Energy level determination in bulk heterojunction systems using photoemission yield spectroscopy: case of P3HT:PCBM

Abstract

Ultraviolet photoelectron spectroscopy (UPS) is commonly used method for energy level determination using planar heterojunction samples in either metal/organic or organic/organic systems. Only some attempts have been made in bulk heterojunction system studies. Photoemission yield spectroscopy (PYS) could be applied as a method for organic compound-organic compound interface studies in bulk heterojunction samples. Contrary to the UPS, PYS method does not require ultra-high vacuum which simplifies experiment setup. Also, scanning depth of PYS is in the range of tens of nanometers which allows studying deeper layers of the sample instead of only surface layer. In this work poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction thin films were studied as a model system. A mass ratio between P3HT and PCBM in the system was varied from 1:0 to 1:50. Ionization energy dependence on this ratio was studied using two methods-UPS and PYS. To study the influence of sample morphology on the PYS measurements and obtainable results, phase separated and homogeneously distributed samples were prepared for analyses. P3HT ionization energy shift of 0.40eV was observed in the samples made from chloroform solution. Experiments showed the need for a low degree of phase separation between P3HT and PCBM to observe P3HT ionization energy shift using PYS. On the contrary, no ionization energy shift of P3HT was observed in the UPS measurements for the same systems.

Influence of organic material and sample parameters on the surface potential in Kelvin probe measurements

“…The obtained surface potential value for P3HT, which does not depend on metal work function, was Φ P3HT = 4.80 eV. Our previous research shows that the ionization energy of P3HT is I P3HT = 4.54 eV [28]. It means that the obtained surface potential value is around 0.25 eV higher than the ionization energy of P3HT molecules and its relation to the Fermi level of P3HT is questionable.…”

The Relation Between Photoconductivity Threshold and Open-Circuit Voltage in Organic Solar Cells

“…For example, ionization energy values for popular electron donor polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) in various scientific papers ranges from 4.7 to 5.3 eV [11]- [15]. In our previous studies, we obtained the value of 4.54 ± 0.03 eV [9], [10]. Correct estimation of electron affinity is even more complicated.…”

“…Photoelectron yield spectroscopy (PYS) is used as an alternative method [7]- [10] as it is simple and does not require an ultrahigh vacuum. Although the principle of these methods is similar, reported results can differ from author to author.…”

“…Additionally, it has been shown that the energy level shift takes place at the organic material interface [10], [21], [22]. It means that even if the correct energy level values for each separate material are obtained, the real gap between donor molecule ionization energy and acceptor material electron affinity can differ from the estimated value when materials are mixed.…”

Green-light wavelength-selective organic solar cells for agrivoltaics: dependence of wavelength on photosynthetic rate

“…The electron affinities (EAs) and ionization potentials (IPs) of TT-FT-ID and TT-T-ID in lms were determined using low-energy inverse photoemission spectroscopy (LEIPS) 40 and photoelectron yield spectroscopy (PYS) 41,42 (Fig. 4).…”