Getting high with quantum dot solar cells

Progress in device engineering and surface passivation strategies has led to steady progress in colloidal quantum dot (QD) solar cells. Bulk homojunction (BHJ) device architecture has several advantages over the conventional planar junction in developing QD solar cells. Herein, surface ligand chemistry is utilized to control the doping type and dispersibility of oppositely doped PbS QDs to develop BHJ solar cells. Thiocyanate and thiol ligand combination is introduced to develop p‐PbS QD ink, which is blended with halide‐passivated n‐PbS QDs to build BHJ solar cells. It is shown that BHJ solar cells are benefited from high energy offset and higher hole mobility. This leads to the superior carrier extraction from a thicker active layer without compromising fill factor and open circuit voltage. Power conversion efficiency has reached 10.7% in 530 nm‐thick BHJ solar cells, a 24% improvement over the best performing planar solar cells. With the help of the 1D solar cell capacitance simulator, it is shown that a 15% efficient QD solar cell can be realized by further improving the hole mobility above 0.1 cm2 V−1 s−1.

We investigated the effects of doping on the photovoltaic efficiency in a GaAs reference cell, and in undoped, n-doped, and p-doped InAs/GaAs quantum-dot (QD) solar cells. We found that the photovoltaic efficiency of the undoped QD solar cell is almost the same as that of the reference cell. However, the efficiency improves monotonically with increasing inter-dot ndoping, while p-doping deteriorates the photovoltaic conversion. We observed a 50 % increase in photovoltaic efficiency in the device n-doped to provide approximately six electrons per dot as compared with the undoped QD cell. In this QD solar cell, the short circuit current density increases to 24.30 mA/cm 2 compared with 15.07 mA/cm 2 in the undoped QD solar cell without deterioration of the open circuit voltage. To identify the physical mechanisms that provide this improvement, we investigated the spectral characteristics of the photovoltaic response and photoluminescence of our QD solar cells. We found that the electron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The electrons trapped in dots enhance IR transitions. The built-in-dot electron charge together with charged dopants outside the dots creates potential barriers, which suppress the fast electron capture processes and at the larger scale form a potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and a radical improvement of the QD solar cell efficiency. Higher efficiencies are anticipated with further increase of doping level and at higher radiation intensity. This makes the QD solar cells promising candidates for use with concentrators of solar radiation.

We studied time-resolved carrier recombination in InAs/GaAs quantum dot (QD) solar cells. The electric field in a p-i-n diode structure spatially separates photoexcited carriers in QDs, strongly affecting the conversion efficiency of intermediate-band solar cells. The radiative decay lifetime is dramatically reduced in a strong electric field (193 kV/cm) by efficient recombination due to strong carrier localization in each QD and significant tunneling-assisted electron escape. Conversely, an electric field of the order of 10 kV/cm maintains electronic coupling in the stacked QDs and diminishes tunneling-assisted electron escape.

Sputtered transparent electrodes for optoelectronic devices: Induced damage and mitigation strategies

The nanoscale properties of CsPbI 3 quantum dots allows for the advantageous manipulation of its phase diagram. We show that these all-inorganic halide perovskite nanocrystals can be used to fabricate colloidal perovskite quantum dot solar cells with low voltage losses. Despite only recently emerging on the photovoltaic scene, CsPbI 3 quantum dot solar cells have already achieved approximately 85% of their Shockley-Quessier limit open-circuit voltage, among the lowest voltage loss achieved for any material with a band gap of 1.7 eV, excepting III-V semiconductors. The low losses in these devices recently led to a world record certified quantum dot solar cell efficiency of 13.4%. While single junction thin film perovskite solar cells have surpassed 22% power conversion efficiency, perovskite-perovskite tandem solar cells offer the potential for power conversion efficiencies exceeding 30%. However, to accomplish this ambitious goal, perovskite solar cells must be designed with low losses and the appropriate band gaps, which differ from the ideal band gap for single junction perovskite solar cells. In this respect, the incredibly low voltage loss and tunable band gap of CsPbI 3 quantum dots make them ideally suited for all-perovskite tandem solar cell applications.

Quantum Dot Solar Cells

We report In0.4Ga0.6As quantum dot (QD) solar cells with In0.2Ga0.8As cap layers, which extends the photoabsorption spectra toward a wavelength longer than those of In0.4Ga0.6As QD solar cells without cap layers. Well-aligned 50-stack In0.4Ga0.6As QD structures with In0.2Ga0.8As cap layers can be grown without using a strain balancing technique. The photoluminescence wavelength of ten-stack In0.4Ga0.6As QDs with an In0.2Ga0.8As cap layer becomes longer, as a result of the reduced strain in the QDs achieved by using the cap layer. The cell characteristics of multistacked In0.4Ga0.6As QD solar cells are improved by employing In0.2Ga0.8As cap layers.

Ten-layer InAs/GaAs quantum dot (QD) solar cells exhibiting enhanced short circuit current (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> ) and open circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> ) comparable to a control GaAs p-i-n solar cell are reported. One-sun J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> is enhanced by 3.5% compared to that of the GaAs control, while the open circuit voltage is maintained at 994 mV. Results were achieved using optimized InAs QD coverage and a modified strain balancing technique, resulting in a high QD density (3.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ), uniform QD size (4 × 16 nm) and low residual strain (103 ppm). The suppression of larger, coalesced InAs islands is shown to reduce non-radiative recombination in the depletion region, reducing dark current densities. The resulting enhanced open circuit voltage is a promising result for the future of InAs QD-enhanced GaAs solar cells. The extrapolation of short circuit current density enhancement with increasing quantum dot layers is evaluated here and potential efficiencies exceed baseline GaAs values. In the final work, a twenty-period QD will be grown. The QD growth improvement used here will be applied in this case assuming a linear enhancement in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> and a continued suppression of voltage degradation typical of these devices. The efficiency value of this device may show the potential to exceed that of a comparable baseline GaAs cell. In order to further evaluate carrier dynamics, temperature dependent PL will be used to extract activation energies with particular focus on barrier layer material. In addition, the choice of depth placement in the i-region has been shown to give varied device results in QW solar cells. This will also be in focus for QD solar cells, as these devices continue to be studied and optimized for photovoltaic energy conversion.

Various solar cells and materials have great potential as future generation solar cells and materials including crystalline Si. Si tandem solar cells such as III-V/Si tandem, II-VI/Si tandem, Chalcopyrite/Si tandem and Perovskite/Si tandem solar cells are expected to have great potential of space and terrestrial application because of high efficiency, light weight and low cost potential. Quantum and Nano structures such as MQWs ‘(Multi Quantum Wells) and QDs (Quantum Dots) are also expected as high-efficiency solar cells. However, there are some problems to be solved. Because point defects in crystalline Si, high density of dislocations and interface recombination in Si tandem solar cell materials and interface recombination in MQWs and QDs affect on solar cell properties, understanding and controlling those defects are very important for realizing higher efficiency. This paper presents analytical results for high efficiency potential of single crystalline Si solar cells, Si tandem solar cells, and MQW and QD solar cells. Although 25.6% efficiency has been attained with hetero-junction and back contact structure and by using bulk minority-carrier lifetime of around 10 msec, high efficiency of more than 27.5% is expected to be realized by increasing bulk lifetime of more than 100 msec Si. Really, high bulk lifetime of around 100 msec has been obtained by the MCZ (Magnetic-field-applied Czochralski) method and decreasing carbon concentration to less than 1014 cm-3 in Si. Understanding behavior of point defects such as Cs-Oi and so forth in Si and decreasing those defects density are suggested for realizing higher bulk lifetime and higher efficiency Si solar cells of more than 27.5%. High efficiency potential of Si tandem solar cells such as III-V/Si tandem, II-VI/Si tandem, Chalcopyrite/Si tandem and Perovskite/Si tandem solar cells is also analyzed by using measure of external radiative efficiency: 43% for III-V/Si 3-J(Junction) tandem, 38% for III-V/Si 2-J tandem, 33% for II-V/Si 2-J tandem, 32% for Chalcopyrite/Si tandem, and 32% for Perovskite/Si tandem solar cells. Effects of dislocations upon minority-carrier lifetime in III-V/Si tandem solar cells are also presented. Our recent result for mechanically stacked InGaP/GaAs//Si 3-junction tandem solar cell with efficiency of 27.4% is shown. Although MQW and QD solar cells are expected as high efficiency solar cells, one of big problems is higher interface recombination near MQWs and QDs because of their lattice mismatching. Lattice mismatch (Δa/a0) dependence of interface recombination velocity (SI) in MQW and QD solar cells is analyzed. Open-circuit voltage drop by interface recombination in MQWs and QDs and their efficiency potential is discussed.

Quantum Dot (QD) Solar Cells have emerged as promising candidates for next-generation photovoltaic technologies due to their tunable bandgaps, solution processability, and potential for low-cost fabrication. However, their photovoltaic performance is often limited by factors such as low light absorption and charge carrier recombination. Plasmonic nanostructures, which exploit the resonant oscillation of conduction electrons in metallic nanoparticles, offer a viable strategy to enhance light absorption and improve charge carrier dynamics in QD solar cells. This paper reviews the integration of plasmonic nanostructures with QD solar cells, examining the mechanisms by which plasmonics can enhance photovoltaic performance. We discuss various types of plasmonic materials, their configurations within solar cell architectures, and the resultant improvements in efficiency. Additionally, the challenges and future prospects of plasmonic-enhanced QD solar cells are explored.

Ligand engineering of perovskite quantum dots for efficient and stable solar cells

Herein, we demonstrate for the first time matrix-free deposition of two dimensional (2D) MoS2 nanosheets as an efficient hole transport layer (HTL) for colloidal lead sulfide (PbS) quantum dot (QD) solar cells. We have developed all-solution-processed n–p–p+ architecture solar cells where ZnO nanoparticles were used as an n-type window layer, a PbS QD layer acted as a light absorbing p-type layer and 2D-MoS2 nanosheets acted as a p+-type hole transport layer. The MoS2 nanosheets allow better interface with the PbS QD layers. The incorporation of the MoS2 hole transport layer leads to superior fill factor, higher open circuit voltage and better performance in colloidal PbS QD solar cells. These results show that one layer of MoS2 nanosheets improves the power conversion efficiency of the device from 0.92% for a hole transport material free device to 2.48%. The present work reveals the development of 2D-MoS2 nanosheets as a new hole transport layer for the fabrication of cost-effective, durable and efficient colloidal PbS quantum dot solar cells.

Multi-stacked GaSb/GaAs quantum dot (QD) solar cells are theoretically studied by two-dimensional simulation with the implementation of Poisson’s equation, continuity equations, drift-diffusion transport model and thermionic emission model. Such physical models are numerically solved by finite element method. QD layers are inserted into the i-region of the GaAs p-in structure. Each QD layer is separated by a GaAs spacer layer. Effects of spacer layer thickness and number of QD layers on the solar cell characteristics are investigated. Spacer layer thickness also determines on the distribution of hole wave functions which are altered by strain and impact absorption properties. Thinner spacer layer results in higher strain which shifts the absorption edge towards shorter wavelengths and is responsible for open-circuit voltage enhancement, while thicker spacer layer increases the Shockley-Read-Hall (SRH) recombination that degrades both open-circuit voltage and fill factor. Increasing the number of QD layers leads to higher short-circuit current density and lowers both open-circuit voltage and fill factor which are derived from higher GaSb amount and SRH recombination. The simulation results suggest that the highest efficiency of 25.82% is achieved by using the 40-stacked QD layers with 15-nm spacer layer.

Formamidinium lead iodide quantum dots (QDs) show great potential for solar cell applications but suffer from restricted charge transfer due to the insulating nature of their ligand shell. Management of the surface properties and resultant energy band alignment is the key to efficient and stable QD solar cells. Herein, an advance in QD surface passivation by introducing tailored multifunctional ligands (glycocyamine (GLA)) to the FAPbI 3 QD surface is demonstrated. The incorporation of GLA ligands can partly substitute the native long‐chain insulating ligands and effectively reduce the non‐radiative recombination loss induced by surface trap states. Notably, the introduction of GLA ligands beneficially shifts the band offsets of the QD films and generates a homojunction structure with a cascading energy band alignment in the QD layers, promoting favorable charge transport and boosting the device's performance. As a result, a record‐high power conversion efficiency (PCE) of 15.34% with improved open‐circuit voltage and fill factor is achieved. Moreover, the GLA‐assisted surface passivation boosts the device stability, in which over 80% and 75% of the original PCEs are maintained after storing the devices for 5500 h in ambient air and 768 h under continuous 1‐sun illumination, respectively.

In order to enhance understanding of the short circuit improvement in InAs/GaAs quantum dot (QD) solar cells, the thermally assisted and tunneling mechanisms of carrier escape from the QD quantum confinement are investigated. The dependence of voltage biased spectral responsivity for QD solar cells at room temperature is studied to analyze carrier extraction through tunneling. Photoexcited carrier confinement and escape were also studied by means of temperature dependent spectral response (TDSR) and temperature dependent photoluminescence (TDPL). Energy required to move a carrier from the ground state to the first excited state, thermal activation energy (E a ), in a quantum dot is calculated from TDPL to be 114 meV. It is found that at room temperature carrier escape from the quantum dot confinement is affected by both thermal assisted escape and tunneling while at low temperature tunneling is the dominant in carrier escape from both wetting layer and QDs. For all temperature ranges, carrier exchange between ground states and excited states and carrier escape from ground states (GS) is first thermal escape to excited states (ES) then tunneling