Planar Solar Cells Research Articles

Effect of cell thickness on the electrical and optical properties of thin film silicon solar cell

In this work Electrical and optical properties of silicon thin films with different thickness were measured. The thickness of the Si films varied from 100 to 800μm. The optical properties of the cell were studied at different thickness. A maximum achievable current density (MACD) generated by a planar solar cell, was measured for different values of the cell thickness which was performed by using photovoltaic (PV) optics method. It was found that reducing the values of the cell thickness improves the open-circuit voltage (VOC) and the fill factor (FF) of the solar cell. The optical properties were measured for thin film Si (TF-Si) at different thickness by using the double beam UV-vis-NIR spectrophotometer in the wavelength range of 300–2000nm. Some of optical parameters such as refractive index with dispersion relation, the dispersion energy, the oscillator energy, optical band gap energy were calculated by using the spectra for the TF-Si with different thickness.

Tailoring Organic Cation of 2D Air‐Stable Organometal Halide Perovskites for Highly Efficient Planar Solar Cells

2D perovskites have recently been shown to exhibit significantly improved environmental stability. Derived from their 3D analogues, 2D perovskites are formed by inserting bulky alkylammonium cations in‐between the anionic layers. However, these insulating organic spacer cations also hinder charge transport. Herein, such a 2D perovskite, (iso‐BA)2(MA)3Pb4I13, that contains short branched‐chain spacer cations (iso‐BA+) and shows a remarkable increase of optical absorption and crystallinity in comparison to the conventional linear one, n‐BA+, is designed. After applying the hot‐casting (HC) technique, all these properties are further improved. The HC (iso‐BA)2(MA)3Pb4I13 sample exhibits the best ambient stability by maintaining its initial optical absorption after storage of 840 h in an environmental chamber at 20 °C with a relative humidity of 60% without encapsulation. More importantly, the out‐of‐plane crystal orientation of (iso‐BA)2(MA)3Pb4I13 film is notably enhanced, which increases cross‐plane charge mobility. As a result, the highest power conversion efficiencies (PCEs) measured from for current density versus voltage curves afford 8.82% and 10.63% for room‐temperature and HC‐processed 2D perovskites based planar solar cells, respectively. However, the corresponding steady‐state PCEs are remarkably lower, which is presumably due to the significant hysteresis phenomena caused by low charge extraction efficiency at interfaces of C60/2D perovskites.

Limiting factors of photon-to-current conversion in polymer/nanocrystal bilayer hybrid solar cells: An analytical quantum efficiency model study

This paper introduces an analytical external quantum efficiency (EQE) model of planar hybrid solar cells (HSCs) based on photon-to-current conversion processes and uses this to investigate the factors that limit the maximum EQE (EQEm) of devices; i.e., the photon absorption coefficient α, exciton diffusion coefficient Dz, exciton lifetime τz, exciton dissociation rate kdis, electron diffusion coefficient De, electron lifetime τe, nanocrystals thickness d, and thickness of the polymer l. Our simulations indicate that relying solely on modifying kdis, De, or τe cannot achieve a breakthrough increase in the EQEm of planar HSCs. However, increasing α, Dz, or τz could potentially lead to a large EQEm (30–100%), especially in the context of high kdis values. Moreover, the calculation results indicate that although both Dz and τz contribute to the exciton diffusion length (Lz) via the equation Lz2 = Dzτz, the EQEm has an asymmetric dependence on these variables. With a small kdis (i.e., <104 cm/s), an increase in Dz results in an initial increase and then decrease in EQEm, resulting in a peak value that increases with increasing kdis. When kdis is sufficiently large (∼105 cm/s), the EQEm becomes saturated after the initial increase. Thus, although an increase in Dz can adversely affect device performance when the kdis is lower than 104 cm/s, increasing τz always improves device performance, regardless of large kdis becomes. This behavior can be attributed to the detrimental effect of excitons accumulating at the D/A interface, and can be used to optimize the material design and device engineering of planar HSCs and related solar cells for maximum photon-to-current conversion performance. In addition, we also demonstrate that the model can fit to the experimental data.

Numerical optical optimization of monolithic planar perovskite-silicon tandem solar cells with regular and inverted device architectures.

We numerically maximize the achievable photocurrent density of planar perovskite-silicon tandem solar cells for different device architectures. For the optimizations we combine the transfer-matrix method with a simulated annealing algorithm. The optimizations are conducted within experimentally accessible and relevant layer-thickness ranges, which allows to extract applicable device guidelines. A comparison between regular and inverted tandem-cell designs reveals that a rear-emitter silicon heterojunction in combination with an inverted perovskite top-cell can yield a photocurrent, which is 1.4 mA/cm2 higher than that of tandem cells with the usual polarity and a front-emitter silicon bottom cell. Switching from the regular to the inverse architecture leads to over 2% (absolute) gain in power conversion efficiency. Finally we show that an efficiency of 30.8% is achievable for such tandem cells with an optimized perovskite band-gap.

300% Enhancement of Carrier Mobility in Uniaxial-Oriented Perovskite Films Formed by Topotactic-Oriented Attachment.

Organic-inorganic perovskites with intriguing optical and electrical properties have attracted significant research interests due to their excellent performance in optoelectronic devices. Recent efforts on preparing uniform and large-grain polycrystalline perovskite films have led to enhanced carrier lifetime up to several microseconds. However, the mobility and trap densities of polycrystalline perovskite films are still significantly behind their single-crystal counterparts. Here, a facile topotactic-oriented attachment (TOA) process to grow highly oriented perovskite films, featuring strong uniaxial-crystallographic texture, micrometer-grain morphology, high crystallinity, low trap density (≈4 × 1014 cm-3 ), and unprecedented 9 GHz charge-carrier mobility (71 cm2 V-1 s-1 ), is demonstrated. TOA-perovskite-based n-i-p planar solar cells show minimal discrepancies between stabilized efficiency (19.0%) and reverse-scan efficiency (19.7%). The TOA process is also applicable for growing other state-of-the-art perovskite alloys, including triple-cation and mixed-halide perovskites.

All-dielectric planar solar cells with multilayer ARC and non-periodic DBR nanolayers based on transmission line equivalent circuit

In this paper, an analytical approach based on transmission line equivalent circuit (TLEC) is used to model planar solar cells. The proposed method has been applied to the all-dielectric planar solar cells with multilayer anti-reflection coating (ARC) and non-periodic distributed Bragg reflector (DBR) nanolayers. This analytical approach is very efficient and easy to be implemented. Consequently, it is straightforward to employ several optimization procedures to determine the optimum thickness for different layers. In this paper, optimization process according to the Fuzzy adaptive modified particle swarm optimization (FAMPSO) algorithm is utilized to maximize the absorption enhancement of these types of solar cells. Results demonstrate the possibility of wideband absorption in all-dielectric planar solar cells and comparable to the conventional light trapping structures.

Triple-cation mixed-halide perovskites: towards efficient, annealing-free and air-stable solar cells enabled by Pb(SCN)2 additive

Organo-metal halide perovskites have suffered undesirably from structural and thermal instabilities. Moreover, thermal annealing is often indispensable to the crystallization of perovskites and removal of residual solvents, which is unsuitable for scalable fabrication of flexible solar modules. Herein, we demonstrate the non-thermal annealing fabrication of a novel type of air-stable triple-cation mixed-halide perovskites, FA0.7MA0.2Cs0.1Pb(I5/6Br1/6)3 (FMC) by incorporation of Pb(SCN)2 additive. It is found that adding Pb(SCN)2 functions the same as thermal annealing process by not only improving the crystallinity and optical absorption of perovskites, but also hindering the formation of morphological defects and non-radiative recombination. Furthermore, such Pb(SCN)2-treated FMC unannealed films present micrometer-sized crystal grains and remarkably high moisture stability. Planar solar cells built upon these unannealed films exhibit a high PCE of 14.09% with significantly suppressed hysteresis phenomenon compared to those of thermal annealing. The corresponding room-temperature fabricated flexible solar cell shows an impressive PCE of 10.55%. This work offers a new avenue to low-temperature fabrication of air-stable, flexible and high-efficiency perovskite solar cells.

Study on the role of additional ions in CH3NH3PbI3−xClx planar solar cells

The present study investigates the influence of the introduced metallic cation and/or halide anion, e.g. Al3+ and Cl−, on the forming mechanism and electro-optical properties of CH3NH3PbI3 films. Our study shows that introducing of Al impurity into perovskite films produces the intermediate AlI3 that improves the film coverage by expressively slowing down the evaporation of CH3NH3Cl. Likewise, a trace amount of hydrolysis product, i.e. Al2O3, homogenously distribute throughout the precursor as a seed, which is proved to be capable of tuning the crystal orientation and the grain size. Consequently, the introduction of metallic cation and/or halide anion enable the realization of highly crystallized thin film with highly reduced defects, which can effectively alleviate the hysteresis effect that was intractable in planar perovskite cells.

Compositional and morphological engineering of mixed cation perovskite films for highly efficient planar and flexible solar cells with reduced hysteresis

We report on compositional and morphological engineering of mixed methylammonium (MA) and formamidinium (FA) lead triiodide (MA1−xFAxPbI3) perovskite absorber layers to produce highly efficient planar and flexible perovskite solar cells (PVSCs) with reduced hysteresis. Incorporation of FA into the MAPbI3 extends the absorption edge of the perovskite to longer wavelengths, leading to enhanced photocurrent of the resultant PVSCs. Moreover, adding a small amount of lead thiocyanate (Pb(SCN)2) additive into mixed perovskite precursor solutions significantly enlarges the grain size and prolongs the carrier lifetime, leading to improved device performance. With optimal compositional and morphological engineering, the average power conversion efficiency (PCE) improves from 15.74±0.74% for pure MAPbI3 PVSCs to 19.40±0.32% for MA0.7FA0.3PbI3 PVSCs with 3% Pb(SCN)2 additive, exhibiting a high reproducibility and small hysteretic behavior. The best PVSC achieves a PCE of 20.10 (19.85)% measured under reverse (forward) voltage scan. Furthermore, the compositional and morphological engineering allowed the fabrication of efficient flexible PVSCs on indium-doped SnO2 (ITO)/polyethylene terephthalate (PET) substrates, with the best PCE of 17.96 (16.10)% with a VOC of 1.076 (1.020) V, a JSC of 22.23 (22.23) mA/cm2 and a FF of 75.10 (71.02)% when measured under reverse (forward) voltage scan. Our approach provides an effective pathway to fabricate highly efficient and reproducible planar PVSCs.

Exploring the Photovoltaic Performance of All-Inorganic Ag2PbI4/PbI2 Blends.

We present an all-inorganic photoactive material composed of Ag2PbI4 and PbI2, which shows unexpectedly good photovoltaic performance in planar junction solar cells delivering external quantum efficiencies of ∼60% and light power conversion efficiencies of ∼3.9%. The revealed characteristics are among the best reported to date for metal halides with nonperovskite crystal structure. Most importantly, the obtained results suggest a possibility of reaching high photovoltaic efficiencies for binary and, probably, also ternary blends of different inorganic semiconductor materials. This approach, resembling the bulk heterojunction concept guiding the development of organic photovoltaics for two decades, opens wide opportunities for rational design of novel inorganic and hybrid materials for efficient and sustainable photovoltaic technologies.

Improvement of minority carrier collection and quantum efficiency in graphene planar silicon solar cell

Graphene planar silicon heterojunction solar cells were investigated using 2D physics-based TCAD simulation. A planar structure consisting of graphene layer as the hole transport material, and n-type silicon as substrate is simulated. Process modeling has been carried out especially for highly boron diffusion. Using this model, the effect of the highly doped surface inverse region located as 0.08, 0.22, 0.35 μm on the photovoltaic performance has been studied. The obtained J–V characteristic is analyzed to study effects of the inverse region depth and doping concentration on Schottky junction modification. The proposed design proves to be highly efficient in 0.2 h annealing, which provides a new platform to further enhance the performance of graphene planar solar cell.

Low-temperature and Ambient Air Processes of Amorphous SnOx-based Mixed Halide Perovskite Planar Solar Cell

Herein, we report an amorphous tin oxide (SnOx)-based planar solar cell using a mixed organic cation mixed halide perovskite ((FAPbI3)0.85(MAPbBr3)0.15) absorber prepared via low-temperature solution processes under the ambient air condition (relative humidity 40%). The device performance demonstrated power conversion efficiency (PCE) up to 16%. Additionally, the device with a SnO2 compact layer (CL) also reveals superior light-harvesting properties under weak incident light in comparison with those of TiO2.

Highly efficient and stable inverted planar solar cells from (FAI)x(MABr)1−xPbI2 perovskites

We report highly efficient and stable inverted planar lead mixed-halide (Br, I) perovskite solar cells with a configuration of ITO/poly(3-bromothiophene)/(FA)y(MA)1−yPbBrxI3−x/C60/BCP/Ag (FA: HC(NH2)2+; MA: CH3NH3+). We found that small changes in the composition of (FA)y(MA)1−yPbBrxI3−x have big impact on the material properties and device performance. Appropriate Br-doping enlarges MAPbBrxI3−x's bandgaps and prolongs the life of the excited charge carrier, which leads to higher device open-circuit voltage (VOC). Replacing “MA” with “FA” extends the absorption of (FA)y(MA)1−yPbBrxI3−x which compensates the JSC loss in MAPbBrxI3−x from Br-doping. The optimized perovskite film with a composite of FA0.8MA0.2PbBr0.2I2.8 shows a lifetime of 670ns and a photoelectric response to 830nm, resulting in an enhanced JSC of 22.2mAcm−2, a high FF of 0.80, and an efficiency of 18.1%. In addition, the inverted device based on FA0.8MA0.2PbBr0.2I2.8 showed long-term stability with 80% efficiency remained after 4 months in a glovebox without encapsulation. Our results demonstrate highly efficient and stable inverted planar perovskite solar cells can be achieved by optimizing absorber material composition, which offer a reference for their applications in flexible or tandem solar cells.

Method of Removing Single-Side Doped Layer While Maintaining Pyramid Textured Surface of n-Type Bifacial Solar Cells

BBr3 and POCl3 thermal diffusion is a widely used technique for p- and n-type emitter formation in bifacial solar cell fabrication. However, single-side doping of BBr3 or POCl3 is difficult to achieve through gas diffusion carried out at high temperatures. Thus, removal of the unexpected doped layer on one side is undertaken in the fabrication of bifacial solar cells. We have implemented a spin-etching technique to remove the doped layer; however, the removal leads to a planar surface, and the related optical losses affect the bifaciality of the cell through a lowering of the short-circuit current. In this paper, we demonstrate a new procedure to maintain the pyramid texture on the side from which the doped layer is removed. The effect of the texture-maintaining removal process was examined using scanning electron microscopy (SEM) and reflectance measurements. SEM images show that the edges of the texture pyramids become wider. The optical losses due to the wider edges were investigated through reflection measurements. To investigate the potential of this new procedure, an n-type bifacial cell was fabricated on a 156 mm × 156 mm 180-μm-thick n-type pseudo Czochralski-Si wafer, and the current–voltage parameters were compared with the single-side planar n-type bifacial solar cell.

High Members of the 2D Ruddlesden-Popper Halide Perovskites: Synthesis, Optical Properties, and Solar Cells of (CH3(CH2)3NH3)2(CH3NH3)4Pb5I16

Here, we present the fifth member (n = 5) of the Ruddlesden-Popper (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 family, which we successfully synthesized in high yield and purity. Phase purity could be clearly determined from its X-ray powder diffraction patterns, which feature the (0k0) Bragg reflections at low 2θ angles. The obtained pure n = 5 compound was confirmed to be a direct band-gap semiconductor with Eg = 1.83 eV. The direct nature of the band gap is supported by density functional theory calculations. Intense photoluminescence was observed at room temperature at 678 nm arising from the band edge of the material. High-quality thin films can be prepared by the hot-casting method from solutions with a pure-phase compound as a precursor. The planar solar cells fabricated with n = 5 thin films demonstrate excellent power-conversion efficiency of 8.71% with an impressive open-circuit voltage of ∼1 V. Our results point to the use of layered perovskites with higher n numbers and pure chemical composition.

Radial junction solar cells prepared on single crystalline silicon wafers by metal‐assisted etching

Radial junction solar cells have been proposed as an alternative device geometry to conventional planar solar cells with its remarkable electrical and optical performance. In this geometry, densely packed nano/micropillars allow minority carrier collection in the radial direction and shorten carrier diffusion length to p–n junction. Besides, reduced reflection from surface and increased light trapping in nano/micropillars enhance solar cell efficiency. In this study, photolithography and metal‐assisted etching (MAE) techniques are used to form well‐ordered silicon micropillar arrays; standard doping, passivation, and metallization steps are followed to form radial junction solar cells. The effect of micropillar length on optical and electrical performance of the solar cells have been investigated. We observed that optical reflection from solar cells surface decreased with increasing micropillar length, hence solar cell short circuit current (Jsc) and efficiency (ɳ) increased. Our best solar cell efficiency is 15.6% and this is one of the highest reported values obtained from the radial junction solar cells prepared by MAE technique.

Influence of Charge Transport and Defects on the Performance of Planar and Mesostructured Perovskite Solar Cells

Photoinduced charge selective carrier extraction by linearly increasing voltage technique allows straightforward assessment of charge transport properties within planar and mesostructured perovskite solar cells with respect to light intensity and signal delay time. Charge sensitive device architecture is realized through implementation of insulating layer between the anode or cathode to prevent extraction of unwanted type of carriers. Resulting behavior of comparatively efficient mesoporous and planar solar cells exhibits well balanced charge transport with slight dependence of charge mobility on applied laser pulse fluence, for given pulse delay times. Very similar charge carrier mobilities are present within mesoporous devices, whereas holes trail approximately half an order of magnitude behind electrons in planar structured specimens. Moreover, dispersive transport is identified in the electron selective devices with titanium oxide electron transporter, suggesting considerable presence of trapping states at the perovskite interface, whereas no such behavior characterizes planar samples. Variation in delay time between laser pulse and extraction ramp only affects initial charge concentration present within the device, while transient outlay remains unchanged, indicating absence of film charging effect.

Efficient and stable TiO2/Sb2S3 planar solar cells from absorber crystallization and Se-atmosphere annealing

Inorganic Sb2S3 thin film solar cells show wide application prospects in low cost, non-toxicity and high stability solar cells comparing with well-developed CIGS, CdTe and organohalide perovskite materials. The present work successfully fabricated Sb2S3 film utilizing rapid thermal evaporation (RTE) method. By optimizing the crystal growth technique, smooth and non-pinhole Sb2S3 film was deposited on compact TiO2 substrate. The corresponding planar TiO2/Sb2S3 solar cells without hole transport material (HTM) assistance achieved an efficiency of 2.5%. To eliminate S-shape J-V curves and improve the fill factor, the as-deposited Sb2S3 film was further annealed in selenium atmosphere. The efficiency and fill factor of Se-atmosphere annealed Sb2S3 devices were improved to 3.2% and 55%, respectively. The device improvement mechanism of Se-annealing process were systematically illustrated by cryogenic measurements, impedance analysis and capacitance-voltage characterizations. The post-annealing could help to improve junction quality, passivate absorber defects and decrease back contact barrier. The Se-atmosphere annealed Sb2S3 devices could stand continuous one sun illumination for 400 h and the efficiency was almost the same. After storing in air for six months, the average efficiencies still hold more than 90% of initial value. The present developed environment-friendly stable devices and efficient treatment technique were expected to enrich the research of thin film solar cell.

Efficient and stable solution-processed planar perovskite solar cells via contact passivation.

Planar perovskite solar cells (PSCs) made entirely via solution processing at low temperatures (<150°C) offer promise for simple manufacturing, compatibility with flexible substrates, and perovskite-based tandem devices. However, these PSCs require an electron-selective layer that performs well with similar processing. We report a contact-passivation strategy using chlorine-capped TiO2 colloidal nanocrystal film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells. We fabricated solar cells with certified efficiencies of 20.1 and 19.5% for active areas of 0.049 and 1.1 square centimeters, respectively, achieved via low-temperature solution processing. Solar cells with efficiency greater than 20% retained 90% (97% after dark recovery) of their initial performance after 500 hours of continuous room-temperature operation at their maximum power point under 1-sun illumination (where 1 sun is defined as the standard illumination at AM1.5, or 1 kilowatt/square meter).

Wide gap microcrystalline silicon carbide emitter for amorphous silicon oxide passivated heterojunction solar cells

Wide gap n-type microcrystalline silicon carbide [µc-SiC:H(n)] is highly suitable as window layer material for silicon heterojunction (SHJ) solar cells due to its high optical transparency combined with high electrical conductivity. However, the hot wire chemical vapor deposition (HWCVD) of highly crystalline µc-SiC:H(n) requires a high hydrogen radical density in the gas phase that gives rise to strong deterioration of the intrinsic amorphous silicon oxide [a-SiOx:H(i)] surface passivation. Introducing an n-type microcrystalline silicon oxide [µc-SiOx:H(n)] protection layer between the µc-SiC:H(n) and the a-SiOx:H(i) prevents the deterioration of the passivation by providing an etch resistance and by blocking the diffusion of hydrogen radicals. We fabricated solar cells with µc-SiC:H(n)/µc-SiOx:H(n)/a-SiOx:H(i) stack for the front side and varied the µc-SiOx:H(n) material properties by changing the microstructure of the µc-SiOx:H(n) to evaluate the potential of such stack implemented in SHJ solar cells and to identify the limiting parameters of the protection layer in the device. With this approach we achieved a maximum open circuit voltage of 677 mV and a maximum energy conversion efficiency of 18.9% for a planar solar cell.