File Name: p type and n type materials .zip
Doping means the introduction of impurities into a semiconductor crystal to the defined modification of conductivity. Other materials are aluminum, indium 3-valent and arsenic, antimony 5-valent. The dopant is integrated into the lattice structure of the semiconductor crystal, the number of outer electrons define the type of doping.
Organic thermoelectrics are attractive for the fabrication of flexible and cost-effective thermoelectric generators TEGs for waste heat recovery, in particular by exploiting large-area printing of polymer conductors.
Here we tackle this problem in a relevant class of electron transporting, naphthalene-diimide copolymers, by substituting the imide oxygen with sulfur. This result highlights the effectiveness of chemical tuning to improve air stability of n-type solution-processable polymer conductors and shows a path toward ambient large-area manufacturing of efficient polymer TEGs.
Energy harvesting with organics sees its most mature example in organic solar cells, which enable lightweight and large-area solar energy converters suitable for distributed energy generation.
At the same time, such development could lead to custom-shaped active coolers, 4 , 5 facilitating their integration into existing electronic appliances. One of the most appealing paths toward such applications is to make use of solution-processed organic compounds in order to enable mass-printed OTEGs, thus drastically limiting production costs.
The efficiency of the conversion of a heat flux into a current by thermoelectric TE materials can be related to the dimensionless material figure of merit zT , defined as. Since in their pristine form conjugated organic materials are typically insulators or semiconductors with low background conductivity, doping is necessary to achieve suitable electrical conductivity. For example, proton transfer from acids or hydride transfer can lead to p-type and n-type doping, respectively.
For the realization of an efficient thermoelectric device, complementary p-type and n-type conducting materials with high PF are needed at the same time. A further strong limitation for n-type materials arises from their air instability, which precludes ambient processing of OTEGs and imposes severe constraints regarding devices encapsulation.
To overcome this issue, both the dopant and the semiconductor must be air-stable. With regard to the dopant, adopting species with very low ionization energy to induce an electron transfer to the host semiconductor is critical, because the donating molecule is very prone to redox reactions with species in the atmosphere. However, all the previous systems did not exhibit air stability or at least this aspect was not investigated. The lowering of the LUMO energy of the semiconductor can be obtained by diverse modifications of the electron withdrawing cores such as in naphthalene-diimide NDI -based copolymers, one of the most studied classes of n-type materials.
In fact molar mass determination by SEC is not straightforward here, as also seen by the very high M w values caused by aggregation. The effect of the substitution of the imide oxygen atoms with sulfur atoms is clearly visible from the UV—visible absorption spectra see Supporting Information, SI, Figure S1.
Since the HOMO is mostly located on the thiophene moieties, 62 while the LUMO is localized on the naphthalene-diimide unit, 63 the energy gap reduction should be ascribable to a deeper LUMO energy level as an effect of thionation. This is confirmed by cyclic voltammetry measurements Figure S2 , as reported by Shin et al.
Following the procedure reported by Schlitz et al. A further increase in dopant concentration is then detrimental for the electrical conductivity. With increasing dopant concentration, W f reduces from 4. X-ray photoemission spectroscopy XPS was used to certify the presence of dopant molecules and to assess their chemical state. Each data point is an average of at least four devices; vertical error bars represent the standard deviation.
The electrical conductivity of PNDIT2 rapidly drops, reaching almost the pristine film value after min. Such strong instability basically prevents any ambient processing of this doped system and its use for OTEGs fabrication.
On the contrary, the electrical conductivity of 2 S - trans -PNDIT2 remains within the same order of magnitude of its initial value for as long as 16 h of air exposure, showing only a reduction factor slightly lower than 2. This is an unprecedented stability for doped n-type organic materials and would clearly leave room for ambient processing of organic thermoelectric devices, strongly desirable in the case of high-throughput printing processes, before encapsulation.
It is interesting to note that the field-effect mobility as measured in bottom-contact, top-gate field-effect transistors based on the pristine thin films, is higher for PNDIT2 than for 2 S - trans -PNDIT2, reaching values of 0. To further investigate the factors that cause the higher conductivity of 2 S - trans -PNDIT2, we followed the doping process through optical absorption and structural measurements on thin films as a function of doping concentration.
The pristine film has a broad band from to nm, with a peak around nm. The strong red shift with respect to the spectrum of molecularly dissolved chains in chloronaphthalene solution is due to aggregation. Overall, the evolution of the UV—vis spectra with doping concentration suggests that the dopant molecule suppresses chain—chain interactions of 2 S - trans -PNDIT2, resulting in an increased amorphous component.
Higher doping concentrations did not lead to further modification of the UV—vis spectra, indicating a weaker interaction of the dopant with PNDIT2. As-cast PNDIT2 films exhibit a pronounced edge-on stacking of chains, with four orders of lamellar stacking peaks evident. We note that this is unusual for PNDIT2 films, likely due to the low molecular weight adopted in this case. Similar to 2 S - trans -PNDIT2, the observed d -spacings are unaffected by doping, indicating that the dopant is not intercalating within the crystals.
In both cases no scattering from the dopant crystals is appearing, with the second-order lamellar stacking no longer evident. Still, at high dopant concentration, miscibility of the two components is a limiting factor on the doping efficacy, with an obvious phase separation, also evidenced by AFM topography measurements Figure S7.
The negative sign of the Seebeck is indicative of electrons being the majority carriers. The improved electrical conductivity, owing to a more favorable interaction of the dopant and the polymer, enables a 4-fold improvement in power factor to be realized.
In conclusion, we have reported an effective strategy to strongly improve ambient stability of n-type doped NDI-based copolymers. Such a pronounced improvement demonstrates that inherent environmental stability of n-type doped polymer conductors can be largely controlled through the proper design of the conjugated semiconductor system.
This result is an important step toward stable n-type doped polymers, which more favorably allow ink formulation for large-area manufacturing through scalable printing technologies, which finally may enable high-throughput fabrication of low-cost and efficient organic thermoelectric generators under ambient conditions based on thermocouples combining n-type and p-type materials.
Aliquots of polymer and dopant solution were mixed and stirred for 10 min at room temperature just before the deposition. Low-alkali F Corning glass was used as substrate. The substrates were cleaned in an ultrasonic bath of Milli-Q water, acetone, and isopropyl alcohol, 10 min for each step, and exposed to O 2 plasma at W for 10 min.
Electrodes were obtained by thermal evaporation, through a shadow mask, of a 1. Metals were deposited by thermal evaporation: 1.
The 40 nm thick gate contact was obtained by thermal evaporation of Al through a shadow mask. The electrical conductivity was extracted through the linear fit of the I — V characteristics, Figure S8. The saturation mobility values were extracted using the gradual-channel approximation.
For the extraction of the work function we used the multiple single-point measurements mode. The work function of the sample was obtained by adding to the measured work function value the contact potential difference CDP of the tip, determined with an initial calibration on standard gold substrate.
Images shown were acquired at an incident angle close the critical angle. Such images were chosen from a series of recorded 2D patterns with incident X-ray angle varying from 0. The X-ray exposure time was 1 s such that no film damage was identified. The sample-to-detector distance was calibrated using a silver behenate sample.
AFM polymer thin film samples were prepared using the same procedure described earlier in the Experimental Section. The surface morphology of the films was obtained with an Agilent atomic force microscope operated in the acoustic mode. For the Seebeck measurements we employed a homemade setup described by Beretta et al. We thank F. Scuratti for help with AFM measurements.
National Center for Biotechnology Information , U. Published online Aug Madan S. Christopher R. Author information Article notes Copyright and License information Disclaimer.
E-mail: ti. E-mail: ed. Received May 16; Accepted Aug This is an open access article published under an ACS AuthorChoice License , which permits copying and redistribution of the article or any adaptations for non-commercial purposes. This article has been cited by other articles in PMC. Keywords: polymer conductors, n-type doping, organic thermoelectrics, air stability, conjugated polymers.
Introduction Energy harvesting with organics sees its most mature example in organic solar cells, which enable lightweight and large-area solar energy converters suitable for distributed energy generation. Open in a separate window. Figure 1. Optical and Electrical Properties The effect of the substitution of the imide oxygen atoms with sulfur atoms is clearly visible from the UV—visible absorption spectra see Supporting Information, SI, Figure S1.
Figure 2. Figure 3. Conclusions In conclusion, we have reported an effective strategy to strongly improve ambient stability of n-type doped NDI-based copolymers. Atomic Force Microscopy AFM polymer thin film samples were prepared using the same procedure described earlier in the Experimental Section.
Seebeck Measurements For the Seebeck measurements we employed a homemade setup described by Beretta et al. Acknowledgments We thank F. Notes The authors declare no competing financial interest. References Huang Y. The Future of Organic Photovoltaics. C , 3 , — Macromolecules , 47 , Review on Polymers for Thermoelectric Applications.
Materials , 7 , — A , 3 , — Energy Mater. Energy Environ. A , 2 , — Nano Energy , 30 , — Energy Sci. C , 2 , B , , —
An extrinsic semiconductor is one that has been doped ; during manufacture of the semiconductor crystal a trace element or chemical called a doping agent has been incorporated chemically into the crystal, for the purpose of giving it different electrical properties than the pure semiconductor crystal, which is called an intrinsic semiconductor. In an extrinsic semiconductor it is these foreign dopant atoms in the crystal lattice that mainly provide the charge carriers which carry electric current through the crystal. The doping agents used are of two types, resulting in two types of extrinsic semiconductor. An electron donor dopant is an atom which, when incorporated in the crystal, releases a mobile conduction electron into the crystal lattice. An extrinsic semiconductor which has been doped with electron donor atoms is called an n-type semiconductor , because the majority of charge carriers in the crystal are negative electrons. An electron acceptor dopant is an atom which accepts an electron from the lattice, creating a vacancy where an electron should be called a hole which can move through the crystal like a positively charged particle.
The various factors like doping element, nature of doping element, the majority and minority carriers in the p-type and n-type semiconductor. The density of electrons and holes, energy level and Fermi level, the direction of movement of majority carriers, etc. The difference between a p-type semiconductor and an n-type semiconductor is given below in the tabulated form. The p-type semiconductor is formed when the Trivalent impurity is added to the pure semiconductor. Similarly, when a Pentavalent impurity is added to the pure semiconductor n-type semiconductor is obtained.
P-n junctions are formed by joining n -type and p -type semiconductor materials, as shown below. Since the n -type region has a high electron concentration and the p -type a high hole concentration, electrons diffuse from the n -type side to the p -type side. Similarly, holes flow by diffusion from the p -type side to the n -type side. If the electrons and holes were not charged, this diffusion process would continue until the concentration of electrons and holes on the two sides were the same, as happens if two gasses come into contact with each other. However, in a p-n junction, when the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move. On the n -type side, positive ion cores are exposed. On the p -type side, negative ion cores are exposed.
This is called an n-type semiconductor. Boron can also be used to dope a pure crystal of silicon. But since boron only offers 3 of the four electrons that a silicon atom needs, each silicon center is left with a hole. Semiconductors made in this manner are called p-type.
Organic thermoelectrics are attractive for the fabrication of flexible and cost-effective thermoelectric generators TEGs for waste heat recovery, in particular by exploiting large-area printing of polymer conductors. Here we tackle this problem in a relevant class of electron transporting, naphthalene-diimide copolymers, by substituting the imide oxygen with sulfur. This result highlights the effectiveness of chemical tuning to improve air stability of n-type solution-processable polymer conductors and shows a path toward ambient large-area manufacturing of efficient polymer TEGs. Energy harvesting with organics sees its most mature example in organic solar cells, which enable lightweight and large-area solar energy converters suitable for distributed energy generation. At the same time, such development could lead to custom-shaped active coolers, 4 , 5 facilitating their integration into existing electronic appliances.
Semiconductors are materials that have properties of both normal conductors and insulators. Semiconductors fall into two broad categories:. In the classic crystalline semiconductors, electrons can have energies only within certain bands ranges of energy levels. The energy of these bands is between the energy of the ground state and the free electron energy the energy required for an electron to escape entirely from the material. The energy bands correspond to a large number of discrete quantum states of the electrons.
The process of purposefully adding impurities to materials is called doping; semiconductors with impurities are referred to as "doped semiconductors".
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