Photogenerated Carrier Transport Properties in Silicon Photovoltaics

Electrical transport parameters for active layers in silicon (Si) wafer solar cells are determined from free carrier optical absorption using non-contacting optical Hall effect measurements. Majority carrier transport parameters [carrier concentration (N), mobility (μ), and conductivity effective mass (m*)] are determined for both the n-type emitter and p-type bulk wafer Si of an industrially produced aluminum back surface field (Al-BSF) photovoltaic device. From measurements under 0 and ±1.48 T external magnetic fields and nominally “dark” conditions, the following respective [n, p]-type Si parameters are obtained: N = [(3.6 ± 0.1) × 1018 cm−3, (7.6 ± 0.1) × 1015 cm−3]; μ = [166 ± 6 cm2/Vs, 532 ± 12 cm2/Vs]; and m* = [(0.28 ± 0.03) × me, (0.36 ± 0.02) × me]. All values are within expectations for this device design. Contributions from photogenerated carriers in both regions of the p-n junction are obtained from measurements of the solar cell under “light” 1 sun illumination (AM1.5 solar irradiance spectrum). From analysis of combined dark and light optical Hall effect measurements, photogenerated minority carrier transport parameters [minority carrier concentration (Δp or Δn) and minority carrier mobility (μh or μe)] under 1 sun illumination for both n- and p-type Si components of the solar cell are determined. Photogenerated minority carrier concentrations are [(7.8 ± 0.2) × 1016 cm−3, (2.2 ± 0.2) × 1014 cm−3], and minority carrier mobilities are [331 ± 191 cm2/Vs, 766 ± 331 cm2/Vs], for the [n, p]-type Si, respectively, values that are within expectations from literature. Using the dark majority carrier concentration and the effective equilibrium minority carrier concentration under 1 sun illumination, minority carrier effective lifetime and diffusion length are calculated in the n-type emitter and p-type wafer Si with the results also being consistent with literature. Solar cell device performance parameters including photovoltaic device efficiency, open circuit voltage, fill factor, and short circuit current density are also calculated from these transport parameters obtained via optical Hall effect using the diode equation and PC1D solar cell simulations. The calculated device performance parameters are found to be consistent with direct current-voltage measurement demonstrating the validity of this technique for electrical transport property measurements of the semiconducting layers in complete Si solar cells. To the best of our knowledge, this is the first method that enables determination of both minority and majority carrier transport parameters in both active layers of the p-n junction in a complete solar cell.

Bias Tunable Photocurrent in Metal-Insulator-Semiconductor Heterostructures with Photoresponse Enhanced by Carbon Nanotubes

Metal-insulator-semiconductor-insulator-metal (MISIM) heterostructures, with rectifying current-voltage characteristics and photosensitivity in the visible and near-infrared spectra, are fabricated and studied. It is shown that the photocurrent can be enhanced by adding a multi-walled carbon nanotube film in the contact region to achieve a responsivity higher than 100 mA W − 1 under incandescent light of 0.1 mW cm − 2 . The optoelectrical characteristics of the MISIM heterostructures are investigated at lower and higher biases and are explained by a band model based on two asymmetric back-to-back Schottky barriers. The forward current of the heterojunctions is due to majority-carrier injection over the lower barrier, while the reverse current exhibits two different conduction regimes corresponding to the diffusion of thermal/photo generated carriers and majority-carrier tunneling through the higher Schottky barrier. The two conduction regimes in reverse bias generate two plateaus, over which the photocurrent increases linearly with the light intensity that endows the detector with bias-controlled photocurrent.

Applied Trace Alkali Metal Elements for Semiconductor Property Modulation of Perovskite Thin Films

With the rapid consumption of energy, clean solar energy has become a key study and development subject, especially the when new renewable energy perovskite solar cells (PSCs) are involved. The doping method is a common means to modulate the properties of perovskite film. The main work of this paper is to incorporate trace amounts of alkali metal elements into the perovskite layer and observe the effects on the properties of the perovskite device and the majority carrier type of the perovskite film. Comparative analysis was performed by doping with Na+, K+, and Rb+ or using undoped devices in the perovskite layer. The results show that the incorporation of alkali metal ions into the perovskite layer has an important effect on the majority carrier type of the perovskite film. The majority carrier type of the undoped perovskite layer is N-type, and the majority carrier type of the perovskite layer doped with the alkali metal element is P-type. The carrier concentration of perovskite films is increased by at least two orders of magnitude after doping. That is to say, we can control the majority of the carrier type of the perovskite layer by controlling the doping subjectively. This will provide strong support for the development of future homojunction perovskite solar cells. This is of great help to improve the performance of PSC devices.

Impact of K+ Doping on Modulating Majority Charge Carrier Type and Quality of Perovskite Thin Films by Two-step Solution Method for Solar Cells

Traditional hetero-junction perovskite solar cells are composed of light-absorbing layers, charge carrier-transporting layers, and electrodes. Recently, a few papers on homo-junction perovskite solar cells have been studied. Here, we studied the effect of K+ doping on TiO2/PbI2 interface quality, perovskite film morphology, photo-physical properties, and majority carrier type. In particular, the K+ extrinsic doping can modulate the majority carrier type of the perovskite thin film. The study indicated that the interface between the perovskite layer and the TiO2 layer deteriorates with the increase of K+ doping concentration, affecting the electron transport ability from the perovskite film to the TiO2 layer and the photo-physical properties of the perovskite layer by K+ doping. In addition, the majority charge carrier type of perovskite thin films can be changed from n-type to p-type after K+ extrinsic doping, and the corresponding hole concentration increased to 1012 cm−3. This approach of modulating the majority charge carrier type of perovskite thin film will pave the way for the investigation of perovskite homo-junction by extrinsic doping for solar cells.

Investigation of dopant centres dominating the conduction process in the bulk of un-doped GaSb

In this paper, first, the theoretical description of the effects of the dopant densities and the activation energies on the ionization densities, the chemical potentials corresponding to each dopant levels, the majority carrier densities and the Fermi-energy levels in one-acceptor-level system, highly compensated system and two-acceptor-level system are described in detail. Upon fitting the theoretical to the experimental results obtained by the temperature-dependent Hall effect measurements for three samples of un-doped GaSb, the dopant densities and the activation energies for a system with different dopants are investigated. The obtained results revealed that the dopant activation energy has less (no) effect on the Fermi-energy level and the majority carrier density in the highest temperature regimes. The doping density has also less (no) effect on the Fermi-energy level in the lowest temperature regimes. Finally, fitting of the theoretical to the experimental Hall effect measurements results confirmed the presence of three acceptor and one donor levels dominating the majority carrier densities at different temperature regions in all the samples of un-doped GaSb semiconductor.

Influence of Solution Deposition Process on Modulating Majority Charge Carrier Type and Quality of Perovskite Thin Films for Solar Cells

In the past ten years, extensive research has witnessed the rapid development of perovskite solar cells (PSCs) and diversified preparation processing craft. At present, the most widely used methods of preparing perovskite solar cells are the one-step method and the two-step method. The main work of this paper is to study the effect of the solution deposition process on the quality of perovskite thin films, as well as modulating majority charge carrier types. Perovskite film was prepared in air by designing different processes, which were then adequately analyzed with corresponding methods. It was demonstrated that the preparation process plays a crucial role in modulating the type of majority carrier and in achieving high-quality perovskite thin film. The one-step prepared perovskite layer is enriched in MA+, leading to a P type majority carrier type thin film. The two-step prepared perovskite layer is enriched in Pb2+, leading to a N type majority carrier type thin film. In addition, we found that the one-step method caused PbI2 residue due to component segregation, which seriously affects the interface and film quality of the perovskite layer. This work aims to modulate the majority carrier type of perovskite film through different preparation processes, which can lay the foundation for the study of homojunction perovskite solar cells to improve the device performance of PSCs.