Subgap Absorption in Organic Semiconductors

Organic semiconductors have found a broad range of application in areas such as light emission, photovoltaics, and optoelectronics. The active components in such devices are based on molecular and polymeric organic semiconductors, where the density of states is generally determined by the disordered nature of the molecular solid rather than energy bands. Inevitably, there exist states within the energy gap which may include tail states, deep traps caused by unavoidable impurities and defects, as well as intermolecular states due to (radiative) charge transfer states. In this Perspective, we first summarize methods to determine the absorption features due to the subgap states. We then explain how subgap states can be parametrized based upon the subgap spectral line shapes. We finally describe the role of subgap states in the performance metrics of organic semiconductor devices from a thermodynamic viewpoint.

Shot noise: At zero volts, the shot noise component is zero. At voltages in the reverse bias, however, shot noise is given by 〈 shot 2 〉 = 2 0 Δ . Hence, narrower gap photovoltaic devices suffer more from this noise source. Ideally, the shunt current does not take part in the shot noise as it is not thermally activated; however, it has been observed that the shot noise scales with the total current (rather than 0 ) so that 〈 shot 2 〉 ≈ 2 Δ . The exact reason behind this observation may vary, but it is more likely because the total current passing through the DUT S4 is passed through the pre-amplifier electronics components involving p-n junctions. Hence the total current can be thermally activated and subject to the shot noise.

Microphonic (pick-up) noise: This noise component is due to the transduction of mechanical
vibrations in the environment to electrical signals. It is, therefore, essential to ensure about mechanical stability of the DUT, wires and the pre-amplifier during the measurement.
Mains Hum noise: this noise is induced due to the 50/60 Hz alternative currents in the mains power cables and can have a dramatic effect. Mains hum not only creates significant noise peaks at 50/60 Hz and its harmonics but also increases the noise floor at low frequencies (<1000 Hz) relevant to EQE measurements. In order to avoid this source, choosing a chopping frequency (of the monochromator light) at a frequency other than the harmonic frequency is necessary but not sufficient. The DUT should be mounted in a closed metallic sample holder acting as a Faraday cage in order to reduce the hum noise. It is also important to employ low-noise coaxial cables to minimize the pick-up noise due to the wires.
Pre-amplifier noise: in order to detect small currents, it is inevitable to pre-amply the current prior to phase-sensitive detection. A high-gain pre-amplifier is required with an input noise level lower than the noise level of the DUT. In the case of measuring sensitive EQE at a nonzero voltage bias, the pre-amplifier must be equipped with an ultra-low noise voltage source, providing a constant voltage with a noise level smaller than the device's shot noise.
S5 Figure S1. The signal-to-noise (SNR) ratio is improved by increasing the integration time (from 10s to 300s) of the US-EQE measurement for a typical PM6:ITIC organic solar cell. Figure S2. The effect of optical interference on the shape of the EQE in the subgap region. Figure S3. (a) Normalised subgap EQE and the respective apparent Urbach energy is plotted at different temperatures. (b) Extracted apparent Urbach energy for blend BHJ and neat material systems is plotted versus temperature. The U app is linear and equals kT ± 2.5 meV at higher temperatures; the offset can be attributed to interference effects. U app eventually deviates from linearity, as the spectral shape is more affected by other absorbing species at lower temperatures.