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Well-Controlled Dielectric Nanomeshes by Colloidal Nanosphere Lithography for Opto-Electronic Enhancement of Ultrathin Cu(In,Ga)Se2 Solar Cells Guanchao Yin, Min Song, Shengkai Duan, Phillip Manley, Dieter Greiner, Christian A. Kaufmann, and Martina Schmid ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10135 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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ACS Applied Materials & Interfaces

Well-Controlled Dielectric Nanomeshes by Colloidal Nanosphere Lithography for OptoElectronic Enhancement of Ultrathin Cu(In,Ga)Se2 Solar Cells Guanchao Yinǁ,*, Min Songǁ,§ , Shengkai Duanǁ, Phillip Manleyǁ, Dieter GreinerΨ, Christian KaufmannΨ, Martina Schmidǁ,§ ǁ

Nanooptische Konzepte für die PV, Helmholtz-Zentrum Berlin für Materialien und Energie

GmbH, Hahn-Meitner Platz 1, 14109 Berlin, Germany. §

Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin, Germany

Ψ

Institute Competence Centre Photovoltaics Berlin (PVcomB), Helmholtz-Zentrum Berlin für

Materialien und Energie, Schwarzschildstraße 3, 12489 Berlin, Germany

ABSTRACT: Ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells pose challenges of incomplete absorption and back contact recombination. In this work, we applied the simple collodial nanosphere lithography and fabricated 2D SiO2 nanomeshes (NMs), which simultaneously benefit ultrathin CIGSe solar cells electrically and optically. Electrically, the NMs are capable of passivating the back contact recombination, and increasing the minimum bandgap of absorbers.

ACS Paragon Plus Environment

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Optically, the parasitic absorption in Mo as a main optical loss is reduced. Consequently, the SiO2 NMs give rise to an increase of 3.5 mA/cm2 in short circuit current density (Jsc) and of 57 mV in open circuit voltage increase (Voc), leading to an absolute efficiency enhancement as high as 2.6% (relatively 30%) for CIGSe solar cells with an absorber thickness of only 370 nm and a steep back Ga/[Ga+In] grading.

KEYWORDS: nanosphere lithography method, ultrathin CIGSe solar cells, SiO2 nanomesh, opto-electronic enhancement, back contact passivation, absorption enhancement INTRODUCTION Cu(In,Ga)Se2 (CIGSe) thin-film solar cells are a promising candidate in the photovoltaic market with record efficiencies beyond 22%.1 However, Indium scarcity may constrain the large scale production and thus reduce the competitiveness of CIGSe solar cells.2,3 Therefore, thinning the CIGSe absorber thickness from typical values of 2-3 µm down to a few hundred nm is highly desirable since it reduces material-related cost and especially releases the concern of Indium scarcity. Additionally, a reduced absorber thickness permits the use of lower-quality CIGSe absorbers with shorter minority-carrier diffusion lengths, which may enhance the tolerance for cell fabrication conditions4 (e.g. lower substrate temperature, non-vacuum deposition) and further reduce the manufacturing cost. However, high efficiencies are presently restrained in ultrathin CIGSe solar cells (with a sub-500 nm absorber thickness) due to two primary challenges: back contact recombination and incomplete absorption.5–7 Back contact recombination, namely the carrier recombination at the rear interface of CIGSe/Mo back contact, is particularly serious for ultrathin CIGSe solar cells since the back contact is within the diffusion length of minority carriers. This issue can be addressed by


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building an increasing [Ga]/[Ga+In] ratio towards the back contact to form a potential for electron diffusion,4–7 and alternatively by applying a point contact structure to lower the CIGSe/Mo interface recombination.8 Incomplete optical absorption is the second major challenge. Ultrathin CIGSe cells typically exhibit a dramatic optical loss, corresponding to a loss of more than 6 mA/cm2 in short circuit current density (Jsc) compared to their thick counterparts. Due to the poor optical reflectivity, Mo back contact dominantly absorbs the light reaching the back interface rather than reflecting back into the CIGSe absorber layer.7,9,10 This makes the parasitic absorption in Mo (AbsMo) a key optical loss. Compared to the comprehensive investigations of applying innovative light trapping nanostructures on Si-based solar cells, light management for CIGSe solar cells is still an emerging field.11–14 Due to their specific electrical property and distinctive cell architecture, CIGSe solar cells pose particular challenges for effective implementation of light trapping nanostructures.12,15 Directly integrated light trapping nanostructures in close proximity to the CIGSe absorber layer at the back interface of CIGSe/Mo has been identified as the most beneficial approach due to being able to reduce the dramatic AbsMo.12,13 Additionally, high temperature fabrication of CIGSe absorbers in a corrosive environment implies that thermally stable inorganic dielectric materials (e.g. SiO2, Al2O3) are favoured over the plasmonic metallic ones (e.g. Ag, Au).15 Since the main optical and electrical limitations of ultrathin CIGSe solar cells come from the back CIGSe/Mo interface, it offers the possibilities to solve the problems simultaneously. Well-defined two-dimensional (2D) ordered nanostructures have been extensively explored as light trapping schemes for photovoltaic applications in the last decade.16–18 They provide great flexibility in achieving light trapping effects in the spectral range of interest for a specific type of solar cells by engineering the nanostructures. A variety of technologies have been applied to


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fabricate 2D ordered nanostructures for photovoltaic applications, mainly including electronbeam lithography19, nanoimprint lithography11 and substrate conformal imprint lithography12. However, they generally require complicated equipments at a high cost and are not easily scalable. Nanoimprint lithography and substrate conformal imprint lithography are in principle capable of large-area fabrication, but the required physical mask is obtained using electron-beam lithography or other sophisticated techniques and tends to wear with usage.20 In comparison, nanosphere lithography (NSL) is a proven low-cost approach since it utilizes self-assembled 2D colloidal nanoparticle monolayers without requiring expensive or complicated equipments.20–22 The 2D nanoparticle monolayers can be straightforwardly prepared by self-assembly of colloidal particles (e.g. silica, polystyrene ) and the covered area can be scaled to the size of meters for industrial applicaiton.23,24 Using NSL to nanostructure glass substrates and then transferring the nanopattern to the overlying layers from conformal growth has been

considered as an effective approach to

enhance absorption in Si thin-film solar cells.25 Nanostructuring glass substrate avoids possible negative effects incurred by bringing a tertiary material to the back contact while maintaining the optical benefits of a patterned back contact and full contacting between back contact and absorber for excellent carrier collection. However, this is optically unfavourable for CIGSe solar cells, because the conventional Mo back contact is poorly reflective and highly absorbing, nanostructured Mo from conformal growth on the nanostructured glass substrate will further increase its parasitic absorption.26 The applied nanostructures from NSL can be directly the colloidal particles themselves13,27 or nanomeshes (NMs) using the particles as sacrificial masks for the wanted materials (mainly noble metallic materials)24,28,29. A direct deposition of CIGSe absorber on the colloidal particles


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will cause peeling-off due to the poor adhesion between nanoparticles and Mo back contact. In contrast, NMs are able to avoid the peeling-off issue since the colloidal particles are removed. Considering the thermal requirement of nanostructures, inorganic dielectric materials are favoured rather than the normally used metallic materials. In this contribution, we took the advantages of NSL and prepared 2D ordered SiO2 NMs directly on Mo back contact. It is demonstrated that the SiO2 NM fabrication process is compatible with the growth and performance of CIGSe solar cells. Remarkably, the SiO2 NMs are simultaneously a point-contact and a light trapping structure, which benefit ultrathin CIGSe solar cells from the joint effects of CIGSe/Mo recombination passivation and light absorption enhancement. Additionally, high NMs are able to increase the minimum bangap (Eg.min) and thus open circuit voltage (Voc) by promoting the inter-diffusion of Ga-In. Via the integration of SiO2 NMs with a peak-to-valley height of 150 nm, the ultrathin CIGSe solar cells with an absorber thickness of only 370 nm exhibit Voc and Jsc enhancements up to 57 mV and 3.5 mA/cm2, respectively. Consequently, the opto-electronic SiO2 NMs contribute to an absolute efficiency improvement of 2.6% (relatively 30%). EXPERIMENTAL SECTION

Fig. 1 Schematic illustration of fabrication process flow of SiO2 NMs using nanosphere lithography method


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SiO2 nanomesh preparation. Fig. 1 illustrates the process flow for the SiO2 NM fabrication using NSL. Hexagonally closely packed polystyrene (PS) nanospheres are first obtained at an air/water interface using self-assembly techniques,30 and Mo substrates are submerged under the monosphere layer, which is then transferred to the substrates by sucking out water. Subsequently plamsa etching treatment is done at an O2 bias pressure of 0.2 mbar to reduce the PS diameter from the original 900 nm down to 630 nm for usage as a mask. This is followed by a direct thermal evaporation of SiO2 on the rotating PS-covered substrates. Finally, the reduced PS spheres are lifted off by ultrasonication in Toluene for 30 min to form SiO2 NMs. Scanning electron microscopy (SEM) images for each step are shown in supporting Figure S1. Solar cell preparation and characterization. Ultrathin CIGSe absorbers are prepared on Mo substrates by the 3-stage co-evaporation process.31 For a high back Ga grading, the Ga-Se precursor is deposited before the In-Se during the 1st stage. Cu-Se and In-Ga-Se precursors are evaporated during the 2nd and 3rd stage, respectively. Substrate temperature is 450 °C, which is significantly lower than the nominally used high temperature at 550 °C. After CIGSe deposition, a 50 nm chemical bath deposition prepared CdS layer is grown on top. Subsequently, a sputtered 130 nm i-ZnO and a 240 nm ZnO:Al layer are coated. The Ni/Al front contact grid is evaporated through a shadow mask for cell completion. Finally, cells are mechanically scribed into 0.5 cm2 for electrical characterization. The absorbers are characterized by X-ray fluorescence analysis (XRF) with an overall [Ga]/[Ga+In] of 0.40, [Cu]/[Ga+In] of 0.94 and a thickness of 370 nm. Scanning electron microscopy (SEM) is used for the morphology characterization of solar cells and nanostructures and atomic force microscope (AFM) is specially for the surface scan of SiO2 NMs, UV-Vis spectrometry is for reflection measurement. Glow discharge optical emission spectrometry


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(GDOES) is used to determine the [Ga]/[Ga+In] depth profiles of CIGSe absorbers. The current density-voltage (J-V) curves are measured under standard test conditions (AM 1.5; 100 mW/cm2; 25°C) by a sun-simulator containing both a Xenon and a Halogen lamp. The external quantum efficiency (EQE) is measured using calibrated Si and Ge diodes as references. Finite element method (FEM) optical simulation. 3D FEM simulations were applied to investigate the optical effects of SiO2 NMs on the CIGSe solar cells using software package JCMsuite.32 The unit simulation structure contains a single mesh at the CIGSe/Mo interface in combination with periodic boundary conditions in hexagonal order. At the top and bottom of the simulation box perfectly matched layers (PMLs) are applied to absorb the light leaving the simulation volume. The layer structure and thicknesses are according to the experimental samples. Optical constants of all layers (except Mo) in solar cells are extracted using transfer matrix method,33 Mo is from ellipsometry method and index of SiO2 is set to 1.5. RESULT AND DISCUSSION

Fig. 2(a) Photography of the SiO2 NMs on a 2.5*2.5 cm2 Mo substrate; (b) the corresponding atomic force microscope (AFM) image and (c) height profiles of Line 1 and 2 in (b)


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SiO2 nanomeshes. Fig. 2(a) shows a photograph of SiO2 NMs on a Mo substrate with the distinctive shiny surface confirming the presence of the NMs. The as-prepared SiO2 NMs shown here is only on a 2.5*2.5 cm2 substrate. However, the area of SiO2 NMs is scalable for industrial module production since the highly ordered PS nanospheres can be readily extended to the size of meters.23,24 The representative atomic force microscope (AFM) image in Fig. 2(b) depicts the ordered nanostructures, where the SiO2 meshes take on the hexagonal periodicity of the (now removed) PS spheres. Interestingly, we observe that the SiO2 framework is surrounding each single mesh (remaining Mo surface) with six higher peaks, each of which corresponds to the gap between three neighbouring PS spheres. This phenomenon is specifically correlated to our preparation process: The SiO2 thermal evaporation source is at an oblique angle below the sample surface. This means that the PS spheres will mask a larger area than their geometrical cross sections. The largest interspace between PS particles is where three particles meet, meaning that shadowing effects are reduced compared to the interspace between two particles. Simultaneously, due to the rotation of substrate, the oblique incidence allows SiO2 material being deposited in the area under the vertical shadow of the PS spheres. This explains the morphological feature of the area of a single NM being smaller in diameter (350 nm) than an etched PS sphere (630 nm), reducing the area coverage of bare Mo to below 20% (calculated using the image-processing software “Image J”). Fig. 2(c) extracts two height profiles, passing the gaps among three neighbouring PS spheres (Line 1, solid) and the ones between two spheres (Line 2, dash), respectively. Line 1 exhibits a maximum peak-to-valley height (h) of around 150 nm, approximately 30 nm higher than Line 2. Cell performance. To investigate the opto-electronic effects of SiO2 NMs for ultrathin solar cells, cells with an absorber thickness of 370 nm were fabricated on the SiO2 patterned Mo


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substrates. NMs-1 is for SiO2 NMs with a maximum h of 40 nm and NMs-2 is for h = 150 nm, which is the sample shown in Fig. 2. Fig. 3(a) presents scanning electron microscope (SEM) cross sections of the nanopatterned CIGSe solar cells. We can observe that the other layers (ZnO:Al/ZnO/CdS/CIGSe) show a conformal growth on top of the SiO2 NMs, which is more obvious for NMs-2 with higher meshes. The CIGSe absorber fully fills the meshes, which geometrically points to a good electrical contact. Fig. 3(b) compares the current density-voltage (J-V) parameters between flat and SiO2 NM patterned cells. From flat cells to NMs-1, patterning contributes to an obvious Jsc increase from 23.7 to 25.5 mA/cm2. Simultaneously, Voc also significantly improves from 532 to 564 mV and fill factor (FF) stays nearly the same. The improved cell performance releases the concern of possible negative effects from the NMs on the opto-electronic properties of cells. The SiO2 NMs prolong the transporting length of holes to the Mo contact and due to defects in the PS monolayer the length may become even longer. However, this will not hinder the hole collection and deteriorate opto-electronic properties of cells, because CIGSe solar cells exhibit an excellent hole diffusion length (> 10µm),34 which is much larger than the distance between the two neighbouring meshes. As h increases to 150 nm, NMs-2 shows a further Jsc enhancement of 2.0 mA/cm2 (25.5 to 27.5 mA/cm2) and Voc gain of 25 mV (from 564 to 589 mV). FF also shows a slight increase from 69.1% to 70.3%. Consequently, SiO2 NMs give rise to a relative efficiency improvement by 30% from flat cells to NMs-2 (8.8% to 11.4%).


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Fig. 3(a) Scanning electron microscope (SEM) cross sections of SiO2 NM patterned ultrathin CIGSe solar cells and (b) current density-voltage (J-V) parameters and (c) external quantum efficiency (EQE) of flat and patterned cells Electrical benefit. According to the simple one-diode equation,35 the expected Voc gain from the increased Jsc can be estimated as follows:

ln

(1)

where A is the diode ideality factor, k is the Boltzmann constant, q is the electron charge, Ea is the activation energy of the dominant recombination mechanism, J00 represents the prefactor of saturation current density, respectively. We start analyzing the Voc gain by comparing NMs-1


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to the flat cells. Assuming the full responsibility of Jsc increase (from 23.7 to 25.5 mA/cm2) for the Voc gain, the patterning can give rise to a maximum Voc increase of around 10 mV at room temperature, where A is set to the maximum value of 2 for thermally activated recombination. However, the experimental Voc gain reaches as high as 32 mV, much larger than the calculated value. This points to other responsible reasons for the Voc gain. As stressed above, the CIGSe/Mo back interface is within the diffusion length of carriers, ultrathin CIGSe solar cells exhibit serious back contact recombination, which deteriorates Voc.6,8 It was demonstrated that a point contact structure, made of a thin planar intermediate layer with nano-openings at the CIGSe/Mo interface, was able to passivate the back contact recombination by minimizing the CIGSe/Mo interface recombination velocity.8 The NMs significantly reduce the contacting area between CIGSe and Mo (less than 20%, see Fig. 2(b)) and leave the mesh area as a channel for carrier collection, which is a point contact structure. Therefore, it is expected, from flat cells to NMs-1, the passivation of back contact recombination at CIGSe/Mo mainly contributes to the Voc increase. Additionally, the restrained CIGSe/Mo back contact recombination is able to benefit Jsc as well because the recombination probability of photogenerated carriers is reduced at CIGSe/Mo.7 This is directly reflected by the EQE increase in the wavelengths below 500 nm, where light absorption is complete even for the ultrathin CIGSe absorbers in this work. The EQE increase in this wavelength range is then assumed to originate from the improved carrier collection arising from back contact passivation effect over absorption enhancement. Notably, when h increases from 40 nm (NMs-1) to 150 nm (NMs-2), there exists a further 25 mV gain in Voc. However, it is speculated that back contact passivation is not responsible because the contacting area of CIGSe/Mo between NMs-1 and NMs-2 is equal and extra back


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contact passivation effect is not expected.

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We took Glowing Discharge Optical Emission

Spectrometry (GDOES) measurements to characterize the Ga/[Ga+In] depth profiles of flat and patterned absorbers and show them in Fig. 4(a). All depth profiles show a characteristic double Ga/[Ga+In] grading towards both front and back with a minimum Ga/[Ga+In] ratio in between (corresponding to Eg,min). CIGSe bandgaps approximately linearly depend on Ga/[Ga+In] ratio from 1.0 eV (CISe) to 1.7 eV (CGSe), in terms of an increase of conductance band. This is why a back Ga/[Ga+In]

grading can generate a potential for restraining electrons from diffusing

towards the Mo back contact and thus reduce back contact recombination.4-7 It is noted here: a back potential of 0.2 eV due to a Ga/[Ga+In] grading is adequate to reduce back contact recombination to a marginal level for thick solar cells.36 Our ultrathin absorbers, showing a steep back Ga/[Ga+In] grading corresponding to a back potential of 0.4 eV, still exhibit back contact passivation effect with the addition of the SiO2 NMs. This, on the one hand, confirms the serious back contact recombination in ultrathin CIGSe solar cells, on the other hand indicates the back potential from a Ga/[Ga+In] grading is not sufficient to restrain the back contact recombination in ultrathin CIGSe solar cells. The Ga/[Ga+In] profile of NMs-1 (blue) is almost identical to that of the flat reference (black), the deviation is within the measurement error. Yet, the absorber NMs-2 (red) shows a relatively flatter Ga/[Ga+In] profile with a higher minimum Ga/[Ga+In] value than the flat absorber and NMs-1, which indicates a higher Eg,min for the absorber NMs-2. During the 3-stage CIGSe deposition process, Ga-Se is intentionally evaporated prior to In-Se in the first stage for a steep back Ga/[Ga+In] grading. (see experimental section for cell preparation details).

As

sketched in Fig. 4(b), on flat substrate, this intentional deposition sequence creates a vertical GaIn gradient and the substrate temperature (450 °C) triggers the inter-diffusion of Ga-In, but is not


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high enough for a homogenous inter-mixing. This is how the steep back Ga/[Ga+In] grading was generated. However, on the patterned substrate, there is a second Ga-in gradient, which is in lateral direction out of the height of NMs. This promotes the inter-diffusion of Ga-In and explains a higher minimum Ga/[Ga+In] ratio for NMs-2. According to Equation 1, Ea is equal to Eg,min. This signifies a higher Voc for NMs-2 cells and explains the further Voc gain from NMs-1 to NMs-2.

Fig. 4(a) Ga/[Ga+In] depth profiles of CIGSe absorbers determined by Glowing Discharge Optical Emission Spectroscope (GDOES) and (b) sketches of Ga-In inter-diffusion on flat and patterned substrate


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Optical benefit. The SiO2 NMs are also able to optically benefit solar cells. Since absorption in each individual layer couldn’t be measured experimentally, we perform 3D simulations to demonstrate the optical benefit of NMs. The cross section of the unit simulation structure is illustrated in Fig. 5(a). To describe the SiO2 meshes in the simulation structure, we approximate the shape of a single mesh as the remaining part of a hemisphere cut by the underlying Mo layer: the diameter (2r) of the contacting area is constant and set to the experimentally determined 350 nm; h is 40 nm and 150 nm, which respectively corresponds to NMs-1 and NMs-2. The NMs are following the experimental geometry with a hexagonal array and a pitch of 900 nm. The volume of the patterned CIGSe absorber is set equally to the flat case and flat interfaces are assumed. We here only intend to demonstrate the optical benefit of SiO2 NMs to the absorption in the absorber (AbsCIGSe) rather than absolute accuracy to experiments, the approximations made for simulations are therefore assumed to be reasonable. Fig. 5(b) plots AbsCIGSe, AbsMo and 1-R. Absorption in ZnO:Al/ZnO/CdS exhibits little change and is not shown here. Despite of a slight AbsCIGSe reduction in the narrow wavelength range of 800-850 nm, due to the shift of Fabry–Pérot interferences, NMs-1 shows an obvious overall AbsCIGSe enhancement beyond the wavelength of 500 nm, where incomplete absorption starts in the bare ultrathin cells. Comparing 1-R and AbsMo between flat and NMs-patterned cells, it is deduced that the AbsCIGSe enhancement is mainly originating from the reduction of AbsMo rather than R reduction. Actually, the parasitic AbsMo is the main optical loss for ultrathin CIGSe solar cells and light reaching Mo will be dominantly absorbed rather than being reflected back into CIGSe absorbers.9 To understand the light trapping mechanism, AbsCIGSe for the case with a flat SiO2 layer is simulated for comparison (see supporting Figure S2(a)). For the case of 40 nm SiO2 height, the flat layer gives almost identical AbsCIGSe to the NMs-1. This implies that it is the low index of SiO2 (thus big refractive index


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contrast to CIGSe), which enhances the interface scattering at CIGSe/Mo and reflects a higher fraction of light back into the CIGSe absorbers.

For the higher NMs-2, Fabry–Pérot

interferences are red-shifted, giving a more pronounced AbsCIGSe enhancement with a further reduction of AbsMo. In addition, the absorption peaks observed at the wavelength of 1080 nm and 1150 nm are linearly shifting as the pitch changes, which are correlated to waveguide modes (see supporting Figure S2(b)). Overall, the trend of AbsCIGSe enhancement is agreeable with the experimental EQE results shown in Fig. 3(c).

Fig. 5(a) Cross section of the patterned cell in the finite element method (FEM) simulation and (b) simulated optical responses (Abs, 1-R) of ultrathin CIGSe solar cells (* indicates the volume of the patterned absorber equivalent to the flat reference at the thickness of 370 nm)


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Combined with the above-analyzed electrical benefits from NMs, it is implied that the experimental Jsc (or EQE) increase from flat cells to NMs-1 is the joint benefit of back contact recombination passivation and absorption enhancement out of the SiO2 NMs, and the increase from NMs-1 to NMs-2 is mainly due to the optical benefit resulting from the increase of NM height. CONCLUSION In this work, we fabricated opto-electronic 2D SiO2 NMs directly on Mo back contact for ultrathin CIGSe solar cells using the simple colloidal nanosphere lithography. Integration of SiO2 NMs with a peak-to-valley height of 150 nm contributes to a Voc increase of 57 mV and a Jsc increase of 3.5 mA/cm2, leading to an absolute efficiency enhancement of 2.6% (relatively 30%) for the ultrathin CIGSe solar cells with an absorber thickness of only 370 nm and a high back Ga/[Ga+In] grading. It is verified that the benefits are simultaneously stemming from NM induced electrical (back contact recombination passivation, Eg,min increase) and optical (AbsCIGSe enhancement by reducing AbsMo) benefits. This proves that NSL is a reliable method to prepare SiO2 NMs for opto-electronic enhancement in ultrathin CIGSe solar cells. AUTHOR INFORMATION *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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The authors would like to thank B. Heidmann, M. Kirsch and J. Albert for technical support, Y. Bao for plottig the 3D images, T. Kodallef for GDOES measurements, W. Raja for reading the paper and Berlin Joint Lab for Optical Simulations for Energy Research (BerOSE) for FEM simulations. The authors acknowledge the funding from the Helmholtz-Association for Young Investigator groups within the Initiative and Networking fund (VH-NG-928). REFERENCES (1)

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