Photovoltaics bulk heterojunction cells

Photovoltaic Devices with OPV4—Ceo- The increased lifetime of the charge-separated state, which extends into the millisecond time domain, opens the possibility of using the OPVrt-Coo dyads as the active material in a photovoltaic device. As an important difference with previous bulk heterojunction cells, the covalent linkage between donor and acceptor in these molecular dyads restricts the dimensions of the phase separation between the oligomer and the fullerene that could freely occur in blends of the individual components. This can be considered as a primitive attempt to obtain more ordered and better-defined phase-separated D-A networks. 

Figure 1.5 Typical organic photovoltaic cell architectures, (a) Bilayer cell (b) bulk heterojunction cell.

Peumans, P. Uchida, S. Forrest, S. R. 2003. Efficient bulk heterojunction photovoltaic cells using small-molecular-weight organic thin films. Nature 425 158-162. 

Besides ruthenium complexes, rhenium complexes were also used as the photosensitizers in photovoltaic cells. Bulk heterojunction photovoltaic cells fabricated from sublimable rhenium complexes exhibited a power conversion efficiency of 1.7%.75,76 The same rhenium complex moiety was incorporated into conjugated polymer chains such as polymer 16a c (Scheme 9). Fabrication of devices based on conjugated rhenium containing polymers 17a c and SPAN by the LbL deposition method was reported.77 The efficiencies of the devices are on the order of 10 4%. 

In a bulk-heterojunction photovoltaic cell with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as an electron acceptor, alternating copolymer 19 (Fig. 9), derived from 2,7-fluorene and 2,5-dithienylsilole, can show impressive performance as the electron donor.31 In a device configuration of ITO/PEDOT/active layer/Ba/Al, the dark current density—bias curve shows a small leakage current, suggesting a continuous, pinhole-free active layer in the device. Under illumination of an AM 1.5 solar simulator at 100 mW/cm2, a high short-circuit current of 5.4 mA/cm2, an open-circuit voltage of 0.7 V, and a fill factor of 31.5% are achieved. The calculated energy conversion efficiency is 2.01%. 

For proper operation of a bulk heterojunction photovoltaic cell, a special alignment of the HOMO and LUMO levels of the bulk heterojunction components must be accomplished, compatible with the electrodes work functions, as depicted in Scheme 5.8. If an exciton is formed in the polymer phase, then the electron is transferred to the NC phase and reaches the aluminum electrode via its percolating pathway. The remaining hole is transported to the ITO electrode through the polymer phase. In the alternative case, that is, the formation of an exciton in the NCs phase, the hole is transferred to the polymer phase and then transported to the ITO electrode, whereas the electron reaches the aluminum electrode through the NCs phase. 

Scheme 5.9 Scheme of a hybrid photovoltaic cell with an active layer consisting of a composite of a conjugated polymer and semiconductor nanocrystals (so-called bulk heterojunction). 

Nowadays the best performing organic photovoltaic cell is represented by a bulk heterojunction (BHJ) solar cell based on the polymer poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM), with reproducible efficiencies approaching 5% [262,263], However, a serious drawback for the preparation of efficient organic photovoltaic cells is represented by the low optical absorbance in the red/near-infrared region of the lightharvesting component(s), as well as their low extinction coefflcient(s). 

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. 

The previous section gave an overview of the transport and junction properties of conjugated materials regarding their importance for photovoltaic devices. In this chapter, the bulk heterojunction device itself will be in the spotlight. Device properties will be discussed and evaluated as for classical inorganic solar cells, concentrating on the short-circuit current /sc, the open-circuit voltage Foc, the fill factor FF, and the spectral sensitivity. 

This work summarizes the physics of a special area of photovoltaic energy conversion, i.e., polymer-based, bulk heterojunction solar cells. With about... 

Fig. 12 Examples of device architectures of conjugated polymer-based photovoltaic cells a single layer b bilayer c disordered bulk heterojunction d ordered bulk heterojunction. (Reproduced with permission from [71], 2005, American Chemical Society)...

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