Investigation on Lanthanum Fluoride as a Novel Cathode Buffer Material Layer for the Enhancement of Stability and Performance of Organic Solar Cell

Md Shahinul Islam,Md Ferdous Rahman,Md Golam Saklayen,Abu Bakar Md Ismail,Hartmut Baerwolff

doi:10.4236/opj.2014.410027

Abstract

This article presents the investigation on very thin Lanthanum Fluoride (LaF3) layer as a new cathode buffer layer (CBL) for organic solar cell (OSC). OSCs were fabricated with poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) polymer blend at 1:1 ratio. Electron-beam evaporation at room temperature was used to deposit 3 and 5 nm thick LaF3 layer. A very smooth surface of LaF3 with an average roughness of 0.2 nm has been observed by the Atomic Force Microscope (AFM) that is expected to prevent diffusion of cathode metal ion through it and thereby enhance the lifetime and stability of OSC. Huge enhancement of JSC and VOC was also observed for 3 nm-thick LaF3 CBL. Several excellent features of the LaF3 layer such as, transporting electron through tunneling, blocking of holes to the cathode, minimizing recombination, protecting the photoactive polymer from ambient oxygen, and reducing degradation/oxidation of any low work function layer at the cathode interface, might have contributed to the performance enhancement of OSC. The experimental findings indicate the promise of LaF3 to be an excellent CBL material for OSC.

Highlights

Solar cells utilizing organic materials have recently become of great interest due to their potential to utilize high throughput and low-cost solution phase processing [1]-[6]
When a buffer layer is used at the electrodes it produces a non-ohmic contact and the VOC is determined by the difference in work-function between the two electrodes instead of the difference in (HOMO) level of the donor (PCBM) and the lowest unoccupied molecular orbital (LUMO) level of the acceptor (P3HT) for ohmic contact [13]
The stability of the organic solar cell is a big concern as the cathode-material ion diffuses through the active layer which can react with the polymer and alter its semi-conducting properties [14]

Summary

Introduction

Solar cells utilizing organic materials have recently become of great interest due to their potential to utilize high throughput and low-cost solution phase processing [1]-[6]. Performances of OSCs, such as the electron extraction and VOC, are found to depend largely on the interfacial layers between a photoactive layer and electrodes [10]-[12], and why it is necessary to develop interfacial layers to further improve the efficiency. The stability of the organic solar cell is a big concern as the cathode-material ion diffuses through the active layer which can react with the polymer and alter its semi-conducting properties [14]. Another degradation phenomenon is the formation of insulating metal-oxide layer at the cathode interface [15]. TiOx, and low work function cathodes have been reported to improve the electron injection/extraction contact [16]-[21]

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