This bachelor thesis presents the fabrication and evaluation of large-scale full-solution roll-to-roll processed, ITO-free flexible organic solar cells in a modified inverted device geometry by gravure printing on a discrete laboratory-scale printing system.
The layer stack is based on flexible PET substrate whereupon the back silver cathode was printed on top. The electron transport layer of ZnO and a double light absorbing photoactive layer of P3HT:PCBM, the hole transport layer of PEDOT:PSS and front silver anode were printed consecutively. All layers were roll-to-roll gravure printed from solution under full ambient vacuum-free conditions at a web speed of 2 m min−1. The completed solar cells were characterized by J-V and comprising layers by light beam induced current measurements. For fast testing and reproducibility experiments the remaining layers of the stack after each gravure printed film were deposited by slot-die coating and flexographic printing on a single roll coating system. Unfortunately functional organic solar cells of a fully gravure printed layer stack could not be found. A power conversion efficiency of 0.15 % of partly roll-to-roll gravure printed and residuary roll-based slot-die coated and flexographic printed organic solar cells under AM1.5G illumination was obtained.
The thesis contains a brief introduction in the topic of renewable energies and organic photovoltaic followed by the state of art in two-dimensional gravure printing organic solar cells and the motivation to particularly foreground this fabrication method. In the fundamentals part the working principle, device geometries, affiliated by the concept of ITO-free organic solar cells and materials in an organic photovoltaic device including characterization methods are presented.
Afterwards large-scale manufacturing techniques of organic photovoltaic comprising coating and printing technologies are reviewed and the roll-to-roll manufacturing strategies are introduced. In the experimental part the design, machinery and equipment used and fabrication of gravure printed flexible organic solar cell are chronologically described in detail in connection with presenting and discussing the results after characterizing the completed solar cells. Challenges that were faced during the studies are described subsequently and solutions of appeared problems are presented. A conclusion and outlook finalizes the thesis.
Table of Contents
1 Introduction
1.1 The need for solar energy
1.2 Generations of solar cell technology
1.3 Research fields of organic solar cells
2 State of art and motivation
3 Fundamentals
3.1 Working principle of OPV device
3.2 Device geometries of OPV device
3.3 ITO-free OPV device
3.4 Materials in OPV device
3.5 Characterization of OPV device
3.5.1 J-V curve
3.5.2 LBIC
3.6 Large-scale manufacturing methods for OPV
3.6.1 Coating technologies
3.6.1.1 Slot-die coating
3.6.1.2 Blade coating
3.6.1.3 Spray coating
3.6.2 Printing technologies
3.6.2.1 Screen printing
3.6.2.2 Gravure printing
3.6.2.3 Flexographic printing
3.6.2.4 Inkjet printing
3.6.3 Summary of coating and printing techniques
3.6.4 R2R concept and manufacturing strategies
4 Experimental
4.1 Gravure printing of silver back cathode
4.2 Gravure printing of ZnO
4.3 Gravure printing of active layer
4.4 Gravure printing of PEDOT:PSS
4.5 Gravure printing of silver front anode
4.6 Deposition of remaining layers
4.7 Characterization
5 Results and discussion
5.1 Gravure printed silver back cathode
5.2 Gravure printed ZnO
5.3 Gravure printed active layer
5.4 Gravure printed PEDOT:PSS
5.5 Gravure printed silver front anode
5.6 Summary of results
6 Challenges
7 Conclusion and outlook
Research Objectives and Core Themes
The primary objective of this thesis is to perform a feasibility study on the fabrication of large-scale, fully solution-processed, vacuum-free, and ITO-free flexible organic solar cells using gravure printing. The study aims to transition from lab-scale production to large-area roll-to-roll (R2R) manufacturing, addressing challenges in printing uniformity, material wetting, and device architecture.
- Manufacturing of large-scale ITO-free flexible organic solar cells.
- Evaluation of gravure printing as a full-solution production technique.
- Analysis of material compatibility and layer morphology in roll-to-roll processing.
- Investigation of printing parameters and their impact on device performance and homogeneity.
- Comparison of different coating and printing strategies for R2R optimization.
Excerpt from the Book
3.1 Working principle of OPV device
The field of solution-processed OPV covers various types of semiconducting polymer donor:acceptor material systems in which the acceptor part can be fullerenes, polymers, semiconductors nanoparticles, -crystals, respectively, or metal oxides. The latter are also referred to as hybrid solar cells. The scope in this chapter is on polymer:fullerene cells working by the bulk heterojunction (BHJ) principle. Clear and brief that means that two different semiconducting materials, one as electron donor (D) and the other as electron acceptor (A), is mixed in an organic solvent such as chlorobenzene and deposited on a conductive substrate. Both components can absorb light. Brought together and after evaporation of the solvent and post-treatment steps microphase separation is taking place. The AL of the solar cell comprising an interpenetrated network of D and A is formed. The interface is randomly and unisotropic dispersed throughout the volume of the AL. The interconnected D and A domains of an OPV cell are continuously linked to the top and bottom electrodes (Figure 3.1b) allowing efficient charge transport to anode and cathode.
The function of an organic solar cell is described in abbreviated version below. For further details references [10, 65–68] are recommended by the author. The working principle of a BHJ OPV device includes four fundamental steps whereby the band diagram and BHJ structure is illustrated in Figure 3.1: 1. Exciton generation: Upon illumination an incident photon with an energy that corresponds at least to the band gap energy of the active material is mainly absorbed in the D material. It excites an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and at the same time a positive charge carrier, so-called hole, remains in the HOMO level. In conjugated polymers the promoted electron has a reduced mobility due to the fact that both charge carriers are attracted to each other and bound by Coulomb forces (binding energy circa 0.4 eV [65]). Hence an electron-hole pair which is regarded as a quasi-particle, the exciton, is formed.
Chapter Summaries
1 Introduction: Provides a rationale for renewable energy and solar power while outlining the three generations of solar cell technologies and the focus on organic solar cells.
2 State of art and motivation: Reviews existing literature on gravure-printed OPV components and establishes the goal of fabricating fully gravure-printed, ITO-free flexible solar cells.
3 Fundamentals: Details the operational principles of BHJ devices, material selections, characterization techniques like J-V and LBIC, and large-scale manufacturing methods.
4 Experimental: Describes the chronological fabrication process of the ITO-free inverted layer stack, including machinery setup, ink properties, and deposition parameters.
5 Results and discussion: Analyzes the printing quality of individual layers, discusses morphological defects like viscous fingering, and presents performance data for completed test cells.
6 Challenges: Identifies technical bottlenecks encountered during R2R gravure printing, such as layer wetting issues, alignment difficulties, and the impact of the discrete manufacturing route.
7 Conclusion and outlook: Summarizes the key findings regarding the feasibility of full-scale gravure printing and suggests future research directions to improve performance and stability.
Keywords
Organic Photovoltaics, Gravure Printing, Roll-to-Roll Processing, ITO-free, Flexible Solar Cells, Bulk Heterojunction, P3HT:PCBM, PEDOT:PSS, Device Geometry, Printing Uniformity, Large-scale Manufacturing, Viscous Fingering, Surface Energy, Layer Deposition.
Frequently Asked Questions
What is the primary focus of this thesis?
The thesis focuses on the feasibility of manufacturing fully solution-processed, vacuum-free, and ITO-free flexible organic solar cells using large-scale gravure printing techniques.
Which type of solar cell technology is investigated?
The research investigates third-generation polymer solar cells, specifically utilizing the bulk heterojunction (BHJ) principle with P3HT:PCBM active layers.
What is the main research question or goal?
The goal is to demonstrate that all layers of an inverted OPV device can be gravure-printed at scale, aiming to replace vacuum-based and ITO-dependent processes with a high-throughput, roll-to-roll (R2R) production method.
What scientific methods were used to evaluate the devices?
The study primarily used Current Density-Voltage (J-V) characterization to measure power conversion efficiency and Light Beam Induced Current (LBIC) to visualize active area coverage and identify morphological defects like viscous fingering.
What are the major challenges addressed in the work?
Key challenges include layer wetting and dewetting issues, achieving uniform layer thickness during printing, managing side registration of the substrate, and the limitations of the discrete manufacturing process compared to an ideal inline workflow.
Which keywords characterize this research?
The research is characterized by terms such as Organic Photovoltaics, Gravure Printing, Roll-to-Roll Processing, ITO-free, and Flexible Solar Cells.
Why was the "IOne" stack chosen for this study?
The IOne stack was chosen because it allows for an ITO-free architecture by using silver grids and PEDOT:PSS, which is better suited for full-solution, large-scale production compared to standard glass/ITO substrates.
How does the "viscous fingering" effect impact the solar cells?
Viscous fingering results in non-uniform, streaky layers with varying thicknesses, which leads to inhomogeneous electrical current generation, as visualized by LBIC imaging, and contributes to lower overall power conversion efficiency.
- Arbeit zitieren
- Johannes Michael Küffner (Autor:in), 2014, Large-scale full-solution, vacuum-free gravure printed ITO-free flexible organic solar cells, München, GRIN Verlag, https://www.grin.com/document/287304
