Chapter 5
Polymer Solar Cells: Development
of π-Conjugated Polymers
with Controlled Energetics
and Structural Orders
Itaru Osaka
5.1 Introduction
Organic solar cells have seen a dramatic progress in the last two decades by the
development of new organic π-conjugated materials that are used as the photoactive
materials [1–4]. Whereas, in the early stage of the area, the photoactive layer(s)
were fabricated by vacuum evaporation of organic pigment dyes [5, 6], recently the
layers are typically fabricated by solution processes of the blend of a polymeric
π-conjugated material, as the p-type organic semiconductor, and a small-molecular
π-conjugated material such as fullerenes and non-fullerenes, as the n-type organic
semiconductor [7, 8]. In particular, π-conjugated polymers have been key materials
for organic solar cells due to the advantage in film-forming property and film stability
over small molecules.
One of the most common π-conjugated polymers is regioregular poly(3hexlylthiophene) (rrP3HT), in which 3-hexylthiophenes are linked to form a conjugated polymer backbone in a head-to-tail manner (Fig. 5.1) [9]. With a regiocontrolled chemical structure, rrP3HT forms a crystalline lamellar structure with
strongly π–π stacked backbones, leading to high charge carrier transport, while
regiorandom polymer (raP3HT) is amorphous and thus has poor charge transport
property. Although the absorption range only covers up to approximately 650 nm
(optical bandgap of ~1.9 eV), rrP3HT, in combination with fullerenes, can give good
power conversion efficiencies (PCEs) of 3–6% with external quantum efficiencies
(EQEs) of ~80% [10, 11], which is believed to be as a consequence of the crystallinity. It is also important to note that rrP3HT affords better performance with
a thick active layer film of ~200 nm, while most of the polymer-based solar cells
show better performance with a thin layer of around 100 nm [12]. The thick layer
I. Osaka (B)
Department of Applied Chemistry, Hiroshima University, Hiroshima, Japan
e-mail: iosaka@hiroshima-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2021
M. Hiramoto and S. Izawa (eds.), Organic Solar Cells,
https://doi.org/10.1007/978-981-15-9113-6_5
89
Polymer Solar Cells: Development
of π-Conjugated Polymers
with Controlled Energetics
and Structural Orders
Itaru Osaka
5.1 Introduction
Organic solar cells have seen a dramatic progress in the last two decades by the
development of new organic π-conjugated materials that are used as the photoactive
materials [1–4]. Whereas, in the early stage of the area, the photoactive layer(s)
were fabricated by vacuum evaporation of organic pigment dyes [5, 6], recently the
layers are typically fabricated by solution processes of the blend of a polymeric
π-conjugated material, as the p-type organic semiconductor, and a small-molecular
π-conjugated material such as fullerenes and non-fullerenes, as the n-type organic
semiconductor [7, 8]. In particular, π-conjugated polymers have been key materials
for organic solar cells due to the advantage in film-forming property and film stability
over small molecules.
One of the most common π-conjugated polymers is regioregular poly(3hexlylthiophene) (rrP3HT), in which 3-hexylthiophenes are linked to form a conjugated polymer backbone in a head-to-tail manner (Fig. 5.1) [9]. With a regiocontrolled chemical structure, rrP3HT forms a crystalline lamellar structure with
strongly π–π stacked backbones, leading to high charge carrier transport, while
regiorandom polymer (raP3HT) is amorphous and thus has poor charge transport
property. Although the absorption range only covers up to approximately 650 nm
(optical bandgap of ~1.9 eV), rrP3HT, in combination with fullerenes, can give good
power conversion efficiencies (PCEs) of 3–6% with external quantum efficiencies
(EQEs) of ~80% [10, 11], which is believed to be as a consequence of the crystallinity. It is also important to note that rrP3HT affords better performance with
a thick active layer film of ~200 nm, while most of the polymer-based solar cells
show better performance with a thin layer of around 100 nm [12]. The thick layer
I. Osaka (B)
Department of Applied Chemistry, Hiroshima University, Hiroshima, Japan
e-mail: iosaka@hiroshima-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2021
M. Hiramoto and S. Izawa (eds.), Organic Solar Cells,
https://doi.org/10.1007/978-981-15-9113-6_5
89