The low energy density of solar illumination necessitates deployment of solar technologies over large surface areas in order to capture enough of the sun’s energy to offset a significant portion of non-renewable energy consumption.  The obstacle of large-area deployment could be overcome with development of a low-cost, transparent, photovoltaic (PV) technology that can be integrated onto window panes in homes, skyscrapers, and automobiles, enhancing the functionality of already utilized transparent surfaces.  Since glass, structural framing, and inverters comprise nearly 40% of thin-film PV module prices ^[1] , integrating solar cells into windows can also reduce effective installation costs.  Presently, window glass used in automobiles and in architectural installations ^[2] requires 70-80% and 55-90% transmission to visible light, respectively.  Previous efforts to construct semitransparent devices have focused on the use of thin active layers (or spatially segmented films) with light absorption focused in the visible spectrum and therefore have been limited to either low efficiencies or low average visible-light transmissivity since both parameters cannot be simultaneously optimized.

We demonstrate an additive transparent organic PV technology  (see Figure 1a) that can similarly retain the glass transparency, where the non-transmitted light in the near-infrared is utilized for power generation.  To do this, we exploit the excitonic character of organic and molecular semiconductors to produce photovoltaic architectures with structured absorption, i.e., exhibiting minima and maxima that are uniquely distinct from the band-absorption of their inorganic counterparts.  Combining these PVs with selective high-reflectivity near-infrared mirrors, we demonstrate a prototype transparent device with a power conversion efficiency of several percent (Figure 1b) while also permitting more than 60% transmission of visible light through the entire device ^[3] .  Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting (see Figure 2), making them highly useful for distributed and point-of-source utilization of solar energy.

1. K. Zweibel, “The terawatt challenge for thin film photovoltaics,” Thin Film Solar Cells, Hoboken, NJ, John Wiley and Sons Ltd., 2006, pp. 427-459.
2. C. Tuchinda, S. Srivannaboon, and H. W. Lim, “Photoprotection by window glass, automobile glass, and sunglasses,” Journal of the American Academy of Dermatology, vol. 54, pp. 845-854, 2006.
3. R. R. Lunt and V. Bulovic, “Transparent, near-infrared organic photovoltaic solar cells for window and energy-scavenging applications,” Applied Physics Letters, vol. 98, pp. 113305:1-3, 2011.

There is emerging interest in the ability to produce low-cost and lightweight solar cells and other electronics on flexible, stretchable, and foldable substrates.  Rigid glass or silicon substrates in the current designs represent a large fraction of the overall module cost ^[1] and also restrict how and where modules can be deployed. Thus, a shift to the design of modules specifically on low-cost substrates could open untapped locations for solar deployment, including formats that are ubiquitous in our society (e.g., textiles, window curtains, printed paper documents, and wall paper). For example, photovoltaic (PV) devices fabricated directly on common fiber-based paper substrates are also foldable and rollable for storage and portability, easily shaped for three-dimensional applications, and able to be stapled to the roof structures or glued onto walls. To this end, there is significant interest in integrating various electronics to low-cost paper substrates, including transistors, storage devices, and displays ^{[2] [3]} .  In our work, we examine the use of oxidative chemical vapor deposition (oCVD) ^{[4] [5]} in conjunction with organic photovoltaics to fabricate PV cells ^[6] directly on fiber-based and paper substrates that are both flexible and foldable ^[7] .  For example, in Figure 1 we show a paper PV cell that was folded into a paper airplane (see corresponding video ^[8] ).   Furthermore, we have designed paper-based, monolithically series-integrated arrays that are capable of powering common electronics such as small-format LCD displays under ambient light as shown in Figure 2, with shelf lifetimes greater than several months.  These demonstrations allow us to rethink how and where lightweight and potentially low-cost photovoltaics can be deployed.

1. K. Zweibel, “The terawatt challenge for thin film photovoltaics,” Thin Film Solar Cells, Hoboken, NJ, John Wiley and Sons Ltd., 2006, pp. 427-459.
2. U. Zschieschang, T. Yamamoto, K. Takimiya, H. Kuwabara, M. Ikeda, T. Sekitani, T. Someya, and H. Klauk, “Organic electronics on banknotes,” Advanced Materials, vol. 23, pp. 654-658, 2011.
3. D. Tobjörk and R. Österbacka, “Paper electronics,” Advanced Materials, vol. 23, pp. 1935-1961, 2011.
4. M. E. Alf, A. Asatekin, M. C. Barr, S. H. Baxamusa, H. Chelawat, G. Ozaydin-Ince, C. D. Petruczok, R. Sreenivasan, W. E. Tenhaeff, N. J. Trujillo, S. Vaddiraju, J. J. Xu, and K. K. Gleason, “Chemical vapor deposition of conformal, functional, and responsive polymer films,” Advanced Materials, vol. 22, pp. 1993-2027, 2010.
5. S. H. Baxamusa, S. G. Im, and K. K. Gleason, “Initiated and oxidative chemical vapor deposition: a scalable method for conformal and functional polymer films on real substrates,” Physical Chemistry Chemical Physics, vol. 11, pp. 5227-5240, 2009.
6. C. W. Tang, “2-Layer Organic Photovoltaic Cell,” Applied Physics Letters, vol. 48, pp. 183-185, 1986.
7. M. C. Barr, J. A. Rowehl, R. R. Lunt, J. J. Xu, A. Wang, C. M. Boyce, S. G. Im, V. Bulovic, and K. K. Gleason, “Paper-thin, organic photovoltaic circuits fabricated directly on ubiquitous, everyday substrates,” Advanced Materials, accepted for publication.
8. Available: http://web.mit.edu/newsoffice/component/mitmultimedia/?type=video&videoid=233