6
M. Hiramoto
between the p-type organic film and a corrosive low work-function metal, such as Al.
However, in the early stages of their development, organic solar cells had minimal
photocurrent—typically less than several micro-amperes [8, 9].
A breakthrough occurred in 1986 when a two-layer organic photovoltaic cell
was developed that had a large photocurrent density of the order of mA cm
−2 and
an efficiency of 1% (Figs. 1.2 and 1.4). This result was obtained using the donor
(D)/acceptor (A) sensitization reported by Tang [10] that has massively influenced
the field of organic solar cells. A blended junction for small-molecule cells was
proposed in 1991 by Hiramoto [11, 12]. In 1992, Sariciftci [13] reported a polymer
heterojunction cell composed of C 60 and MEH-PPV (Fig. 1.5), which was followed
in 1995 by a bulk polymer heterojunction cell reported by Yu [14] (Figs. 1.2, 1.4,
and 1.5). Typical combinations of donors and acceptors for both small molecular
systems and polymer systems are shown in Fig. 1.5. Based on the aforementioned
basic studies, after 2000, the efficiency increased from 1% and started to escalate.
Furthermore, a tandem organic solar cell was reported by Hiramoto in 1990 [15].
The combination of the concepts of blended junction and tandem cell has consistently
boosted efficiencies throughout the research on organic solar cells [3, 16, 17].
Fullerenes and their derivatives, acting as excellent acceptors [13, 14] (Fig. 1.5),
have consistently been indispensable in the development of organic solar cells from
1993 to date. In 2016, a non-fullerene acceptor was developed [18, 19] and it ramped
up the efficiency beyond 17% (Fig. 1.2). An example of a non-fullerene acceptor
(ITIC) is shown in Fig. 1.5. Nowadays, the photocurrent density of organic solar
cells produced by solar radiation exceeds 20 mAcm
−2 and the external quantum
efficiency reached approximately 80%, which is comparable to the values produced
by inorganic solar cells.
Today, researchers have started to focus on the open-circuit voltage (V oc ) since its
increase is crucial to further enhance the efficiency. The main factors that determine
the V oc are the Highest-energy Occupied Molecular Orbital–Lowest-energy Unoccupied Molecular Orbital (HOMO–LUMO) gap (Sect. 1.2.2.1.) and the non-radiative
carrier recombination loss (Sect. 1.2.2.2.).
In summary, the major factors for increasing the efficiency of organic solar cells
thus far are as follows: (i) D/A sensitization; (ii) blended junction; (iii) percolation;
(iv) tandem cells; (v) fullerenes; (vi) HOMO–LUMO gap tuning; (vii) non-fullerene
acceptors; and (vii) non-radiative recombination suppression.
M. Hiramoto
between the p-type organic film and a corrosive low work-function metal, such as Al.
However, in the early stages of their development, organic solar cells had minimal
photocurrent—typically less than several micro-amperes [8, 9].
A breakthrough occurred in 1986 when a two-layer organic photovoltaic cell
was developed that had a large photocurrent density of the order of mA cm
−2 and
an efficiency of 1% (Figs. 1.2 and 1.4). This result was obtained using the donor
(D)/acceptor (A) sensitization reported by Tang [10] that has massively influenced
the field of organic solar cells. A blended junction for small-molecule cells was
proposed in 1991 by Hiramoto [11, 12]. In 1992, Sariciftci [13] reported a polymer
heterojunction cell composed of C 60 and MEH-PPV (Fig. 1.5), which was followed
in 1995 by a bulk polymer heterojunction cell reported by Yu [14] (Figs. 1.2, 1.4,
and 1.5). Typical combinations of donors and acceptors for both small molecular
systems and polymer systems are shown in Fig. 1.5. Based on the aforementioned
basic studies, after 2000, the efficiency increased from 1% and started to escalate.
Furthermore, a tandem organic solar cell was reported by Hiramoto in 1990 [15].
The combination of the concepts of blended junction and tandem cell has consistently
boosted efficiencies throughout the research on organic solar cells [3, 16, 17].
Fullerenes and their derivatives, acting as excellent acceptors [13, 14] (Fig. 1.5),
have consistently been indispensable in the development of organic solar cells from
1993 to date. In 2016, a non-fullerene acceptor was developed [18, 19] and it ramped
up the efficiency beyond 17% (Fig. 1.2). An example of a non-fullerene acceptor
(ITIC) is shown in Fig. 1.5. Nowadays, the photocurrent density of organic solar
cells produced by solar radiation exceeds 20 mAcm
−2 and the external quantum
efficiency reached approximately 80%, which is comparable to the values produced
by inorganic solar cells.
Today, researchers have started to focus on the open-circuit voltage (V oc ) since its
increase is crucial to further enhance the efficiency. The main factors that determine
the V oc are the Highest-energy Occupied Molecular Orbital–Lowest-energy Unoccupied Molecular Orbital (HOMO–LUMO) gap (Sect. 1.2.2.1.) and the non-radiative
carrier recombination loss (Sect. 1.2.2.2.).
In summary, the major factors for increasing the efficiency of organic solar cells
thus far are as follows: (i) D/A sensitization; (ii) blended junction; (iii) percolation;
(iv) tandem cells; (v) fullerenes; (vi) HOMO–LUMO gap tuning; (vii) non-fullerene
acceptors; and (vii) non-radiative recombination suppression.