Organic Solar-Cell Architecture Taps Next Performance Plateau

Fig 1. Integrated into a module, Imec’s inverted architecture for organ photovoltaics employs polymers from Plextronics and materials from Solvay to deliver an efficiency of 5% from an aperture area measuring 25 cm².

Fig 2. Improvements in organic solar-cell technology have been on a consistent upswing since initial research began in the 1970s.

Scientists from Belgian research center Imec, Plextronics, and Solvay have created an organic polymer-based single-junction solar-cell architecture that exhibits a conversion efficiency of 6.9%. Integrated into a module, the end device is the culmination of Imec’s scalable inverted device architecture, Plextronics’ polymers, and materials from Solvay. The end module employing the complete architecture delivers an efficiency of 5% from an aperture area measuring 25 cm² (Fig. 1).

Organic versus Silicon solar Cells

Although they’ve been around for a while, organic solar cells are fairly new contenders for use in a field dominated by silicon photovoltaic components. Many in the field believe these organic cells to be highly desirable, low-cost alternatives set to take off as more performance obstacles are overcome.

An organic photovoltaic cell (OPVC) is made up of conductive organic polymers with a very high optical absorption coefficient, meaning small amounts of the material can absorb large volumes of light. This advantage, and the fact that OPVCs have significantly lower production costs, make the material, at least on the surface, extremely viable and desirable alternatives. However, OPVCs have their limits in the form of lower efficiencies and stability, shorter lifespan, and limited tactile strength.

There are two basic types of OPVC: single layer and multilayer. The simplest version, a single-layer OPVC, comprises a polymer layer between two metal conductor layers. Multilayer cells pack two different polymer layers between the conductive-material layers.

In terms of unique shortcomings, single-layer components exhibit low quantum efficiencies and low conversion efficiencies, in the realm of less than 1% and 0.1%, respectively. Another single-layer pitfall is the lack of electrical field strength between the conductors.

Bilayer OPVCs overcome these issues while having a few of their own, the thickness of the polymer being the major obstacle. To break up charges into carriers, the polymers should be the same thickness as the length of the charges, in the realm of 10 nm. The problem is that the polymer needs to be a minimum of 100 nm thick to absorb sufficient light.

Inverted Architecture May Be The Ticket

Overall, organic photovoltaics are not noted for their functional longevity. In response to this impedance, inverted architectures are developed to extend the lifetime of organic solar cells.

The inverted bulk heterojunction architecture from Imec appears to improve cell performance by approximately 0.5% or better over single-layer and bilayer topologies. The architecture employs a low band-gap p-type polymer with a fullerene derivate from Plextronics.

A unique buffer is used near the active layer to improve and boost light management, enabling the combination of polymer and architecture to achieve a stabile and certified conversion efficiency of 6.9%. This is recorded as the highest performance level for the polymer material and the highest efficiency for inverted architectures to date.

Additionally, the inverted device architecture elicits similar performance boosts from other polymer materials. The researchers report module level efficiencies that confirm the suitability toward upscaling.

Onward And Upward

Based on past developments, right up to Imec’s innovative architecture, the future looks very promising for organic solar cells (Fig. 2).

“With further optimizations to the material as well as to the architecture, for example by introducing a multi-junction featuring different layers of different polymers each capturing another part of the light spectrum, we envision organic solar-cell lifetimes of over 10 years and conversion efficiencies of 10% in two to three years,” says Tom Aernouts, R&D Team Leader Organic Photovoltaics at Imec.

“We believe organic photovoltaics will play a bigger role in the future, when we can boost efficiency and lifetime, at a reduced cost,” says Patrick Francoisse, sustainable energy platform manager at Solvay.

One could easily compare the state of organic photovoltaics with that of LEDs and solid-state lighting in general. The LED makers have the technology under their thumb and need to get the price down, whereas the organic solar-cell researchers have the price point in check and just need to get a handle on the technology—a bit of role reversal, if you will.