RESEARCH & LAB

Figure 1

Solar Energy is known as sustainable energy because of its endless source and huge number of resources.

The development of silicon photovoltaics (PV) and inorganic solar cells based on silicon substrates has been involved for a long time. This photovoltaic technology has stable efficiency performance and has been put into markets. Although such photovoltaics generally has power generation efficiency, i.e., power conversion efficiency (PCE), of more than 20%, their development has been limited for decades due to the high cost of manufacturing processes and dramatic environmental pollution. Dye-sensitized Solar Cells (DSCs), known as the new generation of solar cells, have gained considerable research interest in the past two decades and are considered of more potential as an attractive alternative 333 traditional silicon-based photovoltaics. As shown in Figure 1, the dye-sensitized cells are different from the conventional crystalline silicon photovoltaics. Due to the use of the glass substrate, its appearance is colorful and bright, and it has the characteristics of light transmission. If the plastic substrate is used, the dye-sensitized solar cells can become flexible.

On the way to the new generation of battery technology for life and application, the dye-sensitized solar cells have found a clear application direction, that is, the concept of energy harvesting that can also function in the indoor artificial light source environment. The second picture on the left was taken in a research building of EFPL located in Lausanne, Switzerland. The wall of the building is made up of hundreds of colorful transparent dye-sensitized solar cells and floor-to-ceiling windows, which represent the perfect design of DSCs integrated into modern architecture. This design also shows a great difference between the dye-sensitized solar cells and the silicon photovoltaics—the environment to be employed.

Figure 2

Dyed-sensitized solar cells rely weakly on the angle of incident light and perform better under low light conditions, making them suitable light-harvesting devices for integration with sensors, wireless transmitters in Internet of Things (IoT) systems. As shown in Figure 2 on the left, since the human living environment is mainly surrounded by T5, T8 fluorescent lamps and white LED lights, these features highlight the photo-sensitivity of DSCs to dim lights, as described above. However, unlike sunlight which covers full-wavelength, e.g., panchromatic, the emission spectrum of commercially available fluorescent lamps mainly covers the visible region at 400-640 nm, including three intensive peaks at wavelengths of 430 nm, 550 nm and 620 nm. Thanks to the flexibility of the dye design and the tunable light absorption wavelength, one can create a single dye-sensitized solar cells that is highly efficient under both sunlight and artificial light environments. It is worth noting that there is a huge difference in the intensity of sunlight (typically >100,000 lux during the day) and indoor light sources (usually between 300 and 6,000 lux, depending on the environment). Despite this, with the help of appropriate voltage-boosting components, DSCs is capable to drive microelectronics with power consumption at μW level (0.000001 W).

Figure 3

How does the dye-sensitive solar cell work?

The dye-sensitized solar cells has the characteristics of low material cost, easy process and simple manufacturing equipment. The overall cost is about 1/5~1/10 of the traditional silicon-based solar cell, which has caused the energy industry to actively invest in this research and development. The structure of the DSCs is a very simple sandwich structure as shown in Figure 3, and the two sides are conductive electrodes named photoanode and counter electrode, respectively. Wherein, the photoelectrode is coated with mesoporous TiO2 nanoparticles where dyes are adsorbed on it, and redox electrolytes are fulfilled in the space between the photoelectrode and the counter electrode. The way to intelligently design simple and efficient dyes has become very important since dye is one of the most important components of DSC devices in which it absorbs incident photons and produces photoelectrons that sensitize the semiconductor. Another key material is the electrolyte. The electrolyte plays a very important role in mediating electron transfer processes in terms of regenerating photo-oxidized dyes by counter electrode. The existence of proper redox electrolytes trigger the high performance as well as long-term issues of DSCs, rendering it one of the important keys to mass production.

The dye-sensitized solar cells is mainly composed of a mesoporous semiconductor with nm scale, a dye (also named photo-sensitizer), an electrolyte (redox shuttles), a counter electrode, and a conductive substrate. The working principle is as shown in Figure 4 on the left and detailed as follows: :

1. The dye molecules first undergo photo-excitation from ground state to excited state toward light absorption.

2. The photo-excited dye rapidly injects electrons into the conduction band of the TiO2 semiconductor.

3. The electrons are immediately transferred to the conductive substrate, conducted out and passed through the external line to the counter electrode.

4. After electron injection, the dye in its oxidized form is reduced  to ground state by counter electrode through the mediation of redox electrolytes in terms of various electron transfer processes between electrolyte molecules.

Figure 4

Harvesting solar energy is a promising solution under the growing energy crisis, which is undoubtedly an extremely attractive goal of this century. The emergence of perovskite solar cells (PSCs) has been a matter of the last decade, a very new but familiar area because the cell technology of perovskites comes from dye-sensitized solar cells. Today, the power generation efficiency of perovskite solar cells (ie, photoelectric conversion efficiency or PCE; Photon-to-electron Conversion Efficiency, see Figure 4) has significantly increased by 3.8% over the past few years to over 20%, showing a transcendence Great potential for other organic or hybrid solar cells. Especially in recent years, research on perovskite solar cells has mushroomed, and the current world record has exceeded 25%. This is not only an increase in power generation efficiency, but also an important indicator for the coming generation of solar cells, because this efficiency has caught up with the traditional silicon phovoltaics. Although more than 90% of the solar cell industry's annual production capacity comes from crystal cells, the perovskite solar cells have many advantages, and they are increasingly favored by the academic community and the industry. Material resources are related to related research, and the huge charm of perovskite solar cells is gradually showing up in front of people.

Figure 5

Interestingly, so-called perovskite solar cells, the perovskite material in this name does not contain calcium and does not contain titanium. The reason why it is named as a perovskite battery is because the battery material uses a light absorbing layer material whose structure is called perovskite. Briefly, the dye in DSCs is replaced with a perovskite as a light absorbing material (photosensitive layer). The crystal structure of this perovskite is shown in Figure 5, generally in the form of ABX3, where A is usually methylamine, B is mostly Pb or Sn metal ion, and X is a single or mixed halogen ion such as Cl, Br, I. . Currently, among the high-efficiency perovskite solar cells, the most common perovskite material is iodinated with extremely broad absorption in the ultraviolet-visible (UV-vis) region and a bandgap of about 1.5 eV. Lead methylamine (MAPbI3, molecular formula is CH3NH3PbI3). The HOMO and LUMO energy levels of perovskites are -5.43 eV and -3.93 eV, respectively, making perovskite materials an excellent photosensitizer. At the same time, many characteristics of perovskites such as lower manufacturing costs, longer carrier diffusion length, regulatable energy gap, resistance to solution processes, very low exciton binding energy, and The suppressed charge recombination ability also makes it sufficient as a good electron-transporting materials (ETM). 

Figure 6

As shown in Figure 6, the device structure of perovskite solar cells are layered structure containing FTO glass, electron transport layer (ETM), perovskite layer, hole-transporting materials (HTM) and metal either in the bottom or on the top. Within these, the electron transporting layer is generally a dense nanoparticle to prevent carriers of the perovskite layer from recombining with carriers in the FTO. In research, scientists usually use the design of mixed properties of p-type or n-type materials, the regulation of surface morphology, the doping of elements or the use of other n-type semiconductor materials such as ZnO. Improve the conductivity of the layer to improve battery performance. In particular, in addition to being used as a light absorbing and electron transporting material, perovskites also have a hole transporting capability. Therefore, research has also been conducted to produce a single material perovskite solar cell without HTM or ETM using perovskite as a light absorbing and electron and hole transport layer.

On the other hand, in the choice of hole transport material (HTM), it is necessary to achieve the HOMO and LUMO energy levels of the material to match the perovskite and the good hole mobility requirements of the material itself. The hole is quickly extracted from the perovskite layer and the hole is conducted while suppressing its charge recombination. Based on this demand, scientists have grafted the frequently used design concept of hole-transporting materials in related optoelectronic technologies such as organic light-emitting diode (OLED) and organic field-effect transistor (OFET). The design concept of HTMs is primarily based on aromatic amine groups owing to its good thermal stability and hole mobility.

In dye-sensitized solar cells, most of the redox electrolytes that play a key role are liquid. There also exists many studies using gel-type or solid electrolytes. However, in the technology of perovskite solar cells, early studies have pointed out that the use of liquid electrolytes results in rapid decomposition of the cell, which is also a major problem in early perovskite-based cells. Later, Grätzel et al. have proved that the use of solid Spiro-OMeTAD or PEDOT:PSS can greatly improve device efficiency while remaining good stability. Spiro-OMeTAD (2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene ) is a well-known HTM characterized by a spiro skeleton with multiple electron-rich arylamine substitution. The introduction of Spiro-OMeTAD as an alternative solid-state HTM introduces I-/I3- electrolytes, making the perovskite solar cells an effective candidate for next-generation solar cells. Careful design of perovskite cell components with Spiro-OMeTAD and its cyclopentadiene-based analogs yields high conversion efficiencies of 19.3% and 20.2%. Due to the poor mobility of the original form of the hole, the Spiro-OMeTAD must add Li-TFSI. However, this chemical oxidation process ultimately leads to erosion of the perovskite layer. In addition, the functional adaptability is poor. The tedious synthetic route from the spiral skeleton and the participation of dangerous n-butyllithium in the traditional synthesis process have brought certain obstacles to the practical application of Spiro-OMeTAD. Therefore, considerable efforts have focused on developing alternative hole transport materials with optimized synthesis schemes. Through the improvement of spiral-based materials, sensible molecular engineering has also successfully produced several high-performance HTMs with high carrier mobility and stability.

Figure 7

An OPV cell (Figure 7) is a type of solar cell whose absorber layer is based on an organic semiconductor (OSC)—usually a polymer or a small molecule. To make an organic material conductive or semiconducting, a high level of conjugation (alternating single and double bonds) is required. Conjugation of organic molecules results in delocalization of electrons associated with double bonds over the entire conjugation length. These electrons have higher energy than other electrons in the molecule and are equivalent to valence electrons in inorganic semiconductor materials.

In organic materials, however, these electrons do not occupy the valence band, but are part of the so-called "highest occupied molecular orbital" (HOMO). As in inorganic semiconductors, there are unoccupied energy levels at higher energies. In organic materials, the first is called the lowest unoccupied molecular orbital (LUMO). Between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of an OSC there is an energy gap - often referred to as the material's bandgap. As the conjugation increases, the band gap becomes small enough that visible light can excite electrons from the HOMO to the LUMO.

Figure 8

The steps to control the OPV function can be summarized as shown in Figure 8:

1. Absorption of incident light causes excitons to be generated. Light with a sufficiently high energy level will be absorbed by the OSC and excite electrons from HOMO to LUMO to form excitons. If the absorbed light energy is larger than the bandgap, the electrons will move to a higher energy level than the LUMO and decay. This process is known as "thermalization," in which energy is lost as heat. Thermalization is a key energy loss mechanism in photovoltaics.

2. Diffusion of excitons to the donor-acceptor interface. Once formed, excitons diffuse through the OSC assembly to the donor-acceptor interface, and the exciton dissociation is driven by the shift between LUMO energy levels. This has to happen within a certain amount of time. Otherwise, the excited electrons return to an empty energy state (called a hole), a process known as "recombination." The time this takes is known as the "exciton lifetime" and is usually expressed as the distance an exciton can diffuse during this time (approximately 10 nanometers).

3. Dissociation of excitons at this interface. At the interface, electrons will move to the acceptor material, while holes will remain on the donor. These carriers are still attracted, creating a charge transfer state. When the distance between two people increases, the attraction decreases. Eventually, the binding energy between them is overcome by thermal energy, forming a charge-separated state. While the electron-hole pairs are still attracted in the charge-transfer state, recombination may occur at the interface between the two materials.

4. Carrier transport. The carriers will then diffuse through the relevant interfacial layers to the appropriate electrodes (i.e. holes to anode, electrons to cathode).

5. Charge carrier collection. At the electrodes, the charge carriers are collected and used to do work in the battery's external circuit—producing an electric current.

Recombination. At several stages, the electrons and holes can recombine—at which point the absorbed energy used for the initial excitation is wasted. Recombination can be classified into the following two types: (a) Geminate: the originally generated electron-hole pairs recombine before the excitons dissociate; (b) non-pairing: free electrons and holes can recombine regardless of their origin. Both processes can be radiative (releasing photons) or nonradiative (releasing photons). Non-radiative processes include i) Auger recombination, where recombination energy is transferred to another free electron, which then decays; ii) trap-assisted recombination, where structural defects lead to the formation of energy states in the gap between the HOMO and LUMO.