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Novel light management techniques for thin film solar cells: Nanotextured substrates and transparent conducting upconverters

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Universitat de Barcelona. Departament de Física Aplicada i Òptica

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[eng]  The objective of this work was to study two different light management approaches to enhance the efficiency of thin film Si solar cells and these were the manipulation of the light path (light trapping) and changing the incoming photon energy (upconversion). In the first approach the light path was manipulated by creating either periodic or random textured interfaces. Periodic patterns were created at the front AZO by means of direct laser ablation. Amongst all the patterns assessed, the best result was achieved with a linear pattern of 10 lam of pitch and 360 nm of groove depth, that yielded to an Rs of 11 SI/sq and a haze of 12.7% at 600 nm. However structures in the sub-micrometer range cannot be created because the minimum period is limited by the laser spot. By means of the Aluminum Induced Texturing method (AIT) random textures were performed on glass substrates. In this method, a thin Al film is deposited onto a glass substrate and a redox reaction between the Al and the SiO2 of the glass is induced by high temperature annealing. The reaction products are wet-etched and the result is a uniform and rough glass surface. The process parameters were varied in order to control the resultant glass roughness and it was found that the most critical was the Al deposition method. By using evaporation smooth U-shaped craters morphology and roughness up to 90 nm were created, whereas the sputtered films resulted in rough and porous textures with roughness until 145 nm. AZO grown over the U-shape crater morphology led to a double texture with haze values above 10% at 600 nm, transparency above 84%, and Rs-7 SI/sq whereas AZO over very rough glass resulted in a cauliflower-like surface with haze values >32% at 600 nm, Rs around 9.5 SI/sq and transmittance of 74%. A-Si:H solar cells were deposited on different AIT textures and an improvement of the short circuit current, as well as a reduction of the device reflectivity was achieved in all cases in comparison to the cells deposited on smooth glass textures. The second approach was to create a transparent and conducting upconverter to be used on top of the rear reflector of a thin film Si solar cell. For that purpose, ZnO was doped with Er and Yb ions and was post-annealed under different treatments. The unique spectral properties of rare earth (RE) elements due to their electronic configuration occur as a result of their intra 4f-4f shell transitions. In the case of Er, its excitation takes places at 1500 nm and 980 nm and the upconverted photons are emitted within the Si absorption range. Moreover, codoping with Yb can enhance the Er visible emission because they cooperate together due to the matching of their energy levels for k=980 nm. As deposited ZnO doped with rare earths (RE) was found to be transparent and conducting but not luminescent. RE ions need to be surrounded by 6 oxygen in a distorted octahedron to be optically active and REs replacing zinc in the ZnO lattice do not present this symmetry; hence, a post deposition treatment is needed. When the films were post-annealed in air, visible upconversion (UC) was seen at 660 nm under 980 nm laser excitation, however, the films become almost insulating. When the films were annealed in vacuum, lower UC luminescence was achieved, and the resistivity increased 1 order of magnitude. By using CW laser radiation, the electrical properties were maintained and high UC was observed. UC came from clusters of RE06 as well as from RE203 inside or outside the matrix. When annealing in air, in vacuum or by laser radiation, oxygen from the atmosphere bound to the RE to form RE oxides and/or RE06 complexes but just laser annealing was able to preserve the conductivity while producing optically active centers.

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[cat]   	
L'objectiu d'aquesta tesi és la millora de l'eficiència de les cèl•lules solars de silici en capa prima mitjançant l'estudi de nous mètodes per a l'aprofitament de la llum solar al dispositiu. El primer mètode consisteix en texturar el substrat de vidre per dispersar la llum incident i així incrementar l'absorció en la capa activa. El mètode emprat es la texturització induïda per alumini (AIT); que es basa en una reacció de reducció no uniforme entre el vidre i una capa prima d'alumini gràcies a un tractament tèrmic. Posteriorment els productes de la reacció s'eliminen mitjançant una solució basada en àcid i el resultat és un vidre transparent i texturat. S'ha fet un estudi de la rugositat en funció dels paràmetres del procés i s'ha aconseguit obtenir rugositats controlades i uniformes en superfícies de fins a 10x 10 cm2. Diferents textures s'han provat en cèl.lules solars de silici amorf i s'ha demostrat l'eficàcia d'aquestes en la millora del corrent respecte a les mateixes cèl•lules dipositades sobre vidre pla. El segon mètode estudiat és el fenomen de l'up-conversion que consisteix en la conversió de fotons de baixa energia (EEg) que podran ser absorbits en la zona activa; així doncs s'ha intentat fer una capa conductora, transparent i amb propietats d'up-conversion per utilitzar com a contacte per a cèl• lules solars. Per aquest propòsit s'han estudiat capes conductores i transparents d'òxid de zinc dopat amb erbi i iterbi dipositades per polvorització catòdica sobre vidre. Com que les terres rares han d'estar envoltades d'oxigen per actuar com a centres òptics actius, i en les capes de ZnO:Er:Yb no es dóna aquesta configuració, les capes s'han hagut de sotmetre a diferents tractaments tèrmics, com escalfament en aire, en buit o escalfament amb làser. Escalfant en aire o en buit s'aconsegueix obtenir up-conversion però la conductivitat disminueix notablement, en canvi, escalfant amb radiació làser es possible de mantenir les propietats elèctriques i a més, activar òpticament les terres rares.

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Tesis Doctorals - Departament - Física Aplicada i Òptica

# metadata.dc.title[].value
Novel light management techniques for thin film solar cells: Nanotextured substrates and transparent conducting upconverters