Thickness Effect of Bulk Heterojunction Layers on the Performance and Stability of Polymer:Fullerene Solar Cells with Alkylthiothiophene-Containing Polymer

We report a pronounced thickness effect of bulk heterojunction (BHJ) layers on the performance and stability of inverted polymer solar cells with the BHJ layers of poly­[(4,8-bis­(5-(octylthio)­thiophen-2-yl)­benzo­[1,2-b:4,5-b′]­dithiophene-<i>co</i>-3-fluorothieno­[3,4- b]­thiophene-2-carboxylate] (PBDT-TS1) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). The thickness of the BHJ layers was varied from 40 to 120 nm by changing solution concentrations and spin-coating speeds. The results showed that the film thickness considerably affected the performance and stability of devices. The power conversion efficiency reached ca. 9% at the thickness of 80 nm by the optimized nanoscale phase separation between donor and acceptor components. However, the devices with 120 nm-thick BHJ layers showed better device stability under continuous illumination with a simulated solar light due to the well-maintained surface morphology and nanostructure in addition to the improved morphological volume stability

A Pronounced Dispersion Effect of Crystalline Silicon Nanoparticles on the Performance and Stability of Polymer:Fullerene Solar Cells

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-C₆₁-butyric acid methyl ester (PC₆₁BM). 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:PC₆₁BM), 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)

Liquid Crystal-Gated-Organic Field-Effect Transistors with In-Plane Drain–Source–Gate Electrode Structure

We report planar liquid crystal-gated-organic field-effect transistors (LC-<i>g</i>-OFETs) with a simple in-plane drain–source–gate electrode structure, which can be cost-effectively prepared by typical photolithography/etching processes. The LC-<i>g</i>-OFET devices were fabricated by forming the LC layer (4-cyano-4′-pentylbiphenyl, 5CB) on top of the channel layer (poly­(3-hexylthiophene), P3HT) that was spin-coated on the patterned indium–tin oxide (ITO)-coated glass substrates. The LC-<i>g</i>-OFET devices showed p-type transistor characteristics, while a current saturation behavior in the output curves was achieved for the 50–150 nm-thick P3HT (channel) layers. A prospective on/off ratio (>1 × 10³) was obtained regardless of the P3HT thickness, whereas the resulting hole mobility (0.5–1.1 cm²/(V s)) at a linear regime was dependent on the P3HT thickness. The tilted ordering of 5CB at the LC-P3HT interfaces, which is induced by the gate electric field, has been proposed as a core point of working mechanism for the present LC-<i>g</i>-OFETs

Doping Effect of Organosulfonic Acid in Poly(3-hexylthiophene) Films for Organic Field-Effect Transistors

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

Efficient Deep Red Light-Sensing All-Polymer Phototransistors with <i>p</i>‑type/<i>n</i>-type Conjugated Polymer Bulk Heterojunction Layers

Here we demonstrate deep red light-sensing all-polymer phototransistors with bulk heterojunction layers of poly­[4,8-bis­[(2-ethylhexyl)-oxy]­benzo­[1,2-b:4,5-b′]­dithiophene-2,6-diyl]­[3-fluoro-2-[(2-ethylhexyl)­carbonyl]­thieno­[3,4-<i>b</i>]-thiophenediyl] (PTB7) and poly­[[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)] (P­(NDI2OD-T2)). The device performances were investigated by varying the incident light intensity of the deep red light (675 nm), while the signal amplification capability was examined by changing the gate and drain voltages. The result showed that the present all-polymer phototransistors exhibited higher photoresponsivity (∼14 A/W) and better on/off photoswitching characteristics than the devices with the pristine polymers under illumination with the deep red light. The enhanced phototransistor performances were attributed to the well-aligned nanofiber-like morphology and nanocrystalline P­(NDI2OD-T2) domains in the blend films, which are beneficial for charge separation and charge transport in the in-plane direction

Strong Composition Effects in All-Polymer Phototransistors with Bulk Heterojunction Layers of p‑type and n‑type Conjugated Polymers

We report the composition effect of polymeric sensing channel layers on the performance of all-polymer phototransistors featuring bulk heterojunction (BHJ) structure of electron-donating (p-type) and electron-accepting (n-type) polymers. As an n-type component, poly­(3-hexylthiopehe-<i>co</i>-benzothiadiazole) end-capped with 4-hexylthiophene (THBT-4ht) was synthesized via two-step reactions. A well-studied conjugated polymer, poly­(3-hexylthiophene) (P3HT), was employed as a p-type polymer. The composition of BHJ (P3HT:THBT-4ht) films was studied in detail by varying the THBT-4ht contents (0, 1, 3, 5, 10, 20, 30, 40, and 100 wt %). The best charge separation in the P3HT:THBT-4ht films was measured at 30 wt % by the photoluminescence (PL) study, while the charge transport characteristics of devices were improved at the low THBT-4ht contents (<10 wt %). The photosensing experiments revealed that the photosensivity of all-polymer phototransistors was higher than that of the phototransistors with the pristine P3HT layers and strongly dependent on the BHJ composition. The highest (corrected) responsivity (<i>R</i>_C) was achieved at 20 wt %, which can be attributable to the balance between the best charge separation and transport states, as investigated for crystal nanostructures and surface morphology by employing synchrotron-radiation grazing-incidence wide-angle X-ray scattering, high-resolution/scanning transmission electron microscopy, and atomic force microscopy

Light-Induced Open Circuit Voltage Increase in Polymer Solar Cells with Ternary Bulk Heterojunction Nanolayers

We report a light-induced open circuit voltage (<i>V</i>_OC) increase in polymer solar cells with ternary bulk heterojunction (BHJ) layers that are composed of poly­(3-hexylthiophene) (P3HT), poly­[(4,8-bis­(2-ethylhexyloxy)-benzo­[1,2-b:4,5-b′]­dithiophene)-2,6-diyl-<i>alt</i>-(N-2-ethylhexylthieno­[3,4-<i>c</i>]­pyrrole-4,6-dione)-2,6-diyl]] (PBDTTPD), and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). The ternary BHJ layers were prepared by varying the composition of donor polymers at a fixed ratio (1:1 by weight) of donor (P3HT + PBDTTPD) to acceptor (PC₆₁BM). Results showed that <i>V</i>_OC was gradually increased under continuous illumination of solar light (100 mW/cm²) for ternary solar cells, whereas no <i>V</i>_OC increase was measured for binary solar cells without PBDTTPD. As a consequence, the power conversion efficiency (PCE) of ternary solar cells (except the highest PBDTTPD content) was rather higher after solar light illumination for 10 h, even though the binary solar cell exhibited significantly lowered PCE after 10 h illumination. The <i>V</i>_OC increase has been attributed to the lateral phase segregation between P3HT and PBDTTPD domains in the ternary BHJ layers under continuous illumination of solar light, as evidenced from the analysis result by Raman spectroscopy, atomic force microscopy, transmission electron microscopy, and synchrotron radiation grazing-incidence angle X-ray diffraction measurements

Hybrid Phototransistors Based on Bulk Heterojunction Films of Poly(3-hexylthiophene) and Zinc Oxide Nanoparticle

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_<i>NP</i>). The content of ZnO_<i>NP</i> was varied up to 50 wt % in order to understand the composition effect of ZnO_<i>NP</i> on the performance of HPTRs. The morphology and nanostructure of the P3HT:ZnO_<i>NP</i> films was examined by employing high resolution electron microscopes and synchrotron radiation grazing angle X-ray diffraction system. The incident light intensity (<i>P</i>_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_<i>NP</i> 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>I</i>_D) was proportionally increased with <i>P</i>_IN due to the photocurrent generation in addition to the field-effect current. The highest apparent and corrected responsivities (<i>R</i>_A = 4.7 A/W and <i>R</i>_C = 2.07 A/W) were achieved for the HPTR with the P3HT:ZnO_<i>NP</i> film (50 wt % ZnO_<i>NP</i>) at <i>P</i>_IN = 0.27 μW/cm² (555 nm)

All-Polymer Solar Cells with Bulk Heterojunction Films Containing Electron-Accepting Triple Bond-Conjugated Perylene Diimide Polymer

A triple bond-linked perylene diimide (PDI) conjugated polymer, poly­{[<i>N</i>,<i>N</i>′-dioctylperylene-3,4,9,10-bis­(dicarboximide)-1,7­(6)-diyl]-<i>alt</i>-[(2,5-bis­(2-ethylhexyl)-1,4-phenylene)­bis­(ethyn-2,1-diyl]} (PDIC8-EB), was examined as an electron-accepting component in all-polymer solar cells. As an electron-donating component, poly­[4,8-bis­[(2-ethylhexyl)­oxy]­benzo­[1,2-b:4,5-b′]­dithiophene-2,6-diyl]­[3-fluoro-2-[(2-ethylhexyl)­carbonyl]­thieno­[3,4-<i>b</i>]-thio­phenediyl] (PTB7) and poly­[4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2-b:4,5-b′]­dithiophene-<i>alt</i>-3-fluorothieno­[3,4-<i>b</i>]­thiophene-2-carboxylate] (PTB7-Th) were introduced in order to investigate the feasibility of PDIC8-EB because of their similarity. Results showed that the power conversion efficiency (PCE) was higher for the PTB7-Th:PDIC8-EB solar cells (PCE = 3.58%) than the PTB7:PDIC8-EB solar cells (PCE = 2.81%). The better performance of the PTB7-Th:PDIC8-EB solar cells has been attributed to the formation of a well-defined nanodomain morphology in the PTB7-Th:PDIC8-EB bulk heterojunction layer, as measured with transmission electron microscopy (TEM), atomic force microscopy (AFM), and synchrotron radiation grazing incidence X-ray diffraction (GIXD)

Broadband pH-Sensing Organic Transistors with Polymeric Sensing Layers Featuring Liquid Crystal Microdomains Encapsulated by Di-Block Copolymer Chains

We report broadband pH-sensing organic field-effect transistors (OFETs) with the polymer-dispersed liquid crystal (PDLC) sensing layers. The PDLC layers are prepared by spin-coating using ethanol solutions containing 4-cyano-4′-pentyl-biphenyl (5CB) and a diblock copolymer (PAA-<i>b</i>-PCBOA) that consists of LC-philic block [poly­(4-cyano-biphenyl-4-oxyundecyl acrylate) (PCBOA)] and acrylic acid block [poly­(acrylic acid) (PAA)]. The spin-coated sensing layers feature of 5CB microdomains (<5 μm) encapsulated by the PAA-<i>b</i>-PCBOA polymer chains. The resulting LC-integrated-OFETs (PDLC-<i>i</i>-OFETs) can detect precisely and reproducibly a wide range of pH with only small amounts (10–40 μL) of analyte solutions in both static and dynamic perfusion modes. The positive drain current change is measured for acidic solutions (pH < 7), whereas basic solutions (pH > 7) result in the negative change of drain current. The drain current trend in the present PDLC-<i>i</i>-OFET devices is explained by the shrinking-expanding mechanism of the PAA chains in the diblock copolymer layers