Celle fotovoltaiche di alta efficienza a eterogiunzione silicio amorfo-nanocristallino/silicio cristallino per applicazioni industriali su vasta scala

Dal \u201cPV Status report 2003\u201d della Commissione Europea risulta che la produzione annua mondiale fotovoltaica presenta un incremento esponenziale a partire dal 1998. Estrapolando tale andamento le previsioni indicherebbero gi\ue0 per il 2010 una produzione annua mondiale di 8\uf710 GWatt. Un obbiettivo di tale portata potr\ue0 essere raggiunto solo se saranno disponibili processi di fabbricazione del dispositivo fotovoltaico, che rispondano ad una serie di requisiti fra i quali i principali sono: - Alta efficienza di conversione (>20% AM1.5G) - Processi a pochi stadi e a basso consumo di energia - Lunga durata d'esercizio (>30 anni) - Basso costo - Abbondanza, reperibilit\ue0 e non tossicit\ue0 dei materiali usati I dispositivi fotovoltaici ad eterogiunzione, oggetto della presente ricerca dal 1996, presentano le caratteristiche fisiche e tecnologiche in grado di rispondere, se adeguatamente sviluppate ai requisiti sopra elencati. In particolare tali caratteristiche sono: a) Possibilit\ue0 di raggiungere alte efficienze di conversione (>20% AM1.5G) su dimensioni di interesse industriale, (10x10) cm2. b) Processo di fabbricazione a bassa temperatura (≤ 200 \ub0C) e basso consumo di energia. c) Compatibilit\ue0 del processo con wafer di silicio di grado solare, di basso costo, e di spessore sottile (≤ 200 mm). d) Processo con un numero limitato di stadi con tempi complessivi di esecuzione relativamente bassi. e) Trattandosi di silicio, \ue8 prevista una durata di esercizio, gi\ue0 sperimentata su dispositivi commerciali standard, prossima ai 30 anni. In conclusione si ritiene che la presente attivit\ue0 di ricerca sul dispositivo fotovoltaico ad eterogiunzione possa ragionevolmente sfociare in applicazioni industriali nel breve-medio periodo

Wide band-gap silicon-carbon alloys deposited by very high frequency plasma enhanced chemical vapor deposition

The use of Very High Frequency (VHF) Plasma Enhanced Chemical Vapour Deposition in a capacitive discharge is investigated to fabricate hydrogenated amorphous silicon carbon alloys, using silane and methane as silicon and carbon precursors respectively and hydrogen dilution of the gas mixture. The properties of samples differ significantly from what normally observed for rf deposition. A wide band gap material is obtained, with a carbon ratio ranging from 0.2 to 0.63. An energy gap up to 3.4 eV is measured, indicating a large sp3 content. The most interesting properties are observed using 90% hydrogen dilution and 350\ub0C as substrate temperature. In this case, a Si-C bond concentration up to 6x1022 cm-3 was measured for stoichiometric samples, associated to a highly crosslinked structure and no detectable Si-CH3 bending signal. The role of hydrogen in determining the optical properties of the film is established and is shown to affect mainly the valence electron concentration. Basing on the free energy model, hydrogen bonding is observed to lie in between a random and chemically ordered configuration. The results are obtained at a deposition rate much larger than both rf and ECR deposition, and are associated to a limited gas consumption, both aspects being advantageous for practical applications. The large Si-C bond concentration, associated to a limited silicon and carbon hydrogenation, makes the VHF deposited a-SiC:H a good starting material for subsequent crystallisation

Silicon heterojunction solar cells with microcrystalline emitter

Microcrystalline n-type emitters, that, compared to a-Si:H ones, ensure better electronic properties and better transparency in the visible, were used to fabricate heterojunction solar cells on crystalline silicon. The substrate surface was passivated by the deposition of a very thin intrinsic a-Si:H buffer layer. The microcrystalline n-type emitters were deposited by radio-frequency (rf) Plasma Enhanced Chemical Vapor Deposition, using a high hydrogen diluted gas mixture. The simulation of optical spectra of n/i double layers on c-Si gives a preliminary evidence that the continuity of the intrinsic a-Si:H buffer layer is preserved after the rf deposition. The photovoltaic devices incorporating microcrystalline emitters exhibit a remarkable increase of short circuit current (Jsc) and efficiency (a factor 1.24 and 1.38 respectively) compared to the case of a-Si:H emitters. Noticeable improvements are observed if the structure is applied to textured substrates

Solar cells on porcelain stoneware tiles

Porcelain stoneware tilesare getting a prominent role on the market offering excellent solution either for interiors or for exteriors construction for flooring and coverings. This ceramic material meets the basic requirements to be compatible with different semiconductor deposition technique. Based on these consideration we investigated the possibility of using porcelain stoneware tiles as solar cells substrates for built-integrated photovoltaic applications. The compatibility of the commercial tiles with the solar cell fabrication process was successful proved and a conversion efficiency of more than 4% has been obtained on a small area devices as a preliminary result

Silicon Heterojunction Solar Cells with Epitaxial Buffer Layer on Textured Substrates

In the present work we report recent results for silicon heterojunction solar cells deposited by conventional Plasma Enhanced Chemical Vapor Deposition (PECVD) technique on textured Czochralski (CZ) silicon wafers. A new texturing technique was developed using a wet anisotropic chemical etching with a tetramethyl-ammonium hydroxide (TMAH) solution. An increase of the device photogenerated current, with maximum short circuit current of more then 36 mA/cm2 was attained. A study of the first stages of device layer growth is presented with relation to plasma ignition in PECVD systems. An original passivation technique of the amorphous/crystalline interface defects was implemented for textured wafers. Using this scheme a reproducible efficiency in excess of 16% was obtained on CZ textured wafers

Silicon heterojunction solar cells with p nanocrystalline thin emitter on monocrystalline substrate

In the framework of plasma deposition of silicon heterojunction solar cells, the issue of depositing, by VHF, a microcrystalline emitter, without affecting the passivating properties of the underlying amorphous buffer layer, is addressed. The sequence Deposition-Exposure to H2 Plasma-Deposition was used to fabricate the microcrystalline emitter. Using High Resolution Transmission Electron Microscopy, we give microscopic evidence of the long range effects of hydrogen, already inferred by large area optical techniques. Upon exposure to H2 plasma, it is observed that silicon nanocrystallites are formed within the amorphous layer. Thinner amorphous layers undergo etching, and epitaxial growth takes place from the substrate. Photovoltaic devices with open circuit voltage up to 638 mV were fabricated

Heterojunction solar cells: a new insight in the intrinsic buffer layer concept

In the literature, the thin buffer layers, used to passivate the junction interface in amorphous silicon / crystalline silicon heterojunction solar cells, are often referred to as intrinsic amorphous layers. However, the optical measurements and high resolution TEM cross section observations show that the so-called i a-Si:H layers, grown on (100) crystalline silicon under the standard PECVD conditions used to deposit good quality amorphous layers on glass, can partially regrow epitaxially on the c-Si substrate. Moreover, when a pyramidal textured c-Si substrate is used, the epitaxial regrowth is much more pronounced compared with a flat c-Si substrate. In this paper, we demonstrate that a partial regrowth is detrimental for the device performance, leading to a degradation of the Voc of the cell with respect to the case where no buffer layer is used. A fully epitaxial buffer layer is shown to improve the interface passivation, and consequently the Voc of the device. Using flat c-Si (CZ grown, 1 ohm.cm) substrates and an amorphous emitter, a 605 mV Voc and a 13.5 % efficiency on 1cm2 cells are obtained. A new hypothesis about the structural nature of the intrinsic 'amorphous' buffer layer used in amorphous silicon / crystalline silicon heterojunction is proposed