Acceptor/donor interfaces, solar cell

Vandewal K, Tvingstedt K, Gadisa A, Inganas O, Manca JV (2010) Relating the open-circuit voltage to interface molecular properties of donor acceptor bulk heterojunction solar cells. Phys Rev B 81 125204... 

Tables obtained from the Renewable Resource Data Center website at http //rredc.nrel.gov/solar/spectra/ ami.5/. (b) Energy levels and the harvesting of energy from a photon for an acceptor-donor interface within a photoactive layer of a PV cell. The electron affinity and ionization potential are shown as x and IP, respectively. LUMO and HOMO are the lowest unoccupied molecular orbital and highest occupied molecular orbital, respectively. CB and VB represent the conduction and valence bands, respectively. PC and PAn are photocathode and photoanode, respectively. A schematic of the PV cell design for which the above diagram applies is also shown. (Adapted from Saunders, B.R. et al., A[Pg.479]

Fig. 4 Schematic illustration of the processes leading to photocurrent generation in organic solar cells, (a) Photon absorption in Step 1 leads to excitons that may diffuse in Step 2 to the donor/ acceptor (D/A) interface. Quenching of the exciton at the D/A interface in Step 3 leads to formation of the charge-transfer (CT) state. Note that processes analogous to Steps 1-3 may also occur in the acceptor material, (b) Charge separation in Step 4 leads to free polarons that are transported through the organic layers and collected at the electrodes in Steps 5 and 6, respectively, (c) The equilibria involved in Steps 1-4- strongly influence device efficiency...

It is seen from Table 5.1 that the values of the conversion efficiency in bilayer solar cells also is quite low. As mentioned in the introduction it is difficult to dissociate excitons in the conducting polymers. The Donor/Acceptor (D/A) junction between the polymer and the fullerene is rectifying and can be used for designing photovoltaic cells or photodetectors. In this bilayer cell also the conversion efficiency is low. The cause of the low efficiency is that the charge separation occurs only at the D/A interface that results low collection efficiency. The diffusion length of the exciton is a factor 10, lower than the typical penetration depth of the photon. 

Fig. 5.13. Schematic variation of Voc with acceptor strength (solid double headed arrow, Voci) or/and electrode work function (dotted arrow, V0c2)> m a donor/acceptor BHJ solar cell. The electron transfer, occurring at the donor/acceptor interface after light excitation, is indicated by the bent arrow [134].

At the early development of polymer solar cells, a planar p-n junction structure represented the mainstream in mimicking conventional silicon-based solar cells. However, the obtained devices demonstrated poor photovoltaic performances due to the long distance between the exciton and junction interface and insufficient light absorption due to the thin light absorber. It was not until 1995 that the dilemma was overcome with the discovery of a novel bulk heterojunction in which donor and acceptor form interpenetrated phases. Poly[2-methoxy-5-(2 -ethylhexyloxy)-p-phenylene vinylene] was blended with Ceo or its derivatives to form the bulk heterojunction. A much improved power conversion efficiency of 2.9% was thus achieved under the illumination of 20 mW/cm. (Yu et al., 1995). The emergence of the donor/acceptor bulk-heterojunction structure had boosted the photovoltaic performances of polymer solar cells. Currently, a maximal power conversion efficiency of 10.6% had been reported on the basis of synthesizing appropriate polymer materials and designing a tandem structure (You et al., 2013). The detailed discussions are provided in Chapter 5. 

H. Zhou, L. Yang, S. Stoneking, W. You, A Weak Donor-Strong Acceptor Strategy to Design Ideal Polymers for Organic Solar Cells. ACS Appl. Mater Interfaces 2010,2,1377-1383. 

H. Bai, Y. Wang, P. Cheng, Y. Li, D. Zhu, X. Zhan, Acceptor-Donor-Acceptor Small Molecules Based on Indacenodithiophene for Efficient Organic Solar Cells. ACS Appl. Mater. Interfaces 2014,6,8426-8433. 

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