Organic Bulk Heterojunction Research Articles

Essential relation of spin states, trap states and photo-induced polarization for efficient charge dissociation in a polymer-nonfullerene based organic photovoltaic system

Organic nonfullerene acceptors (NFA) have received tremendous research interests due to the efficient interfacial charge transfer (CT) when they combine with polymeric donors. It is of critical importance to unravel, to which extent, spin-dependent excited states, energetic disorder and photo-induced polarization are essentially correlated, and eventually contribute for the photovoltaic action. Herein, we explore them in a polymer-NFA ITTC based organic bulk heterojunction (BHJ) solar cell. With assist of magneto-photocurrent measurements at the short-circuit current density (Jsc) condition and steady state impedance spectroscopic measurements, the dominant mechanism for the Jsc generation is governed by the triplet-charge reaction. We postulate that long lived triplets are more likely to interact with trapped charges at excited states, giving rise to Jsc. It turns out that the trap states may not always act as passive role for the energy loss in the organic photovoltaic system. More importantly, both applied bias voltage and light can effectively manipulate the magnitude and sign of the magneto-photocurrent. By quenching the photo-generated build-in field, the bias-dependent magneto-photocurrent evidently proves the efficient dissociation of polaron pairs at CT states. In particular, the photo-excitation wavelength has strong impact on the hyperfine field. Lastly, we demonstrate that the double beam photo-excitation generated dipole-dipole interaction assists for the electron-hole dissociation. Our work may pave the way for understanding the inside spin- and photo-physical phenomena in the NFA based organic photovoltaic system.

Spatial Distribution Recast for Organic Bulk Heterojunctions for High‐Performance All‐Inorganic Perovskite/Organic Integrated Solar Cells

AbstractAll‐inorganic CsPbIBr2 perovskite solar cells (pero‐SCs) exhibit excellent overall stability, but their power conversion efficiencies (PCEs) are greatly limited by their wide bandgaps. Integrated solar cells (ISCs) are considered to be an emergent technology that could extend their photoresponse by directly stacking two distinct photoactive layers with complementary bandgaps. However, rising photocurrents always sacrifice other photovoltaic parameters, thereby leading to an unsatisfactory PCE. Here, a recast strategy is proposed to optimize the spatial distribution components of low‐bandgap organic bulk‐heterojunction (BHJ) film, and is combined with an all‐inorganic perovskite to construct perovskite/BHJ ISCs. With this strategy, the integrated perovskite/BHJ film with a top‐enriched donor‐material spatial distribution is shown to effectively improve ambipolar charge transport behavior and suppress charge carrier recombination. For the first time, the ISC is not only significantly extended and enhanced the photoresponse achieving a 20% increase in current density, but also exhibits a high open‐circuit voltage and fill factor at the same time. As a result, a record PCE of 11.08% based on CsPbIBr2 pero‐SCs is realized; it simultaneously shows excellent long‐term stability against heat and ultraviolet light.

Highly efficient near-infrared hybrid perovskite solar cells by integrating with a novel organic bulk-heterojunction

Abstract Extending photoelectric response to near-infrared region (NIR) has become an urgent subject for the research of perovskite solar cells (PSCs). However, it is still a challenge due to the shortage of matching NIR photovoltaic materials and device structure. The rapid development of NIR organic photovoltaic materials and devices (OPVs) in recent years offers new opportunity for developing such PSCs. Herein, to broaden the photoresponse of PSCs, a novel PBDB-TF: BTP-4Cl bulk-heterojunction (BHJ) organic layer was successfully integrated on the PSCs, which extended the photoresponse of the device to 950 nm. And more, to boost the utilization of NIR light, the Au nanorods (Au NRs) were introduced into the organic BHJ layer through localized surface plasmon effect (LSPR). To further improve the device stability and satisfy solution competition, the MoO3 was fabricated as hole transport layer substituting traditional Spiro-OMeTAD. After adopting such a device, the power conversion efficiency (PCE) was increased significantly from 16.67% to 21.55%, which was the optimum among the reported organic/perovskite hybrid solar cells and the devices employing MoO3 as the hole transport layer. The illumination and humidity stability of the perovskite/organic/noble metal integrated solar cells were also significantly improved. The devices under standard sunlight irradiation for 1000 h maintained more than 70% of initial PCE. This work demonstrates that the perovskite/organic/noble metal integrated structure is a novel and powerful approach to obtain efficient and stable NIR-harvesting PSCs.

Hybrid Quantum Dot/Organic Heterojunction: A Route to Improve Open-Circuit Voltage in PbS Colloidal Quantum Dot Solar Cells

We have successfully demonstrated the effect of a thin organic bulk-heterojunction (BHJ) interlayer on improving the photovoltaic performance of lead sulfide (PbS) colloidal quantum dot (CQD) solar...

SWIR Photodetection and Visualization Realized by Incorporating an Organic SWIR Sensitive Bulk Heterojunction.

Short‐wavelength infrared (SWIR) photodetection and visualization has profound impacts on different applications. In this work, a large‐area organic SWIR photodetector (PD) that is sensitive to SWIR light over a wavelength range from 1000 to 1600 nm and a SWIR visualization device that is capable of upconverting SWIR to visible light are developed. The organic SWIR PD, comprising an organic SWIR sensitive blend of a near‐infrared polymer and a nonfullerene n‐type small molecule SWIR dye, demonstrates an excellent capability for real‐time heart rate monitoring, offering an attractive opportunity for portable and wearable healthcare gadgets. The SWIR‐to‐visible upconversion device is also demonstrated by monolithic integration of an organic SWIR PD and a perovskite light‐emitting diode, converting SWIR (1050 nm) to visible light (516 nm). The most important attribute of the SWIR visualizing device is its solution fabrication capability for large‐area SWIR detection and visualization at a low cost. The results are very encouraging, revealing the exciting large‐area SWIR photodetection and visualization for a plethora of applications in environmental pollution, surveillance, bioimaging, medical, automotive, food, and wellness monitoring.

Noise and detectivity limits in organic shortwave infrared photodiodes with low disorder

To achieve high detectivity in infrared detectors, it is critical to reduce the device noise. However, for non-crystalline semiconductors, an essential framework is missing to understand and predict the effects of disorder on the dark current. This report presents experimental and modeling studies on the noise current in exemplar organic bulk heterojunction photodiodes, with 10 donor–acceptor combinations spanning wavelength between 800 and 1600 nm. A significant reduction of the noise and higher detectivity were found in devices using non-fullerene acceptors (NFAs) in comparison to those using fullerene derivatives. The low noise in NFA blends was attributed to a sharp drop off in the distribution of bandtail states, as revealed by variable-temperature density-of-states measurements. Taking disorder into account, we developed a general physical model to explain the dependence of thermal noise on the effective bandgap and bandtail spread. The model provides theoretical targets for the maximum detectivity that can be obtained at different detection wavelengths in inherently disordered infrared photodiodes.

Alternative sequential deposition for optimization-free multi-component organic bulk heterojunctions

Multi-component (ternary, quaternary, and beyond) bulk heterojunction (BHJ) active layers are considered one of the most effective ways of increasing the performance of organic photovoltaics (OPVs). Herein, an alternative sequential deposition method for the development of efficient multi-component OPVs without the need for the complex optimization steps typical for blend parameters is discussed. Simple sequential spin-coating of two binary donor:acceptor blends was applied to produce modified quaternary BHJ layers, making full use of the optoelectronic advantages offered by the two binary blends and their optimized morphological features. A combination of theoretical and experimental analyses revealed the occurrence of molecular reorganization during sequential deposition which facilitated self-optimized molecular stratification and cascade energy level alignment. By using expanded universality test to account for the diverse BHJ systems, this sequential deposition methodology creates new opportunities as an alternative fabrication platform to secure high-performance multi-component BHJ OPVs for versatile (i.e., indoor and outdoor) irradiation environments while overcoming many of the drawbacks of conventional manufacturing procedures. • Alternative manufacturing process of organic photovoltaics (OPVs) is demonstrated. • Sequential spin-coating of two binary blends produces modified multi-component bulk heterojunction layers. • Performance is optimized for both indoor and outdoor light conditions without the need for fine-tuned blend optimization. • Sequential deposition is a universal platform to achieve optimization-free, high-performance multi-component OPVs.

Towards Efficient Integrated Perovskite/Organic Bulk Heterojunction Solar Cells: Interfacial Energetic Requirement to Reduce Charge Carrier Recombination Losses

AbstractIntegrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (Jsc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in Jsc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.

Integrated Perovskite/Organic Photovoltaics with Ultrahigh Photocurrent and Photoresponse Approaching 1000 nm

To enhance the photoresponse of commonly used perovskite materials in the near‐infrared (NIR) region, a fused‐ring electron acceptor (F8IC) with strong NIR absorption and high electron mobility is used to blend with a narrow‐bandgap polymer donor (PTB7‐Th) to construct an organic bulk heterojunction (OBHJ), and this OBHJ is then integrated with the perovskite solar cells (PSCs). The integrated perovskite/OBHJ solar cells exhibit a strong photoresponse approaching 1000 nm and an ultrahigh short‐circuit current density of 28.2 mA cm−2, which is much higher than that of traditional PSCs and organic solar cells.

DFT and TD-DFT studies of new pentacene-based organic molecules as a donor material for bulk-heterojunction solar cells

The performance of organic cells based on bulk heterojunctions (BHJs) has improved recently, but further improvements are necessary. In this work, we have carried out a thorough examination using density functional theory (DFT) and time-dependent (TD)-DFT to investigate the structural and optoelectronic properties of pentacene-based organic molecules (PbOMs) as potential donor material for organic photovoltaic BHJ devices. Our results show that oxadiazole prefers to attach via its nitrogen atoms to the carbon atoms of the pentacene monomer with an adsorption energy about − 32.86 kcal/mol, which means that oxadiazole is efficiently adsorbed on the edge of the pentacene. The HOMO energy level of the PbOM with the lowest bandgap is − 4.00 eV wide, i.e., about 0.86 eV lower and more positive than pentacene, thus providing an ideal open-circuit voltage for photovoltaic devices. The bandgap of the PbOM compounds are about 1.61 and 1.80 eV affording an efficient charge transfer from donor to acceptor. Furthermore, the donor PbOMs are also more stable than the pentacene. We have examined, additionally, the reactivity and absorption properties of individual molecules and PbOM systems. Our results suggest that the PbOM, as a donor material, may significantly improve the efficiency of BHJ solar cells.

In situ Measuring Film-Depth-Dependent Light Absorption Spectra for Organic Photovoltaics.

Organic donor–acceptor bulk heterojunction are attracting wide interests for solar cell applications due to solution processability, mechanical flexibility, and low cost. The photovoltaic performance of such thin film is strongly dependent on vertical phase separation of each component. Although film-depth-dependent light absorption spectra measured by non-in situ methods have been used to investigate the film-depth profiling of organic semiconducting thin films, the in situ measurement is still not well-resolved. In this work, we propose an in situ measurement method in combination with a self-developed in situ instrument, which integrates a capacitive coupled plasma generator, a light source, and a spectrometer. This in situ method and instrument are easily accessible and easily equipped in laboratories of the organic electronics, which could be used to conveniently investigate the film-depth-dependent optical and electronic properties.

Charge Carrier Transport and Generation via Trap-Mediated Optical Release in Organic Semiconductor Devices.

The impact of intermixed donor-acceptor domains in organic bulk heterojunction (BHJ) solar cells, using low-donor-content devices as model systems, is clarified. At low donor contents, the devices are found to exhibit anomalously high open-circuit voltages independent of the donor-acceptor energetics. These observations can be consistently explained by a theoretical model based on optical release of trapped holes, assuming the donors behave as trap sites in the gap of the acceptor. Our findings provide guidelines for reducing the large open-circuit voltage losses in organic solar cells and avoiding morphology-induced losses in state-of-the-art BHJ solar cells and photodetectors.

Direct characterization of vertical molecular distributions of organic bulk heterojunction structure by photoemission spectroscopy combined with argon gas cluster ion beam sputtering

The characterization of the vertical molecular distribution of organic bulk heterojunction (BHJ) structures is crucial to the development of high-performance organic photovoltaic (OPV) devices. Herein, we report a novel and direct method for the characterization of the vertical composition gradient of a BHJ structure. Ar gas cluster ion beam (GCIB) sputtering provided a uniform sputtering yield that preserved the chemical structure of the organic semiconducting materials. The combination of X-ray photoelectron spectroscopy (XPS) and Ar GCIB sputtering facilitated the accurate analysis of the vertical molecular distribution of a regioregular poly(3-​hexylthiophene) (P3HT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) BHJ structure. The J-V characteristics and depth profiles of the as-deposited, ethanol-annealed, and chloroform-annealed devices were examined to demonstrate the usefulness of Ar GCIB sputtering in optimizing the BHJ morphology. Chloroform-annealed device exhibited both the best performance and the most homogeneous morphology among the three prepared samples, and the thickness of the P3HT-rich region was substantially reduced from 20 to 10 nm. Consequently, these results provide important information about the correlation between the vertical molecular distribution of a BHJ blend structure and power conversion efficiency of an OPV device.

Structure-Property Relation in Organic-Metal Oxide Hybrid Phototransistors.

We report an optoelectronic device consisting of a solution-processed indium gallium zinc oxide (IGZO) thin-film transistor and vacuum-deposited small organic molecules. Depending on the configurations of the organic materials, either bulk heterojunction or planar heterojunction (PHJ), the device assumes the functionality of either a photosensor or a photoinduced memory, respectively. Under λ = 625 nm light illumination, the photosensor shows response and recovery time of ∼50 ms, responsivity of ∼5 mA/W, sensitivity above 104, and a linear response. The mechanism of the photoinduced memory is studied experimentally and verified using a device simulation. We find that the memory is due to long charge retention time at the organic PHJ interface which is stable for over 9 days. It is correlated with the low leakage current found in ordered organic junctions having low subgap tail states. The presented integration of the PHJ with the transistor constitutes a new design of write-once-read-many-times memory device that is likely to be attractive for low-cost applications.

Ultrafast and broadband photodetectors based on a perovskite/organic bulk heterojunction for large-dynamic-range imaging

Organic-inorganic hybrid perovskite (OIHP) photodetectors that simultaneously achieve an ultrafast response and high sensitivity in the near-infrared (NIR) region are prerequisites for expanding current monitoring, imaging, and optical communication capbilities. Herein, we demonstrate photodetectors constructed by OIHP and an organic bulk heterojunction (BHJ) consisting of a low-bandgap nonfullerene and polymer, which achieve broadband response spectra up to 1 μm with a highest external quantum efficiency of approximately 54% at 850 nm, an ultrafast response speed of 5.6 ns and a linear dynamic range (LDR) of 191 dB. High sensitivity, ultrafast speed and a large LDR are preeminent prerequisites for the practical application of photodetectors. Encouragingly, due to the high-dynamic-range imaging capacity, high-quality visible-NIR actual imaging is achieved by employing the OIHP photodetectors. We believe that state-of-the-art OIHP photodetectors can accelerate the translation of solution-processed photodetector applications from the laboratory to the imaging market.

Influence of donor:acceptor ratio on charge transfer dynamics in non-fullerene organic bulk heterojunctions

The donor:acceptor (D:A) blend ratio plays a very important role in affecting the progress of charge transfer and energy transfer in bulk heterojunction (BHJ) organic solar cells (OSCs). The proper D:A blend ratio can provide maximized D/A interfacial area for exciton dissociation and appropriate domain size of the exciton diffusion length, which is beneficial to obtain high-performance OSCs. Here, we comprehensively investigated the relationship between various D:A blend ratios and the charge transfer and energy transfer mechanisms in OSCs based on PBDB-T and non-fullerene acceptor IT-M. Based on various D:A blend ratios, it was found that the ratio of components is a key factor to suppress the formation of triplet states and recombination energy losses. Rational D:A blend ratios can provide appropriate donor/accepter surface for charge transfer which has been powerfully verified by various detailed experimental results from the time-resolved fluorescence measurement and transient absorption (TA) spectroscopy. Optimized coherence length and crystallinity are verified by grazing incident wide-angle X-ray scattering (GIWAXS) measurements. The results are beneficial to comprehend the effects of various D:A blend ratios on charge transfer and energy transfer dynamics and provides constructive suggestions for rationally designing new materials and feedback for photovoltaic performance optimization in non-fullerene OSCs

Understanding the Interplay of Transport‐Morphology‐Performance in PBDB‐T‐Based Polymer Solar Cells

Polymer–polymer blends have been reported to exhibit exceptional thermal and ambient stability. However, power conversion efficiencies (PCEs) from devices using polymeric acceptors have been recorded to be significantly lower than those based on conjugated molecular acceptors. Herein, two organic nonfullerene bulk heterojunction (BHJ) blends ITIC:PBDB‐T and N2200:PBDB‐T, together with their fullerene counterpart, PCBM:PBDB‐T, are adopted to understand the effect of electron acceptors on device performance. Free charge carrier properties using time‐resolved microwave conductivity (TRMC) measurements are comprehensively investigated. The nonfullerene devices show an improved PCE of 10.06% and 6.65% in the ITIC‐ and N2200‐based cells, respectively. In comparison, the PCBM:PBDB‐T‐based devices yield a PCE of 5.88%. The optimal N2200:PBDB‐T produced the highest TRMC mobility, longest lifetime, and greatest free‐carrier diffusion length. It is found that such phenomena can be associated with the unfavorable morphology of the all‐polymer BHJ microstructure. In contrast, the solar cells using either the PCBM or ITIC acceptors display a more balanced donor and acceptor phase separation, leading to more efficient free‐carrier separation and transport in the operating device. By sacrificing efficiency for superior stability, it is shown that the improved structure in all‐polymer blend can deliver a more stable morphology under thermal stress.

Organic bulk heterojunction photovoltaics incorporating cyclopenteno[60]fullerene monoadducts as n‐type materials display superior power conversion efficiency than with PC61BM

AbstractOrganic bulk heterojunction photovoltaics, merely incorporating monoadducts of cyclopenteno[60]fullerenes (CPFs) as n‐type materials and P3HT as p‐type materials, display superior power conversion efficiency up to 4.6 ± 0.12%, superseding that with PC61BM/P3HT (3.8 ± 0.20%) for ca. 20%, under AM 1.5G irradiation―primarily attributed to the lack of homo‐conjugation on CPFs and their higher LUMO energy levels.

Efficient Hybrid Tandem Solar Cells Based on Optical Reinforcement of Colloidal Quantum Dots with Organic Bulk Heterojunctions

AbstractWhile colloidal quantum dot photovoltaic devices (CQDPVs) can achieve a power conversion efficiency (PCE) of ≈12%, their insufficient optical absorption in the near‐infrared (NIR) regime impairs efficient utilization of the full spectrum of visible light. Here, high‐efficiency, solution‐processed, hybrid series, tandem photovoltaic devices are developed featuring CQDs and organic bulk heterojunction (BHJ) photoactive materials for front‐ and back‐cells, respectively. The organic BHJ back‐cell efficiently harvests the transmitted NIR photons from the CQD front‐cell, which reinforces the photon‐to‐current conversion at 350–1000 nm wavelengths. Optimizing the short‐circuit current density balance of each sub‐cell and creating a near ideal series connection using an intermediate layer achieve a PCE (12.82%) that is superior to that of each single‐junction device (11.17% and 11.02% for the CQD and organic BHJ device, respectively). Notably, the PCE of the hybrid tandem device is the highest among the reported CQDPVs, including single‐junction devices and tandem devices. The hybrid tandem device also exhibits almost negligible degradation after air storage for 3 months. This study suggests a potential route to improve the performance of CQDPVs by proper hybridization with NIR‐absorbing photoactive materials.

Surface Modification of SnO2 via MAPbI3 Nanowires for a Highly Efficient Non-Fullerene Acceptor-Based Organic Solar Cell.

These days, organic-inorganic hybrid perovskites (OIHP) and non-fullerene acceptor (NFA) molecules are all at the frontiers of research and development in the domain of photovoltaics. A careful design and use of inorganic transparent metal oxides with wide band gaps as electron and hole transport layers are critically important for highly efficient and stable solar cells. As one of the most favorable electron transport materials, tin oxide (SnO2), which has been frequently utilized in highly efficient OIHP solar cells, is rarely seen in the application of NFA organic bulk heterojunction (BHJ) solar cells. To appropriately tailor an interface of SnO2 and an organic blend, while to make them compatible and useful may offer some opportunities for achieving higher efficiencies and longer lifetimes. In fact, there is still a lack of a method to solve the problem. Herein, a unique way is developed by implementing a surface decoration nanostructure such as low dimensional MAPbI3 perovskite nanowires (PeNWs) at the interface of SnO2 and the organic blend such as PBDB-T-SF:IT-4F. Such an interface functions well for the improvement of photovoltaic performance for the organic solar cell of the structure ITO(glass)/SnO2/PeNWs/PBDB-T-SF:IT-4F/MoO3/Ag. Experimental results indicate that the electron-hole dissociation, charge extraction, and photo-absorption ability of the organic solar cell can be improved significantly. The inside generation of the photocurrent is explored by the magneto-photocurrent method. Finally, the solar cell exhibits more than 80% power conversion efficiencies even after 20 days, which suggests the merits of having both SnO2 and PeNWs in the NFA-based organic solar cell.