Jimmy Melskens

The role of oxide interlayers in back reflector configurations for amorphous silicon solar cells

Thin oxide interlayers are commonly added to the back reflector of thin-film silicon solar cells to increase their current. To gain more insight in the enhancement mechanism, we tested different back reflector designs consisting of aluminium-doped zinc oxide (ZnO:Al) and/or hydrogenated silicon oxide (SiOx:H) interlayers with different metals (silver, aluminium, and chromium) in standard p-i-n a-Si:H solar cells. We use a unique inverse modeling approach to show that in most back reflectors the internal metal reflectance is lower than expected theoretically. However, the metal reflectance is increased by the addition of an oxide interlayer. Our experiments demonstrate that SiOx:H forms an interesting alternative interlayer because unlike the more commonly used ZnO:Al it can be deposited by plasma-enhanced chemical vapour deposition and it does not reduce the fill factor. The largest efficiency enhancement is obtained with a double interlayer of SiOx:H and ZnO:Al.

New Insights in the Nanostructure and Defect States of Hydrogenated Amorphous Silicon Obtained by Annealing

Temperature annealing is used as a tool to study the validity of network models for the nanostructure of hydrogenated amorphous silicon (a-Si:H) and its relation to defect states. The changes in the size of the dominant open volume deficiencies have been studied using Doppler broadening positron annihilation spectroscopy and Fourier transform infrared spectroscopy. It is shown that the dominant open volume deficiencies for as-deposited films are divacancies, which appear to agglomerate into larger open volume deficiencies up to 400 °C. Above this temperature, the largest open volume deficiencies are suggested to be released at the surface of the sample. Fourier transform photocurrent spectroscopy results indicate a dramatic increase in the density of various subgap defect state distributions during temperature annealing. In addition, at least four defect states have been identified. These findings cannot be directly explained by assuming solely dangling bonds as the dominant defects in a-Si:H. We discuss that a model based on an anisotropic disordered network with volume deficiencies does explain our findings better than the classical model based on a continuous random network with solely an isotropic distribution of coordination defects. The claim is made that next to dangling bonds not fully hydrogen-passivated vacancies are significantly contributing to the dominant defect states in a-Si:H.

The Nature and the Kinetics of Light-Induced Defect Creation in Hydrogenated Amorphous Silicon Films and Solar Cells

The nature and the kinetics of light-induced defect creation in hydrogenated amorphous silicon (a-Si:H) films and solar cells are investigated by means of Doppler broadening positron annihilation spectroscopy, Fourier transform photocurrent spectroscopy, and J-V characterization. There is a strong correlation between the open volume deficiencies in a-Si:H and the Staebler-Wronski effect (SWE). The carrier generation and recombination profiles in the absorber layer are spatially correlated, and the recombination due to defects in the top and bottom parts of the absorber layer is different. Furthermore, the various defect distributions in the bandgap have different defect creation kinetics. It is demonstrated that the SWE defect formation kinetics in a solar cell are very complex and can impossibly be described by one time scaling ~ tβ as is often claimed.

High pressure processing of hydrogenated amorphous silicon solar cells: Relation between nanostructure and high open-circuit voltage

This study gives a guideline on developing high bandgap, high quality hydrogenated amorphous silicon (a-Si:H) through a carefully engineered nanostructure. Single-junction a-Si:H solar cells with open-circuit voltages (Voc) above 950 mV and conversion efficiencies above 9% are realized by processing the absorber layers at high pressures of 7–10 mbar. The high Voc is a result of an increased bandgap, which is attributed to an increase in the average size of the open volume deficiencies in the absorber layer without a significant increase in the nanosized void density.

Passivating Contacts for Crystalline Silicon Solar Cells: From Concepts and Materials to Prospects

To further increase the conversion efficiency of crystalline silicon (c-Si) solar cells, it is vital to reduce the recombination losses associated with the contacts. Therefore, a contact structure that simultaneously passivates the c-Si surface while selectively extracting only one type of charge carrier (i.e., either electrons or holes) is desired. Realizing such passivating contacts in c-Si solar cells has become an important research objective, and an overview and classification of work to date on this topic is presented here. Using this overview, we discuss the design guidelines for passivating contacts and outline their prospects.

On the solid phase crystallization of In2O3:H transparent conductive oxide films prepared by atomic layer deposition

Hydrogen-doped indium oxide (In2O3:H) has emerged as a highly transparent and conductive oxide, finding its application in a multitude of optoelectronic devices. Recently, we have reported on an atomic layer deposition (ALD) process to prepare high quality In2O3:H. This process consists of ALD of In2O3:H films at 100 °C, followed by a solid phase crystallization step at 150–200 °C. In this work, we report on a detailed electron microscopy study of this crystallization process which reveals new insights into the crucial aspects for achieving the large grain size and associated excellent properties of the material. The key finding is that the best optoelectronic properties are obtained by preparing the films at the lowest possible temperature prior to post-deposition annealing. Electron microscopy imaging shows that such films are mostly amorphous, but feature a very low density of embedded crystallites. Upon post-deposition annealing, crystallization proceeds merely from isotropic crystal grain growth of t...

Correlating the silicon surface passivation to the nanostructure of low-temperature a-Si:H after rapid thermal annealing

Using an inductively coupled plasma, hydrogenated amorphous silicon (a-Si:H) films have been prepared at very low temperatures (<50 °C) to provide crystalline silicon (c-Si) surface passivation. Despite the limited nanostructural quality of the a-Si:H bulk, a surprisingly high minority carrier lifetime of ∼4 ms is demonstrated after a rapid thermal annealing treatment. Besides the excellent level of surface passivation, the main advantage of the low-temperature approach is the facile suppression of undesired epitaxial growth. The correlation between the a-Si:H nanostructure and the activation of a-Si:H/c-Si interface passivation, upon annealing, has been studied in detail. This yields a structural model that qualitatively describes the different processes that take place in the a-Si:H films during annealing. The presented experimental findings and insights can prove to be useful in the further development of very thin a-Si:H passivation layers for use in silicon heterojunction solar cells.

Migration of Open Volume Deficiencies in Hydrogenated Amorphous Silicon During Annealing

The nanostructure of hydrogenated amorphous silicon (a-Si:H) is studied by means of doppler broadening positron annihilation spectroscopy (DB-PAS) and Fourier transform infrared (FTIR) spectroscopy. The evolution of open volume deficiencies is monitored during annealing, demonstrating that small vacancies and other small vacancy clusters that are initially present in the a-Si:H nanostructure agglomerate into larger vacancy clusters. The migration of open volume deficiencies is less pronounced for a-Si:H deposited at higher hydrogen-to-silane gas flow rate ratio, R. FTIR spectroscopy reveals the presence of a peculiar peak in the refractive index in the infrared—and hence the calculated mass density—which occurs just before H effusion from the films starts. The combined results of DB-PAS and FTIR spectroscopy indicate that a stress buildup caused by the accumulation of H2 in agglomerating vacancies during annealing can explain the sudden mass density increase. At higher temperatures, stress is released with the onset of H effusion. The H effusion consists of a two-stage process involving small open volume deficiencies and nanosized voids, contrasting earlier interpretations. The reduced amount of hydrogen migration and enhanced hydrogen passivation degree are suggested as key factors to the reduced light-induced degradation associated with increased R values.

Concepts and prospects of passivating contacts for crystalline silicon solar cells

To further increase the conversion efficiency of crystalline silicon solar cells it is vital to reduce the recombination losses between the photoactive part of the solar cell and the metal contacts. This is ideally achieved by fabricating contacts which passivate defects at the silicon surface while being simultaneously selective for only a single type of charge carrier, i.e. either electrons or holes. Despite the extensive research effort aimed at realizing such contacts, no clear overview of the fundamental physics of passivating contacts has appeared yet. Therefore, we present such an overview, introduce a clear classification of passivating contacts, and discuss their design guidelines and future prospects.

Comparison of thermal and plasma-enhanced atomic layer deposition of niobium oxide thin films

Niobium pentoxide was deposited using tBuN=Nb(NEt2)3 as niobium precursor by both thermal atomic layer deposition (ALD) and plasma-enhanced atomic layer deposition (PE-ALD) with H2O and O2 plasma as coreactants, respectively. The deposition temperature was varied between 150 and 350 °C in both ALD processes. Amorphous films were obtained in all cases. Self-limiting saturated growth was confirmed for both ALD processes along with high uniformity over a 200 mm Si wafer. The PE-ALD process enabled a higher growth per cycle (GPC) than the thermal ALD process (0.56 A vs 0.38 A at 200 °C, respectively), while the GPC decreases with increasing temperature in both cases. The high purity of the film was confirmed using Rutherford backscattering spectrometry, elastic recoil detection, and x-ray photoelectron spectroscopy, while the latter technique also confirmed the Nb+5 oxidation state of the niobium oxide films. The thermal ALD deposited films were substoichiometric due to the presence of oxygen vacancies (VO), of which a more dominant presence was observed with increasing deposition temperature. The PE-ALD deposited films were found to be near stoichiometric for all investigated deposition temperatures.Niobium pentoxide was deposited using tBuN=Nb(NEt2)3 as niobium precursor by both thermal atomic layer deposition (ALD) and plasma-enhanced atomic layer deposition (PE-ALD) with H2O and O2 plasma as coreactants, respectively. The deposition temperature was varied between 150 and 350 °C in both ALD processes. Amorphous films were obtained in all cases. Self-limiting saturated growth was confirmed for both ALD processes along with high uniformity over a 200 mm Si wafer. The PE-ALD process enabled a higher growth per cycle (GPC) than the thermal ALD process (0.56 A vs 0.38 A at 200 °C, respectively), while the GPC decreases with increasing temperature in both cases. The high purity of the film was confirmed using Rutherford backscattering spectrometry, elastic recoil detection, and x-ray photoelectron spectroscopy, while the latter technique also confirmed the Nb+5 oxidation state of the niobium oxide films. The thermal ALD deposited films were substoichiometric due to the presence of oxygen vacancies (VO), ...