Concept: Solar cells
Organometal halide perovskite-based solar cells have recently realized large conversion efficiency over 15% showing great promise for a new large scale cost-competitive photovoltaic technology. Using impedance spectroscopy measurements we are able to separate the physical parameters of carrier transport and recombination in working devices of the two principal morphologies and compositions of perovskite solar cells, viz. compact thin film of CH3NH3PbI3-xClx and CH3NH3PbI3 infiltrated on nanostructured TiO2. The results show nearly identical spectral characteristics indicating a unique photovoltaic operating mechanism that provides long diffusion lengths (1 m). Carrier conductivity in both devices is closely matched, so that the most significant differences in performance are attributed to recombination rates. These results highlight the central role of the CH3NH3PbX3 semiconductor absorber in carrier collection and provide a new tool for improved optimization of perovskite solar cells. We report for the first time a measurement of the diffusion length in a nanostructured perovskite solar cell.
Large area pixelated polymer solar cell design with active encapsulants for down-conversion and redirecting light is demonstrated by K. S. Narayan and Anshuman J. Das on page 2193. The large-area, vacuum-free fabrication is carried out using meltable alloys as cathodes and an architecture which permits channelizing the light back into the device utilizing appropriate fluorescent dye molecules embedded in the encapsulant.
Replacement of the TiO2 layer in a traditional dye sensitized solar cell (DSC) by poly[3-(2-hydroxyethyl)-2,5-thienylene] grafted reduced graphene oxide (PHET-g-rGO) yields an overall power conversion efficiency of 3.06% with the N-719 dye, where the rGO part increases the charge mobility by reducing the backward recombination reaction in the DSC.
Quantum dot sensitized solar cells (QDSSCs) present a promising technology for next generation photovoltaic cells, having exhibited a considerable leap in performance over the last few years. However, recombination processes occurring in parallel at the TiO(2)-QDs-electrolyte triple junction constitute one of the major limitations for further improvement of QDSSCs. Reaching higher conversion efficiencies necessitates gaining a better understanding of the mechanisms of charge recombination in these kinds of cells; this will essentially lead to the development of new solutions for inhibiting the described losses. In this study we have systematically examined the contribution of each interface formed at the triple junction to the recombination of the solar cell. We show that the recombination of electrons at the TiO(2)/QDs interface is as important as the recombination from TiO(2) and QDs to the electrolyte. By applying conformal MgO coating both above and below the QD surface, recombination rates were significantly reduced, and an improvement of more than 20% in cell efficiency was recorded.
Colloid TiO2 nanorods are used for solution processable poly(3-hexyl thiophene): TiO2 hybrid solar cell. The nanorods were covered by insulating ligand of oleic acid (OA) after sol-gel synthesis. Three more conducting pyridine type ligands: pyridine, 2,6-lutidine (Lut) and 4-tert-butylpyridine (tBP) were investigated respectively to replace OA. The power conversion efficiency (PCE) of the solar cell was increased because the electronic mobility of pyridine type ligand modified TiO2 is higher than that of TiO2-OA. The enhancement of PCE is in the descending order of Lut>pyridine>tBP due to the effective replacement of OA by Lut. The PCE of solar cell can be further enhanced by ligand exchange of pyri-dine type ligand with conjugating molecule of 2-cyano-3-(5-(7-(thiophen-2-yl)-benzothiadiazol-4-yl) thiophen-2-yl) acryl-ic acid (W4) on TiO2 nanorods because W4 has aligned bandgap with P3HT and TiO2 to facilitate charge transport. The electronic mobility of two- stage ligand exchanged TiO2 is improved furthermore except Lut, because it adheres well and difficult to be replaced by W4. The amount of W4 on TiO2-tBP is 3 times more than that of TiO2-Lut (0.20 mol% vs. 0.06 mol%). Thus, the increased extent of PCE of solar cell is in the decreasing order of tBP>pyridine>Lut. The TiO2-tBP-W4 device has the best performance with 1.4 and 2.6 times more than TiO2-pyridine-W4 and TiO2-Lut-W4 devices respective-ly. The pKa of the pyridine derivatives plays the major role to determine the ease of ligand exchange on TiO2 which is the key factor mandating the PCE of P3HT:TiO2 hybrid solar cell. The results of this study provide new insights of the signifi-cance of acid-base reaction on the TiO2 surface for TiO2 based solar cells. The obtained knowledge can be extended to other hybrid solar cell system.
Optical tracking is often combined with conventional flat panel solar cells to maximize electrical power generation over the course of a day. However, conventional trackers are complex and often require costly and cumbersome structural components to support system weight. Here we use kirigami (the art of paper cutting) to realize novel solar cells where tracking is integral to the structure at the substrate level. Specifically, an elegant cut pattern is made in thin-film gallium arsenide solar cells, which are then stretched to produce an array of tilted surface elements which can be controlled to within ±1°. We analyze the combined optical and mechanical properties of the tracking system, and demonstrate a mechanically robust system with optical tracking efficiencies matching conventional trackers. This design suggests a pathway towards enabling new applications for solar tracking, as well as inspiring a broader range of optoelectronic and mechanical devices.
Three-dimensional organic-inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley-Queisser limit for a single-junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden-Popper phases-layered two-dimensional perovskite films-have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.
Of the many materials and methodologies aimed at producing low-cost, efficient photovoltaic cells, inorganic-organic lead halide perovskite materials appear particularly promising for next-generation solar devices owing to their high power conversion efficiency. The highest efficiencies reported for perovskite solar cells so far have been obtained mainly with methylammonium lead halide materials. Here we combine the promising-owing to its comparatively narrow bandgap-but relatively unstable formamidinium lead iodide (FAPbI3) with methylammonium lead bromide (MAPbBr3) as the light-harvesting unit in a bilayer solar-cell architecture. We investigated phase stability, morphology of the perovskite layer, hysteresis in current-voltage characteristics, and overall performance as a function of chemical composition. Our results show that incorporation of MAPbBr3 into FAPbI3 stabilizes the perovskite phase of FAPbI3 and improves the power conversion efficiency of the solar cell to more than 18 per cent under a standard illumination of 100 milliwatts per square centimetre. These findings further emphasize the versatility and performance potential of inorganic-organic lead halide perovskite materials for photovoltaic applications.
Morphology control of solution coated solar cell materials presents a key challenge limiting their device performance and commercial viability. Here we present a new concept for controlling phase separation during solution printing using an all-polymer bulk heterojunction solar cell as a model system. The key aspect of our method lies in the design of fluid flow using a microstructured printing blade, on the basis of the hypothesis of flow-induced polymer crystallization. Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes. The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation. We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility.
In this work, we investigate the exciton dissociation dynamics occurring at the donor-acceptor interface in organic and hybrid blends employed in the realization of photovoltaic cells. Fundamental differences in the charge separation process are studied with the organic semiconductor polymer poly(3-hexylthiophene) (P3HT) and either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or titanium dioxide (TiO2) acting as acceptor. By using ultrafast broadband transient absorption spectroscopy with few-fs temporal resolution, we observe that in both cases the incoherent formation of free charges is dominating the charge generation process. From the optical response of the polymer and by tracking the excited state absorption, we extract pivotal similarities in the incoherent energy pathways that follow the impulsive excitation. On timescales shorter than 200 fs we observe that the two acceptors display similar dynamics in the exciton delocalization. Significant differences arise only on longer timescales with only an impact on the overall photocarrier generation efficiency.