Doped and multi-compound ZnO-based transparent conducting oxides for silicon thin film solar cells

Abstract

The objective of the present work is to provide a better understanding of magnetron sputtered transparent conducting oxides based on ZnO in order to use them as electrodes in thin film silicon solar cells at the Grup d'Energia Solar of the Universitat de Barcelona. This thesis presents the properties of magnetron sputtered aluminium and gallium doped ZnO as well as the properties of multi-compound materials deposited by the co-sputtering of zinc oxide and indium tin oxide. The application of ZnO based transparent conducting oxides to the back reflector of pin amorphous solar cells is also discussed.

A set of aluminium doped zinc oxide layers were deposited under different substrate temperature and discharge power. The structural, electrical and optical properties were characterised and discussed. The higher substrate temperatures and discharge powers used during deposition led to highly transparent layers in the visible range with lower resistivities. The polycrystalline layers were oriented with the c-axis perpendicular to the substrate surface and the crystalline quality of the layers improved at higher temperatures and powers. A remarkable increase in mobility was found for temperatures above 300°C and the carrier concentration also rose with temperature reaching 3.71x1020 cm-3 at 420°C. The most remarkable feature found at higher deposition power was the increase in deposition rate (from 0.9 to 9 nm/min).

By means of a high temperature (650°C) annealing process under a capping layer of silicon or alumina, the mobility of aluminium doped zinc oxide layers was considerably raised achieving 68.5 cm2V-1s-1. This process led also to more transparent layers in the near infrared as well as in the ultraviolet part of the spectrum.

Gallium doped zinc oxide films were deposited in order to investigate the suitability of gallium as a dopant in zinc oxide layers. Highly transparent layers with higher carrier concentrations but lower mobilities compared to aluminium doped zinc oxide were obtained. The dependence of the layer properties on the pressure, doping concentration, substrate temperature and oxygen volume concentration during deposition were studied in order to find the adequate layer to be applied as electrode in thin film solar cells. 4 wt.% Ga2O3 doping concentration was found to be optimal for the production of highly conductive ZnO:Ga layers with a high band gap energy. The incorporation of oxygen gas during the sputter deposition led to more transparent layers at wavelengths longer than 1100 nm, but was found to be detrimental for the electrical properties of the studied layers.

By means of co-sputtering, a set of multi-compound layers formed by Zn-In-Sn-O were deposited and carefully characterised. The resulting layers were studied as a function of the Zn content ratio, which varied between 17.1 to 67.3%. The layers were amorphous in nature but presented embedded nanometric crystals. The incorporation of Zn cations into an indium tin oxide matrix favoured the transmittance but did not modify the mobility. The carrier concentration was found to decrease resulting in an increase in resistivity. The electronic band structure was investigated by means of photoelectron spectroscopy. The measurements showed that, with an increase in Zn concentration, the oxygen vacancy concentration of the surface increased resulting in a degenerately n-doped surface layer. The work function of the material was determined by low intensity X-ray photoelectron spectroscopy and the values varied between 4.7 and 4.3 eV with the variation of Zn content.

The final experiments were focussed on the application of ZnO layers in the back reflectors of pin amorphous silicon solar cells. Trials were performed onto pin structures deposited at T-Solar Global SA and the Universitat de Barcelona.

The cells deposited at T-Solar were long exposed to air before a back reflector could be deposited and evidence for the formation of a thin silicon oxide layer at the interface was obtained. The oxide layer was removed using acid etching in dilute HF, but an analysis by X-ray photoelectron spectroscopy showed that the cleaning step resulted in an important amount of carbon contamination on the surface. Both, the silicon oxide and the carbon layer led to devices with S-shaped J-V curves. Later, the n-type interface was protected by a thin ZnO:Al layer at T-Solar to avoid oxidation during transportation. However, the existence of this thin ZnO:Al protective layer determined the growth of the subsequently deposited layers. Thus, the deposition of ZnO layers under different conditions led to similar results. Finally, different back reflectors were tried over the solar cells fabricated at UB. Aluminium and gallium doped zinc oxide layers were deposited on amorphous silicon pin structures, and a clear improvement in performance with respect to devices with only a metal layer as back reflector was observed. Similar performances were observed when Ga doped ZnO or Al doped ZnO was used in the back reflector. It showed that both gallium and aluminium were suitable dopants for the ZnO to be applied in the back reflector.

L'objectiu d'aquest treball rau en l'estudi i optimització dels òxids conductors transparents basats en l'òxid de zinc. Aquests materials, que s'han dipositat mitjançant polvorització catòdica magnetró, es van estudiar amb la finalitat d'emprar-los com elèctrodes en cèl•lules solars de silici en capa prima al Grup d'Energia Solar de la Universitat de Barcelona. En aquesta tesi es presenten les propietats de l'òxid de zinc dopat amb alumini o amb gal•li, així com les propietats de multi-compostos dipositats a partir de la co-polvorització catòdica d'òxid de zinc i d'òxid d'indi dopat amb estany. També es discuteix l'aplicació d'òxids conductors transparents basats en l'òxid de zinc al reflector posterior de cèl•lules solars de silici amorf amb estructura tipus pin. S’ha trobat que l’òxid de zinc dopat amb alumini, a altes temperatures de substrat i altes potències, presenta una elevada transmitància òptica i una baixa resistivitat. La mobilitat de les capes augmentà considerablement fins assolir 68.5 cm2V-1s-1 mitjançant l'aplicació de tractaments tèrmics a alta temperatura previ dipòsit d'una capa protectora de silici amorf o d'alúmina. Pel cas de l'òxid de zinc dopat amb gal•li s'han obtingut una sèrie de capes altament transparents i amb concentracions de portadors superiors, però amb mobilitats inferiors que les de capes dopades amb alumini. Mitjançant co-polvorització d’òxid de zinc i òxid d’indi dopat amb estany s’han obtingut capes amorfes del multi-compost Zn-In-Sn-O amb un contingut de zinc que varia entre el 17.1 i el 67.3%. La incorporació del zinc a l'òxid d'indi dopat amb estany afavoreix l'increment de la transmitància, sense que la mobilitat de les capes es vegi afectada. En canvi, la concentració de portadors disminueix amb la incorporació de Zn. Comparant l'ús del gal•li i de l'alumini com a dopants de l'òxid de zinc del reflector posterior de cèl•lules de silici amorf tipus pin, s'ha observat una gran similitud en el comportament dels dispositius. Això ens ha portat a la conclusió que ambdós materials són adients per ser emprats com a reflectors posteriors.