Molecular-weight-dependent nanoscale morphology in silole-containing cyclopentadithiophene polymer and fullerene derivative blends

We have investigated the effect of polymer molecular weight (MW) on the morphology and efficiency of bulk heterojunction (BHJ) solar cells comprised of poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(5,5′-thienyl-4,4′-dihexyl-2,2′-bithiazole)-2,6-diyl] (Si-PCPDTTBT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM). Striking morphological changes are observed in BHJ films upon the change of the polymer MW. Atomic force microscopy and transmission electron microscopy studies suggest that high MW polymer generated high degree of phase separation, leading to formation of an interpenetrating network for carrier transport. The X-ray diffraction investigation indicated that increased π–π stacking in Si-PCPDTTBT with increasing polymer MWs results in an increase in hole mobility of Si-PCPDTTBT and electron mobility of PCBM as well as the red shift absorption spectrum in BHJ films. The solar cells based on PCBM with high-MW Si-PCPDTTBT deliver power conversion efficiencies of 3.33%.     

Quantifying space charge accumulation in organic bulk heterojunctions by nonlinear optical microscopy

In this work, we apply electric field induced second harmonic generation microscopy to directly observe and quantify space charge accumulation in operating organic bulk heterojunction photovoltaic cells comprised of poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) (PSBTBT) mixed with phenyl-C61-butyric acid methyl ester (PCBM). We adjust the effective electron and hole mobility within these blends by altering the relative composition of PSBTBT and PCBM, and observe dramatic shifts in space charge accumulation. The PSBTBT rich device shows strong electron accumulation (2.8 × 10¹⁴ e⁻/cm³) and the PCBM rich device shows strong hole accumulation (5.5 × 10¹⁴ h⁺/cm³).     

Investigation of donor–acceptor copolymer films and their blends with fullerene in the active layers of bulk heterojunction solar cells by Raman microspectroscopy

Thin films made of three low-band gap donor–acceptor copolymers (CDTF, CDTDP and CDTDOP) composed of 4,6-bis(3′-dodecylthiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole-5′,5′-diyl as an electron-acceptor structural unit and various electron-donor structural units, such as 9,9-bis(2-ethylhexyl)fluorene-2,7-diyl, 2,5-didodecyl-1,4-phenylene and 2,5-didodecyloxy-1,4-phenylene, respectively, and thin films of their blends with various ratios of a soluble fullerene derivative [6,6]-phenyl C₆₁-butyric acid methyl ester ([60]PCBM) as an active layer for bulk heterojunction solar cells were studied by means of UV–vis absorption spectroscopy and Raman microspectroscopy. The molecules of CDTDP and CDTDOP possess the same main chains; they differ in the side-chain oxygen only, which changes the donor strength of the donor units. UV–vis and Raman studies allow us to show differences in the hindering of molecule planarization and aggregation in the blends. Absorption of the polymer films covered the whole visible spectral region and extended up to near infrared for CDTDOP. The absorption behavior of the CDTDP blend films qualitatively differed from the absorption behavior of the blend films of CDTF or CDTDOP. The Raman measurements were performed at two different laser excitation wavelengths (633 and 785 nm), which enabled the photoluminescence of both components in the Raman spectra to be distinguished. The Raman study was performed in different parts of the films, including the separated areas. It was proven that the separated areas in the blend films had higher contents of [60]PCBM than the rest of the films.     

Thermally stable carboline derivative as a host material for blue phosphorescent organic light-emitting diodes

A α-carboline based high triplet energy material, 9,9′-(5′-(carbazol-9-yl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)di-α-carboline (2CbCzT), was designed and synthesized as the thermally stable host material for blue phosphorescent organic light-emitting diodes (PHOLEDs). The 2CbCzT host showed high glass transition temperature of 149 °C and high decomposition temperature of 518 °C at 5% weight loss. In addition, the 2CbCzT exhibited bipolar charge transport properties due to hole transport type carbazole and electron transport type α-carboline units. Blue PHOLEDs were developed using the high triplet energy 2CbCzT host material and a high quantum efficiency of 22.1% was obtained.     

Photo-physics of PTB7, PCBM and ICBA based ternary solar cells

Indene-C₆₀bisadduct (ICBA) is one of the rare acceptors which can supersede commonly used phenyl-C71-butyric acid methyl ester (PCBM70) in enhancing the performance of bulk heterojunction (BHJ) solar cells owing to its shallower lowest unoccupied molecular orbital (LUMO) level. However, ICBA tends to decrease the photocurrent for most of the low band-gap polymers synthesized to date. Here we examine the interaction of ICBA with the one of the popular low band-gap polymers 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-b]thiophenediyl]] (PTB7), at femtosecond level, to explore key dynamics governing the operation of BHJ cells involving ICBA. The photo-physics of binary and ternary systems based on PTB7 coupled with PCBM70 and/or ICBA are studied by means of transient absorption spectroscopy (TAS) and electrochemical impedance spectroscopy (EIS) and supported by morphology analysis. Our study suggests that both inefficient charge-separation and poor charge transport of ICBA is responsible for relatively low photocurrent generation.     

Morphology Inversion of a Non-Fullerene Acceptor Via Adhesion Controlled Decal-Coating for Efficient Conversion and Detection in Organic Electronics

In this study, a promising film formation technique is highlighted, named mold-assisted decal-coating, as a thin film transfer printing process using the polyurethane acrylate-based stamping mold. By optimizing the surface energy of the mold with wetting coefficient theory, the mold-assisted decal-coating process is successfully demonstrated by transferring the photoactive layer composed of the polymer donor, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] and a narrow bandgap non-fullerene acceptor (NFA), 2,2′-[[4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl]bis[[4-[(2-ethylhexyl)oxy]-5,2-thiophenediyl]methylidyne(5,6-difluoro-3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile]. This process induces a well-ordered morphology of photoactive material, prevents damage to the underlying layer by suppressing the solvent penetration. Both photovoltaic cells and photodetectors prepared by the decal-coated photoactive layers containing fluorinated NFAs showed higher performance (power conversion efficiency = 10.69% and specific detectivity = 1.27 × 10¹² A cm Hz^1/2 W⁻¹, respectively) than those of cells prepared by the spin-coating method owing to morphology inversion and smoother interface that led to suppressed internal resistance and enhanced charge flow in normal structure. Thus, the reproducible decal-coating process using a customized elastomeric mediator is an important thin film coating technique for efficient next-generation organic optoelectronic materials.     

Improving performance of polymer solar cells based on PSBTBT/PC71BM via controlled solvent vapor annealing

Controlled solvent vapor annealing (C-SVA) is a powerful tool to control the morphology for high performance polymer solar cells (PSCs). In this work, the PSCs employed a blend of poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) is used to show this case. The solar cells upon C-SVA give Power Conversion Efficiency (PCE) of 5.40%, in contrast to 4.14% for the pristine and 4.70% for the thermally annealed devices. The increased PSBTBT concentration on the bottom surface of the C-SVA treated film favors charge carriers transportation to the anode, which contributes to the increased hole mobility of the photoactive layer and thus the device performance.     

Efficient ternary organic solar cells based on immiscible blends

Organic photovoltaic cells based on ternary blends of materials with complementary properties represent an approach to improve the photon-absorption and/or charge transport within the devices. However, the more complex nature of the ternary system, i.e. in diversity of materials' properties and morphological features, complicates the understanding of the processes behind such optimizations. Here, organic photovoltaic cells with wider absorption spectrum composed of two electron-donor polymers, F8T2, poly(9,9-dioctylfluorene-alt-bithiophene), and PTB7, 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-b]thiophenediyl]), mixed with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) are investigated. We demonstrate an improvement of 25% in power conversion efficiency in comparison with the most efficient binary blend control devices. The active layers of these ternary cells exhibit gross phase separation, as determined by Atomic Force Microscopy (AFM) and Synchrotron-based Scanning Transmission X-ray Microscopy (STXM).     

A review of recent plasmonic nanoparticles incorporated P3HT: PCBM organic thin film solar cells

An optimum thickness of organic active layer of 100 nm or possibly less results in poor optical absorption in organic photovoltaic cells (OPV). The optical absorption can be improved by using a thick organic active layer, but the charge carrier collection efficiency will decrease due to low charge carrier mobility for most of the polymeric organic semiconductor. This phenomenon imposes a trade-off between optical absorption and charge carriers transport inside OPV. Recently, metallic nanostructures such as gold (Au) and silver (Ag) with various sizes and morphologies have been identified as an alternative route to boost the performance of OPV at this specific limited thickness (ie. ≤100 nm). Multiple plasmonic effects such as optical and electrical effects are induced upon introducing metallic nanoparticle(s), NP(s) into OPV. This review highlights recent progress in plasmonic-enhanced poly(3-hexylthiophene-2,5-diyl): phenyl-C61-butyric acid methyl ester (P3HT: PCBM)-based OPV with NP(s) located either inside organic active layer or carrier transport layer (CTL) or at various interfaces within the OPV cell architecture. With understanding of the physical plasmonic effects for Au and Ag in OPV, such plasmonic NP(s) act as a new class of strategy for performance optimization.     

Performance improvement of low bandgap polymer bulk heterojunction solar cells by incorporating P3HT

This work demonstrates a significant improvement of device performance by incorporating the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) into a low bandgap polymer poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b′]dithiophene-siloe 2,6-diyl]] (Si-PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) host system, to form a ternary blend bulk heterojunction solar cell. The P3HT concentration was varied from 1 to 5 wt% in the host system. P3HT functions as a morphology control agent in this ternary system. A small weight percentage of P3HT can enhance the light absorption, polymer phase separation, exciton separation and charge carrier mobilities. These results are supported by UV–vis spectroscopy, X-ray diffraction, photoluminescence analysis and other characterisation methods. The highest average power conversion efficiency improvement of 10% was achieved by adding 1 wt% P3HT to the host system. This study reveals a promising way to achieve high efficiency solar cells using a low bandgap polymer.     

High resolution electrical characterisation of organic photovoltaic blends

In this work, electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM) are applied to perform high-resolution electrical characterisation of organic photovoltaic films. These films are composed of the C₆₀-derivative PCBM blended with hole conductive conjugated polymers PPV derivatives or P3HT. It is demonstrated that both EFM and C-AFM are able to electrically evidence phase separation in the blends, suggesting in addition higher density of carriers along interfaces. Correlation between the EFM contrast and the photovoltaic properties of the blends was observed. Local spectroscopy (I-V curves) completes the C-AFM investigations, analysing charge transport mechanisms in the P3HT:PCBM blend. Significant modifications of the local electrical properties of P3HT are shown to occur upon blending. Space charge limited current is evidenced in the blend and a hole mobility of 1.7 × 10⁻² cm² V⁻¹ s⁻¹ is determined for P3HT.     

Investigation of charge carrier mobility and recombination in PBDTTPD thin layer structures

In this paper we present investigation of hole transport properties in sandwich and OFET structures with single active layer of PBDTTPD (Poly[(5,6-dihydro-5-octyl-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl)[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]]). Sandwich structures were investigated by photo-CELIV and TOF techniques, obtained results showed strongly time dependent hole mobility and non-Gaussian DOS tail. Photogenerated charge carrier decay experiment demonstrated that bimolecular recombination coefficient is smaller than calculated Langevin recombination coefficient and this was explained by faster holes escaping recombination area and not participating in Langevin recombination process. Organic field-effect transistor structure was investigated by current transients technique to find hole mobility near the dielectric layer and to study OTS treatment influence on hole transport. The study of hole mobility dependence on temperature was performed in order to evaluate energetic disorder of interface DOS in the channel of OFET structures.     

A highly efficient PTB7-Th polymer donor bulk hetero-junction solar cell with increased open circuit voltage using fullerene acceptor CN-PC70BM

Modified [6,6]-phenyl-C₇₀-butyric acid methyl ester (CN-PC₇₀BM) is employed as an electron acceptor along with poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) polymer as donor for solution-processed organic solar cells. Inverted device architecture is adopted to fabricate the photovoltaic devices. Using chloronaphthalene as an additive, a high power conversion efficiency (PCE) of 8.2% is achieved with an excellent open circuit voltage (V_oc) of 0.9 V, short-circuit current density (J_sc) of 13.5 mA cm⁻² and fill factor of 0.68 as compared to PCE of 5.4% from the reference solar cell using PC₇₀BM.     

Engineering vertical morphology with nanoparticulate organic photovoltaic devices

Sequential deposition of monolayers, composed of nanoparticles with varied donor-acceptor concentration ratios, has allowed the fabrication of organic photovoltaic (OPV) active layers with engineered vertical morphology. The performance of polymer-polymer poly(9,9-dioctylfluorene-co-bis-N,N-phenyl-1,4-phenylenediamine):poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (PFB:F8BT) and polymer-fullerene poly(3-hexylthiophene):phenyl C61 butyric acid methyl ester (P3HT:PCBM) nanoparticulate (NP), graded nanoparticulate (GNP) and bulk heterojunction (BHJ) OPV devices have been studied. For both material systems the highest device V_OC is observed for the graded structure. Furthermore, thermal treatments can be used to alleviate parasitic series resistance in the GNP devices, thus improving device J_SC and efficiency. Overall, this work shows that the nanoparticle approach provides a new experimental lever for morphology control in OPV devices.     

Enhanced photovoltaic characteristics of solar cells based on n-type triphenodioxazine derivative

Polymer solar cells based on poly (2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV):1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61(PCBM):3, 10-di(trifluoromethane) triphenodioxazine (TFTD) was fabricated using spin coating technology. The absorption spectra of MEH-PPV: PCBM: TFTD film coated from chlorobenzene solution was broadened and showed higher intensity compared with that of the pure MEH-PPV. The TFTD acting as electron acceptors in combination with PCBM induced the photoluminescence (PL) quenching of MEH-PPV, which were associated with the photoinduced charge transfer characteristics of composite film. Further, a photovoltaic conversion efficiency up to 1.03% under 16.7 mW/cm² white light illumination was achieved in the MEH-PPV: PCBM: TFTD system.     

Organic donor-σ-acceptor molecules based on 1,2,4,5-tetrakis((E)-2-(5′-hexyl-2,2′-bithiophen-5-yl)vinyl)benzene and perylene diimide derivative and their application to photovoltaic devices

Donor-σ-acceptor molecules of HPBT-n(PDI) (n = 1, 2, and 4) containing perylene diimide (PDI) and π-extended 1,2,4,5-tetrakis((E)-2-(5′-hexyl-2,2′-bithiophen-5-yl)vinyl)benzene (HPBT) have been successfully synthesized for studying the self-organization of each moiety and their applications in photovoltaic devices. Interesting features were found in these molecules: the aggregation-induced crystallization in the HPBT moieties enhanced the power conversion efficiency (PCE) in the photovoltaic cell. By incorporating HPBT as the donor and PDI as the acceptor moiety, we anticipated that their high degree of independent aggregation-induced crystallization would yield electron/hole transport channels and high mobility in the desired direction of charge transport. In a photovoltaic device, HPBT-1(PDI) gave a PCE of 0.22% with an open circuit voltage ranging from 0.62 to 0.63 V. When the HPBT moiety was more hindered by the PDI moiety, less PCE was observed in HPBT-2(PDI). Addition of methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) resulted in enhancement of the PCE due to enhanced visible absorption. The device bearing HPBT-1(PDI) and PCBM (1:4 mol ratio) demonstrate much higher PCE to be around 1.60%.     

Highly efficient and stable blue quantum-dot light-emitting diodes based on polyfluorenes with carbazole pendent groups as hole-transporting materials

Highly efficient and stable blue quantum-dot light-emitting diodes (QD-LEDs) have been realized by using poly (9,9-bis(N-(2′-ethylhexyl)-carbazole-3-yl)-2,7-fluorene) (PFCz) as hole-transporting layers (HTLs). Due to the carbazole units as substituents at the 9-position of polyfluorene, PFCz shows higher hole mobility and better electrochemical stability than poly (N-vinlycarbazole) (PVK). As a result, the maximum current efficiency (CE) and external quantum efficiency (EQE) of the blue QD-LEDs increased from 4.32 cd A⁻¹ to 7.9% for PVK HTL to 7.38 cd A⁻¹ and 12.61% for PFCz HTL, respectively. Furthermore, the PFCz-based blue QD-LED exhibited lower turn-on voltage and longer device lifetime than the PVK-based device. The improvement performance of blue QD-LED should be attributed to the conjugated fluorene backbone and the substituents of the carbazole active sites, thus enhancing hole mobility and electrochemical stability. This result demonstrates that polyfluorenes with pendent carbazole groups is a promising hole-transporting materials for improving performance of blue QD-LEDs.     

Muon-spin-relaxation study of organic semiconductor spiro-linked compound

We report on muon-spin-relaxation (μSR) experiment of organic semiconductor spiro-linked compound 2′,7′-bis(N,N–diphenylamino)-2-(5-(4–tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-9,9-spirobifluorene (Spiro-DPO) measured at temperatures between 8 and 300 K and at longitudinal fields up to 395 mT in order to study charge transport properties in organic semiconductor. The μSR time spectra were analyzed by using Risch and Kehr (RK) function and it indicates a transition from one-dimensional hopping transport of charge carriers at low temperatures to two- or three-dimensional hopping transport at high temperatures (>75 K).