Synthesis, structural and spectroscopic studies of CuInS2 Quantum Dots for absorption layer of solar cells

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Poster, Magdalena Wozniak, 377
Publication date: 5th February 2015

Semiconducting thin films of the ternary I-III-VI₂ compounds are currently under intensive investigation for highly efficient solar energy conversion. Solution routes to synthesis nanocrystals (NCs) were greatly developed because device fabrications can benefit from low-cost roll-to-roll and inkjet printing. Chalcopiryte selenium free-CuInS₂ Quantum Dots (QDs), with low toxicity, long-term stability (air-insensitive) and high optical absorption coefficient (>10⁵ cm^-1), exhibit band gaps (from 1,5 (bulk) to over 2 eV, well-matched with solar spectrum) that are tunable across a wide range of energy levels by changing the quantum dot size [1–3]. This work focuses on the synthesis of colloidal CuInS₂ quantum dots of different sizes by one pot-wet chemistry route in organic solutions, 1-Dodecanethiol and 1-Octadecene. The temperature and reaction time were modified to obtain differently sized QDs. The resulting material was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Applied synthetic route to high-quality QDs employ long hydrocarbon molecules containing a coordinating headgroup as ligands, which stabilize QDs in solvent. On the other hand, capping molecules create an insulating barrier around each NC and blocks the access of molecular species to the surface, which is detrimental for electronic applications [4]. Thus, the obtained CuInS₂ QDs were extensively purified in order to remove excess organic ligands. Final material was dispersed in chloroform and CuInS₂ QDs layers were fabricated using spin coating. The layers were thermally annealed under N₂ atmosphere. Their optical and electrical properties were investigated and will be presented during the conference. Furthermore the initial solar cells in structure glass/ITO/ZnO/CuInS₂/MoO₃/Ag were fabricated using the CuInS₂ QDs layer as light absorbers. External quantum efficiency (EQE) and absorption spectra of fabricated CuInS₂ solar cell is shown in Fig. 1. Photogenerated current under illumination of wavelength close to 850 nm proves the photovoltaic conversion due to absorption in CuInS₂ QDs layer. Unfortunately, the initially processed solar cells exhibit strong electrical shunts, which limit the photocurrent extraction and thus limits the measured EQE. The main origin of the heavy shunting is believed to be non-uniformities and local defect in the CuInS₂ QDs layer and its limited thickness. Local spiking of evaporated Ag electrode through the active layer is possible, limiting the performance of the fabricated solar cells. Ongoing progress in laboratory work is expected to deliver improved solar cell characteristics, to be presented on the poster.
Figure 1. Left: Photograph of synthesized colloidal CuInS2 quantum dots of different sizes dispersed in chloroform. Right: EQE and absorption spectra of the initially processed solar cells based on CuInS2 QD active layer.
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