M. Abdi-Jalebi thanks Nava Technology Limited for a PhD scholarship. M.I. Dar and M.Gr&#228;tzel thank the King Abdulaziz City for Science and Technology (KACST) and the Swiss National Science Foundation (SNSF) for financial support. The authors would like to thank Dr. Pierre Mettraux in the Molecular and Hybrid Materials Characterization Center, EPFL for carrying out the XPS measurements. A.Sadhanala gratefully acknowledges financial support from the Indo-UK APEX project. S.P. Senanayak acknowledges the Royal Society London for the Newton Fellowship. R.H. Friend, M. Abdi-Jalebi, and A. Sadhanala would like to acknowledge the support from the EPSRC.

1. Synthesis and Deposition of Pristine and Additive-based CH3NH3PbI3
NOTE: All the solutions were prepared inside an argon glove box under moisture- and oxygen-controlled conditions (H2O level: &#60;1 ppm and O2 level: &#60;10 ppm).
<ol>
	<li>Dissolve 553 mg (1.2 M) of PbI2 in 1 mL of N,N-dimethylformamide (DMF) under constant stirring at 80 &#176;C.</li>
	<li>Add 0.02 M of monovalent cation halides to the PbI2 solution.</li>
	<li>Spin-coat the resulting yellow transparent solution onto the substrate (e.g., mesoporous-TiO2) for 30 s at 6,500 rpm with a ramp of 4,000 rpm.</li>
	<li>Bake the films on a hotplate at 80 &#176;C for 30 min.</li>
	<li>Dissolve 40 mg of methylammonium iodide (MAI) in 5 mL of isopropanol.</li>
	<li>Spin-coat a sufficient amount of MAI solution onto the resulting lead iodide films using a two-step protocol that includes 45 s of loading time followed by 20 s of spinning at 4,000 rpm.</li>
	<li>Anneal the spin-coated perovskite films on a hotplate at 100 &#176;C for 45 min.</li>
</ol>
2. Solar Cell Fabrication
<ol>
	<li>Substrate preparation
	<ol>
		<li>Pattern fluorine-doped tin oxide (FTO)-coated glass.
		<ol>
			<li>Cover the active area of the FTO glass with semitransparent adhesive tape.</li>
			<li>Pour the zinc (Zn) powder on the uncovered areas of the FTO substrates.</li>
			<li>Prepare 2 M of hydrochloric acid (HCl) in distilled water.</li>
			<li>Pour the HCl solution onto the part of the FTO glass that is covered with Zn powder.</li>
			<li>Wash the FTO with water and remove the tape.</li>
		</ol>
		</li>
		<li>Cleaning the substrates
		<ol>
			<li>Wash the FTO glass using 2% (w/v) detergent.</li>
			<li>Sonicate the etched FTO substrates in acetone and isopropanol (IPA) for 10 min.</li>
			<li>Treat the FTO substrates with an ultraviolet/O3 cleaner for 15 min.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Deposition of a hole blocking layer
	<ol>
		<li>Add 0.6 mL of titanium diisopropoxide bis(acetylacetonate) (TAA) in 7 mL of IPA.</li>
		<li>Put the cleaned and patterned FTO substrates on a hotplate at 450 &#176;C and cover the contact area before heating.</li>
		<li>Spray pyrolysis the TAA solution onto the uncovered area using O2 as the carrier gas.</li>
		<li>Leave the samples at 450 &#176;C for 30 min.</li>
	</ol>
	</li>
	<li>Deposition of an electron transport layer
	<ol>
		<li>Dilute the commercial TiO2 paste (30-nm particle size) with ethanol (2:7, weight ratio).</li>
		<li>Homogenize the TiO2 dilution by sonicating for 30 min.</li>
		<li>Spin-coat the titania dilution onto the prepared samples with compact TiO2 layers for 30 s at 5,000 rpm with a ramp of 2,000 rpm.</li>
		<li>Anneal the titania films at 500 &#176;C for 30 min.</li>
		<li>Treat the resulting mesoporous TiO2 films in a 40 mM solution of TiCl4 in distilled water&#160;at 70 &#176;C for 20 min.</li>
		<li>Anneal the TiCl4-treated films at 450 &#176;C for 30 min.</li>
	</ol>
	</li>
	<li>Deposition of the perovskite layer 
	NOTE: The FTO substrates with titania layers were transferred to a dry-air box with a humidity of &#60;1% for the rest of the fabrication process.
	<ol>
		<li>Spin-coat the prepared lead iodide solutions (with and without dopants) onto the mesoporous TiO2 for 30 s at 6,500 rpm with a ramp of 4,000 rpm.</li>
		<li>Bake the films on a hotplate at 80 &#176;C for 30 min.</li>
		<li>Spin-coat a sufficient amount of MAI solution into the resulting lead iodide films using a two-step protocol that includes 45 s of loading time followed by spinning for 20 s at 4,000 rpm with a ramp of 2,000 rpm.</li>
		<li>Anneal the spin-coated perovskite films on a hotplate at 100 &#176;C for 45 min.</li>
	</ol>
	</li>
	<li>Deposition of the hole transport layer
	<ol>
		<li>Add 72.3 mg of spiro-OMeTAD to 1 mL of chlorobenzene and shake until the solution becomes transparent.</li>
		<li>Make a stock solution of bis(trifluoromethylsulphonyl)imide (LiTFSI) by adding 520 mg of LiTFSI in acetonitrile.</li>
		<li>Add 17.5 &#181;L of the LiTFSI stock solution and 28.8 &#181;L of 4-tert-butylpyridine (TBP) to the spiro-OMeTAD solution.</li>
		<li>Spin-coat the above solution for 30 s at 4,000 rpm with a ramp of 2,000 rpm.</li>
	</ol>
	</li>
	<li>Thermal evaporation of the top contact
	<ol>
		<li>Mask the samples and put them in the vacuum chamber of the evaporator.</li>
		<li>Evaporate 80 nm of gold at a rate of 0.01 nm/s.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Polman, A., Knight, M., Garnett, E. C., Ehrler, B., Sinke, W. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photovoltaic+materials+-+present+efficiencies+and+future+challenges.">Photovoltaic materials - present efficiencies and future challenges.</a> Science. 352, 307 (2016).</li><li>Green, M. A., Ho-Baillie, A., Snaith, H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+emergence+of+perovskite+solar+cells.">The emergence of perovskite solar cells.</a> Nat. Photonics. 8 (7), 506-514 (2014).</li><li>Stranks, S. D., Snaith, H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metal-halide+perovskites+for+photovoltaic+and+light-emitting+devices.">Metal-halide perovskites for photovoltaic and light-emitting devices.</a> Nat. Nanotechnol. 10 (5), 391-402 (2015).</li><li>Snaith, H. H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Perovskites:+The+Emergence+of+a+New+Era+for+Low-Cost,+High-Efficiency+Solar+Cells.">Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells.</a> The J. Phys. Chem. Lett. 4 (21), 3623-3630 (2013).</li><li>Kojima, A., Teshima, K., Shirai, Y., Miyasaka, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Organometal+halide+perovskites+as+visible-light+sensitizers+for+photovoltaic+cells.">Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.</a> J. Am. Chem. Soc. 131 (17), 6050-6051 (2009).</li><li>Kim, H. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Lead+iodide+perovskite+sensitized+all-solid-state+submicron+thin+film+mesoscopic+solar+cell+with+efficiency+exceeding+9%.">Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%.</a> Sci. rep. 2, 591 (2012).</li><li>Jeon, N. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Compositional+engineering+of+perovskite+materials+for+high-performance+solar+cells.">Compositional engineering of perovskite materials for high-performance solar cells.</a> Nature. 517 (7535), 476-480 (2014).</li><li>Li, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+vacuum+flash-assisted+solution+process+for+high-efficiency+large-area+perovskite+solar+cells.">A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells.</a> Science. 353 (6294), 58-62 (2016).</li><li>Manser, J. S., Kamat, P. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Band+filling+with+free+charge+carriers+in+organometal+halide+perovskites.">Band filling with free charge carriers in organometal halide perovskites.</a> Nat. Photonics. 8 (9), 737-743 (2014).</li><li>Xing, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Long-Range+Balanced+Electron-+and+Hole-Transport+Lengths+in+Organic-Inorganic+CH3NH3PbI3.">Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3.</a> Science. 342 (6156), 344-347 (2013).</li><li>Stranks, S. D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electron-Hole+Diffusion+Lengths+Exceeding+1+Micrometer+in+an+Organometal+Trihalide+Perovskite+Absorber.">Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber.</a> Science. 342 (6156), 341-344 (2013).</li><li>Pazos-Outon, L. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photon+recycling+in+lead+iodide+perovskite+solar+cells.">Photon recycling in lead iodide perovskite solar cells.</a> Science. 351 (6280), 1430-1433 (2016).</li><li>Saliba, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cesium-containing+Triple+Cation+Perovskite+Solar+Cells:+Improved+Stability,+Reproducibility+and+High+Efficiency.">Cesium-containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency.</a> Energy Environ. Sci. 9 (6), (2016).</li><li>Pellet, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mixed-organic-cation+perovskite+photovoltaics+for+enhanced+solar-light+harvesting.">Mixed-organic-cation perovskite photovoltaics for enhanced solar-light harvesting.</a> Angew. Chemie - Int. Ed. 53 (12), 3151-3157 (2014).</li><li>Hao, F., Stoumpos, C. C., Chang, R. P. H., Kanatzidis, M. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anomalous+band+gap+behavior+in+mixed+Sn+and+Pb+perovskites+enables+broadening+of+absorption+spectrum+in+solar+cells.">Anomalous band gap behavior in mixed Sn and Pb perovskites enables broadening of absorption spectrum in solar cells.</a> J. Am. Chem. Soc. 136 (22), 8094-8099 (2014).</li><li>Dar, M. I., Abdi-Jalebi, M., Arora, N., Gr&#228;tzel, M., Nazeeruddin, M. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth+Engineering+of+CH+3+NH+3+PbI+3+Structures+for+High-Efficiency+Solar+Cells.">Growth Engineering of CH 3 NH 3 PbI 3 Structures for High-Efficiency Solar Cells.</a> Adv. Energy Mater. 6 (2), 1501358 (2016).</li><li>Ibrahim Dar, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Understanding+the+Impact+of+Bromide+on+the+Photovoltaic+Performance+of+CH3+NH3+PbI3+Solar+Cells.">Understanding the Impact of Bromide on the Photovoltaic Performance of CH3 NH3 PbI3 Solar Cells.</a> Adv. Mater. 27 (44), 7221-7228 (2015).</li><li>Abdi-Jalebi, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+of+Monovalent+Cation+Halide+Additives+on+the+Structural+and+Optoelectronic+Properties+of+CH+3+NH+3+PbI+3+Perovskite.">Impact of Monovalent Cation Halide Additives on the Structural and Optoelectronic Properties of CH 3 NH 3 PbI 3 Perovskite.</a> Adv. Energy Mater. 6 (10), 1502472 (2016).</li><li>Yin, W. J., Shi, T., Yan, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Unusual+defect+physics+in+CH3NH3PbI3+perovskite+solar+cell+absorber.">Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber.</a> Appl. Phys. Lett. 104 (6), 063903/1-063903/4 (2014).</li><li>Agiorgousis, M. L., Sun, Y. Y., Zeng, H., Zhang, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strong+Covalency-Induced+Recombination+Centers+in+Perovskite+Solar+Cell+Material+CH+3+NH+3+PbI+3.">Strong Covalency-Induced Recombination Centers in Perovskite Solar Cell Material CH 3 NH 3 PbI 3.</a> J. Am. Chem. Soc. 136 (41), 14570-14575 (2014).</li><li>Andaji Garmaroudi, Z., Abdi-Jalebi, M., Mohammadi, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+facile+low+temperature+route+to+deposit+TiO2+scattering+layer+for+efficient+dye-sensitized+solar+cells.">A facile low temperature route to deposit TiO2 scattering layer for efficient dye-sensitized solar cells.</a> RSC Adv. 6 (75), (2016).</li><li>Abdi-Jalebi, M., Mohammadi, M. R., Fray, D. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Double-Layer+TiO2+Electrodes+with+Controlled+Phase+Composition+and+Morphology+for+Efficient+Light+Management+in+Dye-Sensitized+Solar+Cells.">Double-Layer TiO2 Electrodes with Controlled Phase Composition and Morphology for Efficient Light Management in Dye-Sensitized Solar Cells.</a> J. of Clust. Sci. 25 (4), 1029-1045 (2014).</li><li>Abdi-Jalebi, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+of+a+Mesoporous+Titania-Perovskite+Interface+on+the+Performance+of+Hybrid+Organic-Inorganic+Perovskite+Solar+Cells.">Impact of a Mesoporous Titania-Perovskite Interface on the Performance of Hybrid Organic-Inorganic Perovskite Solar Cells.</a> The J. Phys. Chem. Lett. 7 (16), 3264-3269 (2016).</li></ol>

The authors declare that they have no competing financial interests.

A typical architecture of mesoscopic perovskite solar cells was used in this work, where a series of materials were spin-coated between a conductive substrate and a thermally-evaporated metal contact (Figure 1). The mesoporous TiO2 layers were treated with TiCl4, which is reported to passivate the surface traps and to improve the interface between the electron transport layer and the absorber material21,22. The perovskite layer was then deposited using a sequential two-step deposition technique. The full conversion of lead halide into perovskite in the second step is essential to achieve the highest light absorption16,17, and we showed that the monovalent cation halide additives (e.g., NaI and CuBr) result in a complete conversion. Furthermore, the complete coverage of the mesoporous titania layer with the perovskite over-layer is vital to eliminate potential recombination between the hole transport layer (e.g., Spiro OMETAD) and the electron transport layer (e.g., mesoporous TiO2)23. We illustrated that adding the monovalent cation halides (e.g., CuI and AgI) can improve the surface coverage of the perovskite capping layer, which leads to a higher open circuit voltage for the device.
The main advantage of our method is the doping step, where we incorporated monovalent cations into the CH3NH3PbI3 structure to improve the density of charges, the charge transport, and the conductivity of the absorber layer. As stated in the previous section, the aforementioned dopants significantly enhanced both the electron and the hole mobilities. In addition, a remarkable decrease in the charge transport activation energy, as well as in the energetic disorder of the perovskite film, was achieved by monovalent cation doping.
In this work, we have demonstrated a method to dope CH3NH3PbI3 as an absorber layer in the mesoscopic perovskite solar cell structure. Monovalent cation halides were used to tune the morphological, optical, and electrical properties of CH3NH3PbI3 perovskite film in order to enhance the photovoltaic performance. Therefore, we incorporated three different monovalent cations (i.e., Na+, Cu+, and Ag+), which have similar ionic radii to Pb2+, in the lead source in the sequential two-step deposition of CH3NH3PbI3. As a result, a remarkable improvement in the structural and optoelectronic properties of CH3NH3PbI3 occurred in the presence of these additives, leading to higher PCEs for the fabricated solar cells. Therefore, our work highlights a facile way of doping the CH3NH3PbI3 as an absorber layer, which can be used in all other configurations of perovskite solar cells (e.g., planar architecture) in order to further improve the electronic quality of perovskite thin films.
The data underlying this paper are available at: https://www.repository.cam.ac.uk/handle/1810/260187.

Currently, the dominant portion of the world&#39;s energy requirement (i.e., 85%) is being supplied by the combustion of oil, coal, and natural gas, which facilitates global warming and has deleterious effects on our environment1. Therefore, the development of CO2-neutral sources of energy is of paramount interest. Photovoltaics (PV) is an ideal energy conversion process that can meet this requirement. However, cost and efficiency, as the main obstacles to the extensive adoption of PV technology, must be improved. Emerging PV technologies based on new materials, such as perovskite solar cells (PSC), have the combination of lower cost and greater efficiency. This is achieved through the utilization of cheap materials that are readily available, as well as through fast, facile, and low-energy processing routes compared to silicon-based counterparts2,3,4. A remarkable improvement in the power conversion efficiency (PCE), from 3.8% to more than 22%, has been reported for hybrid organic-inorganic lead halide perovskite since its first appearance in PV architecture5,6,7,8. Such a superb performance originates from the strong light absorption with an extremely sharp band-edge, the very low energetic disorder, the weakly-bound excitons that easily dissociate into free carriers with large diffusion lengths, and the photon recycling capability of hybrid organic-inorganic lead halide perovskite9,10,11,12. These materials are categorized in the perovskite family, which are crystallized from organic halide and metal halide salts to form crystals in the ABX3 structure, where X is an anion and A and B are cations of different sizes (A being larger than B). Reported cations for the A site include methylammonium (MA), formamidinium (FA), and cesium (Cs); a combination of these cations shows the highest performance13,14. Furthermore, the main candidate for the divalent cation in the B site is lead, which can be replaced by tin; the bandgap can be successfully red-shifted to over 1,000 nm in a lead-tin mixed perovskite15. Similarly, the X-site occupants have been studied extensively, where a mixture of iodide (I) and bromide (Br) were introduced as the main candidates16,17. Therefore, it is highly plausible to manipulate the structural, morphological, and optoelectronic properties of perovskites by altering their chemical composition.
Despite the fact that the enhanced crystalline quality and the macroscopic uniformity of the perovskite film are key parameters to achieve efficient devices18, the impact of the boundaries between the polycrystalline domains, the origin and role of electronic defects in the perovskite absorbers, and the role of the charge collection layers upon loss processes in the perovskite solar cells are not yet well understood. Regarding the nature of electronic defects in the perovskite structure, it has been reported that many of the defects, such as I or Pb vacancies, result in states that are very close to or within the continuum of states in the conduction and valence bands, which might have a negative electronic impact on the photovoltaic devices19. In addition, a strong covalent bonding interaction between lead cations and iodide anions in the perovskite plane may lead to the existence of intrinsic defects (e.g., under-coordinated Pb dimers and I trimers), which could create sites within the band-edge that act as charge recombination centers during the operation of the device20.
Here, we investigate the impact of doping CH3NH3PbI3 perovskite with monovalent cation halides, including Na+, Cu+, and Ag+, lower-valence metal ions than Pb2+. We therefore incorporate these cations through the addition of a rational amount of their halide-based salts (e.g., NaI, CuBr, CuI, and AgI) into the perovskite precursor solution. These cations have ionic radii similar to Pb2+, so substitutional doping within the crystal is probable. We have shown that the presence of these cations strongly affects both the morphology and coverage of the perovskite layer. In addition, the presence of these cations (e.g., Na+ and Ag+) has been confirmed by X-ray photoelectron spectroscopy (XPS), and a significant change in the Fermi level of perovskite was measured by Kelvin probe force microscopy (KPFM). By incorporating these cations into sequentially deposited perovskite solar cells, we achieved an improvement in the photovoltaic efficiency of PSC (15.6% compare to 14%). Therefore, it is very essential to enhance the structural and optoelectronic properties of the absorber layer (e.g., perovskite) in solar cell architecture to maximize the charge transport and to passivate the surface traps in order to reach the highest PV performance.

<table border="0" cellpadding="0" cellspacing="0" width="573">
	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Fluorine doped Tin Oxide (FTO)-coated glass</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td style="width:119px;">735264-1EA</td>
			<td style="width:167px;">Resistivity&#8776;13 &#937;/sq</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Zinc powder</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td style="width:119px;">96454</td>
			<td>Molecular Weight 65.39&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Hydrochloric acid&#160;</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td style="width:119px;">84415</td>
			<td>&#8805;37 wt. %</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Hellmanex detergent&#160;</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td>Z805939-1EA</td>
			<td>pkg of 1 L&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Titanium diisopropoxide bis(acetylacetonate)&#160;</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td align="right" style="width:119px;">325252</td>
			<td>75&#160;wt. % in isopropanol</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Titania Paste</td>
			<td style="width:71px;">DYESOL</td>
			<td style="width:119px;">MS002300</td>
			<td>30 NR-D Transparent Titania Paste</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Lead (II) iodide</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td align="right" style="width:119px;">211168</td>
			<td>99 wt. %</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">N,N-Dimethylformamide&#160;</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td align="right" style="width:119px;">437573</td>
			<td>ACS reagent, &#8805;99.8%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Methylammonium iodide</td>
			<td style="width:71px;">DYESOL</td>
			<td style="width:119px;">MS101000</td>
			<td>Powder&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">SpiroMeOTAD</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td align="right" style="width:119px;">792071</td>
			<td>99% (HPLC)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Bis(trifluoromethane)sulfonimide lithium salt</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td align="right" style="width:119px;">544094</td>
			<td>99.95% trace metals basis&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">4-tert-Butylpyridine</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td align="right" style="width:119px;">142379</td>
			<td>Purity: 96%</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Chlorobenzene&#160;</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td align="right" style="width:119px;">284513</td>
			<td>anhydrous, 99.8%</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">2-Propanol&#160; (IPA)</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td align="right" style="width:119px;">278475</td>
			<td>anhydrous, 99.5%&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Ethanol</td>
			<td style="width:71px;">Sigma-Aldrich&#160;</td>
			<td align="right" style="width:119px;">2860</td>
			<td>&#160;absolute alcohol, without additive, &#8805;99.8%&#160;</td>
		</tr>
	</tbody>
</table>

monovalent cation doping of ch3nh3pbi3 for efficient perovskite solar cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Field emission scanning electron microscopy (FESEM) was used to record both cross-sectional images of the fabricated perovskite solar cells (Figure 1) and top view images of the deposited PbI2 and CH3NH3PbI3 films (Figure 2). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the structural properties of the perovskite films (Figures 3 and 4). Photothermal deflection spectroscopy (PDS) and Kelvin probe force microscopy (KPFM) were used to probe the optical and electrical properties of the perovskite films, respectively (Figures 5 and 6). Furthermore, temperature-dependent bulk transport measurements based on space charge limited current (SCLC) was performed on the perovskite devices (Figure 7). Finally, a standard photovoltaic measurement of the fabricated devices was performed (Figure 8 and Table 1).
Based on the top-view SEM images of the PbI2 and CH3NH3PbI3 deposited on the mesoporous TiO2 layer (mp-TiO2) shown in Figure 2, the effect of the additives on the morphology of perovskite was illustrated where large branch-shaped crystals of PbI2 were achieved in the NaI-based sample. This led to the formation of larger asymmetric crystals of perovskite. Furthermore, we obtained a uniform and pinhole-free perovskite capping layer for CuI- and AgI-based samples (Figure 2c and 2e). To investigate the effect of monovalent cation halide additives on the crystal structure of CH3NH3PbI3 and on the conversion of PbI2 into CH3NH3PbI3, X-ray diffraction was performed (Figure 3). Although the crystal structure of the final perovskite remained the same for all samples, it is evident that the diffraction peak at 2&#952; = 12.6, which corresponds to the unconverted PbI2, was eliminated in the presence of NaI and CuBr additives. In order to confirm the presence of these monovalent cations within the CH3NH3PbI3 perovskite films, we carried out an XPS analysis, as shown in Figure 4. On the basis of the XPS data, we demonstrated the presence of Na and Ag ions within the perovskite films, whereas the concentration of Cu could not be estimated, probably due to the nearness of iodide (I 3p1/2) and copper (Cu 2p1/2) peaks.
The effect of the monovalent cation additives on the absorption spectrum of the perovskite is shown in Figure 5a, which was measured by PDS. It is evident that the additive-based CH3NH3PbI3 had lower sub-bandgap absorption compared to the pristine sample. Furthermore, an absorption tail was observed for Cu-based samples, which originated from the intrinsic absorption of copper halide (Figure 5b). Although the absorption tail confirms the presence of Cu cations in the final perovskite films, it is evident, based on the comparison between the PDS of CuI-based PbI2 and CH3NH3PbI3, that their incorporation is not complete. In addition, the Urbach energy (Eu), which is a measure of the degree of energetic disorder of a material, was estimated for pristine, NaI-, CuBr-, CuI-, and AgI-based perovskite, and the values are 15.6, 11.8, 12.8, 13.5, and 15.2 meV, respectively (inset of Figure 5a).
To explore the influence of the aforementioned additives on the electronic structure of CH3NH3PbI3, we performed KPFM, where the contact potential difference (CPD) of the line profiles was measured. This corresponds to the surface work function (&#934;) of the perovskite shown in Figure 6. A clear shift in the CPD (i.e., 0.1 V) of additive-based perovskite compared to the pristine one shows that the perovskite Fermi level is shifted towards the valence band. This change in the Fermi level of perovskite can be attributed to either substitutional p-doping (e.g., the replacement of Pb2+ with monovalent cations X+) or surface passivation at the crystalline surfaces of perovskite.
To investigate the effect of doping on the density of the charges and on their transport properties in CH3NH3PbI3, we performed temperature-dependent bulk transport measurements (Figure 7a). We then estimated the bulk carrier mobility (&#956;SCL) based on the space charge limited current (SCLC) of the complete electron and hole-only pristine and additive-based perovskite devices. A remarkable increase in the conductivity and in both the electron and hole mobilities were achieved, particularly for the NaI and CuBr samples compared to pristine perovskite (Table 1). It is notable that the improvement in the charge mobility and conductivity are consistent with the enhancement in the short circuit current (Jsc) and fill factor (FF) of the fabricated solar cells shown in Figure 7b. Furthermore, we estimated the activation energy for charge transport (EA) for both the electron and the hole using temperature-dependent bulk transport measurements, where a clear decrease was achieved for additive-based perovskite. This improvement is attributed to the higher density of carriers because of doping and filling the transport traps, which results in a significant decline in the transport barrier.
We fabricated perovskite solar cells based on the aforementioned monovalent cation halides, the corresponding J-V curves, and the photovoltaic parameters that are summarized in Figure 8a and Table 1. A significant improvement in open circuit voltage was achieved for both CuI- (0.99 V) and AgI- (1.02 V) based solar cells because of the ideal surface coverage (Figure 2c and 2e). Furthermore, a remarkable increase in the short circuit current (&#8776;2 mA cm-2) for CuBr- and NaI-based solar cells was attained, which can be ascribed to the full conversion of PbI2 into CH3NH3PbI3. This improvement was confirmed by the incident photon-to-current conversion efficiency (IPCE) spectra shown in Figure 8b. Finally, higher power conversion efficiency (PCE) levels of 15.2%, 15.6%, and 15.3% were achieved for NaI-, CuBr-, and CuI-based devices, respectively, compare to the 14.0% value for the pristine perovskite solar cell.
<img alt="Figure 1" src="/files/ftp_upload/55307/55307fig1.jpg" /> 
Figure 1: Mesoscopic perovskite solar cell architecture. SEM cross-sectional micrograph of a complete device with the following structure: FTO/compact-TiO2/mesoporous-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au.
<img alt="Figure 2" src="/files/ftp_upload/55307/55307fig2.jpg" /> 
Figure 2: Morphological analysis of lead iodide and perovskite structures. Top-view SEM images of PbI2 (left side) and CH3NH3PbI3 (right side) structures: (a) pristine, (b) CuBr-, (c) CuI-, (d) NaI-, and (e) AgI-based perovskite samples deposited on a mesoporous TiO2-coated FTO. This figure has been reproduced from reference18. <a href="http://cloudflare.jove.com/files/ftp_upload/55307/55307fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/55307/55307fig3.jpg" /> 
Figure 3: Effect of monovalent cation halide additives on perovskite crystallinity. X-ray diffraction spectra of pristine and additive-based CH3NH3PbI3 perovskite that is grown on mesoporous TiO2 film, which is deposited on the FTO-coated glass. This figure has been reproduced from reference18. <a href="http://cloudflare.jove.com/files/ftp_upload/55307/55307fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/55307/55307fig4.jpg" /> 
Figure 4: Trace of monovalent cations in the CH3NH3PbI3 perovskite structure. XPS analysis of pristine, CuBr-, CuI-, NaI-, and AgI-based perovskite films. This figure has been reproduced from reference18. <a href="http://cloudflare.jove.com/files/ftp_upload/55307/55307fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/55307/55307fig5.jpg" /> 
Figure 5: Optical properties of perovskite films. (a) The absorption spectra of perovskite films derived from pristine and additive-based lead sources measured using the PDS technique. The inset shows the corresponding Urbach energies for all samples. The error bar is defined by the SD in fitting the Urbach tail. (b) Comparison of the PDS absorption spectra of pristine and CuBr-based lead iodide and perovskite films, as well as CuBr deposited on ms-TiO2 and CuBr-only films. This figure has been reproduced from reference18. <a href="http://cloudflare.jove.com/files/ftp_upload/55307/55307fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/55307/55307fig6.jpg" /> 
Figure 6: Effect of monovalent cation additives on the surface potential of perovskite films. CPD line profiles recorded from pristine and additive-based perovskite films using KPFM. The AFM topography image is shown on the top. This figure has been reproduced from reference18. <a href="http://cloudflare.jove.com/files/ftp_upload/55307/55307fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/55307/55307fig7.jpg" /> 
Figure 7: Charge transport characteristics of perovskite films. (a) J-V characteristics of hole-only devices (ITO/PEDOT:PSS/Perovskite/Au), utilized for estimating the SCLC hole mobility. Note that the current density (J) is scaled with the thickness of the perovskite layers. (b) The trends in the Jsc, &#181;h, and &#181;e for pristine and additive-based perovskite. This figure has been reproduced from reference18. <a href="http://cloudflare.jove.com/files/ftp_upload/55307/55307fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" src="/files/ftp_upload/55307/55307fig8.jpg" /> 
Figure 8: Photovoltaic performance characteristics. (a) Current-voltage characteristics of devices under an illumination of 100 mW/cm2, obtained using different types of monovalent cation halides added to the lead source solution. (b) Incident photon-to-current efficiency (IPCE) spectra as a function of the wavelength of monochromatic light for the pristine, CuBr-, CuI-, NaI-, and AgI-based perovskite solar cells. This figure has been reproduced from reference18. <a href="http://cloudflare.jove.com/files/ftp_upload/55307/55307fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Type of Sample</td>
			<td>JSC</td>
			<td>VOC</td>
			<td>FF</td>
			<td>PCE</td>
			<td>&#956;e</td>
			<td>&#956;h</td>
			<td>EAe</td>
			<td>EAh</td>
		</tr>
		<tr>
			<td></td>
			<td>(mA cm-2)</td>
			<td>(V)</td>
			<td></td>
			<td>(%)</td>
			<td>(cm2/Vs)</td>
			<td>(cm2/Vs)</td>
			<td>(meV)</td>
			<td>(meV)</td>
		</tr>
		<tr>
			<td>Pristine</td>
			<td>21.03</td>
			<td>0.95</td>
			<td>0.70</td>
			<td>14.01</td>
			<td>0.02</td>
			<td>0.008</td>
			<td>135</td>
			<td>198</td>
		</tr>
		<tr>
			<td>CuBr</td>
			<td>22.92</td>
			<td>0.95</td>
			<td>0.72</td>
			<td>15.61</td>
			<td>0.05</td>
			<td>0.04</td>
			<td>88</td>
			<td>132</td>
		</tr>
		<tr>
			<td>CuI</td>
			<td>21.81</td>
			<td>0.99</td>
			<td>0.71</td>
			<td>15.25</td>
			<td>0.02</td>
			<td>0.036</td>
			<td>94</td>
			<td>157</td>
		</tr>
		<tr>
			<td>NaI</td>
			<td>22.97</td>
			<td>0.9</td>
			<td>0.73</td>
			<td>15.14</td>
			<td>0.04</td>
			<td>0.07</td>
			<td>77</td>
			<td>137</td>
		</tr>
		<tr>
			<td>AgI</td>
			<td>19.24</td>
			<td>1.02</td>
			<td>0.72</td>
			<td>14.18</td>
			<td>0.005</td>
			<td>0.006</td>
			<td>105</td>
			<td>177</td>
		</tr>
	</tbody>
</table>
Table 1: Photovoltaic and charge transport parameters of PSC. Summary of the photovoltaic parameters derived from J-V measurements and charge mobilities, along with the activation energy for the pristine and additive-based perovskite solar cells that show the best performance and were fabricated using the two-step deposition method. It is notable that the statistics of the photovoltaic parameters follow the same trend as the best performing devices. This table has been reproduced from reference18.

Watch this Scientific Journal Video about Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells at JoVE.com

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

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16.5K Views.  University of Cambridge. Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Video: Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.

A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra ...

Ambient Method for the Production of an Ionically Gated Carbon Nanotube Common Cathode in Tandem Organic Solar Cells

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two solution-processed polymeric cells connected in parallel by a transparent carbon nanotubes (CNT) interlayer. This structure includes improvements in fabrication techniques for tandem OPV devices. First the need for ambient-processed cathodes is considered. The CNT anode in the tandem device is tuned via ionic gating to become a common cathode. Ionic gating employs electric double layer charging to lower the work function of the CNT electrode. Secondly, the difficulty of sequentially stacking tandem layers by solution-processing is addressed. The devices are fabricated via solution and dry-lamination in ambient conditions with parallel processing steps. The method of fabricating the individual polymeric cells, the steps needed to laminate them together with a common CNT cathode, and then provide some representative results are described. These results demonstrate ionic gating of the CNT electrode to create a common cathode and addition of current and efficiency as a result of the lamination procedure.

A method of fabricating, in ambient conditions, organic photovoltaic tandem devices in a parallel configuration is presented. These devices feature an air-processed, semi-transparent, carbon nanotube common cathode.

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices ...

Digital Printing of Titanium Dioxide for Dye Sensitized Solar Cells

Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less environmentally damaging with lower processing temperatures presenting a viable and low cost alternative to conventional production. This paper further enhances these environmental credentials by evaluating the digital printing and therefore additive production route for these cells. This is achieved here by investigating the formation and performance of a metal oxide photoelectrode using nanoparticle sized titanium dioxide. An ink-jettable material was formulated, characterized and printed with a piezoelectric inkjet head to produce a 2.6 &#181;m thick layer. The resultant printed layer was fabricated into a functioning cell with an active area of 0.25 cm2 and a power conversion efficiency of 3.5%. The binder-free formulation resulted in a reduced processing temperature of 250 &#176;C, compatible with flexible polyamide substrates which are stable up to temperatures of 350 &#730;C. The authors are continuing to develop this process route by investigating inkjet printing of other layers within dye sensitized solar cells.

This paper investigates the suitability of inkjet printing for the manufacturing of dye-sensitized solar cells. A binder-free TiO2 nanoparticle ink was formulated and printed onto a FTO glass substrate. The printed layer was fabricated into a cell with an active area of 0.25 cm2 and an efficiency of 3.5%.

Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less ...

Electrospinning of Photocatalytic Electrodes for Dye-sensitized Solar Cells

This work demonstrates a protocol to fabricate a fiber-based photoanode for dye-sensitized solar cells, consisting of a light-scattering layer made of electrospun titanium dioxide nanofibers (TiO2-NFs) on top of a blocking layer made of commercially available titanium dioxide nanoparticles (TiO2-NPs). This is achieved by first electrospinning a solution of titanium (IV) butoxide, polyvinylpyrrolidone (PVP), and glacial acetic acid in ethanol to obtain composite PVP/TiO2 nanofibers. These are then calcined at 500 &#176;C to remove the PVP and to obtain pure anatase-phase titania nanofibers. This material is characterized using scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). The photoanode is prepared by first creating a blocking layer through the deposition of a TiO2-NPs/terpineol slurry on a fluorine-doped tin oxide (FTO) glass slide using doctor blading techniques. A subsequent thermal treatment is performed at 500 &#176;C. Then, the light-scattering layer is formed by depositing a TiO2-NFs/terpineol slurry on the same slide, using the same technique, and calcinating again at 500 &#176;C. The performance of the photoanode is tested by fabricating a dye-sensitized solar cell and measuring its efficiency through J-V curves under a range of incident light densities, from 0.25-1 Sun.

The overall goal of this project was to use electrospinning to fabricate a photoanode with improved performance for dye-sensitized solar cells.

This work demonstrates a protocol to fabricate a fiber-based photoanode for dye-sensitized solar cells, consisting of a light-scattering layer made of ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction ...

Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

This manuscript describes how to design and fabricate efficient inverted SMPV1:PC71BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule ...