Bulk heterojunction solar cells

Dennler, G. Scharber, M. C. Brabec, C. ]., Polymer-fullerene bulk-heterojunction solar cells. Adv. Mater. 2009, 21,1323-1338. 

Blom PWM, Mihailetchi VD, Koster LJA, Markov DE (2007) Device physics of polymer fullerene bulk heterojunction solar cells. Adv Mater 19 1551 Onsager L (1938) Initial recombination of ions. Phys Rev 54 554... 

Limpinsel M, Wagenpfahl A, Mingebach M, Deibel C, Dyakonov V (2010) Photocurrent in bulk heterojunction solar cells. Phys Rev B 81 085203... 

Scharber MC, Wuhlbacher D, Koppe M, Denk P, Waldauf C, Heeger AJ, Brabec CL (2006) Design rules for donors in bulk-heterojunction solar cells - towards 10% energy-conversion efficiency. Adv Mater 18 789... 

The incorporation of siloles in polymers is of interest and importance in chemistry and functionalities. Some optoelectronic properties, impossible to obtain in silole small molecules, may be realized with silole-containing polymers (SCPs). The first synthesis of SCPs was reported in 1992.21 Since then, different types of SCPs, such as main chain type 7r-conjugated SCPs catenated through the aromatic carbon of a silole, main chain type cr-conjugated SCPs catenated through the silicon atom of a silole, SCPs with silole pendants, and hyperbranched or dendritic SCPs (Fig. 2), have been synthesized.10 In this chapter, the functionalities of SCPs, such as band gap, photoluminescence, electroluminescence, bulk-heterojunction solar cells, field effect transistors, aggregation-induced emission, chemosensors, conductivity, and optical limiting, are summarized. 

A series of ruthenium(II) phthalocyanines with one or two pyridyl dendritic olig-othiophene axial substituent(s) have also been reported (compounds 50 and 51) [50], The dendritic ligands absorb in the region from 380 to 550 nm, which complements the absorptions of the phthalocyanine core. This combination results in better light harvesting property and enhancement in efficiency of the corresponding solar cells. The solution-processed photovoltaic devices made with these compounds and fullerene acceptor give efficiencies of up to 1.6%. These represent the most efficient phthalocyanine-based bulk heterojunction solar cells reported so far. 

Both phthalocyanines and squaraines are good candidates for bulk heterojunction solar cells. Recently, a supramolecular hetero-array of these functional dyes Pc-Sq-Pc (compound 52) has been reported for the first time, which exhibits a large coverage of the solar spectrum from 250 to 850 nm [51]. This axially held assembly serves as a robust panchromatic sensitizer. Upon excitation, it forms the radical ion pair Pc+-Sq -Pc with a long lifetime of 24 2 p,s. The use of this assembly as a donor material in solution processable bulk heterojunction solar cells has also been briefly studied. 

Semiconductor Aspects of Organic Bulk Heterojunction Solar Cells 161... 

In bulk heterojunction solar cells, the metal/semiconductor interface is even more complex. Now the metal comes into contact with two semiconductors, one p-type (typically the polymer) and one n-type (typically the fullerene) semiconductor. A classical electrical characterization technique for studying the occurrence of charged states in the bulk or at the interface of a solar cell is admittance spectroscopy. If a solar cell is considered as a capacitor with capacitance C, the complex admittance Y is given by... 

Fig. 5.17. One-dimensional device scheme for simulating bulk heterojunction solar cells...

Fig. 5.18. Measurement and simulation of a bulk heterojunction solar cell in the dark (a) and under illumination (b). The dark I/V characteristics are plotted semi-logarithmically, whilst the illuminated characteristics are plotted on a linear scale. The bulk heterojunction was simulated as a diode with the following structure positive electrode/p++/i/n++/negative electrode, (c) Local variation of the energy levels (top) and of the carrier densities for a bulk heterojunction solar cell with balanced mobilities, (d) Local variation of the energy levels (top) and of the carrier densities for a bulk heterojunction solar cell with higher electron mobility...

Further, the model allows us to estimate electrical losses in the device. Figures 5.18c and d show the local variations in the energy levels and the carrier densities for the bulk heterojunction solar cell for different mobilities. In Fig. 5.18c, balanced mobilities for electrons and holes are assumed, while Fig. 5.18d describes the situation for the case where the electron mobility is higher than the hole mobility. In the latter case recombination is enhanced as seen from the carrier densities, and the performance of the device (Jsc) is significantly lowered. 

In general, a large serial resistance and an over-small parallel resistance (shunt) tend to reduce the FF. Strategies for reducing the serial resistivity by improving the quality of the Ohmic contact will be discussed. The insertion of very thin polar layers like LiF have been shown to reduce the interface barrier at the cathode in bulk heterojunction solar cells, if they are evaporated between the photoactive material and an A1 electrode [93,94]. 

The use of low bandgap polymers (ER < 1.8 eV) to extend the spectral sensitivity of bulk heterojunction solar cells is a real solution to this problem. These polymers can either substitute one of the two components in the bulk hetero junction (if their transport properties match) or they can be mixed into the blend. Such a three-component layer, comprising semiconductors with different bandgaps in a single layer, can be visualized as a variation of a tandem cell in which only the current and not the voltage can be added up. 

To obtain a better understanding of the effect of the mobility on the performance of a solar cell, a simplified model is introduced to provide an analytical description of the dependence of the short-circuit on the material parameters of the semiconductor for thin film bulk heterojunction solar cells. The following assumptions are suggested to give separate descriptions of the field current and diffusion current ... 

First, the drift current is calculated in the case of a constant electrical field, as one would expect for very thin bulk heterojunction solar cells. If the width W of the active layer is similar to the drift length of the carrier, the device will behave as a MIM junction, where the intrinsic semiconductor is fully depleted. The current is then determined by integrating the generation rate G = —dP/dx over the active layer, where P is the photon flux ... 

Figure 5.26 sumarizes the findings. Bulk heterojunction solar cells, especially in the thin film limit, are expected to be dominated by the drift current. 

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