Colorful Conducting Polymer Nanocomposites Brighten up Silicon Solar Cells

Deying Luo1, Lei-Ming Yu2, Zheng-Hong Lu1,3* 1Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario, M5G 3E4, Canada 2Henan Key Laboratory of Photovoltaic Materials, School of Physics, Henan Normal University, Xinxiang, 453007, China 3Department of Physics, Center for Optoelectronics Engineering Research, Yunnan University, Kunming, 650091, PR China *Correspondence should be addressed to Zheng-Hong Lu; zhenghong.lu@utoronto.ca


Introduction
Solar cells made of crystalline silicon (c-Si) have dominated the world's solar energy market to date [1,2]. Meanwhile, there is growing interest in adding new features or properties to c-Si solar cells such as vibrant colors that make them particularly attractive for decorative applications [3,4]. Nevertheless, c-Si solar cells with regular p-n junctions present a sizable challenge in generating colors across the visible spectrum due to their fixed device structure and limited materials selection. Hence, typical c-Si solar cells are visually dull and are not suitable for integration with buildings [5], passenger vehicles [6], and so on.
To tackle this issue, several methods such as micro-nano manufacturing techniques, photonic crystal patterning, and thin dielectric films, have been used to achieve colored c-Si solar cells [7][8][9]. These methods, however, require complicated fabrication equipment and are expensive to industrialize. Planar heterojunction c-Si solar cells, consisting of c-Si and novel materials (such as graphene [10], carbon nanotubes [11], and conducting organic semiconductors [12]), provide a very simple way to construct colorful solar panels by a thin-film coating that serves as a light reflection layer and a charge collection contact [13,14]. In particular, researchers have recently made significant progress in making conductive polymer nanocomposites on c-Si solar cells to achieve full colors [15]. In this commentary, we discuss this recent development on brightening up silicon cells by polymer nanocomposite coatings. make colorful c-Si solar cells is to add optical reflective layers on the top of an opaque c-Si wafer [20]. Among the reflective coating strategies, previous studies have shown that introducing particular micro-/nano-structured coating layers on c-Si wafers can scatter the incident light and hence generate structural color to c-Si solar cells ( Figure 1A, left) [3,4,20], and even neutral-colored c-Si solar cells have been enabled by introducing micron-holes on an opaque c-Si wafer [7]. Though microfabrication methods and tools to make micro-/nano-structured coating layers are broadly used in the microelectronics industry, complex manufacturing processes make them prohibitively expensive for the potential solar market.
Coating dielectric films directly on an opaque c-Si wafer offer a more attractive alternative to generate colors by varying the film thickness (i.e., optical paths) ( Figure  1A, right). In this case, the dielectric coatings can be manufactured by using simple film deposition methods [14,17], making cost-effective colorful solar cells possible. Meanwhile, the refractive index and conductivity of the dielectric films can be precisely controlled [21,22], and thus concurrent improvements in device performance and color tunability can be obtained ( Figure 1B). What's more, those materials can be selected from a wealth of polymer materials and inorganic materials, making them useful in practical applications of colorful c-Si solar cells. The challenge is to combine these various materials in a composite form to obtain multi-functional films meeting the targeted tunable electrical and optical properties.

Polymer Nanocomposites for Colorful c-Si Solar Cells
Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, also known as PEDOT:PSS, is a transparent conductive polymer consisting of a mixture of the ionomers poly(3,4ethylenedioxythiophene) and polystyrene sulfonate (PSS). Owing to its unique combination of ductility, transparency, conductivity, and facile fabrication, PEDOT:PSS has become a popular material serving as charge-selective contacts at the front electrode of c-Si solar cells to form a planar heterojunction [14,21]. Most recently, Yu et al. reported that the thickness and the refractive index can be tailored to produce colors on c-Si solar cells [15]. Typically, changes to the thickness of PEDOT:PSS nanocomposites modulate optical phase variations of light propagating in films, resulting in either destructive or constructive interference. The optical destructiveness or constructiveness leads to various color appearances on c-Si wafers ( Figure 1C). In addition, changes in refractive index of PEDOT:PSS nanocomposites can vary the light

High-performance Colorful c-Si Solar Cells
The high device efficiency and high color saturation are the prerequisites to practical applications of colorful c-Si solar cells. There is, however, a trade-off between the optical and electrical properties of the PEDOT:PSS polymer nanocomposites. As for the PEDOT:PSS polymers, previous reports have shown that their conductivities are determined by the PSS-to-PEDOT ratio and the PEDOT chains structure within the films [23,24]. Consequently, varying the PSS-to-PEDOT ratios within the thin films is the most common approach to maximize the conductivity of PEDOT:PSS dielectric films. There is a wealth of literature showing that the PEDOT:PSS polymer is sensitive to polar solvents [21,23], such as Dimethyl sulfoxide (DMSO), methanol, N,N-Dimethylformamide (DMF), etc. This indicates that the conductivity of the PEDOT:PSS layer can be improved by rewetting the surface of as-prepared films using solvent engineering. Regarding vivid c-Si solar To this end, they systematically optimized processes to maximize device performance. And they found that there are negligible changes in both open-circuit voltages and fill factor for a variety of colored c-Si solar cells, whereas a short-circuit current density exhibits a noticeable increase once the color varies from pink to purple. The optimized PEDOT:PSS nanocomposites are found to maintain high device efficiencies ranged from 10.6% to 13.2%, which is among the top planar heterojunction c-Si solar cells [25][26][27].

Conducting Polymer Nanocomposites for Other Applications
As PEDOT:PSS nanocomposites can be made by various solution-phase techniques such as spin coating, spray coating, ink-inject printing, doctor blade coating, slot-die coating, etc., it may find other applications in a variety of devices. For example, PEDOT:PSS with matching energy levels and excellent transparency, has been used to enhance hole-carrier injection or extraction in emerging optoelectronic devices [28], and even in these cases, highly conducive PEDOT:PSS materials are regarded as a potential replacement of other commercial semitransparent electrodes [29]. PEDOT:PSS nanocomposites are expected to receive growing interest in wearable and smart selfpowered flexible solar cells. The operation of flexible solar cells is especially dependent on the mechanical ductility of the functional layers. The PEDOT:PSS nanocomposite also provides robust interfaces with other functional layers. In the future, the polymer nanocomposites could potentially be used to brighten up other solar cells such as perovskite solar cells, organic solar cells, and other thin-film solar cells.
In addition to solar cell applications, it has been demonstrated that a few to tens of nanometers thick organic films on metallic substrates can achieve functionalities such as anti-reflection, high-reflection and dichroism [30]. Various structural color images on metallic substrates have been demonstrated by modifying monochromic conducting polymer nanofilms via vapor phase polymerization and ultraviolet light patterning [31]. Thus, polymer nanocomposites are expected to be useful as optical coatings on metallic substrates.

Conclusion
In summary, polymer nanocomposite films provide a flexible materials platform to tune optical constants and optical paths. Through the selection of a suitable nanocomposite and a film thickness, vivid c-Si solar cells with various color shades have been achieved while maintaining a peak power conversion efficiency of 13.2%, which is comparable to regular planar heterojunction c-Si solar cells.

Conflicts of Interests
The authors declare no conflict of interest.