Semiconductor Nanocrystals as Light Harvesters in Solar Cells

Luminescent Downshifting Silicon Quantum Dots for Performance Enhancement of Polycrystalline Silicon Solar Cells

Silicon quantum dots (Si-QDs) with luminescent downshifting properties have been used for the efficiency enhancement of solar cells. In this study, Phenylacetylene-capped silicon quantum dots (PA Si-QDs) have been fabricated and applied as luminescent downshifting material on polycrystalline silicon solar cells, by dropcasting. The PA Si-QD coated solar cell samples presented an average increase in the short circuit current (Isc) of 0.75% and 1.06% for depositions of 0.15 mg and 0.01 mg on 39 mm × 39 mm pc-Si solar cells, respectively. The increase was further enhanced by full encapsulation of the sample leading to overall improved performance of about 3.4% in terms of Isc and 4.1% in terms of power output (Pm) when compared to the performance of fully encapsulated reference samples. The PA Si-QD coating achieved a reduction in specular reflectance at 377 nm of 61.8%, and in diffuse reflectance of 44.4%. The increase observed in the Isc and Pm is a promising indicator for the use of PA Si-QDs as luminescent downshifting material to improve the power conversion efficiency of pc-Si solar cells.

Insight into morphology dependent charge carrier dynamics in ZnSe-CdS nanoheterostructures

Semiconductor nanoheterostructures (NHSs) are being increasingly used for the photocatalytic conversion of solar energy in which photo-induced charge separation is an essential step and hence it is necessary to understand the effect of various factors such as size, shape, and composition on the charge transfer dynamics. Ultrafast transient absorption spectroscopy is used to investigate the nature and dynamics of photo-induced charge transfer processes in ZnSe-CdS NHSs of different morphologies such as nanospheres (NSs), nanorods (NRs), and nanoplates (NPs). It demonstrates the fast separation of charge carriers and localization of both charges in adjacent semiconductors, resulting in the formation of a charge-separated (CS) state. The lifetime of the charge-separated state follows the order of NSs < NPs < NRs, emphasizing the effect of morphology on the enhancement of photo-induced charge separation and suppression of backward recombination. The separated charge carriers have been utilized in visible light driven hydrogen production and the hydrogen generation activity follows the same order as that for the lifetime of the CS state, underlining the role of charge separation efficiency. Therefore, the variation of the morphology of NHSs plays a significant role in their charge carrier dynamics and hence the photocatalytic hydrogen production activity.

Synthetic Ligand Selection Affects Stoichiometry, Carrier Dynamics, and Trapping in CuInSe2 Nanocrystals

CuInSe₂ nanocrystals exhibit tunable near-infrared bandgaps that bolster utility in photovoltaic applications as well as offer potential as substitutes for more toxic Cd- and Pb-based semiconductor compositions. However, they can present a variety of defect states and unusual photophysics. Here, we examine the effects of ligand composition (oleylamine, diphenylphosphine, and tributylphosphine) on carrier dynamics in these materials. Via spectroscopic measurements such as photoluminescence and transient absorption, we find that ligands present during the synthesis of CuInSe₂ nanocrystals impart nonradiative electronic states which compete with radiative recombination and give rise to low photoluminescence quantum yields. We characterize the nature of these defect states (hole vs electron traps) and investigate whether they exist at the surface or interior of the nanocrystals. Carrier lifetimes are highly dependent on ligand identity where oleylamine-capped nanocrystals exhibit rapid trapping (<20 ps) followed by diphenylphosphine (<500 ps) and finally tributylphosphine (>2 ns). A majority of carrier population localizes at indium copper antisites (electrons), copper vacancies (holes), or surface traps (electrons and/or holes), all of which are nonemissive.

Carbon-Based CsPbBr3 Perovskite Solar Cells: All-Ambient Processes and High Thermal Stability

The device instability has been an important issue for hybrid organic-inorganic halide perovskite solar cells (PSCs). This work intends to address this issue by exploiting inorganic perovskite (CsPbBr₃) as light absorber, accompanied by replacing organic hole transport materials (HTM) and the metal electrode with a carbon electrode. All the fabrication processes (including those for CsPbBr₃ and the carbon electrode) in the PSCs are conducted in ambient atmosphere. Through a systematical optimization on the fabrication processes of CsPbBr₃ film, carbon-based PSCs (C-PSCs) obtained the highest power conversion efficiency (PCE) of about 5.0%, a relatively high value for inorganic perovskite-based PSCs. More importantly, after storage for 250 h at 80 °C, only 11.7% loss in PCE is observed for CsPbBr₃ C-PSCs, significantly lower than that for popular CH₃NH₃PbI₃ C-PSCs (59.0%) and other reported PSCs, which indicated a promising thermal stability of CsPbBr₃ C-PSCs.

Colloidal Precursor-Induced Growth of Ultra-Even CH3NH3PbI3 for High-Performance Paintable Carbon-Based Perovskite Solar Cells

Carbon-based hole transport material (HTM)-free perovskite solar cells (PSCs) have attracted intense attention due to their relatively high stability. However, their power conversion efficiency (PCE) is still low, especially for the simplest paintable carbon-based PSCs (C-PSCs), whose performance is greatly limited by poor contact at the perovskite/carbon interface. To enhance interface contact, it is important to fabricate an even-surface perovskite layer in a porous scaffold, which is not usually feasible due to roughness of the crystal precursor. Herein, colloidal engineering is applied to replace the traditional crystal precursor with a colloidal precursor, in which a small amount of dimethyl sulfoxide (DMSO) is added into the conventional PbI₂ dimethylformamide (DMF) solution. After deposition, PbI₂(DMSO) adduct colloids (which are approximately tens of nanometers in size) are stabilized and dispersed in DMF to form a colloidal film. Compared with PbI₂ and PbI₂(DMSO) adduct crystal precursors deposited from pure DMF and DMSO solvents, respectively, the PbI₂(DMSO) adduct colloidal precursor is highly mobile and flexible, allowing an ultra-even surface to be obtained in a TiO₂ porous scaffold. Furthermore, this ultra-even surface is well-maintained after chemical conversion to CH₃NH₃PbI₃ in a CH₃NH₃I solution. As a result, the contact at the CH₃NH₃PbI₃/carbon interface is significantly enhanced, which largely boosts the fill factor and PCE of C-PSCs. Impressively, the achieved champion PCE of 14.58% is among the highest reported for C-PSCs.

Templating growth of gold nanostructures with a CdSe quantum dot array

In optoelectronic devices based on quantum dot arrays, thin nanolayers of gold are preferred as stable metal contacts and for connecting recombination centers. The optimal morphology requirements are uniform arrays with precisely controlled positions and sizes over a large area with long range ordering since this strongly affects device performance. To understand the development of gold layer nanomorphology, the detailed mechanism of structure formation are probed with time-resolved grazing incidence small-angle X-ray scattering (GISAXS) during gold sputter deposition. Gold is sputtered on a CdSe quantum dot array with a characteristic quantum dot spacing of ≈7 nm. In the initial stages of gold nanostructure growth, a preferential deposition of gold on top of quantum dots occurs. Thus, the quantum dots act as nucleation sites for gold growth. In later stages, the gold nanoparticles surrounding the quantum dots undergo a coarsening to form a complete layer comprised of gold-dot clusters. Next, growth proceeds dominantly via vertical growth of gold on these gold-dot clusters to form an gold capping layer. In this capping layer, a shift of the cluster boundaries due to ripening is found. Thus, a templating of gold on a CdSe quantum dot array is feasible at low gold coverage.

Luminescent CdTe quantum dots incarcerated in a columnar matrix of discotic liquid crystals for optoelectronic applications

The effects of highly luminescent alkylamine-capped semiconductor cadmium telluride quantum dots (CdTe QDs) dispersion on the optical, electrical, thermal properties and supramolecular order of a discotic liquid crystal were studied.

Synthesis, characterization and electrochemical properties of meso-thiocarboxylate-substituted porphyrin derivatives

Considering the versatility of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPPF20) to react with nucleophiles we highlight here the synthesis and characterization of several mono- and tetra-thiocarboxylate derivatives. The selective displacement of the para-fluorine groups in TPPF20 by thiocarboxylic acids demonstrates that TPPF20 is an ideal platform for the rapid formation of thiocarboxylate porphyrins. The optical and electrochemical features of the thiocarboxylate derivatives were also examined thinking on their potential use in photovoltaic devices. From their electrochemical characterization the following parameters were taken into account: (i) electronegative induced effect of the thiocarboxylate dyes owing the presence of the fluorine and sulfur atoms on the molecular structure of the porphyrin; and (ii) the free rotation and flexibility features that such S atom gives to the porphyrin relatively to the semiconductor.

A comprehensive review of the application of chalcogenide nanoparticles in polymer solar cells

In this review the use of solution-processed chalcogenide quantum dots (CdS, CdSe, PbS, etc.) in hybrid organic-inorganic solar cells is explored. Such devices are known as potential candidates for low-cost and efficient solar energy conversion, and compose the so-called third generation solar cells. The incorporation of oxides and metal nanoparticles has also been successfully achieved in this new class of photovoltaic devices; however, we choose to explore here chalcogenide quantum dots in light of their particularly attractive optical and electronic properties. We address herein a comprehensive review of the historical background and state-of-the-art comprising the incorporation of such nanoparticles in polymer matrices. Later strategies for surface chemistry manipulation, in situ synthesis of nanoparticles, use of continuous 3D nanoparticles network (aerogels) and ternary systems are also reviewed.

Semiconductor quantum dot-sensitized solar cells

Semiconductor quantum dots (QDs) have been drawing great attention recently as a material for solar energy conversion due to their versatile optical and electrical properties. The QD-sensitized solar cell (QDSC) is one of the burgeoning semiconductor QD solar cells that shows promising developments for the next generation of solar cells. This article focuses on recent developments in QDSCs, including 1) the effect of quantum confinement on QDSCs, 2) the multiple exciton generation (MEG) of QDs, 3) fabrication methods of QDs, and 4) nanocrystalline photoelectrodes for solar cells. We also make suggestions for future research on QDSCs. Although the efficiency of QDSCs is still low, we think there will be major breakthroughs in developing QDSCs in the future.