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This work studied a series of infrared detectors comprised of organic bulk heterojunctions to explain the origin of their broadband spectral response from the visible to the infrared spanning 1 to 8 μm and the transition from photonic to bolometric operation. Through comparisons of the detector current and the sub-bandgap density of states, the mid- and long-wave infrared response was attributed to charge trap-and-release processes that impact thermal charge generation and the activation energy of charge mobility. We further demonstrate how the sub-bandgap characteristics, mobility activation energy, and effective bandgap are key design parameters for controlling the device temperature coefficient of resistance, which reached up to -7%/K, better than other thin-film materials such as amorphous silicon and vanadium oxide.

Photovoltaic structures of the bulk heterojunction type were fabricated, in which derivatives of N,N-diethylamine-3-Methyl-1-Phenyl-1H-pyrazolo[3,4-b]quinoxalines were used as the active layer. The compounds differed in the position of the electron-donating substituent in the carbocyclic ring. Four isomers were subjected to UV-Vis spectrophotometric measurements in solvents of different polarities. The absorption characteristics were experimentally determined for the tested derivatives. The values of HOMO-LUMO levels were determined by means of quantum chemical calculations using the HyperChem software. The current–voltage and dispersion characteristics of the produced OPV were determined. The spectral characteristics of the refractive indices and extinction coefficients of the active layers were determined using the spectroscopic ellipsometry method. These results were used in the analysis and optimization of photovoltaic structures. It was shown that the location of the N,N-diethylamine substituent affects the photophysical properties of the structure and the photovoltaic properties. The optimization of the OPV_2 photovoltaic structure using the coherent model and the 2 × 2 matrix method can be successfully used in modeling optical multilayer structures, including photovoltaic structures.

We report an investigation into the impact of applying a large reverse bias voltage during current–voltage (J-V) sweeps on the degradation rate of solar cells based on poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6-6]-phenyl C71-butyric acid methyl ester (PC70BM), under air mass 1.5 simulated solar irradiation. The degradation rate was found to be significantly slower when devices were characterized using frequent J-V sweeps with a large reverse bias component (-7 V to 1 V), with a decrease in power conversion efficiency of 24% after ∼ 100 hrs of light exposure. In contrast, devices scanned from −1 V to 1 V degrade by 72% in the same time period. The decay in the photovoltaic performance was found to be related to an increase in the series resistance and a decrease in the shunt resistance of the devices over time. Further characterization of the bulk heterojunction layer under irradiation conditions indicates that this behavior might be caused by the decreasing mobility of the charge carriers due to the formation of the defects or traps in the layer, induced by the photo-oxidation process. The frequent application of a large reverse bias sweep most likely helps de-trap the charge carriers and slows down the device degradation. Impedance analysis indicated that during the decay of PCDTBT:PC70BM devices the bulk layer became more resistive with time, and applying large and frequent reverse bias sweeps during testing could significantly slow down this process. The charge carrier extraction time was found to increase from 7 μs to more than 400 μs after photo-degradation. These findings demonstrate that repeated application of a reverse bias voltage can affect device degradation, and therefore that the specific details of J-V characterization should be reported alongside device lifetime claims.

Two new random donor-acceptor (D-A) copolymers, signed as P1 and P2, were designed and synthesized. Electrochemical and spectroelectrochemical measurements were performed to investigate absorption, energy levels, electronic and optical band gaps for comparison. The polymers were used as donor polymers in the active layer to fabricate non-fullerene, bulk heterojunction (BHJ) organic photovoltaics (OPVs). Investigations were carried out through the conventional BHJ structure; ITO/PEDOT: PSS/Active Layer/LiF/Al, where active layer consists of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC) as the acceptor and thienopyrroledione containing donors. The device based on P1:ITIC(1:1) blend with a thickness of 161 nm gave the best performance with a power conversion efficiency (PCE) of 7.94%, an open-circuit voltage (V-OC) of 0.86 V, a short-current density (J(SC)) of 18.45 mA cm(-2) and a fill factor (FF) of 50.12%. The highest PCE obtained from P2 based organic solar cell is 1.96%. P2 exhibited low solubility attributed to the lack of alkyl groups enhancing polymer solubility, electronic properties, and photovoltaic performances. The research outputs exhibit that introduction of alkyl chains on the polymer backbone can enhance device performance. (C) 2021 Elsevier Ltd. All rights reserved.

Organic photovoltaics are typically composed of at least four different materials, including the donor and acceptor components of the bulk heterojunction, the interfacial layers at each electrode, the electrodes themselves, and solution additives that may persist in the final sandwich structure. The interplay of surface energies between these different layers is profoundly complex, as the deposition and annealing of one layer on top of another may be influenced by the surface energy of these interfaces. While the energy levels of one layer with respect to adjacent layers are important to facilitate charge separation and collection at the electrodes, the relative surface energies of the interfaces in contact with the multicomponent bulk heterojunction can be beneficial or disadvantageous, or be neutral, with respect to the performance of the OPV device. Because the bulk heterojunction is a mixture of donor and acceptor polymers and/or small molecules, the accumulation of one of the components on the underlying electrode interface can be driven by surface energy considerations. A donor- or acceptor-rich interface may affect charge carrier flow to the electrode, thus affecting the overall efficiency. Here, ITO/PEDOT:PSS electrodes in forward organic photovoltaic devices are treated with five different thin interfacial layers to change the relative surface energy of this electrode with respect to the adjacent bulk heterojunction. Contact angle measurements with four probe liquids enable calculation of the surface energies, and the results are compared with the performance of forward-biased organic photovoltaic devices. Time-of-flight secondary ion mass spectrometry results substantiate the predictions of gradients in the bulk heterojunction layers, and grazing-incidence wide-angle X-ray scattering measurements show the impact on the polymer crystallites. Thus, a simple algorithm based on surface energy considerations may inform which interfacial layer for a given bulk heterojunction in an organic photovoltaic device can be the most appropriate.

Conjugated polymers are promising low-cost, lightweight, and flexible candidates for scalable photovoltaic applications to establish decarbonized energy technologies. However, they possess deficiencies in terms of their lower charge mobility and exciton diffusion length compared to their inorganic counterparts, impeding the efficient charge extraction at high active layer thickness values. In this manner, active layer composition should be tuned to improve light harvesting enabling efficient charge transport. This work presents two new approaches to achieve higher photovoltaic performance for organic photovoltaic systems; thiol modification of the polymers for improved morphological features, and incorporation of ligand-free gold nanoparticles with surface plasmon absorption into the active layer to be stabilized by the covalent interaction with the thiol side groups of the polymers. To achieve this goal, a benzoxadiazole bearing polymer (POxT) and its bromine (POxT-Br) and thiol (POxT-SH) comprising derivatives were synthesized, their electrochemical, optical, photovoltaic, and morphological characterizations were performed. For photovoltaic characterizations, conventional device architecture of ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al was utilized, where the POxT-SH showed the highest JSC and PCE values, 6.52 mA/cm2 and 2.71%, respectively. Gold nanoparticles were synthesized via laser ablation method, and upon incorporation, the PCE value was boosted to 3.29%, with an increase of 21.4% compared to POxT-SH comprising organic solar cells.

The properties of organic heterostructures with mixed layers made of arylenevinylene-based polymer donor and non-fullerene perylene diimide acceptor, deposited using Matrix Assisted Pulsed Laser Evaporation on flat Al and nano-patterned Al electrodes, were investigated. The Al layer electrode deposited on the 2D array of cylindrical nanostructures with a periodicity of 1.1 µm, developed in a polymeric layer using UV-Nanoimprint Lithography, is characterized by an inflorescence-like morphology. The effect of the nanostructuring on the optical and electrical properties was studied by comparison with those of the heterostructures based on a mixed layer with fullerene derivative acceptor. The low roughness of the mixed layer deposited on flat Al was associated with high reflectance. The nano-patterning, which was preserved in the mixed layer, determining the light trapping by multiple scattering, correlated with the high roughness and led to lower reflectance. A decrease was also revealed in photoluminescence emission both at UV and Vis excitation of the mixed layer, with the non-fullerene acceptor deposited on nano-patterned Al. An injector contact behavior was highlighted for all Al/mixed layer/ITO heterostructures by I-V characteristics in dark. The current increased, independently of acceptor (fullerene or non-fullerene), in the heterostructures with nano-patterned Al electrodes for shorter conjugation length polymer donors.

Necessary advancements in the area of organic photovoltaic (OPV) devices include the upgrade of power conversion efficiencies (PCE) and stability. One answer to these demands lies in the research into new absorbers. Here, we focus on the development of new small molecule absorbers from the group of squaraines (SQs). These modular absorbers can be applied as donors in organic solar cells and have the ability to utilize a broad range of solar radiation if blended with suitable acceptors. In order to allow for the compatibility and favorable organization of donor and acceptor in the absorber layer, we intend to optimize the structure of the SQ by varying the groups attached to the squaric acid core. For that purpose, we accordingly developed a well-suited synthesis route. The novel alkyl- and benzyl-substituted aryl aminosquaraines were synthesized through an improved and eco-friendly procedure. Special emphasis was placed on optimizing the amination reaction to obtain initial precursors in the synthesis of squaraine, avoiding hitherto common catalytic processes. All SQ precursors and SQ products were completely described. The derived SQs were additionally characterized in thin-film configuration using cyclic voltammetry and UV-VIS spectroscopy and then processed to prepare self-standing bulk heterojunction (BHJ) thin films in conjunction with fullerene-based electron acceptors, which were characterized via profilometry. The comparison between SQ and BHJ solutions and thin films, using atomic force microscopy and UV-VIS spectroscopy, revealed differences in susceptibility for the organization and orientation of the constituting domains.

1-Chloronaphthalene (CN) has been a common solvent additive in both fullerene-and nonfullerene-based organic solar cells. In spite of this, its working mechanism is seldom investigated, in particular, during the drying process of bulk heterojunctions composed of a donor:acceptor mixture. In this work, the role of CN in all-polymer solar cells is investigated by in situ spectroscopies and ex situ characterization of blade-coated PBDB-T:PF5-Y5 blends. Our results suggest that the added CN promotes self-aggregation of polymer donor PBDB-T during the drying process of the blend film, resulting in enhanced crystallinity and hole mobility, which contribute to the increased fill factor and improved performance of PBDB-T:PF5-Y5 solar cells. Besides, the nonradiative energy loss of the corresponding device is also reduced by the addition of CN, corresponding to a slightly increased open-circuit voltage. Overall, our observations deepen our understanding of the drying dynamics, which may guide further development of all-polymer solar cells. Funding Agencies|Knut and Alice Wallenberg Foundation [2016.0059, 2017.0186]; Swedish Government Strategic Research Area in Material Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Swedish Research Council [2017-04123, 2016-06146, 2019-04683, 2021-04798]; China Scholarship Council; Swedish Research Council Formas; Swedish Energy Agency [48598]; Research Grants Council (RGC) of Hong Kong [14303519]

Fused-ring core non-fullerene acceptors (NFAs), designated "Y-series", have enabled high-performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5 to 18% PCE, e.g., in the case of PM6:Y6 OSCs. Here, we introduce a combined solid-state NMR, crystallography, and molecular modelling approach to elucidate the atomic-scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends. We show the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self-assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused-ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular-level understanding of BHJs enabled by this approach will guide the engineering of next-generation NFAs for stable and efficient OSCs. This article is protected by copyright. All rights reserved.

[Abstract] The widespread use of nonfullerene-based electron-accepting materials has triggered a rapid increase in the performance of organic photovoltaic devices. However, the number of efficient acceptor compounds available is rather limited, which hinders the discovery of new, high-performing donor:acceptor combinations. Here, we present a new, efficient electron-accepting compound based on a hitherto unexplored family of well-known molecules: gold porphyrins. The electronic properties of our electron-accepting gold porphyrin, named VC10, were studied by UV−Vis spectroscopy and by cyclic voltammetry (CV) , revealing two intense optical absorption bands at 500−600 and 700−920 nm and an optical bandgap of 1.39 eV. Blending VC10 with PTB7-Th, a donor polymer, which gives rise to an absorption band at 550−780 nm complementary to that of VC10, enables the fabrication of organic solar cells (OSCs) featuring a power conversion efficiency of 9.24% and an energy loss of 0.52 eV. Hence, this work establishes a new approach in the search for efficient acceptor molecules for solar cells and new guidelines for future photovoltaic material design F.L. and P.d.l.C. thank MCI (Spain) (PID2019-105049RB-I00), MICIU (RED2018-102815-T), the Junta de Comunidades de Castilla-La Mancha, and the European Social Fund (SBPLY/17/180501/000254) for financial support. V.C. thanks MECD for an FPU grant (FPU15/02170). G.D.S. thanks the Government of India SERI-DST (DST/TMD/SERI/D05(c)) for financial support. J.M. thanks the MICINN for grant PGC2018-094620-A-I00 Junta de Comunidades de Castilla-La Mancha; SBPLY/17/180501/000254 Government of India; DST/TMD/SERI/D05(c)

The fluorination strategy is one of the most efficient and popular molecular modification methods to develop new materials for organic photovoltaic (OPV) cells. For OPV materials, it is a broad agreement that fluorination can reduce the energy level and change the morphology of active layers. To explore the effect of fluorination on small molecule acceptors, we selected two non-fullerene acceptors (NFA) based bulk heterojunction (BHJ) films, involving PM6:Y6 and PM6:Y5 as model systems. The electron mobilities of the PM6:Y5 and PM6:Y6 BHJ films are 5.76 × 10−7 cm2V−1s−1 and 5.02 × 10−5 cm2V−1s−1 from the space-charge-limited current (SCLC) measurements. Through molecular dynamics (MD) simulation, it is observed that halogen bonds can be formed between Y6 dimers, which can provide external channels for electron carrier transfer. Meanwhile, the “A-to-A” type J-aggregates are more likely to be generated between Y6 molecules, and the π–π stacking can be also enhanced, thus increasing the charge transfer rate and electron mobility between Y6 molecules.

The performance of organic solar cells strongly depends on the bulk-heterojunction (BHJ) morphology of the photoactive layer. This BHJ forms during the drying of the wet-deposited solution, because of physical processes such as crystallization and/or liquid-liquid phase separation (LLPS). However, the process-structure relationship remains insufficiently understood. In this work, a recently developed, coupled phase-field-fluid mechanics framework is used to simulate the BHJ formation upon drying. For the first time, this allows to investigate the interplay between all the relevant physical processes (evaporation, crystal nucleation and growth, liquid demixing, composition-dependent kinetic properties), within a single coherent theoretical framework. Simulations for the model system P3HT-PCBM are presented. The comparison with previously reported in situ characterization of the drying structure is very convincing: The morphology formation pathways, crystallization kinetics, and final morphology are in line with experimental results. The final BHJ morphology is a subtle mixture of pure crystalline donor and acceptor phases, pure and mixed amorphous domains, which depends on the process parameters and material properties. The expected benefit of such an approach is to identify physical design rules for ink formulation and processing conditions to optimize the cell's performance. It could be applied to recent organic material systems in the future.

We investigated the effect of adding p-anisaldehyde (AA) solvent to the ink containing poly[[2,60-4,8-di(5-ethylhexylthienyl)benzo[1,2-b:3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7-Th) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:20,30-d0]-s-indaceno[1,2-b:5,6-b0]-dithiophene(ITIC) on the morphology of the active layer. The present study focuses on determining the effect of the additive on the compositions at the surface of the PTB7-Th: ITIC composite and its morphology, forming one side of the interface of the blend with the MoOX electrode, and the influence of the structural change on the performance of devices. Studies of device performance show that the addition of the additive AA leads to an improvement in device performance. Upon the addition of AA, the concentration of PTB7-Th at the surface of the bulk heterojunction (BHJ) increases, causing an increase in surface roughness of the surface of the BHJ. This finding contributes to an understanding of the interaction between the donor material and high work function electrode/interface material. The implications for the interface are discussed.

Rapid development of organic electrochemical transistor (OECTs)-based circuits bring new opportunities for next-generation integrated bioelectronics. All polymer bulk heterojunction (BHJ) offers an attractive, inexpensive alternative to achieve efficient ambipolar OECTs, and building blocks of logic circuits constructed from them, but have not been investigated till date. Here, we report the first all polymer BHJ-based OECTs, consisting of a blend of new p-type ladder conjugated polymer and the state-of-the-art n-type ladder polymer. The optimized BHJ OECTs exhibit balanced ambipolar transistor performance, having uC* values of 2.72±1.04 and 1.36±0.81 F cm-1 V-1 s-1 as p-type and n-type channels, respectively. The whole ladder-type polymer BHJ also proved that side chains are not necessary for good ion transport. Instead, the polymer nanostructures play a critical role in the ion penetration and transport and thus the device performance. It also provides a facile strategy and simplifies the fabrication process, forgoing the need to pattern multiple active layers. In addition, the development of complementary metal–oxide–semiconductor (CMOS)-like OECTs allows us to pursue advanced functional logic circuitry, including inverter and NAND gate as well as for amplifying electrophysiology signals. Our work opens a new approach in the design of new materials for OECTs and will contribute to the development of organic heterojunctions for ambipolar OECTs towards high performing logic circuits. Ministry of Education (MOE) Submitted/Accepted version W.L.L acknowledges funding support from Ministry of Education (MOE) under AcRF Tier 2 grant (2019-T2-2-106) and National Robotics Programme (W1925d0106).).

Understanding the exciton dissociation process in organic solar cells is a fundamental issue for the design of high-performance photovoltaic devices. In this article, a parameterized quantum theory based on a coarse-grained tight-binding model plus non-local electron-hole interactions is presented, while the diffusion and recombination of excitons are studied in a square lattice of excitonic states, where a real-space renormalization method on effective chains has been used. The Hamiltonian parameters are determined by fitting the measured quantum efficiency spectra and the theoretical short-circuit currents without adjustable parameters show a good agreement with the experimental ones obtained from several polymer:fullerene and polymer:polymer heterojunctions. Moreover, the present study reveals the degree of polymerization and the true driving force at donor-acceptor interface in each analyzed organic photovoltaic device.

In this paper, characterisation of exciton generation is carried out in three bulk-heterojunction organic solar cells (BHJ OSCs)&mdash, OSC1: an inverted non-fullerene (NF) BHJ OSC, OSC2: a conventional NF BHJ OSC, and OSC3: a conventional fullerene BHJ OSC. It is found that the overlap of the regions of strong constructive interference of incident and reflected electric fields of electromagnetic waves and those of high photon absorption within the active layer depends on the active layer thickness. An optimal thickness of the active layer can thus be obtained at which this overlap is maximum. We have simulated the rates of total exciton generation and position dependent exciton generation within the active layer as a function of the thicknesses of all the layers in all three OSCs and optimised their structures. Based on our simulated results, the inverted NF BHJ OSC1 is found to have better short circuit current density which may lead to better photovoltaic performance than the other two. It is expected that the results of this paper may provide guidance in fabricating highly efficient and cost effective BHJ OSCs.

Solution-processed organic photodetectors (OPDs) have a great potential in light sensing and imaging applications. With this emerging technology reaching performances on par with amorphous silicon (a-Si:H) photodiodes, market introduction is imminent. A major R&D focus in further development of OPD technology is therefore to demonstrate the advantages of organic photodiodes in high-resolution, flexible large-area photodetectors. After a survey of the OPD performance, and an introduction of the concepts of active-matrix detectors and their materials, we will present the recent developments at Holst Centre in realizing two prototypical applications, that is, curved X-ray detectors and biometric (fingerprint and palm) scanners.

Four novel benzo[c][1,2,5]thiadiazole comprising monomers namely 5-fluoro-6-((2-octyldodecyl)oxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TBTT), 5-fluoro-4,7-bis(4-hexylthiophen-2-yl)-6-((2-octyldodecyl)oxy)benzo[c][1,2,5]thiadiazole (HTBTHT), 5-fluoro-4,7-di(furan-2-yl)-6-((2-octyldodecyl)oxy)benzo- [c][1,2,5]thiadiazole (FBTF), and 5-fluoro-6-((2-octyldodecyl)oxy)-4,7-bis(thieno[3,2-b]thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TTBTTT) were designed, and synthesized successfully via Stille polycondensation reaction. The structural characterizations of the monomers were performed by 1H and 13C NMR spectroscopy and High Resolution Mass Spectroscopy (HRMS). The monomers were then electropolymerized in a three electrode cell system via cyclic voltammetry. The electrochemical, and spectroelectrochemical characterization of the polymers were reported in detail. Besides, theoretical calculations were performed to elucidate observed experimental properties. According to the cyclic voltammogram of the polymers, HOMO and LUMO energy levels were calculated as −5.68 eV/−3.91 eV, −5.71 eV/−3.72 eV, −5.61 eV/−4.04 eV, and −5.51 eV/−3.71 eV and the electronic band gaps were 1.77 eV, 1.99 eV, 1.57 eV, and 1.80 eV for PTBTT, PHTBTHT, PFBTF, and PTTBTTT, respectively.

In the present study, vacuum evaporation method is used to deposit Bi2Se3 film onto Si nanowires (NWs) to form bulk heterojunction for the first time. Its photodetector is self-powered, its detection wavelength ranges from 390 nm to 1700 nm and its responsivity reaches its highest value of 84.3 mA/W at 390 nm. In comparison to other Bi2Se3/Si photodetectors previously reported, its infrared detection length is the second longest and its response speed is the third fastest. Before the fabrication of the photodetector, we optimized the growth parameter of the Bi2Se3 film and the best Bi2Se3 film with atomic steps could finally be achieved. The electrical property measurement conducted by the physical property measurement system (PPMS) showed that the grown Bi2Se3 film was n-type conductive and had unique topological insulator properties, such as a metallic state, weak anti-localization (WAL) and linear magnetic resistance (LMR). Subsequently, we fabricated Si NWs by the metal-assisted chemical etching (MACE) method. The interspace between Si NWs and the height of Si NWs could be tuned by Ag deposition and chemical etching times, respectively. Finally, Si NWs fabricated with the Ag deposition time of 60 s and the etching time of 10 min was covered by the best Bi2Se3 film to be processed for the photodetector. The primary n-Bi2Se3/p-Si NWs photodetector that we fabricated can work in a self-powered mode and it has a broadband detection range and fast response speed, which indicates that it can serve as a promising silicon-based near- and mid-infrared photodetector.

We report on the fabrication and study of bulk heterojunction (BHJ) solar cells based on a novel combination of a donor&ndash, acceptor poly(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N0-diphenyl)-N,N&prime, di(p-butyl-oxy-pheyl)-1,4-diamino-benzene) (PFB) and [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) blend composed of 1:1 by volume. indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate (PEDOT:PSS)/PFB&ndash, PCBM/Ag BHJ solar cells are fabricated by a facile cost-effective spin-coating technique. The thickness of the active film (PFB&ndash, PCBM) plays an important role in the efficiency of light absorption, exciton creation, and dissociation into free charges that results in higher power conversion efficiency (PCE). In order to optimize the PCE as a function of active layer thickness, a number of solar cells are fabricated with different thicknesses of PFB&ndash, PCBM films at 120, 140, 160, 180, and 200 nm, and their photovoltaic characteristics are investigated. It is observed that the device with a 180 nm thick film demonstrates a maximum PCE of 2.9% with a fill factor (FF) of 53% under standard testing conditions (STC) (25 &deg, C, 1.5 AM global, and 100 mW/cm2). The current&ndash, voltage (I-V) properties of the ITO/PEDOT:PSS/PFB&ndash, PCBM/Ag BHJ devices are also measured in dark conditions to measure and understand different parameters of the heterojunction. Atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) absorption spectroscopy for the PFB&ndash, PCBM film of optimal thickness (180 nm) are carried out to understand the effect of surface morphology on the PCE and bandgap of the blend, respectively. The AFM micrographs show a slightly non-uniform and rough surface with an average surface roughness (Ra) of 29.2 nm. The UV-vis measurements of the PFB&ndash, PCBM blend exhibit a reduced optical bandgap of &asymp, 2.34 eV as compared to that of pristine PFB (2.88 eV), which results in an improved absorption of light and excitons generation. The obtained results for the ITO/PEDOT:PSS/PFB&ndash, PCBM (180 nm)/Ag BHJ device are compared with the ones previously reported for the P3HT&ndash, PCBM blend with the same film thickness. It is observed that the PFB&ndash, PCBM-based BHJ device has shown two times higher open circuit voltage (Voc) and, hence, enhanced the efficiency.

Organic electronics became an attractive alternative for practical applications in complementary logic circuits due to the unique features of organic semiconductors such as solution processability and ease of large-area manufacturing. Bulk heterojunctions (BHJ), consisting of a blend of two organic semiconductors of different electronic affinities, allow fabrication of a broad range of devices such as light-emitting transistors, light-emitting diodes, photovoltaics, photodetectors, ambipolar transistors and sensors. In this work, the charge carrier transport of BHJ films in field-effect transistors is switched from electron to hole domination upon processing and post-treatment. Low molecular weight n-type N,N&prime, bis(n-octyl)-(1,7&amp, 1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN2) was blended with p-type poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene] (PBTTT-C14) and deposited by spin-coating to form BHJ films. Systematic investigation of the role of rotation speed, solution temperature, and thermal annealing on thin film morphology was performed using atomic force microscopy, scanning electron microscopy, and grazing incidence wide-angle X-ray scattering. It has been determined that upon thermal annealing the BHJ morphology is modified from small interconnected PDI8-CN2 crystals uniformly distributed in the polymer fraction to large planar PDI8-CN2 crystal domains on top of the blend film, leading to the switch from electron to hole transport in field-effect transistors.

Made available in DSpace on 2020-12-12T02:41:54Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-07-31 Low-bandgap polymers are widely used as p-type components in photoactive layers of organic solar cells, due to their ability to capture a large portion of the solar spectrum. The comprehension of their supramolecular assembly is crucial in achieving high-performance organic electronic devices. Here we synthezed two exemplar low-bandgap cyclopentadithiophene (CPDT):diketopyrrolopyrrole (DPP)-based polymers, with either a twelve carbon (C12) or a tri etyleneglycol (TEG) side chains on the DPP units (respectively denoted PCPDTDPP_C12 and PCPDTDPP_TEG). We deposited Langmuir-Schaefer films of these polymers blended with the widely used electron donor material [6,6]-phenyl-C61-butyric-acid methyl ester (PCBM). We then characterized the conformational, optical and morphological properties of these films. From the monolayers to the solid films, we observed distinct self-organization and surface properties for each polymer due to the distinct nature of their side chains. Emphasizing their attraction interactions with PCBM and the phase transitions according to the surface pressure. The elements amount on the surface, calculated through the XPS, gave us a good insight on the polymers' conformations. Through UV-visible absorption spectroscopy, the improvement in the PCPDTDPP film ordering upon PCBM addition is evident and we saw the contribution of the polymer units on the optical response. Chemical attributions of the polymers were assigned using FTIR Spectroscopy and Raman Scattering, revealing the physical interaction after mixing the materials. We showed that it is possible to build nanostructured PCPDTDPPs films with a high control of their molecular properties through an understanding of their self-assembly and interactions with an n-type material. Departamento de Física Faculdade de Ci ncias e Tecnologia UNESP, Rua Roberto Simonsen 305 E2S UPPA CNRS IPREM Universite de Pau et des Pays de l'Adour Center for Sustainable Future Technologies Istituto Italiano di Tecnologia, via Livorno 60 Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1 Departamento de Física Faculdade de Ci ncias e Tecnologia UNESP, Rua Roberto Simonsen 305

Organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible-light OPDs are proposed for use in indirect-conversion X-ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near-infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (Jd) is crucial to improve important figures of merits such as the signal-to-noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to Jd in OPDs. Jd reduction is typically addressed by fine-tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental Jd values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.

Incorporating fluorine (–F) substituents along the main-chains of polymer donors and acceptors is an effective strategy toward efficient bulk-heterojunction (BHJ) solar cells. Specifically, F-substituted polymers often exhibit planar conformations, leading to favorable packing, and electronic coupling. However, the effects of fluorine substituents on the charge generation and recombination characteristics that determine the overall efficiency of BHJ active layers remain critically important issues to examine. In this report, two PBDT[2X]T polymer analogs –poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4, 5-b′]dithiophene-thiophene] [PBDT[2H]T] and its F-substituted counterpart poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluoro-thiophene] [PBDT[2F]T]—are studied to systematically examine how –F substituents impact the blend morphology, charge generation, carrier recombination and extraction in BHJ solar cells. Considering the large efficiency differences between PBDT[2H]T- and PBDT[2F]T-based BHJ devices, significant emphasis is given to characterizing the out-of-plane morphology of the blend films as vertical phase-separation characteristics are known to have dramatic effects on charge transport and carrier extraction in polymer-fullerene BHJ solar cells. Herein, we use electron energy loss spectroscopy (EELS) in tandem with charge transport characterization to examine PBDT[2X]T-fullerene blend films. Our analyses show that PBDT[2H]T and PBDT[2F]T possess very different charge generation, recombination and extraction characteristics, resulting from distinct aggregation, and phase-distribution within the BHJ blend films.

Reflectance oximeters based on organic photodiode (OPD) arrays offer the potential to map blood pulsation and oxygenation via photoplethysmography (PPG) over a large area and beyond the traditional sensing locations. Here, an organic reflectance PPG array based on 16 × 16 OPD pixels is developed. The individual pixels exhibit near-infrared sensitivity up to ≈950 nm and low dark current density in the order of 10−6 mA cm−2. This results in high-quality PPG signals. Analysis of the full PPG waveform yields insight on the artery stiffness and the quality of blood circulation, demonstrating the potential of these arrays beyond pulse oximetry and heart-rate calculation.

Minimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias (Jd) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of Jd in state-of-the-art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of Jd are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating Jd to material parameters, insights into the origin of Jd are obtained that enable the future selection of successful OPD materials.

Organic Photovoltaics (OPVs) based on Bulk Heterojunction (BHJ) blends are a mature technology. Having started their intensive development two decades ago, their low cost, processability and flexibility rapidly funneled the interest of the scientific community, searching for new solutions to expand solar photovoltaics market and promote sustainable development. However, their robust implementation is hampered by some issues, concerning the choice of the donor/acceptor materials, the device thermal/photo-stability, and, last but not least, their morphology. Indeed, the morphological profile of BHJs has a strong impact over charge generation, collection, and recombination processes; control over nano/microstructural morphology would be desirable, aiming at finely tuning the device performance and overcoming those previously mentioned critical issues. The employ of compatibilizers has emerged as a promising, economically sustainable, and widely applicable approach for the donor/acceptor interface (D/A-I) optimization. Thus, improvements in the global performance of the devices can be achieved without making use of more complex architectures. Even though several materials have been deeply documented and reported as effective compatibilizing agents, scientific reports are quite fragmentary. Here we would like to offer a panoramic overview of the literature on compatibilizers, focusing on the progression documented in the last decade.

A conjugated, ladder-type multi-fused ring 4,7-dithienbenzothiadiazole:thiophene derivative, named as compound ‘T’, was for the first time incorporated, within the PTB7:PC71BM photoactive layer for inverted ternary organic solar cells (TOSCs) realization. The effective energy level offset caused by compound T between the polymeric donor and fullerene acceptor materials, as well as its resulting potential as electron cascade material contribute to an enhanced exciton dissociation, electron transfer facilitator and thus improved overall photovoltaic performance. The engineering optimization of the inverted TOSC, ITO/PFN/PTB7:Compound T(5% v/v):PC71BM/MoO3/Al, resulted in an overall power conversion efficiency (PCE) of 8.34%, with a short-circuit current density (Jsc) of 16.75 mA cm−2, open-circuit voltage (Voc) of 0.74 V and a fill factor (FF) of 68.1%, under AM1.5G illumination. This photovoltaic performance was improved by approximately 12% with respect to the control binary device.

Polymer semiconductors may have the potential to fully replace silicon in next-generation solar cells because of their advantages such as cheap cost, lightweight, flexibility, and the ability to be processed for very large area applications. Despite these advantages, polymer solar cells are still facing a certain lack of power-conversion efficiency (PCE), which is essentially required for commercialization. Recently, bulk heterojunction of PTB7:PC70BM as an active layer showed remarkable performance for polymer solar cells in terms of PCE. Thus, in this paper, we developed and optimized a novel design using PEDOT:PSS and PFN-Br as electron and hole transport layers (ETL and HTL) for ITO/PEDOT:PSS/PT7B:PC70BM/PFN-Br/Ag as a polymer solar cell, with the help of simulation. The optimized solar cell has a short-circuit current (Isc) of 16.434 mA.cm−2, an open-circuit voltage (Voc) of 0.731 volts, and a fill-factor of 68.055%, resulting in a maximum PCE of slightly above 8%. The findings of this work may contribute to the advancement of efficient bulk-heterojunction-based polymer solar cells.

Enhanced photocurrents were observed by employing zinc oxide doped single wall carbon nanotube (ZnO:SWCNT) in the photoactive medium of thin film organic solar cells (TFOSC). The investigations were conducted based on either poly [[4,8-bis [(2-ethyhexyl)oxy] benzo(1,2-b:4,5-b’)dithiophene-2,6-diyl][3-uoro-2-[(2 ethylhexyl)carbonyl]thieno [3,4-b]thiophenediyl]] (PTB7) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM),PTB7:PCBM or poly (3hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester, P3HT:PCBM blend active layers containing ZnO:SWCNT nanocomposite at various concentrations. The best performances of the devices were found to be dependent on both the concentration of ZnO:CNT and the nature of the host medium. A substantial improvements in power conversion efficiencies (PCE) were observed as high as 68% and 31% for PTB7:PCBM and P3HT:PCBM blend with ZnO:CNT, respectively. Further device improvement were noted by employing co-solvent and solvent additive in the processing of TFOSC. The optical, electrical and morphological properties of photoactive films are discussed based on the experimental results along with the associated effect on device performances.

In recent years, the efficiency of organic solar cells (OSCs) has increased to more than 13%, although different barriers are on the way for reaching higher efficiencies. One crucial barrier is the recombination of charge carriers, which can either occur as the bulk recombination of photogenerated charges or the recombination of photogenerated charges and electrodic induced charges (EICs). This work studies the impact of EICs on the recombination lifetime in OSCs. To this end, the net recombination lifetime of photogenerated charge carriers in the presence of EICs is measured by means of conventional and newly developed transient photovoltage techniques. Moreover, a new approach has been introduced to exclusively measure the bulk recombination lifetime, i.e., in the absence of EICs; this approach was conducted by depositing transparent insulating layers on both sides of the OSC active layer. An examination of these approaches on OSCs with different active layer materials, thicknesses, and varying light intensities determined that the EICs can only reduce the recombination lifetime of the photogenerated charges in OSCs with very weak recombination strength. This work supports that for OSCs with highly reduced recombination strength, eliminating the recombination of photogenerated charges and EICs is critical for achieving better performance. Therefore, the use of a proper blocking layer suppresses EIC recombination in systems with very weak recombination.

Mechanochemically prepared nanopowder of selenium-free kesterite Cu2ZnSnS4 (CZTS) in combination with n-type semiconductors, i.e., CdS, ZnO and TiO2, was tested in planar and bulk-heterojunction solar cells. The samples have been studied by macroscopic current-voltage (I-V) measurements and Kelvin-probe atomic-force microscopy (KPFM). KPFM images taken under light illumination showed the distribution of the potential across the surface, with negative potential on the n-type semiconductor domains and positive potential on the CZTS domains, which indicated charge separation at the interface of the counterparts. The best result was found for the CdS-CZTS composition, which showed a potential difference between the domains up to 250 mV. These results were compared with the planar heterojunctions of CdS/CZTS and TiO2/CZTS, where CZTS nanopowder was pressed/deposited directly onto the surface of films of the corresponding n-type semiconductors. Again, I-V characteristics showed that cells based on CdS/CZTS heterojunctions have the best performance, with a photovoltage up to 200 mV and photocurrent densities up to 0.1 mA/cm2. However, the carrier generation was found to occur mainly in the CdS semiconductor, while CZTS showed no photo-response and served as the hole-transporting layer only. It is concluded that sensitization of the kesterite powder obtained by mechanochemical method is necessary to improve the performance of the corresponding solar cells.

Water-soluble regioregular poly{3-[(6-sodium sulfonate)hexyl]thiophene} (PT6S) and poly{3-[(6-trimethylammoniumbromide)hexyl]thiophene} (PT6N) have been synthesized and employed both as photoactive layers for the assembling of “green” bulk-heterojunction organic solar cells and as charge-collection layers in a cell with “classic” architecture. While the photovoltaic performances obtained with the two aforementioned polymers were lower than the reference cell, their latter use allowed to notably increase the inherent J-V properties, leading to a considerable enhancement in the overall photovoltaic output. The power conversion efficiency of the optimized multilayer BHJ solar cell reached 4.78%, revealing a higher efficiency than the reference cell (3.63%).

Four novel alternating copolymers bearing 5-fluoro-6-((2-octyldodecyl)oxy)benzo[c][1,2,5]thiadiazole as a strong acceptor unit and 9,9-dioctylfluorene as a strong donor unit with bridging units namely, thienothiophene, selenophene, 3-hexylthiophene, and thiophene were designed and synthesized. The polymers were characterized via 1H NMR spectroscopy, and weight average molecular weights were reported via gel permeation chromatography (GPC). For synthesized novel polymers, the bulk heterojunction solar cells were constructed. Besides, the effects of bridging units on electronic, optical, photovoltaic, and morphological properties were investigated. Among the polymers, the thienothiophene containing polymer P1 exhibited the highest PCE as 4.25% under the illumination of AM 1.5 G with 100 mW/cm2.

Organic solar cells (OSCs) are a complex assembly of disparate materials, each with a precise function within the device. Typically, the electrodes are flat, and the device is fabricated through a layering approach of the interfacial layers and photoactive materials. This work explores the integration of high surface area transparent electrodes to investigate the possible role(s) a three-dimensional electrode could take within an OSC, with a BHJ composed of a donor–acceptor combination with a high degree of electron and hole mobility mismatch. Nanotree indium tin oxide (ITO) electrodes were prepared via glancing angle deposition, structures that were previously demonstrated to be single-crystalline. A thin layer of zinc oxide was deposited on the ITO nanotrees via atomic layer deposition, followed by a self-assembled monolayer of C60-based molecules that was bound to the zinc oxide surface through a carboxylic acid group. Infiltration of these functionalized ITO nanotrees with the photoactive layer, the bulk heterojunction comprising PC71BM and a high hole mobility low band gap polymer (PDPPTT-T-TT), led to families of devices that were analyzed for the effect of nanotree height. When the height was varied from 0 to 50, 75, 100, and 120 nm, statistically significant differences in device performance were noted with the maximum device efficiencies observed with a nanotree height of 75 nm. From analysis of these results, it was found that the intrinsic mobility mismatch between the donor and acceptor phases could be compensated for when the electron collection length was reduced relative to the hole collection length, resulting in more balanced charge extraction and reduced recombination, leading to improved efficiencies. However, as the ITO nanotrees increased in height and branching, the decrease in electron collection length was offset by an increase in hole collection length and potential deleterious electric field redistribution effects, resulting in decreased efficiency.

A series of low band gap, planar conjugated polymers, P1 (PFDTBT), P2 (PFDTDFBT) and P3 (PFDTTBT), based on fluorene and benzothiadiazole, was synthesized. The effect of fluorine substitution and fused aromatic spacers on the optoelectronic and photovoltaic performance was studied. The polymer, derived from dithienylated benzothiodiazole and fluorene, P1, exhibited a highest occupied molecular orbital (HOMO) energy level at −5.48 eV. Density functional theory (DFT) studies as well as experimental measurements suggested that upon substitution of the acceptor with fluorine, both the HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of the resulting polymer, P2, were lowered, leading to a higher open circuit voltage and short circuit current with an overall improvement of more than 110% for the photovoltaic devices. Moreover, a decrease in the torsion angle between the units was also observed for the fluorinated polymer P2 due to the enhanced electrostatic interaction between the fluorine substituents and sulfur atoms, leading to a high hole mobility. The use of a fused π-bridge in polymer P3 for the enhancement of the planarity as compared to the P1 backbone was also studied. This enhanced planarity led to the highest observed mobility among the reported three polymers as well as to an improvement in the device efficiency by more than 40% for P3.

Despite the wide use of blends combining an organic p-type polymer and molecular fullerene-based electron acceptor, the proper characterization of such bulk heterojunction materials is still challenging. To highlight structure-to-function relations and improve the device performance, advanced tools and strategies need to be developed to characterize composition and interfaces with sufficient accuracy. In this work, high-resolution X-ray photoelectron spectroscopy (XPS) is combined with very low energy argon ion beam sputtering to perform a nondestructive depth profile chemical analysis on full Al/P3HT:PCBM/PEDOT:PSS/ITO (P3HT, poly(3-hexylthiophene); PCBM, [6,6]-phenyl-C61-butyric acid methyl ester; PEDOT, poly(3,4-ethylenedioxythiophene; PSS, polystyrenesulfonate; ITO, indium tin oxide) bulk-heterojunction solar cell device stacks. Key information, such as P3HT and PCBM composition profiles and Al-PCBM chemical bonding, are deduced in this basic device structure. The interface chemical analysis allows us to evidence, with unprecedented accuracy, the inhomogeneous distribution of PCBM, characterized by a strong segregation toward the top metal electrode. The chemical analysis highresolution spectra allows us to reconstruct P3HT/PCBM ratio through the active layer depth and correlate with the device deposition protocol and performance. Results evidence an inhomogeneous P3HT/PCBM ratio and poorly controllable PCBM migration, which possibly explains the limited light-to-power conversion efficiency in this basic device structure. The work illustrates the high potential of XPS depth profile analysis for studying such organic/inorganic device stacks.

The history of organic photovoltaics has been characterised by the complex interplay between fundamental research, large scale manufacture and commercialization activities. In addition, the field is highly interdisciplinary; ranging across physics, chemistry and engineering. This environment has resulted in a frontier character to the field, with researchers constantly expanding into new areas and confronting new challenges as the area has developed. This article seeks to chart the developments in OPV research, with emphasis on the last two decades, to provide some historical context to current status of the field., Substantia, Vol 3 No 2 Suppl. 1 (2019)

This report demonstrates high-performance infrared phototransistors that use a broad-band absorbing organic bulk heterojunction (BHJ) layer responsive from the visible to the shortwave infrared, from 500 to 1400 nm. The device structure is based on a bilayer transistor channel that decouples charge photogeneration and transport, enabling independent optimization of each process. The organic BHJ layer is improved by incorporating camphor, a highly polarizable additive that increases carrier lifetime. An indium zinc oxide transport layer with high electron mobility is employed for rapid charge transport. As a result, the phototransistors achieve a dynamic range of 127 dB and reach a specific detectivity of 5 × 1012 Jones under a low power illumination of 20 nW/cm2, outperforming commercial germanium photodiodes in the spectral range below 1300 nm. The photodetector metrics are measured with respect to the applied voltage, incident light power, and temporal bandwidth, demonstrating operation at a video-frame rate of 50 Hz. In particular, the frequency and light dependence of the phototransistor characteristics are analyzed to understand the change in photoconductive gain under different working conditions.

La utilització de materials orgànics per a la fabricació de cèl·lules solars és una de les alternatives considerades actualment per a reduir els costos de fabricació de sistemes fotovoltaics. Els materials orgànics tenen coeficients d’absorció elevats que permeten la utilització de capes primes i tècniques de fabricació de baix cost. L’objectiu d’aquesta tesi ha estat contribuir a la millora de les cèl·lules solars orgàniques d’heterounió massiva, mitjançant la incorporació de nous materials acceptors i donadors d’electrons, així com de capes de blocatge de forats depositades a partir de solucions. S’ha dut a terme l’estudi dels efectes de les capes transportadores d’electrons i forats en termes d’eficiència i de degradació dels dispositius fotovoltaics. S’han estudiat i fabricat capes de blocatge de forats amb bathocuproïna (BCP) depositada a través d’una tecnologia de solucions com a substitució del calci. El empleo de materiales orgánicos en la fabricación de celulas solares es una de las alternativas actualmente consideradas para reducir los costes de fabricación de los sistemas fotovoltaicos. Los materiales organicos tienen coeficientes de absorpción elevados, permitiendo el uso de capas finas y técnicas de fabricación de bajo coste. El objectivo de esta tesis ha sido contribuir en la mejora de las células solares orgánicas de heterounión masiva, mediante la incorporación de nuevos materiales aceptores de la familia de los PDI, y de capas bloqueadoras de huecos depositadas a partir de soluciones. Se han estudiado los efectos de las capas transportadoras de electrones y huecos en términos de eficiencia y degradación de dispositivos fotovoltaicos. Se han fabricado capas bloqueadoras de huecos con bathocuproine (BCP) depositada a través de una tecnología de soluciones como sustituyente del Calcio. Se han estudiato y caracterizado las propriedades morfologicas y optoelectronicas de celulas solares con las estructuras ITO/PEDOT:PSS/PTB7:PDI/Ca/Ag y ITO/PEDOT:PSS/PTB7:PCBM/BCP/Ag. Se ha llevado a cabo una caracterización de las propiedades morfológicas y fotofísicas de los dispositivos fotovoltaicos fabricados a través de varios métodos de investigación, entre los cuales medidas J-V, EQE, Difracción de rayos X, medidas espectroscopia UV-Vis, medidas de movilidad de portadores, medidas de extracción de carga y foto-voltaje transitorio, AFM y medidas de ángulo de contacto. The search for low cost photovoltaics has led to the use of organic materials as possible candidates to substitute silicon based solar devices. Organic materials have high absorption coefficients, allowing the use of thin layers and low cost solution phase manufacturing techniques. The objective of this thesis has been to contribute to the improvement of bulk-heterojunction organic solar cells through the incorporation of new acceptor materials of the PDI family and of a film layer of Bathocuproine (BCP) as hole blocking layer. The BCP layer was spin coated from a dilute toluene/methanol solution directly on top of the active layer. The morphological, optoelectronic properties of solar cells with the following structures have been studied and characterized: ITO / PEDOT: PSS / PTB7: PDI / Ca / Ag and ITO / PEDOT: PSS / PTB7: PCBM / BCP / Ag. For this purpose various characterization methods has been carried out, including JV measurements, EQE, X-ray diffraction, UV-Vis spectroscopy, carrier mobility measurements , charge extraction and transient photo-voltage measurements and AFM.

High-responsivity phototransis-tors with a structure of perov-skite-WS2 nanosheet composite optical absorber and a reduced graphene oxide (rGO) channel layer is demonstrated via a facile and low-cost solution-processing method. The WS2 nanosheets are dispersed within the perovskite matrix, forming the perovskite-WS2 bulk heterojunction (BHJ). The hybrid phototransistor exhib-its excellent figures of merit in-cluding high photoresponsivity of 678.8 A/W, high specific de-tectivity of 4.99×1011 Jones, high EQE value of 2.04×105 % and rapid response to photoswitching. The high photoresponsivity could be attributed to the WS2 nanosheets induced photo-generated electron-hole separa-tion promotion effects due to the selective electron trapping effects in the WS2 nanosheets, as well as the high carrier mobility of the rGO channel layer. This work provides a promising platform for constructing high-responsivity photodetectors.

Optimizing the thickness of organic solar cells (OSCs) is a potent way to enhance the power conversion efficiency (PCE). In the present work, we have investigated a novel structure in which poly (9, 9‐dioctylindenofluorene‐co‐benzothiadiazole) (PIF8BT): N′‐bis (1‐ethylpropyl) ‐3, 4, 9, 10‐perylene tetracarboxy diimide (PDI)is used as a photoactive absorber layer. The influence of window layer material such as Zinc oxide (ZnO) and titanium dioxide (TiO2) with various electrode materials including Indium tin oxide (ITO), Fluorine tin oxide (FTO), aluminum(Al) Silver (Ag) and Gold (Au) with different combinations have been investigated with the objective to enhance the absorption and PCE of the cell. Extracted results shows that the proposed scheme of the structure with ITO/Al as top and bottom electrode holds the highest performance parameters including Jsc=9.26 (mA/m2), Voc=0.59 (V), FF=68.86% and ƞ=3.86% respectively as compared to different electrode combination and window layers with the same photoactive absorber material(PIF8BT:PDI). This indicates that the proposed structure can be a good choice for replacing less efficient in-organic cell.

High-responsivity phototransistors with a structure of perovskite-WS2 nanosheet composite optical absorber and a reduced graphene oxide (rGO) channel layer is demonstrated via a facile and low-cost solution-processing method. The WS2 nanosheets are dispersed within the perovskite matrix, forming the perovskite-WS2 bulk heterojunction (BHJ). The hybrid phototransistor exhibits excellent figures of merit including high photoresponsivity of 678.8 A/W, high specific detectivity of 4.99 × 1011 Jones, high EQE value of 2.04 × 105% and rapid response to photoswitching. The high photoresponsivity could be attributed to the WS2 nanosheets induced photo-generated electron-hole separation promotion effects due to the selective electron trapping effects in the WS2 nanosheets, together with the high carrier mobility of the rGO channel. This work provides a promising platform for constructing high-responsivity photodetectors.

The nanoscale morphology of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71. butyric acid methylester (PCBM) blend film is affected by various parameters such as the solvent, coating, and thermal annealing conditions. We investigated the effect of solvents on the performance of inverted solar cells based on the active layer of a P3HT:PCBM bulk heterojunction. P3HT and PCBM (weight ratio 1:0.8) were dissolved in chlorobenzene (CB) and dichlorobenzene (DCB). The difference in the volatility characteristics of the solvents resulted in different P3HT crystallite morphologies. The difference in the P3HT:PCBM film morphology was systemically investigated via atomic force microscopy, ultraviolet (UV)-visible absorption spectroscopy, X-ray diffraction, and electrical impedance spectroscopy. The DCB solvent lead to better P3HT crystallinity and device performance. For example, the short-circuit current density (JSC) and the power conversion efficiency (PCE) of the device using DCB (9.89 mA/cm2 and 3.62%, respectively) were larger than those (9.12 mA/cm2 and 3.01%) of the device using CB.

Two narrow band‐gap copolymers consisting of 2,7‐bis(5‐(trimethylstannyl)thiophen‐2‐yl)‐4,5,9,10‐tetrakis(2‐ethylhexyloxy)‐pyrene (M1) as an electron‐rich unit and diketopyrrolopyrrole (DPP) as an electron‐deficient unit have been synthesized and characterized for polymer solar cells. The two polymers were prepared by Stille coupling reactions. Two solubilizing alkyl chains (ethylhexyl and octlydodecyl) were attached to the DPP unit in order to evaluate their impact upon the optical and electrochemical characteristics of the two polymers. Poly[4,5,9,10‐tetrakis[(2‐ethylhexyl)oxy]pyrene‐alt‐3,6‐bis(thiophen‐2‐yl)‐2,5‐ bis(2‐octyldodecyl)pyrrole[3,4‐c]pyrrole‐ 1,4(2H,5H)‐dione] (PPEHDT‐DPPODo) and poly[4,5,9,10‐tetrakis((2‐ethylhexyl)oxy)pyren‐alt‐3,6‐bis(thiophen‐2‐yl)‐2,5‐bis(2‐ethylhexyl)pyrrole[3,4‐c]pyrrole‐1,4(2H,5H)‐dione] (PPEHDT‐DPPEH) exhibited high thermal stability with decomposition temperatures over 300 °C. Optical properties showed that PPEHDT‐DPPODo and PPEHDT‐DPPEH have optical band gaps of around 1.40 eV. It is believed that both polymers adopt high planar structures in the thin film, leading to more electronic conjugation along the backbone of the conjugated polymers. Powder X‐ray diffraction revealed that PPEHDT‐DPPODo and PPEHDT‐DPPEH seem to have an amorphous nature. The HOMO energy levels of the two polymers are clearly affected by changing alkyl chains. The HOMO levels of PPEHDT‐DPPODo and PPEHDT‐DPPEH were found to be at −5.27 and −5.38 eV, respectively. PPEHDT‐DPPODo showed a HOMO energy level approximately 0.11 eV shallower than that of PPEHDT‐DPPEH, which is probably a consequence of attaching a larger alkyl chain to the DPP moiety reducing its electron accepting ability.

Organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible-light OPDs are proposed for use in indirect-conversion X-ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near-infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (Jd) is crucial to improve important figures of merits such as the signal-to-noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to Jd in OPDs. Jd reduction is typically addressed by fine-tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental Jd values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.

A low-cost and scalable short-wavelength infrared (SWIR: λ = 1–3 μm) photosensor will be widely deployable and transformative for a wide range of spectroscopic systems and optoelectronics that form the foundation for scientific, industrial, medical, and defense applications. Conventional SWIR photosensors are cost-prohibitive due to complex fabrication involving epitaxial growth of inorganic crystals. To make SWIR photodetectors affordable for ubiquitous sensing, this thesis aims to realize low-cost organic SWIR bulk heterojunction (BHJ) photodiodes by using a novel class of modular narrow bandgap conjugated polymers, and to demonstrate the direct solution deposition of organic thin films can replace complex manufacturing processes and produce the comparable performance.While polymeric semiconducting materials have been widely used and extensively studied in organic photovoltaics, different device behaviors accompany the progressive shrinkage of polymer bandgap to extend spectral absorption out to SWIR region. A better understanding of the fundamental properties of SWIR organic photodiode (OPD) is therefore needed to predict and advance performance. This thesis focuses on understanding different aspects of OPD performance that centers around specific detectivity, which ultimately defines the signal-to-noise ratio.Firstly, in Chapter 3, the challenges in the dark noise increase associated with the low-bandgap organic system are discussed, followed by an introduction to different noise suppression methods. An emphasis is made on the importance of direct noise measurement to stay out of the pitfall of overestimating detectivity. Two approaches including interface engineering and electrode work function tuning are demonstrated to suppress the noise current in SWIR OPDs. Secondly, in Chapter 4, an improved model to decouple dissociation and collection efficiency is proposed to pinpoint the limiting factor of SWIR OPDs, and exemplary device engineering methods are shown to improve the dissociation bottleneck. Then, in Chapter 5, two recombination mechanisms limiting the device efficiency are shed light upon by connecting the optoelectronic properties to the materials composition. Lastly, in Chapter 6, three applications are demonstrated to show the practicality of solution-processed SWIR OPDs and the potential in realizing the low-cost and large-scale SWIR sensing.

Minimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias (Jd) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of Jd in state-of-the-art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of Jd are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating Jd to material parameters, insights into the origin of Jd are obtained that enable the future selection of successful OPD materials.

WOS: 000464931800022, In this study, we demonstrate the use of a series of boronic acid functionalized self-assembled monolayers (SAMs) to improve photovoltaic device performance P3HT and PCBM based solar cells. The SAMs treated compact TiO2 (c-TiO2) layer was utilized as an electron transport layer for inverted polymer solar cells (PSCs) with a configuration of FTO/c-TiO2/SAM/P3HT:PCBM/PEDOT:PSS/Ag. The modified with 3,4,5-methoxyphenylboronic acid (3-OMe) SAM shows the best improving due to the enhancement of J(sc) and V-oc. in device, which leads to a 26% improvement (2.8%) over non-modified device (2.2%). The enhancement in the modified devices is achieved by SAM modification reducing recombination of charges and improving charge selectivity. These results prove that the surface and electrical properties of compact TiO2 (c-TiO2) layer can be easily tuned as well as the upper layer morphology can be controlled by SAM modification.