Bulk heterojunction solar cell simulation

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...

Andersson and coworkers have prepared solar cells based on blends of poly(2,7-(9-(2 -ethylhexyl)-9-hexyl-fluorene)-fl/t-5,5-(4, 7 -di-2-thienyl-2, l, 3 -benzothiadiazole) (223) and PCBM [416]. The polymer shows a Amax (545 nm) with a broad optical absorption in the visible spectrum and an efficiency of 2.2% has been measured under simulated solar light. The same group has also reported the synthesis of low bandgap polymers 200 (1 = 1.25 eV) and 224 (1 = 1.46 eV) which have been blended with a soluble pyrazolino[70]fiillerene and PCBM, respectively, to form bulk heterojunction solar cells of PCE of 0.7% [417] and 0.9% [418]. Incorporation of an electron-delident silole moiety in a polyfluorene chain affords an alternating conjugated copolymer (225) with an optical bandgap of 2.08 eV. A solar cell based on a mixture 1 4 of 225 and PCBM exhibits 2.01% of PCE [419]. 

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%. 

Fig. 6 Current density-potential characteristic of the PCBM/copolymer 55 4 1 bulk heterojunction organic solar cell under illumination with AM 1.5 G solar simulated light (dotted line) and in the dark (dashed line) [38]. Wiley. Reproduced with permission...

A further improvement of MDMO-PPV based bulk heterojunctions was achieved by the application of a new C70 fullerene derivative, which was substituted with the same side chains as PCBM and is therefore called [70]PCBM [170]. Due to the reduced symmetry of C70 as compared to the football sphere (icosahedral symmetry) of Ceo. more optical transitions are allowed and thus the visible hght absorption is considerably increased for [70]PCBM. This led to an improved external quantum efficiency (EQE) of MDMO-PPV based solar cells reaching up to 66% (Fig. 30). As a result the power conversion efficiency was boosted to 3% under AM 1.5 solar simulation at 1000 W/m [170]. 

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. 

Overall, the simulations of bulk heterojunctions are very useful fOT relating the performance of polymer solar cells to their structure therefore we shall expect a lot of effort on this area in the next few years. 

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