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Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics
Zhang, Nanlin,Neo, Darren C. J.,Tazawa, Yujiro,Li, Xiuting,Assender, Hazel E.,Compton, Richard G.,Watt, Andrew A. R. American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.33
<P>The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2016/aamick.2016.8.issue-33/acsami.6b01018/production/images/medium/am-2016-01018p_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am6b01018'>ACS Electronic Supporting Info</A></P>
Poly(3-hexylthiophene-2,5-diyl) as a Hole Transport Layer for Colloidal Quantum Dot Solar Cells
Neo, Darren C. J.,Zhang, Nanlin,Tazawa, Yujiro,Jiang, Haibo,Hughes, Gareth M.,Grovenor, Chris R. M.,Assender, Hazel E.,Watt, Andrew A. R. American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.19
<P>Lead sulfide colloidal quantum dot (CQD) solar cells demonstrate extremely high short-circuit currents (<I>J</I><SUB>sc</SUB>) and are making decent progress in power conversion efficiencies. However, the low fill factors (FF) and open-circuit voltages have to be addressed with urgency to prevent the stalling of efficiency improvements. This paper highlights the importance of improving hole extraction, which received much less attention as compared to the electron-accepting component of the device architecture (e.g., TiO<SUB>2</SUB> or ZnO). Here, we show the use of semiconducting polymer poly(3-hexylthiophene-2,5-diyl) to create efficient CQD devices by improving hole transport, removing interfacial barriers, and minimizing shunt pathways, thus resulting in an overall improvement in device performance stemming from better <I>J</I><SUB>sc</SUB> and FF.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2016/aamick.2016.8.issue-19/acsami.5b10228/production/images/medium/am-2015-10228h_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am5b10228'>ACS Electronic Supporting Info</A></P>
High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction
Kim, Byung-Sung,Neo, Darren C. J.,Hou, Bo,Park, Jong Bae,Cho, Yuljae,Zhang, Nanlin,Hong, John,Pak, Sangyeon,Lee, Sanghyo,Sohn, Jung Inn,Assender, Hazel E.,Watt, Andrew A. R.,Cha, SeungNam,Kim, Jong Mi American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.22
<P/><P>Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency.</P>