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Abstract: This paper addresses the influence of the chemical memory effect (CME) of in situ plasma cleaning by using the fluorinated gases on the properties of subsequently deposited thin-film silicon solar cells and discusses methods to avoid or reduce this effect. Secondary ion mass spectrometry (SIMS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) profiles analysis showed a high impurity concentration in the intrinsic (i)-layer of p–i–n solar cells prepared directly after in situ cleaning. With increasing number of cell depositions these contaminations decrease and the solar cell performance recovers to the standard value. Restoring solar cell performance is accompanied by a decrease of contaminants concentration in the i-layer. The intentional variation of the F-content in the i-layer obtained by adding SiF4 to the process gas mixture during a-Si:H i-layer preparation reveals that for solar cells a fluorine content above 1.5×1019 cm−3 is critical. We applied NF3 or SF6 +O2 as cleaning gases and optimized the cleaning procedure. In case of using NF3 as the cleaning gas, the CME was less pronounced as compared to the SF6 +O2 case and by additional procedures, like increasing the total gas flow rate during deposition, hydrogen plasma treatment of reaction chamber, the high solar cell quality could be achieved directly after in situ reactor cleaning. Low concentration of impurities in such cells was observed. Also the long-term illumination test (light-soaking for 1000 h, AM1.5 radiation) shows the same stabilized efficiency as compared to reference cells. [Copyright &y& Elsevier]

Transient depletion of source gases can play an important role in materials processing, particularly during the initial phase of thin film synthesis in which nucleation takes place and the interface is formed. In this paper, we present a zero-order analytical model that allows an estimation of the magnitude and timescale of transient depletion. The model is based on a lumped particle balance for a processing region and reactor volume that are coupled via a directive feed gas flow and diffusive transport. To illustrate the model, an experimental case study is presented on transient depletion during the parallel plate radiofrequency SiH4 +H2 plasma deposition of microcrystalline silicon for solar cells. The SiH4 steady-state depletion was experimentally determined by mass spectrometry, deposition rate and optical emission spectroscopy measurements. The transient depletion of the SiH4 was monitored by time-resolved optical emission spectroscopy measurements. Model and experiment are in good agreement. The implications for materials processing and thin film synthesis, as well as methods to control transient depletion, are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Highly conductive and transparent aluminum-doped zinc oxide (ZnO:Al) films were prepared by reactive mid-frequency (MF) magnetron sputtering at high growth rates. By varying the deposition pressure, pronounced differences with respect to film structure and wet chemical etching behavior were obtained. Optimized films develop good light-scattering properties upon etching leading to high efficiencies when applied to amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon-based thin-film solar cells and modules. Initial efficiencies of 7.5% for a μc-Si:H single junction and 9.7% for an a-Si:H/μc-Si:H tandem module were achieved on an aperture area of 64cm2. [Copyright &y& Elsevier]

Abstract: This contribution discusses recent scientific and technological challenges for the development of highly efficient microcrystalline silicon (μc-Si:H) based thin film solar cells. Aluminium doped ZnO films prepared by sputtering and post deposition etching serve as transparent conductive oxide (TCO) material, which provide excellent light trapping properties. Challenges are the transfer of this approach to cost-effective reactive sputtering from metallic targets and the reduction of optical absorption losses in the front TCO films. We developed μc-Si:H i-layers by plasma-enhanced chemical vapour deposition (PECVD) using 13.56 and 40.68 MHz excitation frequency, high deposition pressures and high RF-powers. These conditions provide sufficiently “soft” deposition for the growth of high quality μc-Si:H material and yield high deposition rates. Stable aperture area module efficiencies of 10.1% and 8.1% were obtained for a-Si:H/μc-Si:H and μc-Si:H modules, respectively, on 10×10 cm2 substrate size. Subsequent up-scaling to ∼1 m2 coating area was performed and a-Si:H/μc-Si:H modules exceeding 10% initial aperture area efficiency were obtained. We discuss the questions of process stability and process reproducibility with respect to large area PECVD production systems. [Copyright &y& Elsevier]

Abstract: Aluminum doped zinc oxide (ZnO:Al, AZO) films were prepared by reactive mid frequency magnetron sputtering. We characterized the electrical and optical properties as well as the surface morphology obtained after wet chemical etching. The carrier mobility could be increased up to 42 cm2/Vs and the transmission between 400 and 1100 nm was enhanced by the reduction of aluminum content in the targets. The working point of the reactive sputtering process strongly influences the etching behavior and was used to optimize the light scattering properties of the ZnO:Al films after wet chemical etching. Finally, the texture-etched ZnO:Al films were successfully applied as substrates for silicon thin film solar cells. [Copyright &y& Elsevier]

Abstract: Thin-film silicon solar cells with up to 11.2% stable cell efficiency have been developed on ZnO coated glass substrates on laboratory scale using solely plasma enhanced chemical vapour deposition (PECVD) and magnetron sputtering as deposition techniques for all thin films. To transfer this concept towards large areas, we developed a novel PECVD electrode which proved its capability of providing homogenous amorphous and microcrystalline silicon films on ∼1 m2 sized substrates. Microcrystalline silicon (μc-Si:H) solar cells were realised with efficiencies up to 7.2%. The high open-circuit voltage V oc and fill factor FF of 520 mV and 71%, respectively, demonstrate that high quality μc-Si:H material can be prepared with this large area electrode. Note that finally the entire thin-film structure–including all silicon, TCO and metal films–can be produced using one single equipment platform which complies with the requirements for a cost-effective and large scale mass production. [Copyright &y& Elsevier]

Influence of front TCO thickness, surface texture and different back reflectors on short-circuit current density and fill factor of thin film silicon solar cells were investigated. For the front TCO studies, we used ZnO layers of different thickness and applied wet chemical etching in diluted HCl. This approach allowed us to adjust ZnO texture and thickness almost independently. Additionally, we used optical modeling to calculate optical absorption losses in every layer. Results show that texture and thickness reduction of front ZnO increase quantum efficiency over the whole spectral range. The major gain is in the red/IR region. However, the higher sheet resistance of the thin ZnO causes a reduction in fill factor. In the back reflector studies, we compared four different back reflectors: ZnO/Ag, Ag, ZnO/Al and Al. ZnO/Ag yielded the best, Al the worst light trapping properties. Furthermore, the Ag back contact turned out to be superior to ZnO/Al for microcrystalline cells. Finally, the smooth ZnO/Ag back contact showed a higher reflectivity than the rough one. We prepared pin cells with rough and smooth ZnO/Ag interface, leaving the roughness of all other interfaces unchanged. [Copyright &y& Elsevier]

We present a comprehensive study of microcrystalline silicon (μc-Si:H) solar cells prepared by plasma-enhanced chemical vapour deposition (PECVD) at 13.56 MHz excitation frequency. In the first step the cell development was performed in a small area PECVD reactor showing the relationship between the deposition process parameters and the resulting solar cell performance. Focus was on the influence of deposition pressure, electrode distance and the application of a pulsed plasma on high rate deposition of solar cells. Subsequent up-scaling to a substrate area of 30×30 cm2 confirmed the suitability of the process for large area reactors. The influence of i-layer deposition parameters on solar cell performance was studied directly in p–i–n cells prepared on textured ZnO. Solar cell efficiencies up to 9% were achieved at deposition rates of 5–6 A˚/s for the i-layer using high plasma powers. Applied as bottom cell in a-Si:H/μc-Si:H tandem cells a stable cell efficiency of 11.2% could be obtained. The excellent homogeneity was proven by the realization of first modules with an aperture area of 689 cm2 and an active area initial efficiency of 10.3% (stable: 8.9%) using an established base technology for laser patterning and back contact sputtering at RWE Solar GmbH. [Copyright &y& Elsevier]

This paper reviews recent efforts to provide the scientific and technological basis for cost-effective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for μc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates of 5 and 9 A˚/s, respectively. These μc-Si:H solar cells were successfully up-scaled to a substrate area of 30×30 cm2 and applied in a-Si:H/μc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%. [Copyright &y& Elsevier]

Abstract. This paper reviews recent developments in the field of amorphous-silicon-based thin-film solar cells and discusses potentials for further improvements. Creative efforts in materials research, device physics, and process engineering have led to highly efficient solar cells based on amorphous hydrogenated silicon. Sophisticated multijunction solar cell designs make use of its unique material properties and strongly suppress light induced degradation. Texture-etching of sputtered ZnO:A1 films is presented as a novel technique to design optimized light trapping schemes for silicon thin-film solar cells in both p-i-n and n-i-p device structure. Necessary efforts will be discussed to close the efficiency gap between the highest stabilized efficiencies demonstrated on lab scale and efficiencies achieved in production. In case of a-Si:H/a-Si:H stacked cells prepared on glass substrates, significant reduction of process-related losses and the development of superior TCO substrates on large areas promise distinctly higher module efficiencies. A discussion of future perspectives comprises the potential of new deposition techniques and concepts combining the advantages of amorphous and crystalline silicon thin-film solar cells. [ABSTRACT FROM AUTHOR]

Abstract: This short review article gives an overview on the current status of polycrystalline silicon thin-film solar cells on glass and discusses the future perspectives of this technology. [Copyright &y& Elsevier]

A time-resolved optical emission spectroscopic study identified transient behavior of the excited SiH emission in a parallel plate SiH4/H2 plasma. The transient behavior could be prevented by filling the background gas with H2 prior to plasma ignition. Applying this condition, state-of-the-art microcrystalline silicon (μc-Si:H) could be deposited irrespective of the applied H2 flow, ultimately demonstrated by a 9.5% efficient solar cell deposited from pure SiH4. The results are discussed in terms of SiH4 back diffusion: an initial diffusion flux of SiH4 from the reactor’s dead volume back into the plasma. [ABSTRACT FROM AUTHOR]

Highlights • Recent progress on liquid phase crystallized silicon is reviewed. • Influence of doping density and BSF finger width on IBC cell is studied. • A roadmap for further development of LPC silicon solar cells is presented. Abstract Liquid phase crystallization of silicon (LPC-Si) offers great potential for high-quality Si films and a cost effective fabrication technique for thin crystalline silicon solar cells on glass. In this work, we report on the progress on LPC-silicon at HZB in the past years. Beginning with a brief description of the fabrication process, we summarize the work on the different contact systems developed for these absorbers before focusing on the interdigitated back contact architecture on which the highest efficiencies were reported. State-of-the art cells form the basis for a detailed discussion of the status of this technology. We investigate the current loss mechanisms and explore the potential for further improvement. Finally, based on this comprehensive quality assessment, we develop a roadmap to increase the cell efficiencies to wafer-equivalent values. [ABSTRACT FROM AUTHOR]

Hybrid solar cells have been fabricated from solution processing of poly-(3-hexylthiophen-2,5-diyl) onto planar poly-crystalline thin film silicon (poly-Si) absorbers on glass. The poly-Si layers were prepared by laser crystallization. Methyl passivation of the poly-Si surface by a one-step grafting process via methyl-Grignard enabled open circuit voltages of up to 552 mV and an overall power conversion efficiency of 6.6% for this device type. The solar cell exhibited significant advantages compared to the wafer-based counterparts. The inverted device structure of the thin film cell lead to an enhancement of the quantum efficiency since the back side contacted cell had less light absorption and reflection losses due to the gold layer which was used as front contact for the c-Si-based hybrid solar cell. As a result, a photo current of 24.3 mA cm−2 was obtained for a 10 μm thin poly-Si layer on glass. Furthermore, the inverted device structure showed a pronounced increase in stability due to the much thicker gold back contact that reduces the diffusion of water and oxygen toward the polymer layer. After 3 months in ambient air, this type of solar cell operated with 86% of its initial efficiency. [ABSTRACT FROM AUTHOR]

The complex refractive indices of formamidinium cesium lead mixed-halide [FA0.83Cs0.17Pb(I1–xBrx)3] perovskite thin films of compositions ranging from x = 0 to 0.4, with both flat and wrinkle-textured surface topographies, are reported. The films are characterized using a combination of variable angle spectroscopic ellipsometry and spectral transmittance in the wavelength range of 190 nm to 850 nm. Optical constants, film thicknesses and roughness layers are obtained point-by-point by minimizing a global error function, without using optical dispersion models, and including topographical information supplied by a laser confocal microscope. To evaluate the bandgap engineering potential of the material, the optical bandgaps and Urbach energies are then accurately determined by applying a band fluctuation model for direct semiconductors, which considers both the Urbach tail and the fundamental band-to-band absorption region in a single equation. With this information, the composition yielding the optimum bandgap of 1.75 eV for a Si-perovskite tandem solar cell is determined. [ABSTRACT FROM AUTHOR]

Liquid phase crystallization of silicon is a promising technology to grow crystalline silicon thin films on glass. It has already been demonstrated that open circuit voltages of up to 656 mV and efficiencies of up to 11.8% can be achieved by this technique. Nevertheless further improvements are required to become competitive with wafer based silicon solar cells. A possibility to improve the quality is to enlarge the grain size and to control the crystallographic orientation of the resulting layers. While a preferential {100} surface orientation can be triggered utilizing suitable crystallization parameters, the in-plane orientation still remains random. Also the grain size stays within the same range. By introducing a local monocrystalline seed at the beginning of the crystallization process, we are able to control both the surface and the in-plane orientation of the silicon films. In addition the grain size is significantly increased in scanning direction and only limited by the substrate size of 5 cm. This high morphological quality is accompanied by an improved electrical quality confirmed by photoluminescence imaging and Hall measurements. This is a big step towards the final goal to directly grow monocrystalline silicon thin films on glass substrates. [ABSTRACT FROM AUTHOR]

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H), known as the Staebler-Wronski effect, has been studied by time-domain pulsed electron-paramagnetic resonance. Electron-spin echo relaxation measurements in the annealed and light-soaked state revealed two types of defects (termed type I and II), which can be discerned by their electron-spin echo relaxation. Type I exhibits a monoexponential decay related to indirect flip-flop processes between dipolar coupled electron spins in defect clusters, while the phase relaxation of type II is dominated by H1 nuclear spin dynamics and is indicative for isolated spins. We propose that defects are either located at internal surfaces of microvoids (type I) or are isolated and uniformly distributed in the bulk (type II). The concentration of both defect type I and II is significantly higher in the light-soaked state compared to the annealed state. Our results indicate that in addition to isolated defects, defects on internal surfaces of microvoids play a role in light-induced degradation of device-quality a-Si:H. [ABSTRACT FROM AUTHOR]

Abstract: Electron-beam evaporation (EBE) of silicon permits the high-rate deposition of photovoltaic thin-film devices at low costs. The directional, non-conformal growth characteristic of EBE is systematically investigated by varying the silicon flux angle of incidence γ on the substrate surface between 0° and 49°. After solid phase crystallization the micro-structural properties of these silicon films are investigated and correlated with the electronic quality of n +/p −/p+-type solar cell stacks. As γ exceeds 30°, the porosity and oxygen content of the silicon films increase significantly coming along with the break-down of the electronic material quality. At γ>40° the silicon crystallization process is even found to be suppressed resulting from a columnar film morphology infiltrated by oxygen-rich pores. The knowledge of this critical angle is essential when textured substrates, consisting of many tilted micro-areas, are used for enhanced light absorption in the silicon film simultaneously ensuring the growth of high-quality material. Furthermore, the inclination angle γ can serve as design parameter for tailored substrate templates for the fabrication of advanced light-harvesting structures by self-organized solid phase crystallization. [Copyright &y& Elsevier]

Abstract: Paramagnetic coordination defects in undoped hydrogenated amorphous silicon (a-Si:H) are studied using multifrequency pulsed electron-paramagnetic resonance (EPR) spectroscopy at S-, X-, Q- and W-band microwave frequencies (3.6, 9.7, 34, and 94GHz, respectively). The improved spectral information extractable from a multifrequency fitting procedure allows us to conclude that the g tensor exhibits a rhombic splitting instead of axial symmetry. Our methods allow for precise and accurate determination of the g tensor principal values g x =2.0079(2), g y =2.0061(2) and g z =2.0034(2) and their distribution parameters (g strain). [Copyright &y& Elsevier]

Abstract: We explore the (near-)interface structure of amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions as employed in high-efficiency heterojunction solar cells. We make use of secondary-ion-mass-spectroscopy profiles and minority carrier lifetime measurements taken on undoped deuterated amorphous silicon [(i)a-Si:D] layers deposited on c-Si from deuterated silane at identical conditions as the hydrogenated layers we have analyzed previously [T. F. Schulze et al., Appl. Phys. Lett. 96 (2010) 252102]. We briefly discuss the implications of the local interface structure for the c-Si surface passivation as well as for the heterojunction band offsets, and identify a route towards optimization of (i)a-Si:H layers as passivating buffers in a-Si:H/c-Si high-efficiency heterojunction solar cells. [Copyright &y& Elsevier]

Multifrequency pulsed electron paramagnetic resonance (EPR) spectroscopy using S-, X-, Q-, and W-band frequencies (3.6.9.7, 34, and 94 GHz, respectively) was employed to study paramagnetic coordination defects in undoped hydrogenated amorphous silicon (a-Si:H). The improved spectral resolution at high magnetic field reveals a rhombic splitting of the g tensor with the following principal values: gx = 2.0079. gy = 2.0061, and gz = 2.0034, and shows pronounced g strain, i.e., the principal values are widely distributed. The multifrequency approach furthermore yields precise 29Si hyperfine data. Density functional theory (DFT) calculations on 26 computer-generated a-Si:H dangling-bond models yielded g values close to the experimental data but deviating hyperfine interaction values. We show that paramagnetic coordination defects in a-Si:H are more delocalized than computer-generated dangling-bond defects and discuss models to explain this discrepancy. [ABSTRACT FROM AUTHOR]

Abstract: Aluminium-doped zinc oxide films were deposited on glass substrates at high rates by reactive mid-frequency sputtering. The in-line sputter system allows oxygen influx along the middle and sides of a dual-cathode system. The effect of varying the oxygen flow from the sides on the electrical and optical properties together with the surface morphology after wet chemical etching was investigated. Increasing the amount of oxygen flow from the sides improved the resistivity profile of static prints and gave highly conductive and transparent films in dynamic deposition mode. The etched films developed rough surface textures with effective light scattering that could be controlled by the oxygen balance between the middle and sides. Optimally textured films were used as front contacts in 1 cm2 single junction microcrystalline solar cells yielding an initial efficiency of 8.4%. The improvement in light trapping led to short circuit densities higher than that of the reference solar cells. [Copyright &y& Elsevier]

Abstract: p-i-n diodes based on amorphous and microcrystalline silicon are used for different applications like sensors or solar cells. However, their long-term stability and dynamic behavior are still under discussion. Therefore, the optical properties of both types of diodes were investigated by a comparative study. The long-term stability of non-encapsulated devices was tested by means of light soaking, damp heat testing and high-temperature treatment of up to 2000 h. The dynamic properties of the thin-film silicon photodetectors were studied by measuring and analysing the admittance under different bias conditions. [Copyright &y& Elsevier]

Liquid phase crystallized Si thin-film solar cells on nanoimprint textured glass substrates exhibiting two characteristic, but distinct different surface structures are presented. The impact of the substrate texture on light absorption, the structural Si material properties, and the resulting solar cell performance is analyzed. A pronounced periodic substrate texture with a vertical feature size of about 1 μm enables excellent light scattering and light trapping. However, it also gives rise to an enhanced Si crystal defect formation deteriorating the solar cell performance. In contrast, a random pattern with a low surface roughness of 45 nm allows for the growth of Si thin films being comparable to Si layers on planar reference substrates. Amorphous Si/crystalline Si heterojunction solar cells fabricated on the low-roughness texture exhibit a maximum open circuit voltage of 616mV and internal quantum efficiency peak values exceeding 90%, resulting in an efficiency potential of 13.2%. This demonstrates that high quality crystalline Si thin films can be realized on nanoimprint patterned glass substrates by liquid phase crystallization inspiring the implementation of tailor-made nanophotonic light harvesting concepts into future liquid phase crystallized Si thin film solar cells on glass. [ABSTRACT FROM AUTHOR]

This study is concerned with the topography of nanostructures consisting of arrays of polycrystalline nanodots. Guided by transmission electron microscopy (TEM) measurements of crystalline Si (c-Si) nanodots that evolved from a "dewetting" process of an amorphous Si (a-Si) layer from a SiO2 coated substrate, we investigate appropriate formulations for the surface energy density and transitions of energy density states at grain boundaries. We introduce a new numerical minimization formulation that allows to account for adhesion energy from an underlying substrate. We demonstrate our approach first for the free standing case, where the solutions can be compared to well-known Wulff constructions, before we treat the general case for interfacial energy settings that support "partial wetting" and grain boundaries for the polycrystalline case. We then use our method to predict the morphologies of silicon nanodots. [ABSTRACT FROM AUTHOR]

Microcrystalline silicon solar cells with an open-circuit voltage surpassing the 600 mV barrier were prepared by combining the use of a hot wire deposited p-i interface with that of an i layer deposited by radio-frequency parallel plate plasma deposition using controlled SiH4 flow profiling. The control of the bulk and interface properties facilitated effective charge carrier collection from i layers with a crystalline volume fraction as low as ∼30%. Judging from the absorption in the infrared and the excellent charge carrier transport, the optoelectronic properties of this material were still dominated by the crystalline rather than the amorphous phase. [ABSTRACT FROM AUTHOR]

An analysis of the contact formation between degenerated n-type transparent conductive oxide (TCO) and p-type amorphous silicon (a-Si:H) as it is used for front side contacts in high efficiency a-Si:H/crystalline silicon (c-Si) heterojunction solar cells is presented. It is shown that the deposition of a TCO on a (p)a-Si:H emitter layer causes a reduction of charge carrier lifetime in low injection levels which leads to a lowering of the implied fill factor. Simulation based analysis of charge carrier lifetime and direct measurements by surface photovoltage reveals that TCO deposition induces a change of the c-Si band bending. The magnitude of this change depends on the (p)a-Si:H doping level. Both observations are explained by the impact of the TCO/a-Si:H work function difference on the c-Si band bending. Based on numerical simulations, the reduced injection-dependent band bending is identified as the reason for the reduced fill factor of final solar cells. [ABSTRACT FROM AUTHOR]

We present temperature-dependent measurements of I-V curves in the dark and under illumination in order to elucidate the dominant transport mechanisms in amorphous silicon-crystalline silicon (a-Si:H/c-Si) heterojunction solar cells. ZnO:Al/(p)a-Si:H/(n)c-Si/(n+)a-Si:H cells are compared with inversely doped structures and the impact of thin undoped a-Si:H buffer layers on charge carrier transport is explored. The solar cell I-V curves are analyzed employing a generalized two-diode model which allows fitting of the experimental data for a broad range of samples. The results obtained from the fitting are discussed using prevalent transport models under consideration of auxiliary data from constant-final-state-yield photoelectron spectroscopy, surface photovoltage, and minority carrier lifetime measurements. Thus, an in-depth understanding of the device characteristics is developed in terms of the electronic properties of the interfaces and thin films forming the heterojunction. It is shown that dark I-V curve fit parameters can unequivocally be linked to the open circuit voltage under illumination which opens a way to a simplified device assessment. [ABSTRACT FROM AUTHOR]

A postdeposition thermal treatment has been applied to sputtered Al-doped zinc oxide films and shown to strongly decrease the resistivity of the films. While high temperature annealing usually leads to deterioration of electrical transport properties, a silicon capping layer successfully prevented the degradation of carrier concentration during the annealing step. The effect of annealing time and temperature has been studied in detail. A mobility increase from values of around 40 cm2/Vs up to 67 cm2/Vs, resulting in a resistivity of 1.4×10-4 Ω cm has been obtained for annealing at temperatures of 650 °C. The high mobility increase is most likely obtained by reduced grain boundary scattering. Changes in carrier concentration in the films caused by the thermal treatment are the result of two competing processes. For short annealing procedures we observed an increase in carrier concentration that we attribute to hydrogen diffusing into the zinc oxide film from a silicon nitride barrier layer between the zinc oxide and the glass substrate and the silicon capping layer on top of the zinc oxide. Both are hydrogen-rich if deposited by plasma-enhanced chemical vapor deposition. For longer annealing times a decrease in carrier concentration can occur if a thin capping layer is used. This can be explained by the deteriorating effect of oxygen during thermal treatments which is well known from annealing of uncapped zinc oxide films. The reduction in carrier concentration can be prevented by the use of capping layers with thicknesses of 40 nm or more. [ABSTRACT FROM AUTHOR]

Polycrystalline silicon (poly-Si) thin films have been prepared by electron-beam evaporation and thermal annealing for the development of thin-film solar cells on glass coated with ZnO:Al as a transparent, conductive layer. The poly-Si microstructure and photovoltaic performance were investigated as functions of the deposition temperature by Raman spectroscopy, scanning and transmission electron microscopies including defect analysis, x-ray diffraction, external quantum efficiency, and open circuit measurements. It is found that two temperature regimes can be distinguished: Poly-Si films fabricated by deposition at low temperatures (Tdep<400 °C) and a subsequent thermal solid phase crystallization step exhibit 1–3 μm large, randomly oriented grains, but a quite poor photovoltaic performance. However, silicon films deposited at higher temperatures (Tdep>400 °C) directly in crystalline phase reveal columnar, up to 300 nm big crystals with a strong <110> orientation and much better solar cell parameters. It can be concluded from the results that the electrical quality of the material, reflected by the open circuit voltage of the solar cell, only marginally depends on crystal size and shape but rather on the intragrain properties of the material. The carrier collection, described by the short circuit current of the cell, seems to be positively influenced by preferential <110> orientation of the grains. The correlation between experimental, microstructural, and photovoltaic parameters will be discussed in detail. [ABSTRACT FROM AUTHOR]

The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (μc-Si:H) has been systematically investigated. Single μc-Si:H layers and complete μc-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of ∼1.2 μm thick μc-Si:H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2×1019 to 2×1019 cm-3; in the case of nitrogen contamination the critical impurity level is lower ([N]critical=6×1018–8×1018 cm-3). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of μc-Si:H films. It was observed that the both contaminant types act as donors. Efficiency losses due to oxygen or nitrogen impurities are attributed to fill factor decreases and to a reduced external quantum efficiency at wavelengths of >500 nm. In the case of an air leak during the μc-Si:H deposition process, the cell performance drops at an air leak fraction from 140 to 200 ppm compared to the total gas flow during i-layer deposition. It is demonstrated that oxygen and nitrogen impurities close to the p/i-interface have a stronger effect on the cell performance compared to impurities close to the n/i-interface. Moreover, thick μc-Si:H solar cells are found to be more impurity-sensitive than thinner cells. [ABSTRACT FROM AUTHOR]

This study addresses the electrical and optical properties of radio frequency magnetron sputtered aluminum doped zinc oxide (ZnO:Al) films. The main focus was on the improvement in carrier mobility μ to achieve simultaneously high transparency for visible and particularly near-infrared light and low resistivity. The influence of Al concentration in the target, film thickness, sputter power, deposition pressure, and substrate temperature on material properties was investigated. The structural, compositional, electrical and optical properties were studied using x-ray diffraction, secondary ion mass spectrometry (SIMS), room temperature Hall effect measurements and spectral photometry, respectively. All ZnO:Al films were polycrystalline and preferentially oriented along [002]. The grain size along the direction of growth increased with higher Al doping and with increasing film thickness. The SIMS measurements revealed that the Al concentration in the film was nearly the same as in the target. Carrier concentration N and mobility μ are determined by the target Al concentration. In addition μ is influenced by the film thickness and the sputter pressure. For each Al concentration, the highest μ was generally observed at low deposition pressures. By using a target with low Al[sub 2]O[sub 3] concentration of 0.5 wt %, μ could be improved up to 44.2 cm[sup 2]/V s while maintaining the electrical resistivity ρ as low as 3.8×10[sup -4] Ω cm. For these films the transparency in the near-infrared wavelength range strongly improved which makes them particularly interesting for the application in optoelectronic devices like thin-film solar cells. The μ-N dependence for films deposited under diverse conditions was studied to identify a practical limit for μ. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A nanoscopic kinetic model of controlled plasma-assisted microcrystallite formation (PAµCF) of Si in pre deposited a-Si:H films at low temperatures is proposed. The model suggests mechanisms for enhancement of the Si crystallization in a-Si:H films at low temperatures by treatment of the films in plasma. The model reveals certain kinetic advantages of hydrogen plasmas for the formation of Si crystalline nuclei in a-Si:H compared to other plasmas (Ar plasma, etc.). These advantages make the hydrogen plasma substantially more efficient in the PAµCF of Si in a-Si:H films. The proposed mechanism for PAµCF of Si is associated with the formation on the surface of the a-Si:H film and in the adjacent nanometer material layer of nanoscale (picosecond) short-lived hot spots of high energy density (or effective temperature). The hot spots are generated in the material by energetic plasma ions of energy ε[sub is] = 20-100 eV accelerated by the electrical field in the thin plasma layer near the solid surface. The hot spots promote Si crystallization in a-Si:H. It is shown how the plasma composition, energy, mass, and fluxes of the plasma ions impinging on the surface of the a-Si:H film determine the Si nucleation rate and density of Si microcrystallization. [ABSTRACT FROM AUTHOR]

Absorption losses at a nanorough silver back reflector of a solar cell were measured with high accuracy by photothermal deflection spectroscopy. Roughness was characterized by atomic force microscopy. The observed increase of absorption, compared to the smooth silver, was explained by the surface plasmon absorption. Two series of silver back reflectors (one covered with thin ZnO layer) were investigated and their absorption related to surface morphology. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The diffusion or transport lengths of photo-generated charge carriers in CH3NH3PbI3 layers (thickness up to 1 lm) and powders have been directly measured with high accuracy by modulated surface photovoltage after Goodman. The values of the diffusion lengths of photo-generated charge carriers ranged from 200 nm to tenths of lm. In thin CH3NH3PbI3 layers, the transport lengths corresponded to the layer thickness whereas in thicker layers and in crystallites of CH3NH3PbI3 powders the grain size limited the diffusion length. For grains, the diffusion length of photo-generated charge carriers depended on the measurement conditions. [ABSTRACT FROM AUTHOR]

The superstrate configuration of Cu(In,Ga)Se 2 (CIGS) photovoltaic devices exhibits the potential for improved light harvesting compared to the standard substrate configuration. Due to the limited power conversion efficiency of superstrate devices caused by the poor hetero-interface between the p -type CIGS and the n -type component, these advantages have not been fully considered so far. This work shows that the superstrate configuration can significantly improve the light management within CIGS devices. Experimentally, it is shown that the electric and optical properties of ZnO window layer strongly improve during the CIGS deposition, allowing the use of structured ZnO layers for efficient light-trapping. A highly reflective and scattering back contact is realized by using the high work function material MoO 3− x for the hole-transport layer in combination with a low work function Ag metal layer. Combined with the unique Ga gradient achievable in the superstrate configuration, simulation results show that a strong reduction of the CIGS layer thickness is possible while increasing the power conversion efficiency. This proof of concept highlights the appeal of the superstrate configuration for the future of cost-effective CIGS solar cell production. [ABSTRACT FROM AUTHOR]

High-mobility hydrogen-doped indium oxide In 2 O 3 :H (IOH) were deposited by magnetron sputtering (radio frequency 13.56 MHz) from an In 2 O 3 target in Ar/O 2 /H 2 O gas mixtures onto unheated substrates. The as-deposited films were amorphous as shown by X-ray measurements and were crystallized in a post-annealing step in vacuum and air at 180 °C for 15 min. The optical, electrical, and morphological properties as well as the crystallization behavior of these transparent conductive oxides films were investigated in detail. A dependence of the annealing behavior in air on the total pressure during deposition could be shown. High carrier mobilities > 100 cm 2 /Vs allow for very low optical absorption and a low resistivity around 350 μΩcm. Amorphous IOH films were crystallized by short-term flash lamp annealing (FLA) in argon atmosphere for around 2.7 ms. Spatial temperature distributions in a typical layer stack (crystalline silicon, amorphous silicon, silicon oxide, and IOH film) were calculated within milliseconds after the FLA-treatment. Electron backscattering diffraction measurements of IOH films crystallized by FLA reveal a polycrystalline microstructure with an average lateral crystallite size of 333 nm. The crystallization process of these IOH films was studied by XRD and Hall measurements. [ABSTRACT FROM AUTHOR]

The crystallization process of hydrogen doped In 2 O 3 :H (IOH) films is investigated with energy-dispersive X-ray diffraction measurements. At annealing temperatures between 125 and 150 °C crystallization of 220 nm thin films occurs within only 2 min, and the percentage of the crystalline phase does not change anymore when the temperature is raised above the crystallization temperature of 150 °C. Maximum electron mobilities above 100 cm 2 /Vs have been reached after crystallization. The IOH films were integrated as front contact into amorphous/crystalline silicon heterojunction cells and compared to In 2 O 3 :Sn (ITO) front contacts. Cells with ITO/IOH bilayer front contacts show a slightly lower open circuit voltage because of the a-Si:H passivation layer degradation caused by the longer annealing process needed for the crystallization of the bilayers, while all cells reach total area efficiencies around 20%. IOH films were also implemented as silver free back contact for μc-Si:H cells, and show higher short-circuit current densities than ZnO:Al back contacts because of the higher near-infra-red transmission of IOH. [ABSTRACT FROM AUTHOR]

Liquid phase crystallization of 10 μm thin silicon layers on glass substrates was performed with a line-shaped continuous wave laser beam. The process window was investigated in terms of the scanning velocity of the laser, the pre-heating of the specimens and the applied laser intensity. We have identified the entire process window, in which large-scale crystallization without deformation or destruction of the substrate and cracking of the silicon layer can be obtained. The grain orientations of the resulting Si layers were analyzed using both electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The influence of the critical crystallization parameters on the grain orientation of the silicon film was examined. EBSD and XRD measurements show that a preferential {100} surface texture and {100} and {101} orientations in scanning direction of the laser can be achieved if appropriate crystallization parameters are used. This texture formation is accompanied with a substantial decrease of high angle grain boundaries. [ABSTRACT FROM AUTHOR]

The heterojunction between amorphous silicon (sub)oxides (a-SiOx:H,O < x <2) and crystalline silicon (c-Si) is investigated. We combine chemical vapor deposition with in-system photoelectron spectroscopy in order to determine the valence band offset ΔEV and the interface defect density, being technologically important junction parameters. ΔEV increases from â0.3 eV for the a-Si:H/c-Si interface to >4 eV for the a-SiO2/c-Si interface, while the electronic quality of the heterointerface deteriorates. High-bandgap a-SiOx:H is therefore unsuitable for the hole contact in heterojunction solar cells, due to electronic transport hindrance resulting from the large ΔEV. Our method is readily applicable to other heterojunctions. [ABSTRACT FROM AUTHOR]

We have developed a microcrystalline silicon oxide (μc-SiOx:H) p-type emitter layer that significantly improves the light incoupling at the front side of silicon heterojunction solar cells by minimizing reflection losses. The μc-SiOx:H p-layer with a refractive index of 2.87 at 632 nm wavelength and the transparent conducting oxide form a stack with refractive indexes which consecutively decrease from silicon to the ambient air and thus significantly reduce the reflection. Optical simulations performed for flat wafers reveal that the antireflective effect of the emitter overcompensates the parasitic absorption and suggest an ideal thickness of about 40 nm. On textured wafers, the increase in current density is still more than 1 mA/cm² for a typical emitter thickness of 10 nm. Thus, we are able to fabricate heterojunction solar cells with current densities significantly over 40 mA/cm² and power conversion efficiency above 20%, which is yet mainly limited by the cell's fill factor. [ABSTRACT FROM AUTHOR]

In order to clarify the origin of the previously reported reduction of sub-band gap absorption of sputtered ZnO:Al films upon thermal annealing and raising deposition temperature, structural characterization using Ra-man spectroscopy was carried out on various films. Correlation of the Urbach energy to the FWHM of the E(high)2 mode was found. Oxygen addition to the film growth did not result in changes of the Urbach energy, despite pronounced changes of the electrical properties. The results suggest that extended defects, rather than intrinsic point defects, give rise to the sub-band gap absorption. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A wide variety of liquid and solid phase crystallized silicon films are investigated in order to determine the performance limiting defect types in crystalline silicon thin-film solar cells. Complementary characterization methods, such as electron spin resonance, photoluminescence, and electron microscopy, yield the densities of dangling bond defects and dislocations which are correlated with the electronic material quality in terms of solar cell open circuit voltage. The results indicate that the strongly differing performance of small-grained solid and large-grain liquid phase crystallized silicon can be explained by intra-grain defects like dislocations rather than grain boundary dangling bonds. A numerical model is developed containing both defect types, dislocations and dangling bonds, describing the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: Liquid phase crystallization (LPC) using e-beam or CW-laser line sources of amorphous or nanocrystalline silicon films has emerged as new method to form high quality absorbers on cheap substrates. A disadvantage of electron beam crystallization was the necessity to use amorphous layers in contact with the silicon to maintain a stable crystallization process, resulting in high surface recombination. In this work we investigate capping layers to enable e-beam induced LPC on alternative interlayer materials especially and characterize the morphological and electrical properties of these layer stacks. Photoluminescence measurements of LPC absorbers on layers exhibit significantly increased radiative recombination compared to . Fabricated hetero-junction cells achieved a new emitter area efficiency record for electron beam induced LPC solar cells of 9% using the FrontERA contact system and H passivation. [Copyright &y& Elsevier]

Abstract: For thin film silicon solar cells with high conversion efficiency and low production costs transparent conducting oxide (TCO) substrates with a high performance are required. TCO layers must have excellent conductivity and high transparency over a wide range in the VIS/NIR wavelength spectrum. Moreover, an advanced light trapping scheme has to be implemented, generally obtained by TCO surface texture. For low production costs it is favorable to keep the TCO and silicon layers as thin as possible. We present methods that accomplish all of these requirements. Tandem solar based on amorphous and microcrystalline silicon deposited on DC magnetron-sputtered zinc oxide (ZnO:Al) coated substrates, which have been capped with silicon and subsequently annealed at 500°C, result in an efficiency improved by 0.4% reaching 12.4%. This is due to the increased carrier mobility, reaching up to 70cm2/Vs, and improved transparency of the ZnO:Al. The high TCO conductivity allows for reducing the layer thickness significantly. In order to demonstrate the potential of combining this improved TCO with alternative light scattering concepts, we show results of a tandem cell on substrates comprising ZnO:Al sputtered on sol–gel coated light-scattering layers with dielectric particles having excellent light trapping properties. Combining both methods will allow for thin film silicon solar cells with high efficiency and potentially low production costs. [Copyright &y& Elsevier]

Abstract: This paper addresses the achievements and challenges in a-Si:H/µc-Si:H tandem solar cell technology including production aspects. As an example we show figures of the module production at Masdar PV, with module conversion efficiencies reaching 10%. The process integration was supported by PVcomB, namely, by developing improved PECVD processes and transferring them to Masdar PV. The important role of the TCO substrate for producing high-efficiency modules at lowest possible costs is discussed. We show the results of tandem cells on thermally annealed ZnO:Al substrates, resulting in an improvement of 0.4–0.5% (abs) as a result of the TCO annealing, reaching a stable efficiency of 12.1% for a 1cm² solar cell and 11.6% for a mini module. The challenges in developing thin film silicon solar modules with higher efficiency than presently reached are discussed. [Copyright &y& Elsevier]

Abstract: The present article gives a summary of recent technological and scientific developments in the field of polycrystalline silicon (poly-Si) thin-film solar cells on foreign substrates. Cost-effective fabrication methods and cheap substrate materials make poly-Si thin-film solar cells promising candidates for photovoltaics. However, it is still the challenge for research and development to achieve the necessary high electrical material quality known from crystalline Si wafers on glass as a prerequisite to harvest the advantages of thin-film technologies. A wide variety of poly-Si thin-film solar cell approaches has been investigated in the past years, such as thermal solid phase crystallization – the only technology that had already been matured to industrial production so far – the seed layer concept where a large-grained seed layer is epitaxially thickened, direct growth of fine grained material, and liquid phase crystallization methods by laser or electron beam. In the first part of this paper, the status of these four different poly-Si thin-film solar cell concepts is summarized, by comparing the technological fabrication methods, as well as the structural and electrical properties and solar cell performances of the respective materials. In the second part, three promising technologies are described in more detail due to their highly auspicious properties regarding material quality and throughput aspects during fabrication: (1) High-rate electron–beam evaporation of silicon for the low-cost deposition of high-quality material, (2) large-area periodic nano- and micro-structuring of poly-Si by the use of imprinted substrates providing a large absorption enhancement by a factor of six at a wavelength of 900nm, (3) liquid-phase crystallization of silicon thin-film solar cells by electron–beam, yielding an excellent poly-Si material quality reflected by an open-circuit voltage of 582mV which has been achieved only very recently. A successful combination of these three complementary technologies is envisaged to be the basis for a prospective low-cost and highly efficient poly-Si solar cell device. [Copyright &y& Elsevier]

The sub bandgap absorption of as deposited and thermally treated ZnO:Al has been investigated and described applying Urbach tail theory. In the as deposited state Urbach energies show a decreasing trend for higher deposition temperatures. Annealing leads to much less band tailing, and resulting Urbach energies are the same as for undoped ZnO. It has been concluded that structural disorder is the main contribution to the bandtails in the as deposited state. In summary, films with excellent electrical properties due to very high carrier mobilities and excellent optical properties due to the strong reduction in sub bandgap absorption were obtained. [ABSTRACT FROM AUTHOR]