René A. C. M. M. van Swaaij

Effect of substrate morphology slope distributions on light scattering, nc-Si:H film growth, and solar cell performance.

Thin-film silicon solar cells are often deposited on textured ZnO substrates. The solar-cell performance is strongly correlated to the substrate morphology, as this morphology determines light scattering, defective-region formation, and crystalline growth of hydrogenated nanocrystalline silicon (nc-Si:H). Our objective is to gain deeper insight in these correlations using the slope distribution, rms roughness (σ(rms)) and correlation length (lc) of textured substrates. A wide range of surface morphologies was obtained by Ar plasma treatment and wet etching of textured and flat-as-deposited ZnO substrates. The σ(rms), lc and slope distribution were deduced from AFM scans. Especially, the slope distribution of substrates was represented in an efficient way that light scattering and film growth direction can be more directly estimated at the same time. We observed that besides a high σ(rms), a high slope angle is beneficial to obtain high haze and scattering of light at larger angles, resulting in higher short-circuit current density of nc-Si:H solar cells. However, a high slope angle can also promote the creation of defective regions in nc-Si:H films grown on the substrate. It is also found that the crystalline fraction of nc-Si:H solar cells has a stronger correlation with the slope distributions than with σ(rms) of substrates. In this study, we successfully correlate all these observations with the solar-cell performance by using the slope distribution of substrates.

Optical Enhancement of Silicon Heterojunction Solar Cells With Hydrogenated Amorphous Silicon Carbide Emitter

In this paper, the electrical and optical properties of p-type hydrogenated amorphous silicon carbide (a-SiC:H) are compared with p-type hydrogenated amorphous silicon (a-Si:H) widely used as emitter material of silicon heterojunction solar cells. The difference in solar-cell performance of the two emitters shows that p-type a-SiC:H emitter is able to enhance the short-circuit current density (Jsc) by reducing the parasitic absorption loss and reflection loss without degrading the electrical performance of devices. The application of the p-type a-SiC:H emitter can lead to a Jsc increase of about 1 mA/cm2, compared with the p-type a-Si:H emitter. Our silicon heterojunction solar cell with p-type a-SiC:H emitter shows an active-area efficiency of 20.8% and the short-circuit current density of 40.3 mA/cm2.

Understanding the thickness-dependent effective lifetime of crystalline silicon passivated with a thin layer of intrinsic hydrogenated amorphous silicon using a nanometer-accurate wet-etching method

This work studies the dependency of the effective lifetime on the a-Si:H layer thickness of c-Si substrates passivated with intrinsic a-Si:H. This is experimentally investigated by using a soft wet-etching method that enables accurate control of the a-Si:H layer thickness. In this way, variations in the effective lifetime down to thicknesses of a few nanometers are studied, while excluding effects originating from the deposition conditions of a-Si:H when samples of different thicknesses are fabricated. For thin passivation layers, results show a strong thickness dependency of the effective lifetime, which is mainly influenced by the recombination at the external a-Si:H surfaces. For thicker passivation layers, the effective lifetime is predominantly determined by the bulk a-Si:H and/or c-Si defect density. During the etching of the a-Si:H passivation layers, a gradient in the Cody gap for our samples is observed. This gradient is accompanied by a stronger decrease in the effective lifetime and is attribut...

Surface passivation of c-Si for silicon heterojunction solar cells using high-pressure hydrogen diluted plasmas

In this work we demonstrate excellent c-Si surface passivation by depositing a-Si:H in the high-pressure and high hydrogen dilution regime. By using high hydrogen dilution of the precursor gases during deposition the hydrogen content of the layers is sufficiently increased, while the void fraction is reduced, resulting in dense material. Results show a strong dependence of the lifetime on the substrate temperature and a weaker dependence on the hydrogen dilution. After applying a post-deposition annealing step on the samples equilibration of the lifetime occurs independent of the initial nanostructure.

Optical and Electrical Simulation of μc-Si:H Solar Cells: Effect of Substrate Morphology and Crystalline Fraction

Hydrogenated microcrystalline silicon (μc-Si:H) is an important material for high-efficiency multijunction solar cells. Due to its complex microstructural properties, it is difficult to describe the electronic behavior clearly. In this study, we measure opto-electronic properties including the mobility gap of μc-Si:H films in solar cells, as well as physical properties such as the crystalline fraction profile. The height distribution function of the ZnO substrates is obtained by AFM scans, which is used for optical simulation. All the parameters that we obtained from measurements were used as input parameters of a model in the ASA simulator. We obtained a good fit between measurements and simulations.

Periodic and Random Substrate Textures for Liquid-Phase Crystallized Silicon Thin-Film Solar Cells

A major limitation in current liquid-phase crystallized (LPC) silicon thin-film record solar cells is optical losses caused by their planar glass-silicon interface. In this study, silicon is grown on nanoimprinted periodically, as well as randomly textured glass substrates, and successfully implemented into state-of-the-art LPC silicon thin-film solar cells. Compared with an optimized planar reference device, both textures enhance absorption of light. Interlayer and process optimization allowed achieving a material quality comparable with the planar reference device. On the random texture, an open-circuit voltage above 630 mV was obtained, as well as an external quantum efficiency exceeding the planar reference device by +3 mA/cm2.

Thin-film silicon technology for highly-efficient solar cells

Photovoltaic (PV) electricity generation is currently the fastest growing technology for energy production. PV technology is for more than 90% based on silicon as source material and this material is one of the most expensive components of silicon based solar cells. Thin-film silicon based solar cells are low-cost alternatives. This contribution discusses the most important features of amorphous silicon based thin-film solar cells. In addition, solar cells based on a heterojunction between amorphous silicon and crystalline silicon are discussed. This solar-cell technology has how attracted much attention due to the high performance and several industrial benefits resulting from low-temperature processing.

Hydrogenated amorphous silicon oxide (a-SiOx:H) single junction solar cell with 8.8% initial efficiency by reducing parasitic absorptions

Hydrogenated amorphous silicon oxide (a-SiOx:H) solar cells have been successfully implemented to multi-junction thin film silicon solar cells. The efficiency of these solar cells, however, has still been below that of state-of-the-art solar cells mainly due to the low Jsc of the a-SiOx:H solar cells and the unbalanced current matching between sub-cells. In this study, we carry out optical simulations to find the main optical losses for the a-SiOx:H solar cell, which so far was mainly optimized for Voc and fill-factor (FF). It is observed that a large portion of the incident light is absorbed parasitically by the p-a-SiOx:H and n-a-SiOx:H layers, although the use of these layers leads to the highest Voc × FF product. When a more transparent and conductive p-nc-SiOx:H layer is substituted for the p-a-SiOx:H layer, the parasitic absorption loss at short wavelengths is notably reduced, leading to higher Jsc. However, this gain in Jsc by the use of the p-nc-SiOx:H compromises the Voc. When replacing the n-a-S...

A Full Scalar Scattering Model for Nano-Textured Interfaces

We present a full scattering model for nano-textured interfaces as they are present in thin film silicon solar cells. The model is based on the scalar scattering theory and predicts measured scattering parameters well.

Passivation mechanism in silicon heterojunction solar cells with intrinsic hydrogenated amorphous silicon oxide layers

In this work, we use intrinsic hydrogenated amorphous silicon oxide layers (a-SiOx:H) with varying oxygen content (cO) but similar hydrogen content to passivate the crystalline silicon wafers. Using our deposition conditions, we obtain an effective lifetime (τeff) above 5 ms for cO ≤ 6 at. % for passivation layers with a thickness of 36 ± 2 nm. We subsequently reduce the thickness of the layers using an accurate wet etching method to ∼7 nm and deposit p- and n-type doped layers fabricating a device structure. After the deposition of the doped layers, τeff appears to be predominantly determined by the doped layers themselves and is less dependent on the cO of the a-SiOx:H layers. The results suggest that τeff is determined by the field-effect rather than by chemical passivation.In this work, we use intrinsic hydrogenated amorphous silicon oxide layers (a-SiOx:H) with varying oxygen content (cO) but similar hydrogen content to passivate the crystalline silicon wafers. Using our deposition conditions, we obtain an effective lifetime (τeff) above 5 ms for cO ≤ 6 at. % for passivation layers with a thickness of 36 ± 2 nm. We subsequently reduce the thickness of the layers using an accurate wet etching method to ∼7 nm and deposit p- and n-type doped layers fabricating a device structure. After the deposition of the doped layers, τeff appears to be predominantly determined by the doped layers themselves and is less dependent on the cO of the a-SiOx:H layers. The results suggest that τeff is determined by the field-effect rather than by chemical passivation.