Investigation of the buried planar interfaces in multi-layered inverted organic solar cells using x-ray reflectivity and impedance spectroscopy

Abstract

The hole and electron extracting interlayers in the organic solar cells (OSCs) play an important role in high performing devices. The present work focuses on an investigation of Zinc oxide/bulk heterojunction (ZnO/BHJ) and BHJ/MoOx (Molybdenum oxide) buried planar interfaces in inverted OSC devices using the optical contrast in various layers along with the electrical measurements. The x-ray reflectivity (XRR) analysis demonstrates the formation of additional intermixing layers at the interfaces of ZnO/BHJ and BHJ/MoOx. Our results indicate infusion of PC71BM into ZnO layer up to ~4 nm which smoothen the ZnO/BHJ interface. In contrast, thermally evaporated MoOx molecules diffuse into PTB7-Th dominant upper layers of BHJ active layer resulting in an intermixed layer at the interface of MoOx/BHJ. The high recombination resistance (~5 kΩ cm2) and electron lifetime (~70 μs), obtained from the impedance spectroscopy (IS), support such vertical segregation of PTB7-Th and PC71BM in the active layer. The OSC devices, processed in ambient condition, exhibit high power conversion efficiency of 6.4%. We consider our results have great significance to understand the structure of buried planar interfaces at interlayers and their correlation with the electrical parameters representing various interfacial mechanisms of OSCs.

Investigating the Role of Cathode Buffer Layers Based on Zinc Oxide with Surface‐Rich Graded Fullerene Isomers in Tuning the Interfacial Properties of Organic Solar Cells

“…Preparation of Photoactive Blend: PTB7-Th: PC71BM photoactive blends were prepared by mixing 10 mg of PTB7-Th and 15 mg of PC71BM in 1 mL of o-DCB containing 3.0 vol% of DIO. [15] PBDB-T-2 F: IT-4F photoactive blends were prepared by mixing 10 mg of PBDB-T-2F and 10 mg of IT-4F in 1 mL of CB containing 0.5 vol% of DIO. [62,65] PBDB-T-2F: BTP-4Cl photoactive blends were prepared by mixing 12 mg of PBDB-T-2F and 12 mg of BTP-4Cl in 1 mL of CB containing 2 vol% of 1-chloronapthelene (CN).…”

“…After CBL deposition, a photoactive layer was deposited. Processing conditions for the different photoactive blends are as follows: 1) PTB7-Th: PCBM photoactive blend was spin-cast at 1000 rpm for 90 s in ambient and was allowed to dry for 30 min on a hot plate in air, [15,68] 2) PBDB-T-2F: IT-4F and PBDB-T-2F: BTP-4Cl photoactive blends were spin-casted at 2000 rpm for 60 s inside a nitrogen-filled glove box, and the films were annealed at 100 °C for 10 min. [62,65] Finally, the devices were loaded into a glovebox filled with inert gas and an integrated thermal evaporation chamber for thermal evaporation of 10 nm MoO 3 and 100 nm Al electrodes as anodes with an active area (A) of 7 mm 2 .…”

“…These interfacial buffer layers help in improving the performance of OSCs by fulfilling several functions such as blocking electrons/holes to minimize charge carrier recombination, providing energy-level alignment between the photoactive layer and an electrode, modifying the surface energy and work function(s) of electrodes, and acting as a guide for the evolution of concentration gradients (donor-rich/acceptor-rich phase) near the electrode. [14,15] Over the years, various types of materials, such as carbonates, metals, acetylacetonates, alkali metal base salts, metal oxides, quantum dots, and organic molecules, have been used as a…”

Preface: fresh perspectives on internal interfaces

“…The investigated systems cover a wide range from ultrathin bilayer structures to buried bulk interfaces. Organic semiconductors are the dominant class of materials [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], reflecting their prominent role in modern (opto)electronic devices. Besides interfaces between two organic materials [4,6,12,[14][15][16]18], also metal/organic [2,7,11,16,17,20,21] and organic/inorganic [5,9,10,12,13] interfaces (especially with ZnO [9,10,12,13]) find much attention.…”

“…Organic semiconductors are the dominant class of materials [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], reflecting their prominent role in modern (opto)electronic devices. Besides interfaces between two organic materials [4,6,12,[14][15][16]18], also metal/organic [2,7,11,16,17,20,21] and organic/inorganic [5,9,10,12,13] interfaces (especially with ZnO [9,10,12,13]) find much attention. Organic molecules are also studied on graphite and graphene [8,19] and are covalently attached to thin oxide films [3] or Si surfaces [5].…”