Near-infrared-sensitive bulk heterojunction solar cells using nanostructured hybrid composites of HgTe quantum dots and a low-bandgap polymer Minwoo Nam a,1 , Sungwoo Kim b,c,1 , Sejin Kim b , Sohee Jeong c , Sang-Wook Kim b , Keekeun Lee a,n a Department of Electrical and Computer Engineering, Ajou University, Suwon 443-749, Republic of Korea b Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Republic of Korea c Nanomechanical Systems Research Division, Korea Institute of Machinery and Materials, Daejeon 305-343, Republic of Korea article info Article history: Received 5 June 2013 Received in revised form 11 December 2013 Accepted 18 March 2014 Keywords: Hybrid solar cell HgTe quantum dot Low-bandgap polymer Nanoimprinting Anodic aluminum oxide abstract Near-infrared (NIR)-sensitive hybrid bulk heterojunction solar cells were developed using NIR-absorbing HgTe quantum dots (QDs) and a low-bandgap polymer, poly[2,6-(4,4 0 -bis-(2-ethylhexyl)dithieno [3,2-b:2 0 ,3 0 -d]silole)-alt-4,7(2,1,3-benzothiadiazole)] (PSBTBT). Hybrid composites of HgTe QDs and PSBTBT facilitate broad-range exploitation of the solar spectrum and efficient carrier dissociation prior to recombination. Nanostructures were formed on the surface of the hybrid composite via a nanoimprinting process using an anodic aluminum oxide (AAO) mold. This contributes to optical light scattering for efficient utilization of light up to the NIR region and enlarged photoactive layer–electrode interfacial areas for improving charge extraction, increasing the overall efficiency from 1.09 to 1.41%. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Near-infrared (NIR) hybrid bulk heterojunction solar cells, which consist of NIR-absorbing colloidal quantum dots (QDs) embedded in an electron-donating conjugated polymer matrix, can exhibit a high conversion efficiency by utilizing a broad spectral response [1–11], considering that half of the sun's energy lies beyond the NIR wavelength region [12]. This kind of hybrid solar cells possess a great capability of utilizing combined merits of two classes of materials, which includes solution-based film- forming property of polymer matrix and size-tunable optical bandgap and carrier multiplication of QDs [5,6]. The efficiency of such hybrid solar cells can exceed those of all-organic solar cells, if several issues such as chemistry of hybrid material interfaces and nanomorphology of hybrid composites can be addressed through further research approaches [13]. Compound semiconductor QDs with narrow bulk energy bandgaps, such as PbS, PbSe, InAs, FeS 2 , and Bi 2 Se 3 , can be used as electron acceptors in the hybrid system [14]. Of these QDs, PbS has been considered as the most promising material owing to its well-known synthesis and intensive studies [1–6]. However, HgTe is expected to perform better as an NIR light-absorber than PbS in solar cells. HgTe has a high electron mobility (26,500 cm 2 V 1 s 1 ) [15], large Bohr radius (39.3 nm) [16], and negative bulk energy bandgap (  0.3 eV) [14], which enable effective bandgap engineering in the NIR wavelength region by the quantum size effect. However, despite these advan- tages, there has been no detailed study on HgTe QD:polymer hybrid bulk heterojunction solar cells. Only a few articles on solid- state HgTe QD-sensitized solar cells have been published, but the efficiencies of HgTe cells are significantly lower than those of PbS QDs [17,18]. This is primarily attributed to the difficult synthesis process and poor stability of HgTe QDs [19]. With respect to the device structure of polymer-based bulk heterojunction solar cells, the optimal thickness in a photoactive film is considered to be approximately 100 nm. This minimizes the charge-carrier recombination effect, which occurs because of intrinsic electrical properties of the conjugated polymers. How- ever, such a thin film has some limits in retaining sufficient solar light in the photoactive layer. To address this issue, plasmonic and nanophotonic structures can be incorporated to enhance light trapping and to realize “optically thick” photoactive layers without increasing the film thickness. Such structures representatively include metal nanoparticles embedded in the photoactive layer Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.03.027 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ82 31 219 1848. E-mail address: keekeun@ajou.ac.kr (K. Lee). 1 These two authors contributed equally to this work. Solar Energy Materials & Solar Cells 126 (2014) 163–169