Progress in perovskite PV; stacking with Si cells promises record efficiency
Most solar cells today are silicon-based, either monocrystalline (most expensive, highest reported efficiency) or amorphous (less expensive, somewhat less efficient). Perovskites are a class of material which are (quoting Wikipedia), “...any material with the same type of crystal structure as calcium titanium oxide (CaTiO3), known as the perovskite structure… the general chemical formula for perovskite compounds is ABX3, where ‘A’ and ‘B’ are two cations of very different sizes, and X is an anion that bonds to both.”. Conversion efficiencies with PV cells built with these materials have historically been low (sub-10% as opposed to the 20+% of silicon) but they offer they offer the potential of very much lower manufacturing costs as they can be produced with solution chemistry (coating and /or printing).
imec, and its partners in the Solliance collaboration (see below) now report achieving power conversion efficiencies of up to 12%. imec regards this as particularly encouraging as it has been working on the technology for only a short time, around two years; the researchers say they have been able to apply many of the lessons learned over many years of refining silicon cell technology to deliver rapid advances in the performance of perovskite cells, and anticipate that the trajectory they have established will yield still higher efficiency in the near future.
By depositing thin active layers, and refining the cell patterning and interconnection (again drawing on its silicon experience) imec reports being able to produce semi-transparent and efficient cells. Cells need electrical connections from the front (top surface) of the structure to the substrate, but also need the maximum ratio of active to total area.
Transparency takes the technology in two directions; it enables semi-transparent PV-windows which are a key towards Zero-Energy Buildings (in compliance with EU and other directives) and building integration. However, the perovskite cells capture and convert a different range of wavelengths to silicon. By stacking a semi-transparent cell over a conventional silicon cell, both can contribute and imec believes overall efficiencies of over 30% are within reach.
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The present work has reached as far as an “unprecedented”20.2% in power conversion efficiency for a perovskite/Si stacked solar module. As well as being made by simple processes, perovskites can also be manipulated to be realised on flexible (plastic films or metal foils) as well as rigid (glass, metal) carriers. Optical and electrical properties of the perovskite solar cells can be varied by tuning the composition of the material components and thereby adjusting colour and transparency. The semi-transparent perovskite modules reported by imec, made using scalable coating techniques, showed efficiencies of 12% on sizes as large as 4 cm² and 10% on sizes as large as 16 cm², a world-best achievement in this domain.
Imec’s novel stacked module concept features a highly transparent perovskite solar module stacked on top of interdigitated back contacted (IBC) silicon solar cells. All devices had the same area and the semi-transparent perovskite top module shows a 70% transmission of light towards the crystalline Si solar cell. The 20.2% figure was reached for the resulting stacked perovskite/Si solar module of relevant sizes of 4 cm²: conversion efficiency of 17.2% was achieved for larger areas of up to 16 cm², employing a Si bottom solar module of four interconnected IBC cells, also representing a record result for this size.
Tom Aernouts, Thin Film PV Technology Manager at imec commented “We are proud about these results as they show we have excellent control over the performance as well as the upscaling capabilities of this technology. Our future work will continue in increasing module sizes and optimizing the perovskite solar cell technology.” Ulrich Paetzold, researcher at the Thin Film PV group at imec added: “With a mm-size perovskite solar cell stacked on our IBC solar cell even efficiency as high as 22% has been obtained. But advancement of the perovskite/Si stacked solar module technology relies on demonstrators of realistic sizes.”
imec; www.imec.be
Solliance is a partnership of R&D organizations from the Netherlands, Belgium and Germany working in thin film photovoltaic solar energy. Various laboratories and pilot production lines are jointly used for dedicated research programs which are executed in close cooperation with the solar business community. Solliance Research Partners are: ECN, imec, TNO, Holst Centre, TU/e, Forschungszentrum Jülich, University Hasselt and Delft University of Technology; www.solliance.eu
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