Enhanced Power Conversion Efficiency of PbS QD/ZnO Solar Cells By Thermal Annealing
Authors: Takahashi Akihiro , Takahira Kazuya , Fukuda Takeshi , Wang Haibin , Kubo Takaya , Segawa Hiroshi , Kamata Norihiko 
Published: February 2020

【Introduction】
Colloidal quantum dots (CQDs) are useful absorbers and transporter for a next-generation solar cell because of size-dependent bandgap tunability and because CQD-based active layers can be formed by solution processes. In spite of recent progress in PbS CQD solar cells1, there are still many challenges left to further improve solar cell performance. Elongation of the carrier diffusion length of the CQD solid layers is a challenge. This is because the typical carrier diffusion length of the layers deposited by a conventional solid-phase ligand exchange is approximately 200 nm, which makes it difficult to achieve thick active layers capable of sufficient light absorption, particularly in the infrared region. In this study, we employed a liquid-phase ligand exchange method to deposit PbS CQD active layers2, and treated them at different temperatures, with the aim of increasing carrier diffusion lengths. The PbS QD/ZnO solar cells fabricated with the active layers that had been treated at 80 °C gave a power conversion efficiency of 10.3%.
【Experimental Methods】
We used PbS CQDs capped with oleic acid (OA) ligands and giving the first exciton absorption peak at 935 nm. The 100 nm thick ZnO layers were first formed on FTO glass substrates by a spin-coating method 3. A liquid-phase ligand exchanged PbS-I CQD ink was deposited on the ZnO layer by a single-step spin-coating method 2. The PbS-I CQD layers were then annealed at five different temperatures (60, 80, 100, 120, 140 ℃) for 30 mins each. Subsequently PbS COD were deposited on top of the PbS-I layers, and treated with 1,2-ethanedithiol (EDT) to replace OA ligands with EDT ligands. Au electrodes were then thermally evaporated onto the layers to complete solar cell fabrication. Photocurrent density vs voltage (J-V) characteristics of the solar cells were measured under one-sun illumination to obtain the short-circuit density (Jsc
), open circuit voltage (Voc
), fill factor (FF), and power-conversion efficiency (PCE). The defect density (Nt
) of the PbS-I layers was estimated from the Mott-Schottoky plots obtained by measuring the built-in potential and the frequency dependence (0.1 MHz - 20 MHz) of the impedance spectra of the solar cells.
【Results and Discussion】
Figure 1(a) shows the active layer thickness dependence of solar cell performance. All the solar cell performance values (Jsc
, Voc
, FF, and PCE) increased as the layer thickness increases. The Jsc
monotonously increases with increasing thickness, and showed no sign of Jsc
's decreasing even at a thickness of approximately 300 nm. This indicates that the carrier diffusion length becomes longer by the liquid-phase ligand exchange. As a result of the increased carrier diffusion length, we succeeded in improving the EQE of the solar cells in the infrared region. The resulting PCE reached 9.41%. We annealed active layers at different temperatures, aiming to increase PbS QD coupling and decrease the defect density in the active layers. Figure 1(b) shows the annealing temperature dependence of the solar cell performance and defect density. The defect density of the active layers gives the lowest value when annealed at 80 ℃, thereby resulting in a maximum power conversion efficiency of 10.3%.
【References】
1) G. H. Carey, A. L. Abdelhady, Z. Ning, S. M. Thon, O. M. Bakr, E. H. Sargent, Chem. Rev.
115
, 12732 (2015).
2) M. Liu, O. Voznyy, R. Sabatini, F. P. Garcia de Arquer, R. Munir, A. H. Balawi, X. Lan, F. Fan, G. Walters, A. R. Kirmani, S. Hoogland, F. Laquai, A. Amassian, E. H. Sargent, Nat. Mater., 16, 258 (2017).
3) H. Wang, V. Gonzalez-Pedro, T. Kubo, F. Fabregat-Santiago, J. Bisquert, Y. Sanehira, J. Nakazaki, H. Segawa, J. Phys. Chem. C, 119, 27265 (2015).

Figure 1