Influence of Mn⁺² incorporation in CdSe quantum dots for high performance of CdS-CdSe quantum dot sensitized solar cells

Venkata-Haritha, M.,V.V.M. Gopi, C.,Thulasi-Varma, C.V.,Kim, S.K.,Kim, H.J. Elsevier Sequoia 2016 Journal of photochemistry and photobiology Chemist Vol.315 No.-

<P>Quantum dot sensitized solar cells (QDSSCs) have attracted considerable attention recently and become promising candidates for realizing a cost-effective and facile fabrication of solar cell with improved photovoltaic performance. QDs were directly grown on the TiO2 mesostructure by the successive ionic layer absorption and reaction (SILAR) technique. QDSSC based on CdS-CdSe photoanode achieves a power conversion efficiency of 3.42% under AM 1.5 G one sun illumination. The loading of Mn+2 metal ions was applied to a CdSe (CdS-Mn-CdSe) photoanode to enhance the absorption in QDSSCs, which greatly improved the power conversion efficiency. Without the passivation layer, the solar cell based on a CdS-Mn-CdSe QD-sensitized TiO2 photoelectrode shows higher J(sc) (14.67 mA/cm(2)), V-oc (0.590 V) and power conversion efficiency (4.42%) comparing to Mn-undoped CdS-CdSe QD sensitized TiO2 (J(sc): 11.29 mA/cm(2), V-oc: 0.568 V, and efficiency: 3.42%), which can be ascribed to superior light absorption, faster electron transport and slower charge recombination for the former. The effective electron lifetime of the device with CdS-Mn-CdSe was higher than those with CdS-CdSe, leading to more efficient electron-hole separation and slower electron recombination. The effects of Mn+2 metal ions on the chemical, physical, and photovoltaic properties of the QDSSCs have been investigated have been investigated by X-ray photon spectroscopy (XPS), UV-vis spectra, photocurrent-voltage (J-V) characteristics and electrochemical impedance spectra (EIS). (C) 2015 Elsevier B.V.All rights reserved.</P>

ZnO nanorods decorated with metal sulfides as stable and efficient counter-electrode materials for high-efficiency quantum dot-sensitized solar cells

Gopi, C. V.,Venkata-Haritha, M.,Lee, Y. S.,Kim, H. J. Royal Society of Chemistry 2016 Journal of materials chemistry. A, Materials for e Vol.4 No.21

<P>As a promising type of new-generation solar cells, the electrocatalytic activity and stability of counter electrodes (CEs) play a key role in the performance of QDSSCs (quantum-dot-sensitized solar cells) at present. Here, a facile solution-processing method for fabricating metal sulfides (CoS, NiS, CuS and PbS) on vertically aligned ZnO nanorods (NRs) has been demonstrated and used to produce efficient CEs in polysulfide electrolyte-based QDSSCs. Compared with bare metal sulfide CEs (CoS, NiS, CuS and PbS), the ZnO NR framework presents a larger surface area for loading more metal sulfide catalysts and easy accessibility of the electrolyte. Additionally, the metal sulfide catalyst with high catalytic activity plays the main role in the reduction of the oxidized polysulfide, white the ZnO NRs offer an excellent electron pathway for shuttling electrons to highly catalytic metal sulfide sites and facilitate charge transport during catalysis. Cyclic voltammetry measurements indicate that the ZnO/PbS CEs still retain good cyclability after 50 cycles, demonstrating super-stability, while the ZnO/CoS, ZnO/NiS, ZnO/CuS, and Pt CEs show obvious fluctuations. Therefore, the ZnO/PbS CE exhibits much higher catalytic activity with the polysulfide electrolyte than ZnO/CoS, ZnO/NiS, ZnO/CuS and Pt CEs. As a result, a QDSSC based on the ZnO/PbS CE achieves a power conversion efficiency (eta) of 4.76%, which is attributed to the high fill factor (FF) of 0.566, and the eta is much higher than that based on ZnO/CoS (2.75%), ZnO/NiS (3.12%), ZnO/CuS (4.10%) and Pt (1.54%) CEs. The excellent catalytic performance along with the facile preparation of ZnO NRs decorated with metal sulfide CE materials make them a distinctive choice among the various CEs studied.</P>

Improved photovoltaic performance and stability of quantum dot sensitized solar cells using Mn-ZnSe shell structure with enhanced light absorption and recombination control

Gopi, Chandu V. V. M.,Venkata-Haritha, M.,Kim, Soo-Kyoung,Kim, Hee-Je The Royal Society of Chemistry 2015 Nanoscale Vol.7 No.29

<P>To make quantum-dot-sensitized solar cells (QDSSCs) competitive, photovoltaic parameters comparable to those of other emerging solar cell technologies are necessary. In the present study, ZnSe was used as an alternative to ZnS, one of the most widely used passivation materials in QDSSCs. ZnSe was deposited on a TiO2-CdS-CdSe photoanode to form a core-shell structure, which was more efficient in terms of reducing the electron recombination in QDSSCs. The development of an efficient passivation layer is a requirement for preventing recombination processes in order to attain high-performance and stable QDSSCs. A layer of inorganic Mn-ZnSe was applied to a QD-sensitized photoanode to enhance the adsorption and strongly inhibit interfacial recombination processes in QDSSCs, which greatly improved the power conversion efficiency. Impedance spectroscopy revealed that the combined Mn doping with ZnSe treatment reduces interfacial recombination and increases charge collection efficiency compared with Mn-ZnS, ZnS, and ZnSe. A solar cell based on the CdS-CdSe-Mn-ZnSe photoanode yielded excellent performance with a solar power conversion efficiency of 5.67%, Voc of 0.584 V, and Jsc of 17.59 mA cm(-2). Enhanced electron transport and reduced electron recombination are responsible for the improved Jsc and Voc of the QDSSCs. The effective electron lifetime of the device with Mn-ZnSe was higher than those with Mn-ZnS, ZnSe, and ZnS, leading to more efficient electron-hole separation and slower electron recombination.</P>

Improving the performance of quantum dot sensitized solar cells through CdNiS quantum dots with reduced recombination and enhanced electron lifetime

Gopi, Chandu V. V. M.,Venkata-Haritha, Mallineni,Seo, Hyunwoong,Singh, Saurabh,Kim, Soo-Kyoung,Shiratani, Masaharu,Kim, Hee-Je The Royal Society of Chemistry 2016 Dalton Transactions Vol.45 No.20

<P>To make quantum dot-sensitized solar cells (QDSSCs) competitive, we investigated the effect of Ni2+ ion incorporation into a CdS layer to create long-lived charge carriers and reduce the electron-hole recombination. The Ni2+ -doped CdS (simplified as CdNiS) QD layer was introduced to a TiO2 surface via the simple successive ionic layer adsorption and reaction (SILAR) method in order to introduce intermediateenergy levels in the QDs. The effects of different Ni2+ concentrations (5, 10, 15, and 20 mM) on the physical, chemical, and photovoltaic properties of the QDSSCs were investigated. The Ni2+ dopant improves the light absorption of the device, accelerates the electron injection kinetics, and reduces the charge recombination, which results in improved charge transfer and collection. The 15% CdNiS cell exhibits the best photovoltaic performance with a power conversion efficiency (eta) of 3.11% (J(SC) = 8.91 mA cm(-2), VOC = 0.643 V, FF = 0.543) under one full sun illumination (AM 1.5 G). These results are among the best achieved for CdS-based QDSSCs. Electrochemical impedance spectroscopy (EIS) and open circuit voltage decay (OCVD) measurements confirm that the Ni2+ dopant can suppress charge recombination, prolong the electron lifetime, and improve the power conversion efficiency of the cells.</P>

A strategy to improve the energy conversion efficiency and stability of quantum dot-sensitized solar cells using manganese-doped cadmium sulfide quantum dots

Gopi, Chandu V. V. M.,Venkata-Haritha, M.,Kim, Soo-Kyoung,Kim, Hee-Je The Royal Society of Chemistry 2015 Dalton Transactions Vol.44 No.2

<P>This article describes the effect of manganese (Mn) doping in CdS to improve the photovoltaic performance of quantum dot sensitized solar cells (QDSSCs). The performances of the QDSSCs are examined in detail using a polysulfide electrolyte with a copper sulfide (CuS) counter electrode. Under the illumination of one sun (AM 1.5 G, 100 mW cm<SUP>−2</SUP>), 10 molar% Mn-doped CdS QDSSCs exhibit a power conversion efficiency (<I>η</I>) of 2.85%, which is higher than the value of 2.11% obtained with bare CdS. The improved photovoltaic performance is due to the impurities from Mn<SUP>2+</SUP> doping of CdS, which have an impact on the structure of the host material and decrease the surface roughness. The surface roughness and morphology of Mn-doped CdS nanoparticles can be characterised from atomic force microscopy images. Furthermore, the cell device based on the Mn-CdS electrode shows superior stability in the sulfide/polysulfide electrolyte in a working state for over 10 h, resulting in a highly reproducible performance, which is a serious challenge for the Mn-doped solar cell. Our finding provides an effective method for the fabrication of Mn-doped CdS QDs, which can pave the way to further improve the efficiency of future QDSSCs.</P> <P>Graphic Abstract</P><P>Better stability and higher performance of Mn-doped CdS QDSSCs (PCE = 2.85%) than those of CdS QDSSCs (PCE = 2.11%). <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4dt03063j'> </P>

Low-temperature easy-processed carbon nanotube contact for high-performance metal- and hole-transporting layer-free perovskite solar cells

Gopi, C.V.V.M.,Venkata-Haritha, M.,Prabakar, K.,Kim, H.J. Elsevier Sequoia 2017 Journal of photochemistry and photobiology. A, Che Vol.332 No.-

Expensive and energy-consuming vacuum process of metal deposition with ambient-unstable hole transporters are incompatible with large-scale and low-cost production of perovskite solar cells (PSCs) and thus hampers their commercialization. For the first time, we demonstrate cost-effective novel carbon nanotube (CNT) paste that was applied to FTO substrate by the facile doctor blade method and processed at low temperature (100<SUP>o</SUP>C). Herein we report a new method of cost-efficient perovskite solar cells with the use of conventional hole transporters by directly clamping a selective hole extraction electrode made of CNT and a TiO<SUB>2</SUB>/perovskite photoanode. Most importantly, under optimized conditions in the absence of an organic hole-transporting material and metal contact, CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB> and CNTs formed a solar cell with an efficiency of up to 7.83%. The PSC devices are fabricated in air without high-vacuum deposition which simplifies the processing and lowers the threshold of both scientific research and industrial production of PSCs. Electrochemical impedance spectroscopy demonstrates good charge transport characteristics of CEs on the photovoltaic performance of devices. The PSCs exhibited good stability over 50h. The abundance, low cost, and excellent properties of the CNT material offer wide prospects for further applications in PSCs.

Electrochemical growth of NiS nanoparticle thin film as counter electrode for quantum dot-sensitized solar cells

Lee, Y.S.,Gopi, C.V.V.M.,Venkata-Haritha, M.,Rao, S.S.,Kim, H.J. Elsevier Sequoia 2017 Journal of photochemistry and photobiology Chemist Vol. No.

The high stability and superior electrocatalytic activity of counter electrodes (CEs) are crucial but important issues in high performance quantum dot-sensitized solar cells (QDSSCs). To address the above issues, nanoparticle-structured nickel sulfide (NiS) thin film electrodes were prepared on F-doped SnO<SUB>2</SUB> glass (FTO glass) substrates using a facile chemical bath deposition method at different growth times and used directly as the CEs for CdS/CdSe/ZnS QDSSCs. The surface morphology and thickness of the resulting NiS films are greatly affected by the deposition time. By optimizing the growth time of the NiS CE materials, a power conversion efficiency up to 3.25% was achieved for CdS/CdSe/ZnS based QDSSCs, which was much higher than that of the Pt CE (0.79%). In addition, a preliminary durability test of the CE in QDSSCs reveals that the NiS CE exhibited good stability than the Pt CE. The improved performance of the QDSSC was attributed to the resulting electrochemical catalytic activity of the NiS CE with efficient charge transfer at the CE/electrolyte interface, which was verified by the electrochemical impedance spectroscopy and the Tafel polarization measurement results. Therefore, the excellent electrochemical performance of NiS highlights its promising application as a CE for high performance QDSSCs.

Recombination control in high-performance quantum dot-sensitized solar cells with a novel TiO₂/ZnS/CdS/ZnS heterostructure

Lee, Young-Seok,Gopi, Chandu V. V. M.,Venkata-Haritha, Mallineni,Kim, Hee-Je The Royal Society of Chemistry 2016 Dalton Transactions Vol.45 No.32

<P>Charge recombination occurring at the TiO2/QDs/electrolyte interface is a crucial factor that limits the power conversion efficiency (eta) of quantum dot-sensitized solar cells (QDSSCs). This paper presents a new approach by inserting a ZnS layer between the TiO2 and CdS/ZnS to prepare a TiO2/ZnS/CdS/ZnS sensitized photoelectrode for QDSSC applications. The CdS QDs and ZnS passivation layers were deposited using a reproducible and controlled successive ionic layer adsorption and reaction method. The TiO2/ZnS/CdS/ZnS based QDSSCs exhibited a power conversion efficiency (eta) value of 3.69%, which is significantly higher than the 3.02% and 2.09% observed for solar cells with a TiO2/CdS/ZnS device and without a passivation layer (TiO2/CdS), respectively. The elevated performance of the TiO2/ZnS/CdS/ZnS-based QDSSCs was attributed to the pre-assembled ZnS layer enhancing the light harvesting and acting as a blocking layer to shield the TiO2 core from the outer QDs and the electrolyte, thereby retarding the interfacial recombination of electrons from the TiO2 with the electrolyte or with the QDs. Electrochemical impedance spectroscopy and open circuit voltage decay measurements showed that the TiO2/ZnS/CdS/ZnS-based QDSSCs inhibit charge recombination remarkably at the photoanode/electrolyte interface and prolong the electron lifetime.</P>

Cost-effective and morphology controllable PVP based highly efficient CuS counter electrodes for high-efficiency quantum dot-sensitized solar cells

Kim, Hee-Je,Myung-Sik, Lee,Gopi, Chandu V. V. M.,Venkata-Haritha, M.,Rao, S. Srinivasa,Kim, Soo-Kyoung The Royal Society of Chemistry 2015 Dalton Transactions Vol.44 No.25

<P>Currently, copper sulfide (CuS) is the most commonly used counter electrode (CE) in high-efficiency quantum dot-sensitized solar cells (QDSSCs) because of its superior electrocatalytic activity in the presence of polysulfide electrolyte. For the first time, CuS thin films were prepared by a facile chemical bath deposition method with different concentrations of polyvinylpyrrolidone (PVP) and directly used as CEs in QDSSCs without any further post treatment. The quantum dot photoanode with the optimized 0.25 mM PVP-based CuS CE exhibits higher short circuit current density (<I>J</I><SUB>sc</SUB>), open circuit voltage (<I>V</I><SUB>oc</SUB>), fill factor (FF), and power conversion efficiency (PCE) of 17.57 mA cm<SUP>−2</SUP>, 0.578 V, 0.514, and 5.22%, respectively, which are much higher values than those of a bare CuS CE (<I>J</I><SUB>sc</SUB>: 12.36 mA cm<SUP>−2</SUP>; <I>V</I><SUB>oc</SUB>: 0.591 V; FF: 0.436; PCE: 3.18%) and Pt CE (<I>J</I><SUB>sc</SUB>: 11.25 mA cm<SUP>−2</SUP>; <I>V</I><SUB>oc</SUB>: 0.464 V; FF: 0.296; PCE: 1.54%) under one-sun illumination (AM 1.5 G, 100 mW cm<SUP>−2</SUP>). Moreover, the 0.25 mM PVP-based CuS CE produces a charge-transfer resistance of only 4.39 Ω with the aqueous polysulfide electrolyte commonly applied in QDSSCs. This value is several orders of magnitude lower than that of a typical Pt electrode (69.75 Ω) and bare CuS electrode (9.27 Ω). This enhancement is mainly attributed to the improved morphology of the 0.25 mM CuS CE with high catalytic activity, which plays a main role in the reduction processes of the oxidized polysulfide electrolyte, as well as the increased sulfur atomic percentage with Cu vacancies. Cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel polarization were performed to study the underlying reasons behind the efficient CE performance.</P> <P>Graphic Abstract</P><P>A maximum efficiency of 5.22% was achieved with the optimized 0.25 mM PVP based Mn–CuS counter electrode. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c5dt01412c'> </P>