Solar cells: Losses made visible on the nanoscale

• Utilizing a conductive atomic force microscopic lense, they scanned the solar cell surfaces and also discovered small, nanometre-sized channels for the destructive dark currents, which are due to disorder in the a-Si: H layer.

Silicon solar cells are now so economical and reliable that they can generate power at rates of less than 2 cent/kWh. The most effective silicon solar cells today are made with less than 10 nanometres thin discerning amorphous silicon (a-Si: H) contact layers, which are accountable for separating the light-generated fees. Effectiveness of over 24% are achieved at HZB with such silicon heterojunction solar cells as well as are also part of a tandem solar cell that lead to a just recently reported effectiveness document of 29.15 % (A. Al-Ashouri, et al. Science 370, (2020 )). The current world record from Japan for a single joint silicon solar cell is additionally based upon this heterocontact (26.6%: K. Yoshikawa, et al. Nature Energy 2, (2017 )).

There is still substantial performance potential pertaining to such heterocontact systems, however, it is not yet comprehended thoroughly just how these layers make it possible for charge carrier splitting up and also what their nanoscopic loss mechanisms are. The a-Si: H call layers are qualified by their innate disorder, which on the one hand enables outstanding finishing of the silicon surface as well as hence reduces the number of interfacial flaws, but on the other hand also has a tiny disadvantage: it can result in local recombination currents and also to the formation of transport barriers.

For the first time, a group at HZB as well as the University of Utah has experimentally measured on an atomic level exactly how such leakage currents create between c-Si as well as a-Si: H, and exactly how they affect the solar cell efficiency. In a collaboration, a group led by Prof. Christoph Boehme at the University of Utah, and also by Prof. Dr. Klaus Lips at HZB, they were able to solve the loss system at the user interface of the above stated silicon heterocontact on the nanometre scale making use of ultrahigh vacuum conductive atomic force microscopy (cAFM).

The physicists had the ability to establish with near atomic resolution where the leak current passes through the discerning a-Si: H contact as well as develops a loss procedure in the solar cell. In cAFM these loss currents appear as nanometre-sized existing networks as well as are the fingerprint of defects connected with the disorder of the amorphous silicon network. "These flaws act as stepping stones for charges to permeate the discerning call and generate recombination, we describe this" as trap-assisted quantum mechanical tunnelling", describes Lips. "This is the very first time that such states have been made visible in a-Si: H which we were able to unwind the loss device under working conditions of the a solar cell of highest quality," the physicist reports enthusiastically.

The Utah/Berlin group was also able to revealed that the transported dark current fluctuates stochastically with time. The outcomes suggest that a short-term existing clog is present, which is caused by local fee that is caught in neighbouring defects which alters the energised positioning of the tunnelling states (tipping rocks). This trapped fee can additionally cause the regional photovoltage at a current network to rise to over 1V, which is much above what one would certainly have the ability to use with a macroscopic get in touch with. "At this transition from the nano to the macro worldwe locate the amazing physics of heterojunctions and also the key on exactly how to additional boost the effectiveness of silicon solar cells in a much more targeted means," states Dr. Bernd Stannowski, who is responsible for the development of industrial silicon heterojunction solar cells at HZB.