Abstract
We have employed state-of-the-art cross-correlation noise spectroscopy (CCNS) to study carrier dynamics in silicon heterojunction solar cells (SHJ SCs). These cells were composed of a light absorbing n-doped monocrystalline silicon wafer contacted by passivating layers of i-a-Si:H and doped a-Si:H selective contact layers. Using CCNS, we are able to resolve and characterize four separate noise contributions: (1) shot noise with Fano factor close to unity due to holes tunneling through the np-junction, (2) a 1/f term connected to local potential fluctuations of charges trapped in a-Si:H defects, (3) generation-recombination noise with a time constant between 30 and 50 μs and attributed to recombination of holes at the interface between the ITO and n-a-Si:H window layer, and (4) a low-frequency generation-recombination term observed below 100 K which we assign to thermal emission over the ITO/ni-a-Si:H interface barrier. These results not only indicate that CCNS is capable of reveling otherwise undetectable relaxation process in SHJ SCs and other multi-layer devices, but also that the technique has a spatial selectivity allowing for the identification of the layer or interface where these processes are taking place.
Highlights
Current-noise spectroscopy has a less obvious spatial selectivity, namely that it tends to magnify a contribution from the most resistive elements of the stacked layers
The technical advantage of our work comes from employment of cross-correlation current noise spectroscopy (CCNS)[24], which provides two to four orders of magnitude improvement in the sensitivity and bandwidth of the measurements, giving access to regions of the noise spectrum which are typically hidden below the input noise floor
The SHJ SC in the tandem devices are illuminated from the n-amorphous silicon (a-Si) side, indicated in Fig. 1A, as the perovskite would be attached to the n-doped hydrogenated amorphous silicon (n-a-Si) layer; we will refer to this side as the front side of the SHJ SC
Summary
Current-noise spectroscopy has a less obvious spatial selectivity, namely that it tends to magnify a contribution from the most resistive elements of the stacked layers. Noise spectroscopy analyzes fluctuations of a signal from its equilibrium or steady-state value and is widely used for the characterization of defects and electronic relaxation processes in semiconducting devices[17,18,19] It has been used in the past to study both doped and undoped a-Si:H20, a-Si:H -based t ransistors[21], light-induced metastable changes in a-Si:H (Staebler-Wronski effect)[22], and more recently to evaluate defect states in crystalline solar cells[23]. The technical advantage of our work comes from employment of cross-correlation current noise spectroscopy (CCNS)[24], which provides two to four orders of magnitude improvement in the sensitivity and bandwidth of the measurements, giving access to regions of the noise spectrum which are typically hidden below the input noise floor It is very suitable for semiconductor devices with planar structure and high capacitance such as a typical photovoltaic cell.
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