Investigation of charge transport in organic polymer donor/acceptor photovoltaic materials (original) (raw)
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Materials, 2020
Electronic transport measurement using modulated photocurrent (MPC) spectroscopy is demonstrated herein in working organic photovoltaics (OPVs) before and after AM1.5G irradiation. OPVs with bulk heterojunction (BHJ) using prototypical donor and acceptor materials, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1–2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl] = hieno [3–4-b]thiophenediyl]] (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), were fabricated. The OPVs had inverted structures (BHJs are formed on transparent conductive oxide substrates). The photovoltaic performance of PTB7:PC71BM OPVs was characterized and the best power conversion efficiency was obtained at PTB7 content of 40 wt%. Electron and hole mobility were determined with MPC spectroscopy in PTB7:PC71BM OPVs and were well balanced at PTB7 content of 40 wt%. Degradation of the photovoltaic performance of PTB7:PC71BM OPVs with PTB7 content of 40 wt% caused by AM1.5G irradiation was studied...
Applied Physics Letters, 2007
Poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3benzothiadiazole]-2 ,2 -diyl) (F8TBT) was developed by Cambridge Display Technology Ltd. The molecular weight of the sample used was M p = 432,000 g/mol. Poly(3hexylthiophene) (P3HT) was supplied by Merck KGaA with molecular weight M w~ 25,000 g/mol and regioregularity of ~ 94 %. (6,6)-phenyl C 61 -butyric acid methyl ester (PCBM) was supplied by Nano C, Inc.
Journal of Photonics for Energy, 2015
Conjugated polymers are potential materials for photovoltaic applications due to their high absorption coefficient, mechanical flexibility, and solution-based processing for low-cost solar cells. A bulk heterojunction (BHJ) structure made of donor-acceptor composite can lead to high charge transfer and power conversion efficiency. Active layer morphology is a key factor for device performance. Film formation processes (e.g., spray-coating, spin-coating, and dip-coating), post-treatment (e.g., annealing and UV ozone treatment), and use of additives are typically used to engineer the morphology, which optimizes physical properties, such as molecular configuration, miscibility, lateral and vertical phase separation. We will review electronic donoracceptor interactions in conjugated polymer composites, the effect of processing parameters and morphology on solar cell performance, and charge carrier transport in polymer solar cells. This review provides the basis for selection of different processing conditions for optimized nanomorphology of active layers and reduced bimolecular recombination to enhance open-circuit voltage, short-circuit current density, and fill factor of BHJ solar cells.
Effect of Charge Mobility for Organic Photovoltaic Devices
Molecular Crystals and Liquid Crystals, 2006
Influence of charge mobility of n-type organic semiconductors for the performance of organic p-n junction type photovoltaic devices has been studied. Two n-type organic semiconductors with same main moiety (i.e., perylenebiscarboximide derivates), which posses similar HOMO, LUMO and band gaps; however with different charge mobility especially electron mobility have been utilized in this study. The electron mobility was measured by time of flight method. The electron mobility difference was caused by molecular packing characteristics since one of the semiconductor showed a liquid crystalline behavior.
2014
Organic solar cell research has attracted scientific and commercial interest in the last decade due to a rapid increase in power conversion efficiencies (PCEs). The most-prominent reported material system in Bulk Heterojunction (BHJ) is the mixture of poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Due to the various research efforts that have been done on the improvement of solar cells, the effect of Hole Transport Layer (HTL) was also noticed as an important factor. This paper gives a effect of different HTL materials like PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) and MoO3 (Molybdenum trioxide) on the photovoltaic performance of BHJ solar cell using blends of P3HT and PCBM in 1:1 ratio using common organic solvents. Prior to these processes, fabrication and designing of solar cells, right from the initial steps such as patterning of ITO coated glass, etching, cleaning and subsequent processes were performed.
Journal of Material Chemistry A 4, 4252 (2016)
We demonstrate that blending fluorinated molecules in PEDOT:PSS hole transport layers (HTL) induces charge transfers which impact on both charge extraction and photogeneration within organic photovoltaic (OPV) devices. OPVs fabricated with modified HTL and two photoactive polymer blends led systematically to power conversion efficiencies (PCE) increases, with PTB7:PC70BM blend exhibiting PCE of ~ 8.3 %, i.e. ~ 15 % increase compared to pristine HTL devices. A reduced device-to-device characteristics variations was also noticed when fluorinated additives were used to modify the PEDOT:PSS. Shading lights onto the effect of HTL fluorination, we show that the morphology of the polymer:PCBM blends remains surprisingly unaffected by the fluorinated HTL surface energy but that, instead, the OPVs are impacted not only by the HTL electronic properties (work function, dipole layer, open circuit voltage, charge transfer dynamic) but also by alteration of the complex refractive indices (photogeneration, short circuit current density, external quantum efficiencies, electro-optic modelling). Both mechanisms find their origin in fluorination induced charge transfers. This work points towards fluorination as a promising strategy toward combining both external quantum efficiency modulation and power conversion efficiency enhancement in OPVs. Charge transfers could also be used more broadly to tune the optical constants and electric field distribution, as well as to reduce interfacial charge recombinations within OPVs.
Internal electric field in organic-semiconductor-based photovoltaic devices
Applied Physics Letters, 2006
The authors performed transient photocurrent measurements under applied bias and electroabsorption spectroscopy on devices based on a pristine poly͑phenylene vinylene͒ derivative as well as its mixture with 1% of a methanofullerene electron acceptor. Combining both techniques allows us to directly determine the internal electric field and to conclude on its implication on the photovoltaic performance of the devices. The electric field is identified as the driving force of the photocurrent, hence indicating the maximum obtainable photovoltage. Acceptor concentrations as low as 1% shift the energetic alignment of the top electrode to the reduction potential of the acceptor, reducing the internal electric field.
Physical Review B, 2013
The ac admittance of solar cells under illumination is investigated under open-circuit conditions. Open-circuit conditions are imposed by inserting a probe capacitor into the circuit. The capacitance and conductance of the cells are investigated as function of frequency and continuous illumination intensity. Results are compared with numerical and analytical modeling of charge recombination and transport. In bulk heterojunction solar cells with [6,6]-Phenyl-C 61 (C 71 )-butyric acid methyl ester as acceptor and poly(3-hexylthiophene) or poly[2methoxy-5-(2 -ethylhexyloxy)-p-phenylene vinylene] as electron donor, the high-frequency capacitance C and conductance G follow a power-law dependence on intensity of white light I, with G(I) ∝ I 3/4 and C(I) ∝ I 1/4 . The modeling shows that these dependencies can be explained in terms of space-charge-limited current in combination with Langevin type recombination of carriers. For poly [2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b ]dithiophene-2,6-diyl]] the capacitance shows a weaker dependence on intensity, indicating fast recombination of photogenerated carriers. Results indicate that the fill factor of relatively well performing polymer solar cells can still be limited by space charge effects and can be improved by enhancing the charge carrier mobility or by reducing the bimolecular Langevin recombination.