Solar Cell Concept Research Articles

Recombination Activity of Crystal Defects in Epitaxially Grown Silicon Wafers for Highly Efficient Solar Cells

Aiming for highly efficient solar cells based on wafers with a low carbon footprint, silicon (Si) EpiWafers are grown epitaxially on reusable, highly doped Si substrates with a stack of porous Si layers (PorSi) for detachment. A state‐of‐the‐art p‐type Si EpiWafer exhibiting a minority charge carrier lifetime of up to 2.2 ms detected at an excess charge carrier density of ≈1 × 1015 cm−3 by photoluminescence (PL) imaging is presented. This translates to a predicted solar cell efficiency of 25.6%, calculated by efficiency limiting bulk recombination analysis (ELBA), and corresponds to losses of less than 1%abs compared to the theoretical limit of the investigated solar cell concept. A detailed loss analysis shows that the major remaining quality limitations are structural defects, specifically stacking faults (SFs). Therefore, the recombination activity of isolated SFs in epitaxially grown reference (EpiRef) wafers on polished substrates without a PorSi is assessed by highly resolved μPL mappings. The recombination activity rises with the number of dislocations within an SF as demonstrated by a comparison to microscope images. When using highly doped substrates, as currently required for EpiWafer fabrication, EpiRef wafers show more SFs exhibiting additionally a higher number of dislocations than SFs in EpiRef wafers on moderately doped substrates.

Lattice battery solar cells: Exceeding Shockley–Queisser limit

AbstractA revolutionary concept of lattice battery solar cell (LBSC) is proposed to leap the conversion efficiency by inherently eliminating two major energy losses of conventional solar cells, namely hot carriers and non‐absorption of the substantial near infrared (NIR) emission. In an LBSC, hot phonon emission will be saved into lattice energy reservoir (LER) through electron–lattice coupling; NIR solar emission is harvested by an NIR‐perovskite composition. The NIR‐generated carriers are upconverted to the conduction band of perovskites driven by LER. The theoretical efficiency of LBSCs is estimated to be over 70%, significantly exceeding the Shockley–Queisser limit. In addition, LBSCs have lower operational temperature, resulting in much improved stability due to the elimination of heating sources from hot carriers. Different from the existing multijunction solar cells, LBSCs will keep the single layer structure with low‐cost fabrication. Therefore, LBSCs could perfectly satisfy the golden triangle of solar cell performance, which prospects great competitive advantage for further commercialization.

Top cell design and optimization of all-chalcopyrite CuGaSe<sub>2</sub>/CuInSe<sub>2</sub> two-terminal tandem solar cells

Solar cells have attracted much attention, for they can convert solar energy directly into electric energy, and have been widely utilized in manufacturing industry and people’s daily life. Although the power conversion efficiency (PCE) of single-junction solar cells has gradually improved in recent years, its maximum efficiency is still limited by the Shockley-Queisser (SQ) limit of single-junction solar cells. To exceed the SQ limit and further obtain high-efficiency solar cells, the concept of tandem solar cells has been proposed. In this work, the chalcopyrite CuGaSe<sub>2</sub>/CuInSe<sub>2</sub> tandem solar cells are studied systematically in theory by combining first-principle calculations and SCAPS-1D device simulations. Firstly, the electronic structure, defect properties and corresponding macroscopic performance parameters of CuGaSe<sub>2</sub> (CGS) are obtained by first-principles calculations, and are used as input parameters for subsequent device simulations of CGS solar cells. Then, the single-junction CGS and CuInSe<sub>2</sub> (CIS) solar cells are simulated by using SCAPS-1D software, respectively. The simulation results for the single junction CIS solar cells are in good agreement with the experimental values. For single-junction CGS cells, the device simulations reveal that the CGS single-junction solar cells have the highest short-circuit current (<i>J</i><sub>sc</sub>) and PCE under the Cu-rich, Ga-rich and Se-poor chemical growth condition. Further optimization in the growth environment with the highest short circuit current (<i>J</i><sub>sc</sub>) shows that the open-circuit voltage (<i>V</i><sub>oc</sub>) and PCE of CGS solar cells can be improved by replacing the electron transport layer (ETL) with ZnSe. Finally, after the optimized CGS and CIS solar cells are connected in series with two-terminal (2T) monolithic tandem solar cell, the device simulation results show that under the growth temperature of 700 K and the growth environment of Cu-rich, Ga-rich, and Se-poor, with ZnSe serving as the ETL, the CGS thickness of 2000 nm and the CIS thickness of 1336 nm, the PCE of 2T monolithic CGS/CIS tandem solar cell can reach 28.91%, which is higher than the ever-recorded efficiency of the current single-junction solar cells, and shows that this solar cell has a good application prospect.

Silver Metallization with Controlled Etch Stop Using SiOx Layers in Passivating Contacts for Improved Silicon Solar Cell Performance

Metallization of polycrystalline‐silicon/silicon oxide (poly‐Si/SiOx) passivating contacts with fire‐through silver paste is a crucial process for implementation of passivating contacts in industrial manufacturing of solar cells. For a microscopic understanding of the metallization process, the contact forming interface between the Ag crystallites and the poly‐Si layer is investigated with high‐resolution transmission electron microscopy. For this purpose, multilayer atmospheric pressure chemical vapor deposition poly‐Si samples with a SiOx layer between the individual poly‐Si layers are fabricated, screen printed with a lead‐free Ag paste, and contact fired. Electron micrographs show that in this process the etching of the paste and the subsequent Ag crystallite formation is stopped by this interface with the SiOx layer. Additionally, energy‐dispersive X‐Ray mapping reveals the presence of an oxide layer around the Ag crystallites. This finding differs significantly from well‐investigated classical cell concepts with contact formation on diffused crystalline silicon. Moreover, an analysis of the Ag crystallite orientation in correlation to the neighboring Si crystallite orientation indicates no direct relationship. Finally, it is shown that the use of this multilayer approach is favorable for integration into a solar cell concept leading to a higher passivation quality at the metallized area and lower contact resistivity.

Thermal interdiffusion, microstructure and contact resistivity of NiOx/Ni/p+ poly-Si layer systems for perovskite/TOPCon tandem solar cells during annealing processes

In a recently published article (Lange et al., 2023) [1], we envisioned and discussed a tandem solar cell concept based on a perovskite/TOPCon architecture in p-i-n configuration, that utilizes a p+/n+ polycrystalline silicon (poly-Si) tunnel junction and NiOx as hole transport material of the perovskite top cell. A direct contact of NiOx to p+ poly-Si is inevitable in such a device. We investigated that junction and reported how the contact resistivity of the NiOx/p+ poly-Si junction can be improved by orders of magnitudes by intentionally inserting a nm-thin metallic Ni interlayer at that interface through the formation of nickel silicide.In this contribution, we present comprehensive data on layer stack morphology, composition and interdiffusion and the impact on the measured contact resistivity based on (scanning) transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) of model samples as well as the real contact structures. Diffusion and reaction processes in the NiOx/Ni/p+ poly-Si layer stack are investigated during the annealing in the temperature range between 100 °C and 500 °C for varying Ni interlayer thicknesses of 0, 1.0 and 2.0 nm. Furthermore, two NiOx syntheses routes are investigated: sputter deposition (s-NiOx) and a wet-chemical approach (wc-NiOx). We correlate the improvement of contact resistivity for both systems and temperatures <300 °C in regard to contacts without metallic Ni interlayer with the observation of pinhole formation within the SiOz. We have observed that annealing the Ag/s-NiOx junctions above 300 °C under ambient air leads to rectifying contacts and high contact resistivities due to excessive interdiffusion of Ni, O and Ag.

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.

RAINBOW Organic Solar Cells: Implementing Spectral Splitting in Lateral Multi-Junction Architectures.

While multi-junction geometries have the potential to boost the efficiency of organic solar cells, the experimental gains yet obtained are still very modest. This work proposes an alternative spectral splitting device concept in which various individual semiconducting junctions with cascading bandgaps are laid side by side, thus the name RAINBOW. Each lateral sub-cell receives a fraction of the spectrum that closely matches the main absorption band of the given semiconductor. Here, simulations are used to identify the important material and device properties of each RAINBOW sub-cell. Using the resulting design rules, three systems are selected, with narrow, medium, and wide effective bandgaps, and their potential as sub-cells in this geometry is experimentally investigated. With the aid of a custom-built setup that generates spectrally spread sunlight on demand, the simulations are experimentally validated, showing that this geometry can lead to a reduction in thermalization losses and an improvement in light harvesting, which results in a relative improvement in efficiency of 46.6% with respect to the best sub-cell. Finally, a working proof-of-concept monolithic device consisting of two sub-cells deposited from solution on the same substrate is fabricated, thus demonstrating the feasibility and the potential of the RAINBOW solar cell concept.

Transport mechanisms in hyperdoped silicon solar cells

According to intermediate band (IB) theory, it is possible to increase the efficiency of a solar cell by boosting its ability to absorb low-energy photons. In this study, we used a hyperdoped semiconductor approach for this theory to create a proof of concept of different silicon-based IB solar cells. Preliminary results show an increase in the external quantum efficiency (EQE) in the silicon sub-bandgap region. This result points to sub-bandgap absorption in silicon having not only a direct application in solar cells but also in other areas such as infrared photodetectors. To establish the transport mechanisms in the hyperdoped semiconductors within a solar cell, we measured the J–V characteristic at different temperatures. We carried out the measurements in both dark and illuminated conditions. To explain the behavior of the measurements, we proposed a new model with three elements for the IB solar cell. This model is similar to the classic two-diodes solar cell model but it is necessary to include a new limiting current element in series with one of the diodes. The proposed model is also compatible with an impurity band formation within silicon bandgap. At high temperatures, the distance between the IB and the n-type amorphous silicon conduction band is close enough and both bands are contacted. As the temperature decreases, the distance between the bands increases and therefore this process becomes more limiting.

Epitaxially Grown p‐type Silicon Wafers Ready for Cell Efficiencies Exceeding 25%

Combining the advantages of a high‐efficiency solar cell concept and a low carbon footprint base material is a promising approach for highly efficient, sustainable, and cost‐effective solar cells. In this work, we investigate the suitability of epitaxially grown p‐type silicon wafers for solar cells with tunnel oxide passivating contact rear emitter. As a first proof of principle, an efficiency limiting bulk recombination analysis of epitaxially grown p‐type silicon wafers deposited on high quality substrates (EpiRef) unveils promising cell efficiency potentials exceeding 25% for three different base resistivities of 3, 14, and 100 Ω cm. To understand the remaining limitations in detail, concentrations of metastable defects Fei, CrB and BO are assessed by lifetime‐calibrated photoluminescence imaging and their impact on the overall recombination is evaluated. The EpiRef wafers’ efficiency potential is tracked along the solar cell fabrication process to quantify the impact of high temperature treatments on the material quality. We observe large areas with few structural defects on the wafer featuring lifetimes exceeding 10 ms and an efficiency potential of 25.8% even after exposing the wafer to a thermal oxidation at 1050 °C.

Ultrafast hot-electron injection at HfN-metal oxide heterojunctions: Role of barrier height

The concept of hot carrier-based solar cell is suggested for high-efficiency photovoltaics by overcoming thermodynamic limit of conversion efficiency. However, efficiency is largely affected by the quality of absorber materials and dynamics of hot carriers (HCs) in the same. Here, we demonstrate the generation of HCs and their transfer dynamics to the metal oxides by performing the femtosecond transient absorption spectroscopy measurements. The role of oxygen composition/content is highlighted on the different physical properties of hafnium nitride (HfN) thin films because the presence of oxygen in HfN deteriorates/quenches the HC generation. All HfN thin films exhibit the polycrystalline nature and the degree of crystallinity decreases with increasing the oxygen concentration as confirmed by X-ray diffraction and high-resolution transmission electron microscopic studies. In addition, the absorption measurements exhibit the broadband light absorption efficiency of the HfN films. The transient absorption maps show a strong photobleaching signal over a wide spectral range. The transient bleach dynamics probed at 650 nm show the faster decay rate (0.77 ps) for the HfN/TiOx in comparison to HfN (1.5 ps) and HfN/MoOx (3.7 ps), revealing the efficient electron transfer from HfN to TiOx layer. These results advance our understanding of hot-electron dynamics in HfN-oxide heterostructures and offers to design an ultra-fast optoelectronics device of HCs.

Design and analysis of multi-layer silicon nanoparticle solar cells

We investigate the concept of nanoparticle-based solar cells composed of a silicon nanoparticle stack as a light trapping absorber for ultrathin photovoltaics. We study the potential of using these inherently nanotextured structures in enhancing the light absorption. For this, a detailed optical analysis is performed on dependency of the cell response to parameters such as the number of particle layers, lattice structure and angle of incidence; Optical response of these cells are then compared with the results in conventional silicon solar cells. Moreover, we propose various configurations to apply these submicron particles as a p–n junction solar cell. We also compute the electrical performance of selected configurations. In doing so, key issues including the effect of contact points between nanoparticles and impact of loss are addressed. In the end, we show how mathrm{SiO}_2 nanoparticles on top of the cell structure can enhance the photocurrent. The appropriate range of mathrm{SiO}_2 particle size is also obtained for the typical cell structures.

Hybrid Solar Cells: A Step Closer to Smart Life

Solar cells come a long way since its invention. The stupendous growth of solar cells has opened many new avenues in many industrial sectors. Sunlight, being the primary source of solar cells is often shadowed by weather conditions such as heavy clouds, fogs, heavy rainfall etc. As such, the performance of solar cells wanes in such weather conditions. To tackle this, the concept of hybrid solar cells emerges which can cater to these weather anomalies and deliver power in bad weather too. This brief opinion article tries to appraise readers regarding the developments of hybrid solar cells

Ferroelectric polarization induced charge separation in BaTiO3/Si: A pathway for non-PN junction photovoltaics

Solar energy is one of the most economical sources of renewable energy. The conventional solar cells are generally configured as PN or Schottky junctions where the absorbed sunlight is used to generate mobile electron-hole pairs and a built-in electric field separates the charge carriers. Further advances in solar photovoltaics are urgently desired to increase the performance at affordable cost. Herein, we introduce a new kind of solar cell concept which utilizes the surface bound charges of a ferroelectric (BaTiO3) for separating the charge carriers in silicon (Si). Based on the new type of charge separation mechanism, a high open circuit voltage (Voc) can be established in this type of cell configuration. The results have shown that after forward polarization, the photovoltaic effect can be achieved in the cell with both Al and Ti- electrodes. The photovoltaic properties on polarized cells of Al–Ti and Au–Ti electrodes were significantly improved where the polarization induced voltage (VP) increased greatly to be 52% of the Voc after poling. Furthermore, the results also indicated that the asymmetric ferroelectric polarization makes the major contribution to the photovoltaic effect. Theoretical simulations were also conducted for the cells, which confirmed that the electric field induced by the polarization of BaTiO3 also extended into the Si layer, resulting in the charge separation in Si. This study provides a new paradigm to design ferroelectric/semiconductor heterostructures for non-PN junction photovoltaic devices and solar energy harvesting.

Interdigitated back-contacted crystalline silicon solar cells fully manufactured with atomic layer deposited selective contacts

The interdigitated back-contacted (IBC) solar cell concept has been extensively studied for single-junction cells and more recently as a good choice for three-terminal tandem devices. In this work, carrier-selective contacts based on transition metal oxides deposited by atomic layer deposition (ALD) technique are applied to IBC c-Si(n) devices. In the first part of the study, we develop a hole-selective contact based on thin ALD vanadium oxide (V2O5) layers without using an amorphous silicon interlayer. The ALD process has been optimised, i.e. number of ALD cycles and deposition temperature, as a trade-off between surface passivation and contact resistivity. Noticeable surface passivation with recombination current densities around 100 fA/cm2, as well as reasonable contact resistivity values below 250 mΩcm2 are reached using 200 ALD V2O5 cycles deposited at a deposition temperature of 125 °C (∼10 nm layer thickness). The optimised ALD V2O5-based contact is combined with both an ALD TiO2-based electron-selective contact and an excellent surface passivation in non-contacted regions provided by ALD Al2O3 films, to form a fully ALD IBC c-Si(n) solar cell scheme. Fabricated devices yield photovoltaic efficiencies and pseudo efficiencies, i.e. calculated without series resistance losses, of 18.6% and 21.1% respectively (3 cm × 3 cm device area). These results reveal the potential of the ALD technique to deposit transition metal oxide (TMO) films as selective contacts on high efficiency devices, paving the way of using low thermal-budget, low cost and highly scalable processes for a highly demanding IBC solar cell architecture in the photovoltaic industry.

Leveraging dewetting models rather than nucleation models: current crystallographic challenges in interfacial and nanomaterials research

Abstract No scientific model has shaped crystallography as much as the classical nucleation theory (CNT). The majority of all growth processes and particle formation processes are attributed to the CNT. However, alternative descriptions exist that may be better suited to explain material formation under certain conditions. One of these alternatives is the dewetting theory (DWT). To describe the possibilities of DWT in more detail, we selected three material systems for three current application areas: Gold particles on silicon as catalysts for nanowire growth, indium particles on molybdenum as precursor material in novel solar cell concepts, and silicon layers on silicon germanium as potential wells in semiconductor quantum computers. Each of these material systems showed particular advantages of DWT over CNT. For example, the properties of surface particles with high atomic mobility could be described more realistically using DWT. Yet, there were clear indications that the DWT is not yet complete and that further research is needed to complete it. In particular, modern crystallographic challenges could serve this purpose, for example the development of semiconductor quantum computers, in order to re-evaluate known models such as the CNT and DWT and adapt them to the latest state of science and technology. For the time being, this article will give an outlook on the advantages of the DWT today and its potential for future research in crystallography.

Reassessment of the intrinsic bulk recombination in crystalline silicon

Characterisation and optimization of next-generation silicon solar cell concepts rely on an accurate knowledge of intrinsic charge carrier recombination in crystalline silicon. Reports of measured lifetimes exceeding the previous accepted parameterisation of intrinsic recombination indicate an overestimation of this recombination in certain injection regimes and hence the need for revision. In this work, twelve high-quality silicon sample sets covering a wide doping range are fabricated using state-of-the-art processing routes in order to permit an accurate assessment of intrinsic recombination based on wafer thickness variation. Special care is taken to mitigate extrinsic recombination due to bulk contamination or at the wafer surfaces. The combination of the high-quality samples with refined sample characterisation and lifetime measurements enables a much higher level of accuracy to be achieved compared to previous studies. We observe that reabsorption of luminescence photons inside the sample must be accounted for to achieve a precise description of radiative recombination. With this effect taken into account, we extract the lifetime limitation due to Auger recombination. We find that the extracted Auger recombination rate can accurately be parameterized using a physically motivated equation based on Coulomb-enhanced Auger recombination for all doping and injection conditions relevant for silicon-based photovoltaics. The improved accuracy of data description obtained with the model suggests that our new parameterisation is more consistent with the actual recombination process than previous models. Due to notable changes in Auger recombination predicted for moderate injection, we further revise the fundamental limiting power conversion efficiency for a single-junction crystalline silicon solar cell to 29.4%, which is within 0.1%abs compared to other recent assessments.

Reaching the Ultimate Efficiency of Solar Energy Harvesting with a Nonreciprocal Multijunction Solar Cell.

The Landsberg limit represents the ultimate efficiency limit of solar energy harvesting. Reaching this limit requires the use of nonreciprocal elements. The existing device configurations for attaining the Landsberg limit, however, are very complicated. Here, we introduce the concept of a nonreciprocal multijunction solar cell and show that such a cell can reach the Landsberg limit in the idealized situation where an infinite number of layers are used. We also show that such a nonreciprocal multijunction cell outperforms a standard reciprocal multijunction cell for a finite number of layers. Our work significantly simplifies the device configuration required to reach the ultimate limit of solar energy conversion and points to a pathway toward using nonreciprocity to improve solar energy harvesting.

High Efficiency (9.60) of CI Perovskites base solar cells with PCBM (ETM)and P3HT(HTM)

Due to their easy manufacturing, low production, excellent light harvest characteristics and high efficiency they have been more preferred in the last few years by organic-inorganic perivoskite solar cells in a photovoltaic research culture.In this research (PCBM) was used as Electron Transport Material (ETM) and (PHT) as Hole Transport Material (HTM). where used with the perovskite (CHNHPbcl3) and we changed the thickness of perovskite, ETM and HTM. And changed effective density of states (CB,VB) with these varibles an efficiency of 9.60 was obtained at 333.15(k).Solar cell concept evaluation is performed with the Solar Cell power simulator (SCAPS). This model optimizes different parameters such as thickness, density of absorber layer of electron transport material (ETM, ND and NA) and concentrations of doping of hole transport material (HTM, for example). This model also optimizes various parameters. Electrons and holes based on the equation of poisson and continuity can be achieved and simulated CH NH Pbcl is efficient in thickness (100nm).

Mathematical modeling of antireflection coating based on double negative index materials

New artificial Negative index materials (NIMs) or metamaterials have rapidly attracted researchers and industry due to their unusual properties. Applications of various NIMs have been found to be used in manufacturing and design some nano devices such as optical sensors and solar cells. The concept of solar cells depends on the maximizing the light transmission and minimizing the reflection. We propose solar cell structure model based on NIMs to enhance the light efficiency in solar cells. The proposed structure consists of four layers including two NIMs layers called Double NIMs. The simulation of the proposed model has been done utilizing the Transfer Matrix Method (TMM). High transmission and low reflection have been obtained. Solar cells based on double NIMs show promising future and could successfully be used to design highly efficient solar cells.

Quantum Transport across Amorphous-Crystalline Interfaces in Tunnel Oxide Passivated Contact Solar Cells: Direct versus Defect-Assisted TunnelingSupported by the National Natural Science Foundation of China (Grant Nos. 61704083 and 61874060), the Natural Science Foundation of Jiangsu Province (Grant No. BK20181388), and NUPTSF (Grant No. NY219030).

Tunnel oxide passivated contact solar cells have evolved into one of the most promising silicon solar cell concepts of the past decade, achieving a record efficiency of 25%. We study the transport mechanisms of realistic tunnel oxide structures, as encountered in tunnel oxide passivating contact (TOPCon) solar cells. Tunneling transport is affected by various factors, including oxide layer thickness, hydrogen passivation, and oxygen vacancies. When the thickness of the tunnel oxide layer increases, a faster decline of conductivity is obtained computationally than that observed experimentally. Direct tunneling seems not to explain the transport characteristics of tunnel oxide contacts. Indeed, it can be shown that recombination of multiple oxygen defects in a-SiO x can generate atomic silicon nanowires in the tunnel layer. Accordingly, new and energetically favorable transmission channels are generated, which dramatically increase the total current, and could provide an explanation for our experimental results. Our work proves that hydrogenated silicon oxide (SiO x :H) facilitates high-quality passivation, and features good electrical conductivity, making it a promising hydrogenation material for TOPCon solar cells. By carefully selecting the experimental conditions for tuning the SiO x :H layer, we anticipate the simultaneous achievement of high open-circuit voltage and low contact resistance.