Incorporation of properly designed nanostructures in solar cells improves light trapping and consequently their power conversion efficiencies. Due to its unique structure, a silicon nanowire (SiNW) matrix provides excellent light trapping and thus offers a promising approach for cost-effective, stable and efficient silicon thin film photovoltaics. Moreover, by decoupling the light absorption and carrier collection directions, radial junction solar cells built around the SiNWs allow the use of very thin active layers. As a matter of fact, radial PIN junctions with 9.2% power conversion efficiency have already been demonstrated on glass substrates with only 100 nm thick intrinsic hydrogenated amorphous silicon layers. The most straightforward way to further improve the short circuit current density is to use an active layer with a lower band gap. In this work, the performances of devices with two different low band gap materials, e.g., hydrogenated microcrystalline silicon (μc-Si:H) and hydrogenated amorphous silicon germanium alloy (a-SiGe:H) are presented. To the best of our knowledge, this is the first demonstration of a-SiGe:H radial junction solar cell.

We propose an implementation of the PCD technique to minority carrier effective lifetime assessment in crystalline silicon at 77K. We focus here on (n)-type, FZ, polished wafers passivated by a-Si:H deposited by PECVD at 200°C. The samples were immersed into liquid N2 contained in a beaker placed on a Sinton lifetime tester. Prior to be converted into lifetimes, data were corrected for the height shift induced by the beaker. One issue lied in obtaining the sum of carrier mobilities at 77K. From dark conductance measurements performed on the lifetime tester, we extracted an electron mobility of 1.1x104 cm².V-1.s-1 at 77K, the doping density being independently calculated in order to account for the freezing effect of dopants. This way, we could obtain lifetime curves with respect to the carrier density. Effective lifetimes obtained at 77K proved to be significantly lower than at RT and not to depend upon the doping of the a-Si:H layers. We were also able to experimentally verify the expected rise in the implied Voc, which, on symmetrically passivated wafers, went up from 0.72V at RT to 1.04V at 77K under 1 sun equivalent illumination.

We summarize our research studies on the synthesis of silicon and germanium nanocrystals and their application to the growth of a variety of thin films, spanning the range from fully disordered amorphous up to fully ordered crystalline. All these films are deposited in a standard radio-frequency glow discharge system at low temperature (~200 °C). We show how the plasma synthesis of silicon nanocrystals, initially a side effect of powder formation, has become over the years an exciting field of research which has opened the way to new opportunities in the field of materials deposition and their application to optoelectronic devices. Our results suggest that epitaxy requires the melting/amorphization of the nanocrystals upon impact on the substrate, the subsequent epitaxial growth being favored on (100) c-Si substrates. As a consequence, the control of the impact energy is a critical aspect of the growth which will require new strategies such as the use of tailored voltage waveforms.

We present a novel ZnO:Al fabrication process consisting of room-temperature vacuum sputtering followed by an excimer laser annealing (ELA). The ELA treatment improves the optical transmission of the films, and the film resistivities (<1 mΩ·cm) remain stable or improve with increasing laser fluence up to 0.6 J/cm2, as the carrier density increases but the carrier mobility is degraded. This process is followed by a standard dilute HCl chemical texturing step, and produces substrates with suitable texture, conductivity, and transparency properties for thinfilm photovoltaic applications. Substrates resulting from this process display elevated haze levels (80% at 600 nm and 50% at 800 nm) after the wet-chemical etching step. Such substrates have been used to make single junction hydrogenated nanocrystalline silicon solar cells, and an increase in the short-circuit current of up to 2.2 mA/cm2 is observed compared to a substrate deposited by a standard room-temperature sputtering + wet-etch process. This gain is primarily due to increased photo-response in the red due to improved light-scattering, as at wavelengths greater than 600 nm, a gain in photocurrent of up to 1.7 mA/cm2 is observed.

Raman spectra of a variety of polymorphous (pm-Si:H) and amorphous silicon (a-Si:H) samples deposited by plasma enhanced chemical vapor deposition (PECVD) at different pressures were recorded in the range of 150 cm-1 to 750 cm-1 using a 514 nm excitation source. A comparison of Raman spectra between a-Si:H and pm-Si:H samples reveals significant differences. The Transverse Optical (TO) peak in case of pm-Si:H is asymmetric and shifted to higher wave numbers. In the literature, discrepancies between predictions of various quantum confinement models and experimental spectra have typically been attributed to either strain or negligible fraction of nanocrystallites. We show that a quantum confinement model along with a Gaussian size distribution is able to accurately predict particle size of nanocrystallites embedded in the amorphous matrix. The crystallite size and size distribution obtained by fitting the TO peak is consistent with high-resolution transmission electron microscopy observations. In our case typical mean crystallite size obtained is about 3 nm with FWHM of the distribution varying in the range 0.2–1 nm. A comparison of ratio of heights in TA and TO peaks of a-Si:H and pm-Si:H material indicates that pm-Si:H as a material has higher medium range order (MRO). This ratio has been used to compare the degree of MRO in pm-Si:H samples prepared under different conditions. Thus, we demonstrate that Raman Spectroscopy along with our model can be used to obtain the crystallite size distribution and provide a measure of degree of medium range order.

Hybrid concepts based on a nanoscale combination of organic and inorganic semiconductors are a promising way to enhance the cost efficiency of solar cells through a better use of the solar spectrum, a higher aspect ratio of the interface, and the good processability of polymers.A new type of solar cells has been investigated. It is based on a heterojunction between regio-regular poly(3-hexylthiophene) as an organic electron donor and silicon as an inorganic electron acceptor. In a first step towards nano-structured devices, cells made of flat thin films of these materials have been studied as a model case of the heterojunction.The materials were characterized through ellipsometry and absorption spectroscopy. The devices were studied by means of their spectral response and their I-V characteristics. By combining these results, the contribution of each layer and the mechanisms of photocurrent generation are explained.The best cells to-date show a power conversion efficiency of 1.6% under AM 1.5 illumination, with a V oc of 0.704 V and a J sc of 4.22 mA/cm2.

Hydrogen diffusion is monitored during plasma conditions corresponding to the growth of polymorphous and microcrystalline silicon thin films in the temperature range from 50 to 300 °C. Structural changes of the films were monitored in situ by Spectroscopic Ellipsometry measurements and interpreted with the Bruggemann Effective Medium Approximation along with the Tetrahedron Model. For both materials, diffusion of hydrogen leading to the formation of a hydrogen-rich subsurface layer during the first minute of exposure to the hydrogen plasma is observed. This initial phase was followed by a steady-sate regime during which the thickness of the subsurface layer stayed relatively constant and the total film thickness decreased as a function of time. This steady-state is explained as the result of the equilibrium between hydrogen diffusion and etching. The hydrogen diffusion coefficient and etching rate were calculated from these measurements and found to be higher in amorphous than in polymorphous silicon. This result is consistent with the growth dynamics of polymorphous silicon. It also provides information concerning the nucleation of micro-crystallites in amorphous silicon under very large atomic hydrogen flow conditions. Moreover, it also explains why polymorphous silicon films do not undergo the phase transition to microcrystalline, even if deposited under relatively similar large atomic hydrogen flow conditions.

Polymorphous Si is a nanostructured form of hydrogenated amorphous Si that contains a small fraction of Si nanocrystals or clusters. Its thermally induced transformations such as relaxation, dehydrogenation, and crystallization have been studied by calorimetry and evolved gas analysis as a complementary technique. The observed behavior has been compared to that of conventional hydrogenated amorphous Si and amorphous Si nanoparticles. In the temperature range of our experiments (650–700 °C), crystallization takes place at almost the same temperature in polymorphous and in amorphous Si. In contrast, dehydrogenation processes reflect the presence of different hydrogen states. The calorimetry and evolved gas analysis thermograms clearly show that polymorphous Si shares hydrogen states of both amorphous Si and Si nanoparticles. Finally, the total energy of the main Si–H group present in polymorphous Si has been quantified.

Thermal crystallization experiments carried out using calorimetry on several a-Si:H materials with different microstructures are reported. The samples were crystallized during heating ramps at constant heating rates up to 100 K/min. Under these conditions, crystallization takes place above 700 °C and progressively deviates from the standard kinetics. In particular, two crystallization processes were detected in conventional a-Si:H, which reveal an enhancement of the crystallization rate. At 100 K/min, such enhancement is consistent with a diminution of the crystallization time by a factor of 7. In contrast, no systematic variation of the resulting grain size was observed. Similar behavior was also detected in polymorphous silicon and silicon nanoparticles, thus showing that it is characteristic of a variety of hydrogenated amorphous silicon materials.

We have studied the evolution of the structure of boron-doped hydrogenated amorphous silicon films exposed to a hydrogen plasma. From the early stages of exposure, hydrogen diffuses and forms a thick H-rich subsurface. At longer times, hydrogen plasma leads to the formation of a microcrystalline layer via chemical transport without crystallization of the initial layer. We observe that the hydrogen content increases in the films during a plasma exposure and once the microcrystalline layer is formed hydrogen diffuses out of the sample accompanied with a decrease in the boron content. This effect can be attributed to the electric field developed within the heterojunction a-Si:H/μc-Si:H that drives the positively charged hydrogen atoms in the boron-doped layer towards the μc-Si:H layer.

Hydrogenated polymorphous silicon (pm-Si:H) has steadily emerged as a potential replacement of hydrogenated amorphous silicon. Possible changes in the density of gap states due to the presence of crystallites is of central importance in understanding steady state and dynamic characteristics of devices using these materials. We have studied a-Si:H and pm-Si:H grown by PECVD at optimized conditions through the measurement of the steady state reverse current and their transients in PIN devices. The transients are analyzed using isothermal spectroscopic techniques such as Time Analyzed Transient Spectroscopy (TATS), and high resolution Laplace DLTS as a function of temperature. In case of a-Si:H, we obtain the expected signature of emission from a broad density of states in the form of stretched exponentials. In contrast the corresponding spectra for pm-Si:H are dominated by nearly exponential fast current decay processes with discrete energies between 0.20 and 0.26 eV. It is shown that the study of the density of states by dynamic methods such as transient techniques reveal features not accessible to steady state measurements.

We present a detailed study of the structure and hydrogen bonding in silicon thin films ranging from amorphous to microcrystalline. We emphasize the results for hydrogenated polymorphous silicon films obtained under plasma conditions close to powder formation where silicon clusters and nanocrystals contribute to growth. Fourier Transform Infra-Red (FTIR) spectroscopy, Raman spectroscopy, X-Ray-Diffraction (XRD), and hydrogen evolution measurements are performed to characterize the hydrogen bonding and the structure of the films in their as-deposited state and after isochronal annealing at increasing temperature in the range of 300 to 600 °C. While Raman spectroscopy and XRD give an average information on the structure of the films, without clear evidence of the presence of crystallites in the polymorphous films, infrared spectroscopy and hydrogen evolution measurements which probe the local hydrogen related structure are shown to be perfectly adapted to characterize polymorphous silicon films. In particular, IR spectroscopy measurements, reveal the presence of a stretching band at 2030 cm−1, associated with a peak at 873 cm−1 in the bending region and a downward shift in the Si-H wagging mode from 640 cm−1 to 622 cm−1. We attribute the 2030 cm−1 mode to the presence of hydrogen bonded at the surface of the plasma produced silicon clusters and nanocrystals. This assignment is supported by hydrogen evolution measurements in which a sharp low-temperature hydrogen evolution peak appears at around 420 °C followed by up to five peaks at higher temperatures. This particular hydrogen bonding in polymorphous silicon films is also supported by isochronal annealing studies which show that the bands at 2030 cm−1 and 873 cm−1 vanish at annealing temperatures corresponding to the low temperature hydrogen evolution peak. Based on these results and their correlation with the hydrogen-related material structure, we propose a picture for the structure of polymorphous silicon films that accounts for the observed features.

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to powder formation, a new type of material, consisting of an amorphous matrix in which silicon nanocrystallites are embedded is obtained. This material, named hydrogenated polymorphous silicon (pm-Si:H), exhibits enhanced transport properties with respect to state-of-the-art hydrogenated amorphous silicon (a-Si:H). In order to understand the origin of such improved properties, we investigated structural characterization of pm-Si:H films. High resolution transmission electron microscopy (HRTEM) micrograph, micro-Raman and infrared spectra of the films are presented. The crystallite sizes deduced from the Raman spectra are supported by the HRTEM measurements. The infrared stretching modes of pm-Si:H films present a band at ~2030 cm−1 attributed to hydrogen platelets. Two approaches are then given in order to explain the enhanced photoconductivity properties. The first one, qualitative, appeals to low density of states at the Fermi level $N(E_{\rm F})$ and low capture cross-section of electrons $\sigma_{\rm c}$ due to the improved amorphous matrix. The second, more quantitative, suggests that recombination mainly occur at dangling bonds at the surface of the crystallites. Considering the dangling bond density at silicon surface passivated by hydrogen and the description of multiphonon carrier capture, both given in the literature, we derive $N(E_{\rm F})$ and $\sigma_{\rm c}$ values in good agreement with experiments.

Polymorphous Silicon (pm-Si:H) deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) has emerged as an alternative material to amorphous silicon (a-Si:H). Deposition parameters of pm-Si:H are such that small crystallites get embedded in a relaxed amorphous silicon matrix, thus improving the optical and electrical properties. We study the size of crystallites and degree of order in pm-Si:H using Raman and photoluminescence (PL) spectra of pm-Si:H and a-Si:H. Raman Spectra of a variety of hydrogenated nanostructured silicon (pmSi:H) and amorphous Silicon (a-Si:H) samples grown at different pressures were analyzed. Deconvolution of observed multiple peaks in photoluminescence spectra and fitting to Gaussian size distribution also yields particle size to be in the range of 2.3 to 3.5nm in agreement with Transmission Electron Microscopy and Raman results.

This work refers to a study performed on polymorphous silicon (pm-Si:H) at excitation frequencies of 13.56 and 27.12 MHz in a large area PECVD reactor. The plasma was characterised by impedance probe measurements, aiming to identify the plasma conditions that lead to produce pm-Si:H films. The films produced were characterised by spectroscopic ellipsometry, infrared and Raman spectroscopy and hydrogen exodiffusion experiments, which are techniques that permit the structural characterisation of the pm-Si films and to study the possible differences between the films deposited at 13.56 and 27.12 MHz. Conductivity measurements were also performed to determine the transport properties of the films produced. The set of data obtained show that the 27.12 MHz pm-Si:H can be grown at higher rates with less hydrogen dilution and power density, being the resulting films denser, chemically more stable and with improved performances than the pm-Si:H films grown at 13.56 MHz.

We compare the deposition rate, hydrogen incorporation and optoelectronic properties of hydrogenated polymorphous silicon films produced either by the decomposition of silane-hydrogen or of silane-helium mixtures. Our results clearly show that He dilution allows to drastically reduce the RF power needed to achieve the same deposition rate as in the case of H2 dilution. Infrared spectroscopy and hydrogen effusion experiments show clear differences in the hydrogen bonding and content in both series of films. Interestingly, both He and hydrogen dilution result in films with improved transport properties, in particular the hole diffusion length, with respect to standard amorphous silicon. These results indicate that He dilution is a good alternative to H2 dilution to prepare intrinsic layers for solar cells.

In this paper, we present results about the vibrational properties of hydrogenated amorphous silicon films. We expect to explain the slight differences observed in the Raman spectra using atomic-scale modeling. In particular, we focuse on the correlation of our results to the density of samples. This should give quantitative structural information which could be correlated to both macroscopic properties and elaboration conditions.

Transport properties of hydrogenated polymorphous silicon layers (pm-Si:H) deposited at 150°C under various pressures in the range 80-293 Pa in sandwich (Schottky and p-i-n diodes) and coplanar structures have been compared to those of hydrogenated amorphous silicon (a-Si:H) samples deposited at the same temperature in standard conditions. The layers have been studied as-deposited, annealed and after light-soaking. With increasing pressure up to 240 Pa: i) the density of states above the Fermi level decreases as determined by means of the modulated photocurrent technique, ii) the mobility-lifetime products of electrons and holes measured by means of steady-state photoconductivity and photocarrier grating techniques both increase. The highest values for the diffusion length of minority carriers exceed 200 nm. Capacitance measurements as a function of frequency and temperature show that the density of states at the Fermi level is lower in the pm-Si:H than in the a-Si:H films. After light-soaking the diffusion length of minority carriers in a-Si:H is reduced by a factor oftwo whereas it is less reduced or not affected in the pm-Si:H layers. Solar cells including this new material present an excellent stability.

Thin films of nanostructured silicon (ns-Si:H) were deposited by plasma-enhanced chemical vapor deposition in the presence of silicon nanoparticles at 100 °C substrate temperature using a silane and hydrogen gas mixture under continuous wave (cw) plasma conditions. The nanostructure of the films has been demonstrated by diverse ways: transmission electron microscopy, Raman spectroscopy, and x-ray diffraction, which have shown the presence of ordered silicon clusters (1–2 nm) embedded in an amorphous silicon matrix. Because of the presence of these ordered domains, the films crystallize faster than standard hydrogenated amorphous silicon samples, as evidenced by electrical measurements during the thermal annealing.

The growth of hydrogenated amorphous silicon films is often explained by the arrival of SiHx radicals on the substrate and the subsequent cross-linking reactions leading to an homogeneous material which can be described by a continuous random network. Here we summarize our recent Work on a new class of silicon thin films produced under plasma conditions where silicon clusters and radicals contribute to the deposition. The main aspects are: i) silicon clusters with sizes of the order of 1-5 nm are easily formed in silane plasmas; ii) these silicon clusters can contribute to the deposition and lead to the formation of films with medium-range order (“polymorphous silicon”); iii) despite their heterogeneity, the films have improved transport properties and stability with respect to a-Si:H. The excellent transport properties are confirmed by the achievement of stable single junction p-i-n solar cells with efficiencies close to 10 %.