Silicon Single-junction Solar Cells Research Articles

Research Progress of Semi-Transparent Perovskite and Four-Terminal Perovskite/Silicon Tandem Solar Cells

Perovskite/silicon tandem solar cells are of great interest due to their potential for breaking the Shockley-Queisser limit of single-junction silicon solar cells. Perovskite solar cells are widely used as the top subcells in perovskite/silicon tandem solar cells due to their high efficiency and lower fabrication cost. Herein, we review the semi-transparent perovskite solar cell in terms of the mechanisms of their translucent structure, transparent electrodes, charge transport layer, and component modification. In addition, recent progress in the research and development of 4T perovskite/silicon tandem solar cells is summarized, with emphasis on the influence of perovskite structure and silicon cells on the progress of tandem solar cells. Finally, we discuss the challenges associated with 4T perovskite/silicon tandem solar cells and suggest directions for the development of perovskite/silicon commercialization.

Perovskite/silicon tandem solar cells based on different layers and structures

Perovskite/silicon tandem solar cells offer great potential for achieving high-efficiency solar energy conversion. Through the absorption of a wide range of wavelength sunlight, they are not limited by the Shockley-Queisser limit in single-junction silicon solar cells. They have shown impressive progress in recent years, reached a power conversion efficiency (PCE) of 32.5%, and have a bright prospect in the future. In light of the condition of the solar industry today, this article mainly introduces the development status and the main progress direction of the perovskite/silicon tandem solar cells in recent years. In terms of efficiency improvement, it primarily uses perovskite unit band gap tuning and recombination layer optimization. Then the typical cell structures (2-terminal and 4-terminal) and some innovations (3-terminal) are introduced. The summary of these frontier researches is valuable and can guide the direction for later improvement. Lastly, the perovskite/silicon tandem solar cells still have many opportunities and challenges and are expected to become more mainstream solar cells.

Nature of contaminants introduced in silicon wafers during molecular beam epitaxy chamber annealing

Epitaxial monolithic III–V/Si tandem solar cells are one of the most promising technologies to be adopted by the industry after the efficiency of the current market dominating single junction silicon solar cell saturates at its fundamental limit. One of the key limitations of this technology is the degradation of silicon wafers during in situ annealing in the molecular beam epitaxy chamber. Determining the nature of contaminants in this process is key to improve the efficiency of epitaxial tandem solar cells. However, to date, the nature of contaminants from molecular beam epitaxy chambers remains unknown. In this work, we use photoluminescence imaging, lifetime spectroscopy, and deep level transient spectroscopy to measure the electronic properties of extrinsic impurities incorporated during annealing in the molecular beam epitaxy chamber. Photoluminescence images reveal that at least two impurities diffuse into silicon wafers during molecular beam epitaxy annealing. One is highly localized, while the other one is distributed uniformly across the whole wafer. Phosphorus diffusion is found to confine the localized impurity within the diffused layer but is ineffective at preventing the indiffusion of other impurities. Lifetime spectroscopy shows that metastable impurities with characteristic similar to Cr and CrB in our molecular beam epitaxy annealed silicon wafers. No evidence of Fe or FeB was observed. The emission rates and concentrations of the electrically active defects were measured with deep-level transient spectroscopy: The emission rates of detected defects do not match that of known Cr-related defects.

Interdigitated Back Contact Technology as Final Evolution for Industrial Crystalline Single-Junction Silicon Solar Cell

We present our own Interdigitated Back Contact (IBC) technology, which was developed at ISC Konstanz and implemented in mass production with and at SPIC Solar in Xining, China, with production efficiencies of over 24%. To our knowledge, this is the highest efficiency achieved in the mass production of crystalline silicon solar cells without the use of charge-carrier-selective contacts. With an adapted screen-printing sequence, it is possible to achieve open-circuit voltages of over 700 mV. Advanced module technology has been developed for the IBC interconnection, which is ultimately simpler than for conventional double-sided contacted solar cells. In the next step, we will realize low-cost charge-carrier-selective contacts for both polarities in a simple sequence using processes developed and patented at ISC Konstanz. With the industrialisation of this process, it will be possible to achieve efficiencies well above 25% at low cost. We will show that with the replacement of silver screen-printed contacts by copper or aluminium metallisation, future IBC technology will be the end product for the PV market, as it is the best performing c-Si technology, leading to the lowest cost of electricity, even in utility-scale applications.

Monolithic perovskite/silicon tandem solar cells with optimized front zinc doped indium oxides and industrial textured silicon heterojunction solar cells

The power conversion efficiency (PCE) of monolithic perovskite/silicon tandem solar cells has surpassed that of silicon single-junction solar cells recently. Industrial textured crystalline silicon heterojunction solar cells are considered as competitive candidate bottom cells because of their high efficiency and simple process flow. Meanwhile, the front transparent conductive window layers with industrial compatibility and low processing temperature are worth investigating in detail. In this work, zinc-doped indium oxide (IZO) thin films achieved by room-temperature sputtering are studied as a function of sputtering powers. The IZO thin film with the resistivity and electron mobility of 7.2 × 10−4 Ω cm and 40.03 cm2/Vs are obtained under 85 W, which is implemented into the monolithic perovskite/silicon tandem solar cells. A PCE of 19.38% is realized based on the industrial textured silicon heterojunction bottom solar cell and optimized IZO films.

Status and challenges of multi-junction solar cell technology

The ongoing energy transition to curb carbon dioxide emissions and meet the increasing energy demands have enhanced the need for integration of renewable energy into the existing electricity system. Solar energy has been gaining an increasing market share over the past decade. Multi-junction solar cells (MJSCs) enable the efficient conversion of sunlight to energy without being bound by the 33% limit as in the commercialized single junction silicon solar cells. III-V semiconductors have been used effectively in space applications and concentrated photovoltaics (CPV) over the past few decades. This review discusses the working and components of MJSCs at cell level as well as module level for space applications and CPV. The fabrication procedure, material acquirement of MJSCs is analyzed before introducing the current challenges preventing MJSCs from achieving widespread commercialization and the research direction in the future where these challenges can be addressed.

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.

Photonic crystals for highly efficient silicon single junction solar cells

The maximum achievable silicon single junction solar cell efficiency is limited by intrinsic recombination and by its limited capability of absorbing sun light. For Lambertian light trapping the maximum theoretical solar cell efficiency is around 29.5%. Recently a new approach for light trapping has been proposed for silicon photovoltaics. Highly regular structures with a size in the range of the wavelengths of the incident light act as so-called photonic crystals. Such structures allow wave-interference light trapping beyond the Lambertian limit. Applying these photonic crystals to silicon solar cells can help to reduce the absorber thickness and thus to minimizing the unavoidable intrinsic recombination. From a simulation study, we can conclude that 31.6% is the maximum possible single junction solar cell efficiency for a 15 μm-thin substrate. Furthermore, we present a process flow for the preparation of regular inverted pyramid structure, that acts as photonic crystal. Finally, regular inverted pyramid structures are prepared on polished and shiny-etched, i. e. on surfaces with a certain roughness, substrates. Surface passivation of these structured surfaces shows as good lifetimes as on conventional randomly pyramid textured surface. Excellent total saturation current densities on asymmetric samples of 4 ± 2 fA/cm2 for n-type and of 4.5 ± 2.2 fA/cm2 on p-type substrates are obtained.

Impact of drying temperature on the photovoltaic performance and impedance spectra of hole transport material free air processed perovskite solar cells

Due to the rapid increase in power conversion efficiency (PCE) of organic–inorganic perovskite solar cells (PSCs) and exceeding the PCE achieved in conventional single-junction silicon solar cells this technology has become the focus of research. The quality of perovskite film plays a vital role in developing the high performance PSCs and depends upon many factors, such as, composition of the perovskite, growth method, drying temperature, etc. In this work, hole transport material free (HTM-free) glass/FTO/c-TiO2/m-TiO2/m-ZrO2/Carbon electrode based PSCs are fabricated. Effect of prevoskite drying temperature on the photovoltaic performance and impedance spectra of these devices is studied by varying temperature from 50 to 70 °C. The photovoltaic and impedance spectra of the devices are observed to be highly dependent on the drying temperature. The best power conversion efficiency is obtained for drying temperature of 60 °C. These results show that determining the optimum drying temperature is crucial to ensure formation of perovskite crystals, highest surface coverage of perovskite, highest light harvesting and successful charge extraction from the fabricated devices in order to achieve highest performance.

Possible top cells for next-generation Si-based tandem solar cells.

Si-based solar cells, which have the advantages of high efficiency, low manufacturing costs, and outstanding stability, are dominant in the photovoltaic market. Currently, state-of-the-art Si-based solar cells are approaching the practical limit of efficiency. Constructing Si-based tandem solar cells is one available pathway to break the theoretical efficiency limit of single-junction silicon solar cells. Various top cells have been explored recently in the construction of Si-based tandem devices. Nevertheless, many challenges still stand in the way of extensive commercial application of Si-based tandem solar cells. Herein, we summarize the recent progress of representative Si-based tandem solar cells with different top cells, such as III-V solar cells, wide-bandgap perovskite solar cells, cadmium telluride (CdTe)-related solar cells, Cu(In,Ga)(Se,S)2 (CIGS)-related solar cells, and amorphous silicon (a-Si) solar cells, and we analyze the main bottlenecks for their next steps of development. Subsequently, we suggest several potential candidate top cells for Si-based tandem devices, such as Sb2S3, Se, CdSe, and Cu2O. These materials have great potential for the development of high-performance and low-cost Si-based tandem solar cells in the future.

InP/Si High Efficiency Heterojunction-Junction Solar Cell Design Using PSO and the GA Algorithms

This paper presents a single junction solar cell using an Atlas code simulation followed by an optimization using MATLAB software. A noticeable interest occurs in the development of a highly efficient solar cell thanks to its flexibility and lightweight. The INP thin- film solar cell is one of the top qualifiers in the thin- film solar cell market that has a significant power conversion efficiency compared to that of other solar cells. Two algorithms methods are applied on this device in order to get high power conversion efficiency, then the results of each method are compared to the others. In addition, an evaluation is performed to the present junction solar cell by comparing the presented device to similar experimental works. As a result of this paper, an optimization of single junction silicon solar cell, a high short-circuit current density of 43.74 mA/cm2, a maximum open-circuit voltage of 804 mV and a high power conversion efficiency of 30.3% are achieved. The flexibility of the proposed methodology is finally demonstrated and explained.

Monocrystalline silicon-based tandem configuration for solar-to-hydrogen conversion

Hydrogen gas produced from a photovoltaic power source for water electrolysis is a promising renewable, low-cost, storable, and transportable energy source. Herein, we propose a simply integrated hydrogenated amorphous silicon germanium (a-SiGe:H)/silicon heterojunction (SHJ) tandem configuration as an integrated photovoltaic-electrolysis device. By optimising the a-SiGe:H top cell for high efficiency using various band-gap engineering techniques, we achieve effective performance enhancement, with a maximum Voc of greater than 1.5 V. Assuming an ideal condition wherein the photovoltaic device operates at maximum power and the output conversion performance is maximized, we estimate a solar-to-hydrogen (STH) conversion performance of 13.1%. The integrated a-SiGe:H/SHJ structure can generate a sufficiently high voltage for water electrolysis while using lesser silicon than single-junction silicon solar cells connected in series. Consequently, it is a viable option for a low-cost, high-efficiency integrated photovoltaic-electrolysis system and an alternative to traditional hydrogen production from fossil fuels.

Performance analysis of cubic silicon carbide solar cell as an appropriate candidate for high temperature application

In this research article, a 3C–SiC-based single-junction solar cell is evaluated using a two-dimensional finite element method. Effects of n + and p + thicknesses and operating temperature on the performance of n + pp + 3C–SiC solar cell are simulated to find its real efficiency. For a cell with a thickness of 5 µm, the efficiencies of 12.52%, 11.2%, 10.3%, and 8.8% are obtained for n + and p + thicknesses of 0.2 µm, 0.3 µm, 0.4 µm, and 0.5 µm, respectively. It is investigated that the conversion efficiency of a Si-based solar cell is very sensitive to temperature variation. It significantly decreases when the temperature increases while 3C–SiC benefits from the best thermal stability. The efficiency gradients of 0.015%/oK and 0.0087%/oK are obtained from numerical and limitation methods for the 3C–SiC-based solar cells, respectively. It is important to mention that this value is 0.0375%/oK for a single-junction silicon solar cell.

Photonic crystal light trapping: Beyond 30% conversion efficiency for silicon photovoltaics

The power conversion efficiency of single-junction silicon solar cells has increased only by 1.5% despite extensive efforts over the past two decades. The current world-record efficiencies of silicon solar cells, within the 25%–26.7% range, fall well below the thermodynamic limit of 32.3%. We review the recent progress in photonic crystal light-trapping architectures poised to achieve 28%–31% conversion efficiency in flexible 3–20 μm-thick, single-junction crystalline-silicon solar cells. These photonic crystals utilize wave-interference based light-trapping, enabling solar absorption well beyond the Lambertian limit in the 300–1200 nm wavelength range. Using experimentally feasible doping profiles, carrier lifetimes, surface recombination velocities, and established Auger recombination losses, we review considerations leading to the prediction of 31% efficiency in a 15 μm-thick silicon photonic crystal cell with interdigitated back-contacts. This is beyond the conversion efficiency of any single-material photovoltaic device of any thickness.

Energy yield analysis of textured perovskite silicon tandem solar cells and modules.

Perovskite silicon tandem solar cells combine potentially low production costs with the ability to surpass the efficiency limit of silicon single junction solar cells. Optical modeling and optimization are crucial to achieve this ambitious goal in the near future. The optimization should seek to maximize the energy yield based on realistic environmental conditions. This work analyzes the energy yield of perovskite silicon tandem solar cells and modules based on realistic experimental data, with a special focus on the investigation of surface textures at the front and rear side of the solar cell and its implication for reflection as well as parasitic absorption properties. The investigation reveals a 7.3%rel higher energy yield for an encapsulated tandem cell with a textured front side compared with an encapsulated high efficiency single junction solar cell with 24.3% harvesting efficiency for irradiance data of the year 2014 in Freiburg/Germany.

Interface Passivation Effects on the Photovoltaic Performance of Quantum Dot Sensitized Inverse Opal TiO₂ Solar Cells.

Quantum dot (QD)-sensitized solar cells (QDSSCs) are expected to achieve higher energy conversion efficiency than traditional single-junction silicon solar cells due to the unique properties of QDs. An inverse opal (IO)-TiO2 (IO-TiO2) electrode is useful for QDSSCs because of its three-dimensional (3D) periodic nanostructures and better electrolyte penetration compared to the normal nanoparticles (NPs)-TiO2 (NPs-TiO2) electrode. We find that the open-circuit voltages Voc of the QDSSCs with IO-TiO2 electrodes are higher than those of QDSSCs with NPs-TiO2 electrodes. One important strategy for enhancing photovoltaic conversion efficiency of QDSSCs with IO-TiO2 electrodes is surface passivation of photoanodes using wide-bandgap semiconducting materials. In this study, we have proposed surface passivation on IO-TiO2 with ZnS coating before QD deposition. The efficiency of QDSSCs with IO-TiO2 electrodes is largely improved (from 0.74% to 1.33%) because of the enhancements of Voc (from 0.65 V to 0.74 V) and fill factor (FF) (from 0.37 to 0.63). This result indicates that ZnS passivation can reduce the interfacial recombination at the IO-TiO2/QDs and IO-TiO2/electrolyte interfaces, for which two possible explanations can be considered. One is the decrease of recombination at IO-TiO2/electrolyte interfaces, and the other one is the reduction of the back-electron injection from the TiO2 electrode to QDs. All of the above results are effective for improving the photovoltaic properties of QDSSCs.

Analytical approach for performance estimation of BSPVT system with liquid spectrum filters

The terrestrial solar spectrum is not monochromatic; it consists of ultraviolet, visible and infrared radiations. The most commonly used single junction silicon solar cell responds to the visible and near infrared part of the solar spectrum. The ultraviolet and the infrared part of the solar spectrum causes thermal losses in the solar cell, thus reduces its electrical conversion efficiency. In order to reduce these thermal losses, and to improve the conversion efficiency of the solar cells, the concept of spectrum filtration is introduced. Use of liquid spectrum filters for photovoltaic thermal systems (PVT) is reported in some of the recent literatures. In this technique, the selective liquids are used to filter the incoming solar radiations; the desired solar spectrum for photo electricity is only allowed to enter in. The unfiltered part of the spectrum is used separately as heat for the thermal systems. The system gives heat and electricity simultaneously and it is known as Beam Split Photovoltaic Thermal System (BSPVT). The experimental feasibility of these systems has been reported in recent literatures. In the current article, an analytical approach for the performance estimation of a BSPVT system with liquid spectrum filters is reported. C-Si PV module with three different liquids as spectrum filters viz; water, silicone oil and coconut oil are considered. The mathematical analysis is based on the estimation of radiation attenuation by the liquid spectrum filters. The ‘Transmissivity Absorptivity product (τα)’ of the liquid spectrum filters is determined using Snell's law and Bouger's law. The maximum average thermal and electrical efficiency of the system are estimated as 79% and 14% respectively. The detailed analytical approach is discussed in this article.

CdCl2 passivation of polycrystalline CdMgTe and CdZnTe absorbers for tandem photovoltaic cells

As single-junction silicon solar cells approach their theoretical limits, tandems provide the primary path to higher efficiencies. CdTe alloys can be tuned with magnesium (CdMgTe) or zinc (CdZnTe) for ideal tandem pairing with silicon. A II-VI/Si tandem holds the greatest promise for inexpensive, high-efficiency top cells that can be quickly deployed in the market using existing polycrystalline CdTe manufacturing lines combined with mature silicon production lines. Currently, all high efficiency polycrystalline CdTe cells require a chloride-based passivation process to passivate grain boundaries and bulk defects. This research examines the rich chemistry and physics that has historically limited performance when extending Cl treatments to polycrystalline 1.7-eV CdMgTe and CdZnTe absorbers. A combination of transmittance, quantum efficiency, photoluminescence, transmission electron microscopy, and energy-dispersive X-ray spectroscopy clearly reveals that during passivation, Mg segregates and out-diffuses, initially at the grain boundaries but eventually throughout the bulk. CdZnTe exhibits similar Zn segregation behavior; however, the onset and progression is localized to the back of the device. After passivation, CdMgTe and CdZnTe can render a layer that is reduced to predominantly CdTe electro-optical behavior. Contact instabilities caused by inter-diffusion between the layers create additional complications. The results outline critical issues and paths for these materials to be successfully implemented in Si-based tandems and other applications.

Efficient Semitransparent Perovskite Solar Cells Using a Transparent Silver Electrode and Four-Terminal Perovskite/Silicon Tandem Device Exploration

Four-terminal tandem solar cells employing a perovskite top cell and crystalline silicon (Si) bottom cell offer a simpler pathway to surpass the efficiency limit of market-leading single-junction silicon solar cells. To obtain cost-effective top cells, it is crucial to develop transparent conductive electrodes with low parasitic absorption and manufacturing cost. The commonly used indium tin oxide (ITO) shows some drawbacks, like the increasing prices and high-energy magnetron sputtering process. Transparent metal electrodes are promising candidates owing to the simple evaporation process, facile process conditions, and high conductivity, and the cheaper silver (Ag) electrode with lower parasitic absorption than gold may be the better choice. In this work, efficient semitransparent perovskite solar cells (PSCs) were firstly developed by adopting the composite cathode of an ultrathin Ag electrode at its percolation threshold thickness (11 nm), a molybdenum oxide optical coupling layer, and a bathocuproine interfacial layer. The resulting power conversion efficiency (PCE) is 13.38% when the PSC is illuminated from the ITO side and the PCE is 8.34% from the Ag side, and no obvious current hysteresis can be observed. Furthermore, by stacking an industrial Si bottom cell (PCE = 14.2%) to build a four-terminal architecture, the overall PCEs of 17.03% (ITO side) and 11.60% (Ag side) can be obtained, which are 27% and 39% higher, respectively, than those of the perovskite top cell. Also, the PCE of the tandem cell has exceeded that of the reference Si solar cell by about 20%. This work provides an outlook to fabricate high-performance solar cells via the cost-effective pathway.

III–V-on-silicon solar cells reaching 33% photoconversion efficiency in two-terminal configuration

Silicon dominates the photovoltaic industry but the conversion efficiency of silicon single-junction solar cells is intrinsically constrained to 29.4%, and practically limited to around 27%. It is possible to overcome this limit by combining silicon with high-bandgap materials, such as III–V semiconductors, in a multi-junction device. Significant challenges associated with this material combination have hindered the development of highly efficient III–V/Si solar cells. Here, we demonstrate a III–V/Si cell reaching similar performances to standard III–V/Ge triple-junction solar cells. This device is fabricated using wafer bonding to permanently join a GaInP/GaAs top cell with a silicon bottom cell. The key issues of III–V/Si interface recombination and silicon's weak absorption are addressed using poly-silicon/SiOx passivating contacts and a novel rear-side diffraction grating for the silicon bottom cell. With these combined features, we demonstrate a two-terminal GaInP/GaAs//Si solar cell reaching a 1-sun AM1.5G conversion efficiency of 33.3%. As silicon solar cells are reaching their optimal efficiencies, below 30%, multi-junctions are being developed to increase the electrical power output over the same area. Here, Cariou et al. use wafer-bonding to fabricate two-terminal silicon III–V tandem cells that reach efficiencies above 33%.