1,720,973 research outputs found
Field Effect versus Driving Force: Charge Generation in Small‐Molecule Organic Solar Cells
No full textEfficient charge generation in organic semiconductors usually requires an interface with an energetic gradient between an electron donor and an electron acceptor in order to dissociate the photogenerated excitons. However, single-component organic solar cells based on chloroboron subnaphthalocyanine (SubNc) have been reported to provide considerable photocurrents despite the absence of an energy gradient at the interface with an acceptor. In this work, it is shown that this is not due to direct free carrier generation upon illumination of SubNc, but due to a field-assisted exciton dissociation mechanism specific to the device configuration. Subsequently, the implications of this effect in bilayer organic solar cells with SubNc as the donor are demonstrated, showing that the external and internal quantum efficiencies in such cells are independent of the donor-acceptor interface energetics. This previously unexplored mechanism results in efficient photocurrent generation even though the driving force is minimized and the open-circuit voltage is maximized.V.C.N. and Y.D. contributed equally to this work. The authors would like to thank Prof. Dr. Dieter Neher for providing measurement time on the electroluminescence setup of his lab at University of Potsdam. They would also like to acknowledge the Optoelectronics group in the University of Cambridge for sharing the global analysis codes. This work was supported by the German Federal Ministry of Education and Research (BMBF) through the InnoProfile project "Organische p-i-n Bauelemente2.2" (FKZ 03IPT602X). A.A.B. is a Royal Society university research fellow.
Open access funding enabled and organized by Projekt DEAL.Nikolis, VC (corresponding author), Tech Univ Dresden, Dresden Integrated Ctr Appl Phys & Photon Mat IAP, Nothnitzer Str 61, D-01187 Dresden, Germany ; Tech Univ Dresden, Inst Appl Phys, Nothnitzer Str 61, D-01187 Dresden, Germany.
Bakulin, AA (corresponding author), Imperial Coll London, Mol Sci Res Hub, London W12 0BE, England.
Vandewal, K (corresponding author), Hasselt Univ, Inst Mat Res IMO IMOMEC, Wetenschapspk 1, B-3590 Diepenbeek, Belgium.
[email protected]; [email protected]; [email protected]
Layer Reduces Nonradiative Losses in Organic Solar Cells”
No full textJ.B. acknowledges the Sachsische Aufbaubank through project 100325708 (Infrakart). M.N. acknowledges funding from the Academy of Finland through project 326000. M.P. acknowledges the Deutsche Akademische Austausch Dienst for the Study Completion Scholarship (April-June 2019). The authors thank Prof. Dr. Koen Vandewal from the University of Hasselt, Belgium, for his input and fruitful discussions, as well as Dr. Oskar Sandberg for fruitful discussions and for providing the drift-diffusion simulations. Stephanie Anna Buchholtz and Carsten Habenicht are kindly acknowledged for the UPS measurements
Excitons dominate the emission from PM6
No full textNon-fullerene acceptors (NFAs) are far more emissive than their fullerene-based counterparts. Here, we study the spectral properties of photocurrent generation and recombination of the blend of the donor polymer PM6 with the NFA Y6. We find that the radiative recombination of free charges is almost entirely due to the re-occupation and decay of Y6 singlet excitons, but that this pathway contributes less than 1% to the total recombination. As such, the open-circuit voltage of the PM6:Y6 blend is determined by the energetics and kinetics of the charge-transfer (CT) state. Moreover, we find that no information on the energetics of the CT state manifold can be gained from the low-energy tail of the photovoltaic external quantum efficiency spectrum, which is dominated by the excitation spectrum of the Y6 exciton. We, finally, estimate the charge-separated state to lie only 120 meV below the Y6 singlet exciton energy, meaning that this blend indeed represents a high-efficiency system with a low energetic offset
Reverse dark current in organic photodetectors and the major role of traps as source of noise
No full textOrganic photodetectors have promising applications in low-cost imaging, health monitoring and near-infrared sensing. Recent research on organic photodetectors based on donor-acceptor systems has resulted in narrow-band, flexible and biocompatible devices, of which the best reach external photovoltaic quantum efficiencies approaching 100%. However, the high noise spectral density of these devices limits their specific detectivity to around 10(13)Jones in the visible and several orders of magnitude lower in the near-infrared, severely reducing performance. Here, we show that the shot noise, proportional to the dark current, dominates the noise spectral density, demanding a comprehensive understanding of the dark current. We demonstrate that, in addition to the intrinsic saturation current generated via charge-transfer states, dark current contains a major contribution from trap-assisted generated charges and decreases systematically with decreasing concentration of traps. By modeling the dark current of several donor-acceptor systems, we reveal the interplay between traps and charge-transfer states as source of dark current and show that traps dominate the generation processes, thus being the main limiting factor of organic photodetectors detectivity. The suppression of dark current in organic photodetectors (OPDs) is important for maximizing the performance of the devices. Here, the authors report the relationship between the high dark saturation current and the presence of mid-gap trap states in OPDs with a donor-acceptor structure.Open Access funding enabled and organized by Projekt DEAL.
Acknowledgements
This work was supported by the German Federal Ministry for Education and Research (BMBF) and by the German Research Foundation (DFG) within the Cluster of excellence Center for Advancing Electronics Dresden (cfaed) and the DFG projects HEFOS (Grant No. FI 2449/1-1) and Photogen (Grant No. VA 1035/5-1). B.K.B. acknowledges funding from DFG Priority Programme FFlexCom under the project FlexARTwo (LE 747/52-2 and EL 506/22-2). J.K. acknowledges funding by the Deutsche Akademische Austausch Dienst (DAAD). J.B. thanks for the financing from Sächsische Aufbaubank through project InfraKart (Grant No. 100325708). Furthermore, we acknowledge N. Sergeeva, Prof. Dr. T. Kirchartz, and Prof. Dr. B. E. Pieters for fruitful discussions.Kublitski, J; Hofacker, A (corresponding author), Tech Univ Dresden, Dresden Integrated Ctr Appl Phys & Photon Mat IAP, Nothnitzer Str 61, D-01187 Dresden, Germany ; Tech Univ Dresden, Inst Appl Phys, Nothnitzer Str 61, D-01187 Dresden, Germany.
Vandewal, K (corresponding author), Hasselt Univ, Inst Mat Onderzoek IMO, Wetenschapspk 1, BE-3590 Diepenbeek, Belgium.
[email protected]; [email protected]; [email protected]
On the Interplay between CT and Singlet Exciton Emission in Organic Solar Cells with Small Driving Force and Its Impact on Voltage Loss
Full text linkThe interplay between free charge carriers, charge transfer (CT) states and singlet excitons (S-1) determines the recombination pathway and the resulting open circuit voltage (V-OC) of organic solar cells.
By combining a well-aggregated low bandgap polymer with different blend ratios of the fullerenes PCBM and ICBA, the energy of the CT state (E-CT) is varied by 130 meV while leaving the S-1 energy of the polymer (ES1) unaffected.
It is found that the polymer exciton dominates the radiative properties of the blend when ECT approaches ES1, while the V-OC remains limited by the non-radiative decay of the CT state.
It is concluded that an increasing strength of the exciton in the optical spectra of organic solar cells will generally decrease the non-radiative voltage loss because it lowers the radiative V-OC limit (V-OC,V-rad), but not because it is more emissive.
The analysis further suggests that electronic coupling between the CT state and the S-1 will not improve the V-OC, but rather reduce the V-OC,V-rad.
It is anticipated that only at very low CT state absorption combined with a fairly high CT radiative efficiency the solar cell benefit from the radiative properties of the singlet excitons
Open-circuit voltage of organic solar cells: interfacial roughness makes the difference
Full text linkOrganic photovoltaics (PV) is an energy-harvesting technology that offers many advantages, such as flexibility, low weight and cost, as well as environmentally benign materials and manufacturing techniques. Despite growth of power conversion efficiencies to around 19 % in the last years, organic PVs still lag behind inorganic PV technologies, mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging, as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here, we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage, photovoltaic gap, charge-transfer state energy, and interfacial morphology. In particular, we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can, however, be circumvented by adjusting the morphology of the donor-acceptor interface.
Organic solar cells, despite their high power conversion efficiencies, suffer from open circuit voltage losses making them less appealing in terms of applications. Here, the authors, supported with experimental data on small molecule photovoltaic cells, relate open circuit voltage to photovoltaic gap, charge-transfer state energy, and donor-acceptor interfacial morphology
Impact of Triplet Excited States on the Open-Circuit Voltage of Organic Solar Cells
No full textThe best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn- and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C-60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T-1) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanced spin-orbit coupling. Only in the latter case, a substantial influence of the triplet state on the nonradiative voltage losses is observed. In contrast, it is found that for a large series of typical OSC materials, the relative energy level alignment between T-1 and the CT state does not substantially affect nonradiative voltage losses
Boron dipyrromethene (BODIPY) with meso-perfluorinated alkyl substituents as near infrared donors in organic solar cells
No full textThree furan-fused BODIPYs were synthesized with perfluorinated methyl, ethyl and n-propyl groups on the meso-carbon. They were obtained with high yields by reacting the furan-fused 2-carboxylpyrrole in corresponding perfluorinated acid and anhydride. With the increase in perfluorinated alkyl chain length, the molecular packing in the single crystal is influenced, showing increasing stacking distance and decreasing slope angle. All the BODIPYs were characterized as intense absorbers in near infrared region in solid state, peaking at similar to 800 nm with absorption coefficient of over 280 000 cm(-1). Facilitated by high thermal stability, the furan-fused BODIPYs were employed in vacuum-deposited organic solar cells as electron donors. All devices exhibit PCE over 6.0% with the EQE maximum reaching 70% at similar to 790 nm. The chemical modification of the BODIPY donors have certain influence on the active layer morphology, and the highest PCE of 6.4% was obtained with a notably high jsc of 13.6 mA cm(-2). Sensitive EQE and electroluminance studies indicated that the energy losses generated by the formation of a charge transfer state and the radiative recombination at the donor-acceptor interface were comparable in the range of 0.14-0.19 V, while non-radiative recombination energy loss of 0.38 V was the main energy loss route resulting in the moderate V-oc of 0.76 V
Energy-gap law for photocurrent generation in fullerene-based organic solar cells
No full textThe involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C-60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state
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