May 27, 2004 Photovoltaics Laboratory Chalcogenide Solar Cells: Choosing the Window Colorado State University Funding: US National Renewable Energy Laboratory.

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Presentation transcript:

May 27, 2004 Photovoltaics Laboratory Chalcogenide Solar Cells: Choosing the Window Colorado State University Funding: US National Renewable Energy Laboratory (NREL) Japanese New Energy Development Organization (NEDO) Special thanks to Markus Gloeckler for assistance with figures Jim Sites Markus Gloeckler, Alex Pudov, and Ana Kanevce (CSU) Falah Hasoon and Miguel Contreras (NREL) Hans Schock (IPE) and Tokio Nakada (AGU) Collaborators: European Materials Research Society – Spring 2004

Approach May 27, 2004 Photovoltaics Laboratory (1) Device-physics approach to the selection of window layers for fabricating high-performance solar cells with CdTe and CIGS absorbers. [Device physics no means the whole story, but may give useful direction even when material structure or other factors play a major role] (2) Large range of possible band gaps will be considered. (3) Attempt to be quantitative. (4) Focus on two areas: (a) Window absorption: how much of an effect? (b) Conduction-band offset: what happens when it changes?

Choosing the Window: Outline May 27, 2004 Photovoltaics Laboratory (1)Photon considerations: Window absorption. (2)Conduction-band offset problem I: Big spikes (and their “red-kink” precursor) that limit current. (3)Conduction-band offset problem II: The cliff problem that limits voltage. (4)How much slack does one get in choosing the window? (5)Conclusions.

Short-Wavelength Current: CdS Windows on CdTe May 27, 2004 Photovoltaics Laboratory Granata, Sites, Contreras- Puente and Compaan, IEEE PVSC-25, 853 (1996) Same current loss should apply for CI(G)S cells. Short-Wavelength Collection

Current Loss with Alternative Windows May 27, 2004 Photovoltaics Laboratory Absorption spectra based on that of CdS, but shifted in energy. Calculated Values

Fractional Current Loss May 27, 2004 Photovoltaics Laboratory For 100-nm window layer Larger fraction with smaller current from larger-gap absorber.

Efficiency Contours May 27, 2004 Photovoltaics Laboratory Record CIGS Cell Parameters for record CIGS cells  E C effects neglected 100 nm window V OC = E g – 550 meV Fill-factor = 80%

Choosing the Window: Outline May 27, 2004 Photovoltaics Laboratory (1)Photon considerations: Window absorption. (2)Conduction-band offset problem I: Big spikes (and their “red-kink” precursor) that limit current. (3)Conduction-band offset problem II: The cliff problem that limits voltage. (4)How much slack does one get in choosing the window? (5)Conclusions.

Sign Convention for  E C May 27, 2004 Photovoltaics Laboratory Smaller Gap Absorber Larger Gap Absorber Some consensus on  E C magnitudes between theory, experiment, and numerical simulations of J-V curves Spike can impede photoelectrons (may be bad) Cliff slows forward electrons in interfacial-recombination region (also may be bad)

Earlier “Red-Kink” (Solarex Cells) May 27, 2004 Photovoltaics Laboratory Also seen In cells from NREL, Boeing, and Siemens/Shell Dark and Red-light J-V Curves

Producing a “Red” Spectrum May 27, 2004 Photovoltaics Laboratory 600-nm high-pass filter Series of high-pass filters with different-wavelength cut-offs Use a high-pass filter Red kink with CdS occurs when no photons are above 2.4 eV

The Red Kink in CdS/CIS May 27, 2004 Photovoltaics Laboratory NREL CdS/CIS J-V Conduction Band at V = 0 (light/dark difference exaggerated) CdS barrier impedes electron transport; blue photons may generate sufficient electron-hole pairs in CdS to alter trap occupation and mitigate the effect. Can be a serious problem if no blue photons present. Usually not a problem with white light, but small “kink” sometimes seen. Compensated CdS

Kink Depends on CdS Thickness (Simulation) May 27, 2004 Photovoltaics Laboratory Weaker kink with thinner CdS. (Also seen experimentally) More generally: strength of kink varies with the carrier densities of CdS and TCO, and with the CdS defect density. Conduction Band. Impact of barrier increases with CdS thickness.

Kink Disappears at Higher E g (NREL Cells) May 27, 2004 Photovoltaics Laboratory E g = 1.11 eV E g = 1.40 eV E g = 1.22 eV Conduction-band offset decreases; changes from spike to cliff

Choosing the Window: Outline May 27, 2004 Photovoltaics Laboratory (1)Photon considerations: Window absorption. (2)Conduction-band offset problem I: Big spikes (and their “red-kink” precursor) that limit current. (3)Conduction-band offset problem II: The cliff problem that limits voltage. (4)How much slack does one get in choosing the window? (5)Conclusions.

Effect of Interfacial Recombination on V OC May 27, 2004 Photovoltaics Laboratory CdS Window Vary  E C by expanding E g (simulated) See Poster P3.9 (Gloeckler) Lack of spike allows significant interfacial recombination Effect of  E C at constant E g discussed by several groups

CdS or Alternative Windows? May 27, 2004 Photovoltaics Laboratory Vary the window and hence the offset

But, kink can return! May 27, 2004 Photovoltaics Laboratory CdS Window (IPE) InS(O,OH) Window (IPE) CIGS Absorber (E g = 1.15 eV) “Red” Cut-off 2.4 eV “Red” Cut-off 2.8 eV See Poster P3.8 (Pudov) Note: ZnS(O,OH) from AGU yields similar curves Good Superposition

Choosing the Window: Outline May 27, 2004 Photovoltaics Laboratory (1)Photon considerations: Window absorption. (2)Conduction-band offset problem I: Big spikes (and their “red-kink” precursor) that limit current. (3)Conduction-band offset problem II: The cliff problem that limits voltage. (4)How much slack does one get in choosing the window? (5)Conclusions.

Efficiency Picture May 27, 2004 Photovoltaics Laboratory Vary the offset independently of E g

Choosing the Window Material May 27, 2004 Photovoltaics Laboratory Big Spike Small Spike or Cliff Offset Values from Zhang,Wei, and Zunger, JAP 83, 3192 (1998) Match absorber and window materials so  E C is in optimal range

Conclusions May 27, 2004 Photovoltaics Laboratory (1)From a device-physics perspective, the optimal choice of window material for chalcogenide solar cells varies with the band gap of the absorber. (2)A general problem for CdS windows is low blue response. (3)A problem for CdS on low-gap absorbers (CIS) is a big spike that impedes current. Mitigated by thin, high-carrier-density, or photoconductive CdS. (4)A problem for CdS on high-gap absorbers (CdTe or CGS) is the lack of a barrier to inhibit interfacial recombination. (5)At room temperature, a single window material is optimal over an approximate 300-meV range of absorber band gap.