Symposium : A
|Silicon Nanocrystals for Photovoltaics : X|
|08:30||Quantum effects in silicon for photovoltaic applications|
Authors : Ph. Ingenhoven 1, A. Anopchenko 1, A.Tengattini 1, D.Gandolfi 1, F. Sgrignuoli 1, G.Pucker 2, Y.Jestin 2, L. Pavesi 1
Affiliations : 1 Nanoscience Laboratory, Department of Physics, University of Trento,Via Sommarive 14, 38123 Povo (Trento) Italy. 2 Advanced Photonics and Photovoltaics Group, Bruno Kessler Foundation, Via Sommarive 18, 38123 Povo (Trento) Italy
Resume : Quantum confinement effects in silicon might help to overcome the theoretical limit of 33% in conversion efficiency of standard silicon cells. The dominant loss mechanism in silicon solar cells is rapid thermalisation of photo-excited carriers to the band gap. Appropriate engineering of the band gap makes it in principle possible to obtain an efficiency of ~44%. Low dimensional silicon is a cheap material which can be deposited by standard manufacturing processes keeping the production cost of the cell low. In our group we currently follow 2 approaches to apply silicon-nanocrystals (Si-NCs) to improve the solar cell efficiency: Firstly SiNC are used for the modification of the solar spectrum in a way that the incoming light is more efficiently converted into electricity by the cell, and secondly for the modification of the silicon band gap in multiple junction cells for minimization of thermalisation losses. In fact properties of Si-NCs can be tuned; energy of the band gap and optical properties depend strongly on the NC size, which can be controlled by the processing parameters. Therefore light management is possible by varying the refractive index of the SiNCs to improve the antireflective coating (ARC) and by conversion of absorbed high energy UV-blue photons re-emitted in the red spectral region of light. Thus in this approach Si-NCs are used as a spectral down-shifters and for ARC optimization. Further, NCs may allow the fabrication of higher band gap solar cells, to be used on top of standard silicon cells as tandem cell elements. Due to the tunability of the band gap it might be possible to construct all silicon multilayer tandem cells. The concepts and research results for both ideas will be presented.
|09:00||Synthesis and properties of Si nanocrystals for photovoltaic applications|
Authors : Corrado Spinella (a), Salvatore Mirabella (b), Maria Miritello (b), Isodiana Crupi (b), Salvatore Lombardo (a), Rosaria Puglisi (a), Giovanni Mannino (a), Rosa Ruggeri (a)
Affiliations : (a) CNR, Istituto per la Microelettronica e Microsistemi, Ottava Strada 5, 95121 Catania (b) CNR, IMM-MATIS, Via Santa Sofia 64, 95123 Catania
Resume : Thin films of the Si:O alloy have been systematically characterized in the structural, optical absorption and electrical transport behaviour, by varying the Si content. Magnetron sputtering or plasma enhanced chemical vapour deposition have been used to deposit a single layer, or multilayer structures of Si–rich–SiO2, followed by pure thermal annealing or by infrared laser irradiation. The optical absorption and the electrical transport of Si:O films can be continuously and independently modulated by acting on different parameters. The light absorption increases with the Si content, determining an optical bandgap which can be continuously modulated into the 2.6–1.6 eV range. The electrical resistivity of Si:O films can be varied among five decades, being essentially dominated by the number of Si grains and by the doping. An extensive study on the electrical properties of the multilayer structure under dark and solar light exposure in AM 1.5G conditions is discussed. The electrical data demonstrate that the current transport in such systems is mediated by tunnel effect, and the lowest effective energy barrier limiting the carrier transport has been found to be 1.7 eV. These data can be profitably used to better implement Si quantum dots for future photovoltaic technologies.
|09:30||Resonance energy transfer from PbS colloidal quantum dots to bulk silicon: A new aspect for hybrid photovoltaics|
Authors : P. Andreakou1, M. Brossard1, M. Bernechea2, G. Konstantatos2, P. Lagoudakis1*
Affiliations : 1 School of Physics and Astronomy, University of Southampton, Southampton, United Kingdom; 2 ICFO - Institut de Ciències Fotòniques, Castelldefels, (Barcelona), Spain
Resume : Semiconductor Nano-Crystals (SNC) are promising materials for photovoltaic applications because their absorption spectrum can be tuned from the visible to near infrared by varying their diameter. They are efficient light absorbers, which can yield multiple excitons upon absorption of a single high-energy photon. However, a significant drawback of these materials is the lack of methods for the efficient charge extraction and carrier transport. We take advantage of the optical properties of SNCs and overcome their drawbacks by using a hybrid photovoltaic device, which is composed of SNCs and bulk silicon. This photovoltaic configuration exploits the absorption of solar photons and the subsequent creation of excitons in the SNCs. Then, excitons are transferred to a silicon p-n junction via Resonance Energy Transfer (RET). We study the efficiency of RET mechanism for this hybrid structure by means of time resolved spectroscopy. As SNCs, we use lead sulphide (PbS) particles with 3nm diameter and emission at 890nm. We study the efficiency of the RET mechanism between the PbS nanocrystals and silicon by varying the separation distance between them. The later results undoubtedly identify RET from colloidal quantum dots to bulk silicon. Temperature measurements are also presented and show that the RET efficiency remains high across a range of temperatures, with a value of 44% at room temperature.
|09:45||Si nanoparticle interfaces in Si/SiO2 solar cell materials|
Authors : J. Slotte, S. Kilpeläinen, F. Tuomisto, Y.-W. Lu, A. Nylandsted Larsen
Affiliations : Department of Applied Physics, Aalto University, P.O.Box 11100, FI-00076 Aalto, Finland; Department of Applied Physics, Aalto University, P.O.Box 11100, FI-00076 Aalto, Finland; Department of Applied Physics, Aalto University, P.O.Box 11100, FI-00076 Aalto, Finland; Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark; Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark
Resume : Silicon has been the mainstream material used in solar cell industry since the introduction of commercially available solar cells. The bottleneck for normal semiconductor solar cells is that each incident solar photon is only able to form a single exciton in the cell and the rest of the photon energy is lost. A few years ago it was discovered that certain semiconductor nanocrystals are able to generate more than one exciton per incident photon, making it possible to greatly improve the efficiency of solar cells. In this work, we have studied one possible candidate for nanocrystalline solar cells, namely Si nanoparticles inserted into SiO2. Multilayer structures consisting of 30 Si/SiO2 bilayers capped with a 50 nm layer of SiO2 were deposited on p-type Si. In the bilayers the SiO2 layer was 4 nm and three different Si layer thicknesses were used, 1, 2 and 4 nm. Each set contained three samples: as-deposited, annealed at 1100 C in N2, and annealed at 500 C in N2 with 5% H2 after the high temperature annealing. The sample sets where studied with both photoluminescence (PL) and positron annihilation spectroscopy (PAS). The PAS measurements were performed in the Doppler broadening mode using a slow positron beam. Results from both measurement techniques show that the most successful nanoparticle formation was found in the sample with 2 nm thick Si layers. Furthermore, the 500 C annealing with H2 was shown to optically passivate defects at the nanoparticle/SiO2 interface.
|10:00||Hierarchical Nano/Microstructures on Silicon Surface with Ultra Low Reflectance for Photovoltaic Applications|
Authors : L. T. Tan, A. H. Lim, Z. Y. Chee, Y. L. Wong, Y. C. Huang, H. W. Ong, Q. X Wee, J. W Ho, S. J. Chua, Rob Steeman
Affiliations : School of Engineering, Republic Polytechnic, 9 Woodlands Ave 9, Singapore 738964; Department of Electrical and Computer Engineering, Centre for Optoelectronics, National University of Singapore, 2 Engineering Drive 3, Singapore 117576; REC Tuas, 20 Tuas South Avenue 14, Singapore 637312
Resume : Silicon nanowires (SiNWs) arrays prepared by silver-assisted chemical etching show ultra-low reflectance over a wide spectral bandwidth from 300 to 1000 nm. In this work, we prepared polycrystalline silicon nanowire arrays by using solar-grade multicrystalline silicon wafers (mc-Si) and single crystal silicon wafers (sc-Si). Reflectance data show an average reflectance over 400 to 700 nm of bare mc-Si is 30 % and decreases to 25 % after acidic texturization. A reduction of reflectance is observed as nanowire network starts to form. The reflectance further reduces in almost 3 times, when the etching duration and volume of HF/AgNO3 solution increases. Reflectance data and field emission scanning microscope (FE-SEM) images provide an estimation of the enhancement of light absorption with respect to the density of nanowire network formation on the substrates.
|Thin Film Technology : S. Pizzini and G. Kissinger|
|10:45||Electrical transport and photocurrent in single and multilayered nanocrystalline Si films|
Authors : S. Gardelis, P. Manousiadis, and A.G. Nassiopoulou
Affiliations : IMEL/NCSR Demokritos, Terma Patriarchou Grigoriou, Aghia Paraskevi, Athens 15310, Greece
Resume : A fundamental result of quantum size effects in silicon nanocrystals (SiNCs) is the widening of the energy band gap with decreasing size. This significant effect can be exploited in order to build solar cells of the third generation. Specifically, one can fabricate tandem solar cells consisted of layers of SiNCs of different sizes, which will be able to absorb and convert different parts of the solar spectrum into electrical energy more efficiently than the first and second generation Si-based solar cells. We have grown single and multilayered nanocrystalline Si (nc-Si) films containing SiNCs of controlled sizes on quartz by low pressure chemical vapour deposition (LPCVD) of Si and subsequent oxidation at high temperature. The aim of the growth of such films is their potential use in tandem Si-based solar cells. The single nc-Si films consisted of a nc-Si layer with a top surface silicon dioxide layer (SiO2). The thickness of the nc-Si layers ranged from less than 2 nm up to 20 nm, by choosing suitable growth conditions. The multilayered films consisted of five nc-Si/SiO2 bilayers. Platinum electrodes were defined by lithography and lift-off technique in order to contact the nc-Si films in both single and multilayered films and to allow for lateral electrical transport measurements. Electrical transport mechanisms were investigated systematically by current-voltage measurements carried out at different temperatures ranging from liquid nitrogen temperature up to 340 K. Photocurrent measurements were performed using the monochromated light of a Xe lamp. From transmission and reflection measurements, energy band gaps of the films were estimated and the results were used to explain electrical transport and photocurrent measurements.
|11:00||Crystallisation of electron beam evaporated amorphous silicon films on oblique-angled substrates|
Authors : Janis Jeanne Merkel, Christiane Becker, Bernd Rech
Affiliations : Helmholtz-Zentrum Berlin f?aterialien und Energie Institut Silizium-Photovoltaik Kekul?ra? 5 12489 Berlin Germany
Resume : Thin-film solar cells based on polycrystalline silicon (poly-Si) can combine the advantages of low cost thin-film technology and high electronic material quality. Electron Beam evaporation (EBE) is considered as emerging deposition technique for silicon thin-film photovoltaics enabling high growth rates up to 1 µm/min and therefore offering a strong cost reduction potential in industrial production. However, the use of textured substrates for light trapping is challenging due to the directional growth during EBE of silicon. Solid phase crystallized Silicon films exhibit a minor material quality on textured substrates, in particular when steep angles are present in the topography. In this study, a systematic analysis of poly-Si thin films deposited on substrates specifically tilted from 0° to 45° was carried out. The structural properties of these solid phase crystallized silicon films were investigated by Raman spectroscopy, FTIR, EDX, and TEM. Solar cell structures were prepared to analyse material quality by implied Voc measurements. It was found that at substrate angles larger than 30° porosity and oxygen content of the films increase while crystalline fraction derived from Raman spectra and solar cell performance decrease. The conclusion of these investigations can serve as design rule for light trapping structures in poly-Si thin-film solar cell devices.  Gall et al, Journal of Crystal Growth 312 (2010) 1277  Werner et al, Proceedings 24th EUPVSEC (2009) 2482
|11:15||Hydrogen induced crystallization in intrinsic hydrogenated amorphous silicon films prepared by RF magnetron sputtering at low temperature|
Authors : D. Senouci1, R. Baghdad1, A. Belfedal2, D. Benlakhel2, X. Portier3, L.Chahed2, S. Charvet4, M. Clin4, P. Roca i Cabarrocas5, K. Zellama4
Affiliations : 1Laboratoire de G?e Physique, Universit?bn-Khaldoun, 14000 Tiaret, Algeria ; 2 LPCMME, D?rtement de Physique, Universit?’Oran Es-s?a, 3100, Oran, Algeria ; 3 SIFCOM-ENSICAEN, UMR 6176, 6 Bvd Mar?al Juin, 14050 Caen Cedex, France ; 4 LPMC, Facult?es Sciences, Universit?e Picardie Jules Verne, 33 rue Saint-Leu,80039 Amiens, France ; 5LPICM, Laboratoire de Physique des Interfaces et Couches Minces, Ecole Polytechnique, 91128 Palaiseau, France.
Resume : We present an investigation on the transition from amorphous to nanocrystalline silicon and the hydrogen effect during the first steps of hydrogenated nanocrystalline silicon growth by reactive radiofrequency magnetron sputtering (RFMS) at substrate temperature as low as room temperature. The microstructure transition from amorphous to nanocrystalline silicon at low temperature deposition is discussed by a large number of studies [1, 2]. In this work, nanocrystalline silicon thin films (40-800 nm) were deposited by RFMS, using undoped single crystalline silicon target of 3-inches diameter and 99.9% purity at total pressure p of 3 Pa and at substrate temperature (TS) equal to room temperature (RT), 100°C. We varied deposition times 3, 7, 15, 30 and 60 minutes. We used an RF power of 220W and a plasma gas mixture of 30% Ar 70% H2, maintained constant for all the films, considered as being conditions to favour the growth of crystallized films . We combine therefore, Fourier transform infrared absorption (FTIR), Raman scattering spectroscopies and conventional transmission electron microscopy (CTEM) in order to correlate between the hydrogen effect and the amorphous to nanocrystalline phase transition. The thermal hydrogen effusion experiment is another tool to check such behavior of hydrogen evolution during the nc-Si:H film deposition. We have interpreted our results on the basis of the model schematized in figure-1, who represent the first steps of nc-Si:H growth. The Raman and CTEM results show clearly that from a few seconds to a few minutes there is a formation of an amorphous H-rich subsurface layer. In fact, from hydrogen dilution in the plasma, H diffuses and forms this H-rich amorphous subsurface layer (phase of incubation), with a volume VH and hydrogen excess H accompanied by an etching of the film surface. When nanocrystalline silicon nc-Si:H layer start to grow by chemical transport, H selective etching or chemical annealing, hydrogen gradually out-diffuses leading to a decrease in both volume VH and H excess H of the H-rich subsurface layer as shown by FTIR and Hydrogen effusion results [1, 4]. References 1. F. Kail, A. Hadjadj and P. Roca I Cabarrocas, Thin Solid Films 126-131 (2005) 487. 2. Saravanapriyan Sriraman, Sumit Agarwal, Eray S. Aydil and Dimitrios Maroudas, Nature, 62 . Vol 418, 4 july 2002. 3. R. Baghdad, D. Benlkhal, X. Portier, K. Zellama, S. Charvet, J.D.Sib, M. Clin and L. Chahed, Thin Solid Films 3965-3970 (2008) 516. 37. N. Pham, A. Hadjadj, P. Roca I Cabarrocas, O. Jbara and F. Kail, Thin Solid Films 517 (2009) 6225-6229. Figure-1: Schematically model for nc-Si:H at its first steps growth.
|11:30||Electrodeposition of a-Si|
Authors : Laetitia Philippe Juan LimonPetersen Mickael Bechelany Johann Michler
Affiliations : EMPA Swiss Federal Laboratories for Materials Science and Technology Laboratory Thun, Switzerland
Resume : We report on the synthesis of amorphous silicon using electrodeposition in two non-oxygenated organic solvents, acetonitril and dichloromethane, under controlled atmosphere. In both solvents, tetraethylammonium chloride has been used as a supporting electrolyte and silicon tetrachloride as a silicon precursor. The porosity and the morphology of the electrodeposited silicon layers have been studied as a function of depositing parameters (solvent, voltage, etc.). We will show the formation from dense to highly porous Si deposits as a function of solvent and experimental parameters. To increase the stability of the deposits before exposure to air, a heat treatment under hydrogen has been performed. The chemical and structural characterizations of the deposit by scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy show the formation of pure amorphous silicon.
|11:45||Closing remarks: G. Kissinger, S. Pizzini, H. Yamada-Kaneta, J. Kang|
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