# Presentation: 258088

## Phase Engineering of High Efficiency a-Si:H Solar Cells

- C. Wronski, R. Collins1, J. Pearce2, J. Deng, G. Ferreira, N. Podraza, M. Albert, C. Chen
- Center for Thin Film Devices, The Pennsylvania State University, University Park, PA 16802
- 1Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606
- 2106 Pierce Science Center, Clarion University of Pennsylvania, Clarion, PA 16214

- Center for Thin Film Devices
- The Pennsylvania State University

*Acknowledgements - NREL*

## Introduction

- High performance a-Si:H based solar cells obtained with hydrogen dilution
- In systematic optimization
**Take into account protocrystalline nature of materials during growth**
**Improve understanding of mechanisms limiting cell performance and stability**
- Develop and apply deposition phase diagrams guided by RTSE
- Clearly identify characterize contributions of:contacts,   p/i interface and bulk recombination, and light induced defects

## Real Time Optics of Silicon Film PECVD

**Real Time Optics of Silicon Film PECVD**

- Developed at PSU –being applied in other laboratories
- Allows in situ characterization of growth (surface roughness) microstructure, optical properties 1.5 to 4.5eV
- Acquisition time ~50ms allows monolayer growth to be characterized

**Suitable for analysis of inhomogeneous films with micro/macro/geometric scale structure**

## Extended Phase Diagram:Si:H Growth on c-Si/oxide Substrates 

**(1)**	**a ******** a**		surface roughening transition
**(2)**	**a ******** (a+********c)**		surface roughening transition
**(a+********c) ******** ********c**	surface smoothening transition
- These transitions provide insights into materials and device optimization

- Microstructure and its evolution is strongly dependent on substrate

***Narrow window for protocrystalline Si:H growth in a thick layer is centered at R=10;******here the  film surface is stable throughout deposition.***

**Obtained from the three transitions detected during Si:H film growth:**

## Deposition Phase Diagram for rf PECVD Si:H Film Growth on R=0 a-Si:H Film Surfaces

**   ****Phase diagrams**
-     describe regimes of db
-     and R within which
-     a-Si:H, (a+c)-Si:H,
-     and c-Si:H films are
-     obtained in the growth
-     process
-   The a  (a+c) transition
-     thickness is deduced
-     by RTSE from the
-     roughening transition
-   The (a+c)  c transition
-     thickness is deduced
-     by RTSE from a
-     smoothening transition

**Phase diagrams depend on the substrate and deposition conditions**

- AFM
- study

- XTEM
- study

- rf PECVD:  13.56 MHz;  Subst. temp.: T=200°C;
- rf power P=0.08 W/cm2;  SiH4 partial pressure: p(SiH4) ~ 0.05 Torr

## Simple Phase Diagrams:Substrate Dependence of a  (a+c) Transition

- Maximum H2-dilution levels for a **200 Å** thick layer (e.g., for a protocrystalline interface i-layer) while remainingbelow the a  (a+c) transition
******c-Si:H substrate:****	****R ~ 7.5**
**c-Si/oxide substrate :****	****R ~ 20**
**a-Si:H substrate (R=0):****	****R ~ 40**

***Under protocrystalline growth conditions, the phase of the nucleating film depends on the substrate:***
***Local epitaxy occurs for************c-Si:H substrates***
***Barrier to ************c-nucleation exists for a-Si:H substrates***

**rf plasma power: P=0.08 W/cm****2**
**Substrate temp.: T=200°C**

## TEM of Si:H Deposited with R=20 on c-Si/SiO2

**TEM of Si:H Deposited with R=20 on c-Si/SiO****2**

## TEM of Si:H Deposited with R=10 on Cr (evap)

**TEM of Si:H Deposited with R=10 on Cr (evap)**

## Optimization Principle for Two-Step i-Layer of a-Si:H p-i-n Solar Cell

**Optimization principle**
-  Prepare interface and bulk
-    i-layers with the maximum
-    R=[H2]/[SiH4] values possible
-    without crossing the a(a+c)
-    transition for the desired
-    thickness  **concept of     **
**   ****protocrystallinity is useful**
**Difficulties**
-  If the a(a+c) transition is
-    crossed accidentally in this
-    process, one must decrease R
-    below 10 (below protocrystalline
-    regime) to suppress continued
-    growth of the microcrystallites 
-    **real time monitoring and **
**   ****control are needed**

**Two step optimization of R=10 bulk i-layers with R=40 p/i 200Å layer**
_**Improvement:**_
**Voc 0.88 to 0.92V**
**Annealed FF same  0.72**
**R=0  p/I** **Voc 0.88 to 0.84**

## Nature of (a+c) Phase and Its Effect on Solar Cell Performance

- From RTSE and AFM for R=40 on R=0 film onset of c nucleation occurs at thickness of 200Å, with complete coalescence of c nuclei within d=400Å.
- Increase in recombination due to reduction in the mobility gaps in R=40 layer to 1.62eV at 300Å and 1.22eV at 400Å.
**Presence of such an a********(a+********c) transition greatly increases carrier recombination and has profound effect on cell characteristics.**

**Increase in d**

**4000Å p(a-SiC:H)-i-n R=10 i-layer**

**Such phase transitions are even more critical in n-i-p structures where the (a+********c) phase is in direct contact with the p-layer.**

## Phase Boundary Determination for the i-Layer ofn-i-p's from Voc ( J. Yang et al., MRS Proc. 2000)

**Phase Boundary Determination for the i-Layer of****n-i-p's from V****oc**** ( J. Yang et al., MRS Proc. 2000)**

- R=1.2R*:  "the edge"
- maximum H2-dilution
- sustainable without
- generating crystallites
- at the i-p interface for
- given thickness

- 1.2R* < R < 2.0R*
- mixed phase (a+c) region
- at the i-p interface
- (irreproducible vol. fraction)

- R  2.0R*
- single-phase c-Si:H
- at i-p interface

**d = 2500  ****Å**

**The "edge" is thickness-specific;  it shifts to lower R with increasing d.**

## Protocrystallinity Concept Applied to p-Si:H Contacts in n-i-p Cells

- Phase diagrams of p-Si:H layers on R=10 a-Si:H were used in optimizing VOC in n-i-p cells.
- The maximum VOC occurs with R=150 and corresponds to a protocrystalline layer terminated at 200Å or close to the (a+μc) phase.
- The lowest VOC is obtained with R=200 where the layer has evolved into a purely μc-Si:H phase – because i/p recombination increases significantly.

- Erroneous conclusions that highest VOC’s are obtained with μc p-Si:H is due to characterizing microstructure on layers >>200Å.

## Two-Transition Phase Diagram: Effect of rf Plasma Power

- Phase diagrams depend on deposition conditions other than R
- Identify effect of deposition parameters on a  a and       a  (a+c) transitions; regimes of protocrystalline Si:H growth
- Large shifts in transitions when the plasma power is increased

**Phase diagrams are a powerful guide in optimizing deposition conditions for fast growth.**

## Protocrystalline a-Si1-xGex:H Alloys

**Protocrystalline a-Si****1-x****Ge****x****:H Alloys**

**Amorphous phase at R=50 with E****g**** ~ 1.4eV**

**c-Si substrate, Si/Ge = 5, R=H****2****/(Si+Ge)**
**T****s**** = 290****o****C**

- Presence of protocrystalline growth regime identified **for first** time in evolution of       Si1-xGex:H thin films prepared with H2 dilution
- Development of deposition phase diagrams **enhances systematic improvement** of their absorber layers

## Characterization of Recombination in Solar Cells From Dark Current-Voltage Characteristics

- Clear separation of p/i interface recombination from that in the bulk of a-Si:H solar cells
- Bulk recombination identified and quantified by systematic reduction of p/i contributions in cell structures
- Cell structures studied where the two components of carrier recombination are **clearly separated****.**

- p(a-SiC:H)-i-n
- 4000Å R=10
-  i-layers

**Cody Gap**
- of 200 Å p/i
- interface layer

**Information about the gap states in the intrinsic layers can be obtained directly from the bulk recombination.**

## Forward Bias Dark Current

-  From **analytical approach** and SRH recombination in bulk and p/i interface region →
-  p/i interface recombination – diode quality factor independent of V, value very close to 1
- Bulk recombination through continuous distribution of gap state – “differential diode quality factor” n(V) = [kT/q(d(lnJD)/dv)]-1
-  n(V) – bias dependence related to energy distribution of gap states with values between 1 and 2; values related to rate of changes in distribution

**Forward Bias Dark Current**

## Importance of Vn, Vp in Determining Cell Characteristics

- Vn: potential barrier in i-layer at n contact
- Vn determined by space charge of electron filled gap states due to large n0 – not large densities of states
- n0 depends on alignment of EF in n layer with Ec in i-layer
- Alignment different for p-i-n cells with   a-Si:H and c-Si:H n contacts
- Bulk recombination same
- Carrier injection clearly lower for n-a-Si:H – higher Vn
- No effect on VOC – zero current flow
- Large effect on FF
- Similar results for Vp

**Bulk**

**Limitations of Carrier Injection**

**Importance of V****n****, V****p**** in Determining Cell Characteristics**

## Defect States in the Intrinsic Layers of a-Si:H Solar Cells with Low p/i Interface Recombination

**Defect States in the Intrinsic Layers of a-Si:H Solar Cells with Low p/i Interface Recombination**

- From clearly identified bulk contributions to JD-V characteristics no evidence found for highly non-uniform distributions of gap states across i-layers in p-i-n and n-i-p cell structures
- Clear dependence on thickness of i-layers
- Recombination consistent with essentially uniform defect states densities
- n(V) characteristics consistent with recombination through these gap states
- Limitations of this recombination on 1 sun Voc can be identified

## Light Induced Defect States in i-layers

- Densities of defect states reflected in increases of JD
- Changes in energy distribution of defect states reflected in evolution of n(V)
- n(V) characteristics can be related to two Gaussian-like distributions –  one at midgap the other away from midgap
- Distinctly different evolution in the broadening of these distributions

**Light Induced Defect States in i-layers**

## n(V) Characteristics for R=0 and R=10 in Degraded States

- Different gap state distributions in annealed state identified
- Evolutions distinctly different in protocrystalline R=10 and undiluted R=0
- Distribution away from midgap much broader in R=0 and R=10
- The difference in these states accompanied by higher stability of protocrystalline    a-Si:H material and cells

**n(V) Characteristics for R=0 and R=10 in Degraded States**

## Importance of Non-midgap Light-Induced Defect States in Solar Cells

**Fill Factor**

- 1 sun Voc – quasi-Fermi level splitting encompasses recombination through states up to ~0.45eV from midgap
- Very big difference in the stability of 1 sun Voc

- Degradation of same 1 sun FF in annealed state
- Dramatic difference in degradation in FF
- Critical role of non-midgap states clearly indicated

**Reduction of these states key to further optimization of stability with microstructure in high performance cells**

**Importance of Non-midgap Light-Induced Defect States in Solar Cells**

## Summary

**Deposition phase diagrams ***extremely useful*** in systematic optimization of Si:H solar cells.**
**Concept of *****protocrystallinity***** useful in:**
***Systematic *****improvement of cell structures**
***Controlling***** deleterious effects of a ******** (a+********c) transition on solar cell characteristics**
**Identifying p/i recombination *****key***** to characterizing i-layers and their contributions to cell characteristics.**
**J****D****-V characteristics a *****new probe***** for characterizing recombination and defect states in i layers of solar cells.**
***Spatially uniform densities***** of defects in the i-layers *****allow***** correlations with corresponding thin film materials.**
**New insights into mechanisms limiting performance and stability of high quality a-Si:H solar cells offer guidance in optimizing a-SiGe:H, microcrystalline/nanocrystalline solar cells**

## Schematic of the structure of Si:H films on a-Si:H (R=0) 

**Schematic of the structure of Si:H films on a-Si:H (R=0)**

- Great attention must be given to the transition a→(a+μc) and its thickness dependence on R- films and cells
- Phase diagrams are a powerful guide in optimizing deposition conditions for fast growth

**     ****R**

- Despite evolutionary nature, protocrystalline a-Si:H has uniform properties over extended regions of thickness