High-performance Inverted Polymer Solar Cells Research Articles

Dramatic improvement in the stability and mechanism of high-performance inverted polymer solar cells featuring a solution-processed buffer layer.

In this study, we demonstrate inverted PTB7:PC71BM polymer solar cells (PSCs) featuring a solution-processed s-MoO3 hole transport layer (HTL) that can, after thermal aging at 85 °C, retain their initial power conversion efficiency (PCE) for at least 2200 h. The T80 lifetimes of the PSCs incorporating the novel s-MoO3 HTL were up to ten times greater than those currently reported for PTB7- or low-band-gap polymer:PCBM PSCs, the result of the inhibition of burn-in losses and long-term degradation under various heat-equivalent testing conditions. We used X-ray photoelectron spectroscopy (XPS) to study devices containing thermally deposited t-MoO3 and s-MoO3 HTLs and obtain a mechanistic understanding of how the robust HTL is formed and how it prevented the PSCs from undergoing thermal degradation. Heat tests revealed that the mechanisms of thermal inter-diffusion and interaction of various elements within active layer/HTL/Ag electrodes controlled by the s-MoO3 HTL were dramatically different from those controlled by the t-MoO3 HTL. The new prevention mechanism revealed here can provide the conceptual strategy for designing the buffer layer in the future. The PCEs of PSCs featuring s-MoO3 HTLs, measured in damp-heat (65 °C/65% RH; 85 °C per air) and light soaking tests, confirmed their excellent stability. Such solution-processed MoO3 HTLs appear to have great potential as replacements for commonly used t-MoO3 HTLs.

Stability Enhancement of High-Performance Inverted Polymer Solar Cells Using ZnO Electron Interfacial Layer Deposited by Intermittent Spray Pyrolysis Approach

In this research work, for the first time, stable high-performance inverted polymer solar cells (iPSCs) have been fabricated utilizing facile and low-cost intermittent spray pyrolysis (SP) techniqu...

Orthogonal Printable Reduced Graphene Oxide 2D Materials as Hole Transport Layers for High-Performance Inverted Polymer Solar Cells: Sheet Size Effect on Photovoltaic Properties.

Creating an orthogonal printable hole-transporting layer (HTL) without damaging the underlying layer is still a major challenge in fabricating large-area printed inverted polymer solar cells (PSCs). In this study, we prepared orthogonal-processable fluorine-functionalized reduced graphene oxide (FrGO) series with various two-dimensional sheet sizes such as large-sized FrGO (1.1 μm), medium-sized FrGO (0.7 μm), and small-sized FrGO (0.3 μm) and systematically investigated the size effect of FrGOs on the hole transport properties of PSCs. The FrGOs exhibit highly stable dispersion without change over 90 days in 2-propanol solvent, indicating very high dispersion stability. Decreasing the sheet size of FrGOs enhanced hole-transporting properties, resulting in power conversion efficiencies (PCEs) of 9.27 and 9.02% for PTB7-Th:EH-IDTBR- and PTB7-Th:PC71BM-based PSCs, respectively. Compared to devices with solution-processed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), a 14% enhancement of PCEs was achieved. Interestingly, the PCEs of devices with the smallest FrGO sheet are higher than the PCE of 8.77% of a device with vacuum-deposited MoO3. The enhancement in the performance of PSCs is attributed to the enhanced charge collection efficiency, decreased leakage current, internal resistance, and minimized charge recombination. Finally, small-sized FrGO HTLs were successfully coated on the photoactive layer using the spray coating method, and they also exhibited PCEs of 9.22 and 13.26% for PTB7-Th:EH-IDTBR- and PM6:Y6-based inverted PSCs, respectively.

Surface modification on nanoripple-like ZnO nanorod arrays using two-dimensional (2D) Bi2OS2 to fabricate high-performance inverted polymer solar cells

In this paper, three-dimensional (3D) nanoripple-like ZnO nanorod arrays (R-ZnO NRAs) are successfully fabricated and modified by two-dimensional (2D) Bi2OS2 material, and inverted polymer solar cells (IPSCs) with R-ZnO modified by Bi2OS2 as electron transmission layer (ETL) are fabricated for the first time. The results show that the surface morphologies of R-ZnO NRAs can be controlled by adjusting the concentration of the modified 2D Bi2OS2 solution. Bi2OS2 modification can not only suppress the surface defects of R-ZnO NRAs, reduce the recombination of photogenerated charges, but also increase crystallinity of the active layer, resulting in effective electron collection. And thus, the performance of IPSCs is obviously improved. The power conversion efficiency (PCE) of PTB7: PCBM based PSCs with R-ZnO NRAs modified by 2% Bi2O2S as ETL is considerably raised to 7.31% from 5.51%. More interestingly, Bi2OS2 modification enhances IPSCs stability to remain 80.9% of their initial PCE after 80 days, yet IPSCs with pristine R-ZnO NRAs remain only 47.4% of their initial PCE. Moreover, this approach can also successfully improve the performance of another IPSC composed of PBDB-T: ITIC blends. The PCE of the device based on 2% Bi2OS2-modified R-ZnO NRAs is improved to 9.94% from 8.03% of the reference device without Bi2OS2 modification. This work not only provides an effective mean of surface modification of R-ZnO NRAs, but also shows the Bi2OS2 material has potential application in PSCs.

Strontium Fluoride and Zinc Oxide Stacked Structure as an Interlayer in High-Performance Inverted Polymer Solar Cells.

Enhanced power conversion efficiency is reported in inverted polymer solar cells when an ultrathin layer of strontium fluoride (SrF2) is evaporated on the surface of the solution-processed zinc oxide (ZnO) electron transport layer. The photoactive layer is made up of bulk heterojunction composites of poly[4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b:4,5- b']dithiopheneco-3-fluorothieno[3,4- b]-thiophene-2-carboxylate] and [6,6]-phenyl-C71-butyric acid methyl ester. The ZnO film acts as an effective electron transport layer, whereas the ultrathin SrF2 layer improves the energy level alignment and enhances the built-in potential via the formation of an interfacial dipole layer at the interfaces between the ZnO film and the photoactive layer, resulting in an enhanced electron extraction efficiency and a decreased carrier recombination loss. Furthermore, the SrF2 layer reduces the inherent incompatibility between the hydrophilic ZnO film and the hydrophobic photoactive layer. As a result, all the photovoltaic performance parameters are remarkably improved, leading to a high efficiency of up to 10.46% (with a fill factor of 71.38%), corresponding to a ca. 21% improvement over the reference device performance (8.64%). The use of a ZnO/SrF2 stacked interlayer provides a simple, but effective, approach to obtain high-efficiency inverted PSCs.

High-Performance Inverted Polymer Solar Cells: Study and Analysis of Different Cathode Buffer Layers

The effect of different cathode buffer layers on the performance of inverted organic solar cells (OSCs) fabric-ated with {poly[[2,6’-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene][3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7-Th):[6,6]-phenyl-C71-butyric acid methyl} (PC70BM) as active blend layer is studied using device simulator Silvaco/ATLAS TCAD. The parameters used in the simulations were extracted in fabricated OSCs, using the metal-insulator-metal model, obtaining very good agreement between measured and simulated current density ( $J_{{\rm{SC}}}$ ), open-circuit voltage ( $V_{{\rm{OC}}}$ ), fill factor, and power conversion efficiency (PCE). The enhanced performance observed for devices where either 10 nm of poly[(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene) (PFN), 0.6 nm of lithium fluoride (LiF) layer, or the combination of these two materials, was very well reproduced by simulation. Results indicate that the described procedure can be used to predict the behavior of new buffer layers in OSCs. The PCEs values obtained by simulation were 10.90%, 10.45%, 10.38%, 9.03%, and 7.60% for OSCs with cathode buffer layer consisting of the stack PFN/LiF, PFN, LiF/PFN, LiF and without cathode buffer layer, respectively, which agree well with measured values.

Niobium doped zinc oxide nanorods as an electron transport layer for high-performance inverted polymer solar cells

The wet chemical synthesis, characterization and applications of Nb doped ZnO nanorods in bulk heterojunction inverted polymer solar cells are presented in this paper. Zn1−xNbxO with x ranging from 0.01 to 0.07 were successfully synthesized using a novel facile solution-processed wet chemical method. The structural and optical properties of the undoped and Nb doped ZnO nanorods were investigated by transmission electron microscopy, X-ray diffraction, UV–visible near infrared, and photoluminescence (PL) spectroscopies. A clear red shift was observed in the absorption spectra of Nb doped samples compared to the undoped ZnO nanorods. The defect emission peak in the PL spectrum of the Nb doped nanorods completely disappeared at the highest dopant concentration of x=0.07. These undoped and Nb doped ZnO nanorods, in amalgamation with PTB7-Th:PC71BM, were used in the inverted polymer solar cells as an electron transport layer which gives better power conversion efficiencies compared to the pure nanorods.

Cross‐Linkable and Dual Functional Hybrid Polymeric Electron Transporting Layer for High‐Performance Inverted Polymer Solar Cells

A cross-linkable dual functional polymer hybrid electron transport layer (ETL) is developed by simply adding an amino-functionalized polymer dopant (PN4N) and a light crosslinker into a commercialized n-type semiconductor (N2200) matrix. It is found that the resulting hybrid ETL not only has a good solvent resistance, facilitating multilayers device fabrication but also exhibits much improved electron transporting/extraction properties due to the doping between PN4N and N2200. As a result, by using PTB7-Th:PC71 BM blend as an active layer, the inverted device based on the hybrid ETL can yield a prominent power conversion efficiency of around 10.07%. More interestingly, photovoltaic property studies of bilayer devices suggest that the absorption of the hybrid ETL contributes to photocurrent and hence the hybrid ETL simultaneously acts as both cathode interlayer material and an electron acceptor. The resulting inverted polymer solar cells function like a novel device architectures with a combination of a bulk heterojunction device and miniature bilayer devices. This work provides new insights on function of ETLs and may be open up a new direction for the design of new ETL materials and novel device architectures to further improve device performance.

Improved performance and stability of inverted polymer solar cells with ammonium heptamolybdate acted as hole extraction layers via thermal annealing method

High-performance Inverted polymer solar cells (PSC) were fabricated with ammonium heptamolybdate (NH4)6Mo7O24−4H2O as hole extraction layer (HEL), and the blend of the poly(3-hexylthiophene) (P3HT):indene-C60 bisadduct (ICBA) as photoactive layer, using the structure of ITO/ZnO/P3HT:ICBA/(NH4)6Mo7O24/Ag. Correspondingly, the influence of different annealing time of ammonium heptamolybdate on carrier mobility and thus device performance were investigated. The power conversion efficiency (PCE) has been optimized from 4.12% to 6.24% after thermal annealing treatment of (NH4)6Mo7O24 layer at 130 °C for 15min. The results of ultraviolet photoelectron spectroscopy (UPS) curve clearly proven the increase of conduction-band minimum (CBM) and valence-band maximum (VBM) energy level of (NH4)6Mo7O24 layer upon thermal annealing, which is caused by the decrease of barrier potential between HOMO of P3HT and HEL. This obviously facilitate hole transport hole transport between HOMO levels of P3HT and HEL block electron transfer between LUMO levels of P3HT and Ag electrode. The space charge-limited current (SCLC) test has further confirmed the enhanced hole collection and transport capability of (NH4)6Mo7O24 after thermal annealing treatment.

Enhanced Electron Extraction Capability of Polymer Solar Cells via Employing Electrostatically Self-Assembled Molecule on Cathode Interfacial Layer.

In this paper, high-performance inverted polymer solar cells (PSCs) with a modified cathode buffer layer, titanium dioxide:polyethylenimine (TiO2:PEI), are demonstrated. The TiO2-O-PEI transport layer was fabricated by electrostatically self-assembled monolayers (ESAM) of PEI molecules. Protonated amine functional groups of PEI can combine protons (H(+)) hydrolyzing from its aqueous solution. Also, PEI could produce ESAM on the surface of hydroxylated TiO2 because of its cationic characteristics. The incorporation of the TiO2-O-PEI layer enhances the photocurrent and power conversion efficiency (PCE) due to the improved interfacial electron transport and extraction of the TiO2-O-PEI surface and the increased light absorption of the active layer. The enhanced PCE, low-cost materials, and solution process of TiO2-O-PEI buffer layers provide a promising method for highly efficient PSCs.

Low-temperature, solution-processed aluminum-doped zinc oxide as electron transport layer for stable efficient polymer solar cells

A simple low-temperature solution-processed zinc oxide (ZnO) and aluminum-doped ZnO (AZO) were synthesized and investigated as an electron transport layer (ETL) for inverted polymer solar cells. A solar cell with a blend of poly(4,8-bis-alkyloxy-benzo[1,2-b:4,5-b′] dithiophene-alt-alkylcarbonyl-thieno [3,4-b] thiophene) and (6,6)-phenyl-C71-butyric acid methyl ester as an active layer and AZO as ETL demonstrates a high power conversion efficiency (PCE) of 7.36% under the illumination of AM 1.5G, 100mW/cm2. Compared to the cells with ZnO ETL (PCE of 6.85%), the PCE is improved by 7.45% with the introduction of an AZO layer. The improved PCE is ascribed to the enhanced short circuit current density, which results from the electron transport property of the AZO layer. Moreover, AZO is a more stable interfacial layer than ZnO. The PCE of the solar cells with AZO as ETL retain 85% of their original value after storage for 120days, superior to the 39% of cells with ZnO ETL. The results above indicate that a simple low-temperature solution-processed AZO film is an efficient and economical ETL for high-performance inverted polymer solar cells. Due to its environmental friendliness, good electrical properties, and simple preparation approach, AZO has the potential to be applied in high-performance, large-scale industrialization of solar cells and other electronic devices.

High electron mobility ZnO film for high-performance inverted polymer solar cells

High-quality ZnO films (ZnO-MS) are prepared via magnetron sputtering deposition with a high mobility of about 2 cm2/(V·s) and are used as electron transport layer for inverted polymer solar cells (PSCs) with polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]:[6,6]-phenyl C71-butyric acid methyl ester as the active layer. A significant improvement of JSC, about 20% enhancement in contrast to the devices built on sol-gel derived ZnO film (ZnO-Sol), is found in the ZnO-MS based device. High performance ZnO-MS based PSCs exhibit power conversion efficiency (PCE) up to 8.55%, which is much better than the device based on ZnO-Sol (PCE = 7.78%). Further research on cathode materials is promising to achieve higher performance.

High-performance inverted polymer solar cells based on thin copper film

We report the fabrication of cost-effective indium-free polymer solar cells (PSCs) with an inverted structure that incorporates an ultrathin copper (Cu) film as a bottom cathode via thermal evaporation. The average optical transmittance of the 15-nm Cu coated glass substrate in the visible region of the spectrum was found to be around 80% with a highest value of 84.5%. The Cu electrode was modified by an interfacial layer of an alcohol-/water-soluble conjugated polymer, poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) interlayer to ensure a very smooth surface. Upon the use of the PFN interfacial layer, the work function of Cu was decreased from 4.68 to 4.31 eV, which can form an Ohmic contact with photoactive layer and facilitate electrode transport and extraction. As a result, a power conversion efficiency of 3.6% was achieved when poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] and a [6,6]-phenyl C71-butyric acid methyl ester blend were utilized as the photoactive layers, demonstrating that the thermally evaporated Cu thin-film electrode can be a promising candidate to replace indium tin oxide for highly efficient PSCs.

Self-Assembled TiO2 Nanorods as Electron Extraction Layer for High-Performance Inverted Polymer Solar Cells

We demonstrate the use of TiO2 nanorods with well-controlled lengths as excellent electron extraction materials for significantly improving the performance of inverted polymer solar cells. The cells containing long nanorods outperform the devices using amorphous TiO2 particles as the electron extraction layer, mainly by a 2-fold increase in short-circuit current and fill factor. The enhanced charge extraction is attributed to the high electron mobility in crystalline nanorods and their preferential alignment during film formation. Furthermore, transient photocurrent studies suggest the presence of fewer interfacial and internal defects in the nanorod interlayers, which can effectively decrease carrier recombination and suppress electron trapping.

Replacing the metal oxide layer with a polymer surface modifier for high-performance inverted polymer solar cells

Replacing ZnO with PEIE, both as a surface modifier for the low work function electrode and as an electron selective layer, enhances the performance and air stability of inverted polymer solar cells by improving electron transport, wettability between the active layer and the cathode, and maximizing light absorption within the active layer without light interference.

Morphologic improvement of the PBDTTT-C and PC71BM blend film with mixed solvent for high-performance inverted polymer solar cells

Tracing the evolution of the bulk heterojunction structure, a dramatic promotion in the efficiency of polymer solar cells has been obtained in recent years. The active layer morphology of low-bandgap polymer solar cells is one of the critical factors for high-efficiency performance. In the past, the relationship between morphology improvement and the device’s characteristics (such as efficiency, fill factor and short-circuit current) in low-bandgap polymer solar cells has been studied intensively with regards to the conventional structure. Here we demonstrate the morphologic improvement of the poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl]/[6,6]-phenyl C71 butyric acid methyl ester (PBDTTT-C/PC71BM) blend film for inverted solar cells. By utilizing a mixed solvent of dichlorobenzene/chlorobenzene with (1,8-diiodooctane) additives, the device efficiency can be significantly enhanced, from 0.92% to 4.43%. This enhancement is attributed to active layer morphologic improvement promoting carrier transport. Furthermore, the thickness optimization of the active layer and the electron blocking layer MoO3 further contributes to efficiency. The device performance could be achieved with an efficiency as high as 5.35%, an open-circuit voltage of 0.70 V, a short-circuit current density of 13.5 mA cm−2, and a fill factor of 57%.

High‐Performance Inverted Polymer Solar Cells with Solution‐Processed Titanium Chelate as Electron‐Collecting Layer on ITO Electrode

High-performance inverted polymer solar cells (PSCs) with solution-processed titanium chelate TIPD as electron collecting layer are reported. The power conversion efficiency (PCE) of the inverted PSC with a-TIPD buffer layer with thermal annealing at 150 °C for 10 min reached 7.4% under the illumination of AM1.5, 100 mW/cm(2), which is increased by 16% in comparison with that (6.4%) of the device in the conventional structure. The PCE of 7.4% is the highest among the inverted PSCs reported so far in the literature.

High-performance inverted polymer solar cells with lead monoxide-modified indium tin oxides as the cathode

The photovoltaic stability of polymer solar cells (PSCs) can be greatly improved by adopting an inverted device structure. This paper reports high-performance inverted PSCs with lead monoxide (PbO)-modified indium tin oxide (ITO) as the cathodes. A thin PbO layer can effectively lower the work function of ITO from 4.5 to 3.8 eV. The optimal inverted PSCs with poly(3-hexylthiophene) (P3HT) as the donor and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) as the acceptor exhibited high photovoltaic performance: open-circuit voltage of 0.59 V, short-circuit current density of 10.8 mA cm −2, fill factor of 0.632, and power conversion efficiency of 4.00% under simulated AM1.5G illumination (100 mW cm −2). The photovoltaic efficiency is significantly higher than that of the control inverted PSCs with unmodified ITO as the cathode. It is even better than that of the control PSCs with normal architecture, which have an optimal efficiency of 3.5%. The lowering in the work function by the PbO modification is attributed to the charge transfer between PbO and ITO, as evidenced by the X-ray photoelectron spectra.