Review of materials and manufacturing options for large area flexible dye solar cells
Introduction
Photovoltaic (PV) is a promising future source of sustainable electricity production. By 2009, close to 30 GW of PV had been installed producing nearly 25 TWh of electricity [1]. By 2050, 15–30% of all electricity could originate from solar energy according to recent scenarios [2]. At present, most of the PV systems are fabricated with mono or multi crystalline silicon (c-Si) having a market share of 90% [3]. One of the main drawback associated with Si based PV systems is their high cost which limits implementation at large scale. Cutback in production cost by developing more cost effective materials and easy manufacturing of solar cells has become an immediate need in order to make them an affordable way of generating renewable energy.
Dye-sensitized solar cells, also called simply dye solar cell (DSC), are among emerging solar technologies that have potential for low cost along with relatively high efficiency [4]. Moreover, the in practice roll-to-roll mass production concepts from existing industries could be employed for DSC due to its compatibility with preparation on flexible substrates as well as low temperature and atmospheric pressure based manufacturing processes. This is an advantage against the thin film solar cells such as CIGS or a-Si requiring very high investment costs for vacuum equipments [5]. Currently, as much as 40% lower manufacturing line cost per MW for DSC compared with Si solar cells has been claimed [6].
For industrial mainstream manufacturing, DSC offers cheap (Table 1) and endlessly abundant materials such as TiO2 particles in comparison with solar grade Si obtained from silicon dioxide (SiO2) by an expensive process [7]. Moreover, unlike for conventional PV technologies, there are basically several different materials to choose from for every DSC component. In addition, several of the materials that can be used for DSCs such as carbon nanotubes [8] have other applications as well which is a clear benefit since there is larger field of scientist working on improving the materials and their preparation methods.
Companies such as Dyesol, Sony, and G24i have established pilot production lines testing different configurations, prototypes and modules. The flexible camouflaged solar panel for military [9], solar lantern [10] and backpacks loaded with flexible dye solar modules [11] have been demonstrated as initial consumer applications. In the future, the applications of DSCs could include for instance their integration to roofing and other building materials.
The main objective of this review is to investigate about the appropriate materials and different design approaches which could be used to scale-up the laboratory sized test cells to solar modules. The review begins with the analysis of different costs related to materials and preparation methods which gives the motivation to focus this work on enabling roll-to-roll mass production. Hence, significant emphasis is given to the preparation of flexible electrodes. All the other cell components such as electrolytes are discussed from the viewpoint of manufacturing and hence having emphasis on the processing of the materials. The different DSC module types are also presented and finally the knowledge is gathered as a vision for the production of DSC modules.
Section snippets
Structure of DSC and working principle
The basic components of a DSC are the photoelectrode (PE), the counter electrode (CE) and electrolyte as illustrated in Fig. 1. The PE consists of a nanocrystalline TiO2 layer deposited on substrate with a conducting layer, conventionally a transparent conducting oxide (TCO) coated glass or alternatively flexible substrate such as TCO plastic or metal foil. The TiO2 layer is sensitized with dye forming a monolayer that absorbs the light. Upon the excitation of a dye molecule by absorbing a
Cost analysis of DSC
In the cost analysis of an early stage technology such as DSC, it is critical to keep in mind learning curve [12] i.e. with higher production the cost will go down. The main issue is not how high the current costs are but rather how quickly they come down. Recently, Nobuo Tanabe from Fujikura Ltd Japan presented that the current material cost is as high as $93/Wp but with 100 MW annual production, it could be lowered down to $0.4/Wp [13]. The current high material costs for DSC are understood by
Substrates
As mentioned in the previous section, the selection of suitable substrates is the key factor affecting the cost but in addition it determines appropriate preparation methods of a DSC and hence also affects the performance and stability. The essential requirements for an ideal substrate are high conductivity, transparency in visible spectral region, non-permeability combined with high stability and low cost [21], [22], [23].
Traditional engineering of DSC utilizes glass sheets coated with
Flexible photoelectrodes
It is possible to adapt modern printed electronics technology for fabrication of DSC by using light weight and flexible sheets. For the production of flexible photoelectrodes (PE), a continuous coating process for deposition of TiO2 layer is required. Several different methods have been presented in particularly for the low temperature preparation of PEs while trying to reach as high efficiency as with high temperature sintering used in the case of TCO-glass substrates. High efficiency (>7%)
Flexible counter electrodes (CE)
The key parameters for an efficient counter electrode (CE) are low charge transfer resistance (RCT) for the tri-iodide ion reduction, chemical stability in the electrolyte solution and mechanical stability [79], [80]. The optical properties also have some effect such as reflectance from the CE. If the cell is reversely illuminated such as when using metal as photoelectrode, the optics plays a critical role as the CE needs to be semitransparent [54]. The CE can be fabricated with different
Dye sensitization methods
The sensitization or staining of the dye can be performed by different methods [27], [120], [122]. Traditional method involves direct immersion of TiO2 coated substrates into dye solution from several hours to overnight in order to obtain good and uniform distribution between dye molecules and TiO2 layer [27]. This technique has been adopted for large area DSC production by using a sealed staining chamber [122], [123]. The concentrated dye solution can be transported to the staining chamber by
Electrolytes
The electrolytes employed in DSC can be categorized into organic liquid, ionic liquid, quasi solid state and solid state electrolytes. The motivation to study other electrolyte compositions besides traditional organic liquids has arisen from the chemical and mechanical stability issues of organic liquid electrolytes which are critical in particular when using alternative flexible substrates. The main advantage of ionic liquid electrolytes over the organic liquids is that ionic liquids have zero
Sealants and encapsulation
The performance challenges to get DSCs to practical applications are mainly associated with their instabilities during long term operation [33], [124]. These instabilities have been attributed, e.g., due to volatility of the solvent present in the electrolyte solution, its thermal stress and vapor pressure [33], [175]. In addition to that the intrusion of water and oxygen, temperature changes and effects caused by UV light harm its basic operation [22], [33]. Therefore there is a vital
Module configurations
Up-scaling small laboratory test cells into large prototypes for industrial manufacturing tackles several issues for instance current collection grid and its protection, sheet resistance of the substrates, electrolyte filling and cell sealing. Numerous cell structure concepts have been developed and illustrated [79], [122], [124], [138]. Mostly the large area DSCs have been fabricated on glass substrates [122], [124], but also DSCs based on ITO-PEN sheets have also been demonstrated [22].
Module production
The fabrication of DSC modules does not essentially require a clean room environment or vacuum conditions [5], [197]. As discussed in the earlier section there are R2R compatible printing methods for the deposition TiO2, catalyst layers and pastes for current collecting grids. Careful optimization is still required. At present, the most popular technique to coat aforementioned materials is screen printing [59], [76], [104] which can be induced in a R2R manufacturing process [198]. In fast R2R
Concluding remarks
In the cost analysis section, it was suggested that via roll to roll production even lower than $1/Wp price may be achieved. This technique can only be adapted with flexible substrates such as metal and plastic sheets. When pushing the cost as low as possible, very cheap metal substrates (e.g., Al) and alternative conducting layers for plastic substrates (e.g., printed extra fine Ag grid) are needed. Moreover, the manufacturing methods should be cheap, easy and fast to reach high volume
Acknowledgements
G.H. is thankful to T. Miyasaka, F.C. Krebs, A.R. Andersen, M. Ikegami and S. Ahmad for the information they have provided. This work was funded by the Academy of Finland.
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