Perovskite solar cells represent an emerging photovoltaic technology being researched and developed by academic and industrial laboratories worldwide because it is able to bring together at the same time low-cost manufacturing (materials/processes) together with high power conversion efficiencies of the end product.

The vast majority of R&D has focused on the technology operating at standard test conditions (i.e. bright sunlight).

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Figure schematizing the demonstration of perovskite solar cell technology with outstanding power outputs under indoor lighting for powering electronics in smart buildings, the internet of things, and wireless sensors. Credit: Thomas M. Brown

The Centre for Hybrid and Organic Solar Energy (CHOSE), Department of Electronic Engineering University of Rome – Tor Vergata, in collaboration with the Department of Physics and Astronomy and London Centre for Nanotechnology at University College London, have developed device architectures with new solution-processed composite electron transport layers that work exceptionally well under artificial indoor lighting (i.e. white LED lamps). Indoor illumination delivers a different spectrum (concentrated in the visible range) and incident power (i.e. 2 to 3 orders of magnitude smaller) compared to that of the sun at standard test conditions. By inserting a thin MgO overlayer over a more conventional SnO2 transport layer, between the perovskite semiconductor film and the bottom transparent electrode, detrimental charge recombination was reduced.

The resulting improvement in the capability of extracting useful electrons by the photovoltaic cell is especially critical at the low illumination levels typically found indoors and has considerable influence on device performance. In fact, the power conversion efficiency improved by 20% and the maximum power density was 20.2 µW/cm2 at 200 lx and 41.6 µW/cm2 at 400 lx (corresponding to a power conversion of 27%) under white LED illumination. To date, these represent the highest output power densities reported for any photovoltaic technology under these typical illumination ranges found in home and office environments.

Ambient indoor conditions represent a milder environment compared to stringent outdoor conditions and are much less demanding on device lifetimes. This, together with their exceptional efficiency under artificial lighting, suggests an initial market for this new photovoltaic technology which is still seeking long-term stability outdoors. Furthermore, all layers of the cells, except for the two electrodes, were solution-processed at low temperatures, making the technology easy to integrate with other printed electronic components on the same substrate, and compatible with low-cost manufacturing. Low-temperature processing means that this technology can be fabricated not only on glass but even on flexible plastic substrate films and thus more-easily integrated with a variety of surfaces and objects.

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Figure: perovskite solar cells on glass (left) and plastic film (right). Republished with permission from Springer from: G. Lucarelli, F. Di Giacomo, V. Zardetto, M. Creatore, T. M. Brown, “Efficient light harvesting from flexible perovskite solar cells under indoor white light-emitting diode illumination”, Nano Research, 10, 2130 (2017)

There are many objects and low-power devices that need energy to operate, including portable electronics, devices that make buildings/homes “smart,” and the fast-rising markets of autonomous indoor wireless sensor networks and the internet of things. Having a power source able to efficiently convert energy from indoor lighting, as well as outdoor sunlight, into electrical energy would enable many of these to do without batteries that require periodic replacing and connections to wires. The unparalleled performance demonstrated by these new solar cell architectures can pave the way for perovskite photovoltaics to contribute strongly to energy harvesting and the powering of indoor electronics of the future.

These findings are described in the article entitled Highly efficient perovskite solar cells for light harvesting under indoor illumination via solution processed SnO2/MgO composite electron transport layers, recently published in the journal Nano Energy. This work was conducted by Janardan Dagar, Sergio Castro-Hermosa, Giulia Lucarelli and Thomas M. Brown from the Centre for Hybrid and Organic Solar Energy (CHOSE), Department of Electronic Engineering, University of Rome Tor Vergata, and Franco Cacialli from The London Centre for Nanotechnology, University College London.

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About The Author

Janardan, Sergio, Giulia, and Thomas are research scientists at the Centre for Hybrid and Organic Solar Energy (CHOSE), Department of Electronic Engineering, University of Rome Tor Vergata.

My research focuses on the study of the optical and electrical properties of organic (carbon-based) and printable semiconductors for optoelectronics and photonics. Research activities in my group span from the study of devices such as light-emitting diodes (LEDs), photovoltaic diodes (PVDs), field-effect transistors (FETs), lasers and sensors, to the investigation of supramolecular architectures for the control of the relevant solid-state properties of conjugated semiconductors. Over the years we have developed a keen interest in the engineering of the electrode-semiconductors interfaces, also with the help of non-invasive optical techniques such as electroabsorption spectroscopy. For example we were first to report an estimate of the work function of the hole-injection layer based on poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) within finished devices, and to correlate this with enhanced device performance in organic LEDs. I also have a strong interest in a variety of high-resolution microscopic techniques and their application to nanopatterning, e.g. by means of the scanning near-field optical microscope (SNOM) and of the Scanning Thermal Microscope (SThM). Recent interests include stretchable electronics, graphene and derivatives, and low-gap printable materials for emission/absorption in the near-infrared region of the electromagnetic spectrum. This is of interest to both biomedical applications (because of the semitransparency window of biological tissue in the window 700-1000 nm) and to photovoltaics (for the potential to extend the absorption of solar energy to such spectral regions).