Ambient Method for the Production of an Ionically Gated Carbon Nanotube Common Cathode in Tandem Organic Solar Cells

Summary

A method of fabricating, in ambient conditions, organic photovoltaic tandem devices in a parallel configuration is presented. These devices feature an air-processed, semi-transparent, carbon nanotube common cathode.

Abstract

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two solution-processed polymeric cells connected in parallel by a transparent carbon nanotubes (CNT) interlayer. This structure includes improvements in fabrication techniques for tandem OPV devices. First the need for ambient-processed cathodes is considered. The CNT anode in the tandem device is tuned via ionic gating to become a common cathode. Ionic gating employs electric double layer charging to lower the work function of the CNT electrode. Secondly, the difficulty of sequentially stacking tandem layers by solution-processing is addressed. The devices are fabricated via solution and dry-lamination in ambient conditions with parallel processing steps. The method of fabricating the individual polymeric cells, the steps needed to laminate them together with a common CNT cathode, and then provide some representative results are described. These results demonstrate ionic gating of the CNT electrode to create a common cathode and addition of current and efficiency as a result of the lamination procedure.

Introduction

Polymer semiconductors are the leading organic photovoltaic (OPV) materials due to high absorptivity, good transport properties, flexibility, and compatibility with temperature sensitive substrates. OPV device power conversion efficiencies, η, have jumped significantly in the past years, with single cell efficiencies as high as 9.1%¹, making them an increasingly viable energy technology.

Despite the improvements in η, the thin optimal active layer thicknesses of the devices limit light absorption and hinder reliable fabrication. Additionally, the spectral width of light absorption of each polymer is limited compared to ....

Protocol

1. Indium Tin Oxide (ITO) Patterning and Cleaning

NOTE: Use 15Ω/□ ITO glass, and purchase or cut the ITO glass into sizes suitable for spin coating and photolithography. It is most efficient to perform steps 1.1-1.7 on a piece of glass as large as possible, and then cut it into smaller devices. Also note that steps 1.1-1.7 require the ITO glass to be oriented with the ITO-side up. This can be checked easily with a multimeter’s resistance setting.

1. Spin coat 1 ml of S1813 positive photoresist onto the ITO-side of the ITO glass at a rate of 3,000 rpm for 1 min. Use more resist for larger pieces of glass, make sure ....

Results

A tandem device formed from differing polymers, particularly polymers of significantly differing band gaps, is of practical interest as these devices can absorb the largest spectral range of light. In this device structure, the PTB7 sub-cell is the back cell and P3HT is the front sub-cell. This is intended to absorb the greatest amount of light as the P3HT sub-cell is largely transparent to the longer wavelength light absorbed by the PTB7 sub-cell. For the sake of clarity, the solar cell parameters, V_OC, J

Discussion

The results highlight a few considerations when designing parallel tandem solar cells. Notably, if one of the sub-cells is performing poorly, tandem performance in negatively affected. The results show that there are two main effects. If one sub-cell is shorted, e.g., shows ohmic behavior, the FF^T will be no higher than the FF of the bad sub-cell. J^T_SC and V^T_OC will be similarly affected. This is the case when V_Gate is low and the P3HT sub-cell ha.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate)	Heraeus	Clevios PVP AI 4083
poly(3-hexylthiophene-2,5-diyl)	Rieke Metals  Inc.	P3HT:  P200
phenyl-C61-butyric  acid methyl  ester	1- Material	PC61BM
Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl})	1- Material	PTB7
phenyl-C61-butyric acid methyl  ester	Solenne	PC71BM
1,8-Diiodooctane	Sigma Aldrich	250295
Chlorobenzene	Sigma Aldrich	284513
Indium Tin Oxide Coated Glass 15 Ohm/SQ	Lumtec
S1813	UTD Cleanroom
MF311	UTD Cleanroom
HCl	UTD Cleanroom
Acetone	Fisher Scientific	A18-20
Toluene	Fisher Scientific	T323-20
Methanol	BDH	BDH1135-19L
Isopropanol	Fisher Scientific	A416-20
CEE Spincoater	Brewer Scientific		http://www.utdallas.edu/research/cleanroom/tools/CEESpinCoater.htm
Contact Printer	Quintel	Q4000-6	http://www.utdallas.edu/research/cleanroom/QuintelPrinter.htm
CPK Spin Processor			http://www.utdallas.edu/research/cleanroom/tools/CPKsolvent.htm
Spin Coater	Laurell	WS-400-6NPP/LITE
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Glove Box	M-Braun	Lab Master 130
Solar Simulator	Thermo Oriel/Newport
Keithley 2400 SMU	Keithley/Techtronix	2400
Keithley 7002 Multiplexer	Keithley/Techtronix	7002
Ultrasonic Cleaner	Kendal	HB-S-49HDT
Micropipette	Eppendorf	200 µl