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Panchromatic All‐Polymer Photodetector with Tunable Polarization Sensitivity
... Each FR consists of N layers of polymer retarder films (r 1 to r N ), with equal thicknesses L and fast axes oriented in a folded manner, i.e., alternating between an angle of ±, as illustrated in Fig. 1F. The polarization sensitivity of the OPV cells can then be introduced by orienting the polymer semiconductors in the plane of the film and exploiting the anisotropic optical character of the polymers (25)(26)(27). Referring to Fig. 1E, OPV1 and OPV2 sense polarization, while OPV3 to OPV6 sense color and also provide a single polarization channel. The light's polarization state is detected by OPV1 and OPV2, which are nominally sensitive to 0° and +45° polarized light relative to the x axis, respectively. ...
... The intrinsic P-OPV cells were realized through orienting the active polymer semiconductors in the plane of the film, as previously described (25,27,33,34). This exploits the optical anisotropy of the polymers, which have their primary optical transition dipole moment (-*) aligned parallel to their backbone (35,36). ...
... The chemical structure of these two polymers is depicted in Fig. 3A. These polymers were chosen because they are both ductile at room temperature and have complimentary spectral absorption, enabling panchromatic P-OPV cells (27,38). However, the strain alignment approach limited the peak dichroic ratios to approximately 2 at the maximum possible strain before film tearing (27). ...
Combining hyperspectral and polarimetric imaging provides a powerful sensing modality with broad applications from astronomy to biology. Existing methods rely on temporal data acquisition or snapshot imaging of spatially separated detectors. These approaches incur fundamental artifacts that degrade imaging performance. To overcome these limitations, we present a stomatopod-inspired sensor capable of snapshot hyperspectral and polarization sensing in a single pixel. The design consists of stacking polarization-sensitive organic photovoltaics (P-OPVs) and polymer retarders. Multiple spectral and polarization channels are obtained by exploiting the P-OPVs’ anisotropic response and the retarders’ dispersion. We show that the design can sense 15 spectral channels over a 350-nanometer bandwidth. A detector is also experimentally demonstrated, which simultaneously registers four spectral channels and three polarization channels. The sensor showcases the myriad degrees of freedom offered by organic semiconductors that are not available in inorganics and heralds a fundamentally unexplored route for simultaneous spectral and polarimetric imaging.
... In this section, the radiometric performance of the Solc-based OPV detectors is quantified using realistic polarization-sensitive OPV materials as described in [29]. Additionally, an optimization scheme for the OPV's transmittance properties is provided in order to enhance the SNR across all five OPVs. ...
... The parameters used in the simulations include a pixel area of 2.5 × 10 −3 cm 2 , a fill factor of 0.9, a detector integration time of 10 ms, an integration capacitance of 100 pf, a holdswitch resistance of 1.5 k , and a reset-switch resistance of 1000 G . The OPV's parameters were obtained from [29] for the OPV cells under applied strain of 60% and included (i) a dark current density of 10 −9 A/cm 2 at 0 V bias, (ii) responsivity values between 0.1 and 0.2 A/W from 400 to 700 nm, and (iii) transmittances (T x and T y ) values as provided in Table 1. ...
Using organic photodetectors for multispectral sensing is attractive due to their unique capabilities to tune spectral response, transmittance, and polarization sensitivity. Existing methods lack tandem multicolor detection and exhibit high spectral cross talk. We exploit the polarization sensitivity of organic photodetectors, together with birefringent optical filters to design single-pixel multispectral detectors that achieve high spectral selectivity and good radiometric performance. Two different architectures are explored and optimized, including the Solc-based and multitwist-retarder-based organic photodetectors. Although the former demonstrated a higher spectral resolution, the latter enables a more compact sensor as well as greater flexibility in device fabrication.
... A rotating AQWP modulator is placed at the entrance of the pixel, followed by 4 P-OPV cells (OPV0 -OPV3) that alternate with 3 LC-based MTR units (MTR1 -MTR3) in series. The OPVs' anisotropy is obtained by aligning the polymer backbone chains uniaxially along the plane of the film [36,37]. In the current configuration of Fig. 1 (d), all OPV cells have their transmission axes parallel to the x-axis. ...
... These materials also have complimentary absorption spectra ( Figure 2a) and appropriate energy level offsets for efficient exciton dissociation and charge collection. 22 In addition to the optoelectronic characteristics, these polymers are found to be ductile, which has been shown to be a good screening tool to asses toughness, 23 as discussed further below. Our primary interest is to explore the impact of the polymer MW and film morphology on mechanical behavior for an All-PSC. ...
Organic solar cells that have all-polymer active layers may have several advantages compared with polymer-small molecule systems including improved mechanical and thermodynamic stability; however, an all-polymer active layer does not guarantee robust mechanical behavior. Here, we consider key parameters that may influence the mechanical behavior and power conversion efficiency of all-polymer solar cells (all-PSCs). Considerations include the thermal transition temperature of the polymers, the molecular weight (MW) of the polymers, and film morphology. The impact these features have on mechanical behavior is probed by measuring the cohesive fracture energy (Gc), crack onset strain, and elastic modulus. We find that the selection of ductile polymers with high MW enhances interchain interactions that improves the mechanical resilience of the films. High MW polymers is also found to maximize power conversion efficiency (PCE). Using this strategy, BHJ films with the best reported combination of Gc (7.96 J m-2) and PCE (6.94 %) are demonstrated. Finally, it is found that increasing film thickness increases the fracture energy of the films but at the cost of PCE. These findings provide a fundamental perspective on the design strategy to achieve high performance and mechanically robust organic solar cells.
Photodetectors that can sense not only light intensity but also light's polarization state add valuable information that is beneficial in a wide array of applications. Polymer semiconductors are an attractive material system to achieve intrinsic polarization sensitivity due to their anisotropic optoelectronic properties. In this report, the thermomechanical properties of the polymer semiconductors PBnDT‐FTAZ and P(NDI2OD‐T2) are leveraged to realize bulk heterojunction (BHJ) films with record in‐plane alignment. Two polymer blends with distinct weight average molar masses (Mw) are considered and either a strain‐ or rub‐alignment process is applied to align the polymer blend films. Optimized processing yields films with dichroic ratios (DR) of over 11 for the high Mw system and nearly 17 for the low Mw system. Incorporating the aligned films into photodetectors results in a polarized photocurrent ratio of 15.25 with corresponding anisotropy ratio of 0.88 at a wavelength of 530 nm, representing the highest reported photocurrent ratio for photodiodes that can operate in a self‐powered regime. The demonstrated performance showcases the ability of polymer semiconductors to achieve BHJ films with exceptional in‐plane polymer alignment, enabling high performance polarization sensitive photodetectors for incorporation into novel device architectures. Polymer semiconductor bulk heterojunction films are aligned using molecular weight dependent strategies to achieve highly oriented blend films. Integrating the films in organic photodetectors, a polarized photocurrent ratio of over 15 is demonstrated at a wavelength of 530 nm. The intrinsically polarization sensitive photodetector is utilized to effectively image a polarized scene.
Organic photodetectors (OPDs) have drawn extensive research efforts due to their tailorable spectral response, ease of processing, compatibility with flexible devices and cooling-free operations. In this review, we outline the promising strategies for constructing high-performance and highly stable photodiodes-based OPDs from the perspectives of molecular engineering, morphology control, and device structure design. Firstly, the impact of molecular design and morphology control on OPD performance is clearly underlined and the molecular design rules and quantitative analysis methods are presented for high-performance OPDs. Subsequently, some striking device designs for multifunctional applications are discussed to elucidate the corresponding mechanism for various responses. What follows are the research efforts of boosting OPD stability for commercial applications. This review not only presents the detailed discussion on various OPD strategies aiming at simultaneously enhancing performance and stability but also provides some insights for the remaining challenges to make further breakthrough of OPDs.
Two photoconductive conjugated polymers (PDTPTT and PCPDTTT) were synthesized to utilize in red and infrared (NIR) organic photodetectors (OPDs). The low bandgap was achieved by stabilizing quinoidal structure of conjugated...
Polymer nanocomposites accommodate the considerable potential for the application of photodetector, on the basis of the abundant nanoscale interface formed between different components in the material system, which can efficiently facilitate the dissociation of photon‐induced excitons. This potential has been keeping fulfilled in the last decades with the boom of the organic electronics and inorganic nanocrystals, which endows the polymer nanocomposites with excellent optoelectronic properties, let alone the inherent flexibility of the materials. To make this field more digestible, this chapter is presented to provide an overview of the development of the polymer nanocomposite‐based photodetectors since the 1990s. The photodetector is systematically introduced in terms of the working mechanism and figure of merit, to reveal the connections between device performance and material features. The numerous reports on polymer nanocomposites‐based photodetectors are categorized regarding the material composition. For each category, the device performances are abstracted in a table for comparison, aiming to explicitly brief the research progress of the particular material system to the readers, and thus inspire the future research in this topic. This chapter would be deemed successful if any novel and valid strategy can be drawn from the narrative to solve the shortcomings of state‐of‐the‐art technology, or to produce a polymer nanocomposites‐based photodetector with a better performance.
Currently available wavelength‐selective p–n junction organic photodetectors (OPDs) nearly exclusively use small molecules. In this study, green (G) selective all‐polymer p–n junction OPDs are developed by engineering the π‐conjugation networks and insulating properties of p‐ and n‐type polymers. Enhanced intermolecular ordering of p–n junction blend films compared to pristine polymer films results in superior hole/electron mobilities and low bimolecular recombination in the devices. Notably, similar G absorption ranges and the huge difference in their absorption coefficients between p‐ and n‐type polymers make excellent G selectivity in the p–n junction OPDs. Thus, narrow selectivity with a full width at half maximum of 155 nm toward green light and high detectivity of ≈1013 Jones are achieved in the all‐polymer p–n junction OPDs. Green selective p–n junction all‐polymer photodetectors are developed by designing p‐ and n‐type conjugated polymers. Narrow selectivity with a full width at half maximum of 155 nm toward green light, high detectivity of ≈1013 Jones, and fast signal responses are achieved.
Conjugated polymers have proven to be an important class of materials for flexible and stretchable electronics. To ensure long term thermal and mechanical stability of associated devices, there is a need to determine the origin of the polymer ductility and toughness. In this work, we investigate a variety of high-performance conjugated polymers and relate their thermomechanical behavior to film toughness. Dynamic mechanical analysis (DMA) is used to probe thermomechanical relaxations of the conjugated polymers. Film ductility is measured as a function of temperature to determine the temperature that corresponds to a significant loss in film toughness. We systematically study polymers with changes to side-chain structure, backbone structure, and crystallinity. We also compare polymers that have a clear glass transition (Tg) to those that do not. It is found that secondary thermal relaxations (sub-Tg) play a critical role in film toughness. This sub-Tg is found to be a local molecular relaxation that appears to relate to side-chain and backbone mobility. We also find that many of the polymers considered continue to show moderate ductility below their sub-Tg, which is attributed to crystallites or aggregates that have active slip systems. These results provide new insights into how conjugated polymer structure and related thermal relaxations influence film toughness that will assist in realizing mechanically robust devices.
The broadband organic photodetectors (OPDs) with photomultiplication (PM) phenomenon are fabricated by using Y6 doped with rather less PBDB-T as active layers having electron-only transport characteristic. The PM-OPDs with PBDB-T:Y6 (3:100, wt/wt) as active layers exhibit the external quantum efficiency (EQE) of 18,000% at 350 nm and 15,000% at 870 nm under 1 V bias. The PM-OPDs exhibits a broad spectral response range from 320 to 950 nm, which is mainly determined by the trapped hole distribution near Al electrode. The working mechanism of PM-OPDs should be attributed to interfacial trap-assisted electron tunneling injection, which can be further confirmed from the EQE spectral shape of PM-OPDs with different active layer thickness. The n-type polymer PZ1 was selected as interfacial layer to be inserted between PEDOT:PSS and active layers, which makes PM-OPDs with PZ1 interfacial layer work well under forward and reverse bias. This work may provide a smart strategy to achieve broadband PM-OPDs by adjusting the distribution of trapped charge near electrode.
We report a new approach for large-scale alignment of micron-sized J-aggregates of a derivative of porphyrin onto planar glass substrates. We applied a unidirectional nitrogen flow to an aqueous dye drop deposited onto a glass substrate to form an, about 5-nm thin film of aligned J-aggregates over macroscopic surface areas up to several mm. The inter-aggregate distance is ~500 nm and it scales with the nitrogen pressure. We verified the film thickness and J-aggregates alignment using multi-modal microscopy and spectroscopic techniques. Our technique is fast, simple, and cost-effective for producing large 2-D arrays of aligned emitters.
Polarization‐sensitive narrowband photodetectors can respond to a narrow spectral range of light together with the ability to sense the polarization of light. Traditionally, expensive filters combined with polarizers are utilized to realize the polarization‐sensitive narrowband photodetections. To reduce the cost and simplify optical system, here a polarization‐sensitive narrowband photodetector based on 2D perovskite single crystals without any additional optical components is reported. The photodetector shows a linear dichroic ratio of 1.56 at 552 nm under the oblique incident angle of 45° and the full width at half maximum of 20 nm. Moreover, the photodetector exhibits a high external quantum efficiency of 120% and a peak specific detectivity of 1.23 × 1010 Jones under the normal illumination. This polarization‐sensitive narrowband photoresponse can be ascribed to the charge collection narrowing mechanism assisted by the enhanced self‐trapped states in 2D perovskites and the large anisotropic crystal structure between different crystal directions. In particular, the device configuration is specially designed so that the absorption anisotropy takes place between the in‐plane and out‐of‐plane directions and photogenerated carriers transport along the in‐plane direct to avoid the large organic barrier along the out‐of‐plane direction, resulting in the polarization‐sensitive narrowband photodetections with high performance. The filterless polarization‐sensitive narrowband photodetectors based on 2D perovskite (iso‐BA)2PbI4 are demonstrated with simultaneously wavelength discrimination and polarization selection. The photodetector shows a linear dichroic ratio of 1.56 at 552 nm, the full width at half maximum of 20 nm an external quantum efficiency of 120% and a specific detectivity of 1.23 × 1010 Jones under the normal incidence.
Photodetectors, converting optical signals from specific wavelengths to electrical signals, have many applications on photoimaging, optical communication, and environmental monitoring. Solution‐processed organic photodetectors (OPDs) based on organic materials emerge promise especially for wearable electronics and smart buildings. In this work, new all‐polymer photodetectors (all‐PPDs) are developed based on bulk‐heterojunction active layers which incorporate a donor polymer and an acceptor polymer. The inverted all‐PPDs exhibit outstanding external quantum efficiency over 70%, low dark current density (J d) of 1.1 × 10−8 A cm−2, and high detectivity (D*) over 3.0 × 1012 Jones with planar response over the entire visible range. It is one of the best‐performing all‐PPDs reported so far and is also comparable with many organic and inorganic photodetectors. By using lamination technique, large‐area, semitransparent, flexible, and “fully” polymeric photodetectors are successfully fabricated for the first time, with D* over 1011 Jones for double‐side light detection. The results highlight the great potential for producing high‐performance all‐PPDs by taking advantages of various device architecture and solution‐processing techniques. Large‐area, semitransparent, and flexible all‐polymer photodetectors are realized by incorporating a pair of donor and acceptor polymers and by using a lamination method. Both sides of these all‐polymer photodetectors respond visible light signals with nearly identical D* over 1.0 × 1011 Jones.
The capability to detect the polarization state of light is crucial in many day‐life applications and scientific disciplines. Novel anisotropic 2D materials such as TiS3 combine polarization sensitivity, given by the in‐plane optical anisotropy, with excellent electrical properties. Here, the fabrication of a monolithic polarization‐sensitive broadband photodetector based on a mixed‐dimensionality TiS3/Si p–n junction is demonstrated. The fabricated devices show broadband responsivity up to 1050 nm, a strong sensitivity to linearly polarized illumination with difference between the two orthogonal polarization states up to 350%, and a good detectivity and fast response time. The discussed devices can be used as building blocks to fabricate more complex polarization‐sensitive systems such as polarimeters. A mixed‐dimensionality TiS3/Si p–n junction with polarization‐sensitive and broadband photodetection is designed and fabricated. The heterostructure device shows a broadband responsivity from 405 to 1050 nm and a high sensitivity to polarized infrared illumination photodetection. The polarized contrast between b‐ and a‐axis directions of the TiS3 lattice is up to 350%.
The field of non-fullerene organic solar cells has experienced rapid development during the past few years, mainly driven by the development of novel non-fullerene acceptors and matching donor semiconductors. However, organic solar cell material development has progressed via a trial-and-error approach with limited understanding of the materials’ structure-property relationships and the underlying device physics of non-fullerene devices. In addition, the availability of hundreds of donor and acceptor semiconductors creates an extremely large pool of possible donor-acceptor combinations, which poses a daunting challenge for rational material screening and matching. This Review describes several important conceptual aspects of the emerging non-fullerene devices by highlighting key contributions that provided fundamental insights regarding rational material design, donor-acceptor pair matching, blend morphology control and the reduced voltage losses in non-fullerene organic solar cells. We also discuss the key challenges that need to be addressed to develop more-efficient non-fullerene organic solar cells.
Solution processed bulk heterojunction organic photodetectors (OPDs) based on a conjugated dimeric porphyrin small molecule as the donor and PC71BM as the acceptor show a broad and strong spectral response from 300 to 1000 nm. At room temperature and zero bias, the panchromatic OPDs responsive to the near infrared (NIR) region exhibit a very low dark current density of 4.25×10−10 A cm−2 with a very high external quantum efficiency (EQE) of 48 % at 850 nm, leading to a responsivity of 0.33 A/W at 850 nm and detectivities over 1013 Jones from 360 to 950 nm with a peak detectivity value of 5.73×1013 Jones at 850 nm. All these parameters are among the best for small-molecule NIR OPDs to date, and comparable to the best NIR OPDs based on polymers, demonstrating the potentials of small molecules on panchromatic photodetectors.
A new fabrication method to realize fully-printed organic photodiode (OPD) arrays capable of RGB light separation is presented. From the photocurrents generated by each pixel type under the light from RGB LEDs, we demonstrate that this “White”, “Yellow” and “Red” array can successfully detect and reconstruct colors in the RGB system, with an average accuracy of 98.5%. A flexible broadband OPD array is printed on PEN substrate by blade-coating PEDOT:PSS, a polyethylenimine cathode interlayer and the photo-active layer, and screen-printing on top a patterned PEDOT:PSS anode. The OPD array achieves an average EQE of ∼37% at −4 V bias over the whole visible spectrum, 5 orders of magnitude of linear dynamic range (LDR), a 0.5 nA/cm2 dark current, and maintains these performances in ambient conditions for more than 30 h. Pixels detecting “White”, “Yellow” and “Red” are fabricated by spray-coating two color filters. The substrate is used as a separator between the filters and OPD array. This physical separation allows solution processing of the filters regardless of their electrical properties or of the compatibility of their solvents with the OPD, thus broadening the choice of filter materials while offering a simple fabrication process. The combination of broadband OPD and broadband filters used in this configuration can significantly simplify the fabrication of spectrally-selective photosensors and full-color imagers.
In very recent years, growing efforts have been devoted to the development of all-polymer solar cells (all-PSCs). One of the advantages of all-PSCs over the fullerene-based PSCs is the versatile design of both donor and acceptor polymers which allows the optimization of energy levels to maximize the open-circuit voltage (Voc). However, there is no successful example of all-PSCs with both high Voc over 1 V and high power conversion efficiency (PCE) up to 8% reported so far. In this work, a combination of a donor polymer poly[4,8-bis(5-(2-octylthio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-(5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione)-1,3-diyl] (PBDTS-TPD) with a low-lying highest occupied molecular orbital level and an acceptor polymer poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-thiophene-2,5-diyl] (PNDI-T) with a high-lying lowest unoccupied molecular orbital level is used, realizing high-performance all-PSCs with simultaneously high Voc of 1.1 V and high PCE of 8.0%, and surpassing the performance of the corresponding PC71BM-based PSCs. The PBDTS-TPD:PNDI-T all-PSCs achieve a maximum internal quantum efficiency of 95% at 450 nm, which reveals that almost all the absorbed photons can be converted into free charges and collected by electrodes. This work demonstrates the advantages of all-PSCs by incorporating proper donor and acceptor polymers to boost both Voc and PCEs.
The nucleation of single macroscopic spherulites at desired positions, as well as ordered arrays of multiple spherulites, is demonstrated by combining the use of crystallizable solvents with local control of solvent evaporation during solution deposition. Moreover, the temperature assisted localized frustration of molecular orientation is shown, enabling the fabrication of samples containing both isotropic areas and spherulites. These macroscopic circular polycrystalline structures are characterized using a range of polarized spectroscopic techniques that allow quantifying their large degree of chain orientation. In order to show the potential of these large conjugated polymer spherulites centered at desired locations, graded bilayer organic photovoltaic devices were fabricated to be used as polarimeters, solid state light polarization detectors with no moving parts, and position sensitive photodetectors.
To explore the advantages of emerging all-polymer solar cells (all-PSCs), growing efforts have been devoted to developing matched donor and acceptor polymers to outperform fullerene-based PSCs. In this work, a detailed characterization and comparison of all-PSCs using a set of donor and acceptor polymers with both conventional and inverted device structures is performed. A simple method to quantify the actual composition and light harvesting contributions from the individual donor and acceptor is described. Detailed study on the exciton dissociation and charge recombination is carried out by a set of measurements to understand the photocurrent loss. It is unraveled that fine-tuned crystallinity of the acceptor, matched donor and acceptor with complementary absorption and desired energy levels, and device architecture engineering can synergistically boost the performance of all-PSCs. As expected, the PBDTTS-FTAZ:PNDI-T10 all-PSC attains a high and stable power conversion efficiency of 6.9% without obvious efficiency decay in 60 d. This work demonstrates that PNDI-T10 can be a potential alternative acceptor polymer to the widely used acceptor N2200 for high-performance and stable all-PSCs.
Polarization-sensitive perovskite photodetectors are realized by crystallographically aligning 1D perovskite arrays. High-quality inorganic perovskite single crystals with crystallographic order are fabricated by strictly manipulating the dewetting process of organic solution, yielding photodetectors with high photoresponsivity and fast response speed.
Polarimetry has widespread applications within atmospheric sensing, telecommunications, biomedical imaging, and target detection. Several existing methods of imaging polarimetry trade off the sensor's spatial resolution for polarimetric resolution, and often have some form of spatial registration error. To mitigate these issues, we have developed a system using oriented polymer-based organic photovoltaics (OPVs) that can preferentially absorb linearly polarized light. Additionally, the OPV cells can be made semitransparent, enabling multiple detectors to be cascaded along the same optical axis. Since each device performs a partial polarization measurement of the same incident beam, high temporal resolution is maintained with the potential for inherent spatial registration. In this paper, a Mueller matrix model of the stacked OPV design is provided. Based on this model, a calibration technique is developed and presented. This calibration technique and model are validated with experimental data, taken with a cascaded three cell OPV Stokes polarimeter, capable of measuring incident linear polarization states. Our results indicate polarization measurement error of 1.2% RMS and an average absolute radiometric accuracy of 2.2% for the demonstrated polarimeter.
Due to the novel optical and optoelectronic properties, 2D materials have received increasing interests for optoelectronics applications. Discovering new properties and functionalities of 2D materials is challenging yet promising. Here broadband polarization sensitive photodetectors based on few layer ReS2 are demonstrated. The transistor based on few layer ReS2 shows an n-type behavior with the mobility of about 40 cm2 V-1 s-1 and on/off ratio of 105. The polarization dependence of photoresponse is ascribed to the unique anisotropic in-plane crystal structure, consistent with the optical absorption anisotropy. The linear dichroic photodetection with a high photoresponsivity reported here demonstrates a route to exploit the intrinsic anisotropy of 2D materials and the possibility to open up new ways for the applications of 2D materials for light polarization detection. Polarization sensitive photodetectors are demonstrated based on anisotropic few layer ReS2. The transistor based on few layer ReS2 shows an n-type behavior with a mobility of about 40 cm2 V-1 s-1 and photoresponsivity of about 103 A W-1. The polarization dependence of photoresponse is ascribed to the unique anisotropi structure. The result demonstrates a route to exploit the intrinsic anisotropy of 2D materials and the possibility to open up new ways of the applications of 2D materials for light polarization detection.
All-polymer solar cells have shown great potential as flexible and portable power generators. These devices should offer good mechanical endurance with high power-conversion efficiency for viability in commercial applications. In this work, we develop highly efficient and mechanically robust all-polymer solar cells that are based on the PBDTTTPD polymer donor and the P(NDI2HD-T) polymer acceptor. These systems exhibit high power-conversion efficiency of 6.64%. Also, the proposed all-polymer solar cells have even better performance than the control polymer-fullerene devices with phenyl-C 61 -butyric acid methyl ester (PCBM) as the electron acceptor (6.12%). More importantly, our all-polymer solar cells exhibit dramatically enhanced strength and flexibility compared with polymer/PCBM devices, with 60- and 470-fold improvements in elongation at break and toughness, respectively. The superior mechanical properties of all-polymer solar cells afford greater tolerance to severe deformations than conventional polymer-fullerene solar cells, making them much better candidates for applications in flexible and portable devices.
We investigate the photocurrent generation mechanisms at a vertical p-n heterojunction between black phosphorus (BP) and molybdenum disulfide (MoS2) flakes through polarization-, wavelength-, and gate-dependent scanning photocurrent measurements. When incident photon energy is above the direct band gap of MoS2, the photocurrent response denmonstrates a competitive effect between MoS2 and BP in the junction region. In contrast, if the incident photon energy is below the band gap of MoS2 but above the band gap of BP, the photocurrent response at the p-n junction exhibits the same polarization dependence as that at the BP-metal junction, which is nearly parallel to the MoS2 channel. This result indicates that the photocurrent signals at the MoS2-BP junction primarily result from the direct band gap transition in BP. These fundamental studies shed light on the knowledge of photocurrent generation mechanisms in vertical 2D semiconductor heterojunction junctions, offering a new way of engineering future two-dimensional materials based optoelectronics devices.
The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodetector using a layered black phosphorus transistor that is polarization-sensitive over a bandwidth from ∼400 nm to 3,750 nm. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material. In this transistor geometry, a perpendicular built-in electric field induced by gating can spatially separate the photogenerated electrons and holes in the channel, effectively reducing their recombination rate and thus enhancing the performance for linear dichroism photodetection. The use of anisotropic layered black phosphorus in polarization-sensitive photodetection might provide new functionalities in novel optical and optoelectronic device applications.
Inorganic semiconductor-based broadband photodetectors are ubiquitous in imaging technologies such as digital cameras and photometers. Herein a broadband organic photodiode (OPD) that has performance metrics comparable or superior to inorganic photodiodes over the same spectral range is reported. The photodiode with an active layer comprised of a poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester bulk heterojunction blend had a dark current < 1 nA/cm2, specific detectivity of ∼1013 Jones, reverse bias −3 dB frequency response of 100 kHz to 1 MHz, and state-of-the-art Linear Dynamic Range for organic photodiodes of nine orders of magnitude (180 dB). The key to these performance metrics was the use of a thick junction (700 nm), which flattened the spectral response, reduced the dark current and decreased performance variations. The strategy also provides a route to large area defect free “monolithic” structures for low noise integrated photo-sensing, position determination, or contact, non-focal imaging.
The effects of the 1,8-diiodooctane (DIO) processing additive on the nanomorphology, charge generation, and charge transport in the PBnDT-FTAZ/PC70BM BHJ solar cells are demonstrated. The optimized nanoscale donor-acceptor morphology is achieved with 3% DIO processing additive in the BHJ film. The BHJ film with 3% DIO exhibits efficient charge generation as indicated by the decreased PL and the associated lifetime compared to the BHJ film without DIO. The balanced charge transport decreased series resistance and balanced hole and electron mobility is also observed in the device fabricated with 3% DIO. Because of the efficient charge extraction, JSC and PCE of the devices are increased from 3.7 mA/cm2 and 1.9% to 13.7 mA/cm2 and 7%, respectively. In summary, optimized fabrication with DIO as a processing additive improves the morphology, faster charge generation, and balanced charge transport in the PBnDT-FTAZ:PC70BM BHJ film, which leads to high performance BHJ solar cells.
We investigate electrical transport and optoelectronic properties of field effect transistors
(FETs) made from few-layer black phosphorus (BP) crystals down to a few nanometers. In
particular, we explore the anisotropic nature and photocurrent generation mechanisms in BP
FETs through spatial-, polarization-, gate-, and bias-dependent photocurrent measurements.
Our results reveal that the photocurrent signals at BP-electrode junctions are mainly attributed
to the photovoltaic effect in the off-state and photothermoelectric effect in the on-state, and
their anisotropic feature primarily results from the directional-dependent absorption of BP
crystals.
In this study, we demonstrate linearly polarized organic photovoltaic cells with a well-controlled level of polarization sensitivity. The polarized devices were created through the application of a large uniaxial strain to the bulk heterojunction poly(3-hexylthiophene):Phenyl-C61-butyric acid methyl ester (P3HT:PCBM) film and printing the plastically deformed active layer onto a PEDOT:PSS and indium tin oxide coated glass substrate. The P3HT:PCBM layer is processed such that it is able to accommodate high strains (over 100%) without fracture. After printing the strained films, thermal annealing is used to optimize solar cell performance while maintaining polarization sensitivity. A dichroic ratio and short circuit current ratio of ≈6.1 and ≈1.6 were achieved, respectively.
The development of flexible and physically robust organic solar cells requires detailed knowledge of the mechanical behavior of the heterogeneous material stack. However, in these devices there has been limited research on the mechanical properties of the active organic layer. Here, two critical mechanical properties, stiffness and ductility, of a widely studied organic solar cell active layer, a blend film composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) are reported. Processing conditions are varied to produce films with differing morphology and correlations are developed between the film morphology, mechanical properties and photovoltaic device performance. The morphology is characterized by fitting the absorption of the P3HT:PCBM films to a weakly interacting H-aggregate model. The elastic modulus is determined using a buckling metrology approach and the crack onset strain is determined by observing the film under tensile strain using optical microscopy. Both the elastic modulus and crack onset strain are found to vary significantly with processing conditions. Processing methods that result in improved device performance are shown to decrease both the compliance and ductility of the film.
Polymer conductors that are solution processable provide an opportunity to realize low-cost organic electronics. However, coating sequential layers can be hindered by poor surface wetting or dissolution of underlying layers. This has led to the use of transfer printing where solid film inks are transferred from a donor substrate to partially fabricated devices using a stamp. This approach typically requires favorable adhesion differences between the stamp, ink, and receiving substrate. Here, we present a shear-assisted organic printing (SHARP) technique that employs a shear load on a post-less polydimethylsiloxane (PDMS) elastomer stamp to print large-area polymer films that can overcome large unfavorable adhesion differences between the stamp and receiving substrate. We explore the limits of this process by transfer printing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films with varied formulation that tune adhesive fracture energy. Using this platform, we show that the SHARP process is able to overcome a 10-fold unfavorable adhesion differential without the use of a patterned PDMS stamp enabling large area printing. The SHARP approach is then used to print PEDOT:PSS films in the fabrication of high performance semitransparent organic solar cells.
The synthesis of an acceptor polymer PIDT-
2TPD, comprising indacenodithiophene (IDT) as the
electron-rich unit and an interconnected bithieno[3,4-c]-
pyrrole-4,4′,6,6′-tetrone (2TPD) as the electron-deficient
unit, and its application for all-polymer photodetectors is
reported. The optical, electrochemical, charge transport, and
device properties of a blend of poly(3-hexylthiophene) and
PIDT-2TPD are studied. The blend shows strong complementary absorption and balanced electron and hole mobility,
which are desired properties for a photoactive layer. The
device exhibits dark current density in the order of 10−5 mA/
cm2, external quantum efficiency broadly above 30%, and nearly planar detectivity over the entire visible spectral range
(maximum of 1.1 × 1012 Jones at 610 nm) under −5 V bias. These results indicate that PIDT-2TPD is a highly functional new
type of acceptor and further motivate the use of 2TPD as a building block for other n-type materials
Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on fullerene acceptors for over two decades. This situation has changed recently, with non-fullerene (NF) OSCs developing very quickly. The power conversion efficiencies of NF OSCs have now reached a value of over 13%, which is higher than the best fullerene-based OSCs. NF acceptors show great tunability in absorption spectra and electron energy levels, providing a wide range of new opportunities. The coexistence of low voltage losses and high current generation indicates that new regimes of device physics and photophysics are reached in these systems. This Review highlights these opportunities made possible by NF acceptors, and also discuss the challenges facing the development of NF OSCs for practical applications.
Polymer semiconductors are an attractive material system for flexible and stretchable electronic devices owing to their potentially favorable mechanical attributes. Establishing the thermomechanical behavior of polymer semiconductors is thus an important consideration to ensure successful operation in these applications. One of the most common mechanical characterization methods for these materials is to manipulate the thin films while on an elastomer substrate. A primary measurement with this approach is the film’s crack onset strain (COS), a measure of ductility. It is simple and effective; however, it is a highly qualitative view of film mechanical stability, particularly in flexible device applications. Alternatively, cohesive fracture energy (Gc) provides a direct quantitative measure of the mechanical integrity of the film. While fracture energy provides important insight into mechanical stability, it typically requires a more complex measurement method than the film on elastomer tests. Here, we compare the COS using film on elastomer testing, with cohesive fracture energy measured using four-point bending for a range of polymer semiconductor films. The polymers considered have a range of molecular structures and molecular packing characteristics providing a broad representative sample set. The values of Gc ranged from 0.4 to 18 J/m² while COS ranged from 2% to over 100%. We show that COS of the films can be correlated with Gc providing support that COS is a valuable measurement to probe the mechanical toughness of polymer semiconductor films. We also discuss the physical characteristics each measurement highlights and the complementary nature of these measurements.
The ability to use infrared imaging systems with multicolor capabilities, high photoresponsivity and polarization sensitivity, is central to practical photodetectors and has been demonstrated with conventional devices based on Ⅲ-Ⅴ or Ⅱ-Ⅵ semiconductors. However, the photodetectors working at room temperature with high responsivity for polarized infrared light detection remains elusive. Here, we first demonstrate a broadband photodetector using a vertical photogate heterostructure of BP-on-WSe2 (black phosphorus-on-tungsten diselenide) in which BP serves as the photogate and WSe2 as the conductive channel. Ultrahigh visible and infrared photoresponsivity at room temperature can reach up to ~10³ A/W and ~5×10⁻¹ A/W, respectively, and ultrasensitive visible and infrared specific detectivity is obtained up to ~10¹⁴ and ~10¹⁰ Jones respectively at room temperature. Moreover, the high sensitivity to infrared polarization is about 40 mA/W with incident light polarized along the horizontal axis (defined as 0° polarization). This performance is due to the strong intrinsic linear dichroism of BP and the device design which can sufficiently collect the photoinduced carriers isotropically, as well as the influence from the orientation of the edge of the BP-on-WSe2 overlapped area which is the same for all polarizations. The high responsivity, good sensitive detectivity and highly polarization-sensitive infrared photoresponse suggest that the photodetectors based on photogate structure afford new opportunities for infrared detecting or imaging at room temperature by using two-dimensional materials.
A series of electron-acceptor polymers, copolymers and blends were used in all-polymer BHJ photodetectors and the effect of acceptor compositions on the key device parameters of dark current density and photocurrent was investigated. Compared with acceptor polymers and polymer blends, the devices based on acceptor copolymer showed lowered dark current and higher photocurrent, due to optimal molecular stacking and morphology of the BHJ active layer. The acceptor blends tend to cause a large phase separation and rough surface of the active layer, thus leading to a low detectivity of the device. Among all the acceptor compositions studied in this work, the all-polymer BHJ photodetector based on a donor polymer (PolyD) and an acceptor copolymer (PolyAA′50) exhibited the highest specific detectivity of over 10¹² Jones in the spectral region of 320–980 nm under -0.1 V bias.
Low noise current is critical for achieving high-detectivity organic photodetectors. Inserting charge blocking layers is an effective approach to suppress the reverse-biased dark current. However, in solution-processed organic photodetectors, the charge transport material needs to be dissolved in solvents that don't dissolve the underneath light-absorbing layer, which is not always possible for all kinds of light absorbing materials developed. Here, we introduce a universal strategy of transfer-printing a conjugated polymer poly(3-hexylthiophene) (P3HT) as the electron-blocking layer to realize highly sensitive photodetectors. The transfer-printed P3HT layers substantially and universally reduced the reverse-biased dark-current by about three orders of magnitude for various photodetectors with different active layers. These photodetectors can detect the light signal as weak as several picowatt per square centimeter, and the device detectivity is over 1012 Jones. The results suggest that the strategy of transfer-printing P3HT films as the electron-blocking layer is universal and effective for the fabrication of sensitive organic photodetectors.
An intrinsic coincident full-Stokes polarimeter is demonstrated by using strain-aligned polymer-based organic photovoltaics (OPVs) that can preferentially absorb certain polarized states of incident light. The photovoltaic-based polarimeter is capable of measuring four Stokes parameters by cascading four semitransparent OPVs in series along the same optical axis. This in-line polarimeter concept potentially ensures high temporal and spatial resolution with higher radiometric efficiency as compared to the existing polarimeter architecture. Two wave plates were incorporated into the system to modulate the S3 Stokes parameter so as to reduce the condition number of the measurement matrix and maximize the measured signal-to-noise ratio. Radiometric calibration was carried out to determine the measurement matrix. The polarimeter presented in this paper demonstrated an average RMS error of 0.84% for reconstructed Stokes vectors after normalized to S0. A theoretical analysis of the minimum condition number of the four-cell OPV design showed that for individually optimized OPV cells, a condition number of 2.4 is possible.
The mechanical properties of organic electronic materials and interfaces play a central role in determining the manufacturability and reliability of flexible and stretchable organic electronic devices. The synergistic effects of mechanical stress and deformation, together with other operating parameters such as temperature and temperature cycling, and exposure to solar radiation, moisture, and other environmental species are particularly important for longer-term device stability. We review recent studies of basic mechanical properties such as adhesion and cohesion, stiffness, yield behavior, and ductility of organic semiconducting materials, and their connection to underlying molecular structure. We highlight thin-film metrologies to probe the mechanical behavior, including when subjected to simulated operational conditions. We also report on strategies for improving reliability through interface engineering and tailoring material chemistry and molecular structure. These studies provide insights into how these metrologies and metrics inform the development of materials and devices for improved reliability.
Power conversion efficiency (PCE) has surpassed 10% for single junction organic solar cells (OSCs) mainly through the design and synthesis of novel donor materials, the optimization of film morphology and the evolution of the devices. However, the development of novel acceptor materials is relatively sluggish compared with the donor compounds. Nowadays, fullerene derivatives, such as PC61BM and PC71BM, are still the dominant acceptors due to their superior charge transporting properties. Unfortunately, these two acceptors suffer from some intrinsic shortcomings such as limited absorption, difficult functionalization, and high production cost. Therefore, developing novel non-fullerene acceptors that can overcome the above-mentioned disadvantages is highly desirable. As a matter of fact, research on non-fullerene acceptors has made considerable progress in the last two years and a highest PCE of around 12% has been achieved. In this review, we will summarize recent research progress in non-fullerene small molecule acceptors and compare these molecules' performances in OSCs employing the same donor materials. Moreover, the acceptors with excellent photovoltaic performance are highlighted and the reasons are elaborated. Finally, the implications and the challenges are proposed.
Low-cost organic photodetectors have shown sensitivity levels comparable to those of inorganic photodetectors, but with response speeds generally limited to the megahertz range due to the low mobility of organic semiconductors. Here, we integrated organic-inorganic hybrid perovskite (OIHP) photoactive layers with low-bandgap organic bulk-heterojunction (BHJ) layers to produce a device that combined the advantages of the two types of photodetectors. Integrating methylammonium lead triiodide (CH3NH3PbI3) with a low-bandgap BHJ layer extended the response of perovskite photodetectors to a wavelength of 1000 nanometers without deteriorating the responsivity and specific detectivity of either type of photodetector. The high mobility of charge carriers in CH3NH3PbI3 allowed the constraints of the resistance-capacitance constant to be relieved so that the device response speed could be increased dramatically. A response time of five nanoseconds was measured for incident infrared light from the device with an active area of 0.1 square millimeters, which represents the state-of-the-art performance for organic-based photodetectors.
Three acceptor–acceptor (A–A) type conjugated polymers based on isoindigo and naphthalene diimide/perylene diimide are designed and synthesized to study the effects of building blocks and alkyl chains on the polymer properties and performance of all-polymer photoresponse devices. Variation of the building blocks and alkyl chains can influence the thermal, optical, and electrochemical properties of the polymers, as indicated by thermogravimetric analysis, differential scanning calorimetry, UV–vis, cyclic voltammetry, and density functional theory calculations. Based on the A–A type conjugated polymers, the most efficient all-polymer photovoltaic cells are achieved with an efficiency of 2.68%, and the first all-polymer photodetectors are constructed with high responsivity (0.12 A W−1) and detectivity (1.2 × 1012 Jones), comparable to those of the best fullerene based organic photodetectors and inorganic photodetectors. Photoluminescence spectra, charge transport properties, and morphology of blend films are investigated to elucidate the influence of polymeric structures on device performances. This contribution demonstrates a strategy of systematically tuning the polymeric structures to achieve high performance all-polymer photoresponse devices.
The reduction of dark current is required to enhance the signal-to-noise ratio and decrease the power consumption in photodetectors. This is typically achieved by introducing additional functional layers to suppress carrier injection, a task that proves to be challenging especially in printed devices. Here we report on the successful reduction of dark current below 100 nA cm−2 (at −1 V bias) in an inkjet printed photodetector by the insertion of an electron blocking layer based on poly[3-(3,5-di-tert-butyl-4-methoxyphenyl)-thiophene], while preserving a high quantum yield. Furthermore, the electron blocking layer strongly increases the surface energy of the hydrophobic photoactive layer, therefore simplifying the printing of transparent top electrodes from water based formulations without the addition of surfactants.
Broad-response and high-detectivity for all-polymer photodetectors based on p- and n-type semiconducting polymers have been achieved through optimization of polymer property and film microstructure. The electron-donating units in the p-type polymers affect a great deal of the polymer properties such as solubility, absorption spectra, and electronic energy levels, which in turn can influence the device performance. The polymer (P3) based on dithienopyrrole and diketopyrrolopyrrole is most promising for photodetector applications, as it possesses the suitable energy level with regard to the n-type polymer and exhibits appropriate film morphology and molecular stacking with aid of 1,8-diiodooctane as an additive during film processing. The photodetector based on P3/poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDI) exhibits good response from 300 to 1100 nm and nearly constant detectivity (D*) above 10¹² Jones at 330–980 nm, rendering a great potential of all-polymer photodetectors for practical applications. (Figure presented.).
The specific optical absorption of an organic semiconductor is critical to the performance of organic optoelectronic devices. For example, higher light-harvesting efficiency can lead to higher photocurrent in solar cells that are limited by sub-optimal electrical transport. Here, we compare over 40 conjugated polymers, and find that many different chemical structures share an apparent maximum in their extinction coefficients. However, a diketopyrrolopyrrole-thienothiophene copolymer shows remarkably high optical absorption at relatively low photon energies. By investigating its backbone structure and conformation with measurements and quantum chemical calculations, we find that the high optical absorption can be explained by the high persistence length of the polymer. Accordingly, we demonstrate high absorption in other polymers with high theoretical persistence length. Visible light harvesting may be enhanced in other conjugated polymers through judicious design of the structure.
Polymer semiconductors based on donor-acceptor monomers have recently resulted in significant gains in field effect mobility in organic thin film transistors (OTFTs). These polymers incorporate fused aromatic rings and have been designed to have stiff planar backbones, resulting in strong intermolecular interactions, which subsequently result in stiff and brittle films. The complex synthesis typically required for these materials may also result in increased production costs. Thus, developing methods to improve mechanical plasticity while lowering material consumption during fabrication will significantly improve opportunities for adoption in flexible and stretchable electronics. To achieve these goals, we consider blending a brittle donor¬-acceptor polymer poly[4(4,4dihexadecyl4Hcyclopenta[1,2b:5,4b’]dithiopen2yl)alt[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT) with ductile poly(3hexylthiophene). We find that the ductility of the blend film is significantly improved compared to neat PCDTPT films, and when employed in an OTFT, the performance is largely maintained. The ability to maintain charge transport character is due to vertical segregation within the blend, while the improved ductility is achieved due to intermixing of the polymers throughout the film thickness. Importantly, applying large strains to the ductile films is then shown to orient both polymers, which further increases charge carrier mobility. These results highlight a processing approach to achieve high performance polymer OTFTs that are electrically and mechanically optimized.
This chapter reviews the status of heterogeneous integration of silicon waveguides and photodetectors. The most commonly used available fabrication technologies, namely germanium growth and III/V-silicon wafer bonding, are described. Design constraints common to both platforms, as well as the key differences between them, are discussed. An overview of demonstrated devices on both platforms is presented.
High efficiency all-polymer solar cells with less thickness-dependent behavior are demonstrated, by using low bandgap n-type conjugated polymer N2200 as acceptor and an absorption-complementary difluorobenzotriazole-based medium bandgap polymer J51 as donor.
All-printed organic photodiode arrays on plastic are reported with average specific detectivities of 3.45 × 10(13) cm Hz(0.5) W(-1) at a bias of -5 V. The blade-coated polyethylenimine cathode interlayer and active layer and screen-printed anode enable precise device performance tunability and excellent homogeneity at centimetric scales. These devices' high operational reverse bias, good linear dynamic range, and bias stress stability make them attractive for implementation in imaging systems.
Bulk heterojunction (BHJ) organic solar cells are fabricated with the polymer semiconductor aligned in the plane of the film to probe charge recombination losses associated with aggregates characterized by varying degrees of local order. 100% uniaxial strain is applied on ductile poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) BHJ films and characterize the resulting morphology with ultraviolet-visible absorption spectroscopy and grazing incidence X-ray diffraction. It is found that the strained films result in strong alignment of the highly ordered polymer aggregates. Polymer aggregates with lower order and amorphous regions also align but with a much broader orientation distribution. The solar cells are then tested under linearly polarized light where the light is selectively absorbed by the appropriately oriented polymer, while maintaining a common local environment for the sweep out of photogenerated charge carriers. Results show that charge collection losses associated with a disordered BHJ film are circumvented, and the internal quantum efficiency is independent of P3HT local aggregate order near the heterojunction interface. Uniquely, this experimental approach allows for selective excitation of distinct morphological features of a conjugated polymer within a single BHJ film, providing insight into the morphological origin of recombination losses.
Water-soluble cadmium telluride (CdTe) quantum dots (QDs) used as an anode interlayer in solution-processed near infrared (NIR) polymer photodetectors (PDs) were demonstrated. Polymer PDs incorporated with CdTe QDs as an anode interlayer exhibited 10-fold suppressed dark current density and analogous photocurrent density relative to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which resulted in enhanced detectivities over 10(11) Jones in the spectral range from 350 nm to 900 nm. Moreover, with the substitution of PEDOT:PSS by CdTe QDs, the stability of unencapsulated NIR polymer PDs was extended up to 650 hours, which is more than 3 times longer than those with PEDOT:PSS as an anode interlayer. These results indicated that CdTe QDs can be utilized as a solution-processable alternative to PEDOT:PSS as an anode interlayer for high performance NIR polymer PDs.
A novel method of strain-aligning polymer films is introduced and applied to regioregular poly(3-hexylthiophene) (P3HT), showing several important features of charge transport. The polymer backbone is shown to align in the direction of applied strain resulting in a large charge-mobility anisotropy, where the in-plane mobility increases in the applied strain direction and decreases in the perpendicular direction. In the aligned film, the hole mobility is successfully represented by a two-dimensional tensor, suggesting that charge transport parallel to the polymer backbone within a P3HT crystal is strongly favored over the other crystallographic directions. Hole mobility parallel to the backbone is shown to be high for a mixture of plane-on and edge-on packing configurations, as the strain alignment is found to induce a significant face-on orientation of the originally highly edge-on oriented crystalline regions of the film. This alignment approach can achieve an optical dichroic ratio of 4.8 and a charge-mobility anisotropy of 9, providing a simple and effective method to investigate charge-transport mechanisms in polymer semiconductors.
Anisotropic charge transport, or polarized photo- and electroluminescence are possible applications for materials combining the optical properties of arylenevinylene polymers with the characteristic orientational order of the liquid-crystalline state. Thermally stale, soluble materials with a broad mesophase, which can be oriented macroscopically using conventional processing techniques and when exhibit highly anisotropic luminescence behavior of use in polarized LEDs are presented.
While organic electronics is mostly dominated by light-emitting diodes, photovoltaic cells and transistors, optoelectronics properties peculiar to organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in the field of light signal detection. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on virtually every substrate, which might enable innovative optoelectronic systems to be targeted for instance in the field of imaging, optical communications or biomedical sensing.
In this review, after a brief resume of photogeneration basics and of devices operation mechanisms, we offer a broad overview of recent progress in the field, focusing on photodiodes and phototransistors. As to the former device category, very interesting values for figures of merit such as photoconversion efficiency, speed and minimum detectable signal level have been attained, and even though the simultaneous optimization of all these relevant parameters is demonstrated in a limited number of papers, real applications are within reach for this technology, as it is testified by the increasing number of realizations going beyond the single-device level and tackling more complex optoelectronic systems. As to phototransistors, a more recent subject of study in the framework of organic electronics, despite a broad distribution in the reported performances, best photoresponsivities outperform amorphous silicon-based devices. This suggests that organic phototransistors have a large potential to be used in a variety of optoelectronic peculiar applications, such as a photo-sensor, opto-isolator, image sensor, optically controlled phase shifter, and opto-electronic switch and memory.
Homogeneous alignment of poly(9,9-dioctylfluorene) films on thin layers of rubbed precursor-route poly(p-phenylenevinylene) allows the construction of light-emitting diodes that emit highly polarized blue light (λem = 458 nm). The rubbed poly(p-phenylenevinylene) acts as an effective hole-injecting alignment layer. Annealing of poly(9,9-dioctylfluorene) in its nematic phase followed by rapid quenching orients the polymer as a glassy monodomain on the alignment layer and gives devices with a polarization ratio of 25:1 and a luminance of up to 250 cd/m2. © 2000 American Institute of Physics.
Field-effect transistors fabricated from semiconducting conjugated polymers are candidates for flexible and low-cost electronic applications. Here, we demonstrate that the mobility of high molecular weight (300 kDa) regioregular, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] can be significantly improved by introducing long-range orientation of the polymer chains. By annealing for short periods, hole mobilities of 6.7 cm2/Vs have been demonstrated. The transport is anisotropic, with higher mobility (approx. 6:1) parallel to the polymer backbone than perpendicular to the polymer backbone.
Highly oriented films of an electron accepting polymer semiconductor, poly{[N,N´-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5´-(2,2´-bithiophene)} (PNDI2OD-T2), are obtained by two different methods, namely directional epitaxial crystallization (DEC) on 1,3,5-trichlorobenzene (TCB) and epitaxy on friction transferred poly(tetrafluoroethylene) (PTFE) substrates. Two distinct polymorphs with unprecedented intra-chain resolution are identified by high resolution transmission electron microscopy (HR-TEM). Form I is obtained by DEC on TCB, whereas highly oriented films of form II are obtained on PTFE substrates after melting at T=300°C and cooling at 0.5K/min. In form I, both electron diffraction and HR-TEM indicate a segregated stacking of bithiophene (T2) and naphthalene diimide (NDI) units forming separate columns. In form II, a ∼c/2 shift between successive π-stacked chains leads to mixed π-overlaps of T2 and NDI. Form I can be transformed into form II by annealing at T>250°C. The different π-stacking of NDI and T2 in the two polymorphs have characteristic signatures in the UV-vis spectra, especially in the charge transfer band around 750nm which is also observed in spin coated films.
Understanding the complex interplay between the 3D structural hierarchy within thin films of conjugated polymers and the properties of devices based thereon is starting to be recognized as an important challenge in the continued development of these materials for a range of applications. As a result, for example, accurate measurements of molecular orientation and elucidation of its influence on optical characteristics are of significant interest. Here we report an improved optical method to determine both the order parameter and the angle between the polymer backbone director and the optical transition dipole moment for the lowest energy π–π* absorption peak in uniaxially aligned thin films of conjugated polymers. The method uses a combination of polarized Raman spectroscopy and UV-vis spectroscopy and is based on a general theoretical treatment to describe the expected Raman and optical absorption anisotropies of such films. It is applied to study the orientation within thermotropically aligned films of the electroluminescent fluorene-based copolymer poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). A more highly axial transition dipole moment is found for the dominant long wavelength absorption peak of F8T2 compared to that of other fluorene-based (co)polymers. The angle between the polymer backbone director and the transition dipole is estimated to be β ≤ 3°, a deduction that helps to explain the relatively large optical dichroism for aligned films of F8T2 and that offers the prospect of highly polarized electroluminescence from F8T2-based light-emitting diodes.
The organic electroluminescent devices emitting polarized light show potential as backlights in conventional liquid-crystal displays. The necessary high polarization ratios require, in turn, highly aligned emitters. This article discusses the various methods of orientation together with the materials to which they are applicable.