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7. Photoconductivity in Materials Research C Stephen Reynolds, Monica Brinza, Mohammed L 151 Photocondu7. Photoconductivity in Materials Research c Stephen Reynolds, Monica Brinza, Mohammed L. Benkhedir, Guy J. Adriaenssens 7.1 Steady-State Photoconductivity (SSPC) . 153 Photoconductivity is the incremental change in 7.1.1 Definitions and Overview .................... 153 the electrical conductivity of a semiconductor or 7.1.2 Example Applications insulator upon illumination. The behavior of pho- in Materials Research .......................... 154 toconductivity with photon energy, light intensity and temperature, and its time evolution and fre- 7.2 Constant Photocurrent Method (CPM) quency dependence, can reveal a great deal about and Related Techniques ..................... 157 carrier generation, transport and recombination 7.2.1 CPM ................................................... 157 processes. Many of these processes now have 7.2.2 Dual Beam Photoconductivity (DBP)...... 158 a sound theoretical basis and so it is possible, 7.2.3 Fourier-Transform Photocurrent with due caution, to use photoconductivity as Spectroscopy (FTPS) ............................. 159 a diagnostic tool in the study of new electronic 7.3 Steady-State Photocarrier Grating materials and devices. This chapter describes the Method (SSPG).................................... 160 main steady-state and transient photoconduc- 7.4 Modulated Photocurrent Spectroscopy tivity techniques applied in the investigation of (MPC) ................................................. 161 semiconductors whose performance is limited by 7.4.1 MPC Background and Experiment ......... 161 the presence of localized electronic states. These 7.4.2 MPC Density of States Analysis.............. 162 materials tend to be disordered, and possess low 7.4.3 MPC Applications ................................ 163 carrier mobilities and short free-carrier lifetimes in comparison with crystalline silicon. They are 7.5 Switch-on and Switch-off Transients .. 164 often prepared as thin films, and are of interest 7.5.1 Switch-on Transient............................ 164 for large-area applications, for example in solar 7.5.2 Switch-off Transient............................ 165 cells, display backplane transistors, photoemis- 7.6 Transient Photocurrent Spectroscopy sive devices such as organic light-emitting diodes (TPC) .................................................. 166 (OLEDs) and medical imagers. However, examples 7.6.1 TPC Principles ..................................... 166 of where these techniques have been useful in the 7.6.2 TPC Experiment................................... 166 Part A | 7 study of defective crystalline semiconductors are 7.6.3 TPC Density-of-States Analysis ............. 167 also given. The approach followed here is by way 7.6.4 TPC Applications.................................. 168 of an introduction to the techniques, the physics 7.7 Time-of-Flight (TOF) supporting them, and their applications, it being and Related Techniques ..................... 168 understood that readers requiring more detailed 7.7.1 TOF Mobility and DOS Measurements..... 168 information will consult the references provided. 7.7.2 Interrupted Field TOF (IFTOF)................. 170 7.8 Other Photoconductivity-Related Techniques ........................................ 171 7.8.1 Surface Photovoltage (SPV)................... 171 7.8.2 Spin-Dependent Recombination .......... 171 7.8.3 Time-Resolved Microwave Conductivity (TRMC)................................................ 171 References................................................... 172 © Springer International Publishing AG 2017 S. Kasap, P. Capper (Eds.), Springer Handbook of Electronic and Photonic Materials, DOI 10.1007/978-3-319-48933-9_7 152 Part A Fundamental Properties Photoconductivity, defined as the increase in conduc- include: tivity of a material resulting from the absorption of Optical absorption coefficient optical photons, has traditionally played a significant Quantum efficiency role in materials research, most notably in the study of Carrier mobility covalently bonded semiconductors and insulators. The Carrier lifetime basic processes that govern the magnitude of the pho- Mobility-lifetime products tocurrent are the generation of free electrons and holes Minority carrier diffusion length through the absorption of incident photons, their trans- Density of localized electronic states and their cap- port through the material under the influence of an elec- ture and emission properties tric field, and their recombination. The study of any of Charge state and energetic location and spatial vari- those aspects as a function of illumination, temperature ations in transport properties. and field strength, and their development over time, will offer insights into the structure and electronic proper- The information obtained need not be specific to ties of the material under investigation. However, given the method, and will depend on the wider context that several processes may be involved in the production of the measurements. Recombination can be studied of a specific photocurrent, a sufficiently comprehensive via photocurrent decay from the steady state, but the dataset is needed to differentiate between alternative in- intensity and temperature dependence of steady-state terpretations. For instance, a low photocurrent may be photoconductivity can also be used to identify different the result of a low optical absorption coefficient at the recombination mechanisms, while details of the density given photon energy, but it may also be due to gemi- of states in the bandgap of a semiconductor can be in- nate recombination of the photogenerated electron–hole ferred both from the spectral response and temperature pairs, or it may reflect the formation of excitons. Photo- dependence of the steady-state photoconductivity and generated charge needs to be transported in a material a detailed analysis of transient photoconductivity. to realize a current, a process which depends upon This chapter focuses on the main photoconductivity the effective carrier mobility. This can vary by up to techniques applied in the investigation of semiconduc- ten orders of magnitude; from around 104 cm2=.Vs/ in tors whose performance is limited by the presence of semiconductors such as crystalline Si and GaAs where localized electronic states. Many are deposited rapidly band transport dominates, to 106 cm2=.Vs/ or less in as thin films, are structurally disordered and tend to disordered inorganic and organic semiconductors where possess low carrier mobilities and short free-carrier transport is via low-mobility extended state conduction lifetimes compared to benchmark crystalline silicon. mediated by carrier traps, or by carrier hopping between Research has been stimulated by their highly successful localized states, or by a combination of such processes commercial application for over 70 years, in xerogra- depending on temperature and excitation regimes. phy, solar energy, large-area displays and medical imag- Given the complexity of these processes, use of ing. Photocopiers are a good example of the evolution Part A | 7 a range of photoconductivity experiments is often ad- of thin-film photoconductors, beginning with photore- visable, as is the combination of photoconductivity with ceptor drums coated with selenium, through amorphous complementary techniques such as optical absorption, silicon, to organic semiconductors. Each of these mate- photoluminescence, dark current activation, charge col- rials has been intensively researched with regard to their lection, electron paramagnetic resonance and various charge generation and retention properties, using photo- pump-probe experiments. A robust model should seek conductivity methods. to satisfy all observations, as rationally as possible. Finally, mention should be made of the use of com- From a slightly different perspective, one of the major puter simulations in photoconductivity studies. This has tasks of the electronic materials scientist is to attempt to been an active complementary field of research for over correlate material (and device) preparation conditions 50 years, and its value in developing a coherent un- and micro/nanostructure with electronic properties. It derstanding of the physics of photoconductivity, and follows that structural information, from analytical electronic transport in general, is considerable. The ex- tools such as electron and scanning probe microscopies, periments described herein furnish data that can be x-ray and electron diffraction and Raman spectroscopy, integrated into, or compared with the predictions of, is also of key importance. computer simulations of physical processes, and it is of- A variety of experimental techniques based on pho- ten by closing the loop in this way that confirmation of toconductivity have evolved. The information that can the validity and limitations of proposed physical mod- be obtained relates to the mechanisms of generation and els or data analysis schemes can best be achieved. In recombination of carriers and their transport by drift a broader context, simulations are now used routinely and diffusion. The physical parameters to be extracted in the development of solar cells and other devices. Photoconductivity in Materials Research 7.1 Steady-State Photoconductivity (SSPC) 153 This book contains several chapters that strongly provided in references [7.5–7]. Background and proce- support this work, and are referred to below. Compre- dures for computer modeling of solar cells, which has hensive accounts of the principles of
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