The Scalability of Third Generation Photovoltaics: Deposition Techniques and Modularity

The development of third-generation photovoltaic (TGPV) technologies promises to address some of the limitations of conventional silicon-based solar cells. However, the scalability of these technologies is a critical factor in their practical application. TGPV include Perovskite Solar Cells (PSC), Organic Solar Cells (OSC), and Dye-Sensitized Solar Cell (DSSC). This paper reviews and summarizes the recent trends and research on the deposition techniques and modularity of TGPV. Various deposition techniques such as slot-die coating, thermal evaporation, and spray pyrolysis are discussed, along with their advantages and limitations. Modularity, which allows for the integration of TGPV cells into different structures, is also examined as a critical factor in scalability. The paper concludes that the scalability of TGPV technologies depends on the development of efficient and cost-effective deposition techniques and modularity, which will facilitate the integration of the TGPV cells into various structures and enable the widespread use of these promising technologies.


Introduction
Third generation photovoltaics (TGPV) encompasses PV technologies that are manufactured through solution processing methods [1,2].The fabrication of first (crystalline silicon) [3] and second (thin film) generation solar cells [4] is considered to energy intensive and bears adverse effects on the environment [5].On the contrary, TGPV offer the advantages of the potential of very low capital and operation cost, ease of processing, the ability to scale up to large area fabrication and the use of flexible substrates.Moreover, TGPV are electrochemical devices that operate and depend on the interactions between stacked layers, and are divided into three subcategories, which are perovskite solar cells (PSC) [6], organic solar cells (OSC) [7], and dye-sensitized solar cells (DSSC) [8].DSSC (Grätzel cell) which was invented by O'Regan et al., [9] mainly consists of a bottom electrode, most commonly fluorine doped tin oxide (FTO), with a titanium dioxide (TiO 2 ) electron transport layer (ETL), a dye-sensitizer, a liquid or a solid electrolyte (as a hole transport layer HTL), and a counter electrode (CE), deposited in that order.OSC follow a similar pattern in terms of structure [10], however the ETL and the HTL are combined into a single blend to form a bulk-heterojunction (BHJ) and sandwiched between two electrodes.All while still having the option of utilizing passivating layers preceding or following the BHJ in order to further enhance performance metrics.Lastly, PSC, which were invented in 2009 by Kojima el al., [11], utilize a bottom electrode/ETL/Passivators/perovskite absorber/HTL/CE [12], with highly achieved efficiencies within the research and development scene and a very high potential for large scale applications.
There are plenty of techniques that are used for the fabrication of TGPV, some of which are spin coating (SC), slot-die coating (SDC), doctor blade coating (DBC), thermal evaporation (TE), electron beam evaporation (EBE), spray pyrolysis (SP), chemical bath deposition (CBD), etc.These techniques vary in defining limitations that affect the overall process of fabrication of TGPV such as cost, complexity, material wastage and scalability [13].In this work, the different techniques that are used to fabricate TGPV, as well as their versatility in terms of scalability are comparatively presented.

Methodology
The data for this research was obtained from literature gathered from Scopus and Google Scholar databases, based on the information presented regarding the type of TGPV, the scalability of the modules/cells that are fabricated, and the performance parameters that were achieved.The search criteria used were based on the keywords and phrases that are commonly associated with third generation solar cells, including but not limited to "perovskite solar cells", "organic solar cells", "Dye-sensitized solar cells", "spin coating", "slot-die coating", "doctor blade coating".The search was mostly constrained to journal articles published between (2019-2023) due to the fact that the TGPV scene is rapidly developing, and recent literature provides a relevant insight on the current performance progress correlated to the utilized deposition technologies.

Perovskite Solar Cells (PSC)
PSC have been set on the track of rapid development since their induction into the TGPV scene in 2009, recently reaching a champion efficiency of 26.1% [14].The scalability of TGPV is an aspect that should be tackled before the commercialization of this technology and its ascension to modularity (perovskite modules PM).Various deposition techniques have been utilized in that regard such as SDC, CBD, TE, SC (associated with moderate PSC areas), etc.Each fabrication technique defines the process of synthesizing the precursors of the different layers involved, as well as an optimization process for the fabrication parameters used.Throughout recent literature, scalability and fabrication of PSC has been put under the spotlight.Kaleli et al. [15], fabricated a PM with an area of 36 cm 2  , and an individual active cell area of 0.30 cm 2 .The PM structure consisted of FTO/TiO 2 /MASnBr 3 /P3HT/Ag, electrode, ETL, perovskite absorber, HTL and CE layers, respectively.All layers were deposited via ultrasonic-spray pyrolysis, achieving a single cell power conversion efficiency (PCE) of 0.10%, an open circuit voltage (V OC ) of 0.37 V, a short circuit current density (J SC ) of 0.460 mA/cm 2 and a fill factor (FF) of 57.20%.Moreover, Fievez et al. [16] fabricated a 100 cm 2 PM with an active single cell area of 0.09 cm 2 , which consisted of FTO/SnO 2 /TiO 2 /CsFAPb(I 0.88 Br 0.12 ) 3 /Spiro OMeTAD/PTAA/Au, electrode, ETL, perovskite absorber, HTL and CE, respectively.The ETL and HTL were both deposited via SP/CBD and SP, respectively, whereas the perovskite absorber layer was deposited via slot-die/air knife coating and the gold CE via TE.A PCE of 18% along with a 1.024 V V OC , a 22.4 mA/cm 2 J SC , and a 78% FF were achieved.Huang et al. [17] also fabricated a high performing PM via slot-die coating with an area of 16 cm 2 at a 10.34% PCE, a 5.73 V V OC , a 2.85 mA/cm 2 J SC and a 63.34% FF.The fabricated PM consisted of FTO/NiO/MAPbI 3 /PCBM/Ag electrode, HTL, perovskite absorber, ETL and CE, respectively.Both the HTL and the perovskite absorber were deposited via slot-die coating, the ETL via SC and the CE via TE.Bi et al. [18] achieved a 20.7% and a 16.5% PCE for PM with areas 0.2 cm 2 and 1 cm 2 , respectively.The fabricated PM had a structure of FTO/SnO 2 PbO/SnO 2 /MAPbI 3 /Spiro OMeTAD/Au, with the ETL and HTL being deposited via SP, the perovskite layer via DBC and the gold CE via thermal evaporation.EBE has also been utilized in upscaling PM, as Liu et al. [19] fabricated a 5 cm 2 PM that achieved 11.20% PCE along with a 1.14 V V OC , a 13.54 mA/cm 2 J SC , and a 72.52% FF, with a structure of ITO/NiO/CsPbI 2 Br/Nb 2 O 5 /Ag, electrode, HTL, perovskite absorber, ETL and CE, respectively.All the layers were deposited via EBE, except for the perovskite absorber which was deposited via SC.Adding to that SC is a prominent fabrication technique in the PSC and PM scene.For example, Qiu et al. [20] fabricated a 22.8 cm 2 PM with the structure of FTO/SnO 2 /CsMAFAPb(I 0.85 Br 0.15 )/Spiro OMeTAD/Au, which achieved a 12.03% PCE, a 5.8 V V OC , a 3.38 mA/cm -2 and a 61.31%FF.Both the HTL and the perovskite absorber were deposited via SC, whereas the ETL was deposited via sputtering and the CE via thermal evaporation.Moreover, Liu et al. [21], was able to successfully fabricate a 10 cm 2 PM using spin coating, except for the CE which was deposited via TE.The fabricated structure consisted of FTO/TiO 2 /MAPbI 3 /Spiro OMeTAD/Ag, electrode, ETL, perovskite absorber/HTL and CE, and achieved a PCE of 15.14%, a 5.11 V V OC , a 4.75 mA/cm 2 J SC and a 63% FF. Figure 1 shows a comparison between the PCE and module area for PSC based on the fabrication technique.Where slotdie coated modules maintain a relatively high efficiency with respect to the module area.Spin coated and slot-die coated modules show comparable performance with similar module areas, however scaling up limitations of spin coating quickly arise.

SDC Retains
High PCE With High "Module" Area 3.2.Organic Solar Cells (OSC) OSC is a well-established TGPV technology that has been around since the 1980s [22].Similar to succeeding technologies, such as PSC, OSC utilizes a structure consisting of a bottom electrode for charge collection, an electron and a hole transporting layer, and an active layer blend.An OSC blend (BHJ) has a unique structure of intermingling electron donor and acceptor materials, that create a network for efficient electron and hole transportation to adjacent ETL and/or HTL.Moreover, scalability has also been thoroughly studied for OSC by implementing flexible substrates, metal grid electrodes, as well as experimenting with various deposition techniques such as SDC, DBC, SC, transfer printing (TP), etc.The current champion OSC efficiency stands at 18.2% [23], and with large area OSC catching up, it has the potential to break through its commercialization stage if stability and degradation issues are overcome [24].Although SC is limited to smaller areas, treatments that are facilitated by SC such as anti-solvent treatment compensate the devices with a high efficiency result due to a well-controlled crystallization of the layer that is being deposited.For instance, Jia et al., [25] fabricated an OSC with the structure ITO/ZnO(ETL)/PM6:Y6:ITIC:PC 71 BM (Active)/MoO(HTL)/Ag, where the active and ETL layers were deposited via SC, while the HTL and the silver CE were deposited via TE.The device achieved an efficiency of 13.25% with an area of 19.34 cm -2 , with a V OC of 6.06 V, a J SC of 3.07 mA/cm -2 and a FF of 66.45%.Additionally, fabricating larger area modules is also viable using SC with the aid of TP.For example, Jahandar et al., [26] fabricated a 50 cm 2 OSC module using spin coating, with the structure ITO/CPEIE (ETL)/PBDB-T-2F:Y6:PC 70 BM (1:1:0.2)/MoO(HTL)/Ag, where the ETL (vacuum dried) and active layer were deposited via spin coating, and the HTL along with the silver CE via TE.They achieved an efficiency of 13.12%, a V OC of 8.74 V, a current density of 2.6 mA.cm -2 , and a FF of 58%.Additionally, and using the SC/TP method, Qin et al. [27] fabricated a 54 cm 2 OSC module with the structure of PET(Ag-grid)/AgNWs:PEI-Zn (ETL)/PBDB-T-2F:Y6:PC 71 BM (Active)/MoO 3 (HTL)/Ag, where the PET(Ag-grid) was deposited via DBC, the ETL and active layer via SC and both the HTL and Ag via TE.The fabricated module had a relatively good performance as the previous example reaching a PCE of 13.20%, a V OC of 7.34V, a J SC 2.57 mA.cm -2 , and a FF 70%.However, upscaling conventional active layer blends solely using SC resulted in relatively poor performances.As reported by Kang et al. [28], a 1.6 cm 2 -OSC of the structure ITO/ZnO (ETL)/P3HT:PCBM (conventional OSC active blend)/MoO 3 (HTL)/Al, where the ETL and the active layer were solely deposited via SC and the HTL along with the Al CE were deposited via TE, had a PCE of 1.40%, a V OC of 0.597 V, a J SC of 7.6 mA.cm 2 and a FF of 30.3%. Figure 2 shows a comparison between the PCE and module area for OSC based on the fabrication technique.It can be seen that utilizing spin coating solely along with conventional OSC materials shows rather poor results on both performance and area spectrums.However, coupling spin coating with techniques such as transfer printing can elevate the modularity as well as performance drastically.Roll-to-roll technologies have also been investigated for the deposition of OSC.Han et al. [29] reported a device of the structure ITO/ZnO (ETL)/AMD2:ITI-Th (Active)/HTL Solar/Ag, where all the layers were deposited via SDC, except for the Ag CE, which was deposited via BC.The 30 cm 2 module had a PCE of 8%, a V OC of 0.789 V, a J SC of 14.8 mA.cm -2 and a FF of 66.4%.Moreover, DBC was also investigated by Fo et al. [30], through its utilization in a 1.2 cm 2 module implementing a conventional active layer blend with the structure of ITO/A-ZO (ETL)/PM6:Y6 (Active)/MoO 3 (HTL)/Al.The ETL was deposited via SC, the active layer via DBC and both the HTL and the CE via TE.The device achieved a relatively good efficiency at 11.69%, a V OC of 0.82 V, a high J SC of 21.68 mA.cm -2 and a FF of 65.78%.Figure 2. Organic Solar Cells -PCE vs Module Area for Different Fabrication Techniques 3.3.Dye-Sensitized Solar Cells (DSSC) DSSC have been extensively studied as their performances depend greatly on each component of the cell.Hence, work has been carried to optimize each element of DSSC from the photoanode to the counter electrode including the electrolyte and dye [31].The electrolyte utilized in the DSSC plays a vital role in its performance as it is responsible for the charge carrier transport between the photoanode and the counter electrode, in addition to the regeneration of the dye.Generally, three categories of electrolytes are present: liquid electrolytes, quasi-solid-state electrolytes, and solid-state electrolytes.In terms of performance, DSSC based on liquid electrolytes have attained the highest efficiencies, with a record efficiency of 14.3%.[32].Hence, large modules of DSSC mainly consist of cells with liquid electrolytes [33].A summary of performance vs module area which accounts for various deposition techniques and electrolyte injection considerations can be seen in Figure 3. Novel natural dyes such as Calotropis and electrolyte composition are the center of recent research, especially electrolytes consisting of copper or cobalt redox shuttles and iodine-free electrolytes [34,35].For instance, a DSSC with a conversion efficiency of 13% was achieved by using Co+2/+3 redox shuttle and porphyrin dye [36].With the aid of co-sensitization, a combination of alkoxysilyl-anchor dye and carboxy-anchor based organic dye integrated with [Co(phen)2]2+/3+ redox couple resulted in a DSSC with a record efficiency of 14.3% [37].Similarly, Cu-based electrolytes have shown great characteristics as potential electrolytes for DSSC especially in the terms of open voltage circuit, where voltages around 1.0 V are obtained [33].A DSSC with an efficiency of 11.3% was achieved by employing a Cu-based redox mediator and a co-sensitized dye [38].The main advantage of Co-based and Cu-based electrolytes is potential for scalability, as these electrolytes do not undergo severe leakage as compared to organic and iodine-based electrolytes.Electrolytes in lab scale DSSC are traditionally introduced through the vacuum back filling method or the electrolyte injection filling method [39,40].In both methods, a drop of electrolyte is dropped through a hole drilled on the counter electrode and photoanode.use of thermoplastic, and a cell channel is created.The advantages of both methods are the potential for scalability, where they have been utilized for the production of large modulus.For instance, DSSC modules with an area of 5x5 cm 2 was assembled using the vacuum back filling method [41].The modules with liquid electrolyte showed a PCE of 2.08%, while modules with polymer electrolytes recorded efficiencies around 1.6%.Through the use of electrolyte injection filling method, a DSSC module of size 30x30 cm 2 achieved an efficiency of 3.5% [42].Another module of the same size also recorded an efficiency of 3.1%.Moreover, a smaller DSSC module of 10x10 cm 2 attained a PCE of 4.3%.Although extremely popular, traditional electrolyte filling methods suffer from certain disadvantages such as leaking, glass breaking during the drilling process, and uneven distribution of the electrolyte.In order to mitigate these issues, methods such as screen printing or inkjet printing are utilized as they contribute to the homogenous electrolyte composition in the device.Printing methods aid to eliminate the leakage of electrolyte, as the viscosity is optimized to successfully apply the methods.For example, a DSSC module achieved a PCE of 4.39% by employing screen printing [43].
Printing methods also show a great advantage in terms of the cell resistance, as the elimination of the drilled holes reduced the overall cell resistances.In addition, these methods allow for the homogenous distribution of the electrolyte and motivate the investigation various methods such as slot die and blade coating.Flexible DSSC can also be produced by the utilization of roll-to-roll methods.

Conclusion
In summary, the scalability of third generation solar cells (TGPV) has been the subject of extensive research and optimization efforts.Various techniques, including slot-die coating (SDC), spin-coating (SC), and doctor blade coating (DBC), have been employed to achieve a balance between large module area and acceptable efficiency levels.Efficiencies reaching 18% for 100 cm 2 cells, 13.2% for 50 cm 2 cells, and 3.1% for 900 cm 2 cells, were achieved for perovskite (PSC), organic (OSC) and dye-sensitized (DSSC) solar cells, respectively.However, scaling up different TGPV technologies presents numerous challenges, including cost and toxicity issues that hinder the commercialization of PSC, degradation problems affecting OSC, and the incompatibility of high-performance liquid electrolyte DSSC with roll-

Figure 1 .
Figure 1.Perovskite Solar Cells -PCE vs Module Area for Different Fabrication Techniques

Figure 3 .
Figure 3. Dye-Sensitized Solar Cells -PCE vs Module Area for Different Fabrication Techniques techniques.As a result, further research and development are required to address these challenges and ensure a seamless transition towards commercialization.Only then can the full potential of third generation solar cells be realized, bringing us one step closer to a more sustainable future.
The electrodes are attached together through the