Session Fr-C3

Nano-and Microcrystalline Silicon: Photovoltaics III

Chair: Stefan Zukotynski, University of Toronto

Fr-C3.1 14:00–14:20

Intensive Luminescence from Laser Heated Freestanding Silicon Nanocrystals

Lihao Han, Arno H. M. Smets, and Miro Zeman

Photovoltaic Materials and Devices Laboratory, Delft University of Technology, P.O. Box 5031, 2600 GA Delft, the Netherlands

Nanocrystals (NCs) have special optical and electrical properties which intermediate between single atoms and bulk materials and lead to promising applications in photovoltaics, memory and luminescence devices. Materials consisting of silicon nanocrystals (Si-NCs) have been extensively studied in recent years for many interesting quantum confinement related properties, such as the bandgap that can be tuned by the size of NCs.

In this work, several characterization methods have been carried out to study various properties of Si-NCs. We propose an original method to determine the size of freestanding Si-NCs, which differs from the conventional techniques based on high resolution transmission electron spectroscopy (HRTEM), X-Ray diffraction (XRD) and Raman Spectroscopy (RS). The size of Si-NCs and their bulk lattice can be measured using HRTEM. Disadvantages are that HRTEM is relative expensive in maintenance and complex in preparing samples for characterization. XRD is another tool to estimate the size, by analyzing the width of diffraction peaks using Scherrer formula. A disadvantage is that XRD only works accurately on highly-crystallized NCs. Another method is the comparatively cheap, fast, non-destructive and easy-to-operate RS. RS is in general utilized to monitor the composition and crystallinity of materials. The size of small (d<10 nm) Si-NCs can be resolved by measuring the red-shift of the first-order Si-Si transverse-optical RS peak (~521 cm–1), which is induced by quantum confinement effects related to the small sizes. A disadvantage is that RS is not sensitive to determine the average size of NCs with a broad size-distribution.

The main focus of this paper is the introduction of an unconventional approach to determine the size of freestanding Si-NCs based on RS. We intentionally heat up Si-NCs using the Raman probe laser (low power 330 μW, sharply focused dL~0.738 μm). The freestanding NCs can be heated up easily, because the heat loss mechanism is dominated by blackbody radiation and the thermal conduction is rather limited. This thermal expansion results in a significant red-shift of up to 30 cm–1 from 521 cm–1 position and broadening of peak width of up to 19.1 cm–1 depending on the laser intensity. By analyzing the ratio of Anti-Stokes-to-Stokes peak intensity, we can determine the temperature of Si-NCs (up to ~953K can easily be achieved) under laser exposure. The average size of large Si-NCs can be determined by measuring the temperature of Si-NCs as a function of laser intensity. This innovative method of estimating the size of large Si-NCs from RS measurements and its relation to the absorption and blackbody emission is discussed in detail.

In addition, we discuss the photoluminance (PL) signals can be resolved using the near-infra-red range of RS. At low laser intensities, PL emission peak depends on the bandgap reflecting the irradiative charge carrier recombination and red-shifts when the average size of the smallest Si-NCs increases from 3.5 nm up to 5 nm. Under high laser illumination (Si-NCs at high temperature), the PL peak becomes independent of bandgap and size. We argue the origin of the latter illumination center lies in the defects of Si-NCs at the interface and the native oxidized surface.

Keywords: silicon nanocrystals, Raman spectrum, photoluminescence spectrum, laser heating effect

Fr-C3.2 14:20–14:40

Fundamental Limits of High-efficiency Microcrystalline Silicon Thin Film Solar Cells: The Role of Interfaces

Simon Hänni, Grégory Bugnon, Gaetano Parascandolo, Jordi Escarré*, Mathieu Boccard, Matthieu Despeisse*, Fanny Meillaud, and Christophe Ballif

Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory (PVLab), Rue A.-L. Breguet 2, CH-2000 Neuchâtel, Switzerland

*now at CSEM SA, CSEM PV-Center, Rue Jaquet-Droz 1, CH-2000 Neuchâtel, Switzerland

State-of-the-art single-junction microcrystalline silicon (μc-Si) solar cells usually exhibit open-circuit voltages (Voc) in the 500–560 mV range [1]. In this contribution, we will discuss the role played by i-n and p-i interfaces to further improve this Voc, demonstrating that the full efficiency potential of μc-Si for photovoltaic applications has not been achieved yet.

Most attention has been given in our laboratory to developing plasma processes allowing for high-quality intrinsic and doped μc-Si material [2]. Here we will show that for very thin microcrystalline intrinsic layer (below 1 μm thick), bulk limitations become small enough compared to interface recombinations to reveal the essential role played by adequate interfaces and doped layers. In particular, we will show the interplay between the thickness of the μc-Si absorber layer and the thickness of an amorphous silicon (a-Si) intrinsic buffer layer applied at the i-n interface. In the case of very thin cells, we propose that the observed gain can be described by passivation effects similar to those which take place in crystalline silicon heterojunction solar cells, i.e. the electronic properties of the device are dominated by the interfaces. This study is performed on superstrates with various roughnesses (going from flat to rough low-pressure-chemical-vapor-deposited zinc oxide), which are known to have a significant impact on the quality of the intrinsic layer. Variable-illumination measurements (VIM) show that the electrical properties of such thin cells are strongly affected both by the roughness of the used superstrate and by the choice of the n-layer (i-a-Si/n-a-Si versus n-SiOx).

Our approach has already been validated by reaching an outstanding high Voc of 608 mV, even at high Raman crystallinity factor of the i-layer (>50%), representing a gain of more than 20 mV compared to our standard design with n-SiOx for the n-layer. Most importantly this high Voc could be achieved together with a very high cell fill factor of 77%, leading to a remarkable cell efficiency of 9.5% for an absorber layer as thin as 600 nm. This is to our knowledge the highest Voc for state-of-the-art devices obtained using SiH4-H2 plasma-enhanced chemical vapor-deposition (PECVD) only. We thus demonstrate that elementary p-i-n μc-Si-based devices with a high-quality intrinsic layer are inherently limited by electronically-dead doped-layers, and that their passivation is required in order to further increase the Voc of such cells. To complete our presentation, our new world-record conversion efficiency for single-junction μc-Si solar cells of 10.69%, independently confirmed at ISE CalLab PV Cells, will be detailed [3]. This cell is only 1.8-μm-thick, with a Raman crystallinity factor of 57±5% and implements our latest progresses in terms of absorber material-quality improvement, process optimization as well as novel anti-reflective textures, paving the road toward efficiencies above 11% for single-junction μc-Si solar cells.

[1] S. Hänni, et al., "On the Interplay Between Microstructure and Interfaces in High-Efficiency Microcrystalline Silicon Solar Cells", IEEE Journal of Photovoltaics 3 (1), pp. 11–16, 2013

[2] G. Bugnon, et al., "A New View of Microcrystalline Silicon: The Role of Plasma Processing in Achieving a Dense and Stable Absorber Material for Photovoltaic Applications", Advanced Materials 22, pp. 3665–3671, 2012

[3] S. Hänni, et al., "High-efficiency microcrystalline silicon single-junction solar cells", accepted in Progress in Photovoltaics: Research and Applications, 2013

Keywords: microcrystalline silicon, thin films, solar cells, high efficiency

Fr-C3.3 (invited) 14:40–15:10

3D Morphologies for Back-Scattering Contacts of a-Si:H and μc-Si:H Thin Film Solar Cells

R. E. I. Schropp (1,2), N. J. Bakker (1), L. van Dijk (3,4), M. Di Vece (4), M. Dörenkämper (1), Y. Kuang (3), P. P. A. C. Pex (1), J. K. Rath (3), W. Soppe (1), L. W. Veldhuizen (2), and C. H. M. van der Werf (1)

1. Energy research Center of the Netherlands (ECN) - Solliance, High Tech Campus Building 5, 5656 AE Eindhoven, The Netherlands

2. Eindhoven University of Technology (TU/e), Department of Applied Physics, Plasma & Materials Processing, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

3. Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science, Physics of Devices, High Tech Campus 5, 5656 AE Eindhoven, The Netherlands

4. Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science, Nanophotonics - Physics of Devices, P.O. Box 80000, 3508 TA Utrecht, The Netherlands

A serious design restriction in shaping the nanostructured metallic back contact for optimal light management in thin film silicon solar cells is the formation of microcracks in the semiconductor layer during deposition, both in μc-Si:H and a-Si:H, though more pronounced in μc-Si:H. These microcracks are formed due to colliding growth zones from the slopes of narrow valleys in the back texture and they form a possible path for diffusing metal atoms from the top and bottom contacts, thus presenting a risk of electrical shunts or even short-circuits in the solar cells.

Various methods to circumvent crack formation have been proposed, such as (i) smoothening treatments, e.g., sol-gel overcoating or plasma treatment of natively rough reflectors, (ii) fabricating geometrically flat, but optically scattering surfaces using filling and polishing steps, (iii) growing controlled protrusions on a flat template, (iv) forming controlled rough surfaces by nano-imprint lithography.

Using these methods, the semiconductor growth mechanism is left unaltered, but the substrate roughness is adjusted, which preferably should not reduce the light trapping effect. While producing designed morphologies, new light management features can be added in: (i) light diffraction at periodic gratings (1D or 2D), (ii) plasmonic gratings inducing lateral photonic modes in the absorber layer.

This presentation will review the above light trapping schemes and highlight results obtained with nanoimprint lithography. This is a promising method for manufacturing 3D nanostructures for solar cells as this is a scalable technique that can be extended to industrial roll to sheet and roll to roll methods.