Synthesis of FeS 2 Nano Crystals for Ink-Based Solar Cells
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Synthesis of FeS2 Nano Crystals for Ink-Based Solar Cells Lakshmi Kanth Ganta, Tara P. Dhakal, Surya Rajendran and Charles R. Westgate Center for Autonomous Solar Power (CASP), Binghamton University, Binghamton, NY 13902, U.S.A.
ABSTRACT Although pyrite (FeS2) is abundant, getting a single-phase pyrite thin film is difficult due to the coexistence of various phases of iron and sulfur in nature. We propose an ink-based process for attaining the pyrite phase of iron sulfide. This work involves degassing Iron (II) chloride in an octadecylamine solution and later reflux with addition of sulfur in diphenyl ether at 200°C. The process yielded phase-pure single crystalline pyrite nanocrystals which were later cleaned and dispersed in chloroform for uniform suspension. Thus obtained nanocrystals were deposited as thin films using drop casting and spin coating. Solar cells were fabricated using CdS as an n-type window layer in a superstrate configuration. When tested, the superstrate type FeS2 nanoparticle cell showed 0.03% with high Voc of 565 mV.
INTRODUCTION FeS2 has been a promising material for thin-film solar cells. With a band gap of 0.95 eV and an absorption coefficient higher than 105 cm-1 [1, 2], FeS2 is an ideal p-type hetero-junction partner for a thin-film solar cell. It is particularly interesting since it consists of a non-toxic and abundant element and can be prepared in the form of single crystals and thin films. It crystallizes in the pyrite structure and is an abundantly occurring material in the earth’s crust. With a quantum efficiency of around 90% and a near ideal band gap, it’s an absorber of interest for thin film photovoltaics [3]. While various methods have been proposed for growth of FeS2 thin films, each has its own shortcomings, such as phase variations and uniformity issues [4]. Hence different methods of producing the films at low temperatures are necessary steps to obtain single-phase pyrite and avoid the formation of impurity phases. One of the best ways to produce phase-pure pyrite nanocrystals is by chemical method [5, 6, 7, 8], which uses simple non-toxic chemicals to obtain nanocrystals which are phase-pure and can be later used to fabricate thin films. The temperatures at which these crystals are produced usually don’t exceed 300°C, and they can be deposited on substrates using drop casting or spin coating or inkjet printing which gives it the versatility to be deposited on any kind of substrates. In this paper, we report the growth of FeS2 nanocrystal ink using a similar chemical reflux method. We also report some preliminary results on the dark and light I-V characteristics of the FeS2/CdS solar cell in superstrate configuration.
EXPERIMENTAL DETAILS The process of producing sulfide nanoparticles is based on reactants like metal chloride and sulfur and solvents like Oleylamine, Octadecylamine, and Diphenyl ether. A typical synthesis of our pyrite nanoparticle ink begins by degassing Iron (II) chloride (FeCl2) in an Octadecylamine solution for one hour at 100°C in a three-neck vesse
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