Bart Macco

Al2O3/TiO2 nano-pattern antireflection coating with ultralow surface recombination

We present a nano-patterned dielectric coating for crystalline Si solar cells that combines excellent anti-reflection and passivation properties. The nano-patterned coating comprises an array of TiO2 nanocylinders placed on top of an ultra-thin Al2O3 layer on a flat Si(100) wafer. The antireflection effect stems from the preferential forward scattering of light through leaky Mie resonances in the TiO2 nanocylinders. The Al2O3 layer provides excellent passivation of the Si surface. We experimentally demonstrate ultralow surface recombination with carrier lifetimes above 4 ms, combined with a reflectivity of 2.8% averaged over a broad spectral range.

Electron Scattering and Doping Mechanisms in Solid-Phase-Crystallized In2O3:H Prepared by Atomic Layer Deposition

Hydrogen-doped indium oxide (In2O3:H) has recently emerged as an enabling transparent conductive oxide for solar cells, in particular for silicon heterojunction solar cells because its high electron mobility (>100 cm(2)/(V s)) allows for a simultaneously high electrical conductivity and optical transparency. Here, we report on high-quality In2O3:H prepared by a low-temperature atomic layer deposition (ALD) process and present insights into the doping mechanism and the electron scattering processes that limit the carrier mobility in such films. The process consists of ALD of amorphous In2O3:H at 100 °C and subsequent solid-phase crystallization at 150-200 °C to obtain large-grained polycrystalline In2O3:H films. The changes in optoelectronic properties upon crystallization have been monitored both electrically by Hall measurements and optically by analysis of the Drude response. After crystallization, an excellent carrier mobility of 128 ± 4 cm(2)/(V s) can be obtained at a carrier density of 1.8 × 10(20) cm(-3), irrespective of the annealing temperature. Temperature-dependent Hall measurements have revealed that electron scattering is dominated by unavoidable phonon and ionized impurity scattering from singly charged H-donors. Extrinsic defect scattering related to material quality such as grain boundary and neutral impurity scattering was found to be negligible in crystallized films indicating that the carrier mobility is maximized. Furthermore, by comparison of the absolute H-concentration and the carrier density in crystallized films, it is deduced that <4% of the incorporated H is an active dopant in crystallized films. Therefore, it can be concluded that inactive H atoms do not (significantly) contribute to defect scattering, which potentially explains why In2O3:H films are capable of achieving a much higher carrier mobility than conventional In2O3:Sn (ITO).

Atomic layer deposition of molybdenum oxide from (NtBu)2(NMe2)2Mo and O2 plasma

Molybdenum oxide (MoOx) films have been deposited by atomic layer deposition using bis(tert-butylimido)-bis(dimethylamido)molybdenum and oxygen plasma, within a temperature range of 50–350 °C. Amorphous film growth was observed between 50 and 200 °C at a growth per cycle (GPC) around 0.80 A. For deposition temperatures of 250 °C and higher, a transition to polycrystalline growth was observed, accompanied by an increase in GPC up to 1.88 A. For all deposition temperatures the O/Mo ratio was found to be just below three, indicating the films were slightly substoichiometric with respect to MoO3 and contained oxygen vacancies. The high purity of the films was demonstrated in the absence of detectable C and N contamination in Rutherford backscattering measurements, and a H content varying between 3 and 11 at. % measured with elastic recoil detection. In addition to the chemical composition, the optical properties are reported as well.

Influence of transparent conductive oxides on passivation of a-Si:H/c-Si heterojunctions as studied by atomic layer deposited Al-doped ZnO

In silicon heterojunction solar cells, the main opportunities for efficiency gain lie in improvements of the front-contact layers. Therefore, the effect of transparent conductive oxides (TCOs) on the a-Si:H passivation performance has been investigated for Al-doped zinc oxide (ZnO:Al) layers made by atomic layer deposition (ALD). It is shown that the ALD process, as opposed to sputtering, does not impair the chemical passivation. However, the field-effect passivation is reduced by the ZnO:Al. The resulting decrease in low injection-level lifetime can be tuned by changing the ZnO:Al doping level (carrier density = 7 × 1019–7 × 1020 cm−3), which is explained by a change in the TCO workfunction. Additionally, it is shown that a ~10–15 nm ALD ZnO:Al layer is sufficient to mitigate damage to the a-Si:H by subsequent sputtering, which is correlated to ALD film closure at this thickness.

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.

Metal–Insulator–Semiconductor Nanowire Network Solar Cells

Metal-insulator-semiconductor (MIS) junctions provide the charge separating properties of Schottky junctions while circumventing the direct and detrimental contact of the metal with the semiconductor. A passivating and tunnel dielectric is used as a separation layer to reduce carrier recombination and remove Fermi level pinning. When applied to solar cells, these junctions result in two main advantages over traditional p-n-junction solar cells: a highly simplified fabrication process and excellent passivation properties and hence high open-circuit voltages. However, one major drawback of metal-insulator-semiconductor solar cells is that a continuous metal layer is needed to form a junction at the surface of the silicon, which decreases the optical transmittance and hence short-circuit current density. The decrease of transmittance with increasing metal coverage, however, can be overcome by nanoscale structures. Nanowire networks exhibit precisely the properties that are required for MIS solar cells: closely spaced and conductive metal wires to induce an inversion layer for homogeneous charge carrier extraction and simultaneously a high optical transparency. We experimentally demonstrate the nanowire MIS concept by using it to make silicon solar cells with a measured energy conversion efficiency of 7% (∼11% after correction), an effective open-circuit voltage (Voc) of 560 mV and estimated short-circuit current density (Jsc) of 33 mA/cm(2). Furthermore, we show that the metal nanowire network can serve additionally as an etch mask to pattern inverted nanopyramids, decreasing the reflectivity substantially from 36% to ∼4%. Our extensive analysis points out a path toward nanowire based MIS solar cells that exhibit both high Voc and Jsc values.

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.