Physics of Thin-Film Photovoltaics

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Victor G. Karpov. Physics of Thin-Film Photovoltaics

Table of Contents

List of Tables

List of Illustrations

Guide

Pages

Physics of Thin-Film Photovoltaics

Dedication

Preface

Part I. General and Thin Film PV. I. Introduction to Thin Film PV

A. The Origin of PV. Junctions

B. Fundamental Material Requirements

C. Charge Transport. Definition of Thin Film PV

D. Distinctive Features of Thin Film PV

References

Part II. One-Dimensional (1D) Diodes and PV. II. 1D Diode

A. Metal-Insulator-Metal Diode

B. Schottky, Reach-Through, and Field-Compensation Diodes. 1. Schottky Diode

2. Reach Through Diodes

3. Field Compensation Diode

C. P-N Homo-Junctions

D. Heterojunctions

E. Other Relevant Types of Diodes

F. Field Reversal Diode: A Counterintuitive Case

G. Cat’s Whisker Diode

III. 1D Solar Cell

A. 1D Solar Cell Base Model

B. Numerical Modeling of 1D PV

1. Governing Equations

2. Device Model Parameters

3. Some Modeling Results

IV. Photovoltaic Parameters

A. Second-Level Parameters

B. Practical Solar Cells and Third-Level Metrics

C. Indicative Facts

D. Phenomenological Interpretation. Ideal Diode with Other Circuitry Elements

V. Case Study

A. Field Reversal PV

1. Analytical Approach

2. Numerical Modeling of the Field Reversal Device Operations

B. Miraculous Back Contact

References

Part III. Beyond 1D: Lateral Effects in Thin Film PV. VI. Examples of Multidimensional Numerical Modeling

VII. Introduction to Random Multidimensional Phenomena

VIII. Lateral Screening Length

A. Shunt Screening

B. Bias Screening

C. Quantitative Approach and Linear Screening Regime

IX. Schottky Barrier Nonuniformities

X. Semi-Shunts

XI. Random Diodes

A. Weak Diodes

B. Random Diode Arrays in Solar Cells

C. Random Diode Arrays in PV Modules and Fields

XII. Nonuniformity Observations

A. Cell Level Observations

B. Module Level Observations

XIII. Nonuniformity Treatment

References

Part IV. Electronic Processes in Materials of Thin Film PV

XIV. Morphology, Fluctuations, and the Density of States. A. The Materials of Thin Film PV are Fundamentally Different

B. Noncrystalline Morphology

C. Long Range Fluctuations of Potential Energy

D. Random Potential in Very Thin Structures

E. Numerical Estimates and Implications

XV. Electronic Transport

A. Band Transport in Random Potential

B. Hopping Transport Through Thin Noncrystalline Films

1. Hopping Between Ideal Electrodes

2. Hopping Between Resistive Electrodes

3. Critical Area and Mesoscopic Fluctuations

XVI. Recombination in Quasi-Continuous Spectrum

XVII. Noncrystalline Junctions

XVIII Piezo and Pyro-PV. A. The Nature of Piezo-PV

B. Piezo-PV Observations

C. The Significance of Piezo-PV

References

Part V. Electro-Thermal Instabilities in Thin Film PV

XIX. The Two-Diode Model

A. Linear Stability Analysis

B. The Two-Diode Modeling: Numerical Estimates and Scaling

XX. Distributed Diode Model. A. Introduction

B. Linear Stability Analysis

XXI. Simplistic Numerical Modeling

XXII. Spontaneous Hot Spots. A. Introduction

B. Observations

C. Numerical Modeling. 1. Electrical Model

2. Thermal Model

D. Modeling Results

E. Approximate Analytical Model

XXIII. Related Work

XXIV. Conclusions on the Electro-Thermal Instabilities in Thin Film PV

References

Part VI. Degradation of Thin Film PV. XXV. Thin Film vs Crystalline PV Degradation Processes

XXVI. Observations

A. Cell Degradation

B. Module Degradation

XXVII. Categories of Degradation. A. General Categories

B. Thin-Film PV Instabilities. 1. Shunting Instability

2. Contact Delamination Instability

XXVIII. Accelerated Life Testing

A. Examples of Very Strong ALT: HALT

1. EBIC HALT

2. LBIC HALT

B. Actuarial Approach to ALT

C. Concluding Remarks on Degradation

References

Appendix. Some Methodological Aspects of Device Modeling

Appendix A: Model of Series Connection

Appendix B: The Diffusion Approximation

Appendix C: Long Range Potential

1. Point Charges

2. Columnar Charges

References

Index

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Another distinctive feature of thin film PV is that their forming layers are noncrystalline, but rather polycrystalline (CdTe and CIGS types of PV) or amorphous (a-Si:H based PV). That type of ‘imperfect’ morphology is inevitable with inexpensive fast deposition techniques not giving enough time for satisfactory crystallization. The polycrystalline films consist of tightly packed individual grains, each having more or less perfect crystalline order inside. They are formed during deposition and subsequent treatments where random atomic configurations are ordered within local regions dictating grain sizes. In the course of such crystallization, material pushes away all ‘foreign’ atoms and structural units towards grain boundaries (GB). (Similar to apples in a freezer where water crystallization breaks and pushes away organic tissues.) As a result, GB material is chemically different from that of interior. These chemical differences entail built-in electric fields that will spatially separate electrons and holes between the grain interior and GB (see Fig. 5) creating local electric field and potential variations. One other general consequence of independently forming grains is that having grown enough they compete for space and exert pressure on each other upon physical contact. The resulted compressive stress can translate into electric potential when the material structure is piezo-active, such as with CdS, ZnO and some others, as explained more in detail next (Sec. XVIII).

Fig. 4 In thin film devices, drift rather than diffusion can determine the kinetics of photogenerated carriers. They readily arrive at the corresponding electrodes avoiding interactions with recombination centers. Having reached the electrodes they can move to the entrances of rare but highly efficient recombination channels shorting between the electrodes and playing the role of recombination highways.

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