Effetto di conversione per aumento dell'efficienza celle fotovoltaiche

Oggigiorno è sempre più importante capire come soddisfare la crescente domanda energetica del nostro mondo. Ci sono paesi come la Cina, l’India e il Brasile che stanno attraversando un notevole periodo di espansione sia a livello industriale che demografico. E in questi paesi, come tempo fa anche nel nostro, sta avvenendo una massiccia migrazione dalle campagne alle città. Questo fenomeno porta a un aumento della classe media e quindi a un aumento della richiesta energetica di queste città che sono ormai metropoli. È facile immaginare l’impatto che si avrà sui consumi energetici, se come si prospetta, questo processo di industrializzazione raggiungerà ogni angolo del nostro pianeta. In questo quadro è importante il ruolo che assume la scienza per poter soddisfare questa domanda energetica e soprattutto farlo senza l’utilizzo di energie non rinnovabili e inquinanti. In questa tesi discuteremo dell’energia solare e in particolar modo di come sia possibile migliorare la produzione di corrente elettrica. Le celle fotovoltaiche prese in studio sono quelle in uso più comune ovvero la cui tecnologia è basata sul silicio e sono capaci di convertire fotoni con lunghezza d’onda compresa tra circa 300 nm e 1100 nm mentre lo spettro solare, misurato al livello del mare, comprende fotoni con lunghezze d’onda che vanno dai 250 nm fino a oltre 2500 nm. L’intento è quello di utilizzare i cristalli come convertitori per poter, appunto, convertire la radiazione non sfruttata dalle celle solari basate sul silicio in radiazione utile alla erogazione di corrente. Per raggiungere questo obiettivo si andranno a sfruttare le particolari proprietà ottiche dei cristalli fluoruri drogati con ioni di terre rare. L’idea di fondo è quella di sfruttare questi cristalli come adattatori tra lo spettro solare e la massima efficienza della cella fotovoltaica. Nel caso specifico questi cristalli sono dei convertitori spettrali nel senso che hanno la proprietà di poter trasformare fotoni con un certa lunghezza d’onda in fotoni con un’altra lunghezza d’onda. Nel dettaglio tale peculiarità si indica con la proprietà del materiale di assorbire fotoni in un spettro di frequenze per poi ottenere una emissione della radiazione in un altra regione spettrale diversa da quella di assorbimento tramite meccanismi di trasferimento di energia interbanda tra ioni terre rare. Questi trasferimenti interbanda sono possibili grazie all’interazione degli ioni con il campo cristallino che è parzialmente schermato. Questo schermaggio è da attribuire ai gusci più esterni degli ioni i quali creano tale effetto sugli elettroni di valenza. Quindi il campo cristallino agisce come una perturbazione e causa la rottura di simmetria del sistema rendendo accessibili transizioni di questo tipo. La tesi è incentrata su una prima parte dedicata allo studio dei meccanismi di conversione della radiazione da parte dei cristalli. Saranno oggetto di studio tre meccanismi di conversione della radiazione. La Downconversion è un meccanismo di conversione che permette l’assorbimento di un fotone con certa energia di cui ne consegue l’emissione di due fotoni a più bassa energia. Il cristallo studiato che sfrutta questo meccanismo di trasferimento per la conversione della radiazione è il bario-yttrio-fluoro(BaY2F8) drogato con ioni di praseodimio e ytterbio. Il secondo meccanismo di conversione è chiamato Downshift è permette la conversione di un fotone in uno meno energetico e in questo caso il cristallo oggetto di studio è BaY2F8 drogato con solo ioni praseodimio. Infine il terzo meccanismo è l’Upconversion il quale consente di assorbire due fotoni e avere una conseguente emissione di un solo fotone più energetico. Il cristallo studiato in questa tesi che converte la radiazione tramite questo fenomeno è il BaY2F8 drogato con erbio. I campioni sono state effettuate misure spettroscopiche di assorbimento e di fluorescenza nell’intento di trovare, al variare di alcune concentrazioni di drogaggio, quelli che hanno la maggiore conversione di radiazione. La seconda parte è dedicata alla misure di efficienza di conversione dei dispositivi cella con cristallo convertitore in opportune configurazioni. Nel caso dell’Upconversion è stata studiata anche l’efficienza di conversione da parte di micro-cristalli

Investigation of Yb-doped LiLuF4 single crystals for optical cooling

Optical cooling of solids, relying on annihilation of lattice phonons via anti-Stokes fluorescence, is an emerging technology that is rapidly advancing. The development of high-quality Yb-doped fluoride single crystals definitely led to cryogenic and sub-100-K operations, and the potential for further improvements has not been exhausted by far. Among fluorides, so far the best results have been achieved with Yb-doped LiYF4 (YLF) single crystals, with a record cooling to 91 K of a stand-alone YLF:10%Yb. We report on preliminary investigation of optical cooling of an LiLuF4 (LLF) single crystal, an isomorph of YLF where yttrium is replaced by lutetium. Different samples of 5% Yb-doped LLF single crystals have been grown and optically characterized. Optical cooling was observed by exciting the Yb transition in single-pass at 1025 nm and the cooling efficiency curve has been measured detecting the heating/cooling temperature change as a function of pumping laser frequency

Characterizing the Sardinia candidate site for the Einstein Telescope

Due to its unique geophysical features and to the low density population of the area, Sos Enattos is a promising candidate site to host the Einstein Telescope (ET), the third-generation Gravitational Wave Observatory. The geophysical characterization of the Sos Enattos former mine, close to one of the proposed ET corners, started in 2010 with the deployment of seismic and environmental sensors underground. Since 2019 a new extensive array of seismometers, magnetometers and acoustic sensors have been installed in three stations along the underground tunnels, with one additional station at the surface. Beside a new geological survey over a wider area, two boreholes about 270 m deep each were excavated at the other two corners, determining the good quality of the drilled granite and orthogneiss rocks and the absence of significant thoroughgoing fault zones. These boreholes are instrumented with broadband seismometers that revealed an outstanding low level of vibrational noise in the low-frequency band of ET-LF (2-10Hz), significantly lower than the Peterson's NLNM and resulting among the quietest seismic stations in the world in that frequency band. The low seismic background and the reduced number of seismic glitches ensure that just a moderated Newtonian noise subtraction would be needed to achieve the ET target sensitivity. Geoelectrical and active seismic campaigns have been carried out to reveal the features of the subsurface, revealing the presence of small-sized fractured areas with limited water circulation. Finally, temporary arrays of seismometers, magnetometers and acoustic sensors are deployed in the area to study the local sources of environmental noise

An increasing cooling efficiency in fluoride crystals co-doped Yb-Tm

Anti-Stokes shift, provided by embedded rare earth (RE) ions in fluoride crystal host, is used to obtain a net cooling of a solid system. Yb3+ is currently the RE ion that presents the best cooling performance when inserted into a suitable host, like yttrium lithium fluoride (YLF). Recently, a new approach to laser cooling has been proposed, in which an enhancement of the cooling efficiency is reached by co-doping with Yb3+ and Tm3+. In this work, we compare, in terms of cooling efficiency, several samples doped with Yb and co-doped Yb-Tm. They were grown by employing the Czochralski technique. This specific technique allows to fabricating defect-free and large samples for such application. Samples possessing the same chemical composition of impurity and defect-free were compared denoting the goodness of the Tm-enrichment for the optical refrigeration. We demonstrate that the presence of a small concentration of Tm ions permits to decrease the background absorption and to increase the cooling efficiency. So far, a sample reached the important results of 4.2% in cooling efficiency overcoming the 3% barrier

Czochralski crystal growth for laser cooling

In laser cooling of crystals in solid-state physics, it is really important to obtain crystals with a large size at a relatively fast growth rate and high-optical quality that is defect-free. To get that, one of the methods to grow crystals is the Czochralski technique. The Czochralski technique will be presented and, in particular, the furnaces in New Materials for Laser Applications Laboratories of Pisa for this application will be discussed. Afterward the parameters for the growth of crystal fluorides are depicted and it is shown how these parameters lead to build samples suitable for optical cooling. All processes that are necessary to avoid contamination inside crystals like OH-ion and how to avoid reduction of Yb3+to Yb2+will be given. Spectroscopy of all samples will be treated in order to obtain the cooling parameters λf and αb for each sample. Afterward, an efficiency model will be discussed and the data efficiency of cooling obtained by a sample's own crystals will be shown

Co-doping of LiYF4crystal: A virtuous effect of cooling efficiency

Anti-Stokes shift, provided by embedded rare earth (RE) ions in fluoride crystal host, is used to obtain a net cooling of a solid system. Yb3+is currently the RE ion that presents the best cooling performance when inserted into a suitable host, like yttrium lithium fluoride (YLF). Recently, a new approach to laser cooling has been proposed, in which an enhancement of the cooling efficiency is reached by co-doping with Yb3+and Tm3+. In this work, we compare, in terms of cooling efficiency, two samples 5%Yb:YLF and 5%Yb-0.0080%Tm:YLF, grown with the same starting material in order to avoid the difference in chemical composition of impurity. Some contaminants, like iron, are very detrimental for cooling efficiency and they make it difficult to compare grown crystals with different raw powders. We demonstrate that the presence of a small concentration of Tm3+ions permits it to decrease the background absorption and to increase the cooling efficiency

Comparative Analysis Of Upconversion Efficiencies In Fluoride Materials For Photovoltaic Application

Infrared to visible upconversion (UC) is a promising way to enhance the efficiency of silicon based solar cells. In this paper, the spectral conversion and recovery of sub-band gap photons of the solar spectrum, from NIR-IR to the VIS-NIR wavelength region, is investigated in two fluorides hosts doped with trivalent erbium ions (Er3+). The efficiency gain due to upconversion in silicon solar cells is compared for single crystal samples of BaY2F8:Er3+ and LiYF4:Er3+ in a dedicated upconverter solar cell device (UCSCD) with monochromatic excitation in the 1.5 µm spectral region. The highest external quantum efficiency due to upconversion was found for the UCSCD using the BaY2F8:30 at% Er3+ single crystal, reaching an EQE of 6.8±0.2% for (1.10±0.12)·105 W m−2 spectral irradiance at 1494 nm. We present a comprehensive spectroscopic study of the crystal samples also taking into account the effects of the different crystal symmetry as well as the different phonon energies. Our findings enable us to explain the higher efficiency of the BaY2F8:Er3+ compared to the LiYF4:Er3+ upconverter in terms of both static and dynamic properties

Operation of a fiber-coupled laser-cooler down to cryogenic temperatures

International audienceWe report on the optical cooling of a 7.5%Yb:LiYF4 crystal down to 125 K in a multi-pass Herriott absorption cell, coupled via a single mode polarization maintaining optical fiber to the laser source. This configuration, never exploited before, is more practical for potential applications, in particular for spaceborne cryogenics setups. Moreover, the temperature reached is exactly the one needed in many setup embarked in small and medium satellites. We evaluate the heat load on the crystal at the minimum attainable temperature, which is comparable to state of the art systems

Crystal growth, spectroscopic characterization, and sub-100 femtosecond mode-locked operation of a Yb:LiLuF4 laser

We present a comprehensive growth, spectroscopic, and laser performance investigation of a Yb:LiLuF4 crystal, including CW laser experiments, and, for the first time to the best of our knowledge, mode-locked laser operation. Pumping with two 400 mW, polarization-combined, single-mode fiber-coupled laser diodes, we demonstrated efficient CW emission with 140 mW maximum output power and up to 59% slope efficiency. Using a semiconductor saturable absorber mirror for inducing the mode-locked regime and a pair of fused silica prisms for intracavity group delay dispersion compensation, we obtained soliton mode-locking pulses with 90 fs duration for the emission with polarization parallel to the crystal c-axis and 113 fs for the orthogonal polarization, with average power exceeding 30 mW in both cases at about a 90 MHz repetition rate

Comparative analysis of upconversion efficiencies in fluoride materials for photovoltaic application

Infrared to visible upconversion (UC) is a promising way to enhance the efficiency of silicon based solar cells. In this paper, the spectral conversion and recovery of sub-band gap photons of the solar spectrum, from NIR-IR to the VIS-NIR wavelength region, is investigated in two fluorides hosts doped with trivalent erbium ions (Er3+). The efficiency gain due to upconversion in silicon solar cells is compared for single crystal samples of BaY2F8:Er3+ and LiYF4:Er3+ in a dedicated upconverter solar cell device (UCSCD) with monochromatic excitation in the 1.5 µm spectral region. The highest external quantum efficiency due to upconversion was found for the UCSCD using the BaY2F8:30 at% Er3+ single crystal, reaching an EQE of 6.8±0.2% for (1.10±0.12)·105 W m−2 spectral irradiance at 1494 nm. We present a comprehensive spectroscopic study of the crystal samples also taking into account the effects of the different crystal symmetry as well as the different phonon energies. Our findings enable us to explain the higher efficiency of the BaY2F8:Er3+ compared to the LiYF4:Er3+ upconverter in terms of both static and dynamic properties