15.++Nanostructured+Organic+PV+cells

15.++Nanostructured+Organic+PV+cells - MSE 302 5/12/08...

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Unformatted text preview: MSE 302 5/12/08 Tandem and bulk heterojunction solar cells Transparent Electrode Active Region Top Electrode Limitations of bilayer cells Most exciton diffusion lengths are in the range of 4-20 nm. The absorption depth is approximately 100 nm. (Transmission = e (-d/100 nm) ) If each layer were only 4-20 nm thick, the internal quantum efficiency ( i ) would be high, but the external quantum efficiency ( e ) would be low because most of the incident photons would not be absorbed. e- h + Approaches to absorbing all of the light and harvesting all of the excitons Increase the exciton diffusion length so that 100-nm thick layers light absorbing layers can be used. Stack thin bilayer cells. Make a bulk heterojunction, which is an interpenetrating network of donors and acceptors, so that all of the excitons are formed within 10 nm of an interface. Find a way to scatter light into waveguide modes so that the distance traveled in the semiconductor is larger. Approach 2: Stacking cells Peter Peumans, Aharon Yakimov, Stephen Forrest, J. of App. Phys. 93 (2003) p. 3693. The current is equal to the current of the diode that produces the least amount of current. The diode at the bottom should be thinner so that each diode absorbs the same number of photons. The voltage is the sum of the voltages generated by each diode. Recently, cells with different bandgaps have been stacked to harvest different regions of the spectrum. (Forrest et al. Appl. Phys. Lett. 85 (2004) 5757.) Approach 3: Bulk heterojunctions The sidegroup makes the C 60 molecule soluble. After the polymer chains absorb light, electron transfer to the PCBM occurs. Then electrons travel along the network of PCBM to one electrode and the holes travel through the polymer to the other electrode. Gang Yu, Alan Heeger et al. Science 270 (1995) p. 1789. PV cells based on blends of PPV and C 60 derivatives Sean Shaheen, Serdar Sariciftci et al., Appl. Phys. Lett. 78 (2001) p. 841. 1:4 ratio by weight Energy conversion efficiency under AM 1.5 solar conditions is 2.5 %. Using chlorobenzene instead of toluene improves the chain morphology, thereby improving charge transport. 100-nm thick MDMO-PPV:PCBM PV cells Sean Shaheen, Serdar Sariciftci et al., Appl. Phys. Lett. 78 (2001) p. 841. The internal quantum efficiency is greater than 60 %. toluene cast chlorobenzene cast How much PCBM is needed? 1 % PCBM quenches the photoluminescence. ~ 18 % PCBM is needed to provide a continuous pathway for electrons to travel to the electrode. (This is called the percolation threshold.) Raising the PCBM concentration beyond 18 % improves the device even more. It is tempting, but incorrect, to conclude that the device is limited by poor electron transport and that adding extra PCBM improves the transport....
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15.++Nanostructured+Organic+PV+cells - MSE 302 5/12/08...

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