3 Solar Cells: Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Arvind Shah
3.1 The Photovoltaic Effect: Interaction of Light and Matter . . . . . . 37
3.2 Conversion of Light into Electrical Carriers
by a Semi-conductor Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.1 Absorption and Energy Conversion of a Photon . . . . . . 38
3.2.2 Direct, Non-direct and Indirect Band-Gaps . . . . . . . . . . 40
3.2.3 Spectrum of the Incoming Light . . . . . . . . . . . . . . . . . 41
3.2.4 Relationship Between Light Spectrum and
Semiconductor Bandgap . . . . . . . . . . . . . . . . . . . . . . . 41
3.3 Separation of Electrons and Holes: The Solar Cell as Diode . . . 44
3.4 Solar Cell Characteristics, Equivalent Circuits and Key
Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.4.1 Dark Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.4.2 Characteristics Under the Influence of Light . . . . . . . . . 49
3.4.3 A Remark About the Theoretical Fundaments
of the Basic Solar Cell Equations . . . . . . . . . . . . . . . . 50
3.4.4 Equivalent Circuits for the Solar Cell . . . . . . . . . . . . . 52
3.4.5 Key Parameters of the Solar Cell . . . . . . . . . . . . . . . . 54
3.5 Solar Cell Efficiency Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.5.1 Limits at STC (Standard Test Conditions) . . . . . . . . . . 58
3.5.2 Variation of Efficiency η in Function
of Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.5.3 Variation of Efficiency η in Function
of Light Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.6 Spectral Response and Quantum Efficiency in Solar Cells . . . . . 66
3.6.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.6.2 Typical Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.6.3 Practical Consequences . . . . . . . . . . . . . . . . . . . . . . . . 69
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4 Solar Cells: Optical and Recombination Losses . . . . . . . . . . . . . . . 73
Sylvère Leu and Detlef Sontag
4.1 Optical Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.1.1 Preliminary Remarks: Reflection, Refraction,
Absorption and Transmission . . . . . . . . . . . . . . . . . . . 74
4.1.2 Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.1.3 Front Side: Avoiding Reflection with Optimized
Surface Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.1.4 Additional Considerations: How to Increase Light
Trapping Further . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.1.5 Wafering and Its Effects on Light Trapping . . . . . . . . . 83
4.1.6 Texturing of the Front Surface . . . . . . . . . . . . . . . . . . 84
4.1.7 Passivation of the Back Surface and Mirror Formation
at the Back . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
xii
Contents
Arvind Shah
3.1 The Photovoltaic Effect: Interaction of Light and Matter . . . . . . 37
3.2 Conversion of Light into Electrical Carriers
by a Semi-conductor Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.1 Absorption and Energy Conversion of a Photon . . . . . . 38
3.2.2 Direct, Non-direct and Indirect Band-Gaps . . . . . . . . . . 40
3.2.3 Spectrum of the Incoming Light . . . . . . . . . . . . . . . . . 41
3.2.4 Relationship Between Light Spectrum and
Semiconductor Bandgap . . . . . . . . . . . . . . . . . . . . . . . 41
3.3 Separation of Electrons and Holes: The Solar Cell as Diode . . . 44
3.4 Solar Cell Characteristics, Equivalent Circuits and Key
Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.4.1 Dark Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.4.2 Characteristics Under the Influence of Light . . . . . . . . . 49
3.4.3 A Remark About the Theoretical Fundaments
of the Basic Solar Cell Equations . . . . . . . . . . . . . . . . 50
3.4.4 Equivalent Circuits for the Solar Cell . . . . . . . . . . . . . 52
3.4.5 Key Parameters of the Solar Cell . . . . . . . . . . . . . . . . 54
3.5 Solar Cell Efficiency Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.5.1 Limits at STC (Standard Test Conditions) . . . . . . . . . . 58
3.5.2 Variation of Efficiency η in Function
of Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.5.3 Variation of Efficiency η in Function
of Light Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.6 Spectral Response and Quantum Efficiency in Solar Cells . . . . . 66
3.6.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.6.2 Typical Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.6.3 Practical Consequences . . . . . . . . . . . . . . . . . . . . . . . . 69
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4 Solar Cells: Optical and Recombination Losses . . . . . . . . . . . . . . . 73
Sylvère Leu and Detlef Sontag
4.1 Optical Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.1.1 Preliminary Remarks: Reflection, Refraction,
Absorption and Transmission . . . . . . . . . . . . . . . . . . . 74
4.1.2 Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.1.3 Front Side: Avoiding Reflection with Optimized
Surface Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.1.4 Additional Considerations: How to Increase Light
Trapping Further . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.1.5 Wafering and Its Effects on Light Trapping . . . . . . . . . 83
4.1.6 Texturing of the Front Surface . . . . . . . . . . . . . . . . . . 84
4.1.7 Passivation of the Back Surface and Mirror Formation
at the Back . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
xii
Contents