(Invited) (AgIn)xZn2(1-x)S2 Quantum Dots with ZnO or TiO2 for Inorganic/Organic Hybrid or Metal Halide Perovskite Solar Cells
Authors: Sagawa Takashi 
Published: February 2020

Nanoparticles made of metal sulfides as quantum dots (QDs) have been prepared as electron transporting, light harvesting, and/or stabilizing parts for solar cells to improve their photovoltaic performance and durability. (AgIn)xZn2(1-x)S2 with its band gap in the range from 1.8 eV to 2.2 eV [1,2] has attractive potentials as the light absorber and the fluorescent material [3-5]. It was also found that the annealing of (AgIn)xZn2(1-x)S2 film at 500 ˚C in air brought the improvement of charge mobilities up to around 6.0 cm2 V-1 s-1[1,6]. In this context, preparation and band gap tuning of (AgIn)xZn2(1-x)S2 QDs through mixing with ZnO or TiO2 as the electron transport layer for inorganic-organic hybrid solar cells or metal halide perovskite solar cells (PSCs) have been investigated.
Monodispersed AgInS2 [viz. x = 0 in (AgIn)xZn2(1-x)S2] QDs with an average value of the diameter of 6.22 ± 0.79 nm were prepared according to the reported procedures [7]. The interplanar spacing of the QDs was 0.33 nm, which corresponds to (112) crystal planes of the tetragonal phase of AgInS2. Introduction of AgInS2 QDs into hybrid solar cells resulted in over doubled enhancement of the photocurrent generation from 400 nm to 450 nm and the improvement of power conversion efficiencies (PCEs) as twice as the device without AgInS2 QDs [8].
While PSCs have been attracting many researchers because of their outstanding photophysical properties for photovoltaic performance. Development of the electron (or hole) transport layer of planar PSCs is one of the most important keys to improve the photovoltaic performance and the stability. Accordingly, development of hole transport layers by using various conjugated polymers for PSCs have been reported [9]. TiO2 and TiO2 mixed with AgInS2 QDs as dual electron transport layers were also introduced in PSCs in order to improve the photovoltaic performance and the stability.
To prepare a decent physical contact between the electron transport layer and the rough surface of fluorine doped tin oxide, convective deposition technique was adopted because it has previously been confirmed that this method is one of the effective solution-based coating methods to deposit some self-assembled monolayer of both micro- and nanoparticles [10].
It was found that the addition of AgInS2 into TiO2 reduced pinholes at the interspace of the grains of the TiO2, enhanced a rectification ratio of the planar PSCs and improved the efficiency of the electron extraction from the active layer of metal halide perovskite through the TiO2/TiO2:AgInS2 QDs electron transport layer. PCE of the device was increased from 16.3% to 17.5% by the TiO2/TiO2:AgInS2 QDs (1.6 mg mL-1) as dual electron transport layers. The device with the dual electron transport layers showed the improvement of the external quantum efficiency in the wavelength region from 300 nm to 750 nm as compared with that of the device with TiO2 single layer. This result probably caused by the enhancement of light harvesting by AgInS2 QDs and the enhancement of the charge transfer from the perovskite layer to the dual electron transport layers.
The long-term stability of the PSCs with the dual electron transport layers was confirmed when 1.6 mg mL-1 of AgInS2 was added, which was followed by the encapsulation and improvement of the retained PCE after the storage of the device in air for 15 days from 11% to 34% was also observed. This result implies that the TiO2/TiO2:AgInS2 QDs as dual electron transport layers not only brought the improvement of photovoltaic performance but also the durability.
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