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Organic Solar Cells

  • Masahiro Hiramoto
  • Yusuke Shinmura
Part of the Springer Handbooks book series (SPRINGERHAND)

Abstract

One of the responsibilities of scientists is to help develop new ways in which we can generate energy, since gaining control over energy resources such as petroleum has been one of the main reasons for conflict between nations. Renewable energy generated by solar cells is one of the potential solutions to the problem of maintaining our energy supply and these have been studied intensively for about half-century. Today, silicon solar cells have already been commercialized and have become an indispensable source of electricity. However, the price of electricity produced by silicon solar cells is still higher than that produced by petroleum. In order to increase the production of energy by solar cells, the price of electricity produced by solar cells needs to be lower than that produced by petroleum. Organic solar cells have the potential to be part of the next generation of low-cost solar cells. There was a steep increase in the power-conversion efficiency of organic solar cells around the year 2000, indicating that the technology needed to bring them to a commercial level would be established by around 2020, taking into consideration the example of organic electroluminescent devices for which scientific breakthroughs were made in 1987 and commercialization occurred around 2010. Now, in 2015, the power-conversion efficiency of organic solar cells has reached 12%.

Organic solar cells have many advantages; they are flexible, printable, light weight, and low cost, can be fashionably designed, and can be fabricated by roll-to-roll production, etc. Printed organic solar cells can be attached to the roofs, windows, and walls of houses and buildings. Automobiles wrapped with colorfully printed organic solar cells can be fabricated. Moreover, they are suitable for constructing solar power plants in space, since their light weight allows them to be easily put into orbit. In this section, the history, fundamental principles, and recent progress in organic solar cells are summarized.

The most essential factor for organic solar cells is the existence of excitons, that is, strongly bound electron–hole pairs. To efficiently generate photocarriers from excitons, donor–acceptor sensitization is used. Fullerenes acting as acceptors are used in present organic solar cells. Since the diffusion length of excitons is extremely small, blended junctions are used. Route formation both for photogenerated electrons and holes to the respective electrodes by phase separation is required for organic blended junctions. The magnitude of the photovoltage that can be obtained is determined by the difference between the lowest unoccupied molecular orbital (LUMO ) of the acceptor molecules and the highest occupied molecular orbital (HOMO ) of the donor molecules. Utilization of tandem cells has been effective in increasing the power-conversion efficiency. Today, the power-conversion efficiency of organic solar cells has reached 12%. For the organic semiconductor films used in organic solar cells, both small molecule films deposited by the dry process of vacuum evaporation and polymer films deposited by the wet process of spin coating are used.

54.1 History

The first organic solar cell was fabricated by Calvin in 1958 (Fig. 54.1 ) [54.1]. For a long while, single films of small molecule organic semiconductors deposited by vacuum evaporation were used. Typical organic semiconductors were phthalocyanines (Fig. 54.2 ) [54.2] and merocyanine [54.3]. Most organic semiconductor films had p-type characteristics since oxygen molecules from the ambient air, acting as acceptors, inevitably doped the films. A photocurrent could be generated at the Schottky junction between the p-type organic film and a low-work function metal, such as Al. However, in the early stages of development, organic solar cells had little photocurrent, typically less than several micro-amperes [54.4, 54.5].
Fig. 54.1

The history of organic solar cells. The main breakthroughs are indicated by arrows. The efficiencies after 2000 (dots) are plotted according to the NREL chart. Grey dots are plotted according to the recent press releases

Fig. 54.2a,b

Chemical structures of typical organic semiconductors acting as donors (a) and acceptors (b)

A breakthrough occurred in 1986, namely, the development of a two-layer organic photovoltaic cell, which had a large photocurrent density, of the order of mA ∕ cm2, and an efficiency of 1% (Fig. 54.1 ). This was as a result of the donor–acceptor sensitization reported by Tang [54.6], which has had a big impact on the field of organic solar cells. A blended junction for small molecule cells was proposed in 1991 by Hiramoto [54.7, 54.8]. In 1992, Sariciftci [54.9] reported a polymer heterojunction cell composed of C60 and MEH-PPV (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) (Fig. 54.2 ), which was followed in 1995 by a bulk polymer heterojunction cell reported by Yu [54.10]. Typical combinations of donors and acceptors both for small molecular systems and polymer systems are shown in Fig. 54.2. Based on these fundamental studies, after 2000, the efficiency rose from 1% and started to increase rapidly and reached 12% by 2013. Nowadays, the photocurrent density of organic solar cells produced by solar radiation is approaching 20 mA ∕ cm2, which is comparable to the values produced by inorganic solar cells.

The main contributory factors to the increase in efficiency of organic solar cells can be summarized as follows:
  1. 1.

    Donor–acceptor sensitization

     
  2. 2.

    The use of blended junctions

     
  3. 3.

    Phase separation in the blended junctions

     
  4. 4.

    The use of tandem cells

     
  5. 5.

    The use of fullerenes acting as acceptors

     
  6. 6.

    Syntheses of new organic semiconductors.

     

54.2 Excitons

The dissociation of photogenerated electron–hole pairs (excitons) is a key factor for generating carriers in organic semiconductors. Exciton dissociation is affected by the relative permittivity of the solid (ε). The attractive force between an electron–hole pair is given by Coulomb’s law
$$F=\left(\frac{1}{4\pi\varepsilon\varepsilon_{0}}\right)\left(\frac{q_{1}q_{2}}{r_{2}}\right).$$
(54.1)
Here, ε0, q1, q2, and r are the absolute permittivity, the elementary charges on the electron and the hole, and the distance between them, respectively. In a solid with a small value of ε, the positive and negative charges experience a strong attractive force, whereas, in a solid with a large value of ε, the positive and negative charges experience a relatively weak attractive force. Inorganic semiconductors have large values for ε. For example, the value for Si is 11.9 and the exciton has a large diameter of 9.0 nm and is localized over about 104 Si atoms (Fig. 54.3 a). The thermal energy at room temperature is sufficient for this Wannier-type exciton to dissociate immediately into a free electron and a hole, thereby generating photocurrent. On the other hand, organic semiconductors have small values for ε. For example, the value for C60 is 4.4 and the exciton diameter is just 0.50 nm and is localized over a single C60 molecule (Fig. 54.3 b). The thermal energy at room temperature is hardly sufficient for these Frenkel-type excitons to dissociate into free electrons and holes and they can easily relax into the ground state (Fig. 54.4a). Therefore, organic semiconductors generate few photocarriers. This is the reason why organic solar cells fabricated before the work of Tang [54.6] had extremely low photocurrents, of the order of less than a microampere.
Fig. 54.3a,b

Size of excitons for (a) an inorganic semiconductor (Si) and (b) an organic semiconductor (C60). The former is a Wannier type and easily dissociates into free carriers. The latter is a Frenkel type and dissociates into free carriers with difficulty

Fig. 54.4a–d

Carrier generation in organic semiconductors. (a) Single molecule solids. (b) Donor(D)/acceptor(A) sensitization for carrier generation by mixing two kinds of organic semiconductor molecules . Efficient free carrier generation occurs from the charge transfer (CT) exciton . (c) Photoinduced electron transfer from the HOMO of the donor molecule (D) to the HOMO of the acceptor molecule (A). (d) Photoinduced electron transfer from the LUMO of the donor molecule (D) to the LUMO of the acceptor molecule (A)

54.3 Donor–Acceptor Sensitization

Today’s organic solar cells have overcome the above problem by combining two kinds of organic semiconductor. An electron-donating molecule (D) and an electron-accepting molecule (A) for which the energetic relationship of the highest occupied molecular orbital (HOMO ) and the lowest unoccupied molecular orbital (LUMO ) are brought together in contact or blended. When the electron-accepting molecule (A) is excited, electron transfer from the HOMO of the electron-donating molecule (D) to the HOMO of the electron-accepting molecule (A) occurs, and as a result, the A molecule charges negatively (A) and the D molecule charges positively (D+) (Fig. 54.4c). On the other hand, when the electron-donating molecule (D) is excited, electron transfer from the LUMO of the electron-donating molecule (D) to the LUMO of the electron-accepting molecule (A) occurs, and as a result, the A and D molecules charge negatively (A) and positively (D+), respectively (Fig. 54.4 d). Irrespective of the excitation of molecules (A) and (D), the charge transferred states (D+A) obtained are the same. Thus, a CT exciton is formed in which the positive and negative charges are separated on neighboring D and A molecules due to photoinduced electron transfer (Fig. 54.4 b). This CT exciton can dissociate to a free electron and a hole due to the thermal energy at room temperature. By utilizing this donor–acceptor (D–A) sensitization, organic semiconductors have become capable of generating photocurrents of significant magnitude; of the order of milliamperes, by solar radiation.

54.4 Exciton Diffusion

A two-layer organic solar cell (Fig. 54.5 ) [54.6] utilizes D–A sensitization at the heterojunction. The width of the photoactive region (shaded brown) is, however, limited to around 10 nm in the vicinity of the heterojunction due to the extremely small exciton diffusion length of just several nanometers [54.11, 54.12]. So, when the thicknesses of the organic layers are increased, the so-called masking effect occurs, namely, a dead region develops in front of the active region in which the incident solar light is absorbed and no photocurrent is generated, and as a result, the magnitude of the photocurrent is severely suppressed. A 10 nm-thick organic film can only absorb a small part of the incident solar light. However, in order to increase the efficiency of organic solar cells, it is necessary for the whole of the incident solar light to be absorbed in the 10 nm-thick active layer.
Fig. 54.5

Schematic illustration of a two-layer cell composed of perylene pigment (Im-PTC) acting as an acceptor molecule (A) and copper phthalocyanine (CuPc) acting as a donor molecule (D). Photocurrent is generated only in the active region (shaded brown) close to the heterojunction and all other parts of the organic films act as dead regions

54.5 Blended Junctions

In order to overcome this problem, blended junctions in organic semiconductors, which contain both donors and acceptors, were fabricated using the codeposition technique for small molecule organic solar cells (Fig. 54.6 a) [54.7, 54.8]. From the standpoint of photocarrier generation occurring at the molecular level, there are donor–acceptor molecule contacts acting as photocarrier generation sites due to donor–acceptor sensitization in the bulk of the codeposited layer. Blended junctions, usually called bulk heterojunctions, have become crucial for polymer organic solar cells [54.10].
Fig. 54.6

(a) A small molecule cell consisting of blended junctions in an organic semiconductor, containing both donor and acceptor molecules. The entire bulk of the blended layer acts as an active layer for photocarrier generation. (b) Energetic structure of a blended junction in a small molecule cell

Blended junctions are fundamentally physically the same as the porous TiO2 in the dye-sensitized solar cells proposed by Grätzel in 1991 [54.13]. For both solid–solid and solid–electrolyte junctions, by transmitting the incident light through a vast number of heterointerfaces , the whole of the incident solar light can be absorbed in extremely thin active layers.

Figure 54.6 b shows the energetic structure of the blended junction in a small molecule cell. Photogenerated electrons in donor molecules transfer from the LUMO of the donor molecule to that of the acceptor molecule and photogenerated holes in acceptor molecules transfer from the HOMO of the acceptor molecule to that of the donor molecule. This D–A sensitization promotes photocurrent generation. The original concept is that the positive and negative charges from ionized donors and acceptors in n- and p-type organic semiconductors, respectively, are compensated by each other, and the resulting codeposited interlayer behaves like an intrinsic semiconductor. With regard to the formation of the built-in potential in a molecular solid, the built-in electric field is distributed across an i-interlayer sandwiched between n- and p-layers, similar to the case of amorphous silicon incorporating a p-i-n junction.

54.6 Route Formation

Even if exciton dissociation occurs, nanostructure control of the codeposited films, that is, route formation for electrons and holes generated by exciton dissociation , is crucial in order to extract a significant portion of the photogenerated charge to the external circuit. Once the two kinds of organic semiconductors acting as donor and acceptor layers are blended, extraction of photogenerated holes and electrons inevitably becomes a tough problem, since they can only move through the electron-donating molecules (D) and the electron-accepting molecules (A):
  1. 1.

    Molecular mixture: When the two kinds of organic molecules are blended by the codeposition technique at room temperature, the codeposited films usually have an amorphous molecularly blended structure (Fig. 54.6a). Only a small number of carriers can be extracted since there are few routes for electron and hole transport.

     
  2. 2.
    Phase-separated structure: Routes for the electrons and holes are formed by percolation due to self-assembly promoted by annealing, while maintaining D–A molecular contacts in the bulk of the organic film (Fig. 54.7a) [54.15, 54.16].
    Fig. 54.7

    (a) Phase-separated structure. (b) Vertical superlattice structure. (c) Interdigitated structure of the round columns of crystalline benzoporphyrine (BP) with diameters of around 20 nm standing almost vertically, used in highly efficient cells. (Reprinted with permission from [54.14]. © American Chemical Society)

     
  3. 3.

    Vertical superlattice structure: An ideal nanostructure is the vertical superlattice structure (Fig. 54.7 b). This structure enables the efficient dissociation of photogenerated excitons at the D–A interfaces within the exciton diffusion length (5–10 nm) and the transport of electrons and holes to the respective electrodes. By changing the layer width, we can determine the exciton diffusion length to several nanometers [54.12]. It is a difficult task to construct such a structure by artificial design over a large area.

     
There are reports of the successful construction of such ideal nanostructures. A pseudo-vertical superlattice structure was fabricated by combining a new type of C60 derivative (bis(dimethylphenylsilylmethyl)[60]fullerene (SIMEF)), Fig. 54.2) with benzoporphyrin (BP, Fig. 54.2) [54.14]. BP is formed by thermal conversion from a precursor. Fabrication by wet processing was adopted, that is, the blended structure (Fig. 54.6 a) was fabricated by a spin-coating technique. After removal of the C60 derivative (SIMEF) by washing in toluene, the desired interdigitated structure is made clearly visible (Fig. 54.7c). The round columns of crystalline BP with diameters of around 20 nm stand almost vertically. This structure allows excitons to dissociate within their diffusion length and ostensibly inhibits the recombination of photogenerated electrons and holes. In 2013, Mitsubishi Chemical Co. Ltd. succeeded in obtaining a power-conversion efficiency of 12% with this system. A commercial version using this system is shown in Fig. 54.8a,b, which shows a flexible see-through organic solar cell module and a demonstration building using these modules as exterior walls and windows [54.17]. This is a clear demonstration of the ideal vertical structure (Fig. 54.7c) showing practical conversion efficiency.
Fig. 54.8

(a) Commercial flexible see-through organic solar cell module. (Courtesy of Mitsubishi Chemical Corporation). (b) Building using these modules on the exterior walls and windows. (Courtesy of Taisei Corporation)

54.7 ππ Stacking

For efficient hole transport in blended junctions, ππ stacking of the organic semiconductor is necessary. The formation of ππ stacking of poly(3-hexylthiophene) (P3HT) in a [6,6]-Phenyl-C61 butyric acid methyl ester (PCBM):P3HT (Fig. 54.2) blend by means of a regular arrangement of long-chain substituents allowed the fabrication of highly efficient polymer solar cells [54.18].

Since the organic films are sandwiched between two electrodes, perpendicular orientation of the ππ stack to the substrate is more favorable to hole transport in the blended junctions. That is, face-on orientation (Fig. 54.9a) is more suitable than edge-on orientation (Fig. 54.9 b). Blended cells using polymers with face-on orientation enabled high photocurrents to be obtained with thicker active layers without reducing the fill factor, resulting in an increase in efficiency [54.19].
Fig. 54.9

(a) Face-on and (b) edge-on orientations of ππ stacking. Molecular planes are indicated in the figure. ππ stackings are formed perpendicular to the molecular planes

54.8 HOMO–LUMO Gap

The maximum open-circuit voltage (Voc) of photovoltaic cells consisting of a combination of donor–acceptor organic semiconductors is determined by the energetic difference between the LUMO of the acceptor molecule and the HOMO of the donor molecule (VLUMO–HOMO) (Fig. 54.10a) [54.20]. In order to accomplish efficient exciton dissociation, about 0.3 eV is reported to be necessary, that is, the observed Voc is around 0.3 V smaller than VLUMO–HOMO.

In other words, in order to obtain high Voc, a combination of donor–acceptor organic semiconductors having a larger VLUMO–HOMO difference is necessary (Fig. 54.10 ). One of the main reasons for the recent increase in power-conversion efficiency is the increase in Voc due to the newly synthesized donor molecules having deeper HOMO levels (Fig.54.10 b) [54.20] and the acceptor molecules having shallower LUMO levels (Fig. 54.10c) [54.21].

Fig. 54.10

(aVLUMO–HOMO difference of donor and acceptor molecules. (bVLUMO–HOMO difference using a donor molecule having a deeper HOMO level. (cVLUMO–HOMO difference using an acceptor molecule having shallower LUMO level

54.9 Tandem Cells

The first organic tandem cell in which two unit cells were connected by metal nanoparticles was reported in 1990 (Fig. 54.11a-d a) [54.22]. Voc can be doubled by connecting two unit cells. Due to the very small diffusion length of excitons, even two very thin donor–acceptor heterojunction cells with identical absorption spectra joined together can increase the total effective thickness of the cells (Fig. 54.11a-d b) [54.23]. Moreover, an efficiency of 5.3% was obtained with a tandem cell comprising two thin blended cells with identical absorption spectra (Fig. 54.11a-dc) [54.24].
Fig. 54.11a–d

Structures of organic tandem cells . (a) Two heterojunction cells connected together. (b) A series of many extremely thin heterojunction cells with identical absorption spectra. (c) Two thin blended cells with identical absorption spectra connected together. (d) Transparent connection layers with the optimum thicknesses

However, a more essential target of cells is to utilize the whole of the solar spectrum by combining cells that absorb light from different parts of the spectrum. Polymer tandem cells having double junctions [54.25] and triple junctions [54.26] utilizing the wide wavelength region from 300 to 900 nm had a Voc of 2.28 V and an efficiency of 11.55%.

More sophisticated small molecule tandem solar cells (Fig. 54.11a-d d) using transparent connecting layers of optimum thickness in which the electric field of light inside the cell is maximized in the photoactive layers of the codeposited films had an efficiency of 12% [54.27].

54.10 Conclusions

The basic components of organic solar cells, that is, excitons, D–A sensitization, exciton diffusion, blended junctions, route formation, ππ stacking, the HOMO–LUMO gap, and tandem cells, are summarized in this section. These components will help us in our search for further breakthroughs in the performance of organic solar cells. For example, (i) if we can utilize organic semiconductors with high ε values, exciton dissociation would no longer be a limiting factor for organic solar cells. (ii) If we can find organic semiconductors with significantly longer exciton diffusion lengths, exciton diffusion would no longer be a limiting factor for organic solar cells. On the other hand, the p-n-control of organic semiconductors by doping has become significant in the fabrication of organic solar cells [54.27, 54.28, 54.29].

Organic solar cells have the potential to provide the next generation of low-cost solar cells following on from silicon solar cells. Organic solar cells are representative of the organic electronics industry, which includes organic electroluminescent devices and organic transistors, and is set to dominate in the twenty-first century, following on from the dominance of the inorganic electronics industry in the twentieth century. The field of organic solar cells is closely related to many other research fields such as device physics, electronics, and synthetic chemistry, in which there are a vast number of researchers. The author fervently hopes that young researchers will take up the challenge of working in the interdisciplinary field of organic solar cells to help develop the next generation of cells for renewable energy. Some suggested books [54.30, 54.31, 54.32, 54.33] are indicated in the references for new and young researchers entering the field of organic solar cells.

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Copyright information

© Springer International Publishing AG 2017

Authors and Affiliations

  1. 1.Dept. of Materials Molecular ScienceInstitute for Molecular ScienceOkazakiJapan
  2. 2.Dept. of Materials Molecular ScienceInstitute for Molecular ScienceOkazakiJapan

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