The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Nano-and Microcrystalline Silicon: Photovoltaics I
Chair: Arno Smets, Delft University of Technology
Fr-C1.1 (invited) 8:10–8:40
Investigation of Metastability Effects in Hydrogenated Microcrystalline Silicon Thin Films by the Steady-state Measurement Methods
1. Department of Physics, Faculty of Sciences, Mugla Sitki Kocman University, Kotekli Yerleskesi, Mugla, TR-48000, Turkey
2. Forschungszentrum Jülich, IEK-5 Photovoltaik, 52425 Jülich, Germany
3. Institut für Physik, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany
Upon exposure of microcrystalline silicon (μc-Si:H) thin films to room ambient, water vapour, deionized water, or to different gas atmospheres, these materials frequently show metastability changes of their electronic properties. These changes are generally detected by using the techniques probing the properties of sample at the steady-state condition. Even though the first published results appeared at the beginning of 1980's by Veprek et al., more extensive investigation has been carried out in the last decade. In most studies, very thin samples deposited on smooth glass substrate were used in measurements due to adhesion problems of thicker samples on the smooth glass substrates. It was reported that conductivities in some materials increased and in others they decreased in the metastable state significantly, showing no clear functional dependence on the crystallinity of the material. These changes were reported to be reversible after heat treatment in short period after deposition and irreversible for longer periods of a few years. Even though such changes were mainly attributed to the band bending at the surface of the film, within cracks or at the grain boundaries, detailed understanding of the metastability is still missing.
In this study, we have developed new standard measurement procedures and applied to investigate the metastability phenomena using temperature dependent dark conductivity, steady-state photoconductivity (SSPC), sub-bandgap absorption spectroscopy as detected by dual beam photoconductivity (DBP) and steady-state photocarrier grating (SSPG) methods for μc-Si:H thin films with a wide range of structure compositions. μc-Si:H films were deposited using VHF-PECVD at 200°C on both smooth and rough glass substrates. The microstructure of the films was changed from amorphous (a-Si:H) to highly crystalline by adjusting the process gas silane concentration during deposition. The crystallinity was evaluated from Raman measurements. Thickness of the samples varies between 200 nm and 1100 nm. Silver coplanar electrodes were evaporated on the samples with 0.5 cm length and 0.5 mm separation. The samples were randomly exposed to atmospheric gases by keeping them in the dark laboratory atmosphere. In addition, a controlled gas treatment was performed in high vacuum cryostat. Annealing was carried out at 430K in high vacuum. All probe measurements were performed at 300K as sample is at the steady-state condition. Metastable changes in dark conductivity due to Fermi level shifts and in majority carrier electron mobility-lifetime products, μnτn, determined from the SSPC measurements were correlated with those in the minority carrier hole mobility-lifetime products, μpτp, obtained from the SSPG measurements. In addition, sub-bandgap absorption coefficient spectra of the samples at the same treated states were carefully measured in order to see the changes in the density of occupied defect states in the bandgap of the microcrystalline silicon which affects both electron and hole transport properties. The results obtained in this study were discussed with those published in the literature on the metastability phenomena of microcrystalline silicon films.
Improvement of Light Trapping in Thin Film Silicon Solar Cells by Combining Periodic and Random Interfaces
IEK5-Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
Light trapping in silicon-based thin-film solar cells is a great issue, since the absorptance near the band gap is quite low for flat devices and the thickness is limited due to material and technological issues. Different concepts are applied to improve light coupling into the absorber material and to provide light guidance in the layer. Mostly, random textures are incorporated that scatters incoming light diffusely prolonging the effective light path. As an alternative, periodic structures like gratings or photonic crystals incorporated at different interfaces of the device are investigated by several groups. The optical design of optimized textures can be done with rigorous optical simulations like, e.g. rigorous coupled wave analysis (RCWA), Finite-Element-Method (FEM) or Finite-Difference Time-Domain (FDTD) method. For periodic structures, RCWA is often applied since the approach is well suited to periodic structures. For the random textures, more computing intensive techniques like FDTD are applied with a large calculation domain to fully describe the optical response of the device.
We recently demonstrated that a simple scalar approach sufficiently describes angular resolved scattering in transmission and reflection inside the silicon absorber material [1–3]. For the pure random texture, light scattering is most efficient in reflection at the back contact with a wide angular distribution, while it is much less efficient in transmission at the front contact with comparably small scattering angles. In contrast, the two-dimensional periodic structure shows a significant specular reflection also for an optimized design leading to a poor light-trapping efficiency. In transmission, the highest intensity for the periodic structure is found for the first diffraction order providing a large scattering angle. Therefore, such periodic structures reveal their light trapping mainly in transmission.
We demonstrate that by the combination of both, random texture and periodic structure, the first diffraction order in transmission still dominates the light scattering, but the resonance is much broader due to the random structure. The light scattering at the back contact shows a very similar behavior as the pure random texture with a broad angular distribution around large angles. The combined texture, therefore, benefits from both resulting in optimal transmission and reflection properties.
Starting with a randomly textured ZnO:Al layer, that is well-known to provide high-efficiency microcrystalline silicon solar cells, we add a two-dimensional periodic structure with optimized period and height on top of the random texture. The design of the structure is done by applying the scalar approach and verified by FDTD simulations taking into account real layer thicknesses and optical constants of all involved materials. We combine light scattering at the front contact and at the back contact to derive a quantity that is well suited to predict external quantum efficiency of the solar cell from the scalar approach. The thus optimized structure outperforms simple pure periodic and random structures.
 M. Schulte, et al., Appl. Phys. Lett. 99, 111107 (2011)
 K. Bittkau, et al., J. Appl. Phys. 111, 083101 (2012)
 M. Ermes, et al., J. Appl. Phys. 113, 073104 (2013)
Keywords: light trapping; photonics; random textures; rigorous optical simulation; scalar scattering theory
Light Management Schemes for n-i-p Thin Film Silicon Solar Cells
Ecole Polytechnique Fédérale de Lausanne, Institute of Microengineering, Laboratory of Photovoltaics and Thin Film Electronics, Rue A.-L. Breguet 2, 2000 Neuchâtel, Switzerland
Thin-film solar cells based on hydrogenated amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon require thin photo-active layers to ensure satisfactory collection of the photogenerated carriers. Because the relevant thicknesses are below the absorption length of light in the long wavelength range for both materials, advanced light-management schemes are required to increase the short-circuit current density (Jsc) of a solar cell. Usually, light absorption is enhanced by using light scattering at textured interfaces that naturally develop as the cell is grown on a textured electrode. This contribution discusses light management schemes in n-i-p solar cells for which the n-doped layer is the first layer to be deposited on the back contact, thus allowing the deposition of solar cells on a wide range of different substrates which can be either transparent or opaque. In particular, two different approaches for light-management are discussed, both leading to the growth of devices with high-efficiencies.
The first approach consists in a traditional rough silver/thin zinc oxide back reflector. We show both experimentally and through simulations that the proper optimization of the thin-dielectric layer thickness allows not only mitigating plasmonic parasitic absorption in the metal, but also mitigating parasitic absorption in the n-doped layer of the solar cell that is grown on the back reflector. We could reproduce such a rough texture on a flexible plastic foil by UV nano-imprinting. Covering this substrate with a silver/thin zinc oxide back reflector, initial and stable efficiencies of 11.1% and 9.2%, were respectively achieved for a tandem a-Si:H/a-Si:H device with a total thickness of silicon of less than half a micron.
Still, the weak point of such traditional scheme that relies on textured interface is that, even if it increases Jsc efficiently, it may induce defective growth of silicon layers with localized porous areas which limit the open-circuit voltage (Voc) and the fill factor (FF) of the solar cell. Thus, the second approach we discuss in this contribution consists in decoupling the optically-rough interface, which allows light scattering and high Jsc, from the growth surface, which is made flat to allow the growth of good-quality silicon and devices with high Voc and FF. By combining this novel type of substrate with an additional anti-reflective coating made by UV nano-imprinting, a triple-junction n-i-p a-Si:H/μc-Si:H/μc-Si:H solar cell with a stable efficiency of 13% was obtained. This innovative approach is very promising as it demonstrates that the usual morphology/light-tapping trade-off can be overcome through the use of a single flat light-scattering substrate that fulfills all necessary requirements to further push thin-film silicon solar cell efficiencies.
Keywords: thin-film silicon solar cell, light management, UV nano-imprinting, parasitic absorption, flat light-scattering substrate
Influence of Plasma Conditions on Properties of the Window Layer and Solar Cell Performance
1. CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa, and CEMOP-UNINOVA, 2829-516 Caparica, Portugal
2. Departamento de Física/ I3N, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Efficiency of thin film silicon solar cells, both single junction and tandem, has reached a plateau in a previous decade, and increased insignificantly since. While the quality of the intrinsic and dopped silicon thin films can be hardly improved further, the new light trapping techniques and better control over the interface areas still offer some potential room for improvement.
High growth rate of intrinsic nanocrystalline silicon (nc-Si:H), used as an absorber layer in tandem and single junction nc-Si:H solar cells, is desired for decreased time (and cost) of solar cell fabrication. However, applied deposition conditions (typically high power, high pressure) lead to enhanced ion damage of the previously deposited doped layers, namely p-type nc-Si:H thin film in the case of the superstrate cell configuration. During the initial stages of deposition of intrinsic nc-Si:H, heavy ion bombardment can easily damage a 20–30 nm thick doped layer, leading to decrease of its crystalline volume fraction and conductivity. This leads to degraded solar cell performance due to increase of the series resistance (RS), and, as a consequence, decrease of the fill factor (FF).
In this work, we studied in detail an influence of various deposition conditions of intrinsic nc-Si:H on electrical and structural properties of the p-type nc-Si:H thin film, and its effect on nc-Si:H solar cell performance. Boron doped nc-Si:H thin films were deposited by standard (13.56 MHz) RF plasma enhanced chemical vapour deposition (PECVD) technique, from the mixture of silane, hydrogen and trimethylboron. Then, in order to simulate initial stages of growth of nc-Si:H on the p-type layer, we performed hydrogen plasma treatment (HPT) at different conditions (fRF = 75 MHz; 1.0 Torr < pressure < 2.13 Torr; 139 mW/cm2 < power density < 433 mW/cm2) for 30 seconds (light ions bombardment). Furthermore, after HPT, a flux of silane was introduced into the deposition chamber in order to initiate heavy ion bombardment and growth. The deposition time was adjusted to grow 5 nm of intrinsic nc-Si:H. The structural changes of the p-type nc-Si:H layers were observed via Raman spectroscopy and interpreted by application of multi-layer modelling of the ellipsometry spectra, and correlated with the optical emission spectroscopy (OES) data acquired during HPT and growth of nc-Si:H. Modelling results confirmed detrimental structural modifications (e.g. partial amorphization of the sub-surface area, etc) of the p-type layer, provoked by the heavy and light ion bombardment.
Structural and electrical changes of the p-type nc-Si:H film due to ion bombardment damage, had a strong influence on the solar cell performance. Namely, it was observed that RS of the p-i-n nc-Si:H solar cell increased from 4.0 to 14.2 Ωcm, thus reducing FF from 63% to 39%, when intrinsic nc-Si:H film with higher growth rate was deposited directly on the p-type layer. Application of a 50 nm thick nc-Si:H buffer layer, deposited at softer conditions (low growth rate) lead to significant decrease of RS (5.3 Ωcm) and, consequently, to increase of FF (up to 63%) and efficiency (by 50%).
Keywords: nanocrystalline silicon, window layer, thin film solar cell, PECVD
The Indium Tin Oxide Films by DC Magnetron Sputtering for Improved Heterojunction Solar Cell Applications
Key Laboratory of Materials Physics of Ministry of Education, School of Physical Engineering, Zhengzhou University, Zhengzhou 450052, China
The indium-tin oxide (ITO) film as antireflection front electrodes is of key importance to obtain high efficiency heterojunction (HJ) solar cell. To achieve high transmittance and lower resistivity ITO films by DC magnetron sputtering, the impacts of the ITO film deposition conditions, such as the oxygen flow rate, pressure and sputter power, on the electrical and optical properties of the ITO films were studied. The resistivity of 3.8x10–4 Ωcm and the average transmittance of 87% in the range of 380–780 nm wavelength were grown under the optimized conditions: oxygen flow rate of 0.1 sccm, a pressure of 0.8 Pa and sputtering power of 110 W. Those ITO film was used to fabricate single-side HJ solar cell. However, the best HJ solar cell conversion efficiency was found to fabricate with sputtering power of 95 W, with the efficiency of 11.5%, an open circuit voltage (Voc) of 0.626 V, fill factor (FF) of 0.50, and short circuit current density (Jsc) of 36.4 mA/cm2. The decrease in performance of the solar cell fabricated with high sputtering power of 110 W is attributed to the ion bombardment to the emitter.
Keywords: indium-tin oxide, DC magnetron sputtering, sputtering power, resistivity, heterojunction solar cell