Fabrication Of Organic Solar Cells Research Articles

Water‐Based Blend Nanoparticles of P3HT and PCBM for the Application in Organic Solar Cells

ABSTRACTIn this work, blend nanoparticles of poly(3‐hexylthiophene) (P3HT) and phenyl‐C61‐butyric acid methyl ester (PCBM) were prepared with different composition ratios, namely 50:50, 60:40, and 70:30. Larger particle size is found for the blend nanoparticles with higher P3HT content, as measured by dynamic light scattering and transmission electron microscopy. The blending of nanoparticles did not affect the optical properties of the two materials as indicated by the UV–vis absorption spectra. The photoluminescence spectra of the blend nanoparticles reveal an efficient charge separation reflected by the quenching of the P3HT emission band by the PCBM. The quenching efficiency increases as the content of the PCBM increases. Good layer morphologies were obtained from casting the blend nanoparticles. The layer morphology was further improved by thermal annealing as indicated from the atomic force microscopy results. The water‐based dispersions offer a promising straightforward strategy toward the large‐scale fabrication of organic solar cells.

Digital fabrication of organic solar cells by Inkjet printing using non-halogenated solvents

Inkjet printing offers versatility and flexibility for the sequential deposition of functional layers for the production of large area organic photovoltaics (OPV). Four layers of an OPV cell are inkjet printed, comprising an ITO-free semi-transparent front electrode (a metal current collecting grid, inkjet printed PEDOT:PSS and ZnO nanoparticle layers), and the photo-active layer combined with PEDOT:PSS as hole transport layer. To render the process R2R compatible, large area printing is performed using a 3.5cm wide printhead and non-halogenated ink formulations only. Similar performance is achieved for the inkjet printed cells as for cells with only spin coated layers and ITO. For the P3HT/PCBM bulk-heterojunction, a mixture of non-halogenated solvents ensured good solubility, proper printing behavior and a blend morphology that yields similar performance to a layer spin-coated from chlorinated solvents. The potential of inkjet printing for large area OPV was demonstrated by the fabrication of a module with 92cm2 active area, which showed an efficiency of 0.98%. Losses due to front and back electrode resistances are modeled and used to explain the recorded I-V curve. Combining these functional layers with inkjet printed electrodes lays out the roadmap toward fully roll-to-roll compatible digital fabrication of OPV.

Versatile third components for efficient and stable organic solar cells

This review highlights the recent progress on the fabrication of organic solar cells with various third components which can improve the power conversion efficiency and stability.

Direct laser patterning of transparent ITO–Ag–ITO multilayer anodes for organic solar cells

Direct laser patterning of transparent ITO–Ag–ITO (IAI) multilayer anodes is investigated using a femtosecond fiber laser for application in organic solar cells (OSC) fabrication. By adjusting laser fluence and scan speed, we successfully patterned the IAI multilayer anode without changing the electrical or optical properties. At an optimized laser fluence of 0.6J/cm2 and a scan speed of 200mm/s, the patterned IAI multilayer was electrically isolated with a clean edge. The metallic Ag interlayer of the IAI multilayer plays an important role in direct laser patterning because it absorbed the laser and increases the maximum temperature in the IAI multilayer. In addition, the Ag layer could effectively decrease the temperature of the IAI multilayer after irradiation of laser. The OSC fabricated on the laser patterned IAI multilayer showed power conversion efficiencies of 3.12% (Ag 8nm) and 2.85% (Ag 12nm). Successful operation of the OSC indicates that direct laser patterning of IAI multilayer anodes is a promising, simple patterning technology for fabrication of IAI-based OSCs.

Enhanced photon harvesting in OPV using optical reflective surface

A new device architecture has been employed in the fabrication of organic solar cell using light-reflective background. We found significant improvement on the short circuit current in bulkheterojunction (BHJ) solar cell using smooth light-reflective surface as the background. The reflective surface presumably acts as an optical spacer in the standard BHJ device structure which enhances the generation and dissociation of exciton with in the photoactive layer. It is found that the power conversion efficiency grew by threefold to that of the value without reflective background.

Face-to-face transferred multicrystalline ITO films on colorless polyimide substrates for flexible organic solar cells

We report on the successful face-to-face transfer of crystalline ITO (c-ITO) films from Si wafers to flexible colorless polyimide (CPI) substrates for application in high-performance flexible organic solar cells (OSCs). By using an Ag sacrificial layer and by coating of a CPI solution onto high-temperature processed c-ITO films, the film can be directly transferred from a Si wafer to a flexible CPI substrate without changing its sheet resistance or optical transmittance. At the optimized thickness of 100nm, the transferred c-ITO film showed sheet resistance of 25.65Ω/square, optical transmittance of 83.8%, and critical bending radius of 5.5mm, which are acceptable values in the fabrication of flexible OSCs. The flexible OSCs fabricated on the transferred c-ITO anode exhibited good cell performance with a fill factor of 68.99 %, a short circuit current of 8.90mA/cm2, an open circuit voltage of 0.821V, and a power conversion efficiency of 5.041 %, comparable to that of OSC with reference ITO/glass (5.67%). This indicating that transferred c-ITO film is a promising flexible and transparent anode for high performance FOSCs.

Nanocomposites composed of P3HT:PCBM and nanoparticles synthesized by laser ablation of a bulk PbS target in liquid

Nanoparticles synthesized by laser ablation of bulk target materials in liquids have ligand-free surfaces since no chemical precursors are used for their synthesis, and thus, they are ideally suited for applications in the fabrication of organic solar cells in which the properties of the interface between the nanoparticles and the polymer blend matrix largerly determine the exciton splitting and transport of carriers to the external electrodes, properties crucial for the device operation and performance. Narrow band gap semiconducting quantum dots can act as sensitizers, increasing the absorption of the device active layer in the infrared part of the solar spectrum. In this work, a bulk PbS target was laser ablated (450 fs, 1,064 nm, 1 kHz) in ethanol for the synthesis of nanoparticle colloidal solutions. The solutions exhibit a broad absorption which extends at the longest wavelength measured of ~1,700 nm and beyond. The nanoparticles were directly mixed with the blend P3HT:PCBM for the formation of nanocomposites. The nanocomposites with the nanoparticles exhibit lower transmission in the whole spectral range as compared to the blend without the nanoparticles.

Solution-processed copper phthalocyanine–gold nanoparticle hybrid nanocomposite thin films

Copper phthalocyanine (CuPc) thin film and gold nanoparticles (Au NPs) of size ~20nm were deposited on conducting indium tin oxide coated glass substrates. Thin films were ~500nm thick having crystalline nature determined by surface profilometer and X-ray diffraction technique. The concentration of Au NPs in the films was varied whereas the concentration of CuPc was constant. It was observed that surface morphology varied with Au NP concentration increase in the films accompanied by changes in the optical spectra over 300-1000nm range and increase in the electrical conduction but no changes in the Fourier transform infra-red spectra. Such nanocomposite films would be useful in the fabrication of organic solar cells.

Synthesis of unstable colloidal inorganic nanocrystals through the introduction of a protecting ligand.

We demonstrate a facile and general strategy based on ligand protection for the synthesis of unstable colloidal nanocrystals by using the synthesis of pure p-type NiO nanocrystals as an example. We find that the introduction of lithium stearate, which is stable in the reaction system and capable of binding to the surface of NiO oxide nanocrystals, can effectively suppress the reactivity of NiO nanocrystals and thus prevent their in situ reduction into Ni. The resulting p-type NiO nanocrystals, a highly demanded hole-transporting and electron-blocking material, are applied to the fabrication of organic solar cells and polymer light-emitting diodes, demonstrating their great potential as an interfacial layer for low-cost and large-area, solution-processed optoelectronic devices.

Inkjet-printing of hybrid transparent conducting electrodes for organic solar cells

We developed metal-grid hybrid transparent conducting electrodes (TCEs) by combining a metal grid with a conducting polymer. Metal-grid hybrid TCEs were prepared by inkjet-printing with both PEDOT:PSS and Ag nanoparticle inks and applied to the fabrication of organic solar cells (OSCs) instead of indium tin oxide (ITO). The electrical and optical properties of the metal-grid hybrid TCEs were optimized by modulating the Ag grid's line-to-line spacing (pitch). With a Ag-grid pitch of 1 mm, the metal-grid hybrid TCEs exhibited a sheet resistance of 10.3 Ω sq−1 and an optical transmittance of 73% which represented ≤10% reduction when compared to the optical transmittance of inkjet-printed PEDOT:PSS films without Ag grids. Ag covering factor (ACF) was defined using the geometry of the Ag-grid and incorporated in a theoretical model to calculate the electrical and optical properties of the inkjet-printed metal-grid hybrid TCEs. It was found that the experimental values of the TCEs' electrical and optical properties were in good agreement with the calculation results based on the ACF-incorporated theoretical model. OSCs fabricated with the metal-grid hybrid TCEs showed a power-conversion cell efficiency comparable to those of conventional OCSs using ITO. This indicates that the inkjet-printed metal-grid hybrid TCE is a promising replacement for ITO to realize cost-effective OSCs.

Controlling the Solidification of Organic Photovoltaic Blends with Nucleating Agents

AbstractBlending fullerenes with a donor polymer for the fabrication of organic solar cells often leads to at least partial vitrification of one, if not both, components. For prototypical poly(3-hexylthiophene):fullerene blend, we show that the addition of a commercial nucleating agent, di(3,4-dimethyl benzylidene)sorbitol, to such binary blends accelerates the crystallization of the donor, resulting in an increase in its degree of crystallinity in as-cast structures. This allows manipulation of the extent of intermixing/ phase separation of the donor and acceptor directly from solution, offering a tool to improve device characteristics such as power conversion efficiency.

Effects of Organic Solvents for Composite Active Layer of PCDTBT/PC71BM on Characteristics of Organic Solar Cell Devices

Bulk heterojunction (BHJ) structure based active layers of PCDTBT/PC71BM were prepared by using different organic solvents for fabrication of organic solar cell (OSC) devices. Mixture of precursor solutions of PCDTBT/PC71BM in three different organic solvents was prepared to fabricate composite active layers by spin-coating process: chloroform; chlorobenzene; o-dichlorobenzene. Four different blend ratios (1 : 3–1 : 6) of PCDTBT: PC71BM were adopted for each organic solvent to clarify the effect on the resulting OSC device characteristics. Surface morphology of the active layers was distinctively affected by the blend ratio of PCDTBT/PC71BM in organic solvents. Influence of the blend ratio of PCDTBT/PC71BM on the OSC device parameters was discussed. Performance parameters of the resulting OSC devices with different composite active layers were comparatively investigated. Appropriate blend ratio and organic solvent to achieve better OSC device performance were proposed. Furthermore, from the UV-Vis spectrum of each active layer prepared using the PCDTBT/PC71BM mixed solution dissolved with different organic solvents, a possibility that the nanophase separation structure inside their active layer could appear was suggested.

Environmentally friendly biomaterials as an interfacial layer for highly efficient and air-stable inverted organic solar cells

Peptides present a low cost and environmentally friendly route to facilitate the fabrication of organic solar cells.

Efficient “Light-soaking”-free Inverted Organic Solar Cells with Aqueous Solution Processed Low-Temperature ZnO Electron Extraction Layers

Low-temperature processes are unremittingly pursued in the fabrication of organic solar cells. The paper reports that the highly efficient and "light-soaking"-free inverted organic solar cell can be achieved by using ZnO thin films processed from the aqueous solution method at a low temperature. The inverted organic solar with an aqueous-processed ZnO thin film annealed at 150 °C shows an efficiency of 3.79%. Even when annealed at a temperature as low as 80 °C, the device still shows an efficiency of 3.71%. With the proper annealing temperature of 80 °C, the flexible device, which shows an efficiency of 3.56%, is fabricated on PET. This flexible device still keeps the efficiency above 3.40% after bent for 1000 times with a curvature radius of 50 mm. In contrast, a low annealing temperature leads to an inferior device performance when the ZnO thin film is processed from the widely used sol-gel method. The device with sol-gel processed ZnO annealed at 150 °C only shows a PCE of 1.3%. Furthermore, the device shows a strong "light-soaking" effect, which is not observed in the device containing an aqueous-processed ZnO thin film. Our results suggest that the adopted aqueous solution method is a more efficient low temperature technique, compared with the sol-gel method.

Organic solar cells using CVD-grown graphene electrodes

We report on the development of flexible organic solar cells (OSCs) incorporating graphene sheets synthesized by chemical vapor deposition (CVD) as transparent conducting electrodes on polyethylene terephthalate (PET) substrates. A key barrier that must be overcome for the successful fabrication of OSCs with graphene electrodes is the poor-film properties of water-based poly(3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) when coated onto hydrophobic graphene surfaces. To form a uniform PEDOT:PSS film on a graphene surface, we added perfluorinated ionomers (PFI) to pristine PEDOT:PSS to create ‘GraHEL’, which we then successfully spin coated onto the graphene surface. We systematically investigated the effect of number of layers in layer-by-layer stacked graphene anode of an OSC on the performance parameters including the open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF). As the number of graphene layers increased, the FF tended to increase owing to lower sheet resistance, while Jsc tended to decrease owing to the lower light absorption. In light of this trade-off between sheet resistance and transmittance, we determined that three-layer graphene (3LG) represents the best configuration for obtaining the optimal power conversion efficiency (PCE) in OSC anodes, even at suboptimal sheet resistances. We finally developed efficient, flexible OSCs with a PCE of 4.33%, which is the highest efficiency attained so far by an OSC with CVD-grown graphene electrodes to the best of our knowledge.

Highly transparent Nb-doped indium oxide electrodes for organic solar cells

The authors investigated the characteristics of Nb-doped In2O3 (INbO) films prepared by co-sputtering of Nb2O5 and In2O3 for use in transparent anodes for organic solar cells (OSCs). To optimize the Nb dopant composition in the In2O3 matrix, the effect of the Nb doping power on the resistivity and transparency of the INbO films were examined. The electronic structure and microstructure of the INbO films were also investigated using synchrotron x-ray absorption spectroscopy and x-ray diffraction examinations in detail. At the optimized Nb co-sputtering power of 30 W, the INbO film exhibited a sheet resistance of 15 Ω/sq, and an optical transmittance of 86.04% at 550 nm, which are highly acceptable for the use as transparent electrodes in the fabrication of OSCs. More importantly, the comparable power conversion efficiency (3.34%) of the OSC with an INbO anode with that (3.31%) of an OSC with a commercial ITO anode indicates that INbO films are promising as a transparent electrode for high performance OSCs.

Photocatalytic Synthesis and Photovoltaic Application of Ag-TiO2 Nanorod Composites

A photocatalytic strategy has been developed to synthesize colloidal Ag-TiO2 nanorod composites in which each TiO2 nanorod contains a single Ag nanoparticle on its surface. In this rational synthesis, photoexcitation of TiO2 nanorods under UV illumination produces electrons that reduce Ag(I) precursor and deposit multiple small Ag nanoparticles on the surface of TiO2 nanorods. Prolonged UV irradiation induces an interesting ripening process, which dissolves the smaller nanoparticles by photogenerated oxidative species and then redeposits Ag onto one larger and more stable particle attached to each TiO2 nanorod through the reduction of photoexcited electrons. The size of the Ag nanoparticles can be precisely controlled by varying the irradiation time and the amount of alcohol additive. The Ag-TiO2 nanorod composites were used as electron transport layers in the fabrication of organic solar cells and showed notable enhancement in power conversion efficiency (6.92%) than pure TiO2 nanorods (5.81%), as well as higher external quantum efficiency due to improved charge separation and transfer by the presence of Ag nanoparticles.

Preparation and characterization of MoO3 hole-injection layer for organic solar cell fabrication and optimization

A facile, solution-processed method to fabricate MoO3 (s-MoO3) thin film as hole-injection layer (HIL) for poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) based organic bulk hetero-junction photovoltaics is presented. The structural, electronic property and surface microstructure of the s-MoO3 thin film are investigated in detail by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, scanning electron microscopy and atomic force microscope. The results indicate that the s-MoO3 thin film possesses appropriate morphological, optical and electronic properties to be suitable for organic photovoltaic applications. The photovoltaic devices have been investigated and optimized in detail by tuning layer thickness, processing temperature and time, annealing conditions of interfacial layers. Using s-MoO3 thin film as hole-injection layer, the device gives open circuit voltage of 0.64V, circuit current density of 9.15mAcm−2, fill factor of 0.67 and power conversion efficiency of 3.92%, which is higher than the controlled device using PEDOT:PSS layer. In addition, the s-MoO3 based devices exhibit good stability.

Optimization of solution-processed oligothiophene:fullerene based organic solar cells by using solvent additives

The optimization of solution-processed organic bulk-heterojunction solar cells with the acceptor-substituted quinquethiophene DCV5T-Bu4 as donor in conjunction with PC61BM as acceptor is described. Power conversion efficiencies up to 3.0% and external quantum efficiencies up to 40% were obtained through the use of 1-chloronaphthalene as solvent additive in the fabrication of the photovoltaic devices. Furthermore, atomic force microscopy investigations of the photoactive layer gave insight into the distribution of donor and acceptor within the blend. The unique combination of solubility and thermal stability of DCV5T-Bu4 also allows for fabrication of organic solar cells by vacuum deposition. Thus, we were able to perform a rare comparison of the device characteristics of the solution-processed DCV5T-Bu4:PC61BM solar cell with its vacuum-processed DCV5T-Bu4:C60 counterpart. Interestingly in this case, the efficiencies of the small-molecule organic solar cells prepared by using solution techniques are approaching those fabricated by using vacuum technology. This result is significant as vacuum-processed devices typically display much better performances in photovoltaic cells.

Transparent Ti-doped In2O3 films grown by linear facing target sputtering for organic solar cells

The electrical, optical, and structural properties of Ti-doped In2O3 (TIO) films grown by linear facing target sputtering (LFTS) were investigated as a function of the DC power and the annealing temperature for use as a transparent anode in organic solar cells (OSCs). By rapid thermal annealing at 600 °C, we obtained optimized TIO anodes with a sheet resistance of 26.08 Ohm/square and an optical transmittance of 84.88%, which are acceptable for OSC fabrication. The effective Ti dopant activation and the crystallization of the TIO film led to a reduced resistivity of the TIO films. The OSC fabricated on the optimized TIO film showed a higher power conversion efficiency of 2.576% compared to the OSC with as-deposited TIO anodes (1.645%) because the fill factors of the OSCs critically depend on the sheet resistance of the anode. Successful fabrication of OSCs with TIO anodes indicates that the LFTS-grown TIO film is a promising transparent anode for OSCs.