Ultra-thin alumina layer encapsulation of bulk heterojunction organic photovoltaics for enhanced device lifetime

Successful organic photovoltaic (OPV) device fabrication is contingent on selecting an effective encapsulation barrier layer to preserve device functionality by inhibiting atmosphere-induced degradation. In this work, ultra-thin AlO_x layers are deposited by atomic layer deposition (ALD) to encapsulate pre-fabricated OPV devices. A summary of ALD recipe effects (temperature, cycling time, and number of cycles) on AlO_x film growth and device longevity is presented. First, AlO_x film growth on the hydrophobic OPV surface is shown to occur by a 3D island growth mechanism with distinct nucleation and cluster growth regions before coalescence of a complete encapsulation layer with a thickness ⩾7 nm by 500 cycles. Encapsulated device performance testing further demonstrates that reducing ALD processing temperature to 100 °C minimizes OPV phase segregation and surface oxidation loss mechanisms as evidenced by improved short circuit current and fill factor retention when compared with the conventional 140–150 °C range. Ultra-thin AlO_x encapsulation by ALD provides significant device lifetime enhancement (∼30% device efficiency after 2000 h of air exposure), which is well beyond other ALD-based encapsulation works reported in the literature. Furthermore, the interfacial bonding strength at the OPV–AlO_x interface is shown to play a crucial role in determining film failure mode and therefore, directly impacts ultimate device lifetime.     

Comparison of short and long wavelength absorption electron donor materials in C60-based planar heterojunction organic photovoltaics

Buckminsterfullerene, C₆₀-based planar heterojunction (PHJ) organic photovoltaics (OPVs) have been created using a short wavelength absorption (λ_max = 490 nm) electron-donating bis(naphthylphenylaminophenyl)fumaronitrile (NPAFN). NPAFN exhibits a hole mobility greater than 0.07 cm² V⁻¹ s⁻¹ as determined by its field-effect transistor. It can be attributed to such hole mobility that enables a thin layer (<10 nm) NPAFN in PHJ OPV, ITO/NPAFN/C₆₀/bathocuproine/Al. Because of the low lying HOMO energy level (5.75 eV) of NPAFN and relatively high ionization potential ITO (∼5.58 eV), such OPVs exhibit a very high open circuit voltage of ∼1.0 V, relatively high fill factor of 0.60, and a relatively high shunt resistance of 1100 Ω cm⁻², which all compensate for a relatively low short circuit current of 3.15 mA cm⁻² due to the short absorption wavelength and inferred short exciton diffusion length of NPAFN. Altogether, NPAFN OPVs display a power conversion efficiency (η_PC) of 2.22%, which is better than other long wavelength absorption materials in similar PHJ OPVs, such as pentacene (λ_max 670 nm, HOMO 5.12 eV, η_PC 1.50%) and copper phthalocyanine (λ_max 624, 695 nm, HOMO 5.17 eV, η_PC 1.43%).     

Energy level alignment and morphology of interfaces between molecular and polymeric organic semiconductors

Ultraviolet photoelectron spectroscopy (UPS) was used to determine the energy level alignment at organic–organic conductor–semiconductor and semiconductor–semiconductor hetero-interfaces that are relevant for organic optoelectronic devices. Such interfaces were formed by in situ vacuum sublimation of small molecular materials [C₆₀ and pentacene (PEN)] and ex situ spin-coating of poly(3-hexylthiophene) (P3HT), all on the common substrate poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). We found that the deposition sequence had a significant impact on the interface energetics. The hole injection barrier (HIB) of C₆₀ on PEDOT:PSS could be changed from 1.0 eV (moderate hole injection) to 1.7 eV (good electron injection) by introducing a layer of P3HT. The HIB of P3HT/PEDOT:PSS was increased by 0.35 eV due to an interfacial PEN layer. However, PEN deposited on PEDOT:PSS and P3HT/PEDOT:PSS exhibited the same value. These observations are explained by material-dependent dipoles at the interfaces towards PEDOT:PSS and substrate dependent inter-molecular conformation.     

Electrical characteristics of lateral organic bulk heterojunction device structures

Lateral structures have been used to characterize charge transport phenomena in organic bulk heterojunctions. Through the analysis of the current vs. voltage relationships and their light intensity dependence, space charge limited extraction currents and injection currents have been observed and characterized. Additionally, the drift length of charge carriers has been estimated by characterizing devices of varying lengths. These studies show that lateral structures are a promising way to study the basic physics of organic bulk heterojunction materials as they offer degrees of freedom unavailable in sandwich structures and such studies complement what can be learned from conventional sandwich structures.     

Correlation between interlayer thickness and device performance in blue phosphorescent organic light emitting diodes with a quantum well structure

We systematically examined the effects of interlayer (ITL) thickness variation in an emission layer (EML) on electrical and optical characteristics of blue phosphorescent organic light-emitting diodes. The EML consisted of a quantum well structure using a hole transport material 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as an ITL. This ITL facilitated the confinement of charge carriers in the recombination zone (RZ), adjusted the charge carrier balance in the EML, and prevented the triplet exciton loss to adjacent transport layers. The thickness variation in the ITL greatly influenced the size and location of the RZ and the exciton density (ED), which is related to charge balance and exciton diffusion in the EML. A micro-cavity effect around 500 nm and the corresponding redshift/blueshift in the electroluminescent spectrum arose from different ITL thicknesses. Remarkably, the device having a 5-nm-thick TAPC ITL showed better current and power efficiencies than those of any other devices because of the rearrangement of the locations of excitons and ED through control of the hole/electron charge density.     

Correlation between photoluminescence data and device performance of p-channel strained-layer materials

The 4-K photoluminescence spectrum and room temperature transconductance for modulation dopedp-type GaAs/(In,Ga)As dual-channel strained-quantum-well field-effect transistors with comparable dopant and 2-D carrier concentrations were studied. All gate sizes were nominally 300 μm wide by 1 μm long. The best sample has a peak normalized extrinsic transconductanceg _moat room temperature of 31 mS/mm and a 4K photoluminescence linewidth of 6 meV. Depending upon the sample,g _movaried from about 0.5 to 31 mS/mm while the 4-K photoluminescence linewidth decreased from 26 to 6 meV. The low-temperature photoluminescence linewidth and room temperature transconductance were correlated. These results indicate that photoluminescence spectroscopy can be used for screening wafers for potential device peformance before processing.     

Energy level alignment regimes at hybrid organic–organic and inorganic–organic interfaces

Ultraviolet photoelectron spectroscopy has been used to determine the energy level alignment at interfaces of molecular hole-transporting materials and various conductive substrates. Depending on the work function of the substrate, ϕ_s, a transition between two different energy level alignment regimes has been observed: namely vacuum level alignment and Fermi level pinning. The transition is associated with spontaneous positive charge transfer across the interface to the organic semiconductors above a certain material-specific threshold value of ϕ_s. The charge transfer results in formation of an interfacial dipole of a magnitude that scales with ϕ_s. In the vacuum level alignment regime, the hole-injection barriers scale linearly with ϕ_s, while in the Fermi level pinning regime, these barriers are constant and independent of ϕ_s.     

High performance near infrared photosensitive organic field-effect transistors realized by an organic hybrid planar-bulk heterojunction

Compared with organic photodiodes, photoresponsive organic field-effect transistors (photOFETs) exhibit higher sensitivity and lower noise. The performance of photOFETs based on conventional single layer structure operating in the near infrared (NIR) is generally poor due to the low carrier mobility of the active channel materials. We demonstrate a high performance photOFETs operating in NIR region with a structure of hybrid planar-bulk heterojunction (HPBHJ). PhotOFETs with the structures of single layer [lead phthalocyanine (PbPc) or copper phthalocyanine (CuPc)], single planar heterojunction (PHJ) of CuPc/PbPc, double PHJ of CuPc/PbPc/3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and HPBHJ of CuPc/PbPc:PTCDA were fabricated and characterized. It is concluded that the photOFET with HPBHJ structure showed superior performance compared to that with other structures, and for NIR light of wavelength 808 nm, the photOFET with HPBHJ structure exhibited a large photoresponsivity of 322 mA/W, a high external quantum efficiency of around 50%, and a maximal photosensitivity of 9.4 × 10². The high performance of HPBHJ photOFET is attributed to its high exciton dissociation efficiency and excellent hole transport ability. For 50-nm thick CuPc layer, the optimal thickness of the PbPc:PTCDA layer is found to be around 30 nm.     

Ionization potential dependent air exposure effect on the MoO3/organic interface energy level alignment

We reported an ionization potential (IP) dependent air exposure effect on the MoO₃/organic interface energy level alignment by carrying out in situ ultraviolet photoelectron spectroscopy and synchrotron light based X-ray photoelectron spectroscopy investigations. The electronic structures at MoO₃/organic interfaces comprising various π-conjugated small organic molecules with different IP on MoO₃ substrate have been systematically investigated. For the molecules with low IP, MoO₃/organic interface electronic structures remained almost unchanged after air exposure. In contrast, for the molecules with high IP, the highest occupied molecular orbital (HOMO) leading edge (or hole injection barrier) increases gradually with the increasing molecule IP after air exposure. For the MoO₃/copper-hexadecafluorophthalocyanine (F₁₆CuPc, IP: ∼6.58 eV) interface, air exposure can induce a significant downward shift of the HOMO level as large as ∼0.80 eV.     

Energy level alignment and interactive spin polarization at organic/ferromagnetic metal interfaces for organic spintronics

Energy level alignment and spin polarization at tetracyanoquinodimethane/Fe and acridine orange base/Fe interfaces are investigated by means of photoelectron spectroscopy and X-ray magnetic circular dichroism (XMCD), respectively, to explore their potential application in organic spintronics. Interface dipoles are observed at both hybrid interfaces, and the work function of Fe is increased by 0.7 eV for the tetracyanoquinodimethane (TCNQ) case, while it is decreased by 1.2 eV for the acridine orange base (AOB) case. According to XMCD results, TCNQ molecule has little influence on the spin polarization of Fe surface. In contrast, AOB molecule reduces the interfacial spin polarization of Fe significantly. Induced spin polarization of the two organic molecules at the interfaces is not observed. The results reveal the necessity of investigating the magnetic property changes of both the OSC and the FM during the process of energy level alignment engineering.     

Interface engineering for enhancement in performance of organic/inorganic hybrid heterojunction diode

Interface engineering through self-assembled monolayer (SAM) is an efficient way for tailoring the work function and electronic property of surface; enhancing the charge injection efficiency and device performance for microelectronics applications. Despite this, there is lack of study on effect of interface engineering of organic/inorganic hybrid heterojunction diode through SAM. Here, we have reported the surface engineering for tailoring the surface work function, electronic property, enhancement in injection efficiency and device performance. Therefore, Zinc Oxide (ZnO) film surface was modified with SAM before formation of hybrid ZnO/poly(3-hexylthiophene) (P3HT) heterojunction diode and compared with unmodified ZnO/P3HT diode. Prior to measurement of J-V heterojunction characteristics, both interfaces were characterized using absorption spectra, grazing incidence X-ray diffraction (GIXD), scanning electron microscopy (SEM), atomic force microscopy (AFM), kelvin probe force microscopy (KPFM). The modification of ZnO with SAM prior to heterojunction formation allows the better fabrication of diodes featuring ∼10 fold enhancement in rectification ratio at ±3 V and ∼32 fold enhancement in forward current density at 3 V with advancement in electronic device parameter. The enhancement in electrical characteristics are also discussed taking into account the absorption spectra, structural analysis, surface morphology, topography, surface potential, barrier height, and the energy band diagram of SAM's modified and unmodified diodes. Our study has cemented a path to further improve the device performance and parameter for electronic applications.     

Performance of ITO-free inverted organic bulk heterojunction photodetectors: Comparison with standard device architecture

We report on the fabrication of Indium Tin Oxide (ITO)-free inverted organic bulk heterojunction (BHJ) photodetectors of poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PCBM). The final inverted device structure is Cr/Al/Cr/P3HT:PCBM/poly-3,4-ethylenedioxythiophene:poly-styrenesulfonate (PEDOT:PSS)/Ag (Zimmermann et al., 2009) [1]. The device is top-absorbing with the light entering through the hole contact grid. We have fabricated standard devices with structure ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al in order to carry out a comparison study. Inverted photodetectors show slightly higher quantum efficiency and responsivity compared to standard devices. Frequency responses at different bias voltages were measured showing a maximum −3 dB cut-off frequency of 780 kHz and 700 kHz at −3 V for the standard and inverted structures respectively. Parameters extracted from the fit of a circuital model to the impedance spectroscopy measurements were used to estimate the photodiode cut-off frequency as function of bias.     

Electronic structure of fullerene derivatives in organic photovoltaics

The electronic structures of the fullerene derivatives [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), [6,6]-diphenyl C₆₂ bis (butyric acid methyl ester) (bisPCBM), C₇₀, [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM), [6,6]-phenyl-C₆₁-butyric acid butyl ester (PCBB), [6,6]-phenyl-C₆₁-butyric acid octyl ester (PCBO), [6,6]-thienyl-C₆₁-butyric acid methyl ester (TCBM), and indene-C₆₀ bisadduct (ICBA), which are frequently used as n-type materials in organic photovoltaics, were studied by ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy. We also performed molecular orbital calculation based on density functional theory to understand the experimental results. The electronic structures near the energy gap of the compounds were found to be governed predominately by the fullerene backbone. The side chains also affected the electronic structures of the compounds. The ionization energy and electron affinity were strongly affected by the number of carbons and functional groups in the side chain.     

The balance between aesthetics and performance in building‐integrated photovoltaics in the tropics

This paper aims at investigating if the balance between aesthetics and performance in building‐integrated photovoltaic (BIPV) systems is possible to be achieved in the tropics. To accomplish this objective, three BIPV systems located in Singapore with photovoltaic (PV) systems playing an aesthetically appealing role in their architecture were analysed in detail for a 1‐year period. The systems were analysed regarding the available solar irradiation, the shading profiles and the resulting yield and performance ratio (PR) in order to compare the performance of different subsystems with variable architectural characteristics and string configurations. All systems are partially shaded in the early morning or late afternoon, a common situation inherent to BIPV systems. Results show that even with a theoretical non‐optimal combination of azimuthal deviations and tilt angles, some PV systems show better performances in terms of yield and PR than those which are installed at more ideal conditions. The high ratio of diffuse irradiation in Singapore, the distance of shading obstacles and times when shading occurs and the subsystem and string configurations strongly influence the systems performance results. With the declining costs of PV systems, BIPV can offer attractive solutions with high integration appeal, architectural sophistication and the promotion of sustainable energy supply. In the tropics, rooftop BIPV systems can perform with relatively small losses and be aesthetically appealing when compared with optimally tilted and oriented PV generators. Copyright © 2013 John Wiley & Sons, Ltd.     

Topographical and morphological aspects of spray coated organic photovoltaics

Herein we discuss the topographical and nanomorphological aspects of spray deposited organic photovoltaics. We show that the solvent properties have a massive impact on the topography, but less on the nanomorphology formation of composites based on the electron donor poly(3-hexylthiophene) (P3HT) and the electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM). An adapted solvent mixture consisting of ortho-dichlorobenzene (oDCB) and 1,3,5-trimethylbenzene (mesitylene) allows us to demonstrate spray coated organic photovoltaic devices with 3.1% power conversion efficiency (PCE). Moreover, we show that spray coating is a feasible technology to deposit all solution processable layers of organic solar cells, including the hole transporting layer poly(3,4-ethylene dioxythiophene) doped with polystyrene sulphonic acid (PEDOT:PSS) as well and demonstrate fully spray coated devices with 2.7% PCE.     

High performance submicrometer pentacene-based organic thin-film transistor using planar bottom-contact structure

This paper presents a pentacene-based organic thin-film transistor (OTFT) with a submicrometer channel length of 0.5 μm that uses a planar bottom-contact (pBC) structure to achieve high electrical performance. The performance of the submicrometer OTFT is dominantly influenced by the growth continuity of pentacene near the edge of the source/drain (S/D). The pBC structure with a bilayer dielectric can provide a continuous plane for improving the growth continuity and quality of pentacene near the edge of the S/D. This results in high electrical performance for the submicrometer OTFT with pBC structure, such as a mobility of 0.14 cm²/V s and an on/off current ratio of 1.9 × 10⁵.     

Photocarrier behavior in organic heterojunction photovoltaic cells

We have investigated the bias dependence of photocurrent in several organic heterojunction cells to elucidate the behavior of photogenerated charge carriers. Both the planar and planar-mixed heterojunction devices are shown to always have negative photocurrent even at large forward biases; this phenomena has been attributed to an increased driving force for carrier diffusion away from the heterointerface as the applied bias increases. In contrast, the drift current generally dominates in mixed heterojunction devices due to distributed nature of charge generation throughout the active layer, leading to a photocurrent that is highly dependent on the internal electric field. This dependence gives rise to the reversal of the photocurrent direction at high biases when compared to that at the short-circuit condition. However, the voltage yielding zero photocurrent shows appreciable wavelength dependence, which is strongly correlated to the detailed charge carrier generation profile within the active layer.     

Cumulative energy demand for small molecule and polymer photovoltaics

Organic photovoltaics (OPVs) are expected to be a low cost, environmentally friendly energy solution with advantageous properties such as flexibility and light weight that enable their use in new applications. Considerable progress in power conversion efficiencies has brought OPV technology closer to commercialization. However, little consideration has been given to potential environmental impact associated with their production. Although environmental life cycle studies of OPV exist, their scope is narrow or too reliant on outdated technologies. Some of the most significant recent improvements are the result of new semiconductors materials, which have not yet been assessed from a life cycle perspective. Therefore, this study calculates life cycle embodied energy for 15 new materials encompassing a variety of donor, acceptor, and interface compounds showing the most promise in organic electronics. With the use of new inventory data, life cycle energy impact associated with production of both single junction and multi‐junction architectures has been calculated including bulk heterojunction polymer, planar small molecule, and planar‐mixed small molecule devices. The cumulative energy demand (CED) required to fabricate small molecule and polymer photovoltaics were found to be similar from 2.9 to 5.7 MJ/Wp. This CED is on average of 50% less than for conventional inorganic photovoltaics, motivating the continued development of both technologies. The use of fullerenes was shown to have a dramatic impact on polymer solar cells, comprising 18–30% of the CED, despite only being present in small quantities. Increases in device efficiency are shown to marginally reduce CED for both small molecule and polymer designs. Copyright © 2012 John Wiley & Sons, Ltd.     

High-quality schottky and ohmic contacts in planar 4H-SiC metal semiconductor field-effect transistors and device performance

We have fabricated planar 4H-SiC, metal-semiconductor field-effect transistors (MESFETs) with high-quality metal/SiC contacts. To eliminate potential damage to the gate region caused by etching and simplify the device fabrication process, gate Schottky contacts were formed without any recess gate etching, and an ideality factor of 1.03 was obtained for these gate contacts. The interface state density between the contact metal and SiC was 5.7×10¹² cm⁻²eV⁻¹, which was found from the relationship between the barrier height and the metal work function. These results indicate that the interface was well controlled. Thus, a transconductance of 30 mS/mm was achieved with a 3-μm gate length as the performance figure of these MESFETs with high-quality metal/SiC contacts. Also, a low ohmic contact resistance of 1.2×10⁻⁶ Θcm² was obtained for the source and drain ohmic contacts by using ion implantation.     

Correlation of device performance and defects in AlGaN/GaN high-electron mobility transistors

Device performance and defects in AlGaN/GaN high-electron mobility transistors (HEMTs) have been correlated. Surface depressions and threading dislocations, revealed by optical-defect mapping and atomic force microscopy (AFM), compromised the effectiveness of the SiN_x surface-passivation effect as evidenced by the gate-lag measurements. The residual carriers in the GaN-buffer layer observed from the capacitance-voltage depth profile have been attributed to the point defects and threading dislocations either acting as donors or causing local charge accumulations. Deep-level transient-spectroscopy measurements showed the existence of several traps corresponding to surface states and bulk-dislocation defects. The formation of electron-accumulation regions on the surface or (and) in the GaN-buffer layer was confirmed by currentvoltage measurements. This second, virtual gate formed by electron accumulations can deplete the channel and cause a large-signal gain collapse leading to degraded output power. A good correlation was established between the device performance and defects in AlGaN/GaN HEMT structure.