Hot Carrier Solar Cells Research Articles

Hot carrier solar cells by adiabatic cooling

Hot carrier solar cell is proposed where charge carriers are cooled adiabatically in the charge transport layers adjoining the absorber. The device resembles an ideal thermoelectric converter where thermopower and therefore also carrier entropy are maintained constant during cooling from the temperature attained in the absorber to the temperature at contacts.

Extending the defect tolerance of halide perovskite nanocrystals to hot carrier cooling dynamics

Defect tolerance is a critical enabling factor for efficient lead-halide perovskite materials, but the current understanding is primarily on band-edge (cold) carriers, with significant debate over whether hot carriers can also exhibit defect tolerance. Here, this important gap in the field is addressed by investigating how intentionally-introduced traps affect hot carrier relaxation in CsPbX3 nanocrystals (X = Br, I, or mixture). Using femtosecond interband and intraband spectroscopy, along with energy-dependent photoluminescence measurements and kinetic modelling, it is found that hot carriers are not universally defect tolerant in CsPbX3, but are strongly correlated to the defect tolerance of cold carriers, requiring shallow traps to be present (as in CsPbI3). It is found that hot carriers are directly captured by traps, instead of going through an intermediate cold carrier, and deeper traps cause faster hot carrier cooling, reducing the effects of the hot phonon bottleneck and Auger reheating. This work provides important insights into how defects influence hot carriers, which will be important for designing materials for hot carrier solar cells, multiexciton generation, and optical gain media.

Modulating hot carrier cooling and extraction with A-site organic cations in perovskites.

Hot carrier solar cells could offer a solution to achieve high efficiency solar cells. Due to the hot-phonon bottleneck in perovskites, the hot carrier lifetime could reach hundreds of ps. Such that exploring perovskites could be a good way to promote hot carrier technology. With the incorporation of large organic cations, the hot carrier lifetime can be improved. By using ultrafast transient spectroscopy, the hot carrier relaxation and extraction kinetics are measured. From the transient kinetics, 2-phenyl-acetamidine cation based perovskites exhibit the highest initial carrier temperature, longest carrier relaxation, and slowest hot carrier relaxation. Such superior behavior could be attributed to reduced electron-phonon coupling induced by lattice strain, which is a result of the large organic cation and also a possible surface electronic state change. Our discovery exhibits the potential to use large organic cations for the use of hot carrier perovskite solar cells.

Evidence of hot carrier extraction in metal halide perovskite solar cells

AbstractThe presence of hot carriers is presented in the operational properties of an (FA,Cs)Pb(I, Br, Cl)3 solar cell at ambient temperatures and under practical solar concentration. Albeit, in a device architecture that is not suitably designed as a functional hot carrier solar cell. At 100 K, clear evidence of hot carriers is observed in both the high energy tail of the photoluminescence spectra and from the appearance of a nonequilibrium photocurrent at higher fluence in light J–V measurements. At room temperature, however, the presence of hot carriers in the emission at elevated laser fluence is shown to compete with a gradual red shift in the PL peak energy as photoinduced halide segregation begins to occur at higher lattice temperature. The effects of thermionic emission of hot carriers and the presence of a nonequilibrium carrier distribution are also shown to be distinct from simple lattice heating. This results in large unsaturated photocurrents at high powers as the Fermi distribution exceeds that of the heterointerface controlling carrier transport and rectification.

Strong Dimensional and Structural Dependencies of Hot Carrier Effects in InGaAs Nanowires: Implications for Photovoltaic Solar Cells.

III-V nanowire structures are among the promising material systems with applications in hot carrier solar cells. These nanostructures can meet the requirements for such photovoltaic devices, i.e., the suppression of thermalization loss, an efficient hot carrier transport, and enhanced photoabsorption thanks to their unique one-dimensional (1D) geometry and density-of-states. Here, we investigate the effects of spatial confinement of photogenerated hot carriers in InGaAs-InAlAs core-shell nanowires, which presents an ideal class of hot carrier solar cell materials due to its suitable electronic properties. Using steady-state photoluminescence spectroscopy, our study reveals that by increasing the degree of spatial confinement and Auger recombination, the effects of hot carriers increase, which is in good agreement with theoretical modeling. However, for thin nanowires, the temperature of hot carriers decreases as the effects of crystal disorder increase. This observation is confirmed by probing the extent of the disorder-induced Urbach tail and linked to the presence of a higher density of stacking defects in the limit of thin nanowires. These findings expand our knowledge of hot carrier thermalization in nanowires, which can be applied for designing efficient 1D hot carrier absorbers for advanced-concept photovoltaic solar cells.

Temperature-dependent ultrafast hot carrier dynamics in the dilute bismide alloy GaSb1−xBix (x ≾ 0.4%)

We report temperature-dependent hot carrier dynamics in liquid-phase epitaxy-grown GaSb1−xBix epilayers with dilute amounts of Bi (x ≾ 0.4%). Degenerate pump–probe (λ = 800 nm) transient reflectivity (PPTR) was used to investigate the carrier dynamics in the epilayers. The PPTR signal consists of two transient processes (fast and slow) at all temperatures for all epilayers. The fast, hot carrier relaxation time, which is attributed to the combined effect of intervalley scattering and thermalization of carriers below cryogenic temperatures (<100 K), is observed to increase with an increase in temperature (≈0.8–2 ps at 6.6 K and ≈4–5 ps at 300 K). However, at higher temperatures (>100 K), the interband CHSH-Auger recombination process affects the band-to-band recombination, leading to an increase in the slower decay time. The findings offer crucial insights for optimizing GaSbBi for hot carrier solar cell applications.

Heat Flow through Nonideal Contacts in Hot-Carrier Solar Cells

Heat Flow through Nonideal Contacts in Hot-Carrier Solar Cells

Thermalization and relaxation mediated by phonon management in tin-lead perovskites

Understanding and control of ultrafast non-equilibrium processes in semiconductors is key to making use of the full photon energy before relaxation, leading to new ways to break efficiency limits for solar energy conversion. In this work, we demonstrate the observation and modulation of slow relaxation in uniformly mixed tin-lead perovskites (MASnxPb1-xI3 and CsSnxPb1-xI3 nanocrystals). Transient absorption measurements reveal that slow cooling mediated by a hot phonon bottleneck effect appears at carrier densities above ~1018 cm−3 for tin-lead alloy nanocrystals, and tin addition is found to give rise to suppressed cooling. Within the alloy nanoparticles, the combination of a newly introduced high-energy band, screened Fröhlich interaction, suppressed Klemens decay and reduced thermal conductivity (acoustic phonon transport) with increased tin content contributes to the slowed relaxation. For inorganic nanocrystals where defect states couple strongly with carriers, sodium doping has been confirmed to benefit in maintaining hot carriers by decoupling them from deep defects, leading to a decreased energy-loss rate during thermalization and an enhanced hot phonon bottleneck effect. The slow cooling we observe uncovers the intrinsic photophysics of perovskite nanocrystals, with implications for photovoltaic applications where suppressed cooling could lead to hot-carrier solar cells.

(Invited) Minimizing Thermal Losses at Quantum Dot-Metal Oxide Interfaces for Solar Energy Conversion

The sensitization of mesoporous oxides by semiconductor quantum dots (QDs) represents an appealing route for the development of low-cost solutions for solar energy production and storage (e.g. solar cells and fuel architectures). In order to boost photoconversion efficiency at these technologically relevant interfaces is important to minimize energy losses upon photon absorption; Energy losses that are intimately linked to kinetic completion at the relevant donor-acceptor interface [1]. Two main thermal energy losses can be identified following the absorption of a photon at the QD-oxide interface: firstly, any excess energy of the impinging photon vs the QD absorption onset is generally wasted as heat within the QD donor; secondly, the excess energy between QD electron donating state and the oxide accepting state (bottom of the conduction band) constitutes as well a thermal loss (now within the oxide) which reduces the available energy in the system to produce work. Reducing thermal losses associated with the latter mechanism is mandatory for allowing these architectures to reach efficiencies up to ~33% (the so called Shockley-Queisser limit). Preventing charge carrier cooling within the QD absorber after photon absorption, e.g. by enabling hot electron transfer toward the oxide, could – in principle - pave the way for developing QD dot based hot carrier solar cells with efficiencies beyond the Shockley-Queisser limit.Here I will present, from a charge carrier dynamics perspective, our studies aiming bypassing thermal losses at QD-oxide interfaces. First I will show how attempts to reduced QD-oxide donor-acceptor interfacial energetics by exploiting QD dipolar molecular capping can be in vain in strongly coupled QD-oxide sensitized systems [2]. This is linked to Fermi level pinning at the QD-oxide interface. I will show how this effect can be bypassed by inserting an insulating layer between QD donor and oxide acceptor. In the second part of my talk, I will discuss results demonstrating room-temperature hot electron transfer (HET) in strongly coupled QD-sensitized mesoporous oxides [3]. We quantify HET rates and show that the HET efficiency is determined by interfacial kinetic competition that can be optimized by both, maximizing QD-oxide coupling and/or suppressing electron cooling within the QD. Implications of these results for the challenging development of ultra-high efficiency solar converters will be discussed.

The Role of Thermalization in the Cooling Dynamics of Hot Carrier Solar Cells

The hot carrier solar cell (HCSC) concept has been proposed to overcome the Shockley Queisser limit of a single p–n junction solar cell by harvesting carriers before they have lost their surplus energy. A promising family of materials for these purposes is metal halide perovskites (MHP). MHPs have experimentally shown very long cooling times, the key requirement of a HCSC. By using ensemble Monte Carlo simulations, light is shed on why cooling times are found to be extended. This article concentrates on the role of thermalization in the cooling process. The role of carrier–phonon and carrier–carrier interactions in thermalization and cooling is specified, while showing how these processes depend on material parameters, such as the dielectric constant and effective mass. It is quantified how thermalization acts as a cooling mechanism via the cold background effect. The importance of a low degree of background doping is to achieve the observed extended cooling times. Herein, it is mapped out how perovskites should be tuned, their material parameters, carrier concentration, and purity, in order to realize a HCSC. It contributes to the debate on the cooling times in MHPs and the suitability of tin perovskites for HCSCs.

Dodecahedron CsPbBr3 Perovskite Nanocrystals Enable Facile Harvesting of Hot Electrons and Holes

This Letter reports the facile harvesting of hot carriers (HCs) in a composite of 12-faceted dodecahedron CsPbBr3 nanocrystal (NC) and a scavenger molecule. We recorded ∼3.3 × 1011 s-1 HC cooling rate in NC when excited with ∼1.4 times the band gap energy (Eg), increasing to >3 × 1012 s-1 in the presence of scavengers at high concentration due to the HC extractions. Since the observed intrinsic charge transfer rate (∼1.7 × 1012 s-1) in our NC-scavenger complex is about an order of magnitude higher than the HC cooling rate (∼3.3 × 1011 s-1), carriers are harvested before their cooling. Further, a fluorescence correlation spectroscopy study reveals NC tends to form a quasi-stable complex with a scavenger molecule, ensuring charge transfer completed (τct ≈ 0.6 ps) much before the complex breaks apart (>600 μs). The overall results of our study highlight the promise shown by 12-faceted NCs and their implications in modern applications, including hot carrier solar cells.

Thermodynamic Performance of Hot-Carrier Solar Cells: A Quantum Transport Model

In conventional solar cells, photogenerated carriers lose part of their energy before they can be extracted to make electricity. The aim of hot-carrier solar cells is to extract the carriers before this energy loss, thereby turning more energy into electrical power. This requires extracting the carriers in a nonequilibrium (nonthermal) energy distribution. Here, we investigate the performance of hot-carrier solar cells for such nonequilibrium distributions. We propose a quantum transport model in which each energy-loss process (carrier thermalization, relaxation, and recombination) is simulated by a B\"uttiker probe. We study charge and heat transport to analyze the hot-carrier solar cell's power output and efficiency, introducing partial efficiencies for different loss processes and the carrier extraction. We show that producing electrical power from a nonequilibrium distribution has the potential to improve the output power and efficiency. Furthermore, in the limit where the distribution is thermal, we prove that a boxcar-shaped transmission for the carrier extraction maximizes the efficiency at any given output power.

The effect of the effective electron mass on the hot electron collection

The dominant factor for hot electron collecting in internally photoemitted hot carrier (IPHC) devices is still not clear under steady-state low intensity light. We here use SnO2 as the electron-collecting layer to replace TiO2 to construct IPHC devices. Almost no photoresponse is observed for the pure SnO2-based IPHC device. However, when an insulating MgO layer or TiO2 covered SnO2, relatively large photocurrent generated from hot electrons can be achieved. The effective electron mass (EEM) is figured out to be the dominate factor in hot electron collection in IPHC devices. The very small EEM of SnO2 results in a small emission cone of hot electrons. Also due to the small EEM of SnO2, the leakage of trapped electrons back to the Au is very large. Because of these two reasons, the SnO2-based IPHC device shows almost no photoresponse. MgO can block the backflow of electrons (leakage), while the larger EEM of TiO2 can increase the emission cone of hot electrons. Our finding is significant for understanding hot electrons collection and will give new directions for hot carrier solar cell applications under low-intensity excitation at steady state.

Effects of impact ionization and Auger recombination on hot-carrier solar cells and hot-carrier photocatalysts

We applied hot-carrier extraction to solar cells and photocatalysts used for artificial photosynthesis including water splitting and CO2 reduction, and elucidated the differences between these two applications: hot-carrier solar cells (HC-SCs) and hot-carrier photocatalysts (HC-PCs) by detailed balance calculations. The hot-carrier effect in the photocatalysts is less significant than that in the solar cells, because of the larger bandgaps required for generating sufficiently high-energy carriers consumed for the reactions. On the other hand, impact ionization and Auger recombination (IA) improves the efficiency of the HC-PCs more notably, because the IA functions like photon upconverters and hence narrows the optimal bandgap. Furthermore, the IA improves the spectral robustness by eliminating the constraint of the particle-number conservation for both the HC-SCs and HC-PCs. These benefits of the IA are contrasting with the well-recognized fact that the IA only reduces the carrier number and consequently lowers the efficiency of the conventional counterparts.

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.

Polarons in perovskite solar cells: effects on photovoltaic performance and stability

Organic–inorganic hybrid perovskites manifest unique photophysical properties in terms of their long carrier lifetime, low recombination rate, and high defect tolerance, enabling them to be promising candidates in optoelectronic devices. However, such advanced properties are unexpected in perovskite materials with moderate charge mobility. Recent investigations have revealed that these appealing properties were endowed due to the formation of large polarons in the perovskite crystals, resulting from the coupling of photogenerated carriers and a polarized crystal lattice, which largely affected the carrier-transport dynamics and structural stability of perovskite solar cells (PSCs). In this review, first the crystal structure of the perovskite lattice and the formation mechanism of polarons are elucidated. Then, the modulation of polaron states in PSCs, including large polaron stabilization, polaron-facilitated charge transport, hot-carrier solar cells, and polaron-related stability issues such as polaron-induced metastable defects, polaronic strain, and photostriction are systematically investigated. Finally, the prospect of further understanding and manipulating polaron-related phenomena, working toward highly efficient and stable PSCs, is suggested.

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 $ZT$

Enhancement of hot carrier effect and signatures of confinement in terms of thermalization power in quantum well solar cell

A theoretical model using electron–phonon scattering rate equations is developed for assessing carrier thermalization under steady-state conditions in two-dimensional systems. The model is applied to investigate the hot carrier effect in III–V hot-carrier solar cells with a quantum well absorber. The question underlying the proposed investigation is: what is the power required to maintain two populations of electron and hole carriers in a quasi-equilibrium state at fixed temperatures and quasi-Fermi level splitting? The obtained answer is that the thermalization power density is reduced in two-dimensional systems compared to their bulk counterpart, which demonstrates a confinement-induced enhancement of the hot carrier effect in quantum wells. This power overall increases with the well thickness, and it is moreover shown that the intra-subband contribution dominates at small thicknesses while the inter-subband contribution increases with thickness and dominates in the bulk limit. Finally, the effects of the thermodynamic state of phonons and screening are clarified. In particular, the two-dimensional thermalization power density exhibits a non-monotonic dependence on the thickness of the quantum well layer, when both out-of-equilibrium longitudinal optical phonons and screening effects are taken into account. Our theoretical and numerical results provide tracks to interpret intriguing experimental observations in quantum well physics. They will also offer guidelines to increase the yield of photovoltaic effect based on the hot carrier effect using quantum well heterostructures, a result critical to the research toward high-efficiency solar cell devices.

Solar cells combining hot carriers and multijunctions: synergies and insights from Maxime Giteau, Samy Almosni, and Daniel Suchet

JPE Editor-in-Chief Sean Shaheen interviewed JPE authors Maxime Giteau, Samy Almosni, and Daniel Suchet about the research presented in their recently published article, “Hot-carrier multijunction solar cells: sensitivity and resilience to nonidealities” (doi https://doi.org/10.1117/1.JPE.12.032208).

Polar methylammonium organic cations detune state coupling and extend hot-carrier lifetime in lead halide perovskites

<h2>Summary</h2> The slow hot-carrier relaxation properties of lead halide perovskites show great promise for hot-carrier solar cells. Here, we discover that the hot-carrier relaxation could be prolonged by two orders of magnitude in lead halide perovskites. By ultrafast spectroscopy, we unravel a slow hot-carrier relaxation process mediated by an intermediate state above the conduction band minimum (CBM). This intermediate state is formed by incorporating polar methylammonium (MA) cations. The first-principle calculation further confirms that the incorporation of MA cations introduces a prolonged relaxation process from CBM + 1 to CBM state, which are the split-off states of the CBM due to the strong spin-orbit coupling. The prolonging effect is interpreted that MA cations largely increase the lattice distortions and detune their inter-state coupling. This work reveals a neglected way to prolong the hot-carrier relaxation processes and provides guidance for the hot-carrier photovoltaics to overcome the current Shockley-Queisser limit.