Elsevier

Materials Letters

Volume 249, 15 August 2019, Pages 37-40
Materials Letters

Boosting the current density in inverted Schottky PbS quantum dot solar cells with conjugated electrolyte

https://doi.org/10.1016/j.matlet.2019.04.067Get rights and content

Highlights

  • Inverted and normal quantum dot solar cells have been fabricated.

  • Non-conjugated PEI and conjugated PFN electrolytes have been examined.

  • In the inverted solar cells, the electrolyte suppresses Fermi level pinning.

  • PEI electrolyte results in higher open-circuit voltages.

  • PFN electrolyte enhances short-circuit current-density.

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/MoOx/Au-Ag. QDSCs of polyethyleneimine (PEI) or poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-1,4-phenylene] (PFN) were fabricated for comparison. The open-circuit voltage (VOC) of QDSCs scaled with the workfunction of electrolyte – modified fluorine-doped tin oxide (FTO). Conjugated PFN electrolyte resulted in lower VOC but it boosted the current density (JSC) of QDSCs by lowering the interfacial potential barrier at FTO-PbS QDs contact.

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. Normal 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) VOC 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 conversion efficiency (PCE) of QDSCs has increased over 10% [5]. However, these high performance QDSCs requires sophisticatedly engineered oxide-QD interfaces [5], [6], [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 undesignable barrier that inhibits electron injection from the QD layer into FTO conduit. Balancing the WF-reduction affinity and the barrier height of electrolyte is critical to improve the cells’ performance. Herein, we used PEI and PFN as the interfacial electrolyte to correlate the chemical structure of electrolyte to the cells’ performance. PFN resulted in higher short-circuit current density (JSC) and fill-factor (FF) but a lower open-circuit voltage (VOC).

Section snippets

Experimental section.

The synthesis of oleic acid capped PbS QDs (OA-QDs) was carried out by using the procedure reported previously [8]. To passivate OA-QDs with Cl, a solution of tetrabutylammonium chloride 0.1 M in ethanol was used in the washing process. To fabricate L-FTO substrates, solutions of PEI or PFN in methoxylmethanol (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

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). From the size optimization [8], we used QDs with a diameter of about 3 nm (Fig. 1d), and a bandgap of 1.32 eV (Fig. 1e) for current study. It is obvious from Fig. 1a that the region close to FTO has a higher carrier concentration then elsewhere. Therefore, inverted Schottky structures, whose working principle is illustrated in Fig. 1b, could extract efficiently carriers

Conclusion

Novel inverted Schottky QDSCs have been examined using non-conjugated PEI and conjugated PFN electrolytes. PEI resulted in higher VOC due to its higher WF reduction affinity while the conjugated electrolyte enhanced JSC and FF. The results pave a way to control cell’s parameters by engineering the interfacial electrolyte and a PCE of 4.5% is very promising for deployment of oxide-free QDSCs.

Conflict of Interest

The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgment

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.99-2016.32.

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