Synthesis of MDMO-PPV capped PbS quantum dots and their application to solar cells

Abstract

Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) capped PbS quantum dots about 3–6 nm in diameter were synthesized with a novel method. Unlike the synthesis of oleic acid capped PbS quantum dots, the reactions were carried out in solution at room temperature, with the presence of a capping ligand species, MDMO-PPV. The quantum dots were used to fabricate bulk heterojunction solar cells with an indium tin oxide (ITO)/polyethylenedioxythiophene/polystyrenesulphonate (PEDOT: PSS)/MDMO-PPV: PbS/Al structure. Current density–voltage characterization of the devices showed that after the addition of the MDMO-PPV capped PbS quantum dots to MDMO-PPV film, the performance was dramatically improved compared with pristine MDMO-PPV solar cells.

Introduction

The increasing demand for energy has already forced us to seek more environmentally clean energy resources. Solar radiation is ideal to meet the demand on energy. At the present time, the solar cells based on semiconductors such as Si and GaAs remain limited due to the high costs imposed by fabrication procedures involving elevated temperature, high vacuum, and numerous lithographic steps [1]. Due to the potential increase in energy conversion efficiency of quantum dot (QD) solar cells to about 66% [2] and the low fabrication cost, hybrid organic/inorganic QD bulk heterojunction (BHJ) solar cells fabricated by a solution process have attracted considerable attentions. Beek et al. [3], [4] used nanocrystal ZnO as an n-type semiconductor to blend with MDMO-PPV as the active layer of BHJ solar cells, and the overall energy conversion efficiency reached 1.6%. The cost is reduced but the efficiency is low for the low charge mobility and low absorption in the near infrared (NIR) spectrum. Sun et al. [5] reported the solar cells based on the blends of CdSe tetrapods and poly(p-phenylenvinylene) derivative OC₁C₁₀-PPV, and the maximum energy conversion efficiency reached 2.8%. This enhanced performance over ZnO hybrid solar cells is due to both the absorption and the charge transport improvement resulting from CdSe's narrow bandgap and tetrapods morphology in nanometer scale. However, CdSe is toxic and can damage the environment and the human body.

With a narrow bandgap of 0.41 eV, a large exciton Bohr radius (18 nm) and a strong quantum-size effect in nanocrystalline form, PbS is widely used in many fields such as solar cells [6], [7], [8], [9], [10], [11], infrared detectors [6], [12], infrared electroluminescent devices [13], [14], and optical switches [15]. Furthermore, an efficient multiple exciton generation has been detected in PbS QD, thus rendering it a promising candidate for highly efficient photovoltaic devices [16], [17]. There are many routes to prepare PbS QDs with different surfactants [18], [19], [20], [21], [22], [23], [24]. The most successful QDs used in solar cells and photodetectors are oleic acid capped PbS QDs or their post-synthetic ligand exchange products [8], [25], [26]. However, the 2.5-nm-long oleic acid ligand inhibits charge transport among the nanocrystals. To improve charge transport, a post-synthetic ligand exchange is used to replace the oleic acid with shorter ligands. However, this adds complexity to the process. Furthermore, the shorter ligands still impede charge transport between QDs or different materials.

Using a polymer as surfactant of the nanocrystals can overcome the disadvantage of the insulator surfactants, and the hybrid materials show different morphologies [27] and applications. Poly(N-isopropylacrylamide) (PNIPAM) has been used as the surfactant in fabrication gold nanoparticles by reduction of HAuCl₄ in aqueous solution [28]. Chausson et al. [29] reported that a nanostructured organic/inorganic hybrid material can be synthesized via an acido-basic reaction and the modification of the titanoniobate oxide KTiNbO₅ by N-alkyl amines. The silica based hybrid materials have also attracted lots of attentions [30], [31], [32], [33] as well as phosphate glass/polymer hybrids [34] due to some unique properties. In this paper, we propose a method to prepare PbS QDs directly in MDMO-PPV solution. High-resolution transmission electron microscopy (HRTEM) images show that the MDMO-PPV capped PbS QDs have a narrow size distribution with diameters in the range of 3–6 nm. Blending MDMO-PPV with MDMO-PPV capped PbS QDs as the active layer, BHJ solar cells were fabricated. The current density–voltage (J–V) measurement shows that the devices with an active layer of MDMO-PPV hybrid with the PbS QDs are better in photovoltaic performance than those with the polymer only.