Bulk Heterojunction Structure Research Articles

Mechanism and Analysis of Thermal Burn-In Degradation of OPVs Induced by Evaporated HTL

In this study, we focus on the analysis of thermal burn-in degradation behavior of organic photovoltaics (OPVs) at 85 °C under nitrogen atmosphere. In this thermal aging as the accelerated test, the thermal burn-in loss (initially rapid drop) in the power conversion efficiency (PCE) of the inverted OPV devices based on ITO/ZnO/PTB7:PC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">71</sub> BM/MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Ag was shown over 40% PCE reduction. We systematically investigate the effects of bulk heterojunction structure and Ag electrode on thermal degradation. Moreover, we adopt two device architectures to investigate the effect of contact between the charge carrier transport layer and active layer on thermal burn-in loss. The degradation mechanism and origin of thermal burn-in loss induced by the evaporated MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> molecules or charge traps are proposed for the first time. The impedance analysis was conducted to verify the mechanism. Insight into the burn-in mechanism will be helpful for developing OPV fabrication strategies and designs to effectively reduce the thermal burn-in loss. The conventional OPV with the structure of ITO/MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /PTB7:PC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">71</sub> BM/ZnO/Ag remains ~94% of its initial PCE after 500 h of thermal aging at 85 °C.

Finely Interpenetrating Bulk Heterojunction Structure for Lead Sulfide Colloidal Quantum Dot Solar Cells by Convective Assembly

Lead chalcogenides colloidal quantum dot (PbS CQD) solar cells employing an ordered bulk heterojunction (OBHJ) structure allow sufficient utilization of solar energy and at the same time ensure efficient charge extractions. However, the interfacial deficiency was determined to be a significant limiting factor for the further improvement of efficiency. Herein, a finely interpenetrating OBHJ structure between zinc oxide nanowire (ZnO NW) arrays and PbS CQDs was achieved by simultaneously controlling the growth orientation of ZnO NWs and introducing convective assembly as the CQD deposition technique. The inherent directionality during the assembly process leads to dense packing and efficient infiltration of CQDs, forming a valid OBHJ structure. Additionally, a self-assembled monolayer was introduced to further improve the Voc deficit. As a result, a record PCE of 9.92% has been demonstrated for OBHJ structured CQD solar cells that are compatible with low-temperature and scalable manufacturing processes.

Sequential Deposition of Organic Films with Eco-Compatible Solvents Improves Performance and Enables Over 12%-Efficiency Nonfullerene Solar Cells.

Casting of a donor:acceptor bulk-heterojunction structure from a single ink has been the predominant fabrication method of organic photovoltaics (OPVs). Despite the success of such bulk heterojunctions, the task ofcontrolling the microstructure in a single casting process has been arduous and alternative approaches are desired. To achieve OPVs with a desirable microstructure, a facile and eco-compatible sequential deposition approach is demonstrated for polymer/small-molecule pairs. Using a nominally amorphous polymer as the model material, the profound influence of casting solvent is shown on the molecular ordering of the film, and thus the device performance and mesoscale morphology of sequentially deposited OPVs can be tuned. Static and in situ X-ray scattering indicate that applying (R)-(+)-limonene is able to greatly promote the molecular order of weakly crystalline polymers and form the largest domain spacing exclusively, which correlates well with the best efficiency of 12.5% in sequentially deposited devices. The sequentially cast device generally outperforms its control device based on traditional single-ink bulk-heterojunction structure. More crucially, a simple polymer:solvent interaction parameter χ is positively correlated with domain spacing in these sequentially deposited devices. These findings shed light on innovative approaches to rationally create environmentally friendly and highly efficient electronics.

Realizing photomultiplication-type organic photodetectors based on C60-doped bulk heterojunction structure at low bias**Project supported by the National Natural Science Foundation of China (Grant No. 61106043), the Natural Science Foundation of Shaanxi Province, China (Grant No. 2015JM6267), and Program of Xi’an University of Technology, China (Grant No. 103-4515013).

Photomultiplication (PM) structure has been widely employed to improve the optoelectronic performance of organic photodetectors (OPDs). However, most PM-type OPDs require a high negative operating voltage or complex fabrication. For obtaining high-efficiency OPDs with low voltage and easy process, here the bulk heterojunction (BHJ) structure of high exciton dissociation efficiency combined with the method of trap-assisted PM are applied to the OPDs. In this paper, we investigate the operating mechanism of OPDs based on poly(3-hexylthiophene) (P3HT): (phenyl-C61-butyric-acid-methyl-ester) (PC61BM), and poly-{[4,8-bis[(2-ethylhexyl)oxy]-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[3-fluore-2-(octyloxy)carbonyl-thieno[3,4-b]thiophene-4,6-diyl]} (PBDT-TT-F):PC61BM doped with C60 as active layer. Furthermore, the influence of C60 concentration on the optoelectronic performances is also discussed. With 1.6 wt.% C60 added, the P3HT:PC61BM:C60 OPD exhibits a 327.5% external quantum efficiency, a 1.21 A·W−1 responsivity, and a 4.22 × 1012 Jones normalized detectivity at −1 V under 460 nm (0.21 mW·cm−2) illumination. The experimental results show that the effective electron traps can be formed by doping a small weight of C60 into BHJ active layer. Thus the PM-type OPDs can be realized, which benefits from the cathode hole tunneling injection assisted by the trapped electrons in C60 near the Al side. The efficiency of PM is related to the C60 concentration. The present study provides theoretical basis and method for the design of highly sensitive OPDs with low operating voltage and facile fabrication.

Bulk Heterojunction Quasi-Two-Dimensional Perovskite Solar Cell with 1.18 V High Photovoltage.

Multicomponent quasi-two-dimensional perovskites (Q-2DPs) have efficient luminescence and improved stability, which are highly desirable for light-emitting diode and perovskite solar cell (PSC). However, the lack of radiative recombination at room temperature is still not well understood and the performance of PSC is not good enough as well. The open-circuit voltage ( VOC) is even lower than that of three-dimensional (3D) PSC with a narrower band gap. In this work, we study the energy transfer of excitons between their multiple components by time-resolved photoluminescence and find that charge transfer from high-energy states to low-energy state is gradually suppressed during elevating temperature owing to trap-mediated recombination. This may reveal the bottleneck of luminescence at room temperature in Q-2DPs, leading to large photovoltage loss in 2D PSC. Therefore, we develop a p-i-n bulk heterojunction (BHJ) structure to reduce the nonradiative recombination and obtain high VOC of 1.18 V for (PMA)2MA4Pb5I15Cl (33.3% PMA) BHJ device, much higher than that of the planar counterparts. The enhanced efficiency is attributed to the improved exciton dissociation via BHJ interface. Our results provide an important step toward high VOC and stable 2D PSCs, which could be used for tandem solar cell and colorful photovoltaic windows.

Highly flexible and solution-processed organic photodiodes and their application to optical luminescent oxygen sensors

Abstract We present a solution-processed flexible organic photodiode (f-OPD) with a bulk heterojunction (BHJ) structure based on a blend of poly (3-hexylthiophene-2,5-diyl) and 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 (P3HT:PCBM). We used Cs2CO3-doped polyethyleneimine ethoxylated (d-PEIE) as the electron transport layer (ETL) material, which significantly improved the electron injection properties of the f-OPD. Compared with f-OPDs with conventional ETL materials such as Cs2CO3, the external quantum efficiency (EQE) of the d-PEIE-based f-OPD was highly improved. Analytical results showed that the d-PEIE reduced the work function of the cathode, thereby facilitating the efficiency of electron injection from the active layer (AL) to the cathode of the f-OPD. In addition, after 10,000 cycles of tensile bending at a bending radius of 5 mm, the normalized ID variation (ID/ID0) in the d-PEIE-based f-OPD remained above 90%, indicating an excellent device bending stability. Finally, f-OPD-based luminescent oxygen (O2) sensors were successfully fabricated consisting of a photoluminescent O2 sensing film, a light source, and an f-OPD. The O2 sensors based on d-PEIE-based f-OPDs showed the highest photocurrent and O2 sensitivity in relation to the O2 concentration compared with O2 sensors based on f-OPDs with conventional ETL materials.

High-efficiency ternary polymer solar cells employing the solid solution as the donor phase

Abstract The solid solutions comprising amorphous conjugated polymer PCDTBT and different fullerene derivatives, i.e. crystalline PC71BM and amorphous NC60BA, have been investigated. It is found that the solid solution based on NC60BA could produce more hole carriers at the room temperature due to a smaller activation energy from trap state to free carriers compared to that based on PC71BM. A higher hole carrier density could effectively fill traps and reduce the recombination loss during charge transport process. The solid solution based on PC71BM shows enhanced exciton dissociation probability at the solid solution/C60 heterojunction interfaces. With combining the advantages of both the solid solutions, a small amount of amorphous NC60BA was introduced into the PCDTBT:PC71BM bulk heterojunction structure and the ternary BHJ PSCs composed of PCDTBT, PC71BM and NC60BA were fabricated. The ternary PSCs display outstanding photovoltaic performance compared to the binary PCDTBT:PC71BM bulk heterojunction PSCs.

Improvement in the performance of inverted planar perovskite solar cells via the CH3NH3PbI3-xClx:ZnO bulk heterojunction

Zinc oxide (ZnO) as an electron transport material has been used in regular planar perovskite solar cells (PSCs). Herein, ZnO nanoparticles serve as an additive directly in perovskite active layer in order to fabricate a bulk-heterojunction inverted planar PSC. The CH3NH3PbI3-xClx:ZnO bulk heterojunction is prepared by mixing ZnO nanoparticles into CH3NH3PbI3-xClx precursor solution. ZnO nanoparticles act as catalytic centers and induce the growth of perovskite grains leading to the formation of large-size perovskite crystals. Remarkably, hysterysis-free power conversion efficiency of 17.26% is achieved when precisely control the concentration of ZnO nanoparticles in CH3NH3PbIxCl3-x:ZnO bulk heterojunction active layer. The presence of ZnO nanoparticles in the perovskite films enhances the conductivity and electron mobility. Efficient charge injection of CH3NH3PbIxCl3-x:ZnO film can improve charge extraction in the PSCs. Moreover, the PSCs with CH3NH3PbIxCl3-x:ZnO bulk heterojunction exhibit high fill factor of 77.3% and short current density of 22.71 mA cm−2, which is attributed to improved perovskite quality. The bulk heterojunction structure is fabricated by combining perovskite and metal oxides, which is of great importance to the development and commercialization of PSCs in the future.

High Performance Flexible Organic Phototransistors with Ultrashort Channel Length

Organic phototransistors with high responsivity and sensitivity to light irradiance have great potential applications in environmental monitoring, space exploration, security, image sensors and healthcare systems. In this manuscript, a novel polymer bulk heterojunction field effect phototransistor with ultrashort channel length (tens of nanometers) and ultrahigh sensitivity to visible light was proposed. Due to the nanoscale channel and bulk heterojunction structure, a high-performance phototransistor with high responsivity of 750 A/W, photosensitivity of 1.0 × 106, and detectivity as high as 4.54 × 1015 Jones was demonstrated under 720 nm light illumination with 0.1 mW/cm2 intensity, which was even better that those lateral organic phototransistors. Moreover, organic phototransistors with ultrashort channel length were investigated for the first time on a flexible substrate, which exhibited outstanding mechanical flexibility due to their unique designs. Further investigation of the correlation between th...

Formation of unique nanocrystalline Cu-In-Se bulk pn homojunctions for opto-electronic devices

Semiconductor pn junctions, integrated in optoelectronic devices require high quality crystals, made by expensive, technically difficult processes. Bulk heterojunction (BHJ) structures offer practical alternatives to circumvent the cost, flexibility and scale-up challenges of crystalline planar pn junctions. Fabrication methods for the current organic or inorganic BHJ structures invariably create interface mismatch and low doping issues. To overcome such issues, we devised an innovative approach, founded on novel inorganic material system that ensued from single-step electrodeposited copper-indium-selenide compounds. Surface analytical microscopies and spectroscopies reveal unusual phenomena, electro-optical properties and quantum effects. They support the formation of highly-ordered, sharp, abrupt 3-dimensional nanoscale pn BHJs that facilitate efficient charge carrier separation and transport, and essentially perform the same functions as crystalline planar pn junctions. This approach offers a low-cost processing platform to create nanocrystalline films, with the attributes necessary for efficient BHJ operation. It allows roll-to-roll processing of flexible devices in simple thin-film form factor.

(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

Nanoscale investigation of bulk heterojunction organic solar cell by scanning capacitance force microscopy

We developed a frequency-modulation atomic force microscopy (FM-AFM) system combined with Kelvin probe force microscopy (KFM) and scanning capacitance force microscopy (SCFM). Using the developed system, we investigated the surface structure and electrical characteristics of a bulk heterojunction (BHJ) sample consisting of a mixture of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT). The BHJ structure consisted of PCBM and P3HT molecules. These materials are often used as the active and charge transport layers in an organic solar cell, respectively. The results suggest that we succeeded in visualizing the phase separation in the BHJ sample using SCFM measurements. We illustrated the energy band diagrams from the KFM results and showed that band bending occurs from the organic material side at the interface with the indium–tin oxide substrate. Furthermore, the KFM and SCFM results suggested that charge separation occurs in the BHJ structure.

Modulate Molecular Interaction between Hole Extraction Polymers and Lead Ions toward Hysteresis‐Free and Efficient Perovskite Solar Cells

AbstractHerein three polymeric hole extraction materials (HEMs), poly(benzene‐dithiophene) (PB2T)‐O, PB2T‐S, and PB2T‐SO are presented for p–i–n perovskite solar cells (PVSCs). This study reveals that the perovskite device hysteresis and performance heavily rely on the perovskite grain boundary conditions. More specifically, they are predetermined through the molecular interaction between Lewis base atoms of HEMs and perovskites. It is revealed that only changing the side chain terminals (OCH3, SCH3, and SOCH3) of HEMs results in effective modulating PVSC performance and hysteresis, due to the effective tune of interaction strength between HEM and perovskite. With an in situ grown perovskite‐HEM bulk heterojunction structure, PB2T‐O with weak binding group (OCH3, −78.9 kcal mol−1 bonding energy) to lead ions allows delivering hysteresis‐free and efficient devices, which is sharp contrast to the strong binding PB2T‐SO (−119.3 kcal mol−1 bonding energy). Overall, this work provides new insights on PVSC hysteresis and the related curing methods via multifunctional HEM design in PVSCs.

Organic Photovoltaic Switches Using DBP/C60 Cells and Blue Light to Trigger Logic and Amplifier Electronics

Apart from ordinary solar cell applications, we have attempted different approaches of device applications. We have fabricated two types of organic photovoltaic (OPV) cells with DBP/C60 heterojunction and measured their spectral photo responses when they are in planar heterojunction (PHJ) and bulk heterojunction (BHJ) type. According to our results, blue light exhibits spectrally outstanding PV effects and the PHJ device showed higher open circuit voltage (VOC) along with ten times faster switching photo-dynamics than those performances of BHJ device, although the mixture type BHJ diode appears much superior to PHJ diode in all spectral ranges of quantum efficiency. Large junction area of mixture type OPV diode is certainly an advantage in photocurrent generation and quantum efficiency, but is regarded as a disadvantage for dynamic PV switching and VOC value because complex route and high density interface traps are unavoidable in BHJ structure. Selecting the blue light illumination on DBP/C60 PHJ diode, we achieve most efficient PV switching and PV gating in logic and amplifier circuits, respectively. Conclusively, bilayer OPV device has its own advantage over mixture type device in dynamic PV switching, and it promises future electronics by exploiting the dynamic harvest of light energy. Figure 1

Solar Cell Polymer Based Active Ingredients PPV and PCBM

A polymer solar cell is a solar cell based on a polymer bulk heterojunction structure using the method of thin film, which can convert solar energy into electrical energy. Absorption of light is carried by active material layer PPV: PCBM. This study aims to make solar cells tandem and know the value of converting solar energy into electrical energy and increase the value of efficiency generated through morphological control, ie annealing temperature and the ratio of active layer mixture. The active layer is positioned above the PEDOT:PSS layer on ITO glass substrate. The characterization results show the surface morphology of the PPV:PCBM active layer is quite evenly at annealing temperature of 165 ° C. The result of conversion of electrical energy with a UV light source in annealing samples with temperature 165 ° C is 0.03 mA and voltage of 4.085 V with an efficiency of 2.61% and mixed ratio variation was obtained in comparison of P3HT: PCBM is 1: 3

Appropriate Donor-Acceptor Phase Separation Structure for the Enhancement of Charge Generation and Transport in Polymer Solar Cells

The morphology of active layer for polymer solar cells is critical to enhance the performance especially for fill factor of the devices. To investigate the relationship between active layer morphology and performance of polymer solar cells (PSCs), 1,8-diiodooctane (DIO) additive, and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) electron acceptor were used to regulate the aggregation morphology of copolymer poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7) electron donor from solution state to solid state. Atom force microscopy (AFM), steady-state absorption (UV-Vis), time-resolved absorption (TA), spectroelectrochemistry (SEC) and current-voltage (J-V) measurements were employed to characterize the morphology, optical and electrical characteristics of active layers and to reveal the relationship among the morphology, photophysical property, and performance of PTB7-based devices. The results show that DIO can refine the aggregation scale of PTB7 during the dissolution process, whereas both the aggregation scale and aggregation behaviors of PTB7 donor are affected by PC71BM acceptor molecules. Furthermore, the bulk heterojunction structure (BHJ) morphology of active layer can be optimized during the DIO evaporation process. TA kinetic data indicate that the population and lifetime of charged species are improved in the DIO-treated BHJ active layer. Moreover, the active layers with DIO treatment have a relative low highest occupied molecular orbital (HOMO) energy level, which makes hole transport more easily in PTB7 donor phase. As a result, the performance of PTB7-based PSCs is enhanced.

Bilateral Interface Engineering toward Efficient 2D–3D Bulk Heterojunction Tin Halide Lead-Free Perovskite Solar Cells

As a promising substitute for toxic lead-based perovskite, tin (Sn)-based halide perovskite has drawn much attention for photovoltaic applications. However, unsatisfied open-circuit voltage (VOC) and fill factor (FF) values of the available Sn-based perovskite solar cells (PSCs) remain a lingering cloud. In this work, we report a bilateral interfacial engineering strategy to fabricate 2D–3D bulk heterojunction Sn-based perovskite solar cells. Specifically, large cation PEAI and bifunctional LiF are evaporated at the bilateral interfaces of a FASnI3 film. The presence of PEAI improves the VOC and FF of the PSCs via improved surface coverage and the formed 2D–3D bulk heterojunction structure, while the bifunctional LiF (i) lowers the work function of PEDOT:PSS and (ii) facilitates hole extraction at the ITO/PEDOT:PSS interface. This strategy enabled a power conversion efficiency (PCE) of 6.98% with a VOC of 0.47 V and FF of 0.74. Our findings will add critical building bricks toward efficient Sn-based PSCs.

High-Performance Organic Bulk-Heterojunction Solar Cells Based on Multiple-Donor or Multiple-Acceptor Components.

Organic solar cells (OSCs) based on bulk heterojunction structures are promising candidates for next-generation solar cells. However, the narrow absorption bandwidth of organic semiconductors is a critical issue resulting in insufficient usage of the energy from the solar spectrum, and as a result, it hinders performance. Devices based on multiple-donor or multiple-acceptor components with complementary absorption spectra provide a solution to address this issue. OSCs based on multiple-donor or multiple-acceptor systems have achieved power conversion efficiencies over 12%. Moreover, the introduction of an additional component can further facilitate charge transfer and reduce charge recombination through cascade energy structure and optimized morphology. This progress report provides an overview of the recent progress in OSCs based on multiple-donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple-acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components.

Ternary System with Controlled Structure: A New Strategy toward Efficient Organic Photovoltaics.

Recently, a new type of active layer with a ternary system has been developed to further enhance the performance of binary system organic photovoltaics (OPV). In the ternary OPV, almost all active layers are formed by simple ternary blend in solution, which eventually leads to the disordered bulk heterojunction (BHJ) structure after a spin-coating process. There are two main restrictions in this disordered BHJ structure to obtain higher performance OPV. One is the isolated second donor or acceptor domains. The other is the invalid metal-semiconductor contact. Herein, the concept and design of donor/acceptor/acceptor ternary OPV with more controlled structure (C-ternary) is reported. The C-ternary OPV is fabricated by a sequential solution process, in which the second acceptor and donor/acceptor binary blend are sequentially spin-coated. After the device optimization, the power conversion efficiencies (PCEs) of all OPV with C-ternary are enhanced by 14-21% relative to those with the simple ternary blend; the best PCEs are 10.7 and 11.0% for fullerene-based and fullerene-free solar cells, respectively. Moreover, the averaged PCE value of 10.4% for fullerene-free solar cell measured in this study is in great agreement with the certified one of 10.32% obtained from Newport Corporation.

Theoretical study on mesoscopic-size impurity effects in the charge separation process of organic photocells.

A bulk-heterojunction structure is often employed to develop high-performance organic photocells, in which the donor and acceptor regions are complexly intertwined. In such situations, mesoscopic-scale islands and peninsulas that compose the donor materials may be formed in the acceptor region. Alternatively, the donor region may extend deeply into the acceptor region. This yields mesoscopic-size impurities that can create obstacles in the charge separation (exciton dissociation) process of organic photocells and prevents the dissociation of excitons (electron-hole pairs). We previously reported on the effect of the cooperative behavior between the hot charge transfer (CT) state and the dimensional (entropy) effect on the charge separation process. In this paper, we discuss the mesoscopic-scale impurity effect on the charge separation process in PCBM acceptor models by considering the hot CT state and dimensional effects. In addition, we discuss atomic-scale effects such as molecular distortions and conformation changes using molecular dynamics (MD) simulations.