(Invited) Nanostructured Metal Sulfides for Solar Cells to Improve the Photovoltaic Performance
Authors: Sagawa Takashi 
Published: May 2020

Nanostructured metal sulfides such as Sb2S3 [1-3], AgInS2 [4], and PbS [5] have been prepared as nanosheet, nanorods, quantum dots (QDs) as electron transporting, light harvesting, and/or stabilizing parts for solar cells to improve their photovoltaic performance and durability.

For further improvement of the metal sulfide–based solar cells, nanostructured Sb2S3 with relatively high crystallinity and appropriate energy diagrams will bring decrease of the interfacial barrier for easy electron extraction and declination of charge recombination. In this context, Sb2S3 nanorods were newly prepared by solvothermal method using antimony chloride with two different sources of sulfur of thioacetamide or potassium o–benzyldithiocarbonate (C7H7OCSSK) in two different solvents of water or ethylene glycol. XRD patterns of the obtained powders were assigned to be Sb2S3, and their crystalline sizes were ranged in 0.2–0.8 nm. It was confirmed by the SEM observations that the minimum sized nanorods of Sb2S3 with their averaged length of 680 nm were obtained when C7H7OCSSK and water were used. Others were ranged in 2–5 mm length. From the uv–vis absorption spectra and the photoelectron yield spectra in air of the thin-films, their band gap, valence band and conduction band were estimated to be ranged in 1.6–1.8 eV, 5.5–5.8 eV, and 3.9–4.1 eV, respectively. Utilization of Sb2S3 nanorods for the planar solar cells composed of glass/F-doped SnO2/TiO2/Sb2S3/poly [(3–hexylthiophene)–2,5–diyl]+Sb2S3 nanorods/MoO3/Ag resulted in 3.5 times enhancement of power conversion efficiency (PCE) through the increment of short circuit current density since the interfacial contacting area of the polymer with Sb2S3 was increased with the use of nanorods as compared to that without the nanorods.

In order to harvest more light both of uv-visible and infrared regions effectively, PbS QDs were prepared and applied as the active layer and/ or the hole transporting layer of solar cells. PbS QDs were prepared by hot injection technique. By varying the injection temperature at 343 K, 353 K, and/or 373 K, the absorption peak was shifted from 917 nm (at 343 K) to 1035 nm (at 373 K), which corresponds to specific band gap of the quantum dots. In addition, with the increase of the injection temperature, sizes of the crystallite and lattice were increased. XRD patterns which were assigned to nanosized crystal of PbS without any other additional peak was observed. The evaluations of solar cell parameters by using the above samples were examined. The superiority of device made of PbS quantum dots from low injection temperature was confirmed. The device made of 6 layers of PbS with methylammonium iodide (MAI) or tetrabutylammonium iodide (TBAI) ligands as active layer, resulted in PCE of 6.87% with short circuit current density of 23 mA cm-2, as the highest among the devices. In addition, comparison of solar cell performance by using the same type of PbS sample before and after ligand exchange with MAI was also examined and resulted in improvement of photovoltaic parameters of open circuit voltage, short circuit current density, and PCE after the ligand exchange with MAI on the PbS quantum dots. The effect of silver doping on PbS as the hole transporting layer for solar cell was also investigated. It was confirmed that the Ag–doped device has better performance for both PbS with TBAI and PbS with 1, 2–ethanedithiol though the device with MAI treatment did not have long stability.

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[2] Yukawa, M.; Hayakawa, A.; Sagawa, T., ECS Trans., 2017, 77, 653-659.

[3] Yukawa, M.; Hayakawa, A.; Sagawa, T., ECS Trans., 2018, 85, 551-555.

[4] Kaewprajak, A.; Kumnorkaew, P.; Sagawa, T. Org. Electron., 2019, 69, 26-33.

[5] Shi, G.; Kaewprajak, A.; Ling, X.; Hayakawa, A.; Zhou, S.; Song, B.; Kang, Y.-w.; Hayashi, T.; Altun, M. E.; Nakaya, M.; Liu, Z.; Wang, H.; Sagawa, T.; Ma, W. ACS Energy Lett., 2019, 4, 960-967.