Efficient inverted bulk-heterojunction solar cells from low-temperature processing of amorphous ZnO buffer layers
AuthorsJagadamma, Lethy Krishnan
El Labban, Abdulrahman
Ngongang Ndjawa, Guy Olivier
Anjum, Dalaver H.
Cha, Dong Kyu
KAUST DepartmentPhysical Sciences and Engineering (PSE) Division
KAUST Solar Center (KSC)
Imaging and Characterization Core Lab
Materials Science and Engineering Program
Chemical Science Program
Advanced Nanofabrication, Imaging and Characterization Core Lab
Organic Electronics and Photovoltaics Group
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AbstractIn this report, we demonstrate that solution-processed amorphous zinc oxide (a-ZnO) interlayers prepared at low temperatures (∼100 °C) can yield inverted bulk-heterojunction (BHJ) solar cells that are as efficient as nanoparticle-based ZnO requiring comparably more complex synthesis or polycrystalline ZnO films prepared at substantially higher temperatures (150-400 °C). Low-temperature, facile solution-processing approaches are required in the fabrication of BHJ solar cells on flexible plastic substrates, such as PET. Here, we achieve efficient inverted solar cells with a-ZnO buffer layers by carefully examining the correlations between the thin film morphology and the figures of merit of optimized BHJ devices with various polymer donors and PCBM as the fullerene acceptor. We find that the most effective a-ZnO morphology consists of a compact, thin layer with continuous substrate coverage. In parallel, we emphasize the detrimental effect of forming rippled surface morphologies of a-ZnO, an observation which contrasts with results obtained in polycrystalline ZnO thin films, where rippled morphologies have been reported to improve efficiency. After optimizing the a-ZnO morphology at low processing temperature for inverted P3HT:PCBM devices, achieving a power conversion efficiency (PCE) of ca. 4.1%, we demonstrate inverted solar cells with low bandgap polymer donors on glass/flexible PET substrates: PTB7:PC71BM (PCE: 6.5% (glass)/5.6% (PET)) and PBDTTPD:PC71BM (PCE: 6.7% (glass)/5.9% (PET)). Finally, we show that a-ZnO based inverted P3HT:PCBM BHJ solar cells maintain ca. 90-95% of their initial PCE even after a full year without encapsulation in a nitrogen dry box, thus demonstrating excellent shelf stability. The insight we have gained into the importance of surface morphology in amorphous zinc oxide buffer layers should help in the development of other low-temperature solution-processed metal oxide interlayers for efficient flexible solar cells. This journal is © the Partner Organisations 2014.
SponsorsAuthors would like to thank the KAUST Office of Competitive Research Funding. Authors thank Dr Liyang Yu for help with the contact angle measurements, as well as Muhammad Rizwan Khan Niazi and Dr Kui Zhao for helpful discussions and training.
PublisherRoyal Society of Chemistry (RSC)
JournalJournal of Materials Chemistry A