Efficient Organic Photovoltaic Device Using a Sublimated Subphthalocyanine as an Electron Donor

Abstract

In this letter, an efficient subphthalocyanine (SubPc)/C 60 heterojunction organic photovoltaic device was demonstrated. The power conversion efficiency was improved from 2.7% to 4.3% by the sublimation of SubPc. The atomic force microscopic images showed that the crystallinity was improved with the sublimation of the SubPc, leading to the significantly enhanced hole mobility of SubPc in orders of magnitude as estimated by the space-charge limited current. The external quantum efficiency spectra further confirmed the improved charge collection efficiency as a result of the increase in the hole mobility of SubPc due to the unchanged spectral shape over the entire spectrum.Recently, considerable progress in organic photovoltaic (OPV) industries has made realistic commercialization possible. 1 The first outstanding breakthrough of OPV devices was demonstrated by Tang with the concept of a donor/acceptor interface, which achieved a power conversion efficiency (PCE) of about 1%. 2 No significant improvement was made until Forrest and his coworkers introduced a new device structure and acceptor material. 3,4 They suggested that for the exciton blocking layer (EBL), the charge collection efficiency can be improved, with the long exciton diffusion length being important to device optimization. After this finding, the choice of materials became an efficient route to the enhancement in device performance. The materials with long diffusion length, such as fullerene (C 60 ) and pentacene, can lead to high external quantum efficiency (EQE) due to the efficient diffusion of excitons and charge collection. 4,5 In addition, to improve the electrical property of OPV devices, light harvest can also be enhanced by combining the material's absorption with compensated spectral coverage. 6,7 These results successfully showed a near superimposition of the individual photocurrents for each single-donor device.Another proposed approach for material modification is the purity of the materials. Jonathan and his coworkers were having difficulties with fabricating devices with acute leakages. 8 They mainly attributed the leaky diodes to the material impurities, which may have caused the formation of local short paths for reverse currents. Wohrle and his colleagues investigated photovoltaic properties as a function of material purities. 9 They revealed that the non-sublimated materials resulted in a rough surface on the thin film. Therefore, pinholes emerged and formed the short circuits that deteriorated the device performance. The Forrest group showed that the PCE of copper phthalocyanine (CuPc)-based OPV devices can be enhanced over five times when the donor material was efficiently purified. 10 They used space-charge limited currents to calculate the hole mobility of CuPc thin films under different purifications. As a result, the less purified thin film exhibited low carrier mobility, leading to a small fill factor (FF).Subphthalocyanine (SubPc) is a promising candidate for a donor material due to its extremely low-lying highest occupied ...

Improving Performance and Lifetime of Small-Molecule Organic Photovoltaic Devices by Using Bathocuproine–Fullerene Cathodic Layer

“…The PCE of the devices with various BCP:C 60 thicknesses was therefore optimized from 2.9% to 3.2% (Supporting Information Figure S2). We have previously reported a series of high efficiency SubPc/C 60 devices through the control of fabrication process and materials preparation, which yielded the PCE approaching 4.0%. − However, in the current study, the standard device showed a PCE of 2.5% only. The reason for the lower PCE here is that SubPc was used as received without further purification.…”

Effect of 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI) as well as bathocuproine (BCP) and Ag interlayer thickness on the performance of organic tandem solar cells

“…9-nm-thick SubPc achieves the highest h PCE of 2.88%, which is likely to be related to the reported L D of 9.4-nm for SubPc [33]. Too thin or too thick SubPc induces lower J SC or S-shaped J-V curve, ascribed to the low hole mobility of about 3 Â 10 À5 cm 2 /V s [34]. However, the reported performance of ClAlPc/C 60 single PHJs shows no significant dependence on the ClAlPc thickness [35].…”