Electron Selective Layer Research Articles

Biopolymer as an electron selective layer for inverted polymer solar cells

In this work, a solution-processable electron selective layer is introduced for inverted polymer solar cells (PSCs). Cationic biopolymer poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) is used as a solution-processable work function modifier of indium-tin-oxide transparent conducting electrode to yield efficient inverted PSCs of 3.3% under AM1.5G illumination, with poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester as the active layer. Devices using PDMAEMA exhibit greater stability in ambient “working conditions” as compared to devices using ZnO, retaining 90% of peak power conversion efficiency after 8 weeks. Therefore, PDMAEMA has great potential as a universal work function modifier material with high robustness.

Solution-Processed LiF-Doped ZnO Films for High Performance Low Temperature Field Effect Transistors and Inverted Solar Cells

This paper reports that high performance metal oxide thin film transistors (TFTs) can be achieved by using LiF-doped ZnO thin films processed from aqueous solution. It was found that LiF doping at an appropriate amount enhanced the oxide film carrier concentration. The TFTs based on the 10 mol % LiF-doped ZnO thin films annealed at 300 °C revealed a good device performance with an average electron mobility of 8.9 cm(2) V(-1) s(-1) and a high on/off current ratio of 4 × 10(7), superior to the devices based on the nondoped ZnO TFTs (1.6 cm(2) V(-1) s(-1)). Even when annealed at 150 °C, the device still showed good transistor operation with an electron mobility of 0.54 cm(2) V(-1) s(-1). The inverted bulk heterojunction solar cells based on P3HT:PCBM blend system fabricated using 10 mol % LiF doped ZnO as electron selective layer showed higher power conversion efficiency (η = 3.3%) than that from undoped ZnO thin films (η = 2.94%) due to enhanced short circuit current (Jsc = 10.55 mA/cm(2)). Our results suggest that LiF incorporation can be a useful technique to produce high performance and low temperature solution-processed oxide TFTs and interface layer for solar cells.

Photovoltaic Properties of Bulk-Heterojunction Organic Solar Cell with Ultrathin Titanium Oxide Nanosheet as Electron Selective Layer

The contributions of ultrathin titania nanosheet (TN) crystallites were studied in both an inverted bulk-heterojunction (BHJ) cell in indium tin oxide (ITO)/titania nanosheet (TN)/poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methylester (PCBM) active layer/MoO3/Ag and a conventional BHJ cell in ITO/MoO3/P3HT:PCBM active layer/TN/Al multilayered photovoltaic devices. The insertion of only one or two layers of poly(diallyldimethylammonium chloride) (PDDA) and TN multilayered film prepared by the layer-by-layer deposition technique effectively decreased the leakage current and increased the open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (η). The abnormal S-shaped curves observed in the inverted BHJ cell above VOC disappeared with the reduction in the work function of the bottom electrode in the inverted cells. The power conversion efficiency was increased nearly twofold, and better photovoltaic performance was observed in the conventional BHJ cells. Although the active layer has a hydrophobic surface, the active layer was fully covered by the insertion of only two or three layers of PDDA/TN multilayered film. The TN layer effectively decreased the leakage current by a factor of 100, and increased the open circuit voltage by 0.25 V probably owing to the complete hole blocking at the P3HT/TN/Al interface.

Development of Inverted Organic Solar Cells with TiO2 Interface Layer by Using Low-Temperature Atomic Layer Deposition

Organic solar cells (OSCs) with inverted structure have attracted much attention in recent years because of their improved device air stability due to the use of stable materials for electrodes and interface layers. In this work, TiO(2) films, fabricated using low temperature (e.g., 130-170 °C) atomic layer deposition (ALD) on ITO substrates, are used as electron selective interface layers to investigate inverted OSCs. It is found that though the as-deposited TiO(2) films are high resistive due to the presence of oxygen defects, the defects can be significantly reduced by light soaking. PV cells with 15-nm-thick amorphous-TiO(2) layers fabricated at low temperature show better performance than those with poly crystal TiO(2) with same thickness deposited at 250 °C. The low temperature ALD-grown TiO(2) films are dense, stable and robust with capability of conformal coating on nanostructural surfaces, showing a promising interface layer for achieving air-stable plastic OSCs with roll-to-roll mass production potential.

Interface-engineering additives for inverted BHJ polymer solar cells

We report on the photovoltaic (PV) effect of inverted polymer solar cells (PSCs) with a bulk-heterojunction (BHJ) PV layer consisting of a donor-conjugated polymer of poly(3-hexylthiophene) (P3HT) and an acceptor of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) mixed with an interface-engineering additive of poly(oxyethylene tridecyl ether) (PTE). The electron-selective layer used is sol–gel processed zinc oxide (ZnO), and the anode layer is silver (Ag). Improved performance was achieved using the PTE additive in the BHJ PV layer of the inverted PSCs. A 1.77wt% mixing level of PTE into a blended P3HT:PCBM PV layer with a ratio of 1.00:0.73 produces a power conversion efficiency (PCE) of 3.14%, which was much higher than the reference device (2.77%) without PTE. This improvement can be attributed to not only the improved carrier transport due to the PTEs at the interfaces between the BHJ PV layer and the electrodes, but also the increased carrier lifetime due to the PTE molecules at the interfaces between the phase-separated BHJ domains. These results suggest that the BHJ PV layer mixed with the PTE interface-engineering additive is a promising functional composite material system for use in efficient inverted polymer PV cells.

MoO3 Thickness, Thermal Annealing and Solvent Annealing Effects on Inverted and Direct Polymer Photovoltaic Solar Cells

Several parameters of the fabrication process of inverted polymer bulk heterojunction solar cells based on titanium oxide as an electron selective layer and molybdenum oxide as a hole selective layer were tested in order to achieve efficient organic photovoltaic solar cells. Thermal annealing treatment is a common process to achieve optimum morphology, but it proved to be damageable for the performance of this kind of inverted solar cells. We demonstrate using Auger analysis combined with argon etching that diffusion of species occurs from the MoO3/Ag top layers into the active layer upon thermal annealing. In order to achieve efficient devices, the morphology of the bulk heterojunction was then manipulated using the solvent annealing technique as an alternative to thermal annealing. The influence of the MoO3 thickness was studied on inverted, as well as direct, structure. It appeared that only 1 nm-thick MoO3 is enough to exhibit highly efficient devices (PCE = 3.8%) and that increasing the thickness up to 15 nm does not change the device performance.

Inverted organic solar cells with TiOx cathode and graphene oxide anode buffer layers

Titanium oxide (TiOx) was introduced as an electron selective layer between the active layer and bottom electrode in inverted organic solar cells with graphene oxide (GO) an anode interfacial layer. The device with TiOx exhibited a significant enhancement in power conversion efficiency compared to that without TiOx, which indicates that TiOx efficiently blocked holes at the active/bottom layer. This device exhibits good stability over 2160h of storage with only 3.67% reduction in efficiency. The dependence of the device performances on TiOx film thickness was also investigated.

Effects of annealing temperature of aqueous solution-processed ZnO electron-selective layers on inverted polymer solar cells

We investigate the effects of ZnO annealing temperature (TA) on the performance of inverted polymer solar cells with ZnO electron-selective layers deposited by spin coating aqueous solutions of an ammine-hydroxo zinc complex. The inverted solar cells based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester with TA as low as 80°C exhibit power-conversion efficiencies of 3.6%, which is equal to those of devices with higher TA. Characterizations of the ZnO films using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, grazing incidence wide-angle X-ray scattering, and optical transmittance measurements show that the abrupt improvement of device performance from TA=60 to 80°C is due to the improvement of energy-level alignment arising from the increases in the relative amount and the crystallinity of ZnO.

All-spray-coated semitransparent inverted organic solar cells: From electron selective to anode layers

We demonstrated an all-solution-processed electron selective layer, active layer and top electrode for large-area inverted organic solar cells. The fabricated devices are semitransparent, fully spray-coated, highly efficient and air-stable, with power efficiencies of 2.41% and 1.0% for cell areas of 0.36 and 15.25cm2, respectively. The shelf life of the cells in air is demonstrated by the ∼80% retention of original cell efficiency after 30days.

Comparison of organic photovoltaic with graphene oxide cathode and anode buffer layers

In this report, we present inverted organic solar cells integrating solution-processed aluminum doped zinc oxide (AZO) and trilayer graphene oxide (GO) as an electron selective and anode buffer layers, respectively. The polymers in this inverted architecture are PCDTBT, PBDTTPD and PCBM as an electron donor and acceptor, respectively and the photovoltaic performance were recorded at 300K under 100mW/cm2 light intensity. The characteristics of PCDTBT and PBDTTPD-based inverted solar cells were: open-circuit voltages (Voc’s) 0.74 and 0.70V, short-circuit current densities (Jsc’s) −12.09 and −12.06mA/cm2, fill factors (FFs) of 60.73% and 60.03%, with an overall power conversion efficiencies (PCEs) of about 5.46% and 5.07%. The fabricated inverted cells show better performances compared to conventional structure reference cells.

Improvement of ITO-free inverted polymer-based solar cells by using colloidal zinc oxide nanocrystals as electron-selective buffer layer

In this study, we report on the improvement of ITO-free inverted polymer/fullerene solar cells by introducing a zinc oxide (ZnO) layer between the active layer and the cathode. The ZnO layers are deposited from solution, using colloidal ZnO nanocrystals with a rodlike shape, which are obtained using a wet-chemical synthesis route at low temperature. The nanocrystals are widely characterized with respect to their structural, optical, and electronic properties. In particular, simulations of powder X-ray diffraction data based on Rietveld refinement are shown to be a suitable method to characterize the average crystallite shape and particle size. Cyclic voltammetry reveals that nanocrystalline ZnO is an appropriate choice as electron-selective buffer layer in organic solar cells based on a bulk heterojunction of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Using ITO-free inverted solar cells in substrate configuration with an opaque Cr/Al/Cr bottom electrode, we demonstrate that introducing a cathodic interlayer of ZnO nanocrystals leads to a notable enhancement in photovoltaic performance. The magnitude of the effect is found to depend on the solvents used to process the active layer. In case of absorber blends processed from o-dichlorobenzene, we show an almost threefold increase in efficiency from 0.8 to 2.2% at an active area of 1cm2.

ITO electrode/photoactive layer interface engineering for efficient inverted polymer solar cells based on P3HT and PCBM using a solution-processed titanium chelate

We report efficient inverted polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) using alcohol-soluble titanium (diisopropoxide) bis (2,4-pentanedionate) (TIPD) as an electron selective layer between the indium tin oxide (ITO) electrode and the photoactive layer. The thermally annealed TIPD layer is highly transparent in the visible range and shows effective electron collection ability. By optimizing the electron-collecting layer, the photoactive layer and the hole-collecting layer, the power conversion efficiency (PCE) of the inverted device with the structure ITO/TIPD/P3HT : PCBM/MoO3/Ag reaches 4.10% under the illumination of AM1.5G, 100 mW cm−2, which is among the highest values for inverted PSCs based on P3HT : PCBM. The PCE of the inverted device is improved in comparison with the conventional device (3.77%) under the same experimental conditions.

Thermal-annealing-free inverted polymer solar cells using ZnO/Cs2CO3 bilayer as electron-selective layer

We report on thermal-annealing-free inverted polymer solar cells using ZnO nanoparticles (NPs)/Cs2CO3 bilayer as an electron-selective layer (ESL) and the blend of poly(3-hexylthiophene) (P3HT): phenyl C61-butryricacid methyl ester(PCBM) as an active light-absorbing layer. These layers were deposited via solution process without additional thermal treatment. The cell gave power conversion efficiency (PCE) of 3.81%, which was higher by about 40% or 80% than that of the cell with single ZnO NPs or Cs2CO3 ESL, respectively. This improved performance could be attributed to the bilayer-structure ESL, whose electron-extraction and leakage-current-suppression abilities were enhanced due to its modified energy level and improved surface morphology. PCE of this device remained 92% of its original value after storage of the as-fabricated device in a N2-filled glove box for 80 day. Furthermore, when the blend of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]: [6,6]-phenyl C71 butyric acid methyl ester replaced P3HT:PCBM as the active layer, higher PCE of 6.45% was achieved in the corresponding cell.

Fully spray-coated inverted organic solar cells

This study evaluated the possibility of utilizing a spray-coating method for all spray-coated inverted organic solar cells (IOSCs). The spray-coating process was applied to deposit electron selective, active and hole selective layers. The IOSCs fabricated by fully spray-coating method showed a power conversion efficiency of 3.17 and 1.33% at a cell area of 0.36 and 15.25cm2, respectively, under AM.1.5 simulated illumination, resulting in similar performances with that of spin-coated devices.

An efficient inverted organic solar cell with improved ZnO and gold contact layers

This paper presents a high efficiency (∼3.8%) inverted organic photovoltaic devices based on a P3HT:PCBM bulk heterojunction (BHJ) blend with improved electron- and hole-selective contact layers. Zinc oxide (ZnO) nanoparticle films with different thicknesses are deposited on the transparent electrodes as a nano-porous electron-selective contact layer. A thin gold film is used between the BHJ photoactive layer and the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), which improves the wettability and significantly enhances the stability of the device (>50days of air exposure). Photovoltaic device parameters such as power conversion efficiency (PCE) and external quantum efficiency (EQE) are systematically examined for inverted devices with different thicknesses of ZnO and gold layers in comparison to the non-inverted and reference inverted devices with no contact layers. The optimized organic devices with ZnO and Au contact layers show exceptional short circuit currents (in excess of 13mA/cm2), in comparison to the reference devices, which is related to increased quantum efficiency of the device observed in measured EQE experiments. These results are important for development of high efficiency and stable all-printed organic solar cells and point out the role of contact layers, in particular, ZnO conductivity and morphology in the device performance.

Functional fullerenes for organic photovoltaics

Tremendous progress has been made on the design and processing of new active and interfacial materials to enable organic photovoltaics to achieve high power conversion efficiencies of >10%. In this Feature Article the development of functional fullerenes as (1) acceptors, (2) electron selective layers, and (3) morphology stabilizers for bulk heterojunction polymer solar cells is reviewed. In addition to the standard PCBM based acceptors, a wide variety of newly developed fullerene-derived molecules have appeared in the past few years and started to show very encouraging photovoltaic performance when they were blended with low bandgap conjugated polymers. New fullerene derivatives with proper molecular design can also serve as electron selective interfacial materials and morphology stabilizers for the bulk heterojunction layer, which are essential to improve the interfacial property and long term stability of polymer solar cells. Although there still are many challenges ahead before practical polymer solar cells will arrive in the market place, the research in functional fullerenes deserves to have more attention in order to expedite this development process.

Inverted polymer solar cells fabricated by a pre-metered coating process

We herein report on the photovoltaic (PV) effect of inverted polymer solar cells (PSCs) that consist of a bulk-heterojunction (BHJ) PV layer and a sol–gel-derived zinc oxide (ZnO) electron-selective layer, produced by a simple pre-metered horizontal-dipping (H-dipping) process. For the BHJ PV layers that consisted of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) (P3HT:PCBM) with an indium-tin-oxide (ITO) cathode and an Ag anode, it was shown that the use of the H-dipping process in the fabrication of inverted PSCs resulted in a reliable and reproducible device performance; furthermore, an inverted P3HT:PCBM PSC fabricated by H-dip-coating had a power conversion efficiency (PCE) of 2.93%, which within the deviations seen was slightly higher than that of a reference device (2.75%) fabricated by conventional spin-coating. This device performance may be attributed to the homogeneous formation of functional ZnO and PV layers on the ITO cathode. Moreover, the H-dipping process was also applied to fabricate PV cells on a large-area substrate (50 × 50 mm2), which also showed that the fabrication of large-area inverted PSCs is relatively straightforward. The use of this pre-metered coating process in the fabrication of ZnO layers could be of considerable benefit in the future development of all-solution- and/or large-area inverted PSCs.

Spray pyrolysed In2S3 thin films: A potential electron selective layer for large area inverted bulk‐heterojunction polymer solar cells

AbstractIn this paper, we report the results of investigations on the potential of spray pyrolysis technique in depositing electron selective layer over larger area for the fabrication of inverted bulk‐heterojunction polymer solar cells. The electron selective layer (In2S3) was deposited using spray pyrolysis technique and the linear heterojunction device thus fabricated exhibited good uniformity in photovoltaic properties throughout the area of the device. An MEH‐PPV:PCBM inverted bulk‐heterojunction device with In2S3 electron selective layer (active area of 3.25 × 3.25 cm2) was also fabricated and tested under indoor and outdoor conditions. From the indoor measurements employing a tungsten halogen lamp (50 mW/cm2 illumination), an open‐circuit voltage of 0.41 V and a short‐circuit current of 5.6 mA were obtained. On the other hand, the outdoor measurements under direct sunlight (74 mW/cm2) yielded an open‐circuit voltage of 0.46 V and a short‐circuit current of 9.37 mA.

An air-stable inverted photovoltaic device using ZnO as the electron selective layer and MoO3 as the blocking layer

An air-stable photovoltaic device based on znic oxide nanoparticles (ZNP) in an inverted structure of indium tin oxide (ITO)/ZnO/poly (3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM)/MoO3/Ag is studied. We find that the optimum thickness of the MoO3 layer is 2 nm. When the MoO3 blocking layer is introduced, the fill factor of the devices is increased from 29% to 40%, the power conversion efficiency is directly promoted from 0.35% to 1.27%. The stability under ambient conditions of this inverted structure device much is better due to the improved stability at the polymer/Ag interface. The enhancement is attributed to the high carriers mobility and suitable band gap of MoO3 layer.

UV light protection through TiO 2 blocking layers for inverted organic solar cells

Fully organic solar cells (OSCs) based on polymers and fullerenes have attracted remarkable interest during the last decade and high power conversion efficiencies (PCEs) beyond 8% have been realized. However, air stability of these cells remains poor. The conventional geometry of OSCs utilizes strongly oxidizing metal top contacts like Al or Ca. These metals are easily oxidized in air resulting in rapid decrease of PCE if cells are not perfectly encapsulated. Using a thin electron-selective hole-blocking bottom layer like TiO 2 enables fabrication of solar cells in a so-called inverted geometry. In this geometry, noble metals like Ag or Au can be used as top contacts, which are less sensitive to ambient oxygen. Thus, air-stability of these inverted solar cells is significantly improved. In this study we investigate inverted polythiophene–methanofullerene solar cells. We find significant influence of the TiO 2 layer thickness on light absorption and illumination stability of the solar cells, as well as the trap filling by photoinduced carriers. Even though TiO 2 layers as thick as 500 nm seem not to be detrimental for charge transport, light intensity losses limit the device performance. In turn, illumination stability is better for thicker TiO 2 layers, which can serve as UV filters and protect the photoactive materials from degradation, when compared to thin TiO 2 layers. Considering these different effects we state that a thickness of 100 nm is the optimization of the TiO 2 layer.