Name: developing high performance gap/si heterojunction solar cells
Uploaded: 2018-11-16T15:00:00.000+00:00
Duration: 10 min 31 s
Description: Watch this Scientific Journal Video about Developing High Performance GaP/Si Heterojunction Solar Cells at JoVE.com

The authors would like to thank L. Ding and M. Boccard for their contributions in processing and testing of the solar cells in this study. The authors acknowledge funding from the U.S. Department of Energy under contract DE-EE0006335 and the Engineering Research Center Program of the National Science Foundation and the Office of Energy Efficiency and Renewable Energy of the Department of Energy under NSF Cooperative Agreement No. EEC-1041895.&#160;Som Dahal at Solar Power Lab was supported, in part, by NSF contract ECCS-1542160.

CAUTION: Please consult all relevant material safety data sheets (MSDS) before dealing with chemicals. Please use all appropriate safety practices when performing a solar cell fabrication including the fume hood and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, closed-toe shoes).
1. Si Wafer Cleaning
<ol>
	<li>Clean Si wafers in Piranha solution (H2O2/H2SO4) at 110 &#176;C.
	<ol>
		<li>To produce Piranha solution, fill the acid bath (high-density polyethylene tank and hereafter) with 15.14 L of H2SO4 (96%) and then 1.8 L of H2O2 (30%).</li>
		<li>Wait for the temperature of the solution to stabilize at 110 &#176;C.</li>
		<li>Place 4-inch-diameter, float-zone (FZ), n-type, and double-side-polished Si wafers in a clean 4&#8221; wafer cassette (polypropylene and hereafter), and place the boat in the Piranha bath for 10 min.</li>
		<li>Rinse for 10 min with deionized (DI) water.</li>
	</ol>
	</li>
	<li>Clean Si wafers with RCA cleaning solution at 74 &#176;C.
	<ol>
		<li>Prepare a diluted solution of HCl:H2O2. Fill the acid bath with 13.2 L of DI H2O and 2.2 L of HCl. Spike the solution with 2.2 L of H2O2 and turn on the heater.</li>
		<li>Wait for the temperature of the solution to stabilize at 74 &#176;C before use.</li>
		<li>Put Si wafers in a clean 4&#8221; wafer cassette and place wafers in the RCA solution for 10 min.</li>
		<li>Rinse with DI water for 10 min.</li>
	</ol>
	</li>
	<li>Clean Si wafers in Buffered Oxide Etch (BOE) solution.
	<ol>
		<li>Pour 15.14 L of BOE solution in the acid bath.</li>
		<li>Place the 4&#8221; wafer cassette in the bath for 3 min.</li>
		<li>Rinse for 10 min with DI water.</li>
		<li>Dry the wafer by dry N2.</li>
	</ol>
	</li>
</ol>
2. P-diffusion in the Diffusion Furnace
<ol>
	<li>Put a cleaned wafer in a diffusion quartz boat.</li>
	<li>Load it into a quartz tube that has a base temperature of 800 &#176;C. Ramp the furnace temperature to 820 &#176;C in the N2 environment. At 820 &#176;C, start flowing the N2 carrier gas that bubbles through phosphorous oxychloride (POCl3) at 1000 sccm. After 15 min, turn off the N2 carrier gas and ramp the temperature down to 800 &#176;C before taking the samples out.</li>
	<li>Place the samples in a BOE solution for 10 min to remove phosphorus silicate glass (PSG) and then perform a 10-min rinse in DI water.</li>
</ol>
3. SiNx coating by PECVD
<ol>
	<li>Put the wafer in a clean boat and dip it in a BOE bath for 1 min to remove the native oxide on the surface.</li>
	<li>Rinse for 10 min with DI water.</li>
	<li>Dry the wafer with a dry N2 gun.</li>
	<li>Place the Si wafer on a clean Si carrier (156 mm monocrystalline Si).</li>
	<li>Load the sample into the plasma enhanced chemical vapor deposition (PECVD) chamber.</li>
	<li>Deposit 150 nm-thick (38.5 s) SiNx at 350 &#176;C in the chamber. Deposit SiNx at 300 W RF power with a base pressure of 3.5 Torr and 60 sccm of SiH4 as a silicon source and 60 sccm of NH3 as a N source (2000 sccm of N2 was used as a diluent).
	<ol>
		<li>Confirm the growth rate of SiNx (3.9 nm/s) by depositing SiNx films with different deposition times on polished wafers and measure the thicknesses by variable angle spectroscopic ellipsometry (VASE).</li>
	</ol>
	</li>
</ol>
4. GaP Growth by MBE
<ol>
	<li>After SiNx deposition, load the wafer into the MBE chamber.</li>
	<li>Outgas in the introductory chamber (180 &#176;C for 3 h), then outgas in the buffer chamber (240 &#176;C for 2 h). Load in to the growth chamber and bake at 850 &#176;C for 10 min.</li>
	<li>Decrease temperature to 580 &#176;C. Increase Ga effusion cell temperature to produce ~2.71&#215;10-7 Torr beam-equivalent pressure (BEP) and Si effusion cell temperature to 1250 &#176;C.</li>
	<li>Adjust the p-valved cracker positioner to achieve ~1.16&#215;10-6 Torr BEP. Open the Ga, P, and Si shutters and grow 25 nm-thick GaP with an interrupted growth method (10 cycles of 5 s open and 5 s closed) followed by 121 s of unshuttered growth (i.e., open Ga and p shutters simultaneously).</li>
	<li>Decrease the substrate temperature to 200 &#176;C and unload the sample from the vacuum chamber.</li>
</ol>
5. Remove Back n+ and SiNx Layers by Wet Etching
<ol>
	<li>Cover the GaP surface with a protective tape to protect it from the HF damage.</li>
	<li>Prepare ~300 mL of 49% HF solution in a plastic beaker.</li>
	<li>Place the sample in the HF solution for 5 min to fully remove the SiNx layer.</li>
	<li>Remove the protective tape, rinse with DI water, and dry by N2.</li>
	<li>Cover the GaP surface with a new protective tape.</li>
	<li>Prepare HNA solution in a plastic beaker (a mixture of hydrofluoric acid (HF) (50 mL), nitric acid (HNO3) (365 mL), and acetic acids (CH&#173;3COOH) (85 mL)) at room temperature. 
	CAUTION: Carefully place the wafer in the solution to avoid HNA penetrating into the GaP surface.</li>
	<li>Put the sample in the HNA solution for 3 min.</li>
	<li>Remove the protective tape and rinse by DI water. Dry by N2.</li>
</ol>
6. Hole-Selective Contact Formation on the Bare Si Side
<ol>
	<li>Cleave the wafer with a diamond pen into four quarters.</li>
	<li>Thoroughly clean the samples in a DI water tank.</li>
	<li>Clean the samples in a BOE bath for 30 s to remove the native oxide from the surface.</li>
	<li>Rinse in the wafers in DI water and then dry by N2.</li>
	<li>Deposit a 50 nm-thick a-Si: H by PECVD on one of the samples to check the Si lifetime.
	<ol>
		<li>Deposit the a-Si: H layer at 60 W RF power with a pressure of 3.2 Torr and 40 sccm of SiH4 as the silicon source (200 sccm of H2 was used as a diluent).</li>
		<li>Confirm the growth rate of a-Si: H (1.6 nm/s) by depositing a-Si films with different deposition times on polished wafers and measuring the thickness with VASE.</li>
	</ol>
	</li>
	<li>Deposit (i)a-Si (9 nm) and (p+)a-Si (16 nm) on the etched (front) side of a separate Si sample by PECVD.
	<ol>
		<li>Deposit the p-type a-Si layer at 37 W RF power with a pressure of 3.2 Torr and 40 sccm of SiH4 as the silicon source and 18 sccm of B[CH3]3 as the boron dopant (197 sccm of H2 was used as a diluent).</li>
		<li>Confirm the growth rate of p-type a-Si (2.0 nm/s) by depositing a-Si films with different growth times on the polished wafers and measuring the thicknesses with VASE.</li>
	</ol>
	</li>
	<li>Deposit a 9 nm-thick MoOx layer at the room temperature by the thermal evaporation from a MoO3 (99.99%) source with a deposition rate of 0.5 &#197;/s.</li>
</ol>
7. External Contact Formation
<ol>
	<li>Deposit 75 nm-thick Indium Tin Oxide (ITO) (In2O3/SnO2 = 95/5 (weight percent), 99.99%) layers on the GaP side of the samples by RF sputtering (RF power of 1 kW and pressure of 5 Torr) with an oxygen flow rate of 2.2 sccm.</li>
	<li>Unload the samples and turn them over. Then use the mesa shadow mask on the samples for ITO mesa deposition.</li>
	<li>Deposit 75 nm-thick ITO by RF sputtering. Deposit 200 nm-thick silver (RF power of 1 kW and pressure of 8 Torr) for the fingers covering the finger shadow mask. Deposit 200 nm-thick silver on the GaP side of the samples as the back contact.</li>
	<li>Anneal the samples in a furnace under atmospheric pressure at 220 &#176;C.</li>
</ol>

<ol>
	<li>Friedman, 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=Progress+and+challenges+for+next-generation+high-efficiency+multijunction+solar+cells.">Progress and challenges for next-generation high-efficiency multijunction solar cells.</a> Current Opinion in Solid State &amp; Materials Science. 14, 131-138 (2010).</li><li>Vadiee, E., 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=AlGaSb-Based+Solar+Cells+Grown+on+GaAs:+Structural+investigation+and+device+performance.">AlGaSb-Based Solar Cells Grown on GaAs: Structural investigation and device performance.</a> IEEE Journal of Photovoltaics. , (2017).</li><li>Wagner, H., 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+numerical+simulation+study+of+gallium-phosphide/silicon+heterojunction+passivated+emitter+and+rear+solar+cells.">A numerical simulation study of gallium-phosphide/silicon heterojunction passivated emitter and rear solar cells.</a> Journal of Applied Physics. 115, 044508 (2014).</li><li>Limpert, 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=Results+from+coupled+optical+and+electrical+sentaurus+TCAD+models+of+a+gallium+phosphide+on+silicon+electron+carrier+selective+contact+solar+cell.">Results from coupled optical and electrical sentaurus TCAD models of a gallium phosphide on silicon electron carrier selective contact solar cell.</a> 2014 IEEE 40th Photovoltaic Specialist Conference (PVSC). , 836-840 (2014).</li><li>Ding, L., 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=On+the+source+of+silicon+minority-carrier+lifetime+degradation+during+molecular+beam+heteroepitaxial+growth+of+III-V+materials.">On the source of silicon minority-carrier lifetime degradation during molecular beam heteroepitaxial growth of III-V materials.</a> 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC). , 2048-2051 (2016).</li><li>Ding, L., 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=Silicon+minority-carrier+lifetime+degradation+during+molecular+beam+heteroepitaxial+III-V+material+growth.">Silicon minority-carrier lifetime degradation during molecular beam heteroepitaxial III-V material growth.</a> Energy Procedia. 92, 617-623 (2016).</li><li>Zhang, C., Kim, Y., Faleev, N. N., Honsberg, C. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improvement+of+GaP+crystal+quality+and+silicon+bulk+lifetime+in+GaP/Si+heteroepitaxy.">Improvement of GaP crystal quality and silicon bulk lifetime in GaP/Si heteroepitaxy.</a> Journal of Crystal Growth. 475, 83-87 (2017).</li><li>Garc&#237;a-Tabar&#233;s, E., 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=Evolution+of+silicon+bulk+lifetime+during+III-V-on-Si+multijunction+solar+cell+epitaxial+growth.">Evolution of silicon bulk lifetime during III-V-on-Si multijunction solar cell epitaxial growth.</a> Progress in Photovoltaics: Research and Applications. 24, 634-644 (2016).</li><li>Varache, R., 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=Evolution+of+bulk+c-Si+properties+during+the+processing+of+GaP/c-Si+heterojunction+cell.">Evolution of bulk c-Si properties during the processing of GaP/c-Si heterojunction cell.</a> Energy Procedia. 77, 493-499 (2015).</li><li>Ishizaka, A., Shiraki, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Low+temperature+surface+cleaning+of+silicon+and+its+application+to+silicon+MBE.">Low temperature surface cleaning of silicon and its application to silicon MBE.</a> Journal of The Electrochemical Society. 133, 666 (1986).</li><li>Zhang, C., Vadiee, E., King, R. R., Honsberg, C. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Carrier-selective+contact+GaP/Si+solar+cells+grown+by+molecular+beam+epitaxy.">Carrier-selective contact GaP/Si solar cells grown by molecular beam epitaxy.</a> Journal of Materials Research. 33, 414-423 (2018).</li><li>Battaglia, C., 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=Hole+Selective+MoOx+Contact+for+Silicon+Solar+Cells.">Hole Selective MoOx Contact for Silicon Solar Cells.</a> Nano Letters. 14, 967-971 (2014).</li></ol>

The authors have nothing to disclose.

A nominal 25 nm-thick GaP layer was epitaxially grown on a P-rich Si surface via MBE. To grow a better quality of GaP layer on Si substrates, a relatively low V/III (P/Ga) ratio is preferable. A good crystal quality of GaP layer is necessary to achieve high conductivity and low density of recombination centers. The AFM root-mean-square (RMS) of the GaP surface is ~0.52 nm showing a smooth surface with no pits, indicative of high crystal quality with a low threading dislocation density (Figure 1a). Further, pendellosung fringes were observed from the &#969;-2&#952; rocking curve (Figure 1b) indicative of smooth interfaces. The full width at half maximum (FWHM) of the GaP peak measured from the triple crystal &#969; rocking curve is ~14 arcsec and the threading dislocation density calculated is ~2&#215;106 cm-2. The RSM (Figure 1c) in the vicinity of (224) diffraction spots of the GaP/Si sample shows coherent GaP and Si peaks, which indicates GaP is fully strained to the Si substrate with good crystalline quality.
The critical step of achieving high-performance Si-based solar cells is to maintain high Si minority-carrier lifetimes throughout the deposition of GaP. It is shown that by inserting the n+ layer before the GaP growth, the Si bulk lifetime can be well-maintained (up to a milliseconds level). In addition, the GaP/Si lifetime was measured to be ~100 &#956;s after GaP growth in the MBE chamber. The achieved high lifetime of Si indicates a promising device performance (as shown in the Figure 2c). The light and pseudo J-V parameters for GaP/Si heterojunctions solar cells (a-Si/Si/GaP (Structure A) and MoOx/Si/GaP (Structure B)) are listed in Table 1, measured under an AM1.5G condition with irradiation intensity of 1 kW m-2. While ITO and Ag were applied as the contact layers to the GaP layer in this work, however, to achieve better performance of GaP/Si solar cells, it is recommended to optimize ITO thickness, transparency, and its conductivity.
In this work, MoOx was also used as a hole selective contact to further improve the carrier collection efficiency at short wavelengths. Benefitting from the higher bandgap of MoOx compared to the a-Si layers, the IQE shows a boost at the short wavelength regime (300 - 600 nm). The MoOx/Si/GaP solar cell demonstrated a better performance than the best performing MoOx/Si solar cells reported in the literature12 without inserting the passivation layer between MoOx and Si interface.
Although a high Si bulk lifetime can be achieved from the aforementioned approach, the minority-carrier lifetime of GaP/Si structure is still not comparable to a-Si passivated structures, which implies that the GaP layer quality should be further improved. The demonstrated approach which requires a diffusion step and SiNx coating layer could affect the surface quality of the Si; hence, the subsequent GaP crystal quality can be impacted. Furthermore, x-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectrometry (SIMS) can be conducted to investigate the P-diffusion profile in this structure.
In this work, we have demonstrated the high-performance GaP/Si heterojunction solar cells by inserting n+ layers into Si substrates before the GaP growth. This protocol can be applied to maintain a high minority-carrier lifetime of Si while epitaxially growing not only GaP (presented here) but also to other III-V or II-VI materials to achieve heterojunction devices. Furthermore, multijunction solar cells with high-performance Si bottom cells can be realized by this approach.

There has been a continuing effort on the integration of different materials with lattice mismatches in order to enhance overall solar cell efficiency1,2. The III-V/Si integration has the potential to further increase the current Si solar cell efficiency and replace the expensive III-V substrates (such as GaAs and Ge) with a Si substrate for multijunction solar cell applications. Among all III-V binary material systems, gallium phosphide (GaP) is a good candidate for this purpose, as it has the smallest lattice-mismatch (~0.4%) with Si and a high indirect bandgap. These features can enable high-quality integration of GaP with Si substrate. It has been theoretically shown that GaP/Si heterojunction solar cells could enhance the efficiency of conventional passivated emitter rear Si solar cells3,4 by benefiting from the unique band-offset between GaP and Si (&#8710;Ev ~1.05 eV and &#8710;Ec ~0.09 eV). This makes GaP a promising electron selective contact for silicon solar cells. However, in order to achieve high-performance GaP/Si heterojunction solar cells, a high Si bulk lifetime and high GaP/Si interface quality are required.
During the growth of III-V materials on a Si substrate by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE), significant Si lifetime degradation has been widely observed5,6,7,8,9. It was revealed that the lifetime degradation mainly happens during the thermal treatment of the Si wafers in the reactors, which is required for surface oxide desorption and/or surface reconstruction before the epitaxial growth10. This degradation was ascribed to the extrinsic diffusion of contaminants originated from the growth reactors5,7. Several approaches have been proposed to suppress this Si lifetime degradation. In our previous work, we have demonstrated two methods in which the Si lifetime degradation can be significantly suppressed. The first method was demonstrated by the introduction of SiNx as a diffusion barrier7 and the second one by introducing the P-diffusion layer as a gettering agent11 to the Si substrate.
In this work, we have demonstrated high-performance GaP/Si solar cells based on the aforementioned approaches to mitigate the silicon bulk lifetime degradation. The techniques used to preserve the Si lifetime can have broad applications in multijunction solar cells with active Si bottom cells and electronic devices such as high-mobility CMOS. In this detailed protocol, the fabrication details of GaP/Si heterojunction solar cells, including Si wafer cleaning, P-diffusion in the furnace, GaP growth, and GaP/Si solar cells processing, are presented.

<table><tbody><tr><td>Hydrogen peroxide, 30%</td><td>Honeywell</td><td>10181019</td><td></td></tr><tr><td>Sulfuric acid, 96%</td><td>KMG electronic chemicals, Inc.</td><td>64103</td><td></td></tr><tr><td>Hydrochloric acid, 37%</td><td>KMG electronic chemicals, Inc.</td><td>64009</td><td></td></tr><tr><td>Buffered Oxide Etch 10:1</td><td>KMG electronic chemicals, Inc.</td><td>62060</td><td></td></tr><tr><td>Hydrofluoric acid, 49%</td><td>Honeywell</td><td>10181736</td><td></td></tr><tr><td>Acetic acid</td><td>Honeywell</td><td>10180830</td><td></td></tr><tr><td>Nitride acid, 69.5%</td><td>KMG electronic chemicals, Inc.</td><td>200288</td><td></td></tr></tbody></table>

developing high performance gap/si heterojunction solar cells

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar cells. In this work, we present high performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime and high crystal quality of epitaxial GaP layers. It is shown that by applying phosphorus (P)-diffusion layers into the Si substrate and a SiNx layer, the Si minority-carrier lifetime can be well-maintained during the GaP growth in the molecular beam epitaxy (MBE). By controlling the growth conditions, the high crystal quality of GaP was grown on the P-rich Si surface. The film quality is characterized by atomic force microscopy and high-resolution x-ray diffraction. In addition, MoOx was implemented as a hole-selective contact that led to a significant increase in the short-circuit current density. The achieved high device performance of the GaP/Si heterojunction solar cells establishes a path for further enhancement of the performance of Si-based photovoltaic devices.

Atomic force microscopy (AFM) images and high-resolution x-ray diffraction (XRD) scans, including the rocking curve in the vicinity of the (004) reflection and the reciprocal space map (RSM) in the vicinity of (224) reflection, were collected for the GaP/Si structure (Figure 1). The AFM was used to characterize the surface morphology of the MBE-grown GaP and XRD was used to examine the crystal quality of GaP layer. The effective minority-carrier lifetime of the GaP/Si structure and Si bulk were measured to examine the effectiveness of the lifetime preserving methods used in this work. External quantum efficiency (EQE), surface reflection, pseudo light J-V (Suns-Voc), and light J-V of the GaP/Si final devices were collected (Figure 2). The internal quantum efficiency (IQE) was generated from the reflection corrected EQE data. The light and pseudo J-V parameters are listed in Table 1. Efficiencies of 13.1% and 14.1% with an open-circuit voltage (Voc) of 618 mV and 598 mV are achieved from Structure A and B, respectively. The MoOx layer in Structure B as a hole-selective contact performed better than better than the a-Si: H in Structure A.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58292/58292fig1.jpg" /> 
Figure 1: Characterization of the GaP layer of the GaP/Si structure. (a) 1 x 1 &#956;m2 AFM image of the 25 nm-thick GaP surface. (b) The coherent double crystal (DC) &#969;-2&#952; rocking curve (black) in the vicinity of Si and GaP (004) reflections (a fitted curve (red) of the structure is also presented). (c) Reciprocal space map of (224) diffraction spots. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/58292/58292fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/58292/58292fig2.jpg" /> 
Figure 2: Electrical characteristics of the GaP/Si heterojunction devices. (a) The effective minority-carrier lifetime of GaP/Si structure (black dots) and Si bulk lifetime (red dots). (b) The IQE and surface reflection spectra of a-Si/Si/GaP (Structure A) (black) and MoOx/Si/GaP (Structure B) (blue). (c) Light J-V (black) and pseudo light J-V (red) of a-Si/Si/GaP device. (d) Light J-V (black) and pseudo light J-V (red) of MoOx/Si/GaP device. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/58292/58292fig2large.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 rowspan="2"></td>
			<td>VOC</td>
			<td>JSC</td>
			<td>FF</td>
			<td>FF0</td>
			<td>WOC</td>
			<td>&#951;</td>
			<td>&#951;0</td>
		</tr>
		<tr>
			<td>(mV)</td>
			<td>(mA/cm2)</td>
			<td>(%)</td>
			<td>(%)</td>
			<td>(mV)</td>
			<td>(%)</td>
			<td>(%)</td>
		</tr>
		<tr>
			<td>Structure A</td>
			<td>618</td>
			<td>33.1</td>
			<td>64</td>
			<td>80</td>
			<td>522</td>
			<td>13.1</td>
			<td>16.5</td>
		</tr>
		<tr>
			<td>Structure B</td>
			<td>598</td>
			<td>34.3</td>
			<td>69</td>
			<td>80</td>
			<td>542</td>
			<td>14.1</td>
			<td>16.9</td>
		</tr>
	</tbody>
</table>
Table 1. Light and pseudo J-V values for GaP/Si heterojunctions solar cells.

Watch this Scientific Journal Video about Developing High Performance GaP/Si Heterojunction Solar Cells at JoVE.com

Developing High Performance GaP/Si Heterojunction Solar Cells

Here, we present a protocol to develop high-performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime.

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar ...

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7.4K Views.  Arizona State University. Here, we present a protocol to develop high-performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime.

Video: Developing High Performance GaP/Si Heterojunction 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 ...

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Atom Probe Tomography Studies on the Cu(In,Ga)Se2 Grain Boundaries

Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the nanoscale and in three dimensions. Indeed, APT possesses high sensitivity (in the order of ppm) and high spatial resolution (sub nm).
Considerable efforts were done here to prepare an APT tip which contains the desired grain boundary with a known structure. Indeed, site-specific sample preparation using combined focused-ion-beam, electron backscatter diffraction, and transmission electron microscopy is presented in this work. This method allows selected grain boundaries with a known structure and location in Cu(In,Ga)Se2 thin-films to be studied by atom probe tomography.
Finally, we discuss the advantages and drawbacks of using the atom probe tomography technique to study the grain boundaries in Cu(In,Ga)Se2 thin-film solar cells.

Atom Probe Tomography Studies on the CuIn,GaSe2 Grain Boundaries

In this work, we describe the use of the atom-probe tomography technique for studying the grain boundaries of the absorber layer in a CIGS solar cell. A novel approach to prepare the atom probe tips containing the desired grain boundary with a known structure is also presented here.

Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the ...

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM). ECCI allows for imaging of defects and crystallographic features under specific diffraction conditions, similar to that possible via plan-view transmission electron microscopy (PV-TEM). A particular advantage of the ECCI technique is that it requires little to no sample preparation, and indeed can use large area, as-produced samples, making it a considerably higher throughput characterization method than TEM. Similar to TEM, different diffraction conditions can be obtained with ECCI by tilting and rotating the sample in the SEM. This capability enables the selective imaging of specific defects, such as misfit dislocations at the GaP/Si interface, with high contrast levels, which are determined by the standard invisibility criteria. An example application of this technique is described wherein ECCI imaging is used to determine the critical thickness for dislocation nucleation for GaP-on-Si by imaging a range of samples with various GaP epilayer thicknesses. Examples of ECCI micrographs of additional defect types, including threading dislocations and a stacking fault, are provided as demonstration of its broad, TEM-like applicability. Ultimately, the combination of TEM-like capabilities &#8211; high spatial resolution and richness of microstructural data &#8211; with the convenience and speed of SEM, position ECCI as a powerful tool for the rapid characterization of crystalline materials.

The use of electron channeling contrast imaging in a scanning electron microscope to characterize defects in III-V/Si heteroexpitaxial thin films is described. This method yields similar results to plan-view transmission electron microscopy, but in significantly less time due to lack of required sample preparation.

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging ...

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu2O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu2O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu2O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu2O interface. Optimization of AP-SALD conditions as well as keeping Cu2O away from air and moisture in order to minimize Cu2O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn1-xMgxO/Cu2O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu2O-based photovoltaics.

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Here we present a protocol for synthesizing Zn1-xMgxO/Cu2O heterojunctions in open-air at low temperature via atmospheric pressure spatial atomic layer deposition (AP-SALD) of Zn1-xMgxO on cuprous oxide. Such high quality conformal metal oxides can be grown on a variety of substrates including plastics by this cheap and scalable method.

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O ...

Chemistry

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 ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to achieve device efficiencies exceeding other thin film device technologies. Yet, large variations in device efficiency and basic physical properties are reported. This is due to unintentional variations during film processing, which have not been sufficiently investigated so far. We therefore conducted an extensive study of the morphology and electronic structure of a large number of CH3NH3PbI3 perovskite where we show how the preparation method as well as the mixing ratio of educts methylammonium iodide and lead(II) iodide impact properties like film formation, crystal structure, density of states, energy levels, and ultimately the solar cell performance.

We present an extensive study on the effects of different fabrication methods for organic/inorganic perovskite thin films by comparing crystal structures, density of states, energy levels, and ultimately the solar cell performance.

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to ...

Morphology Control for Fully Printable Organic&#8211;Inorganic Bulk-heterojunction Solar Cells Based on a Ti-alkoxide and Semiconducting Polymer

The photoactive layer of a typical organic thin-film bulk-heterojunction (BHJ) solar cell commonly uses fullerene derivatives as the electron-accepting material. However, fullerene derivatives are air-sensitive; therefore, air-stable material is needed as an alternative. In the present study, we propose and describe the properties of Ti-alkoxide as an alternative electron-accepting material to fullerene derivatives to create highly air-stable BHJ solar cells. It is well-known that controlling the morphology in the photoactive layer, which is constructed with fullerene derivatives as the electron acceptor, is important for obtaining a high overall efficiency through the solvent method. The conventional solvent method is useful for high-solubility materials, such as fullerene derivatives. However, for Ti-alkoxides, the conventional solvent method is insufficient, because they only dissolve in specific solvents. Here, we demonstrate a new approach to morphology control that uses the molecular bulkiness of Ti-alkoxides without the conventional solvent method. That is, this method is one approach to obtain highly efficient, air-stable, organic-inorganic bulk-heterojunction solar cells.

A method for fully printable, fullerene-free, highly air-stable, bulk-heterojunction solar cells based on Ti alkoxides as the electron acceptor and the electron-donating polymer fabrication is described here. Moreover, a method for controlling the morphology of the photoactive layer through the molecular bulkiness of the Ti-alkoxide units is reported.

The photoactive layer of a typical organic thin-film bulk-heterojunction (BHJ) solar cell commonly uses fullerene derivatives as the electron-accepting ...

Developing High Performance GaP/Si Heterojunction Solar Cells

Summary

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Chapters in this video

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

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Atom Probe Tomography Studies on the Cu(In,Ga)Se₂ Grain Boundaries

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization

Improved Heterojunction Quality in Cu₂O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited
Zn_1-xMg_xO

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

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

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Morphology Control for Fully Printable Organic–Inorganic Bulk-heterojunction Solar Cells Based on a Ti-alkoxide and Semiconducting Polymer