Ji Sun Moon

Processing additives for improved efficiency from bulk heterojunction solar cells.

Two criteria for processing additives introduced to control the morphology of bulk heterojunction (BHJ) materials for use in solar cells have been identified: (i) selective (differential) solubility of the fullerene component and (ii) higher boiling point than the host solvent. Using these criteria, we have investigated the class of 1,8-di(R)octanes with various functional groups (R) as processing additives for BHJ solar cells. Control of the BHJ morphology by selective solubility of the fullerene component is demonstrated using these high boiling point processing additives. The best results are obtained with R = Iodine (I). Using 1,8-diiodooctane as the processing additive, the efficiency of the BHJ solar cells was improved from 3.4% (for the reference device) to 5.1%.

High-Detectivity Polymer Photodetectors with Spectral Response from 300 nm to 1450 nm

Polymer Photodetectors Optical sensing is used in a wide range of applications, such as low-light detection systems in cars and cameras. Most photodetectors have a limited spectral range and can only detect a narrow range of wavelengths. Gong et al. (p. 1665, published online 13 August) developed polymer photodetectors with extremely broad spectral response and exceptionally high sensitivity that can exceed the response of an inorganic semiconductor detector at liquid helium temperature. A key aspect in the device design is the inclusion of blocking layers to reduce significantly the dark current or noise in the devices. Well-designed polymer photodetectors show performance comparable with the best inorganic devices. Sensing from the ultraviolet-visible to the infrared is critical for a variety of industrial and scientific applications. Today, gallium nitride–, silicon-, and indium gallium arsenide–-based detectors are used for different sub-bands within the ultraviolet to near-infrared wavelength range. We demonstrate polymer photodetectors with broad spectral response (300 to 1450 nanometers) fabricated by using a small-band-gap semiconducting polymer blended with a fullerene derivative. Operating at room temperature, the polymer photodetectors exhibit detectivities greater than 1012 cm Hz1/2/W and a linear dynamic range over 100 decibels. The self-assembled nanomorphology and device architecture result in high photodetectivity over this wide spectral range and reduce the dark current (and noise) to values well below dark currents obtained in narrow-band photodetectors made with inorganic semiconductors.

Effect of processing additive on the nanomorphology of a bulk heterojunction material.

The bulk heterojunction (BHJ) material Si-PDTBT:PC(70)BM is sensitive to the use of a small amount of 1-chloronaphthalene (CN) as a processing additive; CN as a cosolvent (e.g., 4% in chlorobenzene) causes in a factor of 2 increase in the power conversion efficiency of BHJ solar cells. The morphology of the BHJ material, prepared with and without the CN additive is studied with top-down transmission electron microscopy, cross-sectional transmission electron microscopy, and atomic force microscopy. The improved performance is the result of changes in the nanoscale morphology. Field-effect transistor measurements are consistent with the observed changes in morphology.

Columnlike structure of the cross-sectional morphology of bulk heterojunction materials.

The cross-sectional morphology of the bulk heterojunction (BHJ) films comprising regio-regular poly(3-hexylthiophene) (rrP3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) was observed with transmission electron microscopy (TEM). The cross-sectional TEM images of the BHJ film provide information on the pathways for charge transport through the film thickness. The length scale of the phase separation was obtained from spatial Fourier transform analysis of the TEM images and from calculations of the autocorrelation function.

End-capping effect of a narrow bandgap conjugated polymer on bulk heterojunction solar cells.

Device performances of BHJ solar cells based on poly[(4,4-didodecyldithieno[3,2-b:2’,3’-d]silole)-2,6-diyl-alt-(2,1,3-benzoxadiazole)-4,7-diyl]and PC₇₁BM improve by capping the chain ends with thiophene fragments. This structural modification yields materials that are more thermally robust and that can be used in devices with thicker films – important considerations for enabling the mass production of plastic solar cells.

Spontaneous Formation of Bulk Heterojunction Nanostructures: Multiple Routes to Equivalent Morphologies

Bulk heterojunction (BHJ) layers based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were fabricated by two methods: codeposition of P3HT/PCBM from a common solvent (conventional BHJ) and by sequential, layer-by-layer deposition of P3HT/PCBM from separate solvents (layer-evolved BHJ). Thermally annealed layer-evolved BHJ solar cells show power conversion efficiencies and electron/hole mobilities comparable to conventional BHJ solar cells. The nanomorphology of both active layers is compared in situ by transmission electron microscopy (TEM) using a multilayer cross-sectional sample architecture. No significant difference is observed between the nanomorphology of the conventional BHJ and layer-evolved BHJ material implying that the bulk heterojunction forms spontaneously and that it is the lowest energy state of the two component system.

Bulk heterojunction solar cells based on a low-bandgap carbazole-diketopyrrolopyrrole copolymer

Bulk heterojunction (BHJ) solar cells fabricated with a phase separated nanomaterial comprising a carbazole-diketopyrrolopyrrole copolymer (PCBTDPP) and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) are demonstrated with power conversion efficiency>3.5%. The PCBTDPP:PC70BM BHJ nanomorphology was controlled by changing the length of the alkyl side-chain of the polymer and by utilizing processing additives.

Encapsulation of organic light-emitting devices using a perfluorinated polymer

Films of Cytop™, a perfluorinated polymer, are spin cast as a single barrier layer for evaluation of barrier properties on organic light-emitting devices and on Ca thin films. Cytop™ is water repellant, resulting in encapsulated organic light-emitting field effect transistors and organic light-emitting diodes (OLEDs), which remain active even after immersion into water or exposure to water droplets on the Cytop™ surface. OLEDs encapsulated with Cytop™ exhibit up to five times longer continuous operation under identical environmental and driving conditions compared with devices that are not encapsulated with Cytop™.

Thermal annealing induced bicontinuous networks in bulk heterojunction solar cells and bipolar field-effect transistors

Polymer bulk heterojunction solar cells fabricated from poly(2,5-bis(3′-dodecyl-2,2′-bithiophen-5-yl)-3,6-dimethylthieno [3,2-b] thiophene):[6,6]-phenyl-C61-butric acid methyl ester (1:1, w/w) blend showed significantly improved power conversion efficiency (PCE), from 0.96% to 2.32%, with post-thermal annealing at 140 °C. Charge transport properties obtained from bipolar field-effect transistors indicated that post-thermal annealing induced the assembly of significantly improved bicontinuous networks and excellently balanced hole (7.2×10−3 cm2 V−1 s−1) and electron (5.8×10−3 cm2 V−1 s−1) mobilities (due, particularly, to improved electron mobility), thereby improving PCE.

Ultrasensitive solution processed polymer photodetectors

We demonstrate a polymer photodetector with spectral response from 300nm to 1450nm by using a narrow-band-gap semiconducting polymer blended with a fullerene derivative. Operating in room temperature, the polymer photodetectors exhibit detectivity greater than 1013Jones (1Jones =1cm Hz1/2/W) from the UV well into the near-infrared out to 1150nm and greater than 1012Jones from 1150nm to 1450nm. The linear dynamic range is over 100dB. To our knowledge, there is no inorganic material system (not even Si-Ge alloys) capable of such high performance photodetectivty over such a wide spectral range.