Interface state recombination in organic solar cells

Street, R. A.; Schoendorf, M.; Roy, Anshuman; Lee, J. H.

doi:10.1103/physrevb.81.205307

Cited by 380 publications

(375 citation statements)

References 30 publications

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“…Given the typical fill factor, efficiency, and other properties of present organic solar cells, D E is unlikely to be less than 0.5 eV, but 0.55 eV is plausible and agrees with other estimates. 23 The 0.55 eV value for D E is therefore evidence that the interface gap energies are correctly located. It has been suggested that values for E GI − eV OC in organic solar cells may be as low as 0.3 eV, 36 which is too small given the typical fill factor, and probably results from inaccurate estimates for the HOMO and LUMO energies.…”

Section: Background and Measurement Methodologymentioning

confidence: 99%

“…The photocurrent spectral response (PSR) technique measures the optical absorption of transitions that generate photocurrent in the BHJ structure. 23 The technique is sufficiently sensitive to measure much lower absorption than standard optical absorption techniques and reveals the CT absorption at energies below the bulk exciton absorption. The measurement technique is described elsewhere and is performed either in a modified FTIR spectrometer 24 or by a standard photoconductivity measurement system with lock-in amplifier.…”

Section: Background and Measurement Methodologymentioning

confidence: 99%

“…23,28 The fact that the intensity of the CT absorption relative to the excitons can vary for the same nominal material indicates that the location of E GI must be found from the shape of the absorption band. Two models for the shape of the CT absorption have been proposed.…”

Section: Background and Measurement Methodologymentioning

confidence: 99%

“…The measurement technique is described elsewhere and is performed either in a modified FTIR spectrometer 24 or by a standard photoconductivity measurement system with lock-in amplifier. 23,25 All the measurements were made at room temperature, and the maximum monochromatic illumination intensity is <1 mW/cm 2 , so that no significant sample heating occurs. Other sensitive optical absorption measurements have confirmed that the PSR data indeed measure the CT optical absorption and hence that the internal quantum efficiency for charge generation is independent of energy.…”

Section: Background and Measurement Methodologymentioning

confidence: 99%

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Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

et al. 2014

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Photocurrent spectroscopy is used to measure both the charge transfer and exciton optical absorption spectra of various bulk heterojunction organic solar cells. The energy difference between the polymer HOMO energy and the fullerene LUMO energy is obtained from the spectra, along with the disorder energy. Combining information from cells with several different polymers and fullerenes allows measurements of the energy differences between HOMO or LUMO energies for about 10 different polymers and fullerenes, with an estimated uncertainty of 50 meV. Heterojunction band offsets are obtained for the various cells, distinguishing between the excitonic and the single-carrier band offsets. The cell open-circuit voltage is shown to be closely correlated with the interface band gap. The exciton disorder energy is directly correlated to the band-tail disorder and we also consider the effects of exciton thermalization on the charge generation mechanism. The data indicate that an energy offset between the polymer exciton and the charge transfer ground state below about 0.25 eV adversely affects the cell performance, while a HOMO band offset below about 0.2−0.3 eV also degrades cell performance but by a different mechanism.

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Section: Background and Measurement Methodologymentioning

confidence: 99%

Section: Background and Measurement Methodologymentioning

confidence: 99%

Section: Background and Measurement Methodologymentioning

confidence: 99%

Section: Background and Measurement Methodologymentioning

confidence: 99%

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Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

et al. 2014

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“…recombination through contacts [92]. The approaches to determine recombination lifetimes include time-resolved vibrational spectroscopy [93,94], impedance spectroscopy [95,96], transient photoconductivity [71], and forward-to-reverse transient response [97].…”

Section: Read-hall (Srh) Interface and Reverse-diffuse [Reverse-diffmentioning

confidence: 99%

Photocurrent Limiting Mechanisms in Organic Bulk Heterojunction Solar Cells

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As we use more energy as a society, there is a greater need to become less reliant on non-renewable energy sources. Photovoltaics (PV), as a method of harvesting sunlight and converting it into electricity, provide a potential way of producing clean renewable energy.Organic bulk heteojunction (BHJ) solar cells featured with low-cost, large-area and thin-film manufacture, have become more viable with the recent demonstration with power conversion efficiencies (PCE) as high as10.7%. However, there is still a room for further improvement to reach the thermodynamic PCE limit of ~22%. For organic BHJ solar cells, the efficiency is essentially limited by the low short-circuit current density, which is determined by four sequential steps that are necessary to transform incident photons to free charge carriers. These steps are photo absorption and exciton formation, exciton diffusion, polaron pair dissociation and free polaron collection. In order to know the extent to which each of these steps limits the efficiency, a systematic study to analyze the limiting mechanisms is beneficial.In this work, we report approaches that are capable of providing insight into these processes by applying several complementary experimental techniques and theoretical calculations. The material system that are studied are poly(3-hexylthiophene) (P3HT): Therefore, it is quite intrigue to explore the reasons behind that.For absorption, the optical transfer-matrix theory is applied on the multi-thinlayer structure of organic BHJ solar cells. The complex refractive index is extracted.Meanwhile, the optical electric field and the absorption efficiency are calculated and iii AcknowledgementThe Ph.D study is a strict but a joyful scientific training. To make through it, I'd like to thank to the people who gave me tremendous support in these years.First and foremost I would like to thank my supervisor, Dr. Joe C. Campbell, for his patient guidance, continual support and encouragement during my Ph.D study. I learned a lot from him, not only about how to tackle the experimental/theoretical problems, but also about the vision, the presentation skills, and the way to keep a good relationship with your collaborators. I also appreciate the research freedom that Dr. Joe C. Campbell gave to me, which allows me to deliberate, to try and to pursuit without worrying.

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