An 8.68% Efficiency Chemically-Doped-Free Graphene–Silicon Solar Cell Using Silver Nanowires Network Buried Contacts

Graphene–silicon (Gr-Si) heterojunction solar cells have been recognized as one of the most low-cost candidates in photovoltaics due to its simple fabrication process. However, the high sheet resistance of chemical vapor deposited (CVD) Gr films is still the most important limiting factor for the improvement of the power conversion efficiency of Gr-Si solar cells, especially in the case of large device-active area. In this work, we have fabricated a novel transparent conductive film by hybriding a monolayer Gr film with silver nanowires (AgNWs) network soldered by the graphene oxide (GO) flakes. This Gr-AgNWs hybrid film exhibits low sheet resistance and larger direct-current to optical conductivity ratio, quite suitable for solar cell fabrication. An efficiency of 8.68% has been achieved for the Gr-AgNWs-Si solar cell, in which the AgNWs network acts as buried contacts. Meanwhile, the Gr-AgNWs-Si solar cells have much better stability than the chemically doped Gr-Si solar cells. These results show a new route for the fabrication of high efficient and stable Gr-Si solar cells

Chemical Vapor Deposition of Graphene on Self-Limited SiC Interfacial Layers Formed on Silicon Substrates for Heterojunction Devices

Direct chemical vapor deposition (CVD) of graphene on any desired substrate is always required to manufacture high-quality heterojunctions with excellent interfacial properties. Herein, the growth of graphene on cubic-silicon carbide (3C-SiC) surfaces using conventional high-temperature direct thermal CVD and plasma-enhanced CVD (PECVD) is explored, which is hardly reported to date. Since 3C-SiC substrates are not available, the controlled self-limited 3C-SiC layers on the Si(100) substrates were grown at different temperatures (900–1200 °C) via thermal-CVD technique to obtain virtual 3C-SiC substrates. The direct production of graphene via thermal CVD could not be achieved on such 3C-SiC surfaces. The density functional theory and molecular dynamics simulations confirm that the carbon atom diffusion over the 3C-SiC surface is extremely low, like over the Si surface, which leads to no graphene growth. A similar growth mechanism may be attributed to their similar crystal structure viz diamond cubic for Si and zinc blend for 3C-SiC. However, graphene nanowalls (GNWs) were successfully grown on both Si and 3C-SiC/Si surfaces at 700 °C via the PECVD technique, where similar surface morphologies were observed because the growth mechanism of GNWs is independent of substrate type. Moreover, I–V characterization was performed for different SiC/Si heterostructures and their corresponding GNWs/SiC/Si heterostructures, respectively. The current conduction improved considerably more for GNW/SiC/Si heterostructures as compared to SiC/Si heterostructures, but the creation of a SiC interfacial layer as well as its quality affected the conductivity of GNWs/SiC/Si heterostructures. The inevitable formation of an interfacial SiC layer during the direct graphene growth via thermal CVD on Si substrates can seriously affect the performance of graphene/Si heterojunction devices. Hence, PECVD growth of graphene is an ideal option to fabricate graphene/Si heterojunction devices with excellent interfacial properties or graphene/3C-SiC/Si heterojunction devices for various electronic/optoelectronic applications such as gas sensors and photovoltaic devices

A 12%-Efficient Upgraded Metallurgical Grade Silicon–Organic Heterojunction Solar Cell Achieved by a Self-Purifying Process

Low-quality silicon such as upgraded metallurgical-grade (UMG) silicon promises to reduce the material requirements for high-performance cost-effective photovoltaics. So far, however, UMG silicon currently exhibits the short diffusion length and serious charge recombination associated with high impurity levels, which hinders the performance of solar cells. Here, we used a metal-assisted chemical etching (MACE) method to partially upgrade the UMG silicon surface. The silicon was etched into a nanostructured one by the MACE process, associated with removing impurities on the surface. Meanwhile, nanostructured forms of UMG silicon can benefit improved light harvesting with thin substrates, which can relax the requirement of material purity for high photovoltaic performance. In order to suppress the large surface recombination due to increased surface area of nanostructured UMG silicon, a post chemical treatment was used to decrease the surface area. A solution-processed conjugated polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was deposited on UMG silicon at low temperature (<150 °C) to form a heterojunction to avoid any impurity diffusion in the silicon substrate. By optimizing the thickness of silicon and suppressing the charge recombination at the interface between thin UMG silicon/PEDOT:PSS, we are able to achieve 12.0%-efficient organic–inorganic hybrid solar cells, which are higher than analogous UMG silicon devices. We show that the modified UMG silicon surface can increase the minority carrier lifetime because of reduced impurity and surface area. Our results suggest a design rule for an efficient silicon solar cell with low-quality silicon absorbers

Ambient Engineering for High-Performance Organic–Inorganic Perovskite Hybrid Solar Cells

Considering the evaporation of solvents during fabrication of perovskite films, the organic ambience will present a significant influence on the morphologies and properties of perovskite films. To clarify this issue, various ambiences of <i>N</i>,<i>N</i>-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and chlorobenzene (CBZ) are introduced during fabrication of perovskite films by two-step sequential deposition method. The results reveal that an ambient CBZ atmosphere is favorable to control the nucleation and growth of CH₃NH₃PbI₃ grains while the others present a negative effect. The statistical results show that the average efficiencies of perovskite solar cells processed in an ambient CBZ atmosphere can be significantly improved by a relatively average value of 35%, compared with those processed under air. The efficiency of the best perovskite solar cells can be improved from 10.65% to 14.55% by introducing this ambience engineering technology. The CH₃NH₃PbI₃ film with large-size grains produced in an ambient CBZ atmosphere can effectively reduce the density of grain boundaries, and then the recombination centers for photoinduced carriers. Therefore, a higher short-circuit current density is achieved, which makes main contribution to the improvement in efficiency. These results provide vital progress toward understanding the role of ambience in the realization of highly efficient perovskite solar cells

High-Performance Ultrathin Organic–Inorganic Hybrid Silicon Solar Cells via Solution-Processed Interface Modification

Organic–inorganic hybrid solar cells based on n-type crystalline silicon and poly­(3,4-ethylenedioxythiophene)–poly­(styrenesulfonate) exhibited promising efficiency along with a low-cost fabrication process. In this work, ultrathin flexible silicon substrates, with a thickness as low as tens of micrometers, were employed to fabricate hybrid solar cells to reduce the use of silicon materials. To improve the light-trapping ability, nanostructures were built on the thin silicon substrates by a metal-assisted chemical etching method (MACE). However, nanostructured silicon resulted in a large amount of surface-defect states, causing detrimental charge recombination. Here, the surface was smoothed by solution-processed chemical treatment to reduce the surface/volume ratio of nanostructured silicon. Surface-charge recombination was dramatically suppressed after surface modification with a chemical, associated with improved minority charge-carrier lifetime. As a result, a power conversion efficiency of 9.1% was achieved in the flexible hybrid silicon solar cells, with a substrate thickness as low as ∼14 μm, indicating that interface engineering was essential to improve the hybrid junction quality and photovoltaic characteristics of the hybrid devices

Enhanced Electronic Properties of SnO₂ <i>via</i> Electron Transfer from Graphene Quantum Dots for Efficient Perovskite Solar Cells

Tin dioxide (SnO₂) has been demonstrated as an effective electron-transporting layer (ETL) for attaining high-performance perovskite solar cells (PSCs). However, the numerous trap states in low-temperature solution processed SnO₂ will reduce the PSCs performance and result in serious hysteresis. Here, we report a strategy to improve the electronic properties in SnO₂ through a facile treatment of the films with adding a small amount of graphene quantum dots (GQDs). We demonstrate that the photogenerated electrons in GQDs can transfer to the conduction band of SnO₂. The transferred electrons from the GQDs will effectively fill the electron traps as well as improve the conductivity of SnO₂, which is beneficial for improving the electron extraction efficiency and reducing the recombination at the ETLs/perovskite interface. The device fabricated with SnO₂:GQDs could reach an average power conversion efficiency (PCE) of 19.2 ± 1.0% and a highest steady-state PCE of 20.23% with very little hysteresis. Our study provides an effective way to enhance the performance of perovskite solar cells through improving the electronic properties of SnO₂

High Performance Nanostructured Silicon–Organic Quasi <i>p</i>–<i>n</i> Junction Solar Cells <i>via</i> Low-Temperature Deposited Hole and Electron Selective Layer

Silicon–organic solar cells based on conjugated polymers such as poly­(3,4-ethylene­dioxy­thiophene):poly­(styrene­sulfonate) (PEDOT:PSS) on <i>n</i>-type silicon (<i>n</i>-Si) attract wide interest because of their potential for cost-effectiveness and high-efficiency. However, a lower barrier height (Φ_b) and a shallow built in potential (<i>V</i>_bi) of Schottky junction between <i>n</i>-Si and PEDOT:PSS hinders the power conversion efficiency (PCE) in comparison with those of traditional <i>p</i>–<i>n</i> junction. Here, a strong inversion layer was formed on <i>n</i>-Si surface by inserting a layer of 1, 4, 5, 8, 9, 11-hexaazatriphenylene hexacarbonitrile (HAT-CN), resulting in a quasi <i>p</i>–<i>n</i> junction. External quantum efficiency spectra, capacitance–voltage, transient photovoltage decay and minority charge carriers life mapping measurements indicated that a quasi <i>p</i>–<i>n</i> junction was built due to the strong inversion effect, resulting in a high Φ_b and <i>V</i>_bi. The quasi <i>p</i>–<i>n</i> junction located on the front surface region of silicon substrates improved the short wavelength light conversion into photocurrent. In addition, a derivative perylene diimide (PDIN) layer between rear side of silicon and aluminum cathodes was used to block the holes from flowing to cathodes. As a result, the device with PDIN layer also improved photoresponse at longer wavelength. A champion PCE of 14.14% was achieved for the nanostructured silicon–organic device by combining HAT-CN and PDIN layers. The low temperature and simple device structure with quasi <i>p</i>–<i>n</i> junction promises cost-effective high performance photovoltaic techniques

CH₃NH₃PbBr₃ Quantum Dot-Induced Nucleation for High Performance Perovskite Light-Emitting Solar Cells

Solution-processed organometallic halide perovskites have obtained rapid development for light-emitting diodes (LEDs) and solar cells (SCs). These devices are fabricated with similar materials and architectures, leading to the emergence of perovskite-based light-emitting solar cells (LESCs). The high quality perovskite layer with reduced nonradiative recombination is crucial for achieving a high performance device, even though the carrier behaviors are fundamentally different in both functions. Here CH₃NH₃PbBr₃ quantum dots (QDs) are first introduced into the antisolvent in solution phase, serving as nucleation centers and inducing the growth of CH₃NH₃PbI₃ films. The heterogeneous nucleation based on high lattice matching and a low free-energy barrier significantly improves the crystallinity of CH₃NH₃PbI₃ films with decreased grain sizes, resulting in longer carrier lifetime and lower trap-state density in the films. Therefore, the LESCs based on the CH₃NH₃PbI₃ films with reduced recombination exhibit improved electroluminescence and external quantum efficiency. The current efficiency is enhanced by 1 order of magnitude as LEDs, and meanwhile the power conversion efficiency increases from 14.49% to 17.10% as SCs, compared to the reference device without QDs. Our study provides a feasible method to grow high quality perovskite films for high performance optoelectronic devices