Solution-processed fullerene-based organic Schottky junction devices for large-open-circuit-voltage organic solar cells

Dilute Donor Organic Solar Cells Based on Non-fullerene Acceptors

The advent of small molecule non-fullerene acceptor (NFA) materials for organic photovoltaic (OPV) devices has led to a series of breakthroughs in performance and device lifetime. The most efficient OPV devices have a combination of electron donor and acceptor materials that constitute the light absorbing layer in a bulk heterojunction (BHJ) structure. For many BHJ-based devices reported to date, the weight ratio of donor to acceptor is near equal. However, the morphology of such films can be difficult to reproduce and manufacture at scale. There would be an advantage in developing a light harvesting layer for efficient OPV devices that contains only a small amount of either the donor or acceptor. In this work we explore low donor content OPV devices composed of the polymeric donor PM6 blended with high performance NFA materials, Y6 or ITIC-4F. We found that even when the donor:acceptor weight ratio was only 1:10, the OPV devices still have good photoconversion efficiencies of around 6% and 5% for Y6 and ITIC-4F, respectively. It was found that neither charge mobility nor recombination rates had a strong effect on the efficiency of the devices. Rather, the overall efficiency was strongly related to the film absorption coefficient and maintaining adequate interfacial surface area between donor and acceptor molecules/phases for efficient exciton dissociation.

Organic Photodiodes with Thermally Reliable Dark Current and Excellent Detectivity Enabled by Low Donor Concentration

Reducing the dark current (J_d) under reverse bias while maintaining a high external quantum efficiency (EQE) is essential for the practical application of organic photodiodes (OPDs). However, the high J_d of OPDs is generally difficult to reduce because its origin in organic photodiodes is still not well understood and is strongly temperature dependent. To address the issues related to high J_d in typical OPDs, we investigate fullerene-based OPDs with various donor concentrations. It is surprising that OPDs with a low donor concentration in the active layer can achieve a very low J_d of 1.68 × 10^-7 mA cm^-2 at a reverse bias of -2 V, which is almost temperature-independent owing to the low polymer content. More importantly, the fullerene-based OPDs with a low donor concentration of 5 wt % can still achieve an external quantum efficiency (EQE) as high as 40%, resulting in a promisingly high detectivity of above 10¹³ Jones at 300-800 nm compared to the OPDs with a standard donor/acceptor ratio. The presented optimized OPD device can also be used for real-time heart rate detection, indicating its potential for practical photon-sensing applications.

Planar Heterojunction Organic Photodetectors Based on Fullerene and Non-fullerene Acceptor Bilayers for a Tunable Spectral Response

Planar heterojunction (PHJ) organic photodetectors are potentially more stable than traditional bulk heterojunction counterparts because of the absence of uncontrolled phase separation in the donor and acceptor binary blend system. This work reports a new class of PHJ organic photodetectors based on the medium-band gap fullerene C₆₀ and low-band gap fused-ring non-fullerene acceptor ID-MeIC bilayer structure, which allows a wide range of spectral response tuning across the UV-visible-near-infrared (UV-vis-NIR) region by tailoring individual layer thickness. The C₆₀ layer not only increases the external quantum efficiency at 745 nm by 57% but also reduces dark currents by two orders of magnitude. More importantly, the p-type poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzi] is found to be the key compound to conduct the layer-by-layer fabrication as combined with n-type ID-MeIC for higher charge extraction efficiency. In light of the above information, PHJ organic photodetectors exhibited a specific detectivity of 6.5 × 10¹⁰ Jones to detect NIR light at 745 nm under -0.1 V. The linear dynamic range was estimated to be 80 dB. This work has demonstrated a feasible approach to develop a PHJ architecture with tunable spectral response in the UV-vis-NIR range toward long-term stable organic photodetectors for potential applications in flexible and wearable biomedical sensors.

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.

Three-Dimensional Spirothienoquinoline-Based Small Molecules for Organic Photovoltaic and Organic Resistive Memory Applications

A new electron-rich spirothienoquinoline unit, ^tBuSAF-Th, has been developed via incorporation of a thienyl unit instead of a phenyl unit into the six-membered ring of the spiroacridine (SAF) and utilized for the first time as a building block for constructing small-molecule electron donors in organic solar cells (OSCs) and as active layers in organic resistive memory devices. The resulting three-dimensional spirothienoquinoline-containing 1-4 exhibit high-lying highest occupied molecular orbital (HOMO) energy levels. By the introduction of electron-deficient benzochalcogenodiazole linkers, with the chalcogen atoms being varied from O to S and Se, a progressively lower lowest unoccupied molecular orbital (LUMO) energy level has been achieved while keeping the HOMO energy levels similar. This strategy has allowed an enhanced light-harvesting ability without compromising open-circuit voltage (V_oc) in vacuum-deposited bulk heterojunction OSCs using 1-4 as donors and C₇₀ as the acceptor. Good photovoltaic performances with power conversion efficiencies (PCEs) of up to 3.86% and high short-circuit current densities (J_sc) of up to 10.84 mA cm^-2 have been achieved. In addition, organic resistive memory devices fabricated with these donor-acceptor small molecules exhibit binary logic memory behaviors with long retention times and high on/off current ratios. This work indicates that the spirothienoquinoline moiety is a potential building block for constructing multifunctional organic electronic materials.

Morphology of a Bulk Heterojunction Photovoltaic Cell with Low Donor Concentration

Atomistic nonequilibrium molecular dynamics simulations have been used to model the morphology of small-molecule bulk heterojunction films formed by vapor deposition as used in organic photovoltaics. Films comprising C₆₀ and 1, 5, 10, and 50 wt % of 1,1-bis[4-bis(4-methylphenyl)aminophenyl]cyclohexane (TAPC) were compared to films of neat C₆₀. The simulations suggest that if holes can hop between donor molecules separated by as little as 1.2-1.5 nm, then a TAPC concentration of 5 wt % is sufficient to form a percolating donor network and facilitate charge extraction. The results provide an explanation for why low donor content organic photovoltaics can still have high efficiencies. In addition, the roughness, porosity, and crystallinity of the films were found to decrease with increasing TAPC content.

High-Efficiency Fullerene Solar Cells Enabled by a Spontaneously Formed Mesostructured CuSCN-Nanowire Heterointerface

Fullerenes and their derivatives are widely used as electron acceptors in bulk-heterojunction organic solar cells as they combine high electron mobility with good solubility and miscibility with relevant semiconducting polymers. However, studies on the use of fullerenes as the sole photogeneration and charge-carrier material are scarce. Here, a new type of solution-processed small-molecule solar cell based on the two most commonly used methanofullerenes, namely [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM), as the light absorbing materials, is reported. First, it is shown that both fullerene derivatives exhibit excellent ambipolar charge transport with balanced hole and electron mobilities. When the two derivatives are spin-coated over the wide bandgap p-type semiconductor copper (I) thiocyanate (CuSCN), cells with power conversion efficiency (PCE) of ≈1%, are obtained. Blending the CuSCN with PC70BM is shown to increase the performance further yielding cells with an open-circuit voltage of ≈0.93 V and a PCE of 5.4%. Microstructural analysis reveals that the key to this success is the spontaneous formation of a unique mesostructured p-n-like heterointerface between CuSCN and PC70BM. The findings pave the way to an exciting new class of single photoactive material based solar cells.

Charge Generation and Recombination in High Fullerene Content Organic Bulk Heterojunction Solar Cells

Organic bulk heterojunction solar cells with a high fullerene content (larger than 70%) have been studied in this work. The device performances of this kind of solar cell could be tuned by adjusting the blend ratio in the active layer. An appropriate amount of p-type semiconductor in the high fullerene content active blend layer is beneficial for light absorbance and exciton dissociation. The proper energy alignment between the highest occupied molecular orbital of a p-type material and an n-type fullerene derivative has a strong influence on the exciton dissociation efficiency. In addition, the mechanism of photogenerated charge recombination in the solar cells has been identified through intensity-dependent current density-voltage (J-V) measurements and results show that the mechanisms governing the recombination are crucial for solar cell performance.

Interface Engineering of Organic Schottky Barrier Solar Cells and Its Application in Enhancing Performances of Planar Heterojunction Solar Cells

In this work, we describe the performance of organic Schottky barrier solar cells with the structure of ITO/molybdenum oxide (MoOx)/boron subphthalocyanine chloride (SubPc)/bathophenanthroline (BPhen)/Al. The SubPc-based Schottky barrier solar cells exhibited a short-circuit current density (Jsc) of 2.59 mA/cm², an open-circuit voltage (Voc) of 1.06 V, and a power conversion efficiency (PCE) of 0.82% under simulated AM1.5 G solar illumination at 100 mW/cm². Device performance was substantially enhanced by simply inserting thin organic hole transport material into the interface of MoOx and SubPc. The optimized devices realized a 180% increase in PCE of 2.30% and a peak Voc as high as 1.45 V was observed. We found that the improvement is due to the exciton and electron blocking effect of the interlayer and its thickness plays a vital role in balancing charge separation and suppressing quenching effect. Moreover, applying such interface engineering into MoOx/SubPc/C₆₀ based planar heterojunction cells substantially enhanced the PCE of the device by 44%, from 3.48% to 5.03%. Finally, we also investigated the requirements of the interface material for Schottky barrier modification.

Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices

Energy level alignment at the organic donor and acceptor interface is a key to determine the photovoltaic performance in organic solar cells, but direct probing of such energy alignment is still challenging especially for solution-processed bulk heterojunction (BHJ) thin films. Here we report a systematic investigation on probing the energy level alignment with different approaches in five commonly used polymer:[6,6]-phenyl-C71-butyric acid methyl ester (PCBM) BHJ systems. We find that by tuning the weight ratio of polymer to PCBM the electronic features from both polymer and PCBM can be obtained by photoemission spectroscopy. Using this approach, we find that some of the BHJ blends simply follow vacuum level alignment, but others show strong energy level shifting as a result of Fermi level pinning. Independently, by measuring the temperature-dependent open-circuit voltage (VOC), we find that the effective energy gap (Eeff), the energy difference between the highest occupied molecular orbital of the polymer donor (EHOMO-D) and lowest unoccupied molecular orbital of the PCBM acceptor (ELUMO-A), obtained by photoemission spectroscopy in all polymer:PCBM blends has an excellent agreement with the extrapolated VOC at 0 K. Consequently, the photovoltage loss of various organic BHJ photovoltaic devices at room temperature is in a range of 0.3-0.6 V. It is believed that the demonstrated direct measurement approach of the energy level alignment in solution-processed organic BHJ will bring deeper insight into the origin of the VOC and the corresponding photovoltage loss mechanism in organic photovoltaic cells.

Disassembly of micelles to impart donor and acceptor gradation to enhance organic solar cell efficiency

A transparent, conducting and low surface energy surface was prepared by disassembly of anionic micelles, which altered the orientation of the donor polymer and imparted gradation between the donor and acceptor. This configuration increased the solar cell device efficiency.

A high performance organic photovoltaic utilizing PEDOT:PSS and graphene oxide

Interface engineering may lead to a high performance organic photovoltaic as well as long lifetime.

A highly sensitive, low-cost, wearable pressure sensor based on conductive hydrogel spheres

Wearable pressure sensing solutions have promising future for practical applications in health monitoring and human/machine interfaces. Here, a highly sensitive, low-cost, wearable pressure sensor based on conductive single-walled carbon nanotube (SWCNT)/alginate hydrogel spheres is reported. Conductive and piezoresistive spheres are embedded between conductive electrodes (indium tin oxide-coated polyethylene terephthalate films) and subjected to environmental pressure. The detection mechanism is based on the piezoresistivity of the SWCNT/alginate conductive spheres and on the sphere-electrode contact. Step-by-step, we optimized the design parameters to maximize the sensitivity of the sensor. The optimized hydrogel sensor exhibited a satisfactory sensitivity (0.176 ΔR/R0/kPa(-1)) and a low detectable limit (10 Pa). Moreover, a brief response time (a few milliseconds) and successful repeatability were also demonstrated. Finally, the efficiency of this strategy was verified through a series of practical tests such as monitoring human wrist pulse, detecting throat muscle motion or identifying the location and the distribution of an external pressure using an array sensor (4 × 4).

Correlation between the Open-Circuit Voltage and Charge Transfer State Energy in Organic Photovoltaic Cells

In order to further improve the performance of organic photovoltaic cells (OPVs), it is essential to better understand the factors that limit the open-circuit voltage (VOC). Previous work has sought to correlate the value of VOC in donor-acceptor (D-A) OPVs to the interface energy level offset (EDA). In this work, measurements of electroluminescence are used to extract the charge transfer (CT) state energy for multiple small molecule D-A pairings. The CT state as measured from electroluminescence is found to show better correlation to the maximum VOC than EDA. The difference between EDA and the CT state energy is attributed to the Coulombic binding energy of the CT state. This correlation is demonstrated explicitly by inserting an insulating spacer layer between the donor and acceptor materials, reducing the binding energy of the CT state and increasing the measured VOC. These results demonstrate a direct correlation between maximum VOC and CT state energy.

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

The bulk heterojunction (BHJ) organic photovoltaic (OPV) architecture has dominated the literature due to its ability to be implemented in devices with relatively high efficiency values. However, a simpler device architecture based on a single organic semiconductor (SS-OPV) offers several advantages: it obviates the need to control the highly system-dependent nanoscale BHJ morphology, and therefore, would allow the use of broader range of organic semiconductors. Unfortunately, the photocurrent in standard SS-OPV devices is typically very low, which generally is attributed to inefficient charge separation of the photogenerated excitons. Here we show that the short-circuit current density from SS-OPV devices can be enhanced significantly (∼100-fold) through the use of inverted device configurations, relative to a standard OPV device architecture. This result suggests that charge generation may not be the performance bottleneck in OPV device operation. Instead, poor charge collection, caused by defect-induced electric field screening, is most likely the primary performance bottleneck in regular-geometry SS-OPV cells. We justify this hypothesis by: (i) detailed numerical simulations, (ii) electrical characterization experiments of functional SS-OPV devices using multiple polymers as active layer materials, and (iii) impedance spectroscopy measurements. Furthermore, we show that the collection-limited photocurrent theory consistently interprets typical characteristics of regular SS-OPV devices. These insights should encourage the design and OPV implementation of high-purity, high-mobility polymers, and other soft materials that have shown promise in organic field-effect transistor applications, but have not performed well in BHJ OPV devices, wherein they adopt less-than-ideal nanostructures when blended with electron-accepting materials.

Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in the same system

The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. It is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.

Time-resolved neutron reflectometry and photovoltaic device studies on sequentially deposited PCDTBT-fullerene layers

We have used steady-state and time-resolved neutron reflectometry to study the diffusion of fullerene derivatives into the narrow optical gap polymer poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) to explore the sequential processing of the donor and acceptor for the preparation of efficient organic solar cells. It was found that when [6,6]-phenyl-C61-butyric-acid-methyl-ester (60-PCBM) was deposited onto a thin film of PCDTBT from dichloromethane (DCM), a three-layer structure was formed that was stable below the glass-transition temperature of the polymer. When good solvents for the polymer were used in conjunction with DCM, both 60-PCBM and [6,6]-phenyl-C71-butyric-acid-methyl-ester (70-PCBM) were seen to form films that had a thick fullerene layer containing little polymer and a PCDTBT-rich layer near the interface with the substrate. Devices composed of films prepared by sequential deposition of the polymer and fullerene had efficiencies of up to 5.3%, with those based on 60-PCBM close to optimized bulk heterojunction (BHJ) cells processed in the conventional manner. Sequential deposition of pure components to form the active layer is attractive for large-area device fabrication, and the results demonstrate that this processing method can give efficient solar cells.

Interplay between efficiency and device architecture for small molecule organic solar cells

Small molecule organic solar cells (OSCs) have experienced a resurgence of interest over their polymer solar cell counterparts, owing to their improved batch-to-batch (thus, cell-to-cell) reliability. In this systematic study on OSC device architecture, we investigate five different small molecule OSC structures, including the simple planar heterojunction (PHJ) and bulk heterojunction (BHJ), as well as several planar-mixed structures. The different OSC structures are studied over a wide range of donor:acceptor mixing concentrations to gain a comprehensive understanding of their charge transport behavior. Transient photocurrent decay measurements provide crucial information regarding the interplay between charge sweep-out and charge recombination, and ultimately hint toward space charge effects in planar-mixed structures. Results show that the BHJ/acceptor architecture, comprising a BHJ layer with high C60 acceptor content, generates OSCs with the highest performance by balancing charge generation with charge collection. The performance of other device architectures is largely limited by hole transport, with associated hole accumulation and space charge effects.

In situ integration of squaraine-nanowire-array-based Schottky-type photodetectors with enhanced switching performance

Organic nanostructure-based photodetectors are important building blocks for future high-performance, low-cost, flexible nano-optoelectronic devices. However, device integration remains a large challenge, and the structure-dependent performance of the device has been seldom studied. Here, we report the in situ integration of 2,4-bis[4-(N,N-dimethylamino)phenyl]squaraine (SQ)-nanowire (NW)-array-based photodetectors by growing the organic NW arrays on prefabricated electrodes through an evaporation-induced self-assembly process. In contrast with ohmic-contact devices, asymmetric electrode pairs of Au-Ti were utilized to achieve the construction of Schottky-type photodetectors on the basis of organic NW arrays. Significantly, the Schottky-type photodetectors exhibited a significantly enhanced performance as compared to the ohmic-type devices in terms of their higher photosensitivity and switching speed. The presence of a strong built-in electric field at the junction interface, which greatly facilitated the separation/transportation of photogenerated electron-hole pairs, was suggested to be responsible for the superior performance of the Schottky-type photodetectors. More importantly, the organic NW-array-based devices also showed a higher sensitivity and reproducibility than that of the single NW-based devices, and they were capable of low-light detection. The investigation of the photodetector circuitry also disclosed a very low pixel-to-pixel variation owing to the average effect of the NW-array-based devices. It is expected that organic NW-array-based Schottky-type photodetectors will have important applications in future organic nano-optoelectronic devices.

A DMF-assisted solution process boosts the efficiency in P3HT:PCBM solar cells up to 5.31%

To achieve ideal phase separation in P3HT:PCBM (1:1, w/w) blend films, a selective second solvent, DMF, which cannot dissolve P3HT but slightly dissolves PCBM, was added into 1,2-dichlorobenzene. Addition of 10% DMF led to better charge transport, better morphology and average power conversion efficiency (PCE) enhancement from 3.75% to 4.29%. To form an acceptor rich layer near the cathode and achieve effective vertical phase separation, PCBM solution in DMF was spin-coated on the top surface of the P3HT:PCBM active layer. The PCBM rich layer enhanced hole blocking and electron transport, leading to an average PCE improvement from 4.29% to 4.83% (for the PCBM rich layer formed by a 2 mg ml(-1) PCBM solution). Finally, to obtain more uniform and smooth films with better contact of Ca/PCBM and BHJ/PCBM interfaces, the films were thermally annealed at 120 ° C for 10 min after spin coating the PCBM rich layer, leading to an average PCE enhancement from 4.83% to 5.17% (the best PCE was 5.31%).