Potential and limitations of colloidal quantum dot solar cells with CuInS2 and ZnO nanocrystals

Rany Miranti^a, Christopher Krause^a, Jürgen Parisi^a, Sebastian Wilken^a, Dorothea Scheunemann^a, Holger Borchert^a

^aUniversity of Oldenburg, Carl-von-Ossietzky-Str. 9-11, Oldenburg, 26129, Germany

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

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Holger Borchert, 321
Publication date: 5th February 2015

Colloidal quantum dot solar cells have made rapid progress in recent years, the power conversion efficiency of devices with a depleted heterojunction formed between layers of lead chalcogenide quantum dots and titanium dioxide reaching now almost 10%, i.e., values that can compete with organic polymer:fullerene solar cells. However, research was strongly focused on lead compounds as absorber material. A more environmentally friendly alternative is copper indium disulfide (CuInS₂, “CIS”). We recently gave a proof of concept that a heterojunction between layers of colloidal CIS and ZnO nanocrystals can also be used in quantum dot solar cells, although efficiencies remained still below 1% in this case [1]. The scheme included in Figure 1 shows the layer sequence of corresponding devices. In the present work we examined limiting factors of this alternative material system as well as its potential for future development. Systematic variations of the ZnO layer thickness revealed a strong impact of this parameter on the external quantum efficiency (EQE, see Figure 1). Tuning the ZnO thickness resulted in pronounced shifts of maxima in the EQE spectra. In order to understand this behavior, optical simulations based on the transfer matrix formalism were performed, and EQE spectra were simulated from the calculated electric field distribution and assumptions for the probability to successfully collect photo-generated charge carriers. The simulations revealed that charges can only be collected from a relatively narrow zone in the absorber layer adjacent to the hetero-contact. This provides an explanation for the limited photocurrent densities. A possible reason for the narrow width of the collection zone might be the charge carrier mobility in the CIS nanocrystal layer. Therefore, we examined also charge transport in CIS nanocrystal films by the fabrication and analysis of single carrier diodes. The hole mobility in CIS layers was found to be about 3 orders of magnitude lower than in typical conductive polymer films like poly(3-hexylthiophene). Strategies for improving the mobility in CIS nanocrystal films as well as improving the efficiency of corresponding quantum dot solar cells are discussed in our contribution.
Figure 1 - External quantum efficiency of CIS/ZnO heterojunction solar cells with varied thickness of the ZnO nanocrystal layer. Inset: sketch of the layer sequence in the devices.
[1] Scheunemann, D.; Wilken, S.; Parisi, J.; Borchert, H. Towards depleted heterojunction solar cells with CuInS2 and ZnO nanocrystals. Appl. Phys. Lett. 2013, 103, 133902.

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