UNIVERSITY of WASHINGTON | BOTHELL

Electrical Engineering | Science & Technology

MRS 2012 FAll

Abstract

MRS – Fall 2012 Boston USA                  Paper #E21.03/H13.03             10:30 AM November 30, 2012

ITO-free Large-area Flexible Organic Solar Cells with an Embedded Metal Grid

Seungkeun Choi*, Yinhua Zhou, Wojciech Haske, Jae Won Shim, Canek Fuentes-Hernandez, and Bernard Kippelen

Center for Organic Photonics and Electronics (COPE)
School of Electrical and Computer Engineering
Georgia Institute of Technology, Atlanta, Georgia 30332

* Current position: Assistant Professor, University of Washington Bothell, WA USA
                               schoi@uwb.edu or schoi5@uw.edu

Roll-to-roll process has been widely used in the printing industry at high process speeds and recently adopted by organic solar cell manufacturers to demonstrate flexible, large-area, low-cost organic solar modules. The so-called stripe geometry is widely used in roll-to-roll process in order to minimize the resistive power loss in large-area solar modules. However, it has its limitations since the area used by the cell-to-cell connections reduces the overall active area of the module, leading to a significant reduction of the power conversion efficiency in modules compared to small-area cells. Recently, we demonstrated improved performance for large-area organic solar cells and modules by integrating metal grids directly with transparent electrodes. However, we found that it was difficult to coat the solution-based organic materials uniformly on top of the substrate due to the thick metal grids.
In this talk, we will report on a new approach to implement a transparent flexible substrate with an embedded thick metal grid. Because the thick metal grid was embedded in the flexible substrate, we were able to spin coat thin organic materials uniformly on top of the large-area substrate where thick metal grids were embedded. For the same reason, organic materials can be coated on this metal-grid-embedded flexible substrate using a roll-to-roll process. Furthermore, the metal lines can be made narrow, reducing the shadowing loss to only 5.5%, and thicker, resulting in lower resistive losses in large-area devices with higher currents.

To make an embedded metal grid on an SU8 thin film, we deposited 200 nm copper on a glass substrate as a seed layer. On top of this seed layer, mold structures were created using SU8 followed by electroplating Cu with a thickness of up to 15 µm. Another layer of SU8 was applied on top of the entire surface. The fabricated SU8 film with embedded metal grids was peeled off from the glass substrate. After removing the seed layer, a high conductivity PEDOT:PSS (PH1000) with 5% DMSO was spin-coated followed by polyethylenimine ethoxylated (PEIE) spin-coating. The active layer of poly(3-hexylthiophene) (P3HT): Indene-C60 Bis-Adduct (ICBA) (1:1, weight ratio) was spin-coated followed by deposition of MoO3 and Ag under vacuum. The completed device had an active area of 9.3 cm2 and a structure of SU8/embedded-metal-grid/PEDOT:PSS(PH1000)/PEIE/P3HT:ICBA/MoO3/Ag. The device exhibited a fill factor of 0.6, a short-circuit current density of 5.8 mA/cm2, and an open-circuit voltage of 0.81 V, yielding a power conversion efficiency of 2.8% under 100 mW/cm2 air mass 1.5G illumination.