Bilayer solar cell

The chapter is organized as follows the second section will discuss the photophysics of conjugated polymer/fullerene composites as a standard model for a charge-generating layer in plastic solar cells. Pristine polymer devices will be discussed in the third section while bilayer and interpenetrating network devices are presented in Sections 4 and 5. Section 6 contains some remarks on large area plastic solar cells and Section 7 conclusions. 

In two-component charge transfer systems, such as in the bulk-heterojuncdon solar cells presented here, deviations of the V,K. from the results of pristine single layer or bilayer devices are expected for two reasons first, some pan of the available difference in electrochemical energy is used internally by the charge transfer to a lower energetic position on the electron acceptor second, the relative posi-... 

Cuiffi J, Benanti T, Nam WJ, Fonash S (2010) Modeling of bulk and bilayer organic heterojunction solar cells. Appl Phys Lett 96 143307... 

In addition to the complexation in solutions, pyridine-appended fulleropyrro-lidines can also form coordination complexes with zinc(II) phthalocyanines in solid-state thin films. Troshin et al. have investigated the photovoltaic behavior of bilayer solar cells fabricated by deposition of solution-processed fulleropy-rrolidines 36-40, which contain chelating pyridyl groups, on vacuum-evaporated films of unsubstituted zinc(II) phthalocyanine (ZnPc) [46], The UV-Vis spectra of these films resemble the spectrum of ZnPc recorded in pyridine, showing a sharp... 

It is the purpose of this chapter to introduce photoinduced charge transfer phenomena in bulk heterojunction composites, i.e., blends of conjugated polymers and fullerenes. Phenomena found in other organic solar cells such as pristine fullerene cells [11,12], dye sensitised liquid electrolyte [13] or solid state polymer electrolyte cells [14], pure dye cells [15,16] or small molecule cells [17], mostly based on heterojunctions between phthalocyanines and perylenes [18] or other bilayer systems will not be discussed here, but in the corresponding chapters of this book. 

The interpenetrating network in bulk hetero junction solar cells [9] helps to overcome the limitations of bilayer systems [25,95] with low mobility materials. However, less is known about the nanometer morphology of an interpenetrating network or the optimum density of donor/acceptor interfacial... 

This view of Voc generation is additionally supported by the fact that the values of the temperature coefficient dUoc/dT = -(1.40-1.65) mVK-1 for the cells under the present study (with bilayer LiF/Al and ITO/PEDOT contacts) coincide with those for polymer/fullerene bulk heterojunction solar cells of the previous generation (with the same components of the active layer but without LiF and PEDOT contact layers) [156]. In this picture, the temperature dependence of Voc is directly correlated with the temperature dependence of the quasi-Fermi levels of the components of the active layer under illumination, i.e., of the polymer and the fullerene. Therefore, the temperature dependence of Voc over a wide range, and in particular V),c(0 K), are essential parameters for understanding bulk hetero junction solar cells. 

It is seen from Table 5.1 that the values of the conversion efficiency in bilayer solar cells also is quite low. As mentioned in the introduction it is difficult to dissociate excitons in the conducting polymers. The Donor/Acceptor (D/A) junction between the polymer and the fullerene is rectifying and can be used for designing photovoltaic cells or photodetectors. In this bilayer cell also the conversion efficiency is low. The cause of the low efficiency is that the charge separation occurs only at the D/A interface that results low collection efficiency. The diffusion length of the exciton is a factor 10, lower than the typical penetration depth of the photon. 

Organic bilayer solar cells have improved steadily since the report by Tang in 1986. Developments have been assisted by detailed modelling and advances in the understanding of molecnlar materials, as well as by innovations in device design and fabrication techniques. 

A major deviation from the above picture was found by Ramsdale et al., who investigated bilayer solar cells based on two polyfluorene derivatives [125]. When compared to the work function of both electrodes, the authors measured a Voc exceeding those values by about 1 eV. This overpotential has been accounted for by a concentration driven diffusion current. 

This Hnear dependence has been confirmed by Halls et al. using the same bilayer structure but employing PPV as the electron donor [44]. The authors estimated the exciton diffusion length of PPV to be in the range of 6-8 nm from both the spectral response and the absolute efficiency [44]. Later Roman et al. demonstrated optical modeling to be a useful tool for the optimization of such bilayer solar cells, which in their case was based on a polythiophene derivative and Ceo [89]. The optical modeling was detailed by Petterson et al. [46]. 

Fig. 44 Light intensity dependence of short circuit photocurrent (filled circles) and open circuit voltage of laminated POPT (MEH-)CN-PPV diffuse bilayer polymer solar cells. The scaling factor of the current calculates as 1.02. (Reprinted with permission from [32], 1998, Macmillan Publishers Ltd)...

The application of polymer precursors, resulting in insoluble PPV and BBL (poly(benzimidazo-benzophenanthroline)) ladder polymers enabled the fabrication of very efficient bilayer polymer solar cells, reaching 49% [33] and even 62% EQE (see Fig. 45) [222]. 

Fig. 45 Chemical structures (a), energy levels (b), and device structure (c) of PPV/BBL bilayers. High EQE (or IPCE) is shown together with the absorption spectrum of PPV/BBL bilayer solar cells. (Left Reprinted with permission from [28], 2000, American Institute of Physics Right Reprinted with permission from [193], 2004, American Chemical...

Kietzke et al. have shown for bilayer solar cells based on M3EH-PPV and several acceptor polymers with varying electron affinities and the fullerene derivative PCBM that the open circuit voltage is linearly related to the respective LUMO levels [225]. While CN-PPV-PPE acceptors resulted in an increased open circuit voltage of about 1.5 V, the fill factor and photocurrent were smaller than those for CN-ether-PPV [225]. 

A logarithmic relationship between light intensity and open circuit voltage had been shown for bilayer polymer solar cells by Ramsdale et al. [125]. This and the observation of an overpotential of the open circuit voltage with respect to the work function difference of the two electrodes—as inferred... 

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