Investigation of the role of Mn dopant in CdS quantum dot sensitized solar cell

Intense photoluminescence from Cu-doped CdSe nanotetrapods triggered by ultrafast hole capture

Brightly photoluminescent Cu-doped CdSe nanotetrapods (NTPs) have been prepared by a modified hot injection method. Their photoluminescence (PL) has a quantum yield of 38% and decays slowly over a few microseconds, while the PL in undoped NTPs has a rather small quantum yield of 1.7% and decays predominantly in tens of picoseconds, with a minor component in the nanosecond time regime. PL spectra of doped NTPs are significantly Stokes shifted compared to the band edge (BE). Efficient PL quenching by a hole scavenger confirms the oxidation state of +I for the dopant ion and establishes hole capture by this ion to be the primary event that leads to the Stokes shifted PL. A fast decay of the photoinduced absorption band, along with a similar decay in PL, observed in a femtosecond optical gating experiment, yields a time constant of about a picosecond for the hole capture from the valence band (VB) by Cu⁺. The remarkably long PL lifetime in the doped NTPs is ascribed to the decrease in the overlap between the wavefunctions of the photogenerated electrons and the captured hole. Hot carrier relaxation processes, triggered by excitation at energies greater than the band gap, leave their signature in a rise time of few hundreds of femtoseconds, in the ground state bleach recovery kinetics. Hence, a complete picture of exciton dynamics in the doped NTPs has been obtained using ultrafast spectroscopic techniques working in tandem.

Photo-Induced Black Phase Stabilization of CsPbI3 QDs Films

α-CsPbI3 quantum dots (QDs) show outstanding photoelectrical properties that had been harnessed in the fabrication of perovskite QDs solar cells. Nevertheless, the stabilization of the CsPbI3 perovskite cubic phase remains a challenge due to its own thermodynamic and the presence of surface defects. Herein, we report the optimization of the CsPbI3 QDs solar cells, by monitoring the structure, the morphology and the optoelectronic properties after a precise treatment, consisting of the conventional solvent washing with a time limited ultraviolet (UV) exposure combination, during the layer-by-layer deposition. The UV treatment compensates the defects coming from the essential but deleterious washing treatment. The material is stable for 200 h and the PCE improved by the 25% compared with that of the device without UV treatment. The photo-enhanced ion mobility mechanism is discussed as the main process for the CsPbI3 QDs and solar cell stability.

Enhanced-performance of self-powered flexible quantum dot photodetectors by a double hole transport layer structure

The usefulness of self-powered quantum dot (QD) photodetectors is increased if they are fabricated on flexible substrates. However, the performance of such photodetectors is typically significantly worse than similar devices fabricated on glass substrates due to poor charge transport performance. Here, a novel flexible self-powered CdSexTe1-x QD photodetector with a double hole transport layer of PEDOT:PSS/P-TPD has been fabricated, which achieves a performance comparable to that of rigid devices. The energy level of the P-TPD layer matches well with that of the PEDOT:PSS and QD layers, which significantly enhances photodetection capability across a spectral region that spans the ultraviolet, visible and near infrared (UV-NIR). A low dark current density (1.03 × 10-6 mA cm-2) and a large specific detectivity of approximately 2.6 × 1012 Jones at a wavelength of 450 nm are demonstrated, significantly outperforming previously reported flexible QD-based detectors. This improvement in performance is attributed to both increased hole transport efficiency and the inhibition of electron transport from the QDs into the PEDOT:PSS layer. The photodetector also exhibits good sensitivity under weak illumination, producing a photocurrent of 196 × 10-6 mA cm-2 under an irradiance of 5 μW cm-2. Moreover, no significant performance degradation is observed after 150 bending cycles to an angle of 60 degrees.

Aqueous synthesis of Mn-doped CuInSe2 quantum dots to enhance the performance of quantum dot sensitized solar cells

Mn-Doped QDs extended light absorption by altering the bandgap and facilitated rapid electron injection and charge separation, which together result in enhanced overall power conversion efficiency (PCE).

Impacts of Mn ion in ZnSe passivation on electronic band structure for high efficiency CdS/CdSe quantum dot solar cells

Surface passivation in quantum dot-sensitized solar cells (QDSSCs) plays a very important role in preventing surface charge recombination and thus enhancing the power conversion efficiency (PCE). ZnSe passivation with dopant in CdS/CdSe co-sensitized QDSSCs has been demonstrated as an effective way to improve the PCE. In the present study, a series of characterizations revealed that a Mn-doped ZnSe passivation layer can not only reduce surface charge recombination, but also enhance light harvesting. By means of density functional theory calculation along with a systematic study of electronic band structure, it has been found that the valence band of ZnSe moves upward on Mn-ion doping which leads to acceleration of charge separation and broader light absorption range. The impact of the Mn ion on charge recombination and light harvesting has been interpreted reasonably and the PCE of CdS/CdSe co-sensitized QDSSCs with Mn-doped ZnSe passivation layer is as high as 6.46%, which is 1.5 times that of the solar cell without the passivation layer.

Carrier transport dynamics in Mn-doped CdSe quantum dot sensitized solar cells

In this work quantum dot sensitized solar cells (QDSSCs) were fabricated with CdSe and Mn-doped CdSe quantum dots (QDs) using the SILAR method. QDSSCs based on Mn-doped CdSe QDs exhibited improved incident photon-to-electron conversion efficiency. Carrier transport dynamics in the QDSSCs were studied using the intensity modulated photocurrent/photovoltage spectroscopy technique, from which transport and recombination time constants could be derived. Compared to CdSe QDSSCs, Mn-CdSe QDSSCs exhibited shorter transport time constant, longer recombination time constant, longer diffusion length, and higher charge collection efficiency. These observations suggested that Mn doping in CdSe QDs could benefit the performance of solar cells based on such nanostructures.

High Efficiency CdS/CdSe Quantum Dot Sensitized Solar Cells with Two ZnSe Layers

CdS/CdSe quantum dot sensitized solar cells (QDSCs) have been intensively investigated; however, most of the reported power conversion efficiency (PCE) is still lower than 7% due to serious charge recombination and a low loading amount of QDs. Therefore, suppressing charge recombination and enhancing light absorption are required to improve the performance of QDSCs. The present study demonstrated successful design and fabrication of QDSCs with a high efficiency of 7.24% based on CdS/CdSe QDs with two ZnSe layers inserted at the interfaces between QDs and TiO₂ and electrolyte. The effects of two ZnSe layers on the performance of the QDSCs were systematically investigated. The results indicated that the inner ZnSe buffer layer located between QDs and TiO₂ serves as a seed layer to enhance the subsequent deposition of CdS/CdSe QDs, which leads to higher loading amount and covering ratio of QDs on the TiO₂ photoanode. The outer ZnSe layer located between QDs and electrolyte behaves as an effective passivation layer, which not only reduces the surface charge recombination, but also enhances the light harvesting.

Recent advances in counter electrodes of quantum dot-sensitized solar cells

The recent progress in the development of counter electrodes (CEs) is reviewed, and the key issues for the materials, structures and performance evaluation of CEs are also addressed.