Silicon based photovoltaic cells are the dominant technology for terrestrial solar energy conversion. After many decades of research and development, efficiencies are today reaching a saturation, with the best devices measuring 25 % in the laboratory. Significantly higher conversion efficiencies up to 38.8 % are so far only reached with multi-junction cells based on III-V semiconductors. However, these materials are too expensive for the use in flat-plat modules on the earth, while for satellite applications they are of high value. NWs allow to significantly reduce material needs without compromising absorption or performance. As an illustration can be mentioned that while the silicon material in a square meter of silicon solar cells weigh about 1 kg, the nanowire material corresponding NW-based square meter of a NW solar cell weigh about 1 g. Combining III-V nanowires (NWs) with today’s silicon photovoltaic technology offers the potential to reach at the same time very high performance devices, efficient use of materials and low cost.
During the Nano-Tandem project, three manufacturing concepts are evaluated against each other (see also Research). After the second year of the project, the three different approaches for NW based tandem junction solar cells will be assessed and a choice will be made on which approach(es) to push further with respect to research and development. This will lead to at least one of the approaches being stopped in development. This choice will be based on evaluation of three factors. The decision will be based on (1) the solar energy harvesting efficiency reached, where the approach with highest efficiency will be given priority. In addition to efficiency, we will take into consideration (2) the up-scalability and (3) cost aspects of the approaches. Should the efficiency be similar for two or more of the approaches, criteria (2) and (3) will be the decisive aspects.
The most important impact of the Nano-Tandem project is to present a path for significant performance improvements of photovoltaic modules. Starting from today’s silicon PV technology, the Nano-Tandem technology allows increasing the efficiency through integration of a single NW pn-junction, and at a later stage possibly even a dual NW pn-junction. In the first step, the theoretical efficiency limit is increased from 34 to 45 % compared to today’s state-of-the-art, and in the second step even up to 50 %. This shows the high potential of the NW tandem solar cell on silicon to reach significantly improved performance of solar cells and photovoltaic modules. Efficiency is the most important factor in reducing the cost of solar electricity, as all area related costs are reduced by the overall power output of a PV module. Therefore, efficiency improvements of solar cells are a key to reach competitiveness with conventional energy sources based on fossil fuels or nuclear power. Furthermore, PV modules with higher efficiency allow harvesting of more energy from a constrained area, like a roof, and therefore having a higher value for the customer. Different approaches towards the implementation of “Building-Integrated Photo-Voltaics – BIPV” will constitute important steps towards our future “electricity-based society”, and will be an enabler for communication, lighting as well as electrical vehicles. High-tech products like the Nano-Tandem cell will help the European industry to have a unique differentiator compared to silicon PV modules, which have become a commodity product. This will allow the European industry to compete on the international market by offering higher value products with higher performance, still at competitive prices. Therefore, the development in the Nano-Tandem project will help the European photovoltaic industry to offer more competitive products and to create employment.
Progress until December 2016
Since the beginning of the project in May 2015 the members of the project have jointly pushed forward the research and development of the three approaches to NW synthesis and solar cell fabrication. The work is divided into work packages (WPs) which run in parallel and in the following a summary structured according to the sequence of work packages is given. We have structured our research during the two first years into three areas, involving different methods for the nanowire fabrication: a) Direct growth of III-V nanowire arrays on (100) silicon, b) Growth of nanowires of InP or GaAs, embedding these in polymer + transfer to silicon, and c) Aerotaxy growth of nanowires, alignment and array formation, embedding in polymer and transfer to silicon. These three thrusts are key for us to be able to make evaluations and decisions after the first 24 months for which approaches to give priority.
In WP1 we analyzed different Si emitter structures for the Si to NW tandem configuration regarding recombination, voltage potential, interconnectivity and compatibility with subsequent processing. Highly doped emitters produced by ion-implantation were identified as most the promising for upcoming experiments. They can tolerate the absence of surface passivation, are robust in further processing and allow formation of a tunnel diode at the Si to NW interface.
In WP2 where we focus on synthesis and process development of large area NW solar cell materials progress has been made in all three areas. A key result achieved and published in January this year is that GaAs nanowire PV-cells have produced world-record results, with efficiencies of 15.3% and also record high open-circuit voltages. This is a very important result since GaAs has clear advantages from a cost perspective and for the intended up-scaling based on Aerotaxy. For the template approach with direct growth on Si substrate ternary GaInP as well as GaAsP NWs were successfully grown and characterized, showing that the method can be used to grow NWs with desired band gap for the tandem approach. Regarding growth on native substrates for peel off, we developed nano imprint lithography (NIL), metal evaporation and lift off for economically viable patterning of large areas of catalyst matrix for NW growth. Parameters affecting the pattern fidelity after growth has been assessed, and synthesis optimized for growth with and without a SiN growth mask. Synthesis of n-type GaInP was investigated, where p-type GaInP was evaluated via esaki tunneling diode characteristics in a InP/GaInP configuration due to difficulties making electrically transparent contact to homogeneously doped p-type NWs. A dramatic up-scaling of the Aerotaxy production technique for GaAs nanowires has been achieved at Sol Voltaics, while ternary GaAsP NWs with intentional n and p type doping has been synthesized by use of aerotaxy at ULUND, achieving material with band gap suited for tandem junction with Si.
In WP3, NIL and microcontact printing (µCP) processes were investigated in order to improve the homogeneity and reproducibility of NW growth on III/V substrates. For the direct growth of III/V NWs on Si, e-beam lithography was applied as standard technique at IBM and NIL was investigated as up-scalable alternative lithographic technique. We found that the NIL processes lead to a more homogeneous template for NW growth than µCP. Therefore, the activities on µCP were discarded. Regarding direct formation of aerosol metal particles in gas phase, SOL has developed an electric arc evaporator capable of producing Au particles up to 200 nm in diameter. ULUND is building a new version of the aerosol generator used for NW seeding in wafer based growth.
In WP4 the structural, optical and electrical properties of NWs grown by MOVPE and by aerotaxy techniques have been analyzed and a feedback has been provided to WP2 for growth optimization. Transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy were employed to analyze the crystallographic structure and composition of InGaP and GaAsP NWs. The impact of p-doping on the crystal structure and composition of the InGaP NWs using Diethylzinc has been analyzed by TEM. The optical properties of NWs was investigated by use of photoluminescence (including time-resolved measurements), cathodoluminescence, and electron beam induced current microscopy.
In WP5 we developed release methods of arrays of MOVPE grown NW arrays, primarily GaAs and InP. We exploration limitations with respect to NW length and diameter of the release process. For GaAs NWs arrays, the requirement of surface passivation by a radial layer imposes a constraint on the release process due to increasing diameter. For InP arrays, there is no need for radial passivation and ideally dimensioned NWs could be released. We have begun process flow development for integration of such membranes into NW arrays and have made the first integrations of NW arrays containing InP and GaAs NWs, resulting in solar cells with a few percent efficiency. We observed that NWs often tilt during peel off and transfer, a so far unsolved challenge.
In WP6 we modelled the electro-optical properties of p-i-n junction NW array solar cells. The optical electron-hole pair generation was calculated by solving Maxwell equations. With the knowledge of the electron-hole pair generation, a stationary drift diffusion calculation is performed to calculate the current in the NW array solar cell. We found that it is advantageous to use a high band gap semiconductor for the top n-doped segment to suppress minority carrier leakage to the contacts. Here we also adapted and prepared our external quantum efficiency (EQE) and current voltage (IV) measurement setups for the NW-on-silicon dual-junction cells under consideration in the project.
In WP7 where we address safety and cost of the different approaches in order to make a life cycle assessment, a literature and lab survey was made to collect existing knowledge on implications of nanomaterials in sustainability. We address the “green and clean” claims of the use of nanomaterials in different technological sectors, resulting in a review manuscript currently under review in the Journal of Cleaner Production.
Based on our project activities we have produced seven peer-review publications (see Results) and twenty-four conference contributions. Moreover, we disseminated our activities for policymakers, investors and the general public (twelve events).
Progress beyond state of the art and expected potential impact
As a consequence of the project we are now able to produce silicon solar cells specifically made for the application of combining these in a tandem junction with NWs. Tandem configuration is necessary for achieving solar cell efficiencies above the theoretical limit of conventional silicon solar cells, helping to reduce the costs of PV electricity. Since high efficiency III-V NW solar cells have well defined geometrical properties for optimal light absorption, high-resolution lithographic techniques are required to obtain a sufficient pattern definition over large areas. These requirements open up a field of potential conflicts: cheap and large scale processing and simultaneously very high control and pattern quality. We think these demands can be fulfilled by nano imprint lithography and processing. We have recently synthesized NWs from nano imprint lithography defined patterns with dimensions corresponding to efficient light absorption with 100% vertically growing NWs in samples areas. These NWs were peeled off from the substrates and made into solar cells, thus opening up the possibility to flexible solar cells and re-use of the substrates. Issues remain to be resolved in the integration of such arrays, including the observed induced tilt of NWs in the polymer, which causes a contacting issue. The sun constantly moves during the day, which is why the efficiency of NW array solar cells under tilt is interesting, and which has been explored. The efficiency remains constant or even increases for angles of tilt until approximately 45 degrees. Even at 60 degrees, the efficiency is 95% of the untilted efficiency. Template assisted NW growth has been developed for NWs with band gap corresponding to band gaps used in tandem configuration with Si at high performance. In parallel development to the template and MOVPE grown nanowires, aerotaxy based NWs offer integration on Si in a path to very low cost high efficiency solar cells. Great progess has been made in the alignment into highly ideal arrays over large areas of GaAs nanowires produced by Aerotaxy.
On the modeling side, detailed electro-optical modeling shows advantage of using a top GaP transparent layer, which advances the knowledge of axial materials composition for optimal energy harvesting beyond the state of the art, which might be implemented by industry and result in more efficient solar cells that can be used to reduce carbon dioxide emission.
Life cycle assessment of the use of nanomaterials in solar/photovoltaic technologies have been reviewed, which show that only two of the nano-based solar technologies perform better compared to conventional technologies in terms of carbon emissions. Nanomaterials have high cradle-to-gate energy demand that results in high carbon emissions due to high energy requirements for the deposition processes. In addition, uncertainties due to scaling up from an early stage of research and development have been identified. Differences in life cycle assessment methodologies from the reviewed studies and challenges related to up-scaling have been identified and valuable knowledge has been extracted to build on the state of the art. We note that none of the studies reviewed have included the effect of toxicity on the impact of nanomaterials in their analysis, due to lack of characterization, where we will be the first.
Here Magnus Borgström explains, why he believes semiconductor nanowires to be one of the most promising candidates for the next generation photovoltaics.
A bit of history:
The Nano-Tandem project builds on achievements of the AMON-RA project, where InP based nanowire solar cells with efficiency of 13.8 % were successfully demonstrated.
The AMON-RA project was a collaboration project of seven partners, financed by the EU 7th Framework Programme. It finished in September 2013.
Overview and results can be found on the project´s former webpage: