Efficiency Enhancement of Organic Solar Cells Using Hydrophobic Antireflective Inverted MothEye Nanopatterned PDMS Films

There is increasing interest in organic solar cells (OSCs) based on conjugated polymers due to their promising potential for future energy sources with low cost, high throughput, light weight, and superior mechanical fl exibility. [ 1–3 ] Recently, Heliateck in Germany demonstrated an OSC with a power conversion effi ciency (PCE) of 12%, which represents the current worldrecord value for OSCs. [ 4 ] However, this value is much lower compared to inorganic multi-junction solar cells, where PCE can exceed 40%. [ 5 ] Thus, further improvements in PCEs of OSCs appear possible. Among various strategies for increasing PCE, one simple approach is to enhance the light collection and trapping in the active layer of OSCs. [ 6–10 ] For this purpose, high-effi ciency broadband antirefl ection (AR) coatings are required on externally facing surfaces of the transparent superstrates (i.e., glasses or plastics), to suppress Fresnel surface refl ections. [ 11 ] Soft imprint lithographic methods have been used to create nanopatterned AR fi lms in fl exible and stretchable polymer-based materials including poly-dimethylsiloxane (PDMS), polycarbonate, polymethyl methacrylate (PMMA), and polyurethane. [ 12–15 ] Such techniques are compatible with continuous processing, which is advantageous for simple, lowcost, and high-throughput production without the need for costly high-vacuum equipment. [ 16 ] Also, the resulting fi lms can be repeatedly used as elastomeric stamps, membranes, or substrates as well as protective layers against mechanical damage to the underlying devices. [ 17–21 ] Although there are several published studies on AR nanostructures in polymer fi lms (i.e., PDMS [ 22 ] and PMMA [ 23 ] ) for OSCs, there is little work on the use of the elastomeric PDMS fi lms with inverted moth-eye nanopatterns. Separately, self-cleaning properties have been found to be useful in removing any surface dirt or dust particles for device applications. [ 24 ] Thus, the wetting behaviors of inverted moth-eye nanopatterned PDMS (IMN PDMS) are also relevant. Here, we fabricate IMN PDMS fi lms by soft imprint lithography using silicon (Si) masters of periodic arrays with conical subwavelength grating patterns. Their wettability and optical properties, together with results of rigorous coupled-wave analysis (RCWA) simulation, on glass substrates are investigated. By laminating the IMN PDMS fi lms as AR layers on glass substrates of encapsulated OSCs, the effects on device characteristics including their stability are also studied. DOI: 10.1002/aenm.201301315 J. W. Leem, S. H. Lee, Prof. J. S. Yu Department of Electronics and Radio Engineering Kyung Hee University 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do, 446–701 , Korea E-mail: [email protected] S. Kim, Prof. E. Kim Department of Chemical and Biomolecular Engineering Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul 120–749 , Korea E-mail: [email protected] Prof. J. A. Rogers Department of Materials Science and Engineering University of Illinois at Urbana-Champaign Urbana , IL 61801 , USA Poly-dimethylsiloxane (PDMS) fi lms with 2D periodic inverted moth-eye nanopatterns on one surface are implemented as antirefl ection (AR) layers on a glass substrate for effi cient light capture in encapsulated organic solar cells (OSCs). The inverted moth-eye nanopatterned PDMS (IMN PDMS) fi lms are fabricated by a soft imprint lithographic method using conical subwavelength grating patterns formed by laser interference lithography/ dry etching. Their optical characteristics, together with theoretical analysis using rigorous coupled-wave analysis simulation, and wetting behaviors are investigated. For a period of 380 nm, IMN PDMS fi lms laminated on glass substrates exhibit a hydrophobic surface with a water contact angle ( θ CA ) of ≈120° and solar weighted transmittance (SWT) of ≈94.2%, both signifi cantly higher than those ( θ CA ≈ 36° and SWT ≈ 90.3%) of bare glass substrates. By employing IMN PDMS fi lms with a period of 380 nm on glass substrates for OSCs, an enhanced power conversion effi ciency (PCE) of 6.19% is obtained mainly due to the increased short-circuit current density ( J sc ) of 19.74 mA cm −2 compared to the OSCs with the bare glass substrates (PCE = 5.16% and J sc = 17.25 mA cm −2 ). For the OSCs, the device stability is also studied.