Organic Electronic Devices

Transcription

1 Organic Electronic Devices Week 4: Organic Photovoltaic Devices Lecture 4.2: Characterizing Device Parameters in OPVs Bryan W. Boudouris Chemical Engineering Purdue University 1

2 Lecture Overview and Learning Objectives Concepts to be Covered in this Lecture Segment Overview of the External Quantum Efficiency (EQE) of a OPV Device and How to Calculate an Air Mass (AM) Value Demonstration of the Equivalent Circuit Diagram Associated with an OPV Device in the Dark and Under Illumination Introduction to the Shockley Equation and How Parameters from This Equation Affect OPV Device Performance Learning Objectives By the Conclusion of this Presentation, You Should be Able to: 1. Explain the benefit of obtaining an EQE spectrum and how it is related to the short-circuit current density of a device. 2. Calculate the Air Mass (AM) value of incoming radiation given certain physical parameters. 3. Estimate the series resistance and shunt resistance of an OPV device given a performance curve.

3 The Type of Light Used is Important in OPV Characterization The Power Input into the Device, Can Be Any Number of Values. η + SC VOC FF P in It is Important to Know Exactly Both the Intensity and the Wavelength of the Light that Are Input During Device Characterization. There are a Number of Standard Spectra for Testing OPV Devices, and We Will Discuss 2: 15 dark light (1) AM0 and (2) AM V max Open circuit voltage (V oc ) max max power point -15 Short circuit current ( sc )

4 External Quantum Efficiency (EQE) Shows Wavelength Dependence Characterizing How the OPV Device Collects Charges Generated from Different Wavelengths of Light is Important to Device Evaluation This is Quantified by the External Quantum Efficiency (EQE), which is Sometimes Called the Incident Photon-to-Current Efficiency (IPCE) Collected Charges at a Given Wavelength EQE Photons in at a Given Wavelength sc q η EQE P0 λ h c If the EQE is Integrated Overall All Wavelengths, It Should Return sc λ 2 q λ Intensity of the Solar Spectrum at a ηeqe ( λ) S( λ dλ S(λ) Given Air Mass Condition hc sc ) λ 1

5 Quantification of the Air Mass If We Take the Earth as a Simple Sphere with an Atmospheric Shell, Then the Air Mass (AM) Can Be Estimated AM R r y E atm ( r cos z) km 9 km 2r r cos z where z Solar Zenith Angle If We Extended the Atmosphere to a Large Value (i.e., to Outer Space) Then the Spectrum in Space is Called AM0. The Standardized Reference for Solar Cell Testing in the Research Laboratory (and for the Comparison of World Record Efficiencies) is AM1.5. This is Because It Is a Reasonable Average for the Lower 48 USA States and Most of Central Europe of the Course of a Year. The intensity of AM1.5 Light is 100 mw cm -2.

6 Amount of Atmosphere Affects Incoming Photons Available Note the dips in the solar spectrum for the AM1.5 versus the AM0. This is due to absorption from oxygen, water, and carbon dioxide in the atmosphere.

7 An OPV Device is ust a Diode-Like Structure Device Schematic Equivalent Circuit Diagram Collecting Contact Organic Semiconductor Transparent Conductor The diode only allows charge to be passed in one direction. The series resistance (R s ) is the resistance to charge flow through the organic semiconductor. This should be low. The shunt resistance (R sh ) is the resistance to charge flow through other pathways besides the organic semiconductor. This should be high. The photocurrent is in the opposite direction of the diode current.

8 Shockley Equation Models Current Density in the Device Device Schematic Equivalent Circuit Diagram Collecting Contact Organic Semiconductor Transparent Conductor The Current in the Circuit Can be Expressed Using the Shockley Equation Total Total Dark S Light q exp ( V R ) n kt s Diode Equation with Infinite R sh V 1 + R R sh s Photocurrent Finite R sh Term Photo s n Reverse Bias Saturation Dark Current Density Diode Ideality Factor

9 The Open-Circuit Voltage Can Be Predicted as Well The Current in the Circuit Can be Expressed Using the Shockley Equation Total Total Dark S Light q exp ( V R ) n kt s 1 + V R R sh s s n Photo Reverse Bias Saturation Dark Current Density Diode Ideality Factor This Equation Can Be Rearranged to Show That: V oc n kt photo ln + 1 q s Therefore, the reverse bias current density in the dark needs to be minimized in order to optimize the open-circuit voltage during OPV operation under solar illumination.

10 Decreasing the Series Resistance Is Good for OPV Devices Increasing the Series Resistance Will: Effect of R s on OPV Curves 1) Decrease the Short-Circuit Current Density and 2) Decrease the Fill Factor One quick way to estimate the series resistance is to measure 2 current density-voltage (-V) curves at different white light input powers (P 1 and P 2 ) Then R s May Be Estimated As: R S ( V ) ( V ) oc 2 oc ( sc ) 1 ( sc ) 1 2 A Simpler Method for Estimating the Series Resistance is to Take the Inverse of the Slope of the -V Curve as the Curve Approaches the Open- Circuit Voltage Value.

11 Increasing the Shunt Resistance Is Good for OPV Devices Increasing the Series Resistance Will: Effect of R sh on OPV Curves 1) Increase the Open-Circuit Voltage and 2) Increase the Fill Factor One quick way to estimate the shunt resistance is to measure the inverse of the slope of the -V curve in reverse basis (RB). Then R sh May Be Estimated As: R Sh V RB RB A Simpler Method for Estimating the Shunt Resistance is to Take the Inverse of the Slope of the -V Curve as the Curve Approaches the Short- Circuit Density Value.

12 Summary and Preview of the Next Lecture The incoming radiation is an important consideration in the design of OPV materials and devices. In order to quantify how the organic semiconductors will respond to different wavelengths of light, external quantum efficiency (EQE) measurements are often taken. Furthermore, the emission spectrum of the Sun (as viewed from various locations on (and above) the Earth will change which wavelengths of light will be of avail to the solar cell during operation. As such, these parameters must be quantified rigorously. Next Time: Nanostructural Impacts in OPV Devices Application of the Shockley equation allows for the performance of the OPV devices to be understood and predicted, in some cases. By controlling the series and shunt resistances, the performance of the OPV device can be improved. This requires being able to optimize both the device structure and the materials of the device as well.