The role of bulk and interfacial morphology in charge generation, recombination, and extraction in non-fullerene acceptor organic solar cells

Lee

et al. 2021

Adv Funct Materials

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The effect of the cross‐coupling catalyst tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) on the performance of a model organic bulk‐heterojunction solar cell composed of a blend of poly([2,6′‐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) donor and 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IOTIC‐4F) non‐fullerene acceptor is investigated. The effect of intentional addition of different amounts of Pd(PPh3)4 on morphology, free charge carrier generation, non‐geminate bulk trap‐ and surface trap‐assisted recombination as well as bimolecular recombination and charge extraction is quantified. This work shows that free charge carrier generation is affected significantly, while the impact of Pd(PPh3)4 on non‐geminate recombination processes is limited because the catalyst does not facilitate efficient trap‐assisted recombination. The studied system shows substantial robustness towards the addition of Pd(PPh3)4 in small amounts.

Section: Resultssupporting

confidence: 60%

“…To demonstrate the applicability to other NFA based systems, we further show the performance changes upon catalyst addition to the PM6:Y6 system, which has gathered much attention due to its performance of over 15%. [ 51–53 ]…”

Section: Introductionmentioning

confidence: 99%

Effect of Palladium‐Tetrakis(Triphenylphosphine) Catalyst Traces on Charge Recombination and Extraction in Non‐Fullerene‐based Organic Solar Cells

Lee

et al. 2021

Adv Funct Materials

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“…The range of light intensities I ≈ 10–100 mW cm −2 is the most suitable and commonly used for testing the dominant recombination mechanisms as it is not influenced by leakage losses and enhanced photodegradation. [ 42–49 ] As shown in previous study, [ 50 ] the parasitic effect of the shunt resistance R sh > 10 kΩ cm 2 (that is the case for our devices: R sh ranges from 0.1 to 1 MΩ cm 2 ) becomes negligible at light intensities I ≥ 10 mW cm −2 since the flux of photogenerated charge carriers significantly exceeds the leakage and thus does not result in errors during the fitting of the experimental data. Light intensities >100 mW cm −2 may result in a significant enhancement of photodegradation processes, rendering any effort to model the recombination dynamics under these conditions pointless.…”

Section: Resultssupporting

confidence: 69%

Temperature and Light Modulated Open‐Circuit Voltage in Nonfullerene Organic Solar Cells with Different Effective Bandgaps

Advanced Energy Materials

et al. 2020

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bandgap systems for the emerging fields of organic semitransparent photovoltaics and near-infrared photodiodes. [9,10] So far, there are no systematic studies on the relationship between the effective bandgap E g,eff , defined by the HOMO of the donor and the LUMO of the acceptor, the density of states (DOS) distribution and multimechanism recombination processes in nonfullerene organic solar cells. However, all mentioned parameters both individually and synergistically are known to be strongly influential on photoelectronic processes and, thus, on the power conversion efficiency (PCE). A specific interest represents the temperature and light intensity dependence of the open-circuit voltage V oc that is selectively sensitive to these parameters and remains immune to the parasitic effects of the series resistance and large shunt. [11-13] Temperature is a macroscopic parameter, which appears in exponential functions in all major equations that describe charge carrier concentration, recombination, and transport processes. This prevalence of the temperature in most equations is mainly caused by the reliance on the Fermi-Dirac distribution or thermally activated processes. [13,14] Hence, temperature is an essential parameter to consider for the analysis of electronic processes in all types of semiconductor materials and devices. [13,15] A controllable change of temperature allows the modulation of basic electronic processes both mechanistically and quantitatively in organic bulk-heterojunction (BHJ) solar cells. In contrast to studies across different devices, uncontrollable fabrication-related deviations in morphology and defect concentrations can be excluded as a parameter of influence. However, in OPVs, the temperature effect is rarely studied and if so, only to inspect the temperature-dependent trends for the main photoelectric parameters (open circuit voltage (V oc), short circuit current density (J sc), fill factor (FF)) and PCE of organic BHJ solar cells. [16-21] Even less attention has been paid to the investigation of temperature-dependent generationrecombination processes in BHJ layers and, specifically, at their interfaces with electrical contacts. [22-24] In this contribution we analyzed temperature and light modulated V oc in a series of nonfullerene BHJ solar cells with conventional or inverted device architecture and a systematically tuned effective bandgap E g,eff in a wide range from 1.0 to 1.62 eV. The temperature-dependent V oc versus light intensity slopes The relationship of the temperature-light intensity dependence of opencircuit voltage V oc in nonfullerene-based organic solar cells with their material characteristics and multimechanism recombination parameters is described. The systematic variation of the effective bandgap E g,eff and the electrode layers allows the observation of different relative contributions of bimolecular, bulk, and surface trap-assisted recombination mechanisms. The complementary advantages of the analytical model and the established voltage-impedance spectroscopy techn...

“…Parameters of influence include the processing and annealing conditions and the resulting morphologies of both layers, the roughness and homogeneity of the interface, the layers’ composition and the resulting changes in surface energy. [ 37–39 ]…”

Section: Resultsmentioning

confidence: 99%

On Optoelectronic Processes in Organic Solar Cells: From Opaque to Transparent

Advanced Optical Materials

Nguyen

2020

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Organic (semi)transparent photovoltaics (ST‐OPVs) promise integrated, sustainable, low‐cost energy harvesting solutions. However, current efficiency limitations have to be overcome to make ST‐OPV a competitive technology. In this simulation‐based work, the effect of the selective transparency on the photoelectronic processes in ST‐OPVs is studied and changes in the generation–recombination dynamics and the extraction efficiency are demonstrated that are causally linked to the increased transparency. The study of five model systems with transparent indium tin oxide (ITO) back electrodes and systematically varied extinction coefficients and an opaque cell with Ag back contact allows to quantify these changes in the photoelectronic processes and to address the role of the series and the shunt resistors while keeping all other parameters of the modeled devices identical. The findings demonstrate the increased importance of the active layer quality in ST‐OPVs and indicate that ST‐OPVs benefit from a wider choice of transparent electrode materials.