Tandem solar cells, in which two solar cells with different absorption characteristics are linked to use a wider range of the solar spectrum, were fabricated with each layer processed from solution with the use of bulk heterojunction materials comprising semiconducting polymers and fullerene derivatives. A transparent titanium oxide (TiO x ) layer separates and connects the front cell and the back cell. The TiO x layer serves as an electron transport and collecting layer for the first cell and as a stable foundation that enables the fabrication of the second cell to complete the tandem cell architecture. We use an inverted structure with the low band-gap polymer-fullerene composite as the charge-separating layer in the front cell and the high band-gap polymer composite as that in the back cell. Power-conversion efficiencies of more than 6% were achieved at illuminations of 200 milliwatts per square centimeter.

http://science.sciencemag.org/content/sci/317/5835/222.full.pdf

Efficient tandem polymer solar cells fabricated by all-solution processing.

An effective way to improve polymer solar cell efficiency is to use a tandem structure, as a broader part of the spectrum of solar radiation is used and the thermalization loss of photon energy is minimized. In the past, the lack of high-performance low-bandgap polymers was the major limiting factor for achieving high-performance tandem solar cell. Here we report the development of a high-performance low bandgap polymer (bandgap 60% and spectral response that extends to 900 nm, with a power conversion efficiency of 7.9%. The polymer enables a solution processed tandem solar cell with certified 10.6% power conversion efficiency under standard reporting conditions (25 °C, 1,000 Wm(-2), IEC 60904-3 global), which is the first certified polymer solar cell efficiency over 10%.

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A polymer tandem solar cell with 10.6% power conversion efficiency

Researchers demonstrate that PBDTT-DPP, a semiconducting polymer with a low bandgap of 1.44 eV, allows tandem polymer solar cells to reach power conversion efficiencies of around 8.6%.

/pdf/tandem-polymer-solar-cells-featuring-a-spectrally-matched-57lor9rliz.pdf

Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer

Solar cells employing a halide perovskite with an organic cation now show power conversion efficiency of up to 22%. However, these cells are facing issues towards commercialization, such as the need to achieve long-term stability and the development of a manufacturing method for the reproducible fabrication of high-performance devices. Here, we propose a strategy to obtain stable and commercially viable perovskite solar cells. A reproducible manufacturing method is suggested, as well as routes to manage grain boundaries and interfacial charge transport. Electroluminescence is regarded as a metric to gauge theoretical efficiency. We highlight how optimizing the design of device architectures is important not only for achieving high efficiency but also for hysteresis-free and stable performance. We argue that reliable device characterization is needed to ensure the advance of this technology towards practical applications. We believe that perovskite-based devices can be competitive with silicon solar modules, and discuss issues related to the safe management of toxic material. Perovskite solar cells have emerged as a potential low-cost alternative to existing technologies. In this Perspective, Park et al. explore a strategy for the commercialisation of perovskite solar cells.

Towards stable and commercially available perovskite solar cells

The first silicon solar cell was reported in 1941 and had less than 1% energy conversion efficiency compared to the 25% efficiency milestone reported in this paper. Standardisation of past measurements shows there has been a 57% improvement between confirmed results in 1983 and the present result. The features of the cell structure responsible for the most recent performance increase are described and the history of crystalline and multicrystalline silicon cell efficiency evolution is documented. Copyright © 2009 John Wiley & Sons, Ltd.

The path to 25% silicon solar cell efficiency: History of silicon cell evolution

Spectral response and I–V measurements of tandem amorphous-silicon alloy solar cells

The accurate measurement of the photovoltaic conversion efficiency under standard test conditions is essential for meaningful comparisons of the device performance of different types of solar cells. A methodology is presented for calibrating reference cells and calculating spectral mismatch factors, and then using these to obtain the corrected short-circuit current with respect to standard test conditions.

Methods for Measuring Solar Cell Efficiency Independent of Reference Cell or Light Source

It is well-known that the maximum power output of photovoltaic devices changes with temperature. Therefore, the temperature coefficients of the basic device performance parameters (open-circuit voltage, short-circuit current, fill factor, and efficiency) are important factors which must be taken into account in the design of a photovoltaic power system, where temperature changes occur throughout the day and year. This paper reports results of experimental temperature coefficient measurements obtained on a wide variety of different photovoltaic devices, many of which have not had temperature coefficient data published previously.

J. Burdick

Papers

Spectral response and I–V measurements of tandem amorphous-silicon alloy solar cells

Methods for Measuring Solar Cell Efficiency Independent of Reference Cell or Light Source

Comparison of the temperature coefficients of the basic I-V parameters for various types of solar cells

A comparison of the errors in determining the conversion efficiency of multijunction solar cells by various methods

A method for determining the conversion efficiency of multiple-cell photovoltaic devices