Device Physics of Nanoscale Interdigitated Solar Cells

Wyatt Metzger and Dean Levi

National Renewable Energy Laboratory • Golden, CO 80401

The information contained in this poster is subject to a government license.

2008 33rd IEEE Photovoltaic Specialists Conference • San Diego, CA • May 11–16, 2008 • NREL/PO-520-43328

CONCLUSIONS

• Interdigitated thin-film solar cells can give good device performance commensurate with comparable planar junction devices. 

• Interdigitated p and n regions increase efficiency relative to a comparable planar junction device for low-quality material when W is about 50 or 100 nm in this study.

• In most cases, device performance is generally not significantly increased relative to a planar junction.

• Performance can be increased for low and high quality material when the band offsets between n and p-type material form a robust type-II band alignment that 

effectively separates electrons and holes even when W is small.

COMPUTATIONAL APPROACH

• Device performance was calculated by solving the Poisson and electron-hole 

continuity equations using the drift-diffusion model and by solving current 

and energy density boundary equations at heterointerfaces described by the 

Anderson model.

• Programming was done within the Sentaurus Device simulation 

environment.

Standard 1D solar cell

DEVICE MODELS: 2 cases of interdigitated cells 

Case 2: Varied band offsets

• offsets between p- and n-type 

regions varied

• Interdigitated α-like and β-like 

phase CIGS

• emitter = 50 nm n-CdS

Case 1: No band offsets

• No band offsets between 

p- and n-type regions

• α-phase CIGS absorber

• emitter = 50 nm n-CdS 

and 200 nm ZnO

Common Elements to both cases

• 3 mm thick CIGS absorber layer

• Interdigitated p- and n- regions of 

equal width W; 10 nm < W < 1 mm

• n-type regions extend 2.5 mm into 

absorber

• n-type doping constant at 6x1017 cm-3

• p-type doping varied from 6x1017 cm-3

to 1.2x1018 cm-3

• Device performance calculated for ‘high 

quality’ and ‘low quality’ CIGS absorber

MOTIVATION

• Nanoscale interdigitated solar cell device architectures are being investigated for organic and inorganic solar cell devices.

• Due to the inherent complexity of these device designs quantitative modeling is needed to understand the device physics.

• Theoretical concepts have been proposed that nanodomains of different phases may form in polycrystalline CIGS solar cells.

• These theories propose that the nanodomains may form complex 3D intertwined p-n networks that enhance device performance.

• Recent experimental evidence offers some support for the existence of nanodomains in CIGS thin films.

• This study utilizes CIGS solar cells to examine general and CIGS-specific concepts in nanoscale interdigitated solar cells.

“Low quality CIGS” “High quality CIGS”

W = 1 um Voc is optimum when there is maximal band bending and minimal dark current, which occurs at large W. 

But the geometry is not ideal for current collection and Jsc is inferior to a comparable planar junction device.

W -> 50 nm As W decreases towards 50 nm, current is collected more efficiently, but increasing dark current and reduced 

band bending lower Voc. PV designs that increase photocurrent collection generlly increase dark current too.

W < 50 nm The strength of the lateral fields diminish and no longer effectively separate charge. Transport and performance 

becomes identical to a planar junction device with a uniform doping equivalent to half of p – n in the CIGS layer.

Material Parameters

Bulk Properties ZnO CdS α-CIGS β-CIGS

Eg (eV) 3.3 2.4 1.15 1.43

χ (eV) 4.6 4.6 4.6 4.62

μe (cm2/V-s) 100 100 100/10 100/10

μh (cm2/V-s) 25 25 12.5/1.25 12.5/1.25

n, p (cm-3) n: 1018 n: 6.0x1017 varied varied

ε/ε0 9 10 13.6 13.6

me /mo 0.2 0.2 0.09 0.09

mh /mo 1.2 0.8 0.72 0.72

Midgap state (cm-3) Na: 1016 Na: 1016 No: 1x1015 No: 1x1015

σe (cm2) 10-16 10-15 2x10-14/-13 2x10-14/-13

σh (cm2) 10-13 10-12 2x10-14/-13 2x10-14/-13

Surface Properties Front Back

Se (cm/s) 107 107

Sh (cm/s) 107 107

Reflectivity 0.05 0

“Low quality CIGS” “High quality CIGS”

∆χ = 0.0 or +0.3 For W greater than 50 nm, the lateral fields and device performance are similar to the case with no offsets. 

For W less than 50 nm, either the valence or conduction band is flat. Hence one carrier is not separated 

laterally and recombination is not effectively reduced,. The results are again similar to the case with no offsets.

∆χ = -0.3 For large W, the lateral fields and device performance are similar to the case with no offsets. However as W is 

reduced, the offsets form a robust type II junction which effectively separates charge and reduces recombination 

even for W = 10 nm. This structure improves device performance relative to a planar junction device, and 

atttains efficiencies greater than 15% even for “low quality” material with a diffusion length of 100 nm. 

Band bending vs W for equal doping

n = p = 6x1017 cm-3

Band bending vs W for unequal doping

n = 6x1017 cm-3 p = 6.2x1017 cm-3

Current flow pathways

W = 50 nm

W = 10 nm

CASE 1: n and p regions both α-phase CIGS

p-doping varies, W varies

Eg(n) = Eg(p) = 1.15 eV

n = 6x1017 cm-3

p = 6.0x1017, 6.2x1017 cm-3 

As W decreases less charge is 

available to create band bending, the 

built-in field at the vertical p-n

junction decreases, and the device 

becomes a 1D solar cell with average 

properties of the n and p regions. 

As W decreases less charge is 

available to create band bending, the 

built-in field at the vertical p-n

junction decreases, and the device 

becomes a 1D solar cell with average 

properties of the n and p regions. 

CASE 2: n regions α-phase, p regions β-phase

Band offset varies, W varies

∆χ = -0.3, 0.0, 0.3 eV

Eg(α) = 1.15 eV Eg(β) = 1.45 eV

Band bending when Dc = 0

n= p = 6x1017 cm-3

For large W (e.g. 250 nm), the lateral band 

alignment is determined by charge transfer and 

band bending (upper left figure). 

For intermediate W (e.g. 50 nm), the lateral band 

alignment is determined both by band bending 

and band offsets (middle left figure). 

For small W (e.g. 10 nm), the lateral band 

alignment is determined by the bandgap and 

conduction and valence band offsets and not by 

band bending (bottom left figure). 

If the bandgap and offsets form a robust type II 

interface (e.g. ∆c = -0.3, middle and upper right), 

charge separation occurs even when W = 10 nm 

(lower right). 

∆χ=-0.3 eV

Type II band alignment

Effective carrier localization

Enhanced device performance