Boosting the current density in inverted Schottky PbS quantum dot solar cells with conjugated electrolyte Van-Tuan Mai a,b , Ngoc-Huyen Duong a , Xuan-Dung Mai c,d,⇑ a School of Engineering Physics, Hanoi University of Science and Technology, 1 Dai Co Viet, Hanoi, Viet Nam b Department of Natural Sciences, Electric Power University, 235 Hoang Quoc Viet, Hanoi, Viet Nam c Department of Chemistry, Hanoi Pedagogical University 2, 32 Nguyen Van Linh, Phuc Yen, Vinh Phuc, Viet Nam d Institute of Research and Development, Duy Tan University, 3 Quang Trung, Da Nang, Viet Nam article info Article history: Received 28 March 2019 Received in revised form 1 April 2019 Accepted 16 April 2019 Available online 17 April 2019 Keywords: Schottky Solar cells Quantum dots Electrolyte Interfaces abstract Herein, we correlate the chemical structure of the electrolyte with the performance of inverted Schottky quantum dot (QD) solar cells (SCs) having a structure of FTO/electrolyte/p-type PbS QDs/MoO x /Au-Ag. QDSCs of polyethyleneimine (PEI) or poly[(9,9-bis(3 0 -(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-1,4 -phenylene] (PFN) were fabricated for comparison. The open-circuit voltage (V OC ) of QDSCs scaled with the workfunction of electrolyte – modified fluorine-doped tin oxide (FTO). Conjugated PFN electrolyte resulted in lower V OC but it boosted the current density (J SC ) of QDSCs by lowering the interfacial potential barrier at FTO-PbS QDs contact. Ó 2019 Published by Elsevier B.V. 1. Introduction Lead sulfide (PbS) QDs have been deployed in various solar cell configurations including Schottky junction, p-n heterojunction (BJH), n-i-p multiple junction, and inverted Schottky QDSCs. Nor- mal Schottky QDSCs, e.g. ITO (indium tin oxide)/p-type PbS QDs/ LiF/Al, operate based on a back Schottky contact formed between a p-type PbS QD layer and a low-workfunction (WF) anode [1,2]. Despite simple fabrication Schottky QDSCs have emerged some limitations including 1) low air-stability [3], 2) V OC deficiency [4], 3) and inefficient carrier extraction [1], and 4) poor harvesting short-wavelength light. These limitations can be overcome by using an n-type, transparent metal oxide layer to create a front junction with the p-type PbS QD layer. As a result, the power con- version efficiency (PCE) of QDSCs has increased over 10% [5]. How- ever, these high performance QDSCs requires sophisticatedly engineered oxide-QD interfaces [5–7]. Lately, metal oxide-free, inverted Schottky QDSCs have been developed [8]. In this novel structure, PEI was used to perform low-workfunction FTO (L-FTO) that creates a front junction with p-type PbS QD layer. The electrolyte layer also results in an undes- ignable barrier that inhibits electron injection from the QD layer into FTO conduit. Balancing the WF-reduction affinity and the bar- rier height of electrolyte is critical to improve the cells’ perfor- mance. Herein, we used PEI and PFN as the interfacial electrolyte to correlate the chemical structure of electrolyte to the cells’ per- formance. PFN resulted in higher short-circuit current density (J SC ) and fill-factor (FF) but a lower open-circuit voltage (V OC ). 2. Experimental section. The synthesis of oleic acid capped PbS QDs (OA-QDs) was car- ried out by using the procedure reported previously [8]. To passi- vate OA-QDs with Cl À , a solution of tetrabutylammonium chloride 0.1 M in ethanol was used in the washing process. To fab- ricate L-FTO substrates, solutions of PEI or PFN in methoxyl- methanol (0.2% by weight) were spin-coated on freshly cleaned FTO. p-type PbS QD films on L-FTO substrates were performed by a layer-by-layer method using 1,2-ethanedithiol (EDT) as the new ligand. MoO x , Au and Ag layers were thermally deposited atop the QD layer through a shadow mask. The WF of substrates was measured by Kelvin probe. JV curves of solar cells were conducted on Keithley 2400 source meter while an Oriel solar simulator oper- ating at 100 mW.cm À2 was used as the light source. 3. Results and discussion The OA-QDs of different sizes with an optical ranging from 1.1 to 1.87 eV were prepared, Fig. S1 (SI: Supporting Information). https://doi.org/10.1016/j.matlet.2019.04.067 0167-577X/Ó 2019 Published by Elsevier B.V. ⇑ Corresponding author. E-mail address: xdmai@hpu2.edu.vn (X.-D. Mai). Materials Letters 249 (2019) 37–40 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue