Ultra-Thin Low Cost Solar Cells

A semi-transparent solar cell based on AgBiS2 nanocrystals (Source: ICFO)

A semi-transparent solar cell based on AgBiS2 nanocrystals (Source: ICFO)

Solar energy is becoming an alter­native source to fossil fuels, however, due to the acce­lerating pace at which we are consuming energy, we need to develop ubiquitous PV techno­logies that can be employed everywhere: on buildings, clothes, consumer electronics and wearables. This necessitates ultra-thin film, low-cost and ideally flexible solar cells without com­promising the environment during production, use, or disposal.

Most of us know that the most common inorganic solar cells, displayed over roof tops and in solar farms, are made of silicon. However, the production of silicon solar cells can be expensive and energy demanding and the final modules are heavy and bulky. Many lower-cost thin film solar cells, alternative to silicon, are composed of toxic elements such as lead or cadmium, or contain scarce elements such as indium or tellurium.

Now researchers of the Institut de Ciències Fotòniques in Barcelona ICFO, led by Gerasimos Konstan­tatos, have found a solution to this increasing problem. They have fabricated a solution-processed, semi-transparent solar cell based on AgBiS2 nanocrystals, a material that consists of non-toxic, earth-abundant elements, produced in ambient conditions at low tempera­tures. These crystals have shown to be very strong pan­chromatic absorbers of light and have been further engineered to act as effective charge-transpor­ting medium for solution-processed solar cells.

Maria Bernechea comments: “They contain AgBiS2 nanocrystals, a novel material based on non-toxic elements. The chemical synthesis of the nano­crystals allows exquisite control of their properties through engineering at the nanoscale and enables their dissolution in colloidal solutions. The material is synthesized at very low tempera­tures (100ºC), an order of magnitude lower than the ones required for silicon based solar cells.”

The team of researchers at ICFO developed these cells through a low tempera­ture hot-injection synthetic procedure. They first dispersed the nanocrystals into organic solvents, where the solutions showed to be stable for months without any losses in the device performance. Then, the nanocrystals were deposited onto a thin film of ZnO and ITO, the most commonly used transparent conductive oxide, through a layer-by-layer deposition process until a thick­ness of approxi­mately 35nm was achieved.

“A very interesting feature of AgBiS2 solar cells is that they can be made in air at low tempera­tures using low-cost solution processing techniques without the need for the sophisti­cated and expensive equipment required to fabricate many other solar cells. These features give AgBiS2 solar cells significant potential as a low-cost alternative to traditional solar cells”, as Nicky Miller states. These cells have already achieved power conversion efficiencies of 6.3 %, which is on par with the early reported effi­ciencies of currently high performance thin film PV technologies. This highlights the potential of AgBiS2 as a solar-cell material that in the near future can compete with current thin film technologies that rely on vacuum-based, high-tempe­rature manufacturing processes.

Konstan­tatos concludes: “This is the first efficient inorganic nano­crystal solid-state solar cell material that simul­taneously meets demands for non-toxicity, abundance and low-tempe­rature solution processing. These first results are very encouraging, yet this is still the beginning and we are currently working on our next milestone towards effi­ciencies > 12 %.” The results obtained from this study, which was financially supported by European Commission within the NanoMat­Cell project, signifies a turning point in the concept and pro­duction of solar cells, moving from silicon cells to low-cost environ­mentally friendly solar cells that will definitely imply a safer and more sus­tainable world for the future. (Source: ICFO)

Reference: M. Bernechea et al.: Solution-processed solar cells based on environmentally friendly AgBiS2 nanocrystals, Nat. Phot., online 

Links: ICFO-Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, Castelldefels (Barcelona), Spain • EU-project NanoMatCell

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