SnO thin films prepared by APCVD for organic solar cells application D.Hatem, M.S Belkaid, M.Pasquenelli LATAGE Laboratory of Advanced Technologies of Genie Electrics Faculty of Electrical and Computer Engineering Mouloud Mammeri University (UMMTO), BP 7 RP 5, Tizi-Ouzou, Algeria IMNP Aix-Marseille University, Institut Matériaux Microélectronique Nanosciences de Provence CNRS UMR 64 Campus de Saint-Jérôme, Avenue Escadrille Normandie Niemen Service 3, F-3397 Marseille Cedex,France *Corresponding author: e-mail: hatemdjedjiga@yahoo.fr, Phone: +3669674 Abstract: The efficiency of organic solar cells depends on many parameters such as absorption, charge transport, interface states, etc.... The amelioration of the efficiency of photovoltaic conversion can be obtained by minimizing losses in reflection and absorption in the transparent electrode involving increased absorption efficiency in the active layer which can be achieved by the use of TCOs with special optical and electrical properties. Tin oxide SnO thin films have been prepared by APCVD method using the SnCl as a starting material on glass. The surface morphology of the films deposited on glass substrates were investigated by scanning electron microscopy (SEM) and ellipsometry was used to determinate the refractive index for the films deposited at 48 C and the sheet resistance was measured using the Four-Point probe. SnO films prepared during minutes present a sheet resistance of 9.57Ωcm-, transmittance of 8% and refractive index of.75 can be used as in organic solar cells application to minimize the reflectivity. The total reflectivity of SnO/P3HT: PCBM obtained by using the films is less than 3%. Keywords organic solar cells, refractive index, reflectivity, conductive transparent oxide (SnO ),APCVD.. Experimental The atmospheric pressure CVD requires a simple apparatus. A precursor is carried into the CVD reactor by an inert gas such as argon. The diagram of our experiment is shown in figure [] : In our case we used the hydrous tin chloride (SnCl, 5H O). The reaction that occurred in this case is: SnCl 48C H O O SnO Cl Fig. Schematic diagram of Deposition of SnO films by APCVD method.
. Results Following curves show measurements of transmittance and reflectance spectra of undoped SnO film deposited on glass by APCVD method (Figure ). Fig. Measurements of transmittance a) and reflectance spectra b) of undoped SnO film deposited on glass APCVD method at T=48 C during minutes. The SnO films deposited by APCVD present a high transmittance and low reflectance The morphology of SnO films is given by the SEM image shown in Fig 3. Figure 3. SEM image of SnO films deposited by APCVD during minutes.
The surface morphology reveals the nano-crystalline SnO grains with pyramidal form.the grains combine each-other to make denser films significantly The pyramidal shape of the grains increases the surface and induces multiple reflections on the facets of the pyramids (a textured surface). Texturing ensures trapping of maximum light reducing reflectivity losses 3. Photovoltaic process The following figure summarizes the basic mechanisms involved during the photovoltaic process in organic solar cells. Absorption de la lumière + - Création d excitons + + Diffusion des excitons - - Relaxation non radiative Transfert des électrons à un complexe du transfert de charges Séparation électron/trou dans le champ induit Transport des électrons vers les électrodes Recombinaison couplée des charges Recombinaison des charges Transfert des électrons aux électrodes Figure 4. Photovoltaic process in organic solar cells It is obvious that these mechanisms are affected by recombination phenomena that reduce the efficiency of the solar cell which can be expressed by the following equation: η global = η A. η TC.FF.eV max /hυ It is observed that the efficiency depends on absorption (ηa), the charge transfer efficiency (ηtc), the fill factor FF and the ratio of photon energy absorbed with the energy extracted evmax / hυ. Improving the efficiency of photovoltaic conversion in the case of organic solar cells (conjugated polymers in our case) requires the improvement of mechanisms involved in this process by making a good choice for the structure of devices in our work we are interested in improving the absorption efficiency by minimizing the total reflectivity of the structure by the adequate choice of the transparent electrode. 4. Choice of thetco forming the electrode The objective of the choice of materials constituting the electrode and the interfacial layers is to contribute in improvement of the effieciency of photovoltaic conversion by incrasing absorption in the photoactive layer which involves the improvement of photo-generated current Iph, therefore the short-circuit that can be done with minimizing losses in reflection at the interface TCO/photoactive layer and the absorption loss in the TCO.
The TCO chosen must have a high transmittance in the visible and must be non-absorbent in this range of solar spectrum. The choice of SnO deposited by APCVD as transparent electrode is based on its optical and electronic properties: - a band gap of 3.6 to 4.6 ev - high transmittance, more than 85% in the visible spectrum - A refractive index n between.7 and.9 -.good conductivity, especially if it is doped fluoride (SnO doped F). - less expensive than ITO whose scarcity of indium is an obstacle to its use - good thermal, chemical and mechanical stability - low toxicity compared to other TCO Knowing that the absorption A, the transmission T and reflection R are related as follows : A+T+R= The determination of refractive indices of various layers that forming the structure and the extinction coefficient of the photoactive layer allows us to make a good choice of the transparent electrode to reduce reflection losses at interfaces. Knowing that the expression of Iph is given by the following equation Iph = [-exp(-αd p )]exp(-αd).f(λ)(-r).dλ d p : the thickness of ZCE d: the thickness of active layer The absorption of the active layer is given by: A=-e-αd α=4πk/λ α: absorption coefficient The expression for the total reflectance R of the structure can be determined as follows : We consider: R : the reflection at the interface glass/tco R : the reflection at the interface TCO/ organic semi-conductor given by : RR RR R R ( R R ) cos( ) R ( ) cos( ) R ( n n ) K ( n n ) K n : refractive index of the TCO n : refractive index of the semi-conductor
Kn tan g n K n 4 nd D : the thickness of TCO : the wavelength K : the extinction coefficient of the semi-conductor material The following figures represent the refractive index of SnO layer deposited by APCVD. Fig 5. Refractive index of undoped SnO films prepared by APCVD method The following figure represent the results giving the total reflectivity R for a structure of: glass/sno /P3HT:PCBM..3 R RRi.. 4 45 5 55 6 65 7 75 8 i Fig 6. Results of total reflectivity of: SnO (deposited by APCVD)/ active layer P3HT:PCBM.
5. Conclusion And Prospects The use of an inorganic oxide semiconductor such as tin oxide (SnO ) as transparent which is indeed more stable in air and humidity and also has the advantage of being transparent in the visible range of the solar spectrum allows us to reduce the total reflectivity Rand increase the absorption. The SnO films deposited by APCVD present a high transmittance and low reflectance The incomplete absorption in the limited thickness of the active layer implies that a part of the light passes through the thickness without being absorbed so we need techniques to capture reflected light which can significantly increase the absorption. We can do this using SnO deposited by APCVD As transparent electrode.refractive index characterizing this layer is a very important parameter for the choice of the layer. These results show that the SnO is thus a good candidate to replace ITO in the achievement of organic solar cells. References [] M.S. Belkaid, «contribution à la réalisation et l'optimisation photovoltaïque d'hétérojonction à semiconducteurs de type silicium-oxyde d'étain», magister thesis, USTHB,984. [] G.DE keser, V.Bayot, B.Olbrechts, V. Wilmart, Electronique organique ELEC 55,(-). [3] S.Alem-Boudjemline, «Réalisation et caractérisation des cellules photovoltaiques plastiques», Doctorat thesis, Université Paris XI Orsay, (4). [4] B. Brousse, «réalisation et caractérisation des cellules solaires organiques obtenues par dépôt physique», Doctorat thesis, Université LIMOGES, (4). [5] S. Sensfuss, M. Al-Ibrahim, optoelectronic properties of conjugated polymere/fullerene binary pairs with variety of LUMO level differences, 534, (4). [6] M.M.Hasan, A.S.M.A.Haseeb, R.Saidur, and H.H.Masjuki, effect of annealing treatment on optical properties of anatase TiO thin films, ISSN, volume 3, (8).