Bulk Heterojunction Systems Research Articles

P3HT Molecular Weight Determines the Performance of P3HT:O‐IDTBR Solar Cells

Large‐scale production of organic solar modules requires low‐cost and reliable materials with reproducible batch‐to‐batch properties. In case of polymers, their (photo)physical properties depend strongly on the polymers’ molecular weight (MW). Herein, the impact of the MW of the donor polymer poly(3‐hexylthiophene) (P3HT) on the photophysics is studied in blends with a recently developed rhodanine‐endcapped indacenodithiophene nonfullerene acceptor (IDTBR), a bulk heterojunction (BHJ) system that potentially fulfills the aforementioned criteria for large‐scale production. It is found that the power conversion efficiency (PCE) increases when the weight‐average MW is increased from 17 kDa (PCE: 4.0%) to 34 kDa (PCE: 6.6%), whereas a further increase in MW leads to a reduced PCE of 4.4%. It is demonstrated that the charge generation efficiency, as estimated from time‐delayed collection field experiments, varies with the P3HT MW and is the reason for the differences in photocurrent and device performance. These findings provide insight into the fundamental photophysical reasons of the MW dependence of the PCE, which is taken into account when using polymer‐based nonfullerene acceptor blends in solar cell devices and modules.

Light Absorption and Photoluminescence Quenching Properties of Bulk Heterojunction Materials Based on the Blend of Poly(n-vinylcarbazole)/poly(n-hexylthiophen)

The enhancement of light absorption and photoluminescence quenching properties of the bulk heterojunction systems which were fabricated using poly(N-vinylcarbazole) (PVK); poly(N-hexylthiophene)(P3HT) and fullerene derivative 1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6] C61 (PCBM) were investigated. The optimized material showed a broad absorption in the region from ultra violet to near infra-red and the luminescence quenching higher than 90%. The obtained results provide further insight into photophysics of the heterojunction system and device performance improvement by using this system as an active layer.

Influence of Solvent Additive 1,8-Octanedithiol on P3HT:PCBM Solar Cells

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Photovoltaic effect in bulk heterojunction system with glass forming indandione derivative DMABI-6Ph

The aim of the work is to evaluate possible use of 2-[[4-(bis(2-trityloxyethyl)amino)phenyl]methylene]indane-1,3-dione (DMABI-6Ph) as light absorbing material for solar cells. DMABI-6Ph is a perspective material due to its good photoelectrical, thermal and chemical properties. The main advantage of DMABI-6Ph is its ability to form amorphous films by wet-casting methods thus allowing using the compound in organic solar cells made from solution. For now most popular materials for solution processable solar cells are polymer P3HT and fullerene derivative PCBM, but lot of investigations are in the field of new low molecular weight materials to replace the polymer. Photoelectrical measurements were made to determine molecule ionization and electron affinity levels of DMABI-6Ph. Difference of 2.06 eV between DMABI-6Ph ionization level and PCBM affinity level was obtained. Accordingly open circuit voltage of system DMABI-6Ph:PCBM was measured up to 0.78 V. The best power conversation efficiency was 0.11 % for the DMABI-6Ph:PCBM mass fraction 2:1. Limiting factor for high efficiency could be low charge carrier mobility which can be increase by additional DMABI-6Ph modification.

Synthesis of Novel Intramolecular Charge-Transfer Molecules Bearing Triphenylamine and Benzothiadiazole Moieties, and Application to Single-Absorber Organic Solar Cells

Organic solar cells (OSCs) have been extensively studied due to their potential for flexible, printable, and low-cost devices. Recently, power conversion efficiency (PCE) has reached around 10% by using bulkhetero junction (BHJ) system which possesses miscible interface between two kinds of materials, electron donor and acceptor. In the BHJ system, LUMO offset is required to promote charge separation at donor-acceptor interface, causing serious photovoltage loss. In order to overcome this problem, we have pursued the possibility of single-absorber organic solar cells, whose active organic layer consists of one component. Generally, photogeneration efficiency in organic bulk film is quite low because of the large exciton binding energy (EBE). In previous work, we found that molecules with intramolecular charge transfer photoabsorption are suitable for the single-absorber OSC. A donor-acceptor (D-A) linked molecule DTDCPB (Fig. 1) was applied to the single absorber OSC, and comparatively good performance was obtained. In this study, we synthesized novel donor-acceptor linked molecules bearing triphenylamine and benzothiadiazole moieties by derivatizing DTDCPB (Fig. 1). DTDCPB was extended by insertion of thienothiophene at the donor-acceptor linkage part to form DTDCPB-TT. DTERPB and DTRCNPB bear an acceptor moiety of 3-ethyl rhodanine and 2-(3-ethyl-4-oxothiazolidin-2-ylidene)malononitrile, respectively, instead of dicyanovinyl moiety. EBEs in the solid state were estimated with quantum chemical calculation to design a suitable molecule for lower EBE. The D-A molecules were applied to a single absorber OSC composed of ITO/PEDOT:PSS/organic/Ca/Al. In the DTDCPB-TT device, FF and hole mobility increased compared with those in the DTDCPB device because the film crystallinity was improved due to thienothiophene. In the DTERPB and DTRCNPB devices, the photovoltage loss was only 0.60 V, that was less than that of the typical BHJ system. The ratio of the voltage loss to the absorbed energy was 33% for DTERPB and 35% for DTRCNPB, which were rather small in comparison with the typical BHJ system, e.g. 65% for P3HT:PC61BM and 56% for PTB7:PC71BM. Figure 1

(Invited) Photocarrier Generation in Single-Absorber Organic Solar Cells

Power Conversion efficiency of organic photovoltaic devices has exceeded 10% by taking advantage of bulk heterojunction (BHJ) structure, where donor and acceptor molecules are microscopically mixed. A remained problem is photovoltage loss from absorbed photon energy. In the BHJ system, LUMO offset is required to promote photodissociation at the donor-acceptor interface, causing serious photovoltage loss. To solve this problem, we are approaching it from single absorber cells, where there is no donor-acceptor interface. Generally, photogeneration efficiency in organic film bulk is quite low because of large exciton binding energy of Frenkel exciton. On the other hand, photogeneration from CT excitons occurs almost spontaneously in the BHJ system. If we could generate charge-transfer (CT) excitons in the single-component film directly, photovoltage loss would be reduced. Recently, we found intramolecular charge-transfer molecules including donor and acceptor units within one molecule show considerable high performance in the single-absorber device. In particular, DTDCPB having triphenyl amine unit (donor) and benzothiadiazole unit (acceptor) was a promising material for the single-absorber system. In this presentation, photogeneration process in the single-component film of CT molecules was investigated from several viewpoints. Molecular design for intramolecular charge-transfer absorption is discussed from the viewpoint of quantum chemical calculation and organic synthesis. The photocarrier dynamics in the single-absorber solar cells is investigated by several techniques such as intensity-modulated photovoltage/photocurrent (IMVS/IMPS) and time delayed collection field (TDCF). Exciton binding energy, that is a key factor for photodissociation in the film bulk, was estimated by Onsager-Braun analysis by using a planar device. Figure 1

Oxidation inhibition of poly(3-hexylthiophene-2,5-diyl) in the bulk heterojunction by an electron acceptor

Poly(3-hexylthiophene-2,5-diyl) (P3HT) is an organic semiconducting polymer that is widely used in organic electronic devices such as organic solar cells. In this work, we studied the oxidation behavior of P3HT on glass under exposure to visible light and dry air. We also compared the oxidation behavior of bare P3HT films with that of P3HT/phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction systems consisting of donor and acceptor. UV–Vis absorption spectroscopy and X-ray photoelectron spectroscopy were employed to provide evidence that optically excited P3HT could undergo oxidation, which was likely initiated via electron transfer from the conduction band of P3HT to O2, forming strongly oxidizing O2– species. By contrast, P3HT + PCBM yielded almost no oxidation of organic materials under the same conditions. PCBM was thus suggested to extract optically excited electrons from P3HT, thereby inhibiting the formation of O2–. PCBM thus played a role in separating electron-hole pairs from the photoactive layers of organic solar cells and affected the passivating oxidation of photoactive conjugate polymers.

Revisiting the Impact of Interfacial Transport Layers on Organic Bulk Heterojunction Systems

The utilization of interfacial transport layers is critically important for extensively elaborating organic photovoltaic efficiencies. Despite some fundamental knowledge regarding their electrical and optical properties, a full realization of their impact on photoactive layers is still lacking. In this work, we report a new insight into the impact of the interfacial transport layers on physical properties of an organic bulk heterojunction (BHJ) system comprising ITO(glass)/PEDOT:PSS/PIDTDTQx:PC70BM/PFN/Al. A combination of multiple measurements was performed either at steady states or at working conditions aiming to deeply understand the role of the interfacial transport layers in the organic BHJ photovoltaics. Experimental results validated our assumption that the optoelectronic properties of the organic blends, such as PIDTDTQx:PC70BM, can be remarkably influenced due to the presence of the electron/hole transport layers. We have looked into this issue by examining the organic blend PIDTDTQx:PC70BM-based BHJ photovoltaic devices from the interfacial trap-related density of states, electron–hole recombination resistance (Rrecomb), recombination lifetime (τrecomb), excitons lifetime (τ), photoinduced electrical polarization, energetic disorder characteristic, and magneto-photocurrent, respectively. Our concept should guide people to reconsider the critical role of the interfacial transport layer in the organic BHJ system, in particular to compensate with two controversial assumptions: (i) properties of the organic blend are nearly unaltered due to the presence of the interfacial transport layers, and (ii) they mainly facilitate the charge carrier extraction and injection. By highlighting the factors that eliminate the performance of the organic photovoltaics, we thus believe this work offers scientifically valuable information for device physicists because the optoelectronic, dielectric, and electronic structure properties of the organic blend are firmly bound to the presence of the interfacial transport layer.

Influence of Solvent Additive 1,8‐Octanedithiol on P3HT:PCBM Solar Cells

AbstractProcessing solvent additives in polymer:fullerene bulk heterojunction systems are known as a promising method to enhance photovoltaic performance. It is generally agreed that solvent additives enable polymers to have a high degree of molecular order which increases the device performance. However, the understanding of the efficiency enhancement is not complete. There is a lack of insight regarding the quantitative determination of the molecular miscibility between polymer and fullerene as well as the inner morphology changes induced by the additives. In this work, understanding of the influence of the solvent additive 1,8‐octanedithiol (ODT) is provided on the classic system poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C61 butyric acid methyl ester (P3HT:PCBM) films. The impact on polymer crystallinity, surface structure, inner morphology, and quantitative molecular miscibility of P3HT and PCBM is studied as a function of ODT volume concentration. The crystallinity is probed with absorption spectroscopy and grazing incidence wide‐angle X‐ray scattering. The morphology and miscibility are characterized via atomic force microscopy and time‐of‐flight grazing incidence small angle neutron scattering. Besides an increased crystallinity and prominent phase separation, ODT increases the solubility of PCBM in P3HT and reduces the size of amorphous P3HT domains. Moreover, solvent processing with a high ODT concentration alters the vertical material composition of the active layer.

Energy level determination in bulk heterojunction systems using photoemission yield spectroscopy: case of P3HT:PCBM

Ultraviolet photoelectron spectroscopy (UPS) is commonly used method for energy level determination using planar heterojunction samples in either metal/organic or organic/organic systems. Only some attempts have been made in the study of bulk heterojunction systems. Photoemission yield spectroscopy (PYS) could be applied as a method for organic compound–organic compound interface studies in bulk heterojunction samples. Contrary to the UPS, PYS method does not require ultra-high vacuum, which simplifies experiment setup. Also, scanning depth of PYS is in the range of tens of nanometers, which allows studying deeper layers of the sample instead of only surface layer. In this work, poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction thin films were studied as a model system. A mass ratio between P3HT and PCBM in the system was varied from 1:0 to 1:50. Ionization energy dependence on this ratio was studied using two methods: UPS and PYS. To study the influence of the sample morphology on the PYS measurements and obtainable results, phase-separated and homogeneously distributed samples were prepared for analyses. P3HT ionization energy shift of 0.40 eV was observed in the samples made from chloroform solution. Experiments showed the need for a low degree of phase separation between P3HT and PCBM to observe P3HT ionization energy shift using PYS. On the contrary, no ionization energy shift of P3HT was observed in the UPS measurements for the same systems.

Semiconductor:Insulator Blends for Speed Enhancement in Organic Photodiodes

AbstractOrganic photodiodes (OPDs) have been studied intensively for a number of years, as they enable low‐cost fabrication of sensor systems for imaging, medical, or industrial applications. While the device performance in terms of responsivity and detectivity is continuously improving, detection speed still lags behind. One of the major limiting factors is the large transit times of the charges. In this work, an approach of blending insulating polymers into a poly(3‐hexylthiophene‐2,5‐diyl) and [6,6]‐phenyl C61 butyric acid methyl ester bulk heterojunction system, which has already shown beneficial effects on the transport properties in organic field effect transistors or organic solar cells, is followed. Starting from a basic morphological study using poly(methyl methacrylate) (PMMA) with different molecular weights, both the steady‐state and dynamic characteristics of the OPDs are examined. Transient photocurrent measurements show that the inclusion of PMMA results in an increase of the −3 dB cut‐off frequency without adversely affecting the device steady‐state performance under illumination or in the dark. This increase is attributed to a reduced transit time in the active layer when PMMA is present.

Ternary bulk heterojunction for wide spectral range organic photodetectors

Ternary bulk heterojunction (BHJ) system, dual electron donors and an acceptor, was studied for developing wide spectral range organic photodetectors (OPDs). With two electron donor polymers with different bandgaps and an efficient electron acceptor of [6,6]-Phenyl-C71-butyric acid methyl ester (PC70BM), different blend ratios for ternary BHJ OPD were examined to achieve high photoresponsivity over a wide spectral range. OPDs based on ternary BHJ showed improved photovoltage response compared to binary BHJ. Current-voltage (J-V) characteristics as a function of external bias and light illumination were measured to reveal the underlying charge recombination mechanism which is found to be dominantly ruled by space charge limit (SCL) effect. Additional in-depth analyses including absorbance, cross-section scanning electron microscope (SEM), incident photon-to-electron conversion efficiency (IPCE) were performed.

Interplay Between Triplet-, Singlet-Charge Transfer States and Free Charge Carriers Defining Bimolecular Recombination Rate Constant of Organic Solar Cells

Despite the myriad of organic donor:acceptor materials, only few systems have emerged in the life of organic solar cells to exhibit considerable reduced bimolecular recombination, with respect to the random encounter rate given by the Langevin equation. Monte Carlo simulations have revealed that the rate constant of the formation of electron–hole bound states depends on the random encounter of opposite charges and is nearly given by the Langevin equation for the domain sizes relevant to efficient bulk heterojunction systems. Recently, three studies suggested that charge transfer states dissociating much faster than their decay rate to the ground state, can result in reduced bimolecular recombination by lowering the recombination rate to the ground state as a loss pathway. A separate study identified another loss pathway and suggested that forbidden back electron transfer from triplet charge transfer states to triplet excitons is a key to achieving reduced recombination. Herein we further explain the reduc...

Analytical theory for charge carrier recombination in blend organic solar cells

An analytical theory is suggested for carrier recombination in organic bulk heterojunction (BHJ) structures taking into account the spatial separation of recombining species. In BHJ systems, electrons and holes are confined to two different material phases, and the nongeminate recombination can happen only at the interfaces between the phases. The carrier concentration in the vicinity of the interface can essentially differ from the average concentration in the bulk. Conditions are considered at which this difference can lead to deviations of the apparent recombination order from the value $\ensuremath{\delta}=2$ that is generally expected for bimolecular reactions. The theory is universal in the sense that it is based solely on the inhomogeneous distributions of the recombining species. It can therefore be applied not only to organic semiconductors but to any system in which recombining species are confined to different material phases and can meet only at the interfaces between these phases.

Charge Transfer Processes in OPV Materials as Revealed by EPR Spectroscopy

Understanding charge separation and charge transport at a molecular level is crucial for improving the efficiency of organic photovoltaic (OPV) cells. Under illumination of Bulk Heterojunction (BHJ) blends of polymers and fullerenes, various paramagnetic species are formed including polymer and fullerene radicals, radical pairs, and photoexcited triplet states. Light‐induced Electron Paramagnetic Resonance (EPR) spectroscopy is ideally suited to study these states in BHJ due to its selectivity in probing the paramagnetic intermediates. Advanced techniques like pulsed EPR and ENDOR spectroscopy allow the determination of hyperfine coupling tensors, while high‐frequency EPR allows the EPR signals of the individual species to be resolved and their g‐tensors to be determined. The magnetic resonance parameters of the various polymer donors reveal details about the delocalization of the positive polaron which is important for the efficient charge separation in BHJ systems. Time‐resolved EPR can contribute to the study of the dynamics of charge separation, charge transfer and recombination in BHJ by probing the unique spectral signatures of charge transfer and triplet states. EPR also has the potential to allow characterization of intermediates and products of BHJ degradation.

An Innovative Postdeposition Annealing Approach Producing Centimeter‐Scale In2O3/In2(TeO3)3 Bulk Heterojunction Thin Film for Room‐Temperature Persistent Photoconductivity

Persistent photoconductivity holds great significance to a variety of interesting applications. However, persistent photoconductivity devices based on large‐scale thin films produced by a facile and efficient technology have rarely been achieved so far. In this work, centimeter‐scale In2O3/In2(TeO3)3 bulk heterojunction thin films are prepared via an innovative and convenient two‐step approach, an initial pulsed‐laser deposition followed by a postdeposition annealing process. A prototype optoelectronic device is fabricated and it exhibits pronounced persistent photoconductivity with positive dependence on both the source–drain voltage and incident power density. This device also shows photoresponse to incident light with wavelengths from ultraviolet to red, revealing great potential for practical device applications such as optoelectronic memory and imaging. A physical model in agreement with the observed phenomena is built to qualitatively illustrate the occurrence of the persistent photoconductivity in the bulk heterojunction system. In addition, the facile two‐step approach is further applied to flexible mica substrates and the persistent photoconductivity is maintained upon tens of bending cycles, which not only demonstrates potential for wearable device but also provides tunable and extended focus range for the photoexcitation. These findings provide an innovative and universal scenario for the development of persistent photoconductivity.

Light Absorption and Luminescence Quenching Properties of Hybrid Bulk Heterojunction Materials Based on the Blend Conducting Polymers.

We have investigated the enhancement absorption light and luminescence quenching properties of the hybrid bulk heterojunction systems which were fabricated using poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV); poly(3-hexylthiophene) (P3HT); fullerene derivative 1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6] C61 (PCBM) and TiO₂ nanocrystals. The optimized material showed a broad absorption in the region of 350 to 670 nm and the luminescence quenching higher 85%. The obtained results provide further insight into photophysics of the heterojunction system and device performance improvement by using this system as an active layer.

Intensity-dependent transient photocurrent of organic bulk heterojunction solar cells

An understanding of the behaviors of photo-generated charge carriers (CCs) has a crucial meaning for establishing a reliable model to describe the operating concept of photovoltaic devices. One of the most-widely used techniques to characterize transport behavior of CCs is a transient photocurrent measurement using a short laser pulse excitation. However, conventional transient photocurrent measurements, often referred to as the time-of-flight method, have a drawback due to the demand for relatively thicker active layers. This is mainly to observe a clear transit time for the CCs; however, some uncertainties can arise when the properties of a thicker active layer are adapted to the properties of conventionally thin active layer of an optimized device configuration. Therefore, we studied two models, one based on a polythiophene derivative and the other based on a narrow band-gap polymer derivative of an organic bulk heterojunction system, by using a transient photocurrent method with a conventional active layer thickness. This comparative study has shown clearly different transient photocurrent behaviors between the two systems when the excitation intensity, as well as the applied electric field, is varied.

Selective Morphology Control of Bulk Heterojunction in Polymer Solar Cells Using Binary Processing Additives.

We report the effect of binary additives on the fabrication of polymer solar cells (PSCs) based on a bulk heterojunction (BHJ) system. The combination of 1,8-diiodooctane (DIO), a high-boiling and selective solvent, for fullerene derivatives and poly(dimethylsiloxane) (PDMS) precursor, a nonvolatile insulating additive, affords complementary functions on the effective modulation of BHJ morphology. It was found that DIO and PDMS precursor each play different roles in the control of BHJ morphology, and thus, the power conversion efficiency (PCE) can be further enhanced to 7.6% by improving the fill factor (FF) from 6.8% compared to that achieved using a conventional device employing only a DIO additive. In the BHJ of the active layer, DIO suppressed the large phase separation of PBDTTT-CF and PC71BM while allowing the formation of continuous polymer networks in the donor polymer through phase separation of the PDMS precursor and BHJ components. The appropriate amount of PDMS precursor does not disturb charge transport in the BHJ despite having insulating properties. In addition, the dependence of photovoltaic parameters on different light intensities reveals that the charge recombination in the device with DIO and PDMS precursor decreases compared to that achieved using the device with only DIO.

Solution-processed small-molecule organic solar cells based on diketopyrrolopyrrole and perlyene derivatives with coplanar structures

ABSTRACTA series of small-molecules (SMs) based on coplanar perylene unit coupled with diketopyrrolopyrrole chromophoric core exhibit broad absorption in the range of 500–800 nm and low bandgap energise of 1.6–1.7 eV. The power conversion efficiency approximately reach at 0.4% under 1.5 G illumination is achieved for organic solar cells based on a small-molecule bulk heterojunction system consisting of SMs as a donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an acceptor.