J. David Cohen

Bulk and metastable defects in CuIn1−xGaxSe2 thin films using drive-level capacitance profiling

The drive-level capacitance profiling technique has been applied to ZnO/CdS/CuIn1−xGaxSe2/Mo solar cell devices, in order to study properties of defects in the CuIn1−xGaxSe2 film. Properties studied include the spatial uniformity, bulk defect response, carrier density, and light-induced metastable effects. These results indicate that previous estimates of carrier densities, from C–V profiling, may be significantly overestimated. In addition, a defect response previously thought to be located at the interface is observed to exist throughout the bulk material. Finally, an infrared light-soaking treatment is demonstrated to induce metastable changes in the bulk CuIn1−xGaxSe2 film. Hence, the drive-level capacitance profiling technique provides valuable insights into these films. Herein, the technique itself is fully explained, compared to other junction capacitance methods, and its utility is demonstrated using numerical simulation.

Electronic properties of the Cu2ZnSn(Se,S)4 absorber layer in solar cells as revealed by admittance spectroscopy and related methods

Admittance spectra and drive-level-capacitance profiles of several high performance Cu2ZnSn(Se,S)4 (CZTSSe) solar cells with bandgap ∼1.0–1.5 eV are reported. In contrast to the case for Cu(In,Ga)(S,Se)2, the CZTSSe capacitance spectra exhibit a dielectric freeze out to the geometric capacitance plateau at moderately low frequencies and intermediate temperatures (120–200 K). These spectra reveal important information regarding the bulk properties of the CZTSSe films, such as the dielectric constant and a dominant acceptor with energy level of 0.13–0.2 eV depending on the bandgap. This deep acceptor leads to a carrier freeze out effect that quenches the CZTSSe fill factor and efficiency at low temperatures.

HIGH PERFORMANCE GLOW DISCHARGE A-SI1-XGEX:H OF LARGE X

Radio frequency glow discharge chemical vapor deposition has been used to deposit thin films of a-Si1−xGex:H which possess optoelectronic properties that are greatly improved over any yet reported in the range of x⩾0.6. These films were deposited on the cathode (cathodic deposition) of an rf discharge. Their properties are assessed using a large variety of measurements and by comparison to the properties of alloys conventionally prepared on the anode (anodic deposition). Steady state photoconductivity measurements yield a quantum-efficiency-mobility-lifetime product, ημτ, of (1–3)×10−7 cm2 V−1 for 1.00⩾x⩾0.75 and (6–10)×10−8 cm2 V−1 for 0.75⩾x⩾0.50, and photocarrier grating measurements yield ambipolar diffusion lengths several times greater than previously obtained for alloys of large x. It is confirmed that the improvements in phototransport are not due to a shift in the Fermi level. In fact, results of recent measurements on lightly doped samples strongly suggest that for these cathodic alloys neither ...

Band offsets and deep defect distribution in hydrogenated amorphous silicon-crystalline silicon heterostructures

Voltage filling pulse measurements taken on a‐Si:H/c‐Si heterostructure Schottky diode samples are used to examine the capture of electrons from the c‐Si substrate into a‐Si:H defect states. These measurements, along with the capacitance versus temperature spectra of these diodes, indicate a nearly zero conduction‐band offset (50±50 meV). In addition, we have observed trapping of holes at the valence‐band discontinuity ΔEv. A clear threshold for the subsequent optical release of these holes yields a value of ΔEv =0.58±0.02 eV. Our measurements also provide the energy and spatial distribution of deep defects within the a‐Si:H layer and indicate a region of anomalously large defect density (1018 cm−3) within roughly 350 A of the a‐Si:H/c‐Si interface.

Mapping out the distribution of electronic states in the mobility gap of amorphous zinc tin oxide

Amorphous zinc tin oxide (ZTO) is a wide-band-gap (transparent) semiconductor which exhibits high electron mobilities irrespective of its disordered nature. Transient photocapacitance (TPC), drive level capacitance profiling (DLCP), and modulated photocurrent spectroscopy (MPC) were used to determine the electronic state distribution within the mobility gap of ZTO. Conduction band-tail and valence band-tail Urbach energies near 10 and 110 meV were obtained by MPC and TPC, respectively. DLCP indicated free carrier densities in the mid-1015 cm−3 range plus a 0.2eV wide band of defects 0.4eV from the conduction band. The MPC spectra for ZTO also disclosed a defect band near the conduction band-tail.

Electronically active defects in the Cu2ZnSn(Se,S)4 alloys as revealed by transient photocapacitance spectroscopy

Transient photocapacitance (TPC) spectra were obtained on a series of Cu2ZnSn(Se,S)4 absorber devices with varying Se:S ratios, providing bandgaps (Eg) between 1 eV and 1.5 eV. Efficiencies varied between 8.3% and 9.3% for devices with Eg ≤ 1.2 eV and were near 6.5% for devices with Eg ≥ 1.4 eV. The TPC spectra revealed a band-tail region with Urbach energies at or below 18 meV for the first group, but in the 25-30 meV range for the higher band-gap samples. A deeper defect band centered near 0.8 eV was also observed in most samples. We identified a correlation between the Urbach energies and the voltage deficit in these devices.

Deep defect structure and carrier dynamics in amorphous silicon and silicon-germanium alloys determined by transient photocapacitance methods

A detailed comparison between transient junction photocurrent and photocapacitance spectra can be used to examine separately the majority and minority carrier processes in amorphous semiconductors. Such methods are employed both on intrinsic samples of hydrogenated amorphous silicon (a-Si:H) and also amorphous silicon-germanium alloys (a-Si, Ge:H) with a Tauc gap near 1.3 eV. It is demonstrated how this method can be used not only to map out the deep defect distribution in such samples, but also to determine the effective μτ products for the minority carrier motion.

Defect bands in a-Si–Ge:H alloys with low Ge content

Abstract We have studied a series of optimized glow discharge a-Si1−xGex:H films with Ge content 0.02⩽x⩽0.2. The drive level capacitance profiling method indicated defect densities 5×10 15 cm −3 in these samples. Modulated photocurrent measurements detected two defect bands at 0.68±0.05 and 0.79±0.05 eV, compared to the single band normally observed in pure a-Si:H. The magnitudes of these bands vary with Ge content and with the state of light-induced degradation. Based on electron spin resonance (ESR) measurements on matched samples we have identified these defect bands as Si and Ge neutral dangling bonds. The Ge dangling bond concentration was found to be larger in the annealed state for all samples, even at 2 at.% Ge. However, in the light-induced degraded state the density of Si dangling bonds was larger for all the alloys in this series.

Chapter 2 Density of States from Junction Measurements in Hydrogenated Amorphous Silicon

Publisher Summary This chapter discusses the determination of the density of states by several techniques: field effect, capacitance, and deep level transient spectroscopy (DLTS). It defines the limitations of each method and describes the sophistication resorted to circumvent the limitations. The field-effect method is very sensitive to surface or interfacial conditions and therefore is not suitable to probe bulk properties. Capacitance–voltage coupled to capacitance–temperature and capacitance–frequency measurements is a powerful technique that is suitable to study undoped a-Si:H. In DLTS, repetitive pulses are applied to a diode from forward or zero bias to reverse bias. During forward bias the traps are filled. They empty thermally during reverse bias, thus changing the capacitance. The transient response of capacitance reflects the emptying of deep levels. The DLTS signal, the difference in capacitance at two different times ∆ t apart, is maximum when ∆ t is equal to the temperature-dependent trap-emptying time. An algorithm that is particularly suitable in the case of a quasi-continuous distribution was developed to convert DLTS data into a density of states. Depth profiling is possible by varying the depletion width via the reverse bias. Several variations of the DLTS method are presented.

Photo-induced changes in the bulk density of gap states in hydrogenated amorphous silicon associated with the Staebler-Wronski effect☆

Abstract We observe a reversible photo-induced modification in the bulk density of gap states in hydrogenated amorphous silicon associated with the Staebler-Wronski effect. Detailed analysis of deep level transient spectroscopy, admittance and thermally stimulated capacitance measurements indicates that as a result of prolonged illumination (1) a lowering of the bulk Fermi level occurs in the mobility gap, (2) a large increase occurs in the density of states below midgap and (3) no discernible change occurs in the midgap defect density normally associated with dangling bond defects. In addition we find that the annealing behavior of the light-induced effects indicates a broad range of activation energies. A comparison of these findings with measurements made by other groups is discussed briefly.