• 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.

© . This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Localized p-doped nanojunctions (200–300 nm in diameter) were formed in n-type crystalline silicon substrates and were characterized using scanning electron microscopy (SEM) and conductive-probe atomic force microscopy (C-AFM). Localized doping was performed by diffusion through sub-micron sized holes in a silicon-oxide mask defined using self-organized polystyrene nanoparticles. After oxide removal, a significant brightness contrast in the SEM top and side view images strongly suggested the successful local doping of these areas. Furthermore, local current-voltage measurements performed by C-AFM revealed an open circuit voltage and a short-circuit current only in the areas defined as nanojunctions. This photovoltaic effect is driven by the laser used to control cantilever deflection in the AFM. We acknowledge the financial support of the InnoEnergy PhD School Program and the European Institute of Technology (EIT). This work was partially funded by the Spanish government under projects TEC2017-82305-R, ENE2016-78933-C4-1-R and ENE2017-87671-C3-2-R, and by the Agence Nationale de la Recherche through the ANR project “APOCALYPSO” (ANR-13- PRGE-0003-01). Peer Reviewed

© . This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Localized p-doped nanojunctions (200–300 nm in diameter) were formed in n-type crystalline silicon substrates and were characterized using scanning electron microscopy (SEM) and conductive-probe atomic force microscopy (C-AFM). Localized doping was performed by diffusion through sub-micron sized holes in a silicon-oxide mask defined using self-organized polystyrene nanoparticles. After oxide removal, a significant brightness contrast in the SEM top and side view images strongly suggested the successful local doping of these areas. Furthermore, local current-voltage measurements performed by C-AFM revealed an open circuit voltage and a short-circuit current only in the areas defined as nanojunctions. This photovoltaic effect is driven by the laser used to control cantilever deflection in the AFM. We acknowledge the financial support of the InnoEnergy PhD School Program and the European Institute of Technology (EIT). This work was partially funded by the Spanish government under projects TEC2017-82305-R, ENE2016-78933-C4-1-R and ENE2017-87671-C3-2-R, and by the Agence Nationale de la Recherche through the ANR project “APOCALYPSO” (ANR-13- PRGE-0003-01). Peer Reviewed

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.

Halide perovskite solar cells demonstrated a dramatic increase in efficiency in the last decade mainly thanks to improvements in the bulk material quality, as well as the choice of suitable selective carrier transport layers (SCTLs). More recently, improvements have come from interface optimization strategies, such as defect passivation. Nevertheless, the interactions between the perovskite (PSK) and SCTLs are still unclear and will become more and more important to improve both efficiency and stability of PSK solar cells.In our study, we quantified the optoelectronic interactions at the SCTL-PSK interfaces employing a lateral heterojunction device (LHJ) (Figure 1A). The device is composed of a glass substrate, on which two different SCTLs (TiOx and NiOx) are deposited in a lateral electrode configuration with a well-defined 100 µm – long channel between them. The two SCTLs sit on two separate ITO electrodes that serve as contacts. We measured a ≈600 meV work function (WF) difference between the SCTLs, which constitutes the built-in voltage (Vbi) of our device. To complete the structure, a Pb-based perovskite layer is deposited on top, thus filling the channel and creating a p-i-n device which develops in the horizontal direction, i.e. in the sample plane. We tracked the surface electrostatic potential variation along the PSK channel employing high-spatial-resolution (≈11 µm) X-ray photoemission spectroscopy (XPS) both in dark short circuit, and under applied bias, by recording the I3d5/2 core level binding energy (BEI) of the PSK.We discovered that the substrate built-in voltage was still present on top of the PSK surface: a linear potential drop (Vdrop) of about 600 meV was recorded across the channel by XPS in dark, short circuit conditions, implying a nearly undoped PSK. Indeed, by fitting the data with a 2D drift-diffusion (DD) model, we found Ndop=1011 cm-3 to be the maximum PSK doping density able to sustain such a long-range linear drop (Figure 1B).Furthermore, we could control (and even invert) the potential drop by applying a voltage bias (Vapp) to the electrodes, which adds up to Vbi (Vdrop= Vbi- Vapp) (Figure 1C). Our results indicate that the tested PSK is nearly intrinsic and its Fermi level is controlled by the substrate over which it is deposited, and can be modulated by an external bias voltage.