Electroabsorption measurements and built-in potentials in amorphous silicon–germanium solar cells

Abstract

We report electromodulated reflectance spectra in n-i-p solar cells with hydrogenated amorphous silicon–germanium alloy absorber layers. At lower photon energies the spectra are determined by bulk electroabsorption, and exhibit peaks near the optical gap of the absorber layers. Voltage scaling of the electroabsorption spectra indicate a built-in potential of Vbi=1.17 V in cells with absorber layer band gaps of 1.50 eV; in conjunction with earlier work, this value argues against a systematic decline in Vbi with an absorber layer band gap. At higher photon energies the spectra are due to direct electroreflectance; the voltage scaling was consistent with model predictions for the electric field at the interface of the p-type and absorber layers.

Hole-mobility limit of amorphous silicon solar cells

“…9 The temperature-dependent photogeneration rate G͑T͒ was determined from the measured, temperature-dependent photocurrent under reverse bias for the thinnest cell ͑186 nm͒. The temperature-dependent bandgap E G ͑T͒ was determined from measurements of the electroabsorption spectrum that are not shown here; this method 10 gave results consistent with earlier reports.…”

Transport, Interfaces, and Modeling in Amorphous Silicon Based Solar Cells: Final Technical Report, 11 February 2002 - 30 September 2006

“…We used idealized p and n layer parameters for which the precise values have little effect on calculated cell properties; prior experiments indicate that doped layers and interfaces are not limiting V OC in contemporary cells [43] The temperature-dependent photogeneration rate G(T) was determined from the measured, temperature-dependent photocurrent under reverse bias for the thinnest cell (186 nm). The temperature-dependent bandgap E G (T) was determined from measurements of the electroabsorption spectrum that are not shown here; this method [44] gave results consistent with earlier reports [45].…”

Drift-mobility characterization of silicon thin-film solar cells using photocapacitance

“…V 0 originates with the built-in potential V bi , which is roughly the difference in Fermi levels between the p and n layers of pin structures. Direct electroabsorption measurements on a-Si:H and a-SiGe:H cells from United Solar indicated built-in potentials in the range |V bi | = 1.0-1.2 V [10,11], which is reasonably consistent with the Fermi-level perspective. For the BP sample and NREL-2 samples, we also did photocarrier time-of-flight measurements that yielded V 0 of −0.2 V and −0.4 V, respectively.…”

“…It is related to the built-in potential, and is negative for the configuration in Fig. 1 [2,10]. Since ΔQ is proportional to the photocurrent i p , we define the linear photocapacitance:…”