Selective Contact Layer Research Articles

Improved electrical contact properties in Indium-free silicon heterojunction solar cells with amorphous SnO2 TCO layers

Silicon heterojunction (SHJ) solar cells are recognized as one of the most efficient architectures in silicon-based photovoltaic devices. However, the reliance on indium (In)-based transparent conductive oxides (TCO) is anticipated to constrain their production capacity due to the critical and economically volatile nature of In. Recently, low-temperature-grown amorphous SnO2 (a-SnO2) films have been explored as an earth-abundant alternative TCO material. In this study, we examine the electrical contact properties of a-SnO2 layers employed as TCO layers in SHJ cells, focusing on their interaction with the underlying carrier selective contact layers. Our findings indicate that a stack of doped amorphous silicon (a-Si:H) and a-SnO2 exhibits relatively high specific contact resistivity, leading to a significant reduction in the device's fill factor. To address this issue, we propose two approaches: the insertion of a thin ZnO-based TCO layer between a-Si:H and a-SnO2, and the use of nanocrystalline silicon layers in place of a-Si:H. Both approaches effectively reduce the contact resistivity, resulting in improvements in fill factor and conversion efficiency comparable to those of benchmark device with In-based TCOs. Based on these findings, we demonstrate a high-efficiency, In-free, SnO2-based SHJ cell.

Unveiling the Role of H2Plasma for Efficient InP Solar Cells

III–V semiconductors are among the highest performing materials for solar energy conversion devices. Exposing III–V semiconductors to a hydrogen plasma can improve optoelectronic properties and is a critical step in fabricating efficient InP solar cells. However, there is a limited understanding of the changes induced by hydrogen plasma exposure to the surface and in the bulk of III–V semiconductors. Herein, it is demonstrated that a 19.3% efficient p‐InP solar cell with a TiO2electron selective contact layer can be achieved by exposing the InP substrate to hydrogen plasma. Detailed investigations employing ultraviolet photoelectron spectroscopy and capacitance–voltage measurement unveil that the hydrogen plasma exposure on p‐InP leads to charge carrier polarity inversion in the near‐surface region (charge inversion layer) while simultaneously reducing the carrier concentration (charge‐depleted layer) in the bulk. The study provides important insights into the impact of hydrogen plasma exposures on InP which may lead to more efficient optoelectronic devices such as solar cells, photodetectors, light‐emitting diodes, and photoelectrochemical cells.

Aerosol-assisted CVD of nickel oxide on silicon for hole selective contact layers

Aerosol-assisted CVD of nickel oxide on silicon for hole selective contact layers

In-situ deposition of tungsten oxide hole-contact by Hot-Wire CVD and its application in dopant-free heterojunction solar cells

The tungsten oxide (WO x ) thin films have been deposited by a novel hot filament oxidation-sublimation process and applied in the dopant-free heterojunction solar cells as the hole selective contacts. The oxygen flow ratio plays a significant role during the deposition process. With increasing the oxygen flow ratio from 1.7% to 6.7%, the morphology of WO x films changes from small cauliflower-like particles to large cluster accumulation, and the ratio of W6+ increases from 76.1% to 91.4% with the ratio of W5+ decreasing from 23.9% to 8.6%. The work function of WO x can be tailored in a range of 5.5–6.1 eV by increasing the oxygen flow ratio. Its optical band gap maintains above 3.2 eV with the conductivity of about 10−5 S·cm−1. We have applied the WO x films in dopant-free silicon heterojunction solar cells as the hole selective contact layer by replacing the p-type amorphous silicon layer. By taking advantage of the highly transparent WO x layer, a high photon-current density of 39.6 mA·cm−2 was achieved with the oxygen flow ratio of 1.7%. It is interesting to find that the optimum cell conversion efficiencies of 14.9% were obtained with the oxygen flow ratio of 1.7% and the thickness of 10–20 nm for the deposition of WO x layer. This work proves the feasibility and good potential of Hot-Wire CVD prepared WO x hole selective contact for efficient dopant-free silicon heterojunction solar cells.

Growth of single-crystalline MAPbI3 perovskite film by a modified space-confined inverse temperature crystallization method

Single crystalline (SC) metal halide perovskite absorber film can outperform its polycrystalline counterpart in power conversion efficiency and stability due to the absence of grain boundaries. However, SC perovskite film's direct growth on charge selective contact layer coated substrates for solar cell applications is not widely explored. In this study, SC methylammonium lead iodide (MAPbI3) perovskite film is directly grown on charge selective (electron and hole transport) contact layer coated substrates (TiO2 coated fluorine doped tin oxide (FTO) glass substrate and poly(triarylamine) (PTAA) coated indium tin oxide (ITO) glass substrate) by inverse temperature crystallization (ITC) method. The PTAA/ITO substrate's hydrophobic nature led to faster ion migration and controlled nucleation, resulting in large-sized SC MAPbI3 perovskite film. X-ray diffraction of the MAPbI3/PTAA/ITO showed SC nature with diffraction from (200) and (400) planes, indicating that the film is majorly (100) oriented. The MAPbI3 film grown on TiO2/FTO is found to be relatively small in size (< 1 mm), majorly oriented in the (110) plane with diffraction also from other planes indicating polycrystalline nature. Long-term stability studies over 1500 h show that SC MAPbI3 films show more endurance to ambient conditions than polycrystalline MAPbI3. The growth time of 50 h was reduced to 30 h by employing antisolvent-assisted ITC, resulting in ultra-stable circular shaped films suitable for photovoltaic applications.

What Can Glow Discharge Optical Emission Spectroscopy (GD-OES) Technique Tell Us about Perovskite Solar Cells?

The emerging broad range of applications of the glow discharge optical emission spectroscopy (GD-OES) technique in the field of perovskite solar cells (PSCs) research is reviewed. It can provide a large palette of information by easily and quickly tracking the depth distribution of light to heavy elements. After a discussion of the advantages and the limitations of the technique and a comparison with other analytical techniques, how GD-OES is employed to give structural information on perovskite solar cells is shown. GD-OES has allowed the full perovskite film formation process investigation, from the initial precursor layers containing soaking and complexed solvent to the final crystallized 3D perovskite layers. The A-site elemental cations distribution is followed-up during the film formation. In addition, this technique gives a deep insight into the action mechanism of additives and their effects on the film formation. It provides fruitful information on optimized light absorbing layers and on the selective contact layers which ensure the charge transport in PSCs. It allows to directly visualize halide ions migration and their blocking by ad-hoc chemical engineering and to study the films and PSCs ageing. GD-OES opens new perspectives to explain the final performances of the devices.

New metric for carrier selective contacts for silicon heterojunction solar cells

Heterojunction carrier selective contacts for solar cells have gained great attention because of the ability of these contacts to efficiently collect majority carriers while hindering the recombination of minority carriers, thus resulting in the highest reported voltage among crystalline silicon technologies. The electrode work-function, doping and thickness of the doped layer remain the key parameters for governing the cell performance. Recently, we have studied the requirements for carrier selective contacts under various illumination levels and reported logarithmic dependence of these parameters. In this work, we define a new metric for describing the ability of a contact to collect the carriers and named it as contact strength. We have developed an analytical approach for contact strength of hole selective contacts which represents the requirements on doping, thickness of the contact layer, and electrode work-function for a given illumination. First, the numerical model is calibrated with the experimental data for a wide range of illumination (0.01 Sun – 1.0 Sun) and then simulations have been fitted with analytical model. For the selective contact layer, we observe that only the total charge, rather than thickness and doping individually, matters. The work-function of the top electrode also contributes strongly to contact strength and can in principle substitute doped layer. This insightful metric will guide the solar cell technologists to better understand the carrier selective contacts and to maximize the cell performance not only at standard test conditions (STC), but also at low light illuminations.

Analysis of Silicon-perovskite Tandem Solar Cells with Transition Metal Oxides as Carrier Selective Contact Layers

Analysis of Silicon-perovskite Tandem Solar Cells with Transition Metal Oxides as Carrier Selective Contact Layers

Electrochemical Screening of Contact Layers for Metal Halide Perovskites

Optimizing selective contact layers in photovoltaics is necessary to yield high performing stable devices. However, this has been difficult for perovskites due to their complex interfacial defects that affect carrier concentrations in the active layer as well as charge transfer and recombination at the interface. Using vacuum thermally-evaporated tin oxide as a case study, we highlight electrochemical tests that are simple yet screen device-relevant contact layer properties, making them useful for process development and quality control. Specifically, we show that cyclic voltammetry and potentiostatic chronoamperometry correlate to key performance parameters in completed devices as well as other material/interfacial properties relevant to devices such as shunt pathways and chemical composition. Having fast, reliable, scalable, and actionable probes of electronic properties is increasingly important as halide perovskite photovoltaics approach their theoretical limits and scale to large-area devices.

Self-assembled donor-acceptor hole contacts for inverted perovskite solar cells with an efficiency approaching 22%: The impact of anchoring groups

Self-assembled molecules (SAMs) have shown great potential in replacing bulk charge selective contact layers in high-performance perovskite solar cells (PSCs) due to their low material consumption and simple processing. Herein, we design and synthesize a series of donor-acceptor (D-A) type SAMs (MPA-BT-CA, MPA-BT-BA, and MPA-BT-RA, where MPA is 4-methoxy-N-(4-methoxyphenyl)-N-phenylaniline; BT is benzo[c][1,2,5]-thiadiazole; CA is 2-cyanoacrylic acid, BA is benzoic acid, RA is rhodanine-3-propionic acid) with distinct anchoring groups, which show dramatically different properties. MPA-BT-CA with CA anchoring groups exhibited stronger dipole moments and formed a homogeneous monolayer on the indium tin oxide (ITO) surface by adopting an upstanding self-assembling mode. However, the MPA-BT-RA molecules tend to aggregate severely in solid state due to the sp3 hybridization of the carbon atom on the RA group, which is not favorable for achieving a long-range ordered self-assembled layer. Consequently, benefiting from high dipole moment, as well as dense and uniform self-assembled film, the device based on MPA-BT-CA yielded a remarkable power conversion efficiency (PCE) of 21.81%. Encouragingly, an impressive PCE approaching 20% can still be obtained for the MPA-BT-CA-based PSCs as the device area is increased to 0.80 cm2. Our work sheds light on the design principles for developing hole selecting SAMs, which will pave a way for realizing highly efficient, flexible, and large-area PSCs.

Relaxation processes in silicon heterojunction solar cells probed via noise spectroscopy

We have employed state-of-the-art cross-correlation noise spectroscopy (CCNS) to study carrier dynamics in silicon heterojunction solar cells (SHJ SCs). These cells were composed of a light absorbing n-doped monocrystalline silicon wafer contacted by passivating layers of i-a-Si:H and doped a-Si:H selective contact layers. Using CCNS, we are able to resolve and characterize four separate noise contributions: (1) shot noise with Fano factor close to unity due to holes tunneling through the np-junction, (2) a 1/f term connected to local potential fluctuations of charges trapped in a-Si:H defects, (3) generation-recombination noise with a time constant between 30 and 50 μs and attributed to recombination of holes at the interface between the ITO and n-a-Si:H window layer, and (4) a low-frequency generation-recombination term observed below 100 K which we assign to thermal emission over the ITO/ni-a-Si:H interface barrier. These results not only indicate that CCNS is capable of reveling otherwise undetectable relaxation process in SHJ SCs and other multi-layer devices, but also that the technique has a spatial selectivity allowing for the identification of the layer or interface where these processes are taking place.

Role of electron carrier selective contact layer of lithium fluoride films with wide bandgap and low work function for silicon heterojunction solar cells

Wide bandgap with low work function-based materials are widely used as an electron transport layer to achieve high efficiency in silicon heterojunction (SHJ) solar cells. We presented transparent Lithium fluoride (LiFx) films with a wide optical bandgap and various thicknesses for the applications of SHJ solar cells. The high-resolution transmission electron microscopic (HR-TEM) analysis was used to confirm the thickness of LiFx layers. The aluminium electrode's work-function was reduced because of the high dipole moment by the LiFx and lowers an energy barrier at back contact. Enhancement of quantum efficiency (QE) at longer wavelength regions and operating voltage was due to lower energy barrier height can improve the short-circuit current density (Jsc), open-circuit voltage (Voc), and hence solar cell efficiency. Thermally evaporated-based LiFx films showed an optical transmittance of 88 ~ 90% in the visible-NIR (380–1100) nm wavelength region. LiFx/a-Si:H(i)/c-Si/a-Si:H(i)/LiFx structure for 20 nm thick LiFx showed lifetime of 1129.73 μs and implied iVoc of 731.7 mV. SHJ solar cells were fabricated using various thicknesses 1, 10 and 20 nm, respectively. The SHJ solar cells fabricated on 20 nm thick LiFx film yield the maximum efficiency of 17.16% and was used for the baseline condition of simulations. The simulated performance of the SHJ solar cell was improved from 0.098 to 22.58% as a function of variable LiFx work function from 3.2 to 3.9 eV.

Potential high efficiency of GaAs solar cell with heterojunction carrier selective contact layers

Advanced configurations of gallium arsenide (GaAs) solar cells are proposed, using hydrogenated amorphous silicon (a-Si: H) and zinc oxide (ZnO) layers as passivation and carrier selective contacts (CSC), as alternatives to the conventional epitaxial GaInP CSC layers. The cell operation is simulated based on the AFORS-HET simulation program. The results show that with a wide-gap ZnO window layer, single-junction GaAs solar cells can reach a higher short-circuit current density (Jsc) and open-circuit voltage (Voc) than that obtained with conventional GaInP CSC layers without affecting the fill factor (FF). Notably, wide-gap and n-type doping ZnO layers show great potential as electron selective contacts in GaAs solar cells with high barriers at the valence band for blocking hole-transport. An efficiency of 30% is reached for a single-junction GaAs cell with a front ZnO electron selective contact. A 4-terminal GaAs/c-Si tandem solar cell configuration with an estimated conversion performance of 35.16% is proposed. These results indicate enormous potential for the development of low-cost and high-efficiency GaAs-based solar cells in the future.

Effects of tunneling oxide defect density and inter-diffused carrier concentration on carrier selective contact solar cell performance: Illumination and temperature effects

In this study, technology computer-aided design (TCAD) was used to investigate carrier selective contact solar cell performance based on the boundary of tunneling oxide and electron selective contact layer properties. The role of tunneling oxide quality in the passivation and inter-diffusion properties of the plasma-enhanced chemical vapor deposition of phosphorus-doped amorphous silicon as an electron selective contact layer is studied for carrier selective contact solar cells. Tunnel oxide quality varies according to the ratio of Si4+ to Si2+ state. For the Si4+/Si2+ ratio of 4, open-circuit voltage of 730 mV and the lowest interface trap density of 5.3 × 1010 cm−2 eV−1 are achieved. The change in diffusion depth of the doped layer with respect to the annealing temperature is analysed by transmission electron microscopy (TEM)/energy dispersive X-ray spectroscopy (EDS) measurement. Carrier selective contact solar cell parameters are optimized by incorporating the experimental values of interface trap density, inter-diffused impurity concentration, and depth in the Quokka 3 TCAD tool. Solar cell conversion efficiency of 24.31% was obtained. To understand the response of carrier selective solar cell to the environmental changes, the output characteristics of the solar cell were studied by varying the illumination temperature by 10% of the standard test conditions (STC) and temperature from 277 K to 377 K (difference of 100 K).

Digital alloy contact layers for perovskite solar cells

Thin film optoelectronic devices commonly require an individual material to perform multiple functions, but the physical properties of single-composition materials often cannot be tuned to optimize these diverse requirements. The electron selective contact layer in perovskite solar cells is a prime example. The material must simultaneously have optimal conduction band alignment, facilitate carrier extraction, prevent recombination, and provide a chemically stable interface with the notoriously volatile perovskite semiconductor. The pulse-by-pulse nature of the thin-film deposition method pulsed laser deposition (PLD) provides an opportunity to form material alloys where the chemical composition is controlled at the nanometer scale. These digital alloys may prove to be a powerful materials class to meet some of the multifunctional needs of thin film devices. Using PLD to make electron transport layers from ZnO and MgZnO targets, we demonstrate that digital alloy gradients can be tuned to significantly outperform either of the parent materials in perovskite solar cells.

Simulation of Silicon Heterojunction Solar Cells for High Efficiency with Lithium Fluoride Electron Carrier Selective Layer

In this work, to ameliorate the quantum efficiency (QE), we made a valuable development by using wide band gap material, such as lithium fluoride (LiFx), as an emitter that also helped us to achieve outstanding efficiency with silicon heterojunction (SHJ) solar cells. Lithium fluoride holds a capacity to achieve significant power conversion efficiency because of its dramatic improvement in electron extraction and injection, which was investigated using the AFORS-HET simulation. We used AFORS-HET to assess the restriction of numerous parameters which also provided an appropriate way to determine the role of diverse parameters in silicon solar cells. We manifested and preferred lithium fluoride as an interfacial layer to diminish the series resistance as well as shunt leakage and it was also beneficial for the optical properties of a cell. Due to the wide band gap and better surface passivation, the LiFx encouraged us to utilize it as the interfacial as well as the emitter layer. In addition, we used the built-in electric and band offset to explore the consequence of work function in the LiFx as a carrier selective contact layer. We were able to achieve a maximum power conversion efficiency (PEC) of 23.74%, fill factor (FF) of 82.12%, Jsc of 38.73 mA cm−2, and Voc of 741 mV by optimizing the work function and thickness of LiFx layer.

Sputter-Deposited Titanium Oxide Layers as Efficient Electron Selective Contacts in Organic Photovoltaic Devices

Organic photovoltaics (OPVs) has recently reached power conversion efficiencies of 17.3%, making it a green technology that not only offers short energy payback times and diverse photovoltaic integration schemes, but also can deliver competitive power outputs. OPVs typically employs electron selective contact layers made from low work function n-type metal oxide semiconductors, such as titanium oxide (TiO2) or zinc oxide (ZnO), developed from a variety of deposition techniques. However, in the case of TiO2 interlayers, the appearance of unwanted s-shape characteristics has been reported extensively in the literature in the past, for a variety of different deposition methods used. It has been shown that the s-shape arises from negatively charged chemisorbed oxygen, and that it can be deactivated by UV light illumination, which, however, is hardly compatible with real-life OPV application. In this work, we introduce sputtered crystalline titanium oxide layers as efficient s-shape-free electron selective extraction layers in organic solar-cell devices. We demonstrate that the onset of crystallization takes place at substrate growth temperatures of approximately 100 °C for the TiOx thin films, and that the crystallization onset temperature correlates well with a strong increase in device performance and the removal of any s-shape characteristics. Optical, structural, compositional, and electronic energy-level characterizations of the TiOx layers are shown in the present work, and point to the formation of an oxide with a low surface-defect density, developed from the sputter–crystallization process. Importantly, well-functioning s-shape-free PTB7:PC70BM devices are demonstrated for TiOx growth temperatures of 155 °C.

High Photovoltage Inverted Planar Heterojunction Perovskite Solar Cells with All-Inorganic Selective Contact Layers.

Inverted planar heterojunction perovskite solar cells based on all-inorganic selective contact layers show great promise for commercialization owing to their competitiveness in terms of cost and stability. However, the power conversion efficiencies (PCEs) of the few reported perovskite solar cells with this type of device structure have been limited by relatively low photovoltages. Here, we propose a new device structure comprising electron beam-evaporated nickel and niobium oxides as the hole and electron selective contact layers, respectively. We demonstrate that a metal oxide material can be directly deposited on a perovskite film by electron beam evaporation without damaging the interface. We propose that the turn-on voltage of the p-n junction formed by the selective contacts represents a quantitative proxy of the charge blocking performance. A high turn-on voltage of 1.36 V is obtained for the NiOx/Nb2O5 p-n junction. An open-circuit voltage of 1.16 V is achieved using a hybrid organic-inorganic perovskite with a band gap of 1.6 eV. The large photovoltage, enabled by the excellent charge extraction and blocking properties of the inorganic selective contact layers, leads to the highest PCE of over 19.0% for this class of device.

Magnesium-doped Zinc Oxide as Electron Selective Contact Layers for Efficient Perovskite Solar Cells.

The electron-selective contact layer (ESL) in organometal halide-based perovskite solar cells (PSCs) determines not only the power conversion efficiency (PCE) but also the thermostability of PSCs. To improve the thermostability of ZnO-based PSCs, we developed Mg-doped ZnO [Zn1-x Mgx O (ZMO)] as a high optical transmittance ESL for the methylammonium lead trihalide perovskite absorber [CH3 NH3 PbI3 ]. We further investigated the optical and electrical properties of the ESL films with Mg contents of 0-30 mol % and the corresponding devices. We achieved a maximum PCE of 16.5 % with improved thermal stability of CH3 NH3 PbI3 on ESL with the optimal ZMO (0.4 m) containing 10 mol % Mg. Moreover, this optimized ZMO PSC exhibited significantly improved durability and photostability owing to the improved chemical/photochemical stability of the wider optical bandgap ZMO.

Hole selective NiO contact for efficient perovskite solar cells with carbon electrode.

In this study, we communicate an investigation on efficient CH3NH3PbI3-based solar cells with carbon electrode using mesoporous TiO2 and NiO layers as electron and hole selective contacts. The device possesses an appreciated power conversion efficiency of 14.9% under AM 1.5G illumination. The detailed information can be disclosed with impedance spectroscopy via tuning the interfaces between CH3NH3PbI3 and different charge selective contacts. The results clearly show charge accumulation at the interface of CH3NH3PbI3. The NiO is believed to efficiently accelerate charge extraction to the external circuit. The extracted charge could improve photovoltaic performance by shifting hole Fermi level down, achieving a high device photovoltage. A fast interfacial recombination at the interface of CH3NH3PbI3/electron selective contact layer (mesoporous TiO2), occurring in millisecond domains, is the critical issue for charge carrier recombination loss.