Effects of CuIn 0,5 Ga 0,5 Se 2 growth by isothermal and bithermal Cu-Poor/Rich/Poor sequence on solar cells properties
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1165-M02-05
Effects of CuIn0,5Ga0,5Se2 growth by isothermal and bithermal Cu-Poor/Rich/Poor sequence on solar cells properties Hakim Marko1, 2, Adam Hultqvist3, Charlotte Platzer-Björkman3, S. Noël2 and John Kessler1 1
Institut des Matériaux Jean Rouxel (IMN), Nantes University, CNRS, 2, rue de la Houssinière, BP 32229, F-44322 Nantes Cedex 3, France. 2 CEA, LITEN, LCRE F-38054 Grenoble, France 3 Ångström Solar Center, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden ABSTRACT Co-evaporated CuIn0,5Ga0,5Se2 thin film solar cells were grown using a sequential CuPoor/Rich/Poor process (CUPRO). During the growth process, the substrate temperature was either kept constant at 570 °C (iso-CUPRO) or decreased during the first step to either 360 or 430 or 500 °C (bi-CUPRO). According to atomic force microscopy (AFM) measurements, the lower the temperature is in the first step the smoother the final CIGS surface becomes. By decreasing the first step temperature, cross-section scanning electron microscopy (SEM) and θ2θ x-ray diffraction (XRD) do not reveal clearly any important changes of morphology and crystallographic preferred orientation. SLG/Mo/CIGS/Buffer layer/i-ZnO/ZnO:Al/grid(Ni/Al/Ni) solar cells with either a chemical bath deposited CdS or an atomic layer deposited Zn(O,S) buffer layer were fabricated. For both buffer layers, the bi-CUPRO processes lead to higher efficiencies. Besides, using Zn(O,S), the electronic collection was improved for the infrared spectrum as well as for the ultraviolet spectrum. This resulted in efficiencies close to 14,5 % for the Zn(O,S) cells. INTRODUCTION Chalcopyrite Cu(In1-xGax)Se2 (CIGS) has an increasing bandgap (Eg) with x as [Ga]/([Ga]+[In]), from 1,04 eV (CIS) to 1,65 eV (CGS). [1,2]. With a gallium content of 30%, leading to Eg~1,2 eV, CIGS-based thin film solar cells have reached energy conversion efficiencies close to 20 % [3]. Even higher efficiencies are expected with larger bandgap CIGS, i.e. Eg > 1,2 eV, thanks to a better matching to the solar spectrum [4]. Also, larger bandgap CIGS leads to higher voltages and lower currents, which is beneficial for the module design. Finally low bandgap CIS and wide bandgap CGS could be integrated in a tandem structure, leading theoretically to efficiencies close to 40 % [5]. Up until now, most experimental attempts using higher Ga contents (> 30%) have led to lower efficiencies [6]. Different assumptions try to explain these suboptimal electrical properties: unfavorable conduction band matching at the interface with the commonly used CBD-CdS buffer layer for the junction formation [7], higher defect densities [8], more efficient recombination center [9] for high Ga-contents or a different dominant recombination mechanism [10]. In this contribution, wide bandgap CIGS with a Ga-content ~ 0,47 has been deposited using the co-evaporation technique with a Cu-Poor/Rich/Poor sequence (CUPRO) [11]. In the previous approaches, the substrate temperature was kept constant throughout the growth, we now call this an isothermal CUPRO (iso-CUP
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