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Record Low Voltage Losses In Efficient Cascade Organic Solar Cells

The sun is a limitless energy source, and photovoltaics is the most commonly used technology for converting solar energy into electrical power. Organic photovoltaics is an emerging thin-film PV technology which aims to revolutionize the solar industry. Due to their high absorption coefficients, organic materials are able to absorb large portions of sunlight even in ultrathin films of a few-tens-of-nm thickness.

This enables a range of key features such as flexibility, light-weight, and semitransparency, which open up new large-area application areas, inaccessible to common inorganic PV technologies. Their integration in “smart” windows, generating electrical power while being semitransparent, is an urban solar application which can lead to fully autonomous buildings in the following years.

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Organic solar cells (OSCs) employ two strongly photoactive organic semiconductor materials, acting as an electron donor (D) and an electron acceptor (A), as a rough equivalent of a conventional p-n junction solar cell. Absorption of photons in OSCs create bound pairs of electrons and holes called excitons, which dissociate into free charge carriers at the interface between D and A. The power conversion efficiency (PCE) of OSCs has been highly optimized over the past decade, reaching values of more than 13%. There are however still some fundamental issues which need to be solved in order to achieve even higher PCEs. The largest efficiency limiting factor is currently related to the poor conversion of the absorbed photon energy into electrical voltage.

Solar cells typically absorb photons having an energy higher than the solar cell’s optical gap (Eg) and, ideally, the absorbed photon energy would be fully transferred to the open-circuit voltage (VOC), which is the maximum extractable voltage from the device.  However, the latter is thermodynamically not allowed and excessive free charge carrier recombination reduces VOC to much lower values. In OSCs, the high charge carrier recombination rate results in an offset between Eg and VOC which is equal to 0.8-0.9 V, being almost twice that of silicon-based solar cells.  This severe loss of voltage currently reduces the PCEs by 30% to 40%, setting the best PCEs of OSCs below 14%. Optimization methods for suppressing these recombination-caused voltage losses in OSCs would boost their PCEs significantly above this limit.

In a recent paper published in Advanced Energy Materials, researchers from TU Dresden in Germany show that free charge carrier recombination can be significantly reduced by engineering the device architecture of the OSCs. According to their approach, the introduction of a third material between D and A reduces the physical contact between D and A and the free charge carrier recombination occurring via that interface. The idea behind this is simple: the charge generating D-A interface is the place in the solar cell where the photo-generated free charge carriers can coexist, encounter and recombine. Therefore, by reducing that interface, the recombination probability is also reduced.

The researchers have investigated a series of interlayer materials and demonstrated that both VOC and PCE can be indeed enhanced. For a cascade OSC employing the small organic molecules alpha-sexithiophene, chloroboron subnaphthalocyanine, and chloroboron subphthalocyanine as photoactive materials, an increase of 0.18 V in VOC is reported, from 0.98 V to 1.16 V. This impressive enhancement is attributed to the reduction of nonradiative recombination, as indicated by the significantly enhanced quantum efficiency of electroluminescence. Moreover, the voltage optimization was combined with a photon-to-electron conversion efficiency which remained high at 79% for the highly-current-contributing photons at the edge of the solar cell’s absorption spectrum.

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The total voltage losses of the investigated solar cells, considered as the Eg – VOC offset, correspond to 0.58 V which is among the lowest reported for OSCs, supported by a minimal driving force for charge separation of less than 10 mV, revealing the high level of optimization of this material system in terms of energy/voltage losses.

This work was from the Institute for Applied Physics at Technische Universität Dresden. The findings are described in the article entitled Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies, published in the journal Advanced Energy Materials.

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