Improved performance of polymer solar cells based on P3HT and ICBA using alcohol soluble titanium chelate as electron collection layer

We demonstrate efficient polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) and fullerene derivatives ether Indene-C₆₀ Bisadduct (IC₆₀BA) or Indene-C₇₀ Bisadduct (IC₇₀BA)) by using solution-processed titanium(IV) oxide bis(2,4-pentanedionate) (TOPD) as electron collection layer (ECL) between the Al cathode and photoactive layer. The TOPD buffer layer was simply prepared by spin-coating isopropanol solution of TOPD on active layer and then baked at 80 °C for 15 min. The short-circuit current density (J_sc) and the open-circuit voltage (V_oc) of the devices can be simultaneously and significantly improved by optimizing the electron collection layer, the photoactive layer and the device fabrication conditions. The power conversion efficiency (PCE) of the P3HT:IC₆₀BA BHJ device with TOPD buffer layer reaches 5.0% under the illumination of AM1.5G, 100 mW/cm², which is increased by 27% in comparison with that (3.9%) of the device without TOPD buffer layer under the same experimental conditions. When IC₇₀BA was chosen instead of IC₆₀BA, the BHJ device could show better performance with PCE of 5.59%. The results indicate that TOPD is a promising electron collection layer for PSCs.     

Solvent-resistant ITO work function tuning by an acridine derivative enables high performance inverted polymer solar cells

In order to avoid an interpenetration of the buffer and the photoactive layers during preparation of polymer solar cells (PSCs), solvent-resistant buffer films were chemically modified on indium tin oxide (ITO) surface. The conjugated aromatics acridine orange base (AOB) was introduced into the films using 3-bromopropyltrimethoxysilane (BrTMS) as coupling agent. Upon ITO surface modification, the respective work functions show a significant decrease. The modified ITO substrates were implemented in inverted PSCs based on PBDTTT-C-T:PC₇₁BM. With the modification, the power conversion efficiency (PCE) was improved significantly from 4.10% (for the inverted PSC without this buffer layer) to 7.56%. The PCE enhancement is mainly caused by the increase of the open-circuit voltage (43%). These results indicate that the solvent-resistant film is able to facilitate electron collection and transportation, thus providing a novel route to high efficient PSCs by surface engineering.     

Inverted polymer solar cells integrated with small molecular electron collection layer

An efficient inverted polymer solar cell (PSC) is reported by integrating a small molecular electron collection layer (ECL) between indium tin oxide (ITO) cathode and the photoactive layer of blended poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM). The ECL is composed of a cesium carbonate-doped tris(8-hydroxyquinolinato) aluminum (Cs₂CO₃:Alq₃) layer. As determined by photoelectron spectroscopy and electrical measurements, the Cs₂CO₃ doping induces suitable energy level alignment at the ITO/Cs₂CO₃:Alq₃/PCBM interface and the increase in bulk conductivity of organic ECL, which are favorable to electron extraction through Cs₂CO₃:Alq₃ to ITO cathode. In addition, optical simulation indicates that the Cs₂CO₃:Alq₃ layer can act as an optical spacer to modulate the region of highest incident light intensity within the photoactive layer, where absorption and charge dissociation are efficient. The inverted PSC with an optimized Cs₂CO₃:Alq₃ ECL exhibits a power conversion efficiency of 4.83%. The method reported here provides a facile approach to achieve high-performance inverted PSCs at low processing temperature.     

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-C₆₁-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.     

Highly conductive Zinc-Tin-Oxide buffer layer for inverted polymer solar cells

Thin-films of Zinc Tin Oxide (ZTO) with an extremely high charge carrier mobility and superior optical transmittance are synthesized using a simple solution method. These ZTO films have been systematically studied for the application in inverted polymer solar cells (PSCs). The Hall effects measurements show that the charge mobility of the ZTO semiconductor is over 16.5 cm².V⁻¹.S⁻¹, which is the highest mobility value ever reported for oxide buffer made by using solution process. By applying the ZTO buffer layer in the inverted PSCs of P3HT:PC₆₁BM, the power conversion efficiency of the device is 30% higher than that of the devices made with other common buffer layers such as ZnO and TiO₂. Light intensity-dependent JV studies and PL measurements also indicate that ZTO buffer layer reduces surface recombination. This work demonstrates that the solution-synthesized ZTO is a promising new buffer layer with superior electron extraction capability for the solar cells.     

Performance enhancement in inverted polymer solar cells incorporating ultrathin Au and LiF modified ZnO electron transporting interlayer

The performance enhancement of inverted polymer solar cells (PSCs), based on the blend system of regioregular poly(3-hexylthiophene) and [6,6]-phenyl C₆₁-butyric acid methylester, due to incorporating ultrathin Au and LiF interlayer between the front transparent indium tin oxide and a ZnO electron transporting layer was analyzed. The results reveal that a 40% increase in PCE, e.g., from 2.62% to 3.67%, was observed for PSCs made with an optimal Au/LiF interlayer as compared to the one having a bare ZnO electron transporting layer. The presence of Au/LiF-modified ZnO interlayer between ITO and the organic layer helps to improve the charge collection. The absorption enhancement arising from the plasmon resonance of Au nanostructures also contributed to the improvement in PCE. It is shown that PSCs with LiF incorporated ZnO electron transporting layer allow improving cell lifetime, demonstrating <50% decrease in PCE compared to that of the ones with a bare ZnO interlayer after 240 day aging test for cells without encapsulation in air.     

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 300 K under 100 mW/cm² light intensity. The characteristics of PCDTBT and PBDTTPD-based inverted solar cells were: open-circuit voltages (V_oc’s) 0.74 and 0.70 V, short-circuit current densities (J_sc’s) −12.09 and −12.06 mA/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.     

Growth and structural properties of crystalline LaAlO3 on Si (0 0 1)

Crystalline LaAlO₃ was grown by oxide molecular beam epitaxy (MBE) on Si (0 0 1) surfaces utilizing a 2 ML SrTiO₃ buffer layer. This SrTiO₃ buffer layer, also grown by oxide MBE, formed an abrupt interface with the silicon. No SiO₂ layer was detectable at the oxide-silicon interface when studied by cross-sectional transmission electron microscopy. The crystalline quality of the LaAlO₃ was assessed during and after growth by reflection high energy electron diffraction, indicating epitaxial growth with the LaAlO₃ unit cell rotated 45° relative to the silicon unit cell. X-ray diffraction indicates a (0 0 1) oriented single-crystalline LaAlO₃ film with a rocking curve of 0.15° and no secondary phases. The use of SrTiO₃ buffer layers on silicon allows perovskite oxides which otherwise would be incompatible with silicon to be integrated onto a silicon platform.     

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.     

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 = 0.3 cm²) 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 (8 cm²) 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.     

Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer

TiO₂ sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO₂ nanoparticles (TiO₂ NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H₂O in the process of peptization both the crystallinity and particle size of TiO₂ NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO₂ NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO₂ NPs/Al showed the short-circuit current (J_sc) of 12.83 mA/cm² and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO₂ buffer layer. The stability measurement showed that PSC devices with a TiO₂ NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200 h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H₂O diffusion into the active layers. The observations indicate that TiO₂ NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs.     

Bright green organic light-emitting devices having a composite electron transport layer

A bright green organic light-emitting device employing a co-deposited Al-Alq₃ layer has been fabricated. The device structure is glass/indium tin oxide (ITO)/ N, N′-diphenyl-N, N′- (3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD)/tris(8-quinolinolato) aluminum (Alq₃)/ Al-Alq₃/Al. In this device, Al-Alq₃ is used as electron transport layer (ETL). The device shows an operation voltage of 6.1 V at 20 mA/cm². At optimal condition, the brightness of a device at 20 mA/cm² is 2195 cd/m² achieved a luminance efficiency of 5.64lm/W. The result proves that the composite Al-Alq₃ layer is suitable for the ETL of organic light-emitting devices (OLEDs).     

High efficient ITO free inverted organic solar cells based on ultrathin Ca modified AZO cathode and their light soaking issue

Aluminum doped zinc oxide (AZO) was used to be the cathode instead of indium-tin-oxide (ITO) in the poly (3-hexylthiophene-2,5-diyl):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) based bulk heterojunction inverted organic solar cells (IOSCs). For the AZO only IOSC, the device shows a poor power conversion efficiency (PCE) of 1.34% and a light soaking issue related to the energy barrier at the AZO/P3HT:PCBM interface. When a 5 nm Ca modifying layer is inserted between AZO and P3HT:PCBM, the obtained AZO/Ca (5 nm) IOSC shows an increased PCE from 1.74% to 2.69% after 15 min illumination. It is thought that the increased photoconductivity of AZO/Ca (5 nm) film upon illumination and the enhanced electron transport across the AZO/Ca interface may be responsible for the light soaking issue. When an ultrathin Ca modifying layer of 1 nm is employed, a further improved PCE of 3.17% is obtained, and remarkably, no light soaking issue is observed in this case. However, this unexpected issue appears after the un-encapsulated AZO/Ca (1 nm) IOSC has been stored in air for several days, which may be due to the energy loss in the electron transport across the interface between partly oxidized Ca and AZO layers induced by the oxidization of Ca. Furthermore, the AZO/Ca (1 nm) IOSC has a comparable PCE to the referenced ITO/Ca (1 nm) IOSC and presents a better air-stability. It is thus concluded that the AZO cathode is a promising alternative of ITO to fabricate the high efficient and long-lifetime IOSCs.     

Polymer solar cells with NiO hole-collecting interlayers processed by atomic layer deposition

We report on the photovoltaic properties of polymer solar cells that use NiO-coated indium tin oxide (ITO) as the hole-collecting electrode. The NiO films were prepared by atomic layer deposition (ALD) on top of ITO with thicknesses varying from 6 to 25 nm. The NiO films increase the work function (WF) of the ITO, allowing NiO-coated ITO to act as an efficient hole-collecting electrode. Devices made with pristine NiO showed poor current–voltage characteristics. However, subsequent O₂-plasma treatment further increased the WF of NiO, tuning NiO-coated ITO into an efficient hole-collecting electrode for polymer solar cells based on the donor poly(3-hexylthiophene-2,5-diyl) (P3HT). The polymer solar cells with the O₂-plasma treated NiO-coated ITO hole-collecting electrodes yield a power conversion efficiency of 4.1 ± 0.2% under simulated air mass 1.5 G 100 mW/cm² illumination, which is comparable to reference devices with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)-coated ITO hole-collecting electrodes.     

Low-cost supercritical CO2/H2O2 treatment on ITO anodes of fluorescent organic light-emitting diodes

A simple and cost-effective approach is proposed as an alternative to conventional oxygen plasma treatment to modify surface property of Indium tin oxide (ITO) anode of a fluorescent organic light-emitting diode (OLED). This was achieved by treating the ITO anode in supercritical CO₂ (SCCO₂) fluids with hydrogen peroxide (H₂O₂). The SCCO₂/H₂O₂ treatment yielded an ITO work function of 5.35 eV after 15 min treatment at 85 °C and 4000 psi, which was significant higher than 4.8 eV of the as-cleaned ITO surface and was slightly less than 5.5 eV of the ITO surface treated by oxygen plasma. The highest work function achieved was 5.55 eV after 45 min SCCO₂/H₂O₂ treatment. The SCCO₂/H₂O₂ treatment can be used to tailor the ITO work function through changing the operation pressure of the treatment. In addition, the correlated dependence of OLED performance on the ITO anodes with and without the treatments was investigated. The maximum power efficiency of 1.94 lm/W was obtained at 17.3 mA/cm² for the device with 15 min SCCO₂/H₂O₂ treatment at 4000 psi. This power efficiency was 19.3% and 33.8% higher than those of the oxygen plasma treatment and as-clean, respectively. The improvement in device efficiency by the SCCO₂/H₂O₂ treatments can be attributed to enhanced hole injection and balance in charge carriers due to increased work function and surface energy of the ITO anodes.     

Efficient and stable inverted polymer solar cells using TiO2 nanoparticles and analysized by Mott-Schottky capacitance

The performance of both inverted and conventional polymer solar cells (PSCs) were examined with a low-temperature, solution-processed synthesized TiO₂ nanoparticles (TiO₂ NPs) as the electron extraction layer. The performance of inverted PSCs based on P3HT:PCBM bulk-heterojunction with a TiO₂ NPs layer was dramatically improved and the highest power conversion efficiency (PCE) of 4.56% was achieved via 24 h exposure in air, which is one of the highest PCEs for P3HT:PCBM bulk-heterojunction PSCs using TiO₂ as electron extraction layer. Meanwhile, the performance of inverted PSCs was superior to regular PSCs. Mott-Schottky capacitance analysis was carried out for both inverted and regular PSCs to obtain the built-in potential, the depletion width, as well as the doping level of the active layer, which all support the performance improvement of PSCs devices with inverted structure. In addition, inverted PSCs show excellent stability in air without encapsulation. The PCE can retain 87% of its original values after 400 h exposure in air, which is much better than that of regular PSCs. The results indicate that solution-processed TiO₂ NPs shows great potential applications in the fabrication of highly efficient and stable inverted PSCs as well as large-area, flexible printed PSCs.     

An organic p-n junction as an efficient and cathode independent electron injection layer for flexible inverted organic light emitting diodes

We demonstrate an organic p-n junction as an efficient electron injection layer for green inverted bottom-emission organic light emitting diodes (IBOLEDs). The organic p-n junction composed of a p-CuPc/n-Bphen layer showed very efficient charge generation under a reverse bias reaching to 100 mA/cm² at 0.3 V, and efficient electron injection from indium tin oxide (ITO) when adopted in IBOLEDs. Moreover, the organic p-n junction resulted in the same current density-voltage-luminance characteristics independent of the work function of the cathode, which is a valuable advantage for flexible displays.     

Improved property in organic light-emitting diode utilizing two Al/Alq3 layers

We reported on the fabrication of organic light-emitting devices (OLEDs) utilizing the two Al/Alq₃ layers and two electrodes. This novel green device with structure of Al(110 nm)/tris(8-hydroxyquinoline) aluminum (Alq₃)(65 nm)/Al(110 nm)/Alq₃(50 nm)/N,N′-dipheny1-N, N′-bis-(3-methy1phyeny1)-1, 1′-bipheny1-4, 4′-diamine (TPD)(60 nm)/ITO(60 nm)/Glass. TPD were used as holes transporting layer (HTL), and Alq₃ was used as electron transporting layer (ETL), at the same time, Alq₃ was also used as emitting layer (EL), Al and ITO were used as cathode and anode, respectively. The results showed that the device containing the two Al/Alq₃ layers and two electrodes had a higher brightness and electroluminescent efficiency than the device without this layer. At current density of 14 mA/cm², the brightness of the device with the two Al/Alq₃ layers reach 3693 cd/m², which is higher than the 2537 cd/m² of the Al/Alq₃/TPD:Alq₃/ITO/Glass device and the 1504.0 cd/m² of the Al/Alq₃/TPD/ITO/Glass. Turn-on voltage of the device with two Al/Alq₃ layers was 7 V, which is lower than the others.     

Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices

Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments – (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSS_ADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSS_IMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSS_IMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSS_IMM electrode out-performed the PEDOT:PSS_ADD electrode, primarily due to an increase in short circuit current density from 6.62 mA cm⁻² to 7.15 mA cm⁻². The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices.     

A reduced electron-extraction barrier at an interface between a polymer poly(3-hexylthiophene) layer and an indium tin oxide layer

Roles of the buried interface between polymer poly(3-hexylthiophene) (P3HT) layer and indium tin oxide (ITO) on the glass substrate have been characterized by transient photovoltage (TPV). Since P3HT is the hole-transporting material, from intuitiveness, ITO/P3HT contact (IPcontact) tends to be hole extracting. However, in this letter, the negative TPV of ITO/P3HT/Al demonstrates that IPcontact dominates the reversed built in electric field, namely pointing from ITO to Al, and is confirmed to be electron extracting. Meanwhile, an interesting biphasic feature of TPV is demonstrated in a device of ITO/P3HT:[6,6]-phenyl-C61-butyric acid methyl ester/Al. The negative component in biphasic TPV shows that IPcontact is one reason resulting in the leakage current for P3HT based solar cells in normal structures. The theoretical study is conducted, and reveals that the interaction between P3HT and ITO reduces electron barrier by 0.5 eV for IPcontact. Band bending and dipole formation are two possible reasons to reduce the electron barrier. By taking advantage of the electron extraction, IPcontact is employed as a composite cathode in an inverted solar cell by pre-coating a pristine P3HT buffer layer between a blended layer and ITO. The study paves a way to characterize the buried interface in solution processable optoelectronics by observing polarity change of TPV, and to fabricate the simplified inverted organic solar cell employing IPcontact to extract electrons.