A novel industrial thin film deposition technology for sustainable CdTe photovoltaics
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A novel industrial thin film deposition technology for sustainable CdTe photovoltaics
P.Nozar3,*, G. Mittica1, S. Milita4, C. Albonetti3, F. Corticelli4, A. Brillante5, I. Bilotti5, G. Tedeschi1 and C. Taliani3,1,2 1
Siena Solar Nanotech, S.p.A., Piazza dell'Abbadia, 4, 53100 Siena, Italy,
2
Organic Spintronics, S.r.l., via Gobetti 52/2, 40129 Bologna, Italy,
3
ISMN-CNR, via Gobetti, 101, 40129 Bologna, Italy,
4
IMM-CNR, via Gobetti, 101, 40129 Bologna, Italy,
5
Dip. di Chim. Fis. e Inorg., Univ. di Bologna, Viale del Risorgimento, 4, 40136 Bologna, Italy
ABSTRACT CdTe and CdS are emerging as the most promising materials for thin film photovoltaics in the quest of the achievement of grid parity. The major challenge for the advancement of grid parity is the achievement of high quality at the same time as low fabrication cost. The present paper reports the results of the new deposition technique, Pulsed Plasma Deposition (PPD), for the growth of the CdTe layers on CdS/ZnO/quartz and quartz substrates. The PPD method allows to deposit at low temperature. The optical band gap of deposited layers is 1.50 eV, in perfect accord with the value reported in the literature for the crystalline cubic phase of the CdTe. The films are highly crystalline with a predominant cubic phase, a random orientation of the grains of the film and have an extremely low surface roughness of 4.6±0.7 nm r.m.s.. The low roughness, compared to traditional thermal deposition methods (close space sublimation and vapour transport) permits the reduction of the active absorber and n-type semiconductor layers resulting in a dramatic reduction of material usage and the relative deposition issues like safety, deposition rate and ultimately cost INTRODUCTION Presently the dominant materials for thick solar cells realization are single crystalline and polycrystalline silicon. The disadvantage of these materials is their indirect optical band gap, which requires large thickness of the high purity active layer (hundreds of micrometers) to assure sufficiently high efficiency of the device. The large thickness of the silicon layer and the large energy required to produce the desired degree of the material purity results into high consumption of the prime material and to high final cost of the resulting solar cell. Grid parity imperative requires the application of thin film solar cells which adopt active material layers exhibiting direct optical band gap like CdTe and CIGS, as the consequence of the direct optical band gap these materials require substantially thinner active layer for the total absorption of the incident light. CdTe is becoming the most successful contender for the realization of thin film solar cells due to its relative simplicity of the composition and the structural and
compositional stability. However, the preparation of this material requires high deposition temperature to achieve the crystalline CdTe and the CdTe/CdS mixture at the interface. Moreover the poor surface quality prevents the use of films thinner than 6 μm. Both t
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