Almantas Pivrikas1,2

*1Physical Chemistry, Linz Institute for Organic Solar Cells, Johannes Kepler University Linz 2School of Chemistry and Molecular Biosciences, Centre for Organic Photonics and Electronics, The University of Queensland, Brisbane Austria* 

### **1. Introduction**

120 Solar Cells – New Aspects and Solutions

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Hybrid nanocrystal/polymer solar cells based on tetrapod-shaped CdSexTe1-x nanocrystals. *Nanotechnology Nanotechnology*, Vol. 17, Nr. 16, pp. 4041-4047, ISSN Global warming and climate change has sparked the interest in alternative energy sources.(Cox et al., 2000) Although solar power reaching the surface of the Earth is able to meet the demands of humanity at the present,(Turner, 1999) an important question remains: how to convert the solar power into electrical power efficiently and at low costs.(Glaser, 1968) Polymer-based organic solar cells offer a possible solution for low cost photovoltaic energy conversion.(Wohrle & Meissner, 1991) In general, organic electronics has created an immense academic interest due to unlimited and flexible molecular engineering possibilities, allowing new organic materials with tailored physical properties to be synthesized.(Forrest, 2005b)

The most promising aspect of organic solar cells is their potential economic advantage due to large-scale production posibilities using continuous and large scale roll-to-rool printing and coating techniques allowing to deposit the active film, electrodes, sealing layers, antireflecting coatings and other components on flexible substrates all-at-once.(Krebs, 2009) Various aesthetic form factors, usually considered to be important for the solar panel integration into buildings can be achieved with this type of solar cell. Desired device form, color and a wide range of applications including solar power stations, roof-tops, portable devices, textile integrated power supplies and other consumer products can be envisioned.

The relation between fabrication costs of photovoltaic modules and power conversion efficiency defines the market success, therefore both factors have to be considered from academic and industrial perspective.(Brabec, 2004) Due to low dielectric constants and weak van der Waals forces binding the organic molecules into a solid, excitons (electron and hole pair strongly bounded by Coulomb attraction) are the primary photoexcitations in organic solids.(Schwoerer & Wolf, 2007) In order to achieve high power conversion efficiency of organic solar cells excitons have to be separated into mobile charge carriers for photocurrent generation.(Forrest, 2005a) The bulk-heterojunction concept is employed to overcome the short exciton diffusion distance. The photoactive film of heterojunction is formed from the donor and acceptor materials which are phase-separated on the nanometer length scale, to facilitate the photo-induced charge transfer as well as create a percolating pathways for charge transport to the electrodes.(Brabec et al., 2001; Halls et al., 1995) Therefore, the

**2.2 Solar cell fabrication techniques**

2008)

printing.

Engineers.

While academic research is highly concentrated on improving the power conversion efficiency, there are other important aspects needed for commercial success, such as cell stability, degradation, low manufacturing costs with rapid large scale production. This has been summarized by the Venn diagram as the unification of challenges when trying to combine power conversion efficiency, processability and stability into final devices.(Jorgensen et al.,

Relation Between Nanomorphology and Performance of Polymer-Based Solar Cells 123

Solution processing is attractive for fabricating organic optoelectronic devices mainly due to its simplicity and applicability for large scale and low-cost production. Thin films can be formed in various ways: a) printing techniques including screen printing, pad printing, gravure printing, flexographic printing and offset printing; b) coating techniques including pin coating, doctor blading, casting, painting, spray coating, slot-die coating, curtain coating, slide coating and knife-over-edge coating. The only technique that in both categories is inkjet

Spin coating has been the most common technique for polymeric solar cell fabrication with numerous reviews and fundamental studies available.(Norrman et al., 2005) This technique, widely used in the microelectronics industry to deposit photoresist on silicon wafers, allows for the reproducible formation of highly homogeneous films over large areas. A typical spin coating process involves application of a solution (with the organic semiconductors dissolved in a solvent) to a substrate which is then either accelerated to the required angular velocity or is already spinning at it, Fig. 2.(Krebs, 2009) A large portion of the solution is wasted leaving a thin film on the substrate. Film thickness, morphology and surface topography strongly depend on the rotational speed, viscosity, volatility, diffusivity, molecular weight

The most important figure of merit describing the performance of a solar cell is the power conversion efficiency, which is determined from the current voltage characteristics of the solar cells under operational conditions. Typical current-voltage characteristics of solar cells under

The accurate measurement of the PCE according to international standards has been described in the literature,(Shrotriya et al., 2006) and is eesential for reproducibility and comparison of results between different laboratories. The Shockley diode equation describes the

Fig. 1. Schematic sandwich-type structure of organic solar cells showing an organic semiconductor active film between two metal electrodes with different work functions (typically ITO/PEDOT-PSS as positive and Ca/Al as negative contacts). Reprinted with permission from (Shaheen, 2007). Copyright 2007 Society of Photo-Optical Instrumentation

and concentration of the solutes and solvents used.(Cohen & Gutoff, 1992)

**2.3 Current-Voltage dependence of solar cells**

illumination is shown in Fig. 3.(Deibel & Dyakonov, 2010)

nanomorphology of polymeric solar cells plays a crucial role for the performance of the devices.

Historically, thermal annealing of the film has been used to induce the phase separation between donor and acceptor in bulk-heterojunction blends.(Padinger et al., 2003) However, thermal treatment creates an additional fabrication step in the whole device fabrication process. Later, various methods have been tested and employed to control the nanomorphology of the blends, namely use of solvents with different boiling points (choice of solvent), reduction of drying speed (rate of drying and vapor annealing), changing the solubility of materials, melting of bilayers and the use of processing additives.(Pivrikas et al., 2010b) The later method has received great academic interest as it removes the need for post-production treatment while at the same time allowing fine control of the nanomorphology in various donor-acceptor blends.(Lee et al., 2008)

In this work the factors limiting the power conversion efficiency of excitonic polymer-based bulk-heterojunction solar cells are discussed. Various methods allowing the film nanomorphology to be controlled are reviewed. The use of processing additives to control the phase separation for the formation of an interpenetrating network and how this impacts the power conversion efficiency is described.
