Organic Solar Cells
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.
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 use of blended junctions
Phase separation in the blended junctions
The use of tandem cells
The use of fullerenes acting as acceptors
Syntheses of new organic semiconductors.
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
54.5 Blended Junctions
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
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.
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.
54.7 π–π Stacking
For efficient hole transport in blended junctions, π–π stacking of the organic semiconductor is necessary. The formation of π–π 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].
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].
54.9 Tandem Cells
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].
The basic components of organic solar cells, that is, excitons, D–A sensitization, exciton diffusion, blended junctions, route formation, π–π 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.
- 54.17Mitsubishi Chemical/Taisei Corporation: www.m-kagaku.co.jp/newsreleases/00018.html, in Japanese (2014)
- 54.27Heliatek: Heliatek consolidates its technology leadership by establishing a new world record for organic solar technology with a cell efficiency of 12% (Press Release), www.heliatek.com/en/press/press-releases/details/heliatek-consolidates-its-technology-leadership-by-establishing-a-new-world-record-for-organic-solar-technology-with-a-cell-effi (2013)
- 54.30C. Brabec, V. Dyakonov, U. Scherf (Eds.): Organic Photovoltaics, Materials, Device Physics, and Manufacturing Technologies (Wiley, Weinheim 2008)Google Scholar
- 54.31F.C. Krebs (Ed.): Stability and Degradation of Organic and Polymer Solar Cells (Wiley, West Sussex 2012)Google Scholar
- 54.33S.-S. Sun, N.S. Sariciftci (Eds.): Organic Photovoltaics, Mechanisms, Materials, and Devices (Taylor Francis, London 2005)Google Scholar