Dependence Of Photovoltaic Performance Research Articles

Insight into the Dependence of Photovoltaic Performance on Interfacial Energy Alignment in Solar Cells with Mobile Ions

A key aspect in the optimization of perovskite (PVK) solar cells is the mitigation of interface losses, which are determined by energy band offsets and recombination. Counterintuitively, recent experimental evidence on inorganic PVK solar cells shows that increasing band offsets may improve open‐circuit voltage at efficiency levels over 20%. In order to improve the knowledge of the impact of interfaces, the authors model solar cells consisting of an intrinsic absorber material containing mobile ions, sandwiched between two wide‐gap charge transport layers. The results show that minimizing band offsets decreases interface losses in various scenarios, but cannot be adopted as a universal optimization rule. For instance, even in the absence of interface recombination, unequal majority carrier band offsets avoid high injection conditions. Moreover, assuming a fixed band offset at one interface, the band offset at the opposite interface should be reduced if it shows the highest interface recombination, or increased if it shows the lowest recombination. Remarkably, it is found that solar cells with mobile ions hold the potential to outperform of ion‐free counterparts, depending on the extent to which the interplay between band offsets and Debye layers result in the establishment of a majority carrier concentration in the intrinsic absorber.

Performance enhancement of perovskite solar cells based on a CdO-quantum dot/TiO2-nanocrystal array electron transport layer

An oriented TiO2 nanocrystal array (TiO2-NA) shows a unique electron extraction capability for perovskite solar cells (PSCs) due to it having fewer grain boundaries and high crystallinity for effective charge collection. Based on these advantages, we prepared PSCs based on TiO2-NA mesoporous film (mp-film), and chose CdO quantum dots (QDs) as the modifier of the TiO2-NA mp-film. The diameter of a single TiO2-NA and the TiO2-NA gap were controlled by controlling the hydrothermal reaction time. The CdO-QDs/TiO2-NA mp-film, as the electron transporting layer (ETL), was prepared by successive ionic layer adsorption and the reaction-sintering method. In this work, oxidation state CdO QDs were first used as the modifier in the ETL of the PSC. We gradually optimized the CdO/TiO2-NA mp-film by controlling the hydrothermal reaction time of TiO2-NA to improve PSC performance. Infiltration of the perovskite absorber–submicrometer-thick rutile TiO2-NA mp-films with Spiro-MeOTAD yielded a strikingly high photoelectric conversion efficiency of 11.94% under AM 1.5G illumination. The dependence of photovoltaic performance and interfacial electronic behavior on the length of the CdO/TiO2-NA was also investigated.

Nanostructure of Porous Si and Anodic SiO2 Surface Passivation for Improved Efficiency Porous Si Solar Cells.

The photovoltaic effect in the anodic formation of silicon dioxide (SiO2) on porous silicon (PS) surfaces was investigated toward developing a potential passivation technique to achieve high efficiency nanostructured Si solar cells. The PS layers were prepared by electrochemical anodization in hydrofluoric acid (HF) containing electrolyte. An anodic SiO2 layer was formed on the PS surface via a bottom-up anodization mechanism in HCl/H2O solution at room temperature. The thickness of the oxide layer for surface passivation was precisely controlled by adjusting the anodizing current density and the passivation time, for optimal oxidation on the PS layer while maintaining its original nanostructure. HRTEM characterization of the microstructure of the PS layer confirms an atomic lattice matching at the PS/Si interface. The dependence of photovoltaic performance, series resistance, and shunt resistance on passivation time was examined. Due to sufficient passivation on the PS surface, a sample with anodization duration of 30 s achieved the best conversion efficiency of 10.7%. The external quantum efficiency (EQE) and internal quantum efficiency (IQE) indicate a significant decrease in reflectivity due to the PS anti-reflection property and indicate superior performance due to SiO2 surface passivation. In conclusion, the surface of PS solar cells could be successfully passivated by electrochemical anodization.

Effect of Additives AX (A = FA, MA, Cs, Rb, NH4, X = Cl, Br, I) in FAPbI3 on Photovoltaic Parameters of Perovskite Solar Cells

Herein, the dependence of photovoltaic performance on the additives of FAX (X = Cl, Br, and I, FA = formamidinium) and ACl [A = methylammonium (MA), Cs, Rb, and NH4] in FAPbI3‐based perovskite solar cells (PSCs) is reported. Effect of concentration on photovoltaic parameters is first screened for each additive, from which optimal concentration is determined with respect to the pristine without additive. Power conversion efficiency (PCE) is significantly improved from 16.55% to 22.51% after adding 20 mol% FACl in the perovskite precursor solution, whereas moderate increase in PCE to 20.08% and 19.97% is observed for FABr and FAI, respectively, indicating an important role of chloride. MACl and CsCl improved PCE to 20.81% and 20.59%, respectively, which is, however, inferior to FACl. A significantly increased carrier lifetime by treating FACl is responsible for the best performance. Energy dispersive X‐ray spectroscopy shows that chloride in the additive FACl is not incorporated in grain but placed on the grain boundary, which plays an important role in passivating iodide‐deficient grain boundary. The FACl additive has benefits over other additives because it cannot change the bandgap of FAPbI3.

Vertical Miscibility of Bulk Heterojunction Films Contributes to High Photovoltaic Performance

AbstractThe power conversion efficiencies (PCEs) of organic photovoltaic cells employing donor (polymer):acceptor (small molecule) bulk heterojunction (BHJ) are being rapidly improved. The phase segregation along vertical (film‐depth) direction of the active layer, referring to vertical immiscibility between donor and acceptor, leads to vertically varied and nonlinearly distributed composition, optical and electronic properties. However, the correlation between vertical miscibility and photovoltaic performance is still confusing. Here, it is semi‐empirically found that such vertical variations induced by vertical immiscibility deteriorate PCE. Subsequently, using PM6:Y6‐based binary and ternary BHJ as examples, a combined statistical, experimental, and numerical investigation on the dependence of photovoltaic performance on vertical miscibility is reported. The vertical phase evolution of BHJ significantly depends on solvents and processing methods. As compared with other donor:acceptor systems, polymer:Y6 deposited from appropriate solvents could have the best vertical miscibility which is nearly independent on film‐depth, leading to a higher PCE. PM6:Y6:fullerene ternary blends also have a good and uniform vertical miscibility, forming spatially well‐mixed ternary BHJ. Consequently, under this design guideline, the optimized film‐depth‐dependent miscibility contributes to optimized vertical distribution of optical and electronic properties, leading to an optimized PCE 17.1% with a low sensitivity to fluctuation of film thickness.

Solar cells sensitised by push–pull azo dyes: dependence of photovoltaic performance on electronic structure, geometry and conformation of the sensitizer

ABSTRACTNovel Azo dyes possessing varying conjugation lengths and different extents of electron cloud delocalisation were synthesised and characterised and explored as sensitizers for dye-sensitised solar cells (DSSC). The envisaged azo dyes comprising of electron withdrawing and electron-donating moieties which are linked through conjugation bridges of varying lengths facilitated the prevalence of push–pull mechanism in the molecules. Optimisation of the geometry were performed for the synthesised compounds using B3LYP/ 6-31 + G (d,p) level of density functional theory and their computed optical absorption and band gaps were validated with experimental results. The dyes exhibited molar extinction coefficients in the range of 3.2 × 104 to 4.2 × 104 mol−1 Lcm−1. The highest occupied molecular orbital (HOMO) was located between −5.53 and −5.03 eV for the various sensitizers synthesised and their lowest unoccupied molecular orbital (LUMO) was located between −2.86 and −3.08 eV. HOMO–LUMO gaps were in the range of 2.02–2.67 eV. Fill factor of the cells varied from 28% to 32% and the power conversion efficiencies ranged from 0.4% to 2.7%. This is the first time reporting of a systematic investigation, correlating the influence of nature and position of substituent, extending of conjugation and geometry of sensitizers on the photo physical properties of the sensitizers and the photovoltaic performance of corresponding DSSC.HighlightsThe article focuses on push–pull azo dyes and were explored as promising candidate sensitizers for low cost dye-sensitised solar cells (DSSC).The effect of chemical structure, extend of conjugation and geometry of the sensitizer on the photo physical properties of the sensitizers and the photovoltaic performance of DSSC were analysed.

Morphological study of F8BT:PFB thin film blends

We have studied the thin film morphology of a semiconducting polymer photovoltaic blend comprising an electron acceptor poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and the donor poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) (PFB). The molecular weight and blend weight ratio of the constituent polymers were used to modify the morphology. The average chemical composition of the bulk of F8BT:PFB blend in thin films was mapped using Raman microscopy at different depths from the air-film interface through controlled successive etching from the upper surface layer using an oxygen plasma. Correlating the lateral to the vertical Raman analysis of the phase separation of the film (blend weight ratio of 50:50) reveals that the μm scale de-mixed lateral phase structure seen on the free surface is present throughout most of the film thickness, though there is also some F8BT content within the PFB-rich wetting layer on the glass substrate, which we consider is due to the F8BT-rich interface at the surface to the substrate. The dependence of photovoltaic performance on morphology is discussed.

Tailoring Absorber Thickness and the Absorber-Scaffold Interface in CdSe-Coated ZnO Nanowire Extremely Thin Absorber Solar Cells

Extremely thin absorber (ETA) solar cells can theoretically provide higher efficiencies than planar cells for low quality absorbers, but only if their nanostructured morphology and interfaces are properly controlled. We report on the dependence of photovoltaic performance on absorber and buffer layer thickness in ETA solar cells containing CdSe-coated ZnO nanowires with CdS buffer. The optimal thickness of electrodeposited CdSe absorber was ∼50 - 80nm, as determined using planar cells. Thicker films lead to poor charge separation efficiency due to bulk recombination, while thinner films suffered from poor light absorption and increased interfacial recombination. In ETA cells, the addition of a sub-5nm CdS buffer layer significantly reduced interfacial recombination and improved fill factor and open circuit voltage. CdS prevents dissolution of the ZnO during CdSe deposition and improves interface quality. With appropriate selection of absorber thickness and interfacial treatment, ETA cells had efficiencies that were 30% larger and short circuit current densities that were two times larger than optimized planar cells. This work presents a rational approach toward designing ETA cell morphology, and it demonstrates a test case where the ETA architecture provides superior performance compared to planar cells made with the same materials and processing conditions.

Dependence of photovoltaic performance of solvothermally prepared CdS/CdTe solar cells on morphology and thickness of window and absorber layers

In the present work a new strategy for straightforward fabrication of CdS/CdTe solar cells, containing CdS nanowires and nanoparticles as a window layer and CdTe nanoparticles and microparticles as an absorber layer, are reported. CdS and CdTe nanostructures were synthesized by solvothermal method. X-ray diffraction analysis revealed that highly pure and crystallized CdS nanowires and nanoparticles with hexagonal structure and CdTe nanoparticles with cubic structure were obtained. Atomic force microscope and field emission scanning electron microscope images showed that CdS nanowires with length of several μm and average diameter of 35 nm, CdS nanoparticles with average particle size of 32 nm and CdTe nanoparticles with average particle size of 43 nm, were uniformly coated on the substrate by the homemade formulated pastes. Based on ultraviolet–visible absorption spectra, the band gap energies of CdS nanowires, CdS nanoparticles and CdTe nanoparticles were calculated 2.80, 2.65 and 1.64 eV, respectively. It was found that, the photovoltaic performance of the solar cells depends on thickness of CdTe and CdS films, reaching a maximum at a specific value of 6 μm and 225 nm, respectively. For such cell made of CdS nanowires and CdTe nanoparticles the VOC, JSC, fill factor and power conversion efficiency were calculated 0.62 V, 6.82 mA/cm2, 59.7 and 2.53 %, respectively. Moreover, photovoltaic characteristics of the solar cells were dependent on CdTe and CdS morphologies. CdS/CdTe solar cell made of CdTe and CdS nanoparticles had the highest cell efficiency (i.e., 2.73 %) amongst all fabricated solar cells. The presented strategy would open up new concept for fabrication of low-cost CdS/CdTe solar cells due to employment of a simple chemical route rather than the vapor phase methods.

ZnO nanorods for solar cells: Hydrothermal growth versus vapor deposition

Performance of dye-sensitized solar cells (DSSCs) based on ZnO nanorods prepared by hydrothermal and vapor-deposition methods has been investigated. In spite of their inferior optical properties, DSSCs based on hydrothermally grown rods exhibit higher power conversion efficiency, which can be attributed to the higher dye adsorption. Hydrothermally grown and vapor deposited nanorods also exhibit different dependence of photovoltaic performance on the annealing conditions of the rods, indicating significant effect of the native defects on the achievable photocurrent and power conversion efficiency. Efficiency of 0.22% is obtained for both as grown hydrothermally grown nanorods and vapor deposited nanorods annealed in oxygen at 200°C.

Stable, High‐Efficiency Ionic‐Liquid‐Based Mesoscopic Dye‐Sensitized Solar Cells

Efficient and stable mesoscopic dye-sensitized solar cells (DSCs) introducing a low-viscosity binary ionic liquid (1-propyl-3-methyl-imidazolium iodide (PMII) and 1-ethyl-3-methyl-imidazolium tetracyanoborate (EMIB(CN)(4))) electrolyte in combination with a new high-molar-extinction-coefficient ruthenium complex, Ru(2,2'-bipyridine-4,4'-dicarboxylic acid)(4,4'-bis(2-(4-tert-butyloxy -phenyl)ethenyl) -2,2'-bipyridine) (NCS)(2), are demonstrated. The dependence of photovoltaic performance, charge transport and electron lifetime on the composition of the binary ionic-liquid electrolyte with different ratios of PMII/EMIB(CN)(4) were investigated by electrochemical impedance and photovoltage transient techniques. A photovoltaic conversion efficiency of 7.6 % was obtained under simulated full sunlight illumination, which is a record for solvent-free DSCs. These devices exhibit excellent stability at 80 degrees C in the dark or under visible-light soaking at 60 degrees C during 1000 h of accelerated tests.

Organic Photovoltaics Interdigitated on the Molecular Scale

Phosphonated porphyrin frameworks that are porous to an electron-accepting perylenediimide are systematically interdigitated with a phosphonated form of the diimide to form bulk heterojunctions. When employed in photovoltaics, these molecularly interlaced heterojunctions provide large junction areas while retaining the phase connectivity of traditional bilayer heterojunctions. Zirconium phosphonate linkages facilitate layer-by-layer chromophore assembly from solution under ambient conditions. The dependence of photovoltaic performance on heterojunction architecture is investigated.