Spray‐Coated Silver Nanowires as Top Electrode Layer in Semitransparent P3HT:PCBM‐Based Organic Solar Cell Devices

Silver nanowire (Ag NW) thin films are investigated as top electrodes in semitransparent inverted organic solar cells. The performance of semitransparent poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) organic solar cells with Ag NW top electrode layers is found to match very closely the performance of reference devices based on thermally evaporated, highly reflective metal silver top electrodes. The optical losses of the semitransparent electrodes are investigated in detail and analyzed in terms of transmission, scattering, and reflection losses. The impact on an external back reflector is shown to increase the light harvesting efficiency of optically thin devices. Further analysis of transparent devices under illumination from the indium tin oxide (ITO) backside and through the Ag NW front electrode open the possibility to gain deep insight into the vertical microstructure related devices performance. Overall, Ag NW top electrodes are established as a serious alternative to TCO based electrodes. Semitransparent devices with efficiencies of over η = 2.0% are realized.     

Non‐destructive lateral mapping of the thickness of the photoactive layer in polymer‐based solar cells

Non‐destructive lateral mapping of the thickness of the photoactive layer in poly(3‐hexyl‐thiophene) : 1‐(3‐methoxy‐carbonyl)propyl‐1‐phenyl‐(6,6)C₆₁ (P3HT : PCBM) solar cells is demonstrated. The method employs a spatially resolved (XY) recording of ultraviolet‐visible spectra in reflection geometry at normal incidence, using a dense raster defined by a circular probe spot of 800‐µm diameter. The evaluation of the thickness of the photoactive layer at each raster point employs an algorithm‐driven comparison of the measured absorption spectrum with spectral features, as compiled from the corresponding simulated spectrum. For the robustness of the applied algorithm toward noise in the recorded absorption data to be increased, a new minimum finder algorithm is described and implemented. The thickness evaluation relies on the correct assignment of extrema in the experimental absorption spectra to the corresponding extrema in the simulated absorption spectra, and a new algorithm for this is also implemented and described. For a level of confidence for the method to be established, first thickness mapping is performed for a set of reference samples consisting of P3HT : PCBM spin‐coated on indium tin oxide‐coated float glass substrates. After this, two application examples for solar cells processed either by spin coating or slot die coating of the P3HT : PCBM layer follow. The spin‐coated solar cells have glass as the substrate with the P3HT : PCBM spun at different spinning speeds. The slot die‐coated solar cells were processed on polyethylene terephthalate foil in a roll‐to‐roll experiment involving a continuously changing P3HT : PCBM concentration along the printing direction. Copyright © 2011 John Wiley & Sons, Ltd.     

All-solution processed organic solar cells with top illumination

All-solution processed organic solar cells with inverted device architecture were demonstrated. Devices contain opaque bottom electrodes and semitransparent top electrodes, resulting in top illuminated devices. Nanoparticles-based Ag ink was used for inkjet printing both top and bottom electrodes. Semi-transparent top electrode consists of high conductivity PEDOT:PSS and Ag current collecting grids. Printed electrodes were compared to evaporated Ag electrodes (both top and bottom) and to ITO electrode in terms of transmittance, roughness, sheet resistance and device performance. All-solution processed devices with top illumination have average PCE of 2.4%, using P3HT:PCBM as photoactive layer. Top-illuminated devices with inverted architecture and bottom-illuminated device with conventional architecture, containing the identical layers, but in the reverse sequence, were then compared. Performed studies have revealed an advantage of inverted cell architecture.     

Effect of Alkyl Side‐Chain Length on Photovoltaic Properties of Poly(3‐alkylthiophene)/PCBM Bulk Heterojunctions

The morphological, bipolar charge‐carrier transport, and photovoltaic characteristics of poly(3‐alkylthiophene) (P3AT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blends are studied as a function of alkyl side‐chain length m, where m equals the number of alkyl carbon atoms. The P3ATs studied are poly(3‐butylthiophene) (P3BT, m = 4), poly(3‐pentylthiophene) (P3PT, m = 5), and poly(3‐hexylthiophene) (P3HT, m = 6). Solar cells with these blends deliver similar order of photo‐current yield (exceeding 10 mA cm^?2) irrespective of side‐chain length. Power conversion efficiencies of 3.2, 4.3, and 4.6% are within reach using solar cells with active layers of P3BT:PCBM (1:0.8), P3PT:PCBM (1:1), and P3HT:PCBM (1:1), respectively. A difference in fill factor values is found to be the main source of efficiency difference. Morphological studies reveal an increase in the degree of phase separation with increasing alkyl chain length. Moreover, while P3PT:PCBM and P3HT:PCBM films have similar hole mobility, measured by hole‐only diodes, the hole mobility in P3BT:PCBM lowers by nearly a factor of four. Bipolar measurements made by field‐effect transistor showed a decrease in the hole mobility and an increase in the electron mobility with increasing alkyl chain length. Balanced charge transport is only achieved in the P3HT:PCBM blend. This, together with better processing properties, explains the superior properties of P3HT as a solar cell material. P3PT is proved to be a potentially competitive material. The optoelectronic and charge transport properties observed in the different P3AT:PCBM bulk heterojunction (BHJ) blends provide useful information for understanding the physics of BHJ films and the working principles of the corresponding solar cells.     

Highly Efficient and Bendable Organic Solar Cells with Solution‐Processed Silver Nanowire Electrodes

Highly efficient and bendable organic solar cells (OSCs) are fabricated using solution‐processed silver nanowire (Ag NW) electrodes. The Ag NW films were highly transparent (diffusive transmittance ≈ 95% at a wavelength of 550 nm), highly conductive (sheet resistance ≈ 10 Ω sq^?1), and highly flexible (change in resistance ≈ 1.1 ± 1% at a bending radius of ≈200 μm). Power conversion efficiencies of ≈5.80 and 5.02% were obtained for devices fabricated on Ag NWs/glass and Ag NWs/poly(ethylene terephthalate) (PET), respectively. Moreover, the bendable devices fabricated using the Ag NWs/PET films decrease slightly in their efficiency (to ≈96% of the initial value) even after the devices had been bent 1000 times with a radius of ≈1.5 mm.     

Enhanced Thermal Stability and Efficiency of Polymer Bulk‐Heterojunction Solar Cells by Low‐Temperature Drying of the Active Layer

This study addresses two key issues, stability and efficiency, of polymer solar cells based on blended poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) by demonstrating a film‐forming process that involves low‐temperature drying (?5 °C) and subsequent annealing of the active layer. The low‐temperature process achieves 4.70% power conversion efficiency (PCE) and ～1250 h storage half‐life at 65 °C, which are significant improvements over the 3.39% PCE and ～143 h half‐life of the regular room‐temperature process. The improvements are attributed to the enhanced nucleation of P3HT crystallites as well as the minimized separation of the P3HT and PCBM phases at the low drying temperature, which upon post‐drying annealing results in a morphology consisting of small PCBM‐rich domains interspersed within a densely interconnected P3HT crystal network. This morphology provides ample bulk‐heterojunction area for charge generation while allowing for facile charge transport; moreover, the P3HT crystal network serves as an immobile frame at heating temperatures less than the melting point (T_m) of P3HT, thus preventing PCBM/P3HT phase separation and the corresponding device degradation.     

Time‐Dependent Morphology Evolution by Annealing Processes on Polymer:Fullerene Blend Solar Cells

Changes in the nanoscale morphologies of the blend films of poly (3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), for high‐performance bulk‐heterojunction (BHJ) solar cells, are compared and investigated for two annealing treatments with different morphology evolution time scales, having special consideration for the diffusion and aggregation of PCBM molecules. An annealing condition with relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization results in poor BHJ morphology because of prevention of the formation of the more elongated P3HT crystals. However, an annealing condition, accelerating PCBM diffusion after the formation of a well‐ordered morphology, results in a relatively stable morphology with less destruction of crystalline P3HT. Based on these results, an effective strategy for determining an optimized annealing treatment is suggested that considers the effect of relative kinetics on the crystallization of the components for a blend film with a new BHJ materials pair, upon which BHJ solar cells are based.     

Abrupt Morphology Change upon Thermal Annealing in Poly(3‐Hexylthiophene)/Soluble Fullerene Blend Films for Polymer Solar Cells

The in situ morphology change upon thermal annealing in bulk heterojunction blend films of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₁ (PCBM) is measured by a grazing incidence X‐ray diffraction (GIXD) method using a synchrotron radiation source. The results show that the film morphology—including the size and population of P3HT crystallites—abruptly changes at 140 °C between 5 and 30 min and is then stable up to 120 min. This trend is almost in good agreement with the performance change of polymer solar cells fabricated under the same conditions. The certain morphology change after 5 min annealing at 140 °C is assigned to the on‐going thermal transition of P3HT molecules in the presence of PCBM transition. Field‐emission scanning electron microscopy measurements show that the crack‐like surface of blend films becomes smaller after a very short annealing time, but does not change further with increasing annealing time. These findings indicate that the stability of P3HT:PCBM solar cells cannot be secured by short‐time annealing owing to the unsettled morphology, even though the resulting efficiency is high.     

Annealing‐Free High Efficiency and Large Area Polymer Solar Cells Fabricated by a Roller Painting Process

Polymer solar cells are fabricated by a novel solution coating process, roller painting. The roller‐painted film – composed of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) – has a smoother surface than a spin‐coated film. Since the roller painting is accompanied by shear and normal stresses and is also a slow drying process, the process effectively induces crystallization of P3HT and PCBM. Both crystalline P3HT and PCBM in the roller‐painted active layer contribute to enhanced and balanced charge‐carrier mobility. Consequently, the roller‐painting process results in a higher power conversion efficiency (PCE) of 4.6%, as compared to that for spin coating (3.9%). Furthermore, annealing‐free polymer solar cells (PSCs) with high PCE are fabricated by the roller painting process with the addition of a small amount of octanedi‐1,8‐thiol. Since the addition of octanedi‐1,8‐thiol induces phase separation between P3HT and PCBM and the roller‐painting process induces crystallization of P3HT and PCBM, a PCE of roller‐painted PSCs of up to 3.8% is achieved without post‐annealing. A PCE of over 2.7% can also be achieved with 5 cm² of active area without post‐annealing.     

Effect of Carbon Chain Length in the Substituent of PCBM‐like Molecules on Their Photovoltaic Properties

A series of [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM)‐like fullerene derivatives with the butyl chain in PCBM changing from 3 to 7 carbon atoms, respectively (F1–F5), are designed and synthesized to investigate the relationship between photovoltaic properties and the molecular structure of fullerene derivative acceptors. F2 with a butyl chain is PCBM itself for comparison. Electrochemical, optical, electron mobility, morphology, and photovoltaic properties of the molecules are characterized, and the effect of the alkyl chain length on their properties is investigated. Although there is little difference in the absorption spectra and LUMO energy levels of F1–F5, an interesting effect of the alkyl chain length on the photovoltaic properties is observed. For the polymer solar cells (PSCs) based on P3HT as donor and F1–F5, respectively, as acceptors, the photovoltaic behavior of the P3HT/F1 and P3HT/F4 systems are similar to or a little better than that of the P3HT/PCBM device with power conversion efficiencies (PCEs) above 3.5%, while the performances of P3HT/F3 and P3HT/F5‐based solar cells are poorer, with PCE values below 3.0%. The phenomenon is explained by the effect of the alkyl chain length on the absorption spectra, fluorescence quenching degree, electron mobility, and morphology of the P3HT/F1–F5 (1:1, w/w) blend films.     

Performance improvement mechanisms of P3HT:PCBM inverted polymer solar cells using extra PCBM and extra P3HT interfacial layers

A novel P3HT:PCBM inverted polymer solar cell (IPSC) was fabricated and investigated. An extra PCBM and an extra P3HT interfacial layers were inserted into the bottom side and the top side of the P3HT:PCBM absorption layer of the IPSCs to respectively enhance electron transport and hole transport to the corresponding electrodes. According to the surface energy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) measurement results, the extra PCBM interfacial layer could let more P3HT to form on the top side of the P3HT:PCBM blends. It revealed that the non-continuous pathways of P3HT in the P3HT:PCBM absorption layer could be reduced. Consequently, the carrier recombination centers were reduced in the absorption layer of IPSCs. The power conversion efficiency (PCE) of the P3HT:PCBM IPSCs with an extra PCBM interfacial layer greatly increased from 3.39% to 4.50% in comparison to the P3HT:PCBM IPSCs without an extra PCBM interfacial layer. Moreover, the performance of the P3HT:PCBM IPSCs with an extra PCBM interfacial layer could be improved by inserting an extra P3HT interfacial layer between the absorption layer and the MoO₃ layer. The PCE of the resulting IPSCs increased from 4.50% to 4.97%.     

Printing high performance reflective electrodes for organic solar cells

In this paper we solve a long term material problem of thin film organic electronics, namely the solution processing of an opaque electrode. Solution processing of opaque metallic top electrodes typically leads to severe shunting problems. We solve this issue by reversing the electrode sequence and print a highly conductive but opaque bottom electrode from metallic precursors. Devices based on these printed bottom electrodes are compared to reference stacks based on evaporated silver. The transparent top electrode is solution processed from silver nanowire inks, which results in highly transparent electrodes with excellent conductivity. The optical, mainly reflective properties of the opaque silver electrode are investigated in comparison to screen-printed silver pastes. The outstanding smoothness of the printed Ag electrode results in high reflectivity and poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based solar cells with η > 2.5% and high fill factors performing on par with reference devices on evaporated silver electrode layers.     

High‐Performance Air‐Processed Polymer–Fullerene Bulk Heterojunction Solar Cells

High photovoltaic device performance is demonstrated in ambient‐air‐processed bulk heterojunction solar cells having an active blend layer of organic poly(3‐hexylthiophene) (P3HT): [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), with power conversion efficiencies as high as 4.1%, which is comparable to state‐of‐the‐art bulk heterojunction devices fabricated in air‐free environments. High‐resolution transmission electron microscopy is combined with detailed analysis of electronic carrier transport in order to quantitatively understand the effects of oxygen exposure and different thermal treatments on electronic conduction through the highly nanostructured active blend network. Improvement in photovoltaic device performance by suitable post‐fabrication thermal processing results from the reduced oxygen charge trap density in the active blend layer and is consistent with a corresponding slight increase in thickness of an ～4 nm aluminum oxide hole‐blocking layer present at the electron‐collecting contact interface.     

Oriented Growth of Al2O3:ZnO Nanolaminates for Use as Electron‐Selective Electrodes in Inverted Polymer Solar Cells

Atomic layer deposition is used to synthesize Al₂O₃:ZnO(1:x) nanolaminates with the number of deposition cycles, x, ranging from 5 to 30 for evaluation as optically transparent, electron‐selective electrodes in polymer‐based inverted solar cells. Al₂O₃:ZnO(1:20) nanolaminates are found to exhibit the highest values of electrical conductivity (1.2 × 10³ S cm^?1; more than six times higher than for neat ZnO films), while retaining a high optical transmittance (≥80% in the visible region) and a low work function (4.0 eV). Such attractive performance is attributed to the structure (ZnO crystal size and crystal alignment) and doping level of this intermediate Al₂O₃:ZnO film composition. Polymer‐based inverted solar cells using poly(3‐hexylthiophene) (P3HT):phenyl‐C₆₁‐butyric acid methyl ester (PCBM) mixtures in the active layer and Al₂O₃:ZnO(1:20) nanolaminates as transparent electron‐selective electrodes exhibit a power conversion efficiency of 3% under simulated AM 1.5 G, 100 mW cm^?2 illumination.     

Photoconductivity of a Low‐Bandgap Conjugated Polymer

The photoconductive properties of a novel low‐bandgap conjugated polymer, poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)], PCPDTBT, with an optical energy gap of E_g ～ 1.5 eV, have been studied. The results of photoluminescence and photoconductivity measurements indicate efficient electron transfer from PCPDTBT to PCBM ([6,6]‐phenyl‐C61 butyric acid methyl ester, a fullerene derivative), where PCPDTBT acts as the electron donor and PCBM as the electron acceptor. Electron‐transfer facilitates charge separation and results in prolonged carrier lifetime, as observed by fast (t > 100 ps) transient photoconductivity measurements. The photoresponsivities of PCPDTBT and PCPDTBT:PCBM are comparable to those of poly(3‐hexylthiophene), P3HT, and P3HT:PCBM, respectively. Moreover, the spectral sensitivity of PCPDTBT:PCBM extends significantly deeper into the infrared, to 900 nm, than that of P3HT. The potential of PCPDTBT as a material for high‐efficiency polymer solar cells is discussed.     

Electrostatic Self‐Assembly Conjugated Polyelectrolyte‐Surfactant Complex as an Interlayer for High Performance Polymer Solar Cells

A simple method is demonstrated to improve the film‐forming properties and air stability of a conjugated polyelectrolyte (CPE) without complicated synthesis of new chemical structures. An anionic surfactant, sodium dodecybenzenesulfonate (SDS), is mixed with cationic CPEs. The electrostatic attraction between these two oppositely‐charged materials provides the driving force to form a stable CPE‐surfactant complex. Compared with a pure CPE, this electrostatic complex is not only compatible with highly hydrophobic bulk‐heterojunction (BHJ) films, e.g. poly(3‐hexylthiophene):[6,6]‐phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM), but also works well with other low bandgap polymer‐based BHJ films. Using this complex as a cathode interface layer, a high power conversion efficiency of 4% can be obtained in P3HT:PCBM solar cells together with improved stability in air. Moreover, ～20% performance enhancement can also be achieved when the complex is used as an interlayer to replace calcium metal for low bandgap polymer‐based BHJ systems.     

Soluble carbon nanotubes/phthalocyanines transparent electrode and interconnection layers for flexible inverted polymer tandem solar cells

In this study, we focus on transparent electrodes for organic solar cells prepared from aqueous solutions consisting of single wall carbon nanotubes (SWCNTs) and a copper-phthalocyanine derivative (TSCuPc). We first investigated their electrical conductivity and optical properties. The high solubility level of the TSCuPc/SWCNTs enables the production of stable inks with high conductivity which allows obtaining flexible photovoltaic devices based TSCuPc/SWCNTs films with good performances. A power conversion efficiency of 3.2% was achieved in a device with a blend of poly (3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as the active layer with a TSCuPc/SWCNTs sprayed electrode on a polyethylene terephthalate (PET) substrate. TSCuPc and TSCuPc/SWCNTs were also employed as the electron transport layer (ETL) and the interconnecting layer (ICL) in an inverted tandem organic solar cell based on front P3HT–ICBA and back [70]PCBM–PCDTBT active layers, achieving an efficiency of 7.40%.     

Three‐Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

Here, an investigation of three‐dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post‐treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (η_IQE ) of the devices. To investigate the 3D BHJ morphology, novel time‐of‐flight secondary‐ion mass spectroscopy studies are employed along with conventional methods, such as UV‐vis absorption, X‐ray diffraction, and high‐resolution transmission electron microscopy studies. The η_IQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high‐performance P3HT:PCBM solar cells using the optimized solvent‐annealing method is reported, and these cells show a mean power‐conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm^?2.     

All‐Solution‐Processed Indium‐Free Transparent Composite Electrodes based on Ag Nanowire and Metal Oxide for Thin‐Film Solar Cells

Fully solution‐processed Al‐doped ZnO/silver nanowire (AgNW)/Al‐doped ZnO/ZnO multi‐stacked composite electrodes are introduced as a transparent, conductive window layer for thin‐film solar cells. Unlike conventional sol–gel synthetic pathways, a newly developed combustion reaction‐based sol–gel chemical approach allows dense and uniform composite electrodes at temperatures as low as 200 °C. The resulting composite layer exhibits high transmittance (93.4% at 550 nm) and low sheet resistance (11.3 Ω sq^‐1), which are far superior to those of other solution‐processed transparent electrodes and are comparable to their sputtered counterparts. Conductive atomic force microscopy reveals that the multi‐stacked metal‐oxide layers embedded with the AgNWs enhance the photocarrier collection efficiency by broadening the lateral conduction range. This as‐developed composite electrode is successfully applied in Cu(In_1‐x,Ga_x)S₂ (CIGS) thin‐film solar cells and exhibits a power conversion efficiency of 11.03%. The fully solution‐processed indium‐free composite films demonstrate not only good performance as transparent electrodes but also the potential for applications in various optoelectronic and photovoltaic devices as a cost‐effective and sustainable alternative electrode.     

Roll‐to‐Roll Printed Silver Nanowire Semitransparent Electrodes for Fully Ambient Solution‐Processed Tandem Polymer Solar Cells

Silver nanowires (AgNWs) and zinc oxide (ZnO) are deposited on flexible substrates using fast roll‐to‐roll (R2R) processing. The AgNW film on polyethylene terephthalate (PET) shows >80% uniform optical transmission in the range of 550–900 nm. This electrode is compared to the previously reported and currently widely produced indium‐tin‐oxide (ITO) replacement comprising polyethylene terephthalate (PET)|silver grid|poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)|ZnO known as Flextrode. The AgNW/ZnO electrode shows higher transmission than Flextrode above 490 nm in the electromagnetic spectrum reaching up to 40% increased transmission at 750 nm in comparison to Flextrode. The functionality of AgNW electrodes is demonstrated in single and tandem polymer solar cells and compared with parallel devices on traditional Flextrode. All layers, apart from the semitransparent electrodes which are large‐scale R2R produced, are fabricated in ambient conditions on a laboratory roll‐coater using printing and coating methods which are directly transferrable to large‐scale R2R processing upon availability of materials. In a single cell structure, Flextrode is preferable with active layers based on poly‐3‐hexylthiophene(P3HT):phenyl‐C61‐butyric acid methylester (PCBM) and donor polymers of similar absorption characteristics while AgNW/ZnO electrodes are more compatible with low band gap polymer‐based single cells. In tandem devices, AgNW/ZnO is more preferable resulting in up to 80% improvement in PCE compared to parallel devices on Flextrode.