We report the influence of the controlled acidity of the hole-collecting buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), on the performance and lifetime of polymer:fullerene solar cells. The acidity was controlled by adding a strong base (NaOH) to the pristine PEDOT:PSS solutions. The NaOH-modified PEDOT:PSS layers were used for fabricating polymer:fullerene solar cells with active layers made from blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The results showed that a small addition of NaOH (0.2 molar ratio) removed 23% of the sulfonic acid groups but did not change the device performance, even though further NaOH addition degraded the device performance owing to an increased sheet resistance and lowered work function, as well as a changed surface morphology. Storage lifetime tests showed that the device with the modified PEDOT:PSS layer (0.2 molar ratio NaOH) was almost not degraded, whereas the pristine PEDOT:PSS layer might affect the deterioration at the interface with the active layer (P3HT:PCBM). Under 1 sun illumination for 10 h, the modified PEDOT:PSS layer (0.2 molar ratio NaOH) resulted in ∼25% improved lifetime, which is in excellent agreement with the extent of reduction of the sulfonic acid groups (∼23%).
We attempted to dope poly(3-hexylthiophene) (P3HT) with 2-ethylbenzenesulfonic acid (EBSA), which has good solubility in organic solvents, in order to improve the performance of organic field effect transistors (OFET). The EBSA doping ratio was varied up to 1.0 wt % because the semiconducting property of P3HT could be lost by higher level doping. The doping reaction was confirmed by the emerged absorption peak at the wavelength of ~970 nm and the shifted S2p peak (X-ray photoelectron spectroscopy), while the ionization potential and nanostructure of P3HT films was slightly affected by the EBSA doping. Interestingly, the EBSA doping delivered significantly improved hole mobility because of the greatly enhanced drain current of OFETs by the presence of the permanently charged parts in the P3HT chains. The hole mobility after the EBSA doping was increased by the factor of 55-86 times depending on the regioregularity at the expense of low on/off ratio in the case of unoptimized devices, while the optimized devices showed ~10 times increased hole mobility by the 1.0 wt % EBSA doping with the greatly improved on/off ratio even though the source and drain electrodes were made using relatively cheaper silver instead of gold.
Organic photodetectors with solution-processed all small-molecular bulk heterojunction layers detect near UV light and are stable under high-intensity lights stronger than sun light.
Developing organic nonvolatile memory devices with a writing/reading/erasing logic function in actual array structures is extremely important for realizing low-cost lightweight/flexible plastic electronic systems. Here, we demonstrate that organic field-effect transistors (OFETs) with a polymer energy well structure (PEW-OFET) exhibit excellent nonvolatile memory performances. The PEW structure is created by sandwiching a self-doped poly(o-anthranilic acid) (SD-PARA) nanolayer (high dielectric constant, k ¼ 14) between two low-dielectric polymer layers (k ¼ 2-4). The primary idea behind this concept is the rapid storage and retrieval of charge carriers in the PEW layer during operation due to the high k feature of the SD-PARA nanolayer, which aids the rapid transport of charge carriers inside, whereas the stored charges are safely trapped due to the two low k layers. The results indicate that the PEW-OFET memory devices exhibit outstanding retention characteristics upon continuous reading up to 2000 s after writing, whereas their excellent writing/reading/erasing/reading cyclability is demonstrated in a test with 43000 cycles. Therefore, the present simple yet cost-effective PEW-OFET concept is expected to significantly contribute to the development of low-cost plastic memory array devices because all processes can be inexpensively performed at low temperatures and additional logic transistors are unnecessary.
Hybrid phototransistors (HPTRs) were fabricated on glass substrates using organic/inorganic hybrid bulk heterojunction films of p-type poly(3-hexylthiophene) (P3HT) and n-type zinc oxide nanoparticles (ZnO(NP)). The content of ZnO(NP) was varied up to 50 wt % in order to understand the composition effect of ZnO(NP) on the performance of HPTRs. The morphology and nanostructure of the P3HT:ZnO(NP) films was examined by employing high resolution electron microscopes and synchrotron radiation grazing angle X-ray diffraction system. The incident light intensity (P(IN)) was varied up to 43.6 μW/cm², whereas three major wavelengths (525 nm, 555 nm, 605 nm) corresponded to the optical absorption of P3HT were applied. Results showed that the present HPTRs showed typical p-type transistor performance even though the n-type ZnO(NP) content increased up to 50 wt %. The highest transistor performance was obtained at 50 wt %, whereas the lowest performance was measured at 23 wt % because of the immature bulk heterojunction morphology. The drain current (I(D)) was proportionally increased with P(IN) due to the photocurrent generation in addition to the field-effect current. The highest apparent and corrected responsivities (R(A) = 4.7 A/W and R(C) = 2.07 A/W) were achieved for the HPTR with the P3HT:ZnO(NP) film (50 wt % ZnO(NP)) at P(IN) = 0.27 μW/cm² (555 nm).
We investigated the dispersion effect of crystalline silicon nanoparticles (SiNP) on the performance and stability of organic solar cells with the bulk heterojunction (BHJ) films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM). To improve the dispersion of SiNP in the BHJ films, we attached octanoic acid (OA) to the SiNP surface via esterification reaction and characterized it with Raman spectroscopy and high-resolution transmission electron microscopy. The OA-attached SiNP (SiNP-OA) showed improved dispersion in chlorobenzene without change of optical absorption, ionization potential and crystal nanostructure of SiNP. The device performance was significantly deteriorated upon high loading of SiNP (10 wt %), whereas relatively good performance was maintained without large degradation in the case of SiNP-OA. Compared to the control device (P3HT:PC(61)BM), the device performance was improved by adding 2 wt % SiNP-OA, but it was degraded by adding 2 wt % SiNP. In particular, the device stability (lifetime under short time exposure to 1 sun condition) was improved by adding 2 wt % SiNP-OA even though it became significantly decreased by adding 2 wt % SiNP. This result suggests that the dispersion of nanoparticles greatly affects the device performance and stability (lifetime).
We report thermally stable diimide nanoclusters that could potentially replace the conventional thick electron transport layer (ETL) in organic light-emitting devices (OLEDs). Bis-[1,10]phenanthrolin-5-yl-bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic diimide (Bphen-BCDI) was synthesized from the corresponding dianhydride and amine moieties, and its purified product exhibited a high glass transition temperature (232 °C) and a wide band gap (3.8 eV). The Bphen-BCDI subnanolayers deposited on substrates were found to form organic nanoclusters, not a conventional layer. The OLED made with a subnanolayer of Bphen-BCDI nanoclusters, instead of a conventional ETL, showed greatly improved efficiency (about 2-fold) compared with an OLED without the diimide nanoclusters. The role of the BPhen-BCDI nanoclusters was assigned to hole trapping and electron injection in the present OLED structure.
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