Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells

Summary

This manuscript describes how to design and fabricate efficient inverted SMPV1:PC₇₁BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.

Abstract

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC₇₁BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

Introduction

Organic photovoltaic (OPV) devices have recently undergone remarkable developments in the application of renewable energy sources. Such organic devices have many advantages, including solution-process compatibility, low cost, light weight, flexibility, etc.^1,2,3,4,5 Up until now, polymer solar cells (PSCs) with a PCE of more than 10% have been developed by utilizing the conjugated polymers blended with PC₇₁BM⁶. Compared to polymer-based PSCs, small molecule-based ....

Protocol

1. Growth of AZO Sputtered Seed Layer on ITO Substrate

1. Stick 4 anti-corrosion tape pieces (0.3 x 1.5 cm) on one side of the indium tin oxide (ITO) substrate to form a square (1.5 x 1.5 cm). Put the ITO into hydrochloric acid for 15 min to etch the exposed area of ITO.
2. Remove the tape and clean the sample using a sonicator; sonicate with deionized (DI) water, acetone, ethanol, and isopropanol in turn for 30 min each. Blow-dry the patterned ITO with a compressed nitrogen gun.
3. Attach the cleaned patterned ITO substrates onto the substrate holder by tape, and load the holder into the main chamber of RF sputtering system. Pump the chamber pr....

Results

The layered structure of the devices consisted of an ITO substrate/AZO (40 nm)/ZnO NRs layer, SMPV1:PC₇₁BM (80 nm)/MoO₃ (5 nm)/Ag (150 nm) as shown in Figure 1. In general, the AZO or ZnO seed layer is widely used to function as the electron transport layer (ETL) in PSCs devices. Apart from PSCs, SM-OPVs usually have a shorter active layer, limited by the shorter diffusion length⁸. Hence, to further improve the li.......

Discussion

By utilizing the NRs interlayer, both the J_sc and the FF of the devices can be improved. However, the surface roughness of NRs will also influence the subsequent processes. Thus, the orientation and the surface morphology of the NRs should be carefully manipulated. For a long time, the sol-gel processed ETL such as TiO₂ and ZnO were commonly used in PSCs due to their simple procedures. However, the crystallization of sol-gel processed layers is generally of the amorphous type, and the surface morpho.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
AZO target	Ultimate Materials Technology Co., Ltd.	none	AZO (2 wt% Al2O3 in ZnO) , 3”ψx 3mmt + 3mmt Cu B/P + Bonding
SMPV1	Luminescence Technology Corp.	1651168-29-4	2,6-Bis[2,5-bis(3-octylrhodanine)-(3,3-dioctyl-2,2':5,2''-terthiophene)]-4,8-bis((5-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene
RF sputtering system	Kao Duen Technology Co., Ltd	none	http://www.kaoduen.com.tw/index.php?action=product
Zinc Acetate Dihydrate	J. T. Baker	5970456	4.39 g
Monoethanolamine	J. T. Baker	141435	1.22 g
2-methoxyethanol	Sigma-Aldrich	109864	40 mL
Zinc Nitrate Hexahydrate	J. T. Baker	10196186	1.49 g
Hexamethylenetetramine	Sigma-Aldrich	100-97-0	0.7 g
Indium tin oxide (ITO)	RiTdisplay	none	coated glass substrates (<10 Ω sq–1)
AFM	Veeco	Innova SPM
SEM	FEI	Nova 200 NanoSEM	operation voltage: 10 kV
XRD	Bruker	D8 X-ray diffractometer	2θ range: 10–90 °; step size: 0.008 °
PL	Horiba	Jobin-Yvon HR800	excitation source: 325 nm UV Laser 20 mW
solar simulator	Newport	91192A	AM 1.5G
Precision Semiconductor Parameter Analyzer	Keysight Technologies	Agilent 4156C	sweep from -1 to +1 V
toluene	Sigma-Aldrich	108-88-3	1 mL
PC71BM	Sigma-Aldrich	609771-63-3	11.25 mg
Thermal evaporation system	Kao Duen Technology Co., Ltd	Kao Duen PVD System	http://www.kaoduen.com.tw/index.php?action=product
HCl	Sigma-Aldrich	7647-01-0
MoO3	Alfa Aesar	1313-27-5	99.50%
silver ingot	ADMAT Inc.	none	100.00%
Thin Film Deposition Controller	INFICON	XTC
anti-corrosion tape (Polyimide Film)	3M Taiwan Corporation	none	http://solutions.3m.com.tw/wps/portal/3M/zh_TW/InsulatingTape/home/product/Polyimide/
spin-coater	Chemat Technology, Inc	KW-4A	http://www.chemat.com/chematscientific/KW-4A.aspx