Cathode Buffer Research Articles

Improved cathode buffer layer to decrease exciton recombination in organic planar heterojunction solar cells

We show that an advanced cathode buffer design, consisting of bathocuproine/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag, increases the short-circuit current of organic planar heterojunction cells and reduces the J-V slope at reverse voltages. We study the physical origin of these effects by measuring reflectivity, voltage dependent external quantum efficiency, and voltage dependent photoluminescence. Our findings suggest that the observed effects are mainly associated with a voltage dependent polaron-induced exciton quenching in the C60 layer. Finally, this improved cathode buffer design is applied to a diindeno[1,2,3-cd:1′,2′,3′-lm]perylene/C70 based cell, leading to a considerable planar heterojunction efficiency of 5.7%.

Inverted quantum-dot light emitting diodes with cesium carbonate doped aluminium-zinc-oxide as the cathode buffer layer for high brightness

We report an inverted structure of quantum-dot light emitting diodes (QLEDs) with cesium carbonate (Cs2CO3) doped aluminum-zinc-oxide (AZO) as the cathode buffer. The Cs2CO3 doped AZO with Cs2CO3 blending ratios (AZO : Cs2CO3) from 4 : 1 to 2 : 1 was used as an electron transport layer in QLEDs. It is found that the conductivity of the AZO : Cs2CO3 blended solution increases with the increase in the concentration of Cs2CO3 until a particular concentration, and the luminance intensity increases sharply from 9949 to 57 350 cd m−2. It is concluded therefore that Cs2CO3 doped AZO could be a good cathode buffer for inverted QLEDs.

Inverted quantum-dot light-emitting diodes with solution-processed aluminium–zinc oxide as a cathode buffer

We report an inverted structure of quantum-dot light emitting diodes (QLEDs) with a metal oxide as a cathode buffer. The Al doped zinc oxide (AZO) with Al concentration from 0 to 30% was used as a charge transport layer in QLEDs which were processed at the temperature of 225 °C. It is found that the conductivity of AZO decreases with increasing Al concentration, but the luminance intensity increases from 6380 to 26 700 cd m−2. The current and power efficiencies at 1000 cd m−2 increase from 3.03 to 4.63 cd A−1 and 2.75 to 3.64 lm W−1, respectively, as the Al concentration increases from 0 to 30%. The luminance intensity for the QLED with 30% AZO increases further to 31 030 cd m−2 and the current efficiency to 5.21 cd A−1 by increasing the thickness of the electron transport layer from 33 to 50 nm. It is concluded therefore that a solution processed AZO can be an effective cathode buffer for an inverted structure of QLEDs.

Thermo-cleavable fullerene materials as buffer layers for efficient polymer solar cells

A new class of thermo-cleavable fullerenes, di-tert-butyl methano[60]fullerene-61,61-dicarboxylate (DBMD), bis(2-methylhexan-2-yl) methano[60]fullerene-61,61-dicarboxylate (BMHMD), and di-tert-butyl methano[60]fullerene-61,61-dicarboxylate bis-adducts (bis-DBMD), was facilely prepared through one-step Bingel reaction from C60. At high temperature, the tertiary alkyl ester groups of the three compounds undergo thermo-cleavage processes to produce insoluble methano[60]fullerene-61-carboxylic acids (MCAs). BMHMD and bis-DBMD were applied in inverted polymer solar cells as a cathode buffer layer. Heating BMHMD or bis-DBMD films on ZnO produced MCA1 and MCA2 interfacial layers, respectively. MCA-modified ZnO possesses a smoother and more hydrophobic surface, and shows excellent solvent-resistance. The MCA buffer layer dramatically improved the device performance by enhancing Jsc and FF of the solar cells. The power conversion efficiency (PCE) increased from 3.44% (control) to 3.79% (MCA1) and 4.10% (MCA2) for a P3HT/PC61BM solar cell, and from 6.95% (control) to 7.13% (MCA1) and 7.57% (MCA2) for a PBDTTT-C/PC71BM solar cell.

High performance polymer solar cells with a polar fullerene derivative as the cathode buffer layer

A highly efficient polymer solar cell was fabricated using a polar fullerene derivative C60 pyrrolidine tris-acid (CPTA) as the cathode buffer layer. By introducing CPTA, the Voc, Jsc and FF were all much enhanced simultaneously. The power conversion efficiency (PCE) was significantly improved to 7.92%, which outperformed the device using Ca/Al as the cathode in our experiment.

Amine group functionalized fullerene derivatives as cathode buffer layers for high performance polymer solar cells

An easy-accessible amine group functionalized fullerene derivative, DMAPA-C60, is explored as a cathode buffer layer (CBL) in polymer solar cells (PSCs) for our presently tested three different material systems, namely P3HT:PCBM, PBDTTT-C:PC70BM and PBDTTT-C-T:PC70BM. The power conversion efficiencies of the three systems with DMAPA-C60 as the CBL reach 3.88%, 6.29% and 7.42%, respectively, which are much higher than those of the corresponding PSCs with the Al-only cathode and even slightly higher than those of the corresponding Ca/Al devices of these systems. The DMAPA-C60 CBL also allows high work function metals (Ag, Cu, and Au) as cathodes.

Efficiency Enhancement of Polymer Solar Cells by Applying Poly(vinylpyrrolidone) as a Cathode Buffer Layer via Spin Coating or Self-Assembly

A non-conjugated polymer poly(vinylpyrrolidone) (PVP) was applied as a new cathode buffer layer in P3HT:PCBM bulk heterojunction polymer solar cells (BHJ-PSCs), by means of either spin coating or self-assembly, resulting in significant efficiency enhancement. For the case of incorporation of PVP by spin coating, power conversion efficiency (PCE) of the ITO/PEDOT:PSS/P3HT:PCBM/PVP/Al BHJ-PSC device (3.90%) is enhanced by 29% under the optimum PVP spin-coating speed of 3000 rpm, which leads to the optimum thickness of PVP layer of ~3 nm. Such an efficiency enhancement is found to be primarily due to the increase of the short-circuit current (J(sc)) (31% enhancement), suggesting that the charge collection increases upon the incorporation of a PVP cathode buffer layer, which originates from the conjunct effects of the formation of a dipole layer between P3HT:PCBM active layer and Al electrodes, the chemical reactions of PVP molecules with Al atoms, and the increase of the roughness of the top Al film. Incorporation of PVP layer by doping PVP directly into the P3HT:PCBM active layer leads to an enhancement of PCE by 13% under the optimum PVP doping ratio of 3%, and this is interpreted by the migration of PVP molecules to the surface of the active layer via self-assembly, resulting in the formation of the PVP cathode buffer layer. While the formation of the PVP cathode buffer layer is fulfilled by both fabrication methods (spin coating and self-assembly), the dependence of the enhancement of the device performance on the thickness of the PVP cathode buffer layer formed by self-assembly or spin coating is different, because of the different aggregation microstructures of the PVP interlayer.

Efficiency and stability enhancement of polymer solar cells using multi-stacks of C60/LiF as cathode buffer layer

We report on the improved performance and stability of bulk heterojunction (BHJ) polymer solar cells using five stacks of C60/LiF as cathode buffer layer, which was prepared by alternating deposition of C60 and LiF layers. The five-stacked C60/LiF film, even with a large thickness ratio of LiF to C60, exhibits good electrical conductivity. The devices with five stacks of C60/LiF buffer layer show a peak power conversion efficiency (PCE) which is 19% higher than that of the conventional devices with only LiF interlayer, primarily due to the improvement in fill factor (FF) of the device. Moreover, the high efficiency of five-stacked C60/LiF based devices is insensitive to changes in the thickness ratio between C60 and LiF layers. Since much thicker LiF can be used in five-stacked C60/LiF film, these devices also show superior air stability as compared to the devices with only LiF interlayer. Therefore, five-stacked C60/LiF is a promising alternative to pristine LiF as a cathode buffer layer in polymer solar cells.

Correction to "Photomask Effect in Organic Solar Cells With ZnO Cathode Buffer Layer" [Oct 12 1480-1482

In the above paper (ibid., vol. 33, no. 10, pp. 1480-1482, Oct. 2012), the acknowledgement for funding is corrected to show that the authors' work was solely supported by the Bilateral International Collaborative R&D Program (Project number: GT-2009-CL-OT-0058, Ministry of Knowledge Economy, Republic of Korea).

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.

Electron Transport Mechanism through a Cathode Buffer in Organic Solar Cells

A new electron transport mechanism through cathode buffer in organic solar cells is proposed with the inverted structure of indium-tin oxide (cathode)/very thin Ag layer (cathode*)/bathocuproine (BCP, buffer)/fullerene/copper phthalocyanine/pentacene/Ag (anode). Electron transport across the BCP occurs not through defect states, but over the LUMO (lowest unoccupied molecular orbital). The electron conductivity is sensitive to the cathode work function (WFc). As WFc is lower, the energy difference between the BCP LUMO and the cathode Fermi level is smaller. This introduces low electronic potential barrier. The low WFc is important to achieve low series resistance.

Effect of Energy States in Buffer Layer on Organic Solar Cells Output Performance

The energy states in cathode buffer layer induced by the metal cathode deposition were studied. The absorption spectra show the existence of energy states in cathode buffer layer, and the energy level of states can be estimated in a quantitative manner. The distribution of state energy levels in the cathode buffer layer is appropriated for the electron transport from C60 to metal cathode. In addition, the Secondary Ion Mass Spectrometer (SIMS) showed the diffusion depth profile of Ag deposition on the bathocuproine (BCP) layer, indicating the relation between the efficiency of organic solar cells (OSCs) and the BCP thickness. As a result, the optimal BCP thickness plays an important role on the OSCs performance for avoiding C60 contact with Ag without affecting the electron collection from C60 to Ag.

Photomask Effect in Organic Solar Cells With ZnO Cathode Buffer Layer

In this letter, we report on the performance of conventional bulk heterojunction organic solar cell (OSC) incorporating a solution-processed zinc oxide (ZnO) spin coated on the photoactive layer regioregular poly(3-hexylthiophene):indene-C60 bis adduct. We show a significant improvement in short-circuit current density (Jsc) upon an introduction of ZnO, and this is further evidenced by the reduction of Jsc leading to a lower PCE in the device without the presence of ZnO. Furthermore, we also examined the “photomask” on the performance of ZnO-based device yielding a decrease in both Jsc and open-circuit voltage Voc (of about 13.42% and 0.73%, respectively) but an increase in filling factor. Results demonstrate that ZnO plays an important role in the improvement of OSCs' performance.

Inverted Organic Solar Cells with Improved Performance using Varied Cathode Buffer Layers

Organic solar cells with inverted planar heterojunction structure based on subphthalocyanine and C60 were fabricated using several kinds of materials as cathode buffer layer (CBL), including tris-8-hydroxy-quinolinato aluminum (Alq3), bathophenanthroline (Bphen), bathocuproine, 2,3,8,9,14,15-hexakis-dodecyl-sulfanyl-5,6,11,12,17,18-hexaazatrinaphthylene (HATNA), and an inorganic compound of Cs2CO3. The influence of the lowest unoccupied molecular orbital level and the electron mobility of organic CBL on the solar cells performance was compared. The results showed that Alq3, Bphen, and HATNA could significantly improve the device performance. The highest efficiency was obtained from device with annealed HATNA as CBL and increased for more than 7 times compared with device without CBL. Furthermore, the simulation results with space charge-limited current theory indicated that the Schottky barrier at the organic/electrode interface in inverted OSC structure was reduced for 27% by inserting HATNA CBL.

Cathode Work Function Dependence of Electron Transport Efficiency through Buffer Layer in Organic Solar Cells

An electron transport mechanism through a cathode buffer layer of organic solar cells is experimentally investigated. Inverted organic solar cells with the structure of indium–tin oxide (ITO)/thin cathode metal/bathocuproine (BCP)/fullerene (C60)/copper phthalocyanine (CuPc)/pentacene/Ag (anode) are examined. A new model, in that electrons are transported across the BCP buffer layer not through defect states but over the lowest unoccupied molecular orbital (LUMO), is proposed. That is, the defect state density in the BCP layer is not important for electron transport, though the hopping transport model via the defect states is widely accepted. The transport efficiency is sensitive to the cathode work function (WFC). As WFC decreases, the energy difference between the BCP LUMO and the cathode Fermi level decreases. This introduces a low electronic potential barrier height from the cathode to the acceptor. The low WFC is thus important to achieve a low series resistance. Furthermore, the dependence of WFC on barrier height is also confirmed for tris(8-hydroxyquinolinato)aluminum buffer.

Fertile Ground

Fertile Ground

Efficient polymer bulk heterojunction solar cells with cesium acetate as the cathode interfacial layer

Abstract The enhanced performance of polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend was achieved by using cesium acetate (CH3COOCs) as cathode buffer layer. Under 100 mW/cm2 white light illumination, the device with 0.8 nm thick CH3COOCs as cathode buffer layer exhibits power conversion efficiency (PCE) as high as (4.16 ± 0.02) %. Compared to the control devices without cathode buffer layer and with LiF as cathode buffer layer, the PCE is enhanced ∼100% and ∼31%, respectively. The introduction of the CH3COOCs buffer layer effectively improves the photo-generated charge collection. The Kelvin Probe measurement shows that the work function of the CH3COOCs is estimated to be −4.0 eV, which has an ideal energy band match with PCBM and a good property for electron collection. The static contact angle results indicated that the CH3COOCs with the hydrophobic CH3COO- group has an improved wettability between the buffer layer and the hydrophobic organic active layer surface, resulting in better interfacial contact and reduced contact resistance. The improved performance may be attributed to the dissociation of semi-conducting CH3COOCs upon deposition to liberate Cs with a low work function, which reduces the interface resistance of the active layer and the cathode and enhances the interior electric field that may result in efficient charge transportation. Therefore, the CH3COOCs interlayer could be a promising alternative to LiF to improve the efficiency of the electron collection of polymer bulk heterojunction solar cells.

A thermal stable cathode buffer based on an inexpensive tetranuclear zinc(II) complex for organic photovoltaic devices

An inexpensive material, i.e., tetranuclear zinc(II) complex, (Zn4O(AID)6) [AID = 7-azaindolate], was utilized as a cathode buffer in organic photovoltaic (OPV) devices, leading to the improvement of device performance. Compared to OPV devices based on a conventional cathode buffer of TPBi (1,3,5-tris(2-N-phenylbenzimidazolyl)benzene), although the freshly prepared devices showed similar performance, when heated to a series of high temperatures under air, the short circuit current and the open circuit voltage of the Zn4O(AID)6 devices dropped more slowly, indicating the superiority of using Zn4O(AID)6 as a cathode buffer over TPBi in OPV devices.

Effect of a thermally evaporated bis (2-methyl-8-quninolinato)-4-phenylphenolate cathode buffer layer on the performance of polymer photovoltaic cells

We investigated the device characteristics of polymer photovoltaic (PV) cells based on a poly(3-hexylthiophene) (P3HT) and [6,6]-phenylC61 butyric acid methyl ester (PCBM) bulk heterojunction with a cathode buffer layer of thermally evaporated bis (2-methyl-8-quninolinato)-4-phenylphenolate (BAlq). A power conversion efficiency (PCE) of 2.46% was obtained with the insertion of a 4-nm-thick BAlq, which was ∼118% increase over that for the cell without a BAlq layer, under Air Mass 1.5 Global (AM 1.5 G) illumination, 100 mW/cm2. Moreover, we examined the charge carrier transport property, and found that the hole mobility of the cell was enhancement due to the insertion of a BAlq layer with a thickness of less than 4 nm, which accounted for the improved in the photocurrent and fill factor (FF) due to the better balance of charge carriers. Finally, the BAlq buffer layer was also demonstrated as an optical spacer that improved the optical absorption of the P3HT:PCBM layer, which accounted for the Jsc enhancement of the device.

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.