Strategies for Performance Improvement of Organic Solar Cells
Introduction
Prologue
Energy issues have always plagued the rapid economy development since the 21st century. First of all, people are now using non-renewable energy sources such as coal, oil, and natural gas. As the energy consumption increases, the demand for traditional energy sources will become more severe. Secondly, the burning of fossil fuels can cause environmental pollution. The Great Smog of London in 1952 was a severe air-pollution event, and 4,000 people died as a direct result of the smog. In December 2015, the severe smog weather in Beijing and North China also made people realize the seriousness of environmental pollution. Solar energy is a kind of clean energy, which is inexhaustible. Solar cell is a device that converts solar energy into electrical energy. The mainstream of solar cells is silicon-based inorganic solar cells because of their stable performance, high photoelectric conversion efficiency, and long service life.
New organic semiconductor materials have been continuously synthesized with the development of organic electronics in recent decades. The mobility, spectral absorption, and solubility of organic materials have been improved gradually. These materials have been applied in the preparation of organic solar cells (OSCs). OSCs have many advantages over inorganic solar cells, such as light weight, solution- based treatment, without high temperature and high vacuum, and flexibility.1 5 The rich source and tunable optical and electronic properties of organic materials have attracted much attention. The potential of the OSCs’ market is huge, not only for solving the current energy crisis and environmental pollution problems, but also for obtaining huge social and economic benefits. Tang6 prepared a donor-acceptor double-layer planar heterojunction solar cell with a photoelectric conversion efficiency of about 1% in 1986. However, the device performance of planar solar cells is mainly limited by two aspects: a limited contact area between donor and acceptor and short diffusion length of carriers. These problems can be solved by introducing a bulk heterojunction (BHJ) of the active layer. The first BHJ was achieved by Hiramoto et al.7 who co-evaporated donor and acceptor molecules at high vacuum condition. The first effective BHJ solar cell was achieved by Heeger group and Friend group in 1995.8-9 Heeger group mainly adopted conjugated polymers and small molecule fullerenes to form a heterojunction, while Friend et al. emphasized the BHJ between polymer and polymer. The BHJ based on polymer and fullerene dominated in the next 20years. An efficiency of 10% was achieved in single-junction heterojunction OSCs in 2015.21011 With the development of new narrow-bandgap donor materials, non-fullerene materials, interface modification, morphology control, and ternary OSCs in recent years, the efficiency of OSCs has now exceeded 18%.12-14 In addition, the products of OSCs can be flexible and semi-transparent. They are getting closer to practical application and commercialization.
Working Mechanism of OSCs
The typical working process of OSCs mainly includes the following four steps: (l) photon absorption, (2) exciton diffusion, (3) charge separation, and (4) charge collection.
10.1.2.1 Photon Absorption
The active layer of the BHJ is composed of conjugated polymers and fullerenes. When the light is incident on the active layer, the electrons will be excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the polymers. Light absorption efficiency depends on the optical characteristics of organic materials. In general, a large absorption coefficient of organic materials is in favor of light absorption. The absorption spectra of organic materials are narrow and always located in the visible regions. Therefore, the absorption efficiency of organic materials is less than 40%, which is one reason why the photoelectric conversion efficiency (PCE) of OSCs is less than inorganic solar cells. Organic semiconductors have low carrier mobility, so the thickness of the active layer is in the range of tens to hundreds of nanometers, w'hich also reduces the light absorption capacity.
10.1.2.2 Exciton Diffusion
The electron-hole pairs are tightly bound together to form a Frenkel exciton due to the low' dielectric constant (er ~ 2-4) of the polymer, and the exciton binding energy is in the range of 0.3-1.OeV.1516 According to the spin state, excitons can be divided into singlet and triplet states. Since excitons are neutral, exciton motion is also described as a diffusion process. After exciton formation, it will rapidly diffuse to the donor-acceptor interface. Due to the low charge mobility of the polymer, the effective diffusion distance of the excitons is within 10 nm.15 When the size of the phase separation between the donor and acceptor is too large, the exciton will recombine, leading to the reduced photocurrent generation.
10.1.2.3 Charge Separation
Exciton separation requires an additional driving force owing to the strong Coulomb interaction of electron-hole pairs and low dielectric constant of organic materials.17-19 The conventional view is that the energy-level difference between the donor and the acceptor provides the driving force for exciton separation. The larger the LUMO difference between the donor and the acceptor, the easier the excitons are separated. However, recent studies have shown that the exciton separation has no absolute relationship with the energy-level difference between donor and acceptor. Friend et al.20-21 used ultrafast spectroscopy to confirm that the driving force of charge separation is to make the excited state delocalized. This process has a short lifetime and can replace Coulomb interaction. Charge separation does not require too much energy to overcome the Coulomb interaction. If the electron energy state generated on the donor overlaps with the fullerene eigenstate, the electrons can be transferred by resonance. Charge separation could be achieved in a long range. Charge transfer efficiencies are different during several polymers even though the similar polymer energy levels.22 Charge transfer efficiencies are caused by the charge transfer state in the molecules. This further confirms that charge separation is not completely determined by the energy difference between the donor and acceptor.
10.1.2.4 Charge Collection
When the excitons are separated to free charges, the electrons are transported in the acceptor, and the holes are transported in the donor. Under the action of the external electric field, the electrons move toward the cathode, and the holes move toward the anode. The continuous transport channel is critical to the charge collection. The donor and acceptor have large contact areas, which facilitate exciton separation. At the same time, the interconnected channels can improve the charge collection efficiency.
