An ammonia modified PEDOT: PSS for interfacial engineering in inverted planar perovskite solar cells

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is one of the most widely used hole transport layers (HTL) in inverted perovskite solar cells (PSCs) due to its simple solution-processed ability, high transparency, and conductivity. However, PEDOT:PSS-based devices suffer a lower open-circuit voltage (V_oc) than devices with the conventional structure. To address this issue, we fabricated ammonia-modified PEDOT:PSS films by simply doping PEDOT:PSS solution with different ratio of ammonia. The acidity of PEDOT:PSS can be neutralized by the doped ammonia, which inhibits the ion-exchange reaction between PSS-H and CH₃NH₃I, thus retarding the reduction of the work function for PEDOT:PSS to some extent. As a result, a superior power conversion efficiency (PCE) of 15.5% was obtained for the device based on the ammonia-doped PEDOT:PSS HTL than that of the pristine PEDOT:PSS-based device. We ascribe the PCE enhancement to the increased V_oc and fill factor (FF), which is attributed not only to the better energy-level alignment between the ammonia-modified PEDOT:PSS film and perovskite layer but also to the increased grain size and crystallinity of perovskite film.     

Moderately reduced graphene oxide/PEDOT:PSS as hole transport layer to fabricate efficient perovskite hybrid solar cells

Graphene oxide (GO) with single layer was moderately reduced at 200 °C for 4 h under N₂. Then the moderately reduced graphene oxide (rGO) water solution was employed as an additive to tune the properties of conventional poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) solution. It's found that the incorporation of rGO into PEDOT:PSS nearly did not change its transparency, hydrophilic property, or the surface roughness. So, the rGO/PEDOT:PSS composite was used as a hole transport layer (HTL) to fabricate perovskite solar cells (PSCs). As a result, PSCs with rGO/PEDOT:PSS as HTL exhibit improved power conversion efficiency than that of PSCs with PEDOT:PSS as HTL. Our findings show that moderately reduced rGO/PEDOT:PSS could be an efficient HTL to improve power conversion efficiency of PSCs.     

A Self-Assembled Vertical-Gradient and Well-Dispersed MXene Structure for Flexible Large-Area Perovskite Modules

Advancing hole transport layers (HTL) to realize large-area, flexible, and high-performance perovskite solar cells (PSCs) is one of the most challenging issues for its commercialization. Here, a self-assembled gradient Ti₃C₂T_x MXene incorporated PEDOT:PSS HTL is demonstrated to achieve high-performance large-area PSCs by establishing half-caramelization-based glucose-induced MXene redistribution. Through this process, the Ti₃C₂T_x MXene nanosheets are spontaneously dispersed and redistributed at the top region of HTL to form the unique gradient distribution structure composed of MXene:Glucose:PEDOT:PSS (MG-PEDOT). These results show that the MG-PEDOT HTL not only offers favorable energy level alignment and efficient charge extraction, but also improves the film quality of perovskite layer featuring enlarged grain size, lower trap density, and longer carrier lifetime. Consequently, the power conversion efficiency (PCE) of the flexible device based on MG-PEDOT HTL is increased by 36% compared to that of pristine PEDOT:PSS HTL. Meanwhile, the flexible perovskite solar minimodule (15 cm² area) using MG-PEDOT HTL achieve a PCE of 17.06%. The encapsulated modules show remarkable long-term storage stability at 85 °C in ambient air (≈90% efficiency retention after 1200 h) and enhanced operational lifetime (≈90% efficiency retention after 200 h). This new approach shows a promising future of the self-assembled HTLs for developing optoelectronic devices.     

Simple and Low-Cost Vanadyl Oxalate as Hole Transporting Layer Enables Efficient Organic Solar Cells

Organic solar cells (OSCs) with the conventional configuration usually use polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) as the hole-transporting layer (HTL); however, its acidity tends to affect the performance and long-term stability of the devices. Therefore, replacing PEDOT:PSS with other more stable HTLs is essential for realizing the practical applications of OSCs. To achieve this goal, a simple and low-cost vanadyl oxalate (VOC₂O₄) is identified as a HTL to facilitate high power conversion efficiencies (PCEs), good stability, and high thickness tolerance to be achieved in OSCs. The VOC₂O₄ thin film can be easily prepared by spin-coating from its aqueous solution onto ITO/glass substrate and thermally annealed at 100 °C to exhibit high transmittance, conductivity, and work function. It can be applied as a robust HTL with wide processing conditions, especially after being heated at 200 °C and treated with UV-ozone (UVO) to afford a very high PCE of 18.94% in OSCs. This value is among the highest PCEs obtained for binary OSCs. In addition, the derived OSCs exhibit high thickness tolerance and better stability than those based on PEDOT:PSS as HTL. These results reveal that VOC₂O₄ is an excellent HTL for OSCs, having great potential for large-area device applications.     

Tuning Hole Transport Layer Using Urea for High‐Performance Perovskite Solar Cells

Interface engineering is critical to the development of highly efficient perovskite solar cells. Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted MAPbI₃ perovskite solar cells (PSCs). This treatment has significantly increased MAPbI₃ photovoltaic performance to 18.8% for the urea treated PEDOT:PSS PSCs from 14.4% for pristine PEDOT:PSS devices. The use of urea controls phase separation between PEDOT and PSS segments, leading to the formation of a unique fiber‐shaped PEDOT:PSS film morphology with well‐organized charge transport pathways for improved conductivity from 0.2 S cm^?1 for pristine PEDOT:PSS to 12.75 S cm^?1 for 5 wt% urea treated PEDOT:PSS. The urea‐treatment also addresses a general challenge associated with the acidic nature of PEDOT:PSS, leading to a much improved ambient stability of PSCs. In addition, the device hysteresis is significantly minimized by optimizing the urea content in the treatment.     

PEDOT:PSS-Free Polymer Non-Fullerene Polymer Solar Cells with Efficiency up to 18.60% Employing a Binary-Solvent-Chlorinated ITO Anode

Despite the tremendous development of different high-performing photovoltaic systems in non-fullerene polymer solar cells (PSCs), improving their performance is still highly demanding. Herein, an effective and compatible strategy, i.e., binary-solvent-chlorinated indium tin oxide (ITO) anode, is presented to improve the device performance of the state-of-the-art photoactive systems. Although both ODCB (1,2-dichlorobenzene) solvent- and ODCB:H₂O₂ (hydrogen peroxide) co-solvent-chlorinated ITO (ITO-Cl_-ODCB and ITO-Cl_-ODCB:H2O2) show similar optical transmittance, electrical conductivities, and work function values, ITO-Cl_-ODCB:H2O2 exhibits higher Cl surface coverage and more suitable surface free energy close to the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-buffered ITO anode (ITO/PEDOT:PSS). As a direct consequence, the performance of ITO-Cl_-ODCB-based PBDB-T-2F:BTP-eC9:PC₇₁BM PSCs is comparable as the bare ITO-based devices. In contrast, the performance of ITO-Cl_-ODCB:H2O2-based devices with both small and the scaled-up areas significantly surpass the ITO/PEDOT:PSS-based devices. Furthermore, detailed experimental studies are conducted linking optical property, blend morphology, and physical dynamics to find the reasons for the performance difference. By applying the ITO-Cl_-ODCB:H2O2 anode to six other photovoltaic systems, the device efficiencies are enhanced by 3.6–6.2% relative to those of the ITO/PEDOT:PSS-based control devices, which validates its great application potential of co-solvent-modified ITO anode employed into PEDOT:PSS-free PSCs.     

Improved performance of perovskite solar cells with a TiO2/MoO3 core/shell nanoparticles doped PEDOT:PSS hole-transporter

We demonstrate improved performance of inverted planar heterojunction CH₃NH₃PbI_3-xCl_x perovskite solar cells with a TiO₂/MoO₃ core/shell nanoparticles (NPs) doped poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) hole-transporting layer (HTL). TiO₂/MoO₃ Core/shell NPs with size of approximately 40 nm are successfully prepared with a simple wet solution method and are interspersed into PEDOT: PSS layer to construct the HTL. The optimized device shows a high power conversion efficiency of 13.63%, which is dramatically improved compared with the reference device with a pristine PEDOT:PSS HTL. The improvement is mainly attributed to the increased crystalline of the CH₃NH₃PbI_3-xCl_x film with large-scale domains and a compact morphology. More interesting, the cells exhibit superior stability in ambient conditions, which is attributed to the inhibited penetration of moisture due to the compact morphology of the CH₃NH₃PbI_3-xCl_x film and the reduced hygroscopicity of the PEDOT:PSS film.     

Improving Performance and Stability of Flexible Planar‐Heterojunction Perovskite Solar Cells Using Polymeric Hole‐Transport Material

For realizing flexible perovskite solar cells (PSCs), it is important to develop low‐temperature processable interlayer materials with excellent charge transporting properties. Herein, a novel polymeric hole‐transport material based on 1,4‐bis(4‐sulfonatobutoxy)benzene and thiophene moieties (PhNa‐1T) and its application as a hole‐transport layer (HTL) material of high‐performance inverted‐type flexible PSCs are introduced. Compared with the conventionally used poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the incorporation of PhNa‐1T into HTL of the PSC device is demonstrated to be more effective for improving charge extraction from the perovskite absorber to the HTL and suppressing charge recombination in the bulk perovskite and HTL/perovskite interface. As a result, the flexible PSC using PhNa‐1T achieves high photovoltaic performances with an impressive power conversion efficiency of 14.7%. This is, to the best of our knowledge, among the highest performances reported to date for inverted‐type flexible PSCs. Moreover, the PhNa‐1T‐based flexible PSC shows much improved stability under an ambient condition than PEDOT:PSS‐based PSC. It is believed that PhNa‐1T is a promising candidate as an HTL material for high‐performance flexible PSCs.     

Boosting Performance of Non‐Fullerene Organic Solar Cells by 2D g‐C3N4 Doped PEDOT:PSS

The power‐conversion efficiency (PCE) of single‐junction organic solar cells (OSCs) has exceeded 16% thanks to the development of non‐fullerene acceptor materials and morphological optimization of active layer. In addition, interfacial engineering always plays a crucial role in further improving the performance of OSCs based on a well‐established active‐layer system. Doping of graphitic carbon nitride (g‐C₃N₄) into poly(3,4‐ethylene‐dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport layer (HTL) for PM6:Y6‐based OSCs is reported, boosting the PCE to almost 16.4%. After being added into the PEDOT:PSS, the g‐C₃N₄ as a Bronsted base can be protonated, weakening the shield effect of insulating PSS on conductive PEDOT, which enables exposures of more PEDOT chains on the surface of PEDOT:PSS core‐shell structure, and thus increasing the conductivity. Therefore, at the interface between g‐C₃N₄ doped HTL and PM6:Y6 layer, the charge transport is improved and the charge recombination is suppressed, leading to the increases of fill factor and short‐circuit current density of devices. This work demonstrates that doping g‐C₃N₄ into PEDOT:PSS is an efficient strategy to increase the conductivity of HTL, resulting in higher OSC performance.     

High efficiency arrays of polymer solar cells fabricated by spray‐coating in air

We present bulk heterojunction organic solar cells fabricated by spray‐casting both the PEDOT:PSS hole‐transport layer (HTL) and active PBDTTT‐EFT:PC₇₁BM layers in air. Devices were fabricated in a (6 × 6) array across a large‐area substrate (25 cm²) with each pixel having an active area of 6.45 mm². We show that the film uniformity and operational homogeneity of the devices are excellent. The champion device with spray cast active layer on spin cast PEDOT:PSS had an power conversion efficiency (PCE) of 8.75%, and the best device with spray cast active layer and PEDOT:PSS had a PCE of 8.06%. The impacts of air and light exposure of the active layer on device performance are investigated and found to be detrimental. Copyright © 2015 John Wiley & Sons, Ltd.     

High performance thermal-treatment-free tandem polymer solar cells with high fill factors

It is an effective way to enhance device performance of polymer solar cells (PSCs) by using a tandem structure that combines two or more solar cells. For tandem PSCs, the buffer layer plays an important role in determining the device performance. The most commonly used buffer layers, such as PEDOT:PSS, TiO_x, and ZnO, need thermal treatments that are not beneficial for reducing the fabrication complexity and cost of tandem PSCs. It is necessary to develop tandem PSCs fabricated by a thermal-treatment-free process. In this paper, we report high performance thermal-treatment-free tandem PSCs by developing PFN as buffer layers for both subcells. A power conversion efficiency (PCE) of 10.50% and a high fill factor of 72.44% were achieved by stacking two identical PTB7:PC₇₁BM subcells. When adopting a rear PTB7-Th:PC₇₁BM subcell, the highest PCE of 10.79% was further obtained for the tandem devices. The thermal-treatment-free process is especially applicable to flexible devices, in which plastic substrates are usually used.     

Mitigating Detrimental Effect of Self-Doping Near the Anode in Highly Efficient Organic Solar Cells

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been one of the most established hole transport layers (HTL) in organic solar cells (OSCs) for several decades. However, the presence of PSS⁻ ions is known to deteriorate device performance via a number of mechanisms including diffusion to the HTL-active layer interface and unwanted local chemical reactions. In this study, it is shown that PSS⁻ ions can also result in local p-doping in the high efficiency donor:non-fullerene acceptor blends – resulting in photocurrent loss. To address these issues, a facile and effective approach is reported to improve the OSC performance through a two-component hole transport layer (HTL) consisting of a self-assembled monolayer of 2PACz ([2-(9H-Carbazol-9-yl)ethyl]phosphonic acid) and PEDOT:PSS. The power conversion efficiency (PCE) of 17.1% using devices with PEDOT:PSS HTL improved to 17.7% when the PEDOT:PSS/2PACz two-component HTL is used. The improved performance is attributed to the overlaid 2PACz layer preventing the formation of an intermixed p-doped PSS⁻ ion rich region (≈5–10 nm) at the bulk heterojunction-HTL contact interface, resulting in decreased recombination losses and improved stability. Moreover, the 2PACz monolayer is also found to reduce electrical shunts that ultimately yield improved performance in large area devices with PCE enhanced from 12.3% to 13.3% in 1 cm² cells.     

Isopropanol-treated PEDOT:PSS as electron transport layer in polymer solar cells

Isopropanol (IPA)-treated poly(3,4-ethylenedioxithiophene):poly(styrene sulfonate) (PEDOT:PSS) was applied as a new electron transport layer (ETL) in P3HT:PCBM bulk heterojunction polymer solar cell (BHJ-PSC) devices for the first time, revealing the electron transport property of IPA-treated PEDOT:PSS in sharp contrast to the well known hole transport property of the untreated PEDOT:PSS. Under the optimized condition for incorporating PEDOT:PSS ETL, the power conversion efficiency (PCE) of the ITO/untreated PEDOT:PSS (HTL)/P3HT:PCBM/IPA-treated PEDOT:PSS (ETL)/Al device (3.09%) is quite comparable to that of the reference ITO/untreated PEDOT:PSS (HTL)/P3HT:PCBM/Al device without any ETL (3.06%), and an annealing treatment of PEDOT:PSS ETL at 120 °C for 10 min led to a PCE of 3.25%, which even slightly surpasses that of the reference device, revealing the electron transport property of IPA-treated PEDOT:PSS. The electron transport property of IPA-treated PEDOT:PSS is interpreted by the lowering of the work function of PEDOT:PSS upon IPA treatment and incorporation as ETL as probed by scanning Kelvin probe microscopy (SKPM).     

Efficient Polymer Solar Cells Based on Poly(3‐hexylthiophene):Indene‐C70 Bisadduct with a MoO3 Buffer Layer

Polymer solar cells (PSCs) with poly(3‐hexylthiophene) (P3HT) as a donor, an indene‐C₇₀ bisadduct (IC₇₀BA) as an acceptor, a layer of indium tin oxide modified by MoO₃ as a positive electrode, and Ca/Al as a negative electrode are presented. The photovoltaic performance of the PSCs was optimized by controlling spin‐coating time (solvent annealing time) and thermal annealing, and the effect of the spin‐coating times on absorption spectra, X‐ray diffraction patterns, and transmission electron microscopy images of P3HT/IC₇₀BA blend films were systematically investigated. Optimized PSCs were obtained from P3HT/IC₇₀BA (1:1, w/w), which exhibited a high power conversion efficiency of 6.68%. The excellent performance of the PSCs is attributed to the higher crystallinity of P3HT and better a donor–acceptor interpenetrating network of the active layer prepared under the optimized conditions. In addition, PSCs with a poly(3,4‐ethylenedioxy‐thiophene):poly(styrenesulfonate) (PEDOT:PSS) buffer layer under the same optimized conditions showed a PCE of 6.20%. The results indicate that the MoO₃ buffer layer in the PSCs based on P3HT/IC₇₀BA is superior to that of the PEDOT:PSS buffer layer, not only showing a higher device stability but also resulting in a better photovoltaic performance of the PSCs.     

In-situ synthesis of metal nanoparticle-polymer composites and their application as efficient interfacial materials for both polymer and planar heterojunction perovskite solar cells

A new approach for the synthesis of gold nanoparticles (Au NPs) via a simple and fast in-situ generation method using an amine-containing polymer (PN4N) as both stabilizer and reducing agent is reported. The application of the Au NPs-PN4N hybrid material as efficient interfacial layer in different types of solar cells was also explored. The synthesized Au NPs show good uniformity in size and shape and the Au NPs doped PN4N hybrid composites exhibit high stability. Amine-containing polymers are good cathode interfacial materials (CIMs) in polymer solar cells (PSCs) and planar heterojunction perovskite solar cells (PVKSCs). The performance of the PSCs with Au NPs doped PN4N CIMs is largely improved when compares to devices with pristine PN4N CIM due to the enhanced electronic properties of the doped PN4N. Furthermore, by incorporating larger Au NPs into PEDOT:PSS to enhance absorption of the light harvesting layer, power conversion efficiencies (PCEs) of 6.82% and 13.7% are achieved for PSC with PCDTBT/PC₇₁BM as the light harvesting materials and PVKSC with a ∼280 nm-thick CH₃NH₃PbI_3−xCl_x perovskite layer, respectively. These results indicate that Au NPs doped into both PEDOT:PSS and PN4N interlayers exhibited a synergistic effect in performance improvement of PSCs and PVKSCs.     

Layer by layer characterisation of the degradation process in PCDTBT:PC71BM based normal architecture polymer solar cells

This work demonstrates the stability and degradation of OSCs based on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′ benzothiadiazole)] (PCDTBT): (6,6)-Phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) photoactive blend layers as a function of ageing time in air. Analysis of the stability and degradation process for the OSCs was conducted under ambient air by using current-voltage (I-V) measurements and x-ray photoelectron spectroscopy (XPS). The interface between photoactive layer and HTL (PEDOT:PSS) was also investigated. Device stability was investigated by calculating decay in power conversion efficiency (PCE) as a function of ageing time in the air. The PCE of devices decrease from 5.17 to 3.61% in one week of fabrication, which is attributed to indium and oxygen migration into the PEDOT:PSS and PCDTBT:PC₇₁BM layer. Further, after aging for 1000 h, XPS spectra confirm the significant diffusion of oxygen into the HTL and photoactive layer which increased from 3.0 and 23.3% to 20.4 and 35.7% in photoactive layer and HTL, respectively. Similarly, the indium content reached to 17.9% on PEDOT:PSS surface and 0.4% on PCDTBT:PC₇₁BM surface in 1000 h. Core-level spectra of active layer indicate the oxidation of carbon atoms in the fullerene cage, oxidation of nitrogen present in the polymer matrix and formation of In₂O₃ due to indium diffusion. We also observed a steady fall in the optical absorption of the active layer during ageing in ambient air and it reduced to 76.5% of initial value in 1000 h. On the basis of these experimental results, we discussed key parameters that account for the degradation process and stability of OSCs in order to improve the device performance.     

Efficient and Stable Perovskite Solar Cells via Dual Functionalization of Dopamine Semiquinone Radical with Improved Trap Passivation Capabilities

Highly efficient planar heterojunction perovskite solar cells (PVSCs) with dopamine (DA) semiquinone radical modified poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) (DA‐PEDOT:PSS) as a hole transporting layer (HTL) were fabricated. A combination of characterization techniques were employed to investigate the effects of DA doping on the electron donating capability of DA‐PEDOT:PSS, perovskite film quality and charge recombination kinetics in the solar cells. Our study shows that DA doping endows the DA‐PEDOT:PSS‐modified PVSCs with a higher radical content and greater perovskite to HTL charge extraction capability. In addition, the DA doping also improves work function of the HTL, increases perovskite film crystallinity, and the amino and hydroxyl groups in DA can interact with the undercoordinated Pb atoms on the perovskite crystal, reducing charge‐recombination rate and increasing charge‐extraction efficiency. Therefore, the DA‐PEDOT:PSS‐modified solar cells outperform those based on PEDOT:PSS, increasing open‐circuit voltage (V _oc) and power conversion efficiency (PCE) to 1.08 V and 18.5%, respectively. Even more importantly, the efficiency of the unencapsulated DA‐PEDOT:PSS‐based PVSCs are well retained with only 20% PCE loss after exposure to air for 250 hours. These in‐depth insights into structure and performance provide clear and novel guidelines for the design of effective HTLs to facilitate the practical application of inverted planar heterojunction PVSCs.     

Decahedral-shaped Au nanoparticles as plasmonic centers for high performance polymer solar cells

The photon harvesting of the photoactive layer within a multilayered polymer solar cells (PSCs) greatly affects the output electric power of the devices. For PSCs, the device performance is very sensitive to the photoactive layer thickness. Therefore, how to enhance the light absorption of the photoactive film with fixed thickness is still a big challenge. Plasmonic enhancement induced by noble metal nanoparticles has been proved to be an effective way to enhance light trapping inside the photoactive film without increasing the thickness of film. By incorporating Au decahedra into the poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) anode buffer layer, high performance plasmonic PSCs based on P3HT:PC₆₀BM and PBDT-TS1:PC₇₀BM were fabricated and the light response of the PSCs are greatly improved in a broadband wavelength, resulting in a remarkable enhancement in short-circuit current density. The calculation results of finite difference time domain (FDTD) confirm that the plasmonic effects induce enhancement in device performance. Upon optimization, the best power conversion efficiency (PCE) of the device based on P3HT:PC₆₀BM and PBDT-TS1:PC₇₀BM reaches 4.14% and 10.29%, respectively, among the best values reported in literature. These results can provide valuable guidelines for the design of metal nanostructures for organic photovoltaic applications.     

Efficient electron-blocking layer-free planar heterojunction perovskite solar cells with a high open-circuit voltage

Perovskite solar cells (PSCs) with a simple device structure are particularly attractive due to their low cost and convenient fabrication process. Herein, highly efficient, electron-blocking layer (EBL)-free planar heterojunction (PHJ) PSCs with a structure of ITO/CH₃NH₃PbI₃/PCBM/Al were fabricated via low-temperature, solution-processed method. The power conversion efficiency (PCE) of over 11% was achieved in EBL-free PHJ-PSCs, which is closed to the value of PSC devices with the PEDOT:PSS as the EBL. It is impressed that the open-circuit voltage (V_oc) up to 1.06 V, an average value of 1.0 V for 43 devices, was obtained in EBL-free PHJ-PSCs. The electrochemical impedance spectroscopy (EIS) results suggested that the high PCE and V_oc are attributed to the relatively large recombination resistance and low contact resistance in EBL-free PHJ-PSCs. The solution-processed, EBL-free PHJ structure paves a boulevard for fabricating high-efficiency and low-cost PSCs.     

Mechanism of the operational effect of black spot growth in OLEDs

When organic light-emitting diodes (OLEDs) are exposed to the ambient atmosphere, water penetration through pinholes in the cathode results in the formation of non-emissive areas (black spots) due to local oxidation of the cathode around the pinhole. This degradation effect severely limits the lifetime of OLED devices and requires high performance encapsulation in order to delay its onset. We have investigated the process of degradation of OLED devices by water penetration through cathode pinholes, and have discovered that this is not just a simple oxidation of the cathode occurring as water diffuses within the device. We have observed that other layers within the device interact with the water. In solution processed OLEDs, the hole-injection layer (PEDOT:PSS) absorbs water due to the presence of the PSS acid (formation of H₃O⁺). This results in a slight local reduction of the luminance of the device (grey spot) around the pinhole location and actually a reduction of the cathode black spot growth rate at the pinhole during a shelf lifetime test. When the device is not operated, the PEDOT:PSS layer is acting as a local reservoir for water keeping it away from the cathode and slowing down the cathode degradation. However, when the device is operated, water that is captured by the PEDOT:PSS layer to form H₃O⁺ can be transported to the cathode under the influence of the applied electric field. This then increases the black spot growth rate. Experimental indications for this operational effect are provided by the threshold voltage behaviour of the effect and transient behaviour of the black spot growth after switching off the OLED. Direct evidence for the role of PEDOT:PSS in the operational effect has been provided by Raman Spectroscopy.