Microwave-synthesized CZTS nanoparticles and their surface modification for the model hybrid photovoltaic devices

Abstract

Environmentally friendly, colloidal nanocrystals (NCs) are promising materials for the next-generation, hybrid organic-inorganic photovoltaic devices obtained by solution-based methods because of their low cost, solution processability, and facile bandgap tunability. In this work, Cu2ZnSnS4 nanocrystals (CZTS NCs) with a diameter of 5 nm and the mixed crystal structure, including the kesterite and wurtzite phases, were synthesized using a “green” microwave-assisted process. The as-synthesized CZTS NCs were characterized with XRD, TEM, UV–Vis spectroscopy, PL spectroscopy, EDS, AFT-IR, XPS, and Raman spectroscopy. The photovoltaic (PV) performance of hybrid devices was studied and it consisted of ITO/PEDOT:PSS/active layer/Al, where CZTS NCs, model polymer PCDTBT, and PC70BM as an acceptor of electrons were in the active layer. In order to improve the charge transport and hydrophobicity of CZTS NCs in hybrid solar cells, the surface of the NCs was modified with four selected aromatic molecules, e.g. 1-aminonaphthalene, o-phenylenediamine, 2-naphthalenethiol, and 2-naphtholic acid, respectively. The performance of the devices significantly depended on the type of the used target ligand for modification of the CZTS NCs and their concentration in the active layer. With a fixed concentration of NCs, the exchange of ligand from initial ethylenediamine, in the best case of o-phenylenediamine, results in a five-time boost in device efficiency. This optimization of the NC ligands also allows achieving c.a. 45 % relative increase in device efficiency compared to the reference system without NCs. The highest average photovoltaic efficiency of 2.82 % with the open circuit voltage (Voc) = 0.86 V, short circuit current density (Jsc) = 8.79 mA/cm2, and fill factor (FF) = 0.41 was noted.