• Energy Research

A model is derived for the frequency and time domain optoelectronic response of perovskite solar cells (PSCs) that emphasizes the role of charge carrier exchange, i.e., extraction and injection, from (to) the perovskite through the transport layer to (from) the collecting electrode. This process is described by a charge carrier exchange velocity that depends on the mobility and electric field inside the transport layer. The losses implied by this process are modeled in an equivalent circuit model in the form of a voltage-dependent transport layer resistance. The analysis of the model predicts that the voltage dependence of the measured time constants allows discriminating situations where the transport layer properties dominate the experimental response. Application of this method to experimental impedance spectroscopy data identifies charge extraction velocities between 1 and 100 cm/s under 1-sun open-circuit conditions for p-i-n PSCs with poly(triaryl amine) as the hole transport layer; this corresponds to transport layer mobilities between 10−4 and 3 × 10−3 cm2 V−1 s−1. The model paves the way for an accurate estimation of the photocurrent and fill factor losses in PSCs caused by low mobilities in the transport layers, using small-perturbation measurements in the time and frequency domain. PRX energy 2(3), 033006 (2023). doi:10.1103/PRXEnergy.2.033006 Published by American Physical Society, College Park, MD

We applied a recently published electroluminescence method for calculating the lateral bandgap distribution to copper indium gallium (di) selenide (CIGS) modules of three manufacturers. First, we used a CIGS module with a known apparent bandgap distribution to calibrate a bandgap imaging method at the electroluminescence setup at the University of Ljubljana. Results of the method newly calibrated at the system are in agreement with the known apparent bandgap distribution. Image analysis of ten CIGS modules of three manufacturers reveals spatial bandgap fluctuations of different types and intensities across the modules. Average apparent bandgap and its standard deviation vary from 0.99 to 1.14 eV and 4 to 22 meV, respectively. Observed fluctuation patterns often match between the modules of the same manufacturer, indicating that the fluctuations originate from systematic spatial variations in the production process. Variety of fluctuations indicates that they should be investigated both between different module types and within a single type. A standard deviation of 22 meV leads to an estimated loss in the efficiency of 0.26% (absolute); thus, such bandgap fluctuations constitute an optimization potential for the manufacturer.

Nanophotonic light management concepts are essential building blocks of advanced thin-film solar cells. These concepts make use of light coupling to waveguide modes that are supported by the photoactive absorber material of the solar cell. In a recent study, we presented a new method based on scanning near-field optical microscopy that enables the direct nanoscale investigation of light coupling to an individual waveguide mode in a nanophotonic thin-film silicon solar cell. Making use of this method, we investigate in this contribution the polarization dependence of the light coupling to a waveguide mode. Based on this polarization dependence, we can attribute the investigated waveguide mode to a transverse electric mode. Moreover, we identify the grating vector, which is responsible for the light coupling to the investigated waveguide mode in the nanopatterned thin-film silicon solar cell.

ABSTRACTLightweight photovoltaic applications are essential for diversifying the solar energy supply. This opens up vast new scenarios for solar modules and significantly boosts the capacity of renewable energy. To ensure high efficiency and stability of the solar modules, several challenges need to be overcome. Degradation due to elevated temperature and/or humidity is a critical concern for silicon heterojunction (SHJ) solar modules. Here, we investigated the stability and degradation mechanism of encapsulated cells with lightweight configurations where the cells are based on three different types of transparent‐conductive oxide (TCO): indium tin oxide (ITO), aluminum‐doped zinc oxide (AZO), and a combination of ITO/AZO/ITO under humid and thermal environmental conditions. A damp heat (DH) test at a temperature of 85°C and relative humidity (RH) of 85% was performed on lightweight modules for 1000 h. Our results show that AZO is the most susceptible to DH degradation. The AZO film was damaged by the combined effects of moisture ingress and delamination of the interconnection foil, resulting in a decrease in the conductivity of the AZO film, leading to a dramatic increase in Rs and a decrease in FF of the modules. Consequently, moisture has a greater chance of percolating through the damaged AZO layer into the a‐Si:H passivation layer, causing passivation degradation, which leads to an increase in recombination, resulting in a decrease in Voc of the modules. In particular, after capping the AZO film with an ITO film, the efficiency loss of the ITO/AZO/ITO module was significantly reduced. This suggests that the ITO film could be a promising protective capping layer for the AZO‐based solar cells.

Abstract This paper shows that the combination of both dark lock-in thermography (DLIT) and electroluminescence (EL) imaging techniques is especially suitable for in-depth shunt analysis of industrial Cu(In,Ga)Se 2 (CIGS) thin-film modules and for the quantitative analysis of the local electrical cell properties, such as the internal junction voltage or the series resistances of the front and back contact. First results obtained for amorphous silicon ( a -Si:H) based thin-film solar modules reveal that the quantitative EL analysis method is applicable to amorphous silicon technology as well. Using the consistent modeling of EL and DLIT images using a SPICE based model with input data generated from EL and DLIT measurements, we demonstrate that we can obtain properties of solar modules like the spatial voltage distribution, the resistances of the front and back contact, and the shunt resistance of local defects. We furthermore discuss two newly developed techniques, namely voltage-modulated lock-in thermography at the maximum power point (MPP-LIT) of an illuminated cell or module as well as differential EL analysis of illuminated cells and modules.

We investigate the influence of surface texture on the effectiveness of reflectance by intermediate reflectors in thin-film tandem solar cells. Two distinct angular reflection regimes are identified. For large surfaces with large aspect ratios, frustrated total internal reflection or photon tunneling is found to be the dominating mechanism. We show that the parasitic reflection losses of commonly used randomly textured surfaces are explained by these distinct regimes and introduce a spectral reflectance parameter, which serves as a guideline for designing optimized spectrally selective intermediate reflectors.

In this work, we present a method to study thermal runaway effects in thin-film solar cells. Partial shading of solar cells often leads to permanent damage to shaded cells and degrades the performance of solar modules over time. Under partial shading, the shaded cells may experience a reverse bias junction breakdown. In large-area devices such as solar cells, this junction breakdown tends to take place very locally, thus leading to very local heating and so-called “hot-spots”. Previously, it was shown that a positive feedback effect exists in Cu(In,Ga)Se2 (CIGS) thin-film solar cells, where a highly localized power dissipation is amplified, which may lead to an unstable thermal runaway process. Furthermore, we introduced a novel characterization technique, laser induced Hot-Spot Lock-In Thermography (HS-LIT), which visualizes the positive feedback effect. In this paper, we present a modified HS-LIT technique that allows us to quantify directly a loop-gain for hot-spot formation. By quantifying the loop-gain we obtain a direct measure of how unstable a local hot-spot is, which allows the non-destructive study of hot-spot formation under various conditions and in various cells and cell types. We discuss the modified HS-LIT setup for the direct measurement of the loop-gain. Furthermore, we demonstrate the new method by measuring the loop-gain of the thermal runaway effect in a CIGS solar cell as a function of reverse bias voltage.

Abstract The paper analyses the electronic transport of high-efficiency silicon solar cells with high-quality back contacts that use a sequence of amorphous (a-Si) and microcrystalline (μc-Si) silicon layers prepared at a maximum temperature of 220 °C. Our best solar cells having diffused emitters with random texture and full-area a-Si/μc-Si contacts have an independently confirmed efficiency of 21.0%. An alternative concept uses a simplified a-Si layer sequence combined with Al-point contacts and yields a confirmed efficiency of 19.3%. Analysis of the internal quantum efficiency (IQE) shows that both types of back contacts lead to effective diffusion lengths Leff exceeding the wafer thickness considerably. Fill factor limitations for the full area contacts result from non-ideal diode behavior, possibly due to the injection dependence of the interface recombination velocity.