Hybrid organic nanocrystal/carbon nanotube film electrodes for air- and photo-stable perovskite photovoltaics.

We report on utilizing free-standing hybrid perylenediimide/carbon nanotube (PDI/CNT) films fabricated in air as back contacts for fully inorganic perovskite solar cells (glass/FTO/dense TiO2/mesoporous TiO2/CsPbBr3/back electrode). The back contact electrode connection is performed by film transfer rather than by vacuum deposition or by wet processing, allowing the formation of highly homogeneous contacts under ambient conditions. The use of this novel electrode in solar cells based on CsPbBr3 resulted in efficiency of 5.8% without a hole transporting layer; it is significantly improved in comparison to the reference cells with standard gold electrodes. Overall device fabrication can be performed on air, using inexpensive processing methods. The hybrid film electrodes dramatically improve the cell photo-stability under ambient conditions and under real-life operating conditions outdoors. The champion unencapsulated device demonstrated less than 30% efficiency loss over 6 weeks of outdoor aging in Negev desert conditions. The CNT/PDI electrodes offer the combination of fabrication simplicity, unique contacting approach, high efficiency and good operational stability for perovskite photovoltaics.

UV-Cross-linkable Donor-Acceptor Polymers Bearing a Photostable Conjugated Backbone for Efficient and Stable Organic Photovoltaics.

High-performance photovoltaic polymers bearing cross-linkable function together with a photorobust conjugated backbone are highly desirable for organic solar cells to achieve both high device efficiency and long-term stability. In this study, a family of such polymers is reported based on poly[(2,5-bis(2-hexyldecyloxy)phenylene)- alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[ c]-[1,2,5]thiadiazole)] (PPDT2FBT), a high-performance photovoltaic donor-acceptor polymer, with different contents of terminal vinyl-appended side chains for cross-linking. The polymers were named PPDT2FBT-V x and prepared by varying the feeding ratio ( x mol %, x = 0, 5, 10, and 15) of the vinyl-appended monomer in polymerization. It was found that the vinyl integration did not sacrifice the original high photovoltaic performance of the polymers, as evidenced by comparable average power conversion efficiencies (PCEs) (6.95, 7.02, and 7.63%) observed for optimized devices based on PPDT2FBT-V0, PPDT2FBT-V5, and PPDT2FBT-V10, respectively, in blending with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Unlike thermal cross-linking that greatly reduced device efficiency, UV cross-linking has proven to be an effective way to achieve both high device efficiency and thermostability for PPDT2FBT-V10 solar cells. UV-cross-linked PPDT2FBT-V10 solar cells displayed an initial average PCE of 5.28% and almost no decrease upon heat treatment at 120 °C for 40 h. Morphology studies revealed that UV-cross-linking did not only alter initial nanophase separation but also suppressed morphology evolution by aggregation in bulk heterojunction blend films. Photo-cross-linking requires material photostability. It is therefore worthwhile to note that these polymers and their blends with PC71BM were found to be extremely photostable, even upon continuous exposure to concentrated sunlight (up to 200 suns), and UV-cross-linking does not hamper this photostability. Further studies found that the devices fabricated with the UV-cross-linked PPDT2FBT-V10/PC71BM active layer can endure continuous light exposure to a solar simulator without deteriorating their performance.

Concentrated Sunlight for Materials Synthesis and Diagnostics.

Herein, the use of highly concentrated sunlight for materials science research is reviewed. Specific research directions include: (1) the generation of inorganic nanostructures, some of which had eluded experimental realization with conventional synthetic processes, and (2) elucidating the processes governing the degradation of organic and perovskite-based photovoltaic materials and devices, along with accelerated assessment of their stability. Both approaches employ solar concentrators capable of producing flux densities exceeding those of terrestrial solar radiation by up to three orders of magnitude, and are geared toward either creating extensive ultrahot reactor conditions conducive to the rapid, safe synthesis of unusual nanomaterials or judiciously interrogating photovoltaic devices.

Band Gap Engineering of Multi-Junction Solar Cells: Effects of Series Resistances and Solar Concentration.

Multi-junction (MJ) solar cells are one of the most promising technologies achieving high sunlight to electricity conversion efficiency. Resistive losses constitute one of the main underlying mechanisms limiting their efficiency under high illumination. In this paper, we study, by numerical modeling, the extent to which a fine-tuning of the different electronic gaps involved in MJ stacks may mitigate the detrimental effects of series resistance losses for concentration-dependent and independent series resistances. Our results demonstrate that appropriate bandgap engineering may lead to significantly higher conversion efficiency at illumination levels above ~1000 suns and series resistance values typically exceeding 0.02 Ω cm2, due to lower operating current and, in turn, series resistance losses. The implications for future generations of solar cells aiming at an improved conversion of the solar spectrum are also addressed.

Effect of Halide Composition on the Photochemical Stability of Perovskite Photovoltaic Materials.

The photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I1-x Brx )3 (MA=methylammonium) compositions (solid solutions) was investigated under accelerated stressing using concentrated sunlight. The relevance of accelerated testing to standard operational conditions of solar cells was confirmed by comparison to degradation experiments under outdoor sunlight exposure. We found that MAPbBr3 films exhibited no degradation, while MAPbI3 and mixed halide MAPb(I1-x Brx )3 films decomposed yielding crystallization of inorganic PbI2 accompanied by degradation of the perovskite solar light absorption, with faster absorption degradation in mixed halide films. The crystal coherence length was found to correlate with the stability of the films. We postulate that the introduction of Br into the mixed halide solid solution stressed its structure and induced more structural defects and/or grain boundaries compared to pure halide perovskites, which might be responsible for the accelerated degradation. Hence, the cause for accelerated degradation may be the increased defect density rather than the chemical composition of the perovskite materials.

Identifying Fundamental Limitations in Halide Perovskite Solar Cells.

The temperature dependence of the principal photovoltaic parameters of perovskite photovoltaics is studied. The recombination activation energy is in good agreement with the perovskite's bandgap energy, thereby placing an upper bound on the open-circuit voltage. The photocurrent increases moderately with temperature and remains high at low temperature, reinforcing that the cells are not hindered by insufficient thermally activated mobility or carrier trapping by deep defects.

Temperature- and Component-Dependent Degradation of Perovskite Photovoltaic Materials under Concentrated Sunlight.

We report on accelerated degradation testing of MAPbX3 films (X = I or Br) by exposure to concentrated sunlight of 100 suns and show that the evolution of light absorption and the corresponding structural modifications are dependent on the type of halide ion and the exposure temperature. One hour of such exposure provides a photon dose equivalent to that of one sun exposure for 100 hours. The degradation in absorption of MAPbI3 films after exposure to 100 suns for 60 min at elevated sample temperature (∼45-55 °C), due to decomposition of the hybrid perovskite material, is documented. No degradation was observed after exposure to the same sunlight concentration but at a lower sample temperature (∼25 °C). No photobleaching or decomposition of MAPbBr3 films was observed after exposure to similar stress conditions (light intensity, dose, and temperatures). Our results indicate that the degradation is highly dependent on the hybrid perovskite composition and can be light- and thermally enhanced.

Reversible degradation in ITO-containing organic photovoltaics under concentrated sunlight.

Stabilities of ITO-containing and ITO-free organic solar cells were investigated under simulated AM 1.5G illumination and under concentrated natural sunlight. In both cases ITO-free devices exhibit high stability, while devices containing ITO show degradation of their photovoltaic performance. The accelerated degradation under concentrated sunlight (of up to 20 suns) in ITO-containing devices was found to be reversible. Dark exposure of degraded samples can partly restore performance. A possible underlying mechanism for such a phenomenon is discussed.

Reversible degradation of inverted organic solar cells by concentrated sunlight.

Concentrated sunlight was used to study the performance response of inverted P3HT:PCBM organic solar cells after exposure to high intensity sunlight. Correlations of efficiency as a function of solar intensity were established in the range of 0.5-15 suns at three different stages: for a pristine cell, after 30 min exposure at 5 suns and after 30 min of rest in the dark. High intensity exposure introduced a major performance decrease for all solar intensities, followed by a partial recovery of the lost performance over time: at 1 sun only 6% of the initial performance was conserved after the high intensity exposure, while after rest the performance had recovered to 60% of the initial value. The timescale of the recovery effect was studied by monitoring the cell performance at 1 sun after high intensity exposure. This showed that cell performance was almost completely restored after 180 min. The transient state is believed to be a result of the breakdown of the diode behaviour of the ZnO electron transport layer by O(2) desorption, increasing the hole conductivity. These results imply that accelerated degradation of organic solar cells by concentrated sunlight is not a straightforward process, and care has to be taken to allow for a sound accelerated lifetime assessment based on concentrated sunlight.

Dielectric microconcentrators for efficiency enhancement in concentrator solar cells.

Metal fingers typically cover more than 10% of the active area of concentrator solar cells. Microfabricated dielectric optical designs that can completely eliminate front contact shading losses are explored. Essentially no microconcentrator optical losses need be incurred, series resistance losses can be reduced, and net efficiency gains of roughly 15% (relative) are realistic.