Raitis Grzibovskis

Improvement of Solar PV Efficiency. Potential Materials for Organic Photovoltaic Cells

Abstract Organic photovoltaic (OPV) cells are considered as a viable alternative to those energy sources currently in use. In this work three derivatives of original N,N’- dimetilaminobenziliden-1,3-indandione (DMABI) material are presented as potential materials for OPV. The photoconductivity threshold energy was evaluated from the perspective of spectral dependence of photoconductivity quantum efficiency, and the optical energy gap was defined to determine the optical absorption spectra. The absorption spectra of derivatives are blue shifted compared to original DMABI. Use of these derivatives in multilayer solar cells with original DMABI makes it possible to broaden the spectral response range of OPV.

Study of the P3HT/PCBM interface using photoemission yield spectroscopy

Photogeneration efficiency and charge carrier extraction from active layer are the parameters that determine the efficiency of organic photovoltaics (OPVs). Devices made of organic materials often consist of thin (up to 100nm) layers. At this thickness different interface effects become more pronounced. The electron affinity and ionization energy shift can affect the charge carrier transport across metal-organic interface which can affect the performance of the entire device. In the case of multilayer OPVs, energy level compatibility at the organic-organic interface is as important. Photoemission yield spectroscopy was used for organic-organic interface study by ionization energy measurements. In this work we studied “sandwich” type samples of two well-known organic photovoltaic materials- poly(3- hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Ionization energy changes at the P3HT/PCBM interface depending on PCBM layer thickness were studied. P3HT layer was obtained by spin-coating while PCBM was deposited on the P3HT by thermal evaporation in vacuum. No ionization energy shift of P3HT was observed. On the contrary, PCBM at the interface with P3HT created additional 0.40eV barrier for hole transport from PCBM to P3HT.

Energy level determination of purine containing blue light emitting organic compounds

Organic light emitting diodes (OLED) have found their applications in the mobile and TV screens. Till now the commercially available diodes are made by expensive thermal evaporation in a vacuum. The costs of OLED fabrication could be decreased by applying low-cost wet casting methods, for example, spin-coating. In this work, we have studied a group of blue light emitting purine derivatives which could potentially be used in OLEDs. The advantage of these compounds is their ability to form amorphous thin films from solutions. All the thin films were prepared by the spincoating method from chloroform solution on ITO glass. The position of hole and electron transport energy levels is important for efficient OLED fabrication. Ionization energy was determined using photoelectron yield spectroscopy. The gap between ionization energy and electron affinity was determined using photoconductivity measurements. Electron affinity (Ea) then was calculated as a difference between ionization energy (I) and photoconductivity threshold value (Eth). Changes in the energy level values depending on the molecule structure were investigated. The position of electron acceptor group strongly affects the gap between ionization energy and electron affinity, while with the help of the attached substitute groups it is possible to alter the ionization energy. Fine “tuning” of the ionization energy values can be achieved by altering the length of the “tail” where the inactive bulky group is attached.

3,3'-Bicarbazole structural derivatives as charge transporting materials for use in OLED devices

In this study we report novel 3,3′-bicarbazole based charge transporting materials mainly designed for a use in systems containing phosphorescent iridium (III) complex emitters. A low-cost oxidative coupling reaction using FeCl3 was employed in the synthesis of 3,3′-bicarbazole compounds. Different derivatives of 3,3′-bicarbazole with 4-ethoxyphenyland ethyl- substituents at 9,9′- positions and (2,2-diphenylhydrazono)methyl- and 4-(dimethylamino)styryl- substituents at 6,6′- positions were synthesized. Obtained (2,2-diphenylhydrazono)methyl- derivatives exhibit glass transition temperatures that are sufficient for applications in electronic devices. Thin amorphous films of good optical quality can be produced from synthesized materials using spin-coating method. The effect of (2,2-diphenylhydrazono)methylsubstituents at 6,6′- and 4-ethoxyphenyl- substituents at 9,9′- positions on the charge transport properties of the 3,3′-bicarbazole derivatives was investigated. With the introduction of both electron acceptor and donor moieties to 3,3′-bicarbazole structure material electron and hole drift mobilities reach approximately 1·10-5 cm2/V·s. Molecule ionization (If) levels and electron affinity (EAf) levels in thin films were determined using photoelectric effect experiment. Depending on the nature of substituents at 6,6′- and 9,9′- positions If levels range from -5.19 to -5.13 eV and EAf levels are from -2.44 to -2.38 eV.

Photoelectrical properties of indandione fragment containing azobenzene compounds

Organic materials are becoming more popular due to their potential application in electronics. Low molecular weight materials possible produce from solution are in special consideration. It gives the possibility to avoid both thermal evaporation in vacuum, and use of polymers in thin film preparation process. Indandione fragment containing azobenzene compounds are one of such materials. These compounds are good candidates for use in design of novel molecular electronic devices due to their possibility to form amorphous structure from solution thus allowing developing flexible, small size systems with low production costs. In this work three indandione fragment containing azobenzene compounds were investigated. Difference between these compounds is bulky groups which assist formation of amorphous thin film. Absorption spectra of the investigated compounds are similar to P3HT but with higher absorption coefficient. Molecule ionization and electron affinity levels of these compounds are around -5.45eV and -3.80eV, respectively. Combining PCBM with investigated compounds could lead to difference between electron affinity levels maximum of 0.15eV. It is several times less compared to ~1eV for P3HT:PCBM system. Higher difference between the donor ionization level and the acceptor affinity level could also be obtained which should lead to the higher open circuit voltage.