Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Summary

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Abstract

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Introduction

Organic photovoltaics (OPV) are a promising technology to produce cost-effective renewable energies in the near future.^1,2,3 Tremendous efforts have been made to develop photo-active polymers and fabricate high efficiency devices. To date, single layered OPV devices have achieved a >10% power conversion efficiency (PCE). These efficiencies have been achieved on laboratory scale devices using spin coating to generate the film, and translation to larger size scale devices has been fraught with significant reductions in the PCE.^4,

Protocol

1. Photon-active Blend Ink Preparation

1. Weigh 10 mg of DPPBT polymer and 10 mg of PC₇₁BM material (chemical structures shown in Figure 1). Mix them in a 4 ml vial.
2. Add 1.5 ml chloroform and 75 µl of 1,2-dichlorobenzene into the mixture.
3. Put a small stir bar into the vial, close the vial with a polytetrafluoroethylene (PTFE) cap, and transfer the vial to a hot plate. Stir at ~400 rpm, and heat at ~50 °C overnight before use.

2. ITO and Wafer Substrate Cleaning and Preparation

1. Load pre-patterned indium tin oxide (ITO) glass substrate (1 inch by 3 inches, w....

Results

Shown in Figure 3 is the mini-slot die coating system. It consists of one coating machine, one syringe pump and a central control box. The coating machine is the essential part, which is made of a slot die head, one horizontal translational stage, and one vertical translational stage. The slot die head is mounted to the base of a vertical translational motor through a 2-D tilting manipulator. Figure 10a shows the printer main body without mounting the pri.......

Discussion

The method described here focuses on developing a film preparation method that can be easily scaled up in industrial production. Thin film printing and synchrotron morphology characterization are the most critical steps with the protocol. In previous lab scaled OPV research, spin coating is used as the dominant method to fabricate thin film devices. However, this process uses high centrifuge force to spread out BHJ solution, which is quite different from industrial based roll-to-roll fabrication. Thus the knowledge and e.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
PC71BM	Nano-C Inc	nano-c-PCBM-SF
DPPBT	The University of Massachusetts	Custom Made
PEDOT:PSS	Heraeus	P VP Al 4083
Mucasol Liquid Cleaner	Sigma-Aldrich	Z637181
Acetone	Sigma-Aldrich	270725
Isopropyl Alcohol	BDH	BDH1133
Chloroform	Sigma-Aldrich	372978
1,2-dichlorobenzene	Sigma-Aldrich	240664
Lithium fluoride	Sigma-Aldrich	669431
Aluminum	Kurt Lesker	EVMAL50QXHD
Glass vials	Fisher Scientific	03-391-7B
Ultrasonic Cleaner	Cleanosonic	Branson 2800
Oven	WVR	414005-118
Cleaning Rack	Lawrence Berkeley National Lab	Custom Made
Shadow Mask	Lawrence Berkeley National Lab	Custom Made
UV-Ozone Cleaner	UVOCS INC	T16X16 OES
Glove Box	MBraun	Custom Made
Evaporator	MBraun	Custom Made
Slot Die Coater	Jema Science Inc	Custom Made
Solar Simulator	Newport	Class ABB
Spin Coater	SCS Equipment	SCS G3
Hot Plate	Thermo Scientific	SP131015Q
X-ray Measurement	Lawrence Berkeley National Lab	Beamline 7.3.3