Synthesis of (CuInS₂)_0.5(ZnS)_0.5 Alloy Nanocrystals and Their Use for the Fabrication of Solar Cells via Selenization

Abstract

As solar cell absorber materials, alloys of CuIn(S,Se) 2 and Zn(S,Se) provide an opportunity to reduce the usage of indium along with the ability to tune the band gap. Here we report successful synthesis of alloyed (CuInS 2 ) 0.5 (ZnS) 0.5 nanocrystals by a method that solely uses oleylamine as the liquid medium for synthesis. The reactive sintering of a thin film of these nanocrystals via selenization at 500 °C results in a uniform composition alloy (CuIn(S,Se) 2 ) 0.5 (Zn(S,Se)) 0.5 layer with micron size grains. Due to the large amount of zinc in the film, the sintered grains exhibit the zinc blende (sphalerite) structure instead of the usual chalcopyrite structure of CuIn(S,Se) 2 films. The use of the selenide films as a p-type absorber layer has yielded solar cells with total area power conversion efficiencies as high as 6.7% (7.4% based on active area). These preliminary results are encouraging and indicate that with further optimization this class of materials has promise as the absorber layer in solar cells.Chalcogenide materials have received significant attention as absorbers for thin film photovoltaic devices. Notably, materials such as CuIn x Ga 1-x Se 2 (CIGSe) 1-6 and CdTe 7 have been successfully used for this purpose and currently provide the most efficient thin-film solar cells. 8 However, recent efforts have focused on reducing the use of costly elements, such as indium and tellurium, in favor of more earth-abundant, lowcost alternatives. To this end, the Cu 2 ZnSn(S z Se 1-z ) 4 (CZTSSe) material system 9-14 has received increased attention for its potential role as a viable earth-abundant alternative to CIGSe and CdTe absorbers, though to-date this technology has been unable to reach the performance level of the best performing CIGSe and CdTe devices.An alternative technology that can be employed for reducing the use of indium in photovoltaic absorbers is the (CuInSe 2 ) y (ZnSe) 1-y (CZISe) material system, where ternary chalcopyrite CuInSe 2 is alloyed with binary cubic zinc blende ZnSe. This alloying has the potential to decrease the net use of indium in the absorber layer. In this alloy material system, the optical band gap of the absorber can be adjusted from 1.05 eV 15 to 3.7 eV 16 by controlling the degree of alloying between CuInSe 2 and ZnSe, as well as through partial or complete substitution of selenium with sulfur. Additionally, the structure of the CZISe alloy can be shifted from tetragonal chalcopyrite to cubic zinc blende by increasing the ratio of ZnSe to CuInSe 2 in the final film. 17 The simpler crystal structure could make the device processing for this material to be more robust. ZnSe is an n-type semiconductor, while CuInSe 2 is a p-type semiconductor; thus, control of ZnSe and CuInSe 2 alloying can potentially lead to control of the defect characteristics of the absorber layer. These attributes could allow for this material system to reach the performance of the best CZTSSe and CIGSSe devices. This material system has previously been utilized for a variety of semiconduct...

Colloidal Wurtzite Cu2SnS3 (CTS) Nanocrystals and Their Applications in Solar Cells

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CuZnInSe3‐based solar cells: Impact of copper concentration on vibrational and structural properties and device performance

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