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Highly efficient hybrid multijunction solar cells are constructed with a wide-bandgap amorphous silicon for the front subcell and a low-bandgap polymer for the back subcell. Power conversion efficiencies of 11.6% and 13.2% are achieved in tandem and triple-junction configurations, respectively. The high efficiencies are enabled by deploying effective optical management and by using photoactive materials with complementary absorption.

The Eley–Rideal reaction scheme in which H abstraction from the a-Si:H surface takes place by SiH3 radicals directly from the gas phase is discussed as the predominant surface dangling bond creating mechanism during a-Si:H growth. As a rate-limiting step, this reaction can account for the experimentally observed temperature-independent SiH3 surface reactivity and the surface silicon–hydride composition –SiHx that changes with substrate temperature. The consequences of H abstraction via the Eley–Rideal mechanism in terms of the surface dangling bond coverage and surface diffusion processes are discussed.

The sub gap absorption at 1.17 eV in hydrogenated amorphous silicon (a-Si:H) has been measured by means of the ex situ cavity ring down (CRD) absorption technique. The evolution of defects has been studied as a function of film thickness and deposition temperatures. A comparison with the dual beam photoconductivity (DBP) technique shows that the CRD results are systematically higher. Furthermore, it is shown that the CRD technique is sensitive to surface defects and typical surface dangling bond coverages of 2×10−4 up to 6×10−3 are obtained for air-exposed a-Si:H samples depending on the deposition temperature.

Time-of-flight (TOF) photocurrent investigations of hydrogenated amorphous silicon (a-Si:H) layers produced by means of an expanding thermal plasma (ETP) reveal a hole drift mobility that is, for depositions near 450 ° C , consistenly one order of magnitude higher than the corresponding mobility in standard PECVD samples. Electron drift mobilities and μτ products do not show such differences. The electronic density of states contains a prominent band of deep traps and the materials show evidence for non-amorphous inclusions.

The kinetic growth model for hydrogenated amorphous silicon (a-Si:H) from SiH 3 radicals in SiH 4 plasmas is reviewed on the basis of recently obtained experimental and computational data. New surface reactions are considered and their implications for the a-Si:H film growth mechanism are discussed. Furthermore, from the experimentally observed substrate temperature-dependence of the bulk hydrogen content and the composition of the a-Si:H surface hydrides, it is concluded that surface processes play an important role in hydrogen elimination from the film during growth.

Time-of-flight measurements on hydrogenated amorphous silicon deposited with a remote expanding thermal plasma at growth rates up to 12 nm/s have revealed a 7 to 10 times larger hole mobility than for films deposited with conventional rf-PECVD. The electron mobility on the other hand is up to 3 times less. Based on a determination of the density of states by post-transit photo-current analysis we suggest a comparable defect density at mid-gap as for films deposited with rf-PECVD. These material properties have been obtained at a substrate temperature of 400°C, which is needed to obtain solar grade material at these growth rates. Possible causes of these particular material properties, which may have application in thin film solar cells, are discussed. Furthermore we show that the high substrate temperature is still a drawback in solar cell preparation when using the standard p-i-n configuration.

An in situ single wavelength HeNe rotating ellipsometry study of high rate (~80 AA s/sup -1/) hydrogenated amorphous silicon (a-Si:H) deposition using an expanding thermal plasma is presented. An optical growth model is used to simulate the measured ellipsometric trajectories similar to models used for low rate a-Si:H growth in the literature. The in situ growth at high growth rates was studied as function of the substrate temperature. The refractive index n (at 632.8 nm) increases with increasing temperature corresponding to an increase in the density of the films. The in situ extinction coefficient k (at 632.8 nm) increases with increasing substrate temperature due to a smaller optical band gap and due to an increase in indirect absorption. It is shown that the ellipsometry setup in combination with optical modeling enables us to monitor the surface roughness evolution during deposition and to obtain the dynamic scaling exponent beta for postinitial growth

In this contribution, the results of ellipsometry measurements on the growth of hydrogenated amorphous silicon (a-Si:H) films using an expanding thermal plasma in Ar/H2/SiH4 are presented. The measurements are simulated with an optical model in which roughness and optical parameters of the bulk are time dependent during deposition. From this simulation it can be concluded that in the first 2–13 s of the deposition, depending on the substrate temperature, a top layer containing voids is created. At substrate temperatures above 400 °C and at a silane flow of 10 secs, corresponding to conditions at which high quality a-Si:H is grown, the smallest roughness of 3 nm is measured.

Mass spectrometric measurements in combination with Langmuir probe measurements reveal that the plasma chemistry of an expanding Ar/C2H2 is dominated by argon-ion-induced dissociation of the precursor gas. Under high arc current conditions complete depletion of acetylene is observed, indicating an efficient consumption of the injected acetylene. A clear correlation between the ion fluence emanating from the arc determined from modeling the mass spectrometry results and Langmuir probe measurements is observed. First measurements by means of cavity ring down and optical emission spectroscopy of the products of the dissociation of acetylene indicate that the dominant dissociation channel is C2H and H.

The scaling behavior of the surface morphology of hydrogenated amorphous silicon deposited from a SiH3 dominated plasma has been studied using atomic force microscopy and in situ ellipsometry. The observed substrate temperature dependence of growth exponent β reflects a crossover behavior from random deposition at 100 °C to a surface diffusion controlled smoothening around 250 °C to full surface relaxation around 500 °C. This crossover behavior has been reproduced by Monte Carlo simulations assuming a site dependent surface diffusion process, revealing an activation energy of ∼1.0 eV for the ruling surface smoothening mechanism. The implications for a-Si:H growth are discussed.

The hydride configurations in the hydrogenated amorphous silicon (a-Si:H) network have been studied by means of infrared absorption spectroscopy. The results on the film mass density of a-Si:H deposited by means of an expanding thermal plasma reveal the presence of two distinct regions in terms of hydrogen content and microstructure: below approximately 14 at. % H a-Si:H contains predominantly divacancies decorated by hydrogen, above 14 at. % H a-Si:H contains microscopic voids. These two distinct regions provide additional information on the origin of the low and high hydride stretching modes at 1980–2010 and 2070–2100 cm–1, respectively.

It is demonstrated that the signature of bulk hydrogen stretching modes in the infrared of microcrystalline silicon (µc-Si:H) deposited at high deposition rates can be used for solar cell optimization in the high pressure depletion regime. A relation between the performance of a p-i-n solar cell and the hydride stretching modes corresponding to hydrogenated crystalline grain boundaries is observed. These crystalline surfaces show postdeposition oxidation and the absence of these surfaces in the µc-Si:H matrix reflects device grade microcrystalline material.

The ion-bombardment induced surface and bulk processes during hydrogenated amorphous silicon (a-Si:H) deposition have been studied by employing an external rf substrate bias (ERFSB) in a remote Ar-H2-SiH4 expanding thermal plasma (ETP). The comparison of the ETP chem. vapor deposition without and with ERFSB enables us to identify some important ion-surface and ion-bulk interactions responsible for film property modifications. Employing ERFSB creates an addnl. growth flux and the low energetic ions deliver an extra 5-10 eV per Si atom deposited at typical deposition rates of 10-42 A/s which is a sufficient ion dose to modify the film growth. It is demonstrated that the extra surface and bulk process during a-Si:H growth, induced by the addnl. ion bombardment, provide an extra degree of freedom to manipulate the a-Si:H microstructure. An ion-film interaction diagram is introduced, which is used to discriminate ion-surface interactions from ion-bulk interactions. According to this ion-film interaction diagram, the a-Si:H grown with ERFSB can be roughly classified in three phases. In phase I the only ion-surface process activated is Si surface atom displacement. In phase II also ion-induced Si bulk atom displacement is sufficiently activated, whereas in phase III ion-induced Si atom sputtering is significant. Phase I is characterized by a redn. in the nanosized void d., a redn. in defect d., and an improvement of the photoresponse. We find that the Si surface displacement is the process responsible for various improvements of the material properties via the enhanced surface migration. Phase II is characterized by an enhancement of vacancy incorporation. In accordance with the introduced ion-film interaction diagram, the Si atom bulk displacement process is responsible for the incorporation of addnl. vacancies. Phase III is characterized by the decrease in growth flux and the increase in void d. The significant contribution of ion-sputtering processes is responsible for the effects obsd. in phase III. [on SciFinder (R)]

A set of 8 rf deposited a-Si:H thin films of various thickness (4-1031nm) have been used to explore the applicability of two optical techniques, thin film cavity ringdown spectroscopy (tfCRDS) and second harmonic generation (SHG), for the measurement of small defect-related absorptions. In this paper we will give a first overview of the different aspects of these techniques, which are novel in the field of amorphous silicon materials. It is shown that tf-CRDS is capable of measuring defect-related absorptions (associated with dangling bonds) as small as 10-7 for a single measurement, without the need for elaborate calibration procedures. The results are compared with photothermal deflection spectroscopy (PDS) for a broad spectral range (0.7 – 1.7 eV) and show good agreement. Furthermore the existence of a defect-rich surface layer with a defect density of 1.1×1012 cm-2 has been proven. The absorption spectrum of a 4 nm thin film has revealed a different spectral signature than a bulk dominated (1031 nm) film. The SHG experiments on a-Si:H films have shown that the second harmonic signal arises from the surface states and polarization dependent studies have revealed that the surface states probed have an ∞m-symmetry. From this it can be deduced that the absorbing surface states are isotropically distributed. A spectral scan suggests that the second harmonic signal, whose origin has not been unrevealed yet, has a resonance at an incident photon energy of 1.22 eV.

The incorporation of hydrogen in vacancies and at void surfaces during hydrogenated amorphous silicon growth from a remote expanding thermal plasma (ETP) is systematically studied by variation of the mass growth flux Γa-Si:H and substrate temperatureTsub. An evident relation between the void incorporation and the growth parameters Γa-Si:H andTsubhas been observed. We speculate on a possible relation with the surface diffusion processes during deposition. An activation energy for surface diffusion during a-Si:H growth of 0.8-1.1 eV is obtained using this assertion, similar to the activation energy deduced from surface roughness evolution studies. For compact films hydrogen is predominantly present at vacancies, and a possible relation with the hydrogen removal mechanism during deposition is discussed.

An interpretation of post-transit photocurrents in a time-of-flight experiment in terms of the underlying density of localized gap states in the sample is presented for the case of hydrogenated amorphous silicon cells prepared by the expanding thermal plasma technique. It is pointed out that part of the observed current is not generated by re-emission of trapped photo-generated charge and should, therefore, not be used for density-of-states calculations.

The properties of hydrogenated amorphous silicon (a-Si:H) deposited at very high growth rates (6–80 nm/s) by means of a remote Ar–H2–SiH4 plasma have been investigated as a function of the H2 flow in the Ar–H2 operated plasma source. Both the structural and optoelectronic properties of the films improve with increasing H2 flow, and a-Si:H suitable for the application in solar cells has been obtained at deposition rates of 10 nm/s for high H2 flows and a substrate temperature of 400 °C. The "optimized" material has a hole drift mobility which is about a factor of 10 higher than for standard a-Si:H. The electron drift mobility, however, is slightly lower than for standard a-Si:H. Furthermore, preliminary results on solar cells with intrinsic a-Si:H deposited at 7 nm/s are presented. Relating the film properties to the SiH4 dissociation reactions reveals that optimum film quality is obtained for conditions where H from the plasma source governs SiH4 dissociation and where SiH3 contributes dominantly to film growth. Conditions where ion-induced dissociation reactions of SiH4 prevail and where the contribution of SiH3 to film growth is much smaller lead to inferior film properties. A large contribution of very reactive (poly)silane radicals is suggested as the reason for this inferior film quality. Furthermore, a comparison with film properties and process conditions of other a-Si:H deposition techniques is presented.

Hydrogenated microcryst. silicon films (micro c-Si:H) deposited at high deposition rates (.apprx.2 nm/s) by means of the very-high-frequency deposition technique in the high pressure depletion regime have been integrated into single junction p-i-n solar cells. It is demonstrated that micro c-Si:H solar cells can be optimized using a twofold approach. First the bulk properties, deposited under steady-state plasma conditions, are optimized by monitoring the presence of cryst. grain boundaries in micro c-Si:H. These hydrogenated cryst. grain boundaries can easily be detected via the cryst. surface hydrides contribution to the narrow high stretching modes by IR transmission spectroscopy. The cryst. grain boundaries suffer from post-deposition oxidn. which results in a reduced red response of the solar cell. The absence of these cryst. surfaces in an as-deposited micro c-Si:H matrix reflects the device grade microcryst. bulk material. Second, the prevention of silane back-diffusion from the background during the initial growth is a necessity to deposit a uniform micro c-Si:H phase over the entire film thickness. The initial growth is optimized while preserving the optimized bulk properties deposited under steady-state conditions, using initial profiling of plasma parameters such as the silane flow and the very-high-frequency power d. Solar cell devices with efficiency of 8.0% at a micro c-Si:H deposition rate of 2.0 nm/s are obtained using the presented approach.

The incorporation of nanosized voids during hydrogenated amorphous silicon film growth is studied by measurements of the film mass density and the hydrogen present at the void surfaces. The observed dependence of the density of nanosized voids on the growth flux and substrate temperature is explained in terms of a surface-diffusion-controlled void incorporation model. From this analysis, an activation energy for surface diffusion in the range of 0.77–1.05 eV has been found, a value which is in agreement with the activation energy obtained from the analysis of the surface roughness evolution during growth in a previous study.