**1.3. Solar-powered photovoltaic paper**

The present development of photovoltaic (PV) devices allows the fabrication of solar cells on a wide variety of low-cost recyclable and disposable substrates such as paper, thereby extending PV, coupled with batteries, to a broad range of consumer-oriented portable applications where autonomous energy harvesting is needed (e.g., wearable PV, solar-powered intelligent packaging, internet-of-things systems).

Two main technological challenges can be identified in the field of PV on paper: (1) the requirement of solar cell (SC) fabrication processes employing low temperatures (below 150°C) and (2) the pronounced surface roughness of the fibrous paper structure, which can create defects in the cell layers and consequently deteriorate their electrical conduction. Such challenges are more relevant for inorganic (e.g., silicon, chalcopyrites, kesterites) based solar cells (given the high temperature, between 200°C and 300°C, typically used in the PECVD deposition of thin film silicon devices) and less crucial for organic or dye-sensitized–based cells as these devices can be processed without high temperature and by nonvacuum methods (e.g., screen printing, doctor blading, spin coating, spray deposition, electrochemical deposition) and their performance is more tolerant to the defect density [45], but at the same time more sensible to the substrate chemistry. Regardless of their lower environmental stability and power conversion efficiency, these PV materials are still considered the most suitable option for the development of printable solar cells [46] given their minimum detrimental impact on the cellulose substrate. Barr et al. successfully integrated solar cells directly onto as-purchased papers without pretreatment to fill interfiber spaces. The paper PV arrays produced more than 50 V, enough to power common electronic displays under ambient indoor lighting, and can be flexed and folded without loss of function [47].

Thin film hydrogenated amorphous silicon solar cells have been already successfully implemented on distinct types of cellulose-based substrates, such as liquid-packaging cardboard [48] and printing paper [49] with sunlight-to-electricity conversion efficiencies of 3–4%. This was attained by developing appropriate low-temperature PECVD processes and by coating the paper surface with a hydrophilic mesoporous layer. Such layer can, not only withstand the cells production temperature, but also provide adequate paper sealing and surface finishing for the cell's layers deposition. A key procedure performed in such works is the continuous monitoring of the substances released from the paper substrates during the cell deposition by mass spectrometry, which allows adapting the fabrication processes to mitigate any contamination from the substrate. Transfer printing or lamination is another class of methods that can also be adapted to inorganic and thin film solar cells. With these methods the cell layers are first deposited on a donor substrate, which is compatible with the optimal fabrication conditions (usually high temperature and under vacuum), then they are detached from the original substrate and transferred to another receiver substrate (e.g., a flexible low-temperature tolerant platform such as paper) [50, 51]. For instance, a transfer printing method is applied in the work of Lee et al. where GaAs photovoltaic modules are transferred to a prestrained, structured substrate of PDMS with a power conversion efficiency (PCE) ≥20% [52]. A similar method was employed on cellulose nanocrystal (CNC) substrates to fabricate SCs with a PCE of 4.0% [53], and 5.9% on optimized transparent paper [54].

Finally, contrary to plastic substrates, paper exhibits high dimensional stability over temperature, which is advantageous for electronic components by not introducing complex thermal parasitic effects in the electronic devices' behavior. For example, an optically transparent paper made of cellulose nanofibers has a coefficient of thermal expansion (CTE) of <8.5 ppm [44], which is much lower than plastic (PEN: 18–20 ppm; PET: 22–25 ppm). Besides, paper can also endure annealing cycles at 200°C for short time periods without any noticeable change in physical characteristics.

The present development of photovoltaic (PV) devices allows the fabrication of solar cells on a wide variety of low-cost recyclable and disposable substrates such as paper, thereby extending PV, coupled with batteries, to a broad range of consumer-oriented portable applications where autonomous energy harvesting is needed (e.g., wearable PV, solar-powered

Two main technological challenges can be identified in the field of PV on paper: (1) the requirement of solar cell (SC) fabrication processes employing low temperatures (below 150°C) and (2) the pronounced surface roughness of the fibrous paper structure, which can create defects in the cell layers and consequently deteriorate their electrical conduction. Such challenges are more relevant for inorganic (e.g., silicon, chalcopyrites, kesterites) based solar cells (given the high temperature, between 200°C and 300°C, typically used in the PECVD deposition of thin film silicon devices) and less crucial for organic or dye-sensitized–based cells as these devices can be processed without high temperature and by nonvacuum methods (e.g., screen printing, doctor blading, spin coating, spray deposition, electrochemical deposition) and their performance is more tolerant to the defect density [45], but at the same time more sensible to the substrate chemistry. Regardless of their lower environmental stability and power conversion efficiency, these PV materials are still considered the most suitable option for the development of printable solar cells [46] given their minimum detrimental impact on the cellulose substrate. Barr et al. successfully integrated solar cells directly onto as-purchased papers without pretreatment to fill interfiber spaces. The paper PV arrays produced more than 50 V, enough to power common electronic displays under ambient indoor lighting, and can be flexed and

Thin film hydrogenated amorphous silicon solar cells have been already successfully implemented on distinct types of cellulose-based substrates, such as liquid-packaging cardboard [48] and printing paper [49] with sunlight-to-electricity conversion efficiencies of 3–4%. This was attained by developing appropriate low-temperature PECVD processes and by coating the paper surface with a hydrophilic mesoporous layer. Such layer can, not only withstand the cells production temperature, but also provide adequate paper sealing and surface finishing for the cell's layers deposition. A key procedure performed in such works is the continuous monitoring of the substances released from the paper substrates during the cell deposition by mass spectrometry, which allows adapting the fabrication processes to mitigate any contamination from the substrate. Transfer printing or lamination is another class of methods that can also be adapted to inorganic and thin film solar cells. With these methods the cell layers are first deposited on a donor substrate, which is compatible with the optimal fabrication condi-

**1.3. Solar-powered photovoltaic paper**

38 Nanostructured Solar Cells

folded without loss of function [47].

intelligent packaging, internet-of-things systems).

Another key point to take into account in the fabrication of solar cells on flexible substrates, such as paper, is the front illuminated contact. This transparent contact must be of particular high performance to realize efficient large-area (centimeters scale) devices. Such contact is conventionally composed of a transparent conductive metal oxide (TCO), but research on alternative and inexpensive methods is essential. Examples of such alternatives are metallic networks engineered by soft lithography, such as colloidal lithography or nano-imprint; and the production of conductive transparent paper, for instance by using cellulose nanofibers and printing silver nanowires[55]. The colloidal lithography approach has allowed engineering innovative ultrathin electrodes with a honeycomb lattice structure [56], known as micromesh electrodes (MMEs) [57, 58], which already shows transmittance (*T* ~91% in visible range) and sheet resistance (*Rs* ~6.2 Ohm/square) better than the state-of-the-art ITO (*Rs* ~10 Ohm/square and *T* ~90%) [59]. Besides, MMEs on flexible polyimide substrates were shown to remain invariant after 1000 cycles of repeat bending to a 2 mm bending radius [64]. As for the conductive paper using cellulose nanofibers and silver nanowires, it presents an optical transparency and electrical conductivity as high as those of ITO glass. The SCs fabricated on such conductive paper exhibited a PCE of 3.2% and generate electrical power even under and after folding.

Lastly, despite all the efforts on solar cell material optimization, efficiencies achieved so far are still considerably below the state-of-the-art (11–13%) on rigid (usually glass) substrates. Advanced light trapping methods employing wavelength-sized photonic structures are regarded as one of the most promising strategy to boost light absorption, allowing thickness reduction while keeping high efficiency [14, 45, 60, 61]. Optically thicker but physically thinner devices imply cheaper and faster fabrication, light-weight and improved flexibility which enables roll-to-roll fabrication on the flexible paper substrates. Besides, thickness reduction can lead to higher open-circuit voltages (and consequently efficiencies) due to lower bulk charge-carrier recombination [62, 63].
