Kwan Hang Lam | GlaxoSmithKline, LLP (original) (raw)

Papers by Kwan Hang Lam

ChemInform, Dec 4, 2014

Phase-Selective Synthesis of Cu 2ZnSnS4 Nanocrystals Through Cation Exchange for Photovoltaic Dev... more Phase-Selective Synthesis of Cu 2ZnSnS4 Nanocrystals Through Cation Exchange for Photovoltaic Devices.-Precursor Cu 2SnS3 (CTS) nanoparticles with zinc blende (ZB) and wurtzite (WZ) derived structure are prepared by dissolving CuCl 2 and SnCl2 in oleylamine (180 C, 30 min) followed by addition of different S sources (ZB-type: elemental S, 280 C; WZ-type: 1-dodecanethiol, 230 C, 2 h). The Zn precursor Zn(S 2CNEt2)2 is precipitated from an aqueous ZnCl2 solution by Na(S2CNEt2); other metal complexes are also prepared (Fe, Co, Ni, Cd for Zn). Finally, Cu 2ZnSnS4 (CZTS) nanocrystals with (ZB)-and (WZ) crystal frameworks are selectively precipitated from solutions of the corresponding CTS precursors and Zn(S2CNEt2)2 in 1-dodecanethiol in the presence of trioctylamine (250 C, 1 h). This method provides a convenient approach for fine-tuning the nanocrystals' cation ratio which enables the optimization of the solar absorber layer compositions and results in a power conversion efficiency of 2.89% in Cu-poor and Zn-rich devices. The capability to synthetically access stable phases with controllable morphologies and compositions demonstrates that the method is powerful as a manufacturing technique for photovoltaic devices.-(WANG, Y.-X.;

Cu₂ZnSnS₄ (CZTS) holds great promises as an absorbe... more Cu₂ZnSnS₄ (CZTS) holds great promises as an absorber material for sustainable and low cost thin film solar cells. Kesterite and wurtzite are two common phases of CZTS. Until now, the synthesis and the growth of both phases are not clearly understood. In this work, kesterite CZTS nanoparticles, wurtzite CZTS nanoparticles as well as CZTS particles with a mixture of both structures were prepared by using elemental sulfur, 1-dodecanethiol, and thioacetamide, respectively. Time dependent studies were conducted and the reaction rate of sulfur source was found to be the key factor in determining the phase formation. Elemental sulfur reacts with oleylamine to produce highly reactive small molecule H₂S, which leads to the formation of kesterite phase. The reaction pathways of the long alkane chain 1-dodecanethiol yield the formation of wurtzite phase via a binary phase. Thioacetamide yields a mixture of kesterite and wurtz...

Inorganic Chemistry, 2014

Cu 2 ZnSnS 4 (CZTS) nanocrystals with two typical structures, i.e., zinc blende (ZB)-derived and ... more Cu 2 ZnSnS 4 (CZTS) nanocrystals with two typical structures, i.e., zinc blende (ZB)-derived and wurtzite (WZ) crystal frameworks, have been selectively synthesized via a solution-based route. Initially, Cu 2 SnS 3 (CTS) nanoparticles with two different phases, i.e., zinc blende-and wurtzite-derived, can be prepared with different S sources and temperatures. Afterward, addition of the Zn precursor to the CTS matrix results in the substitutions of Cu and Sn cations, yielding CZTS with desirable phases. This method can be extended to the synthesis of other similar quaternary chalcogenide nanocrystals. The cation exchange method described here provides a convenient approach for fine-tuning the nanocrystal's cation ratio, which enables us to optimize the solar absorber layer compositions and get a power conversion efficiency of 2.89% in copper-poor and zinc-rich devices. The capability to synthetically access stable phases with controllable morphologies and compositions demonstrates that the developed cation exchange method is powerful as a manufacturing technique for photovoltaic devices.

Organic Electronics, 2014

A p-type small molecule bearing dicarboxylic acid functional group (-COOH) is synthesized 30 and ... more A p-type small molecule bearing dicarboxylic acid functional group (-COOH) is synthesized 30 and evaluated for field-effect transistor properties. We discover and report for the first 31 time, that the-COOH groups assist in the passivation of surface traps on the dielectric layer 32 and simultaneously facilitate the self-assembly of the molecules via inter-molecular hydro-33 gen bonding resulting in crystalline active channels. A 9-fold decrease in the threshold 34 voltage was observed for the transistors made using the-COOH functionalized molecule, 35 QT-DA, compared to its ester analogue, QT-ES, providing an evidence of surface passivation. 36 This resulted in an increase in the hole mobility of QT-DA by up to 2 orders of magnitude. It 37 was shown that QT-DA adopts a vertical alignment with respect to the substrate due to 38 preferential interaction between the-COOH groups and the SiO 2 surfaces. 39 Ó 2014 Published by Elsevier B.V. 40 41 72 function both as the dielectric passivating layer as well 73 as the active channel material. The bi-functional configura-74 tion was selected so that one of the carboxylic acid groups 75 can anchor onto and passivate the dielectric surface via 76 hydrogen bonding or proton transfer onto the lattice 77 oxygen atom while the other acid group can assist in the

ACS Applied Materials & Interfaces, 2013

Small-molecule solar-cell performance is highly sensitive to the crystallinity and intermolecular... more Small-molecule solar-cell performance is highly sensitive to the crystallinity and intermolecular connectivity of the molecules. In order to enhance the crystallinity for the solution-processed small molecule, it is possible to make use of carboxylic acid end-functional groups to drive hydrogen-bonding-induced π-π stacking of conjugated molecules. Herein, we report the synthesis and characterization of quarterthiophenes with carboxylic acid as end groups. The formation of hydrogen bonds between neighboring acid groups gives rise to a pseudo-polymeric structure in the molecules, which leads to substantial improvement in the organization and crystallinity of the active layers. This resulted in a four-fold increase in the hole mobility and a two-fold improvement in the performance of the solar cell device for the acid-functionalized molecule, compared to its ester analogue. More importantly, optimal device performance for the acid-functionalized molecule was achieved for the as-cast film, thereby reducing the reliance on thermal annealing and solvent additives.

ChemInform, Dec 4, 2014

Phase-Selective Synthesis of Cu 2ZnSnS4 Nanocrystals Through Cation Exchange for Photovoltaic Dev... more Phase-Selective Synthesis of Cu 2ZnSnS4 Nanocrystals Through Cation Exchange for Photovoltaic Devices.-Precursor Cu 2SnS3 (CTS) nanoparticles with zinc blende (ZB) and wurtzite (WZ) derived structure are prepared by dissolving CuCl 2 and SnCl2 in oleylamine (180 C, 30 min) followed by addition of different S sources (ZB-type: elemental S, 280 C; WZ-type: 1-dodecanethiol, 230 C, 2 h). The Zn precursor Zn(S 2CNEt2)2 is precipitated from an aqueous ZnCl2 solution by Na(S2CNEt2); other metal complexes are also prepared (Fe, Co, Ni, Cd for Zn). Finally, Cu 2ZnSnS4 (CZTS) nanocrystals with (ZB)-and (WZ) crystal frameworks are selectively precipitated from solutions of the corresponding CTS precursors and Zn(S2CNEt2)2 in 1-dodecanethiol in the presence of trioctylamine (250 C, 1 h). This method provides a convenient approach for fine-tuning the nanocrystals' cation ratio which enables the optimization of the solar absorber layer compositions and results in a power conversion efficiency of 2.89% in Cu-poor and Zn-rich devices. The capability to synthetically access stable phases with controllable morphologies and compositions demonstrates that the method is powerful as a manufacturing technique for photovoltaic devices.-(WANG, Y.-X.;

Cu₂ZnSnS₄ (CZTS) holds great promises as an absorbe... more Cu₂ZnSnS₄ (CZTS) holds great promises as an absorber material for sustainable and low cost thin film solar cells. Kesterite and wurtzite are two common phases of CZTS. Until now, the synthesis and the growth of both phases are not clearly understood. In this work, kesterite CZTS nanoparticles, wurtzite CZTS nanoparticles as well as CZTS particles with a mixture of both structures were prepared by using elemental sulfur, 1-dodecanethiol, and thioacetamide, respectively. Time dependent studies were conducted and the reaction rate of sulfur source was found to be the key factor in determining the phase formation. Elemental sulfur reacts with oleylamine to produce highly reactive small molecule H₂S, which leads to the formation of kesterite phase. The reaction pathways of the long alkane chain 1-dodecanethiol yield the formation of wurtzite phase via a binary phase. Thioacetamide yields a mixture of kesterite and wurtz...

Inorganic Chemistry, 2014

Cu 2 ZnSnS 4 (CZTS) nanocrystals with two typical structures, i.e., zinc blende (ZB)-derived and ... more Cu 2 ZnSnS 4 (CZTS) nanocrystals with two typical structures, i.e., zinc blende (ZB)-derived and wurtzite (WZ) crystal frameworks, have been selectively synthesized via a solution-based route. Initially, Cu 2 SnS 3 (CTS) nanoparticles with two different phases, i.e., zinc blende-and wurtzite-derived, can be prepared with different S sources and temperatures. Afterward, addition of the Zn precursor to the CTS matrix results in the substitutions of Cu and Sn cations, yielding CZTS with desirable phases. This method can be extended to the synthesis of other similar quaternary chalcogenide nanocrystals. The cation exchange method described here provides a convenient approach for fine-tuning the nanocrystal's cation ratio, which enables us to optimize the solar absorber layer compositions and get a power conversion efficiency of 2.89% in copper-poor and zinc-rich devices. The capability to synthetically access stable phases with controllable morphologies and compositions demonstrates that the developed cation exchange method is powerful as a manufacturing technique for photovoltaic devices.

Organic Electronics, 2014

A p-type small molecule bearing dicarboxylic acid functional group (-COOH) is synthesized 30 and ... more A p-type small molecule bearing dicarboxylic acid functional group (-COOH) is synthesized 30 and evaluated for field-effect transistor properties. We discover and report for the first 31 time, that the-COOH groups assist in the passivation of surface traps on the dielectric layer 32 and simultaneously facilitate the self-assembly of the molecules via inter-molecular hydro-33 gen bonding resulting in crystalline active channels. A 9-fold decrease in the threshold 34 voltage was observed for the transistors made using the-COOH functionalized molecule, 35 QT-DA, compared to its ester analogue, QT-ES, providing an evidence of surface passivation. 36 This resulted in an increase in the hole mobility of QT-DA by up to 2 orders of magnitude. It 37 was shown that QT-DA adopts a vertical alignment with respect to the substrate due to 38 preferential interaction between the-COOH groups and the SiO 2 surfaces. 39 Ó 2014 Published by Elsevier B.V. 40 41 72 function both as the dielectric passivating layer as well 73 as the active channel material. The bi-functional configura-74 tion was selected so that one of the carboxylic acid groups 75 can anchor onto and passivate the dielectric surface via 76 hydrogen bonding or proton transfer onto the lattice 77 oxygen atom while the other acid group can assist in the

ACS Applied Materials & Interfaces, 2013

Small-molecule solar-cell performance is highly sensitive to the crystallinity and intermolecular... more Small-molecule solar-cell performance is highly sensitive to the crystallinity and intermolecular connectivity of the molecules. In order to enhance the crystallinity for the solution-processed small molecule, it is possible to make use of carboxylic acid end-functional groups to drive hydrogen-bonding-induced π-π stacking of conjugated molecules. Herein, we report the synthesis and characterization of quarterthiophenes with carboxylic acid as end groups. The formation of hydrogen bonds between neighboring acid groups gives rise to a pseudo-polymeric structure in the molecules, which leads to substantial improvement in the organization and crystallinity of the active layers. This resulted in a four-fold increase in the hole mobility and a two-fold improvement in the performance of the solar cell device for the acid-functionalized molecule, compared to its ester analogue. More importantly, optimal device performance for the acid-functionalized molecule was achieved for the as-cast film, thereby reducing the reliance on thermal annealing and solvent additives.