We present small molecule solar cells with α,ω-bis-(dicyanovinylene)-sexithiophene:C60 mixed heterojunctions, reaching power conversion efficiencies of 4.9±0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%.
In this work we apply a joint experimental and theoretical approach to investigate thin films of side chain substituted dicyanovinyl quaterthiophenes (DCV4T-Et2) and DCV4T-Et2:C60 blends, prototypic absorbers for small molecule organic solar cells. Structural characterization of the morphology of thin films thermally deposited at different substrate temperatures on a silica surface was performed by variable angle spectroscopic ellipsometry, grazing incidence X-ray diffraction, and atomic force microscopy measurements. These methods, combined with full-atomistic molecular dynamic (MD) simulation, provide detailed information about thin film morphology, namely about molecular orientation, absorption, phase separation, and crystallinity, i.e., factors that affect the efficiency of organic solar cells. Using molecular dynamics simulation, we can constitute why the DCV4T-Et2 molecules arrange strongly tilted in pristine (69°to 70°tilt angle to the substrate normal) and DCV4T-Et2:C60 blend films (tilt angle of 65°to 69°).
A low band gap, solution processable oligothiophene with a dialkylated diketopyrrolopyrrole chromophore for use in bulk heterojunction solar cells Appl. Phys. Lett. 94, 103301 (2009); 10.1063/1.3086897Bandgap renormalization in titania modified nanostructured tungsten oxide thin films prepared by pulsed laser deposition technique for solar cell applications J. Appl. Phys.We investigate the end-capped oligothiophene derivative ␣ , -bis-͑dicyanovinylene͒-sexithiophene with ethyl side chains ͑DCV6T͒ as donor material in heterojunctions with C60. The effect of the substrate temperature on the morphology and related photophysical properties of single DCV6T and mixed DCV6T:C60 layers is investigated. Single layers of DCV6T show crystalline features in UV-visible absorption and x-ray diffraction when grown on a substrate heated to 90°C. Investigations of DCV6T:C60 mixed layers by atomic force microscopy, UV-visible absorption, and photoluminescence measurements reveal that the elevated substrate temperature induces an increased phase separation between the two materials with larger domain size and higher surface roughness. Based on these observations, we present mixed heterojunction solar cells where the power conversion efficiency ͑ PCE ͒ is increased from 1.6% to 3.8% by increasing the substrate temperature from 30 to 90°C, respectively.
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