Enhanced conduction and charge-selectivity by N- doped graphene flakes in the active layer of bulk- heterojunction organic solar cells† Gwang Hoon Jun, a Sung Hwan Jin, a Bin Lee, a Bo Hyun Kim, a Weon-Sik Chae, b Soon Hyung Hong * a and Seokwoo Jeon * a Polymeric organic photovoltaic (OPV) cells are promising candidates for low-cost, high-performance energy sources due to their low material and processing costs, flexibility, and ease of manufacturing by solution processes. However, low power-conversion efficiency (PCE) has impeded the development of OPV cells. The low PCE in OPV solar cells has been attributed to low carrier mobility, which is related to the transport length of the charge carriers within active layers. Graphene can be an ideal material for assisting the charge transport in the active layer of OPV cells due to its excellent charger carrier mobility, thermal and chemical stability, and compatibility with the solution process. In this work, we demonstrated for the first time an improvement of the PCE (up to 40%) in OPV bulk-heterojunction (BHJ) cells by incorporating charge-selective graphene flakes into the BHJ active layer. The charge selectivity of graphene flakes was achieved by nitrogen doping (N-doped graphene). The N-doped graphene, when mixed in the active layer (N-doped graphene/polymer:fullerene composites), provided transport pathways exclusively to specific charge carriers through the modulation of band-gap structures. We discuss further the enhancement of the PCE in OPV cells with respect to charge-carrier mobility. Broader context Organic solar cells have received a lot of attention due to their low production costs, easy scalability to large-areas and applicability on exible substrates. One of the main challenges to widespread application in practical devices is their low power conversion efficiency (PCE). This is largely because of the low charge-carrier mobilities and poor charge transfer characteristics in organic materials, resulting in short carrier lifetimes and reduced charge collection efficiencies. In this work, we demonstrate that the use of nitrogen-doped graphene improves the power conversion efficiency of a bulk-heterojunction solar cell system. The nitrogen-doped graphene provides transport pathways to specic charge carriers through the modulation of band structures when mixed into the active layer. We believe that the added functionality of charge selectivity in conductive graphene akes gives a new design parameter for increasing the PCE of bulk-het- erojunction solar cells. 1 Introduction Polymeric organic photovoltaic (OPV) cells are promising candi- dates for low-cost, high-performance energy sources due to their low material and processing costs, exibility, and ease of manufacturing by solution processes. 1–3 However, the low power- conversion efficiency (PCE) of OPV cells due to the extremely short length of exciton diffusion (10 nm) in the organic active layers has impeded their progress toward becoming a leading alternative energy source. 4 Structural advances in solution- processed bulk-heterojunction (BHJ) solar cells have provided a means to maximize the exciton dissociation by matching the typical exciton diffusion length (10 nm). 4,5 In BHJ solar cells, nanoscale phase separation of donors and acceptors within the exciton diffusion length increases the opportunity for excitons to dissociate into charge carriers, resulting in better PCE. 5,6 To date, numerous studies on the PCE of OPV cells have identied the origins of low PCE in OPV-BHJ solar cells: the misalignment of energy levels between the donor and acceptor, insufficient light absorption in the active layer, a short exciton diffusion length, and low carrier mobility in the phase-separated layers of the BHJ. 7,8 Among these, low carrier mobility is the predominant factor underlying low PCE. Due to the low electron and hole mobilities of the polymer materials (<10 4 cm 2 V 1 s 1 ), the separated charge carriers are eliminated by recombination or charge trapping before arriving at the electrodes. 6,9 During the past decade, carbon-based nanomaterials such as carbon a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea. E-mail: jeon39@kaist.ac.kr; Fax: +82 42 350 3310; Tel: +82 42 350 3342 b Gangneung Center, Korea Basic Science Institute, Gangneung 210-702, Republic of Korea † Electronic supplementary information (ESI) available: See DOI: 10.1039/c3ee40963e Cite this: Energy Environ. Sci., 2013, 6, 3000 Received 20th March 2013 Accepted 2nd July 2013 DOI: 10.1039/c3ee40963e www.rsc.org/ees 3000 | Energy Environ. Sci., 2013, 6, 3000–3006 This journal is ª The Royal Society of Chemistry 2013 Energy & Environmental Science PAPER Published on 02 July 2013. Downloaded by Korea Advanced Institute of Science & Technology / KAIST on 26/06/2017 03:24:21. View Article Online View Journal | View Issue