Designs & Architectures for Next Generation of Organic Solar Cells

[tweetmeme Kang-Shyang Liao, Soniya D. Yambem, Amrita Haldar, Seamus A. Curran (Institute for NanoEnergy, Department of Physics, University of Houston) and Nigel J. Alley (School of Physical Sciences, Dublin City University) have published a new article titled „Designs and Architectures for the Next Generation of Organic Solar Cells.“  Organic solar cells show great promise as an economically and environmentally friendly technology to utilize solar energy because of their simple fabrication processes and minimal material usage. However, new innovations and breakthroughs are needed for organic solar cell technology to become competitive in the future. This article reviews research efforts and accomplishments focusing on three issues: power conversion efficiency, device stability and processability for mass production, followed by an outlook for optimizing OSC performance through device engineering and new architecture designs to realize next generation organic solar cells.

The introduction to this paper reads:

Until the 1950s, the US was energy self-sufficient where coal was the main power source. This was changed by the increasing use of oil, nuclear reactors and natural gas. Despite these additions to the energy portfolio, energy consumption in the US began to outstrip production in the 1950s, so the US began importing fuel. On the cost side of the equation, ten years ago oil was selling at slightly over $10 a barrel ($10.35 December 1998), but in the intervening period we have seen that figure climb to an unprecedented high of beyond $140 dollars a barrel. Even though the current economic downturn has changed the demand scenario in the current fossil fuel stock, this is only a short term respite to what will again be a high cost in oil and gas prices as economies begin to grow again. Photovoltaic or solar cells provide some clear advantages to other renewable sources of energy and is an option for energy generation. At present, most of the commercial photovoltaic cells are based on crystalline silicon. Although in recent years an unprecedented effort in solar investment has occurred into new material choices such as inorganic thin films (e.g., CuInGaSe), dye-sensitized cells and of course most recently into organic or carbon based thin film solar cells.

Organic solar cells (OSCs) utilizing semiconducting conjugated polymers have been pursued and studied since the discovery and development of conductive polymers by Heeger, MacDiarmid and Shirakawa. They paved the way for the development of the first thin film OSC surpassing 1% efficiency, reported by Tang in 1986. This technology offers several advantages over silicon solar cells or other inorganic counterparts: the polymers are soluble in common organic solvents allowing the deposition of ultra-thin semiconductor films by simple solution processing technologies such as spin coating, printing and spray coating. It is expected that OSCs will become a viable alternative in certain market sectors where cost and flexibility is an issue to silicon solar cells in the future due to more simple material preparation and potentially easier manufacturing conditions. However, the power conversion efficiency (PCE) of state-of-the-art OSCs (5.4–7.9%) which are laboratory based and not manufactured output, whereas commercially available crystalline silicon solar cells can reach 20%. Consequently, organic based solar cells have some way to go before being a competitive and viable option and this will inevitably require some new disruptive manufacturing technology before they occupy a competitive space. Silicon photovoltaic technology is very mature and is expected to reach ‘grid parity’ with retail prices in Europe around 2015–2020 by reducing the cost through scaling to GWp production. New innovations and breakthroughs are urgently required for OSC technology to become competitive with other photovoltaic technology in the future.

There are three main issues to overcome before the next generation of primarily OSC devices and in some cases all solar cell technologies can be competitive with other energy generation systems. First of all, the crucial efficiency value; OSCs are still inferior to all inorganic counterparts. Assuming the cost for the manufacture processes and materials for OSCs will be much less than crystalline silicon solar cells and in order to lower the costs per watt-peak for OSCs to be competitive, a milestone of 10% PCE of manufactured cells is still needed. This is not a trivial task. Fortunately, one major advantage of using semiconducting conjugated polymers is that there are endless combinations that could possibly be used as potential photoactive layers. By materials engineering of these semiconducting conjugated polymers to finely tune their properties such as bandgap and charge mobility, a trend of incremental PCE improvement has been observed for the last five years and a 10% PCE milestone can be achieved using multiple semiconductor constituents (multi-tandem cells).

The second issue to overcome is device stability under ambient operating conditions. This can also be applicable to many thin film solar cells (organic and inorganic alike). The manufacturers of commercial crystalline silicon solar cells usually assure a 25-year lifetime warranty. However, so far there are few OSC devices that can pass a 1000-hour test in damp heat (85 °C and 85% RH) with less than 10% degradation of PCE. This can be attributed to their sensitivity to oxygen and moisture. While it is possible to protect the devices through cell encapsulation and device packaging, it will no doubt increase overall production costs. An ideal solution is to search for stable materials that are less sensitive to oxygen and moisture, so only minimal encapsulation and packaging is required.

The third issue which is neglected by the majority of researchers is processing technologies for mass production. State-of-the-art OSCs with high efficiencies are usually made in research laboratories. These champion cells usually have very small active areas (less than 1 cm2) and are typically fabricated by multiple steps utilizing many different processing techniques. Indeed, these techniques suitable for research purposes in the laboratory may not be able to scale up to large-area and low-cost mass production. Although OSC technology can potentially offer a credible solution to the problem of high-cost fabrication encountered for other photovoltaic technologies, there are still a lot of issues to be solved. The goal is to find suitable technologies that allow processability and mass production without sacrificing PCE.

In this review, we present research progress mainly on the architectural designs of OSCs in order to provide possible solutions for these three main issues. This review is presented as follows: first, a general introduction to the basic operating principles of OSCs; second, progress of OSC materials and layouts to reach high PCE and stability; third, progress of module designs and processing technologies to realize large-area OSCs; fourth, an outlook on the optimization of OSC performance through device engineering and the development of new architectures.

Read the full paper here in PDF format.

Citation: Liao, Kang-Shyang; Yambem, Soniya D.; Haldar, Amrita; Alley, Nigel J.; Curran, Seamus A. 2010. “Designs and Architectures for the Next Generation of Organic Solar Cells.” Energies 3, no. 6: 1212-1250.

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