Minimizing Current and Voltage Losses to Reach 25% Efficient

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Minimizing Current and Voltage Losses to Reach 25%-Efficient Monolithic Two-Terminal Perovskite-Silicon Tandem Solar Cells Kevin A. Bush, Salman Manzoor, Kyle Frohna, Zhengshan Jason Yu, James A. Raiford, Axel F. Palmstrom, Hsin-Ping Wang, Rohit Prasanna, Stacey F. Bent, Zachary C Holman, and Michael D. McGehee ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01201 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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ACS Energy Letters

Minimizing Current and Voltage Losses to Reach 25%-Efficient Monolithic TwoTerminal Perovskite-Silicon Tandem Solar Cells Kevin A. Bush1, Salman Manzoor2, Kyle Frohna1,3, Zhengshan J. Yu2, James A. Raiford4, Axel F. Palmstrom4, Hsin-Ping Wang1, Rohit Prasanna1, Stacey F. Bent4, Zachary C. Holman2, and Michael D. McGehee1,5* 1

Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA School of Electrical, Computer, and Energy Engineering, Arizona State University, Tempe, AZ 85281, USA 3 School of Physics, Trinity College Dublin, Dublin 2, Ireland 4 Chemical Engineering, Stanford University, Stanford, CA 94305, USA 5 Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309, USA *Corresponding author: Prof. Michael D. McGehee University of Colorado Boulder Department of Chemical and Biological Engineering 596 UCB Boulder, Colorado 80309-0596 e-mail: [email protected] 2

Abstract The rapid rise in efficiency and tunable bandgap of metal-halide perovskites makes them highly attractive for use in tandems on silicon. Recently we demonstrated a perovskite-silicon monolithic two-terminal tandem with 23.6% power conversion efficiency. Here, we present work on optical optimization to improve light harvesting that includes thinning out the top transparent electrode to reduce front-surface reflection and parasitic absorption, introducing metal fingers to minimize series resistance losses, and further minimizing reflection loss with a PDMS stamp with random, pyramidal texture. Additionally, to reduce voltage loss while achieving current matching, we employ PTAA as a hole transport material instead of NiOx and a wider 1.68-eV-bandgap perovskite composition. These optimizations boost the open-circuit voltage to 1.77 V and the short-circuit current density to 18.4 mA/cm2, culminating in a 25%-efficient perovskite-silicon tandem with a 1 cm2 active area.

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Perovskite solar cells have shown a meteoric rise in performance over the past five years with current certified efficiencies for single-junction solar cells above 22%1. The remarkable performance of perovskite solar cells is possible because of a combination of strong absorption across the solar spectrum2, low exciton binding energies and effective masses3, long lifetimes, and moderate carrier mobilities4,5. The bandgap of metal-halide perovskites with formula ABX3 (where A is a monovalent cation, B is a divalent metal cation, and X is a halide anion) can be readily tuned from 1.2 to 3 eV with compositional engineering at all three sites6–10. This makes perovskites ideal candidates for the widebandgap absorber in two-terminal (2T) or monolithic tandems where the two cells are connected in series, either by a tunnel junction or a recombination layer11–14. Wide-bandgap perovskite cells have been paired with bottom cells including silicon15–19, copper indium gallium selenide20,21, and narrow-bandgap Pb/Sn perovskites9,22,23. The market dominance of the incumbent silicon technology combined with its rapidly reducing costs make silicon an ideal tandem partner for wide-bandgap perovskites13. Perovskites in the bandgap range of 1.65–1.75 eV—the optimal bandgap range for 2T tandems with silicon based on annually averaged energy yield calculations24 and Shockley-Queisser detailed balance analysis25,26— have been used as the absorber in high-efficiency single-junction solar cells7,27. We previously reported a 2T perovskite-silicon tandem with an optimised, sputtered indium tin oxide (ITO) transparent top electrode and a pulsed chemical-vapour-deposited (CVD) SnO2/Zn-doped SnO2 (ZTO) bilayer as a sputter buffer15. This high-quality, transparent electrode was deposited on a mixed formamidinium(CH2(NH3)2+, FA) Cs-based 1.63-eVbandgap perovskite incorporated in a p-i-n architecture, and an amorphous silicon/crystalline silicon heterojunction (SHJ) cell optimised for the near infrared (NIR) using a silicon nanoparticle rear reflector. The optimisations allowed us to considerably reduce optical losses and sputter damage compared to the previous state of the art17,28, enabling an efficiency of 23.6% with a matched short-circuit current density (JSC) of 18.1 mA cm-2. However, further improvements can still be made. Most significantly, the open-circuit voltage (VOC) of the champion device was only 1.65 V. With a 0.68 V contribution from the silicon sub-cell at the reduced JSC of the tandem, the perovskite sub-cell only contributed 0.97 V. This equates to a voltage loss of 0.66 V relative to the cell’s bandgap voltage, which is much larger than for state-of-the-art inverted perovskite devices with VOC values of up to 1.16 V and corresponding VOC losses of less than 0.4 V29,30. These high-VOC inverted devices have used the hole selective contact poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and surface trap passivation layers such as quaternary ammonium halides29 or an ultra-thin layer of polystyrene30. Though less significant, the matched JSC of our previous best tandem can also be improved, as there are still optical losses in the device and the bandgap of the perovskite absorber is still below the ideal value24. In this work, we improve the VOC by optimising both the device contacts and the perovskite composition. We also optimise the tandem stack optically, reducing both reflection and parasitic absorption losses, resulting in an improved matched JSC. Taken


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together, these optimisations allow for the fabrication of a 25%-efficient perovskite-silicon 2T tandem. Reducing the Thickness of the ITO Top Electrode to Reduce Reflection Minimizing parasitic absorption and reflectance losses is essential to achieving high matched current densities in efficient tandems, and each layer should be evaluated to ensure it imposes minimal losses to the overall device performance. Our previous champion tandem solar cell failed to capture approximately 5 mA cm-2 across the spectrum due to reflection loss. To determine whether the ITO was having a meaningful impact on the reflection, we characterized the optical constants of each material in the device stack and performed optical simulations of perovskite/silicon tandems with different front ITO thickness (unpublished results). Figure S1 shows that reducing the thickness of the ITO top contact from 150 nm to 60 nm blue-shifts the reflection maximum from 550 nm to 400 nm, which is advantageous because there is considerably less light from the solar spectrum there. Additionally, reducing the thickness of the front ITO decreases parasitic absorption. The summed equivalent current density losses from reflection and parasitic absorption as a function of ITO thickness are plotted in Figure 1, which shows how decreasing the ITO thickness from 150 nm, which we used previously, down to 50–60 nm results in a summed JSC gain of nearly 2 mA/cm2 in the perovskite and silicon absorbers. However, reducing the thickness of the ITO electrode can also introduce resistive losses as the sheet resistance of the sputtered ITO scales inversely with its thickness. Decreasing the thickness by a factor of 2.5 results in an increase in sheet resistance from about 33.3 to 83.3 Ω/sq and a corresponding decrease in the fill factor (FF) of more than 2% absolute, which is why such a thick ITO top contact was used in previous devices. 40 39 2

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Figure 1. Reflectance, parasitic absorptance, and summed JSC, expressed as equivalent current densities, as a function of the front ITO electrode thickness in a perovskite-silicon tandem.



In order to mitigate these losses while maintaining the optical advantage of a thinner ITO layer, we evaporated silver fingers on top of the ITO through a shadow mask31,32. The silver fingers were designed to simultaneously minimize the lateral distance that carriers must travel across the ITO top electrode and the shading losses. Full details of finger optimization are provided in the Supporting Information. Briefly, power losses are minimized with thinner fingers, and the interplay of JSC losses due to shading and FF losses due to ITO resistance determines the optimal finger spacing. Given that the laser cutter used to fabricate the evaporation mask has a resolution of approximately 100 µm, we fabricated two-finger evaporation masks with each finger extending two thirds of the way across the device, resulting in approximately a 1% loss in JSC (Figure S2c). The fabrication of thinner fingers through techniques such as screen printing33, copper plating34, microchannel-contacting35, reactive inks spraying36, and gravure contact printing37 exist and their application could be the subject of future work to further decrease shadow losses. Using a PDMS Scattering Layer to Reduce Reflection Although the thinner ITO top electrode considerably improved the light harvesting of the tandem stack by reducing both parasitic absorption and reflection, considerable optical losses remained. We and others found that the fabrication of a patterned PDMS scattering layer that could be applied on the top surface of a solar cell to reduce reflection and increase JSC38–42. We found that applying the PDMS scattering layer to a silicon solar cell with a planar front surface and textured back resulting in a JSC comparable to that of a doubleside textured silicon solar cell, and applying it to a semi-transparent perovskite solar cell boosted the JSC by almost 2 mA cm-2. Note that, as the PDMS film has a similar refractive index as glass, it is a convenient representation of the performance that may be achieved after cell encapsulation with textured front module glass. Here, we fabricated control tandem devices as described previously15 and added the PDMS scattering layer. As shown in Figure 2, this results in a JSC gain of over 1 mA cm-2 in the perovskite sub-cell, but a JSC loss in the silicon sub-cell, as the PDMS scattering layer reduces reflection considerably at shorter wavelengths but does not help much to reduce reflection at longer wavelengths that reach the silicon sub-cell. This shift drives a considerable current mismatch between the sub-cells and reduces the matched JSC of the tandem. Crucially though, the summed JSC of the two sub-cells increases, meaning that further optical optimizations can increase the matched JSC of the tandem.


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Figure 2. EQE and total absorptance (1-R, with R the reflectance) spectra of control tandem devices using a 1.63 eV band gap Cs0.17FA0.83Pb(I0.17Br0.83)3 composition perovskite without PDMS (solid line) and with PDMS (dashed line).

Matching Sub-Cell JSC While Maximizing Voltage To match the JSC of the sub-cells, there are two primary strategies. First, the thickness of the top cell with a given bandgap can be tuned to enable current matching, or, alternatively, the bandgap of the perovskite top cell can be increased to allow more light to reach the silicon sub-cell. We explored these strategies via the optical simulations displayed in Figure 3, where the JSC of the limiting sub-cell (and thus the JSC of the tandem) is shown as a contour plot against perovskite thickness and bandgap. This was done for both tandem architectures shown in Figure 3c: Figure 3a shows the results for the double-layer antireflection coating strategy employed previously in our 23.6%-efficient tandem, whereas Figure 3b shows the results for the PDMS stamp strategy introduced here. Importantly, consistent with the findings of Figure 2, a PDMS stamp enables higher tandem JSC (by approximately 1 mA cm-2) with proper perovskite absorber design. Increasing the bandgap to achieve current matching is always preferable if the top cell voltage increased proportionally to bandgap. However, this has been rarely the case for perovskites43 and a common problem reported in the literature when widening the bandgap by tuning the halide composition is photo-induced halide segregation, referred to here as the Hoke effect43–48. Under illumination, the halides segregate into bromide- and iodide-rich regions. The iodide-rich regions have a narrower bandgap and act as radiative trap states: carriers funnel into these lower-energy regions and recombine. Photo-induced halide segregation correlates strongly with a loss in the VOC of perovskite solar cells7,45–47,49. This may be the reason why wider-bandgap perovskite solar cells exhibit larger VOC losses than their narrower-bandgap counterparts43,44,50. To address this, we previously demonstrated that widening the bandgap by tuning both the ratio of FA/Cs at the A site, and the ratio of 5 ACS Paragon Plus Environment


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I/Br at the X site (rather than just the I/Br ratio), results in increased VOC7. Using this strategy, we achieve perovskites of a given bandgap that are considerably more stable under illumination than previously reported compositions. In the tandems fabricated here, we used a 1.68-eV-bandgap perovskite composition (FA0.75Cs0.25Pb(I0.8Br0.2)3), referred to here by the percentages of Cs at the A site and Br at the X site, 25%Cs/20%Br. This composition was chosen as it has a bandgap that was previously calculated to be approximately the ideal top-cell bandgap for a silicon tandem in real world conditions taking into account imperfect absorption near the band edge.24 Out of several compositions at this bandgap that we tested, the 25%Cs/20%Br composition showed the lowest VOC loss and nearly the highest JSC7. Thus, from Figure 3b, we targeted a perovskite thickness of roughly 400 nm, denoted by the white star.

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Figure 3. Contour plots of the tandem JSC, which was taken to be the JSC of the limiting sub-cell, in a perovskite-silicon tandem as a function of perovskite thickness and bandgap for two different antireflection strategies: (a) MgF2 and (b) a pyramidally textured PDMS stamp. The solid white lines correspond to the maximum JSC (the current-matched condition) and the stars correspond to (a) our previous tandem and (b) the optimized tandem presented below. The perovskite-silicon tandem architectures used to simulate the contour plots are displayed in (c). We note that the MgF2 layer thickness of 150 nm was not adjusted in these simulations and our optical model does not account for the wrinkled texture of the perovskite surface51, both of which could lead to a wider range of


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ACS Energy Letters

bandgap-thickness pairs in (a) with tandem JSC above 17 mA/cm2. Forward and reverse currentvoltage curves and figures of merit for our champion perovskite solar cells fabricated with NiOx or PTAA hole-selective contacts and Ag rear electrodes are shown in (d). Note, full figures of merit and MPPT data are provided in Table S4 and Figure S4, while figures of merit for the reverse scan are displayed in (d).

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40 39 2

Equivalent Jsc (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

38

sum med Js

37

c

36 35 6 5 4 3 2 1 0

Reflectance

ce sorptan ITO ab 50

100 150 200 250 Front ITO thickness (nm)

300

ACS Paragon Plus Environment

ACS Energy Letters

Perovskite Jsc 2

Silicon Reflectance Jsc 2

2

(mA/cm )

(mA/cm )

(mA/cm )

No PDMS

18.71

18.03

5.51

PDMS

19.87

17.85

1.41

Difference

+1.16

-0.18

-4.10

100

1-Reflectance

90 80

EQE, EQE, 1- R (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

70 60 50 40 30

Perovskite

Silicon

20 10 0 400


1200


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Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(a)

ARC/Flat/Tex

(c) MgF 2 ITO SnO 2 C60 Perovskite PTAA ITO a-Si:H (n) a-Si:H (i) c-Si a-Si:H (i) a-Si:H (p) ITO Ag

(b)

(d)


PDMS/Flat/Tex PDMS

ACS Energy Letters

100

EQE, T, 1-R (%)

90 80

1-Reflectance

70 60 50 40 30

Perovskite 18.7 mA/cm2

20 10

(a)

Transmittance 17.2 mA/cm2

0 400


1200

10

Current Density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5 0 -5 -10

Sweep direction ; efficiency Down ; 15.8% Up ; 15.8% Voc: 1.05 V Jsc: 19.6 mA/cm2 FF: 0.77

-15 (b) -20 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Voltage (V)


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10 1-Reflectance

90 80

Current density (mA/cm2)

EQE, EQE, 1-R (%)

100

70 60 50 40 30 20 10

Perovskite 18.4 mA/cm2

Silicon 18.5 mA/cm2

(a)

0 400


1200

100

Reflectance, Absorption (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Voc: 1.77 V Jsc: 18.4 mA/cm2 FF: 0.77 PCE: 25.0%

0 -5 -10 -15 (b) -20 0.0

0.5

1.0 1.5 Voltage (V)

Loss Gain 2 2 (mA/cm ) (mA/cm ) Reflect 3.75 2.74 PDMS 1.3 0.1 Front ITO 0.7 0.3 0.65 0.28 C60

90 80 70 60 50 40 30 20 10

Champion tandem

5

(c)

0 400


PVK Si Inter ITO Rear ITO

18.9 19.5 0.4 0.5

-

Ag

0.23

0.08

1200


0.1 0.25

2.0

Minimizing Current and Voltage Losses to Reach 25% Efficient

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