Scalable, highly transparent paper with microsized fiber

Abstract

Solar cell substrates require high optical transparency, but also prefer high optical haze to increase the light scattering and consequently the absorption in the active materials. Unfortunately, there is a tradeoff between these optical properties, which is exemplified by common transparent paper substrates exhibiting a transparency of about 90% yet a low optical haze (<20%). In this work we introduce a novel transparent paper made of wood fibers that display both ultra-high optical transparency (˜96%) and ultra-high haze (˜60%), thus delivering an optimal substrate design for solar cell devices. Compared to previously demonstrated nanopaper composed of wood-based cellulose nanofibers, our novel transparent paper has better dual performance in transmittance and haze, but also is fabricated at a much lower cost. This high-performance, low-cost transparent paper is a potentially revolutionary material that may influence a new generation of environmentally friendly printed electronics.

1 . (canceled) 
     
     
         2 . A product comprising:
 a paper comprising a plurality of wood fibers,   wherein the paper has an optical transparency of at least 90% and an optical haze of at least 50%,   wherein each wood fiber comprises cellulose with an introduced carboxyl group, and   each wood fiber has a width of 8-100 μm and a length of 100-1000 μm.   
     
     
         3 . The product of  claim 2 , wherein at least a portion of each wood fiber is cleaved in a direction parallel to an axis of said wood fiber. 
     
     
         4 . The product of  claim 2 , wherein the paper comprises fines filling spaces between adjacent wood fibers. 
     
     
         5 . The product of  claim 4 , wherein the fines comprise cellulose flakes or fragments. 
     
     
         6 . The product of  claim 2 , wherein the paper has a tensile strength greater than 8 MPa, a toughness greater than 0.15 J/m 3 , or both. 
     
     
         7 . An optical system comprising:
 an optical device comprising an active surface; and   a paper covering the active surface of the optical device such that optical radiation received by the active surface passes through the paper, the paper comprising a plurality of wood fibers,   wherein the paper has an optical transparency of at least 90% and an optical haze of at least 50%.   
     
     
         8 . The optical system of  claim 7 , wherein the optical device comprises a photovoltaic device. 
     
     
         9 . A method comprising:
 (a) introducing carboxyl groups into cellulose of a plurality of wood fibers; and   (b) after (a), forming the wood fibers into a paper having an optical transparency of at least 90% and an optical haze of at least 50%,   wherein, after (a), each of the wood fibers has width of 8-100 μm and a length of 100-1000 μm.   
     
     
         10 . The method of  claim 9 , wherein, in (a), the wood fibers are provided in pulp without any beating, grinding, or mechanical refining. 
     
     
         11 . The method of  claim 9 , wherein the introducing of (a) comprises TEMPO oxidation. 
     
     
         12 . The method of  claim 11 , wherein the TEMPO oxidation includes exposing the plurality of wood fibers to NaBr and NaClO. 
     
     
         13 . The method of  claim 9 , wherein (b) comprises: (b1) separating the wood fibers from a solution; and (b2) pressing, drying, or both pressing and drying the separated wood fibers to form the paper. 
     
     
         14 . The method of  claim 13 , wherein (b1) comprises vacuum filtering to separate the wood fibers from the solution. 
     
     
         15 . The method of  claim 9 , wherein:
 in (a), fines are formed, each fine comprises a cellulose flake or fragment, and   in (b), the forming is such that the fines fill spaces between adjacent wood fibers.   
     
     
         16 . The method of  claim 15 , wherein the paper formed in (b) consists essentially of the plurality of wood fibers and the fines. 
     
     
         17 . The method of  claim 9 , further comprising:
 after (b), coupling the paper to an active surface of a photovoltaic device, such that radiation passes through the paper in traveling to or from the active surface.   
     
     
         18 . The method of  claim 17 , further comprising:
 exposing the paper to solar radiation; and   using the photovoltaic device to generate electricity from solar radiation passing through the paper.   
     
     
         19 . The method of  claim 9 , wherein, after (a), at least a portion of each wood fiber is cleaved in a direction parallel to an axis of said wood fiber.