Scientists at Estonia's Tallinn University of Technology (TalTech) have designed a solar cell based on antimony trisulfide (Sb2S3) that uses fluorene-based hole transport materials (HTMs) with terminating thiophene units.
“The HTMs were systemized and developed in the research group of Prof. Vytautas Getautis at the Kaunas University of Technology,” researcher Ilona Oja Acik told pv magazine.
“Sb2S3 is the promising candidate for the photovoltaic community owing to the earth-abundant and environmentally friendly constituent elements alongside appropriate optoelectronic properties such as a desirable band gap of around 1.7 eV, large absorption coefficient (around 105 cm–1) and long-term stability,” the scientists explained, noting that the highest efficiency for such PV devices now stands at 8%. “The overall efficiencies are rather limited by the deficiencies in open circuit voltage (VOC) that are attributed to the self-trapping of carriers due to lattice deformation and/or to the intrinsic defects, which sets the maximum attainable VOC to 0.8 V.”
The research group used the ultrasonic spray pyrolysis (USP) method to build a cell with an active area of 7.06 mm2. The device is based on a substrate made of glass and fluorine-doped tin oxide (FTO), titanium oxide (TiO2) electron transport layer (ETL), an Sb2S3 absorber, the fluorene-based hole transport materials (HTMs) with terminating thiophene units, and gold (Au) metal contacts.
“Sb2S3 solar cells with new HTMs are fabricated in the standard superstrate configuration wherein ETL and absorber are deposited by USP while HTMs are spin coated followed by the thermal evaporation of Au,” they explained. “The as-deposited Sb2S3 films were annealed at around 250–260 C for 5 min under nitrogen.”
The champion device built has a power conversion efficiency of 4.94%, an open-circuit voltage of 0.68 V, a short-circuit current of 13.7 mA/cm2, and a fill factor of 0.53. This cell uses fluorene-based molecules known as V1385. The researchers said this could ensure the plausible fabrication of Sb2S3 solar cells with efficiencies close to 5% at a much lower cost.
By comparison, Sb2S3 solar cells built with conventional hole transporting layers (HTL) made of the polymer Regioregular poly(3-hexylthiophene) (P3HT) have reached efficiencies of up to 4.7% to date at the research level.
“However, they have exhibited lower transparency (26%) owing to parasitic absorption losses in P3HT,” the academics said. “Novel cost-effective and transparent fluorene-based thiophene-terminated hole transport materials are demonstrated, for the first time, in semitransparent Sb2S3 solar cells exhibiting a power conversion efficiency of 4.5% ot 4.9% and an average visible transmittance of 30% to 33% (without the metal) in the 400 nm to 800 nm range.”
They presented the solar cell in “4.9% Efficient Sb2S3 Solar Cells from Semitransparent Absorbers with Fluorene-Based Thiophene-Terminated Hole Conductors,” which was recently published in ACS Publications.
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