INTRODUCTION
Nanomaterials (NMs) are a class of materials attaining remarkable attention in several research areas due to their tunable physical, chemical, and biological properties. NMs exhibit superior electronic, optical, and chemical characteristics in comparison to their bulk counterparts due to their enhanced surface to volume ratio and the quantum confinement effect. Among NMs, quantum-dots (QDs) are one of the most fascinating, as they show improved optoelectronic properties by the quantum confinement effect. QDs have tremendous applications in photovoltaic cells, transistors, LEDs, lasers, photodetectors, quantum computing, and biological imaging.
Quantum dots are based on the theory of quantum confinement, which comes into play when the diameter of the nanoparticle is of the order of the electrons wavelength. Whenever the particles dimension reaches this critical size, the electronic and optical properties of that nanoparticle substantially deviates from its bulk counterpart. In the quantum confinement effect, the particle behaves as a free particle when the confinement dimensions are large as compared to the wavelength of the particle. At this state, the bandgap remains at its original energy due to a continuous energy state. However, when the confining dimension decreases and a specific size limit has been achieved by the particle, the continuous energy state spectrum changes to a discrete energy state spectrum. Resulting in the bandgap dependency on the size of nanoparticle becomes more prominent, as shown inFigure 1. The spectrum of colors in quantum dots relies on this energy, as the size of the quantum dot increases, this results in a red-shift (lower energy), and as the size decreases, there is a blue-shift (higher energy) as shown inFigure 2. In short, quantum confinement is the reason for the increase in the energy difference between bandgap and energy states. InFigure 1, the band structure of bulk semiconductors absorbs light (photons) having > energy bandgap (Eg). In the case of semiconductor cells, photo-generated carriers thermalize to band edges (within about 10−13s) as excess energy is lost as heat, reducing output. This limitation reduces current, while the thermalization reduces the voltage. As a result, semiconductor cells suffer a trade-off between voltage and current, which can be in part alleviated by using multiple junction implementations.
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Figure 1.
Energy-Band diagram of semiconductor
Figure 2.
Representative quantum-dots of a particular
Schematic diagram of a quantum-dot solar-cell..
CLASSIFICATION OF SEMICONDUCTOR QDS BASED SOLAR CELLS
CdSe is a well-known semiconducting material consisting of a direct band of 1.74 eV at room temperature, belongs to the II-VI group of semiconductors, and highly favors wide range optical absorption in the visible range. Most of the earlier research extensively focused on the synthesis of CdSe nanocrystal, especially with the synthesis of QDs due to ease of bandgap tunability with controlled size growth.7,8The outstanding fluorescence of CdSe materials make them ideal for use in various light-harvesting applications, especially photovoltaics. S. N. Sharma et al.9demonstrated a photoinduced charge transfer between CdSe QDs and p-phenylenediamine (ppd). They found that the formation of ppd cation radicals and other charged species at the surface extends the bleaching recovery over several microseconds. To facilitate carrier injection and lightabsorption in QD solar cells, the energy levels of quantum dots should match appropriately, as seen in the cases of CdS/CuInS2, CdSe/CdTe and CdS/CdSe10,11core/shell QD structures. The CdS/ CdSe core/shell structure is one of the most widely explored QD structure owing to its easy synthesis, high stability with an efficiency (PCE) ~ 5%. Currently, the highest performing QD solar cells exhibit a PCE of only 68% due to the severe chargerecombination and low photoanode area coverage. To encounter this situation, mesoporous photoanodes required to employ to enhance electron transport and large area acquisition. The best performing QD solar cells contain TiO2as a mesoporous photoanode because of its low-cost and excellent chemical stability. Linkers, such as MPA-mercaptopropionic acid, have also been used for TiO2film coating, followed by exposures to QDs for QDSC assembly.12
InP (group IIIV) is one of the most promising materials for photovoltaic applications, with an appropriate band gap (1.35 eV), higher mobilities, and a high absorption coefficient. InP shares the lowest intrinsic toxicity as compared to Cadmium based cells. The use of different capping ligands help to overcome the size effect and bandgap tunability. Highly luminescent InP QDs with a bluish-green emission (λ~490 nm) has been reported by synthesis using the hot injection technique, with trioctyl phosphine (TOP) as the source of P instead of the conventional but toxic and expensive tristrimethylsilyylphosphine (P(SiMe3)3) without resorting to any post-synthesis etching to tune the emission to the blue region.13InP QDs are attractive due to their higher photostability and their strong covalent bond. Due to these properties, InP is one of the favorite materials for use in QDSCs. Although the strong covalent bond may cause complications for monodisperse nanoparticle synthesis, which in turn produces defects and cause a lower luminescence efficiency, to overcome the stability situation, modification to time, temperature, and choice of ligands is crucial. Other disadvantages of InP include low photocurrent due to the electron loss at the QD-electrolyte interface, which requires QD loading with enough surface area utilization in a broad range of visible spectra. Furthermore, the size of the QD can be tuned to increase the light-harvesting range. While doping can be useful for tuning the QD to the visible spectra.13,14
PbS QDs are a promising nanostructured material for photovoltaic applications. PbS is known as a photo absorber in the visible and near infra-red regions, due to its bandgap tunability and solution processability and is also considered a low-cost solution-processed photovoltaic material. Rapid advancement in quantum solar-cells by architecting and surface modification led to a power conversion efficiency of 11.3% with outstanding stability. However, the ligand exchange process renders the manufacturing of the QD device. The choice of capping ligand (oleic acid, MPA, thiols, etc.) plays a crucial role in the stability of QDs. A typically used capping ligand, oleic acid, both stabilizes and helps to passivate the surface, making it insulated from surrounding QDs. It also allows interdot distance adjustment, which defines the coupling strength between the inter-related QDs.15An enhanced efficiency of 8.45% was reported for conventionally structured PbS QD solar cells, in which a modified anode buffer layer comprising of a unique conjugated polymer PDTPBT was used. This anode modification improved the device performance due to increased hole extraction to the anode and reduced interfacial carrier recombination which enhanced Voc significantly.16
PbSe is one of the reliable colloidal QDs for the next-generation photovoltaics. Previously, PbS QDs were the primary research focus due to their stability under extreme conditions. There is much interest in expanding the work to PbSe QDs, which shows increased photocurrent due to multiple generations of excitons. The synthesis of a range of sizes is required to overcome the stability issues of using PbSe ambient conditions.17Air stable PbSe QDs were firstly synthesized by the cation exchange method, followed by the solution-phase ligandexchange approach. Then the absorber layers were prepared using the single-step spin coating method, which in turn shows an excellent efficiency of 10.68%, which was nearly 16% higher than earlier recorded data. The stability of that prepared cell remains stable up to 40 days with eight hours of continuous illumination. The performance of PbSe QD solar cells was improved further by introducing SnO2based buffer layer at the interface ETL/PbSe, and this leads to better extraction of charge carriers. Band alignment between the PbSe QD absorber layer and the TiO2buffer layer could be attributed to the performance enhancement of the cell.13,18Figure 4shows the schematic for the fundamental QD base solar-cell structure.
CIS (Copper-Indium/Selenide)
Copper-indium-selenide (CuInSe2) is a p-type semiconductor that has drawn tremendous attraction in the field of photovoltaic applications due to its wide bandgap (1.04 eV) and significant absorption coefficient with high stability.
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It is considered an alternative to the cadmium/lead-free toxic elements. In 1976 a CIS solar cell was fabricated, with an efficiency of ~ 5%, using the evaporation technique.19The parameters of the cell remain stable over a long time, provided the temperature does not exceed 90 C. Unfortunately, even after many precautions, such as placing the assembly in a glass encapsulation, their life does not exceed more than ten years. Moreover, indium resource limitations are a significant concern.20The cell efficiency typically depends on the crystalline nature/structure of the material formed and the share of the Indium and Copper element, In the case of CIS, the hole concentration decreases as the concentration of Copper decreases. The efficiency of CIS increased to 7% when introducing CdS as the absorber layer. Both CIS and CdS show low resistivities when deposited by vapor deposition. Further technological improvement in junctions enables the maximum photon absorption and drags the efficiency to 10%.19Figure 5shows the Schematic diagram representing CIS based Solar-cell.
CIGS (Copper-Indium/Gallium/Selenide)
Several different research technologies emerged, leading to the incorporation of gallium in the CIS cell, also known as CIGS. Following the addition of Gallium (Ga), sodium (Na) has also been incorporated into the absorber layer of CIGS cell assembly with a thin layer of CdS as a buffer layer, which has significantly enhanced the cell efficiency to 22.6%.2022CIGS formation follows two-steps, the optimized CIG precursors are deposited onto a substrate; this could be done by sputtering, vapor deposition, or and electrodeposition. In most of CIG (S, Se) deposition, Ink deposition is trending due to the low material utilization, with less waste and ease of preparation.Figure 5shows the Schematic diagram representing CIS based Solar-cell. Sharma et al. reported an aqueous-free based rapid synthesis (~ 45 mins.) of organically-capped Se-rich CIGSe by the colloidal route.23They reported a process of purification which elevated the charge-transport between CIGSe nanocrystals for the realization of an efficient photovoltaic device without resorting to sodalime- glass (SLG), harsh chemical treatments, or post-deposition thermal selenization.
However, sputtering has proven to be the most efficient method with respect to the cell efficiency. Ga is required to incorporate into Cu targets since Ga has a very low melting point. Stacking sequences of precursor layers can profoundly influence the elemental distribution through the depth of the final film. Most of the time, some Se has already incorporated by precursor layers, which are, in turn, annealed and selenized. The substrate preparation in CIGS solar cells is the most crucial and tedious process, which plays a vital role in the development of the whole device. Molybdenum deposition onto a flexible/rigid substrate defines the condition of selenization. The annealing temperature challenges the ability to create a flexible, CIGS based device. Flexible substrates cannot withstand this high temperature of 500 C, which is a mandatory step for absorber layer crystallization in a CIGS cell. The efficiency recorded using a flexible substrate is ~20%. In CIGS structure, the role of n-semiconductor is fulfilled by ZnO as the buffer layer and CdS as the window layer; both can be replaced by each other. A schematic of the CIGS solar cell has a structure of substrate/ Mo/CIGS/CdS/ZnO/metal grid (Figure 6). Here light enters via a Transparent conductive oxide (TCO) and a back-contact deposited on the substrate.24
Perovskites
Perovskites are a class of minerals with a crystalline cubic/ diamond-like structure. A conventional perovskite is a calcium titanium oxide (CaTiO3) mineral, first found in the Ural Mountains of Russia. The metal halide perovskite, which has a similar structure to oxide perovskites, are potential candidates for solar-cell applications. A halide perovskite has an ABX3crystal structure (Figure 7), where A is a monovalent cation, such as Cs+, MA+, FA+, B is a divalent cation such as Pb2+or Sn2+, and X is an anion such as Cl–, Br–, and I–.25
Perovskites offer several advantages in the solar cell regime. Perovskites offer composition with a tunable bandgap (1.5 2.3eV), which is the most favorable condition for maximum sunlight absorption and hence the maximum conversion of electricity.2628They can be synthesized by solution processing techniques, which makes the fabrication process cost-effective when compared to conventional solar cell technologies.29,30Perovskites can also be used as an electron/hole transport layer to enhance charge transport. It has high charge mobility, high excitation coefficient, long carrier lifetime, carrier diffusion rate, and high absorption coefficient.31Furthermore, the formation of perovskite thin films requires very little material. No rare earth elements are required. Perovskites are highly tolerant of defects, which creates a large manufacturing yield.25The module has the lowest environmental effect, depending upon the manufacturing method used. They can be easily deposited onto a flexible substrate, opening a new area for further research.
The disadvantages of perovskites are their environmental induced (moisture, oxygen, and light) stability issues. However, due to the rapid advancements in cell efficiency, researchers have overcome many of these limitations by providing a standardized packaging assembly. Other stability issues like mechanical durability, thermal influence, applied voltage heating, and current-voltage behavior require further detailed study.
Most efficient metal halide perovskite is composed of lead (Pb) based material, such as, FAPbI3,MAPbI3, and CsPbI3. Figure 8 shows the schematic of a perovskite solar cell. Currently, researchers have achieved very high efficiencies (>25%) with perovskite-based solar cells.32The National Center of Photovoltaics (NCPV) has shown that when carriers are excited with high energy, the cooling rate in perovskite materials slows down during the cooling process. The cooling occurs more slowly when using MAPbI3as compared to other traditional and expensive structures, making this material an excellent candidate for next-generation hot carrier solar cells with high efficiency. Since excitons do not remain stable at room temperature, the coulomb interaction between carriers (electrons and holes) dramatically affects the optical absorption in these materials, and the presence of excitons impacts on recombination dynamics.
