Roll-to-roll manufacturing of organic photovoltaics
Organic solar cells have been the subject of intensive research in both academia and industry during the past decade.1 The lure of organic technology compared to conventional silicon solar cells is based primarily on their promising application to manufacturing large-area, flexible panels using high-throughput (and low-cost) roll-to-roll printing processes.
The most important performance parameter for organic photovoltaics is their power-conversion efficiency (PCE), which is defined as the percentage of electrical power generated with respect to the power of the incident light. Commercially available silicon-based solar panels have PCEs of around 10%. This is the efficiency target that the organic-solar-cell community is trying to achieve. Over the past two years, the laboratory-record PCE of organic solar cells has improved from roughly 5 to approximately 8%. However, there are still substantial challenges that must be addressed for these laboratory-scale, high-efficiency organic photovoltaics to move to commercial roll-to-roll manufacturing. Although efficiencies of 6–8% have been widely achieved in research laboratories, the highest-efficiency roll-to-roll-manufactured organic solar panels have efficiencies of only ~2%.
One of the reasons for this low efficiency is that most current, high-efficiency organic photovoltaics require very thin (~100nm) organic active layers. It is extremely challenging to manufacture organic solar cells with such thin active layers through roll-to-roll printing. On the other hand, when the thickness of the organic active layer is increased to a more practically useful 200–300nm, the series resistance increases and the fill factor decreases dramatically, thus significantly reducing the PCE.
We recently developed a series of polymeric semiconductor materials that offer high PCEs (6–8%) with an organic active-layer thickness (~250nm) that is amenable to larger-scale solar-panel manufacturing, including roll-to-roll processing. Our approach to solve the active-layer-thickness problem involved two aspects. First, we designed a low-band-gap polymeric donor material that could also provide a high open-circuit voltage (Voc). Second, we developed and optimized our materials so that they yield high PCEs for relatively thick active layers.
It is generally accepted that the most important approach to increase organic-solar-cell efficiency is to reduce the band gap of their donor materials, because the wider absorption of low-band-gap materials better matches the solar spectrum. In turn, more solar light will thus be used to generate more electricity. A common problem with this approach is that polymeric donor materials with low band gaps often have high highest-occupied molecular-orbital (HOMO) levels, thus resulting in a low Voc. We have found that precise control of HOMO/lowest-unoccupied molecular-orbital levels of the donor material is the key to simultaneously achieving a low band gap and high Voc. Figure 1(a) shows the external quantum efficiency of a solar cell made using our first-generation donor material (OPV1), while Figure 1(b) shows its representative current density-voltage (J–V) behavior. The absorption onset of OPV1 is at approximately 800nm, which corresponds to an optical band gap of ~1.5–1.6eV. In addition, the J–V plot shows an excellent Voc of 0.74V. This is one of the first examples of a high-performance organic solar cell (PCE = 6.5%) that simultaneously features a low band gap and high Voc.
To address the requirement of a thicker active layer, we developed a polymeric donor material (OPV3) that offers ~70% fill factor for a film thickness of 250nm. The high fill factor is due to the material's excellent charge-transport properties: at 250nm thickness, the series resistance of the solar-cell device is only approximately 1.0–1.5Ωcm2. Solar cells based on OPV3 exhibited relatively wide absorption (onset at 700nm) and high Voc(0.73V) at the same time, resulting in a PCE >7% at AM1.5 (airmass 1.5, sea-level) conditions (see Figure 2). Note that these efficiencies are based on standard solar-cell structure, in which the anode is at the bottom and the cathode at the top. We also achieved approximately 6% PCE using an inverted solar-cell structure. In this case, the more sensitive electrode (cathode) is buried below the active layer. Therefore, the inverted solar-cell structure is significantly more stable than the standard setup, and is more applicable for use in commercial roll-to-roll processes.
In summary, we have developed a series of polymeric semiconductor materials that yield high-PCE (6–8%) organic solar cells with a relatively thick active layer (~250nm). They also yield high efficiencies (~6%) when inverted structures are used. With these features, we believe that these new materials offer great promise for application to roll-to-roll manufacturing of organic solar cells. This represents our future research direction.
Henry Yan is vice president of product development. He obtained his PhD in chemistry at Northwestern University under supervision of Tobin Marks. He has extensive experience in the field of organic electronics, including organic thin-film transistors, photovoltaics, and LEDs. He has published approximately 40 research articles and holds 20 patents.
