Thin film photodiodes fabricated by electrostatic self-assembly of aqueous colloidal quantum dots

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: Thin film photodiodes fabricated by electrostatic self-assembly of aqueous colloidal quantum dots a b s t r a c t a r t i c l e i n f o We demonstrate a thin film photodiode structure consisting of multi layers of colloidal quantum dots (QDs) which has application in photovoltaics and photodetection. The CdTe QDs with either positively or negatively charged capping ligands are self-assembled layer-by-layer on an indium tin oxide (ITO) substrate by electrostatic attraction in aqueous solution. A photolithographically patterned photoresist window defines the device active area and an evaporated aluminum (Al) thin film serves as the top electrode. The built-in electric field due to the work function difference between Al and ITO separates photo-excited electron-hole pairs and generates photocurrent. Since the ligands used for QD synthesis are short (less than 0.5 nm), no additional steps of ligand exchange or annealing is needed for enhancing the thin film photoconductivity. Thiol passivation and self-assembly in an inert environment help reduce surface traps, leading to less fermi-level pinning which also improves the device performance. Colloidal semiconductor nanocrystal/quantum dots (QDs) are of great interest to photovoltaic research due to their size-dependent band gap, solution-processable fabrication and the potential for efficient conversion of solar energy [1,2]. Recently, all-inorganic Schottky junction solar cells based on PbSe or PbS QDs have shown exceptionally high external quantum efficiency up to 65% in the visible wavelength range and power conversion efficiency more than 4% in the infrared region [3–5]. However, in order to achieve efficient charge transport between adjacent QDs, these devices require complicated solution-phase [4] or solid-state [3,6] post-chemical treatment that replaces or removes long alkyl chain ligands, such as oleic acid (OA) and trioctylphosphine oxide (TOPO), on the as-prepared QD surface. Some devices also require high temperature annealing to further remove these insulating organic molecules between QDs [7]. In this work, we demonstrate thin film photodiodes fabricated by electrostatic layer-by-layer self-assembly of QDs that are inherently capped with short ligands during the synthesis. Therefore, no ligand exchange, ligand removal or annealing is needed to enhance the thin film photoconductivity. This method simplifies the preparation of QD thin films, making it easier for …