Pd and Au Contacts to SnS: Thermodynamic Predictions and Annealing Study RAMYA L. GURUNATHAN, 1,3 JOSEPH NASR, 2 JACOB J. CORDELL, 1 RONA A. BANAI, 2 MICHAEL ABRAHAM, 1 KAYLA A. COOLEY, 1 MARK HORN, 2 and SUZANNE E. MOHNEY 1,4 1.—Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA. 2.—Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802, USA. 3.—e-mail: ramya1006@gmail.com. 4.—e-mail: mohney@ems.psu.edu Tin(II) sulfide (SnS) is emerging as an attractive p-type absorber layer material for thin film photovoltaics, which motivates the search for Ohmic, low-resistance contacts for SnS. In this study, Pd and Au contacts were pre- pared on sputter-deposited SnS films and electrically characterized both as- deposited and after annealing of the contacts. Ternary phase diagrams were also calculated to help predict whether the chosen metals would react with the SnS film. Pd was expected to react with the SnS film, while Au was expected to be in thermodynamic equilibrium. The Pd contacts appeared reactive, and their resistance was minimized with post-deposition annealing at 400°C, while the Au contacts showed little change upon annealing and remained unreac- tive. Key words: Tin sulfide, photovoltaics, contact resistance, phase equilibria INTRODUCTION Tin(II) sulfide (SnS) is a natively p-type semicon- ductor that shows promise as the absorber layer material in thin film photovoltaic (PV) devices due to its suitable direct bandgap energy of about 1.1– 1.5 eV, 1 high absorption coefficient of 10 5 cm 1 in the visible and infrared range, 2 and high free carrier concentration of 10 16 cm 3 . 2 Its composi- tion is based on non-toxic, earth-abundant elements in a simple stoichiometry, which could make it attractive for large-scale solar cell production and deployment. However, the highest efficiency yet achieved by an SnS solar cell is 4.4%, and a major hindrance is the high contact resistance and recombination loss at the back contact. 1 Most previously fabricated SnS-based solar cells have utilized Mo as a back contact, which is adapted from other thin film PV materials such as CuIn ð1xÞ Ga x Se 2 (CIGS). 3 How- ever, in Patel et al. 4 Mo contacts were found to be unsuitable, which was attributed to inhomo- geneities at the interface or the formation of the semiconductor MoS 2 . MoS 2 formation would block hole transport, which has already been diagnosed as an issue in Mo-contacted Cu 2 ZnSnS 4 (CZTS) solar cells. 5 A relatively small selection of contact metals to SnS have been studied up until now. Previous studies, for example, have reported Ohmic behavior for Al, 6 Zn, 7 and In 8 contacts, even though these metals have moderate work functions. Causes for these observations could include Fermi level pin- ning, diffusion of the metal into the SnS lattice, or the formation of conductive sulfides at the inter- face. 8 The In contacts, however, showed poor ther- mal stability due to the metal’s low melting temperature. Additionally, whereas Ghosh et al. 6 reported Schottky behavior for Ag contacts and inconclusive results with annealing, Devika et al. 8 reported Ohmic behavior over a narrow voltage range, which improved with annealing. To offer insights into new metal/SnS contacts, this study investigates Pd and Au contacts, with work func- tions of 5.40 and 5.28 eV, respectively. 9 High work (Received July 31, 2016; accepted October 7, 2016) Journal of ELECTRONIC MATERIALS DOI: 10.1007/s11664-016-5042-z Ó 2016 The Minerals, Metals & Materials Society