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A way to lower the manufacturing cost of Cu(In,Ga)Se2 (CIGS) thin film solar cells is to use flexible polymer substrates instead of glass substrates. Because such substrates require a low temperature during absorber deposition, the efficiency of the cells remains slightly lower (18.7%) compared to CIGS on glass substrates (20.3%). Partial compensation of this efficiency loss might be accomplished by Sb doping of the absorber, which is reported to have a positive effect on the morphology of this layer. In this work the defect structure of Sb doped CIGS solar cells is investigated using optical and electrical spectroscopic techniques. Experiments were performed on cells deposited on soda lime glass substrate, adding a thin Sb layer (8, 12 nm) onto the Mo back contact prior to the CIGS absorber deposition. The results are compared with those for cells without Sb doping using the same process. Fourier-Transform near infrared photocurrent measurements in the 10–300K range demonstrate that the band gap of Sb-doped samples is larger than for undoped samples. Photoluminescence spectra in the 5–100K region provide information on shallow-level defects. Deep-Level Transient Spectroscopy spectra of Sb-doped cells exhibit two features not encountered for non-doped cells: 1) a peak at lower temperature than the N1 signal and 2) incomplete charge carrier freeze-out down to 8 K. While the first result appears to be the fingerprint of an extra non-Ohmic contact in the solar cell structure, the second suggests the introduction of a very shallow acceptor by Sb doping. As a salient feature one can accurately monitor the partial hole freeze-out in the 40-60 K range and determine the signature of the intrinsic defects that provide the p-type conductivity of the CIGS absorber using Admittance Spectroscopy.

Deep Level Transient Spectroscopy (DLTS) is often used to identify and quantify defects in semiconductors. In the DLTS spectrum of CIGS (CuInGaSe2) solar cells generally two types of signals are observed : one for which the capacitance transients are measured well below room temperature (RT) labeled N1, and a second corresponding to transients measured close to RT, N2. We have recently shown that the N1 signal can be related to an non-ideal contact in the solar cell. In this work the N2 signal was investigated using isothermal DLTS (capacitance transients at constant voltage) and isothermal constant-capacitance DLTS (CC-DLTS, voltage transients at constant capacitance) experiments at RT. In CC-DLTS a non-exponential factor in the transients due to high trap concentrations is avoided. The effect of filling pulse duration (tp) on the N2 signal is studied here in detail. Both the DLTS and CC-DLTS methods point to an anomalous filling behavior. From a certain tp onwards a peak appears that shifts towards larger emission time as tp increases. The relation between these results and the energy level diagram of the metastable In-on-Cu antisite defect is discussed. DLTS experiments with variation of tp may help to understand the nature of the metastable defects causing the N2 signal.

Here, we propose the usage of dual-interface grating (DIG) supercells with multiperiodicity as shown in Fig. 1(c) and numerically study their extensive mode-coupling possibilities. We consider a-Si solar cell systems which have active layer thickness that is comparable to the size of the grating geometry. At such thickness regime, the grating structure heavily affects the eigenfield profiles of the guided modes. We show how DIG supercells offer much possibility in improving higher order diffraction coupling conditions to guided modes, while maintaining lower order diffraction coupling efficiency. In addition, including symmetry-breaking or blazing properties in DIG structures can lead to excite previously inaccessible modes efficiently, and thus further broadens the enhancement range.

The electronic structure of an organic photovoltaic bulk heterojunction cell strongly deviates from the typical textbook examples of a single sided junction used to explain electrical characterisation of defects in semiconductors. Therefore it is not so straightforward to assign the capacitance of this device or the charge in it to the presence of a depleted layer within this structure. However, conventional electronic spectroscopic techniques could give useful information to understand the electronic behaviour of the device. Therefore, in this work capacitance and charge DLTS have been performed on P3HT:PCBM solar cells. At 1MHz only negligible variation in the capacitance as a function of temperature and bias has been observed. As a result no spectrum could be recorded using a standard DLTS setup, registering the capacitance at this high frequency. To avoid this parasitic effect low frequency capacitance DLTS (40 kHz) has been performed, showing an anomalous signal with negative amplitude and an activation energy of 160meV, and a complementary positive signal could be observed altering the biases. Charge DLTS clearly revealed that both signals transients, conventional and with altered bias have the same time constants. A recent study has shown that such behaviour cannot be explained by the thermodynamic properties of capture and emission of carriers by a defect in bulk semiconductor. The validity of alternative explanations, including interface states, non-ideal ohmic contacts and effects of carrier hopping on charge mobility, will discussed.

This paper discusses how graded absorber structures in CIGS-based solar cell can be studied using the numerical simulation tool SCAPS. A model will be built for an AVANCIS solar cell with double grading which is produced with the laboratory line process. We will first discuss how literature and measurement data should be used to start the buildup of the model and afterwards give an illustration how the model then still has to be optimized. We will draw special attention to the consequences of a graded structure on the model. Moreover, we will show how one can discern the real grading benefit by comparison with a uniform reference model.