Pushing limits of photovoltaics and photodetection using radial junction nanowire devices

Nanowire devices have long been proposed as an efficient alternative to their planar counterparts for different optoelectronic applications. Unfortunately, challenges related to the growth and characterization of doping and p-n junction formation in nanowire devices (along axial or radial axis) have significantly impeded their development. The problems are further amplified if a p-n junction has to be implemented radially. Therefore, even though radial junction devices are expected to be on par with their axial junction counterparts, there are minimal reports on high-performance radial junction nanowire optoelectronic devices. This paper summarizes our recent results on the simulation and fabrication of radial junction nanowire solar cells and photodetectors, which have shown unprecedented performance and clearly demonstrate the importance of radial junction for optoelectronic applications. Our simulation results show that the proposed radial junction device is both optically and electrically optimal for solar cell and photodetector applications, especially if the absorber quality is extremely low. The radial junction nanowire solar cells could achieve a 17.2% efficiency, whereas the unbiased radial junction photodetector could show sensitivity down to a single photon level using an absorber with a lifetime of less than 50 ps. In comparison, the axial junction planar device made using same substrate as absorber showed less than 1% solar cell efficiency and almost no photodetection at 0 V. This study is conclusive experimental proof of the superiority of radial junction nanowire devices over their thin film or axial junction counterparts, especially when absorber lifetime is extremely low. The proposed device holds huge promise for III-V based photovoltaics and photodetectors

Design of Ultrathin InP Solar Cell Using Carrier Selective Contacts

Most recently, III-V based ultrathin solar cells have attracted considerable attention for their inherent advantages, such as increased tolerance to defect recombination, efficient charge carrier separation, photon recycling, flexibility, and reduced material consumption. However, so far, almost all reported devices make use of conventional doped p-i-n kind of structures with a wide-bandgap III-V lattice-matched epitaxial window layer, for passivation and reduced contact recombination. Here, we show that a high-efficiency device can be obtained utilizing an InP thin film of thickness as low as 280 nm, without the requirement of a conventional p-n homojunction or epitaxial window layer. This is achieved by utilizing a wide-bandgap electron and hole selective contacts for electrons and holes transport, respectively. Under ideal conditions [assuming interface recombination velocity (IRV) = 10 3 cm/s and bulk lifetime = 1 micros], the proposed solar cell structure can achieve efficiency as high as 28%. Although, in the presence of bulk and interface Shockley-Read-Hall recombination, the efficiency reduces, still for bulk minority carrier lifetime as low as 2 ns and an IRV as high as 10 5 cm/s, an efficiency of ~22% can be achieved with InP thickness as low as 280 nm. The proposed device structure will be beneficial in cases where the growth of controlled p-n homojunction and window layer can be tedious as in case of low-cost deposition techniques, such as thin-film vapour-liquid-solid and close-spaced vapour transport.This work was supported by Australian Research Council

Indium phosphide based solar cell using ultra-thin ZnO as an electron selective layer

According to the Shockley–Queisser limit, the maximum achievable efficiency for a single junction solar cell is ~33.2% which corresponds to a bandgap (E g) of 1.35 eV (InP). However, the maximum reported efficiency for InP solar cells remain at 24.2%  ±  0.5%, that is  >25% below the standard Shockley–Queisser limit. Through a wide range of simulations, we propose a new device structure, ITO/ ZnO/i-InP/p+ InP (p-i-ZnO-ITO) which might be able to fill this efficiency gap. Our simulation shows that the use of a thin ZnO layer improves passivation of the underlying i-InP layer and provides electron selectivity leading to significantly higher efficiency when compared to their n+/i/p+ homojunction counterpart. As a proof-of-concept, we fabricated ITO/ZnO/i-InP solar cell on a p+ InP substrate and achieved an open-circuit voltage (V oc) and efficiency as high as 819 mV and 18.12%, respectively, along with ~90% internal quantum efficiency. The entire device fabrication process consists of four simple steps which are highly controllable and reproducible. This work lays the foundation for a new generation of thin film InP solar cells based solely on carrier selective heterojunctions without the requirement of extrinsic doping and can be particularly useful when p- and n-doping are challenging as in the case of III–V nanostructures.This research is supported by the Australian Research Council

Passivation of InP solar cells using large area hexagonal-BN layers

Surface passivation is crucial for many high-performance solid-state devices, especially solar cells. It has been proposed that 2D hexagonal boron nitride (hBN) films can provide near-ideal passivation due to their wide bandgap, lack of dangling bonds, high dielectric constant, and easy transferability to a range of substrates without disturbing their bulk properties. However, so far, the passivation of hBN has been studied for small areas, mainly because of its small sizes. Here, we report the passivation characteristics of wafer-scale, few monolayers thick, hBN grown by metalorganic chemical vapor deposition. Using a recently reported ITO/i-InP/ p+-InP solar cell structure, we show a significant improvement in solar cell performance utilizing a few monolayers of hBN as the passivation layer. Interface defect density (at the hBN/i-InP) calculated using C–V measurement was 2 × 1012 eV−1 cm−2 and was found comparable to several previously reported passivation layers. Thus, hBN may, in the future, be a possible candidate to achieve high-quality passivation. hBN-based passivation layers can mainly be useful in cases where the growth of lattice-matched passivation layers is complicated, as in the case of thin-film vapor–liquid–solid and close-spaced vapor transport-based III–V semiconductor growth technique

Carrier Selective Contacts for Thin Film and Nanowire InP Solar Cells

This thesis investigates, both theoretically and experimentally, the use of electron selective contacts for both planar and nanowire InP solar cells. We start this thesis with the optimization of electron selective contact for thin film InP solar cells. We proposed a new device structure, ITO/ZnO/i-InP/p+ InP (p-i-ZnO-ITO) which according to our simulation results in higher efficiency compared to conventional p-i-n junction solar cells because of improved passivation and carrier selectivity. As a proof-of-concept, we fabricated ITO/ZnO/i-InP solar cell on a p+ InP substrate and achieved an open-circuit voltage and efficiency as high as 819 mV and 18.12 %, respectively, along with ~90% internal quantum efficiency. Following the successful optimization of carrier selective contact for InP thin film solar cells, we optimized the electron selective contact for InP nanowire solar cells, both theoretically and experimentally. Using Finite-difference time-domain (FDTD) simulations, we showed that the use of a metal-oxide coating over InP core could significantly increase its absorption, while our device simulation results showed that even for a core minority carrier lifetime of 50 ps, an efficiency of 23% can be obtained, under optimized conditions. Based on simulation results, we fabricated a radial p-n junction nanowire solar cell by etching a heavily doped p-type InP substrate that has an extremely low minority carrier lifetime (less than 100 ps) to form the nanowire core, which was then coated with an aluminium zinc oxide/ ZnO shell. These radial junction nanowire devices exceeded an efficiency of 17%, the best reported value for radial junction nanowire solar cells. Although CuI is a suitable material as a hole selective contact, it can lose its conductivity and transparency with time, which made it less than desirable in real device application. Therefore, a part of this thesis investigates the optimization of CuI to increase its stability in ambient condition without compromising its transparency and conductivity. We showed that the introduction of TiO2 in CuI made it more stable in ambient condition while also improving its conductivity and transparency. A detailed comparative analysis between CuI and CuI-TiO2 composite thin films were performed to ascertain the reasons for improved performance of CuI-TiO2 composite thin films in comparison to pure CuI thin films. Finally, we used simulation to show that a high-efficiency device could be obtained by utilizing an InP thin film of only 280 nm in thickness, without the requirement of a conventional p-n homojunction or epitaxial window layer. This is achieved by utilizing a wide bandgap carrier selective contacts for charge carrier separation. Under ideal conditions, the proposed solar cell structure can achieve an efficiency as high as 28%. Although, the efficiency reduces in the presence of bulk and interface recombination, for a bulk minority carrier lifetime as low as 2 ns and an interface recombination velocity (IRV) as high as 105 cm/s, an efficiency of ~22% can still be achieved with the same thickness of InP. Overall, this thesis presents a thorough study of carrier selective contact in InP planar and nanowire solar cells using a combination of both theoretical simulation and experimental verification. The proposed thin film device structures are particularly beneficial in cases where the growth of controlled p-n junction and heterostructure window layer can be challenging as in case of low-cost deposition techniques, such as thin-film vapour-liquid-solid and close-spaced vapour transport. The results from CuI is promising in terms of its stability, transparency and conductivity but more work is required to utilize it as a hole selective contact in III-V materials. Finally, the work on carrier selective contact for nanowire solar cell is expected to simplify the complications associated with the growth and doping of nanowires

Non-epitaxial carrier selective contacts for III-V solar cells: A review

In the last few years, carrier selective contacts have emerged as a means to reduce the complexities and losses associated with conventional doped p-n junction solar cells. Still, this topic of research is only at its infancy for III-V solar cells, in comparison to other solar cell materials such as silicon, perovskites, chalcogenides, etc. This could be because high quality III-V solar cell materials can be achieved relatively easily using epitaxial growth techniques such as MOCVD (metal organic chemical vapor deposition) and MBE (molecular-beam epitaxy). However, current epitaxial III-V solar cells are very expensive and cannot compete for the terrestrial market, and therefore, researchers are developing alternative growth methods such as thin-film vapor–liquid–solid (TF-VLS), hydride vapor phase epitaxy (HVPE) and closed space vapor transport (CSVT), which are significantly lower in cost compared to epitaxial III-V solar cells. However, at present, these relatively nascent low cost growth methods, face severe optimization issues when it comes to growth of controlled p-n junction, along with heavily doped window and back surface field layers. In such cases, carrier selective contacts can be hugely beneficial. In this review, we cover some of the most recent research on the use of carrier selective contacts for III-V solar cells. Future prospects, challenges, and new device concepts using carrier selective contacts will also be discussed.The Australian Research Council is acknowledged for the financial support and the Australian National Fabrication Facility, ACT node is acknowledged for access to the facilities for some of the work referred to in this revie

Axial vs. Radial Junction Nanowire Solar Cells

Both axial and radial junction nanowire solar cells have their own challenges and advantages. However, so far, there is no review that explicitly provides a detailed comparative analysis of both axial and radial junction solar cells. This article reviews some of the recent results on axial and radial junction nanowire solar cells with an attempt to perform a comparative study between the optical and device behavior of these cells. In particular, we start by reviewing different results on ways in which the absorption can be tuned in axial and radial junction solar cells. We also discuss results on some of the critical device concepts that are required to achieve high efficiency in axial and radial junction solar cells. We include a section on new device concepts that can be realized in nanowire structures. Finally, we conclude this review by discussing a few of the standing challenges of nanowire solar cells

Growth kinetics of indium metal atoms on Si(112) surface

The growth kinetics and desorption behavior of indium (In) atoms grown on high index Si(112) surface at different substrate temperatures has been studied. Auger electron spectroscopy analysis revealed that In growth at room temperature (RT) and high substrate temperature (HT) similar to 250 degrees C follows Frank van der Merve growth mode whereas at temperatures >= 450 degrees C, In growth evolves through Volmer-Weber growth mode. Thermal desorption studies of RT and 250 degrees C grown In/Si(112) systems show temperature induced rearrangement of In atoms over Si(112) surface leading to clusters to layer transformation. The monolayer and bilayer desorption energies for RT grown In/Si(112) system are calculated to be 2.5 eV and 1.52 eV, while for HT-250 degrees C the values are found to be 1.6 eV and 1.3 eV, respectively. This study demonstrates the effect of temperature on growth kinetics as well as on the multilayer/monolayer desorption pathway of In on Si(112) surface

Design Principles for Fabrication of InP-Based Radial Junction Nanowire Solar Cells Using an Electron Selective Contact

Nanowire solar cells hold several advantages over planar solar cells, such as reduced reflection, facile strain relaxation, extreme light trapping, increased defect tolerance, etc. However, because of their large surface-to-volume ratio, nanowires tend to have very low effective minority carrier lifetime. To overcome this issue, a radial junction solar cell was proposed. However, in experimental realization, the efficiency of a radial junction solar cell remains significantly lower than its axial counterpart. This is mainly because of the inability to simultaneously control the doping in both the core and the shell while maintaining low defect density at the interface. To overcome the above-mentioned issues, we propose and simulate a core–shell heterojunction solar cell using p-InP as a core material and ITO/ZnO as a shell material. Using finitedifference time-domain simulations, we show that use of an oxide coating over InP core can significantly increase the absorption in InP nanowire arrays, and for an optimized thickness of oxide layer, InP consumption can be reduced by as much as four folds without sacrificing the ideal short circuit current. In addition, our device simulation results show that even for a core minority carrier lifetime of 50 ps, an efficiency of 23% can be obtained if both core and shell can be heavily doped while maintaining an interface recombination velocity of less than 104 cm/s. Finally, we discuss how the proposed device structure can reduce the fabrication complexities related to epitaxial homojunction/heterojunction core–shell solar cell structure while achieving a high efficiency under optimized conditions

High-Efficiency Solar Cells from Extremely Low Minority Carrier Lifetime Substrates Using Radial Junction Nanowire Architecture

Currently, a significant amount of photovoltaic device cost is related to its requirement of high quality absorber materials, especially in the case of III−V solar cells. Therefore, a technology that can transform a low-cost, low minority carrier lifetime material into an efficient solar cell can be beneficial for future applications. Here, we transform an inefficient p-type InP substrate with a minority carrier lifetime less than 100 ps into an efficient solar cell by utilizing a radial p−n junction nanowire architecture. We fabricate a p-InP/n-ZnO/AZO radial hetero junction nanowire solar cell to achieve a photovoltaic conversion efficiency of 17.1%, the best reported value for radial junction nanowire solar cells. The quantum efficiency of ∼95% (between 550 and 750 nm) and the short-circuit current density of 31.3 mA/cm2 are among the best for InP solar cells. In addition, we also perform an advanced loss analysis of the proposed solar cell to assess different loss mechanisms in the solar cell.This work is supported by the Australian Research Council through the Discovery-Project grants. Access to the fabrication facilities is made possible through the support of the Australian National Fabrication Facility, ACT Node