Solution-Processed Bulk Heterojunction Solar Cells with Silyl End-Capped Sexithiophene

We fabricated solution-processed organic photovoltaic cells (OPVs) using substituted two sexithiophenes, a,w-bis(dimethyl-n-octylsilyl)sexithiophene (DSi-6T) and a,w-dihexylsexithiophene (DH-6T), as electron donors, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor. Solution-processed OPVs using DH-6T and DSi-6T showed good photovoltaic properties in spite of their poor solubility. The best performance was observed on DSi-6T : PCBM 1 : 5 (w/w) blend cell with an open circuit voltage (Voc) of 0.63 V, short circuit current density (Jsc) of 1.34 mA/cm2, fill factor (FF) of 55%, and power conversion efficiency of 0.44% under AM 1.5 G illumination. Although DH-6T has higher hole mobility than DSi-6T, the DSi-6T : PCBM blend cell showed higher hole mobility than DH-6T : PCBM cell. Therefore, DSi-6T cell showed higher device performance than DH-6T cell due to its silyl substitutions, which lead to the increase of the solubility. The incorporation of solution-processed TiO2 interfacial layer in the DSi-6T : PCBM devices significantly enhances FF due to the reduced charge recombination near active layer/Al interface

High performance inverted polymer solar cells using ultrathin atomic layer deposited TiO2 films

Photovoltaic properties of inverted type polymer solar cells (PSCs) using ultrathin TiO2 layers as an adjuvant electron collecting layer were investigated. To prepare the ultrathin (2.5 nm) TiO2 layers on top of TiO2 nanoparticles, atomic layer deposition (ALD) process was conducted at 125, 175, and 200 degrees C. The addition of ALD TiO2 on nanoparticulated TiO2 effectively enhanced the photovoltaic performances of inverted organic solar cells. The inverted PSC device with the thin 200 degrees C-ALD TiO2 layers showed the highest power conversion efficiency of 3.50%, which is an enhancement of approximately 30% compared to the cells without the ALD TiO2 layer (PCE = 2.72%). This work demonstrates that the ALD process plays a critical role in the enhancement of electron extraction efficiency with treating the surface defects of the TiO2 nanoparticles

Morphological investigation of P3HT/PCBM heterojunction and its effects on the performance of bilayer organic solar cells

Organic solar cells utilizing a P3HT/PCBM bilayer (BL) as their photoactive layer are fabricated using a sequential solution process. The film morphology and solar cell performance are investigated by changing the thickness of PCBM layer. The power conversion efficiency (PEC) of a BL solar cell strongly depends on the thickness of the PCBM layer; this value increases significantly after thermal annealing above the glass transition temperature of the P3HT. Based on the water contact angle and photoluminescence (PL) measurements, most of the PCBM was diffused into the amorphous P3HT region after thermal annealing. The external quantum efficiency spectrum reveals that the morphology of the P3HT in the BL solar cell is crystalline, even without thermal annealing, in contrast to the bulk heterojunction films. Moreover, thermal annealing improves charge collection efficiency by generating crystalline P3HT and PCBM domains. The transient photovoltage experiments suggest that the morphology of P3HT and PCBM in the BL solar cell reduces the charge recombination better than that in the BHJ solar cell. The BL solar cell exhibits a PEC of 3.54%, similar to that of the BHJ solar cell. (C) 2014 Elsevier B.V. All rights reserve

Solution processed WO3 layer for the replacement of PEDOT:PSS layer in organic photovoltaic cells

Tungsten oxide layer is formed uniformly by a sol-gel technique on top of indium tin oxide as a neutral and photo-stable hole extraction layer (HEL). The solution processed tungsten oxide layer (sWO(3)) is fully characterized by UV-Vis, XPS, UPS, XRD, AFM, and TEM. Optical transmission of ITO/sWO(3) substrates is nearly identical to ITOs. In addition, the sWO(3) layer induces nearly ohmic contact to P3HT as PEDOT: PSS layer does, which is determined by UPS measurement. In case that an optimized thickness (similar to 10 nm) of the sWO(3) layer is incorporated in the organic photovoltaic devices (OPVs) with a structure of ITO/sWO(3)/P3HT: PCBM/Al, the power conversion efficiency (PCE) is 3.4%, comparable to that of devices utilizing PEDOT: PSS as HEL. Furthermore, the stability of OPV utilizing sWO3 is significantly enhanced due to the air-and photo-stability of the sWO(3) layer itself. PCEs are decreased to 40% and 0% of initial values, when PEDOT: PSS layers are exposed to air and light for 192 h, respectively. In contrast, PCEs are maintained to 90% and 87% of initial PCEs respectively, when sWO(3) layers are exposed to the same conditions. Conclusively, we find that solution processed tungsten oxide layers can be prepared easily, act as an efficient hole extraction layer, and afford a much higher stability than PEDOT: PSS layers

Roughening Conjugated Polymer Surface for Enhancing the Charge Collection Efficiency of Sequentially Deposited Polymer/Fullerene Photovoltaics

A method that enables the formation of a rough nano-scale surface for conjugated polymers is developed through the utilization of a polymer chain ordering agent (OA). 1-Chloronaphthalene (1-CN) is used as the OA for the poly(3-hexylthiophene-2,5-diyl) (P3HT) layer. The addition of 1-CN to the P3HT solution improves the chain ordering of the P3HT during the film formation process and increases the surface roughness of the P3HT film compared to the film prepared without 1-CN. The roughened surface of the P3HT film is utilized to construct a P3HT/fullerene bilayer organic photovoltaic (OPV) by sequential solution deposition (SqSD) without thermal annealing process. The power conversion efficiency (PCE) of the SqSD-processed OPV utilizing roughened P3HT layer is 25% higher than that utilizing a plain P3HT layer. It is revealed that the roughened surface of the P3HT increases the heterojunction area at the P3HT/fullerene interface and this resulted in improved internal charge collection efficiency, as well as light absorption efficiency. This method proposes a novel way to improve the PCE of the SqSD-processed OPV, which can be applied for OPV utilizing low band gap polymers. In addition, this method allows for the reassessment of polymers, which have shown insufficient performance in the BSD process

Lamellar Orientation and Transition Behavior of PS-b-P2VP Copolymers Supported on Physically Adsorbed Layers

A simple approach to utilizing physically adsorbed layers consisting of nonfunctional random and block copolymer chains is suggested for surface neutrality toward lamella-forming polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) films. A chemically identical, low-molecular-weight (LMW) random copolymer of P(S-r-2VP) and block copolymer (BCP) of PS-b-P2VP are used to form the adsorbed layers on an impenetrable solid substrate. We investigate lamellar orientation and transition behavior of the overlying PS-b-P2VP films with respect to the thickness (h(ad)) of the adsorbed copolymer layers. As the h(ad) of P(S-r-2VP ) increases to 4.0 nm, an increasing population of perpendicular lamellae in high-molecular-weight (HMW) PS-b-P2VP films is attributed to the corresponding increase in the surface neutrality toward the BCP films. Meanwhile, an extremely thin adsorbed layer at h(ad) = 1.7 nm, obtained from LMW PS-b-P2VP, is superior in terms of surface neutrality to produce perpendicular lamellae in HMW PS-b-P2VP films. Such a thickness corresponds presumably to the minimum thickness for surface coverage on the solid substrate. The trends for the formation of perpendicular lamellae in HMW PS-b-P2VP films are well consistent with those for lowering the order-to-disorder transition temperature (T-ODT) in LMW PS-b-P2VP films with respect to h(ad). Furthermore, the film thickness dependence on T-ODT of BCP films confined in the best neutral layers reveals that the physically adsorbed layers prepared from the BCP itself provide more balanced interfacial interactions toward the overlying BCP films than the adsorbed random copolymer layers

Immobilization of Conjugated Polymer Domains for Highly Stable Non-Fullerene-Based Organic Solar Cells

To commercialize organic solar cells (OSCs), changes in the optimized morphology of the photoactive layer caused by external stimuli that cause degradation must be addressed. This work improves OSC stability by utilizing the cross-linking additive 1,8-dibromooctane (DBO) and a sequential deposition process (XSqD) to fabricate the photoactive layer. The cross-linking additive in the donor polymer (PTB7-Th) improves polymer crystallinity and immobilizes the crystalline morphology by partial photo-cross-linking. Ellipsometry experiments confirm the increase in the glass transition temperature of cross-linked PTB7-Th. The polymer crystallinity is further improved after removal of non-cross-linked polymer and residual additive by chlorobenzene. The cross-linked polymer layer forms an efficient and stable heterojunction with a nonfullerene acceptor (IEICO-4F) layer via an XSqD process. The OSC based on the immobilized PTB7-Th exhibits excellent stability against light soaking and thermal aging

Facile external treatment for efficient nanoscale morphology control of polymer solar cells using a gas-assisted spray method

A facile and effective treatment method for controlling the morphology of bulk heterojunction (BHJ) structured polymer-based solar cells (PSCs) using a gas-assisted spray (g-spray) technique was demonstrated. High-efficiency BHJ-PSCs were fabricated using a g-spray method that can be adapted to large-scale high-throughput continuous production, and the bulk film morphology and internal nanomorphology of the active layers were well manipulated using a sprayed solvent overlayer (SSO) treatment. The efficient nanomorphology evolution, which is a prerequisite for obtaining high performance BHJ-PSCs, was confirmed by X-ray diffraction, UV-Vis, photoluminescence, and transmission electron microscopy analysis. The SSO treatment was a simple and rapid process that could be carried out at room temperature, unlike conventional external treatment (ET) methods such as solvent-or thermal-assisted treatment, which typically require a prolonged time (> 1 h) or relatively high temperature (> 110 degrees C). After SSO treatment, the PSC performance was enhanced remarkably. The power conversion efficiency (PCE) of the g-sprayed PSCs after SSO treatment was 2.99%, which is higher than that of a solvent vapor treated device (2.42%) and thermally annealed devices (2.61%). Further optimization of the nanomorphology was achieved by sequentially developing P3HT and PCBM. By combining thermal annealing with the SSO treatment, the P3HT/PCBM interfacial area could be enhanced; this enhancement was induced by the PCBM diffusion into the space among pre-assembled P3HT nanofibrils, which in turn promoted their bi-continuity. This means of sequential nanomorphology development further enhanced the PCE (3.35%), which was higher than the other reported values for PSCs using spray methods. Considering that the SSO treatment is a facile room temperature process that requires a short time, these results suggest that the g-spray method can be successfully applied to the continuous production of PSCs