Efficiency Of Single-junction Solar Cells Research Articles

Cation Influence on Hot-Carrier Relaxation in Tin Triiodide Perovskite Thin Films.

Slow hot-carrier cooling may potentially allow overcoming the maximum achievable power conversion efficiency of single-junction solar cells. For formamidinium tin triiodide, an exceptional slow cooling time of a few nanoseconds was reported. However, a systematic study of the cation influence, as is present for lead compounds, is lacking. Here, we report the first comparative study on formamidinium, methylammonium, and cesium tin triiodide thin films. By investigating their photoluminescence, we observe a considerable shift of the emission peak to high energy with the increase of the excited-state population, which is more prominent in the case of the two hybrid organic-inorganic perovskites (∼45 meV vs ∼15 meV at 9 × 1017 cm-3 carrier density). The hot-carrier photoluminescence of the three tin compositions decays with a 0.6-2.8 ns time constant with slower cooling observed for the two hybrids, further indicating their importance.

Scalable Solution-Processed Hybrid Electron Transport Layers for Efficient All-Perovskite Tandem Solar Modules.

All-perovskite tandem solar cells offer the potential to surpass the Shockley-Queisser (SQ) limit efficiency of single-junction solar cells while maintaining the advantages of low-cost and high-productivity solution processing. However, scalable solution processing of electron transport layer (ETL) in p-i-n structured perovskite solar subcells remains challenging due to the rough perovskite film surface and energy level mismatch between ETL and perovskites. Here, scalable solution processing of hybrid fullerenes (HF) with blade-coating on both wide-bandgap (≈1.80 eV) and narrow-bandgap (≈1.25 eV) perovskite films in all-perovskite tandem solar modules is developed. The HF, comprising a mixture of fullerene (C60 ), phenyl C61 butyric acid methyl ester, and indene-C60 bisadduct, exhibits improved conductivity, superior energy level alignment with both wide- and narrow-bandgap perovskites, and reduced interfacial nonradiative recombination when compared to the conventional thermal-evaporated C60 . With scalable solution-processed HF as the ETLs, the all-perovskite tandem solar modules achieve a champion power conversion efficiency of 23.3% (aperture area = 20.25 cm2 ). This study paves the way to all-solution processing of low-cost and high-efficiency all-perovskite tandem solar modules in the future.

Electrochemical Properties of Derivatives of 1,3-Diphenylisobenzofuran – Chromophores for Singlet Fission

Nowadays, there are efforts to apply alternative sources of renewable energy. One of these sources is solar energy. Solar energy represents a renewable free source of energy that is sustainable and totally inexhaustible. The standard efficiency of solar cells is now below 30 %. Singlet fission has attracted rising attention because it promises to increase the maximum of theoretical efficiency of single-junction solar cells. While solar cells after absorption one photon generates one electron, singlet fission is a photophysical process, whereby a singlet exciton generated by irradiation splits into two triplet excitons [1,2]. In this way, one photon can generate theoretically two electrons which should increase the efficiency of inexpensive single-junction solar cells. Therefore, recently substantial attention has been paid to search for suitable chromophores for singlet fission.When looking for new chromophores for efficient SF, their crystalline form and molecular packing, the relative energies of the S0 (ground state), T1 (lowest triplet), and S1 (first excited singlet) states, where energy of the triplet state should be lower than a half of the first excited singlet energy, and the potentials of one-electron reduction and one-electron oxidation are crucial. Redox properties of molecules for SF are thus critical for their use, therefore electrochemical approach is necessary.Molecules on the base of 1,3-diphenylisobenzofuran (DPIBF) attract an interest because of their possible efficiency for singlet fission. In the last few years we have started to investigate various types of derivatives of DPIBF. One of our first studies [3] was focused on a series of fluorinated derivatives of DPIBF where the influence of number and position of fluorine atoms in the molecule on the redox potentials and mechanism is followed. Next to that we studied series of dimer derivatives of DPIBF [4]. The location and the nature of the bridging unit between two DPIBF redox centres attracted our attention because in this way the shape of the molecule, its extent of electron delocalization and, thus, possible ability of formation of biradicaloid can be changed.The most interesting dimeric molecule appeared to be that one with the elongated π-conjugated system, the “quasi dimer” of DPIBF (2). Because of that, quasi dimer was investigated in more details in comparison with other three molecules by standard electrochemical techniques, the in-situ UV-vis and EPR spectroelectrochemical techniques. The three other derivatives for comparison were parent diphenylisobenzofuran (1), direct dimer (3) and methylene bridged dimer (4). The main goal of this deeper study was to characterize properly not only electrochemical properties of the molecules, but also their absorbance and fluorescence properties and stability in solvents. Acknowledgement This work was supported by the grant 21-23261S (Grant Agency of the Czech Republic) and by the institutional support RVO: 61388955.

Singlet Fission in Perylene Monoimide Single Crystals and Polycrystalline Films

Singlet fission (SF) is a spin-allowed process in which a photogenerated singlet exciton down-converts into two triplet excitons. Perylene-3,4-dicarboximide (PMI) has singlet and triplet state energies of 2.4 and 1.1 eV, respectively; thus making SF slightly exoergic and providing triplet excitons that have sufficient energy to raise the efficiency of single-junction solar cells by reducing thermalization losses from hot excitons formed when absorbed photons have energies higher than the semiconductor bandgap. However, PMI SF in the solid state has not been studied previously. Here, we show that 2,5-diphenyl-N-(2-ethylhexyl)perylene-3,4-dicarboximide (dp-PMI) crystallizes into a slip-stacked intermolecular morphology favorable for SF. Transient absorption microscopy and spectroscopy show that dp-PMI SF occurs in ≤50 ps in both single crystals and polycrystalline thin films with a triplet yield of 150 ± 20%. Ultrafast SF in the solid state, the high triplet yield, and its photostability make dp-PMI an attractive candidate for SF-enhanced solar cells.

Bending Pyrenacenes to Fill Gaps in Singlet-Fission-Based Solar Cells

Singlet fission is envisaged to enhance the efficiency of single-junction solar cells beyond the current theoretical limit. Even though sensitizers that undergo singlet fission efficiently are known, characteristics like low-energy triplet state or insufficient stability restrict their use in silicon-based solar cells. Pyrenacenes have the potential to overcome these limitations, but singlet-fission processes in these materials is outcompeted by excimer formation. In this work, bent pyrenacenes with a reduced propensity to stack and thus form excimers are computationally evaluated as singlet-fission materials. The energies of the S1, T1 and T2 states were estimated in a series of bent pyrenacenes by means of time-dependent density functional theory calculations. Our results show the opposite trend observed for perylene diimides, namely, an increase in the energy of the T1 and S1 states upon bending. In addition, we show that the energy levels can be tuned on demand by manipulating the bend angle to match the energy gap of various semiconductors that can be used in single-junction solar cells, making pyrenacenes promising candidates for singlet fission.

Does non-reciprocity break the Shockley–Queisser limit in single-junction solar cells?

The efficiency of single-junction solar cells is bounded by the Shockley–Queisser limit of 41%. However, standard derivation for this limit constrains the system to be reciprocal, and what non-reciprocity can bring for single-junction solar cells remains yet to be clarified. Here, we prove that even with non-reciprocity, the ultimate efficiency of single-junction solar cells is still subject to the Shockley–Queisser limit. We show that the Shockley–Queisser limit does not rely on the detailed balance, but rather is a consequence of the integrated balance between the absorption and emission processes, as required by the second law of thermodynamics.

Effect of thermal radiation entropy on the outdoor efficiency limit of single-junction silicon solar cells

Incoming radiation energy illuminating a solar cell contains a certain amount of entropy, which does not contribute to output electrical work. Entropy has a different spectral distribution from the internal energy of light and consequently affects PV cell performance. Here in this work, we investigate the influence of entropy content of thermally radiated light on the maximum achievable efficiency of single-junction solar cells. We revise the value of the well-known Shockley-Queisser (SQ) limit for various absorber materials and take a deeper look at this effect by re-calculating the efficiency limit of crystalline silicon solar cells considering Meitner-Auger recombination. When considering the entropy content of AM 1.5 standard spectrum, the SQ limit for silicon drops from 33.15% to 30.42%. Further, considering Meitner-Auger recombination and using measured properties of silicon, the efficiency limit lowers to 27.12% from the already established 29.43%. This suggests a 4% thinner silicon absorber, reaching a thickness of ∼101 μm; hinting PV industry that a thinner Si wafer can provide the optimum outdoor energy yield. We further show that the entropy content of terrestrial radiation is less in favor of c-Si technology and most in favor of amorphous silicon. In the end, we discuss a few applications of considering entropy of incoming sunlight for photovoltaics, which range from PV device design to PV module tilt optimization and even PV system electrical standards. • Quality of radiation that a solar cell receives changes under outdoor condition. • Radiation illuminating solar cells in labs has lower entropy, i.e. higher exergy. • Exergy of incoming radiation is used to calculate outdoor efficiency limit. • Single junction solar cells can reach 27.1% efficiency under outdoor condition. • Results suggest 4% thinner silicon absorbers for single junction solar cells.

Two-dimensional Janus Sn2SSe and SnGeS2 semiconductors as strong absorber candidates for photovoltaic solar cells: First principles computations

Two-dimensional materials provide new opportunities for the next generation of effective and ultrathin photovoltaic solar cells. Herein, we propose Janus monolayers of Tin monochalcogenides, especially Janus Sn2SSe (type TA) and SnGeS2 (type TB) nanosheets, as strong absorber candidates for solar energy conversion, referring to their excellent electronic and optical properties. Interestingly, based on the first-principles computations, both Janus Sn2SSe and SnGeS2 monolayers possess semiconductor character with indirect and moderate band gaps of 1.60 and 1.61eV, respectively. Accordingly, the considered systems, Sn2SSe and SnGeS2 single-layers, have high absorption coefficient, reaching up to 49.7 and 62.5μm−1, high optical conductivity of about 4513 and 3559Ω−1cm−1, as well as low reflectivity never exceed 34.6 and 38.5% in visible region, respectively. Additionally, the maximum photovoltaic efficiency of single-junction solar cells based on SnGeS2 and Sn2SSe nanosheets can reach as high as 27.47% and 28.12%, respectively. The present outstanding results would motivate both theoretical and experimental researchers to deepen the study of the potential applications of two-dimensional Janus materials based on Tin monochalcogenides in solar cell technology.

Multiphoton Near-Infrared Quantum Splitting of Er3+

The efficiency of single-junction solar cells is limited to about 30% (the Shockley-Queisser limit). Spectral mismatch losses (transparency to low-energy photons, thermalization of high-energy photons) strongly contribute to lowering the maximum efficiency. To reduce thermalization losses, photon splitting is proposed and observed for a variety of lanthanide-doped materials. For ${\mathrm{Er}}^{3+}$, even a one-to-three photon-splitting process has been reported, yielding three IR photons at around 1530 nm following absorption of one blue-green photon. This is especially beneficial for narrow band gap solar cells, such as crystalline $\mathrm{Ge}$. Here, we report on photon splitting for ${\mathrm{Er}}^{3+}$ in ${\mathrm{YVO}}_{4}$. Following absorption in the ${}^{2}{\mathrm{H}}_{11/2}$ and ${}^{4}{\mathrm{S}}_{3/2}$ levels (520--550 nm), efficient cross-relaxation (CR) yields two excited ${\mathrm{Er}}^{3+}$ ions: one in the ${}^{4}{\mathrm{I}}_{9/2}$ state and one in the ${}^{4}{\mathrm{I}}_{13/2}$ state (CR1). A second CR step from the ${}^{4}{\mathrm{I}}_{9/2}$ state, leaving both ${\mathrm{Er}}^{3+}$ ions in the ${}^{4}{\mathrm{I}}_{13/2}$ excited state (CR2), is crucial in realizing efficient three IR photon splitting. It is demonstrated here that the second step has a low efficiency, as a result of competing fast multiphonon relaxation, ${}^{4}{\mathrm{I}}_{9/2}{\ensuremath{\rightarrow}}^{4}{\mathrm{I}}_{11/2}$, and a large energy mismatch, which makes the CR2 step thermally activated. Based on experiments and theory, a maximum quantum efficiency of 170% is calculated for IR emission, following blue-green excitation in ${\mathrm{YVO}}_{4}:{\mathrm{Er}}^{3+}$. An outlook is presented for three-photon splitting in low-phonon-energy hosts, where nonradiative multiphonon relaxation is suppressed. The anti-Stokes nature of the second CR step makes three-photon splitting unlikely and prevents the realization of IR quantum yields above 200%.

Oxygen-catalysed sequential singlet fission

Singlet fission is the photoinduced conversion of a singlet exciton into two triplet states of half-energy. This multiplication mechanism has been successfully applied to improve the efficiency of single-junction solar cells in the visible spectral range. Here we show that singlet fission may also occur via a sequential mechanism, where the two triplet states are generated consecutively by exploiting oxygen as a catalyst. This sequential formation of carriers is demonstrated for two acene-like molecules in solution. First, energy transfer from the excited acene to triplet oxygen yields one triplet acene and singlet oxygen. In the second stage, singlet oxygen combines with a ground-state acene to complete singlet fission. This yields a second triplet molecule. The sequential mechanism accounts for approximately 40% of the triplet quantum yield in the studied molecules; this process occurs in dilute solutions and under atmospheric conditions, where the single-step SF mechanism is inactive.

High-resolution Scanning Transmission EBIC Analysis of Misfit Dislocations at Perovskite pn-Heterojunctions

Fundamental losses of photovoltaic energy conversion are transmission of sub band gap photons and thermalisation which are the underlying physics of the Shockley-Queisser limit defining maximum conversion efficiency of single-junction solar cells. Strongly correlated materials such as perovskites are promising candidates to exceed this limit by exploiting (i) long wavelength light absorption and (ii) the existence of long-living intraband excitations indicating that harvesting hot excess carriers might be feasible in such systems. In this work, we study pn-heterojunctions produced from Pr1-xCaxMnO3 on SrTi1-yNbyO3 by means of microscopic techniques. Such systems exhibit relevant quantities such as space charge layer width, screening lengths and excess carrier diffusion lengths in the 1-10 nm range which makes the use of standard methods such as electron beam induced current a challenging task. We report scanning transmission electron beam induced current experiments of misfit dislocations at the heterojunction. The dislocation-induced reduction of the charge collection is studied with nanometer spatial resolution. Effects of surface recombination and the heterojunction electric field are discussed.

Perovskite solar cells: On top of commercial photovoltaics

The efficiency of single-junction solar cells is intrinsically limited and high efficiency multi-junctions are not cost effective yet. Now, semi-transparent perovskite solar cells suggest that low cost multi-junctions could be within reach.

Discussion about ultimate efficiency of solar cells

Solar energy is considered as one of the most promising green energy due to clean, safe, long life, and renewable advantages. Solar cell is an electrical device that converts solar energy directly into electric power on the basis of photovoltaic effect. Fritts built the first solar cell in 1883. Although the initial efficiency was only 1%, it has been exciting to know that the power conversion efficiency of solar cells is endlessly improved since it was reported. Up to now, the efficiency of commercial silicon solar cell is between 10%–18%. The latest research shows that the best efficiency of perovskite solar cell has been improved to be over 20% within several years. As it is well known, the maximum power conversion efficiency of single-junction solar cells are only around 33% according to the Shockley- Queisser limit. With the development of new materials and high technologies, one issue is emerging: What is the ultimate efficiency of solar cells? The highest efficiency of solar cell is possible to break the Schockley-Queisser limit? How to further enhance the efficiency of solar cell? All the questions are hard to answer at present. Surrounding these concerns, this article presents a brief review about the efficiency limits of different solar cells. It might help to understand the ultimate efficiency of solar cells. The details address the basic principle, advantages and disadvantages of single-junction, multi-junction and other new concept solar cells regarding the power conversion efficiency. The review also includes the solar cell materials involving silicon, compound, perovskite, quantum dot, hybrid materials, etc. Finally, we look ahead into the prospect of new concept solar cells and the maximum power conversion efficiency.

Conditions for beneficial coupling of thermoelectric and photovoltaic devices

Abstract

Recent progress on quantum dot intermediate band solar cells

In order to surpass the thermodynamic Shockley-Queisser limit of energy conversion efficiency of single-junction solar cells, advanced concepts using multi-junction tandem structure and quantum nanostructure are presently under intense research. Recent developments and future research opportunities in high-efficiency photovoltaics technology based on quantum dot superlattice are reviewed.

Singlet Exciton Fission-Sensitized Infrared Quantum Dot Solar Cells

We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells.

Quantum-dot intermediate-band solar cells with inverted band alignment

The intermediate-band concept was proposed over a decade ago as a possible route to increase the efficiency of single-junction solar cells. Despite a number of experimental attempts to realize this concept, no efficiency improvement over conventional single-junction solar cells has so far been demonstrated. This is likely due to the fact that the intermediate band itself acts to enhance electron–hole recombination. In this work we propose a novel intermediate-band solar-cell architecture based on doped semiconductor nanostructures having an inverted type-I band alignment with the surrounding host. The recombination of carriers in the nanostructures is prevented by ultra-fast charge transfer to the host, thereby removing the main obstacle to achieve high conversion efficiency.

Basic efficiency limits, recent experimental results and novel light-trapping schemes in a-Si:H, μc-Si:H and `micromorph tandem' solar cells

Theoretical: Limits for J sc, V oc, FF and efficiency of single-junction solar cells and tandems are derived, as a function of E g, showing that: (1) double-junction `micromorph' solar cells constitute an optimum combination of bandgap values; (2) main future gain for thin-film silicon solar cells will be increasing J sc, by light trapping. Spectral ranges where light trapping has to act are presented, separately for a:Si:H and μc-Si:H. Experimental: For glass-pin type cells, light trapping is obtained at the front end, by use of rough TCO layers. For plastic-nip type solar cells, light trapping is obtained by texturing of the back reflector, which acts as basic layer for further growth. Both random texturing and periodic gratings have been used, but results presented are limited to single-junction a-Si:H and μc-Si:H solar cells. So far, the highest J sc-enhancement obtained is approximately 20%.