A charged view

WITH so much glass in buildings, cars and the screens of mobile devices, it is understandable why researchers would like to come up with transparent solar cells which could generate electricity and top up batteries.

Solar cells work by absorbing the photons in sunlight and converting them into electrons, which are gathered by electrodes to flow into a circuit. Most solar cells are opaque to absorb all the light they can to maximise their efficiency. So, to look out of a window or use the screen of a smartphone, a layer of solar cells has to let some light through. Yet the more transparent the cells, the less energy they produce—or at least that is how it works with traditional solar technology based on semiconducting materials such as silicon.

An alternative is to make solar cells from substances that absorb light only at wavelengths which are invisible to the human eye, such as those in the infrared (IR) and ultraviolet (UV) spectrum. That would allow visible light to pass through. One company working on this is Ubiquitous Energy, a spin-off from the Massachusetts Institute of Technology (MIT) in 2011. It is developing solar cells using transparent organic materials that absorb IR and UV wavelengths.

Taking light from only part of the spectrum would reduce the percentage of sunlight’s energy that can be converted into electricity. Ubiquitous Energy is hoping, some think optimistically, to exceed 10%. That compares with 20-25% efficiency for a typical non-transparent solar panel.

Last year a team at Michigan State University led by Richard Lunt, formerly at MIT, displayed a variation of the approach using extremely small organic molecules, which Dr Lunt describes as “exceptionally transparent to the human eye”. These molecules absorb specific non-visible wavelengths of light and then glow at a different IR wavelength. This glowing light, which is also invisible to the eye, is guided to the edge of the glass where it is converted to electrical energy by thin strips of photovoltaic cells. The arrangement, known as a transparent luminescent solar concentrator, allows most of the glass to be kept clear of solar components. The first version had a power efficiency of only about 1%. But it is early days and the researchers hope to boost that considerably.

Rolling it out

It should be possible by using materials that absorb non-visible wavelengths of light to produce thin-films of solar cells cheaply using industrial processes that make large rolls, says Rutger Schlatmann, director of the Competence Centre for Thin-Film and Nanotechnology for Photovoltaics, a Berlin-based industry research group. But as he points out: “It is visible light that carries by far most of the energy.” That means, however good they are, transparent solar cells may never rival solar panels designed to capture the maximum amount of light. Nevertheless, what they can trap could still be useful.

Semi-transparent solar cells can be used to produce coloured or tinted glass, which helps when shading is required. Heliatek, a company based in Dresden, Germany, uses organic materials to make solar-cell films which are up to 40% transparent. With a solar efficiency of over 7% they can produce electricity-generating tinted glass in buildings and car sunroofs.

One development that is attracting a lot of interest is the use of a family of crystalline materials called perovskites, which could allow semi-transparent solar cells to be made relatively cheaply in large rolls. A group at Brown University in Providence, Rhode Island, recently reported they had made ultra-thin films with perovskite crystals that are capable of a solar efficiency of over 15%.

Oxford Photovoltaics, spun out of the University of Oxford in 2010 to commercialise thin-film solar cells, reckons perovskites are good for over 20%. The firm calculates that if a 35-storey office block in London was clad with perovskite cells they could generate almost 60% of the building’s energy consumption. When electricity bills are high or batteries are running low, every bit of juice counts.