C60 Bulk Heterojunction Research Articles

Self-powered solution-processed photodetectors utilizing FAPbBr3:C60 bulk heterojunction

Hybrid halide perovskites have emerged as pivotal optoelectronic materials due to their easy and cost-effective solution-based manufacturing processes. In this study, we have combined formamidinium lead bromide (FAPbBr3) with buckminsterfullerene (C60) to create the bulk heterojunction (BHJ) photoactive layer for solution-processed photodetectors. Remarkably, the FAPbBr3:C60 BHJ-based photodetectors exhibited a substantial enhancement in optoelectronic performance, with the responsivity increasing from 0.87 A/W to 1.47 A/W compared to the pristine FAPbBr3. This improvement can be attributed to the superior surface morphology and crystallinity of the FAPbBr3:C60 BHJ thin film, which facilitated enhanced charge transport properties. Furthermore, the fabricated photodetectors demonstrated remarkable detectivity even at 0V, enabling self-powered operation. By employing the space charge limited conduction (SCLC) model, we were able to elucidate the dominant charge transport mechanism. At lower voltages, trap-free SCLC was identified as the primary mechanism, while traps governed charge transport at higher voltages. Additionally, we conducted impedance analysis to investigate the underlying factors contributing to the distinctive optoelectronic performance observed in the photodetectors.

Electrode material and modification dependent performances of lateral photodetectors based on MAPbI3 single crystal

Abstract As new-type optoelectronic materials, organometallic halide perovskites (MAPbX3, X = I, Br, Cl) is an emerging solution-processable material with excellent optoelectronic performances that can be used for photodetectors and solar cells. Here, the effects of electrode materials (Au, Ag and Cu) on MAPbI3 single crystal photodetectors are investigated. Among the three MSCPs, the one with Au electrode exhibits the lowest dark current of 0.01 μA, which indicate that the electron drift current is the dominant component of dark current in MAPbI3 single crystals. While for the 650 nm wavelength illumination, the one with Ag electrode has largest R of 24.5 A/W and highest external quantum efficiency (EQE) of 4676% due to lower electron injection energy barrier at the contact. More importantly, the C60 layer and CuPc:C60 bulk heterojunction layer were used to modify the Au electrode respectively, a large R of 174 A/W and EQE of 33,354% is obtained by C60:CuPc-MSCP and C60-MSCP exhibits the largest PDR of 15,952 and D* of 4.5 × 1013 Jones. These results suggest that lateral photodetectors based on MAPbI3 single crystal modified by heterojunction buffer layer will obtain better optical performance.

Towards high performance visible-blind narrowband near-infrared photodetectors with integrated perovskite light filter

Narrowband near-infrared (NIR) light detection is essential for optical imaging, machine vision, and spectroscopy. In this work, we demonstrate a strategy towards visible-blind narrowband NIR organic photodetectors by integrating perovskite thin film as the light filter. Lead phthalocyanine (PbPc):Fullerene (C60) bulk heterojunction which demonstrates high efficiency in photoexciton dissociation is used as the photosensitive layer due to excellent absorption of PbPc in NIR light region. Perovskite layer which has a strong absorption in the visible region but a weak absorption in the NIR region serves as a light filtering layer. More importantly, we have further improved the device performance by inserting copper phthalocyanine (CuPc) electron blocking layer between the ITO anode and PbPc:C60 bulk heterojunction. A high photoresponsivity (R) of 1.09 A/W, an external quantum efficiency (EQE) of 167%, and a narrow spectral response with the full width of half maximum of ~60 nm for NIR light of 808 nm wavelength is obtained.

Improved performance of small molecule organic solar cells by incorporation of a glancing angle deposited donor layer

Improving the photovoltaic performance directly by innovative device architectures contributes much progress in the field of organic solar cells. Photovoltaic device using different kinds of heterojunction with the given set of organic semiconductors paves the way to a better understanding of the working mechanism of organic heterojunction. Here, we report on the fabrication of a new device structure without employing extra material. A thin film of the donor material (chloroaluminum phthalocyanine (ClAlPc)) is inserted between ClAlPc:C60 bulk heterojunction (BHJ) and C60 layer by glancing angle deposition. A ClAlPc/C60 planar heterojunction co-exists with ClAlPc:C60 BHJ simultaneously in this device. Higher efficiency is obtained with this novel device structure. The effects of this additional ClAlPc layer on open-circuit voltage and fill factor in photovoltaic cells are studied. This work provides a new route to improve the device performance of organic solar cells.

Identifying Molecular Orientation in a Bulk Heterojunction Film by Infrared Reflection Absorption Spectroscopy.

The molecular orientation of organic molecules of zinc phthalocyanine (ZnPc) in single-component films on copper iodide (CuI) substrates can be controlled to achieve a molecular orientation lying flat on the substrate (flat-on) owing to π–d orbital interactions between the ZnPc molecules and the CuI. A 3-fold enhancement in the performance of organic photovoltaic cells has been reported by introducing a CuI interlayer between a ZnPc:fullerene (C60) bulk heterojunction (BHJ) film and the substrate. However, the mechanism underpinning the resultant solar cell performance enhancement was unclear. Herein, we report on the results of using in situ reflection absorption spectroscopy measurements during the vacuum deposition of coevaporated ZnPc:C60 BHJ films on various substrates to investigate the ZnPc molecular orientation. Our results revealed that the flat-on molecular orientation of ZnPc molecules in ZnPc:C60 BHJ films on CuI interlayers and flat-on ZnPc substrates can be successfully identified via the strong π–π interactions between the BHJ film and the substrate. The π–π interactions between individual ZnPc molecules are stronger than the π–d interactions between ZnPc molecules and CuI in coevaporated ZnPc:C60 films, as is evident from the molecular orientation of ZnPc, as determined by in situ reflection absorption spectroscopy. Our findings demonstrate that precisely controlling the molecular orientations of the films could enhance organic photovoltaic (OPV) performance. The present work provides important insights that will enable the design of higher performance OPV cells.

Direct characterization of the energy level alignments and molecular components in an organic hetero-junction by integrated photoemission spectroscopy and reflection electron energy loss spectroscopy analysis

A novel, direct method for the characterization of the energy level alignments at bulk-heterojunction (BHJ)/electrode interfaces on the basis of electronic spectroscopy measurements is proposed. The home-made in situ photoemission system is used to perform x-ray/ultraviolet photoemission spectroscopy (XPS/UPS), reflection electron energy loss spectroscopy (REELS) and inverse photoemission spectroscopy of organic-semiconductors (OSCs) deposited onto a Au substrate. Through this analysis system, we are able to obtain the electronic structures of a boron subphthalocyanine chloride:fullerene (SubPC:C60) BHJ and those of the separate OSC/electrode structures (SubPC/Au and C60/Au). Morphology and chemical composition analyses confirm that the original SubPC and C60 electronic structures remain unchanged in the electrodes prepared. Using this technique, we ascertain that the position and area of the nearest peak to the Fermi energy (EF = 0 eV) in the UPS (REELS) spectra of SubPC:C60 BHJ provide information on the highest occupied molecular orbital level (optical band gap) and combination ratio of the materials, respectively. Thus, extracting the adjusted spectrum from the corresponding SubPC:C60 BHJ UPS (REELS) spectrum reveals its electronic structure, equivalent to that of the C60 materials. This novel analytical approach allows complete energy-level determination for each combination ratio by separating its electronic structure information from the BHJ spectrum.

Organic Position-Sensitive Detectors Based on ZnO:Al and CuPc:C60.

Organic position-sensitive detector (OPSD) based on copper phthalocyanine CuPc:fullerene C60 bulk-heterojunction with an inverted structure have been fabricated using aluminum doped ZnO (ZnO:Al) as a resistive layer, which is prepared by sol-gel method. The resistance length of the one-dimensional PSD is fixed at 5 mm, and the Ag common electrode is fabricated by vacuum evaporation within the 100-µm width. The current density-voltage characteristics with different structures of photodetector, the influence of ZnO:Al resistivity on the thickness and the position characteristics of PSDs are investigated. The experimental results indicate that the architecture, which uses an inverted structure, increases sensitivity under red light illumination compared to the conventional structure. In addition, the thickness of the ZnO:Al has influence on the position characteristics. The resistivity of ZnO:A film with Al doping concentration of 2 mol% prepared in this study is around 150 Ωcm and it increases from less than approximately 400 nm-thickness. These characteristics seem to be correlated with the properties of ZnO:AI resistive layer. For a device with a 620 nm-thick ZnO:Al layer, the measured position values obtained from the output photocurrent agree with the actual position values under red laser light illumination. CuPc:C60 OPSD with an inverted structure exhibits red light sensitivity, high incident-photon-to-current conversion efficiency of above 80% at -3 V and linearity error of 5.9% at -2 V.

Raman study on pentacene:C60 bulk heterojunction films

We measured 785nm excited Raman and infrared spectra of pentacene-d14. The observed spectra were assigned on the basis of the Raman and infrared spectra calculated by the density functional theory (DFT) method at the B3LYP/6⬜311+G** level. We measured 785nm excited Raman spectrum of a pentacne-d14:C60 bulk heterojunction film. The spectrum was assigned on the basis of the wavenumber shifts upon deuteration of pentacene. The assignments of the 1462 and 493cm↙1Ag bands of C60 were confirmed. The 511, 453, and 256cm↙1 bands, which were observed only in pentacene:C60 bulk heterojunction films, did not show large deuteration shifts. This result indicates that the 511, 453, and 256cm↙1 bands are attributed to activation of the silent modes of C60 due to symmetry lowering.

Electronic properties of Mn-phthalocyanine–C60 bulk heterojunctions: Combining photoemission and electron energy-loss spectroscopy

The electronic properties of co-evaporated mixtures (blends) of manganese phthalocyanine and the fullerene C60 (MnPc:C60) have been studied as a function of the concentration of the two constituents using two supplementary electron spectroscopic methods, photoemission spectroscopy (PES) and electron energy-loss spectroscopy (EELS) in transmission. Our PES measurements provide a detailed picture of the electronic structure measured with different excitation energies as well as different mixing ratios between MnPc and C60. Besides a relative energy shift, the occupied electronic states of the two materials remain essentially unchanged. The observed energy level alignment is different compared to that of the related CuPc:C60 bulk heterojunction. Moreover, the results from our EELS investigations show that, despite the rather small interface interaction, the MnPc related electronic excitation spectrum changes significantly by admixing C60 to MnPc thin films.

Why Inverted Small Molecule Solar Cells Outperform Their Noninverted Counterparts

It is shown that the effect of substrate heating on the photo conversion efficiency in vacuum‐deposited small molecule organic solar cells is closely related to the improved free charge generation in ordered C60 regions. The formation of these ordered regions strongly depends on the deposition sequence in the device and differs therefore between inverted and noninverted cells. Substrate‐induced local fullerene ordering is found in small molecule:C60 bulk heterojunctions (BHJs) deposited on pristine C60 at elevated temperatures. This does not occur for BHJs deposited under identical conditions on pristine donor molecule layers, despite similar degrees of phase separation in both cases. These findings point to a hitherto unidentified advantage of inverted over noninverted solar cells that manifests itself in a higher charge separation efficiency.

Highly efficient tandem organic light-emitting diodes based on SubPc:C60 bulk heterojunction as charge generation layer

High efficiency tandem organic light-emitting diodes (OLEDs) were realized using an organic bulk heterojunction (BHJ) as charge generation layer (CGL) consisted of boron subphthalocyanine chloride (SubPc) and fullerene (C60). The results showed that the SubPc:C60 based CGL is a promising connecting unit for the fabrication of high efficiency tandem OLEDs with 2.64 folds enhancement in a current efficiency of 63.6cd/A, compare to the corresponding single-unit OLEDs. The efficiency enhancement of tandem OLEDs is attributed to better charge carrier balance derived from the optimized charge transport pathways and reduced interface energy barrier. It showed that the charge generation ability and charge transport characteristics of CGL were strongly dependent on SubPc:C60 mixing ratio and corresponding film morphology, which was verified by the charge extracting analysis. Additionally, the highest occupied molecular orbital energy level of SubPc is compatible for most of hole transporting layer to minimize the hole injection barrier and suppress the hole accumulation and charge recombination in tandem OLEDs.

The Role of Exciton Ionization Processes in Bulk Heterojunction Organic Photovoltaic Cells

To realize efficient photoconversion in organic semiconductors, photogenerated excitons must be dissociated into their constituent electronic charges. In an organic photovoltaic (OPV) cell, this is most often accomplished using an electron donor–acceptor (D–A) interface. Interestingly, recent work on MoOx/C60 Schottky OPVs has demonstrated that excitons in C60 may also undergo efficient bulk‐ionization and generate photocurrent as a result of the large built‐in field created by the MoOx/C60 interface. Here, it is demonstrated that bulk ionization processes also contribute to the short‐circuit current density (JSC) and open‐circuit voltage (VOC) in bulk heterojunction (BHJ) OPVs with fullerene‐rich compositions. Temperature‐dependent measurements of device performance are used to distinguish dissociation by bulk‐ionization from charge transfer at the D–A interface. In optimized fullerene‐rich BHJs based on the D–A pairing of boron subphthalocyanine chloride (SubPc)–C60, bulk‐ionization is found to be responsible for ≈16% of the total photocurrent, and >30% of the photocurrent originating from C60. The presence of bulk‐ionization in C60 also impacts the temperature dependence of VOC, with fullerene‐rich SubPc:C60 BHJ OPVs showing a larger VOC than evenly mixed BHJs. The prevalence of bulk‐ionization processes in efficient, fullerene‐rich BHJs underscores the need to include these effects when engineering device design and morphology in OPVs.

Temperature and interfacial buffer layer effects on the nanostructure in a copper(II) phthalocyanine: Fullerene bulk heterojunction

The effects of the temperature and the presence of an interfacial buffer layer on the nanostructure of organic bulk heterojunctions (BHJs) based on copper phthalocyanine (CuPc) and fullerene (C60) systems were examined using real-time in situ X-ray measurements. The CuPc:C60 BHJ structures prepared without a buffer layer formed a standing-on configured γ-CuPc phase. Once formed, the γ-phase could be rendered thermally stable by post-annealing at 180°C. The insertion of a CuI buffer layer prior to the deposition of a CuPc:C60 BHJ layer induced the formation of a lying-down configured CuPc phase in the BHJ layer. The CuPc peak intensity and lattice parameters gradually increased upon thermal annealing. This increase in intensity appeared to be related to the strain at the interface between the CuPc:C60 and CuI layers.

Selective observation of photo-induced electric fields inside different material components in bulk-heterojunction organic solar cell

By using electric-field-induced optical second-harmonic generation (EFISHG) measurement at two laser wavelengths of 1000 nm and 860 nm, we investigated carrier behavior inside the pentacene and C60 component of co-deposited pentacene:C60 bulk-heterojunctions (BHJs) organic solar cells (OSCs). The EFISHG experiments verified the presence of two carrier paths for electrons and holes in BHJs OSCs. That is, two kinds of electric fields pointing in opposite directions are identified as a result of the selectively probing of SHG activation from C60 and pentacene. Also, under open-circuit conditions, the transient process of the establishment of open-circuit voltage inside the co-deposited layer has been directly probed, in terms of photovoltaic effect. The EFISHG provides an additional promising method to study carrier path of electrons and holes as well as dissociation of excitons in BHJ OSCs.

Optimization of photovoltaic device based on poly(3-hexylthiophene):C60 bulk heterojunction composites prepared with halogen-free solvent

The combination of halogen-free solvent and neat fullerene is expected to reduce the economical and environmental costs of polymer photovoltaic devices. Recently, it has been found that a naturally produced solvent, 1,2,4-trimethylbenzene, enables the preparation of bulk heterojunction composites with neat C60. However, the power conversion efficiency (PCE) of such devices reported so far was less than 1%. In this research, the annealing effects on and film thickness dependence of the performance of photovoltaic devices based on a composite consisting of equivalent weights of poly(3-hexylthiophene) and C60 have been studied to improve the device performance. The optimized device showed a PCE of 1.21%, suggesting that neat C60 is a promising candidate electron acceptor material for polymer bulk heterojunction composites for environmentally friendly and cost-effective photovoltaic devices.

Optical, electrical, and structural properties of ZrON/Ag/ZrON multilayer transparent conductor for organic photovoltaics application

We investigated the electrical and optical properties of chemically stable nitride-based ZrON/Ag/ZrON multilayers for use in organic photovoltaics. As the ZrON layer thickness increases, the carrier concentration smoothly decreases and the transmission window region becomes wider and gradually shifts toward the lower energy-side. The optical bandgap also decreases with decreasing carrier concentration. The ZrON/Ag/ZrON samples exhibit somewhat lower transmittance in the region of ∼400–800nm than the ITO/Ag/ITO samples. The aging test shows that the resistivity of the ZrON/Ag/ZrON samples remains almost the same even after aging for 1month in air. It is shown that the ZrON/Ag interface is more chemically stable than the ITO/Ag interface. Simulation results exhibit that the ZnPc:C60 bulk hetero-junction organic solar cells fabricated with the ZrON(30nm)/Ag/ZrON(70nm) layers give a short circuit current of 9.54mA/cm2, which is comparable to that (10.54mA/cm2) of the cells with 90-nm-thick ITO electrodes. The ZrON/Ag/ZrON samples experienced a large shift in the absorption edge. This behavior is explained in terms of the material characteristics of the ZrON layers and the higher refractive index of the ZrON/Ag/ZrON samples.

Structural control of bulk heterojunction films based on oligothiophene with sterically-bulky groups

Organic photovoltaic solar cells based on oligothiophene and fullerene have been investigated intensively. Until now, the morphologies of bulk heterojunction (BHJ) structures based on oligothiophene and fullerene have been difficult to control because the oligothiophene molecules have lacked substituent groups and therefore have interacted strongly with one another to the point of aggregating into microfibril structures.We succeeded in fabricating homogeneous oligothiophene:C60 BHJ films by adding sterically-bulky groups to oligothiophene in order to control the extent of aggregation. We found that the aggregation onset and maximum fill factor occurred at higher oligothiophene concentrations in BHJ films with C60 for the substituted molecules – the ratio that provides as much oligothiophene as possible for good photovoltaic performance but not so much as to incur aggregation-fill factor was improved. Here in, we report the morphologies and photovoltaic performances of single-component oligothiophene films and BHJ films based on oligothiophenes.

Impedance model of trap states for characterization of organic semiconductor devices

An equivalent circuit to characterize energy distribution of trap states present in organic semiconductors by impedance spectroscopy is proposed. We analyze the impedance spectra of a small-molecule organic solar cell and observe the contribution of trap states at low frequencies. Starting from previously reported equivalent circuits and a theoretical model based on the integration of a general traps distribution, we develop an equivalent circuit, which is able to describe the energetic distribution of trap states typically observed in organic semiconductors. The experimental data can be reproduced by our equivalent circuit, and we estimate a density of trap states in a Zn-phthalocyanine:C60 bulk heterojunction to be about 1.9 ± 0.6 × 1016 cm–3 eV–1.

Poor man's green bulk heterojunction photocells: A chlorine-free solvent for poly(3-hexylthiophene)/C60 composites

Although C60 is unquestionably not only the cheapest but also the most energy-saving fullerene, lack of suitable solvent to make bulk heterojunction composites with it has prevented its use in photocells. Here, it is shown that a not-so-common organic solvent 1,2,4-trimethylbenzene (TMB) is useful to prepare cost-effective bulk heterojunction composites based on C60. Additionally, chlorine-free TMB is expected to be environmental friendly compared with chloroaromatic solvents commonly used for the preparation of polymer/fullerene composites. A preliminary evaluation indicated that a poly(3-hexylthiophene) (P3HT)/C60 bulk heterojunction photocell prepared from TMB solution shows power conversion efficiency comparable to that of a P3HT/PCBM photocell, suggesting that C60 should be revisited as a promising candidate for an acceptor material in polymer bulk heterojunction composites for low-cost green photocells.

Planar, bulk and hybrid merocyanine/C60heterojunction devices: a case study on thin film morphology and photovoltaic performance

The growth of a thermally deposited merocyanine (HB364) thin film can be modified through insertion of a transition metal oxide layer between an indium tin oxide (ITO) covered glass substrate and the dye film. Pure or V2O5 modified ITO substrates result in highly crystalline HB364 films where the dye molecules adopt an edge-on orientation. Changing the transition metal oxide layer to WO3 or MoO3 results in a less ordered dye film with a mixed growth of edge-on and face-on orientations. Planar HB364/C60 heterojunction (PHJ) solar cells are fabricated using the different transition metal oxides as the anode buffer layers. The devices with a pure ITO or a V2O5 modified anode demonstrate the highest power conversion efficiencies up to 2.7% that also outperform HB364:C60 bulk heterojunction (BHJ) devices (2.5%). Finally, HB364/HB364:C60 hybrid heterojunction (HHJ) cells are fabricated showing the highest power conversion efficiency of 2.9%.