Organic Solar Cells by Annealing Stacked Amorphous and Microcrystalline Layers

Abstract

Organic solar cells were fabricated by stacking aromatic amine and C60 layers. The energy conversion efficiency of these solar cells was low because of poor photoabsorption by these layers and short diffusion length of excitons. However, the photocurrent density was increased by about 3 times by the application of heat treatment to the stacked organic layers at 140 °C, and the maximum energy conversion efficiency reached 1.1 % under AM 1.5, 100 mW cm–2 simulated solar light. The internal quantum efficiency of the photocurrent after the annealing reached about 45 %. When the aromatic amine layer was about 100 nm thick, the organic layers after the annealing showed a wrinkled structure under an optical microscope. The annealing temperature needed for the formation of this structure was in good agreement with the temperature needed for the increase in the photocurrent. The morphological change caused by the annealing was attributed to infiltration of the amorphous aromatic amine compound into grain boundaries of the microcrystalline C60 layer, resulting in expansion of the C60 layer and the wrinkled structure of the organic layers. From observation by electron microscopy, the mixed form of these two compounds near the interface was found to be suited to solar cells because the C60 and aromatic amine phases wedge each other in a direction normal to two electrodes. However, the annealing slightly lowered photovoltage of the solar cell. This effect was attributed to a partial contact of the C60 layer with a counter electrode through the aromatic amine layer.

High‐Performance Organic Photovoltaic Devices Using a New Amorphous Molecular Material with High Hole Drift Mobility, Tris[4‐(5‐phenylthiophen‐2‐yl)phenyl]amine

“…The device exhibited an open circuit voltage ( V OC ) of 0.92 ± 0.01 V, a J SC of 2.6 ± 0.1 mA cm −2 , a fill factor (FF) of 0.71 ± 0.02, and a power conversion efficiency ( η ) of 1.7% for the incident AM1.5G light at 100 mW cm −2 , respectively. As compared with the reported performance ( J SC: 1∼4 mA cm −2 , V OC : 0.3∼0.9 V, FF: 0.3∼0.5, and η : 0.1∼1.3% under white light irradiation) of OPVs using amorphous molecular materials,31, 34, 35, 37–42 Device A exhibited higher V OC , FF and η values⋅ In particular, the FF value for Device A was very high, being comparable to those reported for pn‐heterojunction OPVs using polycrystalline CuPc 1…”

“…Amorphous molecular materials, i.e., small organic molecules that readily form stable amorphous glasses with well‐defined glass‐transition temperatures ( T g s),29, 30 are also expected to be good candidates for materials in OPVs for their practical use and have been receiving growing attention. Amorphous molecular materials which have hitherto been studied as OPV materials include 4,4′,4″‐tris[5‐(dimesitylboryl)thiophen‐2‐yl]triphenylamine,31 4,4′,4″‐tris[4‐nitrophenyl(4‐methylphenyl) amino]triphenylamine,31 4,4′,4″‐tris[3‐methylphenyl(phenyl)amino]triphenylamine ( m ‐MTDATA),32, 33 N , N ′‐diphenyl‐ N , N ′‐bis(3‐methylphenyl)benzidine (TPD),33, 34 N , N ′‐diphenyl‐ N , N ′‐di( α ‐naphthyl)benzidine ( α ‐NPD),35 siloles containing carbazolyl groups,36 π‐conjugated systems consisting of triphenylamine and oligothiophene,37 triarylamine‐substituted carbazole‐based dendrimers with an oligothiophene core,38 perylene diimide derivatives,39 diphenylaminofluorenyl‐capped thiadiazoloquinoxaline,40 π‐conjugated systems consisting of triphenylamine and benzothiadiazole,41 and dithiafulvenyl‐derivatized triphenylamines 42. It has been reported that OPVs using these amorphous materials exhibit power conversion efficiencies of 0.1∼1.3% under white light irradiation with a Xe lamp or under air‐mass (AM) 1.5G illumination at intensities of 97∼100 mW cm −2 31, 34, 35, 37–42.…”

Structural modifications of zinc phthalocyanine thin films for organic photovoltaic applications

“…There are usually two possible techniques to overcome the limitation: preparing bulk heterojunction by co-evaporating two materials to form a blend film; optimizing the thickness, molecular order and the morphology of organic materials. 4,5 In order to modify the structures of bulk heterojunction, controlling the growth condition [6][7][8] or introduction of buffer layers 9,10 has been investigated. High PCEs of 5-6% for single OPV cell have been reported.…”

Aromatic Fluorine as a Versatile Control Element for the Construction of Molecules with Helical Chirality

“…This chemistry should help in the design of novel materials with space‐separated donor–acceptor regions. The simultaneous presence of intrinsic donor–acceptor characteristics in some of these molecules should also allow the investigation of heterojunctions formed in the resulting materials 20b. Along with the possibility for S N Ar fluorine displacement, our method paves the way to the formation of a huge number of helically chiral molecules from a common precursor.…”