Recent Advances in Thin Film Photovoltaics

The solar cell based on Si-wafer is referred to as a first-generation solar cell, whereas solar cells based on thin films are referred to as second-generation solar cells. The basic objective of thin-film technology was to reduce the cost of solar cell modules and the cost of solar cells obtained from Si wafers. The thin-film solar cell technology has been radical advancement for the CISe and CdTe-based materials with an efficiency having more than 20% on a lab scale. Even kesterite-based and other new materials have shown promising results. The recent development in the perovskite solar cells has significantly increased the potential for thin-film technology. The deposition parameters strongly affect the structural, chemical, metallurgical properties. The maximum thicknesses of the films are up to several 10s of a micrometer.

Copper indium gallium selenide (CIGS)-based solar cells have received worldwide attention for solar power generation. It is an efficient thin-film solar cell having achieved more than 23% efficiency on laboratory scale, which is comparable to crystalline silicon (c-Si) wafer-based solar cells. CIGS solar cells have also achieved more than 20% efficiency on flexible polyimide substrate making it most suitable thin-film solar cells. One of the major challenge for small area devices is precise control of stoichiometry and efficiency over CIGS film. For industrial production, apart from stoichiometry and efficiency, high-throughput, reproducibility, low-cost and process tolerance are of much importance in commercializing the technology. In this chapter, after briefly reviewing the history of chalcopyrite system, graded bandgap, effects of sodium distribution in CIGS layer, growth of CIGS layer using various techniques, role of buffer layer and their alternatives, transparent conducting oxides, progress related to flexible solar cells and factors affecting the cell efficiency will be are discussed. Further, options for efficiency improvement, challenges and future prospects of CIGS solar cells will be discussed.

Research on the kesterite (Cu₂ZnSn(S,Se)₄), CZT(S,Se)-based thin film solar cell has been substantially increasing throughout the past decade, reaching the forefront of the photovoltaic (PV) research community. Major advances have been reported at various levels, from the fundamental understanding of the material properties to improvements in the device performances and its exploration for applications other than photovoltaics. Being free of critical raw materials (CRMs), the kesterite-based PV technology was expected to supersede the chalcopyrite CIGS compound as the reference chalcogenide absorber, yet the conversion efficiency at the laboratory scale has been stalling in recent years with devices not able to breach the 13% efficiency mark. An abnormally large V_oc deficit has been pointed out as the main limitation in the kesterite-based thin film solar cells. The origin of this deficit remained for a long time a subject of debate within the community, with various reasons being cited such as native defects existing in the kesterite material itself, as well as a poor morphology and highly defective interfaces (front and back) in the device structure. A broad range of methodologies were developed to address those issues, involving alternative back contact materials, compound alloying, as well as the use of electron transport layers or innovative doping and passivation strategies, but the challenge to reduce the V_oc deficit remains unsolved so far. Following a string of landmark theoretical and experimental studies, the community has recently converged toward the idea that the problem associated with the structural disorder arising from the cationic substitution within the kesterite structure was in all likelihood the main limitation of devices voltage, and that the conversion efficiency could only be improved by markedly mitigating the influence of the resulting defects. To overcome this limitation, various theoretical and experimental studies have proposed replacing the unstable cationic species with other more stable candidate elements. The substitution of Cu by Ag, Zn by Cd, and Sn by Si/Ge could possibly suppress the observed cationic disorder and hence reduce the observed V_oc deficit. Among the different suggested candidates, Ge has been identified as the most promising option for replacement of Sn in the standard CZT(S,Se). Sn is known to exhibit a change in its oxidation state, thus creating deep defects within the band gap and inducing band tailing effects; its substitution is therefore the most sought-after. Owing to its CRM-free constituents and freedom to tune the optical band gap from ~1.4 eV (Cu₂ZnGeSe₄) to ~2.2 eV (Cu₂ZnGeS₄), Ge kesterite is an attractive compound for application in single junction solar cells as well as wide band gap top cell in Si-based tandem solar cell devices. In this chapter, we review the different strategies employed to overcome the bottlenecks of kesterite-based solar cells until now, from the doping/partial substitution of Sn with Ge in (CZT(S,Se)) up to a complete replacement leading to the realization of Sn-free CZGSe thin film solar cells. In addition to this state of the art, a complete assessment of the limitations reported by different studies will be proposed. A specific emphasis will be placed on the description of the Ge-substituted kesterite fundamental properties explored by both theoretical and experimental methods. Finally, approaches to improve the device efficiency for single junction solar cells as well as the feasibility of realizing much anticipated tandem devices with Si will be discussed.

CdTe solar cells are the most successful thin film photovoltaic technology of the last ten years. It was one of the first being brought into production together with amorphous silicon (already in the mid 90 s Solar Cells Inc. in USA, Antec Solar and BP Solar in Europe were producing 60 × 120 cm modules), and it is now the largest in production among thin film solar cells. CdTe solar cells stand out for the robustness of the absorber material: its high chemical stability and the large variety of successful preparation methods available make them suitable for large area module production. Compared to other thin film absorber materials, CdTe has an optimum band gap of 1.5 eV so that it could deliver efficiencies above 27%, with an open-circuit voltage of 1 V and a short-circuit current density of 30.5% mA/cm². In this chapter, we will follow the history of the fabrication process together with each and every improvement explaining the discoveries and achievements that have brought to the record efficiency of 22.1%. Moreover, the environmental impact, the future applications and the possible evolutions of this technology will be also described.

Renewable energy sources are needed to overcome the worldwide increasing energy demands. Conversion of sunlight into electricity belongs to the most abundant and easily accessible renewable energy sources. Over the past decade, hybrid lead halide solution-processed perovskite solar cells have shown great potential for low-cost photovoltaic technology. Till now, the perovskite solar cells efficiency has already surpassed the polycrystalline and thin-film silicon solar cells efficiency. However, material toxicity and the stability of lead are the two foremost concerns toward commercialization that need to be addressed. Therefore, it would be enviable to find stable and lead-free alternatives which keep the unique optical and electronic properties of lead halide perovskite. To date, many new alternative lead-free halide perovskites solar absorbers have been explored and utilized in solar cell devices. This review presents a brief overview of the prospects and critical challenges faced by the lead-free perovskite materials in advancing the advancement of perovskite solar cells.

Dye sensitized solar cells (DSSC) are known as promising candidates as an alternative to the costly crystalline solar cell. Although they lag behind silicon or thin-film (CIGS, etc.)-based solar cells for their relatively low efficiency, in terms of cost DSSC stands much ahead of their other counterparts. According to our experience in the persistent efforts that helped to achieve moderately high efficiency of DSSCs. An in-depth review on major progress of improving the energy conversion efficiency of DSSCs is required which may be useful for future research. There are challenges that affect the performance and marketing of DSSCs such as efficiency, stability, cost, and extension to large area with its flexibility as the world is facing for its commercialization.

Thin-film solar cell technology is now one of the major focuses of research mainly due to CIGS and CdTe solar cells which have efficiency more than 20%. The main limitation is lying with the low abundance of In and Te and the toxicity of Cd, among these materials which is the bottleneck to use these materials as an absorber layer for solar cell. As a replacement, new materials like CZTS, CTS, SnS, and Sb₂S₃/Se₃ are emerging and studied as absorber material for thin-film solar cells by the research community. These semiconductors have the advantages like p-type conductivity, high absorption coefficient, and suitable direct bandgap to be used as a thin-film absorber layer material. CZTS, the most researched material, obtained efficiency of 12.6%, but defects, appearance of secondary phases, and phase complexity are the major drawbacks for this material to improve its efficiency. For the last few years, instead of a lot of research to improve the efficiency, no significant enhancement of efficiency is achieved. So the research community is concentrating now on other binary materials which actually showing improved performance in terms of solar cell efficiency. Thin-film antimony chalcogenides, i.e. Sb₂S₃/Se₃ recently appearing as an emerging topic of interest among the researchers. Sb₂S₃/Se₃, a V-VI semiconductor, possess excellent optoelectrical properties like high absorption coefficient (>10⁵ cm⁻¹), tunable bandgap (1.04–1.67 eV), intrinsic p-type conductivity, decent carrier mobility (9–15 cm²/V.s), low toxicity, moisture, and air stability. Hence, these materials are very much suitable for the application in various optoelectronic fields. Apart from the above advantages, these materials have other added advantages like earth abundance, less structural complexity, less appearance of other secondary phases, and low melting point. Recent studies show that Sb₂S₃/Se₃ obtained considerable momentum in its research and achieved record efficiency of 7.5% and 9.2%, respectively. Though the research is concentrating on its promising aspects of improvement in efficiency, in actual it is still behind the Shockley–Queisser (S-Q) limit of efficiency and also far lower than the reported efficiency of CdTe and CIGS solar cells. The main obstacles in obtaining high efficiency for antimony chalcogenide solar cell are non-ideal series and shunt resistance, deep defects, non-radiative and interface recombination, interdiffusion between layers, etc. This chapter is divided into three sections: firstly different deposition techniques (vacuum and non-vacuum) to have good-quality Sb₂S₃/Se₃ thin film; secondly, progress made till now in terms of photo conversion efficiency, and thirdly, analysing the Sb₂S₃/Se₃-based photovoltaic devices, identifying their main limitations, and providing insights for improving current device performance will be discussed.

This chapter discusses the detailed understanding of metal oxide (MO) thin films and their applications in the field of photovoltaic (PV) solar cell devices. The chapter begins with the literature survey of photovoltaics and metal oxides and explains the utilization, properties, and growth mechanism of metal oxide in the area of thin-film solar cells (TFSCs). Two major TFSC PV technologies, viz. CdTe and CIGS, have highlighted insight into the fabrication, application of the metal oxides layers to enhance the various solar cell parameters and hence the output power of devices. Application of metal oxides such as front and back contacts by using transparent conducting properties and passivation layer by utilizing insulating properties is extensively covered in the following subsections. Zinc (Zn)-, molybdenum (Mo)-, indium (In)-, and vanadium (V)-based metal oxides are explained including synthesis, stability, and applications at the interface level.

An important challenge for lowering the cost of “solar energy” is minimising the required usage of the “active solar absorber material.” Development of ultra-thin solar cells is of paramount importance in ultimate cost reduction of solar cells—without compromising if not increasing, the efficiency of solar cells. Research in ultra-thin solar cells is fast-gaining ground. It was pointed out that basing on Thomas–Reiche–Kuhn sum rule, the amount of material required to achieve maximum optical absorption due to incident light (in the spectral region of interest for solar cells) may well be around 10 nm thick. However, it is important to devise appropriate light manipulation mechanism in conjunction with the semiconductor absorber layer of the solar cells. Plasmonics—the science and technology of confining the electric field energy in low-dimensional systems—is an important route to successfully achieve the “ultra-thin solar cells”. Plasmonics offer two routes for light manipulation—near field and far field. These mechanisms in turn enable more secondary mechanisms such as hot-carrier generation, photon up-conversion, nonlinear effects, etc. When employed optimally, these mechanisms will aid one another producing the amplifying effect on the optical absorption in the active semiconductor absorptive layer of solar cell and consequently on the efficiency of the cell. Availability of methodology and techniques for easily and cost-effectively incorporating plasmonic structure in the immediate vicinity of the semiconductor absorber of the solar cell is one of the limiting factors in achieving the ultra-thin solar cells. Plasmonic metasurfaces—2D analog of plasmonic metamaterials—are found to possess broadband optical properties required for solar cells. There is a growing body of research to implement plasmonic light trapping effects on various inorganic and organic ultra-thin solar cells. We will discuss various plasmonic light trapping mechanisms with respect to solar cells and possible directions to successful implementation in various types of industrially important solar cells.

In this chapter, we will describe the following aspects: (1) concept behind plasmonic photon management, innovative plasmonic structures to confine, scatter light and modify electric field; 2D multilayers, core–shell structures, custom-designed textures and topography; (2) characterisation methods to probe the plasmonic effects, diagnosis tools employing plasmonic effects; (3) solar cell structures, adapting fabrication process of the first-generation and second-generation solar cells in the market, to make effective use of plasmonic light trapping through both far field and near effects, absorber material consideration, assessment of optical gain compared to plasmonic loss and evolution of electronic/structural defects and shunt paths; (4) challenge of upscaling and industrial plasmonic PV fabrication tool; (5) other practical/potential applications: LED, water splitting, third- and future generation solar cells, special emphasis on up-conversion; and (6) industrial viability of the plasmonic-based devices, compared to existing scenario.

This chapter will explain how dimension on optoelectronic devices can be substantially thinned down by increasing light trapping efficiency through plasmonic effects.