Textured interfaces in monolithic perovskite silicon tandem solar cells advanced light management for improved efficiency and energy yield

Efficient light management in monolithic perovskite silicon tandem solar cells is one of the prerequisites for achieving high power conversion efficiencies PCEs . Textured silicon wafers can be utilized for light management, however, this is typically not compatible with perovskite solution processing. Here, we instead employ a textured light management LM foil on the front side of a tandem solar cell processed on a wafer with planar front side and textured back side. This way the PCE of monolithic, 2 terminal perovskite silicon heterojunction tandem solar cells is significantly improved from 23.4 to 25.5 . Furthermore, we validate an advanced numerical model for our fabricated device and use it to optically optimize a number of device designs with textures at different interface with respect to the PCE and energy yield. These simulations predict a slightly lower optimal bandgap of the perovskite top cell in a textured device as compared to a flat one and demonstrate strong interdependency between the bandgap and the texture position in the monolithic stack. We estimate the PCE potential for the best performing both side textured device to be 32.5 for a perovskite bandgap of 1.66 eV. Furthermore, the results show that under perpendicular illumination conditions, for optimized designs, the LM foil on top of the cell performs only slightly better than a flat anti reflective coating. However, under diffuse illumination, the benefits of the LM foil are much greater. Finally, we calculate the energy yield for the different device designs, based on true weather data for three different locations throughout the year, taking direct as well as diffuse illumination fully into account. The results further confirm the benefits of front side texture, even more for BIPV applications. Overall, devices built on a both side textured silicon wafer perform best. However, we show that devices with textured LM foils on the cell s front side are a highly efficient alternativ

Gas-phase hydrosilylation of plasma synthesized silicon nanocrystals with short- and long-chain alkynes

Surface passivation of Si nanocrystals (NCs) is necessary to enable their utilization in novel photovoltaic and optoelectronic devices. Herein, we report the surface passivation of plasma-synthesized, H-terminated Si NCs via gas-phase hydrosilylation using a combination of short- and long-chain alkynes. Specifically, using in situ attenuated total reflection Fourier transform infrared spectroscopy, we show that a sequential exposure of the Si NC surface to acetylene and phenylacetylene results in a surface alkenyl coverage of 58%, which is close to the theoretical maximum of 55% and 60% predicted for alkyl- and alkenyl-terminated Si(111) surfaces, respectively. We attribute this unprecedented high surface hydrocarbon coverage to the combination of short- and long-chain alkynes that reduce the steric hindrance on the surface, higher reactivity of 1-alkynes versus 1-alkenes of the same chain length, and the smaller van der Waals radius of the alkenyl groups compared to the alkyl groups. Unlike 1-alkenes, 1-alkynes also react with the surface to form the 1,1- and 1,2-bridge structures via the bis-hydrosilylation reaction. However, our data clearly show that this reaction pathway cannot account for the enhanced surface coverage in the sequential exposure experiments, since exposure of the surface to just acetylene or phenylacetylene results in an almost identical surface coverage due to the 1,1- and 1,2-bridge sites

Gas-phase hydrosilylation of plasma synthesized silicon nanocrystals with short- and long-chain alkynes

Surface passivation of Si nanocrystals (NCs) is necessary to enable their utilization in novel photovoltaic and optoelectronic devices. Herein, we report the surface passivation of plasma-synthesized, H-terminated Si NCs via gas-phase hydrosilylation using a combination of short- and long-chain alkynes. Specifically, using in situ attenuated total reflection Fourier transform infrared spectroscopy, we show that a sequential exposure of the Si NC surface to acetylene and phenylacetylene results in a surface alkenyl coverage of 58%, which is close to the theoretical maximum of 55% and 60% predicted for alkyl- and alkenyl-terminated Si(111) surfaces, respectively. We attribute this unprecedented high surface hydrocarbon coverage to the combination of short- and long-chain alkynes that reduce the steric hindrance on the surface, higher reactivity of 1-alkynes versus 1-alkenes of the same chain length, and the smaller van der Waals radius of the alkenyl groups compared to the alkyl groups. Unlike 1-alkenes, 1-alkynes also react with the surface to form the 1,1- and 1,2-bridge structures via the bis-hydrosilylation reaction. However, our data clearly show that this reaction pathway cannot account for the enhanced surface coverage in the sequential exposure experiments, since exposure of the surface to just acetylene or phenylacetylene results in an almost identical surface coverage due to the 1,1- and 1,2-bridge sites

Influence of transparent conductive oxides on passivation of a-Si:H/c-Si heterojunctions as studied by atomic layer deposited Al-doped ZnO

In silicon heterojunction solar cells, the main opportunities for efficiency gain lie in improvements of the front-contact layers. Therefore, the effect of transparent conductive oxides (TCOs) on the a-Si:H passivation performance has been investigated for Al-doped zinc oxide (ZnO:Al) layers made by atomic layer deposition (ALD). It is shown that the ALD process, as opposed to sputtering, does not impair the chemical passivation. However, the field-effect passivation is reduced by the ZnO:Al. The resulting decrease in low injection-level lifetime can be tuned by changing the ZnO:Al doping level (carrier density = 7 × 1019–7 × 1020 cm-3), which is explained by a change in the TCO workfunction. Additionally, it is shown that a ~10–15 nm ALD ZnO:Al layer is sufficient to mitigate damage to the a-Si:H by subsequent sputtering, which is correlated to ALD film closure at this thickness

Atomic-layer-deposited transparent electrodes for silicon heterojunction solar cells

We examine damage-free transparent-electrode deposition to fabricate high-efficiency amorphous silicon/crystalline silicon heterojunction solar cells. Such solar cells usually feature sputtered transparent electrodes, the deposition of which may damage the layers underneath. Using atomic layer deposition, we insert thin protective films between the amorphous silicon layers and sputtered contacts and investigate their effect on device operation. We find that a 20-nm-thick protective layer suffices to preserve, unchanged, the amorphous silicon layers beneath. Insertion of such protective atomic-layer-deposited layers yields slightly higher internal voltages at low carrier injection levels. However, we identify the presence of a silicon oxide layer, formed during processing, between the amorphous silicon and the atomic-layer-deposited transparent electrode that acts as a barrier, impeding hole and electron collection

Status and prospects for atomic layer deposited metal oxide thin films in passivating contacts for c-Si photovoltaics

In the field of photovoltaics, atomic layer deposition (ALD) is mostly known for its success in preparing Al2O3-based surface passivation layers for c-Si homojunction cells. In the last years, many novel types of c-Si heterojunctions have appeared, referred to as passivating contacts. In these concepts, metal oxide thin films are used for surface passivation, carrier selectivity and as transparent conductive oxide. This leads to the question whether the success of ALD for homojunctions can be translated into this new field as well. Therefore, this work provides an overview of these new concepts, and highlights both the current role and prospects of ALD in this field

Passivating contacts for crystalline silicon solar cells: from concepts and materials to prospects

To further increase the conversion efficiency of crystalline silicon (c-Si) solar cells, it is vital to reduce the recombination losses associated with the contacts. Therefore, a contact structure that simultaneously passivates the c-Si surface while selectively extracting only one type of charge carrier (i.e., either electrons or holes) is desired. Realizing such passivating contacts in c-Si solar cells has become an important research objective, and an overview and classification of work to date on this topic is presented here. Using this overview, we discuss the design guidelines for passivating contacts and outline their prospects

Influence of transparent conductive oxides on passivation of a-Si:H/c-Si heterojunctions as studied by atomic layer deposited Al-doped ZnO

In silicon heterojunction solar cells, the main opportunities for efficiency gain lie in improvements of the front-contact layers. Therefore, the effect of transparent conductive oxides (TCOs) on the a-Si:H passivation performance has been investigated for Al-doped zinc oxide (ZnO:Al) layers made by atomic layer deposition (ALD). It is shown that the ALD process, as opposed to sputtering, does not impair the chemical passivation. However, the field-effect passivation is reduced by the ZnO:Al. The resulting decrease in low injection-level lifetime can be tuned by changing the ZnO:Al doping level (carrier density = 7 × 1019–7 × 1020 cm-3), which is explained by a change in the TCO workfunction. Additionally, it is shown that a ~10–15 nm ALD ZnO:Al layer is sufficient to mitigate damage to the a-Si:H by subsequent sputtering, which is correlated to ALD film closure at this thickness

Concepts and prospects of passivating contacts for crystalline silicon solar cells

To further increase the conversion efficiency of crystalline silicon solar cells it is vital to reduce the recombination losses between the photoactive part of the solar cell and the metal contacts. This is ideally achieved by fabricating contacts which passivate defects at the silicon surface while being simultaneously selective for only a single type of charge carrier, i.e. either electrons or holes. Despite the extensive research effort aimed at realizing such contacts, no clear overview of the fundamental physics of passivating contacts has appeared yet. Therefore, we present such an overview, introduce a clear classification of passivating contacts, and discuss their design guidelines and future prospect

Atomic Layer Deposition of In2O3:H from InCp and H2O/O2: Microstructure and Isotope Labeling Studies

Atomic layer deposition (ALD), a gas-phase thin film deposition technique based on repeated, self-terminating gas–solid reactions, has become the method of choice in semiconductor manufacturing and many other technological areas for depositing thin conformal inorganic material layers for various applications. ALD has been discovered and developed independently, at least twice, under different names: atomic layer epitaxy (ALE) and molecular layering. ALE, datingback to 1974 in Finland, has been commonly known as the origin of ALD, while work done since the 1960s in the Soviet Union under the name “molecular layering” (and sometimes other names) has remained much less known. The virtual project on the history of ALD (VPHA) is a volunteer-based effort with open participation, set up to make the early days of ALD more transparent. In VPHA, started in July 2013, the target is to list, read and comment on all early ALD academic and patent literature up to 1986. VPHA has resulted in two essays and several presentations at international conferences. This paper, based on a poster presentation at the 16th International Conference on Atomic Layer Deposition in Dublin, Ireland, 2016, presents a recommended reading list of early ALD publications, created collectively by the VPHA participants through voting. The list contains 22 publications from Finland, Japan, Soviet Union, United Kingdom, and United States. Up to now, a balanced overview regarding the early history of ALD has been missing; the current list is an attempt to remedy this deficiency