Fundamentals of Solar Cell Design. Rajender Boddula
Figure 2.1 Schematic representation of photoelectric effect. Einstein equation hf = ɸ + Ek, where h is plank constant (6.63 x 10-34 Js), f is the frequency of the incident light (Hz), ɸ (phi) is the work function (J), Ek is the maximum kinetic energy of the emitted electrons (J).
The major challenges for developing high efficient solar cells are increasing light absorption and minimizing recombination. The tuning in the energy band gap can be useful to overcome these challenges. Recombination loss can be decreased by making thinner solar absorber in PV device. The PV absorber layers are generally thick to allow absorption with near wavelength and collection photocurrent. A major percentage of solar light spectrums are not absorbed in thin film solar cells, and hence, they are facing problem of low efficiency in comparison to traditional Si cells. Nanotechnology has the ability to make more efficient materials and PV devices. Plasmonic nanostructures can be helpful to reduce the thickness of PV absorbers while keeping their optical thickness constant. In plasmonic solar cells, the plasmon resonances in small metal nanoparticles are used to trap light. Then they transfer the energy to thin semiconducting layer to produce electricity through free charge carriers [15–17].
Table 2.1 Various solar cell technologies and its efficiency.
| Solar cell technology | Maximum efficiency | Reference |
| First generation: Single junction Mono-crstalline and poly crystalline silicon, GaAs solar cells | ~20–24% (silicon)~ 38% (GaAs) | [7, 17, 90–92] |
| Second generation: Thin film solar cells such as CIGS, CdTe, CZTSSe, DSSCs | ~ 21% (CIGS)~ 19% (CdTe)~ 13% (CZTSSe)~ 18% (DSSCs, PECs) | [7, 17, 93–101] |
| Third generation: Hybrid DSSCs, PECs, quantum dot, perovskites, plasmonic solar cells | ~ 30% | [7, 102–104] |
| Fourth generation: Multijunction solar cells with GaAs, InP and other intermediate layer | ~ 47% (multijunction GaAs) | [7, 105–112] |
2.1.1 Plasmonic Nanostructure
The specific wavelengths of light spectrum make the conductive electrons to oscillate collectively in the metal. This phenomenon is known as plasmonic resonance or surface plasmon resonance (SPR), which is schematically shown in Figure 2.2. Plasmon resonance of free electrons in metal nanoparticle is basis of the plasmonic effect. SPR wavelength is depending on the medium, size and shape of metal nanoparticles. SPR excitations increase more absorption and scattering of light compared to nonplasmonic nanoparticles [18, 19]. The optical properties can be tuned to absorb wide range of light spectrum of the electromagnetic radiation by tuning the thickness, size, shape, and composition of nanostructure. The reflecting color also can also be tuned by shifting the scattering and absorption, for example, solutions of gold and silver nanoparticles have ruby red and yellow color due to strong scattering, respectively [20, 21]. The resonance wavelength of plasmonic nanostructure is influenced by the refractive index (n) of material, for example, peak resonance of Ag nanoparticles (80 nm in size) in water (n = 1.33) is at 445 nm and in air is about 380 nm. This theory was explored by Mie theory that can be used to analyze the scattering from spherical particle at any wavelength of light [22, 23]. The coupling effect in plasmonic nanoparticles is responsible for the shifting in absorption and color of particles. The scattering by the metal nanoparticles is more important than the absorption of light in plasmonic solar cells and hence plasmonic nanostructures have emerged as promising candidate to enhance the characteristics in solar cells, detectors, and sensors. The plasmonic nanoparticles of silver (Ag) and gold (Au) are mostly used at the top of the surface of PV device because of their SPR exist in the visible range. Al nanoparticles show SPR in UV range but they are unable increase the efficiency in plasmonic solar cells [24].
Figure 2.2 Schematic representation of surface plasmon resonance.
2.1.2 Classification of Plasmonic Nanostructures
SPR are sensitive to the geometry of device, material, and light-matter interaction. The shapes of nanostructure can be classified as sphere, thin film, star, core-shell, disk, cavity, cage, etc. [25]. The plasmonic metal nanoparticles or thin films are used to get maximum absorption of light in absorbing layer. SPRs can be tuned to obtain desired frequency by using specific grating. The plasmonic nanostructures such as metal nanoparticle or thin film are mainly used in plasmonic solar cells that depend on the design and method. The most frequent and widely used design is the deposition plasmonic nanoparticle at the top surface of PV device. The light is scattered by the SPR in all directions. This increases the overall absorption in solar cells. The core (metal)–shell (dielectric) designs are also used to absorb more light. The other design of plasmonic nanostructure is depositing a nano-sized thin film of metal on or below the semiconductor layer. The sunlight generates electric fields inside the absorber layer and electrons can be collected as a flow of current [26–28]. The schematic representation of plasmonic solar cell with nanoparticle and thin film configuration is shown in Figure 2.3.
Figure 2.3 The utilization of plasmonic nanoparticle (upper) and thin film (lower) for solar cells.
2.2 Principles and Working Mechanism of Plasmonic Solar Cells
2.2.1 Working Principle
A plasmonic nanostructure can be used as direct and indirect applications such as active PV material and plasmon modified active semiconductor, respectively [29]. The fundamental operational principle of plasmonic solar cell is scattering through SPR in metallic nanoparticles on semiconductor and surface plasmon polaritons at the metal/semiconductor interface.
Solar cell performance mostly depends on photon energy and active PV layers. When a photon falls on the surface of semiconductor, the photons will pass through material (if photons have energy lower than the semiconductor band gap, Ephotons < Eg) and photons will reflect from the surface or they will be absorbed by the semiconductor layer (if photons have energy higher than the band gap of semiconductor, Ephotons > Eg). The unused sunlight produces heat in the solar cell through lattice vibrations and decreases the performance. An electron is excited into conduction band from the valance band due to the illumination of light and creates hole in semiconductor. Thus, absorbed photon generates e-h pair in the semiconductor. The electrons and holes have tendency to recombine due to their opposite charge, so they can be separated and the electrons can be collected as a flow of current. The fast collection of electrons would produce more electricity. Plasmonic nanostructures decrease the loss of incident photons by increasing optical path length in PV devices [30]. The very thin semiconducting layer absorbs less sunlight and hence the plasmonic nanostructures are used to enhance the absorption through scattering at