ITO-free Polymer Solar Cells Research Articles

ZnO/Ag/ZnO multilayer transparent electrode for highly-efficient ITO-Free polymer solar cells

Ultrathin metal film (UTMF) with a ZnO/Ag/ZnO hybrid structure was used as transparent electrode in a high-efficiency bulk heterojunction system for the fabrication of ITO-free polymer solar cells. The performance of the devices was carefully tuned through optical simulation using transfer matrix method by varying the thickness of ZnO seed layer and thin absorber layer. By employing appropriate device architecture, polymer solar cells fabricated using this UTMF-based electrode show efficiency as high as 9.49%, which is slightly higher compared to that of ITO-based device. From good agreement between the external quantum efficiency and optical modeling, it was found that the optimized microcavity configuration formed in UTMF-based device can greatly enhance the absorbance of the BHJ layer at longer wavelength as well as the favored exciton distribution for better charge transport and collection.

Surface roughness regulation of reduced-graphene oxide/iodine – Based electrodes and their application in polymer solar cells

The current work presents an iodine-mediated fine-tuning method for the electrical and electrochemical properties of reduced-graphene oxide (r-GO)/iodine – based electrodes for application in ITO-free polymer solar cells (PSCs). A multi-technique investigation was applied to correlate the morphological features of GO thin films (GO TFs) with iodine adsorption during the reduction process by HI vapor, electrochemical band gap, Fermi potential, charge carrier mobility and charge density of iodine/r-GO based electrodes. The electrical and electrochemical characteristics of iodine/r-GO electrodes changed considerably by alteration of their surface roughness and iodine content. Iodine/r-GO TFs with the lowest surface roughness and the highest iodine content exhibited the highest charge carrier density and Fermi potential. Electrochemical impedance spectroscopy (EIS) and quantum Hall effect (QHE) results confirmed the p-type conductivity of r-GOs/iodine-based electrodes. PSCs were fabricated using r-GO/iodine electrodes as the photo-anode to follow the influences of iodine content and surface roughness on the photovoltaic performance of the cells. PSCs prepared based on r-GO/iodine electrodes possessing the lowest surface roughness and the highest iodine content provided the lowest charge transfer resistance (Rct) and remarkably higher (∼48%) PCE.

The influences of polyol process parameters on the optoelectronic characteristics of AgNWs-based flexible electrodes and their application in ITO-free polymer solar cells

Abstract Silver nanowires (AgNWs) have been considered as promising materials for fabrication of transparent conducting electrodes (TCEs) due to their suitable optical and electrical conductivity. The current work investigates the effects of the synthesis parameters on the physical and morphological properties of AgNWs-based TCEs and the photovoltaic performances of ITO-free AgNWs-electrode based flexible polymer solar cells (FPSCs). AgNWs were prepared using two different polyol-based methods including modified polyol (MP) and Sigma polyol (SP) processes. These methods differed in the type of applied seed, ratios of PVP/AgNO3 and AgNO3/seed, AgNO3 injection rate and temperature. The reflux condition was used as a novel effective parameter in the MP method. Results showed that MP-AgNWs possessed higher aspect ratio compared to those obtained from the SP process. AgNWs-based TCEs were prepared using spin coating method. The injection volume and the thickness of AgNWs-electrodes were altered to find the optimized electrodes in terms of sheet resistance (Rs) and transparency. The optimized MP-AgNWs and SP-AgNWs electrodes based on H-FoM calculations showed different Rs of 11.3 and 99.3 Ω.sq−1 and transparency of 92.8% and 90.4%, respectively. These electrodes were integrated as the photo-anode in the construction of ITO-free FPSCs. The prepared FPSCs based on the optimized MP-AgNWs electrodes showed PCE of 3.18% which was about 28% higher than that obtained using the best SP-AgNWs electrodes. The photovoltaic performance of MP-AgNWs electrode based FPSC was found to be very close to those of ITO-based reference cell prepared using similar conditions. However, the PCE of these FPSCs was about 18% lower which can be improved by altering the synthesis parameters.

Conductivity enhancement of PEDOT:PSS film via sulfonic acid modification: application as transparent electrode for ITO-free polymer solar cells

3-Hydroxy-1-propanesulfonic acid (HPSA) was applied as a modification layer on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film via spin-coating, resulting in a massive boost of the conductivity of PEDOT:PSS film, and thus the as-formed PEDOT:PSS/HPSA bilayer film was successfully used as a transparent electrode for ITO-free polymer solar cells (PSCs). Under the optimized concentration of HPSA (0.2 mol L−1), the PEDOT:PSS/HPSA bilayer film has a conductivity of 1020 S cm−1, which is improved by about 1400 times of the pristine PEDOT:PSS film (0.7 S cm−1). The sheet resistance of the PEDOT:PSS/HPSA bilayer film was 98 Ω sq−1, and its transparency in the visible range was over 80%. Both parameters are comparable to those of ITO, enabling its suitability as the transparent electrode. According to atomic force microscopy (AFM), UV-Vis and Raman spectroscopic measurements, the conductivity enhancement was resulted from the removal of PSS moiety by methanol solvent and HPSA-induced segregation of insulating PSS chains along with the conformation transition of the conductive PEDOT chains within PEDOT:PSS. Upon applying PEDOT:PSS/HPSA bilayer film as the transparent electrode substituting ITO, the ITO-free polymer solar cells (PSCs) based on poly[ N -9 ² -hepta-decanyl-2,7-carbazole-alt-5,5-(4 ¢ ,7 ¢ -di-2-thienyl-2 ¢ ,1 ¢ ,3 ¢ -benzothiadiazole)]:[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (PCDTBT:PC71BM) active layer exhibited a power conversion efficiency (PCE) of 5.52%, which is comparable to that of the traditional ITO-based devices.

Polymer solar cells spray coated with non-halogenated solvents

Using spray-coating technique, we successfully fabricated conventional ITO-based and inverted ITO-free polymer solar cells (PSCs) based on a conjugated polymer poly[2,3-bis-(3-octyloxyphenyl) quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) as the donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6] -phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. Environment-friendly non-halogenated solvents were used to process the active layers. The influence of substrate temperatures and processing solvents on the photovoltaic performance of the ITO-based TQ1:PC61BM PSCs was systemically investigated. A higher substrate temperature can accelerate the solvent evaporating rate and afford a micro-textured rougher surface, which efficiently reduced light reflectance and enhanced absorption. Furthermore, finer phase separation was observed when using this high substrate temperature, which led to enhanced photocurrent due to the reduced bimolecular recombination. The device performance of spray-processed PSCs using the non-halogenated solvent mixtures was comparable to that of spray-processed PSCs using the halogenated o-dichlorobenzene (oDCB), which demonstrates that the non-halogenated solvents are very promising in spray-processed PSCs. This work sheds new light on developing efficient roll-to-roll compatible spray-coated PSCs with environment-friendly solvents.

ITO-free polymer solar cells with vanadium oxide hole transport layer

Abstract Although highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has emerged as alternative transparent electrodes to indium thin oxide (ITO), cell-performance is inferior to ITO based polymer solar cells (PSCs). To improve PEDOT:PSS based PSCs, we inserts metal-oxide hole transport layer (HTL) between PEDOT:PSS electrodes and photoactive layers for the first time. Device with optimized vanadium oxide HTL exhibits enhanced power conversion efficiency (PCE) of 3.33% (3.03% for conventional ITO-free PSCs). Improved performance is attributed to the increased shunt resistance and reduced surface roughness of PEDOT:PSS electrodes. Furthermore, improvement in device-stability under the atmospheric condition is observed.

Highly conductive, optically transparent, low work-function hydrogen-doped boron-doped ZnO electrodes for efficient ITO-free polymer solar cells

Here the benefits of hydrogen-doped boron-doped ZnO layers for use as efficient bottom cathode electrodes in polymer solar cells are explored.

Highly conductive PEDOT:PSS transparent electrode prepared by a post-spin-rinsing method for efficient ITO-free polymer solar cells

We have proposed a post-spin-rinsing method (PSRM) with polar organic solvent dimethyl sulfoxide (DMSO) to prepare highly conductive and transparent PEDOT:PSS films. The PSRM-prepared PEDOT:PSS film exhibits a conductivity of 1335Scm−1, much higher than 776Scm−1 of the PEDOT:PSS film prepared with the conventional pre-adding method (PAM). In contrast to the PAM, the PSRM with DMSO is more effective to remove the insulating PSS and push the PEDOT chains from coiled to a linear/extended-coil alignment with conformation change from a benzoid structure to a more conductive quinoid structure, leading to an enhanced conductivity of the PEDOT:PSS film. The highly conductive PEDOT:PSS film is used as transparent electrode to fabricate indium tin oxide-free polymer solar cells and a power conversion efficiency of 4.82% is achieved for polymer solar cells based on PCDTBT:PC71BM blend.

An indium tin oxide-free polymer solar cell on flexible glass.

Future optoelectronic devices and their low-cost roll-to-roll production require mechanically flexible transparent electrodes (TEs) and substrate materials. Indium tin oxide (ITO) is the most widely used TE because of its high optical transmission and low electrical sheet resistance. However, ITO, besides being expensive, has very poor performance under mechanical stress because of its fragile oxide nature. Alternative TE materials have thus been sought. Here we report the development of a multilayer TiO2/Ag/Al-doped ZnO TE structure and an ITO-free polymer solar cell (PSC) incorporating it. Electro-optical performances close to those of ITO can be achieved for the proposed TE and corresponding PSC with an additional advantage in their mechanical flexibility, as demonstrated by the fact that the cell efficiency maintains 94% of its initial value (6.6%) after 400 cycles of bending, with 6 and 3 cm maximum and minimum radii, respectively. Instead of common plastic materials, our work uses a very thin (0.14 mm) flexible glass substrate with several benefits, such as the possibility of high-temperature processes, superior antipermeation properties against oxygen and moisture, and improved film adhesion.

Sulfonate Poly(aryl ether sulfone)-Modified PEDOT:PSS as Hole Transport Layer and Transparent Electrode for High Performance Polymer Solar Cells

Polymer solar cells (PSCs) with high short current density (Jsc) have been fabricated through a facile way by using a low-cost polyelectrolyte-modified poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, P VP Al 4083) bilayer film as anode buffer layer. Spin-coating a layer of sulfonate poly(aryl ether sulfone) (SPES) on the surface of PEDOT:PSS hole-transporting layer (HTL) is found to dramatically improve the Jsc value even up to 21.66 mA cm–2. The notable Jsc is demonstrated to be correlated with interaction between the SPES and PEDOT, which removes the insulator of PSS with formation of continuous PEDOT domains, consequently leading to the improved conductivity and more imitate interfacial contact. It should be noted that the notable Jsc also partly results from the effect of a second anode due to the high conductivity of SPES-modified PEDOT:PSS. Through systematically investigation on a series of devices with different areas, it can be found that a real effective area of the devices should be carefully addressed to exclude the effect of a second anode, especially when a highly conductive interfacial material is incorporated. More interestingly, apart from the successful application in HTL, SPES also works well as transparent electrode. Compared with the pristine PEDOT:PSS (PH1000) anode, SPES-modified PH1000 as transparent anode achieves a dramatically increased performance in the ITO-free PSCs together with overall improved parameters, even equal to the one based on ITO anode. These findings indicate that solution-processed SPES shows a great potential in the fabrication of highly efficient PSCs as well as large-area, flexible printable PSCs.

Flexible ITO-free polymer solar cells based on highly conductive PEDOT:PSS and a printed silver grid

We report efficient flexible polymer solar cells based on ITO and ITO-free semi-transparent electrodes consisting of either a highly conductive PEDOT:PSS formulation (PH1000) or a combination of printed silver grids and PH1000. As active layer material a polymer:fullerene bulk heterojunction based on PCDTBT:PC70BM (poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:phenyl-C71-butyric acid methyl ester) was used. We demonstrate that the printed silver grid in combination with PH1000 leads to comparative device performances as ITO-coated PET substrates and has the potential for more.

Highly efficient ITO-free polymer solar cells based on metal resonant microcavity using WO3/Au/WO3 as transparent electrodes

Indium tin oxide (ITO)-free polymer solar cells (PSCs) with the structure of Glass/tungsten trioxide (WO3)/Au/WO3/PCDTBT: PC70BM/LiF/Al was fabricated and studied. The multilayer structure of WO3/Au/WO3 is used as the potential transparent electrode to replace ITO. Metal resonant microcavity, which can enhance light harvesting of active layers, was constructed between Au and Al electrodes. According to the J–V and IPCE characterization with 70nm active layer, power conversion efficiency (PCE) of the ITO-free microcavity device is approaching 4.55%, which is higher than that of the ITO-based device. However, PCE of the ITO-free device is much lower than that of the ITO-based device when the thickness of active layer increases to 130nm. The opposite experimental tendency leads to theoretical research toward the simulation of light absorption and optical electric field and the calculation of maximum short circuit current density (Jsc max) as a function of active layer thickness based on ITO-free and ITO-based devices. The research results show that microcavity effect is closely linked to intrinsic absorption of active layers.

Plasmonic ITO-free polymer solar cell.

The aluminum and sliver multilayered nano-grating structure is fabricated by laser interference lithography and the intervals between nanoslits is filled with modified PEDOT:PSS. The grating structured transparent electrode functions as the anti-reflection layer which not only decreases the reflected light but also increases the absorption of the active layer. The performances of P3HT:PC₆₁BM solar cells are studied experimentally and theoretically in detail. The field intensities of the transverse magnetic (TM) and transverse electrical (TE) waves distributed in the active layer are simulated by rigorous coupled wave analysis (RCWA). The power conversion efficiency of the plasmonic ITO-free polymer solar cell can reach 3.64% which is higher than ITO based polymer solar cell with efficiency of 3.45%.

Highly Conductive PEDOT:PSS Treated with Formic Acid for ITO-Free Polymer Solar Cells

We proposed a facile film treatment with formic acid to enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by 4 orders of magnitude. The effect of formic acid concentration on conductivity was investigated; conductivity increased fast with increasing concentration up to 10 M and then increased slightly, the highest conductivity being 2050 S cm(-1) using 26 M concentration. Formic acid treated PEDOT:PSS films also exhibited very high transmittances. The mechanism of conductivity enhancement was explored through SEM, AFM, and XPS. Formic acid with its high dielectric constant screens the charge between PEDOT and PSS bringing about phase separation between them. Increased carrier concentration, removal of PSS from the film, morphology, and conformation change with elongated and better connected PEDOT chains are the main mechanisms of conductivity enhancement. ITO-free polymer solar cells were also fabricated using PEDOT:PSS electrodes treated with different concentrations of formic acid and showed equal performance to that of ITO electrodes. The concentrated acid treatment did not impair the desirable film properties as well as stability and performance of the solar cells.

Upscaling from single cells to modules – fabrication of vacuum- and ITO-free polymer solar cells on flexible substrates with long lifetime

Fabrication of polymer solar cell (PSC) modules was done on a previously reported compact coating/printing machine and tested in a readily scalable roll process on flexible substrates without applying vacuum, ITO or spin coating. Our aim was to establish loss upon scaling from cells to small modules. We studied from single cells (1 cm2) to modules comprising four serially connected devices with a total active area of 8 cm2. Four different polymers (P3HT, PV-D4610, PDTSTTz-4 and PBDTTTz-4) were applied in the preparation of the modules and efficiencies of more than 3% were achieved which is comparable to single cell devices prepared using the same process. This proves that it is possible to scale up new materials in an ITO free device context to modules without having an efficiency drop, due to reliable and consistent processing. The main loss observed was due to the packaging using barrier materials. The photochemical stability of the materials was therefore studied using intense light along with the operational stability of the corresponding devices according to the ISOS-D-1 and ISOS-L-1 standards. For devices under constant illumination we found that barrier materials from Mitsubishi and 3M provide better operational stability over time, compared to the barrier foil obtained from Amcor, for all the polymers, which is ascribed to the cut-off at higher wavelengths thereby lowering the degree of UV light that reaches the device. When comparing the operational stability of the four polymers under constant illumination, P3HT generally retains its performance better with higher T80 values, while the polymer PV-D4610 shows the highest PCE (1.6%) after 300 hours of operation.

ITO-free Anode with Plasmonic Silver Nanoparticles for High Efficient Polymer Solar Cells

In this work we improved the performance of ITO-free polymer solar cells (PSCs) by incorporating silver nanoparticles (AgNPs) in the highly conductive (HC) PEDOT:PSS anode. The AgNPs were synthetized in-situ in the PEDOT:PSS water dispersion. This anode was used to realize PSCs with the following geometry: glass/HC-PEDOT:PSS/PEDOT:PSS/PBDTTT-C:[70]PCBM/Ca/Al. All the devices were characterized by UV-VIS spectroscopy, impedance spectroscopy, IV light, IV dark and quantum efficiency measurements. The presence of AgNPs in the HC-PEDOT:PSS anode contributes to improve the absorption of the photoactive layer and to lower the resistivity of the anode.

Highly Conductive PEDOT:PSS Electrode Treated With Polyethylene Glycol for ITO-Free Polymer Solar Cells

not Available.

Highly Conductive PEDOT: PSS Electrode Treated with Polyethylene Glycol for ITO-Free Polymer Solar Cells

We used polyethylene glycol (PEG) and enhanced the conductivity of PEDOT:PSS from 0.3 S cm-1 to 805 S cm-1 with only 2% PEG additive. The conductivity was further enhanced to 1100 S cm-1 by combining PEG and methanol treatment. PEG treated PEDOT:PSS films showed better phase separation between PEDOT and PSS with fiber like 3D PEDOT networks. The carrier concentration increased by three orders of magnitude after PEG treatment. ITO-free polymer solar cells with PEG treated PEDOT:PSS anodes were fabricated and optimized. Double layer PEDOT:PSS treated with 2% PEG200 anodes showed power conversion efficiency of 3.71% while and the ITO counterpart showed 3.73%.

High-performance ITO-free spray-processed polymer solar cells with incorporating ink-jet printed grid

Highly efficient ITO-free polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been fabricated by a combination of inkjet-printing and spray processes. A hybrid transparent conducting electrode consisting of printed silver (Ag) grids and highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PH1000) was used as an alternative to indium-tin oxide (ITO). Spray process incorporating with printed Ag grids played a critical role in improving the interfacial contact between Ag grids and photoactive layer, and thus enhanced the performance of ITO-free large-area PSC. The ITO-free PSC (device area=0.3cm2) prepared here has a comparable performance of 2.86%. The average PCE of 2.34% was achieved in the ITO-free PSC with a large electrode area (8cm2) fabricated herein by the combination of inkjet-printed grid and spray processes. This result is much better than ITO-based large-area PSC generally reported.

Efficient ITO-free polymer solar cells with pitch-converted carbon nanosheets as novel solution-processable transparent electrodes

We demonstrate that pitch-converted carbon nanosheet (CNS) films can efficiently function as transparent conducting electrodes for polymer solar cells (PSCs). The solution-processed CNS film was prepared with spin-coating of a cost-effective pitch solution dissolved in dimethylformamide on quartz substrates, followed by stabilization and carbonization treatments to convert the pitch into CNS. The pitch-converted CNS films prepared by the successive heat-treatment process were examined as a novel transparent anode in solar cells, and as a result, PSCs fabricated directly on such CNS electrodes exhibited a high power conversion efficiency of ∼1.7% under 100mWcm−2 illumination and AM 1.5G conditions. This approach could be highly desirable for advancing the realization of fully printable and flexible low-cost transparent electrodes for indium tin oxide (ITO)-free polymer solar cells.