Biased Photoreflectance Spectroscopy for Characterization of Band Bending in Compound Solar Cells

<i><u>Introductions</u></i><br/>The optimization of pn junctions is an inevitable subject to realize high performance of emerging solar cells. Junction capacitance, derived from overall capacitance of a solar cell through an equivalent circuit, widely used to evaluate band bending in a pn junction. However, in emerging solar cells, the electrical properties and band structures of layers or junctions other than the target are often unknown, thereby introducing various unknown factors into the evaluation of specific physical properties.<br/>In this study, we present a characterization technique of pn junctions through Franz-Keldysh oscillations (FKOs). FKOs can be observed in spectra of modulation spectroscopy, such as photoreflectance (PR) and electroreflectance (ER). Analyzing the oscillation periods in spectra provides the internal electric field applied to the pn junctions, offering a means to estimate band bending. Furthermore, we propose applying a bias voltage to solar cells during PR measurement, denoted as biased PR (B-PR) measurement. Through this method, we attempted to evaluate the interface states within pn junction.<br/><i><u>Experimental methods</u></i><br/>We performed PR measurements on pn junction solar cells using ZnSnP₂ (ZTP) bulk crystals and GaInP thin film as absorbing layers, representing emerging and highly efficient solar cells, respectively.<br/>The PR and B-PR measurement was conducted at room temperature using a PR measurement system (Seishin Trading Co. Ltd). A YAG laser (λ=532 nm) was used as a pump beam and its modulation frequency was set at 711 Hz. The probe beam was a monochromatized light from Xenon lamp. In the B-PR measurement, a bias voltage ranging from -5 to +1.4 V was applied to the solar cells using a source meter (Keithly 2400) and Au probes.<br/><i><u>Results and discussions</u></i><br/>In the PR spectra of the ZTP solar cell measured without bias voltage and any contacts, FKOs with up to the 5th extrema were observed at photon energies higher than the bandgap of ZTP (1.64 eV). The internal electric field in ZTP layer around the pn junction was determined to be 87 ± 8 kV cm^-1, derived from the FKOs period following the theory proposed by Aspnes and Studna [2]. Assuming an uniform acceptor concentration of 5.5 × 10¹⁶ cm^-3 in the ZTP layer, we estimated the charge distribution around the pn junction using the Poisson’s equation. As the results, we determined the depletion layer width and the surface potential at the pn junction in ZTP side as 83 nm and +0.47 eV from the valence band maximum (VBM) of ZTP, respectively. It is here emphasized that the above results were obtained with contactless.<br/>FKOs were observed over a bias voltage range from -5 to + 1.4 V also in the B-PR spectra. Following the above-mentioned analysis, we obtained the bias voltage dependence of the surface potential. In the absence of interface states at a pn junction, the surface potential is expected to monotonically increase with rising reverse bias. This behavior is theoretically expressed through the Poisson’s equation. However, the experimental results deviate from the theoretical curve, exhibiting plateau regions around bias voltage of 0 and -4 V. The applying of bias voltage to a solar cell corresponds to the control of the Fermi level. The deviation in the bias dependence of the surface potential from the theoretical curve may be attributed to the occupation (emission) of electrons at interface states. Such transitions of electrons are facilitated by approach of the Fermi level to the interface level. In the ZTP solar cells, the levels of the plateau regions were +0.45 eV and +1.3~+1.5 eV from the VBM of ZTP. In the presentation, we will discuss the consistency between the B-PR measurement and conventional capacitance measurement, using GaInP III-V solar cells together with ZTP solar cells.<br/><i><u>References</u></i><br/>[1] I. Sumiyoshi and Y. Nose, J. Appl. Phys. <b>133</b>, 235702 (2023).<br/>[2] D.E. Aspnes and A.A. Studna, Phys. Rev. B <b>7</b>, 4605-4625 (1973).