Polymer PCBM device, efficiency

The use of Pt-acetylides containing phosphine ligands was extended further by the Schanze group in 2006 [84, 85], In one contribution, they incorporated platinum-acetylide polymers into photovoltaic devices which demonstrate good device efficiency. Transient absorption studies provide definitive evidence for photoinduced electron transfer from the Pt-acetylide to PCBM by the temporal evolution of the TA spectrum, observing the formation of the PCBM radical anion at 1,050 nm. The same system was eventually demonstrated to operate as a bulk heterojunction photovoltaic device [84],... 

Various alternative acceptor components for organic BHJ solar cells have been tried in an attempt to improve cell performance. Fullerene derivatives such as C70 PCBM and Cg4 PCBM have been used in place of Ceo PCBM, because the lower molecular symmetry compared with Ceo PCBM enables stronger light absorption by the fullerene. The C70 PCBM derivatives were relatively successful, leading to 3% power conversion efficiency for devices made with MDMO-PPV polymer (Wienk et al, 2003). The Cg4 derivatives resulted in rather poor device efficiencies, attributed to the unfavourable film morphology resulting from the immiscibility of Cs4 derivatives with typical organic solvents (Kooistra et al, 2006). 

Significant amount of research has been dedicated to two material systems MDMO-PPV PCBM (MDMO-PPV poly(2-methoxy-5-(3, 7 -dimethyl-octyloxy)-l,4-phenylenevinylene)) and RR-P3HT PCBM, which represent the state-of-the-art polymer solar cell technology. Because there are excellent review papers on the MDMO-PPV PCBM system in the literature, in this section we will only focus on the recent improvements in RR-P3HT PCBM system, especially the approaches to optimize the active blend layer for enhancing device efficiency. 

Several processable low bandgap PTs containing isothianaphthene units, such as 226, 227, and 228, have been reported for their use in bulk heterojunction PV devices with PCBM, with efficiencies 0.008%, 0.24%, and 0.31%, respectively, under simulated solar illumination [420—422]. A solar cell made from a mixture of 229/PCBM (1 1) led to an efficiency of 0.09% [423]. Polymer 230 presents a larger optical bandgap (2 eV) than that of 229 (1.3 eV) and a 1 1 mixture of 230 with PCBM leads to a PV cell of 0.024% of PCE under the same illumination conditions than for 229. 

FIGURE 8. The external quantum efficiency of the device ITO/PEDOT PSS/polymer 30 PCBM/A1 as a function of the incident light wavelength. The absorption spectrum of the device is shown for comparison. 

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