CuI/Lead Halide Perovskite Solar Cells
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This study investigates copper iodide (CuI) as an inorganic hole conductor for organolead halide perovskite solar cells. The researchers fabricated solar cells using CuI deposited via an automated drop casting technique. Compared to the conventional organic hole conductor spiro-OMeTAD, the CuI-based devices showed higher photocurrent stability and fill factor but lower open-circuit voltage. Impedance spectroscopy indicated the lower voltage was due to higher recombination rates in the CuI devices. However, CuI exhibited nearly two orders of magnitude higher hole conductivity than spiro-OMeTAD, suggesting its potential as a lower-cost replacement for organic hole transport materials.
CuI/Lead Halide Perovskite Solar Cells
- 1. An Inorganic Hole Conductor for OrganoLead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide. Jeffrey A. Christians1, Raymond C. M. Fung,3 and Prashant V. Kamat*1,2 University of Notre Dame, Notre Dame, Indiana 46556 Laboratory, Department of Chemical and Biomolecular Engineering 2Department of Chemistry and Biochemistry 3Co-op Student, University of Waterloo, Canada 1Radiation DOI: 10.1021/ja411014k ®
- 2. The Need for Photovoltaics • Global energy use is growing at over 1% per year • ® This rate will cause energy consumption to nearly double by 2040 Renewable Energy Potential Hydroelectric Tides and Oceans 2 TW Geothermal 12 TW Wind Power 2-4 TW Source: EIA, International Energy Outlook (2013) 0.5 TW • Solar energy is by far the largest potential source for renewable energy Incident Solar Energy 120,000 TW Source: DOE, Basic Research Needs for Solar Energy Utilization (2005) 2
- 3. Perovskite Solar Cells • • Very rapidly, perovskite solar cells have surpassed 15% efficiency Sprio-OMeTAD and other organic hole conductors are very expensive • ® CuI offers the potential for lower cost Hodes, G. Perovskite-Based Solar Cells. Science 2013, 342, 317–318. 3
- 4. Fabrication & Characterization • • ® CuI is deposited from solution by an automated drop casting technique 0.1 M CuI in 39:1 chlorobenzene: di-n-propyl sulfide Au CuI TiO2/CH3NH3PbI3 FTO 4
- 5. Photovoltaic Performance Hole Conductor Champion CuI Averageα Maximumβ Champion Spiro-OMeTAD Averageα Maximumβ • • • JSC (mA/cm2) 17.8 12.1 ± 3.3 18.9 16.1 14.0 ± 1.7 17.0 VOC (V) 0.55 0.52 ± 0.06 0.62 0.79 0.78 ± 0.02 0.82 FF 0.62 0.60 ± 0.07 0.71 0.61 0.55 ± 0.07 0.65 ® η (%) 6.0 3.7 ± 1.1 -7.9 6.0 ± 1.1 -- Lower Open-Circuit Voltage (avg. 0.52V vs. 0.78V) Better photocurrent stability Higher fill factors (avg. 0.55 vs. 0.60) 5
- 6. Impedance Spectroscopy Equivalent Circuit Model Copper Iodide ® Rs = series resistance Rrec. = recombination resistance Cµ = chemical capacitance RHTM = HTM resistance CHTM = HTM capacitance Spiro-OMeTAD 6
- 7. Why CuI ≠ spiro-OMeTAD? CuI lower Rrec. Low VOC ® CuI higher σHTM High FF 7
- 8. Conclusions • Promising inorganic hole conductor for perovskite solar cells • Better current stability than spiroOMeTAD devices • Impedance Spectroscopy shows that low VOC is caused by high recombination • Nearly two orders of magnitude higher conductivity than spiroOMeTAD • Provides a potential low-cost replacement of spiro-OMeTAD and other organic HTMs 8 ®
- 9. Thank You ® An Inorganic Hole Conductor for Organo-Lead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide. Jeffrey A. Christians1, Raymond C. M. Fung,3 and Prashant V. Kamat*1,2 University of Notre Dame, Notre Dame, Indiana 46556 Chemical and Biomolecular Engineering 2Department of Chemistry and Biochemistry 3Co-op Student, University of Waterloo, Canada 1Radiation Laboratory, Department of DOI: 10.1021/ja411014k This research was supported by the U.S. Department of Energy Visit KamatLab.com for more research from our group or find us on Facebook at facebook.com/kamatlab! 9
Editor's Notes
- #2: Hello, my name is Jeff Christians and I would like to discuss a publication from the Prashant Kamat group entitled, “An Inorganic Hole Conductor for Organo-Lead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide” which was recently published in the Journal of the American Chemical Society.
- #3: The problem of global climate change has put the need for alternative energy at the forefront of public policy discussions. Based on current projections, global energy demand will continue to grow at 1% per year, meaning that energy demand will nearly double by 2040. When comparing various renewable energy technologies, solar energy clearly stands out among the rest as the largest potential renewable energy source.
- #4: Recently organo-metal lead halide perovskites have emerged as one of the most promising materials for the next generation of solar cells with efficiencies rising from less than 5% to over 15% in only a few years. The perovskite solar cells offer the potential for very low cost as well since they are made with simple solution processing techniques. However, sprio-OMeTAD and other organic hole conductors previously used in these solar cells are very expensive and could hinder the commercialization of perovskite solar cells. In this manuscript, we present copper iodide as an inorganic hole conductor for perovskite solar cells that could potentially significantly reduce the cost of these devices.
- #5: Copper iodide is deposited onto mesoporous TiO2 substrates by an automated drop casting technique developed for CuSCN deposition. As pictured, a needle with holes drilled in the side is placed parallel to the TiO2/perovskite film and copper iodide and moved slowly back and forth over the perovskite film to fill the pores of the TiO2 network and create an overlayer of copper iodide to prevent shorting. Finally, a gold contact is evaporated to complete the solar cell.
- #6: Here is shown a summary of the photovoltaic performance of 48 spiro-OMeTAD and 32 copper iodide based solar cells. The champion copper iodide based device exhibited 6.0% power conversion efficiency compared to 7.9% for the champion spiro-OMeTAD solar cell with the primary difference in the performance of these two devices being the lower open-circuit voltage obtained with copper iodide compared to spiro-OMeTAD. Incident Photon to Carrier Efficiency measurements reveal peak external quantum efficiency over 80% for both copper iodide and spiro-OMeTAD solar cells. Copper iodide-based devices also show better photocurrent stability than spiro-OMeTAD-based solar cells when illuminated without encapsulation for 2 hours under AM 1.5G illumination. Another advantage of copper iodide over spiro-OMeTAD was the higher fill factor seen in copper iodide-based solar cells.
- #7: We next used impedance spectroscopy to look more closely at the reasons underlying the lower open-circuit voltage and higher fill factor of copper iodide-based devices. Impedance measurements of representative copper iodide and sprio-OMeTAD devices were conducted under AM 1.5G illumination. Information on several fundamental processes occurring in these solar cells was obtained by fitting the resulting Nyquist plots to the model shown here. This allowed us to separate out the recombination resistance and hole transport resistance from other processes occurring in these solar cells.
- #8: The primary difference between the performance of copper iodide and spiro-OMeTAD devices is the open-circuit voltage. From fitting of the impedance spectra, we find that the copper iodide-based devices have significantly lower recombination resistance than spiro-OMeTAD devices at potentials greater than 200 mV. This lower recombination resistance means that copper iodide solar cells have much higher recombination rates than spiro-OMeTAD solar cells which directly reveals itself in the lower open-circuit voltage seen with copper iodide. Looking at the hole transport material resistance, we are able to calculate the conductivity of the hole conductor as shown in the figure at the right. From this, we see that copper iodide has nearly two orders of magnitude higher conductivity than spiro-OMeTAD. This higher conductivity leads to the higher fill factors seen in copper iodide devices.
- #9: In summary, we have shown copper iodide is a promising inorganic hole conductor for use with perovskite solar cells. The champion copper iodide solar cell exhibited 6% efficiency compared to 7.9% with spiro-OMeTAD. In addition to the promising power conversion efficiency of copper iodide, these solar cells show better photocurrent stability under continuous illumination than spiro-OMeTAD devices and nearly two orders of magnitude higher electrical conductivity. Finally, we see using impedance spectroscopy, that the lower open-circuit voltage of copper iodide-based solar cells is caused by higher recombination. With further study and focus on reducing the high recombination, copper iodide could provide a potential replacement of the expensive organic hole conductors currently found in perovskite solar cells.
- #10: Thank you for watching this webinar presentation on our paper, “An Inorganic Hole Conductor for Organo-Lead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide.” You can find the complete manuscript online published in the Journal of the American Chemical Society. For more information on our research group and our other work and events visit Kamatlab.com or find us on facebook at facebook.com/kamatlab. Thank you.