Trade‐Off between Trap Filling, Trap Creation, and Charge Recombination Results in Performance Increase at Ultralow Doping Levels in Bulk Heterojunction Solar Cells

Shang, Zhengrong; Heumueller, Thomas; Prasanna, Rohit; Burkhard, George F.; Naab, Benjamin D.; Bao, Zhenan; McGehee, Michael D.; Salleo, Alberto

doi:10.1002/aenm.201601149

Cited by 50 publications

(67 citation statements)

References 51 publications

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“…The effects of doping on film morphology—and conversely film morphology on doping—is complex and often difficult to characterize. Doping can strongly affect and be affected by the OSC crystal structure, degree of crystallinity, miscibility of the dopant with crystalline and amorphous phases, differences in the electronic properties of amorphous and crystalline phases, changes in trap density induced by dopants, and changes to the morphology as a function of doping density . At this point, it is difficult to identify even general empirical rules for doping efficiency, as discussed in Section .…”

Section: Fabrication and Morphologymentioning

confidence: 99%

“…Defect states deep within the band gap are highly localized and therefore have high hopping‐transport activation energies; these states are often referred to as trap sites . These trap sites can be filled by doping, typically at very low doping levels, which has the effect of “smoothing out” the energetic landscape for charge transport . Olthof et al studied trap filling in C 60 and observed nonlinear shifts in work function (Figure b), conductivity, and charge carrier mobility at doping levels below 10 −2 mole ratio (MR).…”

Section: Controlling Defects and Impurities With Dopantsmentioning

confidence: 99%

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Controlling Molecular Doping in Organic Semiconductors

2017

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The field of organic electronics thrives on the hope of enabling low-cost, solution-processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution-processed p-type doped polymeric semiconductors. Highlighted topics include how solution-processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication-applications beyond those directly analogous to inorganic doping.

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Section: Fabrication and Morphologymentioning

confidence: 99%

Section: Controlling Defects and Impurities With Dopantsmentioning

confidence: 99%

Controlling Molecular Doping in Organic Semiconductors

2017

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“…The J ‐ V data for 20 different but simultaneously fabricated OPV cells based on PM6:Y6:PC 71 BM BHJs doped with 0.004 wt% of BV, are presented in Figure S16 (Supporting Information), while a summary of cell parameters is given in Table S10 (Supporting Information). In Figure 4c, we summarize the PCE values reported to date for OPVs based on molecularly doped BHJs 17,18,20,22,47,48. Among these studies Xu et al,22 is the only one to describe the impact of n‐type doping in all‐polymer OPVs with the remaining reporting the use of p‐type dopants.…”

Section: Summary Of Operating Parameters Of Solar Cells Based On Pm6:mentioning

confidence: 99%

17.1% Efficient Single‐Junction Organic Solar Cells Enabled by n‐Type Doping of the Bulk‐Heterojunction

et al. 2020

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Molecular doping is often used in organic semiconductors to tune their (opto)electronic properties. Despite its versatility, however, its application in organic photovoltaics (OPVs) remains limited and restricted to p‐type dopants. In an effort to control the charge transport within the bulk‐heterojunction (BHJ) of OPVs, the n‐type dopant benzyl viologen (BV) is incorporated in a BHJ composed of the donor polymer PM6 and the small‐molecule acceptor IT‐4F. The power conversion efficiency (PCE) of the cells is found to increase from 13.2% to 14.4% upon addition of 0.004 wt% BV. Analysis of the photoactive materials and devices reveals that BV acts simultaneously as n‐type dopant and microstructure modifier for the BHJ. Under optimal BV concentrations, these synergistic effects result in balanced hole and electron mobilities, higher absorption coefficients and increased charge‐carrier density within the BHJ, while significantly extending the cells' shelf‐lifetime. The n‐type doping strategy is applied to five additional BHJ systems, for which similarly remarkable performance improvements are obtained. OPVs of particular interest are based on the ternary PM6:Y6:PC71BM:BV(0.004 wt%) blend for which a maximum PCE of 17.1%, is obtained. The effectiveness of the n‐doping strategy highlights electron transport in NFA‐based OPVs as being a key issue.

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“…Dedoping with 10% PA:cyclohexanone for 15 min recovers switching characteristics but results in a slight decrease in charge carrier mobility. This effect may result from increased trap density due to reduced trap filling, 48 Fluorescence spectra of P3HT films as-cast, doped, and dedoped with 10% PA in cyclohexanone for varying times. As in (a), all spectra are normalized by the integrated as-cast fluorescence intensity.…”

Section: Chemistry Of Materialsmentioning

confidence: 99%

Quantitative Dedoping of Conductive Polymers

Jacobs¹,

Wang²,

Hafezi³

et al. 2017

Chem. Mater.

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Although doping is a cornerstone of the inorganic semiconductor industry, most devices using organic semiconductors (OSCs) make use of intrinsic (undoped) materials. Recent work on OSC doping has focused on the use of dopants to modify a material’s physical properties, such as solubility, in addition to electronic and optical properties. However, if these effects are to be exploited in device manufacturing, a method for dedoping organic semiconductors is required. Here, we outline two chemical strategies for dedoping OSC films. In the first strategy, we use an electron donor (a tertiary amine) to act as competitive donor. This process is based on a thermodynamic equilibrium between ionization of the donor and OSC and results in only partial dedoping. In the second strategy, we use an electron donor that subsequently reacts with the p-type dopant to create a nondoping product molecule. Primary and secondary amines undergo a rapid addition reaction with the dopant molecule 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4TCNQ), with primary amines undergoing a further reaction eliminating HCN. Under optimized conditions, films of semiconducting polymer poly(3-hexylthiophene) (P3HT) dedoped with 1-propylamine (PA) reach as-cast fluorescence intensities within 5 s of exposure to the amine, eventually reaching 140% of the as-cast values. Field-effect mobilities similarly recover after dedoping. Quantitative fluorescence recovery is possible even in highly fluorescent polymers such as PFB, which are expected to be much more sensitive to residual dopants. Interestingly, treatment of undoped films with PA also yields increased fluorescence intensity and a reduction in conductivity of at least 2 orders of magnitude. These results indicate that the process quantitatively removes not only F4TCNQ but also intrinsic p-type impurities present in as-cast films. The dedoping strategies outlined in this article are generally applicable to other p- and n-type molecular dopants in OSC films.

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Trade‐Off between Trap Filling, Trap Creation, and Charge Recombination Results in Performance Increase at Ultralow Doping Levels in Bulk Heterojunction Solar Cells

Cited by 50 publications

References 51 publications

Controlling Molecular Doping in Organic Semiconductors

Controlling Molecular Doping in Organic Semiconductors

17.1% Efficient Single‐Junction Organic Solar Cells Enabled by n‐Type Doping of the Bulk‐Heterojunction

Quantitative Dedoping of Conductive Polymers

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