Proceedings of the European Summer School of Photovoltaics 4 â 7 ...

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Organic photovoltaics – chosen aspects Justyna Szostak, Ryszard Signerski, Kamila Żelechowska, Jan Godlewski Gdańsk University of Technology, Department of Physics of Electronic Phenomena, Faculty of Applied Physics and Mathematics Preparation of inorganic solar cells usually requires very costly and complicated high temperature vacuum deposition techniques. Furthermore, materials used for fabrication of these cells are expensive and often dangerous for the environment, not to mention the hazardous and toxic substances used during manufacturing process and its by-products [1]. Hence, organic solar cells (OSCs), that can be fabricated from relatively cheap materials, using simple solution coating techniques, i.e. ink-jet printing or spin and dip coating, which do not require high temperatures nor the usage of hazardous substances, seem to be a promising alternative for the inorganic photovoltaics. Moreover, the number of organic materials is much greater than the number of inorganic ones, so the potential of applications of organic materials in photovoltaics is greater as well. However, relatively low efficiency and fast degradation of organic solar cells [2, 3] are serious drawbacks that hinder commercialization of organic photovoltaic diodes. Designing organic solar cells with higher energy conversion efficiency and stability is impossible without extensive knowledge on all physical and chemical processes taking place inside organic photovoltaic devices in the dark and under illumination. Even though there are many experimental works showing the performance of OSCs, theoretical model for the photovoltaic effect taking place in these devices has not been developed yet. Significant differences in structure of organic and inorganic materials result in different types of interactions between molecules. Thus, even though the mechanisms of photovoltaic effect in inorganic systems are well known and described in literature, processes that affect photovoltaic phenomenon in organic systems still need further understanding. Thus, research we run in our laboratory is focused on analysis of the above mentioned processes. Photovoltaic phenomenon in organic systems In organic semiconductors, absorption of light leads to creation of columbically bound electron-hole pairs – excitons [4]. Low dielectric constants of organic materials result in high exciton binding energies (reaching 1.0 eV), so electron and hole cannot escape each other and they remain bound. Such pairs may diffuse to dissociation sites (e.g. interfaces of two different materials) where charge transfer (CT) states are created and subsequently dissociation of excitons into free charge carriers may occur. To collect these charge carriers a built in electric field, that will drive them towards respective electrodes, is necessary. The performance of organic Fig. 1. Scheme of charge carrier photogeneration at the junction of two organic materials. E F – Fermi level, E g – energy gap; HOMOhighest occupied molecular orbital, LUMO – Lowest Unoccupied Molecular Orbital; I in – incident light intensity; 1 – excitation of material II, 2 – exciton diffusion towards the junction of two organic materials, 3 – exciton dissociation into free charge carriers 118 photovoltaic cells is limited by the efficiency of many processes that constitute the photovoltaic phenomenon. Among others, limitations are inefficient exciton dissociation and poor charge carrier transport resulting from high rates of exciton recombination (short exciton lifetimes) and low charge carrier mobilities. Also potential barriers created in the near-electrode regions stand a serious problem that limits the efficiency of charge carrier collection. Probability of exciton dissociation into free charge carriers is quite high when it reaches an interface with another material of suitable energetic structure – there must be an offset between HOMO (the Highest Occupied Molecular Orbital) of one material and LUMO (the Lowest Unoccupied Molecular Orbital) of the other material (Fig. 1). This is why efficient organic solar cells comprise two layers of different materials and the larger their interface is the higher is the number of photogenerated charge carriers. Molecule that donates an electron in such system is called a donor (D) while the one that accepts this charge carrier is called an acceptor (A). Energy difference between LUMO of the acceptor and HOMO of the donor determines the maximum open – circuit voltage (U oc ) [5]. Results and scope of research Due to high probability of exciton dissociation at the junctions of two different materials future of organic solar cells lays in devices with active layer comprising two or more thin films, blends of organic materials or hybrid systems (based on organic and inorganic materials). Processes that play a significant role in the photovoltaic phenomenon strongly depend on the type of organic materials forming an active layer, this is why a thorough investigation of performance of OSCs comprising various groups of organic materials is inevitable. A particularly interesting new generation of OPV systems utilizes semiconducting conjugated polymers. This type of cells is referred to as ‘polymer solar cells’ or ’plastic solar cells’. The principles of operation of these solar cells have been a topic of various articles, because plastic solar cells are expected to offer several advantages over silicon-based ones due to the fact, that polymers are mechanically flexible, semi-transparent and soluble in common organic solvents. The latter property is a crucial advantage of polymers since it allows deposition of these materials by simple solution processing and printing technologies. Particular attention in the field of polymer solar cells is paid to bulk heterojunction solar cells (BHSC). In this type of cells donor and acceptor materials are mixed, so the active interface exist within the whole bulk of the active layer. However, it is much easier to determine the influence of different factors on the performance of a cell and to analyse the course of observed processes when structure of a cell is less complicated. That is why in the course of our investigations double-layer devices instead of bulk ones were taken into consideration. We investigated the role of anodic and cathodic buffer layers on the performance of polymer solar cells. Buffer layers are thin films put between an anode or cathode and an active layer. They are introduced to organic solar cells to improve the efficiency, but the role they play in various systems is not fully understood. The active layer of our cells comprised MEH-PPV (poly[2-methoxy-5-(2-ethylhexyloxy-p-phenylenevinylen)])) – soluble polymer used commonly as a donor material and F 16 ZnPc (hexadecafluorozincphthalocyanine) – small molecule that is an acceptor in this system. As buffer layers MoO 3 and BCP (bathocuproine) were used. All the investigated systems were sandwiched between ITO (indium tin oxide) and Ag electrodes. There are many theories on the effect of MoO 3 and BCP interlayers, for example, thin films of MoO deposited on the top of ITO electro- 3 Elektronika 6/2012
de are believed to change the ionizing potential of ITO (causing a rise in the work function of the latter, and so an increase in the built in potential) and as a result an increase in the open-circuit voltage [6]. Others think that these layers block an electron transfer to the anode which causes an increase in j sc (short-circuit current), U oc and η (energy conversion efficiency) [7]. BCP is often used due to its exciton blocking ability and its positive effect on j sc , U oc and η [8]. It is also regarded as a protective interlayer that reduces damage caused to the active layer during cathode deposition process [9]. Dark j-V characteristics obtained for our cells show that introduction of BCP buffer layer has a noticeable impact on forward and reverse currents causing a significant rise in the rectification ratio (RR) and decrease in series resistance (R s ) of a cell. We did not observe effect of anodic buffer layer on RR in the absence of BCP layer. However, cells incorporating both MoO 3 and BCP interlayers had much higher rectification ratios than the ones with BCP buffer layer only. Results of our research show, that presence of MoO 3 and BCP layers affects injection of charge carriers at electrode/organic material interface and their recombination. Fig. 2 shows the open-circuit voltage action spectra (U oc as a function of wavelength of incident light) obtained for cells with different set of buffer layers. It is noticeable that the presence of BCP layer causes an increase in shunt resistance (R sh ) improving U oc . Increase in R sh and simultaneous decrease in R s causes an increase in fill factor (FF). Inclusion of MoO 3 buffer layer seems to cause an increase in j sc and FF, but it is not so evident. So far, the best photovoltaic performance was obtained for cells provided with both interlayers. The research on effects of BCP and MoO 3 layers on photovoltage, photocurrent and j-V characteristics under various illumination conditions in polymer/small molecule cells is still in progress. The fact, that we found evidence of beneficial influence of MoO 3 anodic buffer layer rather only in the presence of cathodic buffer layer is very interesting and the origin of such relation needs further investigation. We think that the lack of significant improvement of cell’s performance in the presence of anodic interlayer and the absence of BCP might be related to damage caused to the active layer by silver atoms during cathode deposition. We investigated the role of BCP and MoO 3 buffer layers also in other photovoltaic devices based on copper phthalocyanine (CuPc) and perylene dye (MePTCDI) heterojunction provided with the same electrodes (i.e ITO and Ag) [10]. Strong rectification effect was observed in the presence of both buffer layers, just like in the case of MEH-PPV/F 16 ZnPc cells. Similarly, we noticed that these interlayers have beneficial effects on charge injection ability by limiting potential barriers existing at organic material/electrode interfaces. Reasonable photovoltaic parameters were obtained -U oc [mV] 400 300 200 ITO/MEH-PPV/F16 ZnPc/Ag ITO/MoO3 /MEH-PPV/F16 ZnPc/Ag ITO/MEH-PPV/F16 ZnPc/BCP/Ag ITO/MoO3/MEH-PPV/F16 ZnPc/BCP/Ag thin MEH-PPV layer, toluene+isopropanol 0,8 0,6 0,4 absorbance only in the presence of both buffer layers. Relation between U oc and light intensity (I 0 ) obtained for CuPc/MePTCDI systems shows that BCP layer may hinder exciton dissociation at the Ag electrode (there is no saturation of U oc even at high illumination intensities). MoO 3 also acted favourably on the above mentioned characteristics and we suggested two possible explanations for this effect: MoO 3 increases the rate of hole recombination at the ITO/CuPc interface or in the absence of MoO 3 voltage of polarity opposite to the one generated at CuPc/MePTCDI junction is developed at ITO/CuPc interface. On the other hand, there was hardly any influence of regarded interlayers on the dependence of short-circuit current on I 0 . Apart from working on the role of buffer layers in various organic photovoltaic systems and other research topics, we have been interested in photovoltaic phenomenon taking place in hybrid systems, like cells based on and CdTe (cadmium telluride)/F 16 ZnPc [11], TiO 2 (titanium dioxide)/PdPc (palladium phthalocyanine) [12] or TiO 2 /PBrPc (bromophosporus phthalocyanine) [13], that seem to be promising alternative to inorganic devices. Another class of new materials for photovoltaic applications that need intensive research are carbon nanotubes (CNTs). They possess many advantageous properties – they are optically transparent, flexible, light and environmentally resistant – that make them attractive for improved photovoltaic devices. CNTs act as acceptors of electrons and due to their huge specific area they are expected to be very efficient centres of exciton dissociation. Their high aspect ratio allows the establishment of percolation pathways at low doping levels, providing the means for high carrier mobility and efficient charge transfer to the appropriate electrodes. The CNTs are used as electrodes [14], layered at a desired location [15] or blended with a polymer [16]. The first report of using CNTs as electron acceptor in bulk-heterojunction solar cells concerned SWCNTs (single-walled carbon nanotubes) – polythiophene blends. Authors observed the increase in photocurrent of two orders of magnitude as compared to the polymer film without nanotubes [17]. Later, more reports appeared in the literature, giving examples of CNTs usage in different polymeric matrices [18–20]. However, the advantages of CNTs usage in solar cells construction are hindered due to the difficulties in preparation of homogenous and stable suspensions of nanotubes in solvent or polymer matrices. Chemical functionalization of CNTs is frequently applied to enhance their stable suspension formation [21], however there are only a few papers concerning functionalised CNTs as a material for photovoltaic devices. Functionalized CNTs were used in donor/ acceptor type photovoltaic device by Pradhan et al. [16]. Nogueira and later Stylianakis reported covalent SWCNTs modification by thiophene for use in organic solar cells [22–23]. Other functional groups were also bonded to CNTs. Convenient method for CNTs functionalization was presented by us [24–26]. CNTs were functionalized with different types of chemical residues, e.g. anthraquinone, phthalocyanine. Such modified soluble CNTs were deposited onto electrodes by simple, low cost techniques which can be used in PV manufacturing process. In the nearest future we plan to investigated the course of photovoltaic phenomenon in organic systems incorporating functionalized CNTs in the active layer of a cell (Fig. 3 shows modified CNTs we plan to use in our study). 100 0,2 0 0,0 400 500 600 700 800 900 λ [nm] Fig. 2. Open-circuit voltage action spectra obtained for cells with different set of buffer layers; absorption spectrum of F 16 ZnPc (green line) and MEH-PPV cast from toluene and isopropanol (pink line). MEH-PPV and F 16 ZnPc thickness is ~30 nm and ~100nm respectively. Illumination through ITO electrode, incident light intensity 10 15 photon/cm 2 s Fig. 3. Schematic representation of carbon nanotubes functionalized with anthracenyl residue and cobalt phthalocyanine derivative Elektronika 6/2012 119
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Page 9 and 10: Summary The silver nanopowder paste
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