Towards high efficiency thin film solar cells

As an alternative to single crystal silicon photovoltaics, thin film solar cells have been extensively explored for miniaturized cost-effective photovoltaic systems. Though the fight to gain efficiency has been severely engaged over the years, the battle is not yet over. In this review, we comb the fields to elucidate the strategies towards high efficiency thin films solar cells and provide pointers for further development. Starting from the photoelectron generation, we look into the fundamental issues in photoelectric conversion processes, including light harvesting and charge handling (separations, transportations and collections). The emerging organic-inorganic halide perovskite systems, as well as the rapidly developed polycrystalline inorganic systems, organic photovoltaics and amorphous silicon cells are discussed in details. The biggest bottleneck for the cost-effective polycrystalline inorganic cells is the composition sensitivity and deep defects; for amorphous silicon cells, it is the quantum of the dangling bonds; for organic cells, it is the low charge carrier mobility and high exciton binding energy; and for perovskite cells, it is the environmental degradation and the controversial mechanisms of generation of I-V hysteresis. Strategies of light harvesting and charge handling as well as directions to break the bottlenecks are pointed out.     

The mechanism of burn-in loss in a high efficiency polymer solar cell

Degradation in a high efficiency polymer solar cell is caused by the formation of states in the bandgap. These states increase the energetic disorder in the system. The power conversion efficiency loss does not occur when current is run through the device in the dark but occurs when the active layer is photo-excited.     

Increasing the efficiency of polymer solar cells by silicon nanowires

Silicon nanowires have been introduced into P3HT:[60]PCBM solar cells, resulting in hybrid organic/inorganic solar cells. A cell efficiency of 4.2% has been achieved, which is a relative improvement of 10% compared to a reference cell produced without nanowires. This increase in cell performance is possibly due to an enhancement of the electron transport properties imposed by the silicon nanowires. In this paper, we present a novel approach for introducing the nanowires by mixing them into the polymer blend and subsequently coating the polymer/nanowire blend onto a substrate. This new onset may represent a viable pathway to producing nanowire-enhanced polymer solar cells in a reel to reel process.     

Fabrication of polycrystalline silicon solar cells showing high efficiency

The paper presents a methodology for fabrication of low-costing silicon solar cells with an efficiency of 10%. A polycrystalline silicon wafer, size 100×100 mm and thickness 450 μm, was doped with phosphorus using POCl₃ as the dopant. While, the backside (p-side) of the wafer was printed with a paste of Ag+Al in the ratio of 25 : 1, the front side (n-side) was printed with a paste of silver. It was fired at 720°C for better ohmic contact. Chemical vapour deposition (CVD) method was adopted for antireflection coating. Pure oxygen gas was bubbled through a solution of TiCl₄ at 200°C. The fabricated cells gave a significant increase in efficiency in terms of open circuit voltage (V) 560 mV, short circuit current (I) of 2·7 amp, and fill factor of 0·73. The methods used are inexpensive, and suitable for production of efficient silicon solar on a commercial basis.     

Pentacene nanostructural interlayer for the efficiency improvement of polymer solar cells

In poly(3-hexylthiophene) mixed with phenyl C61-butyric acid methyl ester heterojunction polymer solar cells, organic small molecular pentacene was introduced as the interfacial layer between PEDOT:PSS coated ITO substrates and polymer layer. It is found that the short circuit current density and power conversion efficiency were distinctly improved due to the introduction of the nanostructural pentacene interlayer. The nearly 100% power conversion efficiency improvement was obtained on the cells with a 4 nm pentacene interlayer, which benefits from the increased short circuit current from 2.34 mA/cm² to 5.76 mA/cm². The morphology of different thicknesses of pentacene thin films was observed by atomic force microscopy. The effect of pentacene interlayer's thickness on the distribution of light in the active layer was simulated by using a transfer matrix mode.     

Metamaterial-plasmonic absorber structure for high efficiency amorphous silicon solar cells

We show that a planar structure, consisting of an ultrathin semiconducting layer topped with a solid nanoscopically perforated metallic film and then a dielectric interference film, can highly absorb (superabsorb) electromagnetic radiation in the entire visible range, and thus can become a platform for high-efficiency solar cells. The perforated metallic film and the ultrathin absorber in this broadband superabsorber form a metamaterial effective film, which negatively refracts light in this broad frequency range. Our quantitative simulations confirm that the superabsorption bandwidth is maximized at the checkerboard pattern of the perforations. These simulations show also that the energy conversion efficiency of a single-junction amorphous silicon solar cell based on our optimized structure can exceed 12%.     

Understanding the up-conversion dynamics in high efficiency Yb-Tm systems for solar cells

Up-conversion emission processes have been studied in a transparent oxyfluoride nano-structured glass-ceramic co-doped with Yb³⁺ and Tm³⁺ ions. The decay and rise times, and pump power dependence of the different emission peaks have been analysed. The aim was to achieve a complete picture of the dynamics in this multiphoton excitation system. A method based on balance equations for the involved energy levels has been proposed in order to evaluate the up-conversion and decay rates from these levels. The results are analysed from the point of view of potential application to photovoltaic cells.     

Polypyrrole/Bi2WO6 composite with high charge separation efficiency and enhanced photocatalytic activity

The recombination of photogenerated electrons and holes is a crucial factor that limits the efficiency of photocatalysis and dye-sensitized solar cells. Conducting polymers are known to have high charge carrier mobility. Herein, a polypyrrole (PPy)/Bi₂WO₆ composite with promoted charge separation efficiency was designed by a “photocatalytic oxidative polymerization” method. The photo-degradation of a typical model pollutant, phenol, demonstrated that the PPy/Bi₂WO₆ composite possessed significantly enhanced photo-activity than pure Bi₂WO₆ under simulated sunlight irradiation. The higher photo-activity was attributed to the synergetic effect between PPy and Bi₂WO₆. The photogenerated holes on the valence band of Bi₂WO₆ could transfer to the highest occupied molecular orbital of PPy, leading to rapid photoinduced charge separation and enhancing the photocatalytic activity. This work provided a new concept for rational design and development of highly efficient polymer-semiconductor photocatalysts for environmental purification under simulated sunlight.     

Improved power conversion efficiency of dye-sensitized solar cells using side chain liquid crystal polymer embedded in polymer electrolytes

Side chain liquid crystal polymer (SCLCP) embedded in poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-co-HFP)-based polymer electrolytes (PVdF-co-HFP:side chain liquid crystal polymer (SCLCP)) was prepared for dye-sensitized solar cell (DSSC) application. The polymer electrolytes contained tetrabutylammonium iodide (TBAI), iodine (I₂), and 8 wt% PVdF-co-HFP in acetonitrile. DSSCs comprised of PVdF-co-HFP:SCLCP-based polymer electrolytes displayed enhanced redox couple reduction and reduced charge recombination in comparison to those of the conventional PVdF-co-HFP-based polymer electrolyte. The significantly increased short-circuit current density (J_sc, 10.75 mA cm⁻²) of the DSSCs with PVdF-co-HFP:SCLCP-based polymer electrolytes afforded a high power conversion efficiency (PCE) of 5.32% and a fill factor (FF) of 0.64 under standard light intensity of 100 mW cm⁻² irradiation of AM 1.5 sunlight.     

Carbon-based all-inorganic perovskite solar cells: Progress,challenges and strategies toward 20% efficiency

Organic-inorganic hybrid perovskite solar cells (PSCs) are promising next-generation photovoltaic technology. However, their long-term operation is limited due to thermodynamic instability of hybrid perovskites (loss of organics) and severe migration of constituents (ions and dopants). PSCs have to be free of volatile organics and mobile dopants to become commercially relevant. PSCs based on cesium lead halide inorganic perovskites (CsPbI_3−xBr_x, x = 0 ~ 3) and a carbon electrode, abbreviated here as C-IPSCs, fulfill these requirements: CsPbI_3−xBr_x is stable against decomposition to binary halides and the carbon electrode is inherently moisture-resistive and dopant-free. Since the first report of C-IPSCs in 2016, their power conversion efficiencies (PCEs) have doubled, recently reaching 14.84% with an astonishing stability of over 2000 h at 80 °C and 80% relative humidity (RH). Here we review recent progress of C-IPSCs and analyze the remaining critical issues in the field. We then offer our perspective to address these challenges through morphology, interface, spectral and material engineering. Finally, we argue that C-IPSCs have potential to overcome the 20% efficiency milestone, making them – in combination with their already impressive stability – the most promising PSC architecture for commercialization.     

Achieving high efficiency silicon-carbon nanotube heterojunction solar cells by acid doping

Various approaches to improve the efficiency of solar cells have followed the integration of nanomaterials into Si-based photovoltaic devices. Here, we achieve 13.8% efficiency solar cells by combining carbon nanotubes and Si and doping with dilute HNO(3). Acid infiltration of nanotube networks significantly boost the cell efficiency by reducing the internal resistance that improves fill factor and by forming photoelectrochemical units that enhance charge separation and transport. Compared to conventional Si cells, the fabrication process is greatly simplified, simply involving the transfer of a porous semiconductor-rich nanotube film onto an n-type crystalline Si wafer followed by acid infiltration.     

High efficiency inverted polymer solar cells with the sol-gel derived vanadium oxide interlayer

A solution-processable vanadium oxide film was prepared by sol-gel reaction on a bulk-heterojunction and adopted as the hole collection interlayer of inverted polymer solar cells that were based on a blend of commercial low bandgap polymer and [6,6]-phenyl C₆₁ butyric acid methyl ester as the photoactive layer. The developed cell shows an efficiency of 6.29% compare to 6.24% efficiency from the normal devices, the thus-prepared inverted devices demonstrated the feasibility of this alternative approach for developing an ease all-solution route for fabricating air-stable device using low-temperature processes in air ambient.     

Improved charge separation properties of organic hetero-junction solar cells by self-assembled monolayers anchored Ag nanoparticles

We investigate the effect of self-assembled monolayers and localized surface plasmons of silver nano-particles on an organic solar cell consisting of zinc phthalocyanine as an active layer. The device was fabricated by covalent attachment of silver nanoparticles on n-type silicon substrates using self-assembled monolayer of 4-mercaptophenol. Power conversion efficiency is increased up to 8 times as compared to a reference device with merely 0.13% photo-conversion efficiency containing no self-assembled monolayers and silver nano-particles. We believe that improved conductivity at the interface due to the aromatic self-assembled monolayer and the increased local electric field experienced by the active layer in presence of silver nano-particles act in synergy towards the higher population of excitons and dissipation of charge.     

Influence of thermal annealing on the charge generation and transport in PM6-based non-fullerene solar cells

To study the influence of thermal annealing on the charge generation and transport in PM6-based non-fullerene solar cells. Morphology, optical and electrochemical properties of active layers as well as electrical properties of polymer solar cells were studied. Furthermore, the photoelectric conversion processes of annealed and unannealed devices were also examined by means of time resolved spectroscopy. The results showed that thermal annealing had a weak influence on the dynamics of exciton states. Besides, annealed device is found to suppress bimolecular recombination owing to its higher charge carrier mobility in ordered donor and acceptor aggregation phases, which led to higher photocurrent and power conversion efficiency than unannealed photovoltaic device.

     

Discriminating between bilayer and bulk heterojunction polymer:fullerene solar cells using the external quantum efficiency

The morphology of the active layer in polymer:fullerene solar cells is a key parameter for the performance. We compare bilayer poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT/PCBM) solar cell devices produced from orthogonal solvents before and after thermal annealing with P3HT:PCBM bulk heterojunction solar cells produced from a single solvent. By comparing the spectral shape and magnitude of the experimental and theoretically modeled EQEs we show that P3HT/PCBM bilayers made via orthogonal solution processing do not lead to bilayers with a sharp interface but that partial intermixing has occurred. Thermal annealing of these diffusive P3HT/PCBM bilayers leads to increased mixing but does not result in the same mixed bulk heterojunction morphology that is obtained when P3HT and PCBM are cast simultaneously from single solution. For thicker layers, the annealed bilayers significantly outperform the bulk heterojunction devices with the same nominal composition and same total thickness.