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Publikācija: Investigations of New Organic Materials for PV

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Nosaukums oriģinālvalodā Investigations of New Organic Materials for PV
Pētniecības nozare 2. Inženierzinātnes un tehnoloģijas
Pētniecības apakšnozare 2.2. Elektrotehnika, elektronika, informācijas un komunikāciju tehnoloģijas
Autori Igors Kaulačs
Inta Muzikante
Lilita Gerca
Gunta Šlihta
Jānis Kalnačs
Aleksandrs Murašovs
Māra Plotniece
Peteris Šipkovs
Gaļina Kaškārova
Valdis Kampars
Modris Roze
Vicente Parra
Atslēgas vārdi polymer film, heterojunction, organic solar cell, CT absorption band
Anotācija An attempt was made to extend the photosensitivity spectral range of fullerene/poly(3-hexylthiophene) blend to NIR region by adding extra electron donor – hydroxygallium phthalocyanine (GaOHPc) with a strong and wide intermolecular charge transfer (CT) band around 830 nm. Multilayer cells of ITO/PEDOT: PSS/6 GaOHPc/P3HT: C61(CO2Et)2 have been prepared by spin coating with vacuum evaporated Al or In top electrodes. Significant photosensitivity of the cells was in 370-900 nm spectral range. However charge carrier photogeneration efficiency was 3 times higher for illumination in P3HT absorption band as compared with the GaOHPc CT band at 830 nm. But when GaOHPc was mixed in the blend forming P3HT:C61(CO2Et)2:GaOHPc active layer its CT band shifted to infrared part at 875 nm. At the same time the charge carrier photogeneration efficiency for illumination in GaOHPc CT band increased significantly, exceeding its value for illumination in P3HT absorption band. The bulk heterojunction approach appears to be one of the most promising concepts of creating efficient, low cost and easy producable solar cells. For this purpose one of the best materials is regioregular poly-3-hexylthiophene (P3HT), which is widely used as a donor molecule and a whole transporter, with soluble fullerene derivatives as acceptors and electron transporters. Blends of these molecules in PV cells exhibit the efficiency of light power conversion up to 5%. Still, it is not sufficient to meet realistic requirements for commercialization. The main drawback of this highly efficient blend is its limited spectral range, which covers 350-650 nm interval, allowing only ~ 35% of the full solar spectrum energy to be used. In the present work we tried to extend the spectral range of the blend by adding hydroxygallium phthalocyanine (GaOHPc), which has a strong and wide intermolecular charge transfer (CT) absorption band around 830 nm. The choice of GaOHPc was dictated by the following reasons: 1) high thermal and chemical stability of phthalocyanines as compared with the most of molecular materials, 2) the NIR absorption provides the possibility to extend the photosensitivity spectral range (up to the NIR region) of the blend, 3) the CT character of the IR absorption band, which promises high efficiency of charge carrier photogeneration, 4) the solubility in chloroform, which allows its processing by spin coating. In this work we show that by adding GaOHPc to the P3HT-fullerene blend its photosensitivity spectral range can be extended in the NIR direction beyond 900 nm, while the efficiency of charge carrier photogeneration for illumination in the GaOHPc intermolecular CT band at 880 nm even exceeds its value for illumination in the P3HT absorption band. At the same time the CT band located for pure polycrystaline film at 830 nm shifts to 870-880 in the GaOHPc:P3HT: C61(CO2Et)2 blend with the weight ratio of components being (1:1:2). The paper discusses the problems connected with the electrodes and the built-in electric field in the sample as the cause of low external quantum efficiency (EQE) of photocurrent. For host polymer and hole transporter regioregular poly3-hexylthiophene (P3HT) with an average molecular weight of 87000 (Sigma Aldrich) was chosen. As extra donor for increasing the photosensitivity spectral range of the blend hydroxygallium phthalocyanine (GaOHPc) (Fig.1.) was synthesized according to Yamasaki et al. Fig.1: Chemical structure of used organic molecules For the electron acceptor, the soluble C60 derivative (C61(CO2Et)2) shown in Fig.1 was synthesised. As the sample substrate, ITO-covered glass with Rsu=4-8 Ohm/Sq (purchased from Delta Technologies) was used. The ITO electrode was covered with a 50-100 nm thick PEDOT:PSS layer by spin coating at 4000 rpm, applying the acceleration of 3000 rpm۰s-1, followed by two types of photosensitive organic layers: 1st type – a pure GaOHPc film formed using 6-fold deposition by spin coating of GaOHPc from its solution in chloroform to achieve for the film the optical density of 0.3-0.4. Then this rough surface having ~100 nm high hills was covered by a ~ 100nm thick layer of P3HT: C61(CO2Et)2 with the weight ratio of components 2 : 3, after which the top Al electrode (Rsu ~ 10 Ohm/Sq) was evaporated in vacuum 10-5 Torr. 2nd type - a blend of GaOHPc:P3HT:C61(CO2Et)2, with the weight ratio of components 1 : 1 : 2 was spin-coated from solution in a chloroform-chlorbenzene mixture at ~ 900 rpm applying the acceleration of 200 rpm۰s-1 , which was followed by vacuum evaporation at ~ 10–5 Torr of the Al or In electrode (Rsu ~ 10 Ohm/Sq). For both the types of samples the active cell area was ~0.06 cm2. The experimental setup is shown in Fig.2. The photocurrent measurements were carried out at RT in vacuum ~10-6 Torr. The samples were illuminated by chopper modulated monochromatic light via ITO electrode in the 370 –1300 nm spectral region with intensity 1012 – 1016 phot /(cm2۰s). Fig. 2: Experiment setup. The synchro detection technique with the use of PC-controlled data storage equipment was employed for measuring of the spectral dependences of photocurrent quantum efficiency: EQE = Iphoto /Φ (where Iphoto is photocurrent (el/s) and Φ is the photon flux (phot/s) incident upon the active area of the sample).
Atsauce Kaulačs, I., Muzikante, I., Gerca, L., Šlihta, G., Kalnačs, J., Murašovs, A., Plotniece, M., Šipkovs, P., Kaškārova, G., Kampars, V., Roze, M., Parra, V. Investigations of New Organic Materials for PV. No: RIO-9 World Climate & Energy Event Latin America Renewable Energy Fair (LAREF 2009): Proceedings of the International Conference , Brazīlija, Rio de Janeiro, 17.-19. marts, 2009. Rio de Janeiro: Rio Solar Ltda, 2009, 267.-273.lpp.
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