Heinz-Christoph Neitzert

Characterization of polymer:fullerene solar cells by low-frequency noise spectroscopy

A detailed electric noise investigation of polymer:fullerene solar cells, at 300u2009K under dark conditions, is reported. The experimental noise results are interpreted in terms of a model taking into account the device capacitance and recombination resistance. Relevant parameters of the solar cells can be computed through fluctuation spectroscopy, and the results have been compared with those obtained by alternative techniques. After a thermal treatment at 340u2009K, a modification of the voltage-spectral traces has been observed and related to a strong cell resistance reduction.

Transport and noise spectroscopy of MWCNT/HDPE composites with different nanotube concentrations

Electrical current transport and low-frequency noise spectra of multiwall carbon nanotubes in high-density polyethylene matrix have been measured in a temperature range between 10 and 300K. The dc electrical investigations suggest that these composites can be regarded as a random resistor network, where the resistors are formed by tunnel junctions between carbon nanotubes. A crossover of the conduction from a low-field to a high-field regime is found in current-voltage characteristics. In particular, the high-field regime has a strong dependence on carbon nanotube concentration. Noise measurements reveal a standard 1/f behavior due to resistance fluctuations. However, in samples with different concentration of nanotubes, an unusual temperature dependence of the noise is observed. The samples with higher percentage of nanotubes seem to be the most promising ones for devices application, since their noise level is lower in the whole investigated temperature range.

Unravelling the low-temperature metastable state in perovskite solar cells by noise spectroscopy

The hybrid perovskite methylammonium lead iodide CH3NH3PbI3 recently revealed its potential for the manufacturing of low-cost and efficient photovoltaic cells. However, many questions remain unanswered regarding the physics of the charge carrier conduction. In this respect, it is known that two structural phase transitions, occurring at temperatures near 160 and 310u2009K, could profoundly change the electronic properties of the photovoltaic material, but, up to now, a clear experimental evidence has not been reported. In order to shed light on this topic, the low-temperature phase transition of perovskite solar cells has been thoroughly investigated by using electric noise spectroscopy. Here it is shown that the dynamics of fluctuations detect the existence of a metastable state in a crossover region between the room-temperature tetragonal and the low-temperature orthorhombic phases of the perovskite compound. Besides the presence of a noise peak at this transition, a saturation of the fluctuation amplitudes is observed induced by the external DC current or, equivalently, by light exposure. This noise saturation effect is independent on temperature, and may represent an important aspect to consider for a detailed explanation of the mechanisms of operation in perovskite solar cells.

Universal crossover of the charge carrier fluctuation mechanism in different polymer/carbon nanotubes composites

Carbon nanotubes added to polymer and epoxy matrices are compounds of interest for applications in electronics and aerospace. The realization of high-performance devices based on these materials can profit from the investigation of their electric noise properties, as this gives a more detailed insight of the basic charge carriers transport mechanisms at work. The dc and electrical noise characteristics of different polymer/carbon nanotubes composites have been analyzed from 10 to 300 K. The results suggest that all these systems can be regarded as random resistive networks of tunnel junctions formed by adjacent carbon nanotubes. However, in the high-temperature regime, contributions deriving from other possible mechanisms cannot be separated using dc information alone. A transition from a fluctuation-induced tunneling process to a thermally activated regime is instead revealed by electric noise spectroscopy. In particular, a crossover is found from a two-level tunneling mechanism, operating at low temperatures, to resistance fluctuations of a percolative network, in the high-temperature region. The observed behavior of 1/f noise seems to be a general feature for highly conductive samples, independent on the type of polymer matrix and on the nanotube density.

Epoxy/MWCNT Composite Based Temperature Sensor with Linear Characteristics

An ultra–low cost temperature sensor has been developed that uses as active sensing material an epoxy layer, mixed with a small concentration of multi–walled carbon nanotubes. A stable NTC characteristics has been measured during temperature cycling. Under Joule heating conditions, a linear current–temperature characteristics has been obtained.

Sensitivity of proton implanted VCSELs to electrostatic discharge pulses

The sensitivity of vertical cavity surface-emitting lasers to electrostatic discharge (ESD) pulses has been investigated under human body model test conditions. Very similar degradation behavior has been found for vertical-cavity surface-emitting lasers (VCSELs) from two different manufacturers, both with proton-implantation for lateral current confinement. For all investigated devices we observed during forward bias stress that the optical degradation precedes the electrical degradation and the forward bias damage threshold pulse amplitudes were only slightly higher than the reverse bias values. At the initial stage of the VCSEL degradation, damage of the upper p-DBR mirror region has been observed without modification of the active layer. During the ESD tests we monitored the electrical and the optical parameters of the VCSELs and measured during forward bias stress additionally the optical emission transients. The optical transients during ESD pulsing enable a fast evaluation of the damage threshold and give also an indication of the time scale of the junction heating during ESD pulses.

Lead-free electrical conductive adhesives for solar cell interconnectors

The electrical properties of various lead-free electrically conductive adhesives are investigated. They are intended to replace the solder, which is normally used to connect the interconnector tapes to the busbar. Compared to solder joints conductive adhesives offer the advantage of lower contact formation temperature with reasonable low contact resistances.

Light irradiation tuning of surface wettability, optical, and electric properties of graphene oxide thin films

In this work the preparation of flexible polymeric films with controlled electrical conductivity, light transmission and surface wettability is reported. A drop casted graphene oxide thin film is photo-reduced at different levels by UV light or laser irradiation. Optical microscopy, IR spectroscopy, electrical characterization, Raman spectroscopy and static water contact angle measurements are used in order to characterize the effects of the various reduction methods. Correlations between the optical, electrical and structural properties are reported and compared to previous literature results. These correlations provide a useful tool for independently tuning the properties of these films for specific applications.

Heterojunction solar cell fabricated by spin-coating of a CNT/PEDOT:PSS heteroemitter on top of a crystalline silicon absorber

Crystalline silicon / organic thin film heterojunction based solar cells have been realized using spin-coating deposition. Devices with different organic films, all based on PEDOT:PSS, which in some cases have been mixed with double-walled or multi-walled carbon nanotubes, have been compared. Highest conversion efficiencies have been obtained either with a highly conductive PEDOT:PSS emitter withut nanotubes or with a nanocomposite emitter consisting of low conductive PEDOT:PSS emitter mixed with multi-walled carbon nanotubes. Using the nanocomposite emitter, rather high values for the solar cell shunt resistances have been obtained without any etching procedure in order to improve the lateral current confinement. A comparison with a Schottky diode, realized as reference device by the evaporation of the top metal contact directly on top of the crystalline silicon substrate, showed that the heterodiode characteristics was not dominated by leakage current paths and short circuits through the organic layer.

Bulk and Interface Degradation of Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells Under Proton Irradiation

Two different types of n-type amorphous silicon/p-type crystalline silicon heterojunction solar cells-with and without insertion of a thin intrinsic a-Si:H layer-have been irradiated with proton doses between 5.1010 and 5.1012 protons/cm2 at 1.7 MeV. They have been investigated as well by classical measurement techniques like spectral response and current-voltage characteristics under illumination as well as by electroluminescence measurements of the forward biased solar cell. As another interface sensitive technique, admittance spectroscopy has been applied before and after irradiation. Recently we have shown under which sample preparation conditions this latter technique can be applied to large area solar cells without the need to prepare special test structures [1]. Regarding the insertion of a thin intrinsic silicon layer at the interface between the n-type a-Si:H top layer and the p-type c-Si substrate (HIT structure), we find that this layer does not change the degradation behavior of the effective minority carrier diffusion length (obtained from spectral response measurements) in the crystalline silicon. The resulting damage constant, kL, is 1.2 10-6. Solar cell efficiencies dropped to slightly less than 50% of the original values for irradiation doses of 5 1012 protons/cm2. These results are comparable to the degradation found for crystalline silicon homojunction solar cells [2]. Comparing admittance spectroscopy and electroluminescence efficiency measurements, we found that the latter technique is more sensitive to proton irradiation induced interface modifications. In particular we observed a stronger degradation after irradiation for the heterostructure with the insertion of the intrinsic a-Si:H layer. The electroluminescence is dominated by the crystalline silicon band-to-band recombination and decreases monotonically for increasing irradiation doses