Organic solar cells open-circuit voltage

Rand BP, Burk DP, Forrest SR (2007) Offset energies at organic semiconductor heterojunctions and their infiuence on the open-circuit voltage of thin-film solar cells. Phys Rev B 75 115327... 

Cheyns D, Rand BP, Heremans P (2010) Organic tandem solar cells with complementary absorbing layers and a high open-circuit voltage. Appl Phys Lett 97 3... 

Potscavage WJ, Sharma A, Kippelen B (2009) Critical interfaces in organic solar cells and their influence on the open-circuit voltage. Acc Chem Res 42 1758... 

The influence of interface area on Voc is utilized in high efficiency crystalline silicon solar cells with the so-called point contact concept to increase the open circuit voltage by suppressing area related recombination [149], Recently, the point contact concept has been applied also in pc-Si H solar cells using photolithography or self-organized zinc oxide etch masks [150], However, so far Voc improvement could not be demonstrated. For more details of preparation and application of ZnO etch masks the reader is referred to the original work [118]. 

Mihailetchi [134] investigated the open circuit voltage of the bulk heterojunction organic solar cells based on methanol-fullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as electron acceptor and poly[2-methoxy-5(3 ,7 -dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-PPV) as an electron donor. It is known that a single layer device follows the MIM model [166] and the open circuit voltage V0c is equal to the difference in the work functions of the metal electrodes [134], If charges accumulate in the... 

Mihailetchi et al, 2004), when the electrodes of the device make ohmic contacts with the acceptor and donor materials. Progress in nnderstanding the origin of the open-circuit voltage of organic solar cells and its dependence on device parameters will be discussed in Section 7.6.2 below. 

Furthermore, the production is expected to be easily scalable. This technology is currently developed by many researchers around the world, but has not yet reached the marketplace. In order for polymer solar cells to become economic their efficiency must be improved. The power conversion efficiency of a solar cell is dictated by three factors (i) the fraction of sunlight that can be absorbed, (ii) the fraction of absorbed photons that lead to extracted charges ( internal quantum efficiency ), and (iii) the energy that is retained by the extracted charges (ideally close to the open-circuit voltage ). In this review we will refer often to factors (ii) and (ui). Their interplay is not well understood and at present these have not both been optimized simultaneously even in state-of-the-art organic solar cells. 

Charge Generation and Recombination in Organic Solar Cells with High Open-Circuit Voltages... 

The first application of discotic liquid crystal to organic thin film solar cells is of a binary blend of liquid crystalline phthalocyanine (Pc) as p-type of semiconductor and non- mesogenic PTCDI derivative as n-type [45], where the active layer was not a BHJ type, but had a p-i-n junction (Fig. 8.11). The Pc layer was fabricated by spin-coating and heated up to the isotropic phase followed by cooling to obtain a spontaneously formed homeotoropic alignment on ITO-coated substrate. The n-type layer was deposited by sublimation on Pc layer and finally the counter A1 electrode was deposited in vacuo. The performance was shown simply as short circuit current, Isc = 0.4 mA cm , open circuit voltage, Vqc = 0.3 V and external quantum efficiency, EQE 0.5 %. 

D. Demeter, T. Rousseau, P. Leriche, T. Cauchy, R. Po, J. Roncali, Manipulation of the Open-Circuit Voltage of Organic Solar Cells by Desymmetrization of the Structure of Acceptor-Donor-Acceptor Molecules. Adv. Funct. Mater. 2011,21,4379-4387. 

Keywords Solar cells, organic photovoltaics (OPVs), quantum confinement effect (QCE), conjugated polymers, nanocomposites, blends, quantum dots (QDs), nanocrystals, nanorods, carbon nanotubes (CNTs), graphene, nanoparticles, alternating copolymers, block copolymers, exdton diffusion length, short-circuit current, open-circuit voltage, fill factor, photoconversion efficiency, in-situ polymerization... 

The efficiency of organic photovoltaic devices based on conjugated polymers is limited by the mismatch of their absorption spectrum and the solar spectrum. This problem has motivated the search for low bandgap PT derivatives. While polymers with very low bandgap are known, in the specific case of solar cells the problem is rendered more complicated by the fact that bandgap reduction should not have excessively detrimental effect on the open circuit voltage of the resulting solar cell [414]. 

Cheyns, D. et al. (2008) Analytical model for the open-circuit voltage and its associated resistance in organic planar heterojunction solar cells. Phys. Rev. B., 77 (16), 165-332. 

Kawano, K. et al. (2006) Open circuit voltage of stacked bulk heterojimction organic solar cells. Appl. Phys. Lett., 88 (7), 73-514. 

Figure 7 Open-circuit voltage in organic bulk heterojunction solar cells.

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