One of the most severe challenges man is facing today is to fulfill the need for energy without harmful environmental consequences. This complicated, grand challenge must be met by a wide range of solutions; among these are more efficient use of resources and replacement of fossil fuels by renewable energy sources. Any sustainable, renewable energy system must directly or indirectly rely on solar energy. Photovoltaic or solar cells are already efficient and reliable sources of electricity from solar light, but even though the cost has decreased significantly in recent years, solar cells are still far too costly for a competitive production of bulk grid power. The challenge within the solar cell field is thus to reduce the costs involved in solar cell production without sacrificing efficiency and reliability; actually, the efficiency should better improve towards 25% or more, since the cell efficiency strongly affects the overall economy of a solar cell power plant. Currently, most of the solar cell market is based on 180-300 micrometer thick crystalline silicon wafers, and approximately 50% of the cost is due to the cost of the material. To reduce material costs thin film cells are promising alternatives, but a limitation in thin film solar cell technologies is that the absorbance of light is quite weak in particular for indirect band gap materials like silicon. This limitation may be lifted by application of photon trapping strategies that can increase the absorptivity of thin photo-absorbers by orders of magnitude at longer wavelengths. Another proved approach in solar cell optimization is carrier selective contacts, such as conventional amorphous silicon, or wide bandgap metal oxide semiconductor.
In this project, we will explore several new ideas for novel silicon-based solar cells to develop efficient solar cells that can be fabricated in a low thermal budget, low-cost fabrication procedure using only abundant elements. The main photo-absorber will be lightly doped p-type silicon (1.12 eV band gap) with a thin n-type TiO2 (3.2 eV band gap) film on top. This structure forms a p-n heterojunction that effectively separates the photo-generated electron hole-pairs, since the titania and silicon conduction bands are aligned facilitating electron transport, while a ~2 eV energy barrier will prevent hole transport. The electrons transported through the titania to the surface will be conducted laterally by a metal grid or continuous transparent conductive oxides such as Aluminum Zinc Oxide (AZO) with high conductivity, highly transparent (loss 10 %) electrode layer. On the backside, silicon will be coated with complementary to TiO2 thin film of NiO. NiO is p-type wide bandgap (3.6 eV) semiconductor. In connection to Si it will form p-p isotype heterojunction with excellent valence band matching, and creating hole conducting and electron blocking layer. A back contact will be formed using a high work-function metal to form additional a potential barrier against electron transport, while the holes are easily conducted to the metal. This basic structure will be combined with micro and nanostructuring of the silicon surface prior to fabrication to form a light trapping structure. All fabrication procedures may be done at temperatures close to room temperature with a maximum of ~200oC necessary in a single step, and thus the thermal budget becomes unusually low. At the same time, a high open circuit voltage for the structure is expected due to the efficient carrier separation in the structure.
The overall project will have four main research phases. In the first phase of the project, the basic TiO2-Si heterostructure will be optimized on planar silicon wafers. Here the focus will be on the development of optimized fabrication procedures that results in high-performance junctions and efficient lateral transport. In this phase of the project, we also want to fabricate silicon alumina-titania heterostructures. Alumina has properties similar to titania: a wide bandgap and transparency to visible wavelengths, hole transport blocking and passivation of a silicon surface. In the process, alumina will be deposited on top of silicon and then the lithographic windows will be opened for titania deposition and formation of localized titania-silicon heterojunctions. On top of titania areas aluminum contacts will be introduced. It is planned to fabricate and test such structures since they have a potential to show both high open circuit voltage and short circuit current. Other metal with close by work function will be tested to minimize current blocking effects in diode structure.
In the second stage, NiO-Si isotype heterostructure will be tested and optimized to meet the best ohmic (hole conductive) properties. We will fabricate and characterize NiO-Si structure similar to TiO2-Si structure.
Next micro- and nanostructured silicon surfaces, including “Black silicon”, for light trapping will be developed and characterized. In this stage, nanostructured surfaces should be optimized to obtain the lowest surface recombination velocity in comparison to plain silicon. Then the optimized heterostructure cell fabrication procedure will be ported to these structures.
Finally, after the previous three stages will be developed, different solar cell architectures will be tested for solar cell fabrication and characterization. We consider Pasha, HIT and IBC architecture as the most promising for solar cell test since they showed the world records of efficiencies for conventional silicon solar cells.