Monolithic Tandem Device Research Articles

Enhanced Buried Interface Engineering for Efficient Inverted Perovskite Solar Cells Fabricated via Vapor-Solid Reaction.

Vapor-deposited inverted perovskite solar cells utilizing self-assembled monolayer (SAM) as hole transport material have gained significant attention for their high efficiencies and compatibility with silicon/perovskite monolithic tandem devices. However, as a small molecule, the SAM layer suffers low thermal tolerance in comparison with other metal oxide or polymers, rendering poor efficiency in solar device with high-temperature (> 160°C) fabricating procedures. In this study, a dual modification approach involving AlOx and F-doped phenyltrimethylammonium bromide (F-PTABr) layers is introduced to enhance the buried interface. The AlOx dielectric layer improves the interface contact and prevents the upward diffusion of SAM molecules during the vapor-solid reaction at 170°C, while the F-PTABr layer regulates crystal growth and reduces the interfacial defects. As a result, the AlOx/F-PTABr-treated perovskite film exhibits a homogeneous, pinhole-free morphology with improved crystal quality compared to the control films. This leads to a champion power conversion efficiency of 21.53% for the inverted perovskite solar cells. Moreover, the encapsulated devices maintained 90% of the initial efficiency after 600h of ageing at 85°C in air, demonstrating promising potential for silicon/perovskite tandem application.

Improved Carrier Management via a Multifunctional Modifier for High-Quality Low-Bandgap Sn-Pb Perovskites and Efficient All-Perovskite Tandem Solar Cells.

All-perovskite tandem solar cells (TSCs) hold great promise in terms of ultrahigh efficiency, low manufacturing cost, and flexibility, stepping forward to the next-generation photovoltaics. However, their further development is hampered by the relatively low performance of low-bandgap (LBG) tin (Sn)-lead (Pb) perovskite solar cells (PSCs). Improving the carrier management, including suppressing trap-assisted non-radiative recombination and promoting carrier transfer, is of great significance to enhance the performance of Sn-Pb PSCs. Herein, a carrier management strategy is reported for using cysteine hydrochloride (CysHCl) simultaneously as a bulky passivator and a surface anchoring agent for Sn-Pb perovskite. CysHCl processing effectively reduces trap density and suppresses non-radiative recombination, enabling the growth of high-quality Sn-Pb perovskite with greatly improved carrier diffusion length of >8µm. Furthermore, the electron transfer at the perovskite/C60 interface is accelerated due to the formation of surface dipoles and favorable energy band bending. As a result, these advances enable the demonstration of champion efficiency of 22.15% for CysHCl-processed LBG Sn-Pb PSCs with remarkable enhancement in both open-circuit voltage and fill factor. When paired with a wide-bandgap (WBG) perovskite subcell, a certified 25.7%-efficient all-perovskite monolithic tandem device is further demonstrated.

Investigation of High-Efficiency and Stable Carbon-Perovskite/Silicon and Carbon-Perovskite/CIGS-GeTe Tandem Solar Cells

The primary purpose of recent research on solar cells is to achieve a higher power conversion efficiency with stable characteristics. To push the developments of photovoltaic (PV) technology, tandem solar cells are being intensively researched, as they have higher power conversion efficiency (PCE) than single-junction cells. Perovskite solar cells (PSCs) are recently used as a top cell of tandem solar cells thanks to their tunable energy gap, high short circuit current, and low cost of fabrication. One of the main challenges in PSCs cells is the stability issue. Carbon perovskite solar cells (CPSCs) without a hole transport material (HTM) presented a promising solution for PSCs’ stability. The two-terminal monolithic tandem solar cells demonstrate the commercial tandem cells market. Consequently, all the proposed tandem solar cells in this paper are equivalent to two-terminal monolithic tandem devices. In this work, two two-terminal tandem solar cells are proposed and investigated using the SCAPS-1D device simulator. Carbon perovskite solar cell (CPSC) without hole transport material (HTM) is used as the top cell with a new proposed gradient doping in the perovskite layer. This proposal has led to a substantial enhancement of the stability issue known to be present in carbon perovskite cells. Moreover, a higher PCE, exceeding 22%, has been attained for the proposed CPSC. Two bottom cells are examined, Si and CIGS-GeTe solar cells. The suggested CPSC/Si and CPSC/CIGS-GeTe tandem solar cells have the advantage of having just two junctions, which reduces the complexity and cost of solar cells. The performance parameters are found to be improved. In specific, the PCEs of the two proposed cells are 19.89% and 24.69%, respectively.

Dual-purpose tunnel oxide passivated contact on silicon photoelectrodes with high photovoltages for tandem photoelectrochemical devices enabling unassisted water splitting

Tunnel-oxide-passivated contact on a crystalline Si improves the photovoltage of Si photoelectrode to reach 640–650 mV even after a high-temperature process up to 600 °C, which makes it useful as the bottom cell of a monolithic tandem device.

Efficient monolithic perovskite–Si tandem solar cells enabled by an ultra-thin indium tin oxide interlayer

An ultra-thin indium tin oxide interlayer design was developed for interfacing perovskite solar cells with Si solar cells thereby minimising shunting effects for large area monolithic tandem devices.

Modeling and sensitivity analysis of a two-terminal perovskite on organic tandem solar cell

Two-terminal (2T) perovskite on organic tandem solar cells (PSC/OPV TSCs) are attracting attention due to their fast improvement in power conversion efficiency (PCE). Understanding both the optics and electronics is crucial in monolithic tandem devices. Here, we report an optoelectronic model developed for a 2T PSC/OPV TSC that helps determine the device’s most influential parameters and recombination mechanisms and elucidates the underlying physics. Our simulation results are validated with experiments, and a good agreement is obtained. The effects of surface coverage, bulk, and sheet resistances on charge carrier recombination in the interconnecting layer (ICL) are investigated with the model. When the ICL thickness is increased, we demonstrate that the mechanism in the ICL can change from charge carrier recombination to transport. Finally, a pathway toward the 30% practical PCE limit for PSC/OPV TSCs is identified, which requires the single-junction perovskite and organic sub-cells to reach PCEs of 22.3% and 20.4%, respectively. • An in-depth model for two-terminal perovskite/organic tandem cells is developed • Sensitivity and loss analyses are conducted to determine critical parameters • Layer functionality is interconnected with surface coverage and resistances • Guidelines toward 30% efficiency tandem cell with key parameters quantified Zhao et al. develop a comprehensive optoelectronic model to elucidate the underlying physics of two-terminal perovskite/organic tandem cells. To improve device efficiency, influential parameters and recombination losses are identified. Mechanisms in interconnecting layers concerning surface coverages and resistances are unveiled. This work demonstrates the potential for highly efficient tandem devices.

27.6% Perovskite/c‐Si Tandem Solar Cells Using Industrial Fabricated TOPCon Device

AbstractThe tandem cell structure is the most promising solution for the next generation photovoltaic technology to overcome the single‐junction Shockley–Queisser limit. The fabrication of a perovskite/c‐Si monolithic tandem device has not yet been demonstrated on a c‐Si bottom cell produced from an industrial production line. Here, a c‐Si cell with a tunneling oxide passivating contact (TOPCon) structure produced on a production line as the bottom cell of a tandem device, and a top cell featuring solution‐processed perovskite films to form the tandem device are used. The c‐Si cell features a rough damage etched, but untextured front surface from the wafering processes. To combat the challenge of rough surfaces, several strategies to avoid shunt paths across carrier transport layers, absorber layers, and their interfaces are implemented. Moreover, the origin of reflection loss on this planar structure is investigated and the reflection loss is managed to below 4 mA cm−2. In addition, the source of the voltage loss from the TOPCon bottom cell is identified and the device structure is redesigned to be suitable for tandem applications while still using mass production feasible fabrication methods. Overall, 27.6% efficiency is achieved for a monolithic perovskite/c‐Si tandem device, with significant potential for future improvements.

Strain Modulation for Light‐Stable n–i–p Perovskite/Silicon Tandem Solar Cells

Perovskite/silicon tandem solar cells are promising to penetrate photovoltaic market. However, the wide-bandgap perovskite absorbers used in top-cell often suffer severe phase segregation under illumination, which restricts the operation lifetime of tandem solar cells. Here, a strain modulation strategy to fabricate light-stable perovskite/silicon tandem solar cells is reported. By employing adenosine triphosphate, the residual tensile strain in the wide-bandgap perovskite absorber is successfully converted to compressive strain, which mitigates light-induced ion migration and phase segregation. Based on the wide-bandgap perovskite with compressive strain, single-junction solar cells with the n-i-p layout yield a power conversion efficiency (PCE) of 20.53% with the smallest voltage deficits of 440mV. These cells also maintain 83.60% of initial PCE after 2500 h operation at the maximum power point. Finally, these top cells are integrated with silicon bottom cells in a monolithic tandem device, which achieves a PCE of 26.95% and improved light stability at open-circuit.

The performance of a perovskite-silicon tandem photovoltaic device coupled with the infrared-enhanced response titanium subnitride film

Two kinds of carrier’s passivating contact capping stacks have been fostered for an asymmetric silicon based solar cell, in which ITO/a-SiOx(In) is for hole selection and possesses graded distribution of In, Si, O elements as well as intrinsic- and p-type ultrathin SiOx(In) layer trait, while the SiOx-TiNy layer on rear of n-Si absorber is for electron selection and behaves as a degenerated material with higher electron concentration (1023-1024 /cm3). By using Hall effect, X-ray photoemission and minority carrier lifetime measurements and simulation analyses with AFORS-HET software of TiNy films and ITO/a-SiOx(In)/n-Si/SiOx-TiNy device, respectively, it is found that the performance of the device is strongly correlated to the N deficiency of TiNy film, in addition the simulation results demonstrate that the reduced density of interface state (Dit(E)) and the enhanced near-infrared response (EQE) lead to an increase of open-circuit voltage from 578.6 to 744.1 mV, and power conversion efficiency (PCE) has increased by 30.65%, compared with the prototype of semiconductor-quasi-insulator-semiconductor device (SQIS). Moreover, a monolithic tandem device by n-i-p type perovskite cell tunnel-connected to the modified SQIS cell has achieved a PCE of 31.39%, which is predicted through an available estimation under the maximum values of the Jsc, Voc and pFF.

Contactless Series Resistance Imaging of Perovskite Solar Cells via Inhomogeneous Illumination

A contactless effective series resistance imaging method for large‐area perovskite solar cells that is based on photoluminescence imaging with nonuniform illumination is introduced and demonstrated experimentally. The proposed technique is applicable to partially and fully processed perovskite solar cells if laterally conductive layers are present. The capability of the proposed contactless method to detect features with high effective series resistance is validated by comparison with various contacted mode luminescence imaging techniques. The method can reliably provide information regarding the severeness of the detected series resistance through photoexcitation pattern manipulation. Application of the method to subcells in monolithic tandem devices, without the need for electrical contacting the terminals, appears feasible.

Exploring low-temperature processed multifunctional HEPES-Au NSs-modified SnO2 for efficient planar perovskite solar cells

In the development of high-efficiency n-i-p planar heterojunction perovskite solar cells (PSCs), the development of dense electron-transporting layers (ETLs) with excellent electrical properties and hetero-interface quality is very important for promoting perovskite growth and carrier transport dynamics. Herein, we propose a simple and effective strategy to simultaneously improve the electronic properties and interface quality of a tin oxide (SnO2) ETL by introducing 2-hydroxyethyl modified gold nanostars (HEPES-Au NSs) into a commercial SnO2 colloidal dispersion in order to synthesize mixed ETLs at low temperatures (≤150 ℃). Based on the unique plasmonic structure of the HEPES-Au NSs, we demonstrate that the modified ETLs exhibit higher conductivity and greater efficiency in the extraction, transfer, and collection of photogenerated electrons than conventional SnO2 ETLs. Moreover, the chemical bond interaction between HEPES-Au NSs and the perovskite enhances the affinity of the SnO2/perovskite hetero-interface, which effectively improves the nucleation and crystallization kinetics of the perovskite, thus producing a high-quality absorber with high crystallinity and superior absorbance. Meanwhile, the 2-hydroxyethyl (HEPES) adsorbed onto the SnO2 surface effectively passivates perovskite-related trap states, suppressing the non-radiative recombination and leakage current, and ultimately improving the relative electronic properties and photovoltaic performance output of the modified device. The results show that the power conversion efficiency (PCE) of the modified PSCs is significantly better than that of the original SnO2-based planar devices, and the optimal PCE reached 21.13%, with negligible hysteresis. Additionally, owing to the multifunctional effects of SnO2 modified by HEPES-Au NSs, the functionalized devices without encapsulation also demonstrate reliable reproducibility and good stability, which shows great advantages in the development of high-efficiency flexible PSCs and monolithic tandem devices.

Decay mechanisms in CdS‐buffered Cu(In,Ga)Se2 thin‐film solar cells after exposure to thermal stress: Understanding the role of Na

AbstractDue to their tunable bandgap energy, Cu(In,Ga)Se2 (CIGSe) thin‐film solar cells are an attractive option for use as bottom devices in tandem configurations. In monolithic tandem devices, the thermal stability of the bottom device is paramount for reliable application. Ideally, it will permit the processing of a top device at the required optimum process temperature. Here, we investigate the degradation behavior of chemical bath deposited (CBD) CdS‐buffered CIGSe thin‐film solar cells with and without Na incorporation under thermal stress in ambient air and vacuum with the aim to gain a more detailed understanding of their degradation mechanisms. For the devices studied, we observe severe degradation after annealing at 300°C independent of the atmosphere. The electrical and compositional properties of the samples before and after a defined application of thermal stress are studied. In good agreement with literature reports, we find pronounced Cd diffusion into the CIGS absorber layer. In addition, for Na‐containing samples, the observed degradation can be mainly explained by the formation of Na‐induced acceptor states in the TCO front contact and a back contact barrier formation due to the out‐diffusion of Na. Supported by numerical device simulation using SCAPS‐1D, various possible degradation models are discussed and correlated with our findings.

Investigation of non-Pb all-perovskite 4-T mechanically stacked and 2-T monolithic tandem solar devices utilizing SCAPS simulation

SCAPS simulation was utilized to complement previously published perovskite-on-Si tandem solar devices and explore herein viable all-perovskite 4-T mechanically stacked and 2-T monolithic non-Pb tandem structures. CsSn0.5Ge0.5I3 (1.5 eV) was used as top cell wide bandgap absorber, while CsSnI3 (1.3 eV) was chosen as bottom cell low bandgap absorber. The top cell was simulated with AM 1.5G 1 Sun spectrum, and the bottom cell was simulated with the filtered spectrum from the top cell. To form a 2-T monolithic tandem device, ITO was used as the recombination layer; the current matching condition was investigated by varying the thickness of the absorber layers. For a current-matched device with a Jsc of 21.2 mA/cm2, optimized thicknesses of 450 nm and 815 nm were obtained for the top and bottom absorber layers, respectively. At these thicknesses, the PCEs of the top and bottom cells were 14.08% and 9.25%, respectively, and 18.32% for the final tandem configuration. A much simpler fabricated and simulated 4-T mechanically stacked tandem device, on the other hand, showcased top and bottom cell PCEs of 15.83% and 9.15%, at absorber layer thicknesses of 1300 nm and 900 nm, respectively, and a final overall tandem device PCE of 19.86%.

Lead-free tin perovskite solar cells

Summary The development of efficient and stable lead-free perovskite solar cells (PSCs) is crucial for addressing the concern of environmental pollution from the toxic element lead. In recent years, tin PSCs have emerged as a promising candidate for high-performance, eco-friendly photovoltaic technology with a high certified power conversion efficiency (PCE) of more than 11%, indicating a great potential for future applications. Here, we review the recent efficiency progress of tin PSCs based on the equivalent circuit model of solar cells. We then discuss approaches toward efficiency improvement from the device viewpoint, such as optimizing the band gap, increasing the light-harvesting efficiency and carrier diffusion length, surface passivation, and regulating the interface energy-level alignment. Finally, we point out the possibility of reaching 20% PCE for tin PSCs and issues regarding enlarging the cell size and realizing scalable production in the future. We expect that these perspectives will be helpful for accelerating the commercialization of tin PSCs.

Perovskite/Silicon Tandem Solar Cells: Effect of Luminescent Coupling and Bifaciality

The power conversion efficiency of the market‐dominating silicon photovoltaics approaches its theoretical limit. Bifacial solar operation with harvesting additional light impinging on the module back and the perovskite/silicon tandem device architecture are among the most promising approaches for further increasing the energy yield from a limited area. Herein, the energy output of perovskite/silicon tandem solar cells in monofacial and bifacial operation is calculated, for the first time considering luminescent coupling (LC) between two sub‐cells. For energy yield calculations, idealized solar cells are studied at both standard testing as well as realistic weather conditions in combination with a detailed illumination model for periodic solar panel arrays. Typical experimental photoluminescent quantum yield values reveal that more than 50% of excess electron–hole pairs in the perovskite top cell can be utilized by the silicon bottom cell by means of LC. As a result, LC strongly relaxes the constraints on the top‐cell bandgap in monolithic tandem devices. In combination with bifacial operation, the optimum perovskite bandgap shifts from 1.71 eV to the range 1.60–1.65 eV, where already high‐quality perovskite materials exist. The results are very important for developing optimal perovskite materials for tandem solar cells.

Planar and Nanostructured n‐Si/Metal‐Oxide/WO3/BiVO4 Monolithic Tandem Devices for Unassisted Solar Water Splitting

Planar and Nanostructured n‐Si/Metal‐Oxide/WO₃/BiVO₄ Monolithic Tandem Devices for Unassisted Solar Water Splitting

Exploring low-temperature processed a-WOx/SnO2 hybrid electron transporting layer for perovskite solar cells with efficiency >20.5%

Interfacial engineering strategy between the perovskite absorber and the charge transport layer play a vital role in highly efficient perovskite solar cells. Here, we propose an amorphous tungsten oxides/tin dioxide hybrid electron transport layer to effectively block holes through the pinholes and cracks of tin dioxide to indium tin oxide, resulting in promoting charge extraction and hindering electron-hole recombination process at the hetero-interface. Moreover, owing to the higher mobility of amorphous tungsten oxides and formation of cascade energy level sequence between amorphous tungsten oxides and tin dioxide, better electron transport is obtained compared with the traditional electron transport layer. The PSCs based on amorphous tungsten oxides/tin dioxide hybrid electron transport layer shows a better power conversion efficiency of 20.52% than the single tin dioxide electron transport layer. This study guides design strategies of the electron transport layer to enhance the efficiency of the perovskite solar cells by interfacial engineering. Moreover, the entire devices preparation process are finished at a temperature below 150 °C, promising great potential for the practical use in monolithic tandem devices and providing an avenue for the progress of flexible device.

Low temperature perovskite solar cells with an evaporated TiO2 compact layer for perovskite silicon tandem solar cells

Silicon-based tandem solar cells can overcome the efficiency limit of single junction silicon solar cells. Perovskite solar cells are particularly promising as a top cell in monolithic tandem devices due to their rapid development towards high efficiencies, a tunable band gap with a sharp optical absorption edge and a simple production process. In monolithic tandem devices, the perovskite solar cell is deposited directly on the silicon cell, requiring low-temperature processes (< 200°C) to maintain functionality of under-lying layers of the silicon cell in case of highly efficient silicon hetero-junction (SHJ) bottom solar cell. In this work, we present a complete low-temperature process for perovskite solar cells including a mesoporous titanium oxide (TiO2) scaffold - a structure yielding the highest efficiencies for single-junction perovskite solar cells. We show that evaporation of the compact TiO2 hole blocking layer and ultra-violet (UV) curing for the mesoporous TiO2 layer allows for good performance, comparable to high-temperature (> 500°C) processes. With both manufacturing routes, we obtain short-circuit current densities (JSC) of about 20 mA/cm², open-circuit voltages (VOC) over 1 V, fill factors (FF) between 0.7 and 0.8 and efficiencies (η) of more than 15%. We further show that the evaporated TiO2 layer is suitable for the application in tandem devices. The series resistance of the layer itself and the contact resistance to an indium doped tin oxide (ITO) interconnection layer between the two sub-cells are low. In addition, the low parasitic absorption for wavelengths above the perovskite band gap allow a higher absorption in the silicon bottom solar cell, which is essential to achieve high tandem efficiencies.

Novel Low-Temperature Process for Perovskite Solar Cells with a Mesoporous TiO2 Scaffold.

The most efficient organic-inorganic perovskite solar cells (PSCs) contain the conventional n-i-p mesoscopic device architecture using a semiconducting TiO2 scaffold combined with a compact TiO2 blocking layer for selective electron transport. These devices achieve high power conversion efficiencies (15-22%) but mainly require high-temperature sintering (>450 °C), which is not possible for temperature-sensitive substrates. Thus far, comparably little effort has been spent on alternative low-temperature (<150 °C) routes to realize high-efficiency TiO2-based PSCs; instead, other device architectures have been promoted for low-temperature processing. In this paper the compatibility of the conventional mesoscopic TiO2 device architecture with low-temperature processing is presented for the first time with the combination of electron beam evaporation for the compact TiO2 and UV treatment for the mesoporous TiO2 layer. Vacuum evaporation is introduced as an excellent deposition technique of uniform compact TiO2 layers, adapting smoothly to the rough fluorine-doped tin oxide substrate surface. Effective removal of organic binders by UV light is shown for the mesoporous scaffold. Entirely low-temperature-processed PSCs with TiO2 scaffold reach encouraging stabilized efficiencies of up to 18.2%. This process fulfills all requirements for monolithic tandem devices with high-efficiency silicon heterojunction solar cells as the bottom cell.

Zinc tin oxide as high-temperature stable recombination layer for mesoscopic perovskite/silicon monolithic tandem solar cells

Perovskite/crystalline silicon tandem solar cells have the potential to reach efficiencies beyond those of silicon single-junction record devices. However, the high-temperature process of 500 °C needed for state-of-the-art mesoscopic perovskite cells has, so far, been limiting their implementation in monolithic tandem devices. Here, we demonstrate the applicability of zinc tin oxide as a recombination layer and show its electrical and optical stability at temperatures up to 500 °C. To prove the concept, we fabricate monolithic tandem cells with mesoscopic top cell with up to 16% efficiency. We then investigate the effect of zinc tin oxide layer thickness variation, showing a strong influence on the optical interference pattern within the tandem device. Finally, we discuss the perspective of mesoscopic perovskite cells for high-efficiency monolithic tandem solar cells.