, in order to understand these results and optimize the implant/anneal process, the modeling of the profile shape and the damage evolution would be extremely valuable. In this work, we report on our initial simulation results, and we demonstrate the impact of oxygen on the evolution of the profile shapes of the implanted B-emitters after anneal, taking into account boron diffusion kinetics mechanisms such as Transient Enhanced Diffusion (TED) and Oxidation Enhanced Diffusion (OED). Moreover, we monitor the residual damage and we make a first effort to correlate the increase on the J0e values with the residual damage levels. Throughout this work, we adopt the process simulator tool Sentaurus Process.

In this work we investigate the potential of inline single side wet-etch processing for rear side emitter removal and edge isolation for large area p-type Passivated Emitter and Rear (PERC) silicon solar cells. We focus on the integration of this process in flows leading to shallow emitters and screen printed front contacts and to deep, highly ohmic emitters in association with Cu contacts. The inherent advantage of this process, beside high throughput, is the external gettering performed by full emitter etch at the rear side. This is particularly relevant in the case of two-step emitter formation process, where removal of phosphorus diffused layers is most effective when performed prior to the high temperature drive-in step. Another effect that we make use of is the front-side emitter etch back performed by the etch bath vapors, which can be used to tune the phosphorus surface concentration and thus the emitter sheet resistance. Using this process, we report efficiency up to 19.9% for CZ large-area, homogeneous-emitter, screen printed cells, and 20.4% for CZ, large-area Cu plated cells.

Monolithic solar modules made from thin-film crystalline-silicon layers of high quality on glass substrates could lower the price of photovoltaic electricity substantially. This paper describes three different approaches that we are currently investigating to address the challenge to form high-quality crystalline-silicon layers on glass substrates. The SLiM-Cut approach is a wafering technique that results in 50-micron thick layers with a minimum of material loss. We show that crack initiation can be used as a means to better control the lift-off process. The epifree approach involves the lift-off of ultra-thin monocrystalline films formed by reorganization of cylindrical macropore arrays in silicon upon annealing. So far, films with a thickness of around 1 μm and very simple cells with an efficiency of up to 4.1% have been achieved. Finally, a seed layer approach is presented based on the epitaxial thickening by thermal CVD of monocrystalline silicon layers bonded on glass-ceramic substrates. Very promising cell efficiencies of 11% and Voc values of up to 610 mV have been achieved using a very simple and non-optimized cell structure.

A silicon thin-film technology could lead to less expensive modules by the use of less silicon material and by the implementation of monolithic module processes. A technology based on polycrystalline-silicon thin-films with a grain size between 1 μm and 1 mm (pc-Si), seems particularly promising since it combines the low-cost potential of a thin-film technology with the high efficiency potential of crystalline silicon. One of the possible approaches to fabricate pc-Si absorber layers is metal induced crystallization (MIC). For solar cell applications mainly aluminium is investigated as metal because 1) it forms a eutectic system with silicon instead of a silicide-metal system like e.g. Ni 2) only shallow level defects are formed in the forbidden bandgap of silicon and 3) a layer exchange process can be obtained in combination with a-Si. Aluminum induced crystallization (AIC) of a-Si on non-silicon substrates can results in grains with a preferential (100) orientation and a maximum grain sizes above 50 micrometer. These layers can act as seed layers for further epitaxial growth. Based on this two-step approach (AIC + epitaxial growth) we made solar cells with an energy conversion efficiency of 8%. Based on TEM, EBIC, SEM, defect etch and EBSD measurements we showed that the efficiency is nowadays mainly limited by the presence of electrical intragrain defects.

Silicon photovoltaic (PV) roadmaps indicate the reduction of wafer thicknesses and the need for innovation in wafering method and cell processing. Within this framework, Imec proposes the i2-module device [1], i.e. an heterojunction interdigitated back-contact (HJ i-BC) solar module [2] processed on 40-μm thick epitaxial wafers bonded to carriers by means of silicone. In the i2-module concept, the Rear Side (RS) of the solar cell is passivated while the wafer is bonded to the module glass and the influence of the silicone on the passivation process is reduced by an O2 plasma realized in an Reactive Ion Etching (RIE) chamber [3]. In this contribution, the effect of different post-bonding cleaning sequences on the passivation of wafers/silicone/glass stacks treated with an O2 plasma is investigated and a simplified post-bonding cleaning sequence leading to state-of-the-art passivation is proposed.

In the quest for lowering the manufacturing cost of silicon solar cells, imec has been working successfully on two crystalline Si ‘thin-film’ cell concepts. In a first concept, a 20 µm-thin Si solar cell is epitaxially grown on top of a porous Si-based Bragg-type reflector which is electrochemically etched in a low-cost UMG Si substrate. Large area solar cells with efficiencies of 15.2% have been made, using (semi-)industrial processing tools. This clearly demonstrates that this cell concept has almost reached the stage of industrial application. In a second, longer-term approach, a 1–5 µm-thin, stand-alone mono-crystalline film is created based on the controlled annealing of an ordered macroporous silicon layer (the ‘Epi-free’ process). With this very thin Si layer, a simple proof-of-concept solar cell has been made exhibiting an efficiency of 4%. By optimizing the cell process in terms of light trapping and passivation, efficiencies over 15% can be expected from this technology.

Compared with monolithically stacked multi-junction solar cells, the approach of mechanically stacking junctions allows more freedom on cell design and material choice. Since there is no requirement on current matching, mechanically stacked multi-junction solar cells are not sensitive to spectrum variations, thus have a high performance potential in terms of annual energy output. The trade-off of such an approach is its increased complexity in solar cell processing. At IMEC, we have developed a low temperature thin layer transfer process for the manufacturing of mechanical stack. First Gallium Arsenide-Germanium mechanically stacked dual-junction solar cells have been successfully demonstrated, with the GaAs top junction consisting of only active cell layer of about 4µm. Efficiencies of 23.2% for GaAs junction, 1.9% for Ge junction, have been achieved under AM1.5 global spectrum.

The “Epifree” process involves the lift-off of a high-quality monocrystalline film formed by reorganization upon annealing of cylindrical macropore arrays in silicon, and can thus provide high-quality silicon films without resorting to costly epitaxy. The challenge of this new process lies in etching controlled and regular pores in silicon in a cost-efficient way, and in developing a process compatible with the difficulty of handling a micron-thin material. Proof-of-concept cells have previously been achieved and this paper presents the latest progress, with a first development of thicker films and the inclusion of rear-side passivation. The energy-conversion efficiency of 1-µm-thin Epifree cells was improved from 2.6 to 4.1% by depositing a stack of amorphous silicon (a-Si) layers as rear-side passivation. The increase in Voc was, however, limited and bound to a drop in Jsc. The choice of a-Si was revealed to be unsuitable because of the thinness of the film and the presence of a full aluminum rear contact. The thinness of the film leads to a decrease in rear-side reflectivity by the a-Si absorption, and the aluminum, although not leading to crystallization, partly migrates inside the a-Si stack upon anodic bonding as shown by TEM. These factors indicate that an alternative surface passivation should be developed. In parallel to process developments, the material was thickened by modifying the macropore array dimensions, leading to a 2.4-µm-thick material over 1 cm × 1 cm areas. The efficiency of the next cells is expected to increase with this thicker material. Copyright © 2010 John Wiley & Sons, Ltd.

Cost reduction, expressed commonly as a unit cost/Wp on the level of solar cells, PV modules and systems, is the overarching theme of the major part of worldwide activities in the field of photovoltaics. Obviously, this is the condition sine qua non to be met for reaching economical viability of solar photovoltaic energy. We already outlined 1 earlier that a number of nanotechnologies developed for advanced Si-based nanoscale CMOS transistors might also be very useful for the race toward crystalline Si solar cells with higher efficiency and lower cost. The focus on this cost parameter might, however, obscure the growing need to add more functionality to the PV system if photovoltaic energy is to play an important role in the electricity generation scenarios of the future 2. In the view of the authors much of this added functionality could be integrated at the module or even within the module by using a number of microscale and nanoscale technologies. This added functionality on PV-module level might in first instance aim at an increase of energy yield by adding electronic components allowing to minimize the effect of shadowing losses by allowing a certain degree of reconfigurability of the PV module. Eventually, this might also reach out further by integrating the DC–AC inverter with the module (allowing to reduce DC losses) and in the end even other components integrating control and interaction functions with the electricity grid. In this paper we describe the philosophy behind the concept of the “Smart PV module” as well as two crucial technologies to realize this concept: the embedding technology for cells and components in the so-called U module and the development of a low-cost technology for reliable high-power components based on GaN-on-Si, enabling the high-frequency switching transistors in the DC–DC or DC–AC converters or the low-loss switches in a “reconfigurable” U module. Besides the well-spread conviction that nanotechnological aspects will lead to advances in terms of solar cell performance and lower cell costs, the message which we would like to convey is the fact that the development of miniaturized components using advances in micro- and nanotechnology are also very likely to affect the development of the “smart PV module” concept, providing ground for the statement “linking nanoscale to gigawatts.” Copyright © 2010 John Wiley & Sons, Ltd.

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Stability is one of the key challenges for industrial scale commercialization of perovskite solar cells. In this work, a degradation mechanism that depends on materials and bias conditions of the device during light-soaking is proposed. The observed degradation is linked to the additive 4-tert-butyl pyridine (tBP), crucial to the hole transport layer of most perovskite solar cells, and gold. This conclusion is reached through the statistical analysis of multiple compositional profiles of light-soaked and nonlight-soaked devices and by selective replacement of material layers of the device. Moreover, the rate of the light-induced degradation is enhanced by operation at forward bias, which is required for power generation. Thus, this work stresses the need for the development of transport layers that do not require tBP, and to replace gold to produce high-performing devices that are also stable under operating conditions. ispartof: ADVANCED ENERGY MATERIALS vol:8 issue:23 status: published

The optical performance of thin crystalline silicon solar cells with plasma textured front surface and with porous silicon stack back reflector is analysed. A rigorous analytical ray tracing model, that uses an accurately estimated porous silicon refractive index, is elaborated to predict the optical absorption, carrier generation, external quantum efficiency, and total photogenerated current in the cell. Calculated results best fitted to those measured for an experimental demonstration cell show that surface recombination is a major loss factor in the demonstration cell. The study also concludes that the plasma textured front surface has poor light diffusion properties, and predicts that the maximum gain in the photocurrent from back reflection at the porous silicon stack equals 10% compared to 20% if the back reflector is ideal. However, the proposed cell is realistic and promises excellent performance when state of the art front surface texturing, passivation and front metallization techniques are implemented.

Solar modules made from thin-film crystalline-silicon layers of high quality on glass substrates could lower the price of photovoltaic electricity substantially. One way to create crystalline-silicon thin films on non-silicon substrates is to use the so-called “seed layer approach”, in which a thin crystalline-silicon seed layer is first created, followed by epitaxial thickening of this seed layer. In this paper, we present the first solar cell results obtained on 10-μm-thick monocrystalline-silicon (mono-Si) layers obtained by a seed layer approach on transparent glass-ceramic substrates. The seed layers were made using implant-induced separation and anodic bonding. These layers were then epitaxially thickened by thermal CVD. Simple solar cell structures without integrated light trapping features showed efficiencies of up to 7.5%. Compared to polycrystalline-silicon layers made by aluminum-induced crystallization of amorphous silicon and thermal CVD, the mono-Si layers have a much higher bulk diffusion lifetime.

We 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.

The relatively new “thin-film polycrystalline-silicon (pc-Si) (grain size of 0.1-100 µm) solar cell on foreign substrate” technology aims at low-cost devices with energy conversion efficiencies above 12 %. A very promising technique to obtain thin pc-Si layers is aluminum-induced crystallization (AIC). Solar cell absorber layers can be made by epitaxial thickening of these AIC seed layers. So far, we have reached energy conversion efficiencies of up to 8% with this approach. In contrast to what is expected a performance independent of the grain size is found which is explained by the presence of intragrain defects. In this paper the electrical activity of both the intragrain defects as well as the grain boundaries is investigated with electron beam induced current (EBIC) measurements before and after hydrogen plasma passivation. Metal-insulator-semiconductor contacts were used as collecting junction to eliminate the interference of the junction shape with the EBIC measurement as found when diffused emitters where used. It is shown that both grain boundaries and intragrain defects are electrically active before and after hydrogen plasma passivation. Finally we argue that Leff,mono, the diffusion length inside the grains, is probably much closer to 1µm in our layers than equal to 100µm as often expected in the literature.

We demonstrate an increase of the exciton diffusion length (LD) of a fluorescent donor layer via the process of sensitized phosphorescence, whereby initially generated singlet excitons are transferred to triplet excitons via a properly chosen dopant molecule. Using a poly(p-phenylene vinylene) host doped with the phosphorescent molecule platinum octaethylporphyrin, LD increases from 4 ± 1 to 9 ± 1 nm for an optimal doping of 5 wt%. As a result of the increased LD, the photocurrent from the doped layer increases by 40%. By doping with the fluorescent, non-metalated analogue octaethylporphyrin, a reduction in photocurrent is shown, providing further evidence of the sensitized phosphorescence mechanism.

In 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)

To answer the challenge of obtaining thinner silicon solar cells, we developed a technique based on the transfer of a thin layer of high-quality monocrystalline silicon to a foreign low-cost substrate. The transfer is enabled by the reorganisation of macroporous silicon at high temperature: driven by the minimisation of their surface energy, pores close and, in specific situations, merge to form a large single void, a few micrometers under the wafer surface. As demonstrated in this paper, the micron-thick film above it may be detached and used to process a solar cell. The difficulty of this approach lies in appropriately ordering the macropores, while its originality is to enable processing of a high-quality material directly from porous silicon. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of bathocuproine (BCP) on the optical and electrical properties of organic planar heterounction photovoltaic cells is quantified by current-voltage characterization under 1 sun AM 1.5D simulated solar illumination and spectral response at short-circuit conditions. By inserting a 10 nm BCP layer in an indium tin oxide (ITO)/subphthalocyanine (SubPc)/buckminsterfullerene (C 60 )/BCP/Al thin film structure, an increase in power-conversion efficiency from 0.05 to 3.0 %Ct is observed, mostly reflected in the enhanced open-circuit voltage up to 920mV. Furthermore, the incorporation of a 10-nm BCP layer in an ITO/C 60 /BCP/Al structure leads to an increase in built-in potential from 250 to 850 mV, as demonstrated by electroabsorption. It is argued that BCP passivates C 60 such that in 10-nm layer provides a sufficient buffer layer that prohibits Al contacting the C 60 where it would otherwise create donor states.

Thin films of polysilicon are an attractive material to reduce the cost of photovoltaic energy. Among the different polysilicon techniques, fine-grained polysilicon deposited directly onto a foreign substrate is an interesting option, since no extra seed layer or crystallization step is needed. However, the fabrication of efficient fine-grained polysilicon solar cells is a real challenge due to the large number of defects and the small average grain size of only 0.2 µm. This paper reports on our recent progress with fine-grained polysilicon solar cells. Using a diffuse refracting top surface and a more efficient two-step hydrogenation, we obtained an efficiency of 5.0%.

Thin-film polycrystalline-silicon solar cells might become an alternative to bulk silicon solar cells if sufficiently high efficiencies can be obtained. In this work we made pc-Si layers using aluminium-induced crystallization and thermal CVD on alumina substrates. By using plasma texturing and optimizing the cell structure, we increased the current density of our cells and achieved a cell efficiency of 8.0%. At present, our cell efficiency seems to be mainly limited by the presence in our layers of a high density of electronically active intragrain defects. Intragrain quality improvement will therefore be very important to further increase our pc-Si cell efficiency.

Proceedings on Advanced Materials and Concepts for Photovoltaics EMRS 2007 Conference, Strasbourg, France; International audience; Among the methods for enlarging the grain size of polycrystalline silicon (poly-Si) thin films, aluminium induced crystallization (AIC) of amorphous silicon is considered to be a very promising approach. In the AIC process, a thin a-Si layer on top of an aluminium layer crystallizes at temperatures well below the eutectic temperature of the Al/Si system (Teu = 577 °C). By means of electron backscattering diffraction (EBSD), we have mainly studied the effect of the aluminium layer quality varying the deposition system on the grain size, the defects and the preferential crystallographic orientation. We have found a strong correlation between the mean grain size and the size distribution with the Al deposition system and the surface quality. Furthermore, we show for the first time that more than 50% of the surface of the AIC films grown on alumina substrates are (103) preferentially oriented, instead of the commonly observed (100) preferential orientation. This may have important consequences for epitaxial thickening of the AIC layer into polysilicon absorber layers for solar cells.

Polycrystalline silicon (grain size ~ 0.1–100 μm) solar cells on foreign substrates are a promising approach for the next generation silicon solar cells. Aluminum-induced crystallization AIC in combination with epitaxy is a possible way to obtain such absorber layers. It is believed that Si islands present on the surface of AIC seed layers have a negative effect on the epitaxy. The removal of these islands could therefore lead to an increased absorber layer quality and solar cell performance. In this paper, we present a selective island removal procedure based on the Al layer already present after AIC annealing. By selecting an etchant which removes Si at least as fast as Al (in this paper plasma etching using SF 6 ), the Al layer acts as a perfectly aligned etching mask for the fully developed islands.

One 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.

In this work, we present the structural quality of polycrystalline silicon films formed by high-temperature chemical vapor deposition (CVD) on mullite ceramics coated with spin-on flowable oxides (FO x ) serving as intermediate layers (ILs). The average grain size and the size distribution were investigated by optical microscopy. It is found that more than 65% of the surface of polysilicon films grown on boron-doped FO x is covered by large grains of 5–10 μm. The intra-grain and inner-grain defects as well as the grain orientation were analyzed with the electron backscattering diffraction (EBSD) technique. Twin-type defects such as Σ3 and Σ9 are frequently present in these silicon layers, which are slightly (1 1 0) preferentially oriented. Finally, we present the photovoltaic data on test solar cells made on these CVD polysilicon films. An efficiency of about 3.3% is reported. The limiting factors, as well as possible improvements, are discussed.

The growth of well-defined regular columnar macropores under front-side illumination on medium doped n-type silicon has been investigated. The various morphologies observed, with branched or columnar pores, differ from those reported so far on substrates of both higher and lower resistivities. Unlike stated by the macropore growth model, current density is found to influence the pore growth rate. These observations indicate that, under the conditions used in this work, the pore growth is at the border between meso- and macroporous mechanism. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In order to improve photon harvesting, two small molecule organic solar cells are placed in series on top of each other. These stacked cells need an efficient recombination center in between both cells. In this study we test vacuum deposited metal layers as recombination centers with pentacene and buckminsterfullerene ( C 60 ) as donor and acceptor, respectively. S-shaped curves are visible in the I–V characteristics when using thin layers of aluminum, indicating a barrier for extraction inside the device. Thin metal layers of gold or silver result in an increased open-circuit voltage without the appearance of these S-shaped features.

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from polycrystalline-silicon (pc-Si) thin films on inexpensive substrates. We recently showed promising solar cell results using pc-Si layers obtained by aluminum-induced crystallization (AIC) of amorphous silicon in combination with thermal chemical vapor deposition (CVD). To obtain highly efficient pc-Si solar cells, however, the material quality has to be optimized and cell processes different from those applied for standard bulk-Si solar cells have to be developed. In this work, we present the different process steps that we recently developed to enhance the efficiency of pc-Si solar cells on alumina substrates made by AIC in combination with thermal CVD. Our present pc-Si solar cell process yields cells in substrate configuration with efficiencies so far of up to 8·0%. Spin-on oxides are used to smoothen the alumina substrate surface to enhance the electronic quality of the absorber layers. The cells have heterojunction emitters consisting of thin a-Si layers that yield much higher Voc values than classical diffused emitters. Base and emitter contacts are on top of the cell in interdigitated finger patterns, leading to fill factors above 70%. The front surface of the cells is plasma textured to increase the current density. Our present pc-Si solar cell efficiency of 8% together with the fast progression that we have made over the last few years indicate the large potential of pc-Si solar cells based on the AIC seed layer approach. Copyright © 2007 John Wiley & Sons, Ltd.

In this review, we focus on the field of organic photovoltaic cells based on small molecular weight materials. In particular, we discuss the physical processes that lead to photocurrent generation in organic solar cells, as well as the various architectures employed to optimize device performance. These include the donor–acceptor heterojunction for efficient exciton dissociation, the exciton blocking layer, the mixed or bulk heterojunction, and the stacked or tandem cell. We show how the choice of materials with known energy levels and absorption spectra affect device performance, particularly the open-circuit voltage and short-circuit current density. We also discuss the typical materials and growth techniques used to fabricate devices, as well as the issue of device stability, all of which are critical for the commercialization of low-cost and high-performance organic solar cells. Copyright © 2007 John Wiley & Sons, Ltd.

Intrinsically carbon-doped AlGaAs layers were grown on semi-insulating (001) GaAs (on axis) and Ge (6° misoriented) substrates using tertiarybutylarsine (TBAs). Growth temperatures from 512 to 630 °C were studied and V/III ratios were varied in the range from 9.3 to 0.8. Carrier concentrations saturate at a value of 6 × 10 19 cm -3 . Multi-junction solar cell tunnel diodes were grown, using carbon doping, in different configurations. The highest obtained peak current density was 17.3 A/cm 2 with the tunnel junction embedded in GaAs layers. Using this junction InGaP/GaAs tandem solar cells were grown on 4" Ge which yield a maximum efficiency of 24.3% under the AM1.5 g spectrum.

The removal of secondary phases from the surface of the kesterite crystals is one of the major challenges to improve the performances of Cu2ZnSn(S,Se)(4) (CZTSSe) thin film solar cells. In this Contribution, the KCN/KOH Chemical etching approach, originally developed for the removal of CuxSe phases in Cu(In,Ga)(S,Se)(2) thin films) is applied to CZTSe absorbers exhibiting various chemical compositions. Two distinct electrical behaviors were observed on CZTSe/CdS solar cells after treatment: (i) the improvement of the fill factor (FF) after 30 s of etching for the CZTSe absorbers showing initially a distortion of the electrical characteristic; (ii) the progressive degradation Of the FF after long treatment time for all Cu-poor CZTSe solar cell samples. The first effect can be attributed to the action of KCN on the absorber, that is found to clean the absorber free surface from most of the secondary phases surrounding the kesterite grains (e.g., Se-0, CuxSe, SnSex, SnO2, Cu2SnSe3 phases, excepting the ZnSe-based phases). The second observation was identified as a consequence of the preferential etching of Se, Sn, and Zn from the CZTSe surface by the KOH solution, combined with the modification of the alkali content of the absorber. The formation of a Cu-rich shell at the absorber/buffer layer interface, leading to the increase of the recombination rate at the interface, and the increase in the doping of the absorber layer after etching are found to be at the origin of the deterioration of the FF of the solar cells.

In this contribution, we describe the development and successful implementation of nanophotonic front electrodes in solution processed perovskite solar cells. The nanostructured front electrodes were fabricated by sputtering the ITO front electrode onto a prepatterned transparent glass substrate. Due to the shallow geometry of the employed transparent and conductive nanopatterned front electrodes a straight forward integration into a solution-based baseline process of perovskite solar cells was possible. The nanophotonic electrodes yield a significantly improved light incoupling. Prototype methylammonium lead iodide perovskite solar cells show an improvement of 5% in short-circuit current density and an improvement from 9.6% to 9.9% in power conversion efficiency compared to the flat reference device. These results are confirmed by three-dimensional electromagnetic simulations of the light management in these devices.

In this work, we focus on the replacement of the commonly used but toxic Cd-based buffer layer by In2S3 thin films deposited by co-evaporation for application in Cu2ZnSnSe4 (CZTSe) solar cells. The impact of the deposition conditions of the buffer layer on the electrical behavior of CZTSe/In2S3 devices is first investigated. The best solar cell efficiencies were obtained for relatively thick In2S3 buffer layers (∼100 nm) deposited at low temperature (

Solar energy is a cornerstone of the future global sustainable energy system. This is recognized by the International Energy Agency (IEA) and a wide range of other public organizations, private companies and NGO's. Within the portfolio of solar energy technology options, photovoltaic conversion (PV) for electricity generation plays a special role. It is highly modular and suitable for many applications, ranging from small stand-alone systems through medium-sized integrated grid-connected systems to large power plants. Finally, recent sharp cost and price reductions have made PV competitive on retail price level in most countries which is a crucial step towards the creation of self-sustaining markets. A further reduction by a factor of 2 to 3 will make PV-electricity competitive with electricity generated by fossil fuels or nuclear power plants on wholesale price level. Such a reduction is very likely to be achieved, the main question is how soon and using which technologies. Reduction of costs and prices on the level of costs/Wp alone, however, is not sufficient for PV to reach large-scale penetration. A number of very important challenges has to be tackled in parallel. The progress of PV has been based on upscaling and innovation of the cell technology, but it is clear that the “one size fits all” approach will not be sufficient in the future anymore. It is clear that reliability and qualification of PV-modules is to be shaped according to the location in which these PV-modules will be installed. In addition, given the efficiency increase of PV-cells and modules going to 20% and more, it is quite likely that these high-efficiency devices might suffer more from degradation than in the past. In relation to the further penetration of photovoltaics, the emphasis will also shift from the performance under standard conditions of temperature and irradiation (expressed as Wp) towards energy generation (kWh) under a broad variety of conditions. The paper will outline how performance increase and reliability improvement will have to evolve concurrently to make PV a major electricity source.

In less than 3 years' time, a vast progress in power conversion efficiencies of organometal halide perovskite solar cells has been achieved by optimization of the device architecture, charge transport layers, and interfaces. A further increase in these efficiencies is expected from an improvement in the optical properties via anti-reflection coatings and nanophotonic light management concepts. In this contribution, we report on the development and implementation of a nanophotonic front electrode for perovskite solar cells. The nanostructures were replicated via the versatile and large-area compatible UV-nanoimprint lithography. The shallow design of the used transparent and conductive nanostructures enabled easy integration into our solution-based baseline process. Prototype methylammonium lead iodide perovskite solar cells show an improvement of 5% in short-circuit current density and an improvement from 9.6% to 9.9% in power conversion efficiency compared to the flat reference device.

Polycrystalline CuInSe2 semiconductors are efficient light absorber materials for thin film solar cell technology, whereas printing is one of the promising low cost and non-vacuum approaches for the fabrication of thin film solar cells. The printed precursors are transformed into a dense polycrystalline CuInSe2 semiconductor film via thermal treatment in ambient selenium atmosphere (selenization). In this study, the effect of the selenium content in high purity mechanically synthesized CuInSex (x = 2, 1.5, 1 or 0.5) alloy precursors on the recrystallization of the CuInSe2 phase during the selenization process was investigated. The nanostructure and phase variation of CuInSex nanopowders were investigated by different characterization techniques. The recrystallization process of the 12 μm thick CuInSex coatings into the CuInSe2 phase during selenization in selenium vapor was investigated via in-situ high temperature X-ray diffraction. The CuInSex precursors with lower selenium content showed a more pronounced phase conversion into CuInSe2 compared to the higher selenium content CuInSex precursors. Moreover, the CuInSex (x = 0.5 and 1) precursor resulted in a denser polycrystalline CuInSe2 semiconductor film with larger crystals. This could be attributed to a more intensive atomic interdiffusion within the CuInSex precursor system compared to a CuInSe2 phase precursor, and the formation of intermediate CuSe and CuSe2 fluxing phases during selenization.

The nanoscale morphology of the active layer in bulk-heterojunction solar cells consisting of regioregular poly(3-hexylthiophene-2, 5-diyl) (P3HT) and methanofullerene([6-6]-phenyl C 61 butyric acid methyl ester) (PCBM) was extensively studied using Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and optical microscopy. Different weight ratios of P3HT:PCBM were investigated as a function of annealing temperature and time revealing the occurrence of crystallization of both components. Firstly, the as-prepared films can be described as a semi-crystalline blend. Secondly, it has been demonstrated that for a short annealing time (5 min) at lower annealing temperatures (75–115 °C) an increased crystallization of P3HT occurs. Thirdly, it has been observed that a prolonged annealing at the given temperature range or a short annealing at higher temperatures ( ≥ 120 °C) leads to the formation of a new ordered crystalline structure of PCBM. These new ordered structures, a few μ m up to 100 μ m in length, form a network of needle-like and even fan-shaped crystals. Key-parameters to “tune” this new ordered structure of PCBM are blend ratio and annealing conditions. The growth mechanism of these new PCBM-structures is described by means of diffusion.

The performance of organic solar cells based on the blend of regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is strongly influenced by blend composition and thermal annealing conditions. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) diffraction measurements show that in the considered blends, ordering of P3HT plays a key role in understanding the PV-performance. It is demonstrated that the natural tendency of regioregular P3HT to crystallize is disturbed by the addition of PCBM. Annealing however improves the crystallinity, explaining the observed spectral broadening and is also resulting in a higher mobility of the holes in P3HT.