Intensity-Modulated Spectroscopy Instrument and Its Applications

We have successfully built and tested an intensity-modulated spectroscopy (IMS) instrument that is centered around a commercial lock-in amplifier, which can be used to perform intensity-modulated spectroscopy (IMS) up to a frequency of 250 kHz. We have tested our instrument on a commercial CdS-based light dependent resistor (LDR), a device with well-known physical properties. We found that the dynamic characterizations results of the CdS-based LDR agree with an already well-established knowledge on its physical properties. We have also performed IMS on a state-of-the-art bulk heterojunction (BHJ) organic photovoltaics (OPV) and introduced a new mode of IMS operation where photovoltaic cells operate under a finite load, including at its maximum power point. From our IMS results on BHJ OPV, we have established IMS at maximum power point as the optimum operating condition for IMS on photovoltaics, a much better alternative to the traditional IMS operation, i.e. intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). By using IMS under finite load, we have managed to identify a high-frequency feature that was previously invisible under both IMPS and IMVS. We also found that this feature is ageing-related and is more pronounced after long-term storage. We have also managed to find the origin of this aging feature in the diffusion of indium ions that are etched by a PEDOT layer. In addition, with IMS, we are able to determine the BHJ capacitance of a BHJ OPV without absolute calibration of light intensity. We have also performed IMS on a similar BHJ device but with V2O5 as the hole extraction layer. We found that by using V2O5 as a hole extraction layer in OPV, we have avoided the problem of indium ions diffusion into the BHJ. We have also managed to perform IMS on an OLED and able to identify the causes of low-frequency “hook” feature in an OLED IMS results. This feature is attributable to the presence of parallel resistive- and capacitive-like components in the OLED’s emissive layer, which is also caused by the diffusion of indium ions into the OLED’s emissive layer. In addition, by using IMS, we have managed to determine carrier mobility in an OLED, though only an average mobility in the device. Finally, in the aging study of OLED device with IMS, we found that carrier transit time decreases as the device ages.