Patent Grant
11476378
Aspects and embodiments relate generally to a novel solar cell, methods for making said solar cell, and applications thereof; more particularly to a layered/stacked solar energy collection/conversion apparatus substantially comprising a layered solar cell integrated into a layered coherent-perfect-absorption (CPA), omni-resonant optical cavity, methods for making, and applications thereof, enabling a significant boost (e.g., up to 90%) of the photocurrent generated in an 80 nm NIR region from 660 to 740 nm as compared to a bare solar cell. The enabled wavelength-selective, thin-film PV structure can be useful, for example, in building-integrated energy applications such as transparent windows that harvest NIR solar energy.
The promise held by solar energy as a renewable resource has fueled investigations of all the aspects that may improve the performance of photovoltaic cells. A variety of approaches have been pursued, including surface patterning or modifying the local density of states to enhance light trapping beyond the ray-optic limit, multi junction solar cells, band-splitting, and exploiting plasmonic structures. Thin-film solar-cell technologies lower the cost and weight by utilizing less material, and offer additional benefits of mechanical flexibility coupled with the potential for low-cost manufacturing on large foil substrates via roll-to-roll processing. The inevitable diminishing of optical absorption associated with reduced-thickness solar cells can be overcome by exploiting coherent resonant optical effects that boost the absorption. For example, coherent perfect absorption (CPA) ensures unconditional full absorption of light in a thin film of arbitrarily low intrinsic optical absorption by embedding it in a judiciously structured photonic environment. Indeed, spectrally flat 100% absorption over a full octave of bandwidth in a 2 μm thick polycrystalline silicon film incorporated into a carefully designed planar cavity has been recently reported. However, like most coherent-enhancement schemes, CPA is achieved only at discrete resonant wavelengths. Unavoidably, resonant enhancements within an optical cavity are harnessed over only narrow bandwidths as a result of the fundamental link between the cavity-photon lifetime and the resonant linewidth.
The inventors have recognized the benefits and advantages achievable by an omni-resonance scheme that could extend CPA continuously over a broad spectral bandwidth, thereby rendering this coherent-enhancement arrangement relevant for solar conversion applications. By severing the link between the cavity-photon lifetime and the resonant bandwidth, omni-resonance would lead to the emergence of achromatic resonances that deliver coherently enhanced broadband absorption. The embodiments and aspects described hereinbelow and in the appended claims enable the realization of these benefits and advantages.
A non-limiting embodiment of the invention is a PIN-diode solar cell. A related non-limiting embodiment is a solar energy apparatus including a PIN-diode solar cell incorporated into an unpatterned asymmetric planar Fabry-Pérot cavity produced via standard deposition technology, scalable in principle to large areas. Realizing omni-resonance does not necessitate modifying the cavity structure as in previous work on ‘white-light’ cavities. Rather, incident light is preconditioned using an alignment-free optical arrangement that assigns to each wavelength an appropriate angle of incidence. We introduce into the external field angular dispersion that is equal in magnitude but opposite in sign to that of the cavity. Consequently, angular dispersion cancellation allows all wavelengths within a selected band, which may extend over multiple free spectral ranges of the cavity, to resonate simultaneously, resulting in continuous-wavelength CPA and a concomitant boost in the external quantum efficiency (EQE) of the apparatus. The embodied combination of CPA and omni-resonance in a non-limiting aspect leads to a doubling in the photocurrent harvested in the near-infrared spectral range from 660-740 nm. In a non-limiting aspect, we make use of a thin-film a-Si:H solar cell; however, the combination of CPA and omni-resonance is materials-agnostic and can be exploited with any other form of absorbing photonic structure, such as organic photovoltaics, two-dimensional materials, and perovskites. Complete broadband optical absorption is ensured in the solar cell regardless of its thickness or its intrinsic absorption. This result fulfills a long-standing goal in optical physics whereby the inherent advantages of coherently enhanced optical effects normally restricted to discrete wavelengths is harnessed over a broad continuous spectrum.
An aspect of the invention is a solar cell. In an exemplary, non-limiting embodiment the solar cell includes a top transparent conducting electrode layer; a bottom transparent conducting electrode layer; and a thin-film P-I-N diode disposed intermediate the top and bottom transparent conducting electrode layers. Additional non-limiting embodiments may contain one or more of the following limitations, features, components, characteristics, or adaptations, alone or in combinations as a person skilled in the art would understand:
An aspect of the invention is a layered solar-energy capture and conversion apparatus. In an exemplary, non-limiting embodiment the layered solar-energy capture and conversion apparatus includes a solar cell comprising a top transparent conducting electrode layer, a bottom transparent conducting electrode layer, and a thin-film P-I-N diode disposed intermediate the top and bottom transparent conducting electrode layers; a coherent-perfect-absorption (CPA), omni-resonant optical cavity, wherein the solar cell and the CPA cavity are operationally disposed in a stacked/layered arrangement; and a planar light pre-conditioner disposed on a light input side of the solar cell adapted to intercept broadband incident radiation and assign individual wavelengths to a selected incidence angle into the CPA cavity. Additional non-limiting embodiments may contain one or more of the following limitations, features, components, characteristics, or adaptations, alone or in combinations as a person skilled in the art would understand:
(λ, θ) in
A non-limiting, illustrative solar energy harvesting and conversion apparatus 100 is illustrated in
In the illustrated non-limiting embodiment, the solar cell 102 is sandwiched between a back-reflector 112 formed of a dielectric Bragg mirror deposited on a glass substrate (for support) 113 providing a broad spectral reflection bandwidth maintained over a large angular span, and a dielectric spacer 115 and a partially reflective dielectric front Bragg mirror 116. The back-reflector and front mirror define a planar Fabry-Pérot (FP) cavity whose resonances are determined by the thicknesses and refractive indices of the solar cell and spacer. This cavity is designed to realize the conditions for coherent perfect absorption (CPA), whereupon complete optical absorption occurs in the solar cell regardless of its intrinsic absorption.
CPA is achieved only on resonance, and light at intermediate wavelengths reflects back from the cavity. To extend the impact of resonantly enhanced absorption to a broad continuous spectral band, the aspects and embodiments exploit the phenomenon of omni-resonance, whereby incident broadband light is pre-conditioned by assigning each wavelength of broadband incident light to a prescribed angle of incidence via a tailored dispersive construction. In effect, by endowing the incident radiation with judicious angular-spectral correlations, the structure effectively becomes a so-called ‘white-light cavity’ in which the resonant linewidth is no longer related to the photon lifetime and can extend instead far beyond the ‘bare cavity’ linewidth. All the wavelengths across an omni-resonant bandwidth enter the cavity and are fully absorbed, and the external quantum efficiency (EQE) of the solar cell is consequently enhanced over this spectrum. We have designed the system described here to increase the near-infrared EQE in the vicinity of the electronic bandgap edge of a:Si—H. An example of a fabricated device 120 is shown in
It is to be further especially noted that the illustrated silica spacer and Bragg front mirror are not exclusively required to form the CPA cavity. An enabling omni-resonant, CPA cavity can be realized by a CPA cavity formed by the back reflecting mirror 112 and the upper transparent conducting electrode layer 105 which, in combination with an appropriate light pre-conditioner 130 forms omni-resonant, CPA cavity. For the purpose of clear discussion, the description hereinbelow will refer to the omni-resonant, CPA cavity comprising a silica spacer and Bragg front mirror.
The solar device illustrated in
boosting the photocurrent. The omni-resonant bandwidth is not related to the photon lifetime and can extend far beyond the cavity linewidth. We have designed the exemplary system described here to increase the near-infrared EQE in the vicinity of the electronic bandgap edge of a-Si:H in the spectral range 660-740 nm, but the same strategy can be exploited with other material systems and in any spectral band.
An optically thin layer having low absorption <<1 can always be made to absorb at least 50% of the incident light by engineering its photonic environment, a configuration also known as a Salisbury screen. In the case of a layer of non-negligible thickness, a maximal absorption of
tot=⅛{2+
+
−1}, is achieved in a symmetric cavity (½≤
tot≤⅔) when both mirrors have a reflectivity R=3
−1/
(3−
), where
=1−
. CPA allows for
tot to be raised unconditionally to 100% (independently of
) by arranging for the field to impinge on both sides of the symmetric device after setting the mirrors' reflectivity to R=
and—critically—adjusting the relative phase and amplitude of the two fields. This interferometric configuration is not suitable for solar applications where it is prohibitively difficult to maintain a spectrally varying relative phase between two beams of sunlight. However, complete absorption (
tot=1) can nevertheless be achieved with a single incident field by utilizing an asymmetric structure comprising a back-reflector and a partially reflective front mirror having R=
2 (
The above description of CPA applies only to the resonant free-space wavelengths λm=2nd/m at normal incidence, where m is the resonance order, d is the cavity thickness, and n is its refractive index (
To gain insight into the construction of the required light preconditioning system, we plot in
We now discuss the feasibility of manufacturing such a resonant structure and the impact of fabrication tolerances on its performance. The fabrication steps are outlined in
A transparent conductive oxide layer of aluminum-doped zinc oxide (AZO) is deposited via atomic layer deposition (ALD) at 150° C. as a back contact (step 3). The PIN junction was produced using silane gas (SiH4) and H2 to deposit reduced a-Si:H, and dope the layers accordingly, via plasma-enhanced chemical vapor deposition (PECVD operating at 13.56 MHz; step 4). To enable contacting the bottom AZO layer, we lithographically etch the corners of the a:Si—H layers to expose the AZO (step 5) before depositing an AZO layer via ALD at 130° C. as a top contact (step 6), followed by sputtering a thin gold layer (step 7). We lithographically define an outer ring of gold for contact with the bottom AZO layer and a central feature in the form of a half-circle for contacting the top AZO layer (step 8). The top and bottom AZO layers are joined and envelope the PIN-diode. However, because the vertical dimensions of the layers are orders-of magnitude smaller than the transverse dimensions, etching a trench into the top AZO layer separates the top and bottom contacts and defines the active area (step 9). At this stage we have a fully functional solar cell atop a back-reflector. In a final step, we deposit a silica spacer (of thickness 2, 4, or 1 μm) and a second dielectric Bragg mirror on the solar-cell active area (step 10). Because the optical absorption in the cavity is wavelength-dependent, the required mirror reflectivity must in turn vary with wavelength R=R(λ)=[
(λ)]2, which necessitates an aperiodic multilayer structure. Here we make use of a partially reflective dielectric Bragg mirror that provides a best fit and comprises 3 bilayers of SiO2 and TiO2. Details of the fabrication process and optical characterization of the individual layers is further described below. The thickness of the PIN-diode is ˜360 nm, each contact ˜300 nm, the back-reflector ˜5.7 μm, and front mirror ˜612 nm. The device thickness is therefore dominated by the spacer and back-reflector.
To establish the operating baseline for absorption and EQE, we first characterize a bare solar cell (
To assess the angular dispersion required to achieve omni-resonance, we trace the resonant trajectories in the angular-spectral domain; i.e., the absorption spectrum for all incidence angles tot(λ, θ). The calculated and measured absorption spectra are plotted in
tot=1−Rtot−Ttot. We note the excellent agreement between the calculated and measured spectral trajectories of the resonances and thence the associated angular dispersion β. Moreover, as predicted, increasing the thickness of the dielectric spacer reduces the slope of the spectral trajectories β of the resonances and also reduces the free spectral range.
To achieve omni-resonance, we make use of the measured value of β for a selected resonance, and design a preconditioning system that introduces angular dispersion −β into the field. However, no known optical component can endow broadband light with the desiderata for omni-resonance: angular dispersion that is anomalous and has a large magnitude. A grating typically introduces normal angular dispersion whose magnitude is limited by the ruling density. Nevertheless, tailoring the geometry by tilting the grating with respect to the incident radiation and tilting the cavity with respect to the grating can switch the angular dispersion from normal to anomalous, and a lens can subsequently increase the magnitude of β. We construct such a system with λm≈710 nm to guarantee that each wavelength λ is incident on the cavity at the angle θ(λ) satisfying the omni-resonance condition for specific values of the cavity tilt angle ψ (tot(λ, ψ) shown in
tot(λ, ψ) after implementing the light-preconditioning system in the path of incident light is shown in
We demonstrated that the CPA-enhanced solar device does indeed improve the EQE over the omni-resonant bandwidth through spectrally and angularly resolved EQE measurements. First, using the setup shown in
To highlight the improvement in EQE as a result of the omni-resonant CPA-enhanced solar device, we compare in
This proof-of-principle demonstration validates the feasibility of omni-resonant spectral broadening of CPA beyond the cavity resonance linewidth. However, the particular realization of light preconditioning utilized (diffraction grating plus lens;
Because the role of the lens is to ‘amplify’ the grating's angular dispersion, one may replace the lens with an appropriately tilted second grating (
Our approach offers a new avenue for improving absorption in solar cells that is distinct from light trapping via surface patterning. Fully deterministic coherent enhancement is harnessed in our strategy for incoherent radiation without the traditional drawback of narrow resonant bandwidths. The planar structure we designed and realized exploits a variant of CPA to maximize optical absorption in a multi-layer thin-film structured cavity configuration rather than the previously reported single-absorbing-layer arrangements. Therefore, using purely optical methods that are independent of the specific material system utilized, and without changing the structure of the thin-film solar cell itself, we boost the near-infrared EQE. This capability, which has been demonstrated to date only at specific resonant wavelengths, is extended here via omni-resonance over a continuous targeted bandwidth of 80 nm using a single incoherent optical field in a non-interferometric scheme. This is an enabling step towards utilizing CPA and other resonantly enhanced effects—inherently restricted to discrete wavelengths—in practical energy-harvesting technologies by delivering coherent enhancements over a broad bandwidth. Although CPA guarantees that light is fully absorbed in the solar cell independently of the intrinsic absorption of the material or its thickness, not all the absorbed photons will yield free carriers that contribute to the usable electrical energy. Note that our proof-of-principle system (particularly the back-reflector) was designed to optimize optical absorption in the near-infrared over the spectral range 660-740 nm. We continue to investigate the utility of the combination of CPA and omni-resonance over the visible and near-infrared, rather than only the near-infrared as demonstrated here and, as a potential route to transparent solar cells, we are exploring thinner solar cells with even less absorption in the visible—thus rendering the cell more transparent—to determine the maximum possible boost in the near-infrared photocurrent.
It remains an open question whether a single textured surface (e.g., a metasurface) can provide the requisite angular dispersion to achieve omni-resonance in lieu of the multi-surface systems shown in
Relying on a spectrally selective all-dielectric back-reflector paves the way to thin-film solar-cell devices that are almost transparent except for a specific spectral band (e.g., the infrared) in which optical absorption is maximized. This may establish the viability of transparent solar cells in building-integrated photovoltaics and automotive applications, for instance. These applications will benefit from developing a metasurface realization of the light preconditioning system for omni-resonance that further reduces the thickness of the microprism-based system shown in
We note that the underlying principle of omni-resonance is in the exploitation of correlations introduced into the spatio-temporal spectrum (angle-wavelength) of the incident radiation. This approach therefore falls under the rubric of space-time wave packets, which are propagation-invariant (diffraction-free and dispersion-free), coherent, or incoherent fields whose unique characteristics stem from introducing angular dispersion into their spatio-temporal spectrum to compensate for the angular dispersion intrinsic to free propagation. In our work here, the angular dispersion introduced into the field compensates for that intrinsic to resonant cavity modes. More generally, both omni-resonance and space-time wave packets are ultimately examples of the utility that can be harnessed by ‘entangling’ different degrees of freedom of the optical field rather than remaining independent of each other.
Methods
Structure of the SiO2/TiO2 Bragg Reflector
The back-reflector is a dual band mirror comprising a sequence of two Bragg mirrors, each consisting of 13 bilayers of SiO2 (L: low refractive index) and TiO2 (H: high refractive index). Starting from the substrate, the first Bragg mirror has 13×[115.8 nm (L), 76.2 nm (H)] for lower wavelengths, and 115.8 nm (L), 13×[92.9 nm (H), 141. nm (L)] for the higher wavelengths.
Angular and Spectrally Resolved Optical Absorption Measurements of the CPA-Enhanced Solar Device
Measuring the angular-resolved spectral absorption (λ, θ) is key to estimating the required angular dispersion for omni-resonance. To measure
(λ, θ), we measure the normalized transmission Ttot and reflection Rtot, from which we have
(λ, θ)=1−Ttot−Rtot, as given in
Omni-Resonance Optical Absorption Measurement
The required angular dispersion for the 2, 4, and 10 μm spacer thicknesses are 0.41, 0.35, and 0.31°/nm, respectively. To introduce this angular dispersion into the incident radiation, we use a diffraction grating (Thorlabs GR25-1208; 1200 lines/mm, area 25×25 mm2) that introduces an angular dispersion of ˜0.07°/nm for an angle of incidence of 44° and then ‘magnify’ this angular dispersion via a spherical lens. The required magnification factor MM is 5.9, 5.1 and 4.3 for the 2, 4, and 10 μm spacer thicknesses, respectively. If the distance from the grating to the lens is d1 and the lens focal length is f, then a magnification factor of MM is realized when d1=(M+1)f. We thus implement distances d1 of 172.5, 152.5 and 132.5 mm for the 2, 4, and 10 μm spacer thicknesses, respectively. The sample is placed in the image plane, at a distance d2=d1/M from the lens. Because omni-resonance occurs at particular values of the cavity tilt angle ψ, we mount the sample on a rotational stage (Thorlabs MSRP01) to alter ψ.
The system is designed such that the wavelength 710 nm lies along the optical axis of the imaging system (so that the angle ψ is measured with respect to it. The lens aperture of 25 mm limits the collected bandwidth of the beam reaching the sample to ˜660-760 nm. The back-reflector eliminates transmission through the device in this spectral window, so that Ttot≈0. By measuring Rtot for different tilt angles ψ, we obtain the absorption (λ, ψ)=1−Rtot, as given in
External Quantum Efficiency Measurements
The EQE is measured using a commercial tool (model QEX10) designed for photovoltaic measurements. An ellipsoidal reflector focuses light from a Xenon-arc lamp onto the entrance slit of a dual-grating computer-controlled monochromator (Monochromator 22535 NOVRAM) via a mechanical chopper that modulates the light at a 100 Hz frequency and provides a reference signal to the digital lock-in amplifier. The selected wavelength emerges from the monochromator output slit, and bandpass filters attenuate stray and harmonic light. A portion of the monochromatic light is diverted by a beam splitter to a monitoring photodiode via a lens. The light that passes through the beam splitter is focused by a concave mirror onto the device under test. Integral to the measurement setup is a reference detector whose EQE is calibrated against a NIST reference. A typical EQE measurement is carried out in the following order. First, the reference detector is mounted in lieu of the device under test, and the photocurrent measured at each wavelength is used to assign an EQE value to subsequently measured photocurrents by the device at the same wavelength. Second, the calibration detector is replaced by the device under test and the photocurrent is recorded at each wavelength, and the EQE is estimated in reference to the photocurrent measured in the first (calibration) step. The measured current from the monitoring photodiode is used to compensate for any drift arising from the fluctuations of the lamp's power.
The angular-resolved spectral EQE measurements reported in
The omni-resonance EQE measurements reported in
Angular-Dispersion Measurement
We have measured the angular dispersion produced by four different light preconditioning configurations:
(1) Grating-lens configuration: This is the system described above where a diffraction grating is placed a distance d1 from a spherical lens of focal length f (
(2) A pair of tilted gratings: The two identical transmissive gratings have 1400 lines/mm and an area of 24×24 mm2 (LightSmyth T-1400-800s Series, polarization-independent, 95% maximum diffraction efficiency in the wavelength range of interest), are tilted by an angle 48.7° with respect to each other, and light is incident on the first grating at an angle 23.4° (
(3) A combination of a pair of parallel gratings and a prism: The same pair of transmissive gratings from the previous configuration are used with a polymer prism sandwiched between them (
(4) A combination of a pair of parallel gratings and a microprism array: The same pair of transmissive gratings from the previous configuration are used with a PMMA microprism array sandwiched between them (
In each configuration, we measure the angular dispersion using a cylindrical lens (250 mm focal length, 60 mm width) in a 2f configuration, in which the angle of each wavelength is transformed into a lateral displacement l (with respect to the position of the center wavelength of 710 nm) in the lens focal plane. Over the 100 nm bandwidth of interest (660-760 nm), the angle θ(λ) associated with a wavelength λ is θ(λ)=sin−1(l/f). To measure l in the focal plane, a fiber (Thorlabs M69L02; 300 μm-diameter, 0.39 NA) is displaced along a rail in the focal plane. Detailed layouts of these experimental arrangements are further described below.
Design of the Fabry-Pérot Cavity for Coherent Perfect Absorption
The asymmetric Fabry-Pérot cavity designed to satisfy the conditions for one-sided coherent perfect absorption (CPA) consists of a back-reflector MB and a front mirror MF. The reflectivity of the back mirror is targeted to be unity over the wavelength range of interest to be maintained over a large range of incident angles. The structure of MF is described hereinabove, and simulations of this structure are shown in
For such an asymmetric cavity, CPA requires that the reflectivity of the front mirror be R1=(1−)2, where
is the single-pass absorption through the cavity. Note that R1 is the internal reflectivity of the front mirror for light incident from within the cavity, which may differ significantly from the reflectivity of the same mirror when light is incident from air. Furthermore, the structure incorporated into the cavity (the PIN-diode with AZO contacts and the silica spacer) is characterized by a wavelength-dependent absorption
(λ), which thus necessitates a wavelength dependent front-mirror reflectivity R1(λ)=(1−
(λ))2. If the absorption in each layer within the cavity is
j=1−e−2
(λ)=
+
+
+
+
, where
=1−
j. Using the measured optical parameters of each layer we calculate
(λ) and thence the front-mirror reflectivity R1(λ), which we plot in
Constructing a multilayer dielectric mirror requires an aperiodic structure. Indeed, a 7-layer aperiodic mirror consisting of alternating layers of SiO2 and TiO2 can reproduce the targeted reflectivity, validated with the modeling software package FilmStar (FTG Software Associates). Because aperiodic structures place stringent limits on fabrication tolerances, we found the best fit to the targeted reflectivity R1(λ) that is produced by a periodic Bragg mirror structure comprising three periodic bi-layers (
The absorption (λ, θ) plotted in
Solar Cell Fabrication
We describe here in detail the fabrication steps (depicted in
1. Cleaning of the Substrates
The glass substrates (thickness 1 mm) were cleaned by sonication in acetone for 5 min, followed by sonication in methanol for 5 min, dipping in IPA, and rinsing of each substrate individually with more IPA. The substrates are then nitrogen-dried, followed by oxygen-plasma cleaning (40 SCCM of O2 and a 100 W RF power for 4 min). Approximate step-time is 1 h for 8 samples.
2. Deposition of the Back-Reflector
The dual-band multilayer dielectric Bragg back-reflector is deposited via e-beam evaporation at 200° C. The substrates with deposited back-reflector were cleaned following the same procedure described in step 1.
3. Deposition of Bottom AZO Using an ALD Tool
The bottom transparent conductive oxide (TCO) layer of aluminum-doped zinc oxide (AZO) is deposited at 150° C. This temperature is lower than traditionally used, and was selected to avoid damaging the dielectric mirror. The growth conditions are as follows:
(i) 39 cycles of ZnO are deposited by the pulsing of H2O for 0.015 s followed by a 0.015-s pulse of DEZ with 8.5 s of delay in between.
(ii) 1 cycle of Al2O3 is deposited by the pulsing of H2O for 0.015 s followed by a 0.015-s pulse of TMA with 8.5 s of delay in between.
The above set of 2 cycles was repeated 45 times. Approximate step-time is 9 h. The samples are then cleaned using the same steps used at the end of step 2 (approximate time is 1 h for 8 samples).
4. Deposition of N-I-P Layers Using Plasma-Enhanced CVD
Using SiH4, H2, 1% PH3/Ar as an n-type doping gas, and 10% B2H6/H2 as a p-type doping gas in an STS PECVD tool that operates at 13.56 MHz, we deposit the PIN-diode using the following recipe:
Temperature=200° C., Pressure=200 mTorr.
n-layer: Ignition power: 70 W, t=2 s; Deposition power: 20 W, t=4:20
Recipe: SiH4=10 SCCM, HF power=20 W, H2=0 SCCM, Temperature=200° C., 10% B2H6/H2=0 SCCM, Pressure=200 mTorr, 1% PH3/Ar=20 SCCM
Expected Thickness: 30 nm
i-layer: Ignition power: 70 W, t=2 s; Deposition power: 20 W, t=38:50
Recipe: SiH4=58 SCCM, HF power=20 W, H2=20 SCCM, Temperature=200° C., 10% B2H6/H2=0 SCCM, Pressure=200 mTorr, 1% PH3/Ar=0 SCCM
Expected Thickness: 300 nm
p-layer: Ignition power: 70 W, t=2 s; Deposition power: 20 W, t=1:30
Recipe: SiH4=40 SCCM, HF power=20 W, H2=0 SCCM, Temperature=200° C., 10% B2H6/H2=20 SCCM, Pressure=200 mTorr, 1% PH3/Ar=0 SCCM
Expected Thickness: 30 nm
Approximate step time is 45 m for each run.
5. Photolithography to Expose the Bottom AZO Contact
In this step, we etch away the a-Si:H layer at the corners of the substrate to expose the bottom AZO conductor that will be used subsequently as a bottom contact.
(i) Spin-coat S1818 Shipley photoresist for 5 s at 500 RPM with a 100 RPM/sec ramp, followed by a 45-s spin at 2000 RPM with a 500 RPM/second ramp.
(ii) Bake the resist layer over a hot plate at 115° C. for 70 s.
(iii) Expose the resist through a circular mask placed atop glass using a SUSS MA-6 mask aligner for 1 m.
(iv) Develop in CD-26 for 1 m.
Approximate step-time is 1 h for 8 samples.
5
a. Add a layer of oil within the developed circular photo-resist area. Early trials showed that small pinholes appear within the developed photoresist that result in short-circuiting the top and bottom contacts once the top AZO layer deposited. We avoid this by depositing oil inside the circular area of the developed resist to serve as a barrier that prevents etching of a-Si:H within the circle. Approximate step-time is 1 h for 8 samples.
5b. Use XeF2 to etch away the a-Si:H outside of the circular center. A flow of XeF2 gas was used to etch a-Si:H where exposed outside the central circle, thereby exposing the cell bottom contact. This was done by flowing 10 pulses of the gas for 20 s each. Approximate step-time is 30 m for 3 samples.
5c. Cleaning off the oil and photoresist. In this step, we remove the oil and photoresist used in Steps 5-7. We first squirt acetone on each sample individually and dip the samples in a fresh batch of acetone two times in a row, followed by dipping in methanol, and then IPA. Special care is taken to clean the tweezers used between each dipping step. Approximate step-time is 30 m for 4 samples.
6. Deposit Top-Contact AZO Using ALD
We deposit the top AZO contact following the recipe used in Step 3 but at a temperature of 130° C. Approximate step-time is 9 h.
7. Deposit Au Using the Sputtering Tool
We sputter a 245 nm thick layer of gold on the cells from Step 6 that serves as a top and bottom contact. The sputtering conditions are as follows:
Deposition time=8 m; deposition height=36; deposition power=50% (150 W); deposition pressure=4 mTorr; Ar flow=40 SCCM. Approximate step-time is 30 m.
8. Photolithography and Wet Etching to Define the Au Contacts
In this step, we etch the excess gold and leave intact the top and bottom metal contacts as follows:
(i) Spin-coat S1818 Shipley photoresist for 5 s at 500 RPM with a 100 RPM/s ramp followed by a 45-s spin at 2000 RMP with a 500 RPM/s ramp.
(ii) Bake the resist layer over a hot plate at 115° C. for 70 s.
(iii) Expose the resist through a mask using a SUSS MA-6 mask aligner for 1 m.
(iv) Develop in CD-26 for 1 m.
(v) Wet etch the Au material in an electronics-grade Au etching solution.
Approximate step-time is 3 h for 8 samples.
9. Photolithography and Wet Etching to Disconnect the Top and Bottom AZO Contacts
In this step, we aim to etch an opening in the top AZO contact, thereby disconnecting the top and bottom contacts of the cell. The remaining top and bottom AZO contacts are in direct connection with the Au strips created in Step 8.
(i) Spin coat S1818 SHIPLEY photoresist for 5 s at 500 RPM with a 100 RPM/sec ramp followed by a 45-s spin at 2000 with a 500 RPM/second ramp.
(ii) Bake the resist layer over a hot plate at 115° C. for 70 s.
(iii) Expose the resist through a mask using a SUSS MA-6 mask aligner for 1 m.
(iv) Develop in CD-26 for 1 m.
(v) Wet-etch the AZO material in an electronics-grade A1 etching solution.
Approximate step-time is 3 h for 8 samples.
10. Deposition of the Dielectric Spacer and Front Mirror
In this step, we deposit a silica spacer (of thickness 2, 4, or 10 microns) and a second dielectric Bragg mirror on the solar-cell active area via e-beam evaporation at 200° C. This dielectric Bragg mirror is partially reflective and comprises six alternating layers (3 bilayers) of SiO2 and TiO2 of thicknesses 129 nm and 86 nm, respectively. Approximate step-time is 13 h per run.
Refractive Indices of the Solar Cell Layers
a. Refractive Indices of the Dielectric Mirrors
The multilayer back-reflector and the partially reflective front-mirror are formed of alternating layers of SiO2 and TiO2. The refractive indices of these two materials in the spectral range of interest are shown in
b. Refractive Indices of the CPA Fabry-Pérot Cavity Layers
The refractive indices of the five materials involved in constructing the PIN-diode incorporated into CPA Fabry-Pérot cavity were measured on the Woollam M2000 variable-angle mapping spectroscopic ellipsometer using witness samples (on glass substrates) for each layer produced during the various deposition steps. The materials are the n-type, i-type, and p-type a-Si:H layers, and the front and back AZO contact layers. We plot the real and imaginary parts of the wavelength-dependent refractive indices of these layers in
Omni-Resonance Measurements Configuration
φ(λ)=tan−1 {(d1/d2)tan(γ(λ)−γ0)}, where φ0=φ(λc=710 nm)=0. The distances d1 and d2 are selected such that the illuminated spot on the grating is imaged onto the cavity. If the focal length of L1 is f, then d2=fd1/f−d1.
When the cavity is oriented such that it is perpendicular to the optical axis, the angle of incidence of each wavelength is φ(λ). Upon tilting the cavity by ψ, the angle of incidence with respect to the normal to the cavity is
θ(λ)=φ(λ)+ψ.
The angular dispersion β produced for our dielectric spacer thicknesses and the required distances d1 and d2 in each case are:
2 μm: β=0.410°/nm, d1=172.5 mm, d2=29.2 mm
4 μm: β=0.355°/nm, d1=152.5 mm, d2=29.9 mm
10 μm: β=0.310°/nm, d1=132.5 mm, d2=30.8 mm.
Measurements of the Angular Dispersion
Solar Cell Characterization
The goal of this work is testing the impact of broadband CPA on a solar cell, and not the demonstration of a record bare solar cell. We present here the electrical characterization of the bare solar cell (without the CPA cavity and light preconditioning system) under simulated solar irradiation of AM 1.5 G. It has an efficiency of ≈1% that is limited by the resistance of the transparent AZO contacts.
Bare Cell Bottom AZO: 57 Ω/sq, Thickness=312 nm, n=1.97
While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only and not as a limitation. Numerous changes to the disclosed embodiments can be made in accordance with the specification herein without departing from the spirit or scope of this specification. Thus, the breadth and scope of this specification should not be limited by any of the above-described embodiments; rather, the scope of this specification should be defined in accordance with the appended claims and their equivalents.
The instant application claims priority to U.S. provisional application Ser. No. 62/842,787 filed May 3, 2019, the subject matter of which is incorporated by reference in its entirety.
This work was funded by the US Air Force Office of Scientific Research (AFOSR) under MURI contract FA9550-14-1-0037 and by the US Office of Naval Research under contract N00014-17-1-2458. The U.S. government has certain rights in the invention.
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