Bilayer Heterojunction Research Articles

Improved organic optocouplers based on a deep blue fluorescent OLED and an optimized bilayer heterojunction photosensor

A deep blue fluorescent organic light emitting diode (OLED) and a structurally optimized organic photodiode (OPD) based on fullerene (C60)/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) bilayer heterojunction are coupled into an organic optocoupler (OOC). The demonstrations of overall design strategies show that the selection of OLED wavelength and the elimination of redundant thickness in OPD play important roles in improving the performance of OOC. With these efforts, the current transfer ratio of the OOC is significantly raised from previous 0.17% to 0.58%. The cut-off and following frequencies are 400kHz and 1.25MHz. Quantitative estimations indicate the external quantum efficiencies of OLED and OPD are about 4.3% and 32%, respectively.

3-D modelling of a bilayer heterojunction organic solar cell based on copper phthalocyanine and fullerene (CUPC/C<sub>60</sub>): Evidence of total excitons dissociation at the donor-acceptor interface

A theoretical study of a 3-D modelled bilayer heterojunction organic photovoltaic cell based on copper phthalocyanine (CuPc) as electron donor and fullerene (C60) as electron acceptor is reported. The thin film multilayer stacking theory is applied to the bilayer heterojunction organic solar cell, with the optical matrix of the Abeles theory leading to newexpression of generation rate and density of exciton photogenerated in the organic photoactive layer of CuPc/C60. The excitons density is investigated considering the excitons dissociation parameters at the different interfaces of the organic photoactive region, i.e. the exciton dissociation velocities Sdp, Sdg, Sdj and Sdn respectively associated to thetransparent anode (ITO: Indium thin oxide)/donor (CuPc), the donor (CuPc)/acceptor (C60) and the acceptor (C60)/metallic cathode (Al: aluminium) interfaces. Moreover, the influences of the monochromatic light wavelength ë and the thickness w of the organic photoactive layer (CuPc/C60) on the exciton density are emphasized.KEYWORDS: Bilayer Heterojunction - Organic Solar Cell – Exciton Dissociation

Numerical simulation of photocurrent generation in bilayer organic solar cells: Comparison of master equation and kinetic Monte Carlo approaches

Numerical approaches can provide useful information about the microscopic processes underlying photocurrent generation in organic solar cells (OSCs). Among them, the Kinetic Monte Carlo (KMC) method is conceptually the simplest, but computationally the most intensive. A less demanding alternative is potentially represented by so-called Master Equation (ME) approaches, where the equations describing particle dynamics rely on the mean-field approximation and their solution is attained numerically, rather than stochastically. The description of charge separation dynamics, the treatment of electrostatic interactions and numerical stability are some of the key issues which have prevented the application of these methods to OSC modelling, despite of their successes in the study of charge transport in disordered system. Here we describe a three-dimensional ME approach to photocurrent generation in OSCs which attempts to deal with these issues. The reliability of the proposed method is tested against reference KMC simulations on bilayer heterojunction solar cells. Comparison of the current-voltage curves shows that the model well approximates the exact result for most devices. The largest deviations in current densities are mainly due to the adoption of the mean-field approximation for electrostatic interactions. The presence of deep traps, in devices characterized by strong energy disorder, may also affect result quality. Comparison of the simulation times reveals that the ME algorithm runs, on the average, one order of magnitude faster than KMC.

Bilayer and bulk heterojunction solar cells with functional poly(2,2′-bithiophene) films electrochemically deposited from aqueous emulsion

Ultra thin films of poly(2,2′-bithiophene) with thickness of 11–33nm were electrochemically deposited on poly(3,4-ethylenedioxithiophene):poly(styrene sulfonic acid) modified indium tin oxide substrates using a new approach for electrochemical synthesis. Ultrasonic emulsification of aqueous perchloric acid and 2,2′-bithiophene monomer is an effective method to disperse this water insoluble monomer in micro droplets that improved the contact with the electrode. Rigorous control of the poly(2,2′-bithiophene) thickness was achieved by controlling the potential applied and the charge during the deposition. We studied thin films solar cells based on poly(2,2′-bithiophene) polymer as bilayer with C60, resulting a power conversion efficiency of 0.3%, and as buffer layer in bulk heterojunction solar cells. We show that poly(2,2′-bithiophene) films influenced positively the open circuit voltage and increased about 1.6 times the short circuit current, increasing the efficiency from 1.3% up to 2.9% from devices without buffer layer or with 11nm of poly(2,2′-bithiophene), respectively.

Efficient Exciton Relaxation and Charge Generation in Nearly Monochiral (7,5) Carbon Nanotube/C60 Thin-Film Photovoltaics

We report on photovoltaic diodes based on bilayer heterojunctions between nearly monochiral, polymer wrapped (7,5) semiconducting carbon nanotube photoabsorbing films and C60. The internal quantum efficiencies (IQEs) for exciton dissociation and subsequent charge collection at the nanotubes’ visible E22 and near-infrared E11 and E11 + X resonances are 84% ± 7%, 85% ± 5%, and 84% ± 14%, respectively. The high IQE at each transition shows that recombination losses during relaxation and/or direct dissociation of “hot” E11 + X and E22 excitons are negligible. A peak external quantum efficiency (EQE) of 34% is achieved at the E11 transition. Zero-bias photoresponsivity is invariant up to short-circuit current densities of at least 23 mA cm–2, indicating negligible losses via trion, charge-exciton, and charge–charge recombination relaxation pathways. An open circuit voltage of 0.49 V and power conversion efficiency of 7.1% are achieved in response to monochromatic excitation of the diodes at the E11 transition. The high IQE across multiple spectral windows, invariant photoresponsivity, and attractive open circuit voltage relative to the 1.18 eV optical bandgap demonstrate the future promise of using monochiral and multichiral semiconducting carbon nanotube films for broadband solar photovoltaic applications.

Sensitivity analysis of current generation in organic solar cells—comparing bilayer, sawtooth, and bulk heterojunction morphologies

Organic solar cells (OSC) show great potential as a low-cost energy source. In addition, their mechanical flexibility allows the added advantage of use on a wide variety of surfaces. In recent years, progress in experimental strategies and modeling approaches have enabled enhancing the power conversion efficiencies of OSC. In particular, simulation based analysis has played a significant role in improving our understanding of the charge transport phenomena in the active layer of these devices. The excitonic drift-diffusion (EDD) model has been used widely to simulate the generation and transport properties of bulk heterojunction (BHJ) solar cells. The EDD model – which is derived from the Boltzmann transport is dependent on a number of input parameters such as (1) material properties (mobility and permittivity), (2) operating conditions (illumination and device thickness), and (3) active layer morphology.A comprehensive sensitivity analysis of the short-circuit current, Jsc, to the input parameters is performed. This helps in rank ordering the input parameters and operating conditions – by strength and relevance – on their impact on Jsc. We particularly focus our investigations on understanding how the active layer morphology affects the sensitivity of Jsc. To accomplish this we analyze three classes of morphologies: bilayer, BHJ, and sawtooth. The results show significant differences in sensitivities between BHJ, sawtooth, and bilayer morphologies. Short-circuit current in BHJ structure shows higher sensitivity to material properties than either sawtooth and bilayer structure, suggesting that the necessity for finer control of material properties to counteract the increased disorder in the active layer morphology. The electrode current is found to be most sensitive to illumination intensity for all three morphologies. We report some interesting trends that may help choose the most sensitive parameters to vary for designing OSC's with better performance.

Study of the effect of the charge transport layer in the electrical characteristics of the organic photovoltaics

Significant progress in fabrication and optimization of organic photovoltaics (OPVs) has been made during the last decade. The main reason for popularity of OPVs is due to their low production cost, large area devices and compatibility with flexible substrates [1–3]. Various approaches including optimizing morphology of the active layers [1,2], introducing new materials as the donor and acceptor [3,4], new device structures such as tandem structure [5,6] have been adapted to improve the efficiency of the organic photovoltaics. However, electrical characteristics of the OPVs do not only depend on the active layer materials or device structure. They can also be defined by the interface properties between active layers and the charge transport layers or the metal contacts. Within this paper, the effect of the thickness variation of the charge transport layer in the electrical properties of the bilayer heterojunction OPVs has been studied. Several devices with CuPc/PTCDI-C8 as the donor/acceptor layers have been fabricated with different thicknesses of electron transport layer. MoO3 and Alq3 have been used respectively as the hole transport layer (HTL) and the electron transport layer (ETL). It has been shown that the S-shape effect in the current–voltage curve is attributed to the accumulation of the charge carriers at the interface between the active layer and the charge transport layer [5,7].

Photocurrent generation through electron–exciton interaction at the organic semiconductor donor/acceptor interface

In this work, we report our effort to understand the photocurrent generation that is contributed via electron-exciton interaction at the donor/acceptor interface in organic solar cells (OSCs). Donor/acceptor bi-layer heterojunction OSCs, of the indium tin oxide/copper phthalocyanine (CuPc)/fullerene (C60)/molybdenum oxide/Al type, were employed to study the mechanism of photocurrent generation due to the electron-exciton interaction, where CuPc and C60 are the donor and the acceptor, respectively. It is shown that the electron-exciton interaction and the exciton dissociation processes co-exist at the CuPc/C60 interface in OSCs. Compared to conventional donor/acceptor bi-layer OSCs, the cells with the above configuration enable holes to be extracted at the C60 side while electrons can be collected at the CuPc side, resulting in a photocurrent in the reverse direction. The photocurrent thus observed is contributed to primarily by the charge carriers that are generated by the electron-exciton interaction at the CuPc/C60 interface, while charges derived from the exciton dissociation process also exist at the same interface. The mechanism of photocurrent generation due to electron-exciton interaction in the OSCs is further investigated, and it is manifested by the transient photovoltage characteristics and the external quantum efficiency measurements.

A new method to improve the lifetime stability of small molecule bilayer heterojunction organic solar cells

Unencapsulated small molecular solar cells with a layer of carboxylated copper phthalocyanine (CuPc⁎) introduced between the donor copper phthalocyanine (CuPc) and acceptor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDI), and using ITO and Ag as the cathode and anode were investigated for 35 days. The performance and stability were compared between traditional ITO/PEDOT:PSS/CuPc/PTCDI/Ag cells and the new ITO/PEDOT:PSS/CuPc/CuPc*/PTCDI/Ag cells. The comparison shows that the modified cell has an enhanced stability compared to traditional cells. The improvement of the lifetime is likely due to the CuPc* layer working as buffer layer to prevent humidity and oxygen from diffusing into the active layer. Different thicknesses of CuPc were investigated to study the effects of CuPc layer thickness on the electrical properties of these devices. It was found that the properties of the cell with 20nm thickness of CuPc were the best.

Solvent effect of the fibrillar morphology on the power conversion efficiency of a polymer photovoltaic cell in a diffusive heterojunction

We describe the solvent-dependent morphology of a conjugated polymer, poly(3-hexylthiophene) (P3HT) interfaced with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and the evolution of the resulting inter-connected fibrillar structure (ICFS) in a diffusive bilayer heterojunction (DBHJ). It is found that the fibrillar structure of the P3HT is preferentially developed in a poor solvent and less soluble than an amorphous structure and the ICFS improves the power conversion efficiency (ηp) significantly. Among four different solvents chlorobenzene, dichlorobenzene, chloroform and p-xylene studied, p-xylene results in ηp ≈ 3% whereas chlorobenzene yields only 0.5%. This indicates clearly that the formation of the ICFS plays a critical role to induce high exciton generation/dissociation to produce spatially uninterrupted pathways for the charge transport, and to enhance ηp of a polymer photovoltaic device in the DBHJ configuration.

Hybrid solution/vacuum-processed bilayer heterojunction organic solar cells: Structural characterization and performance

We report on the preparation, structural characterization, and device performance of heterojunction bilayer organic solar cells (OSC) prepared via a combination of solution and vacuum processing. For a functioning OSC, the electron donor layer consisting of unsubstituted copper phthalocyanines (CuPc) is prepared via simple solution processing under ambient conditions. The unsubstituted CuPc layer is wet-deposited via spin coating and drying from a trifluoroacetic acid solution. The other layers are formed by established vacuum evaporation. The internal film structure of the OSC is characterized by atomic force microscopy. The absorption and morphology of the CuPc layer are discussed concerning the power current efficiency of the OSC.

Efficient bilayer heterojunction polymer solar cells with bumpy donor–acceptor interface formed by facile polymer blend

We demonstrated a facile method for the fabrication of bilayer polymer solar cells with a controlled heterojunction structure via simple polymer blends. The spontaneous phase separation of poly(3-hexylthiophene)/polyethylene glycol blends provides a bumpy electron-donor layer with characteristic circular depressions. The diameter and depth of the circular depressions can be controlled by varying the PEG content of the blend. The deposition of [6,6]-phenyl-C61-butyric acid methyl ester as an electron-acceptor layer then creates an interpenetrating donor–acceptor interface for bilayer heterojunction polymer solar cells. The bumpy morphology of the interface results in a significant enhancement in the power conversion efficiency over that of the bilayer polymer solar cells with a typical planar interface, which is mainly due to an increase of photocurrent. An estimation of the field-dependent possibility of charge separation indicates that charge extraction is more efficient than charge recombination in the bilayer devices and the increase in the interfacial area of solar cells with a bumpy-interface leads to generate more electron-hole pairs at the interface.

A Paradigmatic Change: Linking Fullerenes to Electron Acceptors

The potential of Lu(3)N@C(80) and its analogues as electron acceptors in the areas of photovoltaics and artificial photosynthesis is tremendous. To this date, their electron-donating properties have never been explored, despite the facile oxidations that they reveal when compared to those of C(60). Herein, we report on the synthesis and physicochemical studies of a covalently linked Lu(3)N@C(80)-perylenebisimide (PDI) conjugate, in which PDI acts as the light harvester and the electron acceptor. Most important is the unambiguous evidence--in terms of spectroscopy and kinetics--that corroborates a photoinduced electron transfer evolving from the ground state of Lu(3)N@C(80) to the singlet excited state of PDI. In stark contrast, the photoreactivity of a C(60)-PDI conjugate is exclusively governed by a cascade of energy-transfer processes. Also, the electron-donating property of the Lu(3)N@C(80) moiety was confirmed through constructing and testing a bilayer heterojunction solar cell device with a PDI and Lu(3)N@C(80) derivative as electron acceptor and electron donor, respectively. In particular, a positive photovoltage of 0.46 V and a negative short circuit current density of 0.38 mA are observed with PDI/Ca as anode and ITO/Lu(3)N@C(80) as cathode. Although the devices were not optimized, the sign of the V(OC) and the flow direction of J(SC) clearly underline the unique oxidative role of Lu(3)N@C(80) within electron donor-acceptor conjugates toward the construction of novel optoelectronic devices.

A Poly-(3-Hexylthiophene) (P3HT)/[6,6]-Phenyl-C61-Butyric Acid Methyl Ester (PCBM) Bilayer Organic Solar Cell Fabricated by Airbrush Spray Deposition

Airbrush spray deposition is applied to fabricate a bilayer heterojunction solar cell based on P3HT/PCBM. This solar cell device shows an open-circuit voltage of 0.36 V, a short circuit current density of 6.76 mA/cm2, a conversion efficiency of 0.74%, and a fill factor of 30.4%. The results demonstrate that airbrush spray deposition is an effective method to fabricate multilayer or other complex polymer-based organic solar cells. Although spin-coated bulk heterojunction devices have better performance than the airbrushed ones, the airbrush is indeed feasible as a low-cost yet simple process. It is noteworthy that such preliminary results of the airbrush spray solar cell is unoptimized and thus its performance can be further improved with the development of this technology. Furthermore, this method itself has huge potential as it can be used for other polymer-based organic thin film devices.

Electrodes for GaOHPc:PCBM/P3HT:PCBM bulk heterojunction solar cell

The bulk heterojunction approach appears to be one of the most promising concepts of creating efficient, low cost and easily producible organic solar cells. For this purpose one of the best materials was regioregular poly-3-hexylthiophene (P3HT), which is widely used as a donor molecule and a hole transporter, with soluble fullerene derivative (PCBM) as an acceptor and electron transporter. The main drawback of this highly efficient blend is its limited spectral range, covering only a 350–650nm spectral interval. So the main aim of the present work was to extend the spectral range of the cell up to 850nm by adding a second bulk heterojunction layer of complementary absorption spectrum to the P3HT:PCBM layer. Therefore hydroxygallium phthalocyanine (GaOHPc) and PCBM blend was used as an additional layer because GaOHPc has strong and wide intermolecular charge transfer (CT) absorption band of around 830–850nm. Thus an organic bi-layer bulk heterojunction system GaOHPc:PCBM/P3HT:PCBM has been built by the spin coating technique having high charge carrier photogeneration efficiency in the 350–850nm spectral range. The cell annealing temperature influence on the cell performance has been investigated in a 10−6 mbar vacuum within the temperature range from 30°C to 100°C. It was found that annealing at 100°C increases the short-circuit photocurrent external quantum efficiency (EQE) values more than 2 times. Various top electrodes of Al, In and Yb/Al/Se were examined to achieve the best performance of our bi-layer cell. It was found that the best power conversion efficiency (PCE) is provided by an Yb/Al electrode covered with a protective Se layer.

Probing the effect of relative molecular orientation on the photovoltaic device performance of an organic bilayer heterojunction using soft x-ray spectroscopies

The orientation of the constituent molecules in organic thin film devices can affect significantly their performance due to the highly anisotropic nature of π-conjugated molecules. We report here an angle dependent x-ray absorption study of the control of such molecular orientation using well-ordered interlayers for the case of a bilayer heterojunction of chloroaluminum phthalocyanine (ClAlPc) and C60. Furthermore, the orientation-dependent energy level alignment of the same bilayer heterojunction has been measured in detail using synchrotron radiation-excited photoelectron spectroscopy. Regardless of the orientation of the organic interlayer, we find that the subsequent ClAlPc tilt angle improves the π-π interaction at the interface, thus leading to an improved short-circuit current in photovoltaic devices based on ClAlPc/C60. The use of the interlayers does not change the effective band gap at the ClAlPc/C60 heterointerface, resulting in no change in open-circuit voltage.

Efficiency enhancement in organic solar cells by configuring hybrid interfaces with narrow bandgap PbSSe nanocrystals

We hereby present an incorporation technique for inorganic nanocrystals (NCs) in organic solar cells (OSCs) for the improvement of power conversion efficiency (PCE). Ternary PbSSe NCs constitute stable conformations with regular poly(3-hexylthiophene):phenyl-C70 butyric acid methyl ester (P3HT:PCBM) organic composites under two heterojunction systems, and significant solar performance modification was obtained, depending on the incorporation type. Bilayer heterojunction (Bi-HJ) SCs, in which a pristine NC layer is sandwiched between the organic composite and cathode, showed significantly broadened photon-harvesting resulting from combination of both layers and energetic carrier transport as a result of reduced recombination losses. In contrast, bulk heterojunction (BHJ) SCs comprising combined composites of P3HT:PCBM:NCs in a single layer suffered from inefficient charge transport as a result of ubiquitous charge traps. Use of Bi-HJ cells with an NC layer of optimal thickness greatly enhanced the short-circuit current (JSC) to 10.54mAcm−2 and a PCE of 3.12% was achieved; this is a 31% improvement over the conversion efficiency of purely organic cells without NCs. The separate PbSSe NC layer coupled well with the organic composite to provide a broad-range photon-harvesting ability and vertically efficient interfacial junctions for systematic charge transport; this greatly enhances the photovoltaic performances of the OSCs.

Insights from Transient Optoelectronic Analyses on the Open-Circuit Voltage of Organic Solar Cells.

In this Perspective, we review recent progress on the use of transient optoelectronic techniques to quantify the processes determining the open-circuit voltage (VOC) of organic solar cells. Most theoretical treatments of VOC include the effects of both material energetics and recombination dynamics, yet most experimental approaches are based on materials energetics alone. We show that by direct measurement of charge carrier dynamics and densities, the rate of nongeminate charge recombination may be determined within working cells and its impact on achievable VOC determined. A simple fit-free device model utilizing these measurements is shown to agree (to within ±5 mV) with experimentally observed open-circuit voltages for devices comprised of a range of different photoactive layer materials and different processing conditions, and utilizing both bulk and bilayer heterojunctions. This agreement is significantly better than that obtainable from analyzing materials energetics alone, even when employing an in situ analysis of effective electronic band gap. We go on to argue that the precision of our VOC calculations derives from implicitly including the impact of film microstructure on open-circuit voltage. We show that this can modulate VOC by up to 200 mV, and thereby account for the limits of energy-based models in accurately predicting achievable performance.

Combining Ligand-Induced Quantum-Confined Stark Effect with Type II Heterojunction Bilayer Structure in CdTe and CdSe Nanocrystal-Based Solar Cells

We show that it is possible to combine several charge generation strategies in a single device structure, the performance of which benefits from all methods used. Exploiting the inherent type II heterojunction between layered structures of CdSe and CdTe colloidal quantum dots, we systematically study different ways of combining such nanocrystals of different size and surface chemistry and with different linking agents in a bilayer solar cell configuration. We demonstrate the beneficial use of two distinctly different sizes of NCs not only to improve the solar spectrum matching but also to reduce exciton binding energy, allowing their efficient dissociation at the interface. We further make use of the ligand-induced quantum-confined Stark effect in order to enhance charge generation and, hence, overall efficiency of nanocrystal-based solar cells.

Preparation of porous titanium oxide films onto indium tin oxide for application in organic photovoltaic devices

In this work, porous ordered TiO2 films were prepared through sol gel route by using a monolayer of polystyrene spheres as template on indium-tin oxide/glass substrate. These films were characterized by SEM, AFM, Raman spectroscopy, UV–vis absorbance and XRD. The UV–vis absorbance spectrum show a pseudo band gap (PBG) with maxima at 460nm arising from the light scattering and partial or total suppression of the photon density of states, this PBG can be controlled by the size of the pore. We also propose the use of this porous film as electron acceptor electrode in organic photovoltaic cells; we show that devices prepared with porous titania displayed higher efficiencies than devices using compact titania films as electrode. Such behaviour was observed in both bilayer and bulk heterojunction devices.