Bifacial Devices Research Articles

Bifacial perovskite/silicon heterojunction tandem solar cells based on FAPbI3-based perovskite via hybrid evaporation-spin coating

Combining the two technologies of tandem solar cells and bifacial solar cells has a great potential to maximize energy harvesting while minimizing material and surface usage. Mid-bandgap perovskites (1.50–1.60 eV) are important for fulfilling current matching in bifacial perovskite/silicon heterojunction tandem solar cells. Herein, efficient (>20 %) and stable planar FAPbI3-based perovskite (1.54 eV) solar cells have been fabricated via a hybrid evaporation-spin coating process. X-ray diffraction and electron microscopy data reveal the formation of highly crystalline (001) perovskite domains. The fabricated high-quality perovskite films lead to a more homogenized contact potential difference at the film surface and a reduction in non-radiative losses, resulting in a quasi-Fermi-level splitting (half-cell) of 1.16 eV, rendering 120 mV non-radiative losses. Transferring these films into tandem devices atop single-side textured silicon heterojunction bottom cells, we obtain an efficiency of >24 % under AM1.5 G illumination for monofacial devices with an active area of 1.21 cm2. Furthermore, the bifacial devices generate >27 mW cm−2 power output with 15 % rear illumination fraction.

Effect of rear pyramid structures on industrial bifacial PERCs under omnidirectional incidence

The bifacial passivated emitter and rear-side contact cells (PERCs) are capturing a growing photovoltaic market share, with a trend to replace monofacial device. Here, we reported five different rear pyramidal light trapping structures prepared by acid etching, with the monofacial counterparts as a comparison. The impact of the rear pyramid angles on the efficiency between under front and rear illumination shows an opposite trend due to a wavelength-dependent optical performance. To accurately optimize devices in real outdoor environments, the photovoltaic performance of mofacial and bifacial PERCs at different incident angles is simulated and integrated with an omnidirectional efficiency. Consequently, a significant improvement in bifaciality under omnidirectional incidence is obtained compared to vertical incidence, and a rough surface structure yields the maximum bifacial gain when the albedo is greater than 30 %. Furthermore, the power yield of monofacial and bifacial devices was predicted for every day in 2022.

High-performance bifacial perovskite solar cells enabled by single-walled carbon nanotubes

Bifacial perovskite solar cells have shown great promise for increasing power output by capturing light from both sides. However, the suboptimal optical transmittance of back metal electrodes together with the complex fabrication process associated with front transparent conducting oxides have hindered the development of efficient bifacial PSCs. Here, we present a novel approach for bifacial perovskite devices using single-walled carbon nanotubes as both front and back electrodes. single-walled carbon nanotubes offer high transparency, conductivity, and stability, enabling bifacial PSCs with a bifaciality factor of over 98% and a power generation density of over 36%. We also fabricate flexible, all-carbon-electrode-based devices with a high power-per-weight value of 73.75 W g−1 and excellent mechanical durability. Furthermore, we show that our bifacial devices have a much lower material cost than conventional monofacial PSCs. Our work demonstrates the potential of SWCNT electrodes for efficient, stable, and low-cost bifacial perovskite photovoltaics.

Efficient Micrometer Thick Bifacial Perovskite Solar Cells

AbstractPerovskite solar cells have become promising candidates for thin‐film photovoltaics (PV), but many record cells suffer from losses in current (≈3–4 mA cm−2). This is due to the choice of superstrate configurations (i.e., glass‐side illumination) and thin absorber layers, typically on the order of ≈500 nm. Illumination through a top transparent conductive oxide electrode (substrate configuration) using LiF and Al2Ox as anti‐reflective coatings leads to reflectance losses below 1% is demonstrated. When combined with 1 µm thick absorber layers, substrate configurated bifacial devices have power conversion efficiencies >20%, with minimized reflection losses approaching 98% of their detailed‐balance limits and higher Jsc than their monofacial counterparts. Further analysis is conducted to show there is still a significant fraction of current lost due to poor charge‐carrier extraction (e.g., resistive or low mobility contacts). This is studied by a direct comparison of photoluminescence at short‐circuit versus open‐circuit estimating a 4.5% loss in charge‐carrier collection.

N-type CdTe:In for photovoltaics: in situ doping, type verification and compensation effects

We explored the in-situ doping of cadmium telluride thin films with indium to produce n-type absorbers as an alternative to the near-universal choice of p-type for photovoltaic devices. The films were grown by close space sublimation from melt-synthesised feedstock. Transfer of the indium during film growth was limited to 0.0014%–0.014%—unless reducing conditions were used which yielded 14%–28% efficient transport. While chunks of bulk feedstock were verified as n-type by the hot probe method, carrier type of thin film material was only able to be verified by using hard x-ray photoelectron spectroscopy to determine the Fermi level position within the band gap. The assignment of n-type conductivity was consistent with the rectification behaviour of a p-InP/CdTe:In junction. However, chloride treatment had the effect of compensating n-CdTe:In to near-intrinsic levels. Without chloride, the highest dopant activation was 20% of the chemical concentration of indium, this being for a film having a carrier concentration of n = 2 × 1015 cm−3. However, the activation was often much lower, and compensation due to over-doping with indium and native defects (stoichiometry) are discussed. Results from preliminary bifacial devices comprising Au/P3HT/ZnTe/CdTe:In/CdS/FTO/glass are presented.

Understanding and Advancing Bifacial Thin Film Solar Cells under Dual Illumination

There is a great deal of interest in increasing the energy yield from thin film solar cells by implementing bifacial operation. However, deleterious band bending and high interface recombination at the back transparent electrode can cause problems. Herein, it is investigated how bifacial thin film devices perform when illuminated through the front, back, and simultaneously through both interfaces using numerical modeling. It is shown that the downward band bending near the back interface is reduced during illumination when the carrier concentration is low. This effect is not found when the doping is relatively high, but the minority carrier distribution is still modified. Under either condition, the power generated under bifacial illumination exceeds the sum of the power generated when the illumination is solely from the back or the front. It is also shown that the back‐illuminated device performance is independent of the angle at which the light enters the back of the device, which provides accommodation for scattered light. Finally, the power enhancement is calculated for bifacial devices relative to front‐illuminated devices, and is shown that any significant loss in frontside power generation is difficult to overcome with back illumination under real‐world albedo conditions.

A mixed-integer optimization for bifacial two-terminal perovskite-on-perovskite tandem solar cells

We present a mixed-integer optimization for the optics of two-terminal perovskite-on-perovskite tandem solar cells, which accounts for thickness and material variations in all thin-film layers. The model, which is validated with experiments for both monofacial and bifacial devices, optimizes the short-circuit current density under six albedo conditions, identifying the best cell architectures and alternative material combinations that yield optimal and close-to-optimal results. In contrast to monofacial devices and low albedos, tandem devices with top subcells that are not semi-transparent are found to provide the highest short-circuit current densities under high albedos, such as albedos for white sand and snow. According to a sensitivity analysis, the short-circuit current densities in the optimized devices are not significantly affected by small perturbations in layer thicknesses. The device performances are also discussed in the context of the best optical structure. A power conversion efficiency of 34.6% and a bifacial equivalent efficiency of over 50% are predicted for the optimized monofacial device and bifacial device under high albedo values, respectively.

Bifacial CdTe Solar Cells with Copper Chromium Oxide Back‐Buffer Layer

Bifacial solar cells have the potential to maintain energy output higher than monofacial devices under unfavorable weather conditions. A transparent back‐buffer layer which can passivate the interface and improve the minority carrier lifetime is critical in CdTe‐based bifacial devices. Herein, solution‐processed CuxCryOz as a promising back‐buffer for CdTe/CdS solar cells is demonstrated. The carrier lifetimes measured at the front and back of the device are 31.2 and 3.1 ns, respectively, which correspond to an increase of ≈38% and 138%, respectively, compared to the reference device. This dramatic improvement in lifetime results in a 100% increase in short‐circuit current measured with backside illumination. The best bifacial device has efficiencies 7.6% and 12.5%, respectively, from back and front illumination, yielding a bifaciality factor of 0.60.

PEDOT-graphene counter-electrode for solar and improved artificial light conversion in regular, bifacial and FTO-less cobalt mediated DSSCs

Dye-sensitized solar cells (DSSC) are emerging as an attractive indoor photovoltaic technology to address the challenge of sustainable powering wireless sensor nodes for Internet of Things (IoTs) and low power consuming electronics. This study reports on new highly electrochemically active PEDOT:PSS (PP) and graphene nanoplatelets (GNPs) composite layer with protruding morphology as counter-electrode for Co(III/II) mediated DSSCs. The PP:GNP counter-electrode layer displays an apparent activation energy (Ea) for charge transfer with Co(bpy)3+/2+ ions of 6.2 kJ•mol−1, almost twofold lower than pristine PP. The fraction of oxidized EDOT units in the polymer increased upon graphene nanoplatelets doping; the work function of PP:GNPs catalyst was best suited to the Co(bpy)3+/2+ redox potential than pristine PP. Despite the significant difference in the catalytic activity, conventional DSSCs with PP and PP:GNP counter-electrodes display similar power conversion efficiency (PCE) of 11.0% and 10.7%, respectively, under 1-Sun illumination. A considerable advantage of the PP:GNP counter-electrode layer emerges under artificial light; DSSCs with PP:GNP cathode render PCE of 29.7% at 1000 lx, surpassing the counterparts with PP counter-electrode. Protruding from the PP matrix film, GNPs particles shorten the effective diffusion path for Cobalt ions and decrease the device resistance; higher photocurrent is possible under low power illumination. Transparency of ca. 65% in the visible region and superior electrical conductivity of the PP:GNP films allow using them in FTO-less bifacial devices; DSSCs with bare soda-lime glass substrates for the cathode displayed a PCE of 9.4% under 1-Sun and 21.9% and 18.7% under 1000 lx, for front and back illumination respectively. This makes PP:GNP counter-electrode very attractive for the cost-effective DSSCs targeting indoor light conversion.

Copper iodide nanoparticles as a hole transport layer to CdTe photovoltaics: 5.5 % efficient back-illuminated bifacial CdTe solar cells

We report the role of copper iodide (CuI) nanoparticles (NPs) as a hole transport layer (HTL) in cadmium sulfide/cadmium telluride (CdS/CdTe) photovoltaics. These CuI NPs were prepared using solution processing at room temperature and used to fabricate monofacial and bifacial CdTe solar cells with different back contacts. Using CuI/Au as the back contact, the device efficiency reached to 14.8% with outstanding fill factor (FF) of 79.2%. Replacing the gold (Au) electrode with sputtered transparent indium tin oxide (ITO), a CuI/ITO back contact yielded photoconversion efficiencies (PCEs) of 11.6% and 5.5% under front and back illumination respectively. Bifacial devices (CdTe/ITO) without the CuI NP HTL have an efficiency of 7.0% and 1.0% for front and back illumination, respectively. For CuI/ITO, a current collection of 12.0 mAcm−2 was observed upon back illumination which significantly improved over an ITO-only back contact (5.0 mAcm−2). The PCE obtained from back illumination was enhanced when using CuI NPs as the HTL due to the reduced back barrier height, and improved back interface as determined by temperature dependent current vs. voltage characteristics and impedance spectroscopy analysis. The improvement in device performance of the bifacial configuration is a significant step forward toward realizing thin film photovoltaic modules which harvest energy incident on the rear of the module.

Highly crystallized tungsten doped indium oxide film stabilizes silicon heterojunction solar cells in sodium environment

High-efficient silicon heterojunction (SHJ) solar cells are ideal bifacial devices. In this application context, Na + ions induced degradations from encapsulation soda-lime glass are of significance. We conducted accelerated Na + ions aging experiments at 85 °C, 85% relative humidity without encapsulation to simulate damp heat test in soda-lime glass for SHJ solar cells. It is found that Na + ions accelerated the formation of metal In and In(OH) 3 or InOOH in damp heat condition, which destroyed the capping metal oxide, i.e., tungsten doped indium oxide (IWO). Interestingly, it demonstrates that the device stability is highly dependent on the crystallinity of IWO films. IWO films with large grains are more resistive to Na + ions erosion, and consequently, account for the enhanced device stability of SHJ solar cells. We believe this work provides a guide for designing more reliable bifacial SHJ solar cells installed in hot climates and coastal areas. • Accelerated Na + ions aging experiments are performed to simulate damp heat test in soda lime glass for SHJ solar cells. • Na + ions diffuse to a-Si:H layers via the grain boundaries of IWO. • Highly crystallized IWO films are more likely to resist the diffusion and erosion of Na + ions. • This work provides a guide for designing more reliable bifacial SHJ solar cells installed in hot climates.

Review on the Sources of Power Loss in Monofacial and Bifacial Photovoltaic Technologies

An evaluation of the degradation effects on photovoltaic modules is essential to minimise uncertainties in the system operation. Bifacial photovoltaic technology is attracting attention due to the capacity of generating energy from the front and rear sides. This paper presents a review of degradation factors, for both conventional monofacial and bifacial photovoltaic modules, to highlight how the current and voltage characteristics of these technologies are affected by degradation. Microcracking, encapsulant discoloration, and light induced degradation seem to have similar effects on both modules. Contrarily, bifacial modules are more prone to potential induced degradation as the electromagnetic shielding is affected by the bifaciality. Bifacial devices are less affected by light and elevated temperature induced degradation. The degradation (1.3%) is similar for both technologies, up to 40 kWh/m2 of solar radiation. Above this value, monofacial degradation increases faster, reaching values of 7%. For tilted systems, the front side soiling degradation of 0.30% per day is similar for both technologies. For vertical systems, soiling loss for bifacial is considerably lower with values of 0.02% per day.

Plasma etching: a strategy to enhance the photovoltaic conversion efficiency of ultrathin CdTe solar cells

Ultrathin CdTe solar cell is a promising application of using integrated photovoltaics to act as architectural components and bifacial devices to promote the utilization of sunlight. In the preparation process of the ultrathin CdTe solar cells, the chemical solution etching process before back contact deposition has to be refrained due to the high risk of shunting. Therefore, an alternative strategy to get a clean CdTe surface is required. In this work, plasma etching treatment was conducted on sputtered ultrathin CdTe films to eliminate the residual oxides induced by CdCl2 treatment. Furthermore, ultrathin bifacial CdTe solar cells with a structure of glass/FTO/SnO2/CdS/CdTe (∼920 nm)/CuCl/ITO were fabricated to investigate the effect of plasma etching on the device performance. As a result, by employing plasma etching treatment, a bifacial device with a front illuminated efficiency of up to 9.93% and a rear illuminated efficiency of 2.41% was achieved. This work provides important indications for the performance optimization of ultrathin CdTe solar cells.

MoS2/ZIF-8 derived nitrogen doped carbon (NC)-PEDOT: PSS as optically transparent counter electrode for dye-sensitized solar cells

A few-layered MoS2 successfully grown on the nitrogen-doped carbon (NC) derived from zeolitic imidazolate framework (ZIF-8) via the hydrothermal method. A small portions of the obtained MoS2/NC composite blended with Poly (3, 4-ethylenedioxythiophene poly styrene sulfonic acid)PEDOT: PSS(as a conductive binder and utlized as counter electrode (CE) for dye-sensitized solar cells (DSSCs). The spin-coated MoS2/NC-PEDOT: PSS composite on Fluorine-doped Tin Oxide (FTO) glass not only possesses promissing catalytic ability toward the I3-/I-and [Co(bpy)3]2+/3+ redox mediators but also displayed considerable optical transparency in the visible range thus used to fabricate bi-facial DSSCs. The assembled bi-facial devices with MoS2/NC-PEDOT: PSS composite CEs showed an excellent power conversion efficiency (PCE) under both the front and the rear irradiations. The PCE of the device with 5% MoS2/NC-PEDOT: PSS composite CE and I3-/I- redox couple are 7.67 and 4.54%, under the front and rear irradiation respectively. Also, the values of the device with the same composite CE and [Co(bpy)3]2+/3+ redox mediator are 7.87 and 5.34% under the front and rear irradiation, respectively. The optimum performance of the devices with the prepared CEs and both redox systems are comparable with Pt-based devices. Thus, the MoS2/NC-PEDOT: PSS composite CEs are recommend being a promising Pt-free CEs for DSSCs.

Superstrate-type flexible and bifacial Cu(In,Ga)Se2 thin-film solar cells with In2O3:SnO2 back contact

Bifacial and flexible Cu(In,Ga)Se2 (CIGS) thin-film solar cells with superstrate-type structure were fabricated. For this fabrication, the substrate-type solar cells were first transferred from Mo/Glass substrates to 50-µm-thick flexible fluorinated ethylene propylene (FEP) copolymer films as superstrates using a lift-off process. The 300-nm-thick In2O3:SnO2 (ITO) thin-films were subsequently deposited on the rear sides of the CIGS absorber layers as back contacts (BC) of the finished bifacial devices with the structure of FEP/Epoxy/Al/Ni/Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/CdS/CIGS/ITO/Ni/Al. Particular attention was paid to maintain the high optical and electrical transport properties of ITO-BC as well as to improve the Ohmic-like contact behavior at the CIGS/ITO interface to improve cell performances. Under the optimized ITO preparation, the resulting ITO thin film possesses the average optical transparency of ITO (>77%) in the wavelength range of 400–1100 nm, and the resistivity of 6.1 × 10−4 Ωcm with corresponding high carrier mobility of 42.8 cm2/Vs, as well as Ohmic-like characteristics at the CIGS/ITO interface can be realized with shunt resistance as low as 5.5 Ωcm2. Consequently, the flexible and bifacial CIGS thin-film solar cell was fabricated with efficiencies of 9.1% and 2.4% for frontside and backside illuminations, respectively. In this study, highly transparent and conductive ITO thin films have been successfully used as BC in flexible and bifacial CIGS solar cells.

A simulation study to evaluate and compare monofacial Vs bifacial PERC PV cells and the effect of albedo on bifacial performance

Bifacial devices (bifacial cells and modules) have the capability of absorbing irradiation from its front as well as its rear side. This phenomena enables bifacial devices to produce a higher energy yield and consequently a reduction in the levelized cost of energy is achievable. Bifacial PV modules also have a higher energy density as they are capable of producing more energy for the same area. This paper presents a study on the modelling of a PERC bifacial cell performance under STC conditions (AM 1.5G) for different albedos. Along with that, a comparison between monofacial and bifacial cell when exposed to the same conditions is presented. A simulation has been carried out to determine the capability of c-Si PERC bifacial solar cells to derive power compared to its monofacial counterpart. The results show that bifacial solar cell produces more JSC and power relative to monofacial cell. The bifacial cell output is also measured for varying albedo, with higher power being derived at higher values of albedo. These results highlight the beneficial property of bifacial PV technology.

High‐Performance Semitransparent and Bifacial Perovskite Solar Cells with MoOx/Ag/WOx as the Rear Transparent Electrode

AbstractSemitransparent solar cells play a crucial role in such typical photovoltaic applications as smart windows, transparent chargeable devices, tandem and bifacial devices, and so on. Relying on the commercial conductive transparent oxides as the front electrodes, the development of rear transparent electrode (RTE) is especially essential. Here, an efficient semitransparent perovskite solar cell (PSC) with the softly deposited transparent MoOx/Ag/WOx (MAW) as the rear electrode is demonstrated. MoOx enables the continuously grown silver ultrathin film while the high‐refractive‐index WOx capping layer can modulate the whole optical interference and enhance the light transmission throughout the device. As a result, depending on the MAW RTE, the semitransparent normal planar PSC owns the optimal conversion efficiency of 15.40% along with 10.17% in the average visible‐light transmission (AVT) simultaneously, which claims the best conversion efficiency of the semitransparent PSCs at such considerable AVT. Combining the optical characterizations, the bifacial performance test of the same device also reveals the uneven absorption due to the different optical interference depending on the light direction, as well as the typical parasitic absorption by the functional layers. This paper paves an alternative and promising way to fabricate high‐performance semitransparent optoelectronic devices in the future.

Architectural modification coupled with MAI passivation of MAPbI3–MAPbI3 interface for fabrication of highly-responsive broadband bifacial perovskite photodetectors

We present a modified architecture of sandwiched heterojunction bifacial perovskite photodetectors where an incident illumination falls directly on the perovskite layer through a window created on charge transport layers. Furthermore, the effects of passivating sandwiched MAPbI3 active layers with MAI solution was investigated. The fabricated photodetectors were electrically, optically and electrochemically characterized as bifacial devices. The champion device displays excellent performance with maximum detectivity and responsivity of 5.96 × 1013 Jones and 1110 A/W respectively. The photodetectors demonstrate good absorbance between 350 nm and 780 nm wavelength range. A maximum external quantum efficiency of 3000% was recorded for the MAI passivated photodetector at 450 nm. After more than 350 days of storage in ambient environment conditions, the champion device shows remarkable stability retaining about 94% of its initial saturated photocurrent, showing remarkable performance metrics retention. For further study, succinct resistance analysis using the EIS measurements were performed. Based on these performance values, this new architecture may instigate future commercial designs of optoelectronic devices to be bifacial and architecturally modified with perovskite window, thereby improving their photon-absorption property, all scaling up towards building highly efficient classes of optoelectronic devices.

Characterization of sputtered MoOx thin films with different oxygen content and their application as back contact in CdTe solar cells

Transparent MoOx thin films with varied composition were successfully prepared by sputtering and the effects of oxygen contents on the composition stoichiometry, defect states, work functions, electrical and optical properties of MoOx films were systematically studied. These thin films were then utilized in cadmium telluride solar cells to act as back contact buffers. As compared with the CdTe solar cells with Au back contact, deteriorated performance was witnessed in those with MoOx buffers. The deterioration was demonstrated to be caused by the oxidation of CdTe surface, the large valence band offset and the insufficient work function. Finally, by optimizing the back contacts to form a CuCl/MoOx/ITO composite back contact, the efficiency was improved to 11.8%. This work demonstrates feasible approaches to prepare MoOx thin films with varied oxygen concentrations which could be easily applied to many other thin film solar cells and provide important indications in the preparation and optimization of the performance for bifacial devices.

Accuracy of simulated data for bifacial systems with varying tilt angles and share of diffuse radiation

Potential investors in bifacial PV systems are still deterred by the uncertainty of energy yield predictions. Validation of the prediction methods and establishing an accuracy range is therefore a central prerequisite concerning the bankability of the bifacial technology.Because of the sensitivity on multiple additional factors compared to standard monofacial installations, the yield simulation of a bifacial PV array is far more complicated. The available simulation tools for bifacial systems still have to prove their reliability, also for predicting the effect of varying ambient and installation conditions. Moreover, bifacial devices enable new types of applications, such as vertical installations.In this work, we compare the results of simulation tools developed at ISC Konstanz and ECN.TNO, as well as the output of the commercial PVsyst simulation software, to measured data. The measured data is obtained by a test rig that carries out a continuous and automated variation of the tilt angle. The test rig is not a single stand-alone module but an array in order to include shading effects of real extended systems. In addition, the measurements focus on a central module, which represents the conditions of a typical device in an array. Days with differing light intensity and share of diffuse radiation were chosen to determine the impact of the insolation conditions on the simulation accuracy. General trends with regard to the sensitivity to tilt and insolation conditions are analyzed in order to evaluate the quality of state-of-the-art bifacial energy yield simulations.