| Description |
We have developed and implemented a cryostat-based current noise spectrum analyzer based on the cross-correlation technique capable of detecting signals at least 4 orders of magnitude below the input noise floor of a traditional single-channel spectrum analyzer. The apparatus was then used to systematically study charge transport and interfacial dynamics in three different sets of devices The first is a series of high-efficiency heterojunction silicon solar cells utilizing amorphous hydrogenated silicon. We have been able to directly resolve shot and generationrecombination noise generated by these amorphous layers, with implications about charge carrier dynamics at their interface with the crystalline bulk. The near-full-scale shot noise suggests that the noise spectra are dominated by a single interface, the intrinsic amorphous layer with the p-n junction. 1/f noise is shown to mainly originate from fluctuations in current caused by the varying thickness of the same amorphous layer. Finally, several generationrecombination signatures are detected, appearing with changes in illumination wavelength and temperature. In a series of methylammonium lead triiodide perovskite solar cells, we are again able to identify shot noise and generation-recombination noise as generated by interfacial dynamics. As with the silicon devices, we find this noise is dominated by a single interface, likely that between the perovskite bulk and the spiro-OMeTAD hole transport layer. We find a generation-recombination feature which we attribute to bimolecular recombination in the bulk of the material. Finally, we find 1/f noise generated by the interfaces, indicating that iv much of the loss seen in these perovskite devices is interfacial. Finally, we have used the same apparatus to perform immittance spectroscopy on a series of phosphorescent organic light-emitting diodes. Characterization of the I(V) and capacitance-voltage parameters allow us to identify two clear regimes of charge accumulation at different material interfaces in the stack: the first at the electrodes due to the difference in built-in potentials set by the device injection layers; the second occurs at the hole transport layer/emission later interface as charge injection becomes inefficient. A strong negative capacitance effect was also seen at low biases. All in all, this work illustrates how current noise and immittance spectroscopy represent powerful, inexpensive, noninvasive methods to extract important information out of not only materials, but complex devices. |
