Iris Visoly-Fisher

Stability of CdTe/CdS thin-film solar cells

Abstract The recent literature regarding the stability of CdTe/CdS photovoltaic cells (as distinguished from modules ) is reviewed. Particular emphasis is given to the role of Cu as a major factor that can limit the stability of these devices. Cu is often added to improve the ohmic contact to p-CdTe and the overall cell photovoltaic performance. This may be due to the formation of a Cu 2 Te/CdTe back contact. Excess Cu also enhances the instability of devices when under stress. The Cu, as Cu + , from either Cu 2 Te or other sources, diffuses via grain boundaries to the CdTe/CdS active junction. Recent experimental data indicate that Cu, Cl and other diffusing species reach (and accumulate at) the CdS layer, which may not be expected on the basis of bulk diffusion. These observations may be factors in cell behavior and degradation, for which new mechanisms are suggested and areas for future study are highlighted. Other possible Cu-related degradation mechanisms, as well as some non-Cu-related issues for cell stability are discussed.

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.

Direct evidence for grain-boundary depletion in polycrystalline CdTe from nanoscale-resolved measurements

We use scanning probe microscopy-based methods for direct characterization of a single grain boundary and a single grain surface in solar cell-quality CdTe, deposited by closed-space vapor transport. We find that scanning capacitance microscopy can serve to study polycrystalline electronic materials, notwithstanding the strong topographical variations. In this way, we find a barrier for hole transport across grain boundaries, a conclusion supported by the much more topography-sensitive scanning kelvin probe microscopy, with some variation in barrier height between different boundaries.

Photocurrent of a single photosynthetic protein.

Photosynthesis is used by plants, algae and bacteria to convert solar energy into stable chemical energy. The initial stages of this process--where light is absorbed and energy and electrons are transferred--are mediated by reaction centres composed of chlorophyll and carotenoid complexes. It has been previously shown that single small molecules can be used as functional components in electric and optoelectronic circuits, but it has proved difficult to control and probe individual molecules for photovoltaic and photoelectrochemical applications. Here, we show that the photocurrent generated by a single photosynthetic protein-photosystem I-can be measured using a scanning near-field optical microscope set-up. One side of the protein is anchored to a gold surface that acts as an electrode, and the other is contacted by a gold-covered glass tip. The tip functions as both counter electrode and light source. A photocurrent of ∼10 pA is recorded from the covalently bound single-protein junctions, which is in agreement with the internal electron transfer times of photosystem I.

Electronically active layers and interfaces in polycrystalline devices: Cross-section mapping of CdS/CdTe solar cells

Electronic mapping of cross sections of a polycrystalline device, the n-CdS/p-CdTe solar cell, show that the photovoltaic and metallurgical junctions coincide to within experimental resolution (50 nm), which rules out both type conversion of CdS and buried homojunctions. Compositional analysis of the CdS supports this. Mapping was done using scanning capacitance, complemented by scanning Kelvin probe microscopy. Our results explain why a high-resistance transparent conducting oxide layer is needed as contact to the CdS for successful device operation. They define limits on inputs for modeling performance of these devices.

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.

Role of oxygen functional groups in reduced graphene oxide for lubrication

Functionalized and fully characterized graphene-based lubricant additives are potential 2D materials for energy-efficient tribological applications in machine elements, especially at macroscopic contacts. Two different reduced graphene oxide (rGO) derivatives, terminated by hydroxyl and epoxy-hydroxyl groups, were prepared and blended with two different molecular weights of polyethylene glycol (PEG) for tribological investigation. Epoxy-hydroxyl-terminated rGO dispersed in PEG showed significantly smaller values of the friction coefficient. In this condition, PEG chains intercalate between the functionalized graphene sheets, and shear can take place between the PEG and rGO sheets. However, the friction coefficient was unaffected when hydroxyl-terminated rGO was coupled with PEG. This can be explained by the strong coupling between graphene sheets through hydroxyl units, causing the interaction of PEG with the rGO to be non- effective for lubrication. On the other hand, antiwear properties of hydroxyl-terminated rGO were significantly enhanced compared to epoxy-hydroxyl functionalized rGO due to the integrity of graphene sheet clusters.

Porphyrins as ITO photosensitizers: substituents control photo-induced electron transfer direction

Porphyrins have attracted much attention as dyes for photovoltaic applications due to their remarkable light harvesting properties and tunability of electronic behaviour. The photophysical and photochemical properties of porphyrins are influenced by electron-donating or electron-withdrawing substituents that can be attached at the perimeter of the porphyrin macrocycle. The current work shows that changing the porphyrin peripheral substituents can affect the direction of interfacial charge transfer at the interface of porphyrin and Indium tin oxide (ITO), a degenerate n-type semiconductor that is commonly used as a transparent conductive electrode in organic optoelectronic devices. Soret-band excitation resulted in electron injection from the molecular layer to the ITO in all porphyrin derivatives studied, suggesting that electron injection to ITO is faster than relaxation from the porphyrin upper excited state to the lower one. However, the direction of photo-induced electron transfer in the 500–650 nm spectral range (Q-bands excitation in porphyrins) was found to depend on the peripheral substituents. This is highly relevant for photovoltaic devices, as the solar spectrum peaks in this spectral range. The charge transfer behaviour was shown to depend on the composition of the interfacial adsorbed monolayer. Therefore, it is proposed that porphyrin derivatives can be used for modulating photo-induced interfacial transport at ITO/organic layer interfaces in a predefined, controllable way.

Broadband absorption enhancement via light trapping in periodically patterned polymeric solar cells

Improved absorption is needed for thin-film organic solar cells to increase charge generation and/or reduce the thickness of the organic active film. Absorption enhancement by light trapping in a photonic crystal (PC) can be used to enhance the absorption in a patterned photo-active polymer blend. We used the finite difference time domain (FDTD) method to study the effect of combining the PC with different vertical layer configurations, including a hole transporting flash layer, used for charge separation and for increasing the photo-voltage, and a reflective metallic back electrode. The model was applied to P3HT:PCBM and pBBTDPP2:PCBM blends as the active materials, to examine the effect of different absorption spectra on the results. We found that the combination of PC and a reflector electrode is synergistic for enhancing the absorption in such devices, while the effect of the flash layer is minor. The combination of lateral and vertical reflections by the PC and the reflector, respectively, results in...

When, Why and Where are CdTe/CdS Solar Cells Stable?

The role of Cu in CdTe/CdS solar cell instability remains the subject of much debate. The investigation of a range of ‘Cu’-contacted CdTe/CdS cells, which had received various thermal stress treatments, is described. Cells that were stressed in air exhibit strong current-voltage (I-V) rollover and junction degradation. No such degradation was observed for ‘Cu’-contacted cells that had been stressed in dry-N 2 atmosphere. Cu is found to diffuse rapidly through the cell structure during back contact annealing and to accumulate in the CdS layer. With stress, significant levels of Cu dope the grain bulk, producing (with Cl) high resistance, photoconducting CdS. This behavior is independent of stress atmosphere and is, therefore, unlikely to (initially) be a dominating mechanism for cell degradation. Our results suggest simple air oxidation of the back contact interface to be a likely origin of I-V rollover in CdTe/CdS cells.