Effect of Capping-Agents on Performance of Copper Nickel Tin Sulfide (CNTS) Nanocrystals Based Solar Cells

Abstract

In recent years, colloidal semiconductor nanocrystals based solar cells have achieved photo-conversion efficiency closed to 12% which has triggered renewed interest to undertake fundamental investigation on these materials. Among many prospective candidates, quaternary chalcogenides viz. Copper Zinc Tin Sulfide/Selenides (CZTS) and its analogs have been tested extensively due to its high absorption coefficient near 1.1-1.5eV, low elemental cost, non-toxic nature and ability to form alloy structures, without apparent phase segregation. All these characteristics make them very attractive candidate to investigate them for solar cell applications. Their composition dependent band-edge parameters give further opportunity to form tandem heterojunction devices which would absorb broad spectrum of light and give the possibility to push the performance towards betterment. In many such investigation the capping agents used in the preparation of nanocrystals have assumed to be mute candidate in performance of the solar cells. However, our preliminary investigation on copper nickel tin sulfide (CNTS) nanocrystals based solar cell revealed that the nature of capping agents is also an important parameter and impact the solar cell performance. Herein, we report the preparation of CNTS nanocrystals and demonstrated the effect of capping agents on the performance of solar cells. To prepare CNTS nanocrystals, hot-injection method have been used. Two variants have been prepared by varying capping agents; long-chain oleylamine capped CNTS (OLA-CNTS) and short chain Mercaptopropionic acid (MPA) capped CNTS (MPA-CNTS). These were well characterized by UV-Vis-NIR, XRD, Raman, AFM, TEM, FESEM, and EDAX. All these characterizations confirm the formation single phase CNTS. Cyclic voltammetry investigation have been performed on CNTS nanocrystals to estimate the conduction and valence band edge parameters as a function of composition. For the first time, CNTS liquid junction solar cell with an architecture; FTO/TiO2/CdS-CNTS/ZnS/S-/Sn--/CuS/Se/FTO have been prepared. Solar cell prepared out of OLA-CNTS gave very poor power conversion efficiency. While, similar solar cell constructed out of MPA-CNTS displayed better performance having the efficiency and the fill factor in the order of 1.25 % and 0.380, respectively. Furthermore, an electrochemical impedance spectroscopy (EES) measurements revealed that MPA-CNTS has relatively low charge transfer resistance than OLA-CNTS. This property has been attributed to the poor performance of OLA-CNTS. Further investigations to optimize this material and device architecture would expected to increase the efficiency. Figure 1