Exciton Generation and Diffusion in Multilayered Organic Solar Cells Designed by Layer-by-Layer Assembly of Poly(p-phenylenevinylene)

Abstract

Solution-processed organic thin-film solar cells with triple-layered structures were fabricated by combining a holetransporting layer made of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS), a light-harvesting layer assembled by layer-by-layer (LbL) deposition of poly(p-phenylenevinylene) (PPV) and PSS, and an electron-transporting layer of fullerene C 60 dispersed in a polystyrene film. The thickness of the light-harvesting layer was precisely designed and controlled on a scale of nanometers by the LbL deposition technique. The efficiency of exciton generation in the PPV/PSS LbL assembly was estimated for various layered structures by optical simulation considering optical interference effects. For the discussion on the efficiency of exciton diffusion, photoluminescence quenching was measured for the LbL assembly with various thicknesses and analyzed using the onedimensional diffusion model. As a result, the exciton lifetime and diffusion constant were evaluated as 0.67 ( 0.02 ns and 8 × 10 -4 cm 2 s -1 , respectively. On the basis of these analyses, the photocurrent generation in the solar cells was quantitatively explained in terms of the layered structure.

Exciton Diffusion in Conjugated Polymers: From Fundamental Understanding to Improvement in Photovoltaic Conversion Efficiency

“…[25][26][27][28][29]44 Table 1 summarizes the L D reported for various amorphous polymer films evaluated by the PL quenching methods.…”

“…[25][26][27][28][29]44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 As mentioned above, inhomogeneity in conjugated polymers plays a central role in the exciton 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 13 ...…”

Efficient Exciton Harvesting through Long‐Range Energy Transfer

“…In polymer solar cells, excitons (coulombically bound electron-hole pairs) should be transferred to the donor/acceptor interface to be efficiently dissociated into free electron andh ole carriers, because of the strong binding energy. [4][5][6] However,t he exciton diffusion length in mostc onjugatedp olymers is as short as 5-20 nm, [7][8][9][10][11][12][13][14][15] which is not alwayse nough to ensure that all the generated excitons reach the polymer/fullerene interface before deactivating to the ground state. For example, it has been reported that about 10-20 %e xcitons are lost before arriving at the interface even in ab enchmark solar cell based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM).…”

Measurement of Exciton Diffusion in a Well-Defined Donor/Acceptor Heterojunction based on a Conjugated Polymer and Cross-Linked Fullerene Derivative

“…The steady-state PL quenching technique is simple and versatile, as it does not need the high temporal resolution and/or high excitation intensity required for time-resolved PL quenching and exciton− exciton annihilation measurements. [16][17][18]25,26 In this method, the L D can be directly evaluated from the dependence of the steady-state PL quenching efficiency on the polymer donorlayer thickness.…”