Germinant ZnO nanorods as a charge-selective layer in organic solar cells

doi:10.1016/j.jmst.2019.07.027

Abstract

Abstract:

A facile method was introduced and demonstrated to synthesize zinc oxide (ZnO) nanorods (NRs) as an electron transporting layer (ETL) for organic solar cells (OSCs). Hydrothermal synthesis of the NRs showed a constant growth rate of 5.5 nm min^-1 from germination to sub-micrometer length. The properties were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), absorption spectrophotometry and so on. Based on these measurements, the germinant growth mechanism and its corresponding orientation characteristics were investigated. As an ETL of the OSCs, ZnO NRs enhance the charge extraction from the active layer due to their increased interfacial surface area, but there is an optimal length because of the shunt path formation and UV absorption of long ZnO NRs. As a result, the OSC with the ZnO NRs as ETL shows power conversion efficiency (PCE) up to 6.2%. The J-V characteristics and incident photon-to-current conversion efficiency (IPCE) measurement also reveal that the efficiency enhancement is an assembly of individual results from optical, physical and electrical characteristic of the ZnO NRs.

Key words: ZnO nanorods, Solar cells, Self-assembly, Oriented growth mechanism, Hydrothermal synthesis

Kunsik An, Jaehoon Kim, Mohammad Afsar Uddin, Seunghyun Rhee, Hyeok Kim, Kyung-Tae Kang, Han Young Woo, Changhee Lee. Germinant ZnO nanorods as a charge-selective layer in organic solar cells[J]. J. Mater. Sci. Technol., 2020, 55: 89-94.

Figures/Tables 7

Fig. 1. Cross-sectional scanning electron microscopy (SEM) images of ZnO nanorods (NRs) after (a) 60 min, (b) 70 min, (c) 80 min, (d) 90 min, and (e) 100 min growth time and (f) a top-view image of ZnO NRs after 100 min growth time.

Fig. 2. (a) Length profile of the ZnO nanorods (NRs), temperature of precursor solution as a function of growth time and (b) X-ray diffraction (XRD) patterns of the ZnO NRs with respect to growth time (black arrow: ZnO diffraction standards: JCPDS 36-1451) and (c) ratio of (002) peak to (101) peak.

Table 1 X-ray diffraction (XRD) peak ratio of ZnO nanorods (NRs) with respect to growth time.

Peak	Length
Peak	45 nm	85 nm	145 nm	190 nm	265 nm
(002)	583	683	925	1278	1670
(101)	105	103	99	97	102
(002)(101)	5.55	6.63	9.34	13.17	16.37

Fig. 3. UV/Vis absorption spectra of (a) ZnO nanorods (NRs) with respect to length variation and (b) PPDT2FBT:PC70BM layer deposited on ZnO NR arrays of various length (line), ZnO NRs arrays without PPDT2FBT:PC70BM deposition (dotted line).

Fig. 4. (a) Schematic illustration, (b) cross-sectional transmission electron microscopy (TEM) images of ZnO nanorods (NRs) organic solar cells (OSCs), current-voltage (J-V) characteristics and incident photon-to-current conversion efficiency (IPCE) of devices with (c, e) 170-nm- and (d, f) 290-nm-thick active layers, respectively.

Table 2 Performance parametersa of organic solar cells (OSCs) with respect to the active layer thickness and ZnO nanorod (NR) length.

Active layer thickness	ZnO NRs Length	J_SC (mA cm^-2)	V_OC (V)	FF	PCE (%)
170 nm	45 nm	13.3 ± 0.65	0.75 ± 0.02	0.57 ± 0.02	5.62 ± 0.13
	85 nm	13.8 ± 0.85	0.75 ± 0.02	0.57 ± 0.03	5.91 ± 0.19
	145 nm	12.5 ± 1.02	0.71 ± 0.04	0.58 ± 0.02	5.12 ± 0.09
290 nm	45 nm	15.9 ± 0.17	0.71 ± 0.01	0.54 ± 0.01	6.10 ± 0.13
	145 nm	14.5 ± 0.46	0.73 ± 0.02	0.58 ± 0.01	6.20 ± 0.17
	190 nm	14.4 ± 0.29	0.70 ± 0.01	0.56 ± 0.01	5.65 ± 0.01
	265 nm	12.9 ± 0.21	0.58 ± 0.03	0.52 ± 0.01	3.89 ± 0.31

Fig. 5. Power conversion efficiency (PCE) variations of organic solar cells (OSCs) depending on the active layer thickness and ZnO nanorod (NR) length.

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