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Next generation nanowire solar cells

Periodic Reporting for period 1 - NEXTNANOCELLS (Next generation nanowire solar cells)

Reporting period: 2015-08-01 to 2017-07-31

World electricity demand is steadily increasing over years and photovoltaic conversion of solar energy into electricity stands out as a clean, sustainable and reliable way of electricity production. The dominant photovoltaic technology is based on silicon (Si) solar cells. To reduce the cost of photovoltaic electricity and to minimize the semiconductor use, the conversion efficiency of the devices should be increased.

However, after many decades of R&D, the Si technology is mature and no further efficiency improvements are expected (best conversion efficiencies are 25% in the laboratory). Significantly higher conversion efficiencies (e.g. 38.8%) have so far been reached with multi-junction solar cells based on III-V materials, but these devices are too expensive for the use in flat-plat modules for terrestrial applications.

One approach to reduce costs is to grow III-V solar cells on Si substrates, but it is still expensive and technological difficult to achieve. As an alternative, nanowires composed of III-V materials are an innovative way to significantly reduce material needs without compromising light absorption or device performance, given the small dimensions (in the order of 10-9 meters) of such structures. The overall goal of this project is therefore to develop specifically designed III-V nanowire subcells to be subsequently connected to state-of-the-art high-efficiency bottom Si solar cell. Nanowire solar cell development includes synthesis of the materials and development of specific characterization tools of single nanowire solar cells.

In conclusion, within this project a variety of nanowire solar cell structures have been developed to be ready as a building block of a multi-junction solar cell with a Si solar cell. For this to happen, still several challenges need to be addressed in the future, as well as further efficiency increase of the nanowire by, for instance, synthesizing a dual junction solar cell within a nanowire.
The work performed was structured around 4 main tasks:

1) MOVPE growth of nanowires solar cells: A vast variety of core/shell nanowire solar cell structures has been developed by MOVPE such as GaAs/AlGaAs, GaAs/GaInP, GaAs/AlInP and GaAsP/AlInP, by carefully controlling structure, composition and doping of the wires. Prior growth, a well-defined array of Au particles on the GaAs substrates was deposited by means of nanoimprint lithography.
2) Nanowire solar cell characterization: Enabled by a nanoprobe manipulator inside a scanning electron microscope (SEM), optoelectronic characterization at single nanowire level has been developed. In particular, a big piece of work has been done performing and understanding optoelectronic characteristics (e.g. electron beam induced current and dark current-voltage measurements) of single nanowire solar cells and other type of devices such as first multi-junction solar cells prototype structures within nanowires, or InP photodetectors. As a result of these efforts, a new record efficiency of 15% of an InP nanowire solar cell has been obtained.
3) Aerotaxy of nanowire solar cells: In collaboration with Sol Voltaics AB (see next task) a functional single GaAs nanowire solar cell has been grown for the first time by Aerotaxy and a full optoelectronic characterization has been performed
4) Secondment at Sol Voltaics AB: Advanced characterization methods for nanowire based solar cells have been developed, together with mentoring of younger engineers and scientists.

Dissemination of the results was mainly done by publishing so far 6 journal articles and contribution to 6 different major international conferences
Within this project, we have developed GaAs and GaAsP nanowire solar cells. This sets the ground to further develop nanowire based multijunction solar cells by adding either an extra GaInP nanowire solar cell or by connecting the nanowire solar cell to a standard Si solar cell, respectively. In addition, Aerotaxy has proven the production of low–cost and high-throughput GaAs nanowire solar cells. Besides, a new optimization routine in terms of solar cell performance at the nanowire level has been set, by careful interpretation of electron beam induced current and dark current-voltage at single nanowire level, which has led to a new record for InP nanowire solar cell efficiency.
The societal importance and potential impacts to this is to bring nanowire solar cell technology closer to industrialization levels which will benefit society and possibly specific applications (e.g. niche markets) for a clean, accessible and sustainable electricity production.
SEM and EBIC image of a GaAsP NW solar cell array, with the nanoprobe contacting a single NW