Halide Perovskite Photovoltaics
Solar Energy and Photovoltaics
The sun not only gives us light and happy mood, but is also a natural power source for our Earth. It drives the circulation of global wind and ocean currents, the evaporation and condensation of water cycle that creates rivers and lakes, and the biological cycle of photosynthesis that causes the diversity of nature and life (Lewis and Crabtree 2005). It provides us clean and abundant energy. The energy from sunlight to our Earth in 1 hour is more than the total energy consumption by humans in an entire year (Lewis and Crabtree 2005, Lewis and Nocera 2006, Cook et al. 2010). The energy released by the earthquake in San Francisco 1906 with a magnitude of 7.8 is equal to the amount of energy the sun delivers to the Earth in 1 second (Crabtree and Lewis 2007). Solar energy is the largest resource among various renewable energy sources by far. Solar energy can be converted to electricity through photovoltaic cells, fuel through natural or artificial photosynthesis, and thermal energy by heat engines or other techniques (Crabtree and Lewis 2007). Here in this chapter, we focus on how to convert solar energy into electricity through photovoltaic cells, emphasizing the principle of photovoltaics, perovskite materials, and the structure and properties of these materials.
Photovoltaic or solar cell is a device that absorbs light and converts it into electricity. Normally, for a semiconductor with a bandgap of Eg, it absorbs light with energy (hv) higher than its bandgap, and an electron is excited into the conduction band leaving a hole behind in the valence band (Figure 1.1a). If the excited electron could be collected and passed through an outer circuit, electricity is “generated”. Figure 1.1b depicts a typical current-voltage (I-V) curve of a solar cell. The open circuit voltage (V^) is the maximum voltage that a device could obtain. The short circuit current (Jx) is the maximum current that a device could achieve. The power conversion efficiency (//) of one solar cell is the ratio of the output electricity to the input energy of sunlight. In practice, the efficiency ц is determined as the ratio of the maximum power output, Pmax, generated by the solar cell to the power input, Pin, based on the measurement of I-V curve: (Wiirfel 2007)
where Jmp and Vmp are the current density and voltage at the maximum power point (Figure 1.1b). To simplify the calculation and relate the efficiency with practically measurable parameters, fill factor (FF) is introduced, which is defined as the ratio of the areas of two rectangles determined by ./mp and Vmp (blue in Figure 1.1b) and by Voc and Jx (green in Figure 1.1b), respectively. Accordingly, the three parameters of Voc, Jx, and FF combine to determine the efficiency of a device as shown in Eq. (1.1). The input energy of sunlight (Pjn) is 100 (mW cm-2) based on one sun condition as a standard level for comparison of efficiency of devices fabricated in different labs and geographic conditions.
Before going into details of how to improve the efficiency of solar cell, we first look at the spectrum and energy distribution of standard sunlight shown in Figure 1.2 (based on solar cell application). Sunlight is actually electromagnetic radiation.
FIGURE 1.1 (a) Illustration of band structure and photoelectric effect in a bulk semiconductor and (b) a typical current-voltage (I-V) curve of a solar cell device for efficiency calculation (Han et al. 2017). (Reprinted with permission from Elsevier.)
FIGURE 1.2 (a) Energy distribution of solar spectrum and (b) the corresponding energy level and wavelength. (Drawn based on the data of AM 1.5G according to ASTM G173) (Han et al. 2017). (Reprinted with permission from Elsevier.)
According to the wavelength from lower to higher, people divide the spectrum of sunlight into three regions: ultraviolet (UV, wavelength of 300-400 nm), visible (VL, wavelength of 400-700nm), and infrared (IR. wavelength of 700-4,000nm) as presented in Figure 1.2a. The total power of the solar spectrum is integrated to 100 mW cm-2 at AM 1.5 G, which is usually named as standard one sun condition. Currently, most of solar cells utilize UV and visible regions of the solar spectrum. Near infrared (NIR, wavelength of 700-1,400 nm) is also used by some types of solar cells. The rest of the energy in the long-wavelength region is lost as heat. The energy level of photons, as shown in Figure 1.2b, can be calculated by the Planck- Einstein relation (French and Taylor 1978):
where h is the Planck constant, v is the light frequency, c is the speed of light, and X is the wavelength of incident light. This relation accounts for the quantum nature of light. The short-wavelength UV light, i.e. highest energy, only occupies <5% of the total solar energy. While the long-wavelength, low-energy NIR accounts for 52.5% of the solar energy. The visible light located in the middle covers around 43% of the solar energy.
The of the absorber used limits the maximum value of Т(1С. Therefore, the higher the E , the higher the possible value of output Voc. The Jsc is a product of light harvesting efficiency, charge separation efficiency, and charge collection efficiency. The light harvesting efficiency is dependent on the absorbance of a semiconductor. The more photons are absorbed, the more efficient the light harvesting is. In principle, a semiconductor can only absorb a photon whose energy is higher than its Eg. However, photons bearing much higher energy than Eg can excite electrons to energy levels above conduction band minimum (CBM), and subsequently electrons rapidly relax to the CBM by releasing the extra energy as heat. To harvest more photons, the Eg should be as low as possible. Consequently, there exists an optimal bandgap energy for photovoltaic application in consideration of the spectrum losses. Shockley and Queisser calculated a theoretical conversion efficiency of around 31% for single junction solar cells (Shockley and Queisser 1961). M. Green proposed a simple empirical relation to estimate the minimal value of the reverse saturation current density. As such, the optimized Eg should be around 1.5 eV, which agrees well with the experimental data (Shah 2010, Green 1982).
Halide perovskite solar cell is a new type of thin film solar cell. Since its first report in 2009, the efficiency in 2019 already reached to more than 25%, which is the highest among all polycrystalline thin film solar cells. The low-cost solution process and feasibility of bandgap tuning make this material a promising candidate for novel thin film photovoltaics. Section 1.2 elaborates the feature structure and optical and optoelectronic properties of these materials. Section 1.3 explores the stability of halide perovskites including intrinsic thermal stability and strategies developed to stabilize these perovskite structures. A summary follows in Section 1.4.