identification of potential photovoltaic absorbers based on first-principles spectroscopic screening...

Identification of Potential Photovoltaic Absorbers Based

on First-Principles Spectroscopic Screening of Materials

Liping Yu , Alex ZungerPHYSICAL REVIEW LETTERS 108, 068701 (2012)

Kishida Takuto

Ⅰ.Introduction Photovoltaic (PV) absorbers

Shockley and Queisser (SQ) limit

Ⅱ. Discussion & Result Spectroscopic limited maximum efficiency (SLME)

Which materials are good potential PV absorbers.

Ⅲ. Summary

Ⅳ.Future plan

Contents

PV absorbers PV absorbers have attracted attention as clean energy for the next generation.

Solar energy that reaches the earth in one hour is comparable to the energy humanity consumes in a year.(about 1kW/m2)

⭕ Energy source is the sun. CO⭕ 2 is not emitted.

❌ Conversion efficiency is not high. Price is high.❌

Solar cell

Silicon series

Organic series

Compound series

Classification

Si : 24.7 % ( Theoretical limitation 29 % )

GaAs : 28.3 % , CIGS : 20.3 % , CdTe : 16.7 %

Dye sensitization : 11.2% , Organic thin film : 7.9 %

Ⅰ.Introduction

Photovoltaic (PV) absorbers

In 1961,Shockley and Queisseroffered p-n junction PV absorbersefficiency limit by numerical calculation.

Efficiency limit η is expressed as a function of energy gap Eg.

SQ limitEfficiency limit is calculated using the ideal value.△Not taking account of direct or indirect gap.△Radiative recombination is the only recombination process.・・

Ⅰ.Introduction

http://kats.issp.u-tokyo.ac.jp/kats/semiconII/note7.pdf

Shockley and Queisser (SQ) limit

The SLME captures the leading physics of absorption, emission, and recombination characteristics, resolving a spread of different efficiencies for materials having the same gap.

( ) the existence of various energetic sequences of DA, DF and ⅰindirect band gaps

( ) the specific shape of the absorption near the threshold ⅱ

( ) the dependence of radiative recombination losses on the ⅲenergy separation between the minimum gap Eg and Eg

da

Spectroscopic limited maximum efficiency (SLME)

Ⅰ.Introduction

Ⅱ. Discussion & Result

OT1

OT3

OT2

OT4

SLMETaking account of direct or indirect gap→Classifying materials into four ‘‘optical types’’(OT1~4)

Comparison of SQ efficiency and SLME

The power conversion efficiency of a thin film (L) solar cell depends on the fraction of the radiative electron-hole recombination current (fr) and the photon absorptivity [a(E)] .

SQ efficiency

SLME

Radiative recombination is the only recombination processfor all optical types of materials.

× nonradiative recombination dominates

where k is the Boltzmann constant, T is the temperature, and Δ= Egda – Eg


Comparison of SQ efficiency and SLME

SQ efficiency

SLME

For photon absorptivity a(E)

where L is the thickness of the thin film,α(E) is the calculated absorption coefficient from first principles.

SLME improves upon the SQ efficiency formula in the description of both fr and a(E)


GW approximation （ G0W0+HSE06）This method has been widely and successfully applied in firstprinciples quasiparticle electronic-structure calculations for many materials.

Calculation method

Predicted minimum band gaps for some I-III-VI compounds have an average error of less than 12% with respect to experiments.(L=0.5μm)

I = Li , Na , K , Rb , Cs , Cu , Ag

III = B , Al , Ga , In , Tl , Sc , YVI = O , S , Se , Te

Ip-IIIq-VIr chalcopyrite group materials


CuInSe2( - -Ⅰ Ⅲ Ⅵ2)

Chalcopyrite semiconductor.

Experimental energy gap

=1.04[eV] (direct gap)

Lattice parameter

a=5.786[Å]

c/a=2.016

c

a

Cu

In

Se


Chalcopyrite materials

( )ⅰ For OT1, within the same structure type, the band gap of materials decreases withincreasing atomic number of one atom whenthe other two atoms are held fixed.

( )The optical types can change if the ⅱstoichiometry changes within the same element set.

Cu3TlSe2(OT3)→Cu5TlSe3(OT4)

→Cu7TlSe4(OT1)→CuTlSe2(OT2)

GW gaps of 215 compounds


For example

( )For the same compound, the minimum band gap (Eg) may vary by ⅲmore than 2 eV in different crystal structures, whether or not the optical type changes

(OT2、 space group #225) → Eg = 0.07 eV(OT4、 space group #166) → Eg = 2.27 eV

NaTlO2

( ) All reported Iⅳ 3III1VI3 materials have small differences (less than 0.2 eV) between Eg

df, Egda, and Ei

g . (except Li3BO3 which has a 0.43 eV difference between Eg

da and Edfg. )

GW gaps of 215 compounds


The SQ efficiency limit depends only on Eg for all optical types , predicting that the best gap for a PV absorber is 1.34 eV, at which ηSQ = 33.7%

Around the same Eg, ηSQ depends onthe ‘‘optical types’’ and absorption spectra.

　 AgInTe2 (OT1) Cu7TlS4 (OT1) CuYTe2(OT3)

Eg[eV] 1.17SLME[%] 27.6 22.6 7.5

SLME for I-III-VI thin film materials

For example


Which materials are good potential PV absorbers

There are about 25 materials with SLME higher than 20% .

These high-SLME materialshave the band gaps ranging from 0.8 to 1.75 eV.

25 materials with high SLME18 out of 25　→　 OT1 , 7 out of 25　→　 OT3

None of them have been found to be OT2 or OT4→OT2 and OT4 materials are least favorable for PV absorbers.


High-SLME materials ( Ip-IIIq-VIr )

CuInSe2 , CuGaSe2 , CuInS2

CuInTe2 , CuGaTe2 , AgInS2

Which materials are good potential PV absorbers

AgInSe2 , AgInTe2 , AgGaSe2 , AgGaTe that have been found experimentally to be reasonable solar absorbers but are much less studied.

Most of these previously recognized absorber materials within this group have (1:1:2) stoichiometry.

Materials with other stoichiometries also have high SLME.(e.g., AgIn5Se8 , Cs3AlTe3 )


Cu-Tl-Ⅵ materials

All six high-SLME Cu-Tl-VI materials(four Cu7TlS4 , Cu3TlS2 , Cu3TlSe2)

Non-(1:1:2) stoichiometryContain only Tl of +1 oxidation state

Tl is highly toxic → These Tl-containing materials might be disfavored.

Tl1+ → Other nontoxic elements in the 1+ oxidation state.

Cu-Tl-VI → Cup-IIIq-VIr → Cup-( )ⅠⅡ q-VIr

Cup-( )ⅠⅡ q-VIr materials that have SLME more than 20%, not involving Tl.


SLME improves upon the SQ efficiency formula for the initial material screening.

SLME considers different optical types and material-dependent nonradiative recombination loss.

Testing the idea on a couple of hundred generalized I- III-VI chalcopyrites using first-principles spectroscopy calculations and identifying potential PV absorber materials .

→ Cup-( )ⅠⅡ q-VIr materials that have SLME more than 20%,

not involving Tl that is highly toxic

Ⅲ. Summary

Ⅳ.Future plan

Cu-Tl-VI → Cup-( )ⅠⅡ q-VIr

Cu -

(Tl1+)

- VI

Cu -

- VI

Cu - Tl

Tl

Tl - VI

3+ 2+1+

Cu3 - - VIⅠⅡ 3

Ⅰ = Na , K Ⅱ = Fe , Mg Cu3 - K Mg - S3

For example

identification of potential photovoltaic absorbers based on first-principles spectroscopic screening...

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