Enhancement of open-circuit voltage and the fill factor in CdTe nanocrystal solar cells by using interface materials.

Interface states influence the operation of nanocrystal (NC) solar cell carrier transport, recombination and energetic mechanisms. In a typical CdTe NC solar cell with a normal structure of a ITO/p-CdTe NCs/n-acceptor (or without)/Al configuration, the contact between the ITO and CdTe is a non-ohm contact due to a different work function (for an ITO, the value is ~4.7 eV, while for CdTe NCs, the value is ~5.3 eV), which results in an energetic barrier at the ITO/CdTe interface and decreases the performance of the NC solar cells. This work investigates how interface materials (including Au, MoO(x) and C60) affect the performance of NC solar cells. It is found that devices with interface materials have shown higher V(oc) than those without interface materials. For the case in which we used Au as an interface, we obtained a high open-circuit voltage of 0.65 V, coupled with a high fill factor (62%); this resulted in a higher energy conversion efficiency (ECE) of 5.3%, which showed a 30% increase in the ECE compared with those without the interlayer. The capacitance measurements indicate that the increased V(oc) in the case in which Au was used as the interface is likely due to good ohm contact between the Au's and the CdTe NCs' thin film, which decreases the energetic barrier at the ITO/CdTe interface.

A Simple and Effective Phosphine-Doping Technique for Solution-Processed Nanocrystal Solar Cells.

Solution-processed cadmium telluride (CdTe) nanocrystal (NC) solar cells offer the advantages of low cost, low consumption of materials and large-scale production via a roll-to-roll manufacture process. Undecorated CdTe NC solar cells, however, tend to show inferior performance due to the abundant crystal boundaries within the active CdTe NC layer. The introduction of hole transport layer (HTL) is effective for promoting the performance of CdTe NC solar cells. Although high-performance CdTe NC solar cells have been realized by adopting organic HTLs, the contact resistance between active layer and the electrode is still a large problem due to the parasitic resistance of HTLs. Here, we developed a simple phosphine-doping technique via a solution process under ambient conditions using triphenylphosphine (TPP) as a phosphine source. This doping technique effectively promoted the power conversion efficiency (PCE) of devices to 5.41% and enabled the device to have extraordinary stability, showing a superior performance compared with the control device. Characterizations suggested that the introduction of the phosphine dopant led to higher carrier concentration, hole mobility and a longer lifetime of the carriers. Our work presents a new and simple phosphine-doping strategy for further improving the performance of CdTe NC solar cells.

Efficient Nanocrystal Photovoltaics with PTAA as Hole Transport Layer.

The power conversion efficiency (PCE) of solution-processed CdTe nanocrystals (NCs) solar cells has been significantly promoted in recent years due to the optimization of device design by advanced interface engineering techniques. However, further development of CdTe NC solar cells is still limited by the low open-circuit voltage (Voc) (mostly in range of 0.5-0.7 V), which is mainly attributed to the charge recombination at the CdTe/electrode interface. Herein, we demonstrate a high-efficiency CdTe NCs solar cell by using organic polymer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) as the hole transport layer (HTL) to decrease the interface recombination and enhance the Voc. The solar cell with the architecture of ITO/ZnO/CdS/CdSe/CdTe/PTAA/Au was fabricated via a layer-by-layer solution process. Experimental results show that PTAA offers better back contact for reducing interface resistance than the device without HTL. It is found that a dipole layer is produced between the CdTe NC thin film and the back contact electrode; thus the built-in electric field (Vbi) is reinforced, allowing more efficient carrier separation. By introducing the PTAA HTL in the device, the open-circuit voltage, short-circuit current density and the fill factor are simultaneously improved, leading to a high PCE of 6.95%, which is increased by 30% compared to that of the control device without HTL (5.3%). This work suggests that the widely used PTAA is preferred as the excellent HTL for achieving highly efficient CdTe NC solar cells.

A surfactant-free recipe for shape-controlled synthesis of CdSe nanocrystals.

We described surfactant-free recipes for the synthesis of CdSe nanocrystals (NCs) with well-controlled morphologies at a relatively low temperature. Dot-, rod-, tetrapod-and sphere-shaped CdSe NCs were prepared with trioctylphosphine oxide (TOPO) as a non-equilibrium solvent and trioctylphosphine selenide (TOPSe) and cadmium carboxylates as Se and Cd precursors, respectively. It was found that the morphology and stacking pattern of the CdSe NCs were related to the preparation conditions such as the concentration of the injected TOPSe(monomer concentration), reaction temperature and chain length of the cadmium carboxylate precursors. At a reaction temperature of 240 °C, CdSe NCs with a tetrapod selectivity of up to 85% were obtained in the presence of cadmium myristate under high concentrated TOPSe injection, and the in situ-formed myristic acid supplied the best acidic ligand with optimal amount to stabilize the anisotropic growth of the tetrapods. The intentional addition of more myristic acid in the reaction system would block the growth pathway of the tetrapods. Using cadmium laurate, cadmium palmitate and cadmium stearate as the cadmium precursors would reduce the formation of the tetrapods, showing the very low selectivity of the tetrapods.

Efficient PbS Quantum Dot Solar Cells with Both Mg-Doped ZnO Window Layer and ZnO Nanocrystal Interface Passivation Layer.

In this paper, a Mg-doped ZnO (MZO) thin film is prepared by a simple solution process under ambient conditions and is used as the window layer for PbS solar cells due to a wide n-type bandgap. Moreover, a thin layer of ZnO nanocrystals (NCs) was deposited on the MZO to reduce carrier recombination at the interface for inverted PbS quantum dot solar cells with the configuration Indium Tin Oxides (ITO)/MZO/ZnO NC (w/o)/PbS/Au. The effect of film thickness and annealing temperature of MZO and ZnO NC on the performance of PbS quantum dot solar cells was investigated in detail. It was found that without the ZnO NC thin layer, the highest power conversion efficiency(PCE) of 5.52% was obtained in the case of a device with an MZO thickness of 50 nm. When a thin layer of ZnO NC was introduced between MZO and PbS quantum dot film, the PCE of the champion device was greatly improved to 7.06% due to the decreased interface recombination. The usage of the MZO buffer layer along with the ZnO NC interface passivation technique is expected to further improve the performance of quantum dot solar cells.

Efficient Nanocrystal Photovoltaics via Blade Coating Active Layer.

CdTe semiconductor nanocrystal (NC) solar cells have attracted much attention in recent year due to their low-cost solution fabrication process. However, there are still few reports about the fabrication of large area NC solar cells under ambient conditions. Aiming to push CdTe NC solar cells one step forward to the industry, this study used a novel blade coating technique to fabricate CdTe NC solar cells with different areas (0.16, 0.3, 0.5 cm2) under ambient conditions. By optimizing the deposition parameters of the CdTe NC's active layer, the power conversion efficiency (PCE) of NC solar cells showed a large improvement. Compared to the conventional spin-coated device, a lower post-treatment temperature is required by blade coated NC solar cells. Under the optimal deposition conditions, the NC solar cells with 0.16, 0.3, and 0.5 cm2 areas exhibited PCEs of 3.58, 2.82, and 1.93%, respectively. More importantly, the NC solar cells fabricated via the blading technique showed high stability where almost no efficiency degradation appeared after keeping the devices under ambient conditions for over 18 days. This is promising for low-cost, roll-by-roll, and large area industrial fabrication.

Synthesis of Group II-VI Semiconductor Nanocrystals via Phosphine Free Method and Their Application in Solution Processed Photovoltaic Devices.

Solution-processed CdTe semiconductor nanocrystals (NCs) have exhibited astonishing potential in fabricating low-cost, low materials consumption and highly efficient photovoltaic devices. However, most of the conventional CdTe NCs reported are synthesized through high temperature microemulsion method with high toxic trioctylphosphine tellurite (TOP-Te) or tributylphosphine tellurite (TBP-Te) as tellurium precursor. These hazardous substances used in the fabrication process of CdTe NCs are drawing them back from further application. Herein, we report a phosphine-free method for synthesizing group II-VI semiconductor NCs with alkyl amine and alkyl acid as ligands. Based on various characterizations like UV-vis absorption (UV), transmission electron microscope (TEM), and X-ray diffraction (XRD), among others, the properties of the as-synthesized CdS, CdSe, and CdTe NCs are determined. High-quality semiconductor NCs with easily controlled size and morphology could be fabricated through this phosphine-free method. To further investigate its potential to industrial application, NCs solar cells with device configuration of ITO/ZnO/CdSe/CdTe/Au and ITO/ZnO/CdS/CdTe/Au are fabricated based on NCs synthesized by this method. By optimizing the device fabrication conditions, the champion device exhibited power conversion efficiency (PCE) of 2.28%. This research paves the way for industrial production of low-cost and environmentally friendly NCs photovoltaic devices.

High mobility field effect transistor from solution-processed needle-like tellurium nanowires.

Well-dispersed and uniform needle-like tellurium nanowires (NWs) have been fabricated in high yield by an environmentally-friendly hydrothermal method. It is found that beta-cyclodextrin ligands and reaction temperature play a great role on the morphology of Te NWs. Uniform needle-like Te NWs can only be obtained at suitable concentration of beta-CD and reaction temperature. A possible mechanism for the formation of the needle-liked Te NWs is discussed based on the experiment results briefly. High quality single Te NW field effect transistors are prepared through photolithographic patterning. By optimizing electrode and surface treatments, the NW FET has a high carrier mobility of 299 cm2V(-1)s(-1), which is the highest value ever reported for Te NW-based FETs. The performance is influenced by purity, crystallinity, surface species of NWs and metal contacts of NW device.

Hole Transfer Layer Engineering for CdTe Nanocrystal Photovoltaics with Improved Efficiency.

Interface engineering has led to significant progress in solution-processed CdTe nanocrystal (NC) solar cells in recent years. High performance solar cells can be fabricated by introducing a hole transfer layer (HTL) between CdTe and a back contact electrode to reduce carrier recombination by forming interfacial dipole effect at the interface. Here, we report the usage of a commercial product 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro) as a hole transfer layer to facilitate the hole collecting for CdTe nanocrystal solar cells. It is found that heat treatment on the hole transfer layer has significant influence on the NC solar cells performance. The Jsc, Voc, and power conversion efficiency (PCE) of NC solar cells are simultaneously increased due to the decreased contact resistance and enhanced built-in electric field. We demonstrate solar cells that achieve a high PCE of 8.34% for solution-processed CdTe NC solar cells with an inverted structure by further optimizing the HTL annealing temperature, which is among the highest value in CdTe NC solar cells with the inverted structure.

The Effects of ZnTe:Cu Back Contact on the Performance of CdTe Nanocrystal Solar Cells with Inverted Structure.

CdTe nanocrystal (NC) solar cells have received much attention in recent years due to their low cost and environmentally friendly fabrication process. Nowadays, the back contact is still the key issue for further improving device performance. It is well known that, in the case of CdTe thin-film solar cells prepared with the close-spaced sublimation (CSS) method, Cu-doped CdTe can drastically decrease the series resistance of CdTe solar cells and result in high device performance. However, there are still few reports on solution-processed CdTe NC solar cells with Cu-doped back contact. In this work, ZnTe:Cu or Cu:Au back contact layer (buffer layer) was deposited on the CdTe NC thin film by thermal evaporation and devices with inverted structure of ITO/ZnO/CdSe/CdTe/ZnTe:Cu (or Cu)/Au were fabricated and investigated. It was found that, comparing to an Au or Cu:Au device, the incorporation of ZnTe:Cu as a back contact layer can improve the open circuit voltage (Voc) and fill factor (FF) due to an optimized band alignment, which results in enhanced power conversion efficiency (PCE). By carefully optimizing the treatment of the ZnTe:Cu film (altering the film thickness and annealing temperature), an excellent PCE of 6.38% was obtained, which showed a 21.06% improvement compared with a device without ZnTe:Cu layer (with a device structure of ITO/ZnO/CdSe/CdTe/Au).

Efficient CdTe Nanocrystal/TiO2 Hetero-Junction Solar Cells with Open Circuit Voltage Breaking 0.8 V by Incorporating A Thin Layer of CdS Nanocrystal.

Nanocrystal solar cells (NCs) allow for large scale solution processing under ambient conditions, permitting a promising approach for low-cost photovoltaic products. Although an up to 10% power conversion efficiency (PCE) has been realized with the development of device fabrication technologies, the open circuit voltage (Voc) of CdTe NC solar cells has stagnated below 0.7 V, which is significantly lower than most CdTe thin film solar cells fabricated by vacuum technology (around 0.8 V~0.9 V). To further improve the NC solar cells' performance, an enhancement in the Voc towards 0.8⁻1.0 V is urgently required. Given the unique processing technologies and physical properties in CdTe NC, the design of an optimized band alignment and improved junction quality are important issues to obtain efficient solar cells coupled with high Voc. In this work, an efficient method was developed to improve the performance and Voc of solution-processed CdTe nanocrystal/TiO2 hetero-junction solar cells. A thin layer of solution-processed CdS NC film (~5 nm) as introduced into CdTe NC/TiO2 to construct hetero-junction solar cells with an optimized band alignment and p-n junction quality, which resulted in a low dark current density and reduced carrier recombination. As a result, devices with improved performance (5.16% compared to 2.63% for the control device) and a Voc as high as 0.83 V were obtained; this Voc value is a record for a solution-processed CdTe NC solar cell.

Roll-to-Roll Slot-Die-Printed Polymer Solar Cells by Self-Assembly.

Extremely simplified one-step roll-to-roll slot-die-printed flexible indium tin oxide (ITO)-free polymer solar cells (PSCs) are demonstrated based on the ternary blends of electron-donor polymer thieno[3,4- b]thiophene/benzodithiophene, electron-acceptor fullerene [6,6]-phenyl-C71-butyric acid methyl ester, and electron-extracting polymer poly[(9,9-bis(3'-( N, N-dimethylamino)propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN) at room temperature (RT) in ambient air. The flexible ITO-free PSC exhibits a comparable power conversion efficiency (PCE) with the device employing complicated two-step slot-die printing (5.29% vs 5.41%), which indicates that PFN molecules can migrate from the ternary nanocomposite toward the Ag cathode via vertical self-assembly during the one-step slot-die printing process in air. To confirm the migration of PFN, the morphology and elemental analysis as well as charge transport of different active layers are investigated by the in situ transient film drying process, transmission electron microscopy, atomic force microscopy, contact angle and surface energy, X-ray photoelectron spectroscopy, scanning electron microscopy, impedance spectroscopy, transient photovoltage and transient photocurrent, and laser-beam-induced current. Moreover, the good air and mechanical stability of the flexible device with a decent PCE achieved in 1 cm2 PSCs at RT in air suggests the feasibility of energy-saving and time-saving one-step slot-die printing to large-scale roll-to-roll manufacture in the future.

Solvent Engineering for High-Performance PbS Quantum Dots Solar Cells.

PbS colloidal quantum dots (CQDs) solar cells have already demonstrated very impressive advances in recent years due to the development of many different techniques to tailor the interface morphology and compactness in PbS CQDs thin film. Here, n-hexane, n-octane, n-heptane, isooctane and toluene or their hybrids are for the first time introduced as solvent for comparison of the dispersion of PbS CQDs. PbS CQDs solar cells with the configuration of PbS/TiO2 heterojunction are then fabricated by using different CQDs solution under ambient conditions. The performances of the PbS CQDs solar cells are found to be tuned by changing solvent and its content in the PbS CQDs solution. The best device could show a power conversion efficiency (PCE) of 7.64% under AM 1.5 G illumination at 100 mW cm-2 in a n-octane/isooctane (95%/5% v/v) hybrid solvent scheme, which shows a ~15% improvement compared to the control devices. These results offer important insight into the solvent engineering of high-performance PbS CQDs solar cells.

CdTe Nanocrystal Hetero-Junction Solar Cells with High Open Circuit Voltage Based on Sb-doped TiO2 Electron Acceptor Materials.

We propose Sb-doped TiO2 as electron acceptor material for depleted CdTe nanocrystal (NC) hetero-junction solar cells. Novel devices with the architecture of FTO/ZnO/Sb:TiO2/CdTe/Au based on CdTe NC and TiO2 precursor are fabricated by rational ambient solution process. By introducing TiO2 with dopant concentration, we are able to tailor the optoelectronic properties of NC solar cells. Our novel devices demonstrate a very high open circuit voltage of 0.74 V, which is the highest Voc reported for any CdTe NC based solar cells. The power conversion efficiency (PCE) of solar cells increases with the increase of Sb-doped content from 1% to 3%, then decreases almost linearly with further increase of Sb content due to the recombination effect. The champion device shows Jsc, Voc, FF, and PCE of 14.65 mA/cm², 0.70 V, 34.44, and 3.53% respectively, which is prospective for solution processed NC solar cells with high Voc.

Rationally Controlled Synthesis of CdSexTe1-x Alloy Nanocrystals and Their Application in Efficient Graded Bandgap Solar Cells.

CdSexTe1-x semiconductor nanocrystals (NCs), being rod-shaped/irregular dot-shaped in morphology, have been fabricated via a simple hot-injection method. The NCs composition is well controlled through varying molar ratios of Se to Te precursors. Through changing the composition of the CdSexTe1-x NCs, the spectral absorption of the NC thin film between 570-800 nm is proved to be tunable. It is shown that the bandgap of homogeneously alloyed CdSexTe1-x active thin film is nonlinearly correlated with the different compositions, which is perceived as optical bowing. The solar cell devices based on CdSexTe1-x NCs with the structure of ITO/ZnO/CdSe/CdSexTe1-x/MoOx/Au and the graded bandgap ITO/ZnO/CdSe(w/o)/CdSexTe1-x/CdTe/MoOx/Au are systematically evaluated. It was found that the performance of solar cells degrades almost linearly with the increase of alloy NC film thickness with respect to ITO/ZnO/CdSe/CdSe0.2Te0.8/MoOx/Au. From another perspective, in terms of the graded bandgap structure of ITO/ZnO/CdSe/CdSexTe1-x/CdTe/MoOx/Au, the performance is improved in contrast with its single-junction analogues. The graded bandgap structure is proved to be efficient when absorbing spectrum and the solar cells fabricated under the structure of ITO/ZnO/CdSe0.8Te0.2/CdSe0.2Te0.8/CdTe/MoOx/Au indicate power conversion efficiency (PCE) of 6.37%, a value among the highest for solution-processed inversely-structured CdSexTe1-x NC solar cells. As the NC solar cells are solution-processed under environmental conditions, they are promising for fabricating solar cells at low cost, roll by roll and in large area.

Erratum to: Novel Hybrid Ligands for Passivating PbS Colloidal Quantum Dots to Enhance the Performance of Solar Cells.

[This corrects the article DOI: 10.1007/s40820-015-0046-4.].

Novel Hybrid Ligands for Passivating PbS Colloidal Quantum Dots to Enhance the Performance of Solar Cells.

We developed novel hybrid ligands to passivate PbS colloidal quantum dots (CQDs), and two kinds of solar cells based on as-synthesized CQDs were fabricated to verify the passivation effects of the ligands. It was found that the ligands strongly affected the optical and electrical properties of CQDs, and the performances of solar cells were enhanced strongly. The optimized hybrid ligands, oleic amine/octyl-phosphine acid/CdCl2 improved power conversion efficiency (PCE) to much higher of 3.72 % for Schottky diode cell and 5.04 % for p-n junction cell. These results may be beneficial to design passivation strategy for low-cost and high-performance CQDs solar cells.

Solution Grown Se/Te Nanowires: Nucleation, Evolution, and The Role of Triganol Te seeds.

We have studied the nucleation and growth of Se-Te nanowires (NWs), with different morphologies, grown by a chemical solution process. Through systematic characterization of the Se-Te NW morphology as a function of the Te nanocrystallines (NCs) precursor, the relative ratio between Se and Te, and the growth time, a number of significant insights into Se-Te NW growth by chemical solution processes have been developed. Specifically, we have found that: (i) the growth of Se-Te NWs can be initiated from either long or short triganol Te nanorods, (ii) the frequency of proximal interactions between nanorod tips and the competition between Se and Te at the end of short Te nanorods results in V-shaped structures of Se-Te NWs, the ratio between Se and Te having great effect on the morphology of Se-Te NWs, (iii) by using long Te nanorods as seeds, Se-Te NWs with straight morphology were obtained. Many of these findings on Se-Te NW growth can be further generalized and provide very useful information for the rational synthesis of group VI based semiconductor NW compounds.

Field effect transistor from individual trigonal Se nanowire.

Trigonal Se nanowires (NWs) were fabricated through a high-yield chemical solution process. The morphology and structural characterization of the Se NWs were investigated using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and x-ray diffraction (XRD). The results indicated that the Se NWs grow along the crystallographic c-axis, the direction of which is parallel to the helical chains of Se atoms. Single Se NW field effect transistor (FET) devices were prepared through photolithographic patterning. The device performance shows that the Se NWs are p-type semiconductors displaying mobility up to 30 cm(2) V(-1) s(-1). This finding on the Se NW FETs has broad implications and provides very useful fundamental information necessary for future applications in the fabrication of high-quality NW FETs and other electronic devices.

Synthesis of high quality zinc-blende CdSe nanocrystals and their application in hybrid solar cells.

Oleic acid (OA) modified zinc-blende cadmium selenium nanocrystals (NCs) with different diameters, 3-5 nm, have been prepared. We find that the morphology and fluorescent properties of the samples are related to the preparation conditions such as the chain-length and concentration of the cadmium precursor as well as the concentration of OA. The hybrid solar cells based on the obtained spherical CdSe NCs as an acceptor and Poly(2-methoxy-5-(2'-ethylhexoxy)-p-phenylenevinylene) (MEH-PPV) as a donor show an energy conversion efficiency (ECE) as high as 0.85%, three times higher than that reported before for spherical CdSe NCs/conjugated polymer hybrid solar cells. When poly(3-hexylthiophene) (P3HT) is used as the donor phase instead of MEH-PPV, the energy conversion efficiency increases up to 1.08%. The solar cell based on CdSe NCs/conjugated polymer has the potential to open up new production technologies for hybrid solar cells based on semiconductor NCs.