• Energy Research

Metal halide perovskite (MHP) solar cells have experienced a rapid increase in efficiency in the last decade, as well as large interest of the PV scientific community. Perovskite materials are utilized as not-intentionally-doped absorbers inserted between two selective carrier transport layers (SCTL), realizing a p-i-n or n-i-p heterojunction device. The transparency of the SCTLs, the high absorption coefficient of the perovskite, and the carrier selectivity at the heterojunctions are considered to be the key factors to the high power conversion efficiency (PCE) achieved [1][2].Despite the recent advancements in PCE, the role and concentration of defects in perovskites are still questionable. Being polycrystalline materials, MHPs possess defects both in the bulk and at the grain boundaries [3]. Therefore, the combination of surface characterization techniques and modelling could shed light onto defects’ key properties, such as their concentration, type, energy level, and capture rate.In this work, we present a lateral heterojunction device (LHJ) we recently developed in our laboratory. A characterization and modelling study of this device is being conducted to gain insight into perovskite doping levels and defect concentrations. The LHJ device features variable channel lengths (from 10 to 200 µm) between the electron and hole extracting layers made of TiOx and NiOx, respectively. We locally probed the free perovskite surface using Kelvin probe force microscopy (KPFM) across the channels. This allowed us to acquire the electrostatic potential profile, as well as the work function (WF), along the channels. While the extracting layers possess a WF difference of 600 meV, our scans across the 10 μm – long channels revealed the presence of a built-in voltage along the perovskite surface of up to 400 mV. The gradual monotonous decrease of the potential extends beyond the channel, indicating the presence of long-range charge rearrangement.A 2D drift-diffusion model of the LHJ is under development, and it will be employed to explain these findings, together with complementary data acquired by X-ray photoemission spectroscopy and hyperspectral photoluminescence imaging across the channels. [1]N. G. Park, Mater. Today 18 (2015) 65.[2]J. Y. Kim, J. W. Lee, H. S. Jung, H. Shin, N. G. Park, Chem. Rev. 120 (2020) 7867.[3]H. Jin et al., Mater. Horizons 7 (2020) 397.

Metal halide perovskite (MHP) solar cells have experienced a rapid increase in efficiency in the last decade, as well as large interest of the PV scientific community. Perovskite materials are utilized as not-intentionally-doped absorbers inserted between two selective carrier transport layers (SCTL), realizing a p-i-n or n-i-p heterojunction device. The transparency of the SCTLs, the high absorption coefficient of the perovskite, and the carrier selectivity at the heterojunctions are considered to be the key factors to the high power conversion efficiency (PCE) achieved [1][2].Despite the recent advancements in PCE, the role and concentration of defects in perovskites are still questionable. Being polycrystalline materials, MHPs possess defects both in the bulk and at the grain boundaries [3]. Therefore, the combination of surface characterization techniques and modelling could shed light onto defects’ key properties, such as their concentration, type, energy level, and capture rate.In this work, we present a lateral heterojunction device (LHJ) we recently developed in our laboratory. A characterization and modelling study of this device is being conducted to gain insight into perovskite doping levels and defect concentrations. The LHJ device features variable channel lengths (from 10 to 200 µm) between the electron and hole extracting layers made of TiOx and NiOx, respectively. We locally probed the free perovskite surface using Kelvin probe force microscopy (KPFM) across the channels. This allowed us to acquire the electrostatic potential profile, as well as the work function (WF), along the channels. While the extracting layers possess a WF difference of 600 meV, our scans across the 10 μm – long channels revealed the presence of a built-in voltage along the perovskite surface of up to 400 mV. The gradual monotonous decrease of the potential extends beyond the channel, indicating the presence of long-range charge rearrangement.A 2D drift-diffusion model of the LHJ is under development, and it will be employed to explain these findings, together with complementary data acquired by X-ray photoemission spectroscopy and hyperspectral photoluminescence imaging across the channels. [1]N. G. Park, Mater. Today 18 (2015) 65.[2]J. Y. Kim, J. W. Lee, H. S. Jung, H. Shin, N. G. Park, Chem. Rev. 120 (2020) 7867.[3]H. Jin et al., Mater. Horizons 7 (2020) 397.

We briefly review the basic concepts of junction capacitance and the peculiarities related to amorphous semiconductors, paying tribute to Cohen and to his pioneering work. We extend the discussion to very high efficiency silicon heterojunction (SiHET) solar cells where both an amorphous semiconductor, namely hydrogenated amorphous silicon, and heterojunctions are present. By presenting both modeling and experimental results, we demonstrate that the conventional theory of junction capacitance based on the depletion approximation in the space charge region, cannot reproduce the capacitance data obtained on SiHET cells. The experimental temperature dependence is significantly stronger than that of the depletion-layer capacitance, while the bias dependence yields underestimated values of the diffusion potential, leading to strong errors if applied to the determination of band offsets using the procedure proposed precedingly in the literature. We demonstrate that this is not related to the amorphous nature of a-Si:H, but to the existence of a strongly inverted c-Si surface layer that requires minority carriers to be taken into account in the analysis of the junction capacitance.

We briefly review the basic concepts of junction capacitance and the peculiarities related to amorphous semiconductors, paying tribute to Cohen and to his pioneering work. We extend the discussion to very high efficiency silicon heterojunction (SiHET) solar cells where both an amorphous semiconductor, namely hydrogenated amorphous silicon, and heterojunctions are present. By presenting both modeling and experimental results, we demonstrate that the conventional theory of junction capacitance based on the depletion approximation in the space charge region, cannot reproduce the capacitance data obtained on SiHET cells. The experimental temperature dependence is significantly stronger than that of the depletion-layer capacitance, while the bias dependence yields underestimated values of the diffusion potential, leading to strong errors if applied to the determination of band offsets using the procedure proposed precedingly in the literature. We demonstrate that this is not related to the amorphous nature of a-Si:H, but to the existence of a strongly inverted c-Si surface layer that requires minority carriers to be taken into account in the analysis of the junction capacitance.

This article investigates the optimal efficiency of a photovoltaic system based on a silicon thin film tandem cell using polymorphous and microcrystalline silicon for the top and bottom elementary cells, respectively. Two ways of connecting the cells are studied and compared: (1) a classical structure in which the two cells are electrically and optically coupled; and (2) a new structure for which the “current-matching” constraint is released by the electrical decoupling of the two cells. For that purpose, we used a computer simulation to perform geometrical optimization of the studied structures as well as their electrical performance evaluation. The simulation results show that the second structure is more interesting in terms of efficiency.

This article investigates the optimal efficiency of a photovoltaic system based on a silicon thin film tandem cell using polymorphous and microcrystalline silicon for the top and bottom elementary cells, respectively. Two ways of connecting the cells are studied and compared: (1) a classical structure in which the two cells are electrically and optically coupled; and (2) a new structure for which the “current-matching” constraint is released by the electrical decoupling of the two cells. For that purpose, we used a computer simulation to perform geometrical optimization of the studied structures as well as their electrical performance evaluation. The simulation results show that the second structure is more interesting in terms of efficiency.