Charged Grain Boundaries and Carrier Recombination in Polycrystalline Thin-Film Solar Cells

Gaury, Benoit; Haney, Paul M.

doi:10.1103/physrevapplied.8.054026

Cited by 18 publications

(30 citation statements)

References 33 publications

(67 reference statements)

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“…The agreement seems adequate considering the neglect of the doping-dependent depletion region near the CdS/CdTe interface. However, in ref ( 17 ), when the ambipolar diffusion length was longer than the grain width (see Figure 8d in ref ( 17 )), the sign of the doping effect is reversed from our results. Interestingly, when the diffusion length becomes comparable to the grain width, the dependence of V OC on N A has an inverted U-shape, with a maximum value near 3.0 × 10 15 cm –3 .…”

Section: Discussioncontrasting

confidence: 90%

Photovoltages in Polycrystalline Mosaic Solar Cells

Dinca

Schiff²

2021

ACS Appl. Energy Mater.

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In some thin-film solar cells the light-absorbing layer is a mosaic of crystalline grains whose boundaries run from the back to the front of the cell. We used the semiconductor modeling software Sesame to do numerical calculations of the optoelectronic properties of such cells assuming that recombination of minority photocarriers occurs primarily at the grain boundaries. The work complements analytical results for diffusion-limited recombination at grain boundaries and dislocations. We chose idealized n-CdS/p-CdTe solar cells for illustration. We find that the open-circuit voltage, V OC , under illumination declines logarithmically with increasing ratio D /θ 2 , where D is the ambipolar diffusion constant governing minority-carrier transport and θ is the grain size (from 1 to 10 μm). While a decline in V OC as mobility increases is counterintuitive, this finding is consistent with related analytical results and confirms their utility. However, open-circuit voltages are about 0.04–0.10 V lower than the corresponding analytical estimates. We show that the deficit is mostly a consequence of a recombination shortcut. At open circuit, minority photocarrier currents at points closer to the n-CdS interface than to a grain boundary are directed through the conducting front layers and terminate near the “hot spot” at the intersection with the grain boundary. The shortcut lowers open-circuit voltages by about 0.05 V below the analytical estimates.

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Section: Discussioncontrasting

confidence: 90%

Photovoltages in Polycrystalline Mosaic Solar Cells

Dinca

Schiff²

2021

ACS Appl. Energy Mater.

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“…[51][52][53] It is widely recognized that charged GBs impact CdTe solar cells. [48,49,[51][52][53] GB potential or barrier height measurements are difficult, in part because different experimental methods use different excitation/injection conditions and have different probing resolutions. For example, Kelvin probe scanning force microscopy (KPFM) was used to report GB potential histograms in CdTe solar cells, and after CdCl 2 treatment and Cu doping GB barrier heights ranged from 40 to 260 meV.…”

Section: Space Charge Field Screening and Microscopic Carrier Lifetimesmentioning

confidence: 99%

Identification of Recombination Losses in CdSe/CdTe Solar Cells from Spectroscopic and Microscopic Time‐Resolved Photoluminescence

2021

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Due to the lowest‐cost and best reliability, CdTe solar cells are the leading thin‐film photovoltaic technology. Increasing open‐circuit voltage by reducing recombination represents the most promising path toward further improvements. Analysis is needed to identify limitations that cause efficiency losses. To achieve this goal for Cu‐doped CdSe/CdTe solar cells, time‐resolved spectroscopy and microscopy are developed and applied. Recombination lifetimes and radiative efficiency identify that defect‐mediated recombination is the dominant voltage loss mechanism. When carrier lifetimes are averaged over many crystalline grains, they increase from 180 to 430 ns when Al2O3 is applied to the back contact. The quasi‐Fermi‐level splitting correspondingly increases from 880–905 to 906–931 mV, indicating a pathway to overcome the long‐standing 900 mV voltage limitation. However, the dominant recombination losses are attributed to the absorber bulk. From microscopic carrier lifetime measurements, it is identified that space charge fields due to charged grain boundaries (GBs) lead to recombination in the CdTe absorber bulk. At high injection, GB space charge fields are screened, but that occurs above 1 Sun excitation conditions. Alloying with selenium in the near‐interface CdSeTe absorber region reduces GB losses and is identified as one of the factors leading to high radiative and power conversion efficiency.

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“…[ 13,14 ] Conversely, the e–h pair can recombine within the grain or at defects sites, limiting photoactivity. [ 15,16 ] A long‐standing challenge is to design materials with efficient e–h generation (high active volume) but inhibited e–h recombination. Achieving this combination of two requirements simultaneously enables materials with high functionally active volume fraction.…”

Section: Figurementioning

confidence: 99%

“…Grain boundaries and defects generally are considered as recombination centers for electrons and holes, [ 15,16 ] except twin boundaries, which provide “back to back” potentials and facilitate charge separation and transport. [ 22,23 ] No twin boundaries were observed in our samples.…”

Section: Figurementioning

confidence: 99%

Atomic Gradient Structure Alters Electronic Structure in 3D across the Bulk and Enhances Photoactivity

Ren

Song

et al. 2021

Advanced Energy Materials

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Understanding structure–function relationships enables the design of materials with tailored functionalities. The long‐standing challenge is to design materials with high active volume to improve efficiency. Tailored grain boundaries and lattice defects are traditionally used to tune the electronic structure near interfaces or defects to promote electron and hole separation. However, the active volume of point defect sites or interfaces in these traditional photocatalysts is extremely low. This study reports a structure with continuous atomic positional deformation across the bulk, altering the electronic structure in 3D and creating a significantly higher active volume. Such a structure in anatase is obtained and tuned by phase transformation during the heating process. Transmission electron microscopy and density functional theory results reveal that atomic deformations result in continuous band bending across the particles, facilitating electron–hole separation, inhibiting their recombination, and inducing dramatically enhanced photoactivity. These findings enable a different materials design paradigm that can potentially be harnessed for a broad range of applications.

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Charged Grain Boundaries and Carrier Recombination in Polycrystalline Thin-Film Solar Cells

Cited by 18 publications

References 33 publications

Photovoltages in Polycrystalline Mosaic Solar Cells

Photovoltages in Polycrystalline Mosaic Solar Cells

Identification of Recombination Losses in CdSe/CdTe Solar Cells from Spectroscopic and Microscopic Time‐Resolved Photoluminescence

Atomic Gradient Structure Alters Electronic Structure in 3D across the Bulk and Enhances Photoactivity

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