Final Report Summary - SOBONA (Solar Cells Based on Nanowire Arrays)
The primary goal of this joint research program was to design, make and test novel photovoltaic structures based on nanowire arrays and thin-films. A variety of III-V and II-VI semiconductor materials, as well as organic polymer materials, will be used to test new concepts and to create state-of-the-art devices. The project has the following general objectives:
1. To investigate morphological and crystallographic properties of NW arrays (e.g. cubic zincblende or hexagonal wurtzite phase of III-V NWs) in different material systems relevant for applications in photovoltaics.
2. To create optimal p-n junctions and heterostructures within NWs (e.g. in core-shell geometry) to improve the light absorption, carrier collection and finally to significantly increase the conversion efficiency of NW-based photovoltaic devices.
3. To study different combinations of III-V and II-VI inorganic as well organic materials, in order to identify the most suitable methods for novel device architectures.
4. To carry out theoretical investigation of the high temperature growth mechanisms for II-VI nanowires on different substrates. The already developed formalism and software packages for III-V nanowires will be used as a starting platform.
5. To investigate hybrid photovoltaic structures based on combination of nanowire arrays andthin-films. The already established fabrication process for high-efficiency CdTe/CdS or CdTe/CdS/ZnO solar cells (with efficiencies exceeding 10%) will be used as a platform for the hybrid devices.
6. To grow CdTe, CdS and ZnO nanowires by close-space sublimation and magnetron sputtering. These methods will allow a good control and variability in the manufacture of the NW arrays for such hybrids. Electrodeposited ZnO nanowires will also be studied in combination with other materials.
7. To explore new possibilities for using ultra-thin iron pyrite (FeS2) as an effective absorber layer for sensitising NW arrays.
The description of work performed
A great progress have being made towards achievement of milestones and deliverables in the first and second half of the project with the majority of milestones and deliverables already achieved. The project has been going forward without any deviations from the above objectives, as it is evidenced by a large amount of achieved results and high quality publications to date. Each of the research groups involved has being active in research strategy development for sample growth and characterisation within the framework of the program. A large number of growth runs have been carried out successfully and this contributed to the achievements of the all milestones and deliverables from each of the three work packages, planned for the first half of the project, as well as a substantial number of milestones and deliverables scheduled and carried out in the second half of the project. In particular samples with electrically active nanowire arrays have being obtained by three groups on this project.
The joined experiments and material combination involving material exchanges between different groups have being accomplished. The theoretical groups have accomplished large tasks in development of the phenomenological models for the nanowire growth formation, as well as advanced mathematical modelling of the nanowire growth using experimental parameters from the growth labs involved. A great deal of existing, and newly developed (since the beginning of project), experimental techniques and direct hands on knowledge have been passed to early stage researchers/PhD students. This was done during the secondment visits of the researchers (both theorists and experimentalists), as well as during joined discussions and workshops and on project meetings.
A description of main results acheived so far
1. The kinetics as well as morphology of the vapour-liquid-solid growth mechanisms for high temperature CdTe growth have been calculated theoretically as well suitable growth models obtained.
2. A large range of nanowire systems based on III-V and II-VI materials have been obtained with a large number of high quality publications made on this subject.
3. A range of different p-n junctions and heterostructures within NWs (e.g. in core-shell geometry) were created successfully on the basis of existing thin-film platform of CdTe/CdS heterojunction systems, by incorporation of both CdTe and ZnO nanowires. Thus the first hybrid structures planned in the objectives of the project were obtained.
4. A successful templated growth of III-V nanowires using E-beam lythography has been developed and a range of samples with periodic GaAs nanowire arrays were obtained. The resuslts were published in a periodic journal.
5. Electronic properties of nanowire array systems were studied and an exact equivalent circuit model for a specific type of structures was theoretically obtained.
6. A very successful growth mechanism based on non-catalytic (gold-free) processes for growth of CdTe nanowires was obtained using magnetron sputtering. The results were published and more publications on this subject are underway.
7. A successful growth of CdTe nanowire arrays by close-space sublimation was established.
8. Electrodeposited ZnO nanowires was successfully incorporated in thin-film CdTe/CdS p-n junction devices. The first working prototypes were obtained.
9. These prototypes have been considerably improved during the second half of the project and the several devices approaching light conversion efficiency of 10% were successfully obtained. This number is close to the world record for the nanowire based cells. Further improvements within structure are already promising further enhancememnts in performance.
10. Large amount of theoretical and modelling work has been carried out towards development and refinement of the properties of nanowire arrays based on III-V and II-VI semiconductors. Novel structures as well growth mechanisms have been explored in a detail and in great variety of parameters available in experiment thus proving a basis for the subsequent experimental tests and improvement of the existing devices.
11. First prototypes based on hybrid organic/inorganic heterojunctions have been obtained. The rectifying/diode behaviour has been observed successfully within such structures paving the avenue for further work on organic/inorganic solar cells.
The potential impact and use
It is evidenced from all of the results obtained to date that practically all of the milestones and deliverables outlined in the beginning of the program have been achieved by the end of the second reporting period. A significant impact on the solar cell research sector, and potential use by industrial sector, thus provided by the large amount of the published work as well as several patent applications.