Energy Selective Contacts Research Articles

Design and characterization of colloidal quantum dot photoconductors for multi-color mid-infrared detection

This report paves the way for low cost, multi-color, room-temperature, and easily-feasible mid-IR photodetector with distinct output currents. The proposed device is designed and simulated to be fabricated by the solution-process technology thanks to its simplicity and fabrication ease, room-temperature operation capability, low production cost, and excellent photoelectric properties. In this approach, the proposed structure is composed of an array of n × n pixels, where each pixel is designed to enable multi-wavelength detection. The absorber layer consists of different sized quantum dots that absorb distinct wavelengths simultaneously through interband transitions in the mid-infrared region. Selective energy contacts based on quantum well structures are designed to be deposited on interdigitated contacts as a means of extracting the stimulated carriers from the absorber layers to the metal contacts. Through resonant tunneling, the excited carriers are transferred, which generates the output photocurrent and significantly reduces the dark current. In order to model the designed multi-color photoconductor, coupled rate equations and three-dimensional Schrodinger-Poisson equations have been developed and solved self-consistently. As a way of demonstrating the validity of the introduced model, a experimentally fabricated photoconductor and a theoretically modeled photodetector using the nonequilibrium Green’s function (NEGF) model have been simulated; the comparison between the simulation results is quite satisfactory. In order to simplify the theoretical model, two different sizes of InSb/ZnSe core/shell quantum dots have been considered in the theoretical model to detect two MIR wavelengths (3 µm and 4 µm) of the incident light. Simulation results indicate that the peak responsivities are about 3.3(A/W) and 6(A/W), and the high specific detectivities D* are about 9 × 1010(cm.Hz1/2W−1) and 15 × 1010(cm.Hz1/2W−1) for the wavelengths of 4 µm and 3 µm, respectively.

Electron and hole relaxation effect on power conversion efficiency of selective energy contacts solar cells

Electron and hole relaxation effect on power conversion efficiency of selective energy contacts solar cells

Hot carrier-based near-field thermophotovoltaics with energy selective contacts

A model of the thermophotovoltaic device combining a near-field thermal emitter and a hot-carrier solar cell is established. The fluctuating electromagnetic near-field theory for the radiative thermal transport and Landauer's formula for the carrier extraction are introduced. Expressions for the efficiency and the power output of the device are derived. How the voltage and the extraction energy of the energy selective contacts affect the performance of the device is revealed. The results show that the efficiency of the proposed device can be greatly enhanced by exploiting the radiation between the emitter and the cell and extracting carriers through electron tunneling effects.

Thermodynamics of the Monoenergetic Energy-Selective Contacts of a Hot-Carrier Solar Cell

The hot carrier solar cell (HCSC) has the potential for converting solar energy into electrochemical energy with an efficiency of 85.4%. For this, in addition to an idealized light absorber, the HCSC has to be connected to the external load by means of the so-called \emph{mono-energetic energy selective contacts} (ESCs). However, the thermodynamic properties that these types of contact have to exhibit, such as their electric, thermal conductivity and Seebeck coefficient, have not been explored. This paper aims to fill this gap. In this respect, we model electron transport in non-ideal ESCs using the transport theory proposed by Datta and Landauer which has allowed us to calculate the value of these parameters as a function of the temperature and electrochemical potential of operation. Our findings also reveal that, to preserve the HCSC efficiency above 82%, the ESCs could require in the order of $3 \times 10^{19}$ cm$^{-3}$ electron states. As the ESCs depart from ideality, the temperature of the hot carriers at which optimum efficiency is obtained increases to above 2540 K. The mono-enenergetic selective contact characterized by the highest energy demands an electric, thermal conductivity and Seebeck coefficient that, when combined, are characterized by a high thermoelectric figure of merit $(ZT\approx 8)$. We are not aware of any material exhibiting this figure of merit which illustrates the difficulty in putting the HCSC concept into practice. Conversely, our work supports the idea that pursuing materials capable of transporting electrons ballistically through mono-energetic electron channels can provide the key for achieving materials characterized by high $ZT$

Impact of excitation energy on hot carrier properties in InGaAs multi‐quantum well structure

AbstractHot carrier solar cells aim to overcome the theoretical limit of single‐junction photovoltaic devices by suppressing the thermalization of hot carriers and extracting them through energy selective contacts. Designing efficient hot carrier absorbers requires further investigation on hot carrier properties in materials. Although the thermalization of hot carriers is responsible for a large portion of energy loss in solar cells, it is still one of the least understood phenomena in semiconductors. Here, the impact of excitation energy on the properties of photo‐generated hot carriers in an InGaAs multi‐quantum well (MQW) structure at various lattice temperatures and excitation powers is studied. Photoluminescence (PL) emission of the sample is detected by a hyperspectral luminescence imager, which creates spectrally and spatially resolved PL maps. The thermodynamic properties of hot carriers, such as temperature and quasi‐Fermi‐level splitting, are carefully determined via applying full PL spectrum fitting, which solves the Fermi–Dirac integral and considers the band‐filling effect in the nanostructured material. In addition, the impact of thermalized power density and carrier scattering with longitudinal optical phonons on the spectral linewidth broadening under two excitation energies is studied.

Overcoming the temperature effect on a single junction and intermediate band solar cells using an optical filter and energy selective contacts

Overcoming the temperature effect on a single junction and intermediate band solar cells using an optical filter and energy selective contacts

Solar cells efficiency enhancement using multilevel selective energy contacts (SECs)

High-energy incoming photons can be absorbed and concluded to generate Hot Carriers. In normal solar cells, these carriers are scattered by electron–electron and electron-lattice mechanisms and rapidly lose extra energy and approach to conduction band energy edge. This event plus other loss mechanisms cause that the efficiency of the solar cells to be limited to 33% theoretically. If one makes the possibility for carriers that can be extracted rapidly, before scattering and releasing extra energy to the lattice, the efficiency of solar cells is enhanced considerably. This type of solar cell is named hot carrier solar cells (HCSCs). To this end and improvement the conversion efficiency, multilevel energy selective contacts (ESCs) as a new concept and new mechanism in solar cells are used. In the other words, several appropriate energy levels as carrier extraction contacts in the conduction band are introduced. Here, we use multilevel ESCs, and based on our simulation it is shown that the maximum efficiency of 75% is achievable for low bandgap materials. For a typical material such as Si, the maximum efficiency is increased to 60% using ten ESCs.

Selective Energy Contacts and Multi-Wavelength Solution-Processed Quantum Dot Infrared Photodetector

In this work, a three-channel infrared photodetector with three independent related electric signals as a single optoelectronic chip has been introduced. The proposed structure has been implemented by exploiting an array of PbSe/ZnSe quantum dots and the PbSe/ZnS selective energy contacts considering solution-processed nanotechnology. The electron transport system has been designed to allow the transmission of electrons stimulated by three wavelengths simultaneously from different paths to the corresponding output. The high efficiency, simplicity and fabrication ease, cost-effectiveness, and high tunability of the photodetector have been accomplished as a consequence of solution-process technology. Moreover, the dark current has been significantly declined as a consequence of resonance tunneling in the proposed selective energy contacts. Last but not least, our proposed multi-wavelength ( λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> =2.9 μm, λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> =2.25 μm, λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> =1.75 μm) infrared photodetector has been promising candidates for a wideband spectrum analyzer or multi-wavelength infrared camera.

Radiation stability of mixed tin–lead halide perovskites: Implications for space applications

Mixed organic-inorganic halide perovskite solar cells (PSCs) have received considerable attention in recent years for their impressive solar to electrical power efficiency gains and potentially lower material and processing costs for optoelectronic applications. In addition to terrestrial applications, PSCs are of interest to the space power markets for their low cost, low weight, adaptability to flexible architectures, and tolerance to high energy particle irradiation (mainly protons and electrons). Here we investigate the properties of mixed formamidinium-methylammonium tin-lead iodide (FASn)0.6(MAPb)0.4I3 perovskites which lower the lead content as well as the bandgap, making them attractive for the low bandgap absorber material in tandem PSCs. Through current density-voltage (JV) characterization at lower temperatures, majority carrier transport is hindered and a barrier to photogenerated carrier extraction is evident. This is attributed to the thermally induced change of the bandgap of the absorber layer relative to the energy selective contacts in the device. We find that although the architecture used here hinders the performance at temperatures below 225 K, the tolerance to high energy (3.7 MeV) protons is impressive, considerably out-performing commercially available thin-film CIGS. These results suggest further improvements to structural and interface stability as well as lightweight encapsulation could lead to all perovskite flexible tandem arrays deployed for power generation on missions to low Earth orbit, the moon, Mars, and beyond.

Hot‐carrier solar cells and improved types using wide‐bandgap energy‐selective contacts

AbstractI evaluated conversion efficiency of hot‐carrier solar cells (HC‐SCs) of the original type and two improved types: an HC‐SC with intraband transition (HC‐SC+intra) and an intermediate‐band‐assisted HC‐SC (IB‐HC‐SC) that utilize sub‐bandgap photons. For this purpose, three realistic points of great importance were involved in the newly constructed detailed‐balance model: Shockley–Read–Hall (SRH) recombination of photogenerated carriers, characteristics of the energy‐selective contacts (ESCs), and solar spectrum variation. I have revealed that the HC‐SC+intra and IB‐HC‐SC can potentially yield annual electricity production comparable to those of triple‐junction and quad‐junction solar cells (3J‐ and 4J‐SCs) when sub‐bandgap photons are almost perfectly absorbed. The ESCs consisting of wide‐bandgap layers extract the photogenerated carriers more rapidly owing to their larger conductance and consequently yield higher conversion efficiency than the ESCs using resonant tunneling diodes composed of quantum dots and quantum wells. This, in turn, enables the use of an absorber with a short SRH recombination time of the carriers. In other words, the efficiency is less sensitive to SRH recombination than those of the 3J‐ and 4J‐SCs. In particular, for the IB‐HC‐SC, the requisite of the SRH recombination time is 1 ns, and that of the thermalization time is 0.1 ns; the latter is an order of magnitude shorter than the previous requisite. These HC‐SCs are more robust against the solar spectrum variation because they are free from the current‐matching problem unlike the 3J‐ and 4J‐SCs, which also contributes to the large annual electricity production.

Efficiency enhancement in a single bandgap silicon solar cell considering hot-carrier extraction using selective energy contacts.

One of the most important and challenging loss factors of photovoltaics is the heat production of energetic carriers excited by high energy incident photons. The present work shows that if carriers are extracted at their high energies before cooling down due to scattering, the conversion efficiency can be noticeably enhanced. To increase the efficiency of a single-band gap solar cell in this work, selective energy contacts are introduced to a p-i-n structure to extract hot carriers. A selective energy contact solar cell is made up of many collecting contacts with particular energy differences from the conduction band of the cell. In other words, each contact could extract carriers with a special range of energies. The concept of selective energy contact solar cells is to collect high energy carriers, i.e. electrons in this case, within a range of energies onto external electrodes before they cool down. The comparison between conventional solar cells and selective energy contact solar cells shows a significant enhancement in electron collection and efficiency. Based on simulation results, it is observed that the efficiency of the selective energy contact solar cell has been enhanced substantially exceeding almost twice as much as a conventional solar cell's and reaching a significant 34% efficiency.

Optoelectronic reciprocity in hot carrier solar cells with ideal energy selective contacts

AbstractHot carrier solar cells promise theoretical power conversion efficiencies far beyond the single junction limit. However, practical implementations of hot carrier solar cells have lagged far behind those theoretical predictions. Reciprocity relations for electroluminescence from conventional single junction solar cells have been extremely helpful in driving their efficiency ever closer to the theoretical limits. In this work, we discuss how the signatures of a functioning hot carrier device should manifest experimentally when driven in reverse, that is, in electroluminescent mode. Hot carrier properties lead to deviations of the dark I–V from the Shockley diode equation that is typical for conventional single junction solar cells. These deviations are directly linked to an increase in temperature of the carriers and therefore the temperature measured from electroluminescence spectra. We also elucidate how the behaviour of hot carrier solar cells in the dark depends on whether Auger processes play a significant role, revealing a stark contrast between the regime of negligible Auger recombination (carrier conservation model) and dominant Auger recombination (impact ionisation model) for hot carrier solar cells.

Hafnium and zirconium nitrides with rock-salt and Th3P4 structures: electronic and phonon band structure calculations to examine hot carrier solar cell and thermoelectric properties

Hafnium and zirconium mononitrides (HfN and ZrN) have been of interest for the hot carrier solar cell (HCSC) concept as a bulk absorber material and in thermoelectric power generation as a superlattice structure. Slowed hot carrier (HC) cooling is a fundamental requirement in an HCSC device because the carriers must be collected at high energy levels via energy selective contacts before they cool down to the band edges. However, achieving longer thermalization time is challenging in bulk materials. Here, we have calculated electronic and phonon properties of rock-salt HfN and ZrN and the nitrogen-rich thorium phosphide (Th3P4) structures: c-Hf3N4 and c-Zr3N4 using a first principle density functional theory method. Electronic and phonon band structures of alloyed Hf and Zr in the nitrogen-rich phases are presented for the first time. Our calculations show that the mononitrides of Hf and Zr have wide optical phonon (OP) to acoustic phonon (AP) band gap and simultaneous smaller OP dispersion that have the potential to slow down the HCs cooling rates, making them suitable for an HCSC absorber. Further, the results provide a first step towards exploring both HCSC and thermoelectric properties in metal/semiconductor superlattice structure of nitrides of Hf and Zr only.

Switchable Multi-Color Solution-Processed QD-laser

In this paper, for the first time, the switchable two-color quantum dot laser has been realized considering solution process technology, which has both simultaneous and lonely lasing capability exploiting selective energy contacts. Furthermore, both channels can be modulated independently, which is a significant feature in high-speed data transmission. To this end, utilizing superimposed quantum dots with various radii in the active layer provides the different emission wavelengths. In order to achieve the different sizes of QDs, solution process technology has been used as a cost-effectiveness and fabrication ease method. Moreover, at the introduced structure to accomplish the idea, the quantum wells are used as separate selective energy contacts to control the lasing channels at the desired wavelength. It makes the prominent device have simultaneous lasing at different emission wavelengths or be able to lase just at one wavelength. The performance of the proposed device has been modeled based on developed rate equation by assuming inhomogeneous broadening of energy levels as a consequence of the size distribution of quantum dots and considering tunnel injection of carriers into the quantum dots via selective energy contacts. Based on simulation results, the simultaneous lasing in both or at one of two wavelengths 1.31 μm and 1.55 μm has been realized by the superimposition of two different sizes of InGaAs quantum dots in a single cavity and accomplishment of selective energy contacts. Besides, controlling the quantum dot coverage leads to managing the output power and modulation response at the desired wavelengths. By offering this idea, one more step is actually taken to approach the switchable QD-laser by the simple solution process method.

Performance assessment and optimization of a hot carrier solar cell with double energy selective contacts

A novel model of the hot carrier solar cell (HCSC) with double energy selective contacts (ESCs) is established. Expressions of the efficiency and power output are derived analytically. The effects of the cell voltage, extraction energy level, width of the ESCs, band gap energy of the absorber, and energy gap between the two contacts on the performance of the HCSC with double quantum dot (QD)- and quantum well (QW)-ESCs are examined. The results obtained show that the HCSC with double QD- and QW-ESCs can obtain a high efficiency. When the parameters of the HCSC are optimized, the maximum efficiency of the HCSC with double QD-ESCs is higher than that with double QW-ESCs. Moreover, the maximum efficiencies of the HCSC with double QD- and QW-ESCs decrease with the increase of the energy gap between the two contacts.

Potential of HfN, ZrN, and TiH as hot carrier absorber and Al2O3/Ge quantum well/Al2O3 and Al2O3/PbS quantum dots/Al2O3 as energy selective contacts

The hot carrier (HC) solar cell is one of the most promising advanced photovoltaic concepts. It aims to minimise two major losses in single junction solar cells due to sub-band gap loss and thermalisation of above band gap photons by using a small bandgap absorber, and, importantly, collecting the photo-generated carriers before they thermalise. In this paper we will present recent development of the two critical components of the HC solar cell, i.e., the absorber and energy selective contacts (ESCs). For absorber, fabrication and carrier cooling rates in potential bulk materials — hafnium nitride, zirconium nitride, and titanium hydride are presented. Results of ESCs employing double barrier resonant tunneling structures Al2O3/Ge quantum well (QW)/Al2O3 and Al2O3/PbS quantum dots (QDs)/Al2O3 are also presented. These results are expected to guide further development of practical HC solar cell devices.

Fabrication of double barrier structures in single layer c-Si–QDs/a-SiOx films for realization of energy selective contacts for hot carrier solar cells

Thin films of c-Si–QDs embedded in an a-SiOx dielectric matrix forming arrays of double barrier structures have been fabricated by reactive rf-magnetron sputtering at ∼400 °C, without post-deposition annealing. The formation of larger size c-Si–QDs of reduced number density in homogeneous distribution within a less oxygenated a-SiOx matrix at higher plasma pressure introduces systematic widening of the average periodic distance between the adjacent ‘c-Si–QDs in a-SiOx’, as obtained by X-ray reflectivity and transmission electron microscopy studies. A wave-like pattern in the J-E characteristics identifies the formation of periodic double-barrier structures along the path of the movement of charge carriers across the QDs and that those are originated by the a-SiOx dielectric matrix around the c-Si–QDs. A finite distribution of the size of c-Si–QDs introduces a broadening of the current density peak and simultaneously originates the negative differential resistance-like characteristics, which have suitable applications in the energy selective contacts that act as energy filters for hot carrier solar cells. A simple yet effective process technology has been demonstrated. Further initiative on tuning the energy selectivity by reducing the size and narrowing the size-distribution of Si–QDs can emerge superior energy selective contacts for hot carrier solar cells, paving ground for accomplishing all-Si solar cells.

Insights on energy selective contacts for thermal energy harvesting using double resonant tunneling contacts and numerical modeling

Energy selective electrical contacts have been proposed as a way to approach ultimate efficiencies both for thermoelectric and photovoltaic devices as they allow a reduction of the entropy production during the energy conversion process. A self-consistent numerical model based on the transfer matrix approach in the effective mass and envelope function approximation has been developed to calculate the electronic properties of double resonant tunneling barriers used as energy selective contacts in hot carrier solar cells. It is found that the application of an external electric bias significantly degrades the electronic transmission of the structure, and thus the tunneling current in the current-voltage characteristic. This is due to a symmetry breaking which can be offset using finely tuned asymmetric double resonant tunneling barriers, leading to a full recovery of the tunneling current in our model. Moreover, we model the heterostructure using electrons temperature in the emitter higher than that of the lattice, providing insights on the interpretation of experimental devices functioning in hot carrier conditions, especially regarding the previously reported shift of the resonance peak (negative differential resistance), which we interpret as related to a shift in the hot electron distribution while the maximum remains at the conduction band edge of the emitter. Finally, experimental results are presented using asymmetric structure showing significantly improved resonant properties at room temperature with very sharp negative differential resistance.

Photo-thermionic effect in vertical graphene heterostructures.

Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising approach is to drive a current from the thermal energy added to the free-electron bath as a result of light absorption. Successful implementation of this strategy requires a broadband absorber where carriers interact among themselves more strongly than with phonons, as well as energy-selective contacts to extract the excess electronic heat. Here we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath leading to a thermalized hot carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast.

Resonant tunneling diodes as energy-selective contacts used in hot-carrier solar cells

Among the four features unique to hot-carrier solar cells (HC-SCs): (i) carrier thermalization time and (ii) carrier equilibration time in the absorber, (iii) energy-selection width and (iv) conductance of the energy-selective contacts (ESCs), requisites of (i)-(iii) for high conversion efficiency have been clarified. We have tackled the remaining issues related to (iv) in the present study. The detailed balance model of HC-SC operation has been improved to involve a finite value of the ESC conductance to find the required values, which in turn has been revealed to be feasible using resonant tunneling diodes (RTDs) consisting of semiconductor quantum dots (QDs) and quantum wells (QWs) by means of a formulation to calculate the conductance of the QD- and QW-RTDs derived using the rigorous solutions of the effective-mass Hamiltonians. Thus, all of the four requisites unique to HC-SCs to achieve high conversion efficiency have been elucidated, and the two requisites related to the ESCs can be fulfilled using the QD- and QW-RTDs.